NUMERICAL MODELLING OF A SCRAPED SURFACE HEAT EXCHANGERDESIGNED FOR HIGHLY VISCOUS FLUIDS

Scraped Surface Heat Exchangers (SSHEs) are widely used in the process industry especially when highly viscous fluids have to be treated. In these heat exchangers the fluid to be heated/cooled flows axially in an annular section between a stationary outer cylinder and a powered coaxial rotor. The inner wall of the outer cylinder is scraped periodically by blades attached to the rotor. The augmentation of the heat transfer coefficient is due both to the disruption of boundary layer and to the radial mixing of the fluid. Although SSHE are frequently used in several industrial applications, the investigations available in literature about this argument are rare, especially when the laminar flow regime is encountered. Moreover, due to the specificity of each product, it is difficult to generalize the few data available, by making the thermal design of these apparatuses a critical point. The present numerical investigation is aimed to study the heat transfer performance of a concentric SSHE, especially planned for highly viscous fluid foods. A two dimensional model of the heat exchanger is considered and the PDEs governing both momentum and energy transfer in the system are integrated numerically by adopting the finite element method within Comsol Multiphysics® environment. The objective is to investigate the effect of the rotational speed of the blades on the heat transfer performance of the apparatus. The comparison of the numerical results to experimental data available in literature enables to discuss the appropriateness of the two-dimensional approach in describing the behaviour of SSHEs.