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«Title of Document: PREDICTION OF HEAT TRANSFER AND PRESSURE DROP OF CONDENSING REFRIGERANT FLOW IN A HIGH ASPECT RATIO MICRO-CHANNELS Ebrahim ...»

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ABSTRACT

Title of Document: PREDICTION OF HEAT TRANSFER AND

PRESSURE DROP OF CONDENSING

REFRIGERANT FLOW IN A HIGH ASPECT

RATIO MICRO-CHANNELS

Ebrahim AL-Hajri, Doctor of Philosophy, 2009 Directed By: Professor, Michael Ohadi, Department of Mechanical Engineering This thesis presents a detailed study of parametric characterization of twophase condensing flow of two selected refrigerants R134a and R-245fa in a single water-cooled micro-channel of 0.4 mm X 2.8 mm cross-section (0.7 mm hydraulic diameter and 7:1 aspect ratio) and 190 mm in length. To avoid flow mal-distribution associated with typical micro-channel tube banks, a single micro-channel was fabricated utilizing an innovative approach and used for the present study experiments. The study investigated the effects of variations in saturation temperature ranging from 30 oC to 70 oC, mass flux from 50 to 500 kg/m2s, and inlet super heat from 0 oC to 15 oC on the average heat transfer and overall pressure drop coefficient of the micro-channel condenser. In all cases the inlet vapor quality was kept at 100% quality (saturated vapor) and the outlet condition was always kept at 0% quality (saturated liquid). Accuracy of the fabricated channel geometry with careful design and choice of instrumentation of the test setup resulted in energy balance and average heat transfer coefficient uncertainties within +/-11% and +/-12%, respectively. It is observed that saturation temperature and mass flux have a significant effect on both heat transfer coefficient and overall pressure drop coefficient, where as the inlet super heat has little effect. This study provides further understanding of the potential microscale effects on the condensation phenomenon for the tube geometry and the dimensions investigated in the present study combined with flow visualization study.

No previous study has addressed the unique single micro-channel geometry being investigated in the present work combined with the two-phase flow visualization of the flow regimes in the present micro-channel geometry. The letter was a major undertaking of the present work and represents one of the main contributions of the present work. The results of the present work shall prove useful in contributing to better understanding of any micro-scale effects on the condensation flow of the two selected refrigerants (one commonly used high pressure refrigerant, R134a) and the other a new low pressure refrigerant (R245fa). It is also expected that the results of this study will lead to future work in this area, realizing the fast penetration of the micro-channel technology in various compact/ultra compact heat exchangers, including refrigeration, petrochemical, electronics, transportation, and process industries.

PREDICTION OF HEAT TRANSFER AND PRESSURE DROP OF

CONDENSING REFRIGERANT FLOW IN A HIGH ASPECT RATIO MICROCHANNELS

–  –  –

Advisory Committee:

Professor Michael Ohadi, Chair/Advisor Professor Reinhard Radermacher Professor Marino di Marzo Professor Gary A. Pertmer Assistant Professor Bao Yang © Copyright by Ebrahim S AL-Hajri Dedication To my parents, my wife and my kids for their sacrifices, support and unconditional

–  –  –

First and foremost, I would like to thank the CEO of Abu Dhabi National Oil Company (ADNOC) His Excellency Mr. Yousf Bin Omair for his trust on my abilities and for his continuous engorgement.

I would also like to express my deepest appreciation to my advisor and mentor Dr. Michael Ohadi for his dedication, intellectual guidance, support, advices and also for bringing the best out of me which helped making this thesis a success.

I would like to thank my beloved wife Ameera and my two daughters Fatma and Yassa for their unconditional love and support during the long, difficult, challenging and hard working years of my Ph.D. program. Without them, this project never would have come to its conclusion.

I would like to thank Dr. Serguei Dessiatoun and Dr. Amir Shooshtari for their unlimited help and support throughout the course of this research project.

Without them, this project would have never research its final phase.

I am particularly grateful to my best friend more like my brother Mr.

Mohamed Al-Shehhi for his great friendship throughout the years I spent here in the US since 1995 up until this day. This support and competitive attitude was the drive that helped me to finishing my Ph.D.

I am grateful to my many friends and colleagues at the Smart and Small Thermal Systems Laboratory for providing a stimulating, constructive and fun-filled environment to learn and grow. I am especially thankful to Vytenis Benetis, Sourav Chowdhury, Parisa Foroughi, Guohua Kuang, Arman Molki, Jianlin Wu, Lewis

–  –  –

Cetegen, Elnaz Kermani, and Paul Kalinowski.

I am also grateful to my thesis committee members, Dr. Marino Di Marzo, Dr.

Reinhard Radermacher, Dr. Gary Pertmer and Dr. Bo Yang for their constructive comments and suggestions which has brought this work to its final form.





Finally, I wish to thank the America Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) and the Petroleum Institute in (P.I.) in Abu Dhabi, UAE for their invaluable financial support of this project.

–  –  –

DEDICATION

ACKNOWLEDGEMENTS

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

NOMENCLATURE

CHAPTER 1 : INTRODUCTION

1.1 Background

1.2 Objectives of the Present Study

1.3 Organization of Dissertation

CHAPTER 2 : LITERATURE REVIEW

2.1 Condensation in Mini and Micro-Channels

–  –  –

CHAPTER 3 : EXPERIMENTAL APPARATUS AND PROCEDURE................. 21

3.1 Test Section Design and Fabrication

–  –  –

3.2 Micro-channel Design and Fabrication

3.3 Testing Refrigerants

–  –  –

3.4.2 Data Reduction

3.4.3 Uncertainty Analysis

3.5 Experimental Procedure

–  –  –

3.5.3 Visualization Tests

3.5.4 Summary

CHAPTER 4 : RESULTS AND DISCUSSION

–  –  –

4.1.1 Heat Transfer

4.1.2 Pressure Drop

4.2 Pressure Drop Results for Constant Quality Flows

–  –  –

4.3 Comparison with the Literature

4.3.1 Summary

CHAPTER 5 : EMPIRICAL CORRELATIONS

5.1 Heat Transfer Correlations

–  –  –

CHAPTER 6 : FLOW VISUALIZATION

6.1 Flow Regime Mapping

–  –  –

CHAPTER 7 : CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE

WORK

7.1 Conclusions

7.2 Recommendations for Future Work

BIBLIOGRAPHY

–  –  –

Table 2-1 Summary of reviewed literature

Table 3-1 Experimental parametric study table

Table 3-2 Comparison of refrigerants properties

Table 3-3 Range and accuracy of instruments

Table 3-4 Summary of uncertainty

Table 3-5 Local pressure drop test matrix

Table 4-1 Regression statistics for heat transfer coefficient of R134a & R245fa separately

Table 4-2 Regression statistics for heat transfer coefficient for all the data of both refriegerants

Table 4-3 Regression statistics for pressure drop of R134a & R245fa separately..... 68 Table 4-4 Regression statistics for pressure drop for all the data of both refriegerants

Table 4-5 Comparison of average heat transfer coefficients with data from Garimella et al. (2004)

Table 6-1 Visualization test matrix

Table 6-2 Digital Images of two-phase flow inside the 2.8 mm viewing window micro-channel at mass flux of 200 kg/m2s and Tsat = 50 oC

Table 6-3 Digital Images of two-phase flow inside the 0.4 mm viewing window micro-channel at mass flux of 300 kg/m2s and Tsat = 30 oC

–  –  –

Figure 3-1 Schematic of experimental test setup

Figure 3-2 Details of test section: (a) details of the joint between condenser section and sight-glass (b) details of the joint between evaporator Dewar and condenser Dewar (c) view of the arrangement of various parts in the refrigeration loop....... 24 Figure 3-3 Details of test section: (a) details of the joint between condenser section and sight-glass (b) details of the joint between evaporator Dewar and condenser Dewar (c) view of the arrangement of various parts in the refrigeration loop....... 25 Figure 3-4 (a) A drawing of the fabricated micro-channel (b) Electroplated microchannel

Figure 3-5 Visualization test section with viewing window of 2.8 mm

Figure 3-6 Visualization test section with viewing window of 0.4 mm

Figure 3-7 Calibration chart for low-flow meter

Figure 3-8 Calibration chart for high-flow meter

Figure 3-9 Calibration of the Coriolis flow meter with water, R134a and R245fa.... 37 Figure 3-10 Calibration of the differential pressure transducer used for pressure drop measurements

Figure 3-11 Calibration of absolute pressure transducer

Figure 3-12 Water side RTDs temperature measurements calibration

Figure 3-13 Water side thermocouple temperature measurements calibration.......... 40 Figure 4-1 Effect of inlet superheat on average heat transfer coefficient at Tsat = 50 oC for R134a

–  –  –

for R245fa

Figure 4-3 Effect of mass flux on average heat transfer coefficient for R134a at Tsat = 50 oC

Figure 4-4 Effect of mass flux on average heat transfer coefficient for R245fa at Tsat = 50 oC

Figure 4-5 Effect of saturation temperature on average heat transfer coefficient for R134a

Figure 4-6 Effect of saturation temperature on average heat transfer coefficient for R245fa

Figure 4-7 Comparison of experimental HTC vs HTC obtained from equation (4.3) for R134a

Figure 4-8 Comparison of experimental HTC vs HTC obtained from equation (4.4) for R245fa

Figure 4-9 Comparison of experimental HTC for R134a & R245fa vs HTC obtained from equation (4.5)

Figure 4-10 Effect of inlet superheat on pressure drop at Tsat = 50 oC for R134a...... 61 Figure 4-11 Effect of inlet superheat on pressure drop at Tsat = 50 oC for R245fa..... 62 Figure 4-12 Effect of mass flux on pressure drop for R134a at Tsat = 50 oC.............. 64 Figure 4-13 Effect of mass flux on pressure drop for R245fa at Tsat = 50 oC............. 64 Figure 4-14 Vapor & liquid velocities of R134a at Tsat of 30 oC at different vapor qualities

–  –  –

R134a

Figure 4-16 Effect of saturation temperature on average heat transfer coefficient for R245fa

Figure 4-17 The change of liquid and vapor viscosity with saturation temperature for R134a & R245fa

Figure 4-18 Change of liquid and vapor density with saturation temperature for R134a & R245fa

Figure 4-19 Comparison of experimental ∆P vs ∆P obtained from equation (4.6) for R134a

Figure 4-20 Comparison of experimental ∆P vs ∆P obtained from equation (4.7) for R245a

Figure 4-21 Comparison of experimental ∆P for R134a & R245fa vs. ∆P obtained from equation (4.8)

Figure 4-22 Effect of mass flux on local pressure drop for R134a

Figure 4-23 Effect of mass flux on local pressure drop for R245fa

Figure 4-24 Effect of saturation temperature on local pressure drop for R134a........ 74 Figure 4-25 Effect of saturation temperature on local pressure drop for R245fa....... 75 Figure 4-26 All the test results of local pressure drop for R134a

Figure 4-27 All the test results of local pressure drop for R245fa

Figure 4-28 Average condensation heat transfer coefficients as a function of mass flux and quality for a square channel with Dh = 0.76 mm

–  –  –

Shin and Kim (2004)

Figure 4-30 Comparison of pressured drop data from the current study and results from Kim (2003) Shin and Kim (2004)



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