Applications Of Computational Fluid Dynamics In Designing Heat Treatment Furnaces

Gas flow inside the heat treatment furnace influences greatly on the uniformity of heating and cooling of samples, as well as others processing characteristics such as uniformity of nitriding or carburization layers in chemical heat treatment. To study gas flowing numerically is becoming more and more important and convenient as the computational fluid dynamics is developing rapidly.In this work, we construct a 3-D multiple reference frame turbulence numerical model coupling flow velocity fields ,temperature fields and phase transformation fields to simulate the gas flowing in heat treatment furnaces. This model is used to consider detailed implementation of the fan as a driving source and precedes traditional models which treat the fan only as a simplified pressure boundary. The model also featured of Reynolds-averaged Navier-Stokes (RANS) equations and k-εturbulence equations. As natural results of coupling with flow velocity fields and temperature fields, our model is capable to ignore the coefficient of heat transfer which needs sophisticated measurement and hypothetical prerequisites and easily deduces unexpected errors. The model treats the fluids and solids as a whole computing domain and keeps balance of heating flux through the interface between fluids and solids, then calculates the coefficient of heat transfer as output. In order to verify that coupling model, the flow velocity fields inside a tempering furnace were measured by a hot bulb anemoscope and calculated with the model. Further verifications included experimental measurement and simulating calculation of heat transfer coefficient of a single tube flitting by air, and heating and cooling temperature characteristics of large scale samples inside a gas quenching furnace. Good accordance between simulation and experimental results reveal that the 3-D multiple reference frame turbulence coupling model is suitable for simulation of gas flow inside heat treatment furnaces. The model was applied in virtual design of a bell shape nitriding furnace. In order to find rational configuration to generate uniform fluid velocity fields, we simulated several schemes and finally chose one with elliptical coverhead and bottomhead, a fan featured in six long blades and six short blades, a dish-like guiding tube consisted with three parts, and a radial workpiece tray. A guiding scheme with three curved and one cross distributing boards was adopted among 11 different candidates to guarantee gas flowing uniformity in the upper zone of the furnace. Furthermore, simulations for flow velocity fields of whole furnace coupling with temperature fields were performed, and yielded a innovative scheme using hollow flange partially filled with heat insulator, which can dramatically decrease the heat lose from the furnace. The temperature uniformity of our new design was also considered. Experimental results shown that uniformity with empty load fulfilled the national standard. Besides, several heating schemes were studied for fully loaded and one best scheme was adopted which set 650℃as working temperature of heating sources and eliminated one half of the heating resistances on the top of the furnace comparing with the original design. Comparison studying between different loading of workpieces deduced several optimized approaches to improve the uniformity of samples heating, such as avoiding direct contact with workpieces and the tray, adding shield above the workpieces to decrease the radiation and convection in the upper zone of the furnace, etc. An experimental scheme about amounting of thermoelectric couple through the furnace bottom to vicinity of the workpiece tray was also given.In order to further study the application of the 3-D multi reference coordinates turbulence coupling model, other specific cases about the flow velocity and temperature fields in typical heat treatment furnaces were also conducted. The first case was about the distribution of inner gas of a mesh belt furnace. Simulation study revealed that the amount of gas consumption could reduce to 2/3 of the original one by adding an extra shielding board in the front of the chamber. The second case was studying of the flow velocity fields inside a tempering furnace. We successfully optimized the structure to achieve better uniformity of gas flowing by adding guiding bricks and optimizing the fan size. The third case was about a typical high pressure gas quenching vacuum furnace currently used in China and some incorrect designing concepts. The simulation concerning about two actual loading processing, i.e., filled with small size and large amount workpieces and loaded with large size and limited amount workpieces, revealed that inhomogeneous cooling was an inherent property of that kind of furnace. And that property was also underlying reason for large distortion of the processed workpieces by that gas quenching furnace.