Enthalpy change measurements of a mixed refrigerant in a microcryogenic cooler in steady and pulsating flow regimes

Microcryogenic coolers (MCCs) are useful to a number of small electronic devices which require low cooling power. The cooling power of microcryogenic coolers operating with mixed refrigerants in a Joule–Thomson (J–T) cycle can be calculated based on enthalpy change for the mixture, but it has been observed that cooling power depends on flow regime. This article demonstrates a method to measure the isothermal enthalpy change in a mixture undergoing J–T expansion in an MCC for different flow regimes. The enthalpy change for a mixture undergoing steady flow is an order of magnitude below that calculated, whereas the enthalpy change for the mixture undergoing pulsating flow agrees with the calculations below a certain temperature, and is over-predicted above that temperature. For steady flow, an analysis of component separation within the mixture due to annular flow shows good agreement with measured data, and for pulsating flow, the discrepancy is likely due to periods of liquid slug flow interrupted by periods of annular flow.

► We measure the enthalpy change of a refrigerant mixture in a microcooler.
► Enthalpy change of steady flow is a factor of 10 less the design value.
► Enthalpy change of pulsating flow is in reasonable agreement with the design value.
► The discrepancy is analyzed in terms of composition change from liquid hold-up.