Xuegong Hu

Effect of acoustic cavitation on boiling heat transfer

Abstract Boiling heat transfer on a horizontal circular copper tube in an acoustical field is investigated experimentally and the relation between the liquid cavitation, the boiling and the micro bubble radii are analyzed theoretically. The results show that cavitation bubbles have an important influence on the nucleation, growth and collapse of vapor embryo within cavities on the heat transfer surface and that the enhancement of boiling heat transfer by acoustic cavitation mainly depends on whether the vapor embryo is activated by the cavitation bubbles to initiate boiling.

Analysis of Micro Vapor Bubble Growing Process in Open Capillary Microgrooves

The growing process of the individual microbubble in an open rectangular capillary microgroove was theoretically analyzed in this study. Several correlations of bubble growth rate for pool boiling were proved not available for microgroove boiling. A theoretical model based on thermal equilibrium and force balance was developed in this article. The growing process of the individual microbubble was divided into three stages: initial growing stage, normal and axial confined ellipsoidal growing stage, and axial subcylindrical growing stage. Growth period and volume increment of the micro vapor bubble were analyzed. The calculation results indicate that the growth of the micro vapor bubble is confined by the geometric structure of the microgroove. Comparison of the results between calculation and experiment shows that the correlation is available to predict the bubble growth rate for boiling in microgrooves.

Visualization of Microbubble Dynamics Behavior in Rectangular Capillary Microgrooves Under Spray Cooling Condition

Visualization of microbubble dynamics behavior in rectangular capillary microgrooves under spray cooling condition was performed in an open chamber at a nozzle pressure of 0.51 MPa. Distilled water was used as working liquid in this experiment. A high-speed camera with the maximum speed of 100,000 frames per second and a micro lens were used with the combination of VERSION and MATLAB software. The process of microbubble growth was found to be divided into two stages, which are the growth of bubble as semispherical in the grooves, and the growth as semi-ellipsoidal along the groove when constrained by the grooves width until the bubble breaks. The results also show that the bubble life cycle and the bubble breakup equivalent diameter decrease considerably compared with those in rectangular capillary microgrooves without spray cooling, and that they fluctuate as the heat flux rises and are somewhat affected by the placements of the grooves.

Analysis of Axial Meniscus Jump-Like Transition in Rectangular Microgrooves

The meniscus receding process was studied for the axial steady flow in open rectangular microgrooves based on experimental results. Experimental results show that the liquid film recedes remarkably as a cubic trendline from the accommodation stage to the bottom corner-flow stage, but the dead zone and the step change don’t exist. The receding process of the liquid film between the accommodation stage and the bottom corner-flow stage is named jump-like transition in the paper. Characteristics of the axial flow in rectangular microgrooves were theoretically analyzed considering the meniscus receding performance in the jump-like transition, calculation results show that radius of the meniscus curvature decreases along the groove axis, which provides drive for the axial flow; the liquid cross sectional area and the liquid height decrease evidently at the stage of the jump-like transition; the liquid velocity increases along the axis, and increases promptly at the transition stage and the corner flow stage.© 2010 ASME

Study on the Characteristics of Contact Line and Liquid Film in Rectangular Microgrooves Under Vibration Conditions

With the help of a high-speed camera (30000 Frames/second) and a wide-field stereo-microscope, the effects of mechanical vibration on the meniscus film and triple-phase contact line in rectangular microgrooves were experimentally investigated. Distilled water was used as working liquid. The images of the oscillated meniscus film in an oscillation period were captured through the high speed camera and they were analyzed using a MATLAB program. The results show that as the vibration table moves upward, the length of contact line increases; as the vibration table moves downward, the length of contact-line decreases. During the oscillation, the axial liquid film spreads upward further along the microgrooves and the deformation of the contact line becomes more obvious. The increase of the triple-phase contact line length caused by the external mechanical vibration is helpful for contact line heat transfer enhancement. Besides, deformation curve of the contact line with and without heat input under different vibration conditions is similar, while the contact line with heat input is shorter.Copyright

Effect of Microgroove Dimensions on Deformation of the Liquid Film Under Vibration Conditions

Deformation of the triple-phase contact line in various sizes of rectangular microgrooves under vertical vibration conditions was studied in this paper. Width of the rectangular microgroove ranges from 0.2 mm to 0.4mm and depth of the microgrooves is 0.2∼0.6mm. The frequency of vibration is 10Hz, and the amplitude of vibration is approximately 3.5mm. The research results show that oscillation of the liquid film in microgrooves becomes more obvious, and the triple-phase contact line is deformed more greatly when the groove width or the groove depth increases. The main reason is that the flow resistance of the liquid film in microgrooves decreases when the groove width or the groove depth increases.Copyright

Effect of Mechanical Vibration on Heat Transfer Characteristics of Liquid Film in Rectangular Microgrooves

The effects of vertical mechanical vibration on the heat characteristics of liquid film in vertical rectangular microgrooves are observed. The vibration frequencies are 6Hz, 10Hz and 30Hz, respectively; the vibration amplitudes are in the range of 1.95∼3.23mm. Three sizes of rectangular microgrooved plate are used in experiments. The microgrooved plate is vertically mounted on a vibration plane; DC heat load is added on the back wall of the microgrooved plate. Vibration of the liquid film in the microgroove is observed by a high-speed digital camera, and temperature on the back of the plate is recorded by a data acquisition. The experimental results show that temperature on the plate back decreases obviously with the increase of the vibration frequency or amplitude, heat transfer of the microgrooved plate is intensively enhanced. The main reason is that the forced convections on the groove surface and in the liquid film, caused by the mechanical vibration, enhance the heat transfer. The investigation provides more information for the application of the micro-configuration heat sink under fierce vibration conditions.Copyright

Experimental Investigation on Fluid Flow in an Inclined Open Rectangular Microgrooves Heat Sink With Micro-PIV

The paper investigates the fluid flow in an inclined open rectangular microgrooves heat sink using LaVision-assembled micro-PIV system. The images of the accommodation stage and the corner flow stage at inclined 15 degrees angle with heat flux of 0.09W/cm2 were recorded by a CCD camera with the speed of 10 frames per second. The velocity profile at inclined 15 degrees angle with heat flux of 0.14W/cm2 was acquired by using the analyzing software DaVis 8.1. In accommodation stage, X-Y-Z scanning table was adjusted vertically and horizontally to investigate the velocity field of different focused planes. Results show that the experimental images of the accommodation stage and the corner flow stage verify the predictions proposed by Catton and Stroes that there exist accommodation stage and corner flow stage and no jump-like stage proposed by Nilson et al. Flow in microgroove driven by capillary force is very slow and the velocity is less than 1mm per second. In evaporating thin film region, from the left side to the right side of the microgroove, the velocity increases and velocity of the interface is the highest. In lower meniscus region, the velocity increases and then decreases.Copyright

Experimental Investigation on Shape of the Meniscus in Open Rectangular Microgrooves Heat Sinks With Micro-PIV

This paper presents the experimental investigation on shape of the meniscus in cross sections along the axial direction of open rectangular microgrooves heat sinks with input heat flux under pure evaporation heat transfer condition. The images of the shape of the meniscus in rectangular open microgrooves heat sinks were recorded by a CCD camera with the speed of 10 frames per second. Microgrooves heat sinks were placed in four inclined angles, horizon, inclined 15 degrees, inclined 25 degrees and inclined 35 degrees, and heat flux is 0.09W/cm2, 0.14 W/cm2 and 0.25 W/cm2. The results show that the shape of the meniscus is parabolic not round. When the microgrooves heat sinks are placed horizontally, the shape of the meniscus and the curvature of the meniscus in cross sections changes little along the axial direction of the microgrooves. When the microgrooves heat sinks were placed inclined 35 degrees, the meniscus moves downward to the microgroove bottom. At the same distance to the reservoir and the same heat flux, the curvature of the meniscus in inclined 35 degrees is larger than that in inclined 25 degrees, and similarly, the curvature of the meniscus in inclined 25 degrees is larger than that in inclined 15 degrees. While at the same heat flux, the microgrooves heat sink is placed in inclined 35 degrees, the curvature of the meniscus increases with the increase of the distance to the reservoir.© 2013 ASME

Experimental Study of the Heat Transfer Characteristic in Vertical Rectangular Capillary Microgrooves Heat Sink Under an Electric Field

In this study, the EHD (electrohydrodynamic) enhancement of heat transfer in open vertical rectangular capillary microgrooves heat sink was studied, and with the help of a high-speed digital camera with maximum speed of 100,000 frames per second, the nucleate boiling phenomena in microgrooves were also visually investigated. The tests results have shown: (1) the input heat flux has no influences on the threshold voltage basically, while the absolute temperature drop ΔT of the lower or higher input heat fluxes were smaller than that of the midrange ones. (2) the threshold voltage was getting small with the decreasing distance between the electrodes, meanwhile the absolute temperature drop ΔT was growing larger and larger. (3) the threshold voltages of NO. 1 microgrooves (the dimensions are listed in Table 1) were lower than that of NO. 2, and ΔT of NO. 1 microgrooves was larger than that of NO. 2. (4) the bubble growth process was becoming longer with the increasing applied voltage.Copyright