A study of swirling flow in an orifice plate flowmeter

This thesis presents a theoretical and experimental study of swirling flow in an orifice plate flowmeter. Firstly, a solid body rotation was considered as the inlet velocity profile of an orifice plate. The beta ratio considered was 0.4. The Reynolds number was 7.82xl04 . As a result, a theoretical value of the discharge coefficient was obtained. Secondly, Laser doppler anemometry was used to determine the axial and tangential velocity profiles at 3 upstream and 2 downstream cross-sections of an orifice plate with a beta ratio of 0.4. A double right angle bend in two perpendicular planes was placed 6.5 diameters upstream of the flowmeter. The axial velocity profile was found to be slightly asymmetrical. The corresponding tangential profile was found to have a form of a solid body rotation. A 20" mercury manometer was used to measure the pressure difference across the orifice. The results showed that the discharge coefficient decreased. Thirdly, a commercial computer program was used to calculate the swirling flow through an orifice plate. A solid body rotation was considered at the inlet of the orifice. With the pressure field calculated, the corresponding discharge coefficient was determined. An additional simulation was performed with a 1/7 power law inlet profile. The discharge coefficient under swirling flow conditions was smaller than the one under non swirling flow conditions. Based on the experimental and computational results, we can see that the swirling flow produced by a double bend greatly affects the performance of an orifice plate. It is recommended that the flowmeter must be located 70 diameters downstream of a double bend. The BS 1042 does not provide a reasonable straight length for this type of disturbance.