Impingement jets are used in many applications for high convective heat transfer. In order to optimise specialised nozzle systems, a comprehensive understanding of the gas flow is essential. The aim of this work is to investigate high-convective flows at Re = 10,000 to Re = 50,000 for a single slot nozzle (slot width W = 5 mm) and a slot nozzle array (distance between nozzle slots s = 70 mm) consisting of five nozzles. Particle image velocimetry measurements are taken for a distance between strip and nozzle exit of H = 50 mm and are compared to verify if the results from a single slot nozzle are transferable to a nozzle array. The presence of an array of nozzles not only creates a distinct zone where the individual jets interact but also changes the flow characteristics of the respective free jets. The potential core length in the nozzle field is significantly reduced compared to the single nozzle. It is therefore not possible to make a direct transfer of the results. Direct transferability of the results is therefore not possible. This means that further studies on whole arrays are needed to optimise nozzle arrays.

Radiant tubes are highly stressed components, used to separate the atmosphere of the process from the combustion taking place inside the tube. For the identification of critical radiant tube positions in the furnace and possible life-time-promoting adjustments in the design and operation, consideration of the radiation exchange is essential. To address this problem, an already implemented and validated radiation modeling approach is used to determine the temperatures and heat fluxes of single-ended radiant tubes in a horizontal radiant tube furnace. The tube arrangement as well as the emissivities of the components were investigated with respect to their effect on the tubes’ temperature, the furnace wall temperature and the heat flux on the strip.It is shown that a horizontal distance of twice the radiant tube diameter between the tubes is optimal showing low and homogeneous temperatures on the tubes and the furnace wall as well as a homogeneous heat flux on the strip. For the vertical distance between tubes and the wall and between tubes and the strip, a value of half the tubes’ diameter is found to be optimal. With this arrangement, the global temperature difference across a single tube does not exceed 67 K. The common installation recommendations for radiant tubes were reviewed and confirmed. It is also shown that misalignment of the tubes is not beneficial. The radiation exchange within the furnace is sensitive to the emissivities of the components, although it is clear that these can only be influenced to a very limited extent.

In industrial thermal processing plants, metal strips are quenched in cooling zones by impingement jets, with convection being the dominant heat transfer mechanism. To generate the impingement jets, gas is accelerated through a nozzle system and directed onto the material surface, resulting in rapid and uniform cooling. The present work involves the experimental investigation of the heat transfer and associated flow of impingement jets using PIV on a single slot (W = 5 mm) and a single round nozzle (D = 25 mm). These experimental methods form the basis for the evaluation of numerical turbulence models. The turbulence models selected in this work are: SST k-omega model, Generalised k-omega (GEKO) model and the Reynolds Stress Model. The investigations are carried out at a nozzle exit velocity of u approximate to 51 m/s (ReSlot = 34,490, ReRound = 88.780). Compared to other studies with a Reynolds number of below 23,000, the prediction accuracy is less due to the high Reynolds number. The PIV measurement shows that the flow velocities are correctly modelled, but the turbulent kinetic energy can only be poorly predicted.trampe@iob.rwth-aachen.de

The substitution of natural gas with hydrogen is one way to eliminate direct CO2 emissions. However, oxyfuel combustion of hydrogen or hydrogen enriched natural gas leads to different exhaust gas properties due to a changed composition compared to conventional combustion. In combustion simulation, the emissivity of a gas mixture is usually approximated using a Weighted Sum of Gray Gases (WSGG) model. Most of the existing WSGG models have been validated for natural gas combustion with air or oxyfuel and are therefore not applicable to hydrogen-oxyfuel combustion. CFD simulations showed, that none of the investigated WSGG models is able to predict the radiative heat transfer for all considered combustion scenarios with appropriate accuracy. In addition, in container glass manufacturing more than 95% of the heat flux to the glass surface is transferred by radiation because of the high process temperatures. Due to the changed gray gas emissivity, the high content of water vapor leads to a different emission spectrum of the exhaust gas. The influence of the changed emission spectrum on radiative heat transfer and the penetration depth of radiation in the glass melt is investigated using a simulation model of a pilot plant and non-gray modelling of the radiation transport. The CFD simulations show slightly enhanced radiative heat transfer to the glass and a slightly deeper penetration depth especially for wavelength below 2.2 mu m for hydrogen-oxyfuel combustion.

In convection-dominated thermoprocessing plants, the current state of the art is to use centrifugal and axial fans in conjunction with flow straighteners to ensure a homogeneous inflow of the fluid and thus a homogeneous temperature distribution. However, flow straighteners lead to an additional pressure loss in the system that must be overcome. Tangential fans can deliver a homogeneous volume flow over the entire width of the fan even without flow straighteners. Therefore, they offer a possibility to increase the energy efficiency of these systems. Based on this motivation, the possibilities of increasing the thermomechanical long-term stability of tangential fans are being evaluated as part of a current research and development project. To this end, the effects of design changes on the generated flow and the mechanical loads are being investigated with the help of FEM and CFD simulations. The aim is to design a tangential fan that can withstand the mechanical and dynamic loads and deliver a consistently high and homogenous fluid flow at the same time. A geometry based on the results of the simulations was manufactured as a functional sample. This sample was investigated at the hot test stand of the IOB and is compared to a reference tangential fan.

The use of structured tubes in tube bundle recuperators is intended to improve the energy efficiency of gas-fired industrial furnace systems with central air preheating. Currently, smooth tubes as well as smooth tubes with internal twisted tape are commonly utilised. The increased surface area of structured tubes leads to an increased heat transfer between the off-gas and the combustion air. However, structuring leads to an increased pressure loss. The aim is to find an optimal operating range in which the pressure losses are still acceptable and the greatest increase of heat transfer compared to the reference will be achieved. Smooth tubes and also smooth tubes with twisted tapes are considered as reference systems in comparison to two concave structured tubes. CFD simulations were used to investigate the influence of the operating parameters of combustion air velocity and off-gas velocity on heat transfer according to industrial scale. The simulations have shown that the heat flow increases with the honeycomb depth and strongly depends on the operating conditions. Experimental measurements in a test bench were carried out to validate the model and confirm the positive influence of structuring on heat transfer.

In industrial plants, metal strips are quenched using convective heat transfer. This involves accelerating gas through a nozzle system onto the material to be quenched, resulting in a fast and uniform cooling process. The efficiency of the heat transfer is determined by the specific nozzle system. The presented work is focussed on the analysis of the heat transfer by forced convection through impingement jets both theoretically and experimentally. A unique method for determining the forced convective heat transfer coefficient is described and its application to industrial size nozzle arrays of annealing-line cooling sections is presented.

Tube bundle recuperators are generally designed to operate with smooth tubes. Structured tubes can be used to increase the efficiency of recuperators. Compared to smooth tubes, the surface for heat transfer is increased and thus heat transfer is enhanced. This effect is accompanied by an increased pressure loss, which must be kept as low as possible. Four tube geometries with different honeycomb structures are examined. The results are compared with the performance of a smooth tube. The investigations were carried out both numerically and experimentally at different off-gas and combustion air velocities. The experimental results show that the highest heat transfer is achieved with the concave 6 mm structured tube. The greatest pressure loss also occurs here. The validation of the numerical model has shown issues in resolving the turbulence.

In order to achieve the goal of a massive reduction of CO2-emissions, fossil fuels have to be substituted. In Germany, continuous carburizing furnaces for high capacities are almost exclusively fired by natural gas due to the lower energy costs. Electrical heating and hydrogen combustion are obvious alternatives. While electrical heating elements are state-of-the-art for these types of furnaces, hydrogen combustion has not been investigated. Furthermore, these two alternatives strongly depend on the specific energy mix, which determines the CO2-emissions. This case study compares different process heat generation options for continuous ring hearth furnaces for carburizing automotive steel parts by a quantitative approach. The investigated alternatives are natural gas/air heating as the reference, electrical heating and hydrogen/air heating. Besides the energy balances, primary energy consumption and resulting CO2-emissions are calculated. Furthermore, possible developments until 2050 are analysed. The results show that both alternative cases have a high potential to decrease CO2-emissions which strongly depend on the development of the energy mix and, therefore, the future expansion of renewable energy sources.

The interest in ultra-pure metals is steadily growing due to the increasing demand for these materials in modern technology. To be able to meet the increasing demand in the future, it is necessary to implement more efficient and productive processes. As a fractional crystallization method in this application area, the cooled finger method exhibits higher productivity and lower energy requirements when compared to industry well-established methods like zone melting. In this study, the mechanisms and relevant phenomena crucial for a successful implementation of a cooled finger process were investigated using a multidisciplinary approach. With carefully selected process parameters, we present here an experimental setup with a purification potential of approximately 80 pct. Additional micro- and macro-scale simulations demonstrate that the process is sensitive to parameters such as rotation rate, cooling rate, and temperature gradient within the melt, which explains the difficulty in optimizing this process in practice. An analysis and description of various phenomena that characterize the behavior of the cooled finger process are presented within this multi-scale approach. As a result, these approaches can also be transferred to the description of processes for other metals, opening application areas outside of the purification of aluminum.

Gas impingement jets are widely applied in industrial cooling processes. In continuous heat treatment lines of steel, aluminium and copper strips, impingement jet nozzle systems are utilised to achieve rapid cooling or heating. The heat transfer depends on the flow but also on the geometric parameters such as nozzle to strip distance and the nozzle shape. The key challenge while designing cooling sections is to determine the performance of those nozzle systems or their Nusselt number respectively.Jet cooling sections are challenging to model with computational fluid dynamics or in an experimental set up. Yet, RANS-turbulence models are a cost-effective way to predict Nusselt numbers. In this work the capability of the ANSYS generalized k-omega (GEKO) two-equation turbulence model to determine the local and integral Nusselt number of an impinging air jet is evaluated. The results are contrasted to experimental investigations.

The reduction of CO2 emissions in hard-to-abate industries is described in several proposals on the European and National levels. In order to meet the defined goals, the utilization of sustainable, non-fossil fuels for process heat generation in industrial furnaces needs to be intensified. The focus mainly lies on hydrogen (H2) and its derivates. Furthermore, biofuels, e.g., dimethyl ether (DME), are considered. Besides possible changes in the process itself when substituting natural gas (NG) with alternative fuels, the emission of nitrogen oxides (NOx) is a major topic of interest. In current European standards and regulations, the NOx emissions are specified in mg per m(3) of dry off-gas and refer to a reference oxygen concentration. Within this study, this limit specification is investigated for its suitability for the use of various fuel-oxidizer combinations in industrial combustion applications. Natural gas is used as a reference, while hydrogen and DME are considered sustainable alternatives. Air and pure oxygen (O2) are considered oxidizers. It is shown that the current specification, which is built on the use of fossil fuels, leads to non-comparable values for alternative fuels. Therefore, alternative NOx limit definitions are discussed in detail. The most suitable alternative was found to be mg per kWh. This limit specification is finally being investigated for its compliance with current regulations on various aspects of Continuous Emission Monitoring Systems.

Electroslag remelting (ESR) is a well‐known process widely used in the production chain of high‐quality alloys. Herein, numerical investigations of the implementation of a rotating‐electrode (low rotation speed) modification to the flow behavior inside the slag area of a lab‐sized ESR process are shown. Therefore, a coupled simulation approach between a commercial CFD (FVM) and electromagnetics (FEM) software is used. Comparative results for the simulated droplet formation between a stationary and a rotating electrode are shown. The results show a strong effect on the local shaping of the liquid metal film below the electrode and consequently a radial shift of the droplet incidence into the metal pool under electrode rotation. There are no observable positive effects on mean metal film thickness, droplet trajectory, or droplet size, which are suspected as possible explanations for observed material improvements in small‐scale rotating electrode ESR processes. In addition, an overview of current bottlenecks, restrictions, and possible strategies for improved ESR process simulations is given.

Numerical simulation is a vital tool for the continuous improvement of all steelmaking processes. In previous works, the authors have dedicated different approaches to optimize modeling of fluid flow in the ladle. This includes a sensitivity analysis of validation techniques, assembling of a validation database, and a model optimization on the water model scale. Herein, the final part of this project is presented at which the optimized model is scaled to the real process. Associated problems are identified and solutions are proposed. To achieve a tradeoff between large eddy simulation (LES) grid requirements and current computational capacities, a hybrid meshing strategy is optimized. To overcome the intrinsic drawbacks of the Euler–Lagrange model, a mapping approach for phase decoupling is implemented and made publicly available on GitHub. The upscaling of semi‐empiric submodels is discussed. Good agreement with experimental mixing time measurements is found, indicating the success of upscaling. A first time‐averaged flow field of a full‐scale 185 t ladle with the LES model is presented. Due to the extreme computing requirements, these results may currently be used as a benchmark, but the trend toward increasing computing capacities may make LES models state of the art in the future.

Calculating the radiation exchange between arbitrary surfaces using view factors is a common practice in the fields of industrial furnaces, climate modelling, solar power, thermal building design and so on. Here, a new model was developed to calculate the view factors of arbitrary two-dimensional geometries. This model is based on Hottel’s crossed strings method paired with an optimized algorithm to efficiently detect shadowing effects between the surfaces. The model’s accuracy and discretization dependency was tested against an analytical view factor calculation method using an example of two concentric, infinitely extended cylinders. The model can be applied to arbitrary linear discretized two-dimensional geometries. In particular, it guarantees the highest possible accuracy within the chosen discretization. The need for developing customized view factor equations of different two-dimensional geometries is therefore no longer necessary, since the convergence of the model with decreasing mesh size on analytical results can be demonstrated. Additionally, the performance and validity of the newly developed shadowing algorithm was tested against a common brute force approach and significant speedups were achieved. Furthermore, the additional application of the net radiation method for the calculation of the heat fluxes exchanged by radiation within those geometries is shown.

Gas bubbles are of major importance in most metallurgical processes. They promote chemical reactions, homogenize the melt, or float inclusions. Thus, their dynamics are of crucial interest for the optimization of metallurgical processes. In this work, the state of knowledge of bubble dynamics at the bubble scale in liquid metals is reviewed. Measurement methods, with emphasis on liquid metals, are presented, and difficulties and shortcomings are analyzed. The bubble formation mechanism at nozzles and purging plugs is discussed. The uncertainty regarding the prediction of the bubble size distribution in real processes is demonstrated using the example of the steel casting ladle. Finally, the state of knowledge on bubble deformation and interfacial forces is summarized and the scalability of existing correlations to liquid metals is critically discussed. It is shown that the dynamics of bubbles, especially in liquid metals, are far from understood. While the drag force can be predicted reasonably well, there are large uncertainties regarding the bubble size distribution, deformation, and lift force. In particular, the influence of contaminants, which cannot yet be quantified in real processes, complicates the discussion and the comparability of experimental measurements. Further open questions are discussed and possible solutions are proposed.

The aim of a massive reduction of CO2-emissions results in a move away from fossil fuels. In the hot strip production of steel, almost exclusively gas-fired furnaces are currently used due to the lower energy costs. On the contrary, it is imperative to convert existing fossil heated processes to CO2-free (green) technologies in the context of the energy-transition. Obvious alternatives are electrical heating or hydrogen combustion, both strongly dependent on the specific electricity generation mix that determines the CO2-emissions. In this case study, different process heat generation options for continuous reheating furnaces in steel hot rolling mills are discussed by a quantitative approach. A state-of-the-art reheating furnace fired with natural gas is used as reference case, while electrical heating, hydrogen-air heating and hydrogen-oxygen heating are the alternatives investigated. The energy balances, the primary energy consumption and the resulting CO2-emissions are compared for the three countries of France, Poland and Germany with regard to the country-specific electricity generation mix. Additionally, the possible development until 2050 is analysed. The results show the high impact of continuous reheating furnaces in steel hot rolling mills on the total CO2-emissions of downstream steel processing. Furthermore, the massive increase in electrical energy consumption of the whole steel production process is highlighted. Each investigated alternative shows a significant potential to save CO2-emissions, depending on the country specific electricity generation mix and the future expansion of renewable energy sources. An increase in H2-production efficiency will both lead to a lower primary energy consumption and lower CO2-emissions for reheating furnaces.

Self-recuperative burners are a common solution for efficient combustion systems in industrial furnaces. Due to the geometric complexity of the recuperators, a detailed CFD simulation is computationally expensive and not feasible for simulation models of burner-integrated systems such as radiant tubes. Especially in the FSI studies of radiant tubes, the temperature of the radiant tube surrounding the burner is decisive for the final results. The exclusion of the recuperator from the simulation models introduces significant uncertainties in the simulations results. The presented paper describes an innovative, efficient approach to model a fin-type recuperator in which the recuperator is geometrically reduced. The resulting acceleration of the numerical simulation makes a fully dynamic modelling of the recuperator in a radiant tube simulation possible. Specifically designed source terms are used to model pressure loss and heat transfer inside the recuperator to match results obtained with a detailed simulation model. The results show deviations in total heat transfer of less than 1.3% with a 98.5% reduction of numerical mesh size. The computational savings enable comprehensive modelling of air preheat for radiant tube simulations and accurately replicate flow and temperature profiles in the recuperator.