Experimental investigation of non-uniform heating effect on flow boiling instabilities in a microchannel-based heat sink

Copyright @ 2011 ElsevierTwo-phase flow boiling in microchannels is one of the most promising cooling technologies for coping with high heat fluxes produced by the next generation of central processor units (CPUs). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as the cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated. It was observed that boiling inside microchannels with axially non-uniform heating leads to high temperature non-uniformity in the transverse direction.This research was supported by the UK Engineering and Physical Sciences Research Council through grant EP/D500109/1

Confined growth of a vapour bubble in a capillary tube at initially uniform superheat : experiments and modelling

Bubble growth was triggered in a capillary tube closed at one end and vented to the atmosphere at the other and initially filled with uniformly superheated water. Measurements of the rate of axial growth and the varying pressure at the closed end were used to test under these simplified conditions assumptions employed in one-dimensional models for bubble growth applicable to the more complex conditions of confined-bubble flow boiling in micro-channels. Issues included the thickness of the liquid films round confined bubbles and changes in saturation temperature due to the changes in pressure generated by bubble motion. Modelling features requiring further attention were identified, such as the possibility of "roll-up" of the liquid film due to a large dynamic contact angle

Compound effect of EHD and surface roughness in pool boiling and CHF with R-123

This article is a post-print version of the fianl published article which may be accessed at the link below.Saturated pool boiling of R-123 at 1 bar, including the critical heat flux (CHF), was enhanced by modifying the surface characteristics and applying a high intensity electrostatic field, the latter termed electrohydrodynamic (and abbreviated EHD) enhancement. The heat flux was varied from very low values in the natural convection regime up to CHF. Experiments were performed with increasing and decreasing heat flux to study boiling hysteresis without and with EHD. Boiling occurred on the sand blasted surface of a cylindrical copper block with embedded electrical heating elements, with standardized surface parameter Pa = 3.5 μm. The electric field was generated by a potential of 5 kV to 25 kV, applied through a 40 mm diameter circular electrode of ss-304 wire mesh, aperture size 5.1 mm, located at distances of 5 - 60 mm from the surface, with most of the data obtained for 20 mm. The data for the rough surface were compared with earlier data for a smooth surface and indicated a significant increase in the heat transfer rates. EHD produced a further increase in the heat transfer rates, particularly at low heat flux values and near the CHF. Boiling hysteresis was reduced progressively by EHD and eliminated at high field strength.This work was supported by Government of Pakistan under a scholarship programme

Initiation and growth of confined vapour bubbles in micro-channels

State-of-the-art mechanistic models for confined bubble flow boiling in microchannels, such as [1] by Thome et al., currently use sub-models for the initiation of bubble growth that are based on observations of conventional nucleate boiling. Reversed flow towards the micro-channel inlet has been attributed by Kandlikar [2] to a pressure spike at the location of bubble nucleation. Mukherjee and Kandlikar [3] have performed a fully 3-dimensional simulation of growth after nucleation through to the early stages of confined growth at constant saturation temperature, implying constant pressure in the bubble. Detailed observations of the inception of confined bubble growth during flow boiling of water in a rectangular 2 x 1 mm channel without artificial nucleation sites showed that nucleation did not lead immediately to continuous growth [4]. The observations are considered in more detail in this presentation. Inception may be separated from the originating nucleation event in time by up to 10 ms and in axial position by up to 10 mm. The stochastic processes of nucleation and inception are influenced by the local pressure variations and by the flow reversal associated with inlet compressibility. The pressure variations are caused by the confined growth of bubbles pushing liquid slugs along the channel, and not by bubble nucleation. A 1-dimensional simulation of experimental data for confined bubble growth in a capillary tube with no inlet flow and initially at uniform temperature, incorporating the feedback between the varying pressure and the superheat driving growth, has shown that confined growth is sensitive to the initial conditions [5]. Improvements to the model are presented, including an estimation of convective heat transfer to the nose of the bubble, and the difficulties of modelling the growth from nucleation to full confinement are discussed

Subcooled flow boiling at high heat flux

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Comparison of a mechanistic model for nucleate boiling with experimental spatio-temporal data

Mechanistic numerical simulations have been developed for pool nucleate boiling involving large groups of nucleation sites that are non-uniformly distributed spatially and have different activation superheats. The simulations model the temperature field in the heated wall accurately and use approximations for events in the liquid–vapour space. This paper describes the first attempt to compare the numerical simulations with spatio-temporal experimental data at a similar level of detail. The experimental data were obtained during pool boiling of water at atmospheric pressure on a horizontal, electrically heated stainless steel plate 0.13 mm thick. They consist of wall temperature fields measured on the back of the plate by liquid crystal thermography at a sampling rate of 200 Hz over a period of 30 s. Methods of image analysis have been developed to deduce the time, position, nucleation superheat and size of the cooled area for every bubble nucleation event during this period. The paper discusses the methodology of using some of the experimental data as input for the simulations and the remainder for validation. Because of the high-dimensional dynamics and possibly chaotic nature of nucleate boiling, the validation must be based on statistical properties over a large area and a long period. This preliminary study is restricted to a single heat flux

Numerical investigation of nucleate boiling heat transfer on thin substrates

This article has been made available through the Brunel Open Access Publishing Fund.The objective of this paper is to define the guidelines for the design of new boiling test sections with a large number of artificial nucleation sites during nucleate boiling for thin substrates horizontally immersed in a saturated liquid with artificial cavities located on the upper surface. The findings of numerical simulations of pool boiling heat transfer for a single bubble and for a large number of nucleation sites based on the analysis of experimental cases were analysed. Dedicated test sections were used in experiments for the study of boiling mechanisms and interactions between active sites so that the numerical models representing the physics of the problem could be improved. The hybrid nature of the code used in this study, combining the complete solution of the three-dimensional time-dependent energy equation in the solid substrate with semi-empirical models representing the physical phenomena occurring in the liquid side, in a simplified way, allows a large number of simulations in a reasonable computational time. The present paper focuses in the first part on the capability of the model to reproduce the experimental results for various conditions, while in the second part, the results for a large number of nucleation sites are analysed. Regarding the single bubble growth, two series of simulations will be presented in this paper: the first one analyses the mechanisms of nucleate boiling on a silicon substrate immersed in the dielectric fluid FC-72. The second series studies the behaviour of bubbles on metallic substrates, platinum and titanium, in saturated water. In the last section, the effect of the position of a site during simulations of a large population of sites (of the order of 100) on the waiting time, growth time, type and occurrence of coalescence and the thermal characteristics is presented. © 2014 Elsevier Ltd. All rights reserved