Intermediate cooling from pulse tube refrigerator regenerators operating in the real-fluid regime

Under some circumstances, pulse tube refrigerator regenerators operating in the real-fluid regime can absorb a large amount of heat between their warm and cold ends without a decrease to cooling power at their cold heat exchanger. Experiments and analysis show that this extra source of cooling is available because real-fluid thermodynamics modify the temperature profile to direct this additional heat flow toward the warm rather than the cold heat exchanger. Intermediate cooling capacity measurements in a two-stage, dual-inlet pulse tube refrigerator show that the cooling available is larger than commonly appreciated, 2 W at 7.5 K for the particular cryocooler investigated—almost nine times the 0.23 W available at the 3 K cold end. An analytical model is developed that predicts the dependence of injection temperature versus amount of injected heat and location along the regenerator; results from the model agree well with those from experiment and those generated using common numerical codes. This model is also used to explore strategies to increase the available intermediate cooling. For very high amounts of regenerator cooling, the shape of the temperature profile fundamentally changes and power flow changes occur at the cold—rather than warm—heat exchanger. Our results can guide users on how to best utilize this source of cooling and inspire designers to plan for large amounts of intermediate cooling.