Ahti Jaatinen

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.

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

Experimental Study of Pinch in Vaneless Diffuser of Centrifugal Compressor

A centrifugal compressor is often equipped with a vaneless diffuser because the operation range of a vaneless diffuser is wider than the operation range of vaned diffuser, and the geometry of the vaneless diffuser is simple and inexpensive. The flow field after the centrifugal compressor rotor is highly complicated and the velocity is high. A moderate amount of this velocity should be recovered to the static pressure. It is important to study the flow field in the vaneless diffuser in order to achieve guidelines for design and an optimal performance. In this article, the experimental study of the pinch in the vaneless diffuser is conducted. Five different diffuser heights were used, b/b2 = 1 , b/b2 = 0.903 , b/b2 = 0.854 , b/b2 = 0.806 and b/b2 = 0.903 (shroud). In three of the cases, the pinch was made to both walls of the diffuser, hub and shroud, and in one case, the pinch was made to the shroud wall. The total and the static pressure, the total temperature and the flow angle were measured at the diffuser inlet and outlet by using a cobra-probe, kiel-probes and flush-mounted pressure taps. In addition, the static pressure in the diffuser was measured at three different radius ratios. The overall performance, the mass flow, the pressure ratio and the isentropic efficiency of the compressor stage were also monitored. Detailed flow field measurements were carried out at the design rotational speed and at the three different mass flows (close to the surge, design and close to the choke). The isentropic efficiency and the pressure ratio of the compressor stage was increased with the pinched diffuser. The efficiency of the rotor and the diffuser was increased, whereas the efficiency of the volute/exit cone was decreased. The pinch made to the shroud wall was the most effective. The pinch made the flow angle more radial and increased the velocity at the shroud where the secondary flow (passage wake) from the rotor is present.Copyright

Experimental Study of Vaned Diffusers in Centrifugal Compressor

Improving the performance of a centrifugal compressor can lead to decreased energy consumption in industry. Vaned diffusers are known to narrow the operating range but increase the peak efficiency compared to vaneless diffusers. In this study, three vaned diffusers and one vaneless diffuser are studied experimentally to improve compressor performance. Attention is paid to the operating range and performance of the diffuser, specially close to stall, and therefore the vaned diffusers are designed to have large negative incidence (−8°) at the design conditions. Different vane numbers and vane turnings are used. Compressor performance maps are measured at different rotating speeds and diffuser flow fields at the design rotating speed. The results show that the vaned diffusers make the diffuser outlet flow field more uniform than the vaneless one. The peak efficiency of the vaned diffusers occur at lower mass flows, and the efficiency is also higher at the design operation point with the vaned diffusers. In general, the vaned diffusers are better at lower mass flows, but stall earlier.Copyright

Design and Implementation of Problem-Based Learning in a Graduate Engineering Course

In this paper it is described how a graduate engineering course is modified so that it can be taught using problem-based learning (PBL). In the first part of this work, PBL is discussed in general, and in the second part it is described how PBL has been used in the past by others, and what should be taken into account when a PBL course is designed. In the third part there is a description of the old course, an explanation for why it was chosen as an example, and a description of the renewed course. The renewed course was held for the first time in the fall of 2010, and the feedback is based on discussions with the students. On the basis of those discussions, it can be concluded that students like the general idea of PBL, and encourage the faculty to use it in the future and in other suitable courses.Copyright

Effect of diffuser width and tip clearance on the static pressure distributions in a vaneless diffuser of a high-speed centrifugal compressor

Static pressure distributions in the vaneless diffusers, varying in width are studied. Seven different vaneless diffusers were studied. The static pressure before and after the diffuser was measured in four different circumferential locations, and at one circumferential location at five different radii. The pressure distributions were measured at the design rotational speed, and at three different mass flows. The reduction in the diffuser width does not significantly affect the diffuser pressure distribution. The circumferential pressure distributions of the different designs both before and after the diffuser are almost similar in shape between each design.

Blended Education in Turbomachinery and Fluid Dynamics

The paper deals with blended education in turbomachinery and fluid dynamics courses in Finland. The teaching methodology of these courses has been developed to comply with the new challenges of the education in technology. Presently six of the courses in the curriculum are following the schemes explained in this presentation. The courses are studied in the last year of the Bachelor level and in the Masters level quantifying from 2 to 4 ECTS credits. Students get all the material from the teaching platform in the web, which can be accessed freely anytime and practically from anywhere. Before attending the teaching events, the students go through the study material, perform several exercises and take a quiz, which can give them extra points for the exam tally. The contact teaching of the course is divided into four Learning Sessions of four hours. The first half is reserved to deepen the acquired knowledge, and performed in a way to attract a more interactive atmosphere. In the second half the students are divided to groups, where they solve more difficult study exercises compared to those they already have trained on. As the students are expected to study with the material of blended learning in advance, the percentage of the contact hours has to be lowered in comparison to the traditional teaching in order to maintain a balance with the credits in the course. However, with the acquired knowledge, the students are more interactive with the teachers and the teaching becomes more efficient. The students have given positive feedback on the courses. The instructors have found that the blended learning is not necessary an easier task, although there are less contact hours with the student. The efficiency of teaching has increased and the teaching is more rewarding. Both the students and the teachers found that blended education suited best for student groups around 10 students.Copyright

Radial Forces in a Centrifugal Compressor Equipped With Vaned Diffusers

A non-uniform pressure distribution in a volute of a centrifugal compressor causes a radial force on the impeller and the rotating shaft. The diffuser preceding the volute determines how the flow field entering to the volute, and affects the radial forces that are caused by the volute towards the impeller. The radial forces were measured for three different vaned diffuser geometries and one vaneless diffuser with a pinch. The force measurements were made for each assembly with the same compressor using active magnetic bearings. The impeller forces were found to be the smallest near the design flow and higher at choke and at stall for all configurations. The radial forces on the impeller were higher with the vaneless diffuser than with the vaned diffuser. The force distributions with the vaned diffusers were uniform and clearly different from those of the vaneless diffuser. In addition, the radial forces were estimated using static pressure distributions measured at the impeller outlet. The radial force determined from the pressure measurements verified the measured forces obtained in the magnetic bearing measurements.Copyright