Analysis of Heat Transfer in Microchannel Heat Sinks Using Extended Navier-Stoke's Equations and Optimization Techniques

Abstract

The rapid advancement in engineering andtechnology has promptedtheelectronic industries todevelopsmall scaled devices and thusMicro Electro Mechanical Systems(MEMS)have becomeone of the major advances in industrial technologies.Due to its micro sizes, these devises are subjected to highheat generationin them, which endangers its performance. This disastrous situation led to the development of Micro Channel Heat Sinks to eradicate the heat dissipation inMEMS. These heat sinks are efficient devices,designed and developed to upgrade the thermal management in MEMS. At present, theexceptional innovation demands for more efficient heat sinks with high heat transferrate. Thus the current research focuseson the development of advanced methodologies and techniques to effectivelystudy the characteristics and performance of microchannel heat sinks. Heat sinksare designed suchthat,fluids are made to flow through micro sized channels, which are placed above the substrate from whichheat is generated.Though variouskinds of fluidsare preferredfor achieving higher heat dissipationrate, most of the portable electronic devices adapt heat sinks withgas flows, and hence there arises a necessity to analyse the characteristics of gaseous coolants.In the last decade, many authors have done a critical study on micro channel heat sinksusing conventional theories andthey have observed few discrepancies in thenumericalresultswhen compared with experimental values. Later it was justified that such dissimilarities existed due to the fact that micro flows do not obey the same physical fundamental laws asin macroflowmodels.On this account, Durstjustified that,flow through microchannels are subjected to high pressure and density gradient and hence ivhe derived the Extended Navier-Stokes Equations,incorporating the diffusion transport of mass,momentum and energy terms.Gasflows through micro sized channels comprisinghigher Knudsen number are classified under slip and transition region. Microflows falling under this region fails continuum and generatevelocity slip and temperature break at the walls. Henceto studymicro flow modelswith gaseous coolants, it becomes necessary to model themwith Extended Navier-Stokes equations along with velocity slip and temperature jump boundary conditions. Further, determination of accurate valuesof tangential accommodation coefficient and thermal accommodation coefficient occurring in the slip and temperature jump boundary conditions have become an essential factor as these coefficients characterizethe fluid surface interaction. Thus this thesis tries to calculateaccurate values for heat transfer coefficient, wall temperature,pressure dropand accommodation coefficientsinmicrochannelheatsinks with gaseous coolants, which aremodelledusing Extended Navier-Stokes equationswith slip and temperature jump boundary conditionsandsolved using Integral Transform Technique. These extended equationsproduced a uniquethermal diffusivity term which helped to estimate more appropriateheat transfer rate. An infiniteseries solution with faster convergence was obtained. First, the Nusselt number and Prandtl number for various aspect ratiosfor the proposed model werecalculatedand compared with existing numerical and experimental results. Second, assuming all parameters and variables of the solution from experiments,the accommodation coefficients were calculatedand compared with experimental values.Accommodation coefficients were calculatedfor various Knudsen numbersandNusselt numbers with different aspect ratios. All the results vshowed high positive correlation whencompared with experimental measurements. In recent days, in order to satisfy the various designs of electronic gadgets, microchannel heat sinks aredeveloped with different geometries. Onesuch device is themicrochannels heat sinks with secondary oblique channels and rectangular ribs,which havebeenproved to be mostefficient one in heat dissipation. In order to improve the performance of these heat sink,its geometrical parametersnamely, the relative width of the secondary channel, relative rib widthand relative angle 𝛼of the secondary channelwereoptimized with the objective ofmaximizingthe heat transfer and minimizingpressure drop.Opposition Based Antlion Optimization was proposed and employed to optimizethe pressure drop and heat transfer. The results were compared with solutions from Finite Element Method and Antlion Optimizationtechniques. In recent times, nano fluids have replaced the conventional fluids in micro flow models, as they provide with higher heat transfer rate. Thus, analysing the effectsof different nanofluids on heat dissipation has become very essential. Finally,in this work, Al2O3nanofluidwas employed in Microchannel Heat Sink with Secondary Oblique Channels and Rectangular Ribs and its geometric parameters; channel width, relative rib width and relative anglewere optimized by employing Modified Dragonfly Optimization (MDFO) algorithm, andfurtherAdaptive Neuro-FuzzyInference System (ANFIS) was applied to optimizethe heat transferrateand pressure drop. The results obtained were compared with values from Antlion Optimization. Acomparative study on different nanofluid ratio was also executed.