(667d) Estimating Thermal Diffusivity of Bulk Solids

Conductive heat transfer in packed beds is of industrial interest for numerous processes. Packed beds are widely used in catalytic reactors, gas-cooled nuclear reactors, drying, mixing, adsorption processes, pebble bed heaters, high performance cryogenic insulations, absorbers, evaporators and thermal energy storage devices. The large surface area to total volume ratio in packed beds lends itself to excellent heat & mass transfer along with good mixing of the continuous phase. To operate packed bed based processes, one must have a fundamental understanding of packing structure, porosity distribution, fluid flow, pressure drop and heat transfer mechanisms.

For engineering purposes, it is desirable to represent the composite heterogeneous media by an equivalent (or effective) continuous and homogenous media. This approach is justifiable as long as the macroscopic scale of the physical process is significantly larger than the microscopic scale describing the structure of the composite media. Similar approach is taken to describe the thermal conductivity of a packed bed with stagnant fluid. The overall thermal conductivity of a packed bed can be lumped into a single representative value called effective thermal diffusivity.

In steady state methods, the packed bed of well defined simple geometry (cylindrical, cube or parallelepiped) is allowed to achieve steady state in heat flux and temperature gradient. By knowing the precise boundary conditions, temperature gradient and heat flux, and applying Fourier or Laplace equations, the value of thermal conductivity of packed bed can be estimated. The transient method extends the above analysis by imposing a sudden change in boundary condition, either as a step or sinusoidal input, and observing the dynamic thermal response of the packed bed. The thermal conductivity of the packed bed can be determined by fitting the experimental data to the predicted unsteady state thermal response.

In this work, we have measured the effective thermal diffusivity of packed beds (with stagnant fluids) using both steady state and transient methods, and compared the results with available correlations in the literature. Further, the effect of bed voidage has been investigated.