Centrifugal Compressors - volumetric flow and pressure ratio

[fitchie23]

Hi all, I am relatively new to compressor theory and I am looking for a good explanation as to how volumetric flow affects pressure ratio between the stages within a centrifugal compressor. My current knowledge is that as volumetric flow decreases, pressure ratio increases as stated in one of the books I am reading. This doesn't seem intuitive to me and I am looking for a good solid expatiation as to why this occurs. Any help would be greatly appreciated.
Cheers
Anthony

 

[Jobrag]

It depends on how you are measuring your volume flow, if you are using Actual Cubic Metres per Sec then as pressure doubles volume will be halved, (roughly) if using Standard Cubic Metres per Sec then volume flow will remain constant.

 

[fitchie23]

Thanks for the reply, to be honest I didn't know there was a varying definition for volume with the same SI units so that is an alien concept to me. I understand the thermodynamic concept that as volume is reduced in a closed system then pressure must rise to accommodate, but it doesn't seem intuitive when related to centrifugal compressors. Is it perhaps only under the assumption that mass flow rate is kept constant and therefore the density of the fluid must have increased with a reduction in volumetric flow, translating into an increased pressure rise during flow diffusion? Surley a decrease if volumetric flow for the same flow area must mean a reduction in flow velocity and therefore less dynamic pressure before the flow reaches the impeller? Again sorry if my understanding is completely off as i am new to how compressors work aerodynamic.