Surface heat transfer coefficients to stationary spherical particles in an experimental unit for hydrofluidisation freezing of individual foods

Convection heat transfer to spherical particles inside a hydrofluidisation freezing unit was investigated. The unit contained a food tank with a perforated bottom plate to create agitating jets. An aqueous solution of 30% ethanol+20% glucose was used as the refrigeration medium in a temperature range of −20 to 0 °C and flow rates from 5 to 15 l min−1. The lumped capacitance method was applied on cooling profiles of aluminium spheres of 5–50 mm to obtain surface heat transfer coefficients. Coefficients were within a range of 154–1548 W m−2 °C−1, and depended on diameter, flow rate, refrigeration temperature and fluid agitation level. The agitation due to jets was accounted for by means of an agitation Reynolds number in a Nusselt correlation A large variability of measured surface heat transfer coefficients was observed. This could be attributed to non-constant flow and turbulence fields in the refrigeration medium. The value of the heat transfer coefficient was compared to values determined on strawberries.