Performance of a pulsating heat pipe using multimode heat transfer insert

Abstract

Closed Loop Pulsating Heat pipe (CLPHP) is one of the most popular passive heat transfer systems used mostly in electronic cooling. The subtle complexity of internal thermo-fluidic transport phenomena of this device is quite unique. In the present experimental study, a CLPHP is made from a long capillary copper tube with an inner diameter of 2.1mm and an outer diameter of 3.1mm. The tube is bent into eight number of U-turns and divided into three regions; evaporator region (50mm), adiabatic region (100mm) and condenser region (50mm). A rosette like wire fin structure is inserted at the center of the CLPHP to compare the performance with no wire insert (plain tube) and a wire insert. The thermal performance is studied with water, methanol, ethanol and acetone for different fill ratios (20% to 80% with 20% increment) and different heat input (4W to 35W). This experimental study reveals that the thermal resistance of rosette inserts CLPHP perform better compared to a wire insert and no wire insert CLPHPs. Acetone performs better than the other working fluids due to its low latent heat and specific heat. Less quantity of heat is required for acetone which provides early start of nucleation boiling in the evaporator region of the CLPHP. Thermal resistance drops quickly at low heat input range (up to 10-15W) and becomes almost uniform at high heat input. It is observed that the thermal performance also depends on different fill ratios. It has been found from this experimental study that 60% fill ratio of various fluids showed better result compared to other fill ratios. It happens due to the presence of optimum amount of fluid in liquid phase which transfers more heat by nucleation boiling. A comparative study on thermal performance has been carried out for rosette insert CLPHP with respect to no wire insert (Plain tube) and a plain wire insert CLPHPs. Convection and boiling heat transfer are the usual modes of heat transfer in heat pipe. The shape, size and structure of the rosette wire inserts are like that they always conform a solid contact between the fin of the rosette and inner surface of the tube. This solid contact introduces a new mode (conduction) of heat transfer, which ultimately enhances the overall heat transfer in heat pipe. Actually, the introduction of conduction heat transfer in heat pipe technology is the main objective of the present study. An overall decrement of the thermal resistance is approximately 10% for rosette insert compared to plain tube heat pipe and 7% reduction with compared to plain wire insert heat pipe among all the experimental conditions. Acetone with 60% fill ratio shows the best thermal performance in rosette insert CLPHP among all experimental conditions. Thermo-physical properties like latent heat and specific heat of working fluids have an effect on thermal performance of CLPHP. Rosette insert CLPHP performs better for all working fluids than plain tube and plain wire insert CLPHPs.