Zhijun Lei

Effects of Scalloping on the Mixing Mechanisms of Forced Mixers With Highly Swirling Core Flow

This paper presents a detailed experimental and computational investigation of the effects of scalloping on the mixing mechanisms of a scaled 12-lobe turbofan mixer. Scalloping was achieved by eliminating approximately 70% of the lobe sidewall area. Measurements were made downstream of the mixer in a co-annular wind tunnel, and the simulations were carried out using an unstructured Reynolds averaged Navier–Stokes (RANS) solver, Numeca FINE/Hexa, with k-ω SST model. In the core flow, the swirl angle was varied from 0 deg to 30 deg. At high swirl angles, a three-dimensional separation bubble was formed on the lobes suction surface penetration region and resulted in the generation of a vortex at the lobe valley. The valley vortex quickly dissipated downstream. The mixer lobes removed most of the swirl, but scalloped lobes removed less swirl in the region of the scalloped notch. The residual swirl downstream of the scalloped mixer interacted with the vortices and improved mixing rates compared to the unscalloped mixer. Core flow swirl up to 10 deg provided improved mixing rates and reduced pressure and thrust losses for both mixers. As core flow swirl increased beyond 10 deg, the mixing rate continued to improve, but pressure and thrust losses declined compared to the zero swirl case. Lobe scalloping, in high swirl conditions, resulted in better mixing and improved pressure loss over the unscalloped mixer but at the expense of reduced thrust.

Numerical research on the mixing mechanism of lobed mixer with inlet swirl in linear radial distribution

A detailed numerical simulation is presented to investigate the effects of inlet swirl and its radial distribution on the mixing mechanisms of a turbofan mixer with 12 lobes, by using the commercial ANSYS CFX solver and k–ω SST model. The core-to-bypass temperature ratio and pressure ratio were set to 2.59 and 0.97, respectively, giving the Mach number of 0.66 and bypass ratio of 2.65 at mixing nozzle outlet. In the core inlet, the swirl angle was raised from 0° to 30° in a uniform or linear radial distribution manner. The inlet swirl and its radial gradient did enhance the development, interaction, and dissipation of the vortices downstream of lobed mixer, resulting in accelerating the lobed jet mixing. When the inlet swirl was less than 20°, the total pressure and thrust loss increments of lobed jet were acceptable and no more than 0.26% and 1.57%, respectively, compared with the baseline case. The results also showed that the three-dimensional separation bubble on center-body and the backflows along jet axis at the rail of center-body, resulting from the swirling flow between lobes’ trough and center-body, were the dominant sources of total pressure and thrust losses for all cases with inlet swirl. And, reasonable radial distribution of inlet swirl could inhibit the aforementioned 3D separation and backflow, and thus limited the increment of jet mixing loss favorably.

Numerical Research on the Mixing Mechanism of Lobed Mixer With New De-Swirling Structure

A detailed numerical simulation is presented to investigate the new de-swirling methods and their effect on the mixing mechanisms of a turbofan mixer with 12 lobes. The numerical simulation employed a commercial solver, ANSYS CFX, using k-ω SST model. The core-to-bypass temperature ratio and pressure ratio were set to 2.59, and 0.97 respectively, giving the Mach number of 0.66 and bypass ratio of 2.65 at mixing nozzle outlet. The inlet swirl typically accelerates the jet-flow mixing by enhancing the vortices intensity and interaction, but leakage swirling flow can cause a three-dimensional separation bubble and the recirculation zone resulting in the dramatic increasing the total pressure loss and thrust loss. Removal of the leakage swirling flow between the lobes’ trough and centre-body was the key to limit the negative influence of inlet swirl.Two IGV design were investigated, DS1 and DS2. DS1 was installed at the upstream of the lobed mixer, could remove the negative effect of inlet swirl properly, but also inhibited the active role of the inlet swirl. The total pressure and thrust loss reduced by 0.31% and 3.8%, respectively, but the mixing efficiency also decreased by 1.72%. DS2, an integrated strut with the lobed mixer design, not only ensured the structure strength of the lobed mixer, but also reduced the length and weight of the exhaust system. This method suppressed the flow separation bubble on centre-body to some extent, and eliminated the recirculation zone downstream of the cenrebody, resulting in the total pressure loss decrease of 0.31% and thrust gain of 3.63%. On the other hand, the method DS2 also made full use of the inlet swirl to enhance the jet-flow mixing, resulting in the mixing efficiency increased 1.54% compared with that of the DS1 case. Under the off-design conditions with the incidence angle of ±10°, the aerodynamic performance of the DS2 cases didn’t changed too much such as the DS1 cases.Copyright

Parametric studying of low-profile vortex generators flow control in an aggressive inter-turbine duct

This paper presents a study of low-profile vortex generators flow control on the effect of casing boundary layer separation in an aggressive inter-turbine duct. Counter-rotating and co-rotating vortex generators configurations were tested parametrically to obtain the optimum low-profile vortex generators’ setup within the aggressive inter-turbine duct, in terms of the axial location, height, angle of attack and streamwise vortices per swirl vane pitch. Surface oil flow visualization was used to qualitatively examine the low-profile vortex generators eliminating the casing separation and subsequently the detailed seven-hole probe measurements were carried out to evaluate the loss reduction quantitatively. The inter-turbine duct examined for this study was more aggressive compared with the geometries found in the most current modern engine designs. Measurements were made inside the aggressive inter-turbine duct annulus at Reynolds number of 150,000. The flow structures within the aggressive inter-turbine duct were found to be dominated by counter-rotating vortices evolved by the swirl vanes and boundary layer separation in both casing and hub regions. A massive casing boundary layer separation extending from the duct first bend to the second bend was found within the aggressive inter-turbine duct. The primary source of pressure loss in the aggressive inter-turbine ducts was the result of the extensive casing boundary layer separation. Flow control by utilizing the low-profile vortex generators installed on the casing was conducted to eliminate the casing separation. Regardless of the vortex generator mitigating the casing separation associated with consequential loss reduction, a pair of casing counter-rotating vortices was always evident at the duct second bend in all cases. Among the tested low-profile vortex generator configurations, two of the most effective low-profile vortex generator configurations, one counter-rotating and one co-rotating low-profile vortex generator configurations, were acquired as a result of successfully eliminating the casing separation. Despite that the inter-turbine duct is aggressively designed, it is concluded that this duct associated with proper flow control techniques is considered as a potential candidate for weight saving in a high bypass ratio engine.

Influence of Inlet Swirl on the Aerodynamics of a Model Turbofan Lobed Mixer

This paper presents a detailed experimental investigation of the influence of core flow inlet swirl on the mixing and performance of a 12-lobe un-scalloped turbofan mixer. Measurements were made downstream of the mixer in a co-annular wind tunnel. The core-to-bypass velocity ratio was set to 2:1, temperature ratio to 1.0, and pressure ratio to 1.03, giving a Reynolds number of 5.2×105 , based on the core flow inlet velocity and equivalent hydraulic diameter. In the core flow, the background turbulence intensity was raised to 5% and the swirl angle was varied using five vane geometries, with nominally uniform swirl angles of 0°, 5°, 10°, 20° and 30°. Flow measurements captured flow structures involved in the mixing process. Most of mixing took place immediately downstream of the exit nozzle. The vane wake slightly enhanced large scale mixing of streamwise vortices. At low swirl angles, mixing was found to be mainly due to the interaction between streamwise vortices and normal vortices. At high swirl angles, the lobed mixer acted similar to a guide vane and removed most of the inlet swirl between the crest and trough of the mixer. However, the upstream swirling flow persisted in the core region between the center-body and lobed mixer trough, causing a reverse flow zone downstream of the centre-body. As the reversed flow became larger with increasing swirl, the swirling flow in the core region moved radially outwards and further interacted with the outer region flow. The stronger interaction of streamwise vortices with normal vortex improved mixing from the trough to the crest of the lobed mixer. The balance between enhanced mixing and increased reversed flow downstream of the centre-body, resulted in increased overall total pressure losses with increasing inlet swirl angles.Copyright