Conductive and viscous sub-layers on forced convection and mechanism of critical heat flux during flow boiling of subcooled water in a platinum circular tube with 3 mm inner diameter and 32.7 mm heated length at high liquid Reynolds number

Abstract

The turbulent heat transfer, the subcooled boiling heat transfer and the steady state CHF for a Pt-circular test tube of a 3 mm inner diameter and a 32.7 mm heated length are measured with a wide range of inlet subcooling and flow velocity at high liquid Reynolds number, i.e. Red = 2.35 × 104 to 1.12 × 105. For flow velocities ranging from 4.103 to 21.446 m/s, the inner surface temperature of the Pt circular test tube calculated by the steady one-dimensional heat conduction equation is compared with the value obtained from our turbulent heat transfer correlation equation and the numerical solution of the RANS equation (Reynolds mean Navier–Stokes simulation) of the k-ε turbulence model. The conduction sub-layer thicknesses from the non-boiling region to CHF are measured in the conduction sub-layer itself in the forced convection region and the thinner sub-layer dissipated by boiling evaporation in the nucleate boiling region. The nondimensional thicknesses of local and average conductive sub-layers and, the thicknesses and nondimensional thicknesses of local and average viscous sub-layers on forced convection are estimated from the thicknesses of local and average conductive sub-layers and Prandtl numbers evaluated at the calculated temperature of the first control volume on the heated surface. In addition, the thickness of the conductive sublayer at the CHF point is estimated from measurements at various flow velocities. The experimental values of the CHF are also compared with authors’ widely and precisely predictable correlations of critical heat flux during flow boiling of subcooled water and the corresponding theoretical values of the liquid sub-layer dry-out models suggested by other researchers, respectively. The authors’ correlations and other researchers’ theoretical values can represent the subcooled boiling CHFs obtained in this study within the ranges of −22.74 to −6.21% difference and −22.17 to 6.16% one, respectively. Proposals for the main mechanism of critical heat flux during flow boiling of subcooled water on vertical tubes are confirmed again at high liquid Reynolds numbers based on experimental data. The boiling transitions to film boiling at the subcooled water flow boiling in the Pt test tube of d = 3 mm and L = 32.7 mm is not due to heterogeneous spontaneous nucleation or hydro-dynamic instability, but to liquid sub-layer dry-out model at the steady-state CHF. It is similar to those for the Pt test tubes with d = 3 mm and L = 66.5 mm, and d = 3 mm and L = 100 mm.