Presentation given at the International Spaceplanes and Hypersonic Systems and Technology conference in Bremen, Germany in October 2009. Topic was the effect of molecular weight on injectant mixing in supersonic flow

1. 1 The Effect of Injectant Molecular Weight on Transverse Injection Mixing Processes in Supersonic Flow Scott K Burger Joseph A Schetz And Ronald Ungewitter Virginia Tech October 2009

2. Objectives To compare the mixing of gases of widely varying molecular weight in transverse injection into a supersonic flow To perform this comparison at several flow conditions To determine the validity of turbulence modeling in a RANS-based unstructured CRUNCH CFD code by comparison with experimental results 2

3. Overview 3 Two gasses were used for this study Air (MW=28.97 kg/kmol) Methane (MW=16.04 kg/kmol) Helium results from earlier Virginia Tech studies were also available Three transverse injection cases examined Injection into an undisturbed Mach 4 free stream With a shock impinging upstream of injector With a shock impinging downstream of injector center

4. Facility 4 All tests were performed in the Virginia Tech Supersonic Wind Tunnel Blow-down type tunnel with approximately 25 second run time Mach 4 nozzle Plenum pressure PID controlled set point 1035 kPa Supersonic Wind Tunnel.a) Side view, b) Top view

5. Experiment Layout 5 Injector insert mounted flush into tunnel floor plate Slots for traversable probe downstream of injector Wedge mounted on sting to generate impinging shock for case 2) and case 3). Removed for case 1) Experiment set up, tunnel doors open for access. Flow left to right

6. Injection Conditions 6 Single circular hole injector, diameter 3.23 mm, aligned 30 degrees to the wall Sonic injection Dynamic pressure ratio: Mass flow rate 10.0 g/s air 6.7 g/s methane

7. Concentration Probe 7 Three sensors, common housing Temperature (K-type thermocouple) Pressure (tap connected to external transducer) Hot film Forced aspiration by vacuum pump ensures bow shock is swallowed inside housing Schematic diagram of the concentration probe

8. 8 A calibration is performed by sampling known concentrations in a vacuum tank From sensor readings the relative concentrations of species can be determined Calibration can be performed for any binary mixture Calibration curves for methane and air

9. Miniature Five-Hole Probe 9 Five hole probe, left and tip design, right (dimensions in inches) five pressure ports, one at tip center and four equally spaced around Each port contains a fast response internal piezoelectric pressure transducer

10. 10 The five-hole probe is calibrated for Mach number by comparing the ratio PA/P1 at varying Mach number PA=average of outer port pressures P1=center port pressure

11. 11 The five-hole can also be used to measure the flow angle A calibration map was created by recording the ratios of the outer port pressures at a range of pitch and yaw angles.

12. Case 1 12 Undisturbed Mach 4 free stream

13. Case 2 13 Shock impingement 4.5 injector diameters upstream of injector

14. Case 3 14 Shock impingement 2.5 injector diameters downstream of injector

15. Mach Number Contours 15 Mach number data taken at a grid of points encompassing the plume Plume is identified by region of lower Mach number Contours of air and methane compared by maximum height of selected contour Mach number contour for air injection into undisturbed free stream. Data below red line extrapolated

16. Mach Contour Comparison 16 Mach 3 contour was used for case 1, Mach 2 contour used for case 2 and 3 In all cases the air Mach contour penetrates higher than the equivalent methane contour Distances normalized to effective diameter:

17. Mach Number Components 17 Combining the flow angularity with Mach number the magnitude and direction of the Mach number in the transverse direction computed at each point on the grid Counter-rotating vortex pair created by the injection plume can be clearly identified M=0.5 Mach number components for air injection into undisturbed free stream

18. Mach Number Components Comparison 18 The vertical height of the vortex centers, , and horizontal spacing between them, , allows for comparison between surveys The vortices generated by the air injection appear to penetrate slightly higher, and spread slightly wider then the methane injection in all cases Vortices are lower and more closely spaced in the shock impingement cases especially with the shock impinging downstream of the injector

19. Concentration Contours 19 Concentration at each grid point is used to generate concentration contours Note dual plume core related to vortex pair Contours at each case compared by Maximum concentration Height of maximum concentration Width of plume concentration greater then stoichiometric value Concentration contour for methane injection into undisturbed free stream, data below red line is extrapolated

22. CFD 22 Computational comparison performed with RANS based unstructured CFD code CRUNCH CFD Comparison performed for case of undisturbed free stream and case of shock impingement downstream of injector

23. Mach Number Contour CFD Comparison 23

24. Concentration Contour CFD Comparison 24

25. Conclusions Higher molecular weight seems to increase penetration, but the effect is weak Shock impingement before, or shortly after injection reduces penetration Interaction of a shock and an emerging jet increases mixing Modern RANS-based CFD with appropriate turbulence modeling can provide predictions adequate for design More research on turbulence modeling, including better representation of the effects of molecular weight is warranted 25