Research Of Conjugate Heat Transfer Method For Complex Geometry And Its Application In Composite Structure Nozzles

Composite structural nozzles are widely used in large solid rocket motors(SRM),and their thermal protection iusses have raised conern around the world.Focusing on the ablative heat transfer issues of composite structure nozzles,a conjugate heat transfer calculation program of complex surface was developed.From the perspective of heat transfer,the influence of shape change caused by ablation in the composite structure nozzles on the flow field within the nozzles was systematically explored,especially for the influence of the generation of ablation steps on flow field and wall heat transfer,providing an important reference for the thermal protection design and optimization for composite structure nozzles.The main work of this paper is as follows.First,a conjugate heat transfer(CHT)solver was developed to address the conjugate heat transfer issues of complex geometries.Navier-Stokes equations were adopted for both fluid and solid regions,which improves efficiency by avoiding using two different codes and a possible data transmitting code.The direct coupling approach was adopted to keep the energy balance at the fluid/solid interface,in which the common wall temperature was used.S-A and k-co SST turbulent models were used.For the numerical method,the N-S equations were solved by the cell-center based FVM method,the inviscid term was calculated by Van Leer,Roe and AUSMPW+ schemes with reconstruction methods of NND,3 order up-wind,MUSCL;the viscous term was calculated based on Jocabian transformation.For the temporal discretization,TVD Runge-Kutta,implicit LU-SGS,dual time-stepping LU-SGS schemes were adopted.Second,lots of classical validation cases were used to validate the fluid solver and solid solver separately.A coupling approach based on coordinate transform was proposed to address the problem that orthogonal grids are difficult to be established in complex geometries.The coupling approach was proved to be of second order accuracy in the space by the error analysis.Then,three engineer cases with complex geometries were simulated using the proposed coupling approach.Good agreement between simulated results and experiment data was found,indicating that the current approach can address the CHT problem in complex geometries reliably and accurately.Third,a framework was established for the detailed erosion measurement with high accuracy using raster scanning technique.The point cloud of a half nozzle cut along the middle line was obtained and realigned using an in-house reconstruction code.Then the 3D erosion morphology was obtained with detailed information,for example,the shape change after erosion,the erosion rate along in the axial and circumferential directions.Combining with the reconstruction result,the reason for the different erosion behavior on the nozzle inner surface was explained,and special emphasis was put on the two erosion steps.Fourth,the micro structure of the nozzle surface was obtianed using scanning electron microscope.Both the graphite and silica phenolic surfaces were covered by the fluffy material,which could be the cooling products from the commbustion of inhibitor.After the erosion,the graphite surface was relatively smooth,while the silica phenolic surface exhibited phenomenon,for instance,detach of fiber from matrix,form of voids and pores.The thermo-chemical erosion model of graphite was established and the transition of erosion control mechanism during the transient erosion process was investigated.Fifth,in order to study the influence of shape change caused by erosion to the nozzle performance,two models were established.In the first model,the two nozzle shapes were deduced based on the measured average erosion rate,and only fluid domains were considered.The shape change effect on the flow field was more pronounced in the divergent section,in which oblique shock wave arose in the divergent section,changing the flow structure greatly.The second models were CHT models,in which the nozzle profiles before and after the erosion were considered.According to the simulation results,the heat flux was relatively high in the erosion step in the divergent section,indicating that the form of the erosion step enhanced the local heat transfer.In the erosion step in the convergent section,the erosion amount of graphite is abnormally high,which could be attributed to the mechanical erosion.