Analysis of Multi-Stage Joule-Thomson Microcoolers

Presented at the 16th International Cryocooler Conference, held May 17-20, 2008 in Atlanta, Georgia.Microcoolers machined from glass wafers based on Joule-Thomson (J-T) expansion have been investigated for many years at the University of Twente. After successful development of singlestage J-T microcoolers with a cooling capacity of around 10 mW at 100 K, research is being carried out to attain lower temperatures to 30 K using multiple stages. As an initiative in this research effort, a simplified dynamic model of a two-stage microcooler has been developed using MatlabR to analyze the cooler performance. The first stage operates with nitrogen as the working fluid from 80 bar high pressure to 6 bar low pressure and is used to precool the second stage. The second stage operates with neon from a high pressure of 40 bar to a low pressure of 1 bar. The model is composed of single lumped heat capacities at both the evaporators and the precooler. These capacities are linked via thermal resistances and are cooled by the evaporating fluids. The effect of the counterflow heat exchanger (CFHX) length on the performance of the two-stage microcooler is discussed. The length of the CFHX in the second stage between precooler and evaporator is very critical regarding the cool-down time. Based on the model, a preliminary design is made with the envelope dimensions of 8.30 x 37.00 x 0.72 mm that has a cooling power of 5 mW at 27 K. It cools down in less than 3 hours with a corresponding mass flow rate of 2.6 mg/s of nitrogen and 1.5 of mg/s neon