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Mayr B.,University of Graz | Prieler R.,University of Graz | Demuth M.,Messer Austria GmbH Kompetenzzentrum Metallurgie | Hochenauer C.,University of Graz
Energy | Year: 2015

This paper investigates two furnaces which work under oxy-fuel condition with natural gas. One is a 0.8MW furnace where detailed inflame measurements are available. The other furnace is an 11.5kW lab-scale furnace with temperature measurements. The furnaces were investigated by CFD (Computational fluid dynamics) analysis. The main focus was on using combustion models that are not computationally demanding. Therefore the SFM (steady flamelet) approach was used with two detailed mechanisms. The advantage of the SFM is that the calculation time can be reduced from 4 weeks to 4 days on 8 CPU-cores. The applicability of two detailed mechanisms under oxy-fuel condition is pointed out in this paper. The investigation showed that the skeletal25 mechanism and the SFM are in very good accordance with measurements. If the strain rate between CH4 and O2 stream is too low, the SFM fails to predict the flame shape correctly. The influence of three different turbulence models was also investigated. Furthermore simulations with the eddy dissipation model and numerically expensive eddy dissipation concept model were conducted. Different WSGGM (weighted sum of grey gases model) were applied. The comparison of the WSGGMs showed that the difference between them is insignificant for small furnaces. © 2015 Elsevier Ltd. Source


Prieler R.,University of Graz | Demuth M.,Messer Austria GmbH Kompetenzzentrum Metallurgie | Spoljaric D.,Messer Austria GmbH Kompetenzzentrum Metallurgie | Hochenauer C.,University of Graz
Fuel | Year: 2014

In the present work a CFD study of a natural gas fired lab-scale furnace under different oxygen enrichments was done. Results were compared to measured temperatures and heat fluxes on a thermal sink inside the furnace. The main emphasis was to apply the steady flamelet model (SFM) with a detailed chemical mechanism to predict the flow field, temperature and species concentrations in high temperature applications. A skeletal mechanism, which considered 25 reversible reactions and 17 species, showed close agreement with the measurement for all oxygen concentrations in the oxidizer. Turbulence modelling was done by the standard and realizable k-epsilon models as well as the Reynolds stress model (RSM). It was found, that standard k-epsilon model calculates too high temperatures in the vicinity of the burner. Results from realizable k-epsilon model and RSM were in close accordance with marginal differences in flame length and maximum temperatures. The CFD calculation with the SFM was also compared with a time expensive eddy dissipation concept (EDC) simulation with 46 reactions and 17 species. Both approaches calculated similar results for temperature and species concentrations. The calculation time was reduced significantly from 4 weeks (EDC) to 6 days with the SFM although dissociation effects and forming of radicals were taken into account. Considering melting and annealing furnaces, the heat flux to a thermal sink constitutes a major role for combustion processes. Heat flux to the thermal sink was determined by measurement and CFD predictions for different O2/N2 ratios in the oxidizer. CFD calculations revealed a maximum relative error to the measurement for the heat flux of 5% for 25, 30, 45 and 100 Vol% O2 in the oxidizer. The numerical model and experiments highlighted a major increase on the furnace efficiency due to oxygen enrichment. A maximum efficiency of 67% was detected for natural-gas combustion with pure oxygen compared to 44% in case of 25 Vol% O2. It was also found, that high oxygen concentrations lead to a homogeneous temperature distribution on the sink which is desired for melting and annealing applications. © 2014 Elsevier Ltd. Source


Prieler R.,University of Graz | Demuth M.,Messer Austria GmbH Kompetenzzentrum Metallurgie | Spoljaric D.,Messer Austria GmbH Kompetenzzentrum Metallurgie | Hochenauer C.,University of Graz
Fuel | Year: 2014

The present work investigates CFD analysis of an 11.5 kW lab-scale furnace with oxy-natural-gas combustion for high temperature processes. In recent years many researches were done for oxy-fuel combustion with flue gas recycle in power plants. However, this paper considers combustion models which are applicable for oxy-fuel environment in melting and annealing furnaces. The main emphasis was to use the steady laminar flamelet model (SFM) associated with three detailed chemical mechanisms to predict the combustion process in the furnace. This approach was chosen to avoid time expensive chemical calculations in CFD while calculating the flow field, temperature and species concentrations. Just two additional transport equations for mixture fraction and mixture fraction variance have to be solved to determine the reactions in the flow field. It was found, that the skeletal mechanism with 17 species and 25 reversible reactions generated the best results with SFM. CFD calculations with SFM were compared with results from a 2-step eddy dissipation model (EDM) and a numerically expensive eddy dissipation concept (EDC) model with 17 species and 46 reversible reactions. The EDM was exposed to be unsuitable for modelling oxy-fuel combustion due to dissociation effects and forming of radicals in high temperature applications. In contrast to the EDM results, temperatures calculated by EDC and SFM showed close agreement with measured data in the furnace. Also for the species concentrations, obtained with both models, no significant differences were observed. A significant improvement was the reduction of computational time from 3 weeks to 4 days on 8 CPU-cores by using the SFM. The P1 and discrete ordinates (DO) model were also applied to solve the radiative transfer equation. A weighted sum of grey-gases model (WSGGM) was applied to model the radiative properties of the flue gas. Due to the special composition of the flue gas in oxy-fuel combustion, which consists mainly of H2O and CO2, the radiative properties should be treated with a non-grey gas approach. Nevertheless, the WSGGM was used in this study because of the small beam length of the furnace. Simulation with the P1 model overestimates the radiation, which leads to lower temperatures in the furnace. The DO model calculated temperatures in good agreement with the measurement with the SFM and EDC. © 2013 Elsevier Ltd. All rights reserved. Source


Prieler R.,University of Graz | Mayr B.,University of Graz | Demuth M.,Messer Austria GmbH Kompetenzzentrum Metallurgie | Holleis B.,Messer Austria GmbH Kompetenzzentrum Metallurgie | Hochenauer C.,University of Graz
Applied Thermal Engineering | Year: 2016

A three-dimensional analysis of the gas phase combustion and transient heating of steel billets in an industrial walking hearth furnace was done by CFD. An iterative solution procedure was used for the steady-state simulation of the reactive flow and the heat conduction in the billets for air-fired and oxygen enriched combustion with 25 vol% O2 and 75 vol% N2 in the oxidizer. Information about the turbulent flow, species concentrations, temperatures and heat fluxes in the furnace was obtained with low computational demand due to the used solution procedure. Modelling of the gas phase combustion considered 17 species and 25 reversible reactions including radical formation. According to operating conditions of the furnace, a gas saving of 8% was determined due to the oxygen enrichment. Although the fuel input was reduced with increasing oxygen concentration, the calculated heat fluxes to the billets were similar due to the oxygen enrichment. Furthermore, a higher heating rate was achieved in the heating zone compared to air-fuel combustion, as a consequence of the increased radiative heat transfer. The overall efficiency of the reheating process was raised from 57.6% (air-fuel) to 61.4% (oxygen enriched). Surface temperature and temperature uniformity of the billets were calculated using transient simulations and investigated for the two combustion cases. In air-fuel combustion, a faster heating of the billets was detected due to the higher convective heat flux in the pre-heating zone. This effect in the air-fuel case was compensated in the following heating zone in oxygen enriched combustion by radiation. This study shows that higher efficiency of reheating processes can be achieved by means of oxygen enrichment with a similar heating characteristic of the furnace load. © 2016 Elsevier Ltd. All rights reserved. Source


Mayr B.,University of Graz | Prieler R.,University of Graz | Demuth M.,Messer Austria GmbH Kompetenzzentrum Metallurgie | Potesser M.,Messer Austria GmbH Kompetenzzentrum Metallurgie | Hochenauer C.,University of Graz
Fuel | Year: 2015

This paper investigates a natural gas fired lab-scale furnace with a thermal input between 28 and 115 kW under different O2/N2 ratios in the oxidizer by computational fluid dynamics (CFD). Results of the simulation were confronted with temperature measurements inside the furnace and heat flux measurements on a water cooled plate inside the furnace. The main goal of this work was to use a detailed chemical mechanism and reduce the calculation time. This was achieved with the steady flamelet (SFM) approach. The advantage of the SFM approach is that the computational chemical calculation can be pre-processed and stored in look up tables. Only two additional equations have to be solved to determine the chemical reaction in the flow field. For the simulation the detailed mechanism skeletal25 was used with 17 species and 25 reactions. Additionally the furnace was simulated with the eddy dissipation concept model (EDC) which other authors mainly use to describe oxy-fuel combustion. For the EDC simulation a refined version of the 4-step mechanism proposed by Jones and Lindstedt was used. The EDC simulation was used as a benchmark to determine the time saving potential of the SFM approach. With the SFM approach the calculation time could be reduced from 4 weeks to 4 days on 8 CPU cores, although a detailed mechanism was being used. The predicted temperatures of the CFD simulations were in good accordance with the measurements and showed the applicability of the skeletal25 mechanism with the SFM approach under different combustion environments. In metal melting or reheating furnace the right prediction of the heat flux on the goods is crucial. Therefore the heat flux on a water cooled plate inside the furnace was determined by measurements and CFD calculations for different combustion environments. For the heat flux at a temperature level of 1070°C the CFD calculation showed a maximum relative error of 5% to the measurements for 21, 25, 30, 45 and 100 Vol% O2 in the oxidizer. For the temperature level of 1200°C the maximum error increases, especially for O2 concentration in the oxidizer higher than 45% up to 12%. Both the experiments and the numerical model showed an increase in furnace efficiency with increasing oxygen in the oxidizer. A maximum efficiency of 76% was observed for 100 Vol% in the oxidizer compared to 48% at 21 Vol% O2. This shows the fuel saving potential of oxygen or oxygen enriched combustion. © 2015 Elsevier Ltd. All rights reserved. Source

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