The length and bending angle effects on the cooling performance of flat plate heat pipes

The effects of length and bending angle on the cooling performance of flat plate heat pipes (FPHPs) were examined experimentally in this study. The length effect was explored with four lengths of 80, 150, 200, and 300 mm; each FPHP was filled by acetone in cold condition to its optimum filling ratio. The bending angle effect was examined with four angles of 0°, 30°, 60°, and 90°, using the length of 200 mm and the volumetric filling ratio of 31.4%. All FPHPs had the same cross sectional area of 50 mm (width) by 2.5 mm (thickness). Experimental results showed that by increasing the length from 80 to 150 mm, to 200 mm, and to 300 mm, the minimum thermal resistance, Rth(min), increased by the factors of 2.4, 6.0, and 17.9, respectively from that of 0.103 K/W of the 80 mm FPHP. A rapid increase in Rth(min) occurred around the length of 150 mm. For the FPHPs with lengths smaller than 150 mm, Rth(min) could be smaller than 0.252 K/W. The maximum heat transport capability Qmax decreased quickly from 109.5 to 49.6 W (a factor of about 0.452) when the length was increased from 80 to 150 mm, and then slowly decreased to the minimum value of 35 W (a factor of about 0.318) for the length of 300 mm. In contrast, the results of bending angles showed that by increasing the bending angle, the thermal resistance decreased; Rth(min) reduced by a factor of about 3.3 from 0.6207 K/W of 0° bending to 0.1885 K/W of 90° bending. The corresponding maximum effective thermal conductivity, Keff(max), increased from 1933.4 to 6365.6 W/m K and Qmax increased from 45 to 85 W. That is, a short FPHP preformed better than those of longer ones, and the thermal performance of FPHPs could be enhanced by proper bending.