High thrust-to-weight ratio is crucial in aero-engine design. The adverse pressure gradient in compressors limits the maximum diffusion per stage, necessitating more stages to achieve the desired pressure ratio. Tandem airfoils, with their superior diffusion capability compared to conventional single airfoils, can achieve the required pressure ratio with fewer stages. However, the presence of two tip leakage vortices from both the forward and aft rotors creates a more complex tip region compared to conventional rotors. The design and performance aspects of the tandem rotor have been reasonably well documented. However, the stall characteristics of such rotors are yet to be thoroughly investigated. To better understand the stall phenomenon in a tandem rotor, the role of each tip leakage vortex must be investigated separately. Full annulus unsteady analysis of the highly loaded tandem rotor is carried out using the commercially available software Ansys CFX. As the rotor approaches stall, significant changes occur in the trajectory and strength of these vortices, with increased blockage near the stall point. For the tandem rotor, forward rotor spillage is critical. This spillage increases the local incidence near the tip of the forward rotor, resulting in localized flow separation. Small disturbances arising from the leading-edge separation coalesce, forming a rotating stall cell that grows in strength and size as it rotates in the direction opposite to rotor rotation. Even though the aft rotor encounters tip vortex spillage from the forward and aft rotors of the subsequent passage, the nozzle gap effect effectively mitigates flow separation, ensuring stable operation of the tandem rotor system. The leading-edge separation over the forward rotor suction surface evolves into a tornado vortex, with the suction leg on the forward rotor suction surface and the other end connected to the casing. Apart from the tip leakage vortex of the aft rotor, other vortex structures on the aft rotor are intermittent, with some collapsing and new vortices forming. ... Read more

This paper presents experimental and three-dimensional numerical study of gaseous slip flow through diverging microchannel. The measurements are performed for nitrogen gas flowing through microchannel with different divergence angles (4°, 8°, 12° and 16°), hydraulic diameters (118, 147 and 177 μm) and lengths (10, 20 and 30 mm). The Knudsen number falls in the continuum and slip regimes (0.0005 ⩽ Kn ⩽ 0.1; Mach number is between 0.03 and 0.2 for the slip regime) while the flow Reynolds number ranges between 0.4 and 1280. The static pressure drop is measured for various mass flow rates; and it is observed that the pressure drop decreases with an increase in the divergence angle. The viscous component has a relatively large contribution in the overall pressure drop. The numerical solution of the Navier–Stokes equations with the Maxwell’s slip boundary condition shows absence of flow reversal (due to slip at the wall), larger viscous diffusion and lower kinetic energy in the diverging microchannel. The centerline velocity and wall shear stress decrease with an increase in the divergence angle. The numerical results further show three different flow behaviors: a nonlinear pressure variation with rapid flow deceleration in the initial part of the microchannel; uniform centerline velocity with linear pressure variation in the middle part, and flow acceleration with nonlinear pressure variation in the last part of the microchannel. A characteristic length scale for diverging microchannel is also defined. The location of the characteristic length is a function of the Knudsen number and shifts toward the microchannel inlet with rarefaction. Mass flow rate and pressure distribution along the channel are also obtained numerically from the direct simulation Monte Carlo (DSMC) method and compared suitably with the experimental data or Navier–Stokes solutions. Empirical relations for the mass flow rate and Poiseuille number are suggested. These results on gaseous slip flow through diverging microchannels are considerably different than their continuum counterparts, and are not previously available.
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