Evaluation of Reflux Condensation Heat Transfer of Steam-Air Mixtures under Gas-Liquid Countercurrent Flow in a Vertical Tube

Abstract

Reflux condensation in steam generator (SG) is one of the major heat removal mechanisms in a loss of residual heat removal (RHR) system event in mid-loop operation during a pressurized water reactor (PWR) plant outage. In order to evaluate the effectiveness of reflux condensation in SG U-tubes, the condensation heat transfer characteristics in the presence of a non-condensable gas must be clarified. Local temperature data were previously measured for steam-air mixtures under gas-liquid countercurrent flow, in a vertical tube of inner diameter 19.3 mm. In this study, local reflux heat transfer coefficients were calculated by evaluating steam flow rate profile along the tube at low heat fluxes assuming saturated steam conditions and empirical correlations were derived. The correlations are valid over a range 2-9,000 W/m 2 ÁK for 0.1-0.4 MPa, 0.014-0.2 air mass fraction. The axial distributions of the steam-air mixture temperatures calculated using the correlation agreed well with the measured results and validity of evaluation methods and the correlation were verified.

Reflux Condensation Heat Transfer of Steam-Air Mixture under Turbulent Flow Conditions in a Vertical Tube

“…Using the energy balance equation of Eqs. (8) and 12, where the liquid film subccoling is neglected, the liquid-gas interface temperature can be calculated.…”

“…Nagae et al 8) used Eq. 3considering the heat transfer coefficient of the liquid film, h f , and the heat transfer coefficient at the gas-liquid interface, h i , separately for the condensation heat transfer coefficient, h c , because the components reflect different heat transfer mechanisms.…”

“…Equation 18overestimates the heat transfer coefficient at a low film Reynolds number because the compensation value of 1.28 is for the turbulent liquid film, which is discussed later in detail. Nagae et al 8) derived a non-dimensional Nusselt number for the interfacial heat transfer coefficient using the data by Vierow et al 7) Nu where steam represents the thermal conductivity of steam, P air is the partial pressure of air, Re steam is the Reynolds number of steam, j steam is the superficial velocity of steam, and steam is the steam kinematic viscosity. Equation 20is intended for steam-air mixtures under laminar flow conditions and its applicability for turbulent flows is preliminary.…”

“…2. The temperature distribution was calculated with the correlation in RELAP5/ SCDAPSIM/MOD3.2, 9) Moon et al 5) and Nagae et al 8) The correlation in RELAP5/SCDAPSIM/MOD3.2 underestimates the condensation heat transfer and overestimates the steam-air mixture temperature. Moon et al did not measure the heat transfer coefficient in low-temperature range.…”

“…Therefore, the authors conducted experiments for reflux condensation heat transfer of steam-air mixtures under countercurrent flow in a vertical tube having an inside diameter of 19.3 mm and with a pressure range from 0.1 to 0.4 MPa 7) and obtained empirical correlations for the condensation heat transfer coefficients under laminar and turbulent flow conditions. 8) However improvement of the empirical correlation for turbulent flow conditions was required because the database under turbulent flow conditions was insufficient to enable a robust correlation.…”