An experimental study of critical heat flux in very high heat flux subcooled boiling

Abstract

An experimental study of forced convective subcooled boiling heat transfer to water was performed at heat fluxes that ranged beyond 10 8 W m −2. One of the objectives of this study was to obtain predictive ability for the critical heat flux (CHF) at high heat fluxes. Experiments were performed with metallic tubes having inside diameters ranging from 0.3 to 2.7 mm. Mass fluxes ranged from 5000 to 40 000 kg m −2s −1, and exit subcoolings from 40 to 135°C. Exit pressures ranged from 0.2 to 2.2 MPa, and length-to-diameter ratios ranged from 2.0 to 50.0. Single-phase and two-phase pressure drop and heat transfer data were shown to be reasonably well predicted by existing correlations. Over 200 CHF stable data points were obtained. CHF was shown to be an increasing function of both mass flux and subcooling, and an inverse function of diameter. CHF increased for length-to-diameter ratios less than 10, and decreased with increasing exit pressure. Unreasonably low CHF values were obtained for several series of tests; these premature failures are believed to be the result of thermal-hydraulic or nucleation instabilities. A ‘high flux’ CHF data base containing over 700 data points was compiled, and a new statistical correlation was developed. This correlation was tested against a recently published data base, with an average deviation of 19.4%.