Geometry dependence of heating in a U-tube heat exchanger for pasteurization

Abstract

A serpentine tube heat exchanger is physically constrained by its bend radius. The U-tube bend section of a tubular heat exchanger is thus interesting broadly in terms of fluid flow and heat transfer. This paper presents the tube size dependence of heating profile of this U-tube as is commonly encountered in the design of pasteurizers. Analytical and computational modeling were employed in investigating the heat capacity, heat flux, heating rate and energy use of a U-tube at increasing diameters 9.52, 12.70, 19.05 and 25.40 mm, each at constant 1 mm thickness. SolidWorks 2016 was used in geometric modeling while Ansys v16 was used for transient thermal computational fluid dynamics simulation of the conduit (SS316L) and the product (coconut water). The initial conditions were 5,000 W/(m²-K) convective heat supply at 95 and 92 °C surface temperature for the conduit and the product, respectively. Both the tube and the product were initially at 30 °C. The tube, whose heating profile was independent of size, reached 90 °C in 2.1 s. For the product, as the tube size increases, the heat capacity increases exponentially (Q_p = 108.3e^0.1831x), the heat flux drops down 57% within 40 s, while both the heating rate (T_{t =60 s} = -48ln(x) + 183.65) and the energy use (Q_u = -29.65lnx + 235.36) drops logarithmically. These results are beneficial to designers and engineers in sizing of heaters, minimizing fouling and optimizing energy efficiency as well as pasteurizer processing capacity.