Results showing the effect of pool boiling on the surface roughness of copper tubes using deionized water as the working fluid are presented in this paper. To obtain different surface roughness values of each sample, the copper tubes were rotated with an electric rotor and sanded using sandpapers with different grits. The average surface roughness values of the plain copper tubes were in the range 0.032–0.544 µm. All experimental samples were horizontally oriented, and experiments were carried out in ambient conditions up to a moderate heat flux regime (~ 450 kW/m2). Moreover, for a comparative analysis, a sample with a rough surface and hydrophobic patterns was included in this study. Compared with the smoothest surface, the aforementioned rough sample exhibited a heat transfer coefficient that was up to a factor 1.5 higher for the highest evaluated heat flux. These findings show that even small increments in the surface roughness along with the addition of hydrophobic patterns can significantly lower the wall superheat temperature and increase the heat transfer coefficient of copper tubes. Furthermore, supported by high-speed imaging of the experiment, it was observed that increasing the surface roughness caused bubbles to depart when their diameter was larger, and the nucleation site density and bubble departure frequency increased. In contrast, the rough surface with hydrophobic patterns exhibited the best overall enhancement, including the characteristics mentioned above of the rough surfaces along with a uniform distribution of the bubbles around the surface.

Results showing the effect of pool boiling on the surface roughness of copper tubes using deionized water as the working fluid are presented in this paper. To obtain different surface roughness values of each sample, the copper tubes were rotated with an electric rotor and sanded using sandpapers with different grits. The average surface roughness values of the plain copper tubes were in the range 0.032–0.544 µm. All experimental samples were horizontally oriented, and experiments were carried out in ambient conditions up to a moderate heat flux regime (~ 450 kW/m2). Moreover, for a comparative analysis, a sample with a rough surface and hydrophobic patterns was included in this study. Compared with the smoothest surface, the aforementioned rough sample exhibited a heat transfer coefficient that was up to a factor 1.5 higher for the highest evaluated heat flux. These findings show that even small increments in the surface roughness along with the addition of hydrophobic patterns can significantly lower the wall superheat temperature and increase the heat transfer coefficient of copper tubes. Furthermore, supported by high-speed imaging of the experiment, it was observed that increasing the surface roughness caused bubbles to depart when their diameter was larger, and the nucleation site density and bubble departure frequency increased. In contrast, the rough surface with hydrophobic patterns exhibited the best overall enhancement, including the characteristics mentioned above of the rough surfaces along with a uniform distribution of the bubbles around the surface.

Master thesis certification by oral defense committee i
Dedication ii
Acknowledgements iii
Abstract iv
Table of Contents v
List of figures vii
List of tables xi
Nomenclature xii
Abbreviations xiii
1. INTRODUCTION 1
1.1. Background 1
1.2. Pool boiling curve 2
1.2.1. Natural convection boiling 3
1.2.2. Nucleate boiling 4
1.2.3. Transition boiling 4
1.2.4. Film boiling 5
1.3. Thesis structure 5
2. LITERATURE REVIEW 6
2.1. Passive techniques 6
2.1.1. Surface roughness 6
2.1.2. Coating materials 9
2.1.3. Microchannel surfaces 13
2.2. Active techniques 14
2.3. Combination techniques 16
2.4. Comments in the literature review 17
2.5. Objectives 18
3. EXPERIMENTAL APPROACH 19
3.1. Experimental setup 19
3.2. Surface preparation procedures 23
3.3. Preparation of the polymer mixture 26
3.4. Surface characterization of the experimental samples 27
3.5. Data reduction 31
3.6. Uncertainty analysis 33
3.7. Experimental procedures 35
4. RESULTS AND DISCUSSION 40
4.1. Surface wettability behavior 41
4.2. Wall superheat and heat transfer coefficient enhancement 42
4.3. Circumferential surface temperature difference 46
4.4. Visual representation of the bubble dynamics 50
5. CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH 55
5.1. Conclusions 55
5.2. Suggestions for future research 56
References 58
Appendices 63

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