Numerical Investigation on Effect of Geometrical Variations of Microchannel Heat Sink in LED Cooling

Abstract

The heat sink is the last and most influential part of the thermal stack, and is needed to first conduct heat away from the LEDs and then to convect and radiate heat to the ambient air. In some cases, heat sinks are coupled to other heat dissipating devices such as housings, enclosures, etc. For this document, we group these devices under the general term “heat sink”, but this should not be overlooked in system design as it can contribute significantly to the performance of the entire LED system. Microchannel provides the more surface area to volume ratio i.e. compactness, so it preferred in compact electronic devices as a heat sink. Before analysis of Micro-Channel Heat Sink (MCHS), heat transfer phenomena in LED are studied. Mathematical modeling is also done to represent physical system in the form of mathematical equations. Plain microchannel heat sink is analyzed and validated with the published experimental data by comparing Nusselt number and Fanning friction factor at various inlet velocities ranging from 1.54 m/s to 6.17 m/s. In validation it has found that numerical results are in good agreement with experimental results with deviation of 10% to 13%. In plain MCHS six different types of offset ribs are added on sidewall which are rectangular, backward triangular, forward triangular, mix of forward and backward triangular, isosceles and semicircular in shapes. All these geometries are analyzed at Reynolds (Re) number ranging from 200 to 800. The heat flux at the bottom of channel is given as 106 W/m2. All these microchannels are analyzed based on different thermal parameters such as Nu, fe, pressure drop and temperature at the bottom face. From the observation it shows that mix of forward and backward offset ribs MCHS gives better results (At Re 500 and q 106, Nu 15.01, Pressure drop 245.738 KPa, Tb 304.25 K) as compared to other types of channels. So this geometry is selected as best geometry and again simulated with higher heat fluxes. Finally at q 500 W/cm2, Re 500 and selected thickness and thermal conductivity of TIM and PCB temperatures obtained at bottom face of heat flux 73.94 after TIM 78.94 and after PCB i.e. at junction 112.27 . As junction temperature is less than 120 oC which is safe working temperature of LED so, given model of heat sink can be selected for LED.