Study on Convective Heat Transfer Coefficient in the Side Boundary Layer Under the Action of Tunnel Fire Smoke Movement

Abstract

During a tunnel fire, the tunnel’s lining and internal structure suffer significant thermal damage, necessitating an investigation into the heat transfer patterns on the tunnel wall surfaces during such events. Convective heat transfer at the side walls is a crucial aspect. Therefore, it is of great significance to study the longitudinal variation of convective heat transfer coefficient. Through numerical simulation, the thickness of velocity boundary layer is studied first, then the data is analyzed and the corresponding empirical equation is proposed. Based on this relationship, the momentum and energy equations are modified to obtain a Nu-Re correlation for heat release rate ranges of 5 to 20 MW. This correlation represents the relationship between convective heat transfer intensity and smoke flow patterns. Meanwhile, this paper employs the critical Reynolds number to delineate the two stages of convection heat transfer: the stage of rapid intensity reduction and the stage of stablilization. The value of critical Reynolds number increases with the rise in sidewall height. Consequently, the complete Nu-Re correlation is divided into two parts. For Reynolds numbers below the critical value, the Nusselt number (Nu) is correlated with the Reynolds number (Re) using theoretical results from the energy equation. Beyond this threshold, the relationship transitions to a linear one. The accuracy of this correlation model is verified through numerical simulations.Finally, a method for calculating the average Nusselt number is derived, confirming the feasibility of using Re-based dimensionless criteria in studying heat transfer during tunnel fires.