AIAA MHD Flow Control and Power Generation in Low-Temperature Supersonic Air Flows

Transcription

1 AIAA MHD Flow Control and Power Generation in Low-Temperature Supersonic Air Flows Munetake Nishihara, J. William Rich, Walter R. Lempert, and Igor V. Adamovich Dept. of fmechanical lengineering i And Sivaram Gogineni Innovative Scientific Solution, Inc.

2 Support AFOSR grant FA Phase I AFOSR SBIR with ISSI

3 Motivation MHD boundary layer flow separation control in hypersonic inlets Needs relatively l low interaction ti parameter: low electrical lconductivity, it modest magnetic field, use of lightweight permanent magnets Full-scale boundary layers are ~10 cm thick: need to demonstrate MHD control of relatively large cross section area flows MHD power generation on board of hypersonic vehicles Mach number and stagnation temperatures are too high for a power turbine (M=6, T 0 ~2,000 K): MHD may be the only feasible option 0 Typical flow conditions imply the use of low-temperature, nonequilibrium plasmas

4 Objectives Characterize MHD pulser-sustainer discharge plasma (discharge power, flow temperature rise, conductivity, Hall parameter, cathode fall) Isolate Lorentz force effect on core flow Mach number using static pressure measurements: flow acceleration / deceleration Detect MHD power generation in unseeded and seeded low-temperature flows Determine range of applicability for on-board MHD power generation (with simulations of pulser sustainer discharge in magnetic field)

5 MHD test section schematic Static pressure port Optical access window up Flow Sustainer current down Static pressure port Pulsed electrode block DC electrode block Optical fiber location and line of sight Magnet pole B west Flow east Static pressure port Contoured nozzle 12 cm long, 4 cm x 2 cm test section Equipped with pressure ports and Pitot ports Ceramic/copper pulsed and DC electrode blocks Stagnation pressure P 0 = atm Ionization: repetitively pulsed discharge

6 MHD wind tunnel (shown with CPT pulser) CPT pulser: U=20 kv, ν=50 khz, τ=20-30 nsec New FID pulser: U=10-40 kv, ν=100 khz, τ= 3-5 nsec High ionization efficiency at high E/N Excellent plasma stability (duty cycle ~ 1:1000)

7 Repetitively pulsed discharge (40 khz rep rate) + DC sustainer in M=4 air flow 20 Voltage [kv] Current [A] Voltage 10 Current V PEAK =13.2 kv -30 I =31.3 A PEAK Air, B=1.5 T P 0 =1 atm, P test =13 torr, U max =13 kv Time [ns] Plasma always remains uniform and stable for run times of several seconds

8 Pulse voltage and sustainer current in M=3 nitrogen flow P 0 =1/3 atm, P test =8 torr 10 Voltage [kv] 2.0 Current [A] 5 25 μs ns Time [μs] Time [μs] Pulse energy 1-2 mj Time averaged pulsed discharge power W <I> = 0.95 A (top curve) <I>=0.86 A (bottom curve) DC discharge power 1.4 kw

9 N 2 second positive band (C 3 Π u B 3 Π g ) emission in M=3 nitrogen: Flow temperatures with and without 1.4 kw discharge Intensity [arbitrary units] Intensity [arbitrary units] U PS = 0 kv 1.2 U PS = 2 kv Synthetic spectra T = 180 K T = 260 K T = 100 K Wavelength [nm] Wavelength [nm] Line of sight averaged T=180±20 K for both cases ΔT~10 K (~90% of discharge power frozen in N 2 vibrations)

10 Effect of Lorentz force and Joule heat on pressure and Mach number: 1-D theory du dx u 1 γ 1 = F Q& 2 p M 1 γu [( ) ] ( γ 1) γ 1 M + 1 F + dp = 1 M 2 Q& 2 dx M 1 a Lorentz force F = Joule heat j y B z IB A Q& = α j E α y y I( U PS IR) Ah α : Effective Joule heating factor Pressure and velocity changes for two different Lorentz force directions dp / P ± = ( γ 1) 2 1 du / M + ± u ~ 5 ~0.1 due to energy storage in N 2 vibrational mode u ± /u ~ 2% ΔP ± P ~ 10%

11 Analytic expressions for pressure rise and effective Joule heat factor MHD interaction parameter Effective loading parameter j B L α j α E y z 2 y y y η = 10 K = = 4 2 ρu j B u B u y z z E Δp R α Δp Δp A A ( γ 1) 2 M j 2 M 1 + Δp 2 R M 2 1 a y B ( γ 1 ) M j y E y L z L Δp A Δ p R : Pressure change for accelerating force, +F : Pressure change for retarding force, -F

12 Momentum transfer from electrons to neutral flow: how significant is it? Flow B # of momentum transfer collisions i per neutral particle E y n e N ~ ν τ ~ 0.4 v dr φ v dr, m tan φ = β = v dr, m v dr u coll coll res N Neutral velocity change per collision 2m Δucoll ~ ± βvdr ~ ± 15 m / s M Neutral flow velocity change Δ u ± = 2Δucoll Ncoll ~ 12 m / s n e = 10 cm v 7 10 cm/s n e = 1 10 N 7 ν dr = β = coll = 5 10 s τ res = 75 μsec u ± /u ~ 2%

13 Isolation of Lorentz force effect from Joule heating effect Test section Decelerating force j up B B west B east j Flow down j Accelerating force j Magnet pole B B Lorentz force: B, j polarity dependent j j Joule heating: polarity independent

14 Static pressure measurements in M=3 dry air flows Normalized pressure Dry air: B=1.5 T, R=1.0 kω Retarding force B east, j down B west, j up Air, P 0 =250 torr, P test =8.7 torr U PS =2 kv, R=0.5 kω, <I>=1.2 A Pulsed discharge duration 0.5 s 1.1 Accelerating force B east, j up B west, j down Δp R Δp p A = Pulser alone Time [s] 1-D MHD model prediction: α=0.10

15 Static pressure measurements in M=3 N 2 flows Normalized pressure Nitrogen: B=1.5 T, R=0.5 kω Retarding force B east, j down B west, j up Nitrogen, P 0 =250 torr, P test =8.5 torr U PS =2 kv, R=0.5 kω, <I>=0.9 A Pulsed discharge duration 0.5 s Accelerating force 1.1 B east, j up B west, j down Δp R Δp p A = Pulser alone Time [s] 1-D MHD model prediction: α=0.11

16 M=3 room air flows: Comparable pressure rise but no Lorentz force effect 2.0 Current [A] Dry air, <I> = 0.51 A 1.2 Normalized pressure Room air: B=1.5 T, R=0.5 kω 1.5 Room air, <I> = A Retarding force, <I> = A Accelerating force, <I> = A Time [μs] Time [s] Lower current due to rapid electron attachment: e + O2 + H 2O O2 + H 2O Comparable flow heating due to rapid vibrational relaxation N 2 -H 2 O: α=0.40

17 Comparison with quasi-1-d theory 1.3 Normalized pressure Joule heating factor α = Mach number Joule heating factor α = α = α = 0.1 α = ΔM Air Nitrogen Current [A] Current [A] Decelerating force Accelerating force ΔM ± =-0.13 at I = ±1.0 A in air

18 Comparison with quasi-1-d theory (continued) What would it take to increase Mach number? 0.15 Normalized pressure difference ( γ 1) Δp R Δp A 2 2 M 2 M j y B z L Very good agreement with experiment Nitrogen 0.00 Dry air Eq. (9) Eqs. (2-5) Current [A] K eff α jyey α Ey = = 4 ( α = j B u B u y z True flow acceleration (K eff ~1) would require increasing conductivity by a factor of 4 (up to σ=0.3 mho/m) z 0.1)

19 Cathode voltage fall vs. MHD e.m.f. (open voltage): Power generation show stopper 1.0 Average current, A Nitrogen β= Voltage [V] 0.8 B=0T B=0.75T B=1.5T β= B = +1.5 T σ=0.073 mho/m B = 0 T B = -1.5 T Voltage, V Time[μs] U c = V (increasing with B field) U open = ubh = 25-30V (independent of σ) Red flag: Cathode layer not self-sustained in power generation regime

20 Cathode layer bottleneck 0.4 Current [A] Pulse rep. rate: 100 khz Self-sustaining criterion: α d = ln( 1+ 1/ γ ) 0.2 U open <<U c (αd<<1), γ<<1 0.0 U PS = 500 V, <I> = 0.17 A Low secondary emission from cathode Secondary electrons emitted from cathode do not multiply -0.2 U PS = 300 V, <I> = 0.10 A -0.4 U PS = 200 V, <I> = 28 ma U PS = 100 V, <I> = 8.8 ma Time [μs] Extremely low MHD currents (~ma) Not a problem in high-temperature MHD: thermionic emission at relatively high conductivity (σ> mho/m)

21 Pulser-sustainer discharge modeling calculations: kinetic model validation 0.8 Current [A] 0.8 Current [A] Experiment 0.6 Calculation Exp. <V> = 480 V Exp. <V>=530V Voltage [V] 0.0 Calc. <V> = 530 V Time [μs] 2-D time-dependent pulser-sustainer MHD discharge model Reasonably good agreement with experiment can be used for design study calculations

22 MHD discharge modeling calculations: power generation parametric design study (B=0) B=0, U=50 V No field penetration into plasma Extremely low current (0.52 ma) Current w/o cathode layer bottleneck: 83 ma Adding up to 0.1% seed (varying α) and/or using high-emission electrodes (varying γ= ) 1.0) do not help

23 MHD discharge modeling calculations: power generation parametric design study (B=1 1.5T) B=1.5 T, U=50 V No field penetration into plasma Current circles around plasma Extremely low current (0.26 ma) Adding up to 0.1% seed (varying α) and/or using high-emission electrodes (varying γ= ) do not help Is there a way out?

24 Pulser sustainer discharge at higher voltage (B=0 T) B=0T T, U=530 V Greater field penetration into plasma Much higher current at the same conductivity (0.43 A)

25 Pulser-sustainer discharge at higher voltage (B=1.5 T) B=1.5 T, U=530 V Very weak field penetration into plasma Extremely low current (4.5 5mA)

26 Increasing MHD open voltage: three options U open = ubh (h=4 cm) U open /U c ~0.1 Increasing flow velocity: T 0 ~u 2 too low (300 K) Increasing B field: both U open and U c increase with B Increasing MHD electrode separation: can this work? Proposed solution: scale up electrode separation h, run generator in Hall mode (U open = β ubh, β=2-3)

27 Summary Stable high-power MHD pulser/sustainer discharge operation (up to 1.5 kw) Static pressure measurements: Difference in static ti pressure rises by accelerating and retarding Lorentz forces Comparison with 1-D MHD flow model: Good quantitative agreement First experimental evidence of MHD deceleration of cold M=3 nitrogen and air core flows Low-temperature MHD power generation experiments / modeling: Low open voltages reduce MHD current by more than two orders of magnitude (cathode layer bottleneck) This effect cannot be reduced by seeding the flow or by using electrodes with high secondary emission coefficient (γ~1) Need to increase MHD e.m.f. (open voltage) by at least an order of magnitude