Closure to “Discussion of ‘Heat Transfer Mechanisms During Flow Boiling in Microchannels’ ” (2012, ASME J. Heat Transfer, 134, p. 015501)

2011 ◽  
Vol 134 (1) ◽  
Author(s):  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Transfer Mechanisms

Confined Bubble and Heat Transfer during Flow Boiling in a High Aspect Ratio Mini-Channel

2011 ◽  
Vol 312-315 ◽  
pp. 548-553 ◽  
Author(s):  
Yuan Wang ◽  
Khellil Sefiane
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Bubble Growth ◽  
Flow Boiling ◽  
Critical Time ◽  
Heat Fluxes ◽  
Equivalent Radius ◽  
Mass Fluxes ◽  
Transfer Mechanisms ◽  
Working Liquid

Single vapour bubble growth and heat transfer mechanism during flow boiling in a rectangular horizontal mini-channel were experimentally investigated. The hydraulic diameter of the channel was 1454 μm, with an aspect ratio (Win/din) of 10. Degassed FC-72 was used as the working liquid. In this paper, bubble equivalent radius was found to increase linearly till a critical time, beyond which the growth turned into exponential. Bubble growth rate increases with increasing heat flux. Heat transfer mechanisms of the bubble growth at different heat fluxes and mass fluxes were discussed. In addition, the relation between thermal and flow conditions with bubble temporal geometry was explored.

Heat Transfer Mechanisms During Flow Boiling in Microchannels

2004 ◽  
Vol 126 (1) ◽  
pp. 8-16 ◽  
Author(s):  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Contact Line ◽  
Channel Wall ◽  
Flow Boiling ◽  
Reynolds Numbers ◽  
Nucleate Boiling ◽  
Two Phase ◽  
Low Reynolds Numbers ◽  
Low Reynolds ◽  
Transfer Mechanisms

The forces due to surface tension and momentum change during evaporation, in conjunction with the forces due to viscous shear and inertia, govern the two-phase flow patterns and the heat transfer characteristics during flow boiling in microchannels. These forces are analyzed in this paper, and two new nondimensional groups, K1 and K2, relevant to flow boiling phenomenon are derived. These groups are able to represent some of the key flow boiling characteristics, including the CHF. In addition, a mechanistic description of the flow boiling phenomenon is presented. The small hydraulic dimensions of microchannel flow passages present a large frictional pressure drop in single-phase and two-phase flows. The small hydraulic diameter also leads to low Reynolds numbers, in the range 100–1000, or even lower for smaller diameter channels. Such low Reynolds numbers are rarely employed during flow boiling in conventional channels. In these low Reynolds number flows, nucleate boiling systematically emerges as the dominant mode of heat transfer. The high degree of wall superheat required to initiate nucleation in microchannels leads to rapid evaporation and flow instabilities, often resulting in flow reversal in multiple parallel channel configuration. Aided by strong evaporation rates, the bubbles nucleating on the wall grow rapidly and fill the entire channel. The contact line between the bubble base and the channel wall surface now becomes the entire perimeter at both ends of the vapor slug. Evaporation occurs at the moving contact line of the expanding vapor slug as well as over the channel wall covered with a thin evaporating film surrounding the vapor core. The usual nucleate boiling heat transfer mechanisms, including liquid film evaporation and transient heat conduction in the liquid adjacent to the contact line region, play an important role. The liquid film under the large vapor slug evaporates completely at downstream locations thus presenting a dryout condition periodically with the passage of each large vapor slug. The experimental data and high speed visual observations confirm some of the key features presented in this paper.

Flow-Pattern Based Heat Transfer Correlations for Stable Flow Boiling in Micro/Minichannels

2015 ◽  
Vol 138 (3) ◽  
Author(s):  
Zan Wu ◽  
Bengt Sundén
Keyword(s):  
Heat Transfer ◽  
Flow Pattern ◽  
Annular Flow ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Predictive Tool ◽  
Data Points ◽  
Heat Transfer Correlations ◽  
Stable Flow ◽  
Transfer Mechanisms

Flow-pattern based heat transfer correlations for elongated bubbly flow and annular flow in stable flow boiling micro/minichannels were developed separately based on a collected micro/minichannel heat-transfer database by using an improved conventional-to-micro/minichannel threshold Bo = 4 and BoRel0.5 = 200, where Bo is the Bond number and Rel is the liquid Reynolds number. As significant disagreement in experimental trends and heat transfer mechanisms was reported for flow boiling in micro/minichannels in the literature, it is not possible to explain the discrepancy and predict all data points by a single correlation without considering the different flow patterns. The newly developed flow-pattern based predictive tool cannot only present a decent overall accuracy, but also estimate the parametric trends relatively well. Over 95% of the data points can be predicted by the proposed correlations within a ±50% error band for both elongated bubbly flow and annular flow. Therefore, the flow-pattern based correlations can be applied for heat exchanger design to improve the understanding of the underlying heat transfer mechanisms and to guide the development of further enhancement techniques for flow boiling in micro/minichannels.

Heat Transfer Mechanisms During Flow Boiling in Microchannels

2003 ◽  
Author(s):  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Contact Line ◽  
Single Phase ◽  
Flow Boiling ◽  
Reynolds Numbers ◽  
Nucleate Boiling ◽  
Two Phase ◽  
Mass Fluxes ◽  
Transfer Mechanisms

The forces due to surface tension, inertia, and momentum change during evaporation in microchannel govern the two-phase flow patterns and the heat transfer characteristics during flow boiling. These forces are analyzed in this paper, and two new non-dimensional groups, K1 and K2, relevant to flow boiling phenomenon are derived. These groups are able to represent some of the key flow boiling characteristics, including the CHF. The small hydraulic dimensions of microchannel flow passages present a large frictional pressure drop in single-phase and two-phase flows. In order to keep the pressure drop within limits, the channel lengths are generally shorter and the mass fluxes are generally lower than those with conventional channels (Dh>3 mm). The resulting lower mass fluxes, coupled with small Dh, lead to Reynolds numbers in the range 100–1000. Such low Reynolds numbers are rarely employed for flow boiling in conventional channels. In these low Reynolds number flows, nucleate boiling systematically emerges as the dominant mode of heat transfer. Aided by strong evaporation rates, the bubbles nucleating on the wall grow quickly and fill the entire channel. The contact line between the bubble base and the channel wall surface now becomes the entire perimeter at both ends of the vapor slug. Evaporation occurs at the moving contact line of the expanding vapor slug as well as over the channel wall covered with a thin liquid film surrounding the vapor core. The usual nucleate boiling heat transfer mechanisms, including liquid film evaporation and transient heat conduction in the liquid adjacent to the contact line region, play an important role. The liquid film under the large vapor slug evaporates completely at downstream locations thus presenting a dryout condition periodically with the passage of each large vapor slug. The flow boiling correlation by Kandlikar [1, 2] with (i) the nucleate boiling dominant region equation, and (ii) the laminar flow equation for single-phase all-liquid flow heat transfer coefficient hLO was successful in correlating the available R-134a data for parallel microchannels of 190 μm hydraulic diameter.

Simple Flow-Pattern Based Heat Transfer Correlations for Flow Boiling in Micro/Minichannels

2014 ◽  
Author(s):  
Zan Wu ◽  
Bengt Sunden ◽  
Shi-chune Yao ◽  
Vishwas V. Wadekar ◽  
Wei Li
Keyword(s):  
Heat Transfer ◽  
Flow Pattern ◽  
Annular Flow ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Predictive Tool ◽  
Data Points ◽  
Heat Transfer Correlations ◽  
Transfer Mechanisms ◽  
Simple Flow

An improved conventional-to-micro/minichannel threshold was proposed as Bo = 4 and BoRel0.5 = 200, where Bo is the Bond number and Rel is the liquid Reynolds number. The region Bo < 4 and BoRel0.5 < 200 is termed as micro/minichannel because bubbles tend to be confined and elongated in the channel and the conventional two-phase flow theory loses its applicability. Flow-pattern based heat transfer correlations for elongated bubbly flow and annular flow in flow boiling micro/minichannels were developed separately based on a collected micro/minichannel heat-transfer database. As significant disagreement in experimental trends and heat transfer mechanisms was reported for flow boiling in micro/minichannels in the literature, it is not possible to explain the discrepancy and predict all data points by a single correlation without considering the different flow patterns. The newly developed flow-pattern based predictive tool can not only present a decent overall accuracy, but also estimate the parametric trends correctly. Over 95% of the data points can be predicted by the proposed correlations within a ± 50% error band for both elongated bubbly flow and annular flow. Therefore, the simple flow-pattern based correlations can be applied for heat exchanger design, improve understanding of the underlying heat transfer mechanisms, and guide development of further enhancement techniques for flow boiling in micro/minichannels.

Transient Evolution of Heat Transfer Performance During Flow Boiling in a Microchannel

2015 ◽  
Author(s):  
Mrinal Jagirdar ◽  
Poh Seng Lee
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Conduction Problem ◽  
Flow Boiling ◽  
Inverse Heat Conduction Problem ◽  
Working Fluid ◽  
Inverse Heat Conduction ◽  
Underlying Mechanisms ◽  
Transfer Mechanisms ◽  
Frequency Temperature

There is a dearth of understanding about the underlying mechanisms of heat transfer during various flow boiling regimes prevalent during flow boiling in microchannels. In this paper, high frequency temperature data and flow visualization have been captured simultaneously to understand the heat transfer mechanisms. Experiments were performed on a single microchannel with height, width and length of 0.42 mm, 2.54 mm and 25.4 mm respectively. The working fluid was deionized, de-gassed water. The tested heat flux and mass flux were 28 W/cm2 and 180.1 kg/m2s respectively. The flow boiling regime observed was slug flow. Temperature captured was below the wetted surface and hence Inverse Heat Conduction Problem (IHCP) solution methodology had to be used. Its efficacy was first tested and was found to be reasonably good. Transient wetted surface heat flux, temperature and heat transfer coefficient were calculated using this methodology and were then correlated with the visual data. Depending on the flow boiling phenomena, there were significant variations in heat transfer with time. Several insights into the heat transfer mechanisms have been presented.

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