Fraunhofer ISC, Center for High Temperature Materials and Design HTL, Bayreuth
In industrial heat treatment processes, large-volume furnaces are usually used to maximise the throughput of the material to be heated. The heat is transferred from the heaters to the material to be heated by convection and thermal radiation in the furnace atmosphere, as well as by thermal conduction in the kiln furniture used and in the material to be heated itself. Less frequently, volumetric or radiant heating processes are used in which the heat is transferred directly to the material to be heated by electromagnetic radiation (microwaves, induction, infrared light). During cooling, the heat flow is reversed. In any case, the heat distribution in the useful volume plays a major role in the quality of the heat treatment. The cost-efficiency of the heating processes and often also requirements from the material to be heated require the shortest possible temperature cycles, i.e. steep heating and cooling ramps. This results in undesirable temperature gradients and thermal stresses in the useful volume. In this conflict of objectives, careful thermal management is required.
Thermal management in industrial furnaces is optimised at Fraunhofer-Center HTL using finite element (FE) methods and related techniques, e.g. computational fluid dynamics (CFD) simulations. The FE methods apply to all furnace types, e.g. bogie hearth furnaces, roller kilns, bonnet furnaces. They are based on a simplified representation of the complex furnace geometry. Often multi-scale models are used to reduce the computational effort. In this case, parts of the useful volume, e.g. individual firing capsules with the material to be heated inside, are simulated first. Their thermal properties are homogenised and used in models on the next larger scale. By using FE methods, all relevant heat transport processes (thermal radiation, thermal conduction and convection) can be simulated at the HTL.
The quality of the FE simulation depends on the accuracy of the input data. The existing material data for many relevant materials - especially in the high-temperature range - are not sufficient for this. The material data required for the FE simulation are therefore determined at the HTL using specially developed ThermoOptic Measurement (TOM) methods up to temperatures > 2000°C. For example, the thermal diffusivity and emissivity of refractory materials can be measured using special laser-flash and IR methods. The mobile measuring stand for industrial furnaces developed at HTL can also be used to generate input data for FE simulation. For example, gas flows and temperature distributions in the industrial furnace can be measured directly on site. The measurements with the mobile measuring stand can also be used to validate the simulation calculations.
The calculation of the temperature distribution in the material to be heated is used at the HTL to calculate the result of the heat treatment. Robust formal kinetic models are used for this purpose [1]. The formal kinetic models use in-situ measurement data, which are measured directly on the material to be heated during the heat treatment. The industrial heat treatment process is simulated in TOM facilities at the HTL with the relevant furnace atmospheres. The parameterised measurement data are used in special FE models to couple reaction kinetics, temperature calculation and mechanical effects. For example, the process parameters for debinding and sintering of materials produced via the powder route can be optimised in this way.
Another important application area for thermal management in industrial furnaces is the mechanical stresses that occur as a result of temperature gradients or rapid temperature changes in the furnace components, kiln furniture or the components to be treated. For example, the thermal shock occurring during rapid cooling can drastically reduce the service life of kiln furniture, or the temperature differences present in the kiln insulation lead to cracks and heat leaks. At the HTL, the mechanical stresses occurring in the industrial furnace are calculated in coupled thermal-mechanical FE models. To evaluate the FE results, measurements on TOM systems can be used to generate high-temperature gradients or rapid temperature changes up to high temperatures. If required, the FE simulations can be combined with analyses of the service life of refractory materials.
With the help of heat management, it is possible to optimise charge structures concerning low-temperature gradients and the long service life of the kiln furniture. A targeted selection of suitable refractory materials is also possible due to heat management in industrial kilns. The heat capacity of the kiln furniture can be minimised to increase the energy efficiency of the heat treatment. In addition, process parameters such as the temperature cycle or the gas flow can be adjusted in such a way that the conflict of objectives between an economic operation of the kiln plant and the best possible quality of the product in terms of the specific customer requirement is resolved.
One advantage of the thermal management methods used at the HTL is the high degree of abstraction of the geometric FE models, coupled with the very precise experimental data. Usually, FE models for industrial furnaces require a very high computational effort. Therefore, robust models were developed at the HTL, which manage with comparatively short computation times. The precise input data of the FE simulation ensures the high accuracy required for process optimisation. With the help of thermal management, it is also possible to work on special issues within a furnace without having to simulate the entire furnace, which allows a focus on, particularly critical problem areas.
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