Bubble Pump Tesis Stutgar

8/13/2019 Bubble Pump Tesis Stutgar

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Experimental and Theoretical Studies on a BubblePump

for a Diffusion-Absorption Refrigeration System

Project work completed for the award of the degree of

Master of Technologyin

Mechanical Engineering

by

ABHIJIT SATHE

at

Institut fr Thermodynamik undWrmetechnik

Universitt StuttgartGermany

Refrigeration and Air-Conditioning LaboratoryDepartment of Mechanical EngineeringIndian Institute of Technology Madras

India

Homepage: http://www.geocities.com/abhijitsathe

Master's thesis work of Abhijit Sathe (e-mail: [email protected])


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List of Contents

Abstract

Acknowledgement

INTRODUCTIONThe Diffusion-Absorption Refrigeration CycleThe Bubble PumpTwo Phase FlowLiterature Review

MATHEMATICAL ANALYSISThe Maximum Pump Tube DiameterMathematical Model

SELECTION OF PROPERTIESSelection of Working FluidSelection of DimensionsSelection of Other Parameters

EXPERIMENTAL SET-UPOverall Set-up DescriptionDetailed Component DescriptionTest Procedure

RESULTS AND DISCUSSIONSObservations with a Transperant Bubble Pump TubeEvaluation of Bubble Pump Operating ParametersComparison with the Mathematical ModelConclusion

List of Symbols

References

Appendix




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ABSTRACT

A diffusion-absorption refrigeration cycle or a pumpless vapour absorption refrigeration cycle holds a

great significance in noiseless refrigeration applications. The diffusion-absorption cycle is unique in that itruns without any mechanical work input. The cycle utilizes ammonia-water-hydrogen as working fluids.

The diffusion-absorption cycle relies on a bubble pump to pump the solution from the absorber to theboiler. A bubble pump is a fluid pump that operates on thermal energy to pump liquid from lower level tothe higher level. It does not contain any moving parts. The bubble pump operates on the same principlethat lifts coffee to the top of a coffee percolator. The liquid in the liquid reservoir initially fills the tube tothe same level (h). Heat is applied at the bottom of the tube at a rate sufficient to boil some of the liquidin the tube. The resulting vapour bubbles rise in the tube. Due to the small diameter of the pump tube,the vapour bubbles occupy complete cross-section of the tube and are separated by small liquid slugs.Each bubble acts as a gas piston and lifts the corresponding liquid slug to the top of the pump tube. Thebubble pump operates most efficiently in the slug flow regime in which the vapour bubbles areapproximately the diameter of the tube. The important parameters of the bubble pump are pump tube

diameter (dp), driving head (h), pump lift (L) and pump heat input (Q

p).

The bubble pump was built and tested in a test-rig. The test-rig did not comprise of a refrigerationsystem. The working fluid used was methyl alcohol (methanol). Methyl alcohol has a boiling point of 64

C which was suitable for the given set-up. It is non-reactive with copper at all temperatures. Liquidmethanol was stored in a liquid reservoir. It was first pre-heated to the saturation temperature in a liquidpre-heater. Heat was supplied at the bottom of the bubble pump tube by means of an electrical heater. Asmall portion of liquid boiled off and the remaining liquid was lifted to the top by the rising vapourbubbles. The liquid that was pumped by the bubble pump was separated from the accompanying vapourbubbles in a liquid-vapour separator. The vapour was condensed and the flow rate of the condensate wasmeasured. Flow rate of the pumped liquid was also measured separately. The bubble pump was testedextensively for varying heat inputs and different pump tube diameters and driving heads at constantambient pressure. The influence of these parameters on the flow rate of the pumped liquid is discussed indetail. Pumping ratio is another important parameter to judge the performance of the bubble pump. The

variation of the pumping ratio with the pump heat input for different driving heads and different pumptube diameters is also discussed.The frequency of pumping action is observed to increase with increase in pump heat input. The massflow rate of the vapour increases linearly with the heat input while the mass flow rate of the pumpedliquid first increases, attains a maximum value and then decreases with increase in the heat input. Thereexists an optimum value of the heat input for each bubble pump where the pump renders the maximumamount of pumped liquid. This value of heat input increases with increase in the pump tube diameter.The pumping ratio decreases almost linearly with the heat input. Submergence ratio, defined as a ratio ofdriving head to pump lift, also influences the pump behavior. Higher the submergence ratio, more is theamount of the pumped liquid for the same heat input.

A mathematical model of the bubble pump is established by using simple analytical equations such as the

continuity equation and the momentum equation. The model assumes slug flow in the bubble pump. Themodel is compared with the experimental results. A correction factor is necessary to account for thediscrepancies observed between the actual experimental observations and the assumptions made in thetheoretical studies. The correction factor established is a function of the vapour flow rate. It is also afunction of the pump tube diameter and the correlation between the two can be established byconducting more tests.Keywords:- Diffusion-absorption refrigeration, bubble pump, two-phase flow, driving head, pump lift,

pumping action.




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ACKNOWLEDGEMENT

I wish to express my sincere gratitude to Dr. -Ing. K. Spindlerfor his invaluable suggestions andguidance throughout my work at the University of Stuttgart. The eager involvement on his part in my

dissertation work at every step has really been encouraging. I heartily thank Dipl. -Ing. Thomas Brendel for his wonderful guidance throughout the projectwork. He always guided me through the difficulties and made me understand the concepts needed for theproject. His experimental and theoretical knowhow was indeed very helpful.

I express my deep gratitude to Dr. M. P. Maiya for advising and guiding me through e-mails.Without his timely help and advice it would not have been possible for me to complete this project. I amvery much thankful to Prof. S. Srinivasa Murthy for his timely help during the early days and alsothroughout the project work. I am also grateful to Prof. Dr. -Ing. E. Hahnefor his timely encouragement.I also heartily thank all my co-workers in ITW for their co-operation and help.

A special thank is owed to Deutscher Akademischer Austauschdienst (German AcademicExchange Service)or DAAD for providing financial support as well as arranging for our stay and otherrelated things in Germany. I would also like to thank the staff of Internationale Angelegenheiten(Officeof International Affairs) for their kind and timely help during our stay in Germany.

Last but not the least, I thank my family members and friends for giving me moral support andadvice whenever needed.

Abhijit Sathe




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INTRODUCTION

The Diffusion-Absorption Refrigeration Cycle

The diffusion-absorption refrigeration cycle was pioneered around 1920 by two Swedes named von Platenand Munters. The cycle is unique in that it runs without any mechanical work input. This is achieved bypumping the fluids using a bubble pumpdriven by heat. Another unique feature of this cycle is that it isessentially noise free.

The diffusion-absorption cycle utilizes ammonia-water-hydrogen as working fluid. The roles ofammonia and water are familiar from absorption cycle experience. Hydrogen is used as a capping gas toequalize the pressure throughout the cycle to allow the low-head bubble pump to operate as a liquidcirculator. In the diffusion-absorption cycle, the partial pressure of the ammonia gas varies from point topoint instead of the overall system pressure. In reality, there are small variations in the system pressurethat are quite important for operation. The cycle utilizes a regenerative gas heat exchanger between theevaporator and the absorber which is driven by gravity-induced pressure differences. The schematic

arrangement for a typical diffusion-absorption refrigerator is shown in Fig. 1.1The cycle uses a three-component working fluid consisting of the refrigerant (ammonia), the

absorbent (water) and the auxiliary gas (hydrogen). The refrigerant serves as a transporting medium tocarry energy from a low temperature source to a high temperature sink. Water absorbs the refrigerant atlow temperature and low partial pressure and releases it at high temperature against a high partialpressure.

The auxiliary gas provides pressure equalization for the working fluid between the condenserand evaporator. The widest commercial use is ammonia-water-hydrogen. Helium can also be used as theauxiliary gas with a performance penalty.

The circulations in the system are produced solely by gravity and density differences asfollows:

Hydrogen circulates between the absorber and the evaporator because of the greater density of

the ammonia-rich gas column, i.e. the one descending from the evaporator.

Strong liquid coming out of the absorber is carried to the top of boiler by the action of the bubble

pump. Heat applied to the pump causes formation of bubbles and the density of strong solution inthe vertical pump tube is reduced so that the solution is forced to the top by the static head ofsolution in the absorber vessel.

Weak solution flows from the boiler to the absorber because of the difference in height between

the top of the boiler and the top of the absorber.




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Fig. 1.1 Schematic arrangement of a diffusion-absorption refrigerator

The diffusion-absorption cycle has inherent irrreversibilities that are larger than those found intypical vapour absorption cycles. In particular, there is an increased mass transfer resistance on thevapour side due to the presence of hydrogen. There is also an additional heat exchanger called theauxiliary gas heat exchanger. These factors explain why the cycle performance is fairly low.

The Bubble Pump

As discussed in section 1.1, the diffusion-absorption cycle relies on a bubble pump to pump the solutionfrom the absorber to the boiler. A bubble pump is a fluid pump that operates on thermal energy to pumpliquid from lower level to the higher level. It does not contain any moving parts. The bubble pumpoperates on the same principle that lifts coffee to the top of a coffee percolator. The bubble pump, asshown in Fig. 1.2, is nothing but a vertical tube of small circular cross-section.

The liquid in the liquid reservoir initially fills the tube to the same level (h). Heat is applied atthe bottom of the tube at a rate sufficient to evaporate some of the liquid in the tube. The resultingvapour bubbles rise in the tube. Due to the small diameter of the pump tube, the vapour bubbles occupycomplete cross-section of the tube and are separated by small liquid slugs. Each bubble acts as a gaspiston and lifts the corresponding liquid slug to the top of the pump tube. The bulk density of the liquidand vapour mixture in the pump tube is reduced relative to the liquid in the liquid reservoir, thereby

creating an overall buoyancy lift.The energy sources generally used are (i) electric heat and (ii) flame heat. In the latter case,

the entire length of the bubble pump or boiler is heated to increase the heat transfer area.Depending on the bubble pump tube, system pressure and properties of the pumped solution,




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two different kinds of flow are possible, namely slug flow and mixed vapour bubble-liquid(bubbly) flow. At the bottom of the pump tube small bubbles form and join together forming biggervapour bubbles. The rising vapour bubble acts like a piston and lifts a corresponding liquid slug to the topof the bubble pump tube. After a certain pump tube diameter is exceeded, the flow behavior changesfrom the slug flow regime to that of the mixed flow. The important parameters of the bubble pump are

pump tube diameter (dp), driving head (h), pump lift (L) and pump heat input (Q

p). The main

characteristic values to judge the performance of the bubble pump are solution flow rate and the

pumping ratio.

Fig. 1.2 The bubble pump

Two Phase Flow

Basic Definitions

A two-phase flow is defined as a flow of two separate parts of a heterogeneous body or system. Vapour-liquid mixtures, where the vapour and liquid are phases of the same fluid are referred to as two-phasesingle component mixtures (e. g. vapour-liquid mixture in a bubble pump) while gas-liquid mixtureswhere the vapour and liquid are different fluids are referred to as two-phase two component systems (e.g. air-liquid mixture in an air-lift pump). Following are some commonly used terms in two-phase flow.

Dryness fraction:- It is defined as a ratio ofmass flow of gas to the total mass flow.

Void fraction:- The void fraction is the ratio of the gas flow cross-sectional area to the total flowcross-sectional area.




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Mass velocity:- In two-phase flow literature, mass velocity is extensively used. It is the ratio ofmass flow rate to the total flow cross-section area of the mixture.

Pressure drop:- The calculation of two-phase pressure drop involves some complex calculations.Various correlations and charts are used to calculate the pressure gradients developed due tofriction in the flow and change in momentum.

Methods of Analysis

The methods used for analyzing a two-phase flow are extensions of those already well tried for single-phase flows. The procedure invariably is to write down the basic equations governing the conservation ofmass, momentum, and energy, often in a one-dimensional form and to seek to solve these equations bythe use of various simplifying assumptions. Three main types of assumptions have been made, viz.,

1. The homogenous flow model:-In this, the simplest approach to the problem, the two-phase

flow is assumed to be a single-phase flow having pseudo-properties arrived at by suitablyweighting the properties of the individual phases.

2. The separated flow model:-In this approach the two phases of the flow are considered to beartificially segregated. Two sets of basic equations can now be written, one for each phase.

Alternatively, the equations can be combined. In either case information must be forthcomingabout the area of the channel occupied by each phase and about the frictional interaction with thechannel wall. This information is inserted in the basic equations, either from separate empiricalrelationships in which the void fraction and the wall shear stress are related to the primaryvariables, or on the basis of simplified models of the flow.

3. The flow pattern model:-In this more sophisticated approach the two phases are considered tobe arranged in one of three or four definite prescribed geometries. These geometries are based onthe various configurations or flow patterns found when a gas and a liquid flow together in a

channel. The basic equations are solved within the framework of each of these idealizedrepresentations. In order to apply these models, it is necessary to know when each should be usedand to be able to predict the transition from one pattern to another.

Flow Patterns

The flow patterns encountered in vertical upwards co-current flow are shown in Fig.1.3. Following flowpatterns are encountered when a mixture of vapour and liquid flows through a vertical pipe.

1. Bubbly flow.In bubbly flow, the gas or vapour phase is distributed as discrete bubbles in acontinuous liquid phase. At one extreme, the bubbles may be small and spherical and at the other

extreme the bubbles may be large with a spherical cap and a flat tail. In this latter state althoughthe size of bubbles does not approach the diameter of pipe, there may be some confusion withslug flow.

2. Slug Flow.In slug flow the gas or vapour bubbles are approximately the diameter of the pipe. The




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nose of the bubble has a characteristic spherical cap and the gas in the bubble is separated fromthe pipe wall by a slowly descending film of liquid. The liquid flow is contained in liquid slugs whichseparate successive gas bubbles. These slugs may or may not contain smaller entrained gasbubbles carried in the wake of the large bubble. The length of the main gas bubble can varyconsiderably.

Fig. 1.3 Flow patterns in vertical co-current flow

3. Churn flow.Churn flow is formed by the breakdown of the large vapour bubble in the slug flow.The gas or vapour flows in a more or less chaotic manner through the liquid which is mainlydisplaced to the channel wall. The flow has an oscillatory or time varying character, hence thedescriptive name ?churn? flow. This region is also sometimes referred to as semi-annular or slug-

annular flow.4. Wispy annular flow.Wispy-annular flow has been identified as a distinct flow pattern. The flow

in this region takes the form of a relatively thick liquid film on the walls of the pipe together with aconsiderable amount of liquid entrained in a central gas or vapour core. The liquid in the film isaerated by small gas bubbles and the entrained liquid phase appears as large droplets which haveagglomerated into long irregular filaments or wisps. This region occurs at high mass velocities andbecause of the aerated nature of liquid film could be confused with high velocity bubbly flow.

5. Annular flow.. In annular flow a liquid film forms at the pipe wall with a continuous central gasor vapour core. Large amplitude coherent waves are usually present on the surface of the film andthe continuous break up of these waves forms a source for droplet entrainment which occurs invarying amounts in the central gas core. In this case, as distinct from the wispy-annular pattern,the droplets are separate rather than agglomerated.

Literature Review

The most common applications of bubble pumps are electric drip and percolating coffee makers. Bubblepumps are also known as vapour lift pumps. While commonly used, literature on bubble pumps is nearlynon-existent. However, since a bubble pump is really just a pipe containing two phase fluid flow, booksand papers on two phase flow provide more than sufficient information for the analysis of a bubble pump.

An extensive search revealed that the book which provides the best starting point for a bubble pumpanalysis was Two Phase Flow in Pipelines and Heat Exchangers(Chisholm, 1983). Chisholm [2] providesthe basic definitions and terminology, the flow patterns encountered in vertical pipes, and extensivereferences. Convective Boiling and Condensation by Collier and Thome [3] gives some useful information




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about two-phase flow models and two-phase pressure drop correlations.Delano [4] (http://www.me.gatech.edu/energy/andy_phd) has done a mathematical modeling of the

bubble pump in his Ph.D. thesis, Design Analysis of the Einstein Refrigeration Cycle. The model is alsocompared to the bubble pump built and tested in the conceptual demonstration prototype, and it isshown to provide a reasonable estimate of the heat input necessary to provide a given liquid flow rate.Maiya [8] has analyzed the solution circuit of the triple-fluid vapour absorption refrigeration system andhas developed a mathematical model for the solution circuit in his Ph.D. thesis, Investigations on Triple

Fluid Vapour Absorption Refrigerator. He has analyzed the performance of the bubble pump forparameters such as system pressure, pump lift etc. An improved solution circuit is also suggested and theperformance of the bubble pump for simple and improved solution circuits is compared.

A paper on Studies on bubble pump for a water-lithium bromide vapour absorption refrigerator byPfaff, Saravanan, Maiya and Srinivasa Murthy [10] provides a modeling of the bubble pump using themanometer principle. The bubble pump is tested experimentally in a test-rig and the bubble pumpbehavior is analyzed in detail. Another paper by Maiya, [9] Triple Fluid Vapour Absorption Refrigerator:Investigations on Solution Circuit, throws some light on selection of pump tube for a given pumpdischarge and heat input.

Absorption Chillers and Heat Pumps(Herold, 1996) [6] provides a few references to papers whichmention bubble pumps. An old German paper by A. G. Cattaneo [1] with a title ber die Frderung vonFlssigkeiten mittels der eigenen Dmpfe (About the pumping of liquids by means of their own steams)provided a useful information about theconstructional details of the liquid-vapour separator.




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MATHEMATICAL ANALYSIS

The maximum tube diameter

As already discussed, there are four flow regimes for two phase up flow in a fixed diameter vertical pipe.

For low vapour flow rates, small, finely dispersed vapour bubbles will rise in a continuous liquid phase.This is a bubble flow regime. Increasing the vapour flow causes the vapour bubbles to coalesce into bulletshaped slugs of vapour which rise in the liquid phase. This is a slug flow regime. Further increase ofvapour flow causes a highly oscillatory flow with a tendency for each phase alternatively to fill the tube.This is a churn flow regime. The last flow regime, reached by even further increase of vapour flow, isannular flow regime in which the liquid forms a film around the pipe wall and the vapour rises up thecore.

A bubble pump operates most efficiently in the slug flow regime. The maximum diameter tube inwhich slug flow occurs is given by the following equation (Chisholm, 1983):

where vf and v

g are the specific volumes of the liquid and vapour respectively, and is the surface

tension.Note, for a given fluid in a tube of diameter greater than that predicted by the above equation, slug flowwill never occur.

Modelling of the bubble pump

Andy Delano (http://www.me.gatech.edu/energy/andy_phd) has modeled the bubble pump using simpleanalytical equations such as Bernoullis equation, the Continuity equation and the momentumconservation equation.Following assumptions were made in the modeling.

1. The liquid level in the liquid reservoir does not oscillate during the operation.2. All the properties are measured at steady state.3. The variation in the ambient conditions is negligible.4. The liquid is at a saturation temperature at the entry of the bubble pump.5. The liquid is uniformly heated at the bottom of the bubble pump.

Fig. 2.1 Bubble pump schematic




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In Fig. 2.1, point 1 represents the inlet of the bubble pump. Applying Bernoullis equationbetween the surface of the reservoir and point 1 yields:

Next, continuity equation is applied to the control volume to which heat is applied, CV. Assuming that the

mixture of vapour bubbles and liquid exit this control volume at a mixture velocity, V2

, continuity equation

yields:

or,

rearranging the terms,

The specific volume at point 2 is assumed to be the specific volume of a vapour-liquid mixture with aquality x. The specific volume at point 2 can be expressed as

and,

Combining equations 2-5, 2-6 and 2-7,

Now, the mass flow rate of the vapour is assumed negligible relative to the mass flow rate of liquid andthe specific volume of the liquid is assumed negligible relative to the specific volume of the vapour.Equation 2-8 now becomes:

Next, conservation of momentum is applied to CV in Fig. 2-2. Neglecting the friction pressure drop overthis short distance,

Substituting equation 2-9 into equation 2-10,





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Now, applying the conservation of momentum to the bubble pump tube connecting the lower and upperreservoir.

where Bis the perimeter of the bubble pump tube and Wis the weight of fluid in the bubble pump tube.Weight Wcan be expressed as the combined weight of liquid and vapour in the tube.

where Afis the superficial area through which the liquid flows and A

gis the superficial area through which

the vapour flows. Equation 2-14 is simplified by assuming that the density of the vapour phase isnegligible as compared to that of the liquid.

We can also write down the following equations.

Substituting these equations into equation 2-15,


Now substituting equation 2-9 into equation 2-20,

Finally equation 2-21 is equated with equation 2-12,




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where,

where f is a laminar friction factor, since the flow assumed through the bubble pump is laminar. f iscalculated assuming only liquid flow throughout the pipe. The friction factor for laminar flow is

and,

K can be an adjustable parameter to account for losses other than friction in the tube. Pipeelbows and entrance effects may be accounted for by increasing the value of K. Furthermore, Kmay alsobe adjusted to match experimental data since losses are sometimes difficult to quantify analytically.

In the conventional diffusion-absorption refrigeration system, vapour bubbles are produced bythe addition of heat to the lower portion of the bubble pump tube. Assuming the fluid in the lowerreservoir and the tube to be saturated, and no heat transfer over the length of the pump tube, heatingpower required to produce the desired vapour flow rate is,

The amount of the liquid pumped by the bubble pump can be expressed as

This mass flow rate of liquid can be expressed as a function of the heat input using the aboveequations. Neglecting the mass of the condensate, the velocity of the liquid, V1at the entrance of the

bubble pump (point 1) can be calculated as

The pumping ratio is calculated as

Thus all the bubble pump parameters can be calculated mathematically. But the comparison withthe experimental results is necessary for estimation of K. The equations established in this chapter areused in chapter 5 for the comparison with the experimental results.




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SELECTION OF PROPERTIES

Selection of Working Fluid

The most important job was the selection of the working fluid. The working fluid used for testing thebubble pump was methyl alcohol (methanol). Following restrictions were encountered in the test set-up.

1. The system was not capable of operating at pressures higher than atmospheric pressure. Thesealing of the system was not tight enough to tolerate higher pressures.

2. The heating source for the liquid pre-heater was selected as water. So with water used as aheating medium, it was not possible to use a working fluid whose boiling temperature was higherthan 100 C. Higher temperatures were possible only when some other heating medium such asheating oil were used. But with the available system, water heater was used to make the systemsimple and easy to operate.

3. A counter flow heat exchanger was used for heating up the working fluid to its saturationtemperature. So the wall temperature of the heat exchanger had to be necessarily higher than the

saturation temperature by 5-10 C owing to heat losses. For a smooth operation of the system, thesaturation temperature of the working fluid had to be less than 70 C.

4. The aim of the experiments were to measure the amount of liquid pumped by the bubble pump fora given heat input. The saturation temperature of the working fluid had to be above ambienttemperature at all times if the heat supplied to the system were to be measured. Hence use of therefrigerants in the given system was ruled out because the saturation temperature of the workingfluid had to be higher than 40 C.

The working fluid must have the following properties.

1. The boiling point or the saturation temperature must be in the range of 40 to 70 C at atmosphericpressure.

2. The working fluid must be non-reactive and chemically stable at all temperatures.3. It must be non-toxic and non-flammable.4. It should be easily available.

Following fluids were considered for the selection.

1. Methyl Alcohol:- Methyl alcohol (CH3OH) or methanol has a boiling temperature of around 65 C

at atmospheric pressure. It is toxic in nature. It does not react with copper. It is readily availableat low prices.

2. Ethyl Alcohol:- Ethyl Alcohol (C2H

5OH) or ethanol has a boiling temperature of 78.5 C at

atmospheric pressure. It is non-toxic in nature.3. Acetone:- Acetone (C

3H

6O) has a boiling temperature of 56 C at atmospheric pressure. It is

highly flammable. Also it is extremely toxic and presence of 5 % of acetone of vapour in air maycause fire hazards.

Considering the considerable leakage in the set-up and the flammable nature of acetone, the use ofacetone was ruled out. Ethyl alcohol has a boiling temperature of 78.5 C, which meant water cannot beused as a heating medium. Hence methyl alcohol was the only working fluid that can be used in thesystem considering all the constraints. But methyl alcohol is toxic in nature and can be fatal if imbibed orinhaled in excess quantities. Though there were small losses in the system, they were not big enough tocause any danger to the human health. Also the exhaust system of the room was good which meant allthe methanol vapour that leaked out, was removed from the room. Hence methanol was used as aworking fluid for testing the bubble pump without any danger.

Table I in Appendix gives the properties of methyl alcohol.




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Selection of Dimensions

The dimensions of the different components of the test set-up were determined keeping in view severalfactors.

Selection of Pump Lift

The experimental investigation of the bubble pump was a part of a project "Design of a Solar DrivenCooling Unit based on the Diffusion-Absorption Principle" for domestic air-conditioning purposeby European Union. Water was heated using the solar energy and this hot water was used to drive therefrigerator. The maximum allowed height of the cooling unit for a domestic application was 2 m since itwas the height of the room. Also a smaller prototype meant more inaccuracies in the results. Hence theheight of the bubble pump tube was selected as 1.6 m.

Selection of Liquid-Vapour Separator Dimensions

The liquid-vapour separator formed a critical component of the bubble pump test set-up. Followingfactors were considered for the selection of its dimensions.

1. Too small separator meant the pumped liquid got flooded inside the separator body therebyhampering smooth operation of the bubble pump.

2. Too large separator meant the heat losses from the separator were very high. Hence the systembecame thermally inefficient.

Considering these two factors combined by the availability of the material, the diameter of theseparator was chosen as 64 mm and the height was selected as 80 mm

Selection of Other Parameters

The temperature of the sub-cooled methanol liquid was maintained at 20 C for following reasons.

1. The density of methanol was accurately known at 20 C.2. At 20 C, i.e. room temperature, the thermal expansion of the liquid if any, was negligible so the

volume of the liquid measured in the condensate flow meter was accurate.3. It was essential to cool the liquid in order to put some load on the system. Since there was no

evaporator in the system, the thermal load was too small. Hence the liquid was sub-cooled aftercondensation and then again mixed with the liquid in the liquid reservoir.




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EXPERIMENTAL SET-UP

Overall Description

The bubble pump which forms a critical component of the diffusion-absorption refrigeration cycle, was

built and tested in a test-rig. The test-rig did not comprise of a refrigerating system. The working fluidused was methyl alcohol. The full test-rig setup is described in Fig. 4.1.

Fig. 4.1 The Schematic arrangement of the test set-up

The actual experimental set-up with all components is shown in Fig. 4.2. The methanol liquid was storedin a liquid reservoir. It was first pre-heated in a liquid pre-heater and then was boiled at the bottom of

the bubble pump tube using an electrical heater. The liquid that was pumped by the bubble pump wasseparated from the accompanying vapour bubbles in a liquid-vapour separator. Both the phases, i.e.,liquid and vapour were separated. The vapour was condensed in a water-cooled condenser and flow rateof the condensate was measured. The flow rate of the pumped liquid was also measured separately.

After flow measurements, the condensate and the pumped liquid were passed back to the liquidreservoir. All the components where heat leakage was anticipated were insulated by using a black foam(Armaflex) of thickness 10 mm.

Fig. 4.2 The actual experimental set-up

Temperature Measurement:-The temperature was measured at six different locations in the testsetup. (i) temperature of the methanol liquid entering the liquid pre-heater, (ii) temperature of themethanol liquid coming out of the liquid pre-heater, (iii) temperature of the pumped liquid, (iv)




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temperature of the methanol vapour, (v) temperature inside the liquid-vapour separator and (vi)temperature of the condensed liquid.

A NiCr-Ni tube-in-tube thermocouple was used for the temperature measurement. The output wasmeasured in microvolts and was converted in C. The thermocouple wires were inserted in small diametertubes which were placed in flow whose temperature was to be measured (Fig. 4.3). The accuracy intemperature measurement was 0.1 C.

Fig. 4.3 Arrangement of thermocouple

Detailed Component Description

The setup has following components.

1.Liquid Reservoir:-Methyl alcohol was filled in a liquid reservoir made of circular glass tube ofouter diameter 48 mm to a predetermined level h. The liquid level in the reservoir was accuratelymaintained. A vertical scale of accuracy 1 mm was fixed to the reservoir to check the level ofliquid during initial conditions as well as during the tests. The maximum and the minimum levelexperienced by the liquid during the operation was noted down for each test.

2.Liquid Pre-heater:-The liquid methanol then passes through a liquid pre-heater where it washeated to the saturation temperature by a hot water stream. The liquid pre-heater was a counterflow tube-in-tube heat exchanger (Fig. 4.4). Methanol flows through the inner tube while hotwater flows through the outer tube. Both the tubes were made up of copper. The inner tube hadan inner diameter of 10 mm and a thickness of 1 mm while the outer tube had an inner diameterof 20 mm and a thickness of 1 mm. The hot water used for heating the methanol liquid wassupplied from a constant level hot water reservoir. The temperature in the hot water reservoirwas measured to an accuracy of 0.1 C by a mercury thermometer. Water in the reservoir washeated by an electrical heater and circulated by a small water pump. The pump consumedapproximately 300 W. The efficiency of the heat exchanger was assumed to be around 70 %.The heat supplied by the hot water to the methanol was approximately 50 W.




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Fig. 4.4 The Pre-heater

3.The Bubble Pump:-The saturated methanol liquid then entered the bubble pump tube whichwas also made up of copper. An electrical heater was placed at the bottom of the pump tube(Fig. 4.5). The heating element was a cylindrical stick of diameter 8 mm and length 80 mm andwas made of stainless steel. The maximum heating power withstood by the heating element was500 W. The heater was connected to a single phase variable power supply and a power meter.The error in applying and measuring the electrical power was approximately 1 %. The total

height of the bubble pump as measured from the bottom of the electrical heater was 1.6 m. Heatlosses were minimized by insulating the whole vertical pump tube. Plastic flexible tubes wereused to connect the bubble pump with the heating chamber and also with the liquid-vapourseparator.

Fig. 4.5 The bubble pump tube with heater

4.Liquid-Vapour Separator:-The two-phase fluid pumped by the bubble pump entered the

separator. The separator was a hollow cylinder made of copper having a diameter 64 mm and aheight of 80 mm. The detailed separator design is shown in Fig. 4.6. A vertical copper plate wasfixed to the top surface. The liquid pumped by the bubble pump was made to fall down as itforced out of the pump tube into the separator. Only the vapour was allowed to travel upwards.The liquid fell down and was removed from the separator using an outlet situated at the bottomside of the separator. This arrangement ensured that no liquid droplets were present in thevapour section and all the liquid pumped by the bubble pump was measured by the mass flowmeasuring instrument.




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Fig. 4.6 Constructional details of the separator

To estimate and to compensate for the heat losses of the separator, a small electrical heater wasused to heat the separator body. A heating wire was coiled around the whole periphery of theseparator (Fig. 4.7) and an electrical power of 10 W was supplied to the heater. This power wasdetermined from a number of extensive tests which determined the exact point at which the heatlost by the separator equaled the heat supplied to it and the temperature of the separator wasconstant and was equal to the saturation temperature of methanol. This heating ensured that nomethanol vapour condensed in the separator as the separator was fully adiabatic.

5.Pumped Liquid Flow Measurement:-For measuring the mass flow rate and the density of thepumped methanol liquid, a Coriolis flow-meter was used (Fig. 4.10). The flow-meter had anaccuracy of 0.15 % for measuring the mass flow of the liquids. The instrument had a widerange of operation and the mass flow rate was displayed digitally.

Fig. 4.7 Heatingarrangement for the

separator

Fig. 4.8 Separator plate withthermcouple groove Fig. 4.9 Cross-sectionalview of separator




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Fig. 4.10 The mass flow measuring instrument

Measuring PrincipleA mass flow dependent Coriolis force occurs when a moving mass is

subjected to an oscillation perpendicular to the flow direction. The measuring system accurately

determines and evaluates the resulting effects on the measuring tubes. The measuring principleis based on the controlled generation of Coriolis forces. These forces are always present when

both translational (straight line) and rotational (revolving) movement occur simultaneously. The

amplitude of the Coriolis force depends on the moving mass m, its velocity v in the system and

therefore its mass flow. The instrument uses an oscillation instead of a constant angular velocity

and two parallel measuring tubes, with fluid flowing through them, are made to oscillate in

antiphase so that they act like a tuning fork.

Fig. 4.11 Principle of Coriolis flow-meter

The Coriolis forces produced at the measuring tubes cause a phase shift in the tube oscillation

(Fig. 4.11):

When there is zero flow, i.e. with the fluid standing still, both tubes oscillate in phase (1).

When there is mass flow, the tube oscillation is decelerated at the inlet (2) and accelerated

at the outlet (3).

As the mass flow rate increases, the phase difference also increases. The oscillations of themeasuring tubes are determined using electrodynamic sensors at the inlet and outlet. The




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measurement principle operates independently of temperature, pressure, viscosity, conductivityor flow profile.

6.Condenser:-The construction of the condenser was similar to that of the liquid pre-heater. It wasalso a tube-in-tube counter-flow heat exchanger, with cooling water flowing from the inner tubeand methanol vapour flowing from the annular space. Both the tubes were made of copper. Thecondenser operated at atmospheric pressure. The condensed liquid was sub-cooled to a

temperature of 20 C by adjusting the flow rate of the cooling water. The condensed liquid wasthen passed to the flow-meter for flow measurement.

7.Condensate Flow Measurement:-The volume flow rate of the condensed methanol liquid wasmeasured in a calibrated plastic cylinder. Time required for filling 40 ml condensate in themeasuring jar was noted and from this, the mass flow rate of the methanol vapour pumped bythe bubble pump was easily estimated.

Testing Procedure

The bubble pump was tested for three different diameters and three different reservoir levels. Thegeneral test procedure was as follows:

Initial Procedure:-

1.The initial liquid level in the liquid reservoir was adjusted to the pre-determined value. New liquid

was filled to compensate for the losses.

2.The hot water bath temperature for the liquid pre-heater was set to 64 C.

3.The liquid-vapour separator was heated electrically by supplying a power of 10 W.

4.The bubble pump electrical heater was switched on and a steady electrical power was supplied tothe heater by using a variable power supply. The variation in the applied power was minimized bycontinuously monitoring and adjusting the input power.

5.The condenser cooling water was started and the temperature of the sub-cooled liquid wasmaintained at 20 C.

6.The temperatures at all the six locations were measured.

7.When the system achieved a steady state, the test was started.

Test Procedure:-

1.The valve for the condensate flow meter was closed, the stop-watch was started and the initialreading of the liquid mass flow- meter was noted. All the three procedures were conductedsimultaneously.

2.The condensate was allowed to collect in the measuring cylinder.




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3.The fluctuations in the liquid level in the liquid reservoir were noted down by noting the maximumand the minimum level achieved by the liquid.

4.When 40 ml (V)condensate was collected, the stop-watch was stopped and the final reading of

the liquid mass flow-meter was noted.

5.The above procedure was repeated to take 15 readings for a given electrical power.

6.The electrical power was then varied to the next value. The power was changed in the step of 50W from 500 W to the minimum value when the pump stopped.

For each diameter and each reservoir level, two sets of readings were taken, one with increasing powerand the other with decreasing power.

Following formulae were used to calculate the different parameters.

The results that were obtaining from the readings, are presented and discussed in the next chapter.




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RESULTS AND DISCUSSIONS

Observations with a transparent bubble pump tube

The bubble pump test procedure is described in the last chapter. Extensive tests were conducted to

obtain the characteristic curves for the bubble pump. The bubble pump was tested for three differentpump tube diameters and three different levels of liquid in the liquid reservoir. The results are presentedand analyzed in the subsequent discussion.The flow behavior of the fluid inside the pump tube was observed by using a transparent plastic tube of 6mm diameter. Though no readings were taken with this set-up, it was very much useful to visualize theflow of the two-phase fluid inside the pump tube. At very low heating powers (below 20 W), the solutionsimply oscillated inside the pump tube without being lifted to the top. This was due to the inability of thevapour bubbles to lift the solution. As the heating power was increased, more and more liquid wasevaporated and the size of the bubbles formed increased. The maximum height to which the liquidoscillated inside the tube increased with increase in heat input. Though the bubble pump operated at aheating power as low as 50 W, a much higher heat input was needed to start the pumping action whichresembles to a higher starting torque required to start a mechanical pump. The pumping action was notcontinuous, but was intermittent. The time interval between two consecutive pumping actions was notconstant and varied with the heat input. At higher heating powers, this time interval was small whichmeant the liquid was pumped more frequently. In the period between two pumping actions, the liquidoscillated in the tube violently. The level of violence was higher at higher heating powers. The liquid inthe liquid reservoir also oscillated. The amplitude of oscillation was high at high heating powers. At lowheating power, the height to which the liquid raised inside the reservoir was more. With increase in theheating power, this height was reduced though the amplitude of oscillation was increased. The flow ofcondensate started a considerable time after the first pumping action was recorded. The flow of thecondensate, however was not intermittent. At higher heating powers, higher amount of liquid wasevaporated and a huge amount of condensate was observed to flow through the condensate flowmeasurement device.

Evaluation of Bubble Pump Parameters

Following bubble pump parameters were analyzed from the experimental results:

The frequency of pumping actionThe pumping action is intermittent and the frequency of pumping varies with the heat input. A typicalvariation of the pumping frequency with the bubble pump heat input is shown in Fig. 5.1. The pumpingfrequency increases linearly with the heat input. As the boiling rate increases with the heat input, the

duration for each pumping action reduces and the frequency increases. At very low heat inputs, thefrequency of pumping is extremely small. The frequency of pumping action also depends on the geometryof the heating element and other heating onditions. The amount of liquid pumped in a single pumpingaction is also not constant. At higher heat inputs, a large amount of liquid is pumped. Also at low heatinputs, the time interval between the two consecutive pumping actions is not constant for a given heatinput. But it is reasonably constant at higher heat inputs.

The diameter of the pump tube also affects the pumping frequency. Pumping frequency is morefor a smaller diameter pump tube for the same heat input. This is because, the process of forming avapour slug at the surface of the heating element takes a finite time. The smaller the diameter of thetube, smaller is the residence time for formation and transportation of the vapour slug. Since the vapourslugs form quickly, the liquid is lifted more frequently thereby increasing the pumping frequency.

The mass flow ratePerformance of the bubble pump is evaluated on the basis of the amount of liquid pumped by the bubblepump for a given heat input. Many factors influence the performance. The general trend of variation ofthe mass flow rate of the pumped liquid with the pump heat input is shown in Fig. 5.2. At low heat input,




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the mass of liquid pumped by the bubble pump per unit time increases almost linearly with the heatinput. This is due to the fact that, as the heat input to the liquid in the bubble pump increases, more andmore number of vapour bubbles form which lift more and more amount of liquid. This is a slug flowregime, where the liquid is trapped between the slugs of vapour. Hence the liquid flow rate increases asthe heat input increases though the rate of increase decreases. The mass flow reaches a maximum value.This maximum flow occurs when the increase in the frictional pressure drop caused by increased vapourflow rate exceeds the increased buoyancy effect of the vapour to pump the liquid. Further increase in theheat input, results in decrease in the mass flow of the pumped liquid. This is because, at very high heatinputs, the vapour flow rate is very high which means the frictional losses are heavy resulting in loweramount of pumped liquid. Thus the region to the left side of the maximum mass flow rate can be calledas a buoyancy force dominated regionwhile the one to the right side of the maximum mass flow rate canbe termed as pressure drop dominated region.

The mass flow rate of vapour, however, increases proportionately with the heat input. It is muchlower as compared to the mass flow of pumped liquid. If the bubble pump tube is inadequately insulated,some vapour may condense as it travels upwards thus further lowering the vapour flow rate. Thevariation of the mass flow rate of the pumped liquid with that of the vapour should essentially be same asthe variation of pumped liquid flow rate with the heat input.

Effect of driving head on the mass flow rate of pumped liquidThe bubble pump was tested for three different tube diameters and three levels of liquid in the liquidreservoir (driving head h). The height of the pump tube (pump lift L) was not varied. Since the systemwas open system, i.e. it operated at an atmospheric pressure, the variation in the condensingtemperature was not possible. As discussed above, the mass flow rate of the pumped liquid first increaseswith the heat input, reaches a maximum value and then starts decreasing. Fig. 5.3, Fig. 5.4and Fig. 5.5show variation of pumped liquid flow rate with the heat input for different driving heads for the samepump tube. The results indicate that a higher driving head leads to a higher volume flow rate for thesame pump lift, with more or less the same gradients (increase of volume flow rate per increase of heatinput) at any heat input. This is because, at a higher driving head, the force exerted by the liquid columnis higher. This results in increased fluid velocities in the bubble pump tube thereby rendering a higheramount of pumped liquid. A reduced driving head means in the same way a reduced driving forcedeveloped by the bubble pump. Increase in the pump tube height, i.e. the pump lift also results indecrease in the mass flow because the liquid has to be lifted to a higher level. The important parameteris, however, not the driving head or the pump lift alone, but a ratio of the driving head to the pump lift(h/L), i.e. a submergence ratio. If both, the pump lift and the driving head are increased without alteringthe submergence ratio, the mass flow of the pumped liquid should theoretically be the same, because theincrease in frictional pressure drop on the account of increase in the pump tube length is offset by theincreased velocities due to increase in the driving head. The submergence ratio of the bubble pump is ameasure of how far the pump is submerged relative to its length. With increasing submergence ratio, therelative height to which the pump must lift the liquid decreases, so the liquid flow rate increases.

From Fig. 5.3, Fig. 5.4and Fig. 5.5, it is clear that the general trend of variation is the same. Thelower driving heads render lower mass flow rates. The occurrence of the maximum mass flow rate of

pumped liquid for a given pump tube diameter is fairly at a constant heating power. For the pump tube of10 mm diameter, the maximum mass flow rate of the pumped liquid is observed at a heat input ofapproximately 275 W, while for the pump tube of 8 mm diameter, this value is 250 W. For the pump tubeof diameter 6 mm, the maximum mass flow rate occurs at a heat input of 250 W. From all the abovediscussed graphs, it can be seen that at low heat inputs, the behavior of the pump for 0.5 m and 0.45 mdriving heads is approximately the same, i.e. for both of these driving heads, the bubble pump givesnearly the same mass flow rate for a given heating power. At heat inputs higher than 225 W, the twocurves separate. At heat inputs higher than 300 W, all the three curves run almost parallel to each other.Thus at higher heat inputs, the pump behavior is similar for all the three driving heads.

Effect of pump tube diameter on the mass flow rate of pumped liquidThe pump tube diameter played a very important role in the pump behavior. Along with the driving head

(h) and the pump lift (L), the pump tube diameter (dp) also forms a design parameter for the bubble

pump. Fig. 5.6, Fig. 5.7and Fig. 5.8describe the pump behavior for three different tube diameters at the




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same driving head and pump lift. Increase in the diameter of the pump tube results in increased liquidmass flow rate. This is because, as the diameter increases, the frictional pressure drop decreases therebyincreasing the efficiency of the bubble pump which results in increased liquid mass flow rate. Thebehavior of the bubble pump remains the same, i.e., the mass flow rate of pumped liquid increases withheat input, reaches the maximum value and then starts decreasing with further increase in heat input.However, the heat input at which this maximum mass flow rate occurs is not the same for all the pumptubes. For lower diameters, the heat input required to produce the maximum liquid mass flow is less. InFig. 5.7, for the pump tube of 6 mm diameter, the maximum liquid mass flow occurs at a heat input of225 W. For pump tube of 8 mm diameter, the value of heating power required to produce the maximumliquid flow is 250 W while for 10 mm diameter pump tube, this value is 300 W. This behavior may beexplained as - the maximum liquid mass flow rate occurs when the increased buoyancy effect of thevapour to pump the liquid is balanced by the increased frictional pressure drop caused by increasedvapour flow rate. For a pump tube of lower diameter, the frictional drop is higher. So the maximum liquidflow rate occurs at a relatively lower vapour flow rate, i.e., at lower heat input. As the pump diameterincreases, the frictional pressure drop decreases and the occurrence of the maximum liquid flow rate isshifted to the right side, i.e., to the higher heat input side. The bubble pump should always be operatedat this maximum liquid flow rate in order to maximize its performance.

Also observed from the graphs is the fact that at lower heat inputs, the difference in the liquid mass flowrates for different diameters of pump tube is smaller. The difference increases with increase in heat input.It is prominent at higher heat inputs. Also at high heat inputs, the difference between the liquid mass

flow rates for 6 mm pump tube and 8 mm pump tube is much higher (200 %) than the differencebetween liquid mass flow rates for 10 mm pump tube and 8 mm pump tube. Thus in case of 6 mm pumptube, the laminar forces are predominant and the friction factor values are too high which results in amuch reduced liquid mass flow rate.

The pumping ratioThe pumping ratio is the ratio of volume flow rate of the pumped liquid (Vf) to volume flow rate of the

vapour (Vg). The variation of the pumping ratio with the heat input for a constant pump tube diameter

and different driving heads is given in Fig. 5.9 while that for a constant driving head and different pumptube diameters is shown in Fig. 5.10. Irrespective of the diameter of pump tube and the driving head, the

general behavior of the pumping ratio with respect to the heat input seems similar. The pumping ratiodecreases almost linearly with the increase in heat input. This is due to the increased flow rate andconsequently the increased pressure head loss at higher heat inputs. Fig. 5.9reveals that all the threecurves run almost parallel to each other for all the heat inputs. At higher heat inputs, the curves seem toflatten a bit which indicates that that the rate of decrease in pumping ratio is decreased. Also at low heatinputs, a similar behavior is observed, i.e., the rate of decrease in pumping ratio is decreased.From Fig. 5.9, it is clear that for the same heat input, a smaller driving head, gives a lower pumping ratio.When the driving head is reduced, each cycle (pumping action) takes more time and correspondinglymore vapour escapes from the pump. The temperature inside the pump will also increase to a higherlevel before the cycle completes. Thus vapour is vaporized and less solution is pumped, resulting in lowerpumping ratio. The effect of pump tube diameter on the pumping ratio is explained in Fig. 5.10. A biggerdiameter pump tube renders higher pumping ratio for the same heat input. Bigger the diameter, smaller

is the pressure drop, higher is the mass flow of the pumped liquid and so higher is the pumping ratio. Ata heat input of 500 W, the difference in the pumping ratio for 6 mm diameter tube and 8 mm diametertube is as much as twice the difference in the pumping ratio for 10 mm and 8 mm diameter tubes. This isbecause of a very high friction loss experienced by the 6 mm diameter pump tube which results in amuch reduced liquid flow rate and a very low pumping ratio.

The vapour mass flow rateThe vapour mass flow rate is directly proportional to the heat input. It is clear from Fig. 5.11 that thedriving head does not influence the vapour flow rate much. A higher driving head results in a slightlyhigher vapour flow rate. This is because of the increased force offered by the liquid column. The vapourflow rates at higher heat inputs are nearly constant for all the driving heads. Fig. 5.12shows the variationof vapour flow rate with heat input for different pump tube diameters. The curves for 8 mm and 6 mm

diameter pump tubes run parallel to each other for all the heat inputs. The curve for 10 mm diametertube, however, shows a departure from the other two curves.




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Comparison with the Mathematical Model

A mathematical model for a bubble pump has already been established in Chapter 2. It is important tocompare this model with the results obtained from the experiments in order to validate it. Theassumptions made in the modeling are stated in Chapter 2. But in the actual testing, some discrepancieswere noticed with the assumptions. The liquid in the reservoir oscillated violently which was assumed tobe at a constant level. Also as discussed earlier, the flow of the two-phase fluid in the bubble pump tube

was intermittent. In between two consecutive actions, the liquid oscillated in the pump tube. Duringthese oscillations, some vapour escaped from the pump tube without lifting any liquid. Thus all thevapour did not contribute effectively in lifting the solution. This aspect was not included in the analyticalmodel. Also for modeling, the kind of flow in the pump tube was assumed to be always slug flow.However, during the tests, it was clear that the flow pattern was not stable and varied with the heatinput.

As a consequence of this, the model fails to give satisfactory results for all the heat inputs. It is thereforeessential to establish a correction factor that compensates for all the above stated discrepancies. Adimensionless factor K which was defined in Eqn. 2-23 as

is found to have a relation with the mass flow rate of the vapour. The correlation is found out by applyingthe experimental results to the analytical model for a given pump tube diameter and a given drivinghead. The correlation established for one pump tube diameter is not valid for another pump tube

diameter. Fig. 5.13gives the variation of the factor K with the mass flow rate of liquid.

A typical correlation for a given pump tube diameter is of following structure

where A, B, C, D and E are constants and are functions of the pump tube diameter.The values of the constants for the different pump tube diameter are given in Table 5-1.

Table 5-1

From Eqn. 2-22, 2-26, 2-27 and 5-6, it is possible to simulate the bubble pump performance. Thus it ispossible to predict the performance of the bubble pump for a given driving head, pump lift and pumptube diameter. Fig 5.14gives the procedure for finding out the liquid mass flow rate of a given bubblepump. It is also possible to calculate the geometrical parameters of the bubble pump for a given liquidmass flow rate. The procedure for the same is described in Fig. 5.15.

Pump TubeDiameter

A B C D E

10 mm -1.419 x 1015 9.594 x 1011 -1.361 x 108 -2.718 x 104 6.49

8 mm 3.141 x 1015 -3.037 x 1012 1.091 x 109 -1.749 x 105 10.986

6 mm 8.284 x 1014

-7.974 x 1011

2.874 x 108

-4.688 x 104

3.056




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Conclusion

The bubble pump has been tested both analytically and experimentally and the results are presented inthe previous sections. The results reveal that the frequency of pumping action increases with increase inpump heat input. The mass flow rate of vapour increases linearly with the heat input whereas the massflow rate of the pumped liquid first increases, reaches a maximum value and then decreases with theincrease in the heat input. The bubble is best operated at the maximum liquid mass flow rate when the

efficiency is the highest. The pumping ratio decreases almost linearly with the heat input. The importantbubble pump parameters are the driving head (h), pump lift (L) and the pump tube diameter (d

p).

Selection of bubble pump tube

The bubble pump must give the desired pump discharge (mass flow rate of pumped liquid) at the ratedheat input. The important geometrical parameters which govern the bubble pump behavior are the

driving head (h), the pump lift (L) and the pump tube diameter (dp). But as discussed earlier, the driving

head and the pump lift can be combined to form a single parameter known as the submergence ratio(h/L). As seen from the graphs for the bubble pump behavior, for the same value of liquid mass flow ratethere exist different heat inputs depending on the pump tube diameters. The higher the tube diameter,the lesser is the amount of heat to be supplied to the bubble pump to get the required liquid mass flow

rate.Thus it may seem that a large diameter pump tube would always be advantageous. However, increasingthe diameter with a fixed liquid flow will eventually cause transition from the assumed slug flow to bubblyflow. As discussed earlier, the bubble pump operates most efficiently in the slug flow regime and shouldoperate at its maximum liquid flow rate. If the liquid flow rate needs to increase or decrease, then thediameter and vapour flow rate of the pump will be chosen such that this liquid flow rate is the maximum.




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PERFORMANCE CURVES FOR BUBBLE PUMP




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Fig. 5.14 Calculation of liquid mass flow rate




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Fig. 5.15 Calculation of pump tube diameter




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List of Symbols

Subscriptfdenotes liquid while subscript gdenotes vapour.

Symbol Description SI Unit

A Cross-sectional area m2

B Perimeter of the pump tube m

dp Diameter of the pump tube m

f Friction factor

g Acceleration due to gravity m/s2

G Mass velocity kg/m2s

h Driving head m

L Height of the bubble pump tube (Pump lift) m

Mass flow rate kg/s

P Pressure N/m2

Heating power W

Re Reynolds number

s Velocity constant

V Velocity m/s

Volume flow ratem

3

/s

W Weight N

x Dryness fraction

Greek Letters Specific volume m

3/kg

Surface tension N/m

Density kg/m3

Dynamic viscosity N.s/m2

Void fraction




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REFERENCES

1. Cattaneo A. G., 1935, ber die Frderung von Flssigkeiten mittels der eigenen

Dmpfe, Zeitschrift fr die gesamte Klte-Industrie.

2. Chisholm D., 1983, Two-phase Flow in Pipelines and Heat Exchangers, GeorgeGodwin, London and New York.

3. Collier John G. and Thome John R., 1993, Convective Boiling andCondensation, Oxford Science Publications.

4. Delano Andrew, 1998, Design Analysis of the Einstein Refrigeration Cycle,(http://www.me.gatech.edu/energy/andy_phd), Ph.D. Thesis, Georgia Institute of

Technology.

5. Hahne E., Grigull, 1987, Heat Transfer in Boiling, Oxford Science Publications.

6. Herold K. E., R. Radermacher and S. Klein, 1996, Absorption Chillers andHeat Pumps, CRC Press New York.

7. Maczek K. and Zoltaniecki A., 1980, Some Characteristics of Thermal Siphonsfor Ammonia-water Solutions, IIF/IIR-Commissions B1, B2, E1, E2-Mons,Belgium.

8. Maiya M. P., 1998, Investigations on Triple Fluid Vapour AbsorptionRefrigerator, Ph.D. Thesis, Department of Mechanical Engg, Indian Institute ofTechnology Bombay.

9. Maiya M. P., 1999, Triple Fluid Vapour Absorption Refrigerator: Investigations

on Solution Circuit, 20thInternational Congress of Refrigeration, IIR/IIF, Sydney.

10. Pfaff M., Saravanan R., Maiya M. P., Srinivasamurthy S., 1998, Studies onBubble Pump for a Water-Lithium bromide Vapour Absorption Cooler,International Journal of Refrigeration, vol. 21, no. 5: p. 452-462.




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APPENDIX

Properties of Methyl Alcohol

1. General Properties

2. Critical Properties

Chemical name Methanol

Chemical formula CH3OH

Molecular weight 32.04 kg/kmol

Specific gravity 0.7915

Vapour density 1.11 (Air=1)

Melting temperature -97.65 C

Enthalpy of fusion 103 kJ/kg

Boiling temperature 64.7 C

Enthalpy of vapourisation 1100 kJ/kg

Enthalpy of combustion 19,930 kJ/kg

Vapour pressure (at 20 C) 12.3 kPa (97 mm of Hg)

Surface tension at boiling temperature 0.0172 N/m

Flash point 12 C

Solubility Miscible with water

Flammability limitsLowerUpper

6 % in air by volume36% in air by volume

Nature Toxic

Critical temperature 512.6 K (239.6 C)

Critical pressure 81 bar

Critical volume 0.118 m3/kmol




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3. Liquid Properties

4. Vapour Properties

Properties 0 C 20 C 50 C 100 C 150 C Units

Density 813 792 765 714 643 kg/m3

Specific heat 2.386 2.495 2.68 -- -- kJ/kg K

Thermalconductivity

0.207 0.201 0.193 0.178 0.16 W/m K

Dynamic

viscosity

7.77x10-4 5.75x10-4 3.85x10-4 2.28x10-4 1.39x10-4Ns/m2

Properties 0 C 25 C 100 C 200 C 300 C Units

Specific heat 1.33 1.374 1.534 1.798 2.036 kJ/kg K

Thermal

conductivity0.0137 0.0157 0.227 0.034 0.049 W/m K

Dynamic viscosity 0.87x10-5 0.85x10-5 1.22x10-5 1.56x10-5 1.88x10-5 Ns/m2


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