Prediction of the obstacle effect on film-boiling heat transfer

A correlation has been developed to account for the effect of obstacles (simulating the spacing devices in bundles) on heat transfer in dispersed-flow film boiling. The correlation is expressed as a modification factor to the reference geometry without any obstacles. The basic form of the correlation is an exponential decay function that resembles the diminishing effect on turbulence enhancement. The coefficients and constants in the correlation have been optimized with heat-transfer data of low-pressure single-phase (air) flow and high-pressure steam-water flow at film-boiling conditions. An experiment has been set up to obtain validation data with a heated tube of 4.1 mm inside diameter. HFC-134a was used as the working fluid. Three types of obstacles with the same blockage-area ratio of 37.8 % were tested. The results showed that the obstacles exhibited a strong enhancement effect on the film-boiling heat-transfer coefficient at locations downstream of the obstacles. A comparison between predictions of the correlation for the spacing-device effect and experimental data showed an underprediction of the heat-transfer rate at locations downstream of the obstacle. The underprediction is due mainly to the rewetting of the heated surface at the location of an obstacle, beyond which the developing film-boiling effect becomes dominant. The agreement between prediction and data is significantly improved after accounting for the critical heat flux (CHF) enhancement and developing film-boiling effects in the calculations. (C) 2004 Elsevier B.V. All rights reserved.