Pekka Röyttä

Effect of vaneless diffuser width on the overall performance of a centrifugal compressor

Seven different vaneless diffuser designs varying in diffuser width are studied experimentally. One design is the basic design with the diffuser width and impeller exit and tip clearance width ratio of 1.0. The other six diffusers have width ratios of 0.903, 0.854, and 0.806. Three of the narrower diffusers have the width reduced from the shroud, and three from the hub and the shroud, divided evenly. The total-to-total efficiency and pressure ratio over the whole compressor are studied. It is possible to increase the efficiency of the compressor stage by reducing the diffuser width. The efficiency is increased over a wide operating range area at three different rotational speeds. The pressure ratios are increased at the design and low rotational speeds, but decreased at the high rotational speed. The shroud pinch seems to be more beneficial to the performance, while the hub pinch seems to have only a minor effect. The best design is the one with the width ratio of 0.854, with width reduction only at the shroud. A major finding is the experimental confirmation that the pinch influences the performance of the impeller, reducing the work input at the higher rotational speed.

Effects of Different Blade Angle Distributions on Centrifugal Compressor Performance

A centrifugal compressor with three different shrouded 2D impellers is studied numerically. All impellers have the same dimensions, and they only differ in channel length and geometry. Noticeable differences in efficiency are observed. Two different turbulence models, Chiens k- and k- SST, are compared. For this case, k- SST was found more realistic. The hypothesis that pressure losses in a curved duct and in an impeller passage behave similarly is suggested and found inadequate.

Experimental study of centrifugal compressor tip clearance and vaneless diffuser flow fields

Three vaneless diffuser designs, varying in diffuser width, for centrifugal compressor were studied experimentally. Along with the diffuser width, the tip clearance was altered. The compressor overall performance and diffuser flow fields were studied for each of the three diffusers at four different tip clearances. For the diffuser flow fields, the total pressures were measured with probes traversed over the diffuser width both at the diffuser inlet and outlet. Along with the total pressures, the static pressures were measured adjacent to the probes. This enabled the axial flow angle and velocity distributions to be studied. The pinches tested improved the stage efficiency mainly by suppressing the secondary flow region present near the shroud at the impeller outlet. This leads to a lower strain rate, resulting in lower losses. The efficiency decrease due to the increased tip clearance was similar with and without pinch present. This indicates that in the diffuser, the main source of the tip clearance associated losses is the tip jet, and the tip clearance vortices mix out already in the impeller.

Performance and flow fields of a supersonic axial turbine at off-design conditions

The increasing demand of energy efficiency and the utilisation of small-scale energy sources require efficient, small and versatile turbines. Supersonic turbines have a high power density and therefore small size and fewer stages than the subsonic ones. However, the performance of a supersonic turbine can decrease rapidly when operating at off-design conditions. This raises a need for the improvement of the turbine off-design performance. In this article, a supersonic axial flow turbine is studied numerically to find the causes of efficiency decrement. This article presents the most thorough study so far about the reasons that lead to the decreased off-design performance with supersonic axial flow turbines and explains the loss sources individually for the stator and the rotor. Three operating conditions are studied, and it is suggested that at the lower than design pressure ratios, the shock losses of the stator decrease while simultaneously the stator secondary losses increase. The high positive incidence at the lowest modelled pressure ratio, mass flow and rotational speed caused a significant decrease in the rotor and stage performance. This highlights the importance of incidence even in shock-driven supersonic turbine flows.

Experimental Study of the Effect of the Tip Clearance to the Diffuser Flow Field and Stage Performance of a Centrifugal Compressor

The effect of tip clearance to the centrifugal compressor diffuser flow fields and stage overall performance are studied experimentally. The relative tip clearance (tip clearance divided by the impeller exit blade height) is increased by shimming the shroud side casing of a high-speed variable speed driven industrial centrifugal compressor. Four different relative tip clearances are studied: 0.027, 0.053, 0.082, and 0.106. The stage efficiency and pressure ratios are measured, as well as the diffuser flow fields. The diffuser flow fields are measured both at the diffuser inlet and outlet. The total pressure and flow angle are measured with a cobra probe, and the total pressure and temperature with three Kiel probes. Static pressures are measured adjacent to the probe measurements. As expected, increasing the tip clearance leads to lower stage efficiency and pressure ratios. The decrement in the efficiency due to the increasing of the tip clearance is higher with higher mass flows, and at lower rotational speeds. Increasing tip clearance increases the size of the secondary flow region present at the impeller outlet near the shroud.Copyright

Influence of the axial turbine design parameters on the stator–rotor axial clearance losses

The drive towards lower emissions in aerospace engines promotes more efficient and physically smaller engines. One way to decrease the size of the axial turbine is to minimize the distance between successive stator and rotor rows. This can usually have either a positive or negative influence on the turbine performance. The reasons for this behaviour are not currently fully understood. In this study, a novel approach is taken to find new insights into this design question by analysing the influence of different design parameters on the turbine efficiency behaviour. Several different turbines are analysed using the literature. For the first time, the performed analysis reveals the design parameters, which correlate with the different efficiency curve shapes. The correlating parameters are the stator–rotor axial clearance, stator pitch to axial chord ratio, turning velocity Mach number and rotor aspect ratio. The mechanisms behind the found correlations are further analysed to connect the physical phenomena with the design parameters.

Liquid internal cooling of electric machine windings in arctic conditions

Direct water cooling has been used only in large size turbo generators. The method might, however, offer interesting possibilities also in high torque applications. The performance of different cooling fluids for internal liquid cooling of windings is assessed. The arctic operating conditions of machine place special requirements for the fluid thus pure water can not be used for cooling. It is found that use of the anti-freezing substances studied decrease the performance of the cooling system. Fast computational model to asses these effects is proposed.