CONTROL OF INSTABILITIES IN REACTIVE AND NON-REACTIVE FLOWS

Transcription

1 CONTROL OF INSTABILITIES IN REACTIVE AND NON-REACTIVE FLOWS Ann R. Karagozian Department of Mechanical and Aerospace Engineering University of California Los Angeles

2 Propulsion Applications of EPRL Experimental Studies Acoustically-coupled combustion instabilities Improved safety and reliability in rocket engines (launch and spacecraft) Primary Combustion Zone Secondary Combustion Zone Turbine Inlet Guide Vanes Fuel Nozzle Primary Air Jets Dilution Air Jets Controlled dilution and fuel jet injection Gas turbine engines with improved efficiency and reduced emissions AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 2

3 Propulsion Applications of EPRL Experimental Studies Acoustically-coupled combustion instabilities Improved safety and reliability in rocket engines (launch and spacecraft) Controlled diluent injection for LRE preburners Efficient staged combustion cycle engines (RD-180 replacement??) AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 3

4 Fundamental Experimental Studies CROSSFLOW JET Combustion of liquid droplets exposed to external acoustic excitation (Valentini, Vargas, Tran) Variable density, controlled jets in crossflow (Gevorkyan, Shoji, Besnard) AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 4

5 Combustion Instabilities Acoustically coupled combustion instabilities typically understood in terms of a feedback cycle associated with pressure perturbations and reactive flow processes Flow Perturbation Periodic Heat Release Pressure Perturbation Rayleigh s criterion 1 quantifies this coupling in terms of the relationship between pressure perturbations ( ) and heat release ( ) Instabilities damped when and are out of phase Quantification via the Rayleigh Index : 1,, 0 Unstable 0 Stable 1 Lord Rayleigh, The Theory of Sound, Dover, 1945 AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 5

6 Combustion instabilities in liquid rocket engines (LREs) as well as gas turbine engines can lead to large scale failures Coupling of acoustic field to gaseous or condensed phase reactive processes is important in generating large instabilities, but not well understood Example: Combustion instabilities in the F-1 rocket engine led to early failures, requiring hundreds of additional full-scale tests Example: Low NOx gas turbine engines operating lean exhibit thermoacoustic combustion instabilities Shear Coaxial Injection in an LRE (parallel experiments at AFRL)) D. Santavicca, PSU AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 6

7 Movable Speaker CONTROL OF INSTABILITIES IN REACTIVE AND Experimental Setup: Training Facility for UCLA Students Shear Coaxial Jet Studies at AFRL Fuel Droplet Glass Capillary Copper Shroud Syringe Pump Max Velocity Perturbation, u' Max Pressure Perturbation, p' Movable Speaker Pressure Transducer Quartz Window UV Filters Camera Igniter Coil Removable Heating Element Amplifier Visible and Intensified UV Cameras: Filtered to capture phase locked OH* chemiluminescence an indicator of flame location & heat release rate Alternative fuels explored in the past (methanol, JP- 8, Fischer-Tropsch); here we focus on ETHANOL PC DAQ Board Function Generator Standing Acoustic waves: Two speakers out of phase (ϕ 180 ) to create a PRESSURE NODE at the geometric center Resonant conditions achieved at frequencies: 332Hz 898 Hz 1500 Hz AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 7

8 Effects of Acoustic Forcing on Burning Droplets Pressure node (PN) Droplet Velocity Perturbation, u' Pressure Perturbation, p' OH* chemiluminescence indicates bulk flame deflection away from the PN Main effects: Acoustic acceleration* *Tanabe et al. Proc. Combustion Inst. (2000) Burning rate enhancement Flame standoff distance oscillations Flame intensity oscillations [Details in Sevilla, et al, CNF, 2013], tan 4 2 δ f F b f F a F AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 8

9 Ethanol Droplet s OH* Chemiluminescence at different locations x, 898 Hz forcing Flame Switch Highly repeatable qualitative consistency with Tanabe theory [Details in Sevilla, et al, CNF, 2013] AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 9

10 Measured Mean Acoustic Acceleration: All fuels and frequencies Quantitative discrepancy between theoretical and measured g a [Details in Sevilla, et al, CNF, 2013] AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 10

11 Acoustically Coupled Burning Droplets: Phase-locked OH* chemiluminescence 0.46 Ethanol, f=332hz, x=-4 cm, p' max =150 Pa, x/ = I' p' f (I' filt ) / (I' filt ) max p' filt / p' max f Acoustic Phase, [Degrees] 0.00 AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 11

12 Stability of Acoustically Coupled Burning Droplets Pressure from dynamic transducer measurements,, Heat release rate inferred from OH* chemiluminescence 0.46 Non Intrusive Allows for multiple diagnostics Ethanol, f=332hz, x=-4 cm, p' max =150 Pa, x/ = I' p' f Unstable 0 Stable,, Especially strong, in-phase coupling between and occurs at low frequency excitation (e.g., 332 Hz) (I' filt ) / (I' filt ) max p' filt / p' max Acoustic Phase, [Degrees] f AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 12

13 Rayleigh Index Quantification: Moderate Forcing Methanol, 150 Pa Ethanol, 150 Pa Rayleigh index provides quantification of degree of instability for different frequencies or fuels, although care is required in comparing different fuels and conditions due to differences in gain settings, etc. Yet rather significant flame oscillations observed for droplets precisely at the PN, where the Rayleigh index predicts neutral stability [Details in Sevilla, et al, CNF, 2013] AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 13

14 Combustion Instability, Alternative Quantification 1.5,, Unstable 90 o ϕ [deg] Stable ϕ 90 ; ϕ ϕ x/ p' I' Stable Region Unstable Region, Ethanol, f=332hz, p=150pa AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 14

15 [deg] Rayleigh Index, G CONTROL OF INSTABILITIES IN REACTIVE AND Moderate Ethanol Excitation (150 Pa), G and x/ x/ Δϕ Pros Consistent qualitative features between G and Δϕ Focus on phase removes issues of noise, gain setting, etc. for quantification Large flame oscillations at (or near) the PN are better captured by Δϕ than by G Stable Region Unstable Region f=332hz f=898hz f=1500hz Δϕ Cons Does not quantify how vigorously the thermo-acoustic coupling is taking place AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 15

16 Ethanol, 898 Hz, 150 and 200 Pa [deg] Rayleigh Index, G x/ p=150pa p=200pa Stable Region Unstable Region x/ At higher frequencies, phase relationship (Δϕ) is relatively independent of pressure perturbation amplitude, in contrast to G AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 16

17 Local Extinction: Low Frequency Excitation (332 Hz) Ethanol, f=332hz, x=-2 cm, p' max =200 Pa, x/ = , 200, / I' p' f (I' filt ) / (I' filt ) max p' filt / p' max f Acoustic Phase, [Degrees] 0.00 Despite obvious coupling between combustion and acoustics, appears to be out of phase with AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 17

18 Ethanol, 332 Hz, 150 and 200 Pa [deg] Rayleigh Index, G Stable Region Unstable Region x/ p=150pa p=200pa x/ The theoretically stable conditions (G<0, Δϕ 90 ) actually correspond to periodic partial extinction and reignition cases, with significant instability Neither metric represents this instability, unfortunately AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 18

19 Understanding Local, Periodic Extinction, 200, / 0.019, 200, / These oscillating flames are exposed to the same amplitude of disturbance (approximately same / ), and with the same fuel; frequency is the only difference Oscillating flames responding to velocity perturbations are usually interpreted in terms of an oscillatory strain rate,, For a given reactive timescale for ethanol combustion, the flames appear not to respond as vigorously to high frequency excitation (898 Hz) as to low frequency excitation (332 Hz) Consistent with theoretical exploration of periodically strained diffusion flames and timescale limits on flame response (Egolfopoulous & Campbell, 1996; Selerland & Karagozian, 1998) AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 19

20 Conclusions and Ongoing Research The phase difference (Δϕ), as an alternative to Rayleigh index G, removes the effects of noise and optical imaging differences among forcing conditions Δϕ is generally in good agreement with observed combustion instabilities and the Rayleigh index G Large flame perturbations at (or near) the PN are captured more effectively by Δϕ than by G Yet Δϕ does not allow for an immediate quantification of the strength of the oscillations High amplitude excitation levels, where partial extinction and reignition occur, are being explored to understand basic flameacoustic coupling processes Neither metric is capable of adequately addressing periodic local extinction, each suggesting stability where the flame is clearly unstable Oscillatory strain rate and its relation to reaction time scales needs to be incorporated into an adequate model for flame-acoustic coupling Current efforts seek to better understand the meaning of theoretically stable and unstable flames in light of these studies Quantification of velocity perturbations and relevant strain rates AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 20

21 Other Relevant Experimental Research UCLA EPRL: Variable density jets in crossflow, with tailored control strategies (Gevorkyan, Shoji, Besnard) AFRL: Impinging jets of variable geometries under subcritical and supercritical conditions (Roa) AFRL-UCLA CCAS BASIC RESEARCH REVIEW MEETING 21