Heat and Mass Transfer Investigations of Relevance for Food Engineering Applications

This thesis concerns two interrelated subjects of heat and mass transfer processes. The first one concerns mainly experimental measurements, with a subsequent development of a calculation procedure, of the thermal and hydraulic performances, experimental methods and evaluation procedures applied on various plate heat exchangers (PHEs) as evaporators with the refrigerant ammonia as working medium. The second subject concerns the complex physical processes of heat and mass transport during thermal processing of beef hamburgers in a double-sided contact fryer. A mathematical model has been developed and numerical simulations have been carried out for a variety of cases. Successful measurements were performed for a semi-welded PHE and on various versions of the nickel brazed PHE in a so-called direct expansion cooling unit with counter-current vertical upward evaporation of ammonia. The measured data was used in the analysis of the heat and mass transfer for the small cooling unit. Methods for the development of mathematical expressions for the thermophysical properties, heterogeneous void fraction model and all the various pressure drops in a PHE were investigated. The mathematical expressions were adapted, discretised and subsequently included in an in-house developed numerical calculation program for evaluation. A finite difference technique for the steady state problem, based on central difference approximations, was applied. The Gauss?Seidel method found the solution iteratively. With an estimation of the required heat flux for the superheating of the ammonia vapour and by applying a set of governing heat balance equations, a multiple linear regression analysis generated three Nusselt number correlations for the different PHEs. These correlations can predict the measured Nusselt number with sufficient accuracy. The three resulting values for the C-coefficient (in the correlation of the two-phase multiplier) are for higher Reynolds numbers somewhat higher, than those reported in other investigations. An inlet flow distributor in the semi-welded heat exchanger enabled a higher thermal performance. A theoretical model was formulated based on the fundamental governing equations for the coupled heat and mass transfer of both frozen and non-frozen beef hamburgers heated uniformly in a double-sided contact fryer. A literature study provided information on the thermophysical properties of food ingredients and general understanding of the problems associated with the heat and mass transfer process in whole and minced meat products. The different variables that are influencing the heat and mass transfer in a meat patty during a cooking process, e.g., mass fraction of ice, initial freezing/thawing temperature, amount of so-called bound water and salt content were also investigated. Basic heat and mass transfer equations are used and modified to suit the specific conditions in foods. The continuous tracking of the interface, temperature profile in the crust, differences in water, fat and protein contents are accounted for in the model. The in-house developed computer program utilizes a stable and implicit finite difference method. The model was compared with experimental data from the literature and recent experimental measurements. It was able to predict the development of the centre temperature profile for both frozen and non-frozen hamburger patties composed of different types of beef meats and with varying water, fat and protein contents with sufficient accuracy. It was also found that the experience from theoretical and engineering methods in the discipline of heat and mass transfer in plate heat exchangers was beneficial in the development of a simulation tool for the thermal processing of foods, using conventional fundamental governing equations