Liquid-to-particle heat transfer in continuous tube flow: Comparison between experimental techniques

The liquid-to-particle convective heat transfer coefficient (h(fp)) is a critical parameter in the design of safe continuous thermal processes for particulate foods. Three methods of measuring h(fp) values during continuous tube how were compared: moving thermocouple, liquid crystal, and relative velocity. A model error analysis of the heat transfer methods (moving thermocouple and liquid crystal) indicated that measurement of the temperature and particle velocity were important sources of error. For the relative velocity method, the measurement of relative velocity, the rheological properties of the carrier medium, and thermal conductivity of the fluid were found to be most important. The liquid crystal method yielded the highest values of all techniques, largely because of the freedom of rotation allowed to particles under test conditions. The moving thermocouple and relative velocity methods yielded results that were in agreement with each other, the principal reason being the restriction of rotation in the former, and neglect of rotation in the latter. The moving thermocouple approach can be used for conservative but realistic estimation of h(fp) in continuous flow through holding tubes. The relative velocity method does not stand on its own, but can be used to provide a measure of verification for the other methods.