Heat Transfer due to Microscale Thin Film Evaporation From the Steady State Meniscus in a Coherent Porous Silicon Based Micro-Columnated Wicking Structure

2011 ◽  
Author(s):  
Navdeep S. Dhillon ◽  
Jim C. Cheng ◽  
Albert P. Pisano
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Porous Silicon ◽  
Transfer Model ◽  
Thin Film Evaporation ◽  
Wall Superheat ◽  
Rectangular Channels ◽  
Rate Of Evaporation ◽  
Energy Equations ◽  
Liquid Vapor

A numerical fluid flow and heat transfer model is presented in order to study the evaporation characteristics of a stationary thin film liquid-vapor meniscus. The model is used to evaluate the evaporative heat transfer performance of micron-size rectangular channels on the surface of the secondary wick, inside a micro-columnated coherent porous silicon wick design. The shape of the liquid-vapor meniscus in the channel is obtained by solving the Young-Laplace equation, using a surface energy minimizing algorithm. Mass, momentum and energy equations are then solved in the liquid domain using a discrete finite volume method-based approach. The vapor is assumed to be fully saturated and evaporation at the liquid-vapor interface is modeled using kinetic theory. The effect of wall superheat and inlet-liquid subcooling on the rate of evaporation and associated heat transfer from the evaporating meniscus is characterized.

scholarly journals Experimental Study on the Heat Transfer Performance of Pump-Assisted Capillary Phase-Change Loop

2021 ◽  
Vol 11 (22) ◽  
pp. 10954
Author(s):  
Xiaoping Yang ◽  
Gaoxiang Wang ◽  
Cancan Zhang ◽  
Jie Liu ◽  
Jinjia Wei
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Vapor Pressure ◽  
Pressure Difference ◽  
Heat Dissipation ◽  
Vapor Pressure Difference ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Start Up ◽  
Liquid Vapor

To overcome the two-phase flow instability of traditional boiling heat dissipation technologies, a porous wick was used for liquid-vapor isolation, achieving efficient and stable boiling heat dissipation. A pump-assisted capillary phase-change loop with methanol as the working medium was established to study the effect of liquid-vapor pressure difference and heating power on its start-up and steady-state characteristics. The results indicated that the evaporator undergoes four heat transfer modes, including flooded, partially flooded, thin-film evaporation, and overheating. The thin-film evaporation mode was the most efficient with the shortest start-up period. In addition, heat transfer modes were determined by the liquid-vapor pressure difference and power. The heat transfer coefficient significantly improved and the thermal resistance was reduced by increasing liquid-vapor pressure as long as it did not exceed 8 kPa. However, when the liquid-vapor pressure exceeded 8 kPa, its influence on the heat transfer coefficient weakened. In addition, a two-dimensional heat transfer mode distribution diagram concerning both liquid-vapor pressure difference and power was drawn after a large number of experiments. During an engineering application, the liquid-vapor pressure difference can be controlled to maintain efficient thin-film evaporation in order to achieve the optimum heat dissipation effect.

scholarly journals Experimental Study on the Heat Transfer Performance of Pump-assisted Capillary Phase-change Loop

2021 ◽  
Author(s):  
Xiaoping Yang ◽  
Gaoxiang Wang ◽  
Cancan Zhang ◽  
Jie Liu ◽  
Jinjia Wei
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Vapor Pressure ◽  
Pressure Difference ◽  
Heat Dissipation ◽  
Vapor Pressure Difference ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Start Up ◽  
Liquid Vapor

To overcome the two-phase flow instability of traditional boiling heat dissipation technologies, a porous wick was used for liquid-vapor isolation, thus realizing efficient and stable boiling heat dissipation. A pump-assisted capillary phase-change loop with methanol as working medium was established to study the effect of liquid-vapor pressure difference and heating power on its start-up and steady-state characteristics. The results indicated that the evaporator undergoes four heat transfer modes including flooded, partial flooded, thin film evaporation and overheating. The thin film evaporation mode was the most efficient one with the shortest start-up period. Besides, the heat transfer modes were determined by liquid-vapor pressure difference and power. The heat transfer coefficient could be significantly improved and the thermal resistance could be reduced by increasing liquid-vapor pressure difference as long as it did not exceed 8 kPa. However, when the liquid-vapor pressure difference exceeded 8kPa, its influence on the heat transfer coefficient weakened. In addition, a two-dimensional heat transfer mode distribution diagram considering both liquid-vapor pressure difference and power was drawn through a great number of experiments. During engineering application, the liquid-vapor pressure difference can be controlled to maintain efficient thin film evaporation in order to achieve the optimum heat dissipation effect.

Heat Transfer Model for Evaporation of Elongated Bubble Flows in Microchannels

2002 ◽  
Vol 124 (6) ◽  
pp. 1131-1136 ◽  
Author(s):  
Anthony M. Jacobi ◽  
John R. Thome
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Flow Boiling ◽  
Heat Transfer Model ◽  
Transfer Mechanism ◽  
Transfer Model ◽  
Heat Transfer Mechanism ◽  
New Model ◽  
Thin Film Evaporation ◽  
Film Evaporation

Recent experimental studies of evaporation in microchannels have shown that local flow-boiling coefficients are almost independent of vapor quality, weakly dependent on mass flux, moderately dependent on evaporating pressure, and strongly dependent on heat flux. In a conventional (macrochannel) geometry, such trends suggest nucleate boiling as the dominant heat transfer mechanism. In this paper, we put forward a simple new heat transfer model based on the hypothesis that thin-film evaporation into elongated bubbles is the important heat transfer mechanism in these flows. The new model predicts the above trends and quantitatively predicts flow-boiling coefficients for experimental data with several fluids. The success of this new model supports the idea that thin-film evaporation into elongated bubbles is the important heat transfer mechanism in microchannel evaporation. The model provides a new tool for the study of such flows, assists in understanding the heat transfer behavior, and provides a framework for predicting heat transfer.

Theoretical Analysis of Thin Film Evaporation in the Wicks of Loop Heat Pipes

2021 ◽  
pp. 1-14
Author(s):  
Bingyao Lin ◽  
Nanxi Li ◽  
Shiyue Wang ◽  
Leren Tao ◽  
Guangming Xu ◽  
...  
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Pore Size ◽  
Momentum Conservation ◽  
Loop Heat Pipes ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Quantity Of Heat

Abstract In this paper, a thin film evaporation model that includes expressions for energy, mass and momentum conservation was established through the augmented Young-Laplace model. Based on this model, the effects of pore size and superheating on heat transfer during thin film evaporation were analyzed. The influence of the wick diameter of the loop heat pipe (LHP) on the critical heat flux of the evaporator is analyzed theoretically. The results show that pore size and superheating mainly influence evaporation through changes in the length of the transition film and intrinsic meniscus. The contribution of the transition film area is mainly reflected in the heat transfer coefficient, and the contribution of the intrinsic meniscus area is mainly apparent in the quantity of heat that is transferred. When an LHP evaporator is operating in a state of surface evaporation, a higher heat transfer coefficient can be achieved using a smaller pore size.

scholarly journals Capillary-Limited Evaporation From Well-Defined Microstructured Surfaces

2013 ◽  
Author(s):  
Solomon Adera ◽  
Rishi Raj ◽  
Evelyn N. Wang
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Thermal Management ◽  
Latent Heat ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Microstructured Surfaces ◽  
Latent Heat Of Vaporization

Thermal management is increasingly becoming a bottleneck for a variety of high power density applications such as integrated circuits, solar cells, microprocessors, and energy conversion devices. The performance and reliability of these devices are usually limited by the rate at which heat can be removed from the device footprint, which averages well above 100 W/cm2 (locally this heat flux can exceed 1000 W/cm2). State-of-the-art air cooling strategies which utilize the sensible heat are insufficient at these large heat fluxes. As a result, novel thermal management solutions such as via thin-film evaporation that utilize the latent heat of vaporization of a fluid are needed. The high latent heat of vaporization associated with typical liquid-vapor phase change phenomena allows significant heat transfer with small temperature rise. In this work, we demonstrate a promising thermal management approach where square arrays of cylindrical micropillar arrays are used for thin-film evaporation. The microstructures control the liquid film thickness and the associated thermal resistance in addition to maintaining a continuous liquid supply via the capillary pumping mechanism. When the capillary-induced liquid supply mechanism cannot deliver sufficient liquid for phase change heat transfer, the critical heat flux is reached and dryout occurs. This capillary limitation on thin-film evaporation was experimentally investigated by fabricating well-defined silicon micropillar arrays using standard contact photolithography and deep reactive ion etching. A thin film resistive heater and thermal sensors were integrated on the back side of the test sample using e-beam evaporation and acetone lift-off. The experiments were carried out in a controlled environmental chamber maintained at the water saturation pressure of ≈3.5 kPa and ≈25 °C. We demonstrated significantly higher heat dissipation capability in excess of 100 W/cm2. These preliminary results suggest the potential of thin-film evaporation from microstructured surfaces for advanced thermal management applications.

Experimental Investigation of Heat Transfer in Impingement Air Cooled Plate Fin Heat Sinks

2005 ◽  
Vol 128 (4) ◽  
pp. 412-418 ◽  
Author(s):  
Zhipeng Duan ◽  
Y. S. Muzychka
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Thermal Resistance ◽  
Thermal Performance ◽  
Heat Transfer Model ◽  
Heat Sinks ◽  
Transfer Model ◽  
Transfer Coefficients ◽  
Developing Flow ◽  
Rectangular Channels

Impingement cooling of plate fin heat sinks is examined. Experimental measurements of thermal performance were performed with four heat sinks of various impingement inlet widths, fin spacings, fin heights, and airflow velocities. The percent uncertainty in the measured thermal resistance was a maximum of 2.6% in the validation tests. Using a simple thermal resistance model based on developing laminar flow in rectangular channels, the actual mean heat transfer coefficients are obtained in order to develop a simple heat transfer model for the impingement plate fin heat sink system. The experimental results are combined into a dimensionless correlation for channel average Nusselt number Nu∼f(L*,Pr). We use a dimensionless thermal developing flow length, L*=(L∕2)∕(DhRePr), as the independent parameter. Results show that Nu∼1∕L*, similar to developing flow in parallel channels. The heat transfer model covers the practical operating range of most heat sinks, 0.01<L*<0.18. The accuracy of the heat transfer model was found to be within 11% of the experimental data taken on four heat sinks and other experimental data from the published literature at channel Reynolds numbers less than 1200. The proposed heat transfer model may be used to predict the thermal performance of impingement air cooled plate fin heat sinks for design purposes.

Contribution of Microlayer Evaporation During Flow Boiling Inside Microchannels

2008 ◽  
Author(s):  
A. Mukherjee
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Transfer Mechanism ◽  
Heat Transfer Mechanism ◽  
Cross Sectional ◽  
Thin Film Evaporation ◽  
Nucleate Boiling Heat Transfer ◽  
Microlayer Evaporation

Flow boiling through microchannels is characterized by nucleation and growth of vapor bubbles that fills the entire channel cross-sectional area. As the bubble nucleates and grows inside the microchannel, a thin film of liquid or a microlayer gets trapped between the bubble and the channel walls. The heat transfer mechanism present at the channel walls during flow boiling is studied numerically. These mechanisms are compared to the heat transfer mechanisms present during nucleate boiling and in a moving evaporating meniscus. It is shown that the thermal and the flow fields present inside the microchannels around the bubbles are fundamentally different compared to nucleate boiling or in a moving evaporating meniscus. It is explained that how thin film evaporation is responsible for creating an apparent nucleate boiling heat transfer mechanism inside the microchannels.

Sea Water Distillatory Using Rising Film on the Fluted Surface of Horizontal Tubes

2006 ◽  
Author(s):  
Yan Li ◽  
Ning Mei ◽  
Yesheng Sun
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Theoretical Analysis ◽  
Numerical Solution ◽  
Temperature Difference ◽  
Sea Water ◽  
Flow Characteristics ◽  
Horizontal Tube ◽  
Transfer Model ◽  
Horizontal Tubes

The purpose of this study is to investigate the mechanism of the seawater distillatory using rising liquid thin film on the fluted surface of a horizontal tube. By analyzing the formation of the rising film, a process of the HRF evaporators was designed to analysis the efficiency of the system. The numerical solution of heat transfer model shows that the temperature difference of HRF in one effect is lower than that of HFF. The behaviors of the flow characteristics were discussed. The results show that the rising liquid thin film could be formed when the rate of roll equaled 15°. The results from theoretical analysis suggest that seawater distillatory using rising liquid thin film on the fluted surface of a horizontal tube was especially suitable for the wobble environment.

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