Novel Design of a Miniature Loop Heat Pipe Evaporator for Electronic Cooling

Miniature loop heat pipes (mLHPs) are coming up with a promising solution for the thermal management of futuristic electronics systems. In order to implement these devices inside compact electronics, their evaporator has to be developed with small thickness while preserving the unique thermal characteristics and physical concept of the loop scheme. This paper specifically addresses the design and testing of a mLHP with a flat evaporator only $5mm$ thick for the cooling of high performance microprocessors for electronic devices. A novel concept was used to achieve very small thickness for the mLHP evaporator in which the compensation chamber was positioned on the sides of the wick structure and incorporated in the same plane as the evaporator. This is unlike the conventional design of the flat evaporator for mLHP in which the compensation chamber, as a rule, adds to the overall thickness of the evaporator. The loop was made from copper with water as the heat transfer fluid. For capillary pumping of the working fluid around the loop, a sintered nickel wick with $3–5μm$ pore radius and 75% porosity was used. In the horizontal orientation, the device was able to transfer heat fluxes of $50W∕cm2$ at a distance of up to $150mm$ by using a transport line with $2mm$ internal diameter. In the range of applied power, the evaporator was able to achieve steady state without any temperature overshoots or symptoms of capillary structure dryouts. For the evaporator and condenser at the same level and under forced air cooling, the minimum value of $0.62°C∕W$ for mLHP thermal resistance from evaporator to condenser $(Rec)$ was achieved at a maximum heat load of $50W$ with the corresponding junction temperature of $98.5°C$⁠. The total thermal resistance $(Rt)$ of the mLHP was within $1.5–5.23°C∕W$⁠. At low heat loads, the mLHP showed some thermal and hydraulic oscillations in the transport lines, which were predominately due to the flow instabilities imposed by parasitic heat leaks to the compensation chamber. It is concluded form the outcomes of the present investigation that the proposed design of the mLHP evaporator can be effectively used for the thermal control of the compact electronic devices with high heat flux capabilities.