Uncertainties of Soot Measurement in Counterflow Flames using Laser Induced Incandescence: Soot Volume Fraction, Particle Size, and Number Density

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1 Uncertainties of Soot Measurement in Counterflow Flames using Laser Induced Incandescence: Soot Volume Fraction, Particle Size, and Number Density Brendyn G. Sarnacki and Harsha K. Chelliah Department of Mechanical and Aerospace Engineering University of Virginia, Charlottesville VA An absolute irradiance calibrated two-color time resolved Laser Induced Incandescence (LII) technique was utilized to collect quantitative soot incandescence data for dmination of soot particle temperature, primary particle size, soot volume fraction, and number density. The approach requires a comprehensive analysis of the LII nano-scale heat transfer model. In this work, all the sources of uncertainty from both measured experimental variables and assumed heat transfer model variables were considered and perturbed with respect to their uncertainty bounds to obtain overall uncertainty of the measurements. The resulting overall uncertainties of soot volume fraction, primary particle size, and soot number density are reported here. The absorption function was identified as the key source of uncertainty in dmining soot volume fraction. A correlation between extinction measurements and LII signal was analyzed to improve uncertainty of the absorption function. Uncertainty in the primary particle size was dominated by uncertainty in the thermal accommodation coefficient and ambient gas temperature. Since the experiment repeatability uncertainty was much lower than combined total uncertainty due to heat transfer model and other experimental variables, accurate heat transfer model params are needed for reducing the overall uncertainty of LII measurements. Introduction Numerous studies have reported the adverse health, environmental, and climatic effects of aerosol or soot particulate emissions from gas turbines and diesel engines [1-2]. To better understand the soot particle formation and oxidation mechanisms in these combustion devices, especially at high-pressure conditions, concerted experimental and modeling efforts are currently underway [3]. A key objective of these efforts was to obtain fundamental soot data sets with well-defined uncertainties for soot model validations. Such validated models can then be used to efficiently and reliably design future combustion devices through CFD simulations. While many laboratory configurations have been considered for generation of soot data [3], in this work we consider the counterflow geometry. Several studies have reported measurement of temperature, species, and soot characteristics in sooting or barely sooting counterflow flames, both using intrusive and non-intrusive probing techniques [see for example 4-6]. In this work, we have employed the non-intrusive Laser Induced Incandescence (LII) to characterize the soot volume fraction (f v ), effective particle size (d p ), and number density (N) across a non-premixed flame structure. Extensive effort was devoted to better understand the uncertainties associated with LII measurements in counterflow flames, which may not be possible in other laboratory flame configurations that have been considered in the past. Measurements have also being carried out for pressures up to 14 atm by diluting the fuel and the oxidizer streams with He. Experimental Methodology A high-pressure counterflow burner chamber (hydro tested up to 150 atm with fused silica windows for optical access) was developed [7] with co-annular contoured nozzles to minimize the formation of Taylor-Görtler vortices [8]. The inner nozzle diam was 6.0 mm. For the present low strain rate sooting flame experiments, i.e. far from extinction limits, nozzles were separated by 6 mm and the momenta of the two opposed streams were always balanced. Annular curtain flow of nitrogen was used to eliminate any secondary flames in the chamber. The center nozzle flow of fuel, oxygen, and diluents (nitrogen or helium), and the annular curtain flow of nitrogen were med and controlled by a series of Sierra model 100 mass flow controllers interfaced in LabView with a calibrated accuracy of 1% of full scale and repeatability of 0.2% of full scale including linearity at operating conditions. The chamber pressure was controlled by a Stravalve back-

2 pressure regulator, with two pressure relief valves for safety. LII measurements were acquired with a New Wave Research Solo III 50mJ, 532 nm, pulsed dual head Nd:YAG laser with collimated Gaussian sheet optics and a LaVision Imager ProX4M 2048x2048 pixel CCD camera mounted to LaVision Intensified Relay Optics (IRO). The diagnostic equipment was controlled via a LaVision SootMaster LII diagnostics package with timing controlled through a programmable timing unit with 10ns resolution. During LII measurements, a global strain rate was chosen at near sooting regime for the selected fuel-air mixture, and LII image samples were collected from heated soot particles pulsed by the laser and filtered through 450nm and 650nm bandpass filters with 10nm bandwidths and a 10ns intensifier gate. The IRO delay time was varied to collect gated signals temporally along the particle temperature decay in order to characterize soot particle size. The sample corresponding to peak temperature was used to characterize soot volume fraction (see Fig. 1 for this setup). Figure 1: A schematic of the LII setup with the insert showing the measured LII signal of 500 samples. In order to achieve sufficient signal, 500 hundred LII images were collected and averaged in Davis 7.2. The validity of time averaging under the assumption of steady laminar flow has been previously verified with Particle Image Velocimetry (PIV) measurements [9]. For LII data analysis, the PIV derived local strain rate and boundary condition information were used in quasi one-dimensional flame simulation for generating the required local temperature and major species composition profiles. Since the near sooting flames considered were far from extinction, the dependence of the flame structure on the kinetic model was considered to be negligible. Laser Induced Incandescence Methodology Calibration: LII is a widely accepted measurement technique utilized in soot research for extracting soot primary particle size and volume fraction. Several calibration methods are commonly used to dmine soot volume fraction from LII experiments. The first and simplest method employs a standard of known soot volume fraction to calibrate the LII system, yet bears large uncertainties due to variations in soot particle size not accounted for in the calibration [10]. Another common approach employs an inline light extinction measurement as a secondary measure of soot volume fraction, yet suffers from its own uncertainties associated with complex flame structures [10-12]. Because of the axisymmetric nature of the present counterflow flame geometry, this approach in fact can provide a mechanism to obtain a better estimate for the soot absorption function in counterflow ethylene-air flames. A third method, proposed by Snelling et al. [13] relies strictly on the physics of LII by relating the spectral radiance of the soot particle incandescent signal to an absolute irradiance or radiance calibration of the recording device. The latter strategy was that chosen for calibration of our LII system. A calibrated light source of known spectral irradiance (model RS-10D from Gamma Scientific) was used for calibration of the detection system. A sample of 600 images of the irradiance incident on the CCD chip from the calibrated light source was collected over 10ns and 20ns gate widths and through two 10nm bandwidth bandpass filters at 650nm and 450nm. Samples were also collected over three different separations between the calibrated light source energy transfer calibration from a Scientech AC2501 Astral Calorim with 3% documented uncertainty. Measurements from 500 successive laser pulses were recorded for a range of laser power settings covering the params defined by low to high fluence of LII. Recorded images were imported and analyzed in MATLAB where a lookup table was generated of laser fluences and corresponding spatial locations across the excitation laser beam sheet width for a range of

3 laser power settings. The radiometric calibration method thus developed could then be used to dmine properties of interest for each pixel in two dimensions including soot particle temperature, mean particle size, and volume fraction. The experimental temperature was extracted from a standard combination of two-color pyrometry, including the incandescent signal decay. LII data analysis: In order to dmine primary particle size, the experimental particle temperature decay curve dmined was fitted to the results of a LII heat transfer model. The resulting primary particle size was dmined by iterative leastsquares minimization of the modeled and experimental particle temperature decays. Once the primary particle size was dmined, the soot volume fraction could be extracted from the measured incandescent signal and the optimized LII model results. In this study, the volume fraction measurements reported are the average from the two detection wavelengths at 450 and 650nm. Results Because of the need to understand the LII measurement uncertainties, especially at highpressure conditions, a comprehensive analysis was undertaken to dmine all the sources and magnitudes of uncertainty from both measured experimental variables and assumed model variables. These uncertainties include the attenuation of the laser induced soot incandescence signal (I), detector incandescent signal calibration factor (η), the detector solid angle, the CCD pixel area to image area calibration, the ambient gas temperature (T 0 ), the ambient gas heat capacity ratio (γ), the ambient gas thermal conductivity, the ambient gas molar weight, the laser sheet spatial width, the laser fluence profile, the combustion chamber ambient pressure, the heat conduction thermal accommodation coefficient (α T ), the absorption function of soot (E(m)), the density of soot (ρ s ), the heat capacity of soot (c s ), and the heat conduction model (Q cond ). While the uncertainties associated with vaporization of soot particles were considered in the analysis, i.e. the molar weight of vaporized carbon due to sublimation, the vaporization mass accommodation coefficient, the reference pressure used in the Clausius-Clapeyron equation to calculate the equilibrium partial pressure of sublimed carbon, the enthalpy of formation of sublimated carbon clusters, etc., but these uncertainties were negligible for our low fluence experimental conditions (~0.18 J/cm 2 ). Furthermore, because of the near sooting regimes explored with particles in primary size range, the analysis did not include uncertainties due to particle size distribution or particle aggregation. All variables listed above were perturbed in the LII model by their respective uncertainties obtained from our measurements and the literature. The analysis ignored any second order effects. The resulting uncertainties corresponding to extracted variables of soot volume fraction, primary particle size (effective), and soot number density are listed in Table 1 by descending order of importance (only the top 6 are listed for brevity). The cumulative uncertainty listed at the bottom is the square root of the sum-squared error of all uncertainties. It is clear that the absorption function, which directly affects the measurement of incandescent signal and hence volume fraction, gives the largest volume fraction uncertainty. Also as expected, uncertainty in the primary particle size is dominated by uncertainties affecting the conduction heat transfer mechanism. Sublimation terms are found to be negligible for all three extracted variables due to the use of low fluence LII in this work causing minimal soot sublimation. The number density is extracted from the measured values of f v and d p by using the standard volume fraction equation. Hence the uncertainty of the number density follows the square root of the sum-squared error of f v and d p. Table 1: A list of dominant uncertainty params associated with measurement of soot volume fraction, particle size, and number density using LII. Primary Particle Volume Fraction Size Number Density Uncerta inty Uncerta inty Uncertai nty E(m) 29% α T 42% α T 72% T 0 17% T 0 30% T 0 55% I 7% γ 18% γ 30% η 6% ρ s 10% E(m) 29%

4 γ 5% c s 10% ρ s 20% ρ s 5% Q cond 8% c s 20% Total 37% Total 57% Total 106% Irrespective of the above uncertainties, perhaps over estimated, the experimental repeatability uncertainty was found to be less than 15%. Consequently, the measured soot particle temperature, volume fraction, and particle size as a function of the distance along the axis of symmetry of counterflow flames are well described, as seen in Figs The particle number density derived from soot volume fraction equation however show an irregular variation in the soot inception region as seen in Fig. 5. of counterflow ethylene/oxygen/nitrogen nonpremixed flame at a pressure of 14 atm and a strain rate of 160 s. -1 Figure 4: Variation of the normalized soot particle diam, d p (solid black line) along the axis of symmetry of counterflow ethylene/oxygen/nitrogen nonpremixed flame at a pressure of 14 atm and a strain rate of 160 s. -1 Figure 2: Variation of the soot particle temperature, T p (solid red line) along the axis of symmetry of counterflow ethylene/oxygen/nitrogen nonpremixed flame at a pressure of 14 atm and a strain rate of 160 s. -1 Also shown are the numerically computed gas phase temperature T g, velocity v g, and selected species structure. The vertical lines depict the flame location (red) and stagnation planes of gas and particles. Figure 5: Variation of the normalized soot particle number density, N (solid black line) along the axis of symmetry of counterflow ethylene/oxygen/nitrogen nonpremixed flame at a pressure of 14 atm and a strain rate of 160 s. -1 The above results clearly show that the soot particle size grows as they approach the particle stagnation plane, which is distinct from the gas stagnation plane. This distinct particle and gas stagnation planes can be attributed to the thermophoretic forces acting on the particles. The LII data also show a strong dependence on the flow rate and pressure. Figure 3: Variation of the normalized soot volume fraction, f v (solid black line) along the axis of symmetry Concluding Remarks An experimental effort was initiated to obtain fundamental soot data of high-pressure ethylene/oxygen/inert non-premixed counterflow flames, including a comprehensive uncertainty

5 analysis. It was shown that the absolute irradiance calibrated two-color time resolved Laser Induced Incandescence technique provides an excellent method of collecting quantitative soot incandescence data in the counterflow geometry. The detailed uncertainty analysis performed show that the soot volume fraction measurements are mostly influenced by the uncertainties of soot absorption function and local ambient gas temperature, while soot particle size measurements are influenced by the uncertainties of thermal accommodation coefficient, ambient gas temperature, and the ratio of specific heats of ambient gases. Thus, further focused studies are required to reduce the overall uncertainty of LII measurements. Acknowledgements This work was supported by Rolls Royce and the Commonwealth Center for Aerospace Propulsion Systems, with Dr. MS Anand at Rolls Royce as the technical monitor. References [1] A Seaton, W MacNee, K Donaldson, and D Godden. Particulate air pollution and acute health effects. Lancet, 345(8943):176 8, [2] WL Chameides and M Bergin. Climate change - soot takes center stage. Science, 297(5590): , [3] LII Workshop, Le Touquet, France, May 8-11, [4] KTKang, JY Wang, SH Chung, W Lee, Soot zone structure and sooting limit in diffusion flames: comparison f counterflow and co-flow flames, Combust. and Flame 109: (1997). [5] A D Anna, M Commodo, M Sirignano, P Minutolo, R Pagliara. Particle formation in opposed-flow diffusion flames of ethylene: an experimental and numerical study, Proc. Combust. Inst. 32: (2009). [6] L Figura, A Gomez. Laminar counterflow steady diffusion flames under high pressure (p 30bar) conditions. Combust. and Flame 159 (2012) [7] BG Sarnacki, HK Chelliah. Sooting limits of counterflow nonpremixed flames: pressure effects. AIAA 50 th Aerospace Sciences Meeting, Nashville, TN, [8] JM Bergthorson, K Sone, TW Mattner, PE Dimotakis, DG Goodwin, DI Meiron. Impinging Laminar Jets at Moderate Reynolds Numbers and Separation Distances. Physical Review E, Vol. 72, [9] BG Sarnacki, G Esposito, HK Chelliah. Extinction limits and associated uncertainties of non-premixed counterflow flames of methane, ethylene, propylene and n-butane in air, Combust. Flame 159 (2012), pp [10] C Schulz, BF Kock, M Hofmann, HA Michelsen, S Will, B Bougie, R Suntz, and GJ Smallwood. Laser-induced incandescence: recent trends and current questions. Applied Physics B, 83(3): , [11] HA Michelsen, F Liu, BF Kock, H Bladh, A Boiarciuc, M Charwath, T Dreier, R Hadef, M Hofmann, and J Reimann. Modeling laser-induced incandescence of soot: a summary and comparison of lii models. Applied Physics B, 87(3): , [12] RL Vander Wal. Laser-induced incandescence: excitation and detection conditions, material transformations and calibration. Applied Physics B, 96(4): , [13] DR Snelling, GJ Smallwood, F Liu, OL Glder, and WD Bachalo. A calibrationindependent laser-induced incandescence technique for soot measurement by detecting absolute light intensity. Applied Optics, 44(31): , 2005.