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Gopinath R. Warrier

Bio: Gopinath R. Warrier is an academic researcher from University of California, Los Angeles. The author has contributed to research in topics: Heat transfer & Boiling. The author has an hindex of 16, co-authored 39 publications receiving 1449 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, both single-phase forced convection and sub-cooled and saturated nucleate boiling experiments have been performed in small rectangular channels using FC-84 as test fluid.

327 citations

Journal ArticleDOI
TL;DR: In this paper, a correlation for predicting the wall superheat and wall heat flux at ONB has been developed from the data obtained in this study and that reported in the literature.
Abstract: The partitioning of the heat flux supplied at the wall is one of the key issues that needs to be resolved if one is to model subcooled flow boiling accurately. The first step in studying wall heat flux partitioning is to account for the various heat transfer mechanisms involved and to know the location at which the onset of nucleate boiling (ONB) occurs. Active nucleation site density data is required to account for the energy carried away by the bubbles departing from the wall. Subcooled flow boiling experiments were conducted using a flat plate copper surface and a nine-rod (zircalloy-4) bundle. The location of ONB during the experiments was determined from visual observations as well as from the thermocouple output. From the data obtained it is found that the heat flux and wall superheat required for inception are dependent on flow rate, liquid subcooling, and contact angle. The existing correlations for ONB underpredict the wall superheat at ONB in most cases. A correlation for predicting the wall superheat and wall heat flux at ONB has been developed from the data obtained in this study and that reported in the literature. Experimental data are within630 percent of that predicted from the correlation. Active nucleation site density was determined by manually counting the individual sites in pictures obtained using a CCD camera. Correlations for nucleation site density, which are independent of flow rate and liquid subcooling, but dependent on contact angle have been developed for two ranges of wall superheat—one below 15°C and another above 15°C. @DOI: 10.1115/1.1471522#

300 citations

Journal ArticleDOI
TL;DR: In this article, a mechanistic model has been developed for the wall heat flux partitioning during subcooled flow boiling, where the entire energy from the wall is first transferred to the superheated liquid layer adjacent to the wall, while the rest of this energy is utilized for sensible heating of the bulk liquid.
Abstract: In this work a mechanistic model has been developed for the wall heat flux partitioning during subcooled flow boiling. The premise of the proposed model is that the entire energy from the wall is first transferred to the superheated liquid layer adjacent to the wall. A fraction of this energy is then utilized for vapor generation, while the rest of the energy is utilized for sensible heating of the bulk liquid. The contribution of each of the mechanisms for transfer of heat to the liquid—forced convection and transient conduction, as well as the energy transport associated with vapor generation has been quantified in terms of nucleation site densities, bubble departure and lift-off diameters, bubble release frequency, flow parameters like velocity, inlet subcooling, wall superheat, and fluid and surface properties including system pressure. To support the model development, subcooled flow boiling experiments were conducted at pressures of 1.03 ‐3.2 bar for a wide range of mass fluxes ~124‐926 kg/m 2 s!, heat fluxes ~2.5‐90 W/cm 2 ! and for contact angles varying from 30° to 90°. The model developed shows that the transient conduction component can become the dominant mode of heat transfer at very high superheats and, hence, velocity does not have much effect at high superheats. This is particularly true when boiling approaches fully developed nucleate boiling. Also, the model developed allows prediction of the wall superheat as a function of the applied heat flux or axial distance along the flow direction. @DOI: 10.1115/1.1842784#

223 citations

Journal ArticleDOI
TL;DR: In this paper, the growth and departure of single bubbles on two smooth surfaces with very different wettabilities are studied using high-speed video microscopy and numerical simulations, where isolated artificial cavities of approximately 10 μm diameter are microfabricated on both a bare and a Teflon-coated silicon substrate to serve as nucleation sites.
Abstract: The growth and departure of single bubbles on two smooth surfaces with very different wettabilities are studied using high-speed video microscopy and numerical simulations. Isolated artificial cavities of approximately 10 μm diameter are microfabricated on both a bare and a Teflon-coated silicon substrate to serve as nucleation sites. The bubble departure diameter is observed to be almost 3 times larger and the growth period almost 60 times longer for the hydrophobic surface than for the hydrophilic surface. The waiting period is practically zero for the hydrophobic surface because a small residual bubble nucleus is left behind on the cavity from a previous ebullition cycle. The experimental results are consistent with our numerical simulation results. Bubble nucleation occurs on nominally smooth hydrophobic regions with root mean square roughness (R q ) less than 1 nm even at superheat as small as 3°C. Liquid subcooling significantly affects bubble growth on the hydrophobic surface due to increased bubble surface area. Fundamental understanding of bubble dynamics on heated hydrophobic surfaces will facilitate the development of chemically patterned surfaces with enhanced boiling heat transfer performance and novel phase-change based micro-actuators and energy harvesters.

117 citations

Journal ArticleDOI
TL;DR: In this paper, a high-speed CCD camera was used to record the bubble collapse in the bulk subcooled liquid and the experimental data were in turn used to correlate bubble collapse rate and the interfacial heat transfer rate.

100 citations


Cited by
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Journal ArticleDOI
01 Jul 1968-Nature
TL;DR: The Thermophysical Properties Research Literature Retrieval Guide as discussed by the authors was published by Y. S. Touloukian, J. K. Gerritsen and N. Y. Moore.
Abstract: Thermophysical Properties Research Literature Retrieval Guide Edited by Y. S. Touloukian, J. K. Gerritsen and N. Y. Moore Second edition, revised and expanded. Book 1: Pp. xxi + 819. Book 2: Pp.621. Book 3: Pp. ix + 1315. (New York: Plenum Press, 1967.) n.p.

1,240 citations

Journal ArticleDOI
TL;DR: In this article, the effects of the channel size on the flow patterns and heat transfer and pressure drop performance are reviewed in small hydraulic diameter channels, and the fundamental questions related to the presence of nucleate boiling and characteristics of flow boiling in microchannels and minichannels in comparison to that in the conventional channel sizes (3 mm and above) are addressed.

840 citations

01 Jan 2002
TL;DR: In this article, the effects of the channel size on the flow patterns and heat transfer and pressure drop performance are reviewed in small hydraulic diameter channels, and the fundamental questions related to the presence of nucleate boiling and characteristics of flow boiling in microchannels and minichannels in comparison to that in the conventional channel sizes (3 mm and above) are addressed.
Abstract: Flow boiling in small hydraulic diameter channels is becoming increasingly important in many diverse applications. The previous studies addressing the effects of the channel size on the flow patterns, and heat transfer and pressure drop performance are reviewed in the present paper. The fundamental questions related to the presence of nucleate boiling and characteristics of flow boiling in microchannels and minichannels in comparison to that in the conventional channel sizes (3 mm and above) are addressed. Also, the effect of heat exchanger configuration—single-channel and multichannel—on the heat transfer and pressure drop performance is reviewed. The areas for future research are identified.

818 citations

01 Aug 1953
TL;DR: In this paper, a solution for the radius of the vapor bubble as a function of time is obtained which is valid for sufficiently large radius, since the radius at which it becomes valid is near the lower limit of experimental observation.
Abstract: The growth of a vapor bubble in a superheated liquid is controlled by three factors: the inertia of the liquid, the surface tension, and the vapor pressure. As the bubble grows, evaporation takes place at the bubble boundary, and the temperature and vapor pressure in the bubble are thereby decreased. The heat inflow requirement of evaporation, however, depends on the rate of bubble growth, so that the dynamic problem is linked with a heat diffusion problem. Since the heat diffusion problem has been solved, a quantitative formulation of the dynamic problem can be given. A solution for the radius of the vapor bubble as a function of time is obtained which is valid for sufficiently large radius. This asymptotic solution covers the range of physical interest since the radius at which it becomes valid is near the lower limit of experimental observation. It shows the strong effect of heat diffusion on the rate of bubble growth. Comparison of the predicted radius‐time behavior is made with experimental observations in superheated water, and very good agreement is found.

729 citations

Journal ArticleDOI
TL;DR: In this article, a bibliographical review on the convective heat transfer through microchannels is presented, highlighting the main results obtained on the friction factor, on the laminar-to-turbulent transition and on the Nusselt number in channels having a hydraulic diameter less than 1 mm.

647 citations