No abstract

As effective devices to extend the fuel residence time in supersonic flow and prolong the duration time for hypersonic vehicles cruising in the near-space with power, the backward-facing step and the cavity are widely employed in hypersonic airbreathing propulsive systems as flameholders. The two-dimensional coupled implicit RANS equations, the standard k-e turbulence model, and the finite-rate/eddy-dissipation reaction model have been used to generate the flow field structures in the scramjet combustors with the backward-facing step and the cavity flameholders. The flameholding mechanism in the combustor has been investigated by comparing the flow field in the corner region of the backward-facing step with that around the cavity flameholder. The obtained results show that the numerical simulation results are in good agreement with the experimental data, and the different grid scales make only a slight difference to the numerical results. The vortices formed in the corner region of the backward-facing step, in the cavity and upstream of the fuel injector make a large difference to the enhancement of the mixing between the fuel and the free airstream, and they can prolong the residence time of the mixture and improve the combustion efficiency in the supersonic flow. The size of the recirculation zone in the scramjet combustor partially depends on the distance between the injection and the leading edge of the cavity. Further, the shock waves in the scramjet combustor with the cavity flameholder are much stronger than those that occur in the scramjet combustor with the backward-facing step, and this causes a large increase in the static pressure along the walls of the combustor.

The cavity has been widely employed as the flame holder to prolong the residence time of fuel in supersonic flows since it improves the combustion efficiency in the scramjet combustor, and also imposes additional drag on the engine. In this paper, the two-dimensional coupled implicit Reynolds Average Navier-Stokes equations, the RNG k-e turbulence model and the finite-rate/eddy-dissipation reaction model have been employed to numerically simulate the combustion flow field of an integrated hypersonic vehicle. The effect of cavity location on the combustion flow field of the vehicle has been investigated, and the fuel, namely hydrogen, was injected upstream of the cavity on the walls of the first stage combustor. The obtained results show that the viscous lift force, drag force and pitching moment of the vehicle are nearly unchanged by varying the cavity location over the location range and designs considered in this article, namely the configurations with single cavity, double cavities in tandem and double cavities in parallel. The variation of the fuel injection strategy affects the separation of the boundary layer, and the viscous effect on the drag force of the vehicle is remarkable, but the viscous effects on the lift force and the pitching moment are both small and they can be neglected in the design process of hypersonic vehicles. In addition to varying the location of the cavities, three fuel injection configurations were considered. It was found that one particular case can restrict the inlet unstart for the scramjet engine.

In order to design a hypersonic vehicle for a wide-ranged Mach number, a novel parallel vehicle for a wide-speed range has been proposed. In this paper, we employ a numerical method to investigate a parallel vehicle's aerodynamic performance and flow field characteristics. The obtained results show that the aerodynamic performance of the novel parallel vehicle is better than that of the waverider designed with a single Mach number for the wide-speed range. With the increase in Mach number, the lift-to-drag ratio of the novel parallel vehicle first increases and then decreases. When the Mach number is 7 and the angle of attack is 3 • , the lift-to-drag ratio is the largest, and its value is 3.968. When the angle of attack is 3 • , the lift-to-drag ratio is not lower than 3.786 in the range considered in the current study, and the novel parallel vehicle's aerodynamic performance is good. The wing changes the drag performance of the parallel vehicle remarkably, and results in the decrease of the lift-to-drag ratio. Meanwhile, the wing can enhance the pitching moment performance.

Evaporating meniscus of ethanol and ethanol-based nanofluids (0.01vol.%) in micro-channels were experimentally studied. Visualisation and thermographic results of the stationary meniscus confined in high-aspect-ratio rectangular micro-channels (hydraulic diameters are 571 μm, 727 μm and 1454 μm, channel cross sectional aspect ratio is 20, 20, 10 respectively) were obtained. It was found that interface evaporation rate increases with heat flux. The meniscus interface becomes deformed when the evaporation rate increases. The use of nanofluids largely enhances the interface stability even though the particle volume fraction is at a very low level. Besides, a stick-slip and back-jump behaviour of the nanofluids meniscus was captured during the transition from stable to deformed interface. Moreover, sink effect at the liquid-vapour interface was discussed based on the IR results.