Arrival Of Driver Gas Research Articles

Mass spectrometric measurements of driver gas arrival in the T4 free-piston shock-tunnel

Available test time is an important issue for ground-based flow research, particularly for impulse facilities such as shock tunnels, where test times of the order of several ms are typical. The early contamination of the test flow by the driver gas in such tunnels restricts the test time. This paper reports measurements of the driver gas arrival time in the test section of the T4 free-piston shock-tunnel over the total enthalpy range 3–17 MJ/kg, using a time-of-flight mass spectrometer. The results confirm measurements made by previous investigators using a choked duct driver gas detector at these conditions, and extend the range of previous mass spectrometer measurements to that of 3–20 MJ/kg. Comparisons of the contamination behaviour of various piston-driven reflected shock tunnels are also made.

Numerical Analysis of Reducing Driver Gas Contamination in Impulse Shock Tunnels

A numerical analysis was carried out to study the interaction of the reflected shock with incident boundary layer and contact surface in reflected shock tunnels and the driver gas contamination of the hot flow, in tailored conditions. Navier-Stokes equations were solved with a multiblock finite volume method in a shock tube and a nozzle. Different suction devices, such as a corner slit, a corner slit prolongated by a sleeve, and a conical ring, were tested for their capability of delaying this contamination effect. The results show that the driver gas passes through a bifurcated shock foot as a jet and causes premature contamination of the driven gas. They also show why the suction induced by a corner slit is inefficient for delaying the contamination. Located just behind the bifurcated shock foot after the shock/interface interaction, the sleeve device captures driver gas upstream and leads to an improvement in the driver gas arrival time by a factor of 1.8. Finally, based on the present numerical investigation, a new conical ring device is proposed, with a horizontal and vertical suction, which further increases this factor to 2.5

Unstarted Inlet for Direct-Connect Combustor Experiments in a Shock Tunnel

Despite the very short flow duration, it is possible to conduct direct-connect combustor experiments in impulse facilities by using an unstarted converging-diverging nozzle located in the freestream flow. Such an unstarted inlet contains a very large subsonic region, which would be expected to require considerable time to establish and reach steady flow. Pressure measurements indicate that steady flow was achieved after approximately 1.5 ms of freestream flow, which at low tunnel enthalpies leaves sufficient time for meaningful combustor measurements to be made before driver gas arrival.

Gasdynamical Detectors of Driver Gas Contamination in a High-Enthalpy Shock Tunnel

Simple gasdynamical devices consisting of a duct and a wedge have been applied to the detection of driver gas arrival in the test section of a high-enthalpy shock tunnel. Static pressure in the duct has been measured during a shot, and the time of driver gas arrival has been determined by the onset of the pressure rise, which indicates duct flow choking. The ability to detect driver gas in small concentrations is critical to the satisfactory performance of the device. Duct internal flows for various wedge angles have been numerically simulated to clarify the flow choking mechanism. The simulations give an idea of the improvement of detector sensitivity, and modified configurations of the detector are proposed. Flow visualization in the duct leads to a better understanding of pressure traces obtained, and pressure measurement data show a satisfactory degree of sensitivity

Experimental Demonstration of Use of N2O to Increase Shock Tunnel Test Time

An experimental investigation was carried out into the replacement of air in the driven tube of a reected shock tunnel by an N 2O/N2 mixture in order to increase the test time. The ® nal gas mixture obtained is the same with either air or N 2O/N2 in the driven tube. The incident shock velocities were between 2 and 3 km/s. Test times (or times until driver gas arrival) were estimated from light emission histories in the driven tube and in the nozzle and from pressure histories just upstream of the nozzle entrance. The test times estimated from the light emission histories behind the incident shock showed that consistent increases of 60± 100 % were obtained upon substituting N2O/N2 forairinthedriventube.Theseincreaseswereingoodagreementwith theoreticalestimates. Thetesttimes estimated from the light emission histories in the nozzle or pressure histories at the nozzle inlet showed signi® cant improvements with N 2O/N2 only forcases where thefacility was operated at substantially overtailored conditions, probably due to the greater stability of the driver± driven interface at these conditions. At overtailored operating conditions, test times increases of 60± 100 % with N2O/N2 wereobserved with all three diagnostic techniques. These increases were in reasonable agreement with theoretical estimates.

An investigation of the reflected shock interaction process in a shock tube.

Following the first interaction between the reflected shock from the end wall and the advancing contact surface in a shock tube, cold gas has been observed at the end plate at a time much earlier than that predicted on the basis of simple shock tube theory. Investigation of the nature of this cold gas has been made by an infra-red absorption technique. The driver gas hydrogen is tagged with a small quantity of infrared active gas, and shocks are driven with this gas mixture into an infrared inactive test gas. The time of arrival of driver gas at the end plate is measured from the absorption records and calculations based on a modification of the shock bifurcation model agree substantially with these experimental results. Use is also made of an infrared absorption/emission technique to determine the temperature of the infrared source.