DA-00-04-1 -- Diabatic Flow Instabilities in Capillary Tube-Suction Line Heat Exchangers PDF

Three capillary tube-suction line heat exchangers (ctslhx's) were tested under steady and quasi-steady conditions over a range of subcooled inlet temperatures and pressures, and mass flows were measured with a coriolis-type meter. Hysteresis was observed; mass flow rate is multivalued and depends on whether the flash point is moving upstream or downstream. Additional experiments were conducted with capillary tube inlet conditions held constant while suction inlet superheat was varied, and the phenomenon was observed again. The same effect was reproduced in an operating refrigerator, affecting its thermodynamic cycle efficiency by 3%. These experiments provide the first evidence of such behavior in diabatic capillary tubes. The implications for both research and design are important. These results may explain much of the scatter in previously published data, but they imply that the value of such data is limited by lack of information about the path along which the steady test conditions were approached. Designers using steady-state models based on equilibrium thermodynamic assumptions must also live with this uncertainty until more is known about metastable behavior and its path-dependence.

Units: I-P