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.

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.

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.

Study on onset of nucleate boiling with screw tube for fusion reactor application

2021 ◽  
Author(s):  
Ji Hwan Lim ◽  
Minkyu Park
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
Transfer Mechanism ◽  
High Heat Flux ◽  
Heat Transfer Mechanism ◽  
System Parameters ◽  
Smooth Tubes

Abstract The onset of nucleate boiling (ONB) is the point at which the heat transfer mechanism in fluids changes and is one of the thermo-hydraulic factors that must be considered when establishing a cooling system operation strategy. Because the high heat flux of several MW/m2, which is loaded within a tokamak, is applied under a one-side heating condition, it is necessary to determine a correlative relation that can predict ONB under special heating conditions. In this study, the ONB of a one-side-heated screw tube was experimentally analyzed via a subcooled flow boiling experiment. The helical nut structure of the screw tube flow path wall allows for improved heat transfer performance relative to smooth tubes, providing a screw tube with a 53.98% higher ONB than a smooth tube. The effects of the system parameters on the ONB heat flux were analyzed based on the changes in the heat transfer mechanism, with the results indicating that the flow rate and degree of subcooling are proportional to the ONB heat flux because increasing these factors improves the forced convection heat transfer and increases the condensation rate, respectively. However, it was observed that the liquid surface tension and latent heat decrease as the pressure increases, leading to a decrease in the ONB heat flux. An evaluation of the predictive performance of existing ONB correlations revealed that most have high error rates because they were developed based on ONB experiments on micro-channels or smooth tubes and not under one-side high heat load conditions. To address this, we used dimensional analysis based on Python code to develop new ONB correlations that reflect the influence of system parameters.

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.

Sign in / Sign up

Export Citation Format

Share Document