Characterization of Laminar Jet Impingement Cooling in Portable Computer Applications

A thermal characterization study of laminar air jet impingement cooling of electronic components within a geometry representative of the CPU compartment of a typical portable computer is reported. A finite control volume technique was used to solve for the velocity and temperature fields. Convection, conduction and radiation effects were included in the simulations. The range of jet Reynolds numbers considered was 63 to 1500; the applied compartment heat load ranged from 5W-15W. Radiation effects were significant over the range of Reynolds numbers and heat loads considered, while the effect of natural convection was only noticeable for configurations when the ratio Gr/Re exceeded 5. The predicted importance of Re rather than jet size was confirmed with test data. Proof of concept was demonstrated with a numerical model representative of a full laptop computer. Both simulations and lab tests showed that low flow rate JI cooling schemes can provide cooling comparable to a high volume flow rate configuration, while using only a fraction of the air flow. Further, under the conservative assumption of steady state, fully powered components, a hybrid cooling scheme utilizing a heat pipe and laminar JI was capable of cooling the processor chip within to 11C of the vendor specified maximum temperature for a system with a total power dissipation of over 21 W. Index Terms – jet impingement, laminar flow, portable computers, heat pipe NOMENCLATURE Dh Jet hydraulic diameter (m) Gr Grashof number (gβ∆TL/ν) H Compartment Height (m) h Heat transfer coefficient (W/mK) k Thermal conductivity (W/mK) L Characteristic length (m) Nu Nusselt number (q”L/∆Tk) P Power dissipation (W) p Pressure (Pa) Pr Fluid Prandtl number Q Heat flow (W) q” Heat flux (W/m) Re Reynolds number (VDh/ν) T Temperature (C) u Velocity in the x direction (m/s) V Average jet velocity (m/s) v Velocity in the y direction (m/s) w Velocity in the z direction (m/s) W Jet width (m) α Thermal diffusivity (m/s) β Coefficient of volume expansion (1/K) ν Kinematic viscosity (m/s) θ Thermal resistance (C/W)