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Double pipe heat exchangers are commonly used for heat transfer between two fluids. Sizing a heat exchanger depends on factors such as the heat transfer rate required, flow rates and temperatures of the fluids, and their physical properties. The heat transfer rate is usually expressed in terms of the heat transfer coefficient, a measure of the efficiency of heat transfer between the two fluids.

We recently studied the performance of a double pipe heat exchanger with a shear thinning, non-Newtonian fluid. When dealing with such fluids, their varying properties with shear rate and temperature can have a significant impact on the heat transfer rate in a heat exchanger, since their viscosity can vary with flow rate, time, and temperature.

To accurately predict the heat transfer rate in a heat exchanger using non-Newtonian fluids, the properties of the fluid must be fully characterized and non-dimensionalized (the process of removing units from a physical quantity to allow for comparison across different systems). Once this is completed, existing empirical correlations can be used to accurately predict heat transfer rates using for example, the well-known relationship between the Nusselt, Reynolds and Prandtl numbers, with key constants being determined from the experimental data. Their correlated values for turbulent flow inside a pipe (for Newtonian fluids) are shown in this graph. The Nusselt (h·d/k) and Reynolds (ρ·u·d/µ) numbers are calculated at the bulk flow conditions, using the pipe hydraulic diameter as the length scale. This equation is relevant for both the core and the annulus of the double pipe heat exchanger.

Our study concluded that non-Newtonian fluids obey this same relationship, using the same coefficients that have previously been developed for Newtonian fluids, provided that the viscosity of the non-Newtonian fluid is fully characterized with shear rate and temperature, and that the fluid properties are updated for each operating condition (flow rate and temperature).