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Simultaneous water vapor concentration and temperature measurements in unsteady hydrogen flames
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Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 32 (2009) 2527–2534
www.elsevier.com/locate/proci
Simultaneous water vapor concentration and temperature measurements in unsteady hydrogen flames David Blunck a,*, Sumit Basu a, Yuan Zheng a, Viswanath Katta b, Jay Gore a a
Purdue University, Maurice J. Zucrow Laboratories, Chaffee Hall, 500 Allison Rd, West Lafayette, IN 47907, USA b Innovative Scientific Solutions, Inc., 2766 Indian Ripple Road, Dayton, OH 45440, USA
Abstract Techniques applicable to visualization and scalar measurements in large-scale hydrogen fires are highly desired, but lacking. In this research, planar thermal images of a buoyancy-driven unsteady laminar hydrogen/air flame and line images of a thin filament stretched across this flame were obtained using an infrared (IR) camera at a sampling frequency of 348 Hz. A pulsing frequency of 11 Hz was measured. Transient line measurements of temperature (T) and water vapor mole fraction ðXH2 O Þ have been achieved using inverse radiation calculations. Instantaneous XH2 O and T distributions during a flame–vortex interaction cycle were obtained with temporal and spatial resolutions of 3 ms and 1.7 mm, respectively. The instantaneous distributions of XH2 O and T were effected by preferential diffusion and the altered velocity field of the flame. Vortices caused more scalar fluctuations outside the flame surface than inside. The present XH2 O and T measurements are consistent with detailed chemistry calculations of the flame and laser diagnostic measurements of similar flames. With some modifications, the thermal imaging based technique can be extended to large-scale hydrogen fire applications. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Hydrogen flames; Flame radiation; Non-intrusive measurement; Infrared camera
1. Introduction Safety in the production, storage, distribution, and use of hydrogen are critical to a hydrogen economy [1]. Motivated by this, various hydrogen accident scenarios have been studied. Swain et al. numerically simulated the dispersion of hydrogen leaking from a vehicle into a residential garage, to guide garage vent design and hydrogen sensor
*
Corresponding author. Fax: +1 765 494 0503. E-mail address: [email protected] (D. Blunck).
arrangement [2]. Harstad and Bellan qualitatively analyzed gaseous hydrogen burning above a liquid hydrogen pool after an accidental spill. The two dominant factors in the evaporation of the liquid hydrogen were film boiling and radiative heating [3]. Schefer and co-workers have conducted comprehensive investigations on hydrogen fires resulting from accidental storage vessel leaks [4–7]. Visible flame lengths, radiative heat fluxes, and the fraction of the total energy radiated for these large-scale hydrogen jet flames were measured. For further understanding of large-scale hydrogen fire physics and for validation of simulation tools,
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water vapor concentrations and temperature (T) measurements are highly desired but are lacking. What is needed is a technique for laboratory-scale scalar measurements, which can be readily applied to large-scale hydrogen fires. Thermocouples and a gas chromatograph (GC) have been used to measure the average T and water vapor mass fraction ðYH2 O ) distributions in a turbulent hydrogen/air flame [8]. For thermocouple measurements, radiative heat loss from the junction needs to be corrected [9–11] or its effect on T readings should be estimated [8,12]. In typical GC measurements, YH2 O can not be measured directly but is estimated based on the overall mass balance [8] (for hydrogen flames) or on the C/H ratio (for hydrocarbon flames) [12]. Coherent anti-Stokes Raman scattering (CARS) systems have been developed to measure the T of steady and unsteady laminar hydrogen/ air and diluted hydrogen/air flames [13,14]. Simultaneous T and YH2 O measurements of turbulent hydrogen/air and diluted hydrogen/air flames were conducted using a Raman scattering apparatus [15–17]. Raman–Rayleigh scattering techniques were developed for simultaneous T and major species concentration measurements in various diluted turbulent hydrogen and hydrocarbon flames [18]. Line measurements of T and major species concentrations of partially premixed turbulent methane flames have also been reported [19]. The data obtained from these laboratoryscale flames are widely used as benchmarks for the development of numerical simulations of reacting flows. Infrared (IR) emission–absorption spectroscopy is another technique for measuring T and species concentrations in flames [20,21]. Radiation measurements are line-of-sight in nature; therefore, tomographic deconvolution schemes are required for estimating the scalar property distributions in non-homogeneous flows. Measurements of time-varying T and water vapor mole fraction ðXH2 O ) in homogenous turbulent flows have also been achieved using absorption spectroscopy [22] and emission spectroscopy [23]. Recently, Biswas et al. reported line measurements of mean and root mean square (RMS) of T, carbon dioxide mole fraction (XCO2), and soot volume fraction in a turbulent ethylene pool fire using emission spectroscopy [24]. Measured spectral radiation intensities (Ik) at six wavelengths were used to derive the mean and RMS scalar profiles with consideration of turbulence radiation interactions. This technique only requires statistically axi-symmetric scalar fields and can be extended to planar measurements of the mean and RMS of T, and XH2 O , if two-dimensional radiation measurements are available. IR cameras provide planar radiation measurements and images without need of a traverse mechanism, and have high spatial and temporal
resolutions. Information about the flow field can also be obtained from these measurements. These capabilities make emission spectroscopy using IR camera measurements, a strong candidate for laboratory-scale hydrogen fire research with application to larger-scales. A thermal imaging based inversion technique has been used to study an unsteady transitional laminar hydrogen/air jet flame. In this flame, buoyancy-induced vortices form on the air side of the flame [25]. As the vortices are transported downstream, they interact with the flame surface altering the velocity field. The altered flow field and the non-unity Lewis number (Le) of the flame make preferential diffusion important [25, and 26]. The effects of flame–vortex interactions on timevarying T distributions in similar laminar hydrogen flames have been studied experimentally [13,14] and numerically using detailed chemistry [25,27,28]. Radiation and XH2 O measurements of these flames, however, were not reported. The specific objectives of this study are: (1) To obtain IR images and radiance measurements of an unsteady laminar hydrogen flame. (2) To obtain instantaneous line measurements of the T in a flame–vortex interaction cycle using thin filament pyrometry (TFP). (3) To obtain instantaneous line measurements of the XH2 O in a flame–vortex interaction cycle by tomographic deconvolution of the measured radiances. TFP is an established technique [29–33]. In this work TFP, was implemented simultaneously with thermal imaging and tomographic deconvolution of a single IR image, for the first time. Simultaneous measurements of T and XH2 O were obtained using this technique. Numerical simulations of the T and XH2 O for the flame were also conducted. The enhanced confidence in the IR imaging and scalar measurement technique will motivate its application to large-scale fires, as discussed in Section 3.4.
2. Experimental and numerical methods 2.1. Burner arrangement The unsteady hydrogen/air jet flame studied was established on a long axi-symmetric stainless steel tapered tube with an inner diameter (d) of 8 mm. The hydrogen mass flow rate (52 mg/s) was set by controlling the pressure upstream of a choked orifice. The jet exit Reynolds number (Re) of 980 was calculated using cold hydrogen properties. A premixed methane flame anchored to a McKenna burner was used for TFP calibration.
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The McKenna burner [34], made the flame visibly flat for the fuel (0.013 g/s) and air flow rates used (0.30 g/s). The burner was water cooled and had an outer ring of co-flowing air. The IR camera was aligned perpendicular to the thin filament, which was located 0.5 cm above the McKenna burner. After calibration data was collected for the filament and the methane flame, the McKenna burner was carefully removed. The hydrogen flame burner was then moved vertically into the proper location. 2.2. Thermal imaging and radiance measurements Radiance (J) measurements were obtained using an FLIR Phoenix IR camera mounted with a 25 mm lens. The angle of divergence from the center of the flame to the camera was less than 1.5°; therefore the planar images provide line-of-sight (LOS) measurements through the flame. Each of the 256 rows and 320 columns of pixels corresponded spatially to a square with a width and height of 0.24 mm in the center of the flame. The camera integration time was 0.015 ms and the sampling frequency was 348 Hz. This provided a temporal response of 3 ms. A 2.77 ± 0.12 lm band-pass filter was used with the camera. Measurements of the J below 2250 photon counts were discarded because the detector behaved non-linearly below this threshold. The reported J is the measured radiation after being attenuated by the lens, filter, and spectral response of the detector. This was accounted for in the T and XH2 O measurements. Flame radiance (Jf) was determined by averaging the measured J three pixels above and below the filament to avoid interference from the radiation emitted by the filament. Consequently, the spatial resolution of the present scalar measurements in the axial direction is 1.7 mm (7 pixels). The filament radiance (Jfil) was determined by subtracting Jf from the measured peak J along the filament, although it was found that this can lead to a bias in the results due to the dispersion of the intensity emitted by the filament. The calculated optical transmission loss is less than 6% for both the hydrogen and calibration flames. Jfil and Jf were averaged around the jet centerline to compensate for variations in the symmetry of the flame. 2.3. Temperature measurements Thin filament pyrometry consists of placing a small silicon carbide (SiC) fiber (15 lm diameter) in a flame to determine the flame temperature (Tf) using the emitted radiance (Jfil). As the fiber is heated it radiates as a gray body [30] and the Jfil measured by the camera can be described by the equation Z k2 J fil ðT fil Þ ¼ k exp sk Rk ek I b;k ðT fil Þdk ð1Þ k1
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where Tfil is the filament temperature, Rk is the spectral response of the camera, sk is the transmission through the filter and optics, Ib,k is the Planck function, ek is the emissivity, and kexp is the gain of the electronics and the optics efficiency [32]. Following the approach of Vilimpoc and Goss [29], the Jfil was normalized by the radiance ðJ Ref fil Þ at a reference temperature (Tref). Tfil was then determined by cross-referencing the measured ratio to a look-up table with calculated ratios at variousTfil. The uncertainty in J Ref fil was 7% and the uncertainty in Jfil was estimated to be 15% (both 95% confidence), based on averaged J measurements of the filament in the respective flames. This resulted in a 6% uncertainty in Tfil at 1200 K and 10% in Tfil at 2400 K. Tf was determined from Tfil by correcting for radiative heat loss using the correlation [35] T f ¼ T fil þ
erðT 4fil T 4surr Þ hc
ð2Þ
where r is the Stefan–Boltzmann constant and hc is the convective heat transfer coefficient. Following others work, e was set equal to 0.88 [32,35]. The hc was found using a correlation for flow around a cylinder, Nu = 0.43 + 0.48Re0.5 [36]. The Reynolds number (Re) around the filament was estimated using a velocity of 2 m/s, which was based on simulations of the flame velocity. This assumed that the flow velocity is uniform across the flame. In reality the velocity and subsequently the hc vary across the flame [32]. A sensitivity analysis found that for the upper and lower limits of the simulated velocity an assumed constant velocity of 2 m/s resulted in a 1% difference in Tf. It should be noted that Tf at radial distances less than 0.5 cm in this flame were not obtained because the Tfil became too low to give a sufficient J signal. This is a well recognized limitation of the TFP technique [29]. The temperature of the calibration methane flame was measured prior to the J Ref measurefil ment, and was used as Tref. Inverse spectral radiation (Ik) measurements were used to determine a Tref of 1545 ± 60 K. This assumed a uniform chemical equilibrium of species across the burner. The uncertainty in the Tref measurement was estimated based on a 10% uncertainty in the Ik measurements [37]. Based on the uncertainties in Tref measurements, velocity estimates, and J measurements, the overall uncertainty in Tf measurements is estimated to be 7% at 1200 K and 11% at 2400 K. 2.4. Water vapor mole fraction measurements Spectral radiation intensity (Ik) of a nonhomogenous path through non-scattering media may be expressed as follows [38]:
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I k ðsk Þ ¼ I k ð0Þesk þ
Z
sk
0
I bk ðs0k Þeðsk sk Þ ds0k
ð3Þ
0
where Ik(0) is the incident Ik at the boundary and sk is the optical thickness. To numerically evaluate Eq. (3) the cross section of the hydrogen flame was divided into a series of concentric rings as shown in Fig. 1. Each ring was assumed to have a homogenous composition and was three pixels wide; thus the spatial resolution radially of the scalar measurements was 0.7 mm. RADCAL [39], a narrowband radiation model, was used to evaluate Eq. (3) with the known T profile (from TPF) and guessed XH2 O . Beginning with the outer ring, Ibk and sk for three XH2 O guesses were calculated and Jf was found using Eq. (1). Brent’s formula [40], which combines root bracketing, bisection, and quadratic interpolation, was used to find the optimum XH2 O where the error between the calculated and measured Jf was minimized. With XH2 O found for the outer ring, the XH2 O for the next inner ring was solved for using the approach just described and the process was repeated. XH2 O within about 0.5 cm radii could not be determined due to a lack of T measurements. In some instances, the XH2 O would begin to increase sharply near 0.5 cm radii due to the propagation of uncertainty in the deconvolution process, and the uncertainty associated with the T and radiation measurements. These values are not reported. A sensitivity analysis indicated that a 15% uncertainty in J measurements results in an average uncertainty of 15% in XH2 O , and an 11% uncertainty in T measurements results in an average uncertainty of 15% for XH2 O greater than 0.15. Water vapor mole fractions less than this value had higher uncertainties. Based on this analysis, the typical uncertainty in XH2 O data is estimated to be 21%. 2.5. Numerical simulations The flame was modeled using a time dependent axi-symmetric computational fluid dynamics with
Thermal Image
Tomographic Deconvolution
Homogenous rings
Cordlike paths
Spatial Resolution 1.7 mm 0.7 mm Fig. 1. Tomographic measurements.
deconvolution
for
scalar
chemistry code [25,27,28]. The full Navier–Stokes equations were solved along with the species and energy conservation equations, in cylindrical coordinates. Detailed chemical kinetics for 11 species and 40 elementary reactions were used. Body forces were accounted for in the code, as well as a non-unity Lewis number (Le) typical for hydrogen/air reactions. The domain was modeled using a non-uniform staggered orthogonal grid. The measured and simulated J profile for the lower portion of the flame bulge were matched and then used as an origin for the other comparisons. The uncertainty in the matching between experimental and numerical results is estimated to be +/2.5 ms. 3. Results and discussion 3.1. Thermal imaging and radiance profiles The unsteady nature of the flame is shown in Fig. 2. The flame necks and bulges due to the vortical structures which form around the flame [25]. At times 0–35 ms the flame bulges and experiences a broadening of the flame front and at times 53– 76 ms the flame necks down and experiences a narrowing in the flame front. Within the bulge the flame front, which follows the same contour as the thermal images, is both concave and convex with respect to the fuel jet as typified by the curvatures near the bright line (thin filament) at 18 and 41 ms respectively. The thin filament was located 4.5 cm above the burner (HAB). The pulsating frequency (f) of the flame, 11 Hz, was obtained by measuring the time required for the flame to cycle through a thermal image. Malalasekera et al. found the empirical correlation [41], f = 1.68*d0.5, which suggests a pulsating frequency of 18.8 Hz for this flame. This correlation tends to over predict the f for small d flames. This is supported by Hamins et al. who measured frequencies of 12 and 15 Hz for a methane flame at two exit velocities with a 7.4 mm d burner [42]. Figure 3 reports the simulated and measured Jf for the different stages of the flame–vortex interaction. The measured Jf had a maximum (330 W/ m2 sr) and minimum peak (188 W/m2 sr) at 29 and 76 ms respectively, while the simulated Jf had a maximum (338 W/m2 sr) and minimum peak (173 W/m2-sr) at 36 and 76 ms. This is representative of the Jf being larger in the lower portion of the bulge than the upper and both the upper and lower portions of the bulge being larger than the necked region. The simulated and measured peak Jf for all the stages occurred at radii between 0.8 and 1.1 cm. On the fuel side of the Jf peaks, the Jf decayed slightly and then plateaued as it approached the flame centerline. This indicates that the flame has a ring of high radiance surrounding a smaller radiance core.
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Fig. 2. Radiating structures in an unsteady laminar hydrogen flame with a thin filament stretched through the flame. The labels correspond to the time (ms) between images.
Fig. 3. Experimental and calculated LOS flame radiance profiles for a flame–vortex interaction.
Fig. 4. Measured and simulated temperatures for a flame–vortex interaction cycle.
3.2. Temperature profiles The measured and predicted instantaneous T are reported in Fig. 4, and are in good agreement
with previous results. The flame experienced a broadening and increase in T in the bulged section
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(0–35 ms) and a thinning and decrease in the T in the necked region (53–76 ms). Similar results were reported for oscillating hydrogen/nitrogen diffusion jet flames [13,29]. In both the measured and calculated flames, beyond a radius of 1.2 cm the T profile fluctuated greatly while for smaller radii the profile was fairly uniform. This affirms that the vortices surrounding the flame have a stronger vortex flame interaction than any forming inside the flame [28]. The maximum and minimum measured peak T were 2495 and 2273 K in the bulge (35 ms) and the neck (76 ms), respectively. This is a difference of 222 K. The corresponding calculated T were 2433 K (36 ms) and 2330 K (76 ms) for a difference of 103 K. Hancock et al. measured a maximum T of 2400 ± 50 K for a steady laminar hydrogen flame using CARS [13] and Grisch et al. measured a difference of 150 K between the necked and bulged regions in a buoyant hydrogen/nitrogen flame using CARS [14]. The T variation between the different stages is due to the flame-vortex interaction and the nonunity Le of the flame. In the necked region of the flame the surrounding vortex convects fresh air into the flame. This increases the velocity gradient and subsequently the reactant fluxes into the flame front. On the fuel side of the flame the increased velocity gradient has more of an effect on thermal transport than on the mass transport of hydrogen, due to the Le being less than unity. Consequently, thermal losses dominate and the peak T in the stretched region decreases [25]. This is evident at (53 ms) and (76 ms) where the T is the lowest. In the flame bulges, the flame vortex pulls the flame front away from the fuel jet. This causes the flame front to become convex with respect to the fuel jet in the lower region of the bulge and concave in the upper region. The convex curvature has a focusing effect on the diffusion of fuel into the flame front and a defocusing effect on the diffusion of the heat away from the flame front on the fuel side. This acts to increase and decrease the T respectively. However, because the Le is less than one the diffusion of hydrogen is favored and the T is increased [25,26]. This is evident by the peak temperatures occurring at 29 and 35 ms where the flame front is convex with respect to the fuel jet. Likewise a concave curvature has a defocusing effect on the diffusion of fuel into the flame front and a focusing effect on the transport of thermal energy away from the flame front. This acts to decrease and increase the T, respectively [26]. Again, because the Le is less than one the diffusion of hydrogen is favored which results in a drop in the T. This is evident by the peak T at 12 and 18 ms, where the flame front becomes concave, being lower than the T at 29 and 35 ms where the flame front is convex. The before mentioned trends in T just described can also be attributed to laminar flamelet theory, however, Katta et al. reported that these effects are minimal
for this type of flame due to the low flame velocity [27]. 3.3. Water vapor mole fraction profiles The instantaneous measured and simulated XH2 O for the different stages of the flame–vortex interaction are reported in Fig. 5. The XH2 O profiles broadened and increased while moving from the top to the lower regions of the bulge, and in the necked region the XH2 O profile narrowed and decreased. The maximum measured XH2 O , 0.34, occurred at 41 ms while the corresponding calculated maximum XH2 O , 0.32, occurred at 36 ms. The minimum measured peak XH2 O , 0.26, occurred at 0 ms while the minimum calculated peak XH2 O , 0.30, occurred at 76 ms, although the calculated peak values at times 0, 12, and 53.5 were all within 0.005 of the minimum calculated peak XH2 O . It is noteworthy that the peak measured XH2 O in the necked region were higher than most values in the bulged region, while this tendency was not present in the calculated XH2 O . For radii less than 1.2 cm the decay in the measured and simulated XH2 O profiles are the same and do not fluctuate for the different times. Beyond radii of 1.2 cm, both the XH2 O profiles
Fig. 5. Measured and calculated water vapor mole fractions for a flame–vortex interaction cycle.
D. Blunck et al. / Proceedings of the Combustion Institute 32 (2009) 2527–2534
and the magnitudes vary greatly because of flame– vortex interactions. The XH2 O variation between the different stages is also due to flame–vortex interactions and preferential diffusion. The binary diffusion coefficient of water vapor with respect to hydrogen is less than one. When the flame front is convex with respect to the fuel jet, like in the lower region of the bulge, the curvature has a focusing effect on the diffusion of hydrogen into the flame front and a defocusing effect on hydrogen. This acts to increase and decrease the XH2 O , respectively. However, because the relative diffusion coefficient is less than unity, the diffusion of hydrogen into the flame front dominates which leads to an increase in XH2 O in the bulges [27]. This is evident by the maximum XH2 O occurring at 41 ms for the measured results and 36 ms for the calculated results. In the upper regions of the bulge the flame front transitions from being convex to concave and then straight with respect with the fuel jet. Consequently, the diffusion of water vapor away from the flame becomes more dominate and the XH2 O decreases, as seen in the decrease in XH2 O in both the measured and calculated results in the upper region of the bulge (e.g. 0 ms). 3.4. Application to hydrogen fires The thermal imaging based technique developed in this study for determining XH2 O and T can be extended to stationary and non-stationary large-scale hydrogen fires, as long as the timeaveraging LOS radiance is (statistically) axi-symmetric. Radiance measurements using the IR camera at four wavelengths (filters) would need to be conducted since TFP is not applicable to largescale practical fires. The inversion procedure developed by Biswas et al. [24] would then be used to obtain planar measurements of the mean and RMS of T and XH2 O . 4. Conclusions In this study, thermal images of an unsteady laminar hydrogen flame with a pulsating frequency of 11 Hz have been captured. Instantaneous line measurements of T and XH2 O across the flame were obtained using TFP and emission spectroscopy. The measured and simulated T and XH2 O profiles in a flame–vortex interaction cycle experienced broadening in the bulges and narrowing in the necks. The measured and calculated radiance profiles were lower and narrower in the necked portions of the flame while higher and broader in the bulges. Beyond a radial distance of 1 cm, variations in the measured scalar profiles were significant, attesting to strong flame–vortex
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interactions. These interactions stretched the flame in the necked region which, due to preferential diffusion, decreased the T. In the bulges the curvature of the flame front and the less than unity Le and relative binary diffusion coefficient of water led to an increase in the T and the XH2 O in the lower regions of the bulge and a decrease in the upper regions of the bulge. The present T measurements and calculations are consistent with experimental data for previous studies while the XH2 O measurements provide additional data. With some modifications, extension of the present technique to large-scale hydrogen fires is possible.
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Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 32 (2009) 2527–2534
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Simultaneous water vapor concentration and temperature measurements in unsteady hydrogen flames David Blunck a,*, Sumit Basu a, Yuan Zheng a, Viswanath Katta b, Jay Gore a a
Purdue University, Maurice J. Zucrow Laboratories, Chaffee Hall, 500 Allison Rd, West Lafayette, IN 47907, USA b Innovative Scientific Solutions, Inc., 2766 Indian Ripple Road, Dayton, OH 45440, USA
Abstract Techniques applicable to visualization and scalar measurements in large-scale hydrogen fires are highly desired, but lacking. In this research, planar thermal images of a buoyancy-driven unsteady laminar hydrogen/air flame and line images of a thin filament stretched across this flame were obtained using an infrared (IR) camera at a sampling frequency of 348 Hz. A pulsing frequency of 11 Hz was measured. Transient line measurements of temperature (T) and water vapor mole fraction ðXH2 O Þ have been achieved using inverse radiation calculations. Instantaneous XH2 O and T distributions during a flame–vortex interaction cycle were obtained with temporal and spatial resolutions of 3 ms and 1.7 mm, respectively. The instantaneous distributions of XH2 O and T were effected by preferential diffusion and the altered velocity field of the flame. Vortices caused more scalar fluctuations outside the flame surface than inside. The present XH2 O and T measurements are consistent with detailed chemistry calculations of the flame and laser diagnostic measurements of similar flames. With some modifications, the thermal imaging based technique can be extended to large-scale hydrogen fire applications. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Hydrogen flames; Flame radiation; Non-intrusive measurement; Infrared camera
1. Introduction Safety in the production, storage, distribution, and use of hydrogen are critical to a hydrogen economy [1]. Motivated by this, various hydrogen accident scenarios have been studied. Swain et al. numerically simulated the dispersion of hydrogen leaking from a vehicle into a residential garage, to guide garage vent design and hydrogen sensor
*
Corresponding author. Fax: +1 765 494 0503. E-mail address: [email protected] (D. Blunck).
arrangement [2]. Harstad and Bellan qualitatively analyzed gaseous hydrogen burning above a liquid hydrogen pool after an accidental spill. The two dominant factors in the evaporation of the liquid hydrogen were film boiling and radiative heating [3]. Schefer and co-workers have conducted comprehensive investigations on hydrogen fires resulting from accidental storage vessel leaks [4–7]. Visible flame lengths, radiative heat fluxes, and the fraction of the total energy radiated for these large-scale hydrogen jet flames were measured. For further understanding of large-scale hydrogen fire physics and for validation of simulation tools,
1540-7489/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2008.05.046
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water vapor concentrations and temperature (T) measurements are highly desired but are lacking. What is needed is a technique for laboratory-scale scalar measurements, which can be readily applied to large-scale hydrogen fires. Thermocouples and a gas chromatograph (GC) have been used to measure the average T and water vapor mass fraction ðYH2 O ) distributions in a turbulent hydrogen/air flame [8]. For thermocouple measurements, radiative heat loss from the junction needs to be corrected [9–11] or its effect on T readings should be estimated [8,12]. In typical GC measurements, YH2 O can not be measured directly but is estimated based on the overall mass balance [8] (for hydrogen flames) or on the C/H ratio (for hydrocarbon flames) [12]. Coherent anti-Stokes Raman scattering (CARS) systems have been developed to measure the T of steady and unsteady laminar hydrogen/ air and diluted hydrogen/air flames [13,14]. Simultaneous T and YH2 O measurements of turbulent hydrogen/air and diluted hydrogen/air flames were conducted using a Raman scattering apparatus [15–17]. Raman–Rayleigh scattering techniques were developed for simultaneous T and major species concentration measurements in various diluted turbulent hydrogen and hydrocarbon flames [18]. Line measurements of T and major species concentrations of partially premixed turbulent methane flames have also been reported [19]. The data obtained from these laboratoryscale flames are widely used as benchmarks for the development of numerical simulations of reacting flows. Infrared (IR) emission–absorption spectroscopy is another technique for measuring T and species concentrations in flames [20,21]. Radiation measurements are line-of-sight in nature; therefore, tomographic deconvolution schemes are required for estimating the scalar property distributions in non-homogeneous flows. Measurements of time-varying T and water vapor mole fraction ðXH2 O ) in homogenous turbulent flows have also been achieved using absorption spectroscopy [22] and emission spectroscopy [23]. Recently, Biswas et al. reported line measurements of mean and root mean square (RMS) of T, carbon dioxide mole fraction (XCO2), and soot volume fraction in a turbulent ethylene pool fire using emission spectroscopy [24]. Measured spectral radiation intensities (Ik) at six wavelengths were used to derive the mean and RMS scalar profiles with consideration of turbulence radiation interactions. This technique only requires statistically axi-symmetric scalar fields and can be extended to planar measurements of the mean and RMS of T, and XH2 O , if two-dimensional radiation measurements are available. IR cameras provide planar radiation measurements and images without need of a traverse mechanism, and have high spatial and temporal
resolutions. Information about the flow field can also be obtained from these measurements. These capabilities make emission spectroscopy using IR camera measurements, a strong candidate for laboratory-scale hydrogen fire research with application to larger-scales. A thermal imaging based inversion technique has been used to study an unsteady transitional laminar hydrogen/air jet flame. In this flame, buoyancy-induced vortices form on the air side of the flame [25]. As the vortices are transported downstream, they interact with the flame surface altering the velocity field. The altered flow field and the non-unity Lewis number (Le) of the flame make preferential diffusion important [25, and 26]. The effects of flame–vortex interactions on timevarying T distributions in similar laminar hydrogen flames have been studied experimentally [13,14] and numerically using detailed chemistry [25,27,28]. Radiation and XH2 O measurements of these flames, however, were not reported. The specific objectives of this study are: (1) To obtain IR images and radiance measurements of an unsteady laminar hydrogen flame. (2) To obtain instantaneous line measurements of the T in a flame–vortex interaction cycle using thin filament pyrometry (TFP). (3) To obtain instantaneous line measurements of the XH2 O in a flame–vortex interaction cycle by tomographic deconvolution of the measured radiances. TFP is an established technique [29–33]. In this work TFP, was implemented simultaneously with thermal imaging and tomographic deconvolution of a single IR image, for the first time. Simultaneous measurements of T and XH2 O were obtained using this technique. Numerical simulations of the T and XH2 O for the flame were also conducted. The enhanced confidence in the IR imaging and scalar measurement technique will motivate its application to large-scale fires, as discussed in Section 3.4.
2. Experimental and numerical methods 2.1. Burner arrangement The unsteady hydrogen/air jet flame studied was established on a long axi-symmetric stainless steel tapered tube with an inner diameter (d) of 8 mm. The hydrogen mass flow rate (52 mg/s) was set by controlling the pressure upstream of a choked orifice. The jet exit Reynolds number (Re) of 980 was calculated using cold hydrogen properties. A premixed methane flame anchored to a McKenna burner was used for TFP calibration.
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The McKenna burner [34], made the flame visibly flat for the fuel (0.013 g/s) and air flow rates used (0.30 g/s). The burner was water cooled and had an outer ring of co-flowing air. The IR camera was aligned perpendicular to the thin filament, which was located 0.5 cm above the McKenna burner. After calibration data was collected for the filament and the methane flame, the McKenna burner was carefully removed. The hydrogen flame burner was then moved vertically into the proper location. 2.2. Thermal imaging and radiance measurements Radiance (J) measurements were obtained using an FLIR Phoenix IR camera mounted with a 25 mm lens. The angle of divergence from the center of the flame to the camera was less than 1.5°; therefore the planar images provide line-of-sight (LOS) measurements through the flame. Each of the 256 rows and 320 columns of pixels corresponded spatially to a square with a width and height of 0.24 mm in the center of the flame. The camera integration time was 0.015 ms and the sampling frequency was 348 Hz. This provided a temporal response of 3 ms. A 2.77 ± 0.12 lm band-pass filter was used with the camera. Measurements of the J below 2250 photon counts were discarded because the detector behaved non-linearly below this threshold. The reported J is the measured radiation after being attenuated by the lens, filter, and spectral response of the detector. This was accounted for in the T and XH2 O measurements. Flame radiance (Jf) was determined by averaging the measured J three pixels above and below the filament to avoid interference from the radiation emitted by the filament. Consequently, the spatial resolution of the present scalar measurements in the axial direction is 1.7 mm (7 pixels). The filament radiance (Jfil) was determined by subtracting Jf from the measured peak J along the filament, although it was found that this can lead to a bias in the results due to the dispersion of the intensity emitted by the filament. The calculated optical transmission loss is less than 6% for both the hydrogen and calibration flames. Jfil and Jf were averaged around the jet centerline to compensate for variations in the symmetry of the flame. 2.3. Temperature measurements Thin filament pyrometry consists of placing a small silicon carbide (SiC) fiber (15 lm diameter) in a flame to determine the flame temperature (Tf) using the emitted radiance (Jfil). As the fiber is heated it radiates as a gray body [30] and the Jfil measured by the camera can be described by the equation Z k2 J fil ðT fil Þ ¼ k exp sk Rk ek I b;k ðT fil Þdk ð1Þ k1
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where Tfil is the filament temperature, Rk is the spectral response of the camera, sk is the transmission through the filter and optics, Ib,k is the Planck function, ek is the emissivity, and kexp is the gain of the electronics and the optics efficiency [32]. Following the approach of Vilimpoc and Goss [29], the Jfil was normalized by the radiance ðJ Ref fil Þ at a reference temperature (Tref). Tfil was then determined by cross-referencing the measured ratio to a look-up table with calculated ratios at variousTfil. The uncertainty in J Ref fil was 7% and the uncertainty in Jfil was estimated to be 15% (both 95% confidence), based on averaged J measurements of the filament in the respective flames. This resulted in a 6% uncertainty in Tfil at 1200 K and 10% in Tfil at 2400 K. Tf was determined from Tfil by correcting for radiative heat loss using the correlation [35] T f ¼ T fil þ
erðT 4fil T 4surr Þ hc
ð2Þ
where r is the Stefan–Boltzmann constant and hc is the convective heat transfer coefficient. Following others work, e was set equal to 0.88 [32,35]. The hc was found using a correlation for flow around a cylinder, Nu = 0.43 + 0.48Re0.5 [36]. The Reynolds number (Re) around the filament was estimated using a velocity of 2 m/s, which was based on simulations of the flame velocity. This assumed that the flow velocity is uniform across the flame. In reality the velocity and subsequently the hc vary across the flame [32]. A sensitivity analysis found that for the upper and lower limits of the simulated velocity an assumed constant velocity of 2 m/s resulted in a 1% difference in Tf. It should be noted that Tf at radial distances less than 0.5 cm in this flame were not obtained because the Tfil became too low to give a sufficient J signal. This is a well recognized limitation of the TFP technique [29]. The temperature of the calibration methane flame was measured prior to the J Ref measurefil ment, and was used as Tref. Inverse spectral radiation (Ik) measurements were used to determine a Tref of 1545 ± 60 K. This assumed a uniform chemical equilibrium of species across the burner. The uncertainty in the Tref measurement was estimated based on a 10% uncertainty in the Ik measurements [37]. Based on the uncertainties in Tref measurements, velocity estimates, and J measurements, the overall uncertainty in Tf measurements is estimated to be 7% at 1200 K and 11% at 2400 K. 2.4. Water vapor mole fraction measurements Spectral radiation intensity (Ik) of a nonhomogenous path through non-scattering media may be expressed as follows [38]:
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I k ðsk Þ ¼ I k ð0Þesk þ
Z
sk
0
I bk ðs0k Þeðsk sk Þ ds0k
ð3Þ
0
where Ik(0) is the incident Ik at the boundary and sk is the optical thickness. To numerically evaluate Eq. (3) the cross section of the hydrogen flame was divided into a series of concentric rings as shown in Fig. 1. Each ring was assumed to have a homogenous composition and was three pixels wide; thus the spatial resolution radially of the scalar measurements was 0.7 mm. RADCAL [39], a narrowband radiation model, was used to evaluate Eq. (3) with the known T profile (from TPF) and guessed XH2 O . Beginning with the outer ring, Ibk and sk for three XH2 O guesses were calculated and Jf was found using Eq. (1). Brent’s formula [40], which combines root bracketing, bisection, and quadratic interpolation, was used to find the optimum XH2 O where the error between the calculated and measured Jf was minimized. With XH2 O found for the outer ring, the XH2 O for the next inner ring was solved for using the approach just described and the process was repeated. XH2 O within about 0.5 cm radii could not be determined due to a lack of T measurements. In some instances, the XH2 O would begin to increase sharply near 0.5 cm radii due to the propagation of uncertainty in the deconvolution process, and the uncertainty associated with the T and radiation measurements. These values are not reported. A sensitivity analysis indicated that a 15% uncertainty in J measurements results in an average uncertainty of 15% in XH2 O , and an 11% uncertainty in T measurements results in an average uncertainty of 15% for XH2 O greater than 0.15. Water vapor mole fractions less than this value had higher uncertainties. Based on this analysis, the typical uncertainty in XH2 O data is estimated to be 21%. 2.5. Numerical simulations The flame was modeled using a time dependent axi-symmetric computational fluid dynamics with
Thermal Image
Tomographic Deconvolution
Homogenous rings
Cordlike paths
Spatial Resolution 1.7 mm 0.7 mm Fig. 1. Tomographic measurements.
deconvolution
for
scalar
chemistry code [25,27,28]. The full Navier–Stokes equations were solved along with the species and energy conservation equations, in cylindrical coordinates. Detailed chemical kinetics for 11 species and 40 elementary reactions were used. Body forces were accounted for in the code, as well as a non-unity Lewis number (Le) typical for hydrogen/air reactions. The domain was modeled using a non-uniform staggered orthogonal grid. The measured and simulated J profile for the lower portion of the flame bulge were matched and then used as an origin for the other comparisons. The uncertainty in the matching between experimental and numerical results is estimated to be +/2.5 ms. 3. Results and discussion 3.1. Thermal imaging and radiance profiles The unsteady nature of the flame is shown in Fig. 2. The flame necks and bulges due to the vortical structures which form around the flame [25]. At times 0–35 ms the flame bulges and experiences a broadening of the flame front and at times 53– 76 ms the flame necks down and experiences a narrowing in the flame front. Within the bulge the flame front, which follows the same contour as the thermal images, is both concave and convex with respect to the fuel jet as typified by the curvatures near the bright line (thin filament) at 18 and 41 ms respectively. The thin filament was located 4.5 cm above the burner (HAB). The pulsating frequency (f) of the flame, 11 Hz, was obtained by measuring the time required for the flame to cycle through a thermal image. Malalasekera et al. found the empirical correlation [41], f = 1.68*d0.5, which suggests a pulsating frequency of 18.8 Hz for this flame. This correlation tends to over predict the f for small d flames. This is supported by Hamins et al. who measured frequencies of 12 and 15 Hz for a methane flame at two exit velocities with a 7.4 mm d burner [42]. Figure 3 reports the simulated and measured Jf for the different stages of the flame–vortex interaction. The measured Jf had a maximum (330 W/ m2 sr) and minimum peak (188 W/m2 sr) at 29 and 76 ms respectively, while the simulated Jf had a maximum (338 W/m2 sr) and minimum peak (173 W/m2-sr) at 36 and 76 ms. This is representative of the Jf being larger in the lower portion of the bulge than the upper and both the upper and lower portions of the bulge being larger than the necked region. The simulated and measured peak Jf for all the stages occurred at radii between 0.8 and 1.1 cm. On the fuel side of the Jf peaks, the Jf decayed slightly and then plateaued as it approached the flame centerline. This indicates that the flame has a ring of high radiance surrounding a smaller radiance core.
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Fig. 2. Radiating structures in an unsteady laminar hydrogen flame with a thin filament stretched through the flame. The labels correspond to the time (ms) between images.
Fig. 3. Experimental and calculated LOS flame radiance profiles for a flame–vortex interaction.
Fig. 4. Measured and simulated temperatures for a flame–vortex interaction cycle.
3.2. Temperature profiles The measured and predicted instantaneous T are reported in Fig. 4, and are in good agreement
with previous results. The flame experienced a broadening and increase in T in the bulged section
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(0–35 ms) and a thinning and decrease in the T in the necked region (53–76 ms). Similar results were reported for oscillating hydrogen/nitrogen diffusion jet flames [13,29]. In both the measured and calculated flames, beyond a radius of 1.2 cm the T profile fluctuated greatly while for smaller radii the profile was fairly uniform. This affirms that the vortices surrounding the flame have a stronger vortex flame interaction than any forming inside the flame [28]. The maximum and minimum measured peak T were 2495 and 2273 K in the bulge (35 ms) and the neck (76 ms), respectively. This is a difference of 222 K. The corresponding calculated T were 2433 K (36 ms) and 2330 K (76 ms) for a difference of 103 K. Hancock et al. measured a maximum T of 2400 ± 50 K for a steady laminar hydrogen flame using CARS [13] and Grisch et al. measured a difference of 150 K between the necked and bulged regions in a buoyant hydrogen/nitrogen flame using CARS [14]. The T variation between the different stages is due to the flame-vortex interaction and the nonunity Le of the flame. In the necked region of the flame the surrounding vortex convects fresh air into the flame. This increases the velocity gradient and subsequently the reactant fluxes into the flame front. On the fuel side of the flame the increased velocity gradient has more of an effect on thermal transport than on the mass transport of hydrogen, due to the Le being less than unity. Consequently, thermal losses dominate and the peak T in the stretched region decreases [25]. This is evident at (53 ms) and (76 ms) where the T is the lowest. In the flame bulges, the flame vortex pulls the flame front away from the fuel jet. This causes the flame front to become convex with respect to the fuel jet in the lower region of the bulge and concave in the upper region. The convex curvature has a focusing effect on the diffusion of fuel into the flame front and a defocusing effect on the diffusion of the heat away from the flame front on the fuel side. This acts to increase and decrease the T respectively. However, because the Le is less than one the diffusion of hydrogen is favored and the T is increased [25,26]. This is evident by the peak temperatures occurring at 29 and 35 ms where the flame front is convex with respect to the fuel jet. Likewise a concave curvature has a defocusing effect on the diffusion of fuel into the flame front and a focusing effect on the transport of thermal energy away from the flame front. This acts to decrease and increase the T, respectively [26]. Again, because the Le is less than one the diffusion of hydrogen is favored which results in a drop in the T. This is evident by the peak T at 12 and 18 ms, where the flame front becomes concave, being lower than the T at 29 and 35 ms where the flame front is convex. The before mentioned trends in T just described can also be attributed to laminar flamelet theory, however, Katta et al. reported that these effects are minimal
for this type of flame due to the low flame velocity [27]. 3.3. Water vapor mole fraction profiles The instantaneous measured and simulated XH2 O for the different stages of the flame–vortex interaction are reported in Fig. 5. The XH2 O profiles broadened and increased while moving from the top to the lower regions of the bulge, and in the necked region the XH2 O profile narrowed and decreased. The maximum measured XH2 O , 0.34, occurred at 41 ms while the corresponding calculated maximum XH2 O , 0.32, occurred at 36 ms. The minimum measured peak XH2 O , 0.26, occurred at 0 ms while the minimum calculated peak XH2 O , 0.30, occurred at 76 ms, although the calculated peak values at times 0, 12, and 53.5 were all within 0.005 of the minimum calculated peak XH2 O . It is noteworthy that the peak measured XH2 O in the necked region were higher than most values in the bulged region, while this tendency was not present in the calculated XH2 O . For radii less than 1.2 cm the decay in the measured and simulated XH2 O profiles are the same and do not fluctuate for the different times. Beyond radii of 1.2 cm, both the XH2 O profiles
Fig. 5. Measured and calculated water vapor mole fractions for a flame–vortex interaction cycle.
D. Blunck et al. / Proceedings of the Combustion Institute 32 (2009) 2527–2534
and the magnitudes vary greatly because of flame– vortex interactions. The XH2 O variation between the different stages is also due to flame–vortex interactions and preferential diffusion. The binary diffusion coefficient of water vapor with respect to hydrogen is less than one. When the flame front is convex with respect to the fuel jet, like in the lower region of the bulge, the curvature has a focusing effect on the diffusion of hydrogen into the flame front and a defocusing effect on hydrogen. This acts to increase and decrease the XH2 O , respectively. However, because the relative diffusion coefficient is less than unity, the diffusion of hydrogen into the flame front dominates which leads to an increase in XH2 O in the bulges [27]. This is evident by the maximum XH2 O occurring at 41 ms for the measured results and 36 ms for the calculated results. In the upper regions of the bulge the flame front transitions from being convex to concave and then straight with respect with the fuel jet. Consequently, the diffusion of water vapor away from the flame becomes more dominate and the XH2 O decreases, as seen in the decrease in XH2 O in both the measured and calculated results in the upper region of the bulge (e.g. 0 ms). 3.4. Application to hydrogen fires The thermal imaging based technique developed in this study for determining XH2 O and T can be extended to stationary and non-stationary large-scale hydrogen fires, as long as the timeaveraging LOS radiance is (statistically) axi-symmetric. Radiance measurements using the IR camera at four wavelengths (filters) would need to be conducted since TFP is not applicable to largescale practical fires. The inversion procedure developed by Biswas et al. [24] would then be used to obtain planar measurements of the mean and RMS of T and XH2 O . 4. Conclusions In this study, thermal images of an unsteady laminar hydrogen flame with a pulsating frequency of 11 Hz have been captured. Instantaneous line measurements of T and XH2 O across the flame were obtained using TFP and emission spectroscopy. The measured and simulated T and XH2 O profiles in a flame–vortex interaction cycle experienced broadening in the bulges and narrowing in the necks. The measured and calculated radiance profiles were lower and narrower in the necked portions of the flame while higher and broader in the bulges. Beyond a radial distance of 1 cm, variations in the measured scalar profiles were significant, attesting to strong flame–vortex
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interactions. These interactions stretched the flame in the necked region which, due to preferential diffusion, decreased the T. In the bulges the curvature of the flame front and the less than unity Le and relative binary diffusion coefficient of water led to an increase in the T and the XH2 O in the lower regions of the bulge and a decrease in the upper regions of the bulge. The present T measurements and calculations are consistent with experimental data for previous studies while the XH2 O measurements provide additional data. With some modifications, extension of the present technique to large-scale hydrogen fires is possible.
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