Computational Fluid Dynamics (CFD) Analysis of a Flow Straightener for a Heater Chimney

Abstract

This work presents a numerical analysis by means of computational fluid dynamics (CFD) of the flow within and outside a chimney of low length/diameter ratio with a lateral inlet for combustion gases. A partially blocked gases entrance and cyclonic flow present in the chimney in its actual configuration prevent the system to fulfill particulate flow control regulations, in particular concerning the velocity vector inclination in the control plane where monitoring is carried out. A flow straightener device is proposed which reduces, in the numerical simulations, the inclination of streamlines in the flow control plane from more than 35 degrees to less than 5 degrees, meeting the criteria stated in environmental regulations for particle emission control, without introducing excessive pressure losses in the flow. Results of the analysis include the flow configuration, velocity and pressure distributions and helicity distribution, the latter as a measure of the intensity of cyclonic flow. In this problem, gas enters the chimney from a lateral chamber. Numerical simulation of the present situation highlighted three main problems: - Cyclonic flow in the chimney, generating helycoidal streamlines where the velocity vector inclination is beyond acceptable limits, - Flow acceleration in the sector opposite to entrance, due to the effect of the lateral inlet and the deviation that gas suffers after impinging the wall, - Generation of a horizontal vortex in the pre-entrance chamber, due to obstructions in the inlet ducts. From this analysis, possible solutions were proposed and studied numerically. The one presented in this work, a simple straightener put immediately after the chimney entrance, plus the cleaning of all obstructed ducts, show in the results a significant improvement in the flow quality and alignement. The computational domain included the inlet chamber, the chimney and the atmosphere in a cylindrical region of approximately 55 chimney diameters in width and 8 chimney heights in height. The numerical simulation was performed with Ansys Research package. A pressure-based solver was used for compressible flow transient analysis, with second order discretization for space and time, and the “species transport” method was employed for computing the mixing of two species: combustion gas with known properties, and air. The variations with temperature of viscosity, thermal conductivity and specific heats for the gas were approximated with polynomial expressions interpolating known values. Gas density was computed from an equation of state. The k-epsilon turbulence model was used. The mesh was locally refined in order to achieve grid-independent results. The time step for convergence was 1e-5 s.