Dynamic test method to determine the capillary limit of axially grooved heat pipes

The usual experimental method to detect heat pipe capillary limit under different inclinations includes submitting the pipe to a certain tilt and increasing the heat load at the evaporator zone until a sudden rise in temperature at this region (dry out) is observed. According to this method, for each heat load dwell imposed to the pipe, either the dry out phenomenon is detected or steady state temperature is achieved. The complete test is time consuming and the precision of the heat load detection needed to induce the dry out depends on the power step utilized. In addition, sometimes the dry out is difficult to detect accurately in axially grooved heat pipes due to the fact of the liquid phase is distributed in a non-homogeneous way at the evaporator zone. In fact, the grooves at the top of the heat pipe tend to dry faster. At the lower grooves of the evaporator forms a buildup of working fluid (puddle) due to gravitational effects and thus needing a higher heat flux to cause the drying. The dynamic method proposed in this paper to detect dry out in heat pipes consists in applying a given heat load on the heat pipe, which is initially kept in a horizontal position on a rotary table equipped with motor with reducer gearbox and digital inclinometer. After steady state is reached on the heat pipe, which is leveled horizontally, the table is driven causing the pipe to adverse tilt slowly until the dry out occurs. Once dry out initiates, the axial gravity force component, which is permanently increasing due to table rotation, provokes the liquid phase accelerated retreating from the evaporator, including the puddle liquid excess. It assists the fast overheating onset in the dried zone that in its turn allows a very clear detection of the dry out event by a temperature sensor. The pipe is then placed back in the horizontal position. The proposed test method besides requiring less time to obtain the capillary limit curve, permits to detect in a more precision way the exact time when the dry out occurs. The capillary limits obtained from this method were compared against those obtained from conventional methods for ammonia two-core axially-grooved heat pipe. The results show that dynamic test method can be adopted as an effective alternative to determine capillary limit for axially grooved heat pipes.