On heat transfer in gas-solid pipe flows: Effects of collision induced alterations of the flow dynamics

An Eulerian-Lagrangian approach is used to model turbulent gas-solid flows in heated pipes. The key problems of dispersion, turbulence modulation and particle collisions are addressed by means of a complete four-way description. In particular, special care has been brought to the effects of both particle-particle and particle-wall collisions, which are known to be important on the dynamical point of view. Dilute or moderately dense suspensions are considered (loading ratio up to 10). Comparison of the numerical results with available experimental data about suspension Nusselt numbers leads to satisfactory agreement in vertical pipe flows and qualitative accordance in the horizontal case. It is confirmed that the collision treatment is a very crucial and sensitive aspect of the global model, in so far that it strongly affects the flow dynamics. The collision induced alteration of the velocity and concentration profiles leads to significant changes in the heat exchange rate. Since the question may arise of possible effect of conductive heat transfer during collisions, this point has been examined too, confirming that the direct thermal exchanges during solid-solid contacts are negligible under the present conditions.