Determination and analysis of convective heat transfer rates during immersion frying

The first objective of this dissertation was to develop a laboratory method for the measurement of the convective heat transfer coefficient during immersion frying. The method developed was applied to the immersion frying of potato cylinders at an oil temperature of 180°C. The convective heat transfer coefficient was initially 300 W/m2K, it increased sharply to 1100 W/m2K, and gradually decreased to 300 W/m 2K over the duration of the process. Use of this new method allowed study of the effects of oil temperature, oil quality, and interfacial tension on heat transfer during immersion frying.;The relationship between oil temperature, convective heat transfer coefficient, and heat flux during immersion frying was investigated. Potato cylinders were fried in 120, 150, and 180°C oil. The maximum convective heat transfer coefficient and heat flux reached during frying increases as oil temperature increases. The process time to reach these maxima decreased as oil temperature increased.;The relationship between oil quality and heat transfer was studied by degrading canola oil to three quality levels (new, mid and old). Heat flux, convective heat transfer coefficient, oil viscosity, and interfacial tension at an oil/water interface was measured at each oil quality level. As oil was degraded from new to mid a large decrease in interfacial tension with a small increase in oil viscosity resulted in an increase in heat flux and convective heat transfer coefficient. Further degradation of the oil from mid to old level led to a decrease in heat flux and convective heat transfer coefficient. This decrease from the mid level was caused by a continued increase in oil viscosity while interfacial tension remained constant. These findings aided in further understanding the role of surfactants, and oil quality in general, in heat transfer during immersion frying.;The role of surfactant concentration on interfacial tension and its effect on heat transfer rates during immersion frying were studied. A surfactant, glycerol monostearate, was added to canola oil to decrease interfacial tension while holding oil viscosity constant. Interfacial tension at an oil/water interface and heat flux during the boiling phase of immersion frying was measured as a function of surfactant concentration. Interfacial tension decreased as surfactant concentration increased. Interfacial tension became independent of surfactant concentration at a critical concentration. Heat flux increased to a maximum as surfactant concentration was increased to 0.2%. Further increases in surfactant concentration lowered heat flux from its maximum value of 34,600 W/m2. Heat flux was therefore shown to be a function of surfactant concentration. It is theorized that surfactant concentration influenced heat flux by influencing the vapor bubble size, frequency, and resistance to coalescence at departure from the oil/product interface. (Abstract shortened by UMI.).