Effect of Expansion Direction/Area Ratio on Loss Characteristics and Flow Rectification of Curve Diffuser

Curve diffuser is frequently used in applications such as HVAC, wind- tunnel, gas turbine cycle, aircraft engine etc. as an adapter to join the conduits of different cross-sectional areas or an ejector to decelerate the flow and raise the static pressure before discharging to the atmosphere. The performance of the curve diffuser is greatly affected by the abrupt expansion and inflection introduced, particularly when a sharp 90o curve diffuser is configured with a high area ratio (AR). Therefore, the paper aims to numerically investigate the effect of the expansion direction of AR=1.2 to 4.0 curve diffuser on loss characteristic and flow rectification. 90o curve diffuser operated at inflow Reynolds Number, Rein=5.934 × 104 to 1.783 × 105 was considered. Results show that pressure recovery improves when the area ratio increases from 1.2 to 2.16 for both 2D expansion (z- direction) and 3D expansion (x- and z- direction). On the other hand, the increase of inflow Reynolds number causes the flow uniformity to drop regardless of the expansion directions. 3D expansion (x- and z- direction) curve diffuser with AR=2.16, operated at Rein=8.163 × 104, is opted as the most optimum, producing the best pressure recovery up to 0.380. Meanwhile, 2D expansion (z-direction) curve diffuser of AR=2.16, , operated at Rein= 5.934 × 104, is chosen to provide the best flow uniformity of 2.330 m/s. 2D expansion (x- direction) should be as best avoided as it provides the worst overall performance of 90o curve diffuser.

Core-Loss Analysis of Linear Magnetic Gears Using the Analytical Method

In this study, analysis of core-loss occurring in the magnetic flux modulation core of a linear magnetic gear and the core of each mover is presented, using an analytical method. Losses in electric machines were generally calculated and analyzed using the finite element method (FEM). However, in the case of core-loss, the exact loss value could not be calculated using FEM data. Therefore, we considered the harmonic component of the air-gap magnetic flux density waveform with the modified Steinmetz equation, and performed a more accurate core-loss analysis with magnetic behavior analysis. Thus, we performed a calculated core-loss characteristic comparison with the FEM and the modified Steinmetz equation.

Effect of Matching Parameters Between Rotor and Diffuser on the Flow Loss Characteristic in an Axial Turbine

Diffuser is a key component affecting the aerodynamic performance of an axial turbine significantly. However, there is little research on the matching parameters between a rotor and a diffuser. In present study, effect of matching parameters which are normalized axial clearance (ACLR‘) and normalized radial size deviation (RDev‘) between rotor and diffuser on the aerodynamic performance and flow loss characteristic of a typical diffuser is revealed. A validated Computational Fluid Dynamic (CFD) model which couples the cavity between the rotor and diffuser is also proposed. The results illustrate that the isentropic efficiency is decreased with the increase of ACLR‘ and RDev‘. The RDev‘ presents a larger influence on isentropic efficiency. When ACLR‘ is increased from 0.21 to 0.43, the isentropic efficiency is only decreased by 0.3%, while the isentropic efficiency reduction of 0.65% is achieved when RDev‘ is increased from 0.091 to 0.192. A “leakage flow”, which is a result of the egress and ingress of the fluid near diffuser-cavity interface, is formed. It presents limited effect on the main flow in the diffuser when ACLR’ is only included and the flow loss near the guide cone surface is increased slightly. However, obvious back flow caused by the “leakage flow” is observed near the diffuser-cavity interface, when RDev’ is included. A flow separation is thus formed downstream and the flow loss in the diffuser is thus increased. The efficiency reduction caused by actual matching parameters (ACLR‘ with 0.21 and RDev‘ with 0.091) is increased with the decrease of expansion ratio. It can be more than 1.26% when the expansion ratio is less than 2.0. An isentropic efficiency reduction of 0.65% is found when different tip clearance are further adopted, and the mixture loss between tip leakage flow and back cavity flow can be neglected. In summary, matching parameters, especially for radial deviation, should be minimized reasonably in further optimization.