Dynamic simulation on effect of flame arrangement on thermal process of regenerative reheating furna

J. Cent. South Univ. Technol. (2007)02- -0243- -05DOI: 10.1007/s11771-007- 0048 - 6包SpringerDynamic simulation on effect of flame arrangement on thermal process ofregenerative reheating furnaceOU Jian-ping(欧俭平), MA Ai-chun(马爱纯)' , ZHAN Shu-hua(詹树华),ZHOU Jie-min(周孑民)', XIAO Ze qiang(萧泽强)'(1. School of Energy Science and Engineering, Central South University, Changsha 410083, China;2. General Iron and Steel Research Institute, Beijing 100081, China)Abstract: By analyzing the characteristics of combustion and billet heating process, a 3~D transient computer fluid dyamicsimulation system based on commercial software CFX4.3 and some self-programmed codes were developed to simulate the thermalprocess in a continuous heating fumace using high temperature air combustion technology. The effects of different switching modeson injection entrancement of multi burners, combustion and billet heating process in furnace were analyzed numerically, and thecomputational results were compared with on- site measurement, which verified the practicability of this numerical simulation system.The results indicate that the flow patterm and distribution of temperature in regenerative reheating furmace with partialsame-side-switching combustion mode are favorable to satisfy the high quality requirements of reheating, in which the terminalheating temperature of billets is more than 1 460 K and the temperature difference between two nodes is not more than 10 K. Butsince the surface average temperature of bllets apart from heating zone is only about 1 350 K and continued heating is needed insoaking zone, the design and operation of current state are still needed to be optimized to improve the temperature schedule of billetheating. The distribution of velocity and temperature in regenerative reheating fumace with same -side-switching combustion modecannot satisfy the even and fast heating process. The terminal heating temperature of bllts is lower than that of the former case by30 K. . The distribution of flow and temperature can be improved by using Cros8-switching combustion mode, whose terminaltemperature of bllets is about 1 470 K with small temperature difference within 10 K.Key words: high temperature air combustion; recheating furnace; swithed combustion; numerical simulationjets and flames are vertical to the moving direction of1 Introductionbillets， and the inlets and outlets of fluid changeperiodically, it is very difficult to simulate and controlThe reheating fumace is an important equipment ofthis new type reheating furnace. There are dozens ofhot rolled steels for heating the billets to a neededflame pairs in HTAC furnaces, and it is clear thattemperature of high quality steels. The thermal processdifferent reversing combustion modes will causeof se-ololing heating fumace and billet heating processdifferent flows and temperature distributions, which willhave been studied early, and a series of research findingsresult in different billet heating effects. With the aim towere obtained, which promotes the development ofoptimize the combustion control of HTAC reheatingnumerical study on thermal process of steel-rollingfurnaces, based on commercial CFD code CFX4.3, aheating furmnace. Limited by the computation conditions,numerical research on the effect of switching modes onsome important and complex problems were simplified,he thermal process of a HTAC reheating furnace issuch as the billets in the furmace being treated to becarried out to study the effect of flame layouts on themotionlesst- , ignoring the combustion'5 , simplifyingflow and temperature fields. The research will help tothe 3-D process to be 1-D7 or 2-D(8] process. In 1990s,improve greatly the simulating and control optimizingthe technology of High Temperature Air Combustiontechnology and to make a rapid progress in regenerative(HTAC) was applied to reheat furnace in Japan and areheating furmaces.new type HTAC reheating furnace was developed9 -10]. InHTAC furnaces, burners are arranged on the sidewalls2 Numerical simulationand act as combustors and exhausts alternatively.High-speed combustion current brings strong mixing,2.1 Co中国煤化工1sand the dominant direction of current is from one side to:THCNMH Gomestic 80 Vh plateanother with altermative switching. Because the initialmill, as ouowurill T 1g.1, 10 sluicu. The air and gas areFoundation item: Project(200533009) supprted by the Special Foundation for Doctorate Discipline of ChinaReceived date: 2006- 06 -25; Accepted date: 2006- 09-28Corresponding author: OU Jian-ping, PhD; Tel: +86-731-876111; E mail: oujp@mail.csu.edu.cn244J. Cent. South Univ. Technol. 2007, 14(2)preheated simultaneously, and honeycomb ceramics areapr-+V(p;U)=0(1)used as heat exchanger. The chamber and billet size are27 000 mmX4 600 mmX3 000 mm and 2 000 mmXMomentum equation:1 500 mmX 200 mm, respectively. There are 38 pairs ofapjUburners arranged on the sidewalls and acted as+ V(ρrUU)- V(uerVU)=(2)combustors and exhausts alternatively. This furnace isdivided into two combustion spaces by the closed-p'+V(μer(VU)T)+Barrangement of billets. On account of the dynamicEnergy equation:heating process of billets and the large ratio of the即= aptH+V(p{UH)-(v(_工+生5)H) (3)dimension of furmace chamber to nozzles, the upperdCp,f 0日combustion chamber and the up half of the billets areAs to billets, heat-conducting equation isconsidered the computational domain, and symmetricalboundary condition is used to save the memory and toapscpsTs=7(% :VT,)(4)accelerate the calculation. With Body Fitted GridsSystem(BFGS), the computational domain is divided bywhere pris the density of gas, U is the gas velocity,p isblock structure grids with 434 826 non-uniform elements,pressure, p' is modified pressure, B is body force, H isas shown in Fig.2.total enthalpy, OH is Prandtl number, Cp.r is specific heatcapacity of gas, ht is heat conducting coefficient, lerr isefficient viscidity of gas, 1, is the temperature of billet,Upper line spouts: Air nozzles/exhaustsLower line spouts: Fuel nozzles/Ps, Cp,s, and Ag are bilet's density, specific heat capacity,and heat conducting coefficient, respectively.Preheatingone2.3 Boundary conditions and initial conditionsHeating zone 2On account of the on-site testing and producingY:Soaking zonestatistics, the preheated temperatures of preheating zone,beating zone, and soaking zone are 1 180, 1 250, and1 300 K respectively, and the gas flows through eachFig.1 HTAC heating fumace for plate millburner of preheating zone, heating zone, and soakingzone are 280， 270， 245 Nm'/h respectively with(a)excessive air coefficients of these zones being 1.36, 1.22,1.10.The environmental pressure and temperature are setto be 101 325 Pa and 310 K, and mixed gas with lowheat value of 5576 kJNm3 is used as the fuel.(b)Heat- conducting solid boundary condition withoutinner heat source is used to simulate the inner heatingprocess, and the blackness of billet surface and furnacewall is 0.816. The density of billet is 7 800 kg/m', whilethe specific heat capacity and heat conducting cofficientcan be found in Ref.[7].Fig.2 Grids of calculated zoneThe wall function is combined to simulate the(a) Width direction; (b) Length directionvelocity near the wall, and the heat flows of roof andsidewalls of furmace are set to be - -23.2 W/m2 andIn this paper, a transient numerical simulation byusing CFX code in 3-D coordinate system, combined-17.4 W/m2 respectively to simulate the dissipatedwith fluid flow, combustion, and radiation heat transferheat16].In this model, the bottom of the billets is treated asprocess, was performed. The complete heating process ofbillet in furmace is decomposed into many mobile heatingplane symmetrical boundary with heat flow of0.The initial velocity of gases in furnace is 0, and theand stable heating processes, and the temperature resultsof current iteration are set as the initial value for the nexttemp中国煤化工holding temperaturecalculation when a billet's width is added to the pushingafterYHCNMHGratureof bllels isdominaicd uyuscl suUIUuIC. Dy steady simulation, onedistance.can obtain the field distribution of flow, temperature,heat released by combustion, and concentration that are2.2 Governing equations-Iused as the initial condition for later transient calculation,Continuity equation:OU Jan-ping, et al: Dynamic simulation on efet of flame arrangement on thermal process of regenerative reheating furnace 245and the initial temperature of billet being pushed into thedistribution is more stable than that of partialfurnace is 300 K.same-side-switching mode. It is not so turbulent, and3 modes such as partial same-side-switching mode,there is no distinct vortex between heating and soakingsame-side-switching mode, and cross-switching modezones. The interference of the jet flows is aggravatedre considered, as shown in Fig.3. To validate thbecause of the velocity difference of jets from burners,simulation results and make some contrast analysis, theand some nuances of velocity vector distribution appear.actual case is set as reference, as shown in Fig.3(a).There is a clear vortex between heating and preheatingzones, whose style is “w”type, diferent from“O”type(a)of reference case.The velocity vector distribution of gas-gas sectionin furnace of cross-switching mode is shown in Fig.3(c).From this figure, one can see that reverse jets fromadjacent burners cause large vortexes with reversedirection, which almost fill fully two sidewalls an(b)change direction periodically. Though some exhaustedoutlets surtound fluid inlets, it can be seen that even withlower speed, most gases are driven ahead along with↓↑↑7↓mainstreams, and the other gases spread aroundmainstreams. The gases are stirred and mixed by reflux(C)combusted flue. This evidences that the fresh gases willnot be drawn out directly through surrounded exhaustspouts and no short pass problems appear. The sameregularity can be found out in the speed field on air-airsection in furnace. Because of the existence of so manyvortexes, the diffusion area between air and fuel isFig.3 Velocity vector distributions of gas gas section inenlarged, which improves the combustion condition.furmnace(a) Partial same-side-switching mode;3.2 Temperature prfile(b) Same side-swiching mode; (C) Cros-switching modeIn heating furnace producing, more attention is paidto the billet heating process and temperature distribution3 Results and discussionof furnace. The maximum, minimum and averagetemperatures calculated in nodes of billet after heatingare listed in Table 1, where the average temperature is3.1 Flow fieldFig.3 shows the velocity vector distributions ofdefined by Tawo=Er.vEv, where Ti is thegas-gas section in furmace of three different cases. FromFig.3(a), it can be seen that under the condition of partialcalculated temperature of node i and V; is the controlsame- side- switching, because of multi parallel jets'volume of node i. Fig.4 shows the temperatureintervene and interleaving jets between two adjacentdistribution on typical sections for the three cases, andzones, the speed vector distribution in each heating zoneFig.5 shows each average temperature distribution ofis abnormal, and large circumfluence exits, which indeedflue and bllts along the furnace length.overflows within the side walls. The wider the intervalTable 1 Maximum, minimum and average temperaturesbetween burners is, the larger the circumfluence will be._calculated in nodes of bllet after heatingDue to the effect of walls, there are some circumfluencesCaseMaximumMinimumAveragenear end walls, and with different distances from burnerNo.temperature/Ktemperature/K temperature /Kto end wall, the size of circumfluence is different, which148114711474is developed adequately within switching period. When1 4581 4341442 .combustion is switched, the circumfluence will be31 4831 4751 478confused, but some new circumfluence with reverse中国煤化工direction reforms quickly. The circumfluence ofTYHCNMH G; can see that in thecombustion gas can stabilize the further combustion andaperature and billetimprove the temperature distribution of furmace.temperature increase gradually from preheating zone toFig.3(b) shows velocity vector distribution ofsoaking zone. In preheating zone, according to coldgas-gas section in furnace of same-side-switching mode.billets and lower combustion ability of burners requiredBecausesame jet direction in each zone, the flowby the heating technology, the flame temperature and芳芳数据246J. Cent. South Univ. Technol. 2007, 14(2)furnace temperature are lower, and the billets are heatedslowly with less temperature difference in the thicknessdirection of billet. In heating zone, the volume of flamegets large and the temperatures of flame and furmnace(b)increase, and the heating rate of billets becomes higherwith large temperature difference in the thicknessC)direction of billet. While in soaking zone, the furmacetemperature increases and the high temperature zoneenlarges obviously with even temperature distribution.Fig.4 Temperature distribution of y = 850 mm section inThe terminal heating temperature of billets is more thanfurnace1 460 K with little difference. For example, the(a) Partial same-side-switching mode;temperature difference between two nodes is not more(b) Same side-switching mode; (C) Cross switching modethan 10 K, which satisfies the heating process.From Fig.5(a), one can also see that the heating1600system under producing condition is not very rational.a)1 400For example, billets in heating zone are not heateddeeply enough and the surface average temperature ofI 200billets apart from this zone is only about 1 350 K and￥100continued heating is needed in soaking zone. Therefore,800the soaking process is shortened within the last 2 m00passively. This status is validated by productive practice400and needs to be improved in furnace design andoperation in the future.2000120 2530Compared with reference case， the averageL/mtemperature of furmace of case 2 is lower. As shown in1 600b)1Fig.4(b), according to feebler mixing, at y=850 mmsection the flames are relatively thin and short, and the21200flame zone in the connected zone of heating and soakingzone is even invisible. Additionally, the terminal beating必100temperature of billets is 30 K lower than that of referencecase. This modedisadvantageous to heat billets600quickly and uniformly. As shown in Fig.5(b), theterminal temperature of bllts is less than 1 450 K.The results of cross-switching mode are shown in5015202530、Fig.4(c) and Fig.5(c). In this case, the flame shape isperfect and one can see 19 distinct flames in the typical600 [(C)section, which are nearly continuous as a whole. Thetemperature distribution in furmace width direction is2y1 200much more even. And in furnace length direction, the100temperature distribution is reasonable. Within the end32 m of soaking zone, the average furmace temperature is00 tabout I 520 K, which is stable and a lttle higher thanthat of reference case. In this case, when billet movesfrom heating zone to soaking zone, the average transienttemperature of billet surface is about 1 370 K, and that of015202530billet's central section approaches to 1 350 K, which isFig.5 Average temperature distribution along furnace lengthabou中国煤化工ence case. In this case,both1- Furnace temperature of y=l 300 mm section; 2- BilletCNMH Gare improved, and thetermiuas tcuperauc Ul Ulilis 1s about 1 470 K withtemperature of upper surfaces; 3--Billet temperature ofsmall temperaturedifference no more than 10 Ksymmetrical section(a) Partial same- side switching mode;Compared with reference case， the cross-switching(b) Same- side- switching mode; (c) Cross-switching modecombustion mode is favorable to heating process.OU Jian-ping, et al: Dynamic simulation on effect of flame arrangement on thermal process of regenerative reheating furnace 247soaking zone. The design and operation of current state4 Validation of simulation resultsare still needed to be optimized for improving thetemperature schedule of billet heating.The simulation results of furnace temperature in3) The distribution of velocity and temperature inreference case are validated by on-site testing with sixregenerative reheating furnace with same-side-switchingPtRh-Pt thermocouples located in furnace chamber, andcombustion mode cannot satisfy the even and fastterminal temperature of billets is also validated usingheating process for the terminal heating temperature ofsurface thermometers when the billets slide from fumacebillets is more than 30 K, which is lower than that of theonto the roller bed. The results of simulation (Tca) andpartial same-side-switching mode. The distribution oftesting (Tes) are listed in Tables 2 and 3. From theseflow and temperature can be improved by usingtables, it can be seen that the differences of furmacecross-switching combustion mode, whose terminaltemperature between tested results and calculated resultstemperature of billets is about 1 470 K with smallof different points are small. The average difference istemperature difference no more than 10 K.32 K，while the average difference of the surfaceReferencestemperature of billets between tested results andcalculated results is 22 K. It can be concluded that the[1] WU Zhen-kun, YANG Rui, WANG Jian, et al. Numerical simulationcalculated results agree with practical situation well andof a 3-D combustion flow field in a single chamber heating furnacethe simulation model can guide the optimization 0with horizontal tubes[J]. Fire Safety Science, 2001, 10(4)2 213- 217.(in Chinese)producing.2] SHU Zheng-chuan, ZHU Tong, Numerical simulation investigationof the combustion system modifcation scheme for a cell pitfurnace[J]. 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(in Chinese)temperature of billets is more than 1 460 K and the[15]YH. CNMHGfNenfers Mlaourreof Nonferrous Metallurgicaltemperature difference between two nodes is no moreChinese)”.. ..--....(inthan 10 K. But in current, the surface averagetemperature of billets apart from heating zone is onlySteel Plant. References of Industrial Fumace Design for Steel Plantabout 1 350 K and the continued heating is needed in(oL.2)M]. Beijig: Metallurgical Industry Press, 1979. (in Chinese)(Edited by YANG Bing)