The Analogue Research Of Precursor Flow Field In The CVD Reactor For Aluminide Coatings

For keeping the mechanical properties and structural strength of the gas turbine blades in high temperature,CVD(Chemical Vapor Deposition)method was adopted to prepare aluminide coatings on the surface of the inner cavity gas flow channel of the blades for high temperature protection in this paper.Meanwhile,for improving the deposition quality of the aluminide coatings,the deposition technique and the internal structure of the CVD equipment ought to be optimized.But it is not only difficult to measure the changes of the parameters like precursor flow distribution,temperature,and pressure by gauges,but also too costly to improve the technique by repeating experiments.Therefore,Computational Fluid Dynamics(CFD)was chosen to help optimize the deposition technique and the internal structure of the CVD equipment in this paper.In this thesis the CFD software was adopted to build a simulation model for investigating the precursor flow distribution in the CVD reactor.The CFD research object was selected as the CVD reactor.The grids of the 3D reactor model were divided and the quality of the grids was evaluated while the boundary conditions(gas flow distribution,components of precursor,inlet&outlet&wall conditions)were determined according to the former experiments data.In this thesis,the simulation results were used to optimize the CVD deposition technique and the structure of the CVD reactor.The mechanism of how the different internal structures of the reactor,the different dimension scales of the reactor,and the different average precursor flow rate influence the flow distribution of the CVD reactor was studied respectively.The first conclusion was that B-type precursor distribution plate was selected as the internal structure optimized plan of the reactor because it could help the precursor flow into the reactor uniformly.The second conclusion was that the scale,was between 1:2 and 1:2.5,between the bottom diameter of the reactor and the distance of the head cover edge to the bottom of it was adopted as the external structure optimized plan of the reactor because the effective depositing zone was the largest in this scale interval.The third conclusion was that the initial average precursor flow rate V_a was accelerated to more than 2 times the experience flow rate V_e but less than 2.5 times V_e(0.13 m/s?V_a<0.1625 m/s).In other words,the accelerated average precursor flow rate which was between 480 L/h and 600 L/h was the optimized plan of the CVD deposition technique.The preparation experiments on aluminide coatings were carried out in this thesis in order to verify the optimized plan of the CVD deposition technique guided by the CFD simulation results.Meanwhile,the experimental results were analyzed by several property characterizations of the coatings and mathematical statistics method in order to investigate the reason why the excessive flow rate optimized plans of the deposition technique were in failure as well as to verify the availability of the rest optimized plans of the deposition technique.The verification results showed that the?-Ni Al phase aluminide coatings in need were prepared under the optimized deposition technique while the faster the flow rate was,the thicker the coatings were.But once the precursor flow rate was more than2.5 times V_e,the optimized plan of the deposition technique was in failure because the vortex flow in the reactor and the precursor at low temperature contributed to deposition failure.The results also showed that the coating thicknesses of different samples in different positions were not identical under 2.5 times V_e.Therefore,the precursor flow rate under 2 times V_e is the ideal optimized plan,in which the coating thickness of different samples in different places were well-distributed while there was no significant difference between the thickness of the coatings.