Experimental determination of effective thermal conductivity of granular material by using a cylindrical heat exchanger

Granular material in general has lower thermal conductivity than solid material. This is due to the limited contact between particles and the presence of air gaps. In the present study, a cylindrical heat exchanger is utilized to obtain temperature versus time response at the central location for a step change in wall temperature. Steel balls in spherical form are studied for estimation of effective thermal conductivity. Particle sizes studied are 12 mm, 8 mm, 4 mm, 3 mm, 2 mm and 1 mm. It is anticipated that a considerable variation in thermal conductivity would be obtained over this size range of particles. The governing equation for unsteady heat conduction in cylindrical co-ordinates incorporates the thermal diffusivity as a parameter. Hence, an analytical solution to the temperature dynamics is obtained by guessing the value of thermal diffusivity and it is used as predicted profile. The guess value of thermal diffusivity is varied and the standard deviation of error between experimental and predicted temperature profiles is minimized to find the optimum thermal diffusivity value. Later, the thermal conductivity of granular material is calculated using the definition of thermal diffusivity which involves density and specific heat capacity also. The overall temperature-time profile in dimensionless form is again compared to evaluate the deviations if any. The present results of effective thermal conductivity are also compared with prediction by Bruggeman's equation for granular material. (C) 2014 Elsevier Ltd. All rights reserved.