Experimental investigations of the post-CHF heat transfer of R-134a flow boiling in an annulus with spacer grids

An experimental study was performed using R-134a in an annulus channel to investigate the heat transfer characteristics in the post-CHF region. The experiments were conducted under the outlet pressures of 1.1-2.0 MPa, the mass fluxes of 100-400 kg/m(2)s and the inlet temperatures of 25-51 degrees C. Experiments were conducted in three cases of the annular channel test without spacers, the test with I-type spacer grids, and the test with split-swirl type spacer grids. The empirical correlation of the heat transfer in the post-CHF region was developed with 301 data point obtained in the annular test without spacer grids. The heat transfer coefficient was calculated based on the heater-rod temperature and the saturated vapor property. The average and root-mean-square (RMS) errors of the predictions were 0.17 and 3.4%, respectively. I-type spacer grids and split-swirl mixing vanes (with blockage-area ratios of 4.0 and 5.8%) were tested, which showed the enhancing effects on the heat transfer at the downstream location of the spacers and the results from the spilt-swirl-type mixing vane tests were more effective than the I-type grid tests. (C) 2010 Elsevier B.V. All rights reserved.