New techniques for measuring thermal properties and surface heat transfer applied to food freezing

Abstract

This thesis presents two different works. The first part introduces a thermal multimeter which measures heat capacity, thermal conductivity and density. The instrument gives continuous measurement data within a temperature range. With some exceptions this also holds for the prototype of a thermal multimeter which is built and tested. The measuring method is constant heating of one side of a slab. The slab is insulated on all other sides. After some time there will be equilibrium where there is a constant temperature difference over the slab. The thermal conductivity can be calculated from this temperature difference. The heat capacity can be calculated from how fast the temperature rises. Measurements of the slab thickness give density as function of temperature.

The second part discusses a practical method for measuring the heat transfer coefficient (α). The method is based on shell freezing of clear jelly which has the same shape as the product of interest. Transparent jelly is transparent before it freezes and white when frozen. If the sample is removed from the freezer and cut through before it is completely frozen thefreezing front is distinct and the thickness of the frozen layer can be measured. By measuring time the jelly sample was in the freezer and thicknessof the frozen layer the heat transfer coefficient can be calculated by using Plank's equation. The method is suitable for measuring local α because it can be shown that tangential heat flow can be neglected when the frozen layer is thin.

Computer simulations, automated data acquisition and data processing are a considerable part of this thesis, even though it is not obvious from the results presented. There are more lines in the data code written to obtain the results presented here then the number of lines in this thesis. The size of selected simulation results and processed data from the measurements are 6.3 GB.