Research On Thermal-mechanical Behavior Of An Integrated Thermal Protection System With V-pattern Folded Core

The thermal protection system(TPS),as an importmant component of hypersonic vehicles,works mainly to maintain the underlying structure,crew members and equipment within acceptable temperature limits.Requirements of thermal protection and load bearing are usually taken into account separately for conventional TPS,which leads to a design with relatively low structural efficiency and high risk.An integrated thermal protection system(ITPS),which is capable of bearing both mechanical as well as thermal loads,is a promising candidate for future hypersonic vehicles.Folded cores have various cell topologies and design parameters.In addition,the open cell design of a folded core gives it the potential for simultaneous load bearing and thermal protection with either lightweight insulation material inside for passive thermal insulation or internal coolant transport for active cooling.A novel ITPS based on V-pattern folded cores is studied,with its thermal-mechanical behavior investigated for both passive thermal insulation and active cooling.Considering the folded core sandwich panel is a kind of periodic cellular structure,it is possible to obtain the equivalent properties of the sandwich panel from a single unit cell.The homogenized model could significantly reduce the computational expense.First,the constitutive relation of the equivalent plate was derived based on the first-order shear deformation theory.Then periodic boundary conditions were established to obtain the extensional,bending and coupling stiffness matrix,and transverse shear stiffnesses were derived using the shear beam deformation theory.Afterwards,a comparative study was done between the three-dimensional sandwich model and corresponding two-dimensional equivalent model.The displacement results show that the equivalent model is of high accuracy.Finally,the effect of geometric parameters on equivalent stiffnesses was studied and discussed.A passive ITPS is achieved by packing the empty space between the facesheets and folded cores with a lightweight insulation material.A modified form of the rule of mixtures that homogenizes the folded core over its thickness for heat transfer analysis was developed.Temperature responses were compared with a three-dimensional finite element analysis(FEA),which shows that the simplified one-dimensional model predicts temperature with satisfactory accuracy through the ITPS thickness.Then 3D finite element models were established for static and buckling analyses,where thermal load as well as the aerodynamic pressure and in-plane inertial load were considered simultaneously.All these analyses were conducted using the FEA software Abaqus.Python codes were wrote to carry out the pre-and post-processing procedure,which greatly improved the efficiency and made a good preparation for the following optimization study.Normally,the requirements of a load-bearing structure and a TPS for hypersonic vehicles are conflicting.Meanwhile,the weight of a TPS greatly affects the vehicles' flight performance.All those facts make it essential to perform an optimization study considering both thermal and load-bearing constraints.To address this complicated optimization problem,iSIGHT software was employed,and based on which,a combined optimization approach was established using design of experiment and approximate model techniques.Then optimizations of the passive V-pattern folded core ITPS were performed for both thermal and thermal-mechanical constraints.The open cell geometry of V-pattern folded core sandwich panel allows for internal active cooling.Due to the inherent periodic feature of the folded core,a methodology aims to achieve the fully-developed laminar flow and heat transfer state was developed.ANSYS CFX computational fluid dynamics(CFD)software was employed,while CCL and Perl codes were wrote to extract results and impose boundary conditions automatically.All above precedures were integrated using iSIGHT and several iterations were carried out to achieve a thermally developed state.Then the influences of geometric parameters on the coolant flow resistance and heat transfer behavior were evaluated.It is shown that the proposed methodology can simulate periodic heat transfer of V-pattern folded core sandwich structures with less computing resources.A multi-unit conjugate heat transfer model was also established to investigate the heat transfer performance of V-pattern folded core sandwich structures subjected to forced convection,and the effect of Reynolds number on the periodical heat transfer characteristics was also discussed.Normally,an actively cooled structure with a good heat transfer behavior is inclined to suffer a relatively large pressure loss.The heat dissipation efficiency,relative density and the equivalent extesional stiffness along the streamwise direction were taken as objective functions.The previously mentioned iterative method for V-pattern folded core sandwich structure's periodic heat transfer analyses was employed to carry out the single-and multi-objective optimization.Finally,the trends of objective functions along with the geometric parameters were discussed in detail.