(A) study of critical heat flux at low pressure and low velocity conditions

Abstract

The critical heat flux(CHF) phenomenon has been investigated with primary emphasis on the stable flow of water in vertical round tubes at low pressure and low velocity (LPLV) conditions. A low pressure experimental loop was constructed and experiments have been perfomed for water flow in vertical round tubes with diameters of 0.006m and 0.0088m for low mass fluxes under atomspheric pressure. Experimental results show that (1) the behavior of CHF in a vertical round tube appears to be similar in LPLV conditions regardless of the flow direction, inlet subcooling and inlet throttling as far as stable flow condition is maintained, (2) the CHF at near zero flow is predicted well by the Wallis`` flooding correlation, (3) the CHF at LPLV conditions is an increasing function of mass flux, The physical mechansim leading to the CHF at LPLV conditions is discussed, and an expression for the flooding-limited CHF including the non-zero flow condition is derived. The dervied maximum mass flux for flooding and corresponding CHF agrees well to the experimental data. And a new design correlation is developed based on the combination of the present data and other CHF data availabel in the literature. Assessment shows reasonable agreement of predictions to experiments. In addition to the stduy of LPLV CHF, a new methodology for classification of CHF patterns is proposed based on the entropy minimax principle and applied to the Groeneveld et al.``s CHF look-up table. Resulted patterns and comparison with existing correlations show the applicability of this methodology.