Performance degradation of flattened heat pipes

The performance degradation of flattened heat pipes is studied experimentally under a horizontal orientation. The original cylindrical copper/water heat pipes are ?6 mm and 30 cm in length. Tested are the sintered-powder wick and the groove wick. The maximum heat load (Q_max), the evaporator resistance (R_e), the condenser resistance, the overall thermal resistance, and the longitudinal temperature distributions are measured under incremented heat loads. After flattening, R_e is slightly reduced. Q_max is hardly affected when only the evaporator is flattened; but it is greatly reduced for fully flattened heat pipes. Different mechanisms of performance degradation are observed for flattened powdered and grooved heat pipes. With a thicker wick and larger saturate charge, the main degradation mechanism of flattened powdered heat pipes is liquid clogging at the condenser end. This causes malfunction of a powdered heat pipe flattened to 2.5 mm. When flattened to 3 mm, the powdered heat pipe exhibits milder Q_max degradation than a grooved heat pipe because the liquid flow is better protected against the vapor–liquid interfacial shear. In contrast, the serious Q_max degradation of a flattened grooved heat pipe is mainly caused by the interfacial shear which leads to greatly prompted dryout at the evaporator.