Enhancement of convective quenching heat transfer by coated tubes and intermittent cryogenic pulse flows

摘要

This paper reports heat transfer enhancement techniques for the cryogenic quenching process. An experiment was performed to evaluate the enhancement of quenching heat transfer by two techniques: 1. Using intermittent pulse flows and 2. Coating the inner surface of a test tube with layers of low thermal conductivity films. Pulsed liquid nitrogen flows with various duty cycles and periods were applied in the quenching of a room-temperature metal tube coated with four layers of Teflon thin films on the inner surface. In general, the results obtained indicate that the quenching thermal efficiency that measures the effectiveness of heat transfer enhancement increases with decreasing duty cycle, however it is relatively independent of the period. Comparing with non-coated bare surface test tube, the low-conductivity coating substantially improved the thermal efficiency and reduced the total quenching time. Additionally, the thermal efficiency was found to increase with decreasing source inlet pressure or coolant mass flow rate. The savings on the amount of cryogen consumed follow the same trends as those for the thermal efficiency with pulse flows and tube coating. (C) 2019 Elsevier Ltd. All rights reserved.