This paper presents the results of numerical investigation of the flow in a vaneless diffuser of centrifugal compressor stage. Simulations were performed using both a commercial CFD package ANSYS CFX and the own-designed computer program. Steady conditions involving SST turbulence model were used for the calculations using CFX. To consider the interaction between impeller and diffuser, not just a diffuser but the whole stage was calculated. The own-designed methodology is based on solving of conservation equations with assumptions that flow in a diffuser is steady state and axisymmetric. The flow area is divided into the flow core and boundary layers. Results of calculation were compared with experimental data.

The behavior of vaneless diffusers of centrifugal compressors has been widely studied theoretically, experimentally, and numerically. Theoretical analyses have been carried out by Jansen [

This paper presents the results of both experimental and numerical investigation of a centrifugal compressor vaneless diffuser. Simulations were performed using both commercial CFD package Ansys CFX and the own-designed computer program. Investigation has been performed to predict the overall performance of a diffuser and to obtain flow patterns. Finally, the flow structure was analyzed to identify and quantify the sources of losses.

First of all an experimental investigation of the flow has been performed. The test vaneless diffuser is an element of the Sumy Frunze NPO gas compressor end stage. Cross-section of the diffuser is shown in Figure

Cross-section of the test diffuser (a) and the locations of the probe traverses, static pressure taps, and thermocouples (b), and

The air entered the test stage in axial direction through the suction pipe with a filter and an orifice plate. The inlet total and static pressures and temperatures were measured. The operation point was set by a throttle valve installed at the discharge pipe. The mass flow rate was measured with the orifice plate installed at the suction pipe. Also ambient pressure, temperature, and humidity were measured. To obtain the flow fields inside of the diffuser and to estimate its overall performance the following parameters were measured. Total pressures and flow angles were measured with the three-hole probe at eight radial positions downstream of the impeller. The probe was traversed across the diffuser at five points at each of the diameter. The probe was calibrated over a broad Mach number range. Static pressures were also measured at the same diameters through the holes in the diffuser shroud wall. Total temperatures were measured with thermocouples at the diffuser inlet and outlet. To eliminate an influence of the scroll tongue the checkout tests were performed before the main ones. Final location of the static pressure taps, probe traverses, and thermocouples are shown in Figure

The overall performance of the diffuser was evaluated using the static pressure recovery and the total pressure loss coefficients. The static pressure recovery coefficient was defined as

The detailed description of the test system and procedure was given by Kalinkevych and Shcherbakov [

Numerical investigation was performed in addition to experimental investigation for better understanding of the flow structure.

Flow in a diffuser substantially depends on the flow behavior at the impeller outlet. Therefore, to take into account an influence of the impeller, the two-element stage (impeller and vaneless diffuser) was modeled. A steady state model was used for all calculations. The stage and frozen rotor interfaces were used for the calculations involving SST turbulence model. The stage interface was used to calculate the overall diffuser performance and to predict the flow structure at the diffuser, whereas the frozen rotor simulation was used for prediction of the flow patterns at the impeller.

The computational domain contained two different meshes. The impeller mesh is a structured hexahedral mesh, generated using ANSYS TurboGrid. An H/J/C/L-Grid including an O-Grid topologies were used. The mesh was refined to better resolve the flow in the vicinity of the leading and trailing edges of blades. A refinement was also performed near the hub and shroud walls. To prevent a mesh dependent error the grid independence analysis has been performed. Five mesh sizes in the range of 200 to 600 thousand nodes have been generated for the impeller. As a result, the mesh with 430 thousand nodes has been selected for all simulations. Near wall nodes were positioned in the way to ensure the value of y+ not more than 2.5. The resulting mesh is shown in Figure

Impeller mesh: (a) meridional view; (b) midspan view.

The diffuser was meshed with an unstructured tetrahedral mesh with prism layers along the wall surfaces to resolve the near-wall boundary layer flow. Mesh generation was performed using ANSYS CFX-Mesh. On the analogy of the impeller five mesh sizes have been generated for the diffuser to perform the grid independence analysis. Consequently, the mesh with 1.1 million nodes has been selected. The final mesh is presented in the Figure

Diffuser mesh: (a) meridional view; (b) midspan view.

Experimentally measured total pressure and total temperature were specified at the impeller inlet boundary and the mass flow rate at the diffuser outlet. Nonslip and adiabatic conditions were imposed all over the solid walls. The periodic boundaries were specified for the lateral sides of impeller and diffuser domains.

Because surge and rotating stall are unsteady phenomena that cannot be modeled as a steady state problem, simulations were performed for the flow rates at which stall does not initiate.

A new vaneless diffuser calculation procedure based on the boundary-layer theory has been developed and presented by Kalinkevych et al. [

Viscous sublayer velocity profile is

According to Sherstyuk [

At

Turbulent flow region velocity profile (at

The calculated and measured total pressure loss and static pressure recovery coefficients, presented as a function of the inlet flow angle in stationary frame

Diffuser performance: □□□ measured data; ○○○ Ansys CFX data; - - - the own-designed model data (

The diffuser inlet flow angle was defined as

As the diagram shows there is a point with the minimum measured total pressure loss coefficient at

An increase in total pressure loss at low flow angles may be caused by higher flow velocities at the impeller outlet, because of higher specific work at low flow rate, resulting in higher wall friction losses.

To identify other sources of losses, velocity diagrams were considered. The measured radial velocity diagrams are shown in Figure

The measured dimensionless radial velocities.

The measured flow diagrams agree quite well with the numerical results. Figure

Diffuser streamlines in meridional plane.

The own-designed model has also predicted that at

Figure

Calculated and experimental averaged total pressure: ○○○ measured; - ▲ - calculated (Ansys CFX); - - - calculated (the own-designed model).

Total pressure distributions across the diffuser.

An increase in measured total pressure losses at

Impeller streamline patterns in relative frame.

It can be seen that at

Figure

Total pressure distributions in relative frame near the trailing edge.

As it was shown, jet-wake mixing strongly affects the total pressure loss, especially at the off-design point. This feature explains the difference between the measured and calculated total pressure loss coefficients. To predict the true interaction between the impeller and the diffuser the transient simulation has to be used. The main disadvantage of this method is the fact that unsteady simulations are far more computationally expensive.

Summing up the above, it should be noted that at high flow rates the total pressure loss increases because of intensive jet-wake mixing, while at low flow rates the efficiency of the vaneless diffusers decrease mostly because of the flow separation. It was also found that when reverse flow at the diffuser outlet extends close to the entry region the rotating stall is initiated [

Therefore, to decrease total pressure loss coefficient at low flow rates and to improve centrifugal compressor stability boundary layer control techniques should be used. Many researchers have contributed to determine control mechanisms for boundary separation, stall, and surge in centrifugal compressors over the past several years. Some of them [

Experimental and numerical investigations of the vaneless diffuser were performed. As the results show, at low flow rates the total pressure loss increases because of higher frictional, separation, and jet-wake mixing losses, while at higher flow rates it results from the jet-wake mixing.

It was shown that the steady-state models allow to get good results, especially near the best efficiency point. However, these models do not account the true interaction between the rotor and stator. In some cases, modeling these aspects is the key to getting accurate solutions. That is why at off-design points the unsteady simulation seems to be more preferable.

The own-designed model showed a significant qualitative and quantitative correspondence between calculated and experimental data at the design point, and the separation point was estimated rather well.

To decrease the total pressure loss at low flow rates and to improve centrifugal compressor stability, boundary layer control techniques, such as injection and pitched diffusers, should be used.

Radial velocity, circumferential velocity, and absolute velocity,

Averaged radial velocity, averaged circumferential velocity, and averaged absolute velocity,

Diameter,

Relative diameter

Tangential stress,

Averaged static pressure,

Averaged total pressure,

Averaged density,

Diffuser width,

Width ratio

Averaged flow angle,

Static pressure recovery coefficient

Total pressure loss coefficient

Boundary layer thickness,

Tangential stress on the wall,

Kinematic viscosity,

Distance to the wall,

Dimensionless coordinate

Friction velocity,

Dimensionless velocity

Specific Reynolds’ number

Displacement thickness,

Momentum thickness,

The Buri shape-factor

Prandtl number.

Impeller outlet

Diffuser inlet

Diffuser outlet

Viscous sublayer boundary

Boundary of the boundary layer.

The authors declare that there is no conflict of interests regarding the publication of this paper.