Arvind Pattamatta

Heat transport at nanoscales departs substantially from the well established classical laws governing the physical processes at continuum level. The Fourier Law of heat conduction cannot be applied at sub-continuum level due to its inability in modeling non-equilibrium energy transport. Therefore one must resort to a rigorous solution to the Boltzmann Transport Equation (BTE) in the realm of nanoscale transport regime. Some recent studies show that a relatively inexpensive and accurate way to predict the behavior of sub continuum energy transport in solids is via the discrete representation of the BTE referred to as the Lattice Boltzmann method (LBM). Although quite a few numerical simulations involving LBM have been exercised in the literature, there has been no clear demonstration of the accuracy of LBM over BTE; also there exists an ambiguity over employing the right lattice configurations describing phonon transport. In the present study, the Lattice Boltzmann Method has been implemented to study phonon transport in miniaturized devices. The initial part of the study focuses upon a detailed comparison of the LBM model with that of BTE for one dimensional heat transfer involving multiple length and time scales. The second objective of the present investigation is to evaluate different lattice structures such as D1Q2, D1Q3, D2Q5, D2Q8, D2Q9 etc. for 1-D and 2-D heat conduction. In order to reduce the modeling complexity, gray model assumption based on Debye approximation is adopted throughout the analysis. Results unveil that the accuracy of solution increases as the number of lattice directions taken into account are incremented from D2Q5 to D2Q9. A substantial increase in solution time with finer directional resolutions necessitates an optimum lattice. A novel lattice dimension 'Mod D2Q5' has been suggested and its performance is also compared with its compatriots. It is also demonstrated that the inclusion of the center point within a particular lattice structure can play a significant role in the prediction of thermal conductivity in the continuum level. However, as the size of the device comes down to allow high Knudsen numbers, in the limiting case of ballistic phonon transport, the choice of lattice seems to have negligible effect on thermal conductivity Copyright © 2013 by ASME.

This paper brings out the phenomenon of the influence of flow maldistribution on temperature distribution in parallel microchannel system that is supposed to have an adverse effect on hot spot formation in microelectronic devices. An extensive experimental study is carried out where in the parameters affecting the flow maldistribution such as channel hydraulic diameter, channel flow configurations (U, Z, I type) and chip power are varied to study their effect on the pressure drop and temperature distribution across the parallel channels designed for liquid cooling of a CPU using distilled water. It is observed that the flow distribution among the channels improves significantly with a decrease in the channel hydraulic diameter due to higher pressure drop offered by each individual channels simultaneously. This results in a considerable reduction in both the peak temperature and the average temperature of the device with decrease in channel diameter and better temperature distribution. It is observed that a higher pressure drop in d = 88 μm induces more uniform distribution compared to d = 176 μm resulting in a 3 C improvement in the standard deviation of temperature on the chip surface and a reduction in maximum surface temperature. Higher heat fluxes induce a reduction in viscosity of the fluid resulting in higher flow maldistribution. © 2014 Elsevier Ltd. All rights reserved.

In this analysis, forced convective heat transfer characteristics of Al2O3-water nanofluid through a porous channel with several combinations of heaters and coolers is investigated numerically. The two-dimensional equations governing nanofluid flow and heat transfer through porous media are discretized using in-house code with Streamline Upwind Petrov Galerkin-based Finite Element Method. Darcy–Brinkman–Forchheimer’s generalized porous media model is used in this study. The average Nusselt number of basefluid without porous media, nanofluid with and without porous media cases are compared for different Peclet numbers and the effect of Peclet number on stream lines and isotherms are studied for nanofluid with and without porous media cases. In addition to this the effect of Darcy number, porosity, and nanoparticle volume fraction on the performance of average Nusselt number is investigated. From these results, it is observed that the average Nusselt number increases with decrease in Darcy number. From this analysis, it can be concluded that addition of porous media results in enhancement of heat transfer and can be used as a potential technique for electronic cooling applications.

In this paper, a numerical investigation of the phase change characteristics of Taylor-Bubbles (T-B) during flow boiling of FC-72 in a square minichannel is carried out. Multiple Taylor-Bubbles starting from their nucleation, growth and coalescence along with the associated heat transfer mechanisms have been modeled. The temporal variation of bubble coalescence pattern is found to exhibit a good agreement with the in-house experimental measurements conducted in microgravity environment. A detailed parametric study is conducted to understand the effects of Reynolds number (Re), wall superheat (ΔTw), bubble nucleation radii, and the surface tension expressed in terms of Capillary number (Ca) on the T-B nucleation and coalescence characteristics. The parametric study reveals that the nucleating bubbles tend to grow and coalesce faster at Re = 500 compared to Re = 50 due to higher temperature gradients leading to enhanced evaporation rates. The phenomenon of bubble 'roll-off' is observed when the wall and liquid are both superheated to 2 K due to absence of heat transfer between the top wall of the channel and the T-B. Also it is observed that the bubble coalescence time is reduced nearly by a factor of two for the coalescence of unequal bubble sizes. At higher values of Ca, both coalescence and break-up of T-B occur in succession while at lower values no coalescence is observed. The heat flux contours in the vicinity of the T-B contact line region predicted by the numerical model is found to exhibit a good qualitative agreement with the experimental measurement. It is inferred that of the parameters studied, Re and ΔTw are the two most significant factors that influence wall heat transfer during T-B coalescence. © 2014 Elsevier Ltd. All rights reserved.

The objective of this study is to compare the fluid flow and the heat transfer characteristics between a 2-D planar and a 3-D circular wall jet along the stream wise direction. The experiments are performed at a Reynolds number of 5540 for nondimensional streamwise distance ranging from 0 to 40. The hot wire anemometer is used to quantify the velocity distribution on the jet spread and the local maximum velocity decay along the stream wise direction. Liquid Crystal Thermography (LCT) technique is used to map the surface temperature and the semi-infinite approximation methodology is used for extracting the heat transfer coefficient. From the results it is observed that, the 2-D planar wall jet shows lesser distribution of RMS values in the near field and better heat transfer performance than that of the 3-D circular wall jet.

The present work deals with the development of a compressible phase-change solver and implementation toward the numerical modeling and investigation of a part-unit cell of a pulsating heat pipe (PHP). The fundamental understanding of the working of the part-unit cell is imperative in the development of a complete Computational Fluid Dynamics (CFD) model of a PHP. The compressible model developed in the present work is based on the Volume-of-Fluid solver of the open source CFD software, OpenFOAM, in which the contour-based interface reconstruction algorithm and the contact-line evaporation model have been incorporated. Owing to the lack of a single standard benchmark validation case for a compressible phase-change solver, a huge emphasis in the present work is laid on the solver development and validation, the latter part of which is conducted in stages. Furthermore, simulations for the formation of a Taylor-Bubble through a constrained bubble growth are performed and the fallacy of an incompressible solver is shown distinctly. The validated solver is used to model a part-unit cell of a PHP and a parametric study is performed on the part-unit cell. The effect of variation of evaporator length, evaporator superheat, and liquid fill ratio on the performance of the PHP is discussed.

An experimental study using Liquid crystal thermography technique is conducted to study the convective heat transfer enhancement in jet impingement cooling in the presence of porous media. Aluminium porous sample of 10 PPI with permeability 2.48e-7 and porosity 0.95 is used in the present study. Results are presented for two different Reynolds number 400 and 700 with four different configurations of jet impingement (1) without porous foams (2) over porous heat sink (3) with porous obstacle case (4) through porous passage. Jet impingement with porous heat sink showed a deterioration in average Nusselt number by 10.5% and 18.1% for Reynolds number of 400 and 700 respectively when compared with jet impingement without porous heat sink configuration. The results show that for Reynolds number 400, jet impingement through porous passage augments average Nusselt number by 30.73% whereas obstacle configuration enhances the heat transfer by 25.6% over jet impingement without porous medium. Similarly for Reynolds number 700, the porous passage configuration shows average Nusselt number enhancement by 71.09% and porous obstacle by 33.4 % over jet impingement in the absence of porous media respectively.

An experimental study using the liquid crystal thermography technique is conducted to investigate the convective heat transfer performance in jet impingement cooling using various porous media configurations. Aluminum porous foams are used in the present study. Four impinging jet configurations are considered: jet impingement (1) without porous media, (2) over the porous heat sink, (3) with porous obstacle case, and (4) through porous passage. These configurations are evaluated on the basis of the convective heat transfer enhancement for two different Reynolds numbers of 400 and 700. Jet impingement with porous heat sink showed deterioration in the average Nusselt number by 9.95% and 18.04% compared to jet impingement without porous media configuration for Reynolds numbers of 400 and 700, respectively. Jet impingement with porous obstacles showed a very negligible enhancement in the average Nusselt number by 3.48% and 2.73% for Reynolds numbers of 400 and 700, respectively. However, jet impingement through porous passage configuration showed a maximum enhancement in the average Nusselt number by 52.71% and 74.68% and stagnation Nusselt numbers by 58.08% and 53.80% compared to the jet impingement without porous medium for Reynolds numbers of 400 and 700, respectively. Within the porous properties considered, it is observed that by decreasing the permeability and porosity the convective heat transfer performance tends to increase.

Natural convection heat transfer in Al2O3/water nanofluid is analyzed using the single-component nonhomogeneous lattice Boltzmann method (SCNHLBM). There exists a contradictory observation between the numerical and experimental works in the literature with respect to the heat transfer of nanofluids in natural convection. Nanofluid is treated as a single component with nonhomogeneous particle distribution introduced by a concentration transport equation of nanoparticles by considering the Brownian and thermophoretic diffusions. The average Nusselt number is found to deteriorate with increasing nanoparticle volume fraction; thus the trend of the experimental results is captured using SCNHLBM. Addition of Brownian and thermophoretic diffusion results in additional thermal diffusion and hence reduces the convective transport of heat. The contribution of Brownian and thermophoretic diffusions in heat transfer deterioration is revealed.