Measurement of the effective thermal conductivity of particulate materials by the steady-state heat flow method in a cuvette

Abstract

To measure the thermal conductivity of particle beds, a specially designed cuvette is inserted into the chamber of an ITP-MG4 device fitted with a vertical heat flux sensor. The cuvette with a transparent wall makes it possible to reduce the amount of test material to 25 cm3, to monitor visually the uniformity of a charge, to determine the bulk density of the particle bed (and to increase it if necessary using vibrocompaction) and to apply external pressure to the bed from 2.5 to 30 kPa. Using various continuous-solid and particulate materials as references, a calibration equation is obtained for thermal conductivity in the range of 0.03–1.1 W (m K)−1. To eliminate thermal contact resistance when measuring references, the end faces of glass specimens with a departure from flatness of up to 50 μm are wetted with water. To model the calibration, a calculation is carried out by the electrical circuit analogy. The calculated curve is close to the experimental points if a value for the contact thermal resistances r# = 2 × 10−3 m2 K W−1 is taken. Values of r# calculated by the Yovanovich model, based on the known roughnesses of the contact surfaces of the cuvette and the solid specimens, are an order of magnitude lower due to the decisive influence of nonflatness and not surface roughness at the low pressures used. The conditions under which our measurements were made are compared with the instructions of Russian, American and international standards for the measurement of thermal conductivity by the steady-state heat flow method (specimen size, flatness of working surfaces, etc). The sources of measurement inaccuracy and ways to improve the technique are examined.