A. M. Pradeep

Effect of variation in axial spacing and rotor speed combinations on the performance of a high aspect ratio contra-rotating axial fan stage

This article reports a parametric study on the performance of a high aspect ratio, low speed contra-rotating fan stage. Parameters namely speed ratios of the two rotors and axial spacing between the rotors that play a significant role on the overall performance of the contra-rotating fan stage were evaluated. The rotors have a low hub-tip ratio of 0.35 and chord of 45 mm. The two rotors were designed to develop a pressure rise of 1100 Pa and 900 Pa, respectively, when operating at 2400 r/min and developing a mass flow rate of 6 kg/s. In order to evaluate the performance of the designed rotors, measurements of total pressure at the entry of rotor-1, between the rotors and the exit of rotor-2 were taken using total pressure probe rake, 4-hole probe and a Kiel probe rake. The experiments were conducted for different speed combinations of rotor-1 and rotor-2. All these speed combinations were studied separately for different axial spacing. The performance plots revealed the existence of two stall limits namely, partial stall and full stall. For lower throttle positions, rotor-2 was observed to stall. Further lowering the mass flow rate; reduces the pressure rise capacity of the stage due to stalling of rotor-1 as well as rotor-2. For the design speed operation of rotor-1 in combination with an off-design speed of rotor-2, the flow parameters change significantly. A higher rotational speed of rotor-2 generates a stronger suction effect leading to an overall improvement in the performance of the whole stage. The effect of variation in the axial spacing between the rotors was also studied. The strongest suction effect between the rotors was observed at an axial spacing of 0.9 chord.

Secondary Flow Control Using Vortex Generator Jets

Results are presented of an experimental investigation into the effectiveness of vortex generator jets in controlling secondary flows in two-dimensional S-duct diffusers. The experiments were performed in uniform and distorted inflow conditions and the performance evaluation of the diffuser was carried out in terms of static pressure recovery and quality of the exit flow. In the case with inflow distortion, tapered fin vortex generators were employed in addition to vortex generator jets to control flow separation that was detected on the wall with inflow distortion. Detailed measurements including total pressure, velocity distribution, surface static pressure, skin friction, and boundary layer measurements were taken at a Reynolds number of 7.8×10 5 . These results are presented in terms of static pressure rise, distortion coefficient, and total pressure loss coefficient at the duct exit

Experimental study of rarefied gas flow near sudden contraction junction of a tube

An experimental study of nearly isothermal rarefied gas flow near the sudden contraction junction of a tube is presented in this paper. The measurements are performed with nitrogen gas flowing at low pressures in conventional tubes with sudden contraction area ratios of 1.48, 3.74, 12.43, and 64. The flow is dynamically similar to gas flow in a microchannel as the Knudsen number (0.0001 < Kn < 0.032) falls in the slip flow regime. The Reynolds number in the smaller section (Res) ranges between 0.2 and 837. The static pressure measurements are analyzed to understand the flow behavior. The static pressure variation along the wall and uniform radial pressure profile near the junction indicates absence of flow separation and vena contracta. The static pressure variation in both the tubes approaches the pressure variation as that of an isolated straight tube at a certain critical Knudsen number for a given area ratio. The velocity field is obtained through a momentum balance and using the flow measurements. Th...

Slip flow through a converging microchannel: experiments and 3D simulations

An experimental and 3D numerical study of gaseous slip flow through a converging microchannel is presented in this paper. The measurements reported are with nitrogen gas flowing through the microchannel with convergence angles (4°, 8° and 12°), hydraulic diameters (118, 147 and 177 µm) and lengths (10, 20 and 30 mm). The measurements cover the entire slip flow regime and a part of the continuum and transition regimes (the Knudsen number is between 0.0004 and 0.14); the flow is laminar (the Reynolds number is between 0.5 and 1015). The static pressure drop is measured for various mass flow rates. The overall pressure drop increases with a decrease in the convergence angle and has a relatively large contribution of the viscous component. The numerical solutions of the Navier–Stokes equations with Maxwells slip boundary condition explore two different flow behaviors: uniform centerline velocity with linear pressure variation in the initial and the middle part of the microchannel and flow acceleration with nonlinear pressure variation in the last part of the microchannel. The centerline velocity and the wall shear stress increase with a decrease in the convergence angle. The concept of a characteristic length scale for a converging microchannel is also explored. The location of the characteristic length is a function of the Knudsen number and approaches the microchannel outlet with rarefaction. These results on gaseous slip flow through converging microchannels are observed to be considerably different than continuum flow.

Benefits of Nonaxisymmetric Endwall Contouring in a Compressor Cascade With a Tip Clearance

ABSTRACT This paper describes the design of a non-axisymmetric hub contouring in a shroudless axial flow compressor cascade operating at near stall condition. Although, an optimum tip clearance reduces the total pressure loss, further reduction in the loss was achieved using hub contouring. The design methodology presented here combines an evolutionary principle with a three-dimensional CFD flow solver to generate different geometric profiles of the hub systematically. The resulting configurations were preprocessed by GAMBIT

Stall Inception Mechanism in an Axial Flow Fan Under Clean and Distorted Inflows

The present paper describes the use of Morlet wavelet transform in understanding the stall inception mechanism in a single stage axial flow fan. Unsteady pressure data from wall mounted sensors were used in the wavelet transforms. This paper was carried out under undistorted and distorted inflow conditions as well as for slow throttle closure and throttle ramping. It was observed from the wavelet transforms that the stall inception under clean inflow (undistorted) and counter-rotating inflow distortions (in the direction opposing the rotor rotation) incur through short length-scale disturbances and through long length-scale disturbances under static and co-rotating inflow distortions (in the same direction of rotor rotation). Modal activity was observed to be insignificant under clean inflow while under static inflow distortion, long length-scale disturbances evolved due to interaction between rotor blades and the distorted sector, especially near the trailing edge of the distortion screen. The presence of a strong mode was observed under both co- and counter-rotating inflow distortions. With throttle ramping, stall inception occurs through long and short length-scale disturbances under co- and counter-rotating inflow distortions, respectively. Some preliminary flow characteristics were studied using a seven hole probe. A significant increase in flow angle and decrease in axial flow coefficient close to the rotor tip were observed under co-rotating inflow distortion as compared with counter-rotating inflow distortion.

Active flow control in circular and transitioning S-duct diffusers

Performance enhancement of three-dimensional S-duct diffusers by secondary flow and separation control using vortex generator jets is the objective of the current experimental investigation. Two different diffuser geometries namely, a circular diffuser and a rectangular-to-circular transitioning diffuser were studied. The experiments were performed in uniform inflow conditions at a Reynolds number of 7.8 × 10 5 and the performance evaluation of the diffusers was carried out in terms of static pressure recovery and quality (flow uniformity) of the exit flow. Detailed measurements that included total pressure, velocity distribution, surface static pressure, skin friction, and boundary layer measurements were taken and these results are presented here in terms of static pressure rise, distortion coefficient, total pressure loss coefficient, and the transverse velocity vectors at the duct exit

Detection of Separation in S-duct Diffusers using Shear Sensitive Liquid Crystals

In the present experimental investigation, shear sensitive liquid crystals have been successfully used to study the flow characteristics and detect separation in two-dimensional Sduct diffusers of different curvatures. Tapered-fin vortex generators in two different orientations were used to control flow separation that was observed on one of the curved walls of the diffuser. The results were verified by conventional oil flow visualization technique and excellent agreement was observed. In addition to visualization, detailed measurements that included wall static pressure, skin friction, diffuser exit total pressure and velocity distributions were taken in a uniform inlet flow with Reynolds number of 3.49 × 105. These results are presented here in terms of skin friction distribution, distortion and total pressure loss coefficients. The extent of the separation zone (in terms of intensity of red distribution) in the diffuser with and without vortex generators (in both configurations) compared well with the Preston tube measurements. The present study demonstrates that shear sensitive liquid crystals can be efficiently used to study the flow physics in complex internal flows. In addition, the results also indicate that shear sensitive liquid crystals can be effectively used not only as flow visualization tool but also to gain quantitative information about the flow field in internal flows.

PERFORMANCE COMPARISON OF AIR TURBOROCKET ENGINE WITH DIFFERENT FUEL SYSTEMS

An air turbo rocket (ATR) is an air-breathing propulsion system that possesses design, performance, and operation characteristics of the turbojet, ramjet, and rocket in a single unit. In order to assess the capabilities of the ATR, an analysis of a representative liquid oxygen - liquid hydrogen engine is carried out. Complete analysis of the intake, fan, gas generator, turbine and combustion chamber has been carried out for two representative flight conditions, namely, a low speed of Mach number 2 and altitude of 6000m and a high speed of Mach number 3 and altitude of 9000m. The study is aimed at finding oxygen to fuel (O/F) ratio for maximum specific impulse. An off -design analysis of the ATR designed at Mach 1 is also carried out. Finally, to realize these specific impulse values, basic dimensions of the turbo machinery, number of stages for a tip turbine and a conventional inline turbine are proposed. Comparison is made between an ATR operating on different fuels in terms of the number of turbine stages required and the specific impulse variation with different afterburner fuels.

Improvement of Moderately Loaded Transonic Axial Compressor Performance Using Low Porosity Bend Skewed Casing Treatment

This paper presents experimental results of a single stage transonic axial flow compressor coupled with low porosity bend skewed casing treatment. The casing treatment has a plenum chamber above the bend slots. The depth of the plenum chamber is varied to understand its impact on the performance of compressor stage. The performance of the compressor stage is evaluated for casing treatment and plenum chamber configurations at two axial locations of 20% and 40%. Experimental results reveal that the stall margin of the compressor stage increases with increase in the plenum chamber volume. Hot-wire measurements show significant reduction in the turbulence intensity with increase in the plenum chamber volume compared to that with the solid casing at the stall condition. At higher operating speeds of 80% and at 20% axial coverage, the stall margin of the compressor increases by 20% with half and full plenum depth. The improvement in the peak stage efficiency observed is 4.6% with half plenum configuration and 3.34% with the full plenum configuration. The maximum improvement in the stall margin of 29.16% is obtained at 50% operating speed with full plenum configurations at 40% axial coverage.