A COMPARISON OF CRITICAL HEAT-FLUX IN TUBES AND BILATERALLY HEATED ANNULI

This paper examines the critical heat flux (CHF) behaviour for annular flow in bilaterally heated annuli and compares it with that in tubes and unilaterally heated annuli. It was found that the differences in C HF between bilaterally and unilaterally heated annuli or tubes strongl y depend on pressure and quality. The CHF in bilaterally heated annuli can be predicted by tube CHF prediction methods for the simultaneous CHF occurrence at both surfaces, and the following flow conditions: pr essure 7-10 MPa, mass flux 0.5-4.0 mg m(-2) s(-1) and critical quality 0.23-0.9. The effect on CHF of the outer-to-inner surface heat flux r atio, was also examined. The prediction of CHF for bilaterally heated annuli was based on the droplet-diffusion model proposed by Kirillov a nd Smogalev (1969, 1972). While their model refers only to CHF occurre nce at the inner surface, we extended it to cases where CHF occurs at the outer surface, and simultaneously at both surfaces, thus covering all cases of CHF occurrence in bilaterally heated annuli. From the ann uli CHF data of Becker and Letzter (1975), we derived empirical functi ons required by the model. The proposed equations provide good accurac y for the CHF data used in this study. Moreover, the equations can pre dict conditions at which CHF occurs simultaneously at both surfaces. A lso, this method can be used for cases with only one heated surface. ( C) 1997 Elsevier Science S.A.