Heat transfer and rheology of stirred yoghurt during cooling in plate heat exchangers

In the present work an experimental investigation was conducted to obtain a correlation for the determination of convective heat transfer coefficients of stirred yoghurt in a plate heat exchanger. A rheological study was carried out in order to characterise the stirred yoghurt flow behaviour, evaluating its dependency both on shear rate and temperature. A shift in the temperature dependency was evidenced at 25 C. It is also shown that the material shows a complex flow behaviour, changing from a Bingham fluid to a power-law fluid at shear stresses in excess of approximately 6.7 Pa. As regards the heat transfer behaviour of the non-Newtonian stirred yoghurt a correlation for the convective heat transfer coefficient was obtained that reveals the large effects of the thermal entry length due to the high Prandtl numbers and to the short length of the plate heat exchanger. 2002 Elsevier Science Ltd. All rights reserved.