THERMAL ANALYSIS IN FIRE-RESISTANCE FURNACE

Transcription

1 THERMAL ANALYSIS IN FIRE-RESISTANCE FURNACE Ploto, P.A.G. 1, Mesquta, L.M.R. 1 ; Alexandre Perera 2! "#$%"& ' ()! "#$%"& ABSTRACT Fre resstance ratng of buldng constructon elements s defned under fre-resstance test furnace. The geometry and shape of fre-resstance furnaces s not defned by any prescrptve document, beng necessary to comply thermally wth specfed nomnal fre curves, such as ISO 834 or hydrocarbon [1,2]. Ths research work ntends to measure temperatures nsde furnace volume, usng sxteen plate thermocouples to compare average temperature n four planes. Those planes are compared wth reference thermocouple whch s responsble for controllng furnace operaton, see fgure 1. Three tests were performed, the frst two runnng wth ISO 834, durng 45 mnutes and the last one runnng wth hydrocarbon curve, durng 3 mnutes. Expermental results demonstrate that relatve temperature dfferences are smaller than 3 % n the ntal test stage, beng smaller than 5 %, after 5 [s] untl the end of the tests. The numercal smulatons were performed usng Fluent CFD, usng the structured fnte volume mesh method. The Eddy Dsspaton Model (EDM) was used for chemcal speces transport and reactng flow. The governng equatons for mass, momentum and energy were solved for the three dmensonal unsteady ncompressble flow, wth radatve heat transfer and turbulence model. The numercal results agree well wth expermental results, beng the relatve temperature dfference smaller than 5% for each nomnal test. Numercal smulaton also reveals the localzed effect of each burner. 1 INTRODUCTION The thermal performance of fre-resstance furnace s nvestgated. Furnace envronment s normally consdered homogeneous and wth unform temperature dstrbuton, followng specfed nomnal temperature evoluton. Tests were conducted at the Polytechnc Insttute of Bragança laboratory (LERM) usng the 1 cubc meter fre-resstance furnace. Ths small furnace s sutable for ntal valdaton of expermental temperature measurements. The furnace has 4 propane burners wth 9 [kw] maxmum power each, reference Kromschröder BIO 65 HM-1/35-72/8. Ths furnace comples wth European Standard EN , consderng the maxmum devaton between reference temperature and the theoretcal specfed nomnal curve for temperature. Four planes were defned to evaluate the furnace temperature performance. Four plate thermocouples defne each plane. Plane X15 and X85 are parallel to the exhaust zone whle plane Z15 and Z85 are parallel to the transversal plane of burners.

2 a) Furnace model wth four burners B (=1,4). b) Geometrc poston for plate thermocouples nsde furnace. Fg.1. Model for fre-resstance furnace. c) Furnace wth opened door after runnng test 3. Three tests were performed. The frst two tests used ISO834 nomnal fre curve, defned by equaton (1), whle the last one used hydrocarbon nomnal fre curve (2). In these equatons, θ g represents reference furnace temperature for tme t, durng testng condtons. θ ( t ) g = log θ g (º C); t(mn),167 t 2,5t ( ) θg = , 325 e, 675 e θ g (º C); t(mn) (1) (2) The thermal performance s determned by the comparson between average plane temperatures wth reference furnace temperature, whch represents each nomnal fre curve. Numercal smulatons are also performed, usng computatonal flud dynamcs, to evaluate thermal performance and valdate expermental tests. 2 EXPERIMENTAL TESTS The fre-resstance furnace was nstrumented wth 16 plate thermocouples, suspended at nsulated slenderness steel frame, see fgure 2. Plate thermocouples fulfl standards and were postoned accordng to the vertex poston of 7 [mm 2 ] cube, centered nsde furnace. a) Relatve poston for plate thermocouples. b) Plate thermocouple (type K and stanless steel plate A34). Fg.2. Plate thermocouple nstrumentaton.

3 Thermocouple control s acheved by matchng the measured thermocouple temperature wth the prescrbed nomnal fre curve. However, snce thermocouples rradate heat, they adjust themselves to the temperature at whch there s a balance between the convecton and net radatve heat transfer, [3]. Plate thermocouple s a stanless steel plate wth 1 [mm 2 ] and.7 ±.1 [mm] thck. The emssvty was consdered greater than.7. The wre thermocouple s postoned on the back face, screwed wth a small plate and nsulated wth ceramc fbre, [1]. Measurements were performed wth mult-channel data acquston system, MGCplus from HBM, wth frequency equal to.1 [Hz]. Results were averaged from 4 plate thermocouples for each plane. Plane X85 was defned by the average temperature readngs on TF1, TF2, BF1, BF2, Plane X15 was defned by the average temperature readngs on Tb1, Tb2, Bb1, Bb2, Plane Z85 was defned by the average temperature readngs on TFL, BFL, TbL, BbL, whle Plane Z15 was defned by the average temperature readngs on TbR, TFR, BbR and BFR. Fgure 3 represents each measured temperature wth plate thermocouple and the reference temperature for furnace. Temperature [ºC] TF1 TF2 BF1 BF2 TFL TFR BFL BFR Tb1 Tb2 Bb1 Bb2 TbL TbR BbL BbR FURNACE EXHAUST Relatve Temperature Dfference,35,3,25,2,15,1,5 Plane X=85 Plane X=15 Plane Z=85 Plane Z= a) Test 1 wth ISO 834 nomnal curve. b) Test 1 wth ISO 834 nomnal curve. Temperature [ºC] TF1 TF2 BF1 BF2 TFL TFR BFL BFR Tb1 Tb2 Bb1 Bb2 TbL TbR BbL BbR FURNACE Relatve Temperature Dfference,25,2,15,1,5 Plane X=85 Plane X=15 Plane Z=85 Plane Z= c) Test 2 wth ISO 834 nomnal curve. d) Test 2 wth ISO 834 nomnal curve.

4 Temperature [ºC] TF1 TF2 BF1 BF2 TFL TFR BFL BFR Tb1 Tb2 Bb1 Bb2 TbL TbR BbL Relatve Temperature Dfference Plane X=85 Plane X=15 Plane Z=85 Plane Z=15,35,3,25,2,15,1, e) Test 3 wth hydrocarbon nomnal curve. f) Test 3 wth hydrocarbon nomnal curve. Fg.2. Expermental results for temperature measurements and relatve temperature devatons for each plane. The relatve dfference between each average plane temperature and reference temperature from furnace s below 5%, after the ntal testng phase, correspondng to 5 [s]. Hgher relatve dfference s expected before, due to hgher temperature varaton wth tme. The exhaust temperature was also measured for test 1. There was a constant temperature dfference between the reference furnace temperature and the temperature measured at the exhausts. Ths temperature was measured wth an nsulated welded thermocouple wthout plate an nsulaton protecton. Ths explans the oscllatng regstry. 3 NUMERICAL SIMULATION The numercal smulatons were performed wth Fluent software, usng the fnte volume method, [4]. The generalzed Eddy Dsspaton Model was used to smulate the combuston of propane-ar mxture. Ths model s based on the assumpton that chemcal reacton s almost nstantaneous, when compared wth the chemcal speces flow transportaton. The combuston was modelled usng a global one-step reacton mechansm, assumng complete converson of the propane to the product speces of equaton 3. C H + 5 O 3 CO + 4 H O (3) The reacton wll be defned n terms of stochometrc coeffcents, formaton enthalpes and parameters that control the reacton rate. The pressured based solver was used wth unsteady and frst order mplct formulaton. The vscous model K-Epslon wth two equatons and a standard wall functon for near wall treatment was used. The surface to surface radaton model was used wth computed vew factors. The emssvty value for the nternal wall furnace was consdered equal to.7. The use of constant transport propertes for vscosty, thermal conductvty and mass dffusvty coeffcents s acceptable because the flow s fully turbulent, see tables 1 and 2, [4]. The molecular transport propertes wll play a mnor role compared to turbulent transport. The assumpton of temperature dependent specfc heat s an mportant key factor to predct more realstc peak flame temperature.

5 Table 1. Propertes for propane ar msture, [4]. Specfc heat Mxng law [J/kgK] Conductvty.454 [W/mK] Vscosty 1.72x1-5 [Kg/ms] Table 2. Ar propertes, at reference temperature, 298 [K], [4]. Specfc mass [kg/m 3 ] Specfc heat [J/kgK] Conductvty.242 [W/mK] Vscosty x1-5 [Kg/ms] Cp [K/kgK] 6 C3H8 O2 CO2 H2O N Temperatura [K] Fg.4. Varação do calor específco dos produtos e reagentes, [4]. The specfc heat consdered for mxture wll be hgher where the propane s concentrated, near the fuel nlet and where the temperature and combuston product concentraton should be hgher, see fgure 4 for temperature dependence. The governng equatons for mass momentum and energy conservaton are defned by equatons 4-6. t ( ) ( ) ρ Y + ρ v Y = J + R *+, Where ρ represents the specfc mass, v the velocty vector, Y s the local mass fracton that corresponds to the speces, speces formaton by chemcal reacton. J the dffuson flux, whle R represents the rate of each For the momentum conservaton equaton, the statc pressure s defned by p, whle τ represents the stress tensor. t ( ρ v ) + ( ρ v v) = p +. τ (5)

6 For the energy equaton, E represents the energy value, conductvty, T the temperature value, energy value from chemcal reacton. t ( E) + v ( ρe + p) h j the sensble enthalpy, whle k eff the effectve value for S h represents the ρ ( ) = keff T h J + ( τ eff v) + Sh (6) An adabatc condton was assumed n the nternal furnace walls. The nlet thermal condtons were specfed at room temperature. The exhaust temperature products were defned wth nformaton from expermental measured data. The tme dependence nlet velocty for ar and propane were kept n proper rato, durng each test. Those values were defned accordng to the manufacturer lmtng values, because they were not measured. To solve the unsteady soluton, the ntal condtons were defned and an ncremental tme step was specfed. An teratve process was used to solve dscretzed equatons. The numercal model was bult wth a structured mesh, usng hexahedra fnte volumes, each wth.2[m] length sde, see fgure 5. Major smplfcaton was ntroduced nto the four burners, usng the hydraulc dameter as reference value. Four nlet gas zones were defned concentrc wth the same number of ar nlet zones, wth dmenson equal to 2x2 [mm] and 6x6 [mm], respectvely. The exhaust s well dentfed at the bottom of the furnace volume, wth rectangular dmensons equal to 1 x 4 [mm]. a) Fnte volume mesh. b) Sold Model, wth 4 burners and exhausts. Fg.5. Model for fre-resstance furnace. Flame temperature depends on several factors durng the combuston process and affects, sgnfcantly, heat transfer nsde fre-resstance furnace. The rate of heat transfer ncrease wth flame temperature. Fgure 6 represents temperature and velocty, durng the smulaton of test 1, n dfferent defned planes, n partcular X15, X85, Z15, Z85 and exhaust. The burner B3/B4 plane s also represented.

7 a) Contours of temperature [K] n the four planes, tme =3 [s]. b) Vector velocty n plane for burner B3/B4, tme =3 [s]. c) Contours of temperature [K] n the four planes, tme =36 [s]. d) Vector velocty n plane for burner B3/B4, tme =36 [s]. Fg.6. Numercal results for Test 1. The flux of speces s descendent, wth small vortces zones, as can be seen n fgure 6b) and d). The numercal results allow dentfyng the localzed effect of each burner. The expermental measurements corroborate ths evdence. To determne the thermal performance of the numercal smulaton, the mass average temperature value was determned for each plane defned for experments, usng equaton (7). Ths average value s also compared wth the reference value of the fre-resstance furnace, determnng the relatve dfference, see fgure 7. T T ρ v da ρ v da = A A (7)

8 Temperature [ºC] Relatve Temperature Dfference 12 Numercal FURNACE,25 average all 4 planes 1,2 8,15 6 4,1 2, , a) Numercal results, usng nomnal testng curve ISO834 (test1/2). b) Relatve temperature dfference. Fg.7. Average temperature value nsde furnace and relatve temperature dfference. The numercal results for average temperature predcton nsde furnace are close to the reference temperature value for condton test 1/2, beng the relatve dfference smaller than 1 % after 5 [s]. Besdes all the thermal losses assocated wth ths combuston process, an mportant loss s assocated wth the combuston products on exhaust. Ths s dependent on the speces volume, temperature and also on sensble and latent heat contaned n water vapour. 4 CONCLUSIONS Three expermental tests were conducted and measurements made n four representatve plans to establsh the thermal performance nsde the fre-resstance furnace. A comparson was made on the average of temperature measurements wth the reference value. The mean devaton of the temperature n the four plans s less than 5%, for tme nstant greater than 5 [s]. The maxmum devaton of 3% was brefly addressed at 6 [s]. In any case, the devaton of temperature s below 5 [ C]. These results valdate the operatng condtons of the freresstance furnace, and we may assume a nearly unform dstrbuton of temperature, for each nstant of tme, durng tests, for both nomnal fre curves. The numercal results overestmate the value of the average temperature of the four planes under consderaton. Ths dfference may be related to the operatng system condtons. To valdate the numercal procedure, measurements of the nstantaneous nlet ar and gas flow should be performed. Addtonal measurements for speces concentratons n the furnace exhaust should be planned for future work. ACKNOWLEDGMENT The authors acknowledge the fnancal support from the Portuguese Scence and Technology Foundaton, under the reference project PTDC/EME-PME/64913/26. REFERENCES

9 1. AENOR, Norma UNE 1363: Fre resstance test. Part 1: General requrements, AENOR, Norma UNE 1363: Fre resstance test. Part 2: Alternatve and addtonal procedures, October S.Welch, pa Rubn. Three dmensonal smulaton of a fre resstance furnace, Proceedngs of 5th Internatonal Symposum on Fre Safety Scence, Melbourne, Australa, ISBN Internatonal Assocaton for Fre Safety Scence (IAFSS), pp19-12, March FLUENT 6.3 Documentaton, users manual, Gulano Gardolnsk Venson, Glberto Augusto Amado Morera, José Eduardo Mautone Barros, Ramón Molna Valle. Modelagem do Perfl de Temperatura em Câmara de Combustão Tubular Utlzando o Modelo de Combustão Eddy Dsspaton, X Encontro de Modelagem Computaconal, Insttuto Poltécnco/UERJ - Nova Frburgo/RJ, 21 a 23 de Novembro Coelho, P. J. Numercal smulaton of a mld combuston burner, Combuston and flame, 124: , Elsever, Welch, S; Rubn, P.; Three dmensonal smulaton of a fre-resstance furnace, Proceedngs of 5th Internatonal Symposum on Fre Safety Scence, Melbourne, Australa, March 1997, Internatonal Assocaton for Fre Safety Scence, pp , ISBN