Prediction of heat transfer deterioration due to buoyancy

Recently, a so-called apparent Reynolds number (ARN) approach has been proposed to explain the laminarisation of an isothermal turbulent flow caused by an idealised non-uniform body force (He et al., 2016). This concept was further applied to heated flows with a strong influence of buoyancy, for example, an upward pipe flow of air (Marensi et al., 2021) or supercritical CO2 (He et al., 2021). In both cases, turbulent characteristics during the laminarisation process are well explained using the ARN approach. In the present study, this ARN approach, previously used to explain turbulence, is extended to describe heat transfer deterioration. The prediction is based on the determination of the eddy viscosity of a simple reference flow defined by the apparent Reynolds number. The new method has been demonstrated to predict heat transfer deterioration very well in comparison with DNS data. This ARN concept greatly simplifies the calculation of heat transfer in mixed convection flows and offers a better explanation of heat transfer deterioration mechanisms . • New modelling method has been developed for mixed convection using Apparent Reynolds number approach. • The new model prediction of heat transfer deterioration due to buoyancy agrees well with DNS. • The new modelling method is simple and easy to implement.