The Effect of Injectant Molecular Weight on Transverse Injection Mixing

Processes in Supersonic Flow

Virginia TechOctober 2009

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Scott K BurgerJoseph A Schetz

AndRonald Ungewitter

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

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Overview• 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 examined1) Injection into an undisturbed Mach 4 free stream2) With a shock impinging upstream of injector3) With a shock impinging downstream of injector

center

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Facility• 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

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Experiment Layout• 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

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Injection Conditions• 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

1.2)(

)(2

2

u

uq j

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Concentration Probe• 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

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• 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

0 5 10 15 20 25 30 35 402.75

3.25

3.75

4.25

4.75

5.25

% CH4

100%Logarithmic (100%)90%Logarithmic (90%)

Concentration Probe Pressure

Hot

Film

Vol

tage

Calibration curves for methane and air

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Miniature Five-Hole Probe

• five pressure ports, one at tip center and four equally spaced around

• Each port contains a fast response internal piezoelectric pressure transducer

Five hole probe, left and tip design, right (dimensions in inches)

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• 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

0.5 0.55 0.6 0.65 0.71.5

2

2.5

3

3.5

4

4.5

5-Hole Probe Mach Number Calibration

pA/p1

Ma

ch

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• 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.

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Case 1

Undisturbed Mach 4 free stream

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Case 2

Shock impingement 4.5 injector diameters upstream of

injector

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Case 3

Shock impingement 2.5 injector diameters downstream of

injector

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Mach Number Contours

• 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

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Mach Contour Comparison

• 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:

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Case No Shock

Injectant Air CH4

Max Mach 3 contour height

5.0 4.4

Case Shock Upstream

Injectant Air CH4

Max Mach 2 contour height

4.0 3.6

Case Shock Downstream

Injectant Air CH4

Max Mach 2 contour height

3.6 3.3

mmCdd deff 03.3

Mach Number Components• 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

Mach number components for air injection into undisturbed free stream

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M=0.5

Mach Number Components Comparison

• 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

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Case No Shock Shock Upstream Shock Downstream

Injectant Air CH4 Air CH4 Air CH4

2.0 1.9 1.8 1.7 1.7 1.6

3.1 3.0 2.8 2.7 2.1 2.0yVzV

zV

yV

Concentration Contours• 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

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Concentration Comparison

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No Shock Shock Upstream Shock Downstream

Plume width 4.65 4.62 4.29

Vertical location of max concentration

1.45 1.32 0.99

Maximum concentration

0.64 0.47 0.35

• Addition of shock impingement decreases plume height and reduces maximum concentration

• Shock impinging shortly downstream of injector decreases maximum concentration and plume height further

Concentration Contour Comparison

• Shock interaction reduces penetration• Shock impingement downstream increases

mixing

Undisturbed free stream Shock upstream of injector Shock downstream of injector

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CFD

• 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

No Shock Downstream Shock

Shock reflection location

Y= 6 y/Deq

Oblique shock

Symmetry Plane Pressure Contour

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Mach Number Contour CFD Comparison

Experimental Data CRUNCH CFD

No

Shock

No

ShockShock

Downstream

Shock

Z= 6 y/Deq

Z= 6 y/Deq

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Concentration Contour CFD Comparison

Experiment Data CRUNCH CFD

No Shock

Shock Downstream

Z= 6 y/Deq

Z= 6 y/Deq

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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

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