Ejector Systems-Advanced Physics-Project Report | Study Guides, Projects, Research Advanced Physics

Table of Contents

ACKNOWLEDGEMENT .................................................................................................................... V

TABLE OF CONTENTS ..................................................................................................................... VI

LIST OF FIGURES ............................................................................................................................. IX

LIST OF TABLES ............................................................................................................................... XI

ABSTRACT ........................................................................................................................................ XII

1 INTRODUCTION ........................................................................................................................ 1

1.1 MOTIVATION .............................................................................................................................. 1

1.2 OBJECTIVE .................................................................................................................................. 2

1.3 THESIS ORGANIZATION ............................................................................................................... 2

2 LITERATURE REVIEW ............................................................................................................ 4

3 EJECTOR REFRIGERATION SYSTEM ................................................................................. 8

3.1 BASIC WORKING CYCLE ............................................................................................................. 8

3.2 EJECTOR – WORKING PRINCIPLE ................................................................................................ 9

3.3 EJECTOR FLOW MODELS........................................................................................................... 11

3.3.1 Constant Pressure Ejector Model ................................................................................... 11

3.3.2 Constant Area Ejector Model ......................................................................................... 12

3.4 OPERATIONAL MODES OF EJECTOR .......................................................................................... 12

3.4.1 Critical Mode .................................................................................................................. 12

3.4.2 Sub-critical Mode ........................................................................................................... 13

3.4.3 Back Flow Mode ............................................................................................................ 13

3.5 PERFORMANCE PARAMETERS ................................................................................................... 14

3.5.1 Entrainment Ratio ........................................................................................................... 14

3.5.2 Coefficient of Performance ............................................................................................ 14

3.5.3 Pressure Lift Ratio .......................................................................................................... 15

4 WORKING FLUID SELECTION ............................................................................................ 16

4.1 REFRIGERANT CLASSIFICATION ................................................................................................ 16

4.2 ENVIRONMENTAL STANDARDS ................................................................................................. 16

4.3 SAFETY STANDARDS ................................................................................................................. 17

4.4 CHARACTERISTICS OF SATURATED VAPOR LINE IN T-S DIAGRAM ........................................... 17

4.5 REFRIGERANT SELECTION CRITERIA......................................................................................... 20

4.6 DISCUSSION ON REFRIGERANT SELECTION ............................................................................... 22

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vi

ACKNOWLEDGEMENT .................................................................................................................... V

TABLE OF CONTENTS .....................................................................................................................VI

LIST OF FIGURES .............................................................................................................................IX

LIST OF TABLES ...............................................................................................................................XI

ix

1 INTRODUCTION ABSTRACT XII
- 1.1 MOTIVATION
- 1.2 OBJECTIVE
- 1.3 THESIS ORGANIZATION
2 LITERATURE REVIEW
3 EJECTOR REFRIGERATION SYSTEM
- 3.1 BASIC WORKING CYCLE
- 3.2 EJECTOR – WORKING PRINCIPLE
- 3.3 EJECTOR FLOW MODELS...........................................................................................................
  - 3.3.1 Constant Pressure Ejector Model
  - 3.3.2 Constant Area Ejector Model
- 3.4 OPERATIONAL MODES OF EJECTOR
  - 3.4.1 Critical Mode
  - 3.4.2 Sub-critical Mode
  - 3.4.3 Back Flow Mode
- 3.5 PERFORMANCE PARAMETERS
  - 3.5.1 Entrainment Ratio
  - 3.5.2 Coefficient of Performance
  - 3.5.3 Pressure Lift Ratio
4 WORKING FLUID SELECTION
- 4.1 REFRIGERANT CLASSIFICATION
- 4.2 ENVIRONMENTAL STANDARDS
- 4.3 SAFETY STANDARDS
- 4.4 CHARACTERISTICS OF SATURATED VAPOR LINE IN T-S DIAGRAM
- 4.5 REFRIGERANT SELECTION CRITERIA.........................................................................................
- 4.6 DISCUSSION ON REFRIGERANT SELECTION
5 EJECTOR DESIGN AND ANALYSIS..................................................................................... vii
- 5.1 GOVERNING EQUATIONS FOR PRIMARY NOZZLE
  - 5.1.1 Primary Throat Calculations
  - 5.1.2 Exit Conditions
- 5.2 GOVERNING EQUATIONS FOR SECONDARY NOZZLE
  - 5.2.1 Secondary Choked Flow at Section X-X
  - 5.2.2 Primary Flow Properties at Section X-X
  - 5.2.3 Properties of Mixed Flow
  - 5.2.4 Properties after Shock Wave
  - 5.2.5 Properties at Diffuser Exit
- 5.3 CFD ANALYSIS OF EJECTOR
  - 5.3.1 Mesh Independence
  - 5.3.2 3D Vs Axi-symmetric Approach
  - 5.3.3 Flow Solver
  - 5.3.4 Turbulence Modeling
  - 5.3.5 Near Wall Treatment
  - 5.3.6 User Defined Material
  - 5.3.7 Boundary Conditions
  - 5.3.8 Discretization Scheme
  - 5.3.9 Convergence Monitoring
6 FLOW PATTERNS IN EJECTOR
- 6.1 SHOCK/BOUNDARY LAYER INTERACTION
  - 6.1.1 Shock Train
  - 6.1.2 Pseudo Shock
  - 6.1.3 Normal and Oblique Shock Train
- 6.2 INTERACTION OF SHOCK WAVE WITH SECONDARY STREAM
  - 6.2.1 Double Choking Mode
  - 6.2.2 Single Choking Mode
- 6.3 COMPARATIVE ANALYSIS OF ANALYTICAL AND FLUENT RESULTS
7 EFFECTS OF OPERATING CONDITIONS ON EJECTOR PERFORMANCE
- 7.1 GENERATOR PRESSURE
  - 7.1.1 Effect of Generator Pressure on Flow Structures
- 7.2 EVAPORATOR PRESSURE...........................................................................................................
  - 7.2.1 Effect of Evaporator Pressure on Flow Structures
8 EFFECTS OF GEOMETRIC PARAMETERS ON EJECTOR PERFORMANCE
- 8.1 EJECTOR AREA RATIO
- 8.2 CONSTANT AREA SECTION LENGTH
- 8.3 PRIMARY NOZZLE POSITION viii
- 8.4 SECONDARY CONVERGENCE ANGLE
9 SUMMARY AND CONCLUSION
FUTURE RECOMMENDATIONS
REFERENCES
APPENDIX A RESULTS VALIDATION
VITA
FIGURE 3.1 EJECTOR REFRIGERATION CYCLE. List of Figures
FIGURE 3.2 SCHEMATIC DIAGRAM OF EJECTOR.
FIGURE 3.3 MOLLIER‟S CHART OF AN EJECTOR.
FIGURE 3.4 CONSTANT PRESSURE MIXING EJECTOR...............................................................................
FIGURE 3.5 CONSTANT AREA MIXING EJECTOR......................................................................................
FIGURE 3.6 OPERATIONAL MODES OF EJECTOR.
FIGURE 4.1 CHARACTERISTIC CURVE OF DRY WORKING FLUID.
FIGURE 4.2 CHARACTERISTIC CURVE OF ISENTROPIC WORKING FLUID.
FIGURE 4.3 CHARACTERISTIC CURVE OF WET WORKING FLUID.
FIGURE 5.1 SCHEMATIC DIAGRAM OF PRIMARY NOZZLE.
FIGURE 5.2 SCHEMATIC DIAGRAM OF COMPLETE EJECTOR.
FIGURE 5.3 GEOMETRIC ILLUSTRATIONS OF SECTIONS FOR MESH INDEPENDENCE (UNITS ARE MM).
FIGURE 5.4 PLOT OF STATIC PRESSURE AT SECTION A-A FOR VARYING MESH.
FIGURE 5.5 PLOT OF MACH NUMBER AT SECTION A-A FOR VARYING MESH.
FIGURE 5.6 PLOT OF STATIC PRESSURE AT SECTION B-B FOR VARYING MESH.
FIGURE 5.7 PLOT OF MACH NUMBER AT SECTION B-B FOR VARYING MESH.
FIGURE 5.8 PLOT OF STATIC PRESSURE AT SECTION C-C FOR VARYING MESH.
FIGURE 5.9 PLOT OF MACH NUMBER AT SECTION C-C FOR VARYING MESH.
FIGURE 5.10 PLOT OF STATIC PRESSURE AT SECTION D-D FOR VARYING MESH.
FIGURE 5.11 PLOT OF MACH NUMBER AT SECTION D-D FOR VARYING MESH.
FIGURE 5.12 VARIOUS SECTIONS OF MESHED EJECTOR.
FIGURE 5.13 ISOBARIC VARIATION OF SPECIFIC HEAT CAPACITY (CP)...................................................
FIGURE 5.14 ISOBARIC VARIATION OF GAMMA.
FIGURE 5.15 ISOBARIC VARIATION OF THERMAL CONDUCTIVITY.
FIGURE 5.16 ISOBARIC VARIATION OF VISCOSITY.
FIGURE 5.17 BOUNDARY CONDITION TYPES FOR EJECTOR ANALYSIS.
FIGURE 6.1 SHOCK WAVE/TURBULENT BOUNDARY LAYER INTERACTION IN CONSTANT AREA DUCT...
FIGURE 6.2 STATIC PRESSURE ALONG DUCT CENTERLINE AND WALL SURFACE FOR PSEUDO SHOCK.
FIGURE 6.3 EFFECTS OF BOUNDARY LAYER AND UPSTREAM MACH NUMBER ON SHOCK TRAIN.
FIGURE 6.4 FLOW STRUCTURES INSIDE EJECTOR ILLUSTRATING SHOCK TRAIN AND PSEUDO SHOCKS.
FIGURE 6.5 MACH NO. AND STATIC PRESSURE VARIATION ALONG CENTERLINE FOR CRITICAL MODE.
FIGURE 6.6 FLOW PATTERNS IN EJECTOR OPERATING IN SINGLE CHOKING MODE.
FIGURE 6.7 MACH NO. AND STATIC PRES. VARIATION ALONG CENTERLINE FOR SUB-CRITICAL MODE.
FIGURE 6.8 SCHEMATIC EJECTOR HIGHLIGHTING THE CONCERNED EJECTOR SECTIONS
FIGURE 7.1 SCHEMATIC DIAGRAM OF TESTED EJECTOR (UNITS: MM).
FIGURE 7.2 EFFECT OF GENERATOR PRESSURE ON ENTRAINMENT RATIO.
FIGURE 7.3 VARIATION OF REFRIGERATION CAPACITY WITH CONDENSER PRESSURE.
FIGURE 7.4 VARIATION OF REFRIGERATION CAPACITY WITH GENERATOR PRESSURE. x
FIGURE 7.5 VARIATION OF PRESSURE LIFT RATIO WITH GENERATOR PRESSURE....................................
FIGURE 7.6 EFFECT OF COMPRESSION RATIO ON CRITICAL ENTRAINMENT RATIO.
FIGURE 7.7 EFFECT OF DRIVING PRESSURE RATIO ON CRITICAL ENTRAINMENT RATIO.
FIGURE 7.8 SHOCK TRAIN TRANSFORMATION AND FLOW AREA VARIATION..........................................
FIGURE 7.9 MACH NUMBER PLOTS FOR DIFFERENT GENERATOR PRESSURES.
FIGURE 7.10 EFFECT OF EVAPORATOR PRESSURE ON ENTRAINMENT RATIO.
FIGURE 7.11 VARIATION OF REFRIGERATION CAPACITY WITH EVAPORATOR PRESSURE.
FIGURE 7.12 VARIATION OF COP WITH EVAPORATOR PRESSURE.
FIGURE 7.13 EFFECT OF EVAPORATOR PRESSURE ON PRESSURE LIFT RATIO..........................................
FIGURE 7.14 SHOCK TRAIN AND FLOW AREA IN EJECTOR.
FIGURE 7.15 MACH NUMBER PLOTS FOR DIFFERENT EVAPORATOR PRESSURES.
FIGURE 8.1 SCHEMATIC DIAGRAM OF EJECTOR HIGHLIGHTING THE AREA RATIO(UNITS: MM).
FIGURE 8.2 VARIATION OF ENTRAINMENT RATIO WITH AREA RATIO.
FIGURE 8.3 EFFECT OF AREA RATIO ON REFRIGERATION CAPACITY.
FIGURE 8.4 PRESSURE LIFT RATIO VARIATION WITH AREA RATIO.
FIGURE 8.5 SCHEMATIC EJECTOR HIGHLIGHTING CONSTANT AREA SECTION LENGTH (UNITS: MM).
FIGURE 8.6 TRENDS OF ENTRAINMENT RATIO FOR VARYING CONSTANT AREA SECTION LENGTH.
FIGURE 8.7 TRENDS OF REFRIGERATION CAPACITY FOR VARYING CONSTANT AREA SECTION LENGTH.
FIGURE 8.8 TRENDS OF CRITICAL CONDENSER PRESSURE FOR VARYING CONSTANT SECTION LENGTH.
FIGURE 8.9 TRENDS OF PRESSURE LIFT RATIO FOR VARYING CONSTANT AREA SECTION LENGTH.
FIGURE 8.10 TRENDS OF COP FOR VARYING CONSTANT AREA SECTION LENGTH.
FIGURE 8.11 SCHEMATIC EJECTOR HIGHLIGHTING THE PRIMARY NOZZLE POSITION (UNITS: MM).
FIGURE 8.12 VARIATION OF ENTRAINMENT RATIO WITH PRIMARY NOZZLE POSITION.
FIGURE 8.13 SCHEMATIC ILLUSTRATION OF SHOCK TRAIN INSIDE EJECTOR.
FIGURE 8.14 SHOCK TRAIN EFFECTS ON SECONDARY FLOW AREA.
FIGURE 8.15 VARIATION OF PRESSURE LIFT RATIO WITH PRIMARY NOZZLE POSITION.
FIGURE 8.16 MACH NO PLOTS FOR DIFFERENT PNP...............................................................................
FIGURE 8.17 SCHEMATIC EJECTOR HIGHLIGHTING SECONDARY CONVERGENCE ANGLE (UNITS: MM).
FIGURE 8.18 VARIATION OF ENTRAINMENT RATIO WITH SECONDARY CONVERGENCE ANGLE.
FIGURE 8.19 VARIATION OF PRESSURE LIFT RATIO WITH SECONDARY CONVERGENCE ANGLE.
FIGURE 8.20 CONTOURS OF MACH NUMBER FOR DIFFERENT CONVERGENCE ANGLES.

xii

Abstract

This work aims to design an ejector for refrigeration system, having 0.15 KW cooling

capacity, using one dimensional (1D) ejector theory. Dichlorodifluoromethane (R12)

is selected as working fluid. Performance of an ejector is greatly influenced by the

operational and geometric parameters. Parametric analysis is done for two operational

parameters, namely, generator pressure and evaporator pressure, and four geometric

parameters, namely, ejector area ratio, constant area section length, primary nozzle

position and secondary convergence angle, using commercial software FLUENT. The

ejector geometries are modeled and meshed in GAMBIT. Mesh independent results

are ensured through the mesh convergence. Analytical/experimental results are used

to validate the simulated results. The generator and evaporator pressures are varied

over a range of 5 – 8 bar and 1.5 – 2.4 bar respectively. It is observed that

entrainment ratio decreases and critical condenser pressure increases by increasing

generator pressure whereas both can be increased by increasing evaporator pressure.

The area ratio is limited by the condenser pressure, whereas constant area section

length does not significantly affect the performance parameters. The variation of

primary nozzle position and secondary convergence angle shows that, an optimum

value of these parameters can be obtained, for which system show peak performance.

These optimum values are found out to be 3 mm and 25 deg for the designed ejector.

xiii

Nomenclature

Alphabetic Letters

A Area, m^2

C Specific heat capacity, J/Kg-K

d Diameter, mm

h Enthalpy, KJ/Kg

L Primary nozzle length, mm

l Length of secondary nozzle, mm

M Mach number

m ^ Mass flow rate, Kg/sec

P Pressure, bar, Pa

Q Heat exchange rate, J/sec

R Gas constant, J/Kg-K

s Entropy, KJ/Kg-K

T Temperature, K

t Time, sec

v Velocity, m/sec

W Work done, J

Greek Letters

α Secondary nozzle angles, under-relaxation factor

β Primary nozzle angle

γ Specific heat ratio

Δ Change or difference

ω Entrainment ratio

ρ Density, Kg/m

ε Area ratio

Φ Friction loss coefficient

Subscripts

1 Primary inlet

01 Stagnation condition at generator

1x Primary fluid at section X-X

1 Introduction 1.1 Motivation

Most industrial processes use a significant amount of thermal energy, mostly by

burning fossil fuels. Some part of the energy released in combustion is inevitably

rejected to the surroundings as waste [1]. This waste heat can be utilized for various

productive purposes in daily life applications. Greater emphasis has now been placed

on the renewed research for the utilization of renewable energy sources such as solar

energy, geothermal energy and waste heat because of the present environmental and

energy crises.

Refrigeration and air conditioning applications share a considerable portion of

the overall electricity consumed all around the world. This poses a heavy load for the

electricity generation companies. With the use of renewable energy sources for

refrigeration and air conditioning, the amount of electricity purchased from utility

companies, required for conventional vapor compression in the refrigeration cycle,

can be reduced. Also, utilization of the waste heat in refrigeration systems promotes

mitigating the problems related to the environment, particularly by reduction of CO 2

emission from combustion of fossil fuels in boilers of utility power plants [1],[2].

Ejector refrigeration system or jet refrigeration system is a non mechanical,

thermally activated system that uses ejector instead of a mechanical compressor to

compress the refrigerant vapor from the evaporator to the condenser. Ejector-based

heat pumps and refrigerators have been known for a long time, but their potential has

not been fully exploited and despite their relatively low efficiency, they remain very

attractive mainly because of their simplicity and reduced overall cost [3].

A lot of research is going on to improve the efficiency of the ejector

refrigeration system. The main focus is on understanding the mixing process and the

flow structures that are produced during mixing in the ejector. A series of oblique

shocks and expansion waves called shock train is produced at the exit of the primary

nozzle that affects the entrainment of the secondary flow from the evaporator [4].

Researchers are deeply investigating the effects of operating conditions and suction

chamber geometry on the system efficiency by varying the operational and geometric

parameters. The parametric analysis is done for various refrigerants and valuable

results are obtained but to achieve reliable general trends that satisfy all the

refrigerants, require more research yet. The accomplishment of this task would be

revolutionary and will pave the way for the development of high performance ejectors

for refrigeration and air-conditioning systems.

1.2 Objective

This thesis aims to design an ejector for a refrigeration system of 0.15 KW cooling

capacity for selected working fluid. The ejector is core part of the ejector refrigeration

system. The ejector performance depends on working fluid, operating conditions like

generator pressure, evaporator pressure, condenser pressure, and the geometric

parameters like ejector area ratio, length of constant area section, primary nozzle

position and the secondary nozzle convergence angle.

The designed ejector will be modeled and meshed in GAMBIT.

Mesh independent results will be ensured through the mesh convergence procedure.

CFD software package FLUENT will be used to investigate the effects of operational

and geometric parameters on the ejector performance. The plots of performance

parameters like entrainment ratio, pressure lift ratio, refrigeration capacity and COP

will be used to analyze and select the optimum geometry for the designed ejector. The

FLUENT results will be validated through analytical calculations using one

dimensional gas dynamics theory.

A thorough discussion on the flow structures inside ejector and their

dependence on the above discussed parameters will also be presented using schematic

diagrams and figures.

1.3 Thesis Organization

Chapter 1 describes the motivation and objective.

Chapter 2 contains brief literature review identifying the development stages of

ejector refrigeration system.

Chapter 3 describes a simple ejector refrigeration cycle along with its operating

characteristics, performance parameters, flow models and its operational modes.

Chapter 4 deals with the refrigerant classification and the criteria for the working

fluid selection. The advantages of the selected refrigerant are also listed.

Chapter 5 explains the design and analysis of primary and secondary nozzle of the

ejector. The setup procedure for the analysis of the ejector in FLUENT is illustrated

step by step. The turbulence model used and mesh independence is also discussed

here.

2 Literature Review

The ejector was invented by Sir Charles Parsons around 1901. Its first application was

in a steam engine condenser. Maurice Leblanc was the one, who at first introduced the

ejector in the field of refrigeration in 1910. This system was appreciated and applied

for air-conditioning purposes in 1930s [1]. The introduction of vapor compression

refrigeration system was a setback for the ejector refrigeration system due to its

excellent performance and efficiency as compared to ejector refrigeration system.

Due to energy shortage in some regions, especially after the energy crisis of

the 1970‟s, renewable energy sources have once again become field of interest for the

researchers. Research and development in the solar energy field has grown rapidly,

along with research in solar cooling.

Valuable literature is available on the review of application of ejectors in the

refrigeration and air-conditioning systems [1],[2]. K. Chunnanond, S. Aphornratana et

al [1] have described major steps in the evolution and development of ejector theory

and its application in refrigeration technology along with its performance

characteristics.

The modifications in the ejector refrigeration system are also been introduced

by various researchers to compete this system with widely used and more efficient

vapor compression system. Compression enhanced jet refrigeration systems using

booster, compressor or jet pump are introduced to decrease the backpressure of the

ejector, and hence the entrainment ratio and the coefficient of performance (COP) of

the new system could be increased [1],[5],[9]. Hybrid ejector-absorption and

adsorption refrigeration systems are proposed to improve the performance of single-

effect absorption and adsorption refrigeration systems respectively [1],[5],[7],[8]. A

single ejector can operate over a limited range of operating conditions and its

efficiency will be reduced if it is operated in the off-design conditions. To tackle this

problem, multistage ejector systems are proposed [5],[21]. In addition to these

systems, vast options of the solar refrigeration technologies are presented in [2],[5].

Another system is introduced by B.J. Huang et al that utilize a multi-function

generator (MFG) to eliminate the mechanical pump [10]. The MFG serves as both a

pump and a vapor generator. A similar system is proposed by Zhang and Shen in

which the mechanical pump of a single ejector refrigeration cycle is replaced by a

vapor-liquid ejector [6]. In a study by Y.J. Chang and Y.M. Chen, it was found out

that the compression ratio and the entrainment ratio can be enhanced if the petal

nozzle is used in an ejector instead of conventional conical nozzle [11].

A thermodynamic shock is produced in the constant area section of the ejector

which causes a loss of the stagnation pressure and hence pressure lift ratio decreases.

I.W. Eames introduced a theory called “Constant Rate of Momentum Change”,

CRMC, which allows the static pressure to rise gradually from entry to exit producing

no shock and hence no loss of total pressure associated with shock process [12].

The ejector refrigeration system has been tested for various working fluids. A lot of

work has been done on steam jet refrigeration system [21],[22],[23]but due to high

temperatures required at the boiler and inability to achieve sub-zero temperatures;

other refrigerants become a nice option for use.

The experimental work has been done on various working fluids including

Ammonia [24], R134a [5],[25], R141b [26], R113 [27], R-744 [28], methanol [29],

R142b and R600a [30], and CO 2 -NH 3 Cascade [31]. A valuable work has been done

on the comparative study of the performance of an ejector refrigeration cycle with

various refrigerants by D. Sun [32]. These refrigerants include R718, R11, R12, R113,

R21, R123, R142b, R134a, R152a, RC318 and R500. COP and Entrainment Ratios of

the above mentioned refrigerants are compared and the optimum Area Ratio is

determined. Later on, another comparative analysis was done for environment

friendly refrigerants including R134a, R152a, R290, R600a and R717 [33].

The operational and geometric parameters greatly affect the performance of

the ejector refrigeration system. There is no single ejector that can operate under all

working conditions because they are very sensitive to the generator, evaporator and

condenser pressures and temperatures. The continued research in this field has

successfully provided us with the general trends of the performance parameters under

varying operating conditions [3],[23].

The effects of geometric parameters are much more confusing and complex

and are difficult to understand. In a study about the Influence of geometrical factors

on steam ejector performance [35], three geometrical factors – the area ratio, constant

area section length and the primary nozzle position, were considered. The results

indicated the existence of an optimal area ratio, depending on operating conditions.

The location of primary nozzle position had a little influence on entrainment ratio,

whereas a longer constant area section resulted in a higher critical back pressure.

supersonic air ejector contradict first of the above two results. It was concluded that

over the whole range of operating conditions, the overall deviation is below 10% for

the k-epsilon model, while the results for the k-omega-sst model are in a less

agreement to the experimental results [46].

Axi-symmetric CFD model was used in literature to determine ejector

efficiencies for the primary nozzle, suction, mixing and diffuser sections [47]. Water

was considered as working fluid and the operating conditions were selected in a range

that would be suitable for an air-conditioner powered by solar thermal energy. Ejector

performance was estimated for different nozzle throat to constant section area ratios.

The results indicated the existence of an optimal ratio, depending on operating

conditions. Ejector efficiencies were calculated for different operating conditions. It

was found that while nozzle efficiency can be considered as constant, the efficiencies

related to the suction, mixing and diffuser sections of the ejector depend on operating

conditions.

S. He et al presented a new approach for the performance analysis of ejector

refrigeration system using Grey system theory [48]. This theory use the multivariate

grey model combined with grey relational analysis. It was observed that the proposed

combined method provides significant improvement over the traditional methods in

model simplification, time requirement and universal application.

In addition to refrigeration, ejectors have application in air conditioning

systems also. A. Arbel and M. Sokolov have designed solar-powered ejector air-

conditioner using R142b as working fluid [49]. Pridasawas has listed existing solar-

driven refrigeration technologies that are applied to air conditioning applications as

well [5]. Work has also been done on solar drive steam jet ejector chiller [50]. The

investigation showed that the cooling water temperature as well as the cold water

temperature has a strong influence on the coefficient of performance of a steam jet

ejector chiller. The coefficient of performance reaches high values in part load and at

good re-cooling conditions, so that the mean efficiency is clearly higher than the

nominal efficiency of the system.

J. Guo and H.G. Shen have done the modeling of the solar-driven ejector air

conditioning system for office building. Compared with traditional compressor based

air conditioner, the system can save up to 80% electric energy when providing the

same cooling capacity for office buildings [51].

3 Ejector Refrigeration System 3.1 Basic Working Cycle

Ejector refrigeration system uses an ejector instead of a compressor to achieve

compression. Schematic diagram of ejector refrigeration system, illustrating the basic

working cycle, is shown in figure 3.1. There are six main components of this system

that include: generator or boiler, condenser, evaporator, ejector, liquid recirculation

pump and an expansion device.

High pressure and high temperature vapor is produced in the generator by

absorbing waste heat or the solar energy. This vapor is called primary vapor and acts

as actuating vapor in the system. The primary vapor „1‟ flows to the ejector, where it

expands in the primary converging-diverging nozzle to supersonic velocities and

hence producing low pressures at the primary nozzle exit.

Figure 3.1 Ejector Refrigeration Cycle.

High pressure primary vapor from the generator enters the ejector at „1‟. It

accelerates to supersonic velocities in the primary converging-diverging nozzle and

produce low pressures at the exit of primary nozzle. High pressure secondary vapor

stream enters from evaporator „2‟ due to pressure difference. The primary vapors fans

out of the primary nozzle producing a convergent duct for the secondary vapor. This

converging duct causes the secondary stream to accelerate to sonic velocity at

hypothetical throat area called “effective area” [1]. After this point, the mixing of two

streams start. After complete mixing „3‟, a normal shock occurs in constant area

section. This shock wave produce the compression effect by increasing the pressure

and consequently makes the mixed stream subsonic „4‟. The mixed stream is further

compressed in the diffuser section before going to condenser „5‟.

It has been observed that shock wave, which produces major compression

effect, causes loss in total pressure of the mixed stream. The process is described

using Mollier‟s chart and is shown in figure 3.3 [1]. If the mixed stream is brought to

stagnation state isentropically (without a normal shock), the exhaust pressure will be

as high as „P‟ as shown in figure 3.3. This can be considered as an ideal ejector,

which can be considered as an isentropic compressor driven by an isentropic turbine.

Figure 3.3 Mollier’s Chart of an Ejector.

This loss can be avoided using a new technique called “Constant Rate of

Momentum Change Method (CRMC)” [12]. According to this method the static

pressure of the mixed flow increases gradually from entry to exit while passing

through the ejector. It must be noted that the model is not a reversible system even if

the loss due to the shock is eliminated. Another loss caused by the mixing of two fluid

streams (primary and secondary flows) remains. Not only the shear mixing, but the

shear force is also introduced to the flowing process by the shear stress layer. These

two factors are considered as the cause of entropy generation, and hence, the

irreversibility of an ejector [1].

3.3 Ejector Flow Models 3.3.1 Constant Pressure Ejector Model

Ejector Systems-Advanced Physics-Project Report, Study Guides, Projects, Research of Advanced Physics

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vi

Table of Contents

ACKNOWLEDGEMENT .................................................................................................................... V

TABLE OF CONTENTS .....................................................................................................................VI

LIST OF FIGURES .............................................................................................................................IX

LIST OF TABLES ...............................................................................................................................XI

ix

xii

Abstract

This work aims to design an ejector for refrigeration system, having 0.15 KW cooling

capacity, using one dimensional (1D) ejector theory. Dichlorodifluoromethane (R12)

is selected as working fluid. Performance of an ejector is greatly influenced by the

operational and geometric parameters. Parametric analysis is done for two operational

parameters, namely, generator pressure and evaporator pressure, and four geometric

parameters, namely, ejector area ratio, constant area section length, primary nozzle

position and secondary convergence angle, using commercial software FLUENT. The

ejector geometries are modeled and meshed in GAMBIT. Mesh independent results

are ensured through the mesh convergence. Analytical/experimental results are used

to validate the simulated results. The generator and evaporator pressures are varied

over a range of 5 – 8 bar and 1.5 – 2.4 bar respectively. It is observed that

entrainment ratio decreases and critical condenser pressure increases by increasing

generator pressure whereas both can be increased by increasing evaporator pressure.

The area ratio is limited by the condenser pressure, whereas constant area section

length does not significantly affect the performance parameters. The variation of

primary nozzle position and secondary convergence angle shows that, an optimum

value of these parameters can be obtained, for which system show peak performance.

These optimum values are found out to be 3 mm and 25 deg for the designed ejector.

xiii

Nomenclature

Alphabetic Letters

A Area, m^2

C Specific heat capacity, J/Kg-K

d Diameter, mm

h Enthalpy, KJ/Kg

L Primary nozzle length, mm

l Length of secondary nozzle, mm

M Mach number

m ^ Mass flow rate, Kg/sec

P Pressure, bar, Pa

Q Heat exchange rate, J/sec

R Gas constant, J/Kg-K

s Entropy, KJ/Kg-K

T Temperature, K

t Time, sec

v Velocity, m/sec

W Work done, J

Greek Letters

α Secondary nozzle angles, under-relaxation factor

β Primary nozzle angle

γ Specific heat ratio

Δ Change or difference

ω Entrainment ratio

ρ Density, Kg/m

ε Area ratio

Φ Friction loss coefficient

Subscripts

1 Primary inlet

01 Stagnation condition at generator

1x Primary fluid at section X-X

Most industrial processes use a significant amount of thermal energy, mostly by

burning fossil fuels. Some part of the energy released in combustion is inevitably

rejected to the surroundings as waste [1]. This waste heat can be utilized for various

productive purposes in daily life applications. Greater emphasis has now been placed

on the renewed research for the utilization of renewable energy sources such as solar

energy, geothermal energy and waste heat because of the present environmental and

energy crises.

Refrigeration and air conditioning applications share a considerable portion of

the overall electricity consumed all around the world. This poses a heavy load for the

electricity generation companies. With the use of renewable energy sources for

refrigeration and air conditioning, the amount of electricity purchased from utility

companies, required for conventional vapor compression in the refrigeration cycle,

can be reduced. Also, utilization of the waste heat in refrigeration systems promotes

mitigating the problems related to the environment, particularly by reduction of CO 2

emission from combustion of fossil fuels in boilers of utility power plants [1],[2].

Ejector refrigeration system or jet refrigeration system is a non mechanical,

thermally activated system that uses ejector instead of a mechanical compressor to