Pruthvik A. Raghupathi

Pool boiling enhancement through contact line augmentation

In addition to transient conduction, microconvection, and microlayer evaporation, contact line region heat transfer has been identified as an important mode of heat transfer during boiling. In this work, we demonstrate that generating additional contact line regions within the base of a nucleating bubble leads to critical heat flux (CHF) enhancement. The creation of a liquid meniscus adjacent to 10–20 μm deep microgrooves in the bubble base area was responsible for the generation of the additional contact line regions. The depth of the microgrooves was determined such that a sufficient reservoir of liquid is present in the meniscus to sustain evaporation in the contact line region throughout the bubble cycle. The effective contact line length at the base of the bubble was seen to be a good indicator of the CHF (wetted area) over the surface. The microgroove geometry played a significant role in influencing the bubble dynamics and bubble departure diameter during boiling. It was seen that the bubbles were ...

Characterization of Pool Boiling of Seawater and Regulation of Crystallization Fouling by Physical Aberration

ABSTRACT As fresh water becomes increasingly scarce desalination has become an important technique to meet the portable water requirements around the world. Thermal distillation continues to be one of the most important and widely used methods of desalination currently used. Scale formation, corrosion of the heater surface, and the subsequent degradation of the heat exchanger is one of the biggest challenges in thermal desalination. In this paper, pool boiling of seawater is characterized using standard artificial sea water. Various boiling characteristics such as critical heat flux and heat transfer coefficient are analyzed. The nature of the scales formed on the heater surface and their effect on the heat transfer efficiency are studied. A passive method to reduce the thermal resistance due to scale buildup from crystallization fouling using stainless steel beads is examined.

Bubble induced flow field modulation for pool boiling enhancement over a tubular surface

We demonstrate the efficacy of using a strategically placed enhancement feature to modify the trajectory of bubbles nucleating on a horizontal tubular surface to increase both the critical heat flux (CHF) and the heat transfer coefficient (HTC). The CHF on a plain tube is shown to be triggered by a local dryout at the bottom of the tube due to vapor agglomeration. To mitigate this effect and delay CHF, the nucleating bubble trajectory is modified by incorporating a bubble diverter placed axially at the bottom of the tube. The nucleating bubble at the base of the diverter experiences a tangential evaporation momentum force (EMF) which causes the bubble to grow sideways away from the tube and avoid localized bubble patches that are responsible for CHF initiation. High speed imaging confirmed the lateral displacement of the bubbles away from the diverter closely matched with the theoretical predictions using EMF and buoyancy forces. Since the EMF is stronger at higher heat fluxes, bubble displacement increas...

Evaporation Momentum Force on a Bubble Under Asymmetric Temperature Conditions

i Acknowledgements ii List of Figures v List of Tables vii Nomenclature viii 1.0 INTRODUCTION 1 1.1 Boiling 2 1.2 Pool boiling 2 1.3 Boiling curve 2 1.3.1 Free convection 3 1.3.2 Nucleate boiling 4 1.3.3 Transition boiling 4 1.3.4 Film boiling 5 2.0 LITERATURE REVIEW 6 2.1 Bubble nucleation 6 2.2 Modes of heat transfer 7 2.3 Bubble growth rate 8 2.4 Surface enhancements 12 2.5 Evaporation momentum force 13 3.0 Objectives 19 4.0 Model 20 4.1 Expression for EMP 20 iv 4.2 Effect of EMF on bubble growth 21 4.3 Assumptions 24 4.4 Effect of Temperature distribution 24 4.6 Asymmetric temperature distribution 32 4.7 Bubble motion 33 5.0 Experimental Procedure 36 5.1 Experimental setup 36 5.2 Experimental procedure 38 5.3 Data acquisition 39 5.4 Test section 39 6.0 Results 42 6.1 Effect of EMF on bubble growth 42 6.2 Distribution of EMF and bubble displacement 44 6.3 Visualization and Comparison 47 7.0 Conclusion 50 8.0 Future work 52 9.0 References 53

Flow Regimes and Transition Criteria during Passage of Bubbles through a Liquid–Liquid Interface

The passage of a single bubble or a stream of bubbles through a liquid-liquid interface is a highly dynamic process that can result in a number of different outcomes. Previous studies focused primarily on a single bubble and single flow regime, and very few investigations have considered bubble streams. In the present work, six different liquid combinations made up of water, ethanol, a perfluorocarbon liquid, PP1, and one of three different viscosity silicone oils are tested with air bubbles from 2 to 6 mm in diameter rising between 5 and 55 cm/s. Both single bubbles and bubble streams varying in frequency from 5 to 40 bubbles/s are tested. High-speed imaging is used to capture and classify the flow regimes associated with each flow type. Four different flow regimes are identified for single-bubble passage, and six are found for bubble stream passage. On the basis of theoretical considerations, nondimensional numbers are developed for characterizing the flow regimes and maps are generated that distinguish them and define flow regime transitions.

Preliminary Results of Pool Boiling of Seawater

As the availability of fresh water is becoming scarce, desalination of seawater is increasingly important to meet the fresh water (portable water) requirements of the world. Thermal distillation continues to be one of the most important and widely used methods of desalination currently used. Scale formation and corrosion of the heater surface are some of the challenges in thermal desalination. In this paper, pool boiling of seawater is characterized using standard artificial sea water. The nature of the scales formed on the heater surface and its effect on the heat transfer efficiency is studied. A passive method to reduce the rate of scale formation during boiling is studied. Particularly, steel beads are introduced to prevent the growth of scales on the heater surface and the corresponding boiling performance is evaluated.Copyright

Heat Transfer During Evaporation and Boiling at the Three Phase Contact Line Region: A Critical Review

A fundamental understanding of the various modes of heat transfer and their contributions is critical in the development of enhanced surfaces to augment boiling performance. Recently, a number of studies have highlighted the importance of contact line region in boiling-especially in applications involving thin film evaporation and wicking structures. Contact line region also plays an important role during heat transfer around a nucleating bubble, especially at higher bubble frequencies near critical heat flux (CHF). In this work, a review of the characteristics of the contact line region, the forces at play, and the associated heat transfer mechanisms is conducted. Experimental and analytical works on the contact line region are explored to develop a comprehensive picture of its physical and heat transfer behavior. Various optical and thermal measurement techniques employed by researchers to understand evaporation in the contact line region are also reviewed. The interaction of different forces in this region and the analytical models for predicting the forces is studied. Finally, the contribution of microlayer and contact line heat transfer in nucleate boiling is also presented.Copyright