Numerical simulation of the heat transfer process of a coiled tube for viscous fluids

This study presents the results of a numerical simulation in a helical coiled tube for Newtonian and non-Newtonian fluids in laminar regime. As a practical contribution, the numerical simulations use non-Newtonian fluids, the rheological properties of which are experimentally measured.In both non-Newtonian fluids, at the helical section, the heat transfer rate was higher than that at the straight section. For the fruit juice, the heat transfer coefficient (h(w)) reached values of 1000 and 1350 W/m(2) K at the coil, which were 73% and 126% higher than in the straight section, for an Re-g of 255 and 634, respectively. For the carboxyl methyl cellulose solution, these differences ranged between 76% (Re-g = 255) and 210% (Re-g = 634), reaching h(w) values at the coil of 1950 and 2700 W/m(2) K.In all non-Newtonian cases, fluid mixing was improved at the coil section. This influenced the viscosity variability along the coil, especially for the fruit juice, with a pronounced pseudoplastic behaviour (flow behaviour index of 0.5). This viscosity variation was more influenced by the strain rate than the viscosity change with temperature, as the fruit juice showed a moderate energy activation (8.2 kJ/mol).