Modeling of the process of non-stationary heat exchange in the lining of a rotating unit

Abstract

Rotating heat units, or rotary kilns, are widely used in many industries where they are used for heat treatment of bulk materials. The efficiency of use largely depends on the reliable operation of the main structural elements. This is especially true of the lining of the furnace, the reliability of which significantly affects the performance, safety and long-term and reliable operation of the furnace unit. Therefore, the work devoted to the improvement of the theoretical model of heat exchange in a rotary kiln is extremely relevant as it contributes to the intensification of its work and makes it possible to implement more intensive modes of operation of the heat unit. The calculation of the temperature regime in the lining of the rotating furnace and filled with the processed material is performed. The differential equation for no stationary thermal conductivity is used. Numerical calculation is performed iteratively using the finite difference method according to an explicit scheme. AutoLisp programs in the AutoCAD environment are used to visualize and display graphical information. The movement of the material is modelled by changing the value of the heat transfer coefficient to the inner surface of the lining in the place where it is located. This takes into account the movement of the material along the inner surface of the lining. The change in position depends on the speed of rotation and the amount of material. The shift of the material layer by the angle Δφ occurs after determining by iteration the total time period for which the indicated displacement occurs. At the beginning of the rotation of the furnace, the temperature of the lining area free of material is evenly heated to the maximum value. After contact of the lining with the material begins the redistribution of temperatures in the surface layers. The inner surface of the lining of the furnace is heated by a gas stream, and when it gets under the layer of material - is cooled to 1246 °C, compared with the initial 1465 °C. At the time of exit, the temperature of the lining rises quite rapidly, after which the temperature rise slows down. After one revolution, the lining reaches the initial temperature level. This phenomenon explains the essence of the process of heat recovery by lining. In general, the amplitude of temperature fluctuations on the outer surface of the lining is 218.7 °C. At the same time intensive change of temperatures promotes emergence of the largest gradients and accordingly thermal stresses especially in a zone where there is a processed material. The depth of penetration of the variable temperature into the lining is defined as the ratio at which the amplitude of temperature fluctuations is 0.01 from the amplitude of oscillations on the surface. It is possible to determine that in this case the temperature change occurs at a depth of up to 70 mm, with a total lining thickness of 230 mm. The layers of the lining, which are deeper, have significantly smaller temperature fluctuations. Determining the intensity of heat flow allows to characterize the influence of technological parameters of the unit on the temperature and thermal regime of the lining and the rotary kiln as a whole. Analysis of the data of temperature fields and heat flux shows that the output to the quasi-stationary heat regime occurs after 4-5 revolutions of the furnace. The result of this research is the development of a mathematical model, algorithms and software for calculating and studying the process of non-stationary heat transfer in the lining of rotating units. Numerical calculations are performed and the evolution and distribution of temperatures and heat fluxes in the lining are determined. The depth of penetration and the amplitude of alternating temperature fluctuations that lead to the destruction of the surface layer of the lining refractorines due to thermal stresses are determined. The results of the calculation are also given in the electronic appendix to the article in the form of video files.