Numerical study of buoyant flows in straight and converging chimneys

Abstract

We numerically investigate the buoyancy-induced laminar flow in an adiabatic vertical chimney with straight and converging walls. The study is done with air as the fluid for different radii of the chimney, R = 15–50 mm, aspect ratios, L/R = 5–25, and inlet temperature, Ti = 350–550 K. In addition to these parameters, the converging chimney analysis are performed with different converging angles, θ = 1–5 degrees. The commercial CFD code Fluent is used to solve the equations governing the flow taking temperature effects on fluid properties into consideration. The effect of the above parameters on the buoyancy-induced mass flow rate has been presented. We see that the mass flow rate increases with an increase in Ti due to increased buoyancy with both straight and converging chimneys. With an increase in L/R, the mass flow rate increases for straight chimney and this is due to the frictional pressure drop characteristics of the chimney. On the other hand, the converging chimney shows a varying trend with L/R depending on the converging angle: with the increase in L/R the mass flow rate initially increases and then decreases for lower θ (<1o) and continuously decreases for higher θ (>1 o). The varying combined effect of the frictional and acceleration pressure drop characteristics of the chimney causes this change in trend. For the given inlet conditions and aspect ratio, the straight chimney would drive the highest mass flow rate. Based on the simulated data, we present correlations to predict the induced mass flow rate. For generality, we present correlations for the non-dimensional mass flow rate in terms of the relevant non-dimensional parameters i.e., the modified Gr number, the temperature ratio, and the area ratio. The correlations we present are applicable over a range that can be useful in designing chimneys for bio-mass gasification, household utilities, and medium-scale industries.