Proposal for Bachelor Thesis in Department of Applied Mechanics

Projektförslag för kandidatarbete inom Tillämpad mekanik

Proposal for bachelor thesis in Department of Applied Mechanics

Title

Numerical study of the flow in a conical diffuser with an annular inlet

Background

In many practical applications of conical diffusers, the flow is fed by an annular flow passage formedby a center body. Conical diffusers are commonly used to reduce the velocity and increase the static pressure of a flow. The water turbines, gas turbines, bounded fans, pumps are of expamples.

Figure 1. The center body section and the diffuser section. Dimensions are in millimeters. The flow is from left to right.

Diffuser flow is susceptible to flow separation due to the adverse pressure gradient. Flow separation is undesirable because it reduces the pressure recovery, increases flow instability, and reduces flow uniformity into downstream components. Flow separation, which occurs if thecenter body ends abruptly, is undesirable because itdegrades the diffuser performance.

In order to optimize diffuser performance, it is important to understand the flow structure in the diffuser. In the past decades, numerous studies have been carried out on flow in conical diffusers with circular inlets. However, in many practical applications, the conical diffuser is fed by an annular flow passage. In the case of the water turbine application, the turbine hub acts as a center body that creates an annular inlet to the diffuser. A central separation bubble forms at the base of the center body if it ends abruptly. This results in two possible regions of separation in the diffuser. The first is the separated wake of the center body, and the second occurs if the boundary layer on the diffuser outer wall separates due to the adverse pressure gradient. Each of the two separation regions can affect the pressure distribution in the diffuser, which couples their development. For example, if the outer wall boundary layer separates, it relieves the adverse pressure gradient and helps the central separation bubble to close.

Problembeskrivning

Description of the Problem

The aim is to studythree component mean velocity and validate with measurements for a set of conical diffusers with an annular inlet using a CFD code, ANSYS or OpenFoam. The numerical study aims to show strongcoupling between the diffuser wall boundary layer development and the wake of the center body using unsteady Reynolds Averaged Navier-Stokes (URANS). The geometry of the conical diffuser with a central body is shown in Figure 1. The effect of swirl (rotating flow) at inlet and on the separation of the flow on central body is of particular interest. For non-swirl flow, the flow can rotate in either direction, while when a slight swirl is included, the precessing vortex counter-rotates to the direction of the swirl.

Genomförande

Students will go through the thesis in following steps:

1. Study the physics of the flow and the geometry.

2. Getting familiar with a CFD code.

3. Draw the geometry preferably with ICEM Hexa software.

4. Generate the mesh preferably with ICEM Hexa software.

5. Do simulation with a CFD code.

6. Take the results and study.

7. Write thesis :)!

Speciella förkunskapskrav:

Good knowledge in Fluid dynamics.

Möjlig målgrupp:

The group working will be on agreement!

Gruppstorlek:

2-5 persons

Handledare: Dr.Ardalan Javadi

Examinator: Prof. Håkan Nilsson