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Nuclear Engineering and Design 285 (2015) 158–170

Contents lists available at ScienceDirect

Nuclear Engineering and Design

jou rn al hom epage : www.elsev ier .com/ locate /nucengdes

isualization of boiling flow structure in a natural circulation boilingoop

rnab Karmakar, Swapan Paruya ∗

hemical Engineering Department, NIT Durgapur, Mahatma Gandhi Avenue, Durgapur 713209, India

i g h l i g h t s

Vapor–liquid jet flows in natural circulation boiling loop.Flow patterns and their transitions during geysering instability in the loop.Evaluation of the efficiency of the needle probe in detecting the vapor–liquid and boiling flow structure.

r t i c l e i n f o

rticle history:eceived 28 June 2014eceived in revised form0 November 2014ccepted 7 January 2015

a b s t r a c t

The present study reports vapor–liquid jet flows, flow patterns and their transitions during geyseringinstability in a natural circulation boiling loop under varied inlet subcooling �Tsub (30–50 ◦C) and heaterpower Q (4–5 kW). Video imaging, voltage measurement using impedance needle probe, measurementof local pressure and loop flow rate have been carried out in this study. Power spectra of the voltage, thepressure and the flow rate reveal that at a high �Tsub the jet flows have long period (21.36–86.95 s) andthey are very irregular with a number of harmonics. The period decreases and becomes regular with a

◦

decrease of �Tsub. The periods of the jet flows at �Tsub = 30–50 C and Q = 4 kW are in close agreementwith those obtained from the video imaging. The probe was found to be more efficient than the pressuresensor in detecting the jet flows within an uncertainty of 9.5% and in detecting a variety of bubble classes.Both the imaging and the probe consistently identify the bubbly flow/vapor-mushrooms transition or thebubbly flow/slug flow transition on decreasing �Tsub or on increasing Q.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

Natural circulation boiling loop (NCBL) is widely used for con-rolled heat removal in boiling water reactors (BWRs), simplifiedoiling water reactors (SBWRs), nuclear steam generators, ther-al drum-type boilers and all the thermosyphon reboiler units

f chemical, refining and petrochemical plants, etc. NCBL experi-nces various nonlinear flow instabilities (Durga Prasad et al., 2007;ayak and Vijayan, 2008; Goswami and Paruya, 2011) during its

tartup operation. One of the major instabilities is geysering phe-omenon which was first observed by Griffith (1962) in a verticaloiling channel at a low heat flux and a low pressure. The phe-omenon is induced by wall-superheating at boiling incipience. The

eysering in a vertical channel is classically characterized by theeriodic water jets (geysers) from the channel. The jet is initiatedy the formation of big bubbles and the expansion of the bubbles.

∗ Corresponding author. Tel.: +91 3432754086.E-mail address: swapanparuya@gmail.com (S. Paruya).

ttp://dx.doi.org/10.1016/j.nucengdes.2015.01.005029-5493/© 2015 Elsevier B.V. All rights reserved.

The water ejection causes a decrease of the hydrostatic head. Asa result, vigorous boiling occurs and more geysers form. The gey-sering in an NCBL occurs in three successive stages (Yadigaroglu,1981)—(1) refilling of heater with the liquid from riser due to thecondensation of vapor in the riser resulting in an increase of thestatic head that stops boiling, (2) incubation to initiate bubble for-mation, and (3) a jet of liquid and vapor from the heater as a result ofaggressive self-evaporation due to a decrease of the static head. Thejet is usually identified by the surge of the loop flow rate in NCBLmeasured by a flow meter. A significant amount of efforts has beendevoted in the recent past to the research on the nonlinear identi-fications of the geysering instability in NCBL (Ozwaw et al., 1979;Hands, 1979; Aritomi et al., 1993; Kyung and Lee, 1994; Jiang et al.,1995; Wu et al., 1996; Subki et al., 2003; Baars and Delgado, 2006;Watanabe et al., 2008; Paruya et al., 2009; Karmakar and Paruya,2012a). Periodic and chaotic dynamics of the overall loop during

geysering are reported in the literature. There are not sufficientinformation on the local bubble dynamics and flow structure thatplay a vital role in deriving a more realistic mechanism of geyseringoccurring in NCBL. Very few literatures contribute to revealing the

A. Karmakar, S. Paruya / Nuclear Engineeri

Nomenclature

Notationsic circuit current (Amp)p pressure (N/m2, psia)pHi heater inlet pressure (N/m2)Q heating rate (kW)Rmc resistance of metal conductor (�)T temperature (◦C)td bubble departure time (s)tw bubble waiting time (s)�Tsub inlet subcooling (◦C)�Tw wall superheating (◦C)Va applied voltage (V)Vloop loop volume (m3)VPP voltage amplitude signal from probe (V)z axial length (m)Wloop loop flow rate (kg/h)

Greek˛ void fraction, dimensionless� differential

initial liquid volume (m3)

Subscriptsa appliedb bulkd departureHi heater inletHo heater outletsat saturationw wall

AbbreviationsBWR boiling water reactorCLTC cold-leg temperature controllerCPDF cumulative probability density functionDFT discrete Fourier transformDPT differential pressure transmitterNCBL natural circulation boiling loopPDF probability density functionPMB picture motion browserPSD power spectral densitySBWR simplified boiling water reactorsSD standard deviation

lecbmors(ifpbaesr

2.1. Natural circulation test facility

ocal flow structure through flow visualization of geysering. Hsieht al. (1997) applied dynamic imaging technique through a CCDamera and a Sony Hi-8 video camera (30 frame/s) for visualizingoiling flow in NCBL. They observed the periodic loop flow rate ofedium amplitude in bubbly flow regime and the periodic flow rate

f large magnitude or chaotic flow rate in slug flow or churn flowegime. They also reported flow reversal, periodic bubbly/slug tran-ition and bubbly/churn transition in the loop. de Mesquita et al.2012) investigated to recognize the flow patterns of chugging-typenstability in NCBL using a CCD camera and derived a number ofuzzy principles to predict the flow patterns based on a grayscalerofile of the images. Khodabandeh and Furberg (2010) observedubbly/slug boiling flows in low heat fluxes and slug/annular flowt high heat fluxes in a thermosyphon loop. They also studied theffect of the channel height on the flow instability, the flow rever-

al and the heat transfer coefficient. Karmakar and Paruya (2012b)eported a brief photographic study on the bubble growth in NCBL.

ng and Design 285 (2015) 158–170 159

In this paper, we present our experimental observations on thelocal bubble dynamics (bubble growth and collapse), the jet flowfrom the heater section of our NCBL, the flow patterns and theirtransitions in the heater captured using the videos. The frequency(the cycle time) and the duration of the jet flows were determinedby analyzing the videos. Attempts were also made to detect thelocal flow structures using an impedance needle probe designedand developed for the study. The performance of the probe wasinvestigated by comparing its voltage signals with the local pres-sure signals and the video images. The pressure was measuredat the heater inlet. The video images provide the benchmarks ofthe local flow structures and the time periods of the jet flows forthe probe and the pressure sensor. The local pressure measure-ments also give some interesting insights of the bubble dynamics.To the authors’ knowledge, the identification of the flow patternsof boiling flow using impedance probes is hardly reported in lit-eratures. The effects of heater power Q and inlet subcooling �Tsub(=Tsat − THi) on the flow structure and the associated jet flows havenot also been investigated. The experiments were carried out atQ = 4 kW (50.955 kW/m2) and 5 kW (75.932 kW/m2). �Tsub wasvaried from 30 ◦C to 50 ◦C (where a low �Tsub means a high temper-ature of water at the heater inlet and vice versa). The loop pressurewas controlled at 1.15 bar. These conditions are suitably chosen toobtain measurable quantities of bubble growth, bubble size, flowoscillations and the parametric variations of those phenomena.

The pattern recognition of two-phase air–water flow usingimpedance probes or conductivity probes is well established (Jonesand Zuber, 1975; Ceccio and George, 1996; Cheng et al., 1998;Paglianti and Pintus, 2001; Le Corre et al., 2003; Schlegel et al.,2009; Paranjape et al., 2012; Ghosh et al., 2012). Probability den-sity function (PDF) of the normalized signals determines the flowregimes. However, the nature of the PDFs depends on the type ofprobes (ring probe or two-wire parallel probe) and the diameter offlow channel. For the slug flow regime, the PDFs with a single peakare obtained using the ring probe or the pipes of large diameter,whereas the parallel probes or the pipes of small diameter give riseto the PDFs with two peaks. The guidelines for identifying the flowregime of air–water flow based on PDFs of the non-dimensionaladmittance � are given for the flow channel of very small flow areaof 0.6078 mm2 and a length of 50.8 mm (Paranjape et al., 2012):

(1) for a bubbly flow, the PDF contains a sharp peak at a high �,(2) for a cap-bubbly flow, the PDF contains two very distinct peaksthat are very closely located; the peak corresponding to higher � isdue to liquid phase while the peak at lower � is due to cap bubbles,(3) for a slug flow in which a liquid slug and a gas slug flow alter-nately, the PDF contains two distant peaks—the one at very high �for liquid slug and the other at very low � for gas slug, and (4) along gas slug has a peak at low �.

In a channel of large diameter (150 mm) and length of 4.4 m(Schlegel et al., 2009), the PDFs of � significantly differ from thosein the microchannel. The PDFs contain a single peak in all regimes;the PDFs for the slug regime are wider than those for the bubblyregime and the churn regime. The bubbly flow yields a very sharppeak of PDF compared to the churn flow. In a pipe of large diam-eter, a bubble cannot occupy the entire pipe diameter because ofthe inability of its curved interface to sustain itself and resorts to alarge turbulence and recirculation around the bubble. Such recircu-lation results in the PDFs with a low peak. The PDF becomes widercompared to bubbly flow.

2. Experimental setup and measuring devices

The NCBL mainly consists of six major parts including a heater,an adiabatic riser, a condenser, a down comer, an upper plenum

160 A. Karmakar, S. Paruya / Nuclear Engineering and Design 285 (2015) 158–170

Fig. 1. Experimental setup of natural circulation boiling loop (NCBL).

Table 1Dimensions of the various sections of the NCBL.

Section Orientation Diameter (m) Length (m)

Heated section (AB) Vertical 0.02096 (ID)0.02671 (OD) 1.00Adiabatic riser section (BC) Vertical 0.02096 (ID)0.02671 (OD) 2.00Upper plenum (CD) Horizontal 0.02096 (ID)0.02671 (OD) 1.20Lower plenum (JK) Horizontal 0.02096 (ID)0.02671 (OD) 1.20

ailothlfac(aotsl

Condenser (EH) Vertical

Condenser tubes (4 Nos., FG) Vertical

Down comer (HI) Vertical

nd a lower plenum shown in Fig. 1. The hot fluid from the risers cooled on the shell side of the condenser. The entire rectangularoop was fabricated of SS 316. The sizes of the various componentsf the loop are mentioned in Table 1. Eight resistance tempera-ure detectors (RTDs) T1 − T8 shown in Fig. 1 are installed at theeater inlet, the heater outlet, the riser section, the condenser out-

et for process fluid, at the condenser inlet and the condenser outletor cooling water. The RTDs are PT-100 (sensitivity of 0.384 �/◦Cnd uncertainty of less than 0.5%) with a range of 0–400 ◦C. Threeapacitive pressure sensors P1 − P3 (see Fig. 1) made of SS 310WIKA make, Germany, uncertainty < 0.25%, response time—1 msnd range—0–10 bar) are located at the heater inlet, the heater

utlet, and the top of the riser. A capacitive differential pressureransmitter (DPT) with a range of 0–10 bar measures the pres-ure drop (uncertainty <0.25% and response time—1 ms) over aength of 0.6 m in the heater. The electromagnetic flow meter (EFM)

– 0.600.010 (ID) 0.450.02096 (ID)0.02671 (OD) 2.35

with 20 mm polytetrafluoro ethylene lining (range—0–1.8 m3/hand uncertainty—<0.5%) measures the flow rate of water in thelower plenum. We recognize this flow rate as the loop flow rate.EFM is preferred because of its ability to measure an extremely lowflow and a reverse flow encountered in NCBL.

Six segmental heater-coils (Kanthal super: MoSi2) are woundaround 1 m long aluminum tube for uniform and safe heating. Thetest section is the process SS-tube in the heater section AB in Fig. 1(the top view of the heater section). The process tube is also 1 mlong. Inside diameter (ID) and outside diameter (OD) of the tubeare 20.96 mm and 26.71 mm. The tube has the pressure sensors P1and P2 at its inlet and exit, respectively. RTDs T1 and T2 are installed

at the inlet and exit of the tube. There is a RTD for measuring thesurface temperature of tube. The pressure drop across the tube ismeasured by DPT. The video and impedance probe are installed tostudy the bubble phenomena at the exit of the tube. The aluminum

ineering and Design 285 (2015) 158–170 161

tflto

madacPbcsasoitAttcruttft

(1(Wfbsi

2

itaftoiiFcdtltnwt(spntdo

Fig. 2. Schematic of impedance needle probe-channel-DSO assembly.

A. Karmakar, S. Paruya / Nuclear Eng

ube is placed outside the process tube through which the workinguid flows. Fig. 1 also shows the heater configuration. The heat isransferred from the surface of the aluminum tube to the surfacef the process tube by radiation and convection.

�Tsub, loop pressure and Q were regulated during the experi-ents by a cold-leg temperature controller, a pressure controller

nd a heater power controller, respectively. In order to maintain aesired cold-leg temperature, the temperature controller was oper-ted in proportional-integral (PI) mode to manipulate the flow ofooling water to the condenser. While minimizing the offset, theI mode may lend a possibility of intensifying system oscillationsecause of the oscillatory nature of the mode. We also noted theontrolled variable such as cold leg temperature oscillating withmall amplitude of 2.62 ◦C at a low �Tsub of 30 ◦C. The derivativections did not improve the situations in presence of severe mea-urement noise. We have discussed the measurement uncertaintiesf the oscillations later in this paper. The pressure was controlledn proportional (P) mode by manipulating air injection in the loophrough a solenoid valve. A compressor was used for this purpose.n accurate control of the heater power was achieved by a thyris-

or power controller acting in PID mode. The data acquisition andhe control operations were made centrally through a supervisoryontrol and data acquisition (SCADA) system. The data samplingate is 0.5 Hz. The loop was filled up with distilled water (preparedsing the ultra quartz distiller) as the working fluid. The heater washen put on for heating the water in the loop. It is required to men-ion that the two-phase valve controlling the loop resistance wasully open in all the experiments. Karmakar et al. (2014) providehe further details of the setup and the experimental procedure.

The flow structures were captured using Sony Handy CamHDR-XR160, frame rate—24 frame/s, shutter speed—1/6 (min) and/6000 (max), 3.3 mega pixels, 2112 × 1584) through the view glass25 mm diameter) just above the heater outlet, as shown in Fig. 1.

ith the help of picture browser motion (PMB), we extracted therames from the videos and analyzed them to understand the bub-le dynamics, the flow patterns and their transitions. The PMB is aoftware tool used for extracting frames from the videos and is anntegral component provided with the handy cam.

.2. Impedance needle probe

In this section, we describe the design and operation of thempedance needle probe used to determine the flow patterns andhe jet flows from the fluctuation of the voltage signals. The designnd fabrication of the impedance needle probe have been adoptedrom Paruya et al. (2009). An extensive use of the probes in the fastransients of multiphase systems is due to their very high speedf response and high sensitivity. They can be suitably operatedn either of conductance mode and capacitance mode by choos-ng a proper frequency of the AC-signal applied to excite the probe.ig. 2 presents a schematic of the probe and the associated electricalomponents for the voltage measurement. A copper wire of 0.8 mmiameter was insulated by varnish and wrapped with an ampereape. The wire was inserted concentrically in a 135 mm long stain-ess steel tube of 3 mm OD and has a 3 mm long leading edge outsidehe tube to sense the presence of bubbles. The leading edge haso insulation. The annular gap between the tube and the copperire was completely insulated by epoxy resin. The copper wire of

he probe was excited by a rectangular AC-voltage signal of 1 kHzapplied voltage Va is 3.16 V) from a function generator. The outerurface of the tube was insulated by ampere tape and varnish. Therobe was inserted horizontally (normal to the upflow in the chan-

el) to the center of the test channel and was installed just abovehe view glass fitted at the heater outlet (see Fig. 2). The inner con-uctor (copper wire) was excited by an AC-signal while the outerne (stainless steel tube) was grounded. A change in the voltage

Fig. 3. Effect of �Tsub on liquid temperature at heater outlet at Q = 4 kW�Tsub = 50 ◦C—�Tsub = 40 ◦C—�Tsub = 30 ◦C.

signal due to phase change at the tip of the probe was analyzedusing a digital storage oscilloscope. The voltage signal was stabi-lized by applying a suitable trigger voltage. The modulations of thefrequency and the amplitude of the voltage signal were recordedby the oscilloscope during flow oscillations.

3. Jet flow of vapor–liquid mixture

Before the boiling initiates, the heating of liquid at a constantpressure takes place to attain the nucleation point. The liquid isalmost passive and the stage is, therefore, regarded as incubation.At the nucleation point, the liquid gets superheated to initiate aphase change (boiling) and the bubbly flow is observed at the topof the heater. Within a few seconds, the bubbles form violently dueto a decrease of the local static head and thereby a transition ofthe bubbly flow to the distorted slugs of vapor takes place leadingto an expulsion (or ejection) of both bubble and liquid (geysers)from the heater. The expulsion associates to a high-velocity flow ofvapor–liquid mixture in the form of a jet. The bubble growth andthe bubble expulsion give rise to the high velocity of the mixture.We recognize the expulsion as jet flow in this work. The intensity ofjet flow observed by us is discussed later in Section 3.1. After sometime, the vapor is condensed in the long riser containing a largeamount of subcooled liquid. The condensed vapor flows back torefill the heater and the flow reversal is encountered. The flow meterregisters a negative flow. In the refilling stage, the static head in theheater increases to stop the occurrence of phase change and the jetflow; the liquid in the heater becomes almost stagnant. A full cycleof geysering oscillation ends here and the next cycle starts with theincubation stage. The flow reversal depends on �Tsub. Fig. 3 shows

the effect of �Tsub on the oscillations of liquid temperature THoat the heater outlet at Q = 4 kW. THo drops with �Tsub. The largestdrop of THo is observed at �Tsub = 50 ◦C. This gives the signature ofmore severe flow reversal from the riser to the heater at a higher

162 A. Karmakar, S. Paruya / Nuclear Engineering and Design 285 (2015) 158–170

Fig. 4. Jet flow duration and cycle time at various Q and �Tsub.

Table 2Jet flow duration and cycle time from the videos.

Q (kW) �Tsub (◦C) Jet flow durations (s) Cycle time (s)

4 50 5.24–31.44 23.04–73.52

�s

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4 30 3.48–11.8 9.44–31.485 30 3.92–5.80 11.68–15.42

Tsub. This is due to the condensation of vapor expedited by highubcooling in the riser in the refilling stage.

.1. Effect of �Tsub and Q on jet flow

The frames extracted from the videos using the PMB were ana-yzed to determine the cycle time (the frequency of jet flow) andhe duration of jet flow (the jet flow lasts for a few seconds) at dif-erent Q and �Tsub. Fig. 4 shows the durations of jet flows and theycle times for 24 consecutive cycles at different Q and �Tsub. At

= 5 kW and �Tsub = 30 ◦C, the points are close to each other com-ared to Q = 4 kW. The variations of the cycle time and the durationre not much at Q = 5 kW. It means that the jet flow occurs almostt a regular interval and it lasts for almost uniform durations. Thiss because of the fact that the big bubbles responsible for the jetows form at a regular interval at a higher Q.

For a decrease of Q to 4 kW from 5 kW at �Tsub = 30 ◦C, the jetow duration and the cycle time vary widely by an average factorf 5.0 (see Table 2). For an increase of �Tsub to 50 ◦C from 30 ◦C at

= 4 kW, their variations are found to be much wider by an averageactor of 13.5. The evidence clearly shows that at a higher �Tsub,oth jet flow duration and its frequency are highly irregular. How-ver, the effect of �Tsub on the duration and the frequency is moreignificant compared to the effect of Q. After a bubble departs fromhe heated surface, the subcooled liquid fills the region. The timeequired to generate the next bubble in the region increases withTsub. The departure time of the bubble also increases with �Tsub.ue to a low growth rate of the bubble at a high �Tsub, the bubble

eleasing time increases for a given Q. Table 2 shows a significanteduction of the jet flow duration and the cycle time when Q isncreased from 4 kW to 5 kW at �Tsub = 30 ◦C. The short durationf the jet flow is due to high rate of bubble release. The releaseate is given by 1/(tw + td); tw and td are bubble waiting time andubble departure time, respectively. Both tw and td decrease with Q.ith an increase of Q, the wall superheat �Tw (=Tw − Tsat) increases

nd the radius of cavities at nucleation sites decreases. The num-

er of the active nucleation sites per unit area of the heated surfacelso increases and so does the heat transfer coefficient. It conse-uently accelerates the mass transfer from the microlayer of liquido the bubble. As a result, the growth rate of the bubble increases

Fig. 5. Effect of �Tsub on the variations of Wloop and pHi at Q = 4 kW ········ pHi — Wloop.

and the bubble diameter increases rapidly with Q. Dhir et al. (2007)reported that the growth rate and the departure diameter increasedwith heat flux. The bubble departure time decreased from 45 msto 30 ms for an increase of �Tw from 7 to 9 ◦C while the depar-ture diameter increased from 3.2 mm to 3.4 mm. With an increasein �Tsub, the condensation rate increases appreciably leading toa significant reduction of the bubble growth rate. Dhir et al. alsoobserved the similar effect of subcooling on the bubble growth rateand the bubble departure time in the boiling flow. At a higher �Tsub,the frequency of bubble release decreases, as tw increases.

We captured the intensity of jet flow by measuring the loopflow rate Wloop. The time series of Wloop and pHi at different �Tsubfor Q = 4 kW are shown in Fig. 5. The peaks of Wloop are the sig-nature of the jet flows. The peaks are higher at lower �Tsub. Anextended reverse flow and high amplitude of pHi-oscillations arenoted at higher �Tsub. The incubation stage, the jet flow stage andthe refilling stage are indicated in Fig. 5(a) by ab, cd and de, respec-tively. The bubbly flow is observed in bc and Wloop starts to increasetherein. The region cd indicates the violent jet flow from the heater(where the peaks of Wloop are observed). In the region de, Wloopdecreases suddenly due to the collapse of bubbles in the riser ofNCBL. Reverse flow or flow termination occurs and a non-boilingcondition is reached in the heater. After the region de, the incuba-tion one again starts (where the initial stage of nucleate boiling isobserved). This stage is indicated by the flat line just after the regionde. At a lower �Tsub, the frequency of flow oscillations increasesand becomes more regular as evident in Fig. 5((b) and (c)). Boththe amplitude and the frequency of the oscillations increase with adecrease in �Tsub.

Discrete Fourier transform (DFT) of Wloop was done to obtainthe power spectral density (PSD)-frequency plot. Fig. 6 shows thePSDs of Wloop at different Q and �Tsub with a filling ratio of 0.75

(˚/Vloop = 0.75). We see that the dominant frequency increaseswith a decrease in �Tsub or with an increase in Q. At �Tsub = 50 ◦C,40 ◦C and 30 ◦C, the dominant frequencies are 0.0115 Hz, 0.029 Hzand 0.055 Hz, respectively, for Q = 4 kW, whereas the respective

A. Karmakar, S. Paruya / Nuclear Engineering and Design 285 (2015) 158–170 163

and Q

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Fig. 6. Effect of �Tsub

ominant frequencies are 0.021 Hz, 0.0395 Hz and 0.046 Hz for = 5 kW. The PSDs indicate an increased chaotic nature of theloop-oscillations at a lower Q or a higher �Tsub. As already men-

ioned, the oscillations are relatively regular at a lower �Tsub andhe same is confirmed by PSDs at a lower �Tsub. At high �Tsub,

any harmonics of the fundamental frequencies attribute to thehaotic oscillations of bubble surface. The harmonic frequencies arehe integeral multiple of the fundamental or dominant frequencyt which the peaks appear in the PSDs. We describe the oscillationso be chaotic with the help of Hurst exponent (>0.5) and correlationimensions (2–3) computed for Wloop.

At �Tsub = 50 ◦C, the harmonic frequencies are 0.0415 Hz and.080 Hz for Q = 4 kW, whereas they are 0.042 Hz and 0.0635 Hzor Q = 5 kW, respectively. The PSD at Q = 5 kW and �Tsub = 40 ◦Chows no harmonic frequency while the harmonic frequencies of.0575 Hz and 0.084 Hz are observed at Q = 4 kW and �Tsub = 40 ◦C.herefore, either lowering Q or increasing �Tsub produces morearmonic frequencies. Wu et al. (1996) also observed many num-ers of harmonic frequencies at a high �Tsub in their NCBL.

. Visualization of flow patterns and pattern transitions

This section describes the visualization of flow structures dur-ng natural circulation boiling. The grayscale frames presented inigs. 7 and 8 capture the changes of bubble shape during bubbleucleation, bubble merger, mushrooming, slugging and bubble col-

apse. The figures also show the effect of Q and �Tsub on the bubblehape. The dotted lines/contours indicate the bubble shapes and theotted lines the bubble–water interfaces and the bubble–bubble

nterfaces. The video imaging was performed for more than 1 min.owever, the analysis of the images reveals that the local bub-le dynamics and the local pattern transitions continued for 3–5 sepending on Q and �Tsub, during which the jet flow mostly occurs.

on the PSDs of Wloop.

We, therefore, have presented the images for only 2–3 s duringwhich a fast bubble growth or bubble collapse occurs.

Fig. 7 presents the effect of Q on the shape of the bubbleand the growth history at �Tsub = 30 ◦C at Q = 4 kW and 5 kW.Fig. 7((a)–(h)) and Fig. 7((i)–(p)) show the frames at several instantsat Q = 4 kW and 5 kW, respectively. The frames reveal several com-plex phenomena that take place in succession during the bubblegrowth—(1) initially, the spherical bubble (see Fig. 7(a)) forms anddeforms to an ellipsoid one (Fig. 7(b)) due to the pressure differ-ence across its diameter, (2) an axial bubble merger takes places(Fig. 7((c)–(d)); the interfaces of two or more bubbles merge to acommon interface. As the time progresses, the merged bubbles giverise to the vapor mushroom of large vapor mass with bubble stems(Fig. 7(e)). The merged bubbles also lead to form a column of bub-bles (Fig. 7(f)). At Q = 4 kW and �Tsub = 30 ◦C, the vapor mushroomsare umbrella-type mass with a long bubble stem. The vapor mush-rooms attached to the column of bubbles are also noted (Fig. 7(g)).Bubbly/vapor mushroom transition during transition boiling wasobserved by many authors in the past (Gaertner, 1965; Haramuraand Katto, 1983; Das et al., 2006). In our study, we also observed theformation of vapor mushrooms leading to the transition boiling atQ = 4 kW and �Tsub = 30 ◦C. The occurrence of transition boiling canbe established based on the periodic oscillation of the heater-walltemperature Tw (thermal oscillations) presented in Fig. 9. Fig. 9 hasbeen adapted from Karmakar and Paruya (2012a). The oscillations,as we see in the figure, have a very long period (about 8.3 min) andvery low amplitude. The long period (3.5 min) of the thermal oscil-lations during the transition boiling was also reported by Liu et al.(1994) using Freon in the heater of almost similar configuration.

In our photographic study, we observed bubbly/vapor mush-

room transition during the transition boiling. The transition boilingresults in the thermal oscillations. When Q is increased to 5 kW, thegrowth phenomena significantly differ from that at Q = 4 kW. Thenumber of the active nucleation sites becomes sufficiently high

164 A. Karmakar, S. Paruya / Nuclear Engineering and Design 285 (2015) 158–170

Fig. 7. Effect of heater power Q on bubble growth.

n bub

dTFt

Fig. 8. Effect of �Tsub o

ue to increased Q and the small bubbles coalesce to a big one.he vapor–liquid interfaces of the big slug bubbles are shown inig. 7((k)–(p)). They look glossy white. The interfaces are more dis-inct and less complex. The length of the interface is almost equal

ble growth at Q = 4 kW.

to the diameter of our test channel (20.96 mm ID). It indicates thatsuch bubble covers the entire cross-section because the large diam-eter of the channel. The interface of the bubble is almost flat witha large radius of curvature due to the action of high pressure on

A. Karmakar, S. Paruya / Nuclear Engineering and Design 285 (2015) 158–170 165

heater-wall temperature [17].

tstaqbf

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5b

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V

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Table 3Standard deviation of voltage amplitude for different trigger voltage at Q = 5 kW and�Tsub = 30◦C.

Trigger voltage (V) Period (s) Standard deviation

1.0 16.00 0.547030172

Fig. 9. Effect of �Tsub on

he interface. As shown in Fig. 7((m)–(p)), the interfaces are mostlytable and the shape of the interface does not change much withime. It, therefore, confirms the formation of the slug bubbles. Welso noted that the slug flow lasted for 2–3 s with a mean slug fre-uency of 5–6 s−1 during a jet flow. At Q = 4 kW, the shape of theubbles continuously changes with time and the possibility of slugormation is relatively less.

The effect of �Tsub on the bubble growth and the bubble shapet Q = 4 kW is presented in Fig. 8. The frames at �Tsub = 50 ◦C, 40 ◦C,5 ◦C and 30 ◦C are shown in the figure. At �Tsub = 50 ◦C, small bub-les form continuously (Fig. 8((a)–(d)) and they cannot grow muchue to the high inlet subcooling which induces bubble condensa-ion aggressively. Thus, the bubble merger and the coalescence areot favored by a high �Tsub of 50 ◦C. Due to the large tempera-ure gradient at high �Tsub, the bubble-size distribution becomesider. Decreasing �Tsub, as we see, expedites the axial bubbleerger and the coalescence. Thus, it causes the formation of large

apor bubbles. Eventually, fully developed vapor mushrooms with long vapor stem and the columns of bubbles (Fig. 8((l)–(m))t �Tsub = 30 ◦C) form. The bubble condensation and the bubbleerger occur at �Tsub = 40 ◦C and are shown in Fig. 8((e) and (f))

nd ((g) and (h)), respectively. The big bubbles merging to thenderdeveloped slugs at �Tsub = 35 ◦C (Fig. 8((j) and (k))) are alsobserved. The vapor–liquid interfaces become gradually more dis-inct as �Tsub is gradually decreased. During bubble condensationt �Tsub = 40 ◦C, as we see in Fig. 8((e) and (f)), the size of the risingubbles decreases until their bottom becomes hollow. The bubble

ooks “dimpled ellipsoid” and “skirt”. These observations closelygree to those reported by Kamei and Hirata (1990).

. Analysis of voltage signals and local pressure signals forubble dynamics

In our electric circuit, the applied voltage Va (=3.16 V) is the sumf voltage amplitude VPP, the voltage drop across the probe and theoltage drop across the metal conductors (copper wire and stainlessteel electrodes). The sum is expressed as:

PP + ic (Rmc + Gm) = Va (1)

Eq. (1) is rewritten in the following to find VPP:

PP = Va − ic (Rmc + Gm) (2)

c is the circuit current; Rmc is the resistance of the metal conductor;m is the impedance of vapor–liquid mixture at the test location.ne can ignore Rmc(Gm � Rmc). A high impedance results in a lowPP and vice versa, as seen in Eq. (2). Therefore, VPP is low for a high

2.0 15.58 0.5099328943.0 14.43 0.156120951

void fraction and high at a low ˛. Fig. 10 shows the output voltagesignals from the probe for the two-phase bubble–water flow andthe single-phase saturated water flow at Q = 5 kW and �Tsub = 30 ◦C.Due to the high impedance of the bubble–water flow, the amplitudeof the signal drops and so does VPP. Because of very low impedanceof the saturated water flow compared to bubble–water flow, theoutput voltage signal remains almost close to the applied signal of1 kHz frequency and 3.16 V amplitude without much deformation.

Fig. 11 shows the variations of the VPP-signal and its frequencywith time at the trigger voltages of 1 V, 2 V and 3 V. During the for-mation of bubbles, VPP drops while its frequency increases. At ahigh ˛, the VPP drops to a minimum and the frequency increasesto a maximum. At a low (the water fraction is significant), VPPincreases to a maximum and its frequency drops to a minimum.The mean period of the VPP-signal does not change significantlywith the trigger voltage while the standard deviation (SD) of theVPP-signal is very sensitive to the trigger voltage. Table 3 showsthat the SD of the VPP-oscillations at a trigger voltage of 1 V ishigher than the SDs at 2 V and 3 V. At the trigger voltage of 1 V,the probe can efficiently track the bubbles. Increased sensitivityof the probe at a lower trigger voltage can be explained by thefact that the flow of electrons in a gas–liquid system of very highimpedance (about 104–105 �) is different from that in a metal con-ductor (about 1–10 �). In the gas–liquid system, the weak electriccurrent is due to limited availability of electrons. The current doesnot increase in proportion to the phase fraction when the appliedvoltage across the gas–liquid mixture is increased.

Since the fluctuations of the VPP-signal are due to the alter-nate presence of bubble–water and water hitting the tip of theprobe, the fluctuations are equivalent to the fluctuations of local˛. Fig. 12((a)–(c)) presents the effect of �Tsub on the VPP-signals atQ = 4 kW and the trigger voltage of 1 V. The chaotic VPP-oscillationsat a high �Tsub of 50 ◦C are due to a wide range of bubblesizes resulting from an irregular bubble growth and collapse. We

describe the chaotic oscillations with the help of Hurst exponent(>0.5) and correlation dimensions (1–2) computed for the signal.The periods of the VPP-signals presented in Table 4 are in closeagreement with periods (cycle time) of jet flows determined from

166 A. Karmakar, S. Paruya / Nuclear Engineering and Design 285 (2015) 158–170

Fig. 10. Voltage amplitude signals from the probe observed in DSO (a) bubble

Table 4Comparison of periods of pHi signals, Wloop-signals and VPP-signals at Q = 4 kW.

Signals Periods (s)(�Tsub = 50 ◦C)

Periods, (s)(�Tsub = 30 ◦C)

pHi 27.78 500.0, 18.5Wloop 86.95, 24.09 18.10, 500.0

ttbmsrtrsT

5W

aoVia�s(rfqaqba�tfcmitP

VPP 21.36, 64.10 16.0, 8.54, 21.36Photographic measurement 23.04–73.52 9.44–31.48

he videos (refer Table 2). As �Tsub is decreased to 30 ◦C, the ampli-ude and the frequency of the VPP-signals increase and the peaksecome sharper and more regular, as seen in Fig. 12, because ofore or less uniform size of bubbles. The large amplitude of the

ignal at �Tsub = 30 ◦C is due to the presence of large vapor mush-ooms. (see Fig. 7((e)–(h))). The SDs of the VPP-signals are estimatedo be 0.3086, 0.3378 and 0.4309 at �Tsub = 50 ◦C, 40 ◦C and 30 ◦C,espectively. An increase of the SD with a decrease of �Tsub alsoignifies higher amplitude of the VPP-oscillations at a lower �Tsub.he SDs at �Tsub = 50 ◦C and 40 ◦C do not differ appreciably.

.1. PSDs of voltage signals and comparisons with pHi-signals and

loop-signals

Fig. 13 presents the PSDs of the VPP-signals at different �Tsubnd Q = 4 kW. The PSD gives a feel of the chaotic nature of VPP-scillations when there is a continuous decrease of PSD of thePP-signal with frequency and the peaks in the PSD appear at

rregular intervals of frequencies (many fundamental frequenciesre present). We see the decrease of PSD with frequency in Fig. 13 atTsub = 50 ◦C. With decrease in �Tsub, both the frequency and the

trength of the oscillations increase. The short and regular periods8.54–21.36 s) of the VPP-signals at �Tsub = 30 ◦C are also compa-able with the periods (9.44–31.48) of the jet flows determinedrom the videos (see Table 4). At �Tsub = 50 ◦C, the dominant fre-uency is 0.0468 Hz. At �Tsub = 30 ◦C, the fundamental frequenciesre 0.0468 Hz and 0.0625 Hz (dominant). Two fundamental fre-uencies at a lower �Tsub make sure the presence of two classes ofubbles. Usually the low frequency corresponds to a large bubblend the high frequency to a small bubble. The frames at Q = 4 kW andTsub = 30 ◦C in Fig. 7((e)–(h)) show that the bubbles are substan-

ially mushrooms with stem, and a column of some small bubblesollows the mushrooms. These two classes of bubbles seem toontribute to the two fundamental frequencies. It is important to

ention that these two classes of bubbles are very difficult to be

dentified by the pHi-signals or Wloop-signals. The effect of �Tsub onhe VPP-signal is comparable with the effect on the pHi-signal. TheSDs of pHi at different �Tsub for Q = 4 kW are presented in Fig. 14.

–water flow and (b) saturated water flow at Q = 5 kW and �Tsub = 30 ◦C.

The dominant frequency increases with �Tsub. The fundamentalfrequencies are 0.002 Hz (dominant), 0.054 Hz & 0.004 Hz (domi-nant), and 0.016 Hz & 0.036 Hz (dominant) at �Tsub = 30 ◦C, 40 ◦Cand 50 ◦C, respectively. The frequency distribution is the widestone at �Tsub = 50 ◦C indicating the presence of several classes ofbubbles that lead to a complex bubble dynamics at �Tsub = 50 ◦C.At �Tsub = 50 ◦C, the counteraction of evaporation and condensa-tion through the bubble surface causes more intense oscillations ofbubble surface leading to a high-frequency fluctuations of the localpressure. The PSD also shows a significantly high dominant fre-quency of pHi (0.036 Hz). Many other frequencies at �Tsub = 50 ◦Calso attribute to the chaotic bubble oscillations. The videos in Fig. 8also show strong oscillations of the bubble surface at Q = 4 kWand �Tsub = 50 ◦C. As reported by Karmakar and Paruya (2012a),the dominant frequency of 0.002 Hz is equal to the frequency ofTw-oscillations (thermal oscillations) at �Tsub = 30 ◦C. The ther-mal oscillations occur due to switching between nucleate boilingregime and the transition boiling regime.

We also compare the VPP-signals with those of the pHi-signalsand the Wloop-signals at different �Tsub for Q = 4 kW presented inTable 4. We found that the signals could measure the cycle time(periods) of the jet flows close to the periods obtained from thevideos. The VPP-signals have the periods of 8.54–21.36 s (the dom-inant period is 16.0 s) at �Tsub = 30 ◦C. The periods are very closeto those obtained from the videos (9.44–31.48 s and its logarithmicmean is 18.30 s). At �Tsub = 30 ◦C, the pHi-signals have the domi-nant period of 500 s for the Tw-oscillations. They have also anotherfundamental period of 18.5 s (of very low amplitude) for the flowoscillations, which is close to the mean period obtained from thevideos. Unlike the pressure sensor, the probe has the ability todetect the jet flow with the dominant period (16 s) close to themean period (18.3 s) and the jet flow with an extremely low period(8.54 s) close to the lowest value (9.44 s) obtained from the videos.The pHi-signal does not have the periods close to 9.44 s. It maybe due to a large measurement error of the pressure sensors. TheWloop-signals have the dominant period of 18.18 s with a deviationof 0.6% and have no periods close to 9.44 s obtained from the videos.These deviations are with respect to the logarithmic time period(18.30 s) derived from the videos. VPP-signals have the period of8.54 s having a deviation of (−) 9.5% from 9.44 s obtained from thevideos.

At a higher subcooling, the jet flows are not so fast due to avery low bubble growth. At �Tsub = 50 ◦C, the dominant periodsof the VPP-signal, the pHi-signal and the Wloop-signal are 21.36 s,

27.78 s and 86.95 s, respectively. The videos provide the cycle timeof 23.04–73.52 s at the same condition. Table 4 shows that theVPP-signals and the Wloop-signals have the periods in the rangeof 21.36–64.10 s and 24.09–86.95 s, respectively. The pHi-signals

A. Karmakar, S. Paruya / Nuclear Engineering and Design 285 (2015) 158–170 167

= 5 kW

dcvnt

Fig. 11. Variations of voltage amplitude and frequency at Q

o not have the period close to 73.52 s. We conclude from the

omparisons that the voltage signals can successfully catch bothery high and very low frequency triggers of jet flows, which can-ot be detected using the pHi-signal. It is worthwhile to mentionhat the response time of the pressure sensors and the DPT is

, �Tsub = 30 ◦C and trigger voltage (a) 1 V (b) 2 V and (c) 3 V.

sufficiently low (∼10−3 s) so that they can capture the dynam-

ics during the delay time. The delay time (∼10−1–101 s) variesdepending on �Tsub and Q. However, the periods of jet flows,we determined, are also comparable with those reported by Wuet al. (1996) for their heater and riser with a total height of

168 A. Karmakar, S. Paruya / Nuclear Engineering and Design 285 (2015) 158–170

Fig. 12. Effect of �Tsub on voltage amplitude signals with a trigger voltage of 1 V atQ = 4 kW.

F

2aQ�ir

5W

ofWts

Fig. 14. Effect of �Tsub on the PSDs of pHi at Q = 4 kW.

Fig. 15. Variation of cumulative SDs with time at various �Tsub and Q = 4 kW.

Table 5Experimental uncertainties of loop flow rate and inlet pressure.

Heating rate (Q) 4 kW 5 kW

�Tsub (◦C) 30 40 50 30 40 50

ig. 13. PSDs of voltage amplitude signals with a trigger voltage of 1 V at Q = 4 kW.

.1 m. They found the periods to be in the range of 11.88–42.97 snd 13.96–57.06 s at �Tsub = 44 ◦C and 56 ◦C, respectively for

= 4 kW. At Q = 4 kW, we obtained the periods of 10.75–64.10 s atTsub = 40 ◦C and 21.36–86.95 s at �Tsub = 50 ◦C. The longer periods

n our NCBL are due to a relatively large height of the heater andiser (the total height is 3.0 m).

.1.1. Uncertainties of the VPP-signals, the pHi-signals and

loop-signalsIt is highly relevant to discuss the uncertainties of measurement

f the voltage, pressure and loop flow rate. The VPP-signals were

ound to be highly non-stationary compared to the pHi-signals and

loop-signals. Fig. 15 presents the variations of cumulative SDs ofhe voltage signals with time at various �Tsub for Q = 4 kW. The VPP-ignals at �Tsub = 50 ◦C and 40 ◦C look stationary as the cumulative

Uncertainties of Wloop (%) 2.0% 3.7% 5% 4.47 4.88% 5.24%Uncertainties of pHi (%) 1.36% 6.0% 6.68% 2.0% 9.3% 9.14%

SDs remain almost unchanged with time. The difference betweenthe cumulative SDs of the VPP-signals at �Tsub = 50 ◦C and 40 ◦Cis very small. This indicates that the flow patterns at these condi-tions seem to be very close to each other. Fig. 15 also shows thatthe cumulative SD at �Tsub = 30 ◦C is appreciably high comparedto that at �Tsub = 40 ◦C and 50 ◦C. The uncertainties of the signalswere computed to be within 20%. The uncertainty increases withdecrease in �Tsub.

All the instruments including pressure sensors, RTDs and elec-tromagnetic flow meter have the uncertainties less than 0.5%. We

noted from the variations of the SDs of Wloop and pHi with time thatthe times series are almost stationary with the maximum uncer-tainties within 10.0%. Table 5 shows the uncertainties of Wloop and

A. Karmakar, S. Paruya / Nuclear Engineering and Design 285 (2015) 158–170 169

phhtst2twbrtasp

5

ceflciot

I�ibia(tCoaasopit�ucpd

Fig. 16. PSDs of voltage amplitude signals at various �Tsub.

Hi at different Q and �Tsub. The uncertainties are large a bit atigher Q, indicating a slight non-stationary nature of the signals atigher Q. These uncertainties are due to the controllability of theemperature controller and the pressure controller. The loop pres-ure controller has the uncertainties within 0.5% of full scale whilehe uncertainty of the temperature controller varies in between.8% and 3.7% depending on the �Tsub. A small loss of water fromhe loop and the air injection into the loop for the control of ploopere noted. The loss of water for one-hour experiment varied in

etween 1.5 and 5.0% depending on the operating conditions. Aelatively large loss of fluid from the loop at higher Q is due tohe difficulty of controlling the loop pressure. We also experience

small heat loss. Although the setup is well insulated outside, ituffers from a small amount of heat loss (about 3.5% of the heaterower).

.2. PDFs for the voltage signal

The statistical analysis of the VPP-signals, such as PDF, has beenarried out to reveal the characteristics of the flow patterns (Ghosht al., 2012; Hills, 1976). As mentioned in Section 1, the differentow regimes in a two-phase air–water flow including bubbly flow,ap-bubbly flow, slug flow, churn flow and annular flow can bedentified based on the nature of PDFs of impedance signals. Basedn the guidelines outlined in Section 1, we attempted to identifyhe flow structure in the heater section of our NCBL.

Fig. 16 presents Kernel-PDFs at various �Tsub for Q = 4 kW.t has been noted that the Gaussian-PDFs look almost same at

Tsub = 30 ◦C and 40 ◦C, while the Kernel-PDF at �Tsub = 30 ◦C hasts trailing part at a high VPP. It argues why the Kernel-PDFs haveeen preferred in the present study. At �Tsub = 40 ◦C, the PDF shown

n Fig. 16 has a narrow distribution with a sharp and long peak at higher VPP (lower impedance) indicating developed bubbly flowsee Fig. 8((e)–(h)) for videos). The peak at a higher VPP revealshat the vapor bubbles are dispersed in a large volume of liquid.ompared to the PDF at �Tsub = 40 ◦C, the PDF at �Tsub = 30 ◦C isbserved to have a wider distribution with a relatively low peak at

lower VPP (higher impedance) and to have a flat trailing edge at higher VPP. The peak indicates the formation of underdevelopedlugs or fully developed vapor-mushrooms with a long vapor stem,r the column of bubbles (see Fig. 7((e)–(h)) for videos). The singleeak for the underdeveloped slugs or fully developed mushrooms

s due to the presence of the liquid-microlayers at the bottom or onhe sides of the mushroom indicated in Fig. 8((l)–(m)). Our PDF at

Tsub = 30 ◦C looks similar to that of Schlegel et al. (2009) obtained

sing two arc-electrodes for slug flow of air–water mixture in thehannel of large diameter (150 mm); their PDF has a single blunteak at a lower admittance (higher impedance) and has a wideristribution. Ghosh et al. (2012) obtained the PDF with two peaks

Fig. 17. CPDFs of voltage amplitude signals at various �Tsub.

for slug flow of air and water in a channel of 25.2 mm using two-wire parallel electrode and the PDF with one peak at higher voltageusing a ring-type electrode. The ring-type conductivity probe ofGhosh et al. detected the thin layer of liquid around a big bubblein addition to the liquid slug resulting in a peak of PDF at a highvoltage. Our probe is sensitive to vapor phase because it was oper-ated in the impedance mode with an AC-excitation frequency. Thebehavior is very similar to the arc-type impedance probe of Schlegelet al. which was very sensitive to vapor slug resulting in a peak atlow admittance.

As already discussed, the videos confirm the presence of under-developed slugs, fully developed mushrooms with a long bubblestem, and column of bubbles. The bubble shapes observed byus using the videos and the voltage signals are very consistent.Following Schlegel et al. (2009), the cumulative PDFs (CPDF) at�Tsub = 30 ◦C and 40 ◦C are presented in Fig. 17. The CPDFs alsosupport the conclusions drawn from the PDFs presented in Fig. 16.However, the variation of CPDF is consistent with the results ofSchlegel et al. (2009). The sharper rise of the CPDF with non-dimensional voltage amplitude (VPP/VPPmax) at �Tsub = 40 ◦C alsoconfirms the bubbly flow that occurs in the channel and the risestarts at higher value of VPP/VPPmax. The high value of VPP/VPPmax

indicates the presence of large volume of liquid. The CPDF at�Tsub = 30 ◦C does not increase as fast as that at �Tsub = 40 ◦C andeventually becomes flat at high VPP/VPPmax. However, the CPDF at�Tsub = 30 ◦C starts rising faster at low VPP/VPPmax indicating thepresence of large volume of vapor compared to that at �Tsub = 40 ◦C.So, one can confirm the possibility of forming bubbles of largerdiameter at �Tsub = 30 ◦C.

6. Conclusions

In this paper, we have discussed our experimental observationson the flow instabilities, the flow structures and the jet flows withthe help of the videos and the impedance probe on the parameterspace of Q and �Tsub. The loop flow rate and the local pressure werealso measured. The major findings are:

1. Either of lowering Q and increasing �Tsub produces higher har-monic frequencies leading to chaotic oscillations. The same hasbeen demonstrated with the PSDs of Wloop for varied �Tsub atQ = 4 kW and 5 kW. The flow reversal becomes more aggres-sive due to bubble condensation at a higher �Tsub at whichthe jet flow has a longer period (21.36–86.95 s) and is highlyirregular (chaotic). The time periods are close to those obtainedfrom video imaging (23.04–73.52 s). The jet flow becomes moreregular (periodic) with lower periods (9.44–31.48 s) and higher

amplitudes at a lower �Tsub.

2. The video imaging reveals that the flow patterns include bub-bly flow, vapor mushrooms with long vapor stem, the columnof bubbles and the slug flow depending on Q and �Tsub. The

1 ineeri

3

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70 A. Karmakar, S. Paruya / Nuclear Eng

patterns, as we observe, are due to the complex bubble dynam-ics such as bubble collisions, bubble merger and coalescence.The vapor mushrooms form at a relatively low Q for a given�Tsub and at a relatively low �Tsub for a given Q while a highQ and a low �Tsub favor the formation of slug bubbles. The slugsform vigorously at Q = 5 kW and �Tsub = 30 ◦C with a frequencyof 5–6 s−1.

. The sensitivity of the probe strongly depends on the trigger volt-age. An optimal trigger voltage yields a desired sensitivity. Theoverall comparisons of the VPP-signals and the pHi-signals revealthat the probe performs better in detecting both the very highfrequency and the very low frequency triggers of jet flows withinan uncertainty of 9%. The probe can also sense a variety of bubbleclasses.

. The voltage signals from the probe can successfully detect theflow patterns and their transitions observed with the help ofvideo imaging technique. The PDFs and the CPDFs of the voltagesignals and the videos confirm the transition from the bub-bly flow to undeveloped vapor-mushrooms to fully developedvapor-mushrooms as a result of decreasing �Tsub. The bub-bly/vapor mushroom transition at �Tsub = 30 ◦C and Q = 4 kW isassociated with the transition boiling. A further increase of Q to5 kW causes the bubbly/slug transition.

It is also relevant to point out some shortcomings of the presentork. Experiments at additional heating powers would have helpedraw better conclusions. We could not increase Q above 5 kWecause experimental uncertainty would be large due to loss ofass from the loop. The study provides mostly the qualitative

nformation on the local flow structure and pattern transitions.he quantitative data would have a potential use in validating theelated numerical models. Mathematical formulation of the loop isnother challenging issue that can be explored in future. We plano replace the present control schemes with a centralized DCS, inhich we can test the loop using any of control modes (P, PI and

ID).

cknowledgement

The authors are thankful to the reviewers for their valuable sug-estions to improve the technical content and the presentations.he financial support of Department of Science and Technology,ndia under SERB (Sanction No. SR/S3/CE/089/2009 and SE-SB-3-CE-006-2013) scheme required for this work is gratefullycknowledged. The authors also thank Oinam Bidyarani Devi foreading the manuscript carefully.

eferences

ritomi, M., Chiang, J.-H., Mori, M., 1993. Geysering in parallel boiling channels. Nucl.Eng. Des. 141 (1–2), 111–121.

aars, A., Delgado, A., 2006. Non-linear effects in a natural circulation evapora-tor: geysering coupled with manometer oscillations. Heat Mass Transfer 43,427–438.

eccio, S.L., George, D.L., 1996. A review of electrical impedance techniques for themeasurement of multiphase flows. J. Fluid Eng. 118, 391–399.

heng, H., Hills, J.H., Azzorpardi, B.J., 1998. A study of the bubble-to-slug transition

in vertical gas–liquid flow in columns of different diameter. Int. J. MultiphaseFlow 24, 431–452.

as, A.K., Das, P.K., Saha, P., 2006. Heat transfer during pool boiling based onevaporation from micro and macrolayer. Int. J. Heat Mass Transfer 49, 3487–3499.

ng and Design 285 (2015) 158–170

de Mesquita, R.N., Masotti, P.H.F., Penha, R.M.L., Andrade, D.A., Sabundjian, G., Torres,W.M., Macedo, L.A., 2012. Classification of natural circulation two-phase flowpatterns using fuzzy inference on image analysis. Nucl. Eng. Des. 250, 592–599.

Dhir, V.K., Abarajith, H.S., Li, D., 2007. Bubble dynamics and heat transfer during pooland flow boiling. Heat Transfer Eng. 28, 608–624.

Durga Prasad, G.V., Pandey, M., Kalra, M.S., 2007. Review of research on flowinstabilities in natural circulation boiling systems. Prog. Nucl. Energy 49, 429–451.

Gaertner, R.F., 1965. Photographic study of nucleate pool boiling on a horizontalsurface. J. Heat Transfer 87, 17–29.

Ghosh, S., Pratihar, D.K., Maiti, B., Das, P.K., 2012. Identification of flow regimes usingconductivity probe signals and neural networks for counter-current gas–liquidtwo-phase flow. Chem. Eng. Sci. 84, 417–436.

Goswami, N., Paruya, S., 2011. Advances on the research on nonlinear phenomenain boiling natural circulation loop. Prog. Nucl. Energy 53, 673–697.

Griffith, P., 1962. Geysering in liquid-filled lines. ASME Paper 62-HT-39.Hands, B.A., 1979. Flow stability of a liquid-nitrogen thermosyphone with 8 mm bore

riser. AIChE Symp. Ser. 189, 177.Haramura, Y., Katto, Y., 1983. New hydrodynamic model of critical heat flux appli-

cable widely to both pool and forced convection boiling on submerged bodiesin saturated liquids. Int. J. Heat Mass Transfer 26, 389–399.

Hills, J.H., 1976. The operation of a bubble column at high throughputs, I. Gas holdupmeasurements. Chem. Eng. J. 12, 89–99.

Hsieh, C.C., Wang, S.B., Pan, C., 1997. Dynamic visualization of two-phase flow pat-terns in a natural circulation loop. Int. J. Multiphase Flow 23, 1147–1170.

Jiang, S.Y., Yao, M.S., Bo, J.H., Wu, S.R., 1995. Experimental simulation study onstartup of the 5 MW nuclear heating reactor. Nucl. Eng. Des. 158, 111–123.

Jones Jr., O.C., Zuber, N., 1975. The interrelation between void fraction fluctuationsand flow patterns in two-phase flow. Int. J. Multiphase Flow 2, 273–306.

Kamei, H., Hirata, M., 1990. Condensing phenomena of a single vapor bubble intosubcooled water. Exp. Heat Transfer 3 (2), 173–182.

Karmakar, A., Paruya, S., 2012a. Nonlinear analysis of chaotic time series in a naturalcirculation boiling loop. Ind. Eng. Chem. Res. 51, 16467–16481.

Karmakar, A., Paruya, S., 2012b. Bubble growth and expulsions in natural circulationboiling loop. In: AIChE Annual Meeting, Paper ID—277994, Pittsburgh.

Karmakar, A., Goswami, N., Paruya, S., 2014. Subcooled boiling oscillations in naturalcirculation boiling loop at low pressure. AIChE J. 60 (1), 375–386, 2014.

Khodabandeh, R., Furberg, R., 2010. Instability, heat transfer and flow regime in atwo-phase flow thermosyphon loop at different diameter evaporator channel.Appl. Therm. Eng. 30, 1107–1114.

Kyung, I.S., Lee, S.Y., 1994. Experimental observations on flow characteristics in anopen two-phase natural circulation loop. Nucl. Eng. Des. 150, 163–174.

Le Corre, J.-M., Hervieu, E., Ishii, M., Delhaye, J.-M., 2003. Benchmarking and improve-ments of measurement techniques for local-time-averaged two-phase flowparameters. Exp. Fluids 35, 448–458.

Liu, H.T., Kakac , S., Mayinger, F., 1994. Characteristics of transition boiling and ther-mal oscillation in an upflow convective boiling system. Exp. Therm Fluid Sci. 8,195–205.

Nayak, A.K., Vijayan, P.K., 2008. Flow instabilities in boiling two-phase natural cir-culation systems: a review. Sci. Technol. Nucl. Install., 1–15, Article ID 573192.

Ozwaw, M., Nakanishi, S., Ishigai, S.M.Y., Tarui, H., 1979. Flow instabilities in boilingchannels, Part 2: Geysering. Bull. Jpn. Soc. Mech. Eng. 22 (170), 1119–1126.

Paglianti, A., Pintus, S., 2001. An impedance probe for measurements of liquid hold-up and mixing time in two/three-phase stirred tank reactor. Exp. Fluids 31,417–427.

Paranjape, S., Ritchey, S.N., Garimella, S.V., 2012. Electrical impedance-based voidfraction measurement and flow regime identification in microchannel flowsunder adiabetic conditions. Int. J. Multiphase Flow 42, 175–183.

Paruya, S., Saha, A.K., Bhattacharya, P., 2009. Validations of thermohydraulic modelsfor geysering in a natural circulation loop using an impedance needle probe. Ind.Eng. Chem. Res. 48, 2020–2033.

Schlegel, J.P., Sawant, P., Paranjape, S., Ozar, B., Hibiki, T., Ishii, M., 2009. Void fractionand flow regime in adiabatic upward two-phase flow in large diameter verticalpipes. Nucl. Eng. Des. 239, 2864–2874.

Subki, M.H., Aritomi, M., Watanabe, N., Kikura, H., Iwamura, T., 2003. Transportmechanism of thermohydraulic instability in natural circulation boiling waterreactors during startup. J. Nucl. Sci. Technol. 40, 918–931.

Watanabe, N., Aritomi, M., Kikura, H., 2008. Thermal Hydraulic Flow Oscillation Char-acteristics in Multiformed Channels under Natural Circulation and Low-PressureConditions. J. Nucl. Sci. Technol. 45 (2), 160–170.

Wu, C.Y., Wang, S.B., Pan, C., 1996. Chaotic oscillation in a low-pressure two-phasenatural circulation loop under low power and high inlet subcooling conditions.

Nucl. Eng. Des. 162, 223–232.

Yadigaroglu, G., 1981. Two-phase flow instabilities and propagation phenomena. In:Delhaye, J.M., Giot, M., Riethmuller, M.L. (Eds.), Thermodynamics of Two-PhaseSystems for Industrial Design and Nuclear Engineering. McGraw-Hill, New York,NY, p. 353.