Optical investigation of a long throated Venturi conveying inert spherical particulate with size range similar to pulverised coal

摘要

This experimental and numerical calculation work demonstrates the behaviour of a two-phase gas-solid Venturi. The type of two-phase flow studied is that encountered in conveying coal to the furnaces in power stations - a light to moderate powder loading at near atmospheric pressure. A glass Venturi with a long throat (three throat diameters) is inserted into a pneumatic conveying test rig, which has similar pipe configuration, and powder and air dynamic similarity to a coal fired power station. The Reynolds number (Re) based on throat diameter is 460,000 and the particles have a size range up to 140 μm. Laser sheet visualisation and particle image velocimetry (PIV) reveal the behaviour of the fine powder in the Venturi. The PIV registers velocities less than the gas velocity in the throat with scatter in the order of 5% around the mean. There is a region next to the walls on the entire circumference in the throat section and diffuser which contains few particles. The particle lean region appears to correspond to the velocity boundary layer in the throat (according to standard flat-plate boundary layer theory) and the limit of the wake in the diffuser based on a numerical calculation. The experimentally determined particle velocities are compared with a numerical calculation, which includes a calculation of the wake using initial boundary layer assumptions at the end of the throat. The initial condition which corresponds best to the PIV results uses the assumption of a turbulent flat plate boundary layer on the length of the throat. The PIV velocity corresponds approximately to the calculated velocity of the mass mean diameter particle. This particle size is approximately 50% of the pixel resolution size on the recorded images. The observation of Payne and Crowe (1984) that the pressure relationship with mass flow rate is unaffected by particle size, provided that the Stokes number for the particles is greater than 7, is supported here. Particles with St < 7 achieve the throat velocity, and the proportion that has St > 7 continues accelerating beyond the end of the throat. It is likely that in power station operation there is a significant proportion of coal particles having St < 7 over the operational range.