Contemporary correlations for heat and momentum transfer in irrigated packed beds applied to the design of cooling equipment for horticultural produce

If it is to remain fresh, horticultural produce such as fruits and vegetables must be cooled as soon after harvest as possible. An effective way of achieving this is to spray chilled water through beds of the produce, after which the water is treated and re-cycled around the system. Designers need guidelines on the effects of design parameters such as the flow rate and temperature of the chilled water, the depth of the beds of produce and the size of the produce on the operation of cooling systems. This three-phase system remains somewhat mathematically intractable on the length scales of the inter-produce pores. Hence, contemporary correlations of heat and momentum transfer in the pores of packed beds have been used in conjunction with a model of conduction heat transfer within the individual pieces of produce. This gives rise to a 'semi-continuum' model. Parameters used in the analysis include the fraction of water within the bed of produce, the degree of wetting of the produce, the produce/water heat transfer coefficient, and an expression has been invoked for the thermal dispersion within the bed. The governing equations for each phase are formulated and a scaling analysis is used to simplify them. Results from the analysis suggest that the conventionally used flow rate of water, namely 10 kg/(m2.s), appears to be technologically sound. At this flow rate the fraction of produce that is wetted exceeds 0.95, and the water/produce heat transfer coefficient is sufficiently high to not be the rate limiting step in cooling the produce. It is shown that the power consumption of hydrocoolers can be several hundred kilowatts, and that the chilled water must be recycled and treated to maintain its potability. These requirements provide opportunities for engineers to make contributions in an area usually dominated by biologists and chemists