Infrared thermography of a pulsating heat pipe: Flow regimes and multiple steady states

Understanding of the operational characteristics of a closed loop Pulsating Heat Pipe (PHP) with non-intrusive spatial and temporal measurement of tube wall temperature by high resolution infra-red thermography (IRT) has been attempted in this work. The PHP is made of a copper capillary tube of internal diameter of 2.0 mm and has eight turns each in the evaporator and condenser section. It is operated in vertical heater down position with water as the working fluid. The local temperature field of the PHP surface is measured by IRT while a micro-thermocouple is used to measure the local fluid temperature. IRT field information throws light on several important operational features and helps, not only to correlate the temperature foot-print with thermal performance but also to qualitatively identify the internal flow characteristics as the heat power input to the system is increased. The PHP displays different flow behaviour, viz., no-flow, intermittent oscillatory flow, oscillatory flow with bulk circulation, with corresponding unique thermal performance. In addition, this study clearly reveals that PHPs exhibit multiple steady states at intermediate heat input powers. While this phenomenon has been observed for single-loop PHP system earlier, it is reported here for the first time in a multi-turn device. The overall thermal resistance of the device decreases from ∼1.90 K/W to 0.24 K/W, with increasing input heat power from 30 W to 500 W. The overall thermal performance of the device makes is quite an attractive solution for many potential high heat flux thermal management applications.