Thermal investigation of a signal chopping device within a cryogenic test chamber

This paper details the development of a computational model used to Investigate the transient thermal response of an infrared signal filtering device located in a cryogenic test chamber. The device, a revolving disk called a chopper, Is used to differentiate a blackbody radiation source from the "noise level" of chamber radiation. When the temperature of the disk rises due to radiation exchange with the blackbody source and frictional heating from the support bearings. it begins to radiate energy which contaminates the sensor reading. When this occurs, the test run must be terminated and a cool down period allowed.

The computational model is formulated in standard transient finite-difference equations and a computer program developed to solve the resulting algorithms. The chopper system, consisting of a disk, a shaft, and the enclosure front plate, is represented by finite elements, and three main thermal loads, which are the blackbody radiation source, support bearing frictional heat, and convective helium cooling, are applied at the appropriate elements. The computer program allows modeling of a variety of configurations and can provide guidance toward optimal chopper designs. Several short runs verified capabilities and options found In the program algorithms. Initial Investigation runs are also Included In this study and reveal some general guidelines.

With a radiation source of 300°K the major contributor to thermal loading In a typical chopper system is the bearing frictional heat. The helium coolant applied to the enclosure front plate readily maintains chamber conditions on the plate, and the effect of thermal expansion of the shaft on the bearings is insignificant up to bearing temperatures of 50°K. The results of initial runs tend to support the validity of the model since the resultant thermal profiles agree with reports from actual operations. Incorporating experimental data into the program could enhance accuracy, however. An initial parametric study reveals the significance of the proper use of materials in the chopper design as an aluminum disk mounted to a stainless steel shaft operates with a lower chopper blade temperature than systems made entirely of either material.