查看更多>>摘要：In order to clarify the effect of a buoyancy force on conduction-convection coupled heat transfer in a hollow cylinder,the flow and thermal characteristics were analyzed using an RNG k-εturbulence model.The Reynolds number was fixed at 1.014 ×106,and the Rayleigh number varied from 1.122 × 1010 to 1.088 × 1011.Results have shown that,when considering the effect of an opposed buoyancy force,increasing the Rayleigh number has a positive impact on the rate of change and uniformity of the cylinder temperature.The temperature distributions along the axial and circumferential directions are similar for different Rayleigh numbers,but extreme values differ.Along the axial direction,the maximum temperature is obtained at the interface between the variable-diameter part and the constant-diameter part.The maximum dimensionless temperature value decreases to 0.12 when the Rayleigh number increases to 1.088 × 1011.Along the circumfer-ential direction,the temperature distribution is affected by the buoyancy force,which results in the temperature of the upper part being higher than that of the lower part.After non-dimensionalization of the temperature and time,a correlation was proposed to illustrate the tran-sient heat transfer process quantitatively.The standard deviation of the maximum relative temper-ature,representing the temperature uniformity,was also calculated.It was found that the difference in the direction of the buoyancy force made a huge difference.Compared with the opposed buoy-ancy force,the maximum dimensionless temperature is almost two times higher with an assisted buoyancy force.Similarly,the heat transfer coefficient with an assisted buoyancy force is half of that with an opposed buoyancy force.Overall,an assisted buoyancy force plays a negative role in terms of thermal characteristics.The flow field around the hollow cylinder was also illustrated to reveal the mechanism of the buoyancy force on magnitude and direction aspects.
查看更多>>摘要：Hydrazine is toxic and carcinogenic,which greatly increases the difficulty of application and no longer meets the needs of green aerospace.As a green propellant,the Ammonium Dinitra-mide(ADN)-based liquid propellant has the advantages of higher specific impulse,being non-toxic,pollution-free,and easy storage.However,an ADN-based space engine in orbit has exposed the problems of high-temperature deactivation of catalysts and cold-start failure.An active ignition technology—electric ignition technology was explored in this paper to break through the technical bottleneck of catalyst deactivation and the inability to a cold start.An experimental system of a constant-volume combustor for the ADN-based liquid propellant based on the electric ignition method was established.The electric ignition and combustion characteristics of the ADN-based liq-uid propellant in a volume combustor with an electric ignition method were studied.The influencing mechanisms of the ignition voltage and the electrode structure on the electric ignition characteristics of the ADN-based liquid propellant were investigated.An elevation of the ignition voltage could facilitate the ignition process of the ADN-based liquid propellant,curtail electric energy input and heating effect,while exerting an adverse impact on the combustion process of the propellant.An increase in the ignition voltage enhanced the ignition process of the propellant while simultane-ously suppressing its combustion process when utilizing mesh electrodes.Compared to the strip electrodes,the mesh electrodes increased the contact area between the electrodes and the propellant,increased the electric energy input power in the electric ignition process,and reduced the ignition delay time.The mesh electrodes could promote the combustion process of the propellant to a cer-tain extent.
查看更多>>摘要：Ammonia(NH3)is currently considered to be a potential carbon-free alternative fuel,and its large-scale use as such would certainly decrease greenhouse gas emissions and meet increas-ingly stringent emission requirements.Although the low flame propagation speed and high NO pro-duction of NH3 hinder its direct application as a renewable fuel,co-combustion of NH3-H2 is an effective way to overcome these challenges.In this study,the combustion characteristics of NH3-H2 swirling flames under different equivalence ratios and H2 blending ratios conditions are both numerically and experimentally investigated.Numerically,the One-Dimensional(1D)laminar flame computation presents a comparison base and the Three-Dimensional(3D)numerical simula-tion yields detailed flame property distributions.Experimentally,the high-speed camera takes instantaneous swirl flame images and the gas analyzer measures the NO emission at the exit plane of the flame chamber.Qualitative and quantitative analysis is performed on the flame structure and NO emission for a series of NH3-H2 swirl flames.The variation trends of the NO emission calcu-lated using different techniques agree very well.The quantitative results show that the NO emissions are much higher at lean equivalence ratios than those at rich equivalence ratios,and such difference is closely related to the combustion flame structure.Moreover,it is shown that the utilization of secondary air injection can achieve a significant reduction in NO emissions at the exit of the combustion chamber at equivalence ratios less than or equal to 0.9.
查看更多>>摘要：This paper aims to tackle the calculation efficiency problem raised in the cavitation-flow simulation of the aviation centrifugal pump due to the fading-away interface resulting from the dis-sipation of numerics used in the phase-change control equation for unstructured-grid multiphase flow,and due to the limitation of flow time-step in whole flow regimes,the control equation of vapor-liquid two-phase flow considering cavitation mass transport is established firstly,modifying the momentum equation by introducing the surface tension,and adding the artificial convective flow to the phase equation to solve the numerical dissipation problem.Secondly,in consideration of the local time step principle and based on the multi-dimensional general limiter algorithm with explicit solutions under the OpenFOAM platform,a solution method of steady-state VOF(Volume of Fluid)model considering cavitation two-phase change is constructed,and the feasibility of this method is verified by NACA hydrofoil and NASA flat plate inducer.Finally,based on the platform developed,the cavitation performance of an aviation centrifugal pump inducer is analyzed.The research results show that the error of the calculated cavitation pressure distribution for NACA hydrofoil between the simulation test and the experimental-test is less than 5％,and the maximum error of calculated cavitation number at pump head dropping for NASA high-speed flat plate indu-cer between the simulation test and the experimental-test is 2.1％.The cavitation area observed in the simulation test is the same as that obtained in the high-speed photography test.Based on the OpenFOAM simulation method,the position of pump head dropping of the fuel centrifugal pump can be accurately captured.The error of the calculated cavitation number at pump head dropping between the simulation test and the experimental test is about 3.7％,showing high calculation accuracy.
查看更多>>摘要：As an integral part of the internal air system of aero-engines,the axial throughflow of the cooling air can interact with the cavity flow between the rotating compressor disks,forming a three-dimensional,unsteady,and unstable flow field.The flow characteristics in an engine-like rotating multi-stage cavity with throughflow were investigated using particle image velocimetry,flow visual-ization technology and three-dimensional unsteady Reynolds-Averaged Navier-Stokes(RANS)simulations.The focus of current research was to understand the distribution of the mean swirl ratio and its variation with a wide range of non-dimensional parameters in the co-rotating cavity with high inlet pre-swirl axial throughflow.The maximum axial Reynolds number and rotational Reynolds numbers could reach 4.41 × 104 and 1.24 × 106,respectively.The velocity measurement results indicate that the mean swirl ratio is greater than 1 and decreases with an increase in the radial position.The flow structure is dominated by the Rossby number,and two different flow pat-terns(flow penetration and flow stratification)are identified and confirmed by flow visualization im-ages.In the absence of buoyancy,the flow penetration caused by the precession of the throughflow makes it easier for the throughflow to reach a high radius region.Satisfactory consistency of results between measurements and numerical calculations is obtained.This study provides a theoretical basis and data support for toroidal vortex breakdown,which is of practical significance for the design of high-pressure compressor cavities.
查看更多>>摘要：The mass application of Hall thrusters poses the need for a diagnostic method of ioniza-tion mechanism in the discharge channel to boost the iteration and optimization of thruster design.This work presents an Optical Emission Spectroscopy(OES)method for diagnostics of the contri-bution of different ionization mechanisms and the flux of ions in different valences in the discharge channel of a Hall thruster.The emission spectra in the discharge channel are analyzed by jointly utilizing a collisional-radiative model,an ionization-excitation model,and a flux-conservation model.It is found that the intensities of some spectral lines can be converted into the reaction rates of collision processes,e.g.,electron-induced excitation and ionization processes.The latter can fur-ther be used to determine the evolutions of particle fluxes by utilizing the conservation law of mat-ter.The novel method is demonstrated on a kilo-watt Hall thruster.The evolutions of several parameters are determined using this method along the discharge channel,including the ionization rates of different mechanisms,particle fluxes,particle densities,and particle velocities.This novel method can be further developed by being jointly utilized with spectral imaging and tomography techniques,enabling diagnostics of multi-dimensional distributions of the above-mentioned parameters in the discharge channel and near-field plume.
查看更多>>摘要：It has been found that the static pressure distribution along the axial direction of liquid kerosene is lower than that of the gaseous kerosene under the same flow condition and overall equivalent ratio from previous studies.To further investigate this phenomenon,a compressible two-phase parallel simulation method is utilized to analyze the mixing and combustion character-istics of gaseous and liquid kerosene jets in a cavity-based supersonic combustor.The numerical results are consistent with the experiments and demonstrate that gaseous injection leads to a cavity shear layer that dives deeper into the cavity,forming two recirculation zones in the front and rear of the cavity.In contrast,the cavity shear layer is closer to the mainstream during liquid injection,and only a large recirculation zone is formed in the rear of the cavity.As a result of the cavity shear layer and the recirculating flow,the fuel vapor of gaseous injection accumulates in the front of the cavity,while for the liquid injection,the fuel vapor disperses in the cavity,cavity shear layer,and the region above,and the rear of the cavity has a higher fuel vapor concentration than the front.This unique fuel distribution causes the combustion area to be concentrated in the cavity during the gaseous injection but dispersed inside and downstream of the cavity during the liquid injection.As a result,forming a thermal throat under the same conditions is more challenging during liquid injection,and the generated static pressure distribution is lower than that during the gaseous injection.
查看更多>>摘要：This work is devoted to the aeronautical application of topology optimization for mod-ular structures with multiple assemblies that consist of repeated standard modules and optional reinforcements.These kinds of structures are widely used owing to their transportability,reconfig-urability,low manufacturing and service costs.In this work,the design of airborne shelves with modular structures characterized by the standard module configuration is formulated for the first time as a topology optimization problem of multiple assemblies and multiple load cases subjected to the volume constraint.It is shown that the weighted compliance design of multiple assemblies is a compromising solution compared to the optimization result of each individual assembly of standard modules.Meanwhile,the performance of optimized airborne shelves with the modular structures can effectively be ameliorated with the help of reinforcements.
查看更多>>摘要：In the current soft-landing system,the lander-leg and the buffer are two independent parts which may cause weight problem.The traditional mechanical buffers also have their own shortcomings such as small buffer stroke and being non-reusable.A Novel Buffer Lander-leg with Yoshimura core(NBLY)is proposed in this paper,inspired by the Buckling Restrained Brace(BRB).The design details of NBLY and parameter definitions of the core are presented.The finite element analysis modes have been employed to analyze the nonlinear mechanical behaviors of NBLY subjected to the cyclic loading.NBLY with four different core styles of hexagon,octagon,dodecagon and hexadecagon are analyzed and compared.The restraining ratio and cumulative dis-sipation energy ratio have been obtained to evaluate the energy absorption properties.The results show that the origami pattern has significant influences on the deformation ductility,plastic energy absorbing and axial load transferring to the core.The NBLY with octagon core has outstanding properties,especially the hysteretic energy dissipation and enhancement of compression capacity.The interaction mechanism between the core and encasing member constrains the deformation extensions and contributes to the frictional energy dissipation of the NBLYs.This paper provides a new conceptual design of buffer lander-leg for soft-landing missions.It can realize the reuse func-tion as a buffer lander-leg with better loading-bearing and energy absorption behaviors.
查看更多>>摘要：Structural nonlinearities such as freeplay will affect the stability and even flight safety of the fin-actuator system.There is a lack of a practical method for designing Active Flutter Suppres-sion(AFS)control laws for nonlinear fin-actuator systems.A design method for the AFS controller of the nonlinear all-movable fin-electromechanical actuator system is established by combining the inverse system and the Immersion and Invariance(I&I)theory.First,the composite control law combining the inverse system principle and internal model control is used to offset the nonlinearity and dynamics of the actuator,so that its driving torque can follow the ideal signal.Then,the ideal torque of the actuator is designed employing the I&I theory.The unfavorable oscillation of the fin is suppressed by making the output torque of the actuator track the ideal signal.The simulation results reveal that the proposed AFS method can increase the flutter speed of the nonlinear fin-actuator system with freeplay,and a set of controller parameters is also applicable for wider free-play within a certain range.The power required for the actuator does not exceed the power that can be provided by the commonly used aviation actuator.This method can also resist a certain level of noise and external disturbance.
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