During the initial stage of biomass combustion (so-called devolatilization), biomass particles exchange mass and heat at the surface with surrounding environment; receiving heat from hot gas flow while releasing gas. At low Biot number (non-dimensional number representing the ratio of the heat transfer resistances inside of and at the surface of particle), the heat flux from gas flow to particles is governed by external heat transfer. It is common approach to utilize Rantz-Marshall correlation for Nusselt number of sphere particles in forced flow as Nu = 2 + 0.6*Re^(1/2)*Pr^(1/3) where Re expresses particle Reynolds number and Pr is Prandtl number. However, outgoing gas could affect Nusselt number by changing the conditions in boundary layer. Therefore, this study will investigate if and in what extent outgoing gas flow from sphere particles can affect Nusselt number. The goal of this project to propose the modified Nusselt number correlation that account for the effect of Stefan flow. The study will be carried out with resolved particle direct numerical simulation (RP-DNS) in OpenFOAM platform. To make the problem simple, we will impose constant or time-dependent Stefan flow while varying gas flow rate and turbulent intensity, particle size, and gas temperature. At the moment, the parameter range could be Re_p<20, d_p<2 mm, and T_g=900-1900 K. Verification of the model will be carried out by two methods. First, the results of the temperature field obtained in quiescent air will be comared to analytical solution. Second, we will analyze the experimental data and obtain drag coefficient and Nusselt number without Stefan flow, which will be compared with well-known correlation.
This is an initiation part of project "Chemical interaction of closely located reactive particles in gas flow", supported by Swedish Research Council (VR) during 2016-2019. After the success of this initiation project, we are planning to extend the simulation platform into reacting particle arrays to investigate the interaction among a number of closely located particles via volatile gases formed during devolatilization.