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Joule–Thomson refrigerators operating with mixtures require heat exchangers with an effectiveness of over 96% for the system to function. The hot (high pressure) stream enters the heat exchanger in a superheated condition and leaves in a subcooled state. The cold (low pressure) stream, on the other hand, enters the heat exchanger in a two-phase state and leaves in a superheated state. The streams undergo a temperature change of over 200 K in the heat exchanger. The above requirements mandate the use of innovative heat transfer surfaces in the heat exchanger. Coiled wire-finned heat exchangers have been employed successfully in this work. In this paper, we present the performance of the J–T refrigerator with two different wire-finned heat exchangers (wire-fin pitches of 8 mm and 12.7 mm) measured in our test stand. About 20 K lower temperature could be obtained when the wire-fin pitch was changed from 12.7 to 8 mm. The exergy efficiency of the refrigerator also improved. [ABSTRACT FROM AUTHOR]

Abstract: Accurate and reliable dimensionless heat transfer characteristic is very essential for the analysis of heat exchangers. It is also required for the rating and sizing problems of heat exchangers. One of the important experimental methods used to determine the heat transfer coefficient between the heat transfer surface of the heat exchanger and the flowing fluid is transient test techniques. The transient test techniques are usually employed to establish Colburn factor versus Reynolds number characteristics of a high NTU heat exchanger surfaces like compact or matrix heat exchangers. In those situations, a single-blow test, where only one fluid is used, is employed to conduct the transient test. The transient technique may have the fluid inlet temperature having a step change, periodic or an arbitrary rise/drop. In this paper, various transient test techniques that are used for the determination of heat transfer characteristics of high NTU heat exchanger surfaces are discussed. [Copyright &y& Elsevier]