Fluorescence tracer technique for simultaneous temperature and equivalence ratio measurements in Diesel jets

Transcription

1 Renewable energies Eco-friendly production Innovative transport Eco-efficient processes Sustainable resources Fluorescence tracer technique for simultaneous temperature and equivalence ratio measurements in Diesel jets Gabrielle Tea 1,2, G. Bruneaux 1, J. Kashdan 1, C. Schulz 2 1 IFP Energies nouvelles, France 2 IVG, University of Duisburg-Essen, Germany Towards Clean Diesel Engines, TCDE2011, June 8th, 2011

2 Scientific context Reduction of fuel consumption and exhaust emission IC engine development : Need to understand fundamental physical phenomena inside the combustion chamber Experimental data required to validate CFD models Quantitative and qualitative study of physical parameters inside the combustion chamber Temperature, fuel/air ratio,... 2

3 Scientific context Temperature and fuel/air ratio are key parameters for combustion in IC engines Mixture preparation, auto-ignition, combustion, pollutants formation New combustion modes: CAI, HCCI, LTC (High EGR: significant oxygen stratification) Challenges to temperature, [fuel], [O2] measurements High precision required Processes of interest are extremely sensitive to temperature and fuel/air ratio Spatially resolved information required Resolve spatial fluctuations in ignition and combustion In an engine environment (large variation in p, ρ fuel, O 2 ) Laser-induced fluorescence (LIF) of tracers 3

4 Tracer LIF Objectives Tracer fluorescence signal after UV excitation 1 st order approximation 2 nd order approximation [fuel] Temperature [O 2 ] 4 Requirements: Tracers Requirements: : organic molecules low sensitivity with T, [O2] high sensitivity with T Ketones : acetone, 3-pentanone... adiabatic mixing model low sensitivity with [O2] independent Aromatics T, [O2] : toluene, naphthalene... measurement for correction One-tracer LIF technique: toluene (high sensitivity with T) T results independent of [fuel] development and validation of the technique in oxygen-free environment Requirements: high sensitivity with [O2] low sensitivity with T Two-tracer LIF technique: toluene + naphthalene (cf. literature) development and validation of the technique

5 Normalized toluene emission Strongly reducing LIF signal with increasing temperature One-tracer LIF technique Background Strong temperature dependence of the fluorescence emission spectra of toluene after UV excitation Toluene fluorescence emission spectra 248 nm excitation Can be used for 1-color LIF thermometry (1 detector only) in an homogeneous environment only (constant tracer concentration) Red-shift of the LIF emission spectrum with increasing temperature Can be applied in non-homogeneous environments Based on 2-color LIF technique (2 detectors) Local tracer concentration cancels; ratio is a function of temperature only Wavelength / nm 5

6 Normalized toluene emission One-tracer LIF technique Background Toluene fluorescence emission spectra 248 nm excitation Collection Filter 2 Collection Filter 1 Wavelength / nm 2-color toluene LIF thermometry uses the red-shift of the LIF emission spectra with increasing temperature Collection of the fluorescence signal on two spectral ranges (collection filter 1 & 2) Ratio of the collected signal (R LIF ) is a function of temperature R LIF must be calibrated at known temperatures in a high pressure cell 6

7 One-tracer LIF technique Experimental set-up High pressure high temperature cell: Wide optical access (4 windows of 100 mm diameter) Reproduces the thermodynamic conditions in Diesel engines during injection High temperature and high pressure provided by pre-combustion Start of injection at the desired ambient temperature and pressure after a specified delay during cooldown Here: Inert, i.e. oxygen-free atmosphere during injection 7

8 One-tracer LIF technique Experimental set-up LIF image - HP300 Fuel spray n-dodecane with 10%Vol toluene Collection filter HP300 UV excitation at 248 nm High temperature high pressure cell LIF image - BP280/20 Measurements in the vaporized part of the spray: NO LIQUID Collection filter BP280/20 8

9 One-tracer LIF technique Measurement methodology LIF signal intensity / Counts Filter HP300 2 Filter BP280/20 1 LIF ratio / - Temperature / K CALIBRATION Ref. : G.Tea, G. Bruneaux, J.T. Kashdan, C. Schulz, "Unburned gas temperature measurements in a surrogate Diesel jet via two-color toluene-lif imaging", Proc. Comb. Inst. 33, (2011) MEASUREMENT CONDITIONS Injection duration = 600 µs Image acquired 1500 µs after start of injection T Ambient = 624 K P Ambient = 46 bar 9

10 One-tracer LIF technique Development and validation Experimental methodology developed to Optimize data processing Improve and evaluate precision and accuracy Develop calibration-free methodology Pixel intensity correlation plot Collection Filter BP280/20 One-tracer LIF thermometry Shows a limitation of toluene two-color LIF thermometry to T < 700 K at high pressure with the given strategy Provides results on Precision: ±18 K(σ) (single-shot) Accuracy: ±30 K below 700 K Collection Filter BP280/20 σ I 10

11 LIF of tracers Objectives Tracer fluorescence signal after UV excitation 1 st order approximation 2 nd order approximation [fuel] Temperature [O 2 ] 11 Requirements: low sensitivity with T, [O2] adiabatic mixing model independent T, [O2] measurement for correction Requirements: good sensitivity with T low sensitivity with [O2] One-tracer LIF technique: toluene (good sensitivity with T) T results independent of [fuel] development and validation of the technique in oxygen-free environment Requirements: good sensitive with [O2] low sensitivity with T? Two-tracer LIF technique: toluene + naphthalene (cf. literature) development and validation of the technique

12 Two-tracer LIF technique Tracers Temperature [O2] Photophysical interaction Excitation wavelength TOLUENE NAPHTHALENE ? 266nm or 248nm? Red-shift of the LIF emission spectrum with increasing temperature: Toluene : 2nm/100K Naphthalene : 4nm/100K + R LIF less sensitive to [O2] 12

13 Two-tracer LIF technique Tracers Spectral characterization of toluene and naphthalene after UV excitation: Using the high-temperature, high-pressure cell (University of Duisburg-Essen) Temperature : K Pressure : 1-10bar Gas composition : N 2, AIR, N 2 /O 2 Fluorescence spectra + lifetime measurement : - Toluene - Naphthalene - Toluene + Naphthalene 13

14 Measurement strategy: Two-tracer LIF technique Strategy Collection of 2 tracer-lif signal on 3 spectral ranges (collection filter 1, 2, 3) Toluene Naphthalene R LIF_naph/tol R LIF_naph = I 2 /I 1 = I 3 /I 2 = g([o 2 ]) Collection filter 1 Collection filter 2 Collection filter 3 = f(t) 14 Fluorescence emission spectra (after 266nm excitation) of toluene and naphthalene at 375K, 1bar in N2

15 Two-tracer LIF technique Validation Validation of the two-tracer LIF technique : Application in well controlled conditions High pressure, High temperature cell (IFP Energies nouvelles) In the vapor phase of a surrogate Diesel jet Optimization of the experimental set-up Proportion of naphthalene and toluene Different collection filters Selection of optimum wavelength excitation (248nm or 266nm?) Optimization of the data processing: Impact of corrections (temperature, oxygen concentration) on results Estimation of the accuracy and precision 15

16 Two-tracer LIF technique Experimental set-up Fuel spray n-dodecane with 10%Vol tracer (Naphthalene/Toluene) High temperature high pressure cell LIF image different filters 16 Measurements in the vaporized part of the spray: NO LIQUID average on 10 images

17 Conclusions (1/2) Development of tracer-lif technique for measuring simultaneously : Temperature field Local oxygen concentration field Local fuel concentration field Temperature field Development of a technique independent of local fuel concentration and in oxygen-free environment 1 tracer with good temperature sensitivity (toluene) 1 UV excitation (248nm), 2 collections Results on Precision: ±18 K(σ) (single-shot) Accuracy: ±30 K below 700 K 17

18 Conclusions (2/2) Local oxygen concentration field Development of tracer-lif technique Independent of local fuel concentration Simultaneous temperature measurement 2 tracers (toluene/naphthalene) : Toluene and naphthalene are good candidates Tracers spectral characterization Verify the photophysical interaction using 2 tracers simultaneously Work in progress : Application of the technique in a the vapor phase of a Diesel jet Well controlled conditions Optimization and validation of the technique Ongoing experiments Applications in engine conditions 18

19 Renewable energies Eco-friendly production Innovative transport Eco-efficient processes Sustainable resources

20 Two-tracer LIF technique Validation Dependence of the fluorescence emission spectra of toluene and naphthalene after UV excitation on temperature, [O2] and [fuel] Temperature [O 2 ] [fuel] Red-shift of the LIF emission spectrum (naphthalene) with increasing temperature Strongly reducing LIF signal with increasing [O2] with different sensitivity between naphthalene and toluene LIF signal dependence on [fuel] (naphthalene or toluene) 20 Correction for temperature effect on LIF signal Fuel/Air ratio Correction for temperature AND [O2] effect on LIF signal

21 1- Single-tracer LIF technique Measurement methodology LIF signal intensity / Counts LIF ratio / - Temperature / K Filter HP300 2 Filter BP280/20 1 CALIBRATION 21 Raw image Raw image Numerical Adjustment Background Spatial filtering of signal the adjustment camera (median removed sensitivities or mean) Image processing usually required in imaging technique Background signal removed from raw image Correction for camera shot noise (numerical filter) Correction for variations in pixel-to-pixel sensitivity (flat-field correction) Image processing specific to 2-color imaging technique: Both images must be Spatially well adjusted before calculating the ratio Corrected for differences in the overall sensitivity between the two cameras MEASUREMENT CONDITIONS Injection duration = 600 µs Image acquired 1500 µs after start of injection T ambient = 624 K p ambient = 46 bar

22 1- Single-tracer LIF technique Measurement methodology Collection filter BP280/20 Collection filter BP280/20 22 Comparison of LIF images with the same filter Images should be identical Observed differences are due to Imperfect superposition Variations in sensitivity between the two cameras Variations in the sensitivity of individual pixels Camera shot noise Use of all these differences to correct or determine each effect. (optimization process) No in-situ calibration required Residual differences are analyzed to quantify random error (measurement precision)

23 1- Single-tracer LIF technique Error analysis measurement precision Camera 1 Camera 2 BP 280/20 BP 280/20 Filter BP280/20 Analysis of differences σ R calibration σ TM 1σ temperature measurement precision of ±18 K for single-pulse measurement 23 σ T

24 1- Single-tracer LIF technique Error analysis measurement precision Effect of numerical filter box size on measurement precision Dependence of the maximum statistical error On the filter box size On the filter type (for two numerical filters) Trade-off between measurement precision and spatial resolution σ TM Temperature / K 24 σ TM ~18 K with a resolution of 2 mm (26² pixels) (1 pixel = 0.08 mm)

25 1- Single-tracer LIF technique Error analysis measurement accuracy Results (T LIF ) are compared with independent T-measurements Bath-gas temperature T TC (measured by thermocouple) in the center of the cell Temperature from LIF thermometry in of the the cell homogeneously burned gas after filled the with fuel/toluene pre-combustion vapor (T LIF ) Burned Gas gas temperature measured by a thermocouple in after the the cell heated pre-combustion to 453 K (T TC ) Temperature from LIF thermometry in the evaporated spray (T LIF ) Temperature information propagated into the spray based on T TC using an adiabatic mixing model (T AM ) 25

26 Single-tracer LIF technique Error analysis measurement accuracy Results (T LIF ) are compared with independent T-measurements Straight line (y = x) Below 700 K: T LIF in agreement with reference temperature (T LIF T AM ) < ±30 K (systematic error) Technique limited to T < 700K due to low signal-to-noise ratio 26