Analytical study of critical heat flux in natural convective boiling in a vertical tube

Abstract

An analytical study has been made for the prediction of Critical Heat Flux (CHF) of natural convective boiling in a vertical uniformly heated tube, submerged in a saturated liquid bath by a flow model in which the CHF has treated to be occurred under annular co-current flow of liquid and vapor. Mass, momentum and energy balances for the two-phase (liquid-vapor) flow in a vertical tube are used to construct the flow model together with the criterion that the CHF is assumed to be taken place at the exit of the heated tube when void fraction is unity and mass flux is maximum. CHF data are calculated for saturated Water, RIB and RI2 at different pressures ofP = 0.1 to 3.0 MPa for the tube diameter ofD = 1.12 to 18.4 mm, and the tube heating length of Lhe = 23 to 960 mm. The effects of the pressure which corresponds to the density ratio P,/PI ranging from 0.622x I0" to 0.2095 and of the ratio of LhJD = 5 to 857 on the CHF, are mainly discussed. The characteristics of the CHF are satisfactorily explained against different parameters. About 300 predicted CHF data are compared with available experimental data to verify the basic performance of the model. The comparison shows that the predicted results by the present model agree with the experimental data within '" 50% for the Lh,/D > 20.