Abstract:

In this work， two methods were adopted to calculate heat transfer coefficients at the internal surface of a cylindrical rotational mold in the heating process. In the first method， the test temperatures at the mold surface and inside were adopted to calculate heat transfer coefficients at the internal surface of the mold via a heat equilibrium equation for the internal space of the mold. In the second method， the formula for the heat transfer coefficients between the powder material in the rotary kiln and the kiln wall was used to calculate the coefficients at the internal surface of rotational mold. Both of the methods were used to calculate instantaneous and average heat transfer coefficients at the internal surface of the mold in the cases of no powders and powders of 0.21 kg loaded. The boundary layer theory of fluid mechanics was applied to explain the variation of heat transfer coefficients at the internal surface with time. The average heat transfer coefficients obtained by the first method were processed in the form of a dimensionless correlation of N_u with Pr （N_u= 45.73 P_r^-0.55）. The correlation holds for a cylindrical rotational mold with its axial length larger than inner diameter， which rotates along its central axis. The results indicated that the average heat transfer coefficients at the internal surface decreased with an increase of the volume percentage of powders inside the mold. The results calculated by the above two methods were found to be in good agreement with each other. The relative error of calculated results by the second method was less than 20 % as long as the volume percentage of powders was higher than 62.4 vol %.

Key words: rotational mold, heating process, heat transfer coefficient at internal surface, equation of heat equilibrium, dimensionless correlation