Institute Of Mechanics,Chinese Academy of Sciences
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Pure ammonia combustion and decomposition in an autothermal recirculating combustor
No full textTwo-stage pure ammonia combustion significantly improves combustion efficiency while reducing NOx emissions. However, at the primary stage, the low temperature and poor fuel-air mixing result in flame instability. An autothermal recirculating combustor (ARC) is proposed, and it is used as the primary-stage combustor under fuel-rich conditions (ER > 1) in the study. The mixing, preheating and reaction characteristics of NH3 and air in ARC were investigated by numerical simulation. The low-temperature NH3 and air introduced are rapidly preheated and efficiently mixed, this widens the upper limit of the combustion equivalence ratio, and stable flame can be maintained even at an equivalence ratio of 1.83. In the 1020-1350 K regime, both oxidation and decomposition of NH3 proceed rapidly. Hydrogen radicals in flame act as a key chain carrier, enabling OH radical production and NH3 dehydrogenation, thus controlling the reaction kinetics at ER > 1. With the increase of equivalence ratio from 1.30 to 1.66, H-2 concentration and conversion rate at ARC outlet increase from 5.33 % to 9.28 % and from 14.56 % to 21.91 %, respectively. An optimal equivalence ratio exists for maximizing H-2 concentration at ARC outlet. NOx concentration decreases from 71 ppm to 6 ppm when the equivalence ratio increases from 1.30 to 1.83
Linear stability of rotating pipe flow with non-ideal fluid
No full textA linear stability analysis is performed on rotating pipe flow with a non-ideal fluid. The study focuses on supercritical CO2 near its vapor-liquid critical point, where thermodynamic properties deviate significantly from ideal gas. Different wall temperatures are considered, ensuring centerline temperatures span subcritical, transcritical, and supercritical conditions. The modal analysis reveals that at low rotation speeds, unstable mode only exists at rotational speed Omega < 0. Also multiple unstable modes emerge, introducing a more complex instability mechanism compared to non-rotating pipe flow. As rotation speed increases, viscous dissipation plays a key role in flow stabilization, while thermodynamic effects remain secondary. The non-modal analysis further demonstrates that optimal system response under fixed-frequency forcing shifts due to rotation, with stronger deviations from incompressible behavior at high compressibility. In rotating pipe flow, the dependence of transient energy growth on the azimuthal wavenumber (n) is inherently nonlinear, which stands in stark contrast to the approximately linear relationship typically observed in non-rotating pipe flow. This nonlinearity arises primarily due to the influence of azimuthal velocity components introduced by rotation. These findings highlight the intricate coupling between rotation, compressibility, and thermodynamics, providing new insights into instability mechanisms in non-ideal fluid systems
Pure ammonia combustion and decomposition in an autothermal recirculating combustor
No full textTwo-stage pure ammonia combustion significantly improves combustion efficiency while reducing NOx emissions. However, at the primary stage, the low temperature and poor fuel-air mixing result in flame instability. An autothermal recirculating combustor (ARC) is proposed, and it is used as the primary-stage combustor under fuel-rich conditions (ER > 1) in the study. The mixing, preheating and reaction characteristics of NH3 and air in ARC were investigated by numerical simulation. The low-temperature NH3 and air introduced are rapidly preheated and efficiently mixed, this widens the upper limit of the combustion equivalence ratio, and stable flame can be maintained even at an equivalence ratio of 1.83. In the 1020-1350 K regime, both oxidation and decomposition of NH3 proceed rapidly. Hydrogen radicals in flame act as a key chain carrier, enabling OH radical production and NH3 dehydrogenation, thus controlling the reaction kinetics at ER > 1. With the increase of equivalence ratio from 1.30 to 1.66, H-2 concentration and conversion rate at ARC outlet increase from 5.33 % to 9.28 % and from 14.56 % to 21.91 %, respectively. An optimal equivalence ratio exists for maximizing H-2 concentration at ARC outlet. NOx concentration decreases from 71 ppm to 6 ppm when the equivalence ratio increases from 1.30 to 1.83
Dynamics of molten droplet/pool fusion in additive manufacturing
No full textThis mini-review first clarifies a very basic process of the fusion-solidification between molten droplet/pool in additive manufacturing (AM) from its technical principle and various experimental observations on the material defects, such as porosity, separation and splashing, erosion and surface concave, and balling effects. Then, the dynamical similarities between the droplet/pool coalescence at normal temperature and the molten droplet/pool fusion at high temperature are illustrated to reveal the fusion-solidification-induced mechanism of material defects, where the droplet/pool fusion phenomena in the early stage have significant influences on the solidification of molten pool in the late stage for the AM process. Finally, some thoughts on the modeling of the fusion-solidification process between molten droplet/pool are stated by referring to the droplet-pool dynamics, which is benefit to the Eulerian-Lagrangian simulation on the prediction of AM process, so as to change the traditional trial-and-error methodology and improve the manufacturing technique
Creep strain and stress state-dependent creep asymmetry during early-stage room-temperature creep in a titanium alloy
No full textRoom-temperature (RT) creep may happen below yield stress in titanium alloys, while the creep asymmetry remains pending under various stress states. The creep behavior and plastic damage were investigated during the early-stage RT-creep up to 60 h in a titanium alloy. The investigated TC4 ELI Ti-alloy is a high-purity ("Extra-Low-Interstitial") version of Ti-6Al-4V with a near- alpha type microstructure after thermomechanical treatment. Three kinds of creep testing were conducted, including axial tension, compression, and torsion, respectively. The microstructure and especially, dislocation behaviors were analyzed in detail by using electron backscattered diffraction, transmission electron microscopy, and X-ray diffraction after an interrupted and terminated creep testing. The creep strain differs, which is 5 %, 1 %, and 0.5 % under tension, compression, and torsion, respectively. It undoubtedly indicates the presence of creep asymmetry. To clarify the creep mechanism, the slip system was then analyzed. It is shown that the prismatic slip is dominant during tensile creep, while the pyramidal slip appears most during compressive creep. The limited slip transmission and immobile dislocations result in lower creep strain. The reason behind the creep asymmetry is attributed to the stress state, producing the activation of various slip systems, along with the evolution of true stress. Finally, the mechanistic origins are also discussed as to the distinctive creep rate under various stress states. (c) 2025 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology
Development of a two-domain-approach-based multi-scale model for the two-phase flows in space accumulators in microgravity
No full textManagement of cryogenic fluid is critical for space accumulators in both loop heat pipe (LHP) and mechanically pumped two-phase loop (MPTL) system. To achieve proper fluid transport in the extreme environments in space, some complex structures are used in these apparatuses including porous meshes and porous vanes. The coexistence of free flow regions and porous medium regions results in a common cross-scale two-phase flow in the multi-scale structures. However, there is a lack of reliable mathematical methods to describe such flows, and thus the flow dynamics in space accumulators are hard to analyze. To solve this problem, we build a coupled multiscale two-phase flow mathematical model based on the two-domain approach: on Onsager's variational principle, minimizing the energy dissipation of the system to derive fluid dynamic equations and interface evolution equations. Navier-Stokes equations for the free flow region and Darcy equation for the porous medium region are derived separately. To account for capillary-driven flow in microgravity, the Darcy equation is modified by explicitly including the capillary force. The boundary conditions that couple fluid dynamic equations and the interface capture methods are incorporated into the model. After validation, the model is applied to analyze transient two-phase flow behavior inside two typical space accumulators in microgravity: one for LHP and the other for MPTL. The flow characteristics are demonstrated, and different porous structures are compared for geometric optimization purposes. The results show that the primary wick of the LHP accumulator with a small pore radius (rc1 = 20 mu m) generates substantial capillary pressure (-170 Pa) to maintain fluid circulation, and the secondary wick with a large pore radius (rc2 = 50 mu m) enables efficient liquid delivery. In the accumulator of MPTL system, the implementation of porous mesh with a large pore radius (rc = 80 mu m) significantly enhances the liquid replenishment rate (i.e., 0.017 m/s)
Data-driven enhanced rough contact mechanics: PINN estimation of gap distribution across length scales for partial contacts
No full textIn this study, we employ Green's function molecular dynamics (GFMD) to simulate non-adhesive elastic contact between a half-space and a randomly rough counterface in (1+1) dimensions, obtaining gap distributions across varying length scales and Hurst exponents. Using the GFMD-generated dataset and incorporating the convection-diffusion equation form (derived in prior and current work) as a physical constraint, we predict gap distributions via Physics-Informed Neural Network (PINN). Results demonstrate that under partial contact conditions-where analytical solutions are unavailable-PINN predictions assuming drift and diffusion coefficients scale with length exhibit high agreement with GFMD. Furthermore, PINN successfully predicts gap distributions and relative contact areas at larger scales using small-scale training data, closely matching GFMD benchmarks. This establishes PINN as an effective tool for rough surface contact problems, particularly when analytical solutions are absent or computational models are prohibitively expensive
Study on the effect of the fuselage profile on aerodynamic characteristics of HCW configuration
No full textThe High-pressure Capturing Wing aerodynamic configuration (abbreviated as HCW) can enhance the aircraft's lift and lift-to-drag ratio through the effective utilization of favourable aerodynamic interference between the fuselage and the capturing wing (abbreviated as CW). Consequently, the fuselage shape significantly influences not only the aerodynamic characteristics of the fuselage but also those of the CW. In this study, based on the concept of a fuselage of revolution combined with CW, the effects of different fuselage profile shapes-specifically, conical profile, 3/4-power profile, and Karman profile-on the aerodynamic characteristics of the HCW configuration are systematically investigated using a numerical simulation technique. The results indicate that while the 3/4-power and Karman configurations differ from the conical configuration by employing non-uniform compression, which increases the fuselage volumes by 18.5 % and 48.1 %, respectively, they achieve drag reductions of 6.7 % and 1.1 % under the design condition at Ma = 6. Moreover, due to the influence of different fuselage profiles on the flow characteristics, the peak pressure on the lower surface of the CW is reduced for both the 3/4-power and Karman configurations, while the area of the high-pressure zones is increased. Overall, compared to the conical configuration, the 3/4-power configuration exhibits a slight decrease in lift, whereas the Karman configuration demonstrates a significant increase in lift. Both configurations achieve an increase in the lift-to-drag ratio. Additionally, an analysis of the effects of different Mach numbers (Ma = 7, Ma = 8) that as the Mach number increases, the lift becomes more sensitive for both the 3/4-power and Karman configurations. However, compared to the conical configuration, both configurations keep exhibiting a decrease in overall drag and an increase in the lift-to-drag ratio. Furthermore, under different Mach number conditions, the centers of pressure for both configurations shift rearward
Machine learning analysis for condensation flow heat transfer in mini/ micro-channels
No full textMiniature condensers have emerged as an efficient solution for thermal management of compact high-power devices due to their exceptional heat dissipation capability. However, accurate prediction of heat transfer coefficient(HTC) remains challenging due to complex flow and thermal behaviors in two-phase heat transfer. This study employed explainable machine learning to develop condensation HTC prediction models in mini/micro channels. A multidimensional feature database containing 4003 experimental data points across 19 fluids in hydraulic diameter 0.1mm <= D <= 4.8 mm was constructed. Four machine learning models, including Artificial Neural Network (ANN), Random Forest (RF), Extreme Gradient Boosting (XGBoost), and Support Vector Regression (SVR), were developed utilizing the database to explore their potential in predicting condensation HTC. The models were validated through internal and external datasets, with comparison against six traditional correlations. The SHapley Additive exPlanations (SHAP) method was subsequently applied to explain the XGBoost prediction mechanism. Results demonstrate all machine learning models achieved satisfactory performance compared to traditional correlations, with XGBoost exhibiting optimal accuracy and generalization. It attained a coefficient of determination (R2) of 0.993 and a mean absolute relative deviation (MARD) of 3.6 % across the database, with strong generalization even for new fluid datas. SHAP explanation revealed Froude number and dimensionless vapor velocity were critical features, while the influence of features such as thermal conductivity and mass flux on the model's prediction aligned with the trend of physical laws and experimental results, effectively enhancing predictive rationality of "black-box" models. This work shows machine learning's significant potential for two-phase heat transfer prediction, providing an efficient predictive tool for mini/microchannel condenser design
Investigation of solidification parameters and microstructure evolution in directed energy deposition with laser beam oscillation
No full textLaser beam oscillation offers significant potential to enhance process stability, control solidification parameters, and tailor microstructure in directed energy deposition. A coupled mesoscopic-microcosmic numerical model is utilized in this work to investigate the effect of oscillating laser beam on the solidification parameters and microstructure evolution during the directed energy deposition with laser beam oscillation (DED-LBO) process. The dynamics solidification conditions induced by the oscillating laser beam are considered in the mesoscopic thermal-fluid model. Based on the solidification parameters, the columnar-to-equiaxed transition of the microstructure is discussed, and microstructure evolution is analyzed using the microcosmic phase-field model. The results show that temperature gradient (G) and cooling rate (GR) vary transiently with the position along the laser oscillation trajectory. The microstructure is predominantly characterized by columnar grain growth, with a relative probability exceeding 85.37 %. An increase in oscillation amplitude and frequency effectively reduces both G and GR, resulting in a coarser microstructure. Good agreement is achieved between the simulated and experimental dimensions and microstructural morphologies of the deposited layers, demonstrating the validity of the developed model. The findings of this work provide valuable insight into revealing the dynamic solidification parameters under the oscillating laser beam and elucidating the physical mechanisms governing microstructure evolution under varying oscillation conditions