Design of supersonic compressor impeller

Ever since the use of small gas turbine engines, axial and mixed flow compressors have been inserting in a variety of aviation applications. These type of compressors are in applications such as rockets, unmanned aerial vehicles with high performance and low weight demand (with high power density). In recent years, there is much more developments for axial compressors but the requirement of multiple stages due to low pressure ratios has made axial compressors disadvantageous for small gas turbine engine applications. For this reason, the use of mixed flow compressors in small gas turbine engines with high power density is essential, considering the high performance and low weight criteria. Mixed flow compressors are the intermediate form of axial and centrifugal compressors, as the name implies. In this context, the reason why it is defined as mixed flow is that the flow direction at the outlet of the impeller is not axial or radial, but between the both of them. Such compressors, due to their small diameters and high compliance with the high mass flow rates and also with respect to the axial compressors they have more total to total pressure ratios and compared to centrifugal compressors, they can operate efficiently at high flow rates, they have more advantageous. In this study, design and optimization studies of a mixed flow compressor with high flow rate and pressure ratio are discussed. The compressor outlet had to be supersonic and therefore worked on different geometries, as more energy would have to be transferred to the limited area of the mentioned mixed flow compressor. In addition, information about these type of mixed flow compressors available in the literature also is given. The mixed flow compressor designed within the scope of the thesis has the disadvantages as well as the advantages of being super sonic. Numerous losses occur due to instantaneous changes in Mach number and pressure. In this context, shock losses which are very important in supersonic compressors have been observed within the thesis study. The first step in designing a high flow and high pressure mixed flow supersonic compressor is to create the meridional geometry. The most important parameters determining the meridional geometry are the axial length, the direction of the exit of the channel, and the shapes of the hub and the shroud. These parameters are closely related to the specific speed of the machine. The effects of all of these on machine performance are considered in this study. Another important parameter in the design of mixed flow compressors is the shape of the impeller. In this context, angular change along the blade is important with respect to the blade inlet and exit angles as well as the blade angle. Another impeller geometry parameter that affects compressor performance is blade sweep. The shape of the sweep can be made in the tangential direction and the meridional direction according to the impeller. Tangential sweep affects the shock structure and the size of the wake zone. At this point, the operation point of the impeller are also changed. Tangential displacement also affects the structural resistance of the blade. The sweep of the impeller blades in the meridional direction changes the shock characteristic of the impeller and thus affects the flow and pressure points and therefore affects the operation point and efficiency of the impeller. The creation of pre-swirl at the impeller inlet and therefore the use of the inlet guide vane is another parameter that must be examined from the point of view of the efficiency of the impeller. Accordingly, it is advantageous in the high flow rate compressor impellers that the axial component of the absolute velocity's angle, which is the component of flow rate, is close to zero. Hence, there is no need to pre-swirl in the high flow rate compressor impellers, so the inlet guide vane is not required. Another parameter that affects the blade geometry of the supersonic compressors is the thickness distribution of the impeller blades. The blade thickness is indirectly affected by the losses. Thickness values will also affect the angle of attack because it affects the choking of the compressor. The smaller the attack angle, the lower the losses especially at high speeds. Accordingly, blade thickness and efficiency values will change. The supersonic compressor also influences the flow behavior of the impeller blades' leading edge. In order to reduce the losses caused by the interaction of the shock wave with the boundary layer, it is necessary to make the leading edge as sharp as possible. In addition to these, the use of splitters is an effective parameter in the performance of centrifugal and mixed flow compressors as it affects the solidity. However, even though splitters are studied in this thesis, the aimed result can not be achieved under supersonic outlet conditions and therefore the rotor formed in the compressor design does not have splitters. The diffuser also affects the performance of the compressor stage as well as the compressor impeller. Accordingly, diffusers can be manufactured as vaned or vaneless. The vaned diffusers are used mostly in the desired application of axial flow into the combustion chamber. However, due to frictional losses especially in supersonic applications, diffusers can reduce aerodynamic performance. The first parameter that should be considered in the design of vaned diffusers is the spacing that must be left between the impeller outlet and the vaned diffuser inlet, that is, the vaneless region. The amount of vaneless region is especially important for off-design conditions. Another way to reduce diffuser losses in a supersonic compressor impeller is to optimize the axial length of the vaned diffuser. Here, friction losses will be high due to the high velocity in the diffuser inlet. Therefore, in order to reduce the losses in the diffuser, the shock characteristic should also be observed. Another design parameter is the angle at which the flow will exit the diffuser exit before entering the combustion chamber. This angle is "namely diffuser outlet angle" generally preferred to be close to the axial direction, but may vary with various applications In this study, an initial design was obtained by using some empirical approaches in the literature. Computer aided design tools used to obtain geometry are described in this thesis. Design methods are also explained. Subsequently, the meshing tools have been specified to subject them to CFD analysis. Since the target values could not be obtained in the initial design, the energy transfer values from the hub to shroud were examined and the blade exit angles were optimized accordingly. The CFD results of both designs are compared. The target values of the mixed flow compressor to be designed within the scope of this thesis are mass flow rate of 5 kg/s, total pressure ratio of 6, maximum external diameter of 232 mm and maximum axial length of 260 mm. These goals have been reached as a result of the studies made.