Theoretical Model of Electric Field Effects on the Enhancement of Critical Heat Flux

2004 ◽  
Author(s):  
Takeshi Yajima ◽  
Akira Yabe ◽  
Hiroshi Maki
Keyword(s):  
Surface Tension ◽  
Heat Flux ◽  
Film Thickness ◽  
Liquid Film ◽  
Critical Heat Flux ◽  
Applied Voltage ◽  
Liquid Film Thickness ◽  
Flux Enhancement ◽  
The Relationship ◽  
Lines Of Force

Critical heat flux enhancement by the electrohydrodynamic (EHD) effect has been analyzed quantitatively based on the increased frequency of liquid-vapor interface oscillations around the edge of the bubble. The majority of heat transfer occurs when the liquid film thickness becomes less than 50 μ m, which only occurs once per period. The main mechanism of heat flux enhancement induced by the EHD effect would be a result of an increase in surface tension due to the effect of electric lines of force. By representing the terms of the forces for a change in curvature and the surface tension resulting from the electric lines of force, the equation of the liquid-vapor instability was obtained and analyzed. Experimentally it has been shown that as the applied voltage increased, the periodic time interval of the thickness change was shortened. This effect reduces the potential for dryout of the liquid film by making the minimum thickness time period shorter. By measuring the pressure oscillation on the boiling surface, the change of the thin liquid film thickness and the dynamic shape of bubbles, the relationship among the pressure, the liquid film thickness and the bubble shape was clarified. Consequently, this model successfully explains the relationship between the applied voltage and the enhancement of the critical heat flux.

The Influence of Liquid Film Thickness on Airblast Atomization

1980 ◽  
Vol 102 (3) ◽  
pp. 706-710 ◽  
Author(s):  
N. K. Rizk ◽  
A. H. Lefebvre
Keyword(s):  
Film Thickness ◽  
Liquid Film ◽  
Drop Size ◽  
Porous Plate ◽  
Liquid Films ◽  
Liquid Sheet ◽  
Liquid Film Thickness ◽  
Air Stream ◽  
And Control ◽  
The Relationship

The influence of initial liquid film thickness on mean drop size and drop-size distribution was examined using two specially designed airblast atomizers. Both were constructed to produce a flat liquid sheet across the centerline of a two-dimensional air duct with the liquid sheet exposed on both sides to high velocity air. In one case a thin film of uniform thickness was produced by injecting the liquid through a porous plate located just upstream of the atomizing edge. The film thickness, t, was then measured by a needle contact device. In the second design the fuel entered the air stream through a thin slot whose height could be adjusted accurately to vary and control the initial film thickness. Drop sizes were measured by the well-established light-scattering technique. From analysis of the processes involved, and from correlation of the experimental data, it was found that high values of liquid viscosity and liquid flow rate result in thicker films. It was also observed that thinner liquid films produce better atomization, according to the relationship, SMD ∝ t0.38.

scholarly journals Analytical Solutions of Heat Transfer and Film Thickness with Slip Condition Effect in Thin-Film Evaporation for Two-Phase Flow in Microchannel

2015 ◽  
Vol 2015 ◽  
pp. 1-15 ◽  
Author(s):  
Ahmed Jassim Shkarah ◽  
Mohd Yusoff Bin Sulaiman ◽  
Md. Razali bin Hj Ayob
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Film Thickness ◽  
Liquid Film ◽  
Two Phase Flow ◽  
Current Model ◽  
Liquid Film Thickness ◽  
Phase Flow ◽  
Two Phase ◽  
Analytical Results

Physical and mathematical model has been developed to predict the two-phase flow and heat transfer in a microchannel with evaporative heat transfer. Sample solutions to the model were obtained for both analytical analysis and numerical analysis. It is assumed that the capillary pressure is neglected (Morris, 2003). Results are provided for liquid film thickness, total heat flux, and evaporating heat flux distribution. In addition to the sample calculations that were used to illustrate the transport characteristics, computations based on the current model were performed to generate results for comparisons with the analytical results of Wang et al. (2008) and Wayner Jr. et al. (1976). The calculated results from the current model match closely with those of analytical results of Wang et al. (2008) and Wayner Jr. et al. (1976). This work will lead to a better understanding of heat transfer and fluid flow occurring in the evaporating film region and develop an analytical equation for evaporating liquid film thickness.

High Heat Flux With Small Scale Monodisperse Sprays

2012 ◽  
Vol 134 (12) ◽  
Author(s):  
Sergio Escobar-Vargas ◽  
Jorge E. Gonzalez ◽  
Drazen Fabris ◽  
Ratnesh Sharma ◽  
Cullen Bash
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Film Thickness ◽  
Liquid Film ◽  
Heat Dissipation ◽  
High Heat ◽  
Heat Fluxes ◽  
Liquid Film Thickness ◽  
Small Scale ◽  
High Heat Flux

This work is aimed at cooling small surfaces (1.3 mm × 2 mm and 3 mm × 5 mm) using spray from thermal ink jet (TIJ) atomizers. Particular interests in this work include obtaining heat fluxes near the critical heat flux (CHF), understanding the correlation between the heat dissipation efficiency (η) and the liquid film thickness (δ) through experimental data, and understanding the primary mode of heat transfer on spray cooling at different liquid film thickness. Current experimental results indicate that high heat fluxes (∼4 × 107 W/m2) are obtained for controlled conditions of cooling mass flow rate, higher efficiencies are achieved at smaller liquid film thickness (δ ≈ 5 μm → η ≈ 0.9), and the heat transfer by conduction through the film becomes dominant as δ decreases.

Liquid Film Features Analysis on Mechanical Seals with Micro-Pore Face by CFD

2011 ◽  
Vol 354-355 ◽  
pp. 575-578
Author(s):  
Wei Zheng Zhang ◽  
Shu Rong Yu ◽  
Yan Wang ◽  
Xue Xing Ding
Keyword(s):  
Film Thickness ◽  
Liquid Film ◽  
Optimization Design ◽  
Theoretical Data ◽  
Least Square ◽  
Liquid Film Thickness ◽  
Stable Operation ◽  
Mechanical Seals ◽  
Film Stiffness ◽  
The Relationship

The six kinds of different models were simulated by FLUENT software under certain circumstances. As a result, their pressure distributions and liquid film opening force with different thickness were obtained. The relationship between liquid film thickness and opening force was obtained by least square fitting. And then the relationship between stiffness and liquid film thickness was calculated and analyzed. The result shows that: seal opening force and liquid film stiffness decrease as the liquid film thickness increase, this simulation results is identical with the theoretical data. In larger film thickness range, the opening force is larger, and so was the liquid film stiffness, which provide the basis for seal optimization design and the stable operation.

Liquid Film Thickness in Micro Tube Under Flow Boiling Condition

2009 ◽  
Author(s):  
Youngbae Han ◽  
Naoki Shikazono
Keyword(s):  
Surface Tension ◽  
Film Thickness ◽  
Liquid Film ◽  
Capillary Number ◽  
Flow Boiling ◽  
Thin Liquid Film ◽  
Empirical Correlation ◽  
Liquid Film Thickness ◽  
Bubble Velocity ◽  
Wall Superheat

Slug flow is one of the representative flow regimes of flow boiling in micro tubes. It is well known that the thin liquid film formed between the tube wall and the vapor bubble plays an important role in micro scale heat transfer. In the previous study [1], liquid film thickness under adiabatic condition was investigated and an empirical correlation for the initial liquid film thickness based on capillary number, Reynolds number and Weber number was proposed. In the present study, the effects of wall superheat and bubble acceleration on the liquid film thickness are investigated. Under flow boiling condition, bubble velocity is not constant but accelerated, and it is necessary to consider this acceleration effect on the liquid film thickness, since it may affect the viscous, surface tension and inertia forces in the momentum equation. In addition, viscous boundary layer develops, and it may also affect the liquid film thickness. Besides, viscosity and surface tension coefficient are sensitive to temperature change. If wall superheat is high, it is crucial to consider the property change according to the temperature variation. In order to investigate these effects, laser focus displacement meter is used to measure the liquid film thickness. Ethanol, water and FC-40 are used as working fluids. Circular tubes with three different diameters, D = 0.5, 0.7 and 1.0 mm, are used. It is observed that when the wall superheat is larger than 5°C, liquid film thickness becomes thinner than the adiabatic case due to the decrease of viscosity near the wall. The increase of liquid film thickness with capillary number is restricted by bubble acceleration. Finally, an empirical correlation is proposed for accelerated flows in terms of capillary number and Bond number based on bubble acceleration.

CRITICAL HEAT FLUX ENHANCEMENT ON NANOPILLAR ARRAY SURFACES

2018 ◽  
Author(s):  
Tuan Tran ◽  
Dongdong Liu ◽  
Binh-Thien Nguyen
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Flux Enhancement
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