CFD Simulation and Analysis of Thermal Diffuser for High Temperature Flue Gases

Experience with vane-less diffuser has shown a large setback in the diffusion process of high temperature working fluids. Without distribution vanes, a thermal jet like flow is created, while the aim of the diffuser is to produce uniform distribution at the exit of the diffuser. The need of guide vanes in a diffuser is an idea born through an experimental problem faced when passing a turbulent high temperature flue-gas through a large angle diffuser. It was expected to have approximate uniform discharge at the exit of the diffuser but was found to create a jet diffused at the rate of 1:2, of inlet to exit, thus the high temperature flue gas concentrated at the centre of the exit thereby affecting the insulation and the experimental result. To solve this problem, a CFD simulation was employed to analyze the process and redesign the system for improved performance. GAMBIT software was used to model the diffuser in two-dimension and FLUENT software was used to simulate the 2-D fluid flow and thermal field, using different arrangement of guide vanes as distributors. The CFD simulation without, and then with guide vane to create uniform flow at the exit of a short diffuser with diverging angle of 57.72o from centre line (diffuser angle, α = 115.44o), inlet to exit aspect ratio of 1:20 and inlet to exit surface area ratio of 1:25 is analyzed in this work. Different guide vanes shapes where employed such as sharp/straight shapes, oval shapes and airfoil shapes. It was found that the airfoil shaped guide vanes offered the best result with most uniform distribution at the exit of the diffuser.