MEASURING THE TEMPERATURE OF A FLAME PROPAGATING THROUGH A QUARTZ TUBE USING HIGH SPEED COLOUR IMAGING AND THIN SIC FILAMENT BASED TWO COLOUR METHOD

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1 MEASURING THE TEMPERATURE OF A FLAME PROPAGATING THROUGH A QUARTZ TUBE USING HIGH SPEED COLOUR IMAGING AND THIN SIC FILAMENT BASED TWO COLOUR METHOD M.Z.*, Ebieto C. E., Zhng Y., nd Woolley R. *Author for correspondence Deprtment of Mechnicl Engineering, University of Sheffield, Sheffield, S1 3JD, United Kingdom, E-mil: zm1@sheffield.c.uk ABSTRACT Blckbody rdition from thin filment of SiC cn be pplied to mesure hot gses temperture. The technique, Thin- Filment Pyrometer(TFP), exhibits fst temporl response nd high sptil resolution owing to the very fine dimeter nd low het conductivity. The dvntge of this pproch is the low cost nd simplicity, nd it is pplicble to the situtions where other pproches re difficult to pply. In this study, the investigtion of methne-ir flme propgting through tube of 2mm internl dimeter qurtz tube with open ends hs been crried out. SiC fibres were instlled t both ends nd the midsection of the tube s rdition emitters. The required volume of methne t ech equivlence rtio ws mesured nd injected into the rig nd the mixture ws ignited t one end (left). The propgting flme ws recorded using two highspeed cmers t frme rte of 2 fps with one trcking the speed of the flme nd nother cpturing the imges of the glowing filment. The imges of the glowing filment were then processed to determine the surrounding gs temperture using the two-colour technique. Unlike the two-colour technique using bem splitting nd nrrow bndpss filtering pproch, which often results in mislignment between two grey level imges, complicted configurtion nd high cost, the current technique uses the cmer built-in colour-bnded filter. The rtio between two of the RGB primry chnnels ws used for the clcultion of the temperture. The mximum speed nd temperture were observed t equivlence rtio 1.1 with bellshped curve. For cses where the flmes were subjected to instbilities, the tempertures were found to fluctute ccordingly. The ccurcy of TFP mesurement ws verified by commercil Infr-pyrometer. INTRODUCTION Temperture is mong the most frequently mesured properties in combustion study. A temperture mesurement technique hving less-intrusive, fst-responding fetures, nd being esily set-up, is desirble. A thin filment pyrometer is one of the cndidtes tht mtches severl of the bove chrcteristics, especilly for high-temperture mesurement in closed spce. This technique hs been proven to be useful pproch to mesuring flme temperture. It provides onedimensionl profile of temperture mesurement long the length of the filment. NOMENCLATUREFE C 2 [-] Second Plnck s constnt G(λ,T) [-] Grey-level of Imge k [W/mK] Therml Conductivity h [W/m 2 K] Het Trnsfer Coefficient Re [-] Reynolds Number Nu [-] Nusselt Number T [m] True Temperture S [-] Instrument Fctor of Imging System λ R λ G D Pr ε T Subscripts g f G R Green [m] [m] [m] [-] [K] Wvelength in R Chnnel of the Imging System Wvelength in G Chnnel of the Imging System Dimeter of SiC Fibre. Prndtl Number SiC fibre Emissivity Surfce Temperture Surrounding gs of the SiC fibre SiC fibre Green chnnel of the imging system Red chnnel of the imging system Green chnnel of the imging system Ambient environment Typiclly, TFP uses β-sic fibre of 15μm in dimeter. It hs good strength, reltively constnt emissivity[1], resistnce to oxidtion with no ctlytic effect, nd high melting point (3325K). For temperture mesurement, the fibre is plced in therml environment nd the fibre glows nd emits nerly blck body rdition, which is detected by n imging system, nd the determintion of the fibre temperture is bsed on the twocolour method. Finlly, this temperture is correlted to the locl gs temperture through n energy blnce on the fibre. Vilimpoc[2] pioneered the demonstrtion of the potentil of mesuring temperture using TFP nd estimted the temporl response to be round 1.5ms. Bedt[3] pplied the TFP mesurement in wekly turbulent flme nd extended the limit of temperture to 55 K. Pitts[4] performed the TFP in flickering lminr diffusion flme. Struk[5] compred the temperture mesured by thermocouple with the temperture mesured by TFP within 3.5% difference for tempertures bove 12 K. Mun[6] pplied digitl colour imging bsed TFP mesurement with n estimted uncertinty of ±6K in the rnge of 14-22K. Kuhn[7] compred the thin-filment-

2 R / G derived gs temperture with the computtionl result, showing strong similrity. Bin[8] compred lser-bsed technique nd thermocouple with TFP mesurement, showing good greement in mesured tempertures. For ll TFP mesurements, energy blnce is n importnt step to determine the surrounding gs temperture. The devition between surrounding gs temperture nd TFP determined temperture depends much on the ccurcy of the energy blnce procedure. SiC fibre is typiclly considered s stble emitter nd fibre temperture is determined from the incndescence of the glowing fibre by using two-colour method. This technique needs pre-clibrtion on known temperture source, typiclly tungsten lmp. Once this is done, it is free of further temperture clibrtion. In this pper, TFP is pplied to mesure propgting flme temperture in n open end tube. The vrition of temperture nd the hot gs velocity through the tube cn be observed. The ccurcy of two-colour method hs been compred with n Infr-pyrometer. The min originlity here is to explore the possibility of mesuring fst developing hot gs by tking dvntge of the good therml properties of thin SiC fibre. EXPERIMENTAL SETUP The experimentl rig consisted of tube with n inner dimeter of 2 mm nd length of 12 mm, opened t both ends. The centrl 7mm length of the tube ws mde of qurtz to provide opticl ccess(fig. 1). The qurtz ws used becuse of its high trnsmissivity nd the bility to survive in hightemperture environment. SiC fibres were instlled t the midsection nd both ends of the qurtz tube. The experiment ws conducted t different equivlence rtios of.9, 1.,1.1 nd 1.2. The required volume of methne t ech equivlence rtio ws mesured nd injected into the rig using syringe, nd two fns ttched to the rig were used to crete homogeneous mixture. The propgting flme ws recorded using two highspeed cmers t frme rte of 2 fps with one trcking the speed of the bluish flme nd the other cpturing the imges of the only glowing filments for flme temperture mesurement using higher shutter speed to eliminte the disturbnce of the flme illumintion nd void imge sturtion s result of the fibre strong emission. Ignition ws initited using pilot flme. This ws done by opening port t the left end of the tube fter switching off the fn nd directing the pilot flme into the tube. The gs phse temperture cnnot be directly mesured by the thin filment pyrometer. Insted, the surfce temperture of the fibre is determined first, followed by n energy blnce nlysis to evlute the temperture difference between the fibre nd its surrounding gs. It involves solving the energy conservtion equtions on frgment of the fibre. In this reserch, two-colour method ws used to determine the fibre temperture; it will be discussed in the next section. THE METHOD OF TWO-COLOUR PYROMETER Two-colour method hs been widely used over the lst decde, becuse of its bility to mesure the surfce temperture of grey body without knowing the emissivity. This principle cn be found elsewhere [9-11]. In this study, the fibre temperture cn be clculted by deriving the rtio of the colour-bnded imges cptured by the high-speed colour digitl cmer, insted of utilising the nrrow bndpss filtering nd bem-splitting pproch in order to void the complex configurtion nd opticl ttenution. Ech imge cptured by the RGB cmer cn be seprted into three different chnnels corresponding to the red, green nd blue sub-imges in the visible wvelength bnd. For this reserch, we hve used the R nd G chnnel in the two colour technique for determining the fibre temperture becuse the rdition detected in the B chnnel ws much weker thn tht detected in the R nd G chnnels (Eq. 1). Where S rtio ws determined through the clibrtion procedure using pre-clibrted tungsten lmp rnging from 1225 to 2497 K; the RG Rtio responses of the imging system in this rnge cn be found in Fig (1) T e m p e r t u r e ( K ) Figure 1 Experimentl setup for flme temperture mesurement; there re two fibres t ech end of the tube nd one in the middle. Figure 2 Temperture clibrtion between the RG rtio nd the true temperture Accurcy of the two-colour pproch Prior to mesuring the flme temperture in the qurtz tube, the fibre tempertures mesured by the two-colour method nd the Infr-pyrometer hve been compred. This comprison test used stble Methne/Air premixed flme on Bunsen burner. Four fibres were plced in the flme t different heights nd hung tutly on U-shpe frme. An IMPAC Infr-pyrometer ws pplied here to compre with the two-colour method; this pyrometer cn offer single-spot temperture mesurement with declred ccurcy of 1%, nd only ble to mesure grey-body

3 T e m p e r t u r e ( K ) temperture with known emissivity. The spot size is function of the distnce between the mesured object nd the Infrpyrometer. Fig3 shows the mesured re of ech fibre mrked by blck circles. The time-dependent temperture vrition of three single points hs been mesured by both the Infr- nd two-colour pyrometer in.1 secs with.1 secs intervl. The comprison of mesured tempertures is presented in Fig 4. tempertures by the two-colour nd the Infr-pyrometer on ech SiC fibre.,b nd c present the compred mesured tempertures t position P1, P2 nd P3, respectively. The two-colour method mesured fibre tempertures re slightly higher thn tht mesured by the Infr-pyrometer. The mesured temperture differences between these two pproches re 6%, 2.7% nd 4.7% t P1, P2 nd P3 respectively. Figure 3 The position of mesured points on ech glowing SiC fibre; ech spot is equivlent to 4mm in length of fibre corresponding to 3pixels in the imge I n f r - P y r o m e t e r T w o - C o l o u r THERMAL BEHAVIOUR OF THE FILAMENT The ctul locl gs temperture nd the fibre temperture re different nd re relted by the stedy-stte energy blnce eqution (Eq. 2). It is typicl het trnsfer problem s the fibre is heted by its surrounding hot gs nd cooled by rdition loss. For the cse of the thin filment, the conduction loss or gin in the xil direction is negligible becuse the rtio of the circumferentil to cross-sectionl re is lrge; thus, the het trnsfer rte by rdition nd convection is much higher thn the xil conduction het trnsfer. The gs temperture clcultion cn be obtined by εσ T T g + f (2) It is obvious tht the first term on the right-hnd side represents the difference between the gs nd the fibre tempertures. In order to determine this difference, the het trnsfer coefficient must be determined. This coefficient cn be determined using the equtions below: 1 4 Re ρvd μ (3) T i m e ( s e c s ) b c Nu Re.52 Pr.3 (4) h Nu k D Where the het trnsfer coefficient comes from Nusselt number correction for forced convection over cylinder, this number is vlid for 1-1 <Re <1 5 [12].The fluid properties must be determined by the verge of gs nd mbient tempertures Becuse the gs temperture is unknown, n itertive method hs to be pplied for the temperture correction. The lgorithm utilised to estimte the het loss is reviewed below. The fibre temperture determined by Eq.(1), is ssumed to be the gs temperture. The mole frction of mjor species concentrtion clculted vi n dibtic equilibrium chemicl rection t corresponding equivlence rtio in ech cse. Then the mole frction bsed trnsport properties cn be found nd used to clculte the Reynolds number nd the Nusselt number by the reltionship in Eq.(3) nd Eq.(4), respectively. Since the quntity required is the verge het trnsfer coefficient, the verge flme velocity pssing through the fibre ws used to clculte the Reynolds number. Once the Reynolds Number is found, the verge het trnsfer rte cn be computed nd used to clculted the corrected T g in Eq.(2). The entire process is repeted twice to obtin more ccurte corrected temperture. (5) Figure 4 Comprison of mesured time-dependent

High-speed imging visulistion hs shown tht the flme developed with reltively wek oscilltion t the left side of the tube, then, t the middle the flme oscilltion ws stronger; finlly, the oscilltion

4 T e m p e r t u r e ( K ) I n t e n s i y (. u ) RESULTS AND DISCUSSIONS In order to hve better understnding of the flme temperture vrition to be presented lter on, the behviour of the flme propgtion hs to be clrified first. High-speed imging visulistion hs shown tht the flme developed with reltively wek oscilltion t the left side of the tube, then, t the middle the flme oscilltion ws stronger; finlly, the oscilltion experienced reduction towrds the end of the tube. In Fig.5, the imge shows the emission vrition t the middle of the tube, cptured by mono-colour cmer t 2 frmes per second (fps). It cn be seen tht the middle fibre emitted rdition t its centre first, then it grdully incresed towrds ech end of the fibre. This implies tht higher temperture is observed t the mid-section of the glowing fibre compred to those nerer the tube wlls. different equivlence rtio settings. It ws not possible to imge ll fibres simultneously becuse tht would hve cused significnt drop in sptil resolution. Fig.7 shows the loclly derived gs temperture vrition t the different fibre positions. Fig. 7b illustrtes stronger oscilltion in gs temperture thn tht shown in Figs. 7,c. Fig. 7 shows the gs temperture vrition t the loction ner the ignition point, with two fibres plced 3mm prt. It depicts similr vrition of the gs temperture, which indictes smll therml grdient between these two fibres. At the end of the tube Fig. 7c shows moderte gs temperture vrition. For ll cses, obvious fibre deflection hs not been observed; the flme temperture vritions re minly due to flme instbilities which re stronger t the middle of the tube. 3 F r o n t F i b r e 5 R e r F i b r e M i d d l e F i b r e Figure 5 The glowing fibre s the flme crossed through tube middle section nd time-dependent pixel brightness intensity response Fig. 6 illustrtes the behviour of the strong flme oscilltion in the middle of the tube. It cn be observed tht the flme boundry lyer chnged from being irregulr to semicirculr shpe for complete cycle of oscilltion; these oscilltions continued nd grdully dimmed towrds the tube end; this finding my ccount for the pttern of the timedependent temperture vrition shown in Fig b F r o n t F i b r e R e r F i b r e c Figure 6 The behviour of flme oscilltion inside the tube With reference to the gs temperture mesurement, the imging system of TFP pyrometer seprtely cptured ech glowing fibre plced t the middle nd ech end of the tube. At ech loction, the gs temperture ws mesured t three Figure 7 Time-dependent gs temperture vrition t equivlence rtio = 1.1. () Temperture vrition t the left end of the tube ner ignition plce, (b) Temperture vrition t the middle of the tube, (c) Temperture vrition t the right end of the tube. In ddition, the time-dependent pressure oscilltion of the flme trvelling through the tube from the left to the right end of the tube ws lso recorded. As cn be seen in Fig.8, the mplitude of oscilltion ws reltively moderte before.1s, fterwrds, the pek mplitude occurred round.2s which

5 I n t e n s i t y (. u ) A m p l i t u d e F l m e v e l o c i t y ( m / s ) T e m p e r t u r e ( K ) A c c o u s t i c p r e s s u r e ( B r ) correspond to the position of the middle section of the tube, nd finlly, the mplitude ws grdully tiled off by the tube right end. This behviour ws similr to the time-vried vritions of gs tempertures detected by the two-colour pyrometer Left-end, Middle nd Right end of the tube, d,e nd d present the min frequency of intensity oscilltion t the corresponding positions of the tube, using FFT. The fibre nd the corrected gs tempertures vrying with equivlence rtio re shown in Fig. 1, where ech point verges 162 smples. The common trend shows tht the tempertures incresed from ø=.9, peked t ø=1.1, nd then decreses until ø=1.2, which is similr to the flme velocity vritions shown in Fig. 11 Both the highest flme velocity nd gs temperture were found t ø=1.1. The difference between ech gs-fibre temperture combintion ws evluted s shown in Fig. 12. This difference is lrger for flmes t the middle section of the tube L e f t e n d G s Figure 8 The pressure redings inside the tube t equivlence rtio = 1.1. Furthermore, by using the illumintion from ech fibre the vrition of the flme oscilltion could lso be observed. The cptured RGB imges of ech glowing fibre were converted to grey imges, nd then only brightness intensity of pixels within the glowing fibre ws summed up s the totl intensity. Fig. 9,b,c shows the cycle of heting up nd cooling down s the flme pssed ech fibre t similr cycling time. The cooling rte incresed from left to right of the tube. Fig. 9 d,e,f shows the plots of the flme mplitude ginst the frequency, with the highest flme frequency occurring t the middle section of the tube E q u i v l e n c e r t i o L e f t e n d F i b r e M i d d l e G s M i d d l e F i b r e R i g h t e n d G s R i g h t e n d F i b r e Figure 1 The verge temperture of flme trvelling through the fibre t different equivlence rtios L e f t e n d 2 4 d H Z L e f t e n d M id d le 5 1 R i g h t e n d M i d d le b 3 e H Z E q u i v l e n c e r t i o R i g h t e n d 2 c 4 f Figure 11 Flme velocity vrying with equivlence rtio H Z F r e q u e n c y ( H Z ) Figure 9,b nd c Experimentl het-up nd cool-down cycle t

6 T g s / T f i b e r E q u i v l e n c e r t i o R i g h t e n d L e f t e n d M id d le Figure 12 The rtio of the gs nd fibre temperture vrying with equivlence rtio Finlly, the temperture correction between the fibre nd its surrounding temperture is derived from the energy blnce. This blnce involves the convective het trnsfer from the gs to the fibre, het loss vi rdition from the fibre to its mbient environment. The conductive het trnsfer long the fibre is discounted becuse it hs limited effect in the het trnsfer process. Since the gs temperture is not directly mesured, it is importnt to consider the potentil errors nd their effect on the ccurcy of the gs temperture mesurement. The min uncertinties in the mesurement of the gs temperture re in the clcultion of the fibre temperture, the emissivity of the fibre, nd the convective het trnsfer coefficient. Moreover, there re severl sources of uncertinty in the determintion of Nu number; firstly, the correltion my not fully fit to the flow conditions used in the experiment. Secondly, knowing the Reynolds number (Re) is required to clculte Nu number, which implies the knowing of the gs velocity perpendiculr to the fibre. In this study, only the verged velocity pssing the fibre ws estimted by the imging-bsed techniques. Finlly, the therml properties of the gs re functions of temperture nd gs composition, which re both estimted in the study. Regrding the other vribles, the tolernce of the fibre dimeter nd its emissivity depends on oxidtion resistnce nd the roughness of the fibre which probbly vry in the hightemperture environment. The obvious uncertinty exists in the determintion of the fibre temperture. Fig. 2 shows the mesured temperture difference between using the two-colour nd the Infr pyrometers. This error could rise from the prtil mislignment of the mesured spot re using the two techniques, nd the fibre emissivity my not be consistent long the length of the fibre. CONCLUSION In the present work, we compred single point fibre mesurement using the TFP- nd the commercil Infrpyrometer. Mesured tempertures by the two pyrometers were found to be in good greement with cceptble difference in temperture. The clibrted TFP ws then pplied to mesure the temperture of flme propgting through 2mm dimeter qurtz tube by instlling SiC fibres t different positions of the tube. Both the time-dependent gs temperture nd the pressure oscilltions reched their peks t the middle section of the tube. The gs tempertures found t the midsection of the glowing fibre were higher thn tht nerer the tube wlls. Finlly, the mjor sources of uncertinty in gs temperture hve been discussed. Bsed on our results, it hs been shown tht the TFP pproch hs the potentil to trck the temperture vrition of fst moving hot gses. Hence, it could be promising tool for fst temperture mesurement. REFERENCES [1] D. Alfno, Spectroscopic properties of crbon fibre reinforced silicon crbide composites for erospce pplictions 211. [2] V. V.. L.P.Goss. (1988). SiC-sed thin-filment pyrometry:theory nd the therml properties. [3] T. f. i. p. i. t. profile nd m. i.. w. t. h. p. flme, "<Sic Bedt.pdf>." [4] W. M.pitts, "Temperture Uncertins for Bre_bed nd Aspirted Thermocouple Mesuremnets in fire Environment," THe Foundtion of Fire Stndrds. [5] W. M.PITTS, "Thin-filment pyrometry in flickering lminr diffusion flmes," presented t the Twenty-Sixth Symposium on Combustion, [6] P. B. S. Jinesh D.Mun, nd Dvid L.Urbn, "<Thin-filment pyrometry with digitl still cmer.pdf>," APPLIED OPTICS, vol. 4, pp , 27. [7] P. B. Kuhn, B. M, B. C. Connelly, M. D. Smooke, nd M. B. Long, "Soot nd thin-filment pyrometry using color digitl cmer," Proceedings of the Combustion Institute, vol. 33, pp , 211. [8] B. M, G. Wng, G. Mgnotti, R. S. Brlow, nd M. B. Long, "Intensity-rtio nd color-rtio thin-filment pyrometry: Uncertinties nd ccurcy," Combustion nd Flme, vol. 161, pp , 214. [9] D. Sun, G. Lu, H. Zhou, nd Y. Yn, "Flme stbility monitoring nd chrcteriztion through digitl imging nd spectrl nlysis," Mesurement Science nd Technology, vol. 22, p. 1147, 211. [1] Z. M nd Y. Zhng, "High temperture mesurement using very high shutter speed to void imge sturtion," pp , 214. [11] Y. H.. Y.Yn, "Trnsient two-dimensionl temperture meurement of open flmes by dul_spectrl imge nlysis," SAGE, vol. 22, pp , 2. [12] J. P. Holmn, Het Trnsfer. New York: McGrw, 28.

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