Numerical Investigation Into the Effects of CoFlow Air on a Self-Excited Helium Jet

A numerical investigation is carried out on a helium jet having Reynolds number 150, and Richardson number 6.11. The effect of air co-flow on a self-excited helium jet is studied in the near field using commercial software ANSYS Fluent V18.1. The co-flow velocity ratio varied in the range of 0.17–0.87. The contours of the helium mole fraction along with the streamlines show the interaction of the toroidal vortex with the jet. The suppression of toroidal vortex is observed as the air co-flow velocity induced to the jet flow. Due to the suppression of vortices, radial spread/diffusion is limited, resulting in large gradients at the shear layer. The flickering frequency increases with the air co-flow. The amplitude of the oscillation at axial locations of higher z/d increases up to a certain co-flow velocity and then drops significantly at high co-flow velocity ratios. However, at upstream (near jet exit plane), oscillation amplitude decrease with increase in air co-flow. The velocity difference in the shear layer to the ambient elucidates the stabilization mechanism of the self-excited helium jet.

Caustics of Non-Paraxial Perfect Optical Vortices Generated by Toroidal Vortex Lenses

In this paper, we consider the comparative formation of perfect optical vortices in the non-paraxial mode using various optical elements: non-paraxial and parabolic toroidal vortex lenses, as well as a vortex axicon in combination with a parabolic lens. The theoretical analysis of the action of these optical elements, as well as the calculation of caustic surfaces, is carried out using a hybrid geometrical-optical and wave approach. Numerical analysis performed on the basis of the expansion in conical waves qualitatively confirms the results obtained and makes it possible to reveal more details associated with diffraction effects. Equations of 3D-caustic surfaces are obtained and the conditions of the ring radius dependence on the order of the vortex phase singularity are analyzed. In the non-paraxial mode, when small light rings (several tens of wavelengths) are formed, a linear dependence of the ring radius on the vortex order is shown. The revealed features should be taken into account when using the considered optical elements forming the POV in various applications.

Трехмерная структура течения в окрестности импульсного поверхностного дугового разряда в магнитном поле

The flow structure around pulsed surface arc discharge in a transverse magnetic field is described. It is shown that the motion of the plasma channel leads to the formation of a toroidal vortex. In this case, the characteristic rate of gas inflow to the model wall in the aerodynamic wake of the arc was 30–50 m/s and corresponded to a value of up to 40% of the maximum gas expansion rate in the loading phase.

Conditioning of Leakage Flows in Gas Turbine Rotor-Stator Cavities

Ingress is the penetration of hot mainstream fluid into the cavity formed between the turbine disc (rotor) and its adjacent casing (stator). Gas turbine engine designers use rim seals fitted at the periphery of the discs and a superposed sealant flow — typically fed through the bore of the stator — is used to reduce, or in the limit prevent, ingress. Parasitic leakage enters the cavity through pathways created between mating interfaces of engine components. Owing to the aggressive thermal and centrifugal loading experienced during the turbine operating cycle, the degree of leakage and its effect on ingress are difficult to predict. This paper considers the potential for leakage flows to be conditioned in order to minimise their parasitic effect on disc cooling, and ultimately engine, performance. Measurements of static and total pressure, swirl and species concentration were used to assess the performance of a simple axial clearance rim-seal over a range of non-dimensional leakage flow-rates. A computational model was used to provide flow visualisation to support the interpretation of flow structures derived from the experiments. Data is presented to investigate the effects of swirling the leakage flow in accordance with, and counter to, the disc rotation. The injected momentum from the leakage created a toroidal vortex in the outer part of the cavity. Co-swirl was found to improve the sealing effectiveness by up to 15% compared to the axially-introduced baseline and counter-swirled configurations. Varying the momentum of the leakage flow was considered by passing consistent mass-flows through a range of leakage outlet areas. Increasing the momentum was seen to increase the influence of the toroidal vortex on the flow structure in the cavity, which in turn influenced the sealing effectiveness.

Flow Visualization of Axisymmetric Impinging Jet on a Concave Surface

Flow visualization was performed to give a physical insight with vortical structures of an axisymmetric impinging jet on a concave surface. High-speed imaging was employed to get clear images with a laser light sheet illumination. An axisymmetric jet is issued into quasi-ambient air through a straight pipe nozzle with fully-developed velocity profile. A regular vertical pattern of an axisymmetric jet was observed with different flow entrainment rate. While an impinged jet turns to convert a wall jet along a concave surface, the flow interaction between the large-scale toroidal vortex and the concave surface was observed in the transition between the stagnation and wall jet zone. The ring-shaped wall eddies induced from a pair of toroidal vortices were also appeared to diverge into the radial direction along the concave surface. As the jet Reynolds number increases, small-scale vortices can be developed to a large-scale toroidal vortex. The location in which a large-scale toroidal vortex strikes is generally identical to the location where the secondary peak in heat transfer occurs. The frequency of large scale toroidal vortex on concave surface is found to be nearly similar as that of wall jet on flat surface. As the nozzle-to-target spacing (L/D) increases, it becomes shorter due to the loss of jet momentum. The flow behavior of axisymmetric impinging jet on a concave surface can be helpful to design the internal passage cooling for gas turbine blade.