ANALYSIS OF SWIRLING FLOW IN ANNULAR DIFFUSER

Abstract

In today’s rapidly advancing world, there is an increasing demand for devices that are energy-efficient, safe, and cost-effective. Available usable energy resources are depleting quickly, creating an urgent need to address this challenge. One possible solution is to develop techniques that can convert otherwise unusable energy into useful forms. However, creating such methods often requires significant effort, and in many cases, the outcomes may not justify the cost or may lack economic feasibility. An alternative and more practical approach is to conserve existing resources by designing energy-efficient systems that minimize energy losses. A diffuser is one such device that plays an important role in energy conservation. It converts the kinetic energy of a flowing fluid—energy that would otherwise be lost—into an increase in static pressure. In turbomachinery systems used for power generation, annular diffusers are commonly employed. These diffusers typically operate under conditions where the incoming flow may contain varying degrees of swirl. Therefore, improving their performance is essential and requires systematic investigation. Experimental research on annular diffusers is often challenging due to the need for advanced instrumentation and complex, time-intensive procedures, making such studies costly and limiting the extent of research in this field. The present study combines both experimental and analytical approaches to investigate the aerodynamic behaviour of axial annular diffusers. A specialized test setup was developed to introduce different levels of inlet swirl. Measurements were conducted v using a three-hole cobra probe to determine static pressure distribution, axial velocity, and swirl velocity profiles at various sections along the diffuser length. In addition to experimental work, computational analysis was carried out using CFD modelling. The study includes grid independence testing and the selection of an appropriate turbulence model that closely matches experimental observations as well as results reported in existing literature. After validation, the CFD model was used to analyse flow characteristics in two types of annular diffusers: one with a parallel hub and diverging casing, and another with both hub and casing diverging at equal angles. Both diffuser configurations were studied for equivalent cone angles of 10° and 20°, and area ratios of 2 and 3. The influence of inlet conditions—specifically velocity profiles with and without swirl angles of 7.5°, 12°, 17°, and 25° — was examined to evaluate diffuser performance. Detailed flow behaviour was analysed, and key performance parameters were calculated. The development of the flow was studied to identify regions of flow separation and reversal within the diffuser. The effects of various factors, including inlet swirl, area ratio, diffuser geometry, and cone angle, were systematically analysed to understand their impact on flow separation and overall performance. The results indicate that inlet swirl has an optimal value that maximizes diffuser performance. This optimal swirl level depends on the geometry and configuration of the diffuser.