Investigation Of Several Critical Issues In Screen Mesh Heat Pipe Manufacturing And Operation Andreas Engelhardt, MEng (Hons), Dipl. Ing. (FH) Thesis submitted to for the Degree of Doctor of Philosophy February 2010 Abstract PhD Thesis Andreas Engelhardt The PhD thesis with the title “Investigation of several critical issues in screen mesh heat pipe manufacturing and operation” presented hereafter describes work carried out in four main areas. Initially the relevant literature is reviewed and presented, followed by the presentation of theoretical work regarding screen mesh heat pipe fill calculations, heat pipe processing and an investigation into the capillary or lifting height for screen mesh heat pipes. Further, the possibility of tailoring screen mesh heat pipes towards certain applications was investigated and it was found that further work is required in this area to allow a conclusive judgement whether a coarser or finer wick at the wall provides a distinguish advantage over two wraps of a medium coarse type. Within this approach a calculation method for the determination of the optimum working fluid fill of a screen mesh heat pipe based on geometrical parameters of the wick was developed and successfully implemented for the production of the later tested samples. The investigation into the effects of bending on the heat pipe performance, both using single phase flow CFD as well as experimentation, was performed using five different geometrical cases, each with five samples. These were tested in order to minimise the effects of sample variation. The test cases investigated contained the deformation angles of 0° (straight), 45°, 90°, 135° and 180°. During all test cases the orientation of the samples was kept constant at 0° to minimise additional influences like the effects of gravity on the reduction of available power handling capability. The test results show in deviation from CFD results that screen mesh heat pipe performance is - i - significantly affected when bends are introduced and the reduction in power handling capability can be up to nearly 50% of a straight heat pipe value. Finally this thesis advances into the field of water heat pipe freeze thaw and the possibility of screen mesh heat pipes with changed shapes to withstand multiple freeze thaw cycles. It was found that correctly engineered screen mesh copper water heat pipes can be used in applications where they are subjected to multiple freeze thaw cycles. The fluid charge for water heat pipes subjected to these conditions needs to be adjusted in such a way, that accumulation of working fluid in certain areas, regardless of orientation or process variation during filling, is avoided. - ii - ACKNOWLEDGEMENTS The author would like to take the opportunity to thank Xudong Zhao of the School of Built Environment at the University of Nottingham and David Mullen of Thermacore Europe for their continuous support and offering of advice when required over the entire duration of this PhD. Further thanks have to go to Saffa Riffat of the School of Built Environment at the University of Nottingham and Kevin Lynn of Thermacore Europe for their understanding and their support during the period of the work conducted. Since most of the experimental work was carried out at the sponsoring company, Thermacore Europe, an additional large thank you has to go to all the co-workers there for their support and help. But a special thank you has to go to Ian Davies for the production of the test samples and advice when required. Without his knowledge and skills the production of test samples with certain characteristics would have been more time consuming and less successful then it was. Last but not least the author would like to thank the EPSRC (Engineering and Physical Sciences Research Council), which made this PhD thesis possible through their financial support under the Grant number EP/P501733/1. - iii - COPYWRITER DECLARATION I declare that this thesis is the result of my own work and has not, whether in the same or a different form, been presented to this or any other university in support of an application for any degree other than that of which I am now a candidate. - iv - TABLE OF CONTENTS Abstract i Acknowledgements iii Copyright Declaration iv Nomenclature xi List of Figures xv List of Tables xxiii CHAPTER 1 – INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 BACKGROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 DESCRIPTION OF THE RESEARCH . . . . . . . . . . . . . . . . . . . . . 4 1.3 WORK INVOLVED IN THE RESEARCH. . . . . . . . . . . . . . . . . . . 7 CHAPTER 2 - LITERATURE REVIEW HEAT PIPE TECHNOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.1 HEAT PIPE CONSTRUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.1.1 Container Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.1.2 Working Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.1.3 Performance Limits Of Heat Pipes . . . . . . . . . . . . . . . . . . 24 2.1.4 Types Of Heat Pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.2 HEAT PIPE APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . 36 2.2.1 Typical Heat Pipe Applications . . . . . . . . . . . . . . . . . . . . . 36 - v - 2.2.2 Detail Focused Investigation Of Heat Pipe Behaviour (Start- Up; Freeze- Thaw) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 2.3 MATHEMATICAL MODELLING OF HEAT PIPES . . . . . . . . 44 2.3.1 General Approaches To Heat Pipe Performance Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 2.3.2 Heat Pipe Modelling Using Commercial CFD Codes . . . 52 2.3.3 Modelling Of Multiphase Flow Using Commercial CFD Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 2.3.4 Numerical Investigation Of The Effects Of Bends And Shape-Changes In Related Areas . . . . . . . . . . . . . . . . . . . . 57 2.4 EXPERIMENTAL TECHNIQUES WITH FOCUS ON HEAT PIPES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 2.4.1 Test Rig Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 2.4.2 Temperature Measurement Techniques . . . . . . . . . . . . . . 66 2.4.3 Previous Experimental Work On The Subject Of Bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 2.5 LATEST TECHNOLOGY DEVELOPMENTS IN THE HEAT PIPE AREA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 2.6 SCOPE OF THE WORK OF THIS THESIS AND ITS NOVEL ASPECTS TO THE SCIENTIFIC COMMUNITY . . . . . . . . . . . .70 2.7 AREAS OF FURTHER RESEARCH REQUIRED . . . . . . . . . . . 73 CHAPTER 3 – INVESTIGATIONS OF HEAT PIPE FILLING AND VENTING TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . 75 - vi - 3.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 3.2 MESH HEAT PIPE FILL CALCULATIONS . . . . . . . . . . . . . . . 77 3.2.1 Weight Of Water Required For Generating Saturated Steam Within The Vapour Space . . . . . . . . . . . . . . . . . . . . 80 3.2.2 Mathematical Foundations Of The Porosity Calculation For Screen Meshes Used As Heat Pipe Wicks . . . . . . . . . . . . . 83 3.2.3 Development And Implementation Of VB Based Screen Mesh Wick Heat Pipe Fill Calculation Software . . . . . . . . 88 3.3 ANALYTICAL INVESTIGATION OF THE CAPILLARY OR LIFTING HEIGHT FOR DIFFERENT WICK STRUCTURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 3.4 VENTING OR PROCESSING OF HEAT PIPES. . . . . . . . . . . . 100 3.4.1 Venting Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .100 3.4.2 Venting Technique Applied To The Heat Pipes Used For This Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 3.4.3 Most Common Venting Failure Modes . . . . . . . . . . . . . . 107 3.5 SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . .113 CHAPTER 4 – INVESTIGATION OF THE USE OF DISSIMILAR SCREEN MESHES AS HEAT PIPE WICKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . 115 4.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 4.2 DISSIMILAR SCREEN MESH WICK HEAT PIPE INVESTIGATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 - vii - 4.2.1 Design And Manufacturing Considerations For Dissimilar Mesh Wick Heat Pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 4.2.2 Experimental Results And Analysis Of Dissimilar Mesh Wick Heat Pipe Investigations. . . . . . . . . . . . . . . . . . . . . . 121 4.3 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 CHAPTER 5 – BENDING OF SCREEN WICK HEAT PIPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 5.1 FLOW SIMULATION THROUGH PIPE BENDS USING CFD SOFTWARE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .131 5.1.1 Introduction Into Separate Phase Modelling Theory . . . 132 5.1.2 Separate Phase Modelling . . . . . . . . . . . . . . . . . . . . . . . . . .134 5.1.3 Calculation Of Flow Parameters . . . . . . . . . . . . . . . . . . . .135 5.1.3.1 Liquid Film Height Calculation . . . . . . . . . . . . . . . . . . . . 135 5.1.3.2 Reynolds Number Calculation . . . . . . . . . . . . . . . . . . . . . .137 5.1.4 Theory Used For Obtaining The CFD Settings . . . . . . . . 141 5.1.4.1 Flow Models and Boundary Conditions used . . . . . . . . . . 145 5.1.4.2 Settings as per Heat Pipe Theory disregarding Validity of Mathematical Models for certain Flow Conditions . . . . . .145 5.1.5 CFD Results And Result Plots . . . . . . . . . . . . . . . . . . . . . . 150 5.1.5.1 Results And Plots From Runs Of Annular Flow Per Heat Pipe Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 5.1.5.2 Results And Plots From Runs Of Full Pipe Flow At Flow Velocity From Heat Pipe Theory. . . . . . . . . . . . . . . . . . . . . 152 - viii - 5.1.6 Creating The Link Between The Pressure Results From The CFD Model And The Experimental Results . . . . . . . . . . 155 5.2 TEST RIG DESIGN FOR BENT HEAT PIPES . . . . . . . . . . . . . 161 5.2.1 Purpose Of The Test Rig And Introduction To The Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 5.2.2 Instrumentation Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 5.2.3 Water Side And Cold Plates . . . . . . . . . . . . . . . . . . . . . . . 164 5.2.4 Test Side And Heater Blocks . . . . . . . . . . . . . . . . . . . . . . .166 5.2.5 Adaptability For Different Heat Pipe Diameters And Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 5.3 EXPERIMENTAL TRIALS WITH BENT HEAT PIPES . . . . . 169 5.3.1 Experimental Methodology . . . . . . . . . . . . . . . . . . . . . . . 170 5.3.2 Figures Of Experimental Procedure For One Single Heat Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 5.3.3 Failure Mode Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 5.3.4 Analysis Of The Thermal Load Transmitted By The Heat Pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 5.3.5 Comparison Of The CFD Simulation Results To Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 5.4 DEVELOPMENT OF A MATHEMATICAL APPROXIMATION FOR PERFORMANCE LOSSES DUE TO THE BEND ANGLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 - 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