RADIATION THERMOMETRY OF METAL IN HIGH TEMPERATURE FURNACE

Transcription

1 XVII IMEKO World Congre Metrology in the 3rd Millennium June 22 27, 2003, Dubrovnik, Croatia RADIATION THERMOMETRY OF METAL IN HIGH TEMPERATURE FURNACE Tohru Iuchi, Tohru Furukawa and Nobuharu Sato Deartment of Mechanical Engineering, Toyo Univerity, Kawagoe, Jaan Abtract Thi tudy aim to develo a ractical radiation thermometry ytem of metal moving through in a high temerature furnace. In order to achieve thi tudy, two roblem; emiivity comenation of a target and elimination of background radiation noie filled in a furnace mut be orted out. We have uccefully develoed a method for imultaneou meaurement of emiivity and temerature by ue of olarized directional roertie of the radiance from the target, in thi cae, tainle teel, and moreover a technique to eliminate background radiation noie uing a eudo blackbody intalled in a furnace. Keyword : emiivity, olarization, furnace 1. INTRODUCTION Temerature meaurement and it control are imortant for roduction line to enable the imrovement of the quality of roduct a well a the economy of energy conumtion. Thi tudy aim to develo a ractical radiation thermometry ytem of metal moving through in a high temerature furnace uch a a continuou annealing furnace. In order to achieve thi tudy, two roblem mut be orted out; that i, emiivity comenation of a target and elimination of background radiation noie filled in a furnace. Radiation thermometry i a non-contact temerature meaurement method that i uitable for moving target. When the emiivity of the target varie, however, it caue eriou temerature error in radiation thermometry. In addition, in a high temerature furnace, the temerature of the furnace wall i nearly equal to or higher than the temerature of the target, in general. That mean that the furnace i filled with enormou amount of background radiation noie. Therefore, a radiometer i unable to detect true radiance of the target without excluion of background radiation noie. Thi aer decribe a method for imultaneou meaurement of emiivity and temerature by ue of olarized directional roertie of radiance of the target, in thi cae, tainle teel, and moreover a technique to eliminate background radiation noie by ue of a eudo blackbody intalled in a furnace, and ubequent exerimental reult of the method and the technique. Stainle teel a a ecimen i oxidized and the oxide film i grown on it urface when it i heated. Large variation of -olarized and -olarized emiivitie are caued by the multile reflection of radiation between the ecimen and film urface [1]. Under ome ectral and directional condition, it i found that there i one-to-one correondence between an emiivity and a ratio of - and -olarized radiance of tainle teel during heating. Thi relation ha uccefully led to the formation of emiivity-comenated radiation thermometry for tainle teel, even though large emiivity change occur. Introduction of a eudo blackbody made of a heatreitant material like alumina into a high temerature furnace enable to remove background radiation noie filled in the furnace a well a it ulie contant reference radiance to a radiometer. Thu, the rincile for emiivity comenated radiation thermometry i maintained even inide the furnace a well. According to exerimental reult achieved at the laboratory, the meaurement of emiivity and temerature were oible with about 9,0 % and 1,3 % error, reectively at temerature region over 1000 K. 2. MEASUREMENT PRINCIPLE AND EXPERIMENT 2.1. Emiivity comenation method Let T be the temerature of the ecimen. Let E (θ) be the -olarized radiance ignal that i emitted by the ecimen and detected by a radiometer with a -olarizer from a direction θ to the normal of the ecimen urface. Then the following equation hold. E ( θ ) = k L λ ( T), (1), b where ε (θ) i -olarized emiivity of the ecimen at a wavelength λ and a direction θ; k i a contant factor that convert the -olarized radiance into the electrical outut ignal E (θ); L λ,b (T) i a ectral blackbody radiance at temerature T and wavelength λ. Similarly, the -olarized radiance ignal E (θ) i obtained a E ( θ ) = k L λ ( T). (2), b The ratio of (1) and (2) i hown a E( θ) k ε ( θ) = ( )( ) = k R E ( θ) k. (3)

2 A k i contant, the ratio R of -olarized and - olarized emiivitie i derived from the meaurement of E (θ) and E (θ). If there i one-to-one correondence between a ectral emiivity ε λ (θ) and the ratio R for tainle teel during heating [2], the emiivity ε λ (θ) of the ecimen can be obtained by meauring the ratio R. Fig.1 how an exerimental arrangement to find a characteritic curve between ε λ (θ) and R. In the exeriment, everal radiometer that are enitive at 0,9 µm (Si enor), at 1,55 µm (InGaA enor) and at wide range from 3 to 4 µm (PbSe enor) are ued. meaurement were carried out with increaing temerature of the ecimen. It i oberved from Fig.2 that ocillating change of both - and -olarized emiivitie occur at a high temerature more than 1200 K and the change tart at a hort wavelength (λ=0,9 µm) at firt, then at λ=1,55 µm, and finally at λ=3~4 µm. Uing the exerimental reult of Fig.2, the characteritic curve between ε λ (θ) and R were obtained a hown in Fig.3, where (a) R veru ε λ, and (b) R veru ε λ,. (a) Relation for -olarized emiivity Fig.1 Exerimental arrangement to find a characteritic curve between ε λ (θ) and R. (b) Relation for -olarized emiivity Fig.3 Characteritic curve between R and ε λ (θ) derived from the exerimental reult of Fig.2. (ecimen: tainle teel (SUS430)). Fig.2 Exerimental reult of ectral olarized emiivitie of a ecimen with increaing temerature in air. (ecimen: tainle teel (SUS430)). Fig.2 how meaurement reult of ectral olarized emiivitie at 0,9, 1,55 and 3~4 µm and at a direction of θ=70 uing Si, InGaA and PbSe enor, reectively, where (a) -olarized and (b) -olarized emiivitie. The From everal characteritic curve between R and ε λ (θ) hown in Fig.3, the mot reliable and reroducible relation i the characteritic curve between R at λ=3~4 µm and - olarized emiivity ε λ, (θ) at λ=1,55 µm that meet the requirement mot for one-to-one correondence between the ratio R and the emiivity. The econd referable relation i the one between R at λ=3~4 µm and -olarized emiivity ε λ, (θ) at λ=0,9 µm. According to exerimental reult achieved in air, the meaurement of emiivity and temerature were oible with about 9,0 % and 1,3 % error, reectively at temerature region over 1000 K Meaurement method in high temerature furnace Fig.4 how a chematic of a ytem for emiivity comenated radiation thermometry in a high temerature furnace baed on the rincile decribed above. In order to

3 exclude background radiation noie filled in the furnace and uly the reference radiation i introduced to the furnace. The eudo-blackbody that i intalled inide the furnace and a radiometer whick i et outide the furnace are ymmetrically dioed each other at an angle θ to the normal n of a ecimen urface. From the thermal equilibrium condition, the following relation can be guided, 1 2 1, + = (8) = (9) A diffuely reflecting coefficient 2 and 2 are negligibly mall comared with ecularly reflecting coefficient 1 and 1 for bright tainle teel uch a SUS430, the third term of the right ide of (4) and (6) can be neglected [3]. Then equation (4) and (6) are recontructed a follow, E( θ) kl 1 λ, b( T2 ) = kε( θ){ Lλ, b( T1) L 1 λ, b( T2)} (10) E( θ ) kl ( T) = k{ L ( T) L ( T)} (11) 1 λ, b 2 λ, b 1 1 λ, b 2 Taking the ratio of (10) and (11), Fig.4 Schematic of a ytem for emiivity comenated radiation thermometry in a high temerature furnace Let T 1, T 2 and T 3 be temerature of the ecimen, the blackbody radiator and the wall of the furnace, reectively. The radiance ignal E (θ) detected by the radiometer equied with a -olarizer i decribed a follow, E ( θ) = k { L ( T ) + (1 ) L ( T ) λ, b 1 1 λ, b 2 + (1 ε ( θ)) L ( T )} 2 λ, b 3 = E ( T ) + (1 ) E ( T ), λ, b 1 1, λ, b 2 + (1 ) E ( T ), 2, λ, b 3 E ( T ) = k L ( T ), i = 1,2,3., (5), λ, b i λ, b i where the econd term of the right ide of (4) i the radiance ignal that leave the blackbody at temerature T 2 and i reflected on the ecimen and detected by the radiometer; The third term i the radiance ignal that leave the wall of the furnace at temerature T 3 and i reflected on the ecimen and detected by the radiometer; 1 and 2 (0< 1, 2 1) are coefficient that rereent degree of ecularly and diffuely reflecting characteritic of the ecimen for - olarization, reectively. Similarly, the radiance ignal E (θ) detected by the radiometer equied with a -olarizer i decribed a E ( θ ) = k { L ( T ) + (1 ) L ( T ) λ, b 1 1 λ, b 2 + (1 ε ( θ)) L ( T )} 2 λ, b 3 = E ( T ) + (1 ) E ( T ), λ, b 1 1, λ, b 2 + (1 ) E ( T ), 2, λ, b 3 E ( T ) = k L ( T ), i = 1,2,3., (7), λ, b i λ, b i where 1 and 2 (0< 1, 2 1) are coefficient rereenting degree of ecularly and diffuely reflecting characteritic of the ecimen for -olarization, reectively. (4) (6) E ( θ ) k L ( T ) N 1 λ, b 2 E ( θ ) k L ( T ) 1 λ, b 2 k ε Lλ, b( T1) 1 Lλ, b( T2) =. k ε L ( T ) L ( T ) λ, b 1 1 λ, b 2 (12) When urface irregularity of a ecimen i random, jut a the cae of tainle teel (SUS430), coefficient 1 i equal to 1. Thu, equation (12) i exreed a E( θ ) 1 E, λ, b( T2) k ε = = k R E ( θ) E ( T ) k ε 1, λ, b 2, = 2(1 ε ( θ )), (15). (13) A each term of the left ide of (13) can be meaured and comuted if coefficient 1 i known, the ratio R of the right ide of (13) can be obtained. Thi mean that the rincile of emiivity comenation a hown in (3) i available even in a high temerature furnace. If once the emiivity ε (θ) i derived from a relation between ε (θ) and R by ue of (13), the following equation are derived from dividing (4) by ε (θ). E( θ) (1 ) = k L T + L T ) r 1 { λ, b( 1 ) λ, b( 2 2 (1 ε ( θ)) + L λ, b 3 1, λ, b 1, λ, b 2 ε ( θ), λ, b 3 ( T )} (1 ) = E ( T ) + E ( T ) N r, + E ( T ), (14) where N,r i a noie factor. The temerature T 1 of the ecimen i calculated from the right ide term of following equation (16) by ue of comenated -olarized emiivity ε (θ).

4 E, λ, b 1 E ( θ ) ( T ) = 1 (1 ε ( θ)) - E, λ, b( T2). (16) which i heated at 1200 K and oxidized. E i meaured at T 3 =1273 K. The temerature T 1 derived from (16) include the error T caued by N,r E,λ,b (T 3 )/ε (θ), the third comonent of (14). In order to reduce the meaurement error T, the olid angle dω, aerture of the eudo blackbody ubtended by a meauring oint of the ecimen mut be deigned to be wide a much a oible. Similarly, the calculation of temerature T 1 can be derived a well by ue of comenated -olarized emiivity ε (θ) a hown in (17). E, λ, b 1 E ( θ ) ( T ) = 1 (1 ε ( θ)) - E, λ, b( T2). (17) The calculation of T 1 by (17) include the error T caued by background radiation noie N,r E,λ,b (T 3 )/ε (θ). N = (1 ε ( θ )). (18) r, Meaurement of olid angle dω and noie factor N r In order to etimate background radiation noie, an exerimental aaratu hown in Fig.5 ha been deigned, which can invetigate a relation between olid angle dω and noie factor N r. In Fig.5 (a), ide wall and a ceiling wall excet a mall aerture made in one of ide wall for oberving radiance by ue of radiometer are attached with black ainted late having effective emiivity of 0,95. The temerature of thee wall i maintained at T 3. A ubtrate of tainle teel (SUS430) whoe urface i coated with BaSO 4 article i et at a floor of the furnace, which i utilized a a comletely diffuely reflecting urface having effective reflectivity of 0,98 at viible and near infrared range. The rear ide of the ubtrate i water cooled from the bottom ide to maintain the urface at room temerature. In Fig.5 (b), mall aerture are rovided at both ide wall, one of them i aumed to be a eudo-blackbody at room temerature, and contitute olid angle dω. The other i ued a an aerture from which background radiation noie i detected by a radiometer. Secimen of a-grown (commercial bae roduct) and oxidized tainle teel are et at a floor, ucceively. P-olarized radiance detected by the radiometer in (a) rereent radiance E,λ,b (T 3 ) of the furnace wall and - olarized radiance derived from (b) how the third comonent E =N,r E.λ.b (T 3 ) of the right ide of (4). Thu, a noie factor N,r i obtained by calculating E /E,λ,b (T 3 ). Similarly, noie exreion for -olarized comonent are given a E =N,r E,λ,b (T 3 ) and N,r, reectively. Fig.6 how an exerimental relation between olid angle dω and noie factor N r for tainle teel (SUS430) Fig.5 Exerimental aaratu for meaurement of olid angle dω and noie factor N r. Fig.6 Exerimental relation between olid angle dω and noie factor N r for tainle teel (SUS430) which i heated at 1200 K and oxidized.

5 Now, the rocedure to etimate temerature error T=T a - T 1 caued by background radiation noie N r E b (T 3 )/ε(θ) i calculated with the following equation for -olarized and -olarized radiance, reectively hown in (19) and (20), N E T = E T + E T ) (19) r,, λ, b( a), λ, b( 1 ), λ, b( 3 ε ( θ) N E T = E T + E T ) (20) r,, λ, b( a), λ, b( 1 ), λ, b( 3 ε ( θ) Fig.7 how a relation between temerature error T and olid angle dω for -olarized emiivity comenation. T i calculated by ue of Fig.6 and (19), where both T 1 and T 3 are 1273 K. The ecimen i SUS430 tainle teel which i oxidized by heating 1200 K. According to Fig.7, when olid angle i et to dω=0,01π r, temerature error T caued by background radiation noie N r E b (T 3 )/ε (θ) i maintained to about 1 K under the condition T 1 =T 3 =1273 K regardle of emiivity value of the ecimen for λ=0,9 and 1,55 µm. In cae of λ=3,9 µm, however, T dilay more than 4 K due to low ectral emiivity and long wavelength effect. temerature T 1 by 100 K. For examle, under the condition of dω=0.01π r when -olarized emiivity comenation i ued, T=3 K for λ=0,9 µm, T=4 K for λ=1,55 µm and 25 K for λ=3,9 µm a hown in Fig.8; by ue of -olarized emiivity comenation, T=4 K for λ=0,9 µm, T=9 K for λ=1,55 µm and 62 K for λ=3,9 µm a hown in Fig.9; and by ue of unolarized emiivity comenation, T=4 K for λ=0,9 µm, T=6 K for λ=1,55 µm and 35 K for λ=3,9 µm a hown in Fig.10, reectively. Judging from quantitative analyi decribed above, the bet choice i to ue the relation between -olarized emiivity at 0,9 µm and R at 3.9 µm. Solid angle of dω=0.02π r (that correond to 8º of field view angle) i quite good deign to avoid background radiation noie. Fig.8 Relation between T and dω for -olarized emiivity comenation, where both T 1 i 1273 K and T 3 i 1373 K. The ecimen i tainle teel (SUS430) which i oxidized by heating 1200 K. Fig.7 Relation between T and dω for oxidized tainle teel (SUS430), which i calculated by ue of Fig.6 and (19). Both T 1 and T 3 are 1273 K. Fig.8 how a relation between T and dω for - olarized emiivity comenation, where both T 1 i 1273 K and T 3 i 1373 K, that i, the wall temerature T 3 i higher than the ecimen temerature T 1 by 100 K. The ecimen i tainle teel (SUS430) which i oxidized by heating 1200 K. Similarly, Fig.9 and Fig.10 how relation between T and dω for -olarized emiivity and for unolarized emiivity comenation, reectively under the ame condition with Fig.8. From Fig.8~10, we can ee that T become large when the wall temerature T 3 i higher than the ecimen Fig.9 Relation between T and dω for -olarized emiivity comenation. The condition i the ame with Fig.8.

6 4. CONCLUSION Fig.10 Relation between T and dω for unolarized emiivity comenation, where both T 1 i 1273 K and T 3 i 1373 K. The ecimen i tainle teel (SUS430) which i oxidized by heating 1200 K. 3. CONSIDERATIONS FOR PRACTICAL USE Fig.11 how chematic of a ractical radiation thermometry ytem intended to be intalled in a high temerature furnace for annealing of tainle teel. Following requirement have been mentioned by feaibility tudy, which i baed on exerimental reult, (1) a eudo blackbody intalled in the furnace and radiometer 1 hould be ymmetrically et at an angle more than θ=70, (2) radiometer 1 hould oe combined enor that can detect -olarized and -olarized radiance at λ=3.9 µm to obtain R, and radiance at 0,9 µm or 1,55 µm to comenate ε λ (θ) a well, (3) radiometer 2 equied with a Si enor enitive at 0,9 µm detect the temerature T 2 of a eudo-blackbody which i naturally heated by the radiation in the furnace. A reliable and reroducible relation between R and emiivity ε λ (θ) ha been obtained for emiivitycomenated radiation thermometry of tainle teel (SUS430) in laboratory exeriment. The meaurement of emiivity and temerature are oible with about 9,0 % and 1,3 % error, reectively at temerature region over 1000 K. In order to aly thi method to ecimen in a high temerature furnace, a quantitative relation between olid angle dω that rereent ize of a blackbody aerture, and temerature error T caued by background radiation noie from urrounding wall of the furnace ha been found. Under the condition of T 1 =1273 K, T 3 =1373 K (T 3 - T 1 =100 K) and dω=0,02π r, temerature error T i ureed within 4 K for both λ=0,9 µm and λ=1.55 µm. Radiation thermometry ytem of tainle teel in a high temerature furnace ha been deigned, which i baed on the above relation. The eudo-blackbody which i naturally heated inide the furnace oee uch feature that it can remove background radiation noie filled in the furnace without cooling a ecimen and alo rovide the reference radiation that i ueful for ignal roceing to obtain emiivity of the ecimen. REFERENCES [1] T. Iuchi, Modeling of emiivitie of metal and their behavior during the growth of the oxide film, TEMPERATURE, 2002, in rint. [2] T. Iuchi and T. Furukawa, Emiivity-comenated radiation thermometry, Proc. of IMEKO 2000, vol.iv, , Setember [3] T. Iuchi and R. Kuaka, Two method for imultaneou meaurement of temerature and emittance uing multile reflection and ecular reflection, and their alication to indutrial rocee, TEMPERATURE, AIP, , Author: Tohru Iuchi, Profeor, Det. of Mechanical Engineering, Toyo Univerity, 2100 Kujirai, Kawagoe, Saitama , Jaan, hone , fax , iuchi@eng.toyo.ac.j. Tohru Furukawa, YRE03770@nifty.ne.j. Nobuharu Sato, nobu.atou@aoc.entax.co.j. Fig.11 Schematic of a ractical radiation thermometry ytem for tainle teel in a high temerature furnace.