John Gary and Abbie O'Gallagher, ACMD
Ray Radebaugh, Chemical Science and Technology Laboratory
Over the past several years John Gary and Abbie O'Gallagher, in collaboration with Ray Radebaugh, have developed a numerical model of the regenerator portion of a cryocooler. A cryocooler is a low temperature refrigerator used to cool, for example, infrared detectors, medical instruments, and superconducting devices. This is a one-dimensional time dependent numerical model for oscillating, compressor driven flow through a porous media with heat transfer between the porous media and the fluid. It is a difficult computation. Thus far, no general purpose algorithm has provided a satisfactory solution.
NIST is in a good position to perform this model development because the measurements that Ray's group makes are generally more extensive than those done at the companies which produce the devices. For example, in 1992 Wayne Rawlins did a PhD thesis with Ray and Prof. Timmerhaus at CU which provides careful measurements of a pulse tube cryocooler which can be used to validate a model. The results of this work can be reported more freely than those obtained within a company. Because Ray is in contact with so many different companies, the model can be easily tested on different problems, which gives an advantage over model work in a typical university, or in a single company.
They have recently, for the first time, obtained reasonably good results from a model of a full cryocooler, one which includes the compressor and heat exchangers in addition to the regenerator. Within the last year they have also modified a regenerator model to study the effect of heat exchanger mesh size on the operation of a device being constructed jointly by NIST, LASL and Tektronix. Numerical experiments with the FIDAP fluid code were conducted in order to predict the effect of geometry on the flow in a device being built as part of a joint project between NIST and Cryenco. In 1994 the older regenerator model was used at NIST to assist with designs for the Navy, NASA, Hughes, Tektronix, Alabama Cryogenics (a small business), Lockheed, Cryenco, and Yaron (a small business). A PC version of the older model has been distributed to a 8 sites this year, including both companies and universities. A joint paper with Ray was given at the Cryocooler conference in June.
They will improve the model of a full cooler so that it can be used as a design tool and start development of a two dimensional model which can be used to better understand the operation of pulse tube cooler.