Fully Developed Heat Transfer to a Gaseous Suspension of Particles Flowing Turbulently in Ducts of Different Size

Abstract

This paper is a sequel to two earlier studies of friction (I)† and eddy diffusion (2) carried out for the same flow conditions as in the present study of heat transfer. Heat was transferred to an upward flowing suspension of 0-40 μ zinc particles in pipes of 1, 2 and 3 in bore. The solids/gas flow rate was in the range 0 < W_s/W _g < 17 and the pipe Reynolds number in the range 3·5 × 10⁴< Re < 10⁵.

It was found that the fractional increase in heat transfer coefficient due to the presence of solids was always less than the increase in the friction factor. Minimum values of both these parameters are often observed in the range 1 < W _s /W _q < 2·5. This paper gives further evidence that turbulence is suppressed by the particles, particularly when the duct is small and Re is large; in this case the wall Nusselt number, Nu _s , is markedly reduced below the value when gas flows alone. However, when the pipe is large and Re is low, Nu _s can be substantially higher than the value for gas alone. In this case the suspension is a superior coolant to the flow of gas alone.