Abstract
This paper aims to study numerically the laminar convective heat transfer of ionized water flow inside rectangular heat sinks with periodic expansion-constriction cross-section; each heat sink consists of parallel microchannels system with 4 mm wide and 0.1 mm deep in constant cross-section segment.  Two-dimensional laminar numerical simulations, based on Navier-Stoks equations and energy equation, are obtained under the same boundary conditions for different microchannels. In this study, the heat transfer and pressure drop inside microchannels with cross-section (cylindrical grooves and triangular cavities) are compared with that of simple smooth microchannel at Reynolds number ranging from 150-1500; an increase in pressure drop of 44% for all microchannels  is observed with Reynolds number increasing.  The obtained results indicate an enhancement in Nusselt number for all microchannels at all Reynolds number values with a maximum enhancement of 36%, these ameliorated thermal parameters attribute to enhance the heat transfer efficiency of proposed microchannels. Which improve the effect of periodic expansion-constriction cross-section on the heat transfer performance for microelectromechanical systems (MEMS) cooling phenomena.  
Highlights
Because of the miniaturization of the recent industrial systems, Micro-Electro Mechanical systems (MEMS), [1] are used to solve the problem of heat evacuation for electronic devices, to maintain their temperatures in a safe range in order to ensure their performance
Tuckerman and Pease [2] were the first persons who examined the earliest work on microchannel heat sink, they showed that microchannel heat sink gave a good heat dissipation rate from the electronic chips, and this result guaranteed the potential of this technology
In order to study the effect of those added cross-sections on the dynamic and thermal behavior of water flow used for cooling systems
Summary
Because of the miniaturization of the recent industrial systems, Micro-Electro Mechanical systems (MEMS), [1] are used to solve the problem of heat evacuation for electronic devices, to maintain their temperatures in a safe range in order to ensure their performance. In order to calculate heat transfer, Samalam [3] studied analytically and numerically a forced convective flow in microchannel for electronic devices cooling, he showed the optimum dimensions for width and microchannel spacing. Peng et al [4, 5, 6] investigated the single-phase forced convective heat transfer of water in rectangular channels with hydraulic diameter ranging from 133 to 367 μm. Their results indicated that the Reynolds number for transition from laminar to turbulent flow became much smaller than in the macroscale channels, and the aspect ratio of the channel had a significant effect on the convective heat transfer. Margot et al.[7] has elaborated a numerical modelling of cavitation implemented in CFD code to investigate the three dimensional flow within Diesel injectors, they found that cavitation model was able to predict the onset of cavitation, they find that in both the injection rate and the occurrence of chocked flow are satisfactory agreement when compared with experiments
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