Using Tip Gaps on Tandem Diffuser to Broaden the Operation Range of a Centrifugal Compressor

Gaps in compressors are generally unfavorable because they lead to energy loss.However,gaps on diffuser blades can broaden the operation range of a compressor.To broaden the operation range of a centrifugal compressor with a tandem diffuser,gaps with different sections and widths on the diffuser blades were examined to determine their influence on the compressor performance.The results show that the compressor’s surge margin increased from 33.28% to 49.2% for a 2.5%tip gap ratio,while its stability margin increased from 44% to 60.5%.However,pressure ratio and efficiency losses of 1.16% and 1.14%,respectively,were incurred.Gap widths less than 5%of the height of the diffuser blade are adequate for broadening the operation range,whereas gap ratios greater than 5%cause more energy loss without broadening the operation range further.Gaps can also reduce the Mach number in the diffuser blade channels and can be influenced by the impeller-tip gap.Moreover,diffuser gaps on the shroud side yield better performance than those on the hub side.An experiment was conducted to investigate the reliability of the calculation and conclusions in this study,and the results proved that the conclusions are valid for different types of centrifugal compressors.

THE UNSTABLE FLOW FROM THE GAP BETWEEN TWO CIRCULAR CYLINDERS

The interference of two circular cylinders has been researched for many years. In this study, the research work was concentrated on two cylinders in some special staggered arrangement. The velocity signal was measured with a hot wire anemometer and recorded with a TEAC portable data recorder. The relationship between spectrum and time was established by making 200 spectrum analysis in se- quence. The gap flow is unstable when two cylinders are arranged in L/D=2, T/D=1. Mainly, it can be looked as flow past a single blunt body and the Strouhal number is 0.15, The second peak arises in the power spectrum when the gap flow appears at irregular time interval. Sometimes it may be higher than the first one. The Strouhal num- ber corresponding to the second peak is 0.33. But at most of the time it is lower than the first one and even disap- peared. This is meaningful for exploring the mechanisms of interference induced oscillations in flow past two cylin- ders.

Flow Analysis of Tandem Blades as the Outlet Vane of a Highly Loaded Compressor

A tandem blade configuration is a significant flow control method that delays the onset of flow separation.This study numerically investigates the effects of diffusion factor and percentage pitch on the flow structure of tandem blades.Diffusion factors vary from 0.328 to 0.484.Percentage pitches change from 0.80 to 0.92.Results show that the loss coefficient increases with diffusion factor and decreases with percentage pitch.There is a hub-corner stall of the forward blade in all cases.Gap flow determines the rear blade corner separation.Varying the percentage pitch and diffusion factor changes the momentum distribution of the gap flow.Corner separation of the rear blade is inhibited as low-momentum gap fluids are involved in the passage vortex along with the hub-corner stall of the forward blade.Increasing diffusion factor causes a change in incidence at the leading edge of the rear blade,resulting in a variation at the corner separation of the rear blade.A tandem blade is compared with the reference outlet vane.The performance of the tandem blade is superior to that of the reference outlet vane in all incidences,with a 26.35%reduction in the loss coefficient and a 7.89%enhancement in the pressurization at the designed incidence.Tandem blades stall at positive incidence because of the hub-corner stall of the forward blade.The intensity of the gap flow increases with incidence,preventing corner separation of the rear blade at positive incidences.