Study On Temperature Uniformity Of Heat Pipe Used For Low Temperature Standard Blackbody Source

The needs of radiation thermometer calibration has increased with the widely use of radiation thermometry technology, as the standard radiation thermometer calibration device, blackbody radiation source has subjected to more and more attention. For standards cavity blackbody source, the uniformity of the cavity temperature has important impact on the blackbody source’s performance to trace temperature. There are three main methods to keep the temperature uniformity of the blackbody. They are resistance wire wound method, constant temperature oil bath method and heat pipe method respectively. According to the temperature uniformity defects of the resistance wire wound blackbody source and the temperature control difficulty of the constant temperature oil bath blackbody, the heat pipe blackbody source has the characteristic of easy to control the temperature, rapid warming and high temperature uniformity. Due to its good isothermal and stability, heat pipe has achieved good results in the use of the blackbody source, but now days, the studies of heat pipe are focused on its heat transfer performance; studies of factors that have influence on blackbody source heat pipe temperature uniformity are very less.In this paper, we have designed a water-stainless steel heat pipe used for low temperature blackbody source and tested it with touching temperature measurement method to study the influence of capillary structure, operating temperature, inclined angle and filling ratio on its temperature uniformity. We also have compared touching temperature measurement method with the method that drag thermometer through a hole. We find that, there is a big difference between two temperature measurement methods. As the distance from the bottom increases, the temperature of the wall reduces, and the greater the distance the faster the temperature changes. The temperature uniformity of heat pipe deteriorates with increased operating temperatures. Heat pipe without capillary structure show a better performance on temperature uniformity than those with capillary structure. We also found that, when the filling ratio is small, the temperature uniformity of heat pipe changes little with the incline angle, but when filling ratio is high enough, the heat pipe temperature uniformity at first deteriorated with the angle increases and then turns to changes better with the angle increases.After the analysis we believe that due to changes of the heat pipe working fluid level occur when inclined, the fluid may approach or flooding cavity wall that caused wall temperature uneven. To solve this problem, we designed a copper -R134a heat pipe, the outer cylinder and the inner cylinder arranged eccentrically. When placed horizontally, the inner cylinder bias up direction. We experimentally tested the copper-R134a heat pipe to investigate the influence of inclined angle, operating temperature and filling ratio on the temperature uniformity of the heat pipe. The regional temperature standard deviation was proposed as the heat pipe temperature uniformity metrics. Experiments show that eccentric arrangement of the heat pipe has solved the problem that temperature uniformity deteriorated when the heat pipe inclined. Experiments also verify that the temperature uniformity of heat pipe deteriorates with increased operating temperatures. Under the experimental filling ratio, the impact of filling ratio on temperature uniformity is not obvious.We have established the thermal resistance model for the cavity of the blackbody source. Thermal resistance analysis shows that, for low temperature blackbody source, the radiation heat transfer between the inner cylinder wall and the cavity hole can be ignored. There are two main factors that influence the wall temperature uniformity, firstly, the heat loss of the surface with cavity hole caused the temperature difference between the axial; secondly, the thickness difference of the liquid film due to uneven steam flow and uneven heat transfer caused the axial temperature difference. The temperature difference caused by heat loss only performance in a small area, mainly around the cavity hole. And the liquid film thickness difference will cause the temperature difference in a larger area, so the film thickness difference can be an important factor to the temperature uniformity.