MEASURED AND PREDICTED FLOW INSIDE LOGARITHMIC SPIRAL CURVED DIFFUSERS

Abstract

The diffuser represents an important fluid-mechanical element in many flow devices. The main purpose of the diffuser is to increase the pressure energy and reduce the kinetic energy of the flow. In centrifugal turbomachines, two common types of radial diffusers are employed: the vaneless and vaned diffusers. A vaned diffuser uses blades with different forms (straight, wedge. circular, airfoil. and logarithmic spiral) to better guide the flow and obtain a rate of diffusion higher than that can be obtained with a vaneless diffuser for the shortest possible length. In the present work, the turbulent flow inside a single passage with

logarithmic spiral profile for the vaned radial diffuser cascade is investigated experimentally and modeled numerically. The effects of three important parameters were investigated experimentally. These include the diffuser enlargement rate (which gives diffusers with various area and length ratios), the flow inlet incidence angle and the inlet Reynolds number. A test rig is built to achieve the experimental plan. Three curved diffusers with enlargement rate of k' = 0, 0.5, and 0.8 are designed and constructed from smooth thin-galvanized sheet metal. Several static pressure taps with (I mm) inner diameter were fixed along mid-height of both curved side walls. An approaching channel, ended with inlet contraction nozzle, was connected to the inlet of the tested diffuser channel. It can be set parallel or oblique to the diffuser axis at the inlet in order to achieve a flow with positive and negative incidence angles at the diffuser inlet. The mean flow velocity components and turbulent quantities (turbulent intensities and Reynolds shear stress) are measured at a series of stations within the diffuser passage from inlet to exit using hot-wire anemometer (CTA). In addition, the local static pressure along both curved side walls is also measured through the wall pressure taps using the electronic micromanometer.