No abstract

Zur Auslegung von Bauteilen unter wechselnden Beanspruchungen werden häufig Lebensdaueranteilregeln verwendet, um eine Aussage über die erwartete Verformung zu treffen. Im Falle von Ofenkomponenten hat sich gezeigt, dass die Kriechverformung unter wechselnder Temperaturbeanspruchung bei niedrigen, konstanten mechanischen Lasten mit der Lebensdaueranteilregel stark unterschätzt wird. Temperaturwechsel scheinen einen sehr viel schädlicheren Einfluss zu haben, als bisher angenommen. Um die höhere Kriechverformung durch Temperaturwechselbeanspruchungen berechnen zu können, wurden verschiedene isotherme und anisotherme Kriechversuche durchgeführt und daraus ein Kriechmodell entwickelt. Gekoppelte Fluid-Struktur-Interaktions (FSI) Berechnungen auf Basis dieses Kriechmodells an einem typischen Ofenelement, einem metallischen P-Strahlheizrohr, wurden mit weiteren Versuchen validiert. Schlagwörter Kriechen, Temperaturwechsel, Strahlheizrohr, Fluid-Struktur-Interaktion

The influence of surrounding (or proximal) radiation on radiant tubes inside a continuous hot-dip galvanizing line was investigated. The furnace chamber, the strip and the neighbouring tubes were considered as the surroundings. A coupled heat transfer model was developed and subsequently validated against experimental measurements. This model was used to calculate the radiation exchange between the tube and its surroundings, and to give the new temperature distribution on the tubes as its output. This result served as an input to an already validated FEM model, which was used to assess the creep behaviour and the corresponding stresses on the tube. Basis of the investigated setups were Alloy 602 p-type tubes operating under burner on/off firing. The results show an increase in creep deformation of the tube when the surroundings were taken into account. Highest creep deformations were observed for setups with a strip, even though these cases showed the lowest maximum tube temperatures. Furthermore, an opposing effect between creep deformation and stresses acting on the tube exists. This is supported by the fact that no definite pattern relating the creep and stresses of a tube was found. Local tube temperature gradients and transient cyclic loading due to burner on/off firing were observed to have a significant influence on the tube's service life.

The effects of surrounding radiation—emanating from radiation exchange with neighboring partners in indirect-fired vertical strip annealing furnaces, such as the other radiant tubes, the passing strip, and the enclosing furnace chamber—on the radiant tube lifetime were studied. In-house developed and validated numerical models were used to calculate the thermomechanical behavior, especially creep deformations and the corresponding stresses as lifetime indicating parameters. Different setups of recirculating P-type radiant tubes were investigated, including a reference case of an isolated tube. The investigations could be broadly classified into the study of the effects of different tube arrangements, burner operations (synchronous/asynchronous on/off firing), and changes of strip parameters (width/temperature). Results showed higher creep deformation of the central radiant tube in the setup with three tubes arranged horizontally in a row compared to three tubes stacked in a vertical column, even though the respective characteristic temperature values in a firing cycle were similar. Furthermore, the cases with asynchronous burner firing resulted in lower creep rates than other cases, where the burners were operating in synchronous on/off firing modes. In addition, the change of strip width had a higher impact on radiant tube lifetime compared to locally changing strip temperatures across the furnace. Alternating temperatures, caused by burner operation or process changes, such as change of strip’s speed or cross-section, and local temperature gradients were observed to be the main factors influencing the tube’s service life.

Shell and Tube heat exchanger is widely used as heat-transfer equipment in various process industries. The paper attempts to understand the flow, heat transfer characteristics and the effectiveness of the divided-flow type heat exchanger. The study is based on the CFD based simulation methods and the experimental methods. The study provides the flow and heat-transfer characteristics of the divided-flow type heat exchanger by using validated Computational Fluid Dynamics (CFD) model. Results are obtained for the temperature, velocity and turbulence intensity distributions in the flow domain. The results from the simulation and experimental study are used in determining the effectiveness (ξ) of the heat exchanger using Logarithmic Mean Temperature Difference (LMTD) and Number of Transfer Units (NTU). The simulation and experimental performance are found to be in good agreement highlighting the minimization of losses in the experimental setup. This confirms the accuracy of the flow-based heat and turbulence distributions. The effectiveness value shows that the heat exchanger can be used for compact applications