Jari Backman

Loss Definition of Electric Drives by a Calorimetric System With Data Processing

An accurate measurement of power or heat losses of high-efficiency electrical devices is difficult with input and output powers. In the calorimetric method, these losses are measured directly. In this paper, a functional heat loss measurement concept with adequate data processing methods is suggested for heat losses up to 2 kW. Such a heat loss range can be applied to present-day power electronic converters with the rated power of up to 110 kW and motors up to 37 kW. The measurement system does not require a complex mechanical structure or a large footprint area on the measurement site. The concept is scalable to different sizes. New features to the balance type calorimeter data processing are introduced. These include a mass flow correction, a technique to detect the thermal equilibrium, and a method to correct measured heat losses to improve the measurement repeatability and reduce the measurement uncertainty. The measurement uncertainty of the proposed calorimetric concept is 0.4%. Laboratory measurements are carried out both for a commercial frequency converter and an electric motor. These are measured at the same time with two calorimeters. The losses of these devices are measured both by the input-output and calorimetric methods.

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.

Conceptual Design of a Multistage High-Speed Motor Driven Air Compressor

Centrifugal compressors operate with relatively high rotational speeds. A high rotational speed can be reached by a gearbox-coupled standard motor or by a high-speed motor with a frequency converter. The motivation for using high-speed motors is usually related to features like performance, size and total life cycle costs. A comparison of these features for the high-speed motor and gearbox driven air compressors are presented. The delivered pressure of oil-free plant air compressors is 600–900 kPa(g). Traditional gearbox driven turbo compressors have typically two or three compressor stages. In these machines, the cost impact for the additional stages is not as high as in the case of high-speed motor compressors, where only two stages can easily be coupled to one motor. The performance characteristics of a two-, three- and four-stage high-speed motor compressors are compared to each other. Based on the feasibility study, the two-stage compressor was selected for further analysis. The aim was to design a 250 kW plant air compressor. The optimization process to obtain the best performance for the compressor is discussed. This includes careful analysis of stage-specific speeds, electric motor performance, bearing performance, rotor dynamics, as well as material strengths and cooling. The predicted performance of the high-speed motor and compressor is presented.Copyright

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

Investigation of the Stage Performance and Flow Fields in a Centrifugal Compressor with a Vaneless Diffuser

The effect of the width of the vaneless diffuser on the stage performance and flow fields of a centrifugal compressor is studied numerically and experimentally. The diffuser width is varied by reducing the diffuser flow area from the shroud side (i.e., pinching the diffuser). Seven different diffuser widths are studied with numerical simulation. In the modeling, the diffuser width is varied within the range 1.00 to 0.50. The numerical results are compared with results obtained in previous studies. In addition, two of the diffusers are further investigated with experimental measurement. The main finding of the work is that the pinch reduces losses in the impeller associated with the tip-clearance flow. Furthermore, it is shown that a too large width reduction causes the flow to accelerate excessively, resulting in a highly nonuniform flow field and flow separation near the shroud.

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

Importance of the vane exit Mach number on the axial clearance-related losses

More efficient and physically smaller axial turbine designs are promoted to lower emissions and increase revenue. The physical size and the weight of the axial turbine can be minimised by adjusting the distance between successive stator and rotor rows. The influence of changing stator–rotor axial clearance can usually have either a positive or a negative influence on the turbine performance, and the reasons for this varying behaviour are not currently fully understood. A previous study revealed several design parameters that correlate with the efficiency curve shape. However, the effects of two parameters, namely the stator outlet Mach number and the reduced blade passing frequency, still remained unclear. Therefore, a novel approach is taken to analyse the correlations between the two design parameters and the axial clearance efficiency curve shape. Several different turbines are analysed using data available in the literature, and also new data are presented. The study suggests that the stator outlet Mach number correlates reasonably well with the efficiency curve shape, and it was further linked to five loss mechanisms and the rotational speed. Although the unsteady interaction plays an important role in the loss share, the level of unsteadiness did not correlate with the efficiency curve shape.

Numerical Investigation of Centrifugal Compressor Tip Clearance

In this paper, the effect of the axial tip clearance on the performance and flow fields of a centrifugal compressor is studied numerically. The compressor is equipped with a pinched vane-less diffuser. Six different axial clearances were modelled and the relative axial tip clearance was varied from 0.027 to 0.154. The tip clearance was changed by transferring the shroud in the axial direction. The modelled results are compared to measured results obtained in previous projects.The results indicate that the effect of tip clearance to the impeller performance is linear, even though the clearance is increased to such amounts which are seldom used in traditional compressors. The clearance has significant effect on the flow fields. Even though the mass flow averaged tangential velocity remained constant with the increasing clearance, the velocity fields show increase in tangential velocity in the passage wake between the suction side of the splitter blade and pressure side of the full blade and decreased in the other blade passage with increasing clearance. Radial velocity contours showed that the backflow region near the shroud increased in size when the clearance is increased. The growth was stronger on the suction side of the splitter blade.Even though there is quite a difference in the stage efficiency between the highest and lowest clearance, the difference in velocity triangles at the impeller exit is small. This should be taken into a consideration when compressors with high relative axial tip clearances are designed.Copyright