Heat transfer during thermal processing of a temperature dependent non-Newtonian fluid in a tubular heat exchanger

Abstract

Heat transfer is highly dependent upon fluid properties and operating conditions for a particular heat exchanger. Determination of heat transfer coefficients in aseptic processing of a temperature dependent non-Newtonian fluid (banana puree) as a function of steam temperature, flow rate and heat exchanger's length/diameter ratio is essential for modeling the thermal process. A commercial acidified aseptic banana puree with an average pH of 4.49 and average soluble solids concentration of 22.1 °Brix was processed in a tubular heat exchanger with two heating sections. Three flow rates (2.5 × 10 −5, 3.7 × 10 −5 and 4.7 × 10 −5 m 3 s −1), three steam temperatures (110.0, 121.1 and 132.2 °C) and two length ( L)/diameter ( D) ratios (250 and 500) were studied. Density, thermal conductivity and specific heat capacity were considered constant and heat transfer coefficients were calculated from the temperature data obtained. For an L/ D ratio of 500, heat transfer coefficients varied from 654.8 to 842.2 W m −2 K −1, while for an L/ D ratio of 250 heat transfer coefficients varied from 735.5 to 1070.4 W m −2 K −1. An empirical correlation was proposed and verified, which explained the experimental data within 10% error.