ISSN 1611-616X VULKAN-VERLAG · ESSEN 22007

HHEEAATTPPRROOCCEESSSSIINNGGINTERNATIONAL MAGAZINE FOR INDUSTRIAL FURNACES · HEAT TREATMENT PLANTS · EQUIPMENT

CFD-simulation of melting furnaces for secondary aluminum

Dr.-Ing. Stefan Murza, Ingenieurbüro für technische und wissenschaftliche Berechnungen, Bochum, GermanyDr.-Ing. Björn Henning, Dipl.-Ing. Hans-Dieter Jasper, Jasper GmbH, Germany

Published in HEAT PROCESSING 2/2007

Vulkan-Verlag GmbH, Essen (Germany)

Editor: Dipl.-Ing. Stephan Schalm, Tel. +49 (0) 201/82002-12, E-Mail: s.schalm@vulkan-verlag.de

http://www.heatprocessing-online.com

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ne of the major challenges ofindustry is to increase production

at a simultaneous reduction of energyuse and environmental protection incombination with reduced investmentand operation costs. In parallel thedemand for increased availability atreduced maintenance is growing – fol-lowing continuous rationalisation activi-ties. Measures are tightened by codeson environmental protection and morerestrictive requirements on the emissionof contaminants. These challenges andthe quality requirements mentionedafore call for considerable research anddevelopment efforts and a high readi-ness for innovation from plant construc-tors.

State-of-the-art melting furnaces –respectively heat treatment plants –require well adjusted burner systems.Highest energy savings can be achievedwith regenerative heat recovery. Theselection of a suitable (regenerative)burner system is based on a number ofcriteria which have to be tailored to therespective process technology. Anextremely interesting aspect is related tothe fact that state-of-the-art regenera-tor systems are not only able to achieveconsiderable economic advantages

(energy savings), but also to accomplishconsiderable improvements in processtechnology. There is proven evidencefrom the field of the aluminium industrywhich demonstrate that the melting lossof aluminium is reduced. A combinationof regenerators and an alteration of thefurnace technology have been proven tobe very successful also during recon-struction of furnaces in the steel indus-try. Modifications of rotary heard fur-naces resulted in increases in productionof up to 33 % at a simultaneous reduc-tion of energy consumption of up to 20% [1, 2].

Firm insight resulting from experimentas well as the utilisation of mathematicalcomputing models is inevitable to meetthe requirements for further develop-ment and optimisation of melting fur-naces and heat treatment plants. Ele-mentary calculations reflecting theactual condition of the plant can be per-formed on the basis of experimentaldata. Simulations of different variationsprovide information on temperatureprofiles, flow patterns and radiative andconvective heat input. The actual plantcondition of a melting furnace for alu-minium shall be described in the follow-ing.

Numerical simulation with CFDComputational fluid dynamics (CFD) hasbecome an important part in modernproduct development. Due to the factthat most natural flows (e.g. air, water)cannot be seen directly people canhardly relate to them. Using computa-tional fluid dynamics we are able to visu-alise and analyse flows and the relatedphysical processes like turbulence orheat distributions. CFD acts as an eye inthe flow and, so to speak, provide theengineer with an additional 'sense'.They can be used to interpret technicalflows already during design processesand also for product optimisation. CFDanalyses consist of three main parts. Theprecise definition of the tasks combinedwith the module selection (pre-process-ing), the actual calculation (processing),and the analysis of the results includingdata visualisation (post-processing).Unfortunately, in the review of CFD pro-jects it sometimes becomes evident thatinsufficient attention was paid to thepre-processing with regard to thepotential results and their significance.The time required for the second workstep, the actual calculation time for theprocessing, depends on the computingpower of the system which is availablefor the simulation. Using actual CFDtools most analyses can be solved onparallel computers or computer clustersystems. The performance of moderncomputers has grown dramatically, butthis does not automatically result inshorter computation time in that grow-ing computational power typically leadsto the application of more complexmodels. Additionally, CFD engineers usefiner grids to describe the considereddomain. Ten years ago a massive parallelcomputer had to be used in order tosolve a simulation of reacting flows withone million grid points in acceptabletime. Now, using the same models anddiscretisation the simulation can bedone on a normal personal computer inapproximately the same time. Doing

CFD-simulation of melting furnacesfor secondary aluminum Stefan Murza, Björn Henning, Hans-Dieter Jasper

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Hall 4 / A55

The optimal utilisation of fuel per product quantity in combination with areduction of investment costs has always been a permanent challenge forevery manufacturer of furnaces and burners for industrial use – and thisnot only owing to the current increase in energy cost. The dimensioningand the design of melting furnaces depend on the one hand on the typeof scrap metals and on the other hand on the specification of the sub-stance to be produced (e.g. casting or forgeable alloys for the aluminiumindustry). Today mathematical tools are used in the further developmentand optimisation of melting furnaces. CFD (computational fluid dynamics)simulation is a tool which has been successfully established in many diverse sectors of industry. It is the intention of this article to present thesimulation of a melting furnace which has been done using the commer-cial software FLUENT®. Numerical simulation shall provide the plant constructor with information on the guiding of flows, temperature distri-bution, heat input and heat losses in a furnace.

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Fig. 1: Numerical simulation of an aluminium melting furnace

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post-processing, the third work step, theCFD engineer should keep in mind thatit is sometimes better to focus theresults to the answer to the originalquestion, instead of showing any possi-ble graphical representation.

The presented simulation of an alu-minium melting furnace was done byusing the CFD program FLUENT® in ver-sion 6.2. The analysis focussed on theinfluence of different geometries andthe fuelling parameters effecting theefficiency and the output of melting fur-naces for secondary aluminium. Due tothe fact that multiple varying simula-tions had to be made – amongst othersfor the definition and validation ofboundary conditions – the calculationscould not be solved on a very fine grid.Otherwise the computational effortwould have gone far beyond the scopeof project time and cost. There was noexplicit need for highest academic accu-racy of single results. The focus wasrather put on a couple of relevant para-meters which are of practical impor-tance for the furnace design and opera-tion, in order to help the manufacturerto continually optimise his product.

The entire calculation domain was setup completely three dimensional withunstructured grids, using the programGAMBIT® in version 2.3. The domainshad a size of 110 to 160 thousand vol-ume elements. The standard k-ε-modelwas used to describe the effect of tur-bulence onto the flow field. For thedescription of the gas phase combustiona model was used which solved thetransport phenomena of every relevantgas phase species. To take into accountthe effects of turbulence on the chemi-cal conversion rates an eddy-break-up

model was used. The radiative heattransfer in the melting furnace wassolved by using the 'discrete-transfer'model. For this analysis it was particu-larly important to use common modelswith standard parameters to assurecomparability and traceability of the cal-culation results. Due to the fact thatheat transfer in melting furnaces ismainly dominated by radiation it wasvery important to determine the mostaccurate emission/absorption coefficientfor the surface of the liquid aluminium.This was done in numerous pre-calcula-tions. Based on empirical data obtainedfrom literature a coefficient was deter-mined which was used for all subse-quent simulations. To ensure compara-bility and traceability this was the onlyadjustment of the standard models andtheir parameters.

Different geometrical variations werecalculated. For each of them a newcomputational grid had to be gener-

ated. Varying arrangements of theexhaust gas exit were investigated aswell as geometric modifications of theburners used and their arrangement. Inaddition domains with different shapesof the melting furnace covers were sim-ulated.

A further analysis discussed the effectsof different air flap settings in the feedpipe of the burners with special empha-sis on the asymmetric momentum andits consequence on the heat input intothe liquid aluminium.

ResultsFig. 1 shows some selected figures ofthe computation results of one versionof an aluminium melting furnace with astraight cover of the furnace cover anda centred exhaust gas exit in the middleof the roof. The computational domainwith the numerical grid is shown inupper left figure. Temperature andvelocity drawings show a distinctiveflow nearby the aluminium surface. Asintended by the manufacturer the flowis cooled down and ascends in the rearsection of the furnace where it is redi-rected towards the exhaust gas exit. Thissteady flow through the furnace gener-ates a good convective heat transferwhich has an amount of 17 percent ofthe total heat transfer into the liquidaluminium. The radiative heat transfer -shown in the right lower figure of pic-ture 1 – dominates in the first third ofthe furnace where the regions with highflame temperatures are located. This dis-tribution of heat input caused by radia-tion is beneficial for the liquid alu-minium which is in counter flow withthe gas phase.

Fig. 2:Temperature distribution in anaxial plane (high impulse)

Fig. 3:Temperature distribution in anaxial plane (low impulse)

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Fig. 2 and Fig. 3 show a comparison ofthe temperature distribution along aburner axis for two variant simulationsof the melting furnace to show theeffect of different exit momentumcaused by geometrical alterations at theburners. In combination with the Fig. 4and Fig. 5 for the same variant calcula-tion a significant increase in radiativeheat transfer can be observed. This isshown by the white areas in the figurescaused by a heat transfer in a magni-tude which is beyond the colour-map.For comparison purpose the colour-mapwas set consistently for all variant calcu-lations. The low momentum leads toshorter flames with a higher energydensity, which is responsible for the veryhigh radiative heat input in smallregions. Looking at the results of the

lead to poorer melting capacities causedby a significant lower convective heattransfer into the liquid aluminium. Acomparison of the results for the simula-tions of melting furnaces with differentcover shapes and the same energy inputshow an up to two percent higher melt-ing output achieved by a convex covershape. It can be seen that the increase inmelting capacity is mainly caused by themodified flow field and the adjunctiveconvective heat transfer. A similar effectcan be found for variants with displacedexhaust gas exits which also lead to amodified flow field inside the meltingfurnace. A comparison of the variantsdemonstrates that even minor alter-ations – for example a different settingof the air flap position – have a non-negligible effect on the heat input andthe melting capacity.

Literature[1] Henning, B.; Jasper, R.: MultiMelter© -

The new generation of aluminium melt-ing furnaces, Heat Processing 2/2004,Vulkan-Verlag

[2] Henning, B.; Klatecki, P.; Jasper, R.: Opti-mierte Regeneratortechnologie an indus-triellen Feuerungssystemen, Gaswärme-International 6/2005, Vulkan-Verlag

[3] von Starck, A.; Mühlbauer, A.; Kramer,C.: Praxishandbuch Thermoprozesstech-nik, Vulkan-Verlag 2003 �

calculation it can be seen that this vari-ant with low momentum is inappropri-ate. In addition to the higher NOx emis-sions a lower total heat input into thealuminium and an unbalanced tempera-ture distribution in the liquid can bedemonstrated.

ConclusionThis article describes a CFD-Simulationof a melting furnace for secondary alu-minium with the commercial avail-abletool FLUENT®.

Looking at the calculated melting capac-ities, a difference up to five percentbetween the variant simulations pro-voked by geometrical modifications canbe demonstrated. In particular, variantswith low flow impulse at the burner exit

Dr.-Ing. Stefan Murza

Tel. +49 (0)234 / 3387996stefan.murza@murza.net

Ingenieurbüro für technischeund wissenschaftlicheBerechnungen, Bochum Germany

Dr.-Ing. Björn Henning

Tel. +49 (0)4106 / 8043-0b.henning@jasper-gmbh.de

Jasper GmbH Germany

Dipl.-Ing. Hans-DieterJasper

Tel. +49 (0)4106 / 8043-0hd.jasper@jasper-gmbh.de

Jasper GmbH Germany

Fig. 4:Radiative heattransfer into thealuminium bath(high impulse)

Fig. 5:Radiative heattransfer into thealuminium bath(low impulse)