# Review of Convective Heat Transfer Modelling in CFD Simulations of Fire-Driven Flows
## 1 Introduction  
The paper reviews the most commonly used approaches for convective heat transfer modelling in the context of fires. Focus is given on numerical modelling using Computational Fluid Dynamics (CFD) with Large Eddy Simulations (LES) considering both pool fires and turbulent wall fires (i.e., involving inert and/or combustible solid surfaces). Progress in fire safety science, nowadays, strongly relies on the use of CFD for modelling scenarios that would be either too expensive to conduct experimentally (e.g., large-scale tests and/or performed repeatedly) or too complex, in terms of geometries, to analyse with analytical solutions and zone model approaches. Nevertheless, most real-life fire scenarios (e.g., involving industrial facilities or high-rise buildings) pose extreme challenges for CFD not only due to the different physical processes that need to be modelled (i.e., turbulence, combustion, radiation, soot, heat transfer, pyrolysis and flame spread) but also due to the wide range of length and times scales involved [1]. Boundary layer combustion [2] is a canonical configuration in fire research and applicable to various fire scenarios encountered in everyday life. A good understanding of the physics involved in such scenarios is essential and a pre-requisite for accurate and predictive fire modelling. Modelling of such scenarios (e.g., involving evaporation of liquid fuels and pyrolysis in the solid phase) can be quite challenging given the coupling between the gas and solid phases [3] which greatly affect the (convective and radiative) heat transfer to the fuel surface and the resulting fuel gasification rate [4]. The difficulties in convective heat transfer modelling lie in the fact that the convective transfer coefficient is dependant on various aspects including, e.g., the geometry of the problem (i.e., vertical, horizontal,inclined or complex surface), the type of scenario (i.e., involving natural, forced or mixed convection) as well as on the type of fluid involved (i.e., gas, liquid). Given the above mentioned aspects, no universal approach exists that can be used for modelling convective heat transfer within the context of fire scenarios. Within the context of boundary layer flows (i.e., momentum or buoyancy-driven), it is well accepted in literature that the grid sizes in the vicinity of surfaces must be millimetre-sized (i.e., values of \(y^{+}\approx 1\) for wall-resolved simulations) for accurate convective heat transfer modelling [5]. On top of that, there are additional challenges involved in such scenarios including that the flows have low-to-moderate Reynolds numbers, for which the theory for turbulent boundary layers does not strictly apply, and that multi-physics phenomena (i.e., turbulence, combustion, radiation and mass transpiration) require accurate near-surface modelling. Accurate modelling of the physical processes involved near the surface of pool fires or (flammable) walls is an active area of research in the literature [6]. When it comes to liquid pool fires, the influence of convection and radiation is often comparable for pool diameters up to \(D<0.3\) m [7,8]. The importance of convection decreases for \(0.3\) m \(<D<1.0\) m until radiation becomes the dominant mode of heat transfer, with respect to the heat feedback to the fuel surface, when \(D>1.0\) m [9] due to mass transpiration effects (i.e., blockage in the convective heat fluxes due to fuel vapours leaving from the pool surface) [10]. Convection can also be important in scenarios involving flame spread, more specifically in the downstream flaming zone as well as on the early stages when pre-heating of the flammable surface occurs [11]. Recently, heat transfer modelling was also shown to greatly affect the burning behaviour of flammable walls depending on the approach used to model convection [12]. Nevertheless, the influence that convective heat transfer modelling can have in flame spread scenarios has not yet been extensively reported in literature.