Structure of turbulent non-isothermal flow in a pipe with a sudden expansion

The article studies a mathematical model of turbulent non-isothermal flow of non-Newtonian fluid. At the inlet, the fluid is Newtonian and, due to a decrease in temperature, it becomes non-Newtonian due to increased viscosity and yield strength. The system of turbulent motion and heat transfer equations is solved by the numerical control volume method in variables of the velocity and pressure components. The calculations yielded average and pulsation characteristics of the non-isothermal motion of non-Newtonian fluid in a pipe with sudden expansion. The calculations show a sharp reduction in the structure of the recirculation zone and a decrease in its parameters with an increase in the Bingham number Bn. In this zone, the maximum negative value of the average velocity, equal to‒Umax/Um1 ≈ 0.2 for a Newtonian fluid, decreases to ‒Umax/Um1 ≈ 0.1 at the Bingham number Bn = 17. A decrease in the turbulent characteristics of the non-Newtonian fluid flow is also observed with an increase in the Bingham number. Heat exchange characteristics in the flow region of turbulent non-Newtonian and Newtonian fluids are qualitatively similar. The location of the flow attachment and maximum heat exchange of non-Newtonian fluid does not exceed 10%. The length of the recirculation zone of viscoplastic fluid is shorter by up to 66% compared to Newtonian fluid.