Modification And Application Of Turbulent Diffusion Flamelet Model In Supersonic Combustion Flow

Aiming at the supersonic turbulent diffusion combustion phenomenon,this thesis investigated the turbulent flamelet databases generated from different probability density functions.Then,a modified flamelet model was proposed for studying supersonic combustion with pressure variation considering that pressure was far from homogenous in a supersonic combustor.Finally,the representative interactive flamelet model for turbulent diffusion combustion in supersonic flow was introduced.Firstly,Flamemaster software was used to generate the laminar flamelet database.Different probability density functions were selected to generate turbulent flamelet databases.Combined with DLR strut numerical simulation,the different turbulent flamelet databases were evaluated.The effects of the different scalar dissipation rates,pressures and temperatures on the database were analyzed.The scalar dissipation caused by turbulent flow made the flame surface twist,deform.And the twisting and deformation would consume chemical energy generated from chemical reaction,resulting in the flame surface temperature decrease.Pressure change had a great impact on intermediate products,but little effect on the main components.High temperature could had a great impact on all products.The DLR strut numerical simulation was introduced to evaluate different probability density functions.From statistical average aspect of velocity field and temperature field,the calculation results of mixture fraction submit to ? distribution were better than the ? one,in description of the local characteristics of the combustion flow field.Secondly,based on the ? distribution probability density functions,a modified flamelet model was proposed for studying supersonic combustion with pressure variation considering that pressure is far from homogenous in a supersonic combustor.In this model,the flamelet database is tabulated at a reference pressure,while quantities at other pressure are obtained using a six order polynomial in pressure.Attributed to merit of the modified model which compute coefficients for the expansion only,and the brought less requirements for memory and table lookup time,expensive cost is avoided.The performance of modified model was much better than the approach of using a flamelet model based method with tabulation at different pressure values.Two types of hydrogen fueled scramjet combustors were introduced to validate the modified flamelet model.It was observed that the temperature is sensitive to the choice of model in combustion area,which in return will significantly affect the pressure.Additionally,the combustion characteristics in supersonic combustor with ethylene injection are investigated using a modified flamelet model.For distributed injection,due to the relatively sufficient jet mixing,the high pressure generated from the intense combustion in the upper cavity pushed the main flow downward.Due to insufficient jet mixing,the concentrated injection upstream of bottom cavity leaded to lower temperature and pressure,which indicated a weaker combustion.It was found that the pressure variation of distributed injection is significant as result to the results of modified model were in good agreement with the experimental data compared with the isobaric flamelet model.It was concluded that the modified flamelet model was more effective for cases with a wide range of pressure variation.Finally,based on the above modified model,considering the unsteady phenomenon due to the high scalar dissipation change gradient,representative interactive flamelet model was introduced.The coupling between the CFD code and the flamelets was performed using the RIF concept.In the CFD code,the equations for the flow,turbulence,the enthalpy,the mixture fraction,and its variance are solved.The flamelet parametersst?,p were passed to the flamelet code.During one time step of the CFD code,the flamelet code solves the unsteady flamelet equations with time steps that can be much smaller.In this way also,the time scales of the fluid dynamics and the chemistry are decoupled.At the same time,according to the former introduction,the modified representative interactive flamelet model was compared with Flamemaster software.The property of modified representative interactive flamelet model was validated.In final,two types of hydrogen fueled scramjet combustors were introduced to validate the modified representative interactive flamelet model.It was concluded that the modified RIF model on the whole was in good agreement with the experimental results.