Review of Enhancement Techniques With Vapor Extraction During Flow Boiling in Microchannels

The present work shows the potential of using a diverging channel with in-situ vapor extraction as a means to reduce flow instability in microscale flow boiling. It has been shown that diverging channel helps stabilize convective boiling flow. In-situ vapor extraction is proposed as an alternative method that helps further stabilize flow boiling. The main concept of in-situ vapor extraction is to reduce the vapor available inside the channel where it forms by locally extracting vapor through a hydrophobic porous membrane that forms a wall of the channel to stabilize the flow and reduce pressure drop along the channel. In-situ vapor extraction of boiling flow in a microchannel also has the potential to reduce the required pressure to drive the flow through the channel without losing the benefit of convective boiling heat transfer. In this study, four microchannels, each with a range extraction pressures, are evaluated using a one-dimensional predictive model. Each channel is 50 mm and has a mid-channel width of 500 microns and height of 500 microns. The half angle of divergence of the channels varies: 0, 0.11, 0.23, and 0.34 degrees. Wall heat flux values range from 13.3 to 133 W/cm2. Extracted mass flow rates, global pressure drop across the channels, and quality at the channel outlet are presented as a function of heat flux and extraction pressure. Local variations of pressure, quality and bulk fluid temperature are also presented. Stability is predicted by a newly proposed criterion applicable for a diverging channel with in-situ vapor extraction. The results show that in-situ vapor extraction significantly reduces pressure drop for all channel configurations. Although the drop in pressure across the diverging channels is less influenced by vapor extraction than the non-diverging channel, the coupling of diverging channels and vapor extraction considerably improves the predicted stability of the flow.

Download Full-text

Review of Enhancement Techniques With Vapor Extraction During Flow Boiling in Microchannels

ASME 2020 18th International Conference on Nanochannels, Microchannels, and Minichannels ◽

10.1115/icnmm2020-1068 ◽

2020 ◽

Author(s):

Debora C. Moreira ◽

Gherhardt Ribatski ◽

Satish G. Kandlikar

Keyword(s):

Heat Transfer ◽

Boiling Heat Transfer ◽

Flow Boiling ◽

High Heat ◽

Design Guidelines ◽

Heat Sinks ◽

Transfer Coefficients ◽

Vapor Extraction ◽

High Heat Transfer ◽

Flow Boiling Heat Transfer

Abstract Flow boiling heat transfer in microchannels can remove high heat loads from restricted spaces with high heat transfer coefficients and minimum temperature gradients. However, many works still report problems with instabilities, high pressure drop and early critical heat flux, which hinder its possible applications as thermal management solutions. Much comprehension on the phenomena concerning flow boiling heat transfer is still missing, therefore many investigations rely on empirical methods and parametric studies to develop novel configurations of more efficient heat sinks. Nevertheless, investigations involving vapor extraction have successfully addressed all these previously reported issues while also increasing the heat transfer of heat sinks employing flow boiling in microchannels. In this sense, the objective of this review is to identify the main techniques employed for vapor extraction in microchannels-based heat sinks and analyze the physical mechanisms underneath the observed improvements during flow boiling, such that some design guidelines can be drawn. Three main strategies can be identified: passive vapor extraction, active vapor extraction, and membrane-based vapor extraction. All these strategies were able to dissipate heatfluxes higher than 1 kW/cm2, with the best performance achieved by a membrane-based heat sink, followed by active and passive designs. According to the present experimental and numerical data available in the literature, there is still room for improvement.

Download Full-text

Vapor Extraction From Flow Boiling in a Fractal-Like Branching Heat Sink

ASME 2007 InterPACK Conference, Volume 2 ◽

10.1115/ipack2007-33423 ◽

2007 ◽

Cited By ~ 6

Author(s):

Mario Apreotesi ◽

Deborah Pence ◽

James Liburdy

Keyword(s):

Pressure Drop ◽

Flow Rate ◽

Heat Sink ◽

Single Phase ◽

Driving Forces ◽

Flow Boiling ◽

Average Pore Size ◽

Working Fluid ◽

Vapor Extraction ◽

Flow Through

Increases in pressure due to vapor generation during boiling in microchannels can be reduced by extraction of vapor at its point of inception. Ultimately, this local vapor extraction decreases the pressure drop required to drive the flow through the microchannel network within the heat sink. Indeed, by lowering the overall flow rate by vapor extraction, the pressure drop can, in principle, be lowered below that of single-phase flow. In this present study the relative driving forces necessary for vapor extraction and for flow through the microchannels are investigated. The concept also has the potential to separate flow independent of orientation or gravity. The fractal-like flow network used here is one that has been previously shown to reduce pressure drop and yield a more uniform surface temperature distribution for single-phase flows. The disk shaped heat sink was covered with a porous Nylon membrane with an average pore size of 0.45 microns. Water was used as the working fluid with inlet subcooling of approximately 2.5 K and flow rates ranging between 8 and 12 g/min. The vapor extraction pressure was varied and maintained between 0 and 56 kPa below the average pressure between the inlet and exit of the heat sink. Heating varied from 18 to 30 Watts. Actual vapor extraction data are correlated with the exit quality predicted with no vapor extraction, the value of which is dependent upon mass flow rate, heat input and degree of subcooling. Network pressure drop data are correlated with the membrane pressure difference data.

Download Full-text

Modeling Criteria for Extraction Regime Transitions for Microscale In-Situ Vapor Extraction Application

Volume 8B: Heat Transfer and Thermal Engineering ◽

10.1115/imece2013-65764 ◽

2013 ◽

Author(s):

Saran Salakij ◽

Deborah V. Pence ◽

James A. Liburdy

Keyword(s):

Two Phase Flow ◽

Flow Boiling ◽

Flow Regimes ◽

Bubble Collapse ◽

Phase Flow ◽

Vapor Extraction ◽

Two Phase ◽

Extraction Mechanism ◽

Microscale Flow ◽

Partial Extraction

A major detriment of two-phase microscale flow systems is a relatively high pressure drop, as well as the potential for flow instabilities. A possible mechanism to overcome these disadvantages is vapor extraction through a hydrophobic porous wall in the channel to reduce vapor content and suppress vapor expansion. The vapor extraction may occur either as evaporation, bubble extraction or a mix of both. For the design of vapor extraction systems, it is important to accurately predict extraction regimes, extraction rates and the effect of extraction on the heat transfer and flow conditions. This study focuses on two parts: the development of physic-based models for the transition criteria among (i) the extraction mechanism regimes, and (ii) the extraction flow regimes for microscale flow boiling. The identification and conditions for the various extraction regimes are discussed and criteria for transition are developed based on physical concepts. Six potential extraction mechanism regimes are identified: (a) no extraction, (b) pure evaporation, (c) pure bubble extraction, (d) bubble extraction with partial liquid blockage, (e) bubble extraction with evaporation, and (f) liquid breakthrough. Based on the criteria for the extraction mechanism regimes, the rate of vapor extraction is modeled and used to analyze the effects of vapor extraction on the dynamics of two-phase flow boiling. The results show six extraction flow regimes for two-phase flow boiling: (i) single-phase evaporation, (ii) two-phase evaporation – bubble collapse, (iii) full extraction – stable, (iv) full extraction – unstable, (v) partial extraction – stable and (iv) partial extraction – unstable.

Download Full-text