Heat transfer performance of fruit juice in a heat exchanger tube using numerical simulations

Abstract

Enhancing heat transfer rates in heat exchangers is essential in many applications, such as in the food industry. Most fluids used in the food industry are non-Newtonian, whose viscosity is not uniform, and depends on the shear rate and temperature gradient. This is important in the selection of equipment and type of processing. The aim of this work was to numerically simulate, with a non-Newtonian fluid in laminar regime, the heat transfer process in a tube with a curved elbow. The numerical model was validated with published correlations using water as heat transfer fluid. A commercially available fruit juice was used as a non-Newtonian fluid. Its rheological properties were measured using a Modular Compact Rheometer, as well as the activation energy. The difference between outlet temperature and inlet temperature was higher for the laminar simulation (approximately 4 °C) than for the turbulent one (approximately 0.7 °C). The highest dynamic viscosity values were found at the centre of the pipe (between 0.05 and 0.09 Pas), with the lowest values at the wall (0.0076 Pas). This behaviour is explained by the pseudoplastic condition of the fruit juice. The activation energy did not yield high values, showing a moderate viscosity variation with the temperature change.