Ahti Jaatinen-Värri

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.

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.

Direct Liquid Cooling Method Verified With an Axial-Flux Permanent-Magnet Traction Machine Prototype

Efficient cooling is needed, for example, in traction motors which face regularly high torque peaks and generate high stator Joule losses. This paper studies the feasibility of the direct liquid cooling in the thermal management of a low-power low-voltage permanent-magnet machine. A tooth-coil axial-flux permanent-magnet double-stator-single-rotor test machine was first equipped with indirect liquid cooling using water cooling jackets and then with direct winding cooling. The winding material used is a hybrid conductor comprising a stainless steel coolant conduit tightly wrapped with stranded Litz wire. The performance of the motor is examined at various power levels using oil or water as the cooling fluid. The results confirm that the proposed direct cooling method is practical also in small machines, and furthermore, it offers significant improvements in the machine thermal management, especially, in cases where stator Joule losses dominate.

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.

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

Study of the sensorless operating point estimation for turbocompressors

Frequency converters enable energy savings and sensorless operation monitoring in pump and fan systems. This is possible with model-based estimation methods that apply frequency converter estimates of the rotational speed and the shaft power. Since compressors produce air flow similarly as fans, the same estimation methods should be applicable, if the fluid density change can be detected and analyzed. In addition, suction side air conditions have an impact on the estimation of the compressor operating point and should therefore be considered accordingly. This paper studies the use of the QmP and Qφm estimation methods for a radial-flow turbocompressor and the accuracy of the affinity laws in the case of compressors. The results show that the Qφm method could be applied to turbocompressors, and the affinity laws are accurate enough if the performance curves are measured with sufficiently small rotational speed steps.

Laboratory testing of an active magnetic bearing supported permanent magnet 3.5 kW blower prototype

A commissioning and laboratory tests of an active magnetic bearing supported PM 3kW blower are presented. The motor operation is evaluated in no-load and load tests. An identification of AMB-rotor system, testing of H∞ centralized control and mass-flow of the blower for recirculation of anode gas in a SOFC are discussed. The key results include comparison between analytical and measured values of an output sensitivity function of different MIMO controllers, measured efficiency and output motor power, and blower measurements.

Influence of Reynolds number variation method on centrifugal compressor loss generation

The path to instability of a small centrifugal turbo-compressor included in an air supply system developed by Liebherr-Aerospace was experimentally investigated thanks to high-frequency unsteady static pressure sensors. At the lowest speed line, rotating stall is observed both at impeller inlet and vaned diffuser inlet. It is characterized by one stall cell rotating at 6.4% of the impeller speed. At higher rotational speeds, the compressor experiences deep surge. The surge frequency decreases as the rotational speed increases and as the downstream volume increases. The peak-to-peak pressure amplitude increases as the rotational speed increases but do not depend on the downstream volume. A detailed analysis of the instability onset was performed thanks to Fourier and wavelet transforms showing that the instability inception is disclosed by pressure spikes. Nevertheless the spikes cannot be considered as precursors but as the embryo of the developing instability, which jeopardizes the chances of a control system to damp the perturbation.

Design of a 400 KW Gas Turbine Prototype

Decentralized power and heat generation is a growing trend throughout the world. In smaller applications, electrical power output less than few megawatts, reciprocating engines have dominated the market. In recent years, small sized gas turbines have emerged as challengers for the reciprocating engines. The small gas turbines have a growing share of the decentralized energy market, which itself is rapidly growing. Hence, improvements in small gas turbine efficiency have a significant impact from the economic and environmental perspective.In this paper, the design of a high efficiency 400 kW gas turbine prototype is described and discussed. The prototype is a two-spool, recuperated and intercooled gas turbine where both spools comprise of a radial compressor and turbine, a permanent magnet electric generator, an axial and two radial active magnetic bearings and two safety bearings.The prototype design was divided into five categories and each of the categories are discussed. The categories were: the process design, the turbomachinery design, the generator and electrical design, bearing design and rotor dynamic analysis, and mechanical design. The design of recuperator, intercooler, and combustion chamber were outsourced. Hence, they are not discussed in this paper.The prototype design process showed the readiness of the chosen technological selections, as well it showed that the type of machine under discussion can be designed and manufactured.© 2016 ASME