Objective The angle factor is a crucial parameter in the calculation of radiation heat transfer between surfaces. The commonly used methods for calculating radiation heat transfer in diffuse gray surface system, such as the net heat method and network method, utilize energy flux density (emissive power) and angle factor to describe the energy relationship during the process of radiation heat transfer. Whether it is the net heat method or network method, it is necessary to calculate the angle factor between any two surfaces. With the development of large focal plane array, long-linear-array, dual/multi-band, digital infrared detector technology, the size and structure complexity of dewar gradually increased. The traditional methods calculating the radiation heat generally simplified dewar to the coaxial cylinder model, resulting in significant calculation errors. Therefore, the accurate calculation of dewar radiation heat transfer is essential. The inside of the dewar is a high vacuum, and its radiative heat transfer belongs to the category of radiative heat transfer between surfaces. To calculate the radiation heat transfer, the angle factor should be calculated first. This paper mainly studied the calculation of the angle factor of dewars.

Methods The Monte Carlo method was used to calculate the angle factors of the dewar. 3ds Max was used to extract the surfaces involved in radiation for modeling and triangulating to enhance the universality of the calculation (Fig.1, Fig.17). A calculation program was developed. The calculation process mainly included randomly arranging the well-distributed emission points (Fig.4), generating the emission direction constrained by Lambert's law (Fig.7), and tracing energy bundle based on the whole dewar model.

Results and Discussions The angle factors and relative errors of three typical models were calculated to verify the correctness of the program. The calculation result showed that the relative error could be controlled between 1% and −1% with the increase of the number of energy bundles(Fig.12, Fig.13, Fig.14, Fig.15). The angle factors of a certain type of dewar were calculated and 12 of them were provided (Tab.4). Monte Carlo method is an efficient calculation method for compact dewars and statistically significant results could be obtained when the number of energy bundles reaches the level of ten thousand.

Conclusions A general calculation program for calculating the angle factors between the radiation surfaces of dewar was obtained. The angle factors of one surface to multiple surfaces could be calculated at one time. The angle factors of a certain type of dewar were calculated. The angle factor of the window to the optical filter was 0.5617, the angle factor of the window base to the cold shield was 0.4875, the angle factor of the main cylinder to cold finger was 0.3069, and the angle factor of the getters to the cold finger was 0.1879. After obtaining the accurate angle factors, the radiant heat transfer of dewar can be calculated accurately by combining the calculation methods of radiant heat transfer between surfaces, such as the net heat method and the network method.