An accurate calorimeter-based method for the thermal characterization of heat pipes
Joseph P. Mooney, Jeff Punch, Nick Jeffers, Vanessa Egan
Abstract
This study presents a method that can be used to accurately determine the thermal performance of a cylindrical heat pipe. In the method, the heat pipe is placed between two stainless steel 304 cylindrical blocks, configured as radial calorimeters that achieve thermal contact with the evaporator and condenser sections of the pipe. A flexible isothermal electrical heater mat surrounds the evaporator block, and a liquid-cooled copper pipe wrapped around the condenser block is used to remove heat. High precision thermistors (±0.01 K) positioned at fixed radial locations within the calorimeters are used to measure the heat supplied to the evaporator and the heat extracted from the condenser. One-dimensional radial conduction is assumed to occur within each calorimeter, and this enables the quantification of heat flows from the temperature readings. This assumption is verified by a steady-state analysis of the radial, axial and circumferential temperature differences within the evaporator calorimeter, based on data recorded for the lowest and highest heat inputs. Furthermore, a numerical model is used to confirm that end effects have a negligible influence on radial conduction within each calorimeter. This study concludes that the most commonly used characterization techniques for heat pipes can greatly overestimate thermal performance (15–32% for input powers of 7.5–25 W respectively) due to inaccurate quantification of heat flows into the evaporator and from the condenser. The calorimetric technique reported here achieves uncertainties in thermal resistance of <7.5% for low thermal loads (<12.5 W) and <6% for higher loads (>12.5 W). Moreover, the method achieves a significant improvement in the experimental thermal efficiency, with values of >75% recorded for all heat inputs in this study. The use of radial calorimeters in the current study obviates the requirement for calculating the losses from the heater to ambient, hence achieving low uncertainties in thermal resistance and effective thermal conductivity for a range of heat inputs.