Showing papers in "Journal of Thermal Science and Engineering Applications in 2017"

Effects of Nanoparticle Shapes on Slot Jet Impingement Cooling of a Corrugated Surface with Nanofluids

Abstract: Numerical study of jet impingement cooling of a corrugated surface with water - SiO$_2$ nanofluid of different nanoparticle shapes was performed. The bottom wall is corrugated and kept at constant surface temperature while the jet is emerged from a rectangular slot with cold uniform temperature. The finite volume method is utilized to solve the governing equations. The effects of Reynolds number (between 100 and 500), corrugation amplitude (between 0 and 0.3), corrugation frequency (between 0 and 20), nanoparticle volume fraction (between 0 and 0.04) and nanoparticle shapes (spherical, blade, brick, cylindrical) on the fluid flow and heat transfer characteristics were studied. Stagnation point and average Nusselt number enhance with Reynolds number and solid particle volume fraction for both flat and corrugated surface configurations. An optimal value for the corrugation amplitude and frequency was found to maximize the average heat transfer at the highest value of Reynolds number. Among various nanoparticle shapes, cylindrical ones perform the best heat transfer characteristics in terms of stagnation and average Nusselt number values. At the highest solid volume concentration of the nanoparticles, heat transfer values are higher for a corrugated surface when compared to a flat surface case.

Thermodynamics Analyses of Porous Microchannels With Asymmetric Thick Walls and Exothermicity: An Entropic Model of Microreactors

Abstract: This paper presents a study of the thermal characteristics and entropy generation of a porous microchannel with thick walls featuring uneven thicknesses. The system accommodates a fully developed flow while the solid and fluid phases can include internal heat sources. Two sets of asymmetric boundary conditions are considered. The first includes constant temperatures at the surface of the outer walls, with the lower wall experiencing a higher temperature than the upper wall. The second case imposes a constant heat flux on the lower wall and a convection boundary condition on the upper wall. These set thermal models for micro-reactors featuring highly exothermic or endothermic reactions such as those encountered in fuel reforming processes. The porous system is considered to be under local thermal non-equilibrium (LTNE) condition. Analytical solutions are, primarily, developed for the temperature and local entropy fields and then are extended to the total entropy generation within the system. A parametric study is, subsequently, conducted. It is shown that the ratio of the solid to fluid effective thermal conductivity ratio and the internal heat sources are the most influential parameters in the thermal and entropic behaviours of the system. In particular, the results demonstrate that the internal heat sources can affect the entropy generation in a non-monotonic way and, that the variation of the total entropy with internal heat sources may include extremum points. It is, further, shown that the asymmetric nature of the problem has a pronounced effect on the local generation of entropy.

Two-Phase Analysis on the Conjugate Heat Transfer Performance of Microchannel With Cu, Al, SWCNT, and Hybrid Nanofluids

Abstract: The present numerical study has been carried out by developing two phase mixture model with conjugate heat transfer. Pure and hybrid nanofluids (HyNF) with particle as well as base fluid hybridization are used in analyzing the performance of micro-channel under forced convection laminar flow. The flow as well as heat transfer characteristics of pure water, copper (Cu), Aluminum (Al), single walled carbon nanotube (SWCNT) and hybrid (Cu+Al, water+methanol) nanofluids with various nanoparticle volume concentrations at different Reynolds numbers are reported. Pure nanofluids such as Al, Cu and SWCNT with 3 vol.\% nanoparticle concentration enhanced the average Nusselt number by 21.09\%, 32.46\% and 71.25\% in comparison with pure water at Re = 600. Where as, in the case of hybrid nanofluids such as 3 vol.\% HyNF (0.6\% Cu + 2.4\% Al) and 3 vol.\% SWCNT (20\% Me + 80\% PW), the enhancement in average Nusselt number is observed to be 23.38\% and 46.43\% in comparison with pure water at Re = 600. The study presents three equivalent combinations of nanofluids [1 vol.\% Cu and 0.5 vol.\% SWCNT], [2 vol.\% Cu, 1 vol.\% SWCNT and 3 vol.\% HyNF (0.6\% Cu + 2.4\% Al)] as well as [2 vol.\% SWCNT and 3 vol.\% SWCNT (20\% Me + 80\% PW)]. The study also shows that by dispersing SWCNT nanoparticles one can enhance heat transfer characteristics of base fluid containing methanol as antifreeze. The developed numerical model is validated with the numerical and experimental results available in literature.

Electrothermal transport in biological systems : an analytical approach for electrokinetically-modulated peristaltic flow

Abstract: A mathematical model is developed to investigate the combined viscous electro-osmotic flow and heat transfer in a finite length micro-channel with peristaltic wavy walls. The influence of Joule heating is included. The unsteady two-dimensional conservation equations for mass, momentum and energy conservation with viscous dissipation, heat absorption and electro-kinetic body force, are formulated in a Cartesian co-ordinate system. The Joule heating term appears as a quadratic function of axial electrical field in the energy conservation equation. The axial momentum and energy equations are coupled via the thermal buoyancy term. The peristaltic waves propagating along the micro-channel walls are simulated via a time-dependent co-sinusoidal wave function for the transverse vibration of the walls. Both single and train wave propagations are considered. Constant thermo-physical properties are prescribed and a Newtonian viscous model is employed for the fluid. The electrical field terms are rendered into electrical potential terms via the Poisson-Boltzmann equation, Debye length approximation and ionic Nernst Planck equation. The dimensionless emerging linearized electro-thermal boundary value problem is solved using integral methods. A parametric study is conducted to evaluate the impact of isothermal Joule heating term on axial velocity, temperature distribution, pressure difference, volumetric flow rate, skin friction and Nusselt number. The modification in streamline distributions with Joule heating and electro-osmotic velocity is also addressed to elucidate trapping bolus dynamics.

Design and Simulation of Passive Thermal Management System for Lithium-Ion Battery Packs on an Unmanned Ground Vehicle

Abstract: The transient thermal response of a 15-cell, 48 V, lithium-ion battery pack for an unmanned ground vehicle (UGV) was simulated using ANSYS FLUENT. Heat generation rates and specific heat capacity of a single cell were experimentally measured and used as input to the thermal model. A heat generation load was applied to each battery, and natural convection film boundary conditions were applied to the exterior of the enclosure. The buoyancy-driven natural convection inside the enclosure was modeled along with the radiation heat transfer between internal components. The maximum temperature of the batteries reached 65.6 C after 630 s of usage at a simulated peak power draw of 3600 W or roughly 85 A. This exceeds the manufacturer’s maximum recommended operating temperature of 60 C. We present a redesign of the pack that incorporates a passive thermal management system consisting of a composite expanded graphite (EG) matrix infiltrated with a phase-changing paraffin wax. The redesigned battery pack was similarly modeled, showing a decrease in the maximum temperature to 50.3 C after 630 s at the same power draw. The proposed passive thermal management system kept the batteries within their recommended operating temperature range. [DOI: 10.1115/1.4034904]

Ionic Wind Heat Transfer Enhancement in Vertical Rectangular Channels: Experimental Study and Model Validation

Abstract: This paper presents the results of an experimental study of ionic wind heat transfer enhancement in internal rectangular channels. Ionic wind is a potential technique to enhance natural convection cooling noise-free and without using moving part and thus ensuring a high reliability and a long lifetime. The goal of the present study is twofold: first, the multiphysics numerical model of ionic wind developed in previous work is validated experimentally. Second, the potential of the heat sink concept combining a fin array with an ionic wind generator is demonstrated by building a technology demonstrator. The heat sink presented in this work dissipates 240 W on a baseplate geometry of 200 263 mm. It is shown that the baseplate temperature can be reduced from 100 C under natural convection to 81 C when the ionic wind generator is turned on. [DOI: 10.1115/1.4035291]

Experimental Investigation and Empirical Correlations of Heat Transfer in Different Regimes of Air–Water Two-Phase Flow in a Horizontal Tube

Abstract: The present article reports on experimental investigation of heat transfer to co-current airwater two phase flow in horizontal tube. The idea is to enhance heat transfer to the coolant liquid by air injection. Experiments were conducted for different air water ratios in constant temperature heated tube. Visual identification of flow regimes was supplemented. The effects of the liquid and gas superficial velocities and the flow regimes on the heat transfer coefficients were investigated. The results showed that the heat transfer coefficient generally increases with the increase of the injected air flow rate and the enhancement is more significant at low water flow rates. A maximum value of the two-phase heat transfer coefficient was observed at the transition to wavy-annular flow as the air superficial Reynolds number increases for a fixed water flow rate. It was noticed that the Nusselt number increased about three times due to the injection of air with at low water Reynolds. Correlations for heat transfer by air-water two phase flow were deduced in dimensionless form for the different flow regimes.

Geometric Size Optimization of Annular Step Fin Using Multi-Objective Genetic Algorithm

Abstract: Although an annular stepped fin can produce better cooling effect in comparison to an annular disc fin, it is yet to be studied in detail. In the present work, one-dimensional heat transfer in a two-stepped rectangular cross-sectional annular fin with constant base temperature and variable thermal conductivity is modeled as a multi-objective optimization problem. Taking cross-sectional half thicknesses and outer radii of the two fin steps as design variables, an attempt is made to obtain the efficient fin geometry by simultaneously maximizing the heat transfer rate and minimizing the fin volume. For further assessment of the fin performance, three more objective functions are studied, which are minimization of the fin surface area and maximization of the fin efficiency and effectiveness. Evaluating the heat transfer rate through the hybrid spline difference method, the well-known multi-objective genetic algorithm, namely NSGA-II, is employed for approximating the Pareto-optimal front containing a set of trade-off solutions in terms of different combinations of the considered five objective functions. The Pareto-optimal sensitivity is also analyzed for studying the influences of the design variables on the objective functions. As an outcome, it can be concluded that the proposed procedure would give an open choice to designers to lead to a practical stepped fin configuration.