MRF (Moving Reference Frame): Centrifugal Compressor

This project simulates a centrifugal compressor fitted with a diffuser using ANSYS Fluent, with the Moving Reference Frame (MRF) technique as the central theme. The centrifugal compressor is among the most widely used in industry: it raises gas pressure by combining positive pressure with centrifugal force. As the impeller rotates, low-pressure air is drawn in along the central axis, its pressure rises, and the compressed air is discharged radially through the diffuser surrounding the impeller. Representing the rotating impeller is the core modelling challenge here, and it is handled through the MRF approach rather than a physically moving mesh.

Because the compressor is rotationally symmetric and its blades are geometrically identical, only a single blade is modelled to simplify the problem and reduce computational cost. Each blade's domain consists of an inlet block connected to the input and a passage connected to the output, bounded on either side by two covers — the hub and the shroud — between which the blade sits. The impeller rotates about the central z-axis at 800 rpm.

The role of the diffuser is to convert the kinetic energy of the fast-moving discharge into pressure. By the Bernoulli relation, pressure change is inversely related to the square of the fluid velocity, so reducing the velocity of the flow leaving the blades increases the outlet pressure. The diffuser achieves this by enlarging the cross-sectional area of the passage: as the area grows the flow slows, and as the velocity falls the outlet pressure rises — improving the compressor's working efficiency.

The geometry was created in Design Modeler and meshed in ANSYS Meshing with an unstructured grid of 303,600 cells.

The heart of the methodology is how the rotation is imposed. Instead of physically moving the mesh, the rotation is applied through cell-zone conditions using the Frame Motion (MRF) method, which solves the flow in a reference frame attached to the rotating component. The rotating elements of the passage and the attached hub are assigned a rotational speed of 800 rpm in the frame-motion settings, while the blade itself is treated as a boundary within that frame. This lets the simulation capture the effect of rotation on the steady flow field at a fraction of the cost of a fully transient moving-mesh calculation.

After solving, the simulation produces contours of pressure and stress on the blade surface, along with contours of pressure, temperature, velocity and turbulent kinetic energy on the blade and velocity vectors around it. The pathlines clearly reveal the centrifugal action of the machine, with the flow moving radially outward from the central region, and the variation of pressure and velocity around the blade reflects the influence of the rotation. As a study in MRF modelling, the project demonstrates how a moving reference frame can represent the rotation of an impeller — reproducing the pressure rise, the centrifugal flow pattern and the diffuser's velocity-to-pressure conversion — without the expense of physically rotating the mesh.