Effect of Inlet/Outlet Fillets on the Clearance Flow in a Rotor-Casing Assembly Bassam Abu-Hijleh , Jiyuan Tu, Aleksander Subic, and Leigh Fostineo School of Aerospace, Mech. and Manufacturing Engineering, RMIT University, Victoria, Australia. Abstract: The performance of a Rotor-Casing Assembly is influenced more by the internal air clearance/leakage paths than by any other thermo-fluid aspect of its behavior. The pressure difference drives the air across the different clearance paths at a rate that is not the same for every path. So the distribution of clearance flow through the various clearance paths within the machine is important for the improvement of the performance. The numerical analysis was conducted using the FLUENT Computational Fluid Dynamics (CFD) package. Geometry definition, mesh generation, boundary and flow conditions, and solver parameters have all been investigated as the part of the numerical analysis. This analysis was conducted for static 2D rotors. Several configurations of inlet only fillets, outlet only fillets, and combined inlet/outlet fillets were investigated. The effects of the fillets on the total clearance flow rate, distribution of the flow among the different paths, and the turbulent kinetic energy within the rotor-casing assembly are reported and discussed. In general, the use of outlet only fillets had little effect on the flow field. The use of small fillets (2 mm) on both inlet and exit provided significant reduction in the turbulent kinetic energy indicating improved system efficiency. Keywords: Supercharger, Clearance flow, Leakage, CFD, Numerical. 1. INTRODUCTION Environmental issues have been highlighted in recent years on a global level, and motor vehicles are required to meet severe requirements for fuel consumption and exhaust gas control while satisfying various driving requirement of the users. To meet these requirements, motor engines equipped with high-response superchargers are being introduced to the market (Yoshiyuki et al., 2001). The Twin Screw Supercharger (TSSC) is a positive displacement compressor, the working cavity of which is enclosed by the casing bores, casing end plates and the helical surfaces of the male and female rotors. As the rotors rotate, the volume of the working cavity varies from zero to its maximum and from its maximum to zero periodically in a manner determined uniquely by the geometry of the supercharger. As a consequence of this periodic variation, the supercharger completes its suction, compression and discharge processes. This paper demonstrates a two-dimensional model of a supercharger. Due to the geometry of the rotor-casing assembly, the clearances between two rotors and the clearances between rotors and casing, air clearance flow paths exist within the supercharger assembly. The internal flow rate is dependent on three different clearances: between the casing and male rotor (C-M), between the two rotors (M-F), and between the female rotor and the casing (F-C) (Takei and Takabe, 1997; Flemin and Tang, 1995). It is very important to know the total clearance flow rate as well as the distribution of the flow among each clearance path for the purpose of design procedures in general and for improving the rotor lobe profile. CFD analysis is performed using the FLUENT CFD package, which is employed to simulate the flow passage through the supercharger. The flow is simulated as a steady 2-D turbulent incompressible air using the Realizable k-ε turbulence model. The Realizable k-ε turbulence model is employed because it has shown substantial improvements over the standard k-ε model where the flow features include strong streamline curvature, vortices, and rotation (Shih et al., 1995). 2. COMPUTATIONAL TECHNIQUE The 2-D geometric model is produced from the AutoCAD engineering drawings of the Sprintex supercharger. This supercharger assembly consists of a male rotor, a female rotor and a casing. Clearances are formed between the two rotors, and between the rotors and casing. Figure (1) shows the “reference” position, when the two rotors are fully meshed. The placement of the inlet (I) / outlet (O) fillets can also be seen in Figure (1). The distance between the centres of the two rotors is 65.8 mm, resulting in a nominal clearance of 0.2 mm. The clearance between the rotors and the casing is also 0.2 mm. Two other