A. K. M. M. Morshed

Numerical Analysis of Single Phase Multi Layered Micro-Channel Heat Sink With Inter-Connects Between Vertical Channels

Micro-channels embedded in solid matrix have already proven to be a very efficient way of electronics cooling. Traditional micro-channel heat sinks consist of single layer of parallel channels. Although micro-channel heat sink can achieve very high heat flux, its pumping requirement for circulating liquid through the channel increases very sharply as the flow velocity increases. The pumping requirement can be reduced by stacking multi layers of micro-channels. By introducing multi layers of channels, the flow velocity through each channel is reduced for the same total mass flow rate of the coolant. A novel approach to take advantage of multi layered channel is proposed in this study where the vertical channels are interconnected to allow cross flow of the coolant. The cross-flow between the channels disrupts boundary layer enhancing heat removal capacity of the heat sink. A CFD model has been developed using commercially available software package FLUENT to evaluate overall thermal performance of multi layered micro-channel heat sink. A parametric study of the flow rates and the effect of the number of layers and interconnections have been performed. Significant reduction in thermal resistance has been observed for multiple layers, it is also observed that this reduction in thermal resistance is dependent on the thermal conductivity of the heat sink material.Copyright

Enhanced Thermal Performance of Ionic Liquid-Al2O3 Nanofluid as Heat Transfer Fluid for Solar Collector

Next generation Concentrating Solar Power (CSP) system requires high operating temperature and high heat storage capacity heat transfer fluid (HTF), which can significantly increase the overall system efficiency for power generation. In the last decade several research going on the efficacy of ionic liquids (ILs) as a HTF in CSP system. ILs possesses superior thermophysical properties compare to currently using HTF such as Therminol VP-1 (mixture of biphenyl and diphenyl oxide) and thermal oil. However, advanced thermophysical properties of ILs can be achieved by dispersing small volume percentage of nanoparticles forming nanofluids, which is called Nanoparticle Enhanced Ionic Liquids (NEILs). In the present study NEILs were prepared by dispersing 0.5% Al2O3 nanoparticles (spherical and whiskers) in N-butyl-N, N, N-trimetylammonium bis(trifluormethylsulfonyl)imide ([N4111][NTf2]) IL. Viscosity, heat capacity and thermal conductivity of NEILs were measured experimentally and compared with the existing theoretical models for liquid–solid suspensions. Additional, the convective heat transfer experiment was performed to investigate thermal performance. The thermal conductivity of NEILs enhanced by ∼5%, heat capacity enhanced by ∼20% compared to the base IL, which also gives 15% enhancement in heat transfer performance.Copyright

Heat Transfer and Flow Behavior of Nanoparticle Enhanced Ionic Liquids (NEILs)

Experimental investigations were carried out to characterize forced convection behavior of Nanoparticle Enhanced Ionic Liquids (NEILs). 1-butyl-3-methylimidazolium bis{(trifluoromethyl) sulfonyl} imide ([C4mim][NTf2]) was used as the base ionic liquid (IL) with 0.5% (weight%) loading of Al2O3 nanoparticles. Flow experiments were conducted in a circular tube in the laminar flow regime. Convection results from IL without nanoparticles were used as the base line data for comparison with convection results with NEIL. Viscosity and thermal conductivity of the NEIL and base IL were also measured. NEIL displayed superior thermal performance compared to the base IL. An average of 13% enhancement in heat transfer coefficient was found for the NEIL compared with that of the base IL. Probable reasons of these enhancements are discussed in the paper.Copyright

Effect of Cross Groove on Flow Boiling in a Microgap

Flow boiling performance of a microgap channel has been investigated experimentally. Experimental studies were carried out on a bottom surface heated single microgap channel having 5×0.372 mm cross sectional area using DI water as coolant. Four rectangular grooves were cut along the flow direction and surface morphology of the microgrooves was modified by depositing ZnO nanoparticles from a nanofluid using electrophoresis deposition technique. Flow boiling experiments were conducted for different mass flux. The results from the microgap channel having no cross grooves have been used as the baseline data. Cross grooves have been found effective in reducing boiling incipience temperature and enhancing heat transfer coefficient. Up to 50% enhancement in two-phase heat transfer coefficient was observed for the cross groove. Nanoparticles deposition reduces boiling incipient temperature but does not show any significant effect on two-phase heat transfer coefficient.Copyright

Effect of the orientation of doped nanoparticles on thermal transportation of a solid: A molecular dynamics study

Interstitial phenomena like different types of atoms, doping and their orientation in nano scale influence the heat transfer and these effects become very significant with the reduction of material size. Non equilibrium Molecular Dynamics (NEMD) simulation was employed in this study to understand the effects of interfacial thermal resistance named kapitza resistance on solid. Argon like solid was considered in this study and LJ potential model was employed for the calculation of atomic interaction. Doping of nanoparticles at different orientation was inserted into the solid. From the simulation, it was observed that a large interfacial mismatch due to change in orientation in homogenous solid causes distortion of phonon frequency causing an increase in thermal resistance. The position of doped particles has a profound effect on thermal conductivity of solid. Interfacial atoms positioned perpendicular to heat flow direction causes sharp reduction in thermal conductivity. Phonon scattering at the material i...

A molecular dynamics study of thermal transport in nanoparticle doped Argon like solid

Interfacial phenomena such as mass and type of the interstitial atom, nano scale material defect influence heat transfer and the effect become very significant with the reduction of the material size. Non Equilibrium Molecular Dynamics (NEMD) simulation was carried out in this study to investigate the effect of the interfacial phenomena on solid. Argon like solid was considered in this study and LJ potential was used for atomic interaction. Nanoparticles of different masses and different molecular defects were inserted inside the solid. From the molecular simulation, it was observed that a large interfacial mismatch due to change in mass in the homogenous solid causes distortion of the phonon frequency causing increase in thermal resistance. Position of the doped nanoparticles have more profound effect on the thermal conductivity of the solid whereas influence of the mass ratio is not very significant. Interstitial atom positioned perpendicular to the heat flow causes sharp reduction in thermal conductivi...

Natural Convection in Rectangular Cavity With Nanoparticle Enhanced Ionic Liquids (NEILs)

A systematic natural convection heat transfer experiment has been carried out of nanoparticle enhanced ionic liquids (NEILs) in rectangular enclosures (lengthxwidthxheight, 50×50×50mm and 50×50×75mm) heated from below condition. In the present experiment NEIL was made of N-butyl-N-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl} imide, ([C4mpyrr][NTf2]) ionic liquid with 0.5% (weight%) Al2O3 nanoparticles. In addition to characterize the natural convection behavior of NEIL, thermophysical properties such as thermal conductivity, heat capacity, and viscosity were also measured. The result shows that the thermal conductivity of NEIL enhanced ∼3% from the base ionic liquid (IL), heat capacity enhanced ∼12% over the measured temperature range. The natural convection experimental result shows consistent for two different enclosures based on the degrading natural convection heat transfer rate over the measured Rayleigh number range. Possible reasons of the degradation of natural convection heat transfer may be the relative change of the thermophysical properties of NEIL compare to the base ionic liquid.Copyright

Numerical investigation of natural convection of nanoparticle enhanced ionic liquids (NEILs) in enclosure heated from below

The paper presents the numerical simulation of natural convection heat transfer of Al2O3 nanoparticle enhanced N-butyl-N-methylpyrrolidinium bis{trifluoromethyl)sulfonyl} imide ([C4mpyrr][NTf2]) ionic liquid. The simulation was performed in three different enclosures (aspect ratio: 0.5, 1, and 1.5) with heated from below. The temperature dependent thermophysical properties of base ionic liquids (ILs) and nanoparticle enhanced ionic liquids (NEILs) were applied in the numerical simulation. The numerical results were compared with the experimental result. The numerical results show that at a certain Rayleigh number NEILs has a lower Nusselt number compared to the base IL which are consistent with the experimental results. But the percentage of degradation is much less on the numerical results compared to the experimental. However the numerical results match well with the predicted model of using thermophysical properties of NEILs. From these observations it can be concluded that the extra degradation in the...

Enhanced heat sink with geometry induced wall-jet

Mini-channels embedded in solid matrix have already proven to be a very efficient way of electronic cooling. Traditional mini-channel heat sinks consist of single layer of parallel channels. Although mini-channel heat sink can achieve very high heat flux, its pumping requirement for circulating liquid through the channel increase very sharply as the flow velocity increases. The pumping requirements of the heat sink can be reduced by increasing its performance. In this paper a novel approach to increase the thermal performance of the mini-channel heat sink is proposed through geometry induced wall jet which is a passive technique. Geometric irregularities along the channel length causes abrupt pressure change between the channels which causes cross flow through the interconnections thus one channel faces suction and other channel jet action. This suction and jet action disrupts boundary layer causing enhanced heat transfer performance. A CFD model has been developed using commercially available software pa...

A molecular dynamics study of thermal transportation of graphene sheet with various temperature

Temperature variation has a dominating effect on thermal transportation and phonon wave propagation. Non-equilibrium molecular dynamics (NEMD) simulation has been employed in this study to investigate the effects of thermal transportation in graphene sheets and it is found that both in-plane and out-of-plane thermal conductivity of graphene sheet reduces with increasing temperature. Optimized Tersoff potential has been applied to observe the decreasing trend of conductivity with increasing temperature. Quantum correction has been introduced for the prediction of thermal conductivity as a function of temperature to include quantum effects below Debye temperature. The study shows that up to Debye temperature conductivity increases with increasing temperature, attains a peak and then falls off. In plane conductivity is much higher than out of plane conductivity. This high in plane conductivity is due to the sp2 bonding between C atoms whereas weak van der waals interaction force limits the cross-plane conduc...