High energy requirement for electronic cooling is a major problem to operate high performance computers and data centres. Developing low cost thermal management systems for micro-electronic devices and micro-electro-mechanical systems (MEMS) is a cutting edge research area. A heat pump system associating micro-gap evaporator with internal micro-fins is a potential candidate for two-phase cooling of these advanced devices. Micro-fins induce pseudo-turbulence in the flow field, which escalates heat transfer rate. In this paper, the system performance of a heat pump using micro-gap evaporator has been investigated numerically and experimentally. As heat transfer rate in the micro-gap evaporator is influenced by turbulence generation, flow field in the inlet and outlet manifolds have been visualized in the numerical simulation to observe fin-induced pseudo-turbulence at the entrance and outlet of the micro-gap evaporator. The simulation has been performed using FLUENT 14.5 release. Experimental work has been carried out to validate numerical results. For experimentation purpose, a test rig has been developed, which contains a test section accommodating the micro-gap evaporator. A heater is provided at the bottom of the evaporator to supply uniform heat flux ranging 1 ~ 8 kW/m2. A pre-heater is installed at the compressor outlet to vary refrigerant temperature at the condenser inlet. The range of pre-heater temperature is 93 ~ 159°C. A variable speed compressor is used. The input frequency to the compressor is varied within the range of  20 ~ 50 Hz to run the compressor at different speeds. Experimental data show good agreement with numerical results. It is observed that in transient state, temperatures and pressures at different locations of the test apparatus fluctuate due to quasi-periodic dry out and surface rewetting nature of the flow. When pre-heater temperature is set at 159⁰C and compressor frequency is increased from 20 Hz to 30 Hz, evaporator wall heat flux escalates 118.2% and heat transfer rate of the condenser increases 65.2%. However, heat transfer rate declines with the further increment of compressor frequency. Coefficient of performance (COP) of the heat pump also increases with the frequency increment from 20 Hz to 30 Hz and declines after surpassing 40 Hz frequency.

In recent years, researchers are investigating several potential applications of two-phase flow in micro-gap heat sinks; electronic cooling is one of them. Further, internal micro-fins are used to enhance the heat transfer rate. However, the pressure drop penalty due to small gap height and fin surfaces is a major concern. Hence, minimization of thermal resistance and pressure drop is required. In this paper, effects of operating conditions, e.g., wall heat flux, pumping power, and inlet void fraction, on total thermal resistance and pressure drop in a micro-gap heat sink with internal micro-fins of rectangular and triangular profiles have been investigated by numerical analysis for the R-134a coolant. Furthermore, optimization of these parameters has been carried out by response surface methodology. Simulation results show that rectangular micro-fins show superior performance compared to triangular fins in reducing thermal resistance. Finally, for an optimum condition (7.1202×10-5 W pumping power, 1.2×107 Wm-2 heat flux, and 0.03 inlet void fraction), thermal resistance and pressure drop are reduced by 56.3% and 87.2%, respectively.

Abstract.A numerical study is conducted by using ANSYS CFX 14.5, a commercial computational fluid dynamics program to predict the thermal performance of a counter and parallel flow on triangular double layered microchannel heat sink for various channel aspect ratios. Findings reveal that the counter flow configuration leads to a better heat transfer performance for low channel aspect ratio (α < 4) and higher Reynolds Number (Re > 700). For the parallel flow configuration, improved performance is normally shown when channel aspect ratio, α is more than 4 and lower Reynolds Number (Re < 700).

The ability to use different parameters and finishing techniques in fused deposition modelling (FDM) depends largely on part geometries, materials, printing processes like Z-resolution and post processing to some extent. Low quality poor surface finish due to layer ovality, improper Z-resolution parameter selection and fill of the empty shell in three-dimensional (3D) printing, results unexpected texture and appearance. An investigation is carried out on the effects of Z-resolution (0.15 mm to 0.40 mm) and Fill parameters on flat and curved surface objects manufactured using the FDM process. Moreover, post surface treatment was performed using acetone. It was found that average surface roughness increases with increasing Z-resolution. Solid Fill can create a smoother surface as oppose to Sparse Fill. Surface roughness improves significantly after post treatment with organic solvent acetone.

Evaporation in the micro-gap heat sink has a very high heat transfer coefficient. As a result, it is significant for high heat flux management. Heat transfer rate can be enhanced further by including internal micro-fins. However, the pressure drop penalty due to the small gap height and fin surfaces is a major concern. Wall shear stress development is responsible for pressure drop. This paper investigates the effects of operating conditions, e.g., wall heat flux, pumping power, and inlet void fraction, on evaporation rate and wall shear stress development in a micro-gap heat sink with internal micro-fins of rectangular and triangular profiles, while the cross-sectional area (21.8 mm2) is kept constant. R-134a is considered as coolant. Results show that the evaporation rate from per unit volume increases with the increment of wall heat flux and decreases with the enhancement of pumping power. However, after a threshold value of the pumping power (2×10-4 W), the decrement rate falls. Again, the wall shear stress rises with the increasing wall heat flux and pumping power while reduces for escalating inlet void fraction.