Absolute concentration of the chemiluminescent radical OH* was determined in an axisymmetric hydrogen-air premixed flame adjacent to a resistively-heated graphite surface. Two-dimensional images of the axisymmetric chemiluminescence from the excited-state of OH were recorded by an ICCD camera with a narrow-band filter at approximately 310 nm. A temperature of around 1800 K was achieved on the graphite surface using an electrical heating power of 5.5 kW.  Surface temperatures were measured using a two-color ratio pyrometry (TCRP) technique. The line-of-sight-integrated chemiluminescent emissions that were imaged using the ICCD device were transformed to radial distributions through an Abel inversion method. A new method for calibration of the absolute number density of the radiating radical OH* is proposed based on the intensity ratio of the measured OH* chemiluminescence and the radiation emitted from the hot graphite surface. This is a convenient approach in the present work because adequate signal magnitudes from both these phenomena are acquired by the ICCD device simultaneously during testing

Buttsworth, D. R.

Choudhury, R.

Malpress, R.

Zhao, M. M.

English

University of Southern Queensland ePrints

Absolute concentration measurements of OH* in an axisymmetric hydrogen-air premixed flame adjacent to a hot graphite model

20th Australasian Fluid Mechanics ConferencePerth, Australia5-8 December 2016Absolute Concentration Measurements of OH* in an Axisymmetric Hydrogen-air PremixedFlame Adjacent to a Hot Graphite ModelM.M. Zhao, R. Choudhury, R. Malpress and D.R. ButtsworthSchool of Mechanical and Electrical EngineeringUniversity of Southern Queensland, Toowoomba, Queensland 4350, AustraliaAbstractAbsolute concentration of the chemiluminescent radical OH*was determined in an axisymmetric hydrogen-air premixedflame adjacent to a resistively-heated graphite surface. Two-dimensional images of the axisymmetric chemiluminescencefrom the excited-state of OH were recorded by an ICCD cam-era with a narrow-band filter at approximately 310 nm. Atemperature of around 1800 K was achieved on the graphitesurface using an electrical heating power of 5.5 kW. Surfacetemperatures were measured using a two-color ratio pyrometry(TCRP) technique. The line-of-sight-integrated chemilumines-cent emissions that were imaged using the ICCD device weretransformed to radial distributions through an Abel inversionmethod. A new method for calibration of the absolute numberdensity of the radiating radical OH* is proposed based on theintensity ratio of the measured OH* chemiluminescence andthe radiation emitted from the hot graphite surface. This is aconvenient approach in the present work because adequate sig-nal magnitudes from both these phenomena are acquired by theICCD device simultaneously during testing.IntroductionChemiluminescence refers to the spontaneous light emissionfrom chemically excited species by an electronic exchangeprocess and is a frequently-used diagnostic in combustion re-search for detecting the location of flame fronts [8] and heatrelease [10, 6]. The OH* chemiluminescence was recently usedas a supersonic combustion diagnostic in [9], and is conve-nient because the detected radiation is brought about directlyby inherent chemical reactions within the oxidation system andthe diagnostic avoids the need for expensive and maintenance-prone laser instruments. OH* provides a ready diagnostic forflame and combustion phenomena analysis due to its simplicityand non-intrusive nature.While there have been numerous experimental investigationsand applications of chemiluminescence in flames, most previ-ous research involving the radical’s chemiluminescence is lim-ited to qualitative or relative measurements. The measurementof absolute concentration of excited species has been achievedusing chemiluminescence measurements with a Raman andRayleigh scattering calibration by several researchers [14, 11],and [13] appears to be the only published measurements of ab-solute OH* concentrations in hydrogen/air flames. In H2/O2combustion, the observed self-luminescent emission of UV ra-diation at a wavelength of around 306 nm is attributed to theOH (A2Σ+X2Π) transition from its electronically excited state(typically denoted OH*) to its ground state.OH* chemiluminescence has been extensively studied [4, 7]and the primary pathways for OH* formation and depletion arecommonly proposed as a set of elementary reactions,H +O+M ⇀↽ OH∗ (R1)OH∗→ OH +hv (R2)OH∗+M→ OH +M (R3)where M is a third body species.Experimental ArrangementAn external axisymmetric configuration was adopted in an ef-fort to generate high quality data from optical diagnostics thatwill be suitable for validation of future computational simula-tions. Combustion experiments where performed in a nominallyquiescent test section environment of the free-piston wind tun-nel of the University of Southern Queensland – the ‘TUSQ’ fa-cility, and a detailed description of TUSQ has been reported in[3].The arrangement of the hot surface axisymmetric model, thefuel delivery system and the optical instruments in the TUSQfacility are illustrated in Figure 1. The fuel supply system canbe operated to give either pure hydrogen or premixed hydrogen-air delivery, by the control of two fast-action normally closedsolenoid valves (PROCESS SYSTEMS B35). The solenoidvalve 1© is kept closed if an experiment is conducted with de-livery of pure hydrogen fuel.A K-type fine wire thermocouple (OMEGA, dia. 0.001 inch)with an estimated time constant of 0.01 s and a DRUCK pres-sure transmitter (PTX1400, 25 Bar) were mounted on the de-livery line at a location upstream of the hot model. The massflow rates of hydrogen and air were measured by an OMEGAflow controller (FMA-2600A) and a ROTA mass meter (YOKO-GAWA, RCCS32), respectively. Check valves were mounted ineach line to ensure mixing of air and hydrogen does not oc-cur upstream of these devices. Two bespoke flame arrestersutilising stainless steel metal mesh with an aperture of 0.132mm were installed upstream of the check valves. The aperturesize of the flame arrestors was chosen based on the quenchingdistance of about 0.3 mm for stoichiometric hydrogen-air mix-tures within the target operation pressures range of 100 to 200kPa [15]. The flame arresters will prevent flame propagationupstream if the premixed hydrogen-air mixture is ignited acci-dently and the check valves do not close fast enough. A flexiblebag with a volume of approximately 2 liters was utilized as ahydrogen reservoir and was contained in a stainless steel vessel(20 liters approximately).The hot surface model consisted of a cylindrical graphite tube(dia. 15 mm), a cone with a 9◦ half angle and other model sup-port components. These were arranged to allow a large amountof electrical power (supplied by a Miller Dynasty 700 weldingunit) to be passed through the model during the heating process,generating a large and rapid temperature rise. The water cool-ing system was designed to remove heat from the metallic stingassembly so that the integrity of soldered joints was maintainedduring the model heating operation.Chemiluminescent OH* measurements were recorded using aPrinceton Instruments PI-MAX intensified CCD (ICCD) cam-era mounted above a CaF2 window on the top side of the testsection with an object distance of about 800 mm and an expo-Figure 1: View of hot surface model and instruments arrangement in the TUSQ facility.sure time of 25 ms. The camera was equipped with an apoc-hromatic, 105 mm focal length UV lens (CoastalOpt 105mmf/4 UV-Macro-Apo) and a narrow-band-pass filter centered at310 nm with 10 nm FWHM (ASAHI XBPA310) was positionedin front of the lens to capture the radiation emitted in the wave-length range of interest around 308 nm. The resolution of theCCD array used in the camera is 1024× 256px (each pixel26×26µm in size). The sensitivity of the ICCD array was notuniform because of previous damage, so a calibration of the in-dividual pixels relative to an arbitrarily chosen reference pointwas performed to determine the relative efficiency of each pixelin counts per unit of radiation emitted from a source, as shownin Figure 2.Figure 2: Realtive spatial sensitivity of ICCD pixels.The two Colour Ratio Pyrometry (TCRP) method used forsurface temperature measurement works on the principlethat the ratio of any two wavelength intensities emitted bya grey body is unique to a particular temperature and thusthe temperature can be determined by the measured ratio ofradiation emitted at two wavelengths, without knowledge ofthe absolute values of radiant intensity or the emissivity. Theoperating wavelengths can be chosen arbitrarily as long as theyare known and significant radiation can be received at thesewavelengths for the temperature range of interest. Althoughgraphite does not behave strictly as grey body, the investigationinto the wavelength dependent emissivity of graphite hasshown a maximum difference of 5 % in the wavelength rangeof 500 ∼ 1000 nm [12, 1]. For the present study, TCRP withthe wavelength ratio of I(850nm)/I(700nm), fast responsetimes (10 kHz) and uncertainties of about ±50 K was used fortime-resolved temperature determination of the heated modelduring the combustion testing.Results and DiscussionFigure 3 illustrates the OH* chemiluminescent emission sig-nals in pixel counts from the H2-air premixed flame. For thisfigure, the background image that was taken with the flame ex-tinguished as shown in Figure 4 has been subtracted. The con-ditions of the premixed H2-air combustion testing are listed inTable 1.m˙ Tf uel Tgraphite Ptest section φ(kg/s) (K) (K) (kPa)1.40×10−4 320 1798 15 1.03Table 1: Conditions for the premixed H2-air combustion testing.A smoothing process was performed on the image using a 10-pixel moving average filter in order to reduce the noise of theraw OH* chemiluminescence signals, and the smoothed resultis shown in Figure 5 for a zoomed-in view. Figure 6 illustratesthe raw and smoothed radial signals at X = 9.2 mm. The two-dimensional imaged line-of-sight-integrated chemiluminescentemissions recorded by the ICCD camera can be transformedto radial distributions by the inverse Abel transformation. TheAbel-inverted results for the smoothed data are plotted in Fig-ure 7. A three-point Abel inversion method based on work byDasch [5] was used in this case.The deviation of the transformed results that occurs at the outer-most radius (see Figure 7) is caused by the non-zero value andderivative at the outer boundary due to a relatively small viewfield of ICCD camera. In order to assess the effect of non-zerovalues at the imaged outer boundary on the Abel-inverted re-sults, the smoothed data was extrapolated to zero based on thesecond-order polynomial fitting as shown in Figure 6. The re-sults reconstructed from this extrapolation are also plotted inFigure 7. It is clear that the anomalous inverted results at theouter-most radius (r = 16 mm) can be corrected using extrap-olated data, whereas the peak value and the other values awayfrom the outer-most radius are not affected significantly in thepresent case. Figure 8 illustrates the radial distribution of theOH* chemiluminescence signals obtained via the 3-points Abelinversion method in the region of interest.Negative values which are not physically possible appear withinthe radial range from 0 to 8.4 mm in Figure 7. For radius valuesof 7.5 mm or less, the ADU/mm value should be zero theoreti-cally since this radius corresponds with the graphite surface. Atleast two factors contribute to this error: (1) the hot surface usedas a calibrating source for radiation deduction has a slightly dif-ferent temperature between the backgound image case and theexperiment case resulting in a different radiating graphite back-ground intensity; and (2) the slight displacement of the modelduring the testing made it very difficulty to match the graphiteedges perfectly when processing the background radiation de-duction. As the Abel inversion is sensitive to the derivative val-ues close to the point being processed, the error at the locationsnear the graphite edge is amplified. However, the Abel inver-sion method is effectively marched from r to ∞, the invertedresults at locations greater than the graphite edge would not beaffected by the graphite edge and background errors.Figure 3: Counts from the ICCD image showing OH* chemilu-minescence. The rectangle in red indicates the region of interestfor application of the Abel inversion processing. The data alongthe black line at location of X = 9.2 mm is extracted for a moredetailed illustration of the Abel inversion analysis.Figure 4: Counts from the ICCD image showing hot sur-face emission at temperature of 1783 K. The location marked‘TCRP’ represents the temperature measurement location(scale: mm).Figure 5: Smoothed OH* chemiluminescence signals in the re-gion of interest.The absolute number density of the radiating OH* was calcu-lated based on the intensity ratio of the simultaneously mea-sured OH* chemiluminescence and the radiation emitted fromthe hot graphite surface which is assumed a blackbody. An il-lustration of the optical detection system is presented in Figure9. Pixels that collect radiation from ∆A at the intersection ofthe graphite surface and the axis through its center are chosenas the calibration sources. ∆V represents an observed volumein the path of the line-in-sight integration for a pixel. The ra-tio of signal intensity Sg (units of ADU) and the Abel invertedFigure 6: Extracted OH* chemiluminescence signals of pixelsat X = 9.2 mm from the region of interest.Figure 7: Abel-inverted radial signal distributions at X =9.2 mm.Figure 8: Abel-inverted OH* chemiluminescence signals in theregion of interest.OH* chemiluminescence SOH∗(Abel) (units of ADU/mm) can berelated to the number ratio of photons collected by the opticalsystem that are emitted from the hot graphite surface (Ng) andthose emitted by excited-state OH* chemiluminescence (NOH∗ )within ∆V .SgSOH∗(Abel)=NgNOH∗=∫ 330λ=290Ibλhν ηdλΩ1 ξ1 ε∆t[OH∗]em4pi η(λ=308nm)Ω2 ξ2 ε∆t=∫ 330λ=290Ebλpihν ηdλΩ1 ξ1 ε∆t[OH∗]em4pi η(λ=308nm)Ω2 ξ2 ε∆t(1)where Ibλ W/(m2 µmsr) and Ebλ W/(m2µm) are the monochro-matic radiation intensity and emissive power from ∆A into sur-rounding hemisphere space in which Eb = piIb (Lambert’s co-sine law), [OH∗]em (N/m3 · s) is the OH* number density in ∆Vthat emit photons into surrounding sphere space per second, hνis the photon energy, η is the Asahi bandpass filter transmission,Ω is the solid angle over which the light is collected, ξ is thepixel efficiency in counts per photon, ε is the efficiency of thecollection optics which is treated as a constant within the nar-row bandpass range of the optical filter. ∆t is the ICCD exposuretime. The difference of solid anglesΩ1 andΩ2 can be neglectedsince the effective pixel size (0.126× 0.126 mm) is very smallcomparing to the object distance (800 mm approximately). Therelative sensitivity of the pixels has been determined, see Fig-ure 2. An assumption is made that a constant filter transmissionat λ = 308nm is applied to OH* emission spectra and this isa reasonable assumption because spectrally resolved measure-ments of OH* chemiluminescence [2] demonstrate that most ofits emission occurs within the wavelength range of 306 nm and310 nm. Thus the number density of excited-state OH* emittingradiation can be computed.[OH∗]em =4SOH∗(Abel) ξ1Sgη(λ=308nm) ξ2∫ 330λ=290Ebλhνηdλ (2)The calculated number density of radiating OH* based on theAbel-inverted radical distributed signals is shown in Figure 10.Figure 9: Optical arrangement of OH* chemiluminescence de-tection system.Figure 10: Number density of the radiating OH*.ConclusionsA number density map of radiating OH* was determined for anaxisymmetric hydrogen-air premixed flame based on a calibra-tion method proposed in this paper. For highest quality mea-surements of axisymmetric chemiluminescence, the followingprecautions should be observed. (i) An optical configurationwith a large depth of field should be used to approximate line-in-sight collection along parallel rays. This can also reduce the‘circle of confusion’ effect in which collected light blurs theneighboring pixels. (ii) The radius of the field of view shouldextend beyond the emissions of interest so that the backgroundlevel at sufficient radius is acquired.References[1] Balat-Pichelin, M., Robert, J. and Sans, J., Emissivitymeasurements on carbon–carbon composites at high tem-perature under high vacuum, Applied Surface Science,253, 2006, 778–783.[2] Brieschenk, S., Lorrain, P., Capra, B., Boyce, R., McIn-tyre, T. J., Kleine, H. and O’Byrne, S., Chemilumi-nescence imaging in supersonic combustors operating inradical-farming mode, in 18th AIAA/3AF InternationalSpace Planes and Hypersonic Systems and TechnologiesConference, Tours, France, AIAA 2012-5854, 2012.[3] Buttsworth, D. 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Absolute concentration measurements of OH* in an axisymmetric hydrogen-air premixed flame adjacent to a hot graphite model

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