Numerical Research On Gas-gas Mixing Characteristics Based On Impinging Mass Transfer Principle

The uniform mixing of fuel/air is a key factor in achieving low NOx emissions in DLN combustion technology.The technology of "fuel side regulation" is the mainstream mixing technology.However,the disadvantage of this regulation method are the complex mechanism structure of fuel injection and great loss of flow resistance.Therefore,this paper puts forward the idea of "air side regulation",which simplifies the fuel injection mechanism structure,and adopts the principle of impinging mass transfer to make fuel/air mixed fast and efficiently.Although the impinging stream has been widely used in industrial processes such as drying,extraction,pulverization,whether it can be applied to the rapid and uniform gas-gas mixing with large flow rate ratio is still to be further studied.In order to determine the applicability of impinging mass transfer principle on gas-gas uniform mixing process,the flow and mixing characteristics of impinging streams was examined numerically,so as to analyze the effects of velocity,jet diameter and initial turbulent intensity on the mixing uniformity and deepen our understanding of the impact mass transfer process.Results show that:(1)Within the range of 1.5 times the diameter of the nozzle from the axis of the jet after impinging,the mixing non-uniformity UMs could be reduced to 3% because of the strong turbulent intensity;(2)the total loss coefficient caused by impinging is low;(3)the mixing characteristics is enhanced when velocity is increased,but the total pressure loss increases linearly as the jet momentum increases;(4)small jet diameter can achieve uniform mixing within a short distance;(5)the mixing process has not been further improved because of limited increase of turbulent intensity when the flow velocity ratio increases.After confirming the feasibility of the impact mass transfer principle,different impinging mixers were designed from the aspects of mixing uniformity,pressure loss and potential applications,and the influence of wall on flow and mixing under different impact forms was analyzed by numerical simulation.Results show that in the axial impinging model,the outer wall surface does not affect the development of the radial jet after the first impinging,and the secondary impinging between the wall and radial jet further strengthens the mixing process,while in the tangential model,a large-scale vortex is induced by the impinging of four injections under the guidance of the outer wall.Further,in the radial impinging model,the fuel is still wrapped in the air jet because the outer wall surface limits the radial development of the jet,which is adverse to the diffusion of the fuel jet.The impinging strength and the contact area of the two gases are increased with the secondary annular flow path,which improves the mixing process Finally,it is found that the axial impinging mixer has the best comprehensive performance by comparing mixing non-uniformity in outlet and total pressure loss coefficient in different mixers.The research provides a basis for the following practical engineering design.