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Detection of Nitrogen Flow Condensation in a Hypersonic Wind-Tunnel Using a Static Pressure Probe

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Detection of Nitrogen Flow Condensation in a Hypersonic Wind-Tunnel Using a Static Pressure Probe Detection of Nitrogen Flow Condensation in a Hypersonic Wind-Tunnel using a Static Pressure Probe Guillaume Grossir∗ and Patrick Rambaudy von Karman Institute for Fluid Dynamics, Chauss´eede Waterloo 72, 1640 Rhode-St-Gen`ese,Belgium Measurements with a slender static-pressure probe in the free-stream of the Longshot hypersonic wind-tunnel have recently been performed. They have revealed pressures larger than the theoretical values obtained with the assumption of an isentropic nozzle flow. The presence of flow condensation during the nozzle expansion is now investigated as a possible source of non-isentropicity to explain the free-stream static pressure mismatch. Different stagnation temperatures are investigated which either delay or promote flow nucleation. Standard operating conditions of the Longshot wind-tunnel are demonstrated to be free of condensation. Experiments performed with lower stagnation temperatures have success- fully promoted the condensation of nitrogen which could be detected by the static pressure probe. A weak amount of flow supersaturation has been achieved in agreement with het- erogeneous nucleation theory. The accurate performances of static pressure probes and their usefulness for the characterization of hypersonic flows are demonstrated. Nomenclature Symbols Greek c Specific heat, J=(kg:K) α Angle of attack, degrees D Static pressure probe diameter, mm ρ Flow density, kg=m3 h Flow enthalpy, J=kg σ Standard deviation k Thermal conductivity, W=(m:K) Length of the last characteristic line l projected along the nozzle axis, m Subscripts Distance along the static pressure probe L s At the stagnation point from its nosetip, mm t Stagnation condition m Mass, kg 0 p Flow pressure, Pa In the reservoir 1 Upstream a normal shock wave P_ Nozzle expansion rate parameter, s−1 2 Downstream a normal shock wave Q_ Heat flux, W=m2 1 Free-stream value t Time, ms T Flow temperature, K u Flow velocity, m=s I. Introduction he von Karman Institute for Fluid Dynamics (VKI) owns the Longshot hypersonic wind-tunnel. A Trecent flow characterization using static pressure probes1 revealed discrepancies between the measured free-stream pressure and the one estimated from theory with the assumption of an isentropic nozzle expansion. A general assessment of the wind-tunnel is now under way to determine the origin(s) of these differences. This paper is devoted to the eventuality of flow condensation as a possible non-isentropic source which could explain the free-stream pressure being larger than expected. ∗PhD candidate, Aeronautics and Aerospace Dept., AIAA Student Member, [email protected] yAssociate Professor, Aeronautics and Aerospace Dept., [email protected] 1 of 15 American Institute of Aeronautics and Astronautics The expansion of a gas through a nozzle to achieve supersonic flow velocities is associated with a severe decrease of the flow temperature. This is resulting from the energy conservation, which can be expressed u2 as h0 = 1=2 + h1 if the expansion is adiabatic. Therefore, any increase of the flow kinetic energy must be balanced by a decrease of the thermal energy h1 of the flow. It is then obvious that very low flow temperatures may easily be reached in supersonic/hypersonic wind-tunnels which accelerate the flow up to large velocities. The static temperature of the flow may be lowered so much that it reaches the saturation temperature of some of its constituents, and potentially resulting in a partial condensation of the test gas. The nucleation process is non-isentropic and the presence of droplets into the flow no longer allows to use classical compressible equations developed for adiabatic and isentropic single gaseous phase flows. Some free-stream properties, such as the free-stream pressure, are severely influenced and this justifies the present efforts to determine whether or not condensation takes place within the Longshot nozzle. Interpretation of experimental data obtained in a partially condensed flow is meaningless. Test condi- tions should therefore be defined so that the flow static temperature always remain above the condensation temperature. This can be done based on a theoretical description of the nozzle flow although a direct flow characterization is more reliable. The present paper first describes the Longshot wind-tunnel and the probes which are used to detect the presence of flow condensation. Then, a short review of the condensation phenomenon and the different parameters influenced are given. Theoretical results from the literature are used to approximate the non- equilibrium temperature at which the flow will condense according to free-stream flow properties in the Longshot wind-tunnel. Finally, experiments performed in the wind-tunnel are reported for a standard operating condition and additional ones which varied the stagnation temperature of the flow. The use of static pressure probe is demonstrated to be particularly useful to detect the onset of flow condensation. II. Wind tunnel, probes, instrumentation and test conditions II.A. Wind-tunnel The VKI Longshot hypersonic wind-tunnel was designed in the late 1960's in order to obtain flows at large Mach numbers and high Reynolds numbers.2 It has long been used at the VKI and successively modified to improve its performance3 including the design of a Mach 14 contoured nozzle. This facility is based on the principle of a gun tunnel with a piston used to compress the test gas. It allows for experiments at large Mach numbers within an environment quite free of high-enthalpy effects and at relatively large Reynolds numbers. A sketch of the wind-tunnel is given in figure1. It is composed of five main elements: Figure 1: Sketch of the VKI Longshot hypersonic wind-tunnel • a driver tube, initially filled with nitrogen at high pressure (up to ≈ 345 × 105 Pa for the present tests). • a driven tube, containing the test gas: nitrogen for the present experiments, initially at 1:5 × 105 to 5 × 105 Pa, and at ambient temperature. Nitrogen is supplied from commercially available pressurized bottles and pure to 99.998%. Residual flow pollutants amount to less than 3 ppm of water and less than 5 ppm of dioxygen. 2 of 15 American Institute of Aeronautics and Astronautics • a piston, initially located at the interface between the driver and driven tubes and with a mass of 1:5 to 5 kg in the present case. It is the interface between the driver and the driven gases. • a contoured nozzle. • a test section, vacuumed prior to an experiment to p < 1 Pa, in which the probes are located. When the piston is set free by the controlled rupture of a diaphragm, the high pressure gas of the driver tube pushes the piston through the driven tube at supersonic velocities and compresses almost adiabatically the nitrogen located ahead in the driven tube. The kinetic energy of the piston together with the flow being processed by a shock wave contribute to increase the internal energy of the test gas. The peculiarity of the Longshot wind tunnel is the presence of a set of 48 check valves between the driven tube and the 320 cm3 reservoir of the nozzle. They are used to trap the gas in a constant volume reservoir and prevent it from flowing backwards to the driven tube. Test duration is thus increased by at least an order of magnitude and is on the order of 20 ms at the nozzle exit.4 Once the piston has compressed the gas, it bounces in the driven tube, eventually compressing some additional gas into the reservoir, until it stops by itself. It is argued in Ref. 5 that the characteristic time of the decay of the reservoir conditions is long compared to the time required to establish a flow over the probes as long as flow recirculation zones or base flows are not considered. This justifies analysis of the measurements as quasi-steady ones. A 3 m long contoured nozzle (between throat and nozzle exit) with an exit diameter of 426 mm is currently used. The nozzle was designed for Mach 14 using the method of characteristics6 assuming an equilibrium flow and taking into account the dense gas phenomena present in the reservoir using an equation of state described in Refs. 7,8. The inviscid contour obtained was then corrected for the boundary layer displacement thickness based upon the correlation of Edenfield.9 II.B. Probes and instrumentation II.B.1. Reservoir measurements The total pressure is measured in the reservoir using a Kistler type 6215 piezoelectric sensor. It is connected to a Kistler 568 charge amplifier and uncertainties are given as 2σ = ±2%. The stagnation temperature in the reservoir cannot be measured accurately due to the short test time. It is estimated for each test based on measurements performed at the nozzle exit. II.B.2. Stagnation pressure/heat flux probe A hemispherical probe with a nose radius of 12:7 mm is equipped with a dual surface temperature and pressure measurement at its stagnation point. It is used to measure the test section stagnation point heat flux and stagnation pressure which are both required for the estimation of the free-stream total enthalpy. A coaxial thermocouple of type E with a microsecond response time measures the surface temperature.10 Its thermal sensitivity is taken from the standard laws for a type E thermocouple and the material thermal properties ρck (where ρ is the density, c the specific heat of the material and k its thermal conductivity) are calibrated by the manufacturer with an accuracy of 2σ = ±10 %. A thin tube runs through the axis of the thermocouple to allow for the stagnation pressure measure- ment. The pressure is measured by a differential Kulite sensor XCQ-093-25psi calibrated with a mercury manometer. Its measurements are accurate to 2σ = ±300 Pa.
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