Partial Dryout in Enhanced Evaporator Tubes and Its Impact on Heat Transfer Performance

Recent studies have demonstrated that enhanced ribbed and finned surfaces used in compact automotive A/C evaporators can exhibit dryout behavior in the latter stages of the vaporization process that is distinctly different from that observed in round tubes. In particular, because of the spatial nonuniformity of the flow field, dryout at first occurs preferentially at a few locations on the passage wall. Dry regions generally get larger with increasing downstream distance until the wall is entirely dry. To accurately predict the heat transfer for such circumstances, heat exchanger designers must be able to reliably predict the conditions at which the onset of partial dryout occurs and the variation of the heat transfer coefficient as the wall becomes progressively drier. In this paper, the results of recent experimental studies are summarized which document the dryout characteristics of two idealized enhanced surfaces: a cross-ribbed passage and a large-scale offset fin surface. R113 was used for the working fluid over the following ranges of flow conditions: 1≤ p ≤ 1.5 atm., 30 ≤ G ≤ 115 kg·m^-2·s^-1, 10 ≤ q″ ≤ 89 kW·m^-2. Data for these two passage types is presented from which the conditions for the onset of dryout for vertical flow boiling can be inferred. The physical mechanisms affecting the onset of dryout are discussed, and a methodology for predicting the onset of dryout is explored. Heat transfer data in the post-dryout regime is also presented and discussed for these two model surfaces. The trends in these data are compared with data for round tubes under similar conditions and previously published results for other enhanced surfaces. Methods for predicting heat transfer in the post dryout regime are also examined. The implications of enhanced surface dryout characteristics for the design of automotive A/C evaporators is discussed in some detail.