Internal Temperature Distributions in an Operational Heat Pipe

Abstract

In recent years there has been a great deal of research conducted on various aspects of heat pipe design and operation. However, experimental evaluations have been limited by a lack of sufficient data, most notably the absence of temperature measurements internal to the heat pipe container. In this paper results of internal temperature measurements in an operational heat pipe are reported utilizing both water and methanol as working fluids. Although Winter and Barsch * have reviewed most of the significant heat pipe research, the work of Phillips is more pertinent to our results since he also dealt with water and methanol as working fluids. He found that heat fluxes were over twice as large for water than for methanol although it must be pointed out that his experiments were not conducted in an operational heat pipe. At this point it is also important to note that in all the heat pipe literature to date, burnout is defined as that point at which the external heater temperature no longer increases in a linear fashion with power. As we shall show this is not necessarily indicative of heat pipe failure as implied in previous works. The only investigator to report the actual measurement of an internal temperature in an operational heat pipe was McS weeney. Using a single thermocouple which could be traversed along the longitudinal axis of the pipe, McS weeney claimed that burnout could be detected by a large temperature decrease in the vapor section of the pipe. As Winter and Barsch point out, this puzzling result might be due to the presence of noncondensables in the condenser portion of his pipe. Convinced that only through internal measurements could we achieve the required degree of insight into heat pipe operation, we instrumented a 1.5 in. heat pipe with twenty-six internal thermocouples at the locations shown in Fig. 1. In addition we also measured the temperature of the outer heat pipe wall at each of the ten axial stations. Since the thermocouple bundle occupied a maximum of 7.5% of the vapor cross section in the condenser section and only 2% in the heater section, no obvious interference of the thermocouples with the heat pipe performance was noted