Two-Phase Flow in Refrigeration Systems

Junjie Gu • Shujun Wang •

Zhongxue Gan

Two-Phase Flow inRefrigeration Systems

123

Junjie GuDepartment of Mechanical and Aerospace

EngineeringCarleton UniversityOttawa, ONCanada

Shujun WangAtomic Energy of Canada LimitedDeep River, ONCanada

Zhongxue GanENN Intelligent Energy Co., Ltd.ENN GroupLangfangPeople’s Republic of China

ISBN 978-1-4614-8322-9 ISBN 978-1-4614-8323-6 (eBook)DOI 10.1007/978-1-4614-8323-6Springer New York Heidelberg Dordrecht London

Library of Congress Control Number: 2013943709

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Preface

This book presents recent developments in our research on two-phase flow inrefrigeration systems. It is a monograph on vapour compression refrigerationsystems, provided for university postgraduate students as a textbook, students inself-study, researchers and professors as an academic reference book, and engi-neers and designers as a guide for efficient system development. The primary goalis to investigate the performance of an air-conditioning system through experi-mental and theoretical results, with consideration of liquid–vapour two-phase flowand lubrication oil concentration in circulation. This important and unique aspectof the vapour compression refrigeration systems—‘‘two-phase’’ provides potentialimprovement in energy efficiency.

The book includes seven parts: (1) description of an automotive air-condi-tioning test system with real-size components, which operates as a Clutch CyclingOrifice Tube system, where vapour quality and oil concentration data can bemeasured; (2) a Stream Analysis Model for an accumulator in R134-a cycles,including the effects of the accumulator structure and oil concentration on vapourquality and pressure drop, parametric study and comparison with the experimentalresults; (3) an investigation of a swash plate compressor of automotive air-con-ditioning, including the effects of two-phase flow and oil concentration on thecompression process, the compressor efficiencies and the system performances; (4)an investigation of capillary tube in R134-a cycles, including the phase changeduring expansion process, the effects of capillary tube size, condensation pressure,cubcooling degree, surface roughness and oil concentration; (5) a model of non-adiabatic capillary tube in trans-critical CO2 cycles, including heat transfer andfluid flow of a trans-critical throttling process, determination of the optimum highpressure using the gas cooler outlet temperature, the effectiveness of internal heatexchanger and the vapour quality at the suction line inlet; (6) an investigationof Internal Heat Exchangers in R134-a cycles, including the effects of two kinds ofInternal Heat Exchangers on two-phase flow, compression ratio, coefficient ofperformance and compressor efficiencies; (7) two-phase flow and system perfor-mance of R134-a cycles, including the effects of refrigerant charge, air tempera-ture to evaporator, condenser temperature and compressor speed on the two-phaseflow, coefficient of performance and cooling capacity.

v

In the efforts of this book, the following issues have been resolved. They are: (i)measuring techniques of the vapour quality (two-phase flow) and oil concentrationin a refrigeration cycle consisting of real-size components, (ii) looking into howthe system components affect the two-phase flow, (iii) how the two-phase flowaffects the performances of refrigeration systems, (iv) the complicated expansionprocess of a non-adiabatic capillary tube in trans-critical CO2 cycles is revealed.Therefore, the unique engineering contributions of this book are demonstrated inthe understandings on two-phase flow and its effects on performance of vapourcompression refrigeration systems, and a new methodology in design of efficientsystems.

Here, we would like to express our sincere appreciation to Dr. Ying Chen, whooffered his great help for the development of the related mathematical models ofthe capillary tubes, which is included in this book.

We would like to thank everyone in the Department of Mechanical andAerospace Engineering at Carleton University, for their generous help, directlyand indirectly, for providing convenience to our work. We are thankful to OntarioCentres of Excellence for providing funding to several projects directly related tothis work as well as to Halla Climate Control Canada Inc. for providing theexperimental assistance.

Finally, we would like to express our appreciation to our wives, Jane, Youwenand Xiaoshu, and to our children for their continued patience, understanding andsupport throughout the preparation of this book.

Junjie GuShujun Wang

Zhongxue Gan

vi Preface

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Vapour-Compression Automotive Air-Conditioning Systems . . . 3

1.2.1 Types of Automotive AC Systems . . . . . . . . . . . . . . . . 31.2.2 Characteristics of Automotive AC Systems . . . . . . . . . . 6

1.3 Analysis of Automotive AC Systems . . . . . . . . . . . . . . . . . . . . 71.4 New Refrigerants to Replace R-134a . . . . . . . . . . . . . . . . . . . . 10

1.4.1 Carbon dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.4.2 R-1234yf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2 Experimental Facility for Two-Phase Flow Measurement . . . . . . . 152.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2.1 The Compressor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2.2 The Subcooled Condenser . . . . . . . . . . . . . . . . . . . . . . 172.2.3 The Expansion Device. . . . . . . . . . . . . . . . . . . . . . . . . 172.2.4 The Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.2.5 The Accumulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.2.6 Vapour Quality Measurement Device . . . . . . . . . . . . . . 192.2.7 Oil in Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.8 Data Acquisition and Processing Device . . . . . . . . . . . . 202.2.9 Graphical User Interface . . . . . . . . . . . . . . . . . . . . . . . 21

2.3 Experimental Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3 Properties of Refrigerant, Oil and Their Mixture . . . . . . . . . . . . . 253.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2 Properties of Refrigerant R-134a . . . . . . . . . . . . . . . . . . . . . . . 263.3 Properties of Oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.4 Properties of Refrigerant/Oil Mixture . . . . . . . . . . . . . . . . . . . . 28References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

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4 Two-Phase Flow in Accumulator . . . . . . . . . . . . . . . . . . . . . . . . . 314.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.2 Stream Analysis Model for Accumulator . . . . . . . . . . . . . . . . . 35

4.2.1 Physical Effects and Mathematical Description. . . . . . . . 364.2.2 Descriptions of Accumulator . . . . . . . . . . . . . . . . . . . . 374.2.3 Flow Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.2.4 Flow Streams in an Accumulator . . . . . . . . . . . . . . . . . 404.2.5 Pressure Drop Along Streams . . . . . . . . . . . . . . . . . . . . 414.2.6 MATLAB program . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.3 Accumulator Model Validation and Results . . . . . . . . . . . . . . . 484.3.1 Model Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484.3.2 Parametric Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4.4 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5 Two-Phase Flow in Compressor . . . . . . . . . . . . . . . . . . . . . . . . . . 575.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575.2 Investigation of Compressor Performance. . . . . . . . . . . . . . . . . 60

5.2.1 Thermodynamic Properties . . . . . . . . . . . . . . . . . . . . . . 605.2.2 Effects of Vapour Quality and Oil Concentration . . . . . . 625.2.3 Efficiencies of the Compressor . . . . . . . . . . . . . . . . . . . 63

5.3 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665.3.1 Effects of Vapour Quality on Volumetric Efficiency . . . . 665.3.2 Effects of Vapour Quality on Isentropic Efficiency . . . . . 675.3.3 Evaporation Ratio in Compression Process. . . . . . . . . . . 69

5.4 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

6 Two-Phase Flow in Adiabatic Capillary Tubefor R-134a Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 736.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 736.2 Model Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

6.2.1 Assumptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746.2.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . 746.2.3 Single-Phase Region . . . . . . . . . . . . . . . . . . . . . . . . . . 766.2.4 Two-Phase Region . . . . . . . . . . . . . . . . . . . . . . . . . . . 766.2.5 Judgment of Extreme Flow Condition . . . . . . . . . . . . . . 79

6.3 Parametric Study and Discussion. . . . . . . . . . . . . . . . . . . . . . . 796.3.1 Validation of the ACAM . . . . . . . . . . . . . . . . . . . . . . . 806.3.2 Influence of Tube Inner Diameter . . . . . . . . . . . . . . . . . 846.3.3 Influence of Tube Length. . . . . . . . . . . . . . . . . . . . . . . 856.3.4 Influence of Condensation Pressure . . . . . . . . . . . . . . . . 856.3.5 Influence of Subcooling Degrees . . . . . . . . . . . . . . . . . 86

viii Contents

6.3.6 Influence of Surface Roughness . . . . . . . . . . . . . . . . . . 876.3.7 Influence of Oil Effect. . . . . . . . . . . . . . . . . . . . . . . . . 88

6.4 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

7 Two-Phase Flow in Non-Adiabatic Capillary Tubefor CO2 Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 917.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 917.2 A New Transcritical Refrigeration Cycle . . . . . . . . . . . . . . . . . 927.3 Model Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

7.3.1 Assumptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 937.3.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . 947.3.3 Single-Phase Region . . . . . . . . . . . . . . . . . . . . . . . . . . 957.3.4 Two-Phase Region . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

7.4 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 977.4.1 Case 1: Constant Surrounding Heat

Transfer Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . 987.4.2 Case 2: Varying Surrounding Heat

Transfer Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . 1057.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

8 Two-Phase Flow in Internal Heat Exchangers . . . . . . . . . . . . . . . . 1118.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1118.2 New AC System with an Accumulator-Internal

Heat Exchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158.3 Comparison Investigation of Different AC Systems . . . . . . . . . . 116

8.3.1 Experimental Conditions . . . . . . . . . . . . . . . . . . . . . . . 1168.3.2 Vapour Quality at Inlet of Compressor . . . . . . . . . . . . . 1188.3.3 Compression Ratio in Compression Process . . . . . . . . . . 1198.3.4 Coefficient of Performance . . . . . . . . . . . . . . . . . . . . . 1218.3.5 Compressor Efficiencies. . . . . . . . . . . . . . . . . . . . . . . . 1228.3.6 P-h Diagram of Refrigeration Cycles. . . . . . . . . . . . . . . 122

8.4 Parametric Studies of the New System. . . . . . . . . . . . . . . . . . . 1248.4.1 Experimental Conditions for ACTSLHX Systems . . . . . . 1248.4.2 Effect of Capillary Tube Length . . . . . . . . . . . . . . . . . . 1248.4.3 Effect of Capillary Tube Diameter . . . . . . . . . . . . . . . . 124

8.5 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

9 Two-Phase Flow and System Performance . . . . . . . . . . . . . . . . . . 1319.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1319.2 Effects of Refrigerant Charge . . . . . . . . . . . . . . . . . . . . . . . . . 1329.3 Effect of Inlet Air Temperature to Evaporator. . . . . . . . . . . . . . 135

Contents ix

9.4 Effect of Condenser Water Temperature. . . . . . . . . . . . . . . . . . 1389.5 Effect of Compressor Speed . . . . . . . . . . . . . . . . . . . . . . . . . . 1389.6 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

x Contents

Symbols

A Empirical constant of calculation of compressor isentropic efficiencya Empirical constants of R-134a/oil mixture temperatureB Empirical constant for calculating compressor volumetric efficiencyC Empirical constants for calculation of oil viscosityc Oil concentrationcp Specific heat (kJ kg-1 K-1)D Diameter of smaller size (m)d Diameter of larger size (m)E Coefficient 1 for calculation of dimensionless degree of superheatF Coefficient 2 for calculation of dimensionless degree of superheatec Compression efficiencyei Isentropic efficiencyem Mechanical efficiencyev Volumetric efficiencyf Friction factorH Height (m)h Specific enthalpy (kJ kg-1)K Loss coefficientL Length (m)M Molecular weight (g mol-1)_m Mass flow rate (kg s-1)N Mole number (mol)P Pressure (kPa)DP Pressure drop (kPa)Q Cooling load (kW)r RatioRe Reynolds numberT Temperature (oC)T* Dimensionless degree of superheat for R-134a/PAG mixturesV Linear speed (m s-1)v Compressor rotation speed (rpm)

xi

_Vd Compressor displacement (m3 rev-1)W Power (kW)w Liquid refrigerant fractionx Refrigerant vapour qualityy Mole number fraction

xii Symbols

Greek Symbols

qf Liquid density (kg m-3)qg Vapour density (kg m-3)qo PAG oil density (kg m-3)qmix Density of R-134a/PAG mixture (kg m-3)lf Liquid viscosity (Pa s)lg Vapour viscosity (Pa s)lo PAG oil viscosity (Pa s)e Surface roughness (mm)er Relative roughness (–)m Kinematic viscosity (m2 s-1)u2 Friction factor multiplier (–)/ Heat flux (W m-2)g Effectiveness for heat exchangers (–) or efficiency for others (–)n Local pressure drop coefficient (–)D Difference

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Subscripts

A Point A of the flow streamacc AccumulatorB Point B of the flow streamC Point C of the flow streamC Cold reservoir, evaporatorchar Refrigerant chargecomp Compressioncond CondensationD Point D of the flow streamdyn DynamicE Point E of the flow streamevap Evaporationf Liquid phaseg Vapour phasehot Hot reservoir, condenseri Isentropicin InletJ J-tubeliq liquidm Mechanicalmix R-134a/PAG mixtureo Oilout Outletr Refrigerantr Relativesat Saturatedsl Suction linesiph Anti-siphonsp Single-phase

xv

stag Stagnationsub Subcoolingsup Superheating or supercritical fluid regiont Total (including refrigerant and oil)tp Two-phaseV Volumetric

xvi Subscripts

Abbreviations

AAC Automotive air-conditioningAACTS Automotive air-conditioning test systemACAM Adiabatic capillary modelACTSLHX Accumulator-capillary tube-suction line heat exchangerAXE Accumulator-heat-exchanger-expanderCCOT Clutch cycling orifice tubeCFC ChlorofluorocarbonCOP Coefficient of performanceCTSLHX Capillary tube-suction line heat exchangerHFC HydrofluorocarbonIHX Internal heat exchangerLLSLHX Liquid line-suction line heat exchangerNIHX No internal heat exchangerPAG Polyalkylene glycolTXV Thermal expansion valve

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