Abstract:
In the current research work, a computational analysis of a high-speed centrifugal
compressor stage for turbocharger applications is presented. A detailed investigation
about the interactions between
backswept impeller and downstream vaneless and
vaned diffusers is carried out. '
A unshrouded
backswept impeller with splitters was combined with a vaneless
diffuser or a number of different designs of vaned diffusers.
The CFD solver CFX-TASCow was used. The
three-dimensional Reynolds-
Averaged Navier-Stokes equations are solved and a pressure correction method is
employed to solve the system of equations.
A
steady simulation and analysis of the interactions between the impeller and the
vaneless diffuser is carried out, emphasis is focused on the comparisons of the
different interactions at different conditions
regarding the flow structures at different
radius ratios, effect of rotational speed, mass flow rate and impeller tip clearance. The
predicted results were also compared with the available experimental results in terms
of radial
Velocity, tangential Velocity and flow angle. In general, the predicted results
show a reasonable agreement with the experimental data.
A
steady state simulation and analysis regarding the interaction between the impeller
and various vaned diffusers is carried out. For the interface
between the rotational
impeller outlet and the stationary vaned diffuser inlet, the stage averaging condition
is used. A detailed
comparison between the predicted and the available experimental
data is
performed in terms of static pressure rise, total pressure ratio, choking mass
flow and
efficiency characteristics, and very good agreement is accomplished. In
addition, detailed flow distributions are compared, assessed and critically analysed,
regarding different number of diffuser vanes, rotational speed, gap between the
leading edge of the vaned diffuser and impeller tip, mass flow rate. Emphasis is
focused on the
steady state study of the effect of the number of diffuser vanes on the
stage operating range.
Further more, unsteady simulation and analysis regarding the interactions between
backswept impeller and downstream vaned diffusers is carried out. In the unsteady
simulation, a geometry scaling method is used to modify the diffuser geometry to the
nearest
integer pitch ratio while keeping the throat area, flow direction and area ratio
unchanged in order to deal with the unequal pitch ratio problems which exist in the
unsteady simulation.
The
unsteady investigation was undertaken regarding different number of diffuser
vanes, rotational speed, gap between the leading edge of the vaned diffuser and
impeller tip, mass flow rate and impeller tip clearance. The detailed interactions at
different conditions are
compared, assessed and analysed. The studies focus on the
analyses of the effect of the different interactions on the stage operating range, peak
efficiency, total pressure ratio, level of unsteadiness, flow structures, flow angle or
incidence
angle, etc. In addition, the' predicted results are compared with available
experimental data and a quite good agreement is achieved although the geometry is
scaled.
On the other hand, a detailed investigation on the differences between the time
averaged unsteady simulation results and steady simulation results was performed at
different conditions. The
comparisons were carried out regarding static pressure, total
pressure, speed, flow angle (or incidence angle) and isentropic efficiency. The
investigation confirms that unsteady simulation is still quite important, since some of
the
steady state simulation results are still not similar to the time averaged ones.
Designers should take into account the influence of the unsteadiness on the flow fields
when
they employ the steady state model in the design process.