Eckehard Specht

Enhancement and Local Regulation of Metal Quenching Using Atomized Sprays

In atomized spray quenching, the spraying water is atomized into fine droplets of the size of about 20 μm by compressed air and sprayed onto a hot surface. Only single droplets touch the surface, become deformed, and transfer heat. The drops partially evaporate and then move away from the superposed airflow. Thus, the vapor film is avoided as it is the case for other quenching techniques. It is demonstrated that the unintended collapses of the vapor film (Leidenfrost problem) at edges, corners, roughness peaks, etc., is eliminated by this technique. In this way, mass lumped regions of work pieces can be cooled more intensively than the edges. Consequently, a uniform temperature distribution with reduced thermal stresses can be obtained. The heat transfer was measured by infrared thermography. One side of the metallic sheet of 3 mm thickness was quenched by atomized spray and the surface temperature was measured on other side by an IR camera. The heat transfer coefficient is mainly determined by the impingement density. Heat transfer coefficient of 10,000 W/m2/K can be achieved within film boiling regime. This value is about three times higher than that of conventional spray quenching.

Simulation of the Distortion of Long Steel Profiles During Cooling

Quenching processes are applied to long steel profiles after the rolling process to produce profiles with reliable service properties. The typical problem is the distortion that is occurring upon residual stresses. Profile distortion should be eliminated as it leads to rework expenditure and creates problems associated with bearing and bundling. Moreover, the residual stresses should be controlled to prevent the crack initiation. The modeling of the cooling process is difficult due to the complicated couplings among different physical and mechanical processes 1. The main reason for the distortion is the inhomogeneity of the temperature profile during the cooling process which results in residual stresses, varying phase fractions, etc. 2. The cooling distortion of railway profiles has already been extensively investigated by experiments. An overview of the works can be found in the literature 3, where the experimental results for the residual stresses and deformations after quenching are presented. The distortion and residual stresses in carburized thin strips are investigated by Prantil et al. 4. The thermoplasticity and metallurgical transformations establish the fundamental part of the problem. The distortion is due to transformation-induced stresses and thermal stresses. The phase transitions, temperature, and stresses must be accurately simulated to determine the distortion. If the temperature and cooling conditions are constant in the axial direction of the profile, all the other fields are also constant. Consequently, a 2D mathematical model with fewer degrees of freedom is enough for the simulations. This reduction of the dimension simplifies not only the visualization of the fields, but also the determination of the optimal cooling conditions. This work indicates that the distortion can be significantly reduced by an intensified cooling of the mass lumped parts of the cross section. The atomized spray quenching technique 5,6, where the spraying water is atomized into fine droplets by means of compressed air, is suitable to intensify the cooling of the mass lumped regions, since the vapor film, which occurs in other quenching techniques, is avoided. In this way the mass lumped regions of the workpieces can be cooled more intensively than the edges. Consequently, a uniform temperature distribution on the surface and a reduced stress distribution inside the body with reduced distortion can be obtained.

Limit value of the Nusselt number for particles of different shape

Abstract The limit value of the Nusselt number for pure heat conduction was numerically determined for different particle shapes. In the case of nonspherical particles, substantial errors can result in the calculation of the heating time when the Nusselt number of the sphere Nu =2 is applied and the sieve diameter is used as the characteristic dimension. That a uniform description of the Nusselt number is possible for all considered particle shapes is demonstrated. However, this Nusselt number, determined using the sieve diameter, is then not constant, but depends exponentially on the ratio of the Sauter diameter to the sieve diameter.

Numerical analysis of multiple jets discharging into a confined cylindrical crossflow

Abstract A row of jets discharging normally into a confined cylindrical crossflow is numerically investigated using the control-volume-based finite difference method. Interest is focused on determining the relationship between the temperature trajectory and the upstream flow and geometric variables. Parameter variations studied include nozzle diameter, number of nozzles, duct radius, jet and mainstream volume-flow, temperature ratio, and dynamic pressure ratio. The dynamic pressure ratio, the number of nozzles, and nozzle spacing are found to be significant variables. A logarithmic function describing the relationship between penetration depth and dynamic pressure divided by the square of the number of nozzles is derived by fitting the data of the computation results. The values for penetration depth and nozzle spacing are described for optimum mixing. A suggested design procedure is presented, which can be used as a first approach in configuration design.

Moisture Diffusion Coefficients for Modeling the First and Second Drying Sections of Green Bricks

A model has been developed that describes the dependence of the moisture diffusion coefficient on the water fraction. Until the end of shrinkage has been achieved, the moisture diffusion coefficient is proportional to the second power of the water fraction. Due to shrinkage, the relevant capillary spaces available for water transport become smaller. Consequently, the moisture diffusion coefficient decreases continually. After the end of shrinkage, the flow resistance to the water moving toward the surface increases sharply due to penetrating air. This leads to a steep drop of the moisture diffusion coefficient by several powers of ten. Measurements were carried out with specimens of defined geometry to determine the moisture diffusion coefficient. On the basis of a specified limiting value, the model is capable of calculating the moisture diffusion for all initially specified raw materials moistures. The moisture can also be determined if the degree of drying shrinkage is known. Using the determined moisture diffusion coefficient, the first and the second drying section can be located. Drying tests were carried out in a laboratory dryer and the experimental results obtained were compared to the simulation results. The simulation results are in good agreement with the experimental results.

Optimization of Vapor Compression for Cost Savings in Drying Processes

Energy saving from vapor compression entails additional costs for the vapor compressors and larger heating surfaces of the drier. Thus, they are offset to a certain degree. An analytically solvable mathematical model is presented for determining the maximum cost savings with the optimum pressure ratio of the compression. According to this, the maximum cost savings depends only on five parameters, namely, the specific cost ratios of drier to fuel and compressors to fuel, the price ratio of power and fuel, the difference between the moisture contents of the inlet and outlet of the drier and the pressure level inside the drier. Above dy = 1 kg H2O /kg DS (DS-dry substance) the specific cost ratio of drier to fuel proves to be dominant and thus decisively determines the optimum pressure ratio. Hence, one can explain why, e.g., in the case of brown coal drying higher pressure ratios are required than in the case of sludge drying and here again higher ones than in the case of evaporization processes for liquids.

TEMPERATURE HOMOGENIZATION OF REACTIVE AND NON-REACTIVE FLOWS AFTER RADIAL JET INJECTIONS IN CONFINED CROSS-FLOW

Abstract Multiple jets injected radially into a reactive and a non-reactive cross-flow in a cylindrical chamber has been studied numerically using FLUENT CFD code. The chamber diameter was varied from 0.3 m to 3 m and the number of nozzles from 8 to 32. The maximum temperature difference over the chamber’s cross-sectional area was defined as the parameter to evaluate the mixing quality. The optimum mixing conditions for both reactive and nonreactive flows were obtained at a normalized momentum flux ratio (J/n2) of 0.3 with a penetration depth (h/R) of 0.6. This condition is valid for all number of nozzles. However, increasing the number of nozzles will also improve the mixing quality. It has been observed that for higher number of nozzles, the optimum mixing quality is nearly independent of J/n2 in the range greater than 0.3. This is important for the processes with ever-changing conditions which is typical in chemical processing industry.

An analytical model for free convection film boiling on immersed solids

Abstract An analytical model has been developed for the film boiling of immersed slabs with free convection. The heat flows calculated with the model agree well with the numerically calculated and experimentally determined values. For liquid temperatures significantly below the saturation, the Nusselt number is dependent on the Galilei number, the Prandtl number and on the ratio of the mean heat conductivities and the temperature differences in the vapour and the liquid, respectively. The Nusselt function also applies to other solid geometries, provided the flow length is inserted as a characteristic dimension. Under consideration of the limiting value T∞ = Ts, the analytical model yields the known Nusselt function for saturated liquids. Furthermore, it is possible to calculate the vapour film thicknesses using this model.

Ähnlichkeitskennzahlen zur Beschreibung des Einflusses der Temperaturabhängigkeit von Stoffwerten beim Wärmeübergang an überströmten Körpern

Dimensionless groups were derived to describe the influence of temperature-dependent properties with heat transfer on affluxed bodies generally. The isothermal Nusselt-function was extended by terms of these numbers to present the results quantitatively. Furthermore the isothermal Nusselt-function was modified to enlarge the range of validity toPr< 0.6.ZusammenfassungEs wurden dimensionslose Kennzahlen hergeleitet, mit denen der Einfluß der Temperaturabhängigkeit von Stoffwerten beim Wärmeübergang an überströmten Körpern allgemeingültig beschrieben werden kann. Zur quantitativen Darstellung wurde die isotherme. Nusseltfunktion mit Termen dieser Kennzahlen erweitert. Die isotherme Nusseltfunktion wurde ferner modifiziert und damit ihr Gültigkeitsbereich aufPr < 0,6 ausgedehnt.

Optimum Strategies to Reduce Residual Stresses and Distortion during the Metal Quenching Process

Quenching is a complex thermo-mechano-metallurgical problem. Unexplored or without an optimized cooling strategy, a quenching process can end up with high residual stresses and distortion. This paper presents the mathematical formulation of the physics behind the quenching process, the numerical technique and optimization of the cooling strategies for the selected geometries. The finite element method (FEM) is used to solve the coupled partial differential equations in the framework of an isothermal-staggered approach. The solid-solid phase transformations are modeled using a linear iso-kinetic law with Schiel’s additivity rule and Koistinen-Marburger (KM) law. The thermoplastic material model is formulated on the basis of J2-plasticity theory with a temperature and phase fraction-dependent yield limit together with the appropriate mixture rule. The coupling effects such as phase transformation enthalpy, transformation-induced plasticity and dissipation are considered. The local heat transfer coefficient (HTC) during the quenching process plays a crucial role for the evolution of the distortion and residual stresses. It is demonstrated that with an enhanced quenching at the mass lumped regions, the distortion can be reduced. It is always possible to find an HTC profile which eliminates the distortion completely on the expense of an increased residual stress state. Therefore, an optimum quenching strategy has to be found to reduce the distortion and the residual stresses simultaneously. It is shown that with an enhanced quenching at the mass lumped regions and with a reduced quenching at the edges and corners, stresses and distortion can be minimized simultaneously. Examples are given for different kinds of metals and geometries such as long profiles (L and T) and disk with a hole.