Numerical study of the convective heat transfer coefficient for steel column surrounded by localized fires

Abstract

The convective heat transfer coefficient hc characterizes the heat transfer capacity between the fluid-solid interface in a thermal convection phenomenon. At present, values adopted from specifications are mostly used to define the hc coefficients in localized fire researches. In this paper, the specific scenario of the steel column surrounded by fire plume is selected as a typical case to explore the surface hc change rules of the steel column under the action of the fire source at the bottom. Different from the application of the concept of adiabatic surface temperature (AST) to achieve fluid-solid thermal coupling, this paper takes the scenario with the fire source heat release rate (HRR) being 81 kW as an example and conducts a two-way coupling simulation analysis of the fire and the thermal models by means of CFD codes. First, the validity of this simulation methodology is verified by comparing the results of the numerical model and the experiment. Following this, the distribution laws of the spatial velocity and temperature fields are analyzed, and the hc coefficient of the steel column surface is explored in detail using theoretical formula and traditional empirical formula respectively. It is noted that the hc results obtained based on theoretical formula can comprehensively consider the velocity boundary layer and temperature boundary layer situations above the column surface, which is a direct result of the definition of the convective heat transfer coefficient. It is found that the hc‾ is lower at the bottom of the column and increases with height, with relative maximum value in the lower and middle parts of the column, reaching around 40 W/m2 K. The hc‾ results decrease along the axial direction, with the value of upper part of the column being about 20 W/m2 K. In contrast, the hc‾ distributions along the height obtained by the empirical formulas are similar to the distribution of fire plume velocity along the height. In other words, it considers only the effect of the velocity boundary layer, while the effect of significant changes in fluid temperature on convective heat transfer behavior is less considered. Compared to the results based on the theoretical definition, the hc‾ results obtained by empirical formulas are significantly smaller in the lower and middle parts of the column, and higher at the top of the column. Finally, a new hc‾ practical formula for the column surfaces surrounded by fire plume is proposed, which agrees well with the results based on the theoretical definition in several cases.