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TERM PAPER

OF ENGINEERING THERMOYNAMICS

Topic: First law applied to flow process

SUBMITTED TO SUBMITTED BY

Tukesh soni Anush charak

Section b4912

Roll no.A01

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ABSTRACT

This report is based on the how First law applied to flow process. I am discuss in this report how first law applied to steady and unsteady flow process .due to this I am find the mass and energy balance for a general steady-flow system and un-steady flow process

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Table of content

Introduction of first law of thermodynamics………………………..5 Derivation of general energy equation…………………………5-7 Steady flow process…………………………………………..7 Mass Balance for a Steady flow process……………………..7-8 Characteristic of steady flow process……………………………..8 Energy analysis of steady flow process………………………….8-9 Steady Flow Engineering Devices……………………………….9-10 Energy analysis of unsteady flow process…………………………11 References………………………………………………………12

ACKNOWLEDGEMENT

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I great thank to a great many people who helped and supported me during the writing of this term paper.

My deepest thanks to Lecturer, Tukesh soni the Guide of this term paper for guiding and correcting various documents of mine with attention and care. I also extend my heartfelt thanks to my family and well wishers.

INTRODUCTION OF FIRST LAW OF THERMOYNAMICS……a

The first law of thermodynamics is an expression of the principle of conservation of energy. It states that energy can be transformed (changed from one form to another), but cannot be created or destroyed.

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The first law is usually formulated by saying that the change in the internal energy of a close thermodynamic system is equal to the difference between the heat supplied to the system and the amount of work done by the system on its surroundings. It is important to note that internal energy is a state of the system whereas heat and work modify the state of the system. In other words, a specific internal energy of a system may be achieved by any combination of heat and work; the manner by which a system achieves a specific internal energy is path independent.

Some points of control volume……………………b

Denote with CV subscript (e.g., mCV) Mass and energy cross system boundary

DERIVATION OF GENERAL ENERGY EQUATION…………c

Consider a system which is identical to a control volume.  Let us consider the change in the system over a finite time interval dt.

We have allowed the flexible system boundary to enclose some matter in the inlet "pipe' at time t, and to enclose some other matter in outlet "pipe' at time t + dt. Apart from these places, at specified times, the system boundary is identical to the control surface.

Thus over a time interval dt, dmi mass of fluid enters the control volume and dme leaves the control volume. Heat and work interactions also occur during time dt , so by first law:

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Let us now consider each of the terms of the first law as it is written for the system and transform each term into an equivalent form that applies to the control volume.  (i )    Energy        Let   Et     = the energy in the control volume at time t.               Et+ dt = ..         ..          ..           ..            ..          ..       t + dt.        Then;               E1 =      Et + ei dmi        =  Energy of system dt               E2 =      Et+ dt + ee dme  =  Energy of system dt + dt        Thus:               dE = E2 - E1 = (Et+ dt - Et) + (eedme - ei dmi)                                                                                (I)                        (II) (I):   Change in the total energy inside the control volume in time dt. (II):  The net flow of energy that crosses the control surface during dt as a result of the masses dmi and dme crossing the control surface. (ii)   Work Done        W in the equation (1) is not identical with Wx (external work) shown on the diagram.  Since displacement work is done by the moving parts of the system boundary.

So Wx is all the work done at the control surface other than that associated with normal forces at places where material crosses the surface e.g. shaft work, shear forces, etc.  Thus we can write the energy equation for the control volume as:

But

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This is the GENERAL ENERGY EQUATION

It states that the rate of heat transfer into the control volume plus rate of energy flowing in as a result of mass transfer is equal to the rate of change of energy inside the control volume plus rate of energy flowing out as a result of mass transfer plus power output associated with shaft, shear and electrical effects.

THE STEADY FLOW PROCESS………………….e

A steady flow process is one in which matter and energy flow steadily in and out of an open system. In a steady flow process, the properties of the flow remain unchanged with time, that is, the properties are frozen in time. But, the properties need not be the same in all points of the flow. It is very common for a beginner to confuse the term steady with the term equilibrium. But, they are not the same. When a system is at a steady state, the properties at any point in the system are steady in time, but may vary from one point to another point. The temperature at the inlet, for example, may differ from that at the outlet. But, each temperature, whatever its value, remains constant in time in a steady flow process. When a system is at an equilibrium state, the properties are steady in time and uniform in space. By properties being uniform in space, we mean that a property, such as pressure, has the same value at each and every point in the system.

Mass Balance for a Steady flow process

Since a steady flow process can be considered as a special process experienced by the open system, we may start from the mass balance for open systems. The steady flow process is that the mass of the open system experiencing a steady flow process remains constant. This is achieved if the mass flow rate at the inlet equals the mass flow rate at the exit

mi = me

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Where the subscript i denotes the inlet and the subscript e denotes the exit

CHARACTERISTICS OF STEADY FLOW PROCESS………..e

Characteristic 1:No property at any given location within the system boundary changes with time. That also means, during an entire steady flow process, the total volume vs of the system remains a constant, the total mass ms of the system remains a constant, and that the total energy contents ES of the system remains a constant.

Characteristic 2:Since the system remains unchanged with time during a steady flow process, the system boundary also remains the same.

Characteristic 3:No property at an inlet or at an exit to the open system changes with time. That means that during a steady flow process, the mass flow rate, the energy flow rate, pressure, temperature, specific (or molar) volume, specific (or molar) internal energy, specific (or molar)Enthalpy and the velocity of flow at an inlet or at an exit remain constant.

Characteristic 4:Rates at which heat and work are transferred across the boundary of the system remain unchanged.

ENERGY ANALYSIS OF STEADY FLOW PROCESS…………..f

During a steady-flow process, no intensive or extensive properties within the control volume change with time. Thus, the volume V, the mass m, and the total energy content E of the control volume remain constant. As a result, the boundary work is zero for steady-flow systems (since VCV constant), and the total mass or energy entering the control volume must be equal to the total mass or energy leaving. These observations greatly simplify the analysis. The fluid properties at an inlet or exit remain constant during a steady flow process. The properties may, however, be different at different inlets and exits. They may even vary over the cross section of an inlet or an exit. However, all properties, including the velocity and elevation, must remain constant with time at a fixed point at an inlet or exit. It follows that the mass flow rate of the fluid at an opening must remain constant during a steady flow process. As an added simplification, the fluid properties at an opening are usually considered to be uniform (at some average value) over the cross section. Thus, the fluid properties at an inlet or exit may be specified by the average single values. Also, the heat and work interactions between a steady-flow system and its surroundings do not change with time. Thus, the power delivered by a system and the rate of heat transfer to or from a system remains constant during a steady-flow process.

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The mass balance for a general steady-flow system

mi = me

Under steady-flow conditions, themass and energy contents of a control volume remain constant

During a steady-flow process, the total energy content of a control volume remains constant and thus the change in the total energy of the control volume is zero Therefore, the amount of energy entering a control volume in all forms (by heat, work, and mass) must be equal to the amount of energy leaving it. Then the rate form of the general energy balance reduces for a steady-flow process to

Energy balance for steady flow process: Ein = Eout

Steady Flow Engineering Devices………………………e

Nozzles and diffusers

Nozzles and diffusers are properly shaped ducts which are used to increase or decrease the speed of the fluid flowing through it. Schematics of a typical nozzle and a typical diffuser. Nozzles are used for various applications such as to increase the speed of water through a garden hose, and to increase the speed of the gases leaving the jet engine or rocket. Diffusers are used to slow down a fluid flowing at high speeds, such as at the entrance of a jet engine.

Turbine

A turbine is a device with rows of blades mounted on a shaft which could be rotated about its axis. In some water turbines used in hydroelectric power stations, water at high velocity is directed at the blades of the turbine to set the turbine shaft in rotation. The work delivered by the

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rotating shaft drives an electric generator to produce electrical energy. In steam turbines, steam at high pressure and temperature enters a turbine, sets the turbine shaft in rotation, and leaves at low pressure and temperature. In gas turbines, gaseous products of combustion at high pressure and temperature set the turbine shaft in rotation. The rotating shaft of a turbine is not always used for electric power generation. It is also an essential part of a jet engine in an aircraft which generates the thrust required to propel the aircraft.

Compressor

A compressor is a device used to increase the pressure of a gas flowing through it. The rotating type compressor functions in a manner opposite to a turbine. To rotate the shaft of a compressor, work must be supplied from an external source such as a rotating turbine shaft. The blades that are mounted on the shaft of the compressor are so shaped that, when the compressor shaft rotates, the pressure of the fluid flowing through the compressor increases. The rotating type compressors are used to raise the pressure of the air flowing through it in the electricity generation plants and in the jet engines. In a reciprocating type compressor, a piston moves with in the cylinder, and the work needed to move the piston is generally supplied by the electricity obtained from a wall socket. Household refrigerators use the reciprocating type of compressors to raise the pressure of the refrigerant flowing through them.

Throttling valve

A throttling valve is a device used to cause a pressure drop in a flowing fluid. It does not involve any work. The drop in pressure is attained by placing an obstacle such as a partially open valve, porous plug or a capillary tube in the path of the flowing fluid. The pressure drop in the fluid is usually accompanied by a drop in temperature, and for that reason throttling devices are commonly used in refrigeration and air-conditioning applications where a drop in the temperature of the working fluid is essential.

Heat Exchangers

In the industries, there is often a need to cool a hot fluid stream before it is let out into the environment. The heat removed from cooling of a hot fluid can be used to heat another fluid that has to be heated up. This can be achieved in a heat exchanger, which in general is a device where a hot fluid stream exchanges heat with a cold fluid stream without mixing with each other. The simplest type is the double-pipe heat exchanger which has two concentric pipes of different diameters. One fluid flows in the inner pipe and the other in the annular space between the two pipes.

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ENERGY ANALYSIS OF UNSTEADY FLOW PROCESS…………f

During a steady-flow process, no changes occur within the control volume; thus, one does not need to be concerned about what is going on within the boundaries. Not having to worry about any changes within the control volume with time greatly simplifies the analysis.Many processes of interest, however, involve changes within the control volume with time. Such processes are called unsteady-flow, or transient flow, processes.Another difference between steady- and unsteady-flow systems is that steady-flow systems are fixed in space, size, and shape. Unsteady-flow system. They are usually stationary; that is, they are fixed in space, but they may involve moving boundaries and thus boundary work.

min – mout =∆msysteam

The energy content of a control volume changes with time during an unsteady-flow process. The magnitude of change depends on the amount of energy transfer across the system boundaries as heat and work as well as on the amount of energy transported into and out of the control volume by mass during the process. When analyzing an unsteady-flow process, we must keep track of the energy content of the control volume as well as the energies of the incoming and outgoing flow streams

EIN – EOUT =∆E

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REFERENCES Www.me.utexas.edu/~me326/Schmidt/TnChp4.ppt...................................b www.rshanthini.com/tmp/ThermoBook/ThermoChap10.................................e https://wiki.ucl.ac.uk/display/.../First+law+applied+to+flow+processes.................c en.wikipedia.org/wiki/Thermodynamics……………………………….a

REFERENCES FROM BOOK

YUNUS A.CENGEL MICHAEL A. BOLES ………………………………………………f