10. Heat Flux and Thermal Resistance

Heat flux is a major issue that must be considered for evacuation, fire spread and structure protection in tunnel fires. The three heat transfer mechanisms – convective, radiative and conductive heat transfer – are described with a focus on correlations related to tunnel fires. The Reynolds-Colburn analogy is introduced as a basis for the calculation of convective heat transfer. Characteristics of the absorbing, emitting and scattering gases are summarized, together with radiation between multiple surfaces. Analytical solutions for heat conduction into tunnel walls are summarized for different types of simplified boundary conditions. The overall heat transfer from flames and gases to the tunnel structure involves all three heat transfer mechanisms; their correlations are illustrated using an electrical circuit analogue. Simple models for calculating heat flux in small and large tunnel fires are presented with a focus on radiation. Correlations for incident heat flux are proposed and verified for small and large fires in tunnels, taking radiation from both flames and smoke into account. Different radiation models used for jet flames in an open environment are also discussed.