Question

If flow is already swirling at the inlet of a centrifugal impeller, how will that affect compressor stability?

Answer 1

When the flow passes through a straight pipe to enter inside centrifugal compressor the flow is initially straight, uniform and has no swirling motion, so the swirl angle is α₁ = 0°. As the flow passes through the centrifugal impeller, the impeller forces the flow to spin at high speed as it gets further from the rotational axis. According to Euler's fluid dynamics equation, known as the pump and turbine equation, the energy input to the fluid is proportional to the local spinning velocity of flow multiplied by the local impeller tangential velocity.

In other cases, the flow leaving the centrifugal impeller is travelling attains the speed of sound. It then flows respectively to a stationary compressor causing it to deaccelerate. The stationary compressor is ducting with increasing flow-area where energy transformation happens. If the flow has to be turned in a backward direction to enter the other part of the machine, eg another impeller or a combustor, flow losses can be deducted by directing the flow with stationary turning vanes or individual pipe diffusers. As described in Bernoulli's principle, the reduction in velocity causes an increase in pressure.