Numerical Simulation of the Grille-Type Gas Flow Meter in a Short Duct

Abstract

Based on Bernoulli law, a average flow meter for short duct is usually designed with a rectifying grille, a series of sectional distributed pressure points and a large pressure-averaging vessel. As rectification is one of the key characteristics of this method, the effect of grille on rectifying is numerically simulated. The predicted results show that the grille in the 800mm×500mm section is very effective on regulating the originally disordered flow in the wind box branch duct. The velocity distribution tends to be uniform when the arrayed pitch is decreased while the rectifying effect is not closely associated with the relative location of the grille. The grille thickness plays a determining role as the rectifying becomes gradually weakened when the thickness is over 200mm. When a 5×4 grille with a thickness of 200mm is mounted at 200mm from the main duct outlet horizontally in downstream duct of the main wind box, the averaged pressure of each subsection is in good linear relation with the average gas velocity in the branch duct. The linear relationship is well maintained when the static pressure is measured at specific position and the dynamic pressure point is installed with error of 4o. This indicates that the grille-type flow meter is very reliable on measuring average gas velocity in short ducts. The application of this method on a 200MW industrial boiler shows that it possess promising potentials on optimizing combustion and emission, which could bring admirable environmental, economic and social benefits.