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Numerically investigated the electric double layer (EDL) Effects on the performance of the square microchannel heat exchanger (MCHE) at combined electro-osmotic and pressure-driven flow with compared pure pressure-driven with a hydraulic diameter (10 – 50) μm. We defined at any size (Dh) of microchannel heat exchanger become the impact of EDL very slight with the studied effect of electric double layer thickness λ. The diluted water 1:1 potassium chloride (KCl) solution is used as a working fluid at an ionic concentration , , silicon microchannel at zeta potential of surface -0.2 volt. A three-dimensional (3D) Poisson-Boltzmann equations and Naiver-stoke equations with applied electric field solved by using the finite volume scheme in this work. The results show an increase in pressure drop of the microchannel heat exchanger at combined flow electroosmotic and pressure-driven flow with a percentage of 31.09 % at an ionic concentration and 42.71 % at increase in pumping power, especially at low ionic concentration. Slight enhancement in average heat transfer rate and effectiveness due to an increase in average temperature difference. Decrease in overall performance at combined electroosmotic and pressure-driven flow compared with pure pressure driven.

In the recent time the EAHE is widely used as a passive cooling and heating technique due to the hug thermal potential of earth as and as a result of energy crisis. In the past years many researches published including studying this type of heat exchanger and using it in many applications and many locations with different climate conditions. Also these researches investigated the most affecting parameters. In this paper a review is made to survey the published literature is this filed to shed a light on the research in this subject

In this paper, an experimental electrical distribution transformer was studied and a new technique was proposed to improve the performance of a new mixed cooling consisting of pure transformer oil, paraffin wax and nanoparticles. The experiment was carried out on a small transformer that was done by taking a model with dimensions (15 * 10 * 10) cm to facilitate calculations. Paraffin wax absorbs the heat generated in the transformer due to the smelting process that can be used to cool electrical appliances. Nanoparticles have good thermal properties and lead to increased oil insulation to thermal improvements in transformer oil with dispersal of solid nanoparticles and their effects on transformer cooling. Three types of solid nanoparticles were used in this experiment (Al2O3, TiO2, and Sic) with a different volume concentration (1%, 3%, and 5%) and 4% paraffin wax as a certified added percentage for each process. The obtained results showed that when mixing paraffin wax and solid nanoparticles with transformer oil, the transformer cooling performance is improved by reducing the temperature. The best selected nanoparticles were found to be Sic and the reason for this is that Sic has a higher thermal conductivity compared to (Al2O3 and TiO2). The proposed hybrid oil reduces the temperature by 10 ° C (in the case of PCM and Sic) and it is possible to improve the cooling performance of electrical transformers.

In this paper the effect of surface roughness on the performance of counter flow microchannel heat exchanger has been numerically investigated. The studied Microchannel heat exchanger is a square shape and made of aluminum as substrate material with different values of hydraulic diameters (20, 50, 110, 150 ) μm. The working fluid used is water at constant properties. Roughness- viscosity model has been used to study the roughness effect with 0.14 ratio of roughness to hydraulic diameter. The results obtained indicate that pressure drop of (CFMCHX) increased with increasing surface roughness and decrease hydraulic diameter also the results showed that there is a slight increasing in thermal performance with increasing the surface roughness.

The effect of the hybrid suspension on the intrinsic characteristics of microencapsulated phase change material (MEPCM) slurry used as a coolant in counterflow microchannel heat exchanger (CFMCHE) with different velocities is investigated numerically. The working fluid used in this paper is a hybrid suspension consisting of nanoparticles and MEPCM particles, in which the particles are suspended in pure water as a base fluid. Two types of hybrid suspension are used (Al2O3 + MEPCM and Cu + MEPCM), and the hydrodynamic and thermal characteristics of these suspensions flowing in a CFMCHE are numerically investigated. The results indicated that using hybrid suspension with high flow velocities improves the performance of the microchannel heat exchanger while resulting in a noticeable increase in pressure drop. Thereupon, it causes a decrease in the performance index. Moreover, it was found that the increment of the nanoparticles’ concentration can rise the low thermal conductivity of the MEPCM slurry, but it also leads to a noticeable increase in pressure drop. Furthermore, it was found that as the thermal conductivity of Cu is higher than that for Al2O3, the enhancement in heat transfer is higher in case of adding Cu particles compared with Al2O3 particles. Therefore, the effectiveness of these materials depends strongly on the application at which CFMCHE is employed.

The aim of this paper is to study the microencapsulated phase change materials (MEPCM) suspension as a coolant instead of pure fluid in the micro channel heat sink (MCHS) with different core and wall materials as well as different base fluids. The MEPCM suspension using in this study consist of microcapsules constructed from n-octadecane, RT44 as a phase change materials (PCM) and shell materials are poly-methylmthacrylate (PMMA), poly-alpha olefin (PAO), these capsules are suspended in (pure water, ethylene glycol and pure oil) in a concentration of (0–20)% and have been used as a cooling mediums at different ambient temperatures. Constant heat flux is applied at base of heat sink. The results obtain show that, using MEPCM suspensions as a coolant in micro heat sink instead of conventional cooling with pure fluid lead to enhance the cooling performance of micro heat sink is be 20.91% with (RT44+PMMA) pure oil, 16.91% with (RT44+PMMA) for ethyleneglycol and 12.91% with (RT44+PMMA) for pure water at concentration 2% and = 0.28 m/s. The phase change material in MEPCM suspension should be select according to its melting temperature corresponding to the certain application since different MEPCM suspension concentration caused different values of reduction in heat sink temperature in range of ambient temperature due to different in its melting temperatures

In this paper the axial heat conduction in an isosceles right triangular microchannel heat exchanger is numerically investigated, for laminar, 3D, incompressible, single-phase, steady state flow. The behaviour of axial heat conduction in the separating wall under different conditions is studied. The solution was obtained by solving the continuity and Navier–Stokes equations for the hot and cold fluids by using the pressure-correction method to obtain the velocity distribution, and then the energy equations were solved for the two fluids and the separating wall simultaneously to obtain the temperature distribution. The governing equations are discretized using finite-volume and the hybrid differencing scheme with FORTRAN code was used. Various parameters that can have effect on the axial heat conduction were investigated. The results showed that, the axial heat conduction plays an important role in a parallel flow microchannel heat exchanger and the factors affecting the local and average axial heat conduction are; Reynolds number (Re), thermal conductivity ratio (Kr), hydraulic diameter (Dh), thickness of separating wall (ts) and channel volume. Increasing of Re, Kr and ts leads to an increase in the axial heat conduction while increasing of Dh and channel volume leads to a decrease in the axial heat conduction.

The rectangular microchannel heat exchanger performance is numerically investigated in this paper. Hydrodynamic and heat transfer characteristics in a laminar, 3-D, incompressible, slip flow, steady state and counter flow are proposed. The Navier–Stokes equations and energy equation for the hot and cold fluids are solved with the slip velocity and temperature jump conditions. The governing equations are discretized using finite-volume method-upwind differencing scheme, and then these are solved using SIMPLE algorithm method on staggered grid with FORTRAN code. From the obtained results it was found that the factors affecting the effectiveness are: Reynolds number Re, thermal conductivity ratio Kr, Knudsen number Kn and aspect ratio α. Increasing of Re, Kn and α separately lead to decrease the effectiveness. While the effectiveness increases with increasing Kr until it reach a certain optimal value which gives the maximum effectiveness at Kr = 90. Also, it is found that Nusselt number decrease with increases Knudsen number. Increasing of Re and Kn separately lead to increase pressure drop.

Microchannel heat exchangers (MCHE) can be made with channels of various geometries. Their size and shape may have considerable effect on the thermal and hydraulic performance of a heat exchanger. In this paper numerical simulation is carried out to solve 3D developing flow and 3D conjugate heat transfer of a balanced counter flow microchannel heat exchanger (CFMCHE) to evaluate the effect of size and shape of channels on the performance of CFMCHE for the same volume of heat exchanger. The effect of shape of the channels on its performance is studied for different channel cross-sections such as circular, square, rectangular, iso-triangular and trapezoidal. Results show that for the same volume of a heat exchanger, increasing the number of channels lead to increase in both effectiveness and pressure drop. Moreover circular channels give the best overall performance (thermal and hydraulic) among various channel shapes. New correlations are developed to predict the value of heat exchanger effectiveness and performance index as a function of relative size of channels with overall heat exchanger volume, Reynolds number and thermal conductivity ratio.