Effect of external factors on the operating characteristics of a copper–water loop heat pipe

The paper presents operating characteristics of a copper–water loop heat pipe (LHP) developed for the use in cooling systems of servers for heat transfer from heat-tensioned elements of electronics to peripheral sections or an outer circulation cooling loop beyond the server. The LHP effective length was 400 mm. The device was provided with a flat-oval evaporator. Its thickness, width and length were equal to 7 mm, 42 mm and 80 mm, respectively. The evaporator was equipped with a thermal interface whose heating zone measured 30 mm × 30 mm. The aim of the research work was to determine the effect of external factors such as the device orientation, the condenser cooling temperature and the condition of heat exchange with the surroundings, on the LHP operating performances. The tests were conducted at the unfavorable LHP slopes from 0° to +60° and heat-sink temperatures from 20 °C to 80 °C, and also in different conditions of heat exchange with the outside ambient. The investigation results are presented in the range of heat loads from 20 to 600 W. It has been shown that the slope dependence of the LHP heat-transfer capacity decreases with increasing heat-sink temperature and practically disappears at a value of the latter of 80 °C. In this case the LHP thermal resistance decreases too and reaches a minimum value of 0.02 °C/W in the range of heat loads from 400 to 600 W.