The Study Of The Flow Field In A Cold Model Of The Kiln Rear Predecomposition System

Dry-process precalciner kiln technology is advanced in conctret manufacture at present. The performance of cyclone preheater and precalciner in the precalciner kiln system has great effect on the energy consumption and the discharge of pollutants of the system. The multi-phase flow in the system is complex as well as the reactions of fuel combustion and raw materials decomposing, which are influenced by each other, are taken place in suspension of the flow. For these reasons, it is difficult to research and develop the cyclone preheater and precalciner system. In the traditional way, the improvement of system design was depended on the data from experimentation and empirical formula. But the experimentation method and test which may be limited by different conditions can not sufficiently reflect the pyrology and technical process in the precalciner system as well as the inter-coupling and interaction among system's structure parameters, technical parameters and physical parameters.The numerical simulation based on the computational fluid dynamics (CFD) is applied widely in the engineering field due to its accurate numerical prediction for the flow, heat exchange, chemistry reaction and multiphase flow. In the simulation, the spatio-temporal change of parameters in the whole system can be computerized. In this way, it can reflect the whole internal flow field structure without any "blind spot" which can not be tested by the experiment because of the limits of test methods. So the numerical simulation on the turbulence flow is adopted to investigate the flow character in the preheater and precalciner to optimize system structure and improve system performance. This method can save the R&D cost and time so that it is significant in the engineering application.This dissertation integrates the cold model test with numerical simulation of 2500t/d five-stage preheater and precalciner system to establish the numerical model and relative boundary conditions. The simulation result is accordant to the cold modle test.The research conclusions are:(1) The "dynamic similarity" rule is for the first time to be applied successfully in the design, manufacturing, test and data disposal for the 2500t/d five-stage cyclone preheater and precalciner system cold model test. Improvement suggestions for the system are proposed in this paper.(2) According to the test data from cold model test, the flow in the preheater andprecalciner is belong to incompressible turbulent flow. In terms of the structure of preheater and precalciner, the mix mesh is adopted to establish the computational model for the flow in the whole system. In this method, the boundary condition and relative solution algorithm of incompressible turbulent flow is determined.(3) By analysis, we get the conclusion that "spiral up flow in the boundary layer and spiral down flow in the center" is the typical flow field structure of cyclone. The flow field structure of outlet pipe of cyclone is "high-speed spiral up flow in the boundary layer and low-speed up small local eddy in the center". The high-speed spiral up flow in the outlet pipe is the main reason for pressure-drop loss. The spiral up flow in the center, especially the flow in the cone part of cyclone is the main cause to affect the preheater efficiency.(4) The pressure-drop reduction effect of pressure-drop board in the preheater is limited by the following two aspects: the increase pressure drop (positive value) caused by the high-speed spiral flow overcoming the pressure-drop reduction board; the decrease pressure drop (negative value) caused by the commutated flow passing through the residual part of outlet pipe of cyclone as well as the other cyclones along the flow direction. The sum of these two values is the real pressure-drop reduction effect.(5) It is the first time that the multiphase flow theory is applied in flow field of precalciner. According to the structure and working parameter of 2500t/d RSP precalciner, the computerized parameters of flow field distribution and particle movement not only reflect the internal flow field of precalciner but also help to improve the precalciner performance.(6) According to the numerical simulation of multi-phase flow in the precalciner, there are several problems in this precalciner: the direction of tertiary air is not appropriate; the velocity of secondary air is too high; the fuel doesn't mix well with the materials in the combustion chamber; the residual time in the combustion chamber is fairly short and the detached high-speed layer and low-speed layer in the ascending flue made against material decomposing. Based on these problems, this dissertation puts forwards corresponding measures to optimize the precalciner structure.