Detailed flowfield measurements in a vaneless diffuser model

Velocity measurements have been taken in the vaneless diffuser of a centrifugal blower at steady flow conditions using miniature X-probes. The impeller speed of 1000 rpm was maintained constant at optimum operating flow coefficient of 0.373 to ensure that the diffuser is running at stall-free conditions. The flow angle measured (at half of the diffuser width) did not exceed the value of 65° (this is well below the critical stall angle of 78° reported in the open literature). Radial and tangential velocity profiles and their statistics are reported. Although stall was not present, a small inward flow region was detected on the shroud side of the diffuser wall. This is a confirmation that the presence of inward flow zones on the diffuser walls does not automatically indicate the presence of stall. The results also showed that the flow exiting the impeller is not constant across the width of the diffuser. Due to rapid mixing, the velocity distribution across the diffuser becomes more uniform as one moves out in the radial direction toward the diffuser exit. Profiles of Reynolds stresses showed that turbulent mixing decreases with diffuser radial distance and is higher in the vicinity of the shroud compared to the hub. This type of detailed and accurate flow data is a useful tool for developing turbulent flowfield models of radial diffusers.