High-speed Gas Flow Research Articles

A quasi-one-dimensional model for an outwardly opening poppet-type direct gas injector for internal combustion engines

Direct injection of compressed natural gas in internal combustion engines is a promising technology to achieve high indicated thermal efficiency and, at the same time, reduce harmful exhaust gas emissions using relatively low-cost fuel. However, the design and analysis of direct injection–compressed natural gas systems are challenging due to small injector geometries and high-speed gas flows including shocks and discontinuities. The injector design typically involves either a multi-hole configuration with inwardly opening needle or an outwardly opening poppet-type valve with small geometries, which make accessing the near-nozzle-flow field difficult in experiments. Therefore, predictive simulations can be helpful in the design and development processes. Simulations of the gas injection process are, however, computationally very expensive, as gas passages of the order of micrometers combined with a high Mach number compressible gas flow result in very small simulation time steps of the order of nanoseconds, increasing the overall computational wall time. With substantial differences between in-nozzle and in-cylinder length and velocity scales, simultaneous simulation of both regions becomes computationally expensive. Therefore, in this work, a quasi-one-dimensional nozzle-flow model for an outwardly opening poppet-type injector is developed. The model is validated by comparison with high-fidelity large-eddy simulation results for different nozzle pressure ratios. The quasi-one-dimensional nozzle-flow model is dynamically coupled to a three-dimensional flow solver through source terms in the governing equations, named as dynamically coupled source model. The dynamically coupled source model is then applied to a temporal gas jet evolution case and a cold flow engine case. The results show that the dynamically coupled source model can reasonably predict the gas jet behavior in both cases. All simulations using the new model led to reductions of computational wall time by a factor of 5 or higher.

New slip boundary condition in high-speed rarefied gas flow simulations

In this paper, we propose a new slip boundary condition in hypersonic gas flow simulations. It is derived by considering the Langmuir isotherm adsorption into the Kaniadarkis et al. model of the kinetic theory of gas. Moreover, the motion of the adsorbed molecules over the surface (i.e. surface diffusion) is considered for the calculation of the mean free path in new slip condition. Three aerodynamic configurations are selected for evaluating new slip condition such as (1) the sharp-leading-edge flat plate, (2) circular cylinder in cross-flow, and (3) the sharp 25°–55° biconic cases. Hypersonic gas flows have the Mach number ranging from 6.1 to 15.6, and the working gases are argon and nitrogen. The simulation results show that new slip condition predicts better slip velocity than the Maxwell slip condition and gives good agreement with the direct simulation Monte-Carlo data for all cases considered in the present work.

Similarity of Polyatomic Gas Flows in the Kinetic Shock Layer

The two-dimensional problem of high-speed nonequilibrium homogeneous polyatomic gas flow past a surface formulated on the basis of macrokinetic 13-moment equations with the use of the two-layer thin viscous shock layer (TVSL) approximation in the neighborhood of non-slender bodies is considered. A class of similarity variables which make it possible to reduce the complex kinetic problem considered to the well-known Navier—Stokes problem of thin viscous shock layer is suggested.

Implementation of Knudsen Layer Phenomena in Rarefied High-Speed Gas Flows

AbstractA numerical investigation of Knudsen layer effects in high-speed flows in a rarefied flow regime has been carried out. The conventional compressible flow computational fluid dynamics (CFD) ...

Perspective Heat-Resistant Coating for Protection of Cf/SiC Composites in Air Plasma Hypersonic Flow

The investigation results on development and testing of heat-resistant coating with experimental composition of Si–TiSi2–MoSi2–TiB2–CaSi2 for protection of Cf/SiC composites from oxidation and high-speed erosion in high-speed gas flows are presented. This coating was formed by slip firing technology out of slip compositions with filler based on heterophase powder of the system of interest. The results of fire bench tests of samples with coating under conditions of flow-around and off balance heating by air plasma streams with M = 5.5–6.0 and enthalpy of 40–45 MJ/kg. The effectiveness of the protective action of coating at surface temperature Tw = 1810–1820°C is kept not less than 920–930 s, at Tw ≥ 1850–1860°C—not less than 420–430 s. It was found that the effectiveness of coating is provided by structural-phase state of the base coating and formation of heterogeneous oxide film on its surface formed by borosilicate glass with titanium and calcium segregation heterogeneity and TiO2 reinforce micro-needles in the form of rutile. The decrease in saturated steam pressure level in oxide film–coating system induced by increase in the level of external layer heterogeneity was experimentally confirmed.

Investigation of non-equilibrium boundary conditions considering sliding friction for micro/nano flows

Purpose Accurate prediction of temperature and heat is crucial for the design of various nano/micro devices in engineering. Recently, investigation has been carried out for calculating the heat flux of gas flow using the concept of sliding friction because of the slip velocity at the surface. The purpose of this study is to exetend the concept of sliding friction for various types of nano/micro flows. Design/methodology/approach A new type of Smoluchowski temperature jump considering the viscous heat generation (sliding friction) has recently been proposed (Le and Vu, 2016b) as an alternative jump condition for the prediction of the surface gas temperature at solid interfaces for high-speed non-equilibrium gas flows. This paper investigated the proposed jump condition for the nano/microflows which has not been done earlier using four cases: 90° bend microchannel pressure-driven flow, nanochannel backward facing step with a pressure-driven flow, nanoscale flat plate and NACA 0012 micro-airfoil. The results are compared with the available direct simulation Monte Carlo results. Also, this paper has demonstrated low-speed preconditioned density-based algorithm for the rarefied gas flows. The algorithm captured even very low Mach numbers of 2.12 × 10−5. Findings Based on this study, this paper concludes that the effect of inclusion of sliding friction in improving the thermodynamic prediction is case-dependent. It is shown that its performance depends not only on the slip velocity at the surface but also on the mean free path of the gas molecule and the shear stress at the surface. A pressure jump condition was used along with the new temperature jump condition and it has been found to often improve the prediction of surface flow properties significantly. Originality/value This paper extends the concept of using sliding friction at the wall for micro/nano flows. The pressure jump condition was used which has been generally ignored by researchers and has been found to often improve the prediction of surface flow properties. Different flow properties have been studied at the wall apart from only temperature and heat flux, which was not done earlier.

Effect of gaseous coolant temperature on the transpiration cooling for porous wall in the supersonic flow

The problems of the transpiration cooling for porous wall in the supersonic flow is considered. The experimental results are presented for the porous wall temperature distribution along the mainstream, the Mach number of the stream was 2, and the total temperature was +20 °C. The temperature of the air injected through the porous surface is varied in a wide range from -12 °C to +77 °C. It is established that depending on the temperature of injected gas several characteristic regimes can be distinguished. If the temperature of injected gas is less than a certain value then the temperature of the porous wall decreases along the flow. On the other hand, if the injected gas temperature is higher than a certain value, then the temperature of the porous wall increases downstream. In the intermediate range of temperatures of injected gas, the temperature profile along the flow has a minimum. The minimal temperature of the wall can be lower than the temperature of injected gas. The criteria for realization of different types of temperature profiles on the permeable porous surface in a high-speed gas flow are obtained on the base of simple phenomenological model.

Comparison of the Shakhov kinetic equation and DSMC method as applied to space vehicle aerothermodynamics

The paper is devoted to the comparative study of Shakhov model kinetic equation and Direct Simulation Monte Carlo (DSMC) solutions as applied to high-speed flow of a monatomic gas over generic three-dimensional space vehicle under angle of attack. The corresponding calculations are carried out using Nesvetay and SMILE software packages, developed by the authors. Satisfactory agreement is shown for surface distribution of pressure, friction and heat transfer coefficients despite very different modeling approaches and computational meshes employed. It is thus shown that the use of the model kinetic equations with approximate collision integrals provides accurate results for high-speed external monatomic gas flows over complex three-dimensional geometries at acceptable computational cost.

Comprehensive assessment of newly-developed slip-jump boundary conditions in high-speed rarefied gas flow simulations

In this paper we numerically evaluate the recently developed Aoki et al. slip and jump conditions in high-speed rarefied gas flows for the first time. These slip and jump conditions are developed to be employed with the Navier–Stokes–Fourier equations. They were derived based on the Boltzmann equation with the first order Chapman–Enskog solution, and the analysis of the Knudsen layer. Four aerodynamic configurations are selected for a comprehensive evaluation of these conditions such as sharp-leading-edge flat plate, vertical plate, wedge and circular cylinder in cross-flow with the Knudsen number varying from 0.004 to 0.07, and argon as the working gas. The simulation results using the Aoki et al. boundary conditions show suitable agreement with the DSMC data for slip velocity and surface gas temperature. The accuracy of these boundary conditions is superior to the conventional Maxwell, Smoluchowski and Le boundary conditions.

Gas flow characteristics analysis of SF 6 circuit breaker using similarity theory

In this paper, the gas flow model of high voltage SF6 circuit breaker arc quenching chamber was established to analyse the nozzle high speed gas flow variation characteristics using finite volume method, and k-ɛ turbulent equation combined with the wall function was analysed. According to the similarity theory, the similarity criteria and the equivalent model for the arc quenching chamber was established and simulated for reflecting the aerodynamic behavior under cold gas flow interruption condition.

Excellent corrosion resistant amorphous coating prepared by gas atomization followed by AC-HVAF spray technology

A novel Fe49.9Mo32.9Cr9.32Co4.55Si2.28Al1.55 amorphous coating was synthesized successfully on 45 steel by gas atomization followed by activated combustion high-velocity air fuel (AC-HVAF) spray technology. During the fabrication process, Fe-base alloy was firstly atomized into amorphous powders by high-speed gas flow, and then the amorphous powders are used to prepare amorphous coating by AC-HVAF spray technology. The research focused on the microstructure and corrosion behavior in the prepared coatings. Results show that an ultra-high volume fraction of amorphous phase over 90% was obtained in the prepared coatings, which significantly improves the corrosion resistance of the coatings under extreme service conditions.

Numerical study on the jet formation of simple-geometry heavy gas inhomogeneities

The jet formation of gas inhomogeneities under the impact of a planar shock wave is an interesting phenomenon that is closely related to shock convergence. In this study, a series of heavy gas inhomogeneities with very simple geometries (square, rectangle, circle, and triangle) are numerically reproduced to trace the source of the jet formation. Special attention is given to the wave patterns that lead to the formation of specific jet forms. The shock-accelerated multicomponent flow is simulated by solving inviscid compressible Euler equations. An up-wind characteristic space-time conservation element and solution element scheme is adopted, and a five-equation model is used to treat the gas interface. The jet types that emerge in the experimental images are explained based on the numerical results, and a typical shock pattern that ensures the jet formation is uncovered. It is found that, physically, the jet is initiated by the impact of the internal Mach stem, and the jet growth is nourished by the high speed gas flow induced by this Mach stem. The width of the jet is determined by the height of the internal Mach stem. Geometrically, a focal wedge enveloped by slip lines emerges in the gas inhomogeneity, in which the gas is accelerated. It is found that the existence of the focal wedge and the coordinates of the wedge tip can be used as qualitative criteria to illustrate the mechanism of the jet formation. These criteria provide a more intuitive basis for understanding the direction, scale, and process of jet formation.

Conduction, convection, and radiation thermal exchange with discretized fixed, contacting particles in a high speed compressible flow

A novel technique is proposed to simulate the thermal interactions occurring within a compressible fluid flow with particles. Thermal exchanges between particles, fluid, and flow paths including particle-particle and particle-solid body contact are included. Contact conduction with a contact resistance, convection, and radiation modes of heat transfer are accounted for without the need for heat transfer coefficient correlations or other simplifying assumptions since the particles are discretized within the fluid domain. The discretization of the particles in the flow allows for the local temperature and flow field data to be utilized in calculating the thermal interactions and facilitates the exploration of temperature dependent phenomena based on local instead of bulk interface conditions. So that these developed techniques can be applied to computational fluid dynamics simulations with moving objects, a fluid gap space is retained around the particles while the thermal conductive contact effects are simulated. The method is applied to an eleven fixed particle configuration with the flow driver strengths and limited radiative emissivities varied. The results demonstrate that for the conditions considered, the conductive contact dominates the temperature field development, with the radiative and convective cooling taking a lesser role. The new method clearly improves capabilities for the study of the thermal interaction in particle laden high speed gas flow and thus of temperature dependent phenomena for a more comprehensive simulation.

Robust Cascade Temperature Control for a HSNWT

Applies programmable logic controller (PLC) to configure control hardware system, addresses the temperature control for a high speed nitrogen gas wind tunnel (HSNWT) testing, in order to obtain variable high temperature control with variable high speed gas flow, a coordinating factor of the gas flow is proposed between the tunnel’s heating and water cooling systems, moreover, considering the temperature modeling nonlinearities, uncertainties and disturbances, establish robust cascade fuzzy PID and expert predictive control strategies for the inner-loop water cooling subsystem and the outer-loop heating system, respectively, which effectively overcomes the influences of large inertia and transport time delay on the temperature responses. Furthermore, designs human-machine integrated user control interface (HMI), achieves fast and accurately control for user operating. The designed system are simulated and tested in the application, which results demonstrate that the system runs stable and reliable, has strongly robustness when the temperature changes with different loading heat modes, and has excellent capability of variable high speed nitrogen gas flow.

Flow characteristics in the water-cooled channel of the MHD accelerator

Many problems related with the development of a hypersonic vehicle and development of new thermal protection materials are directly related to the possibility of conducting a wide range of laboratory researches under conditions that are close to the real flight conditions as possible. In this connection, the work on the formation of high-enthalpy and high-speed gas flows in ground-based installations is of special importance. It is shown in this paper that the flow characteristics required can be achieved using the experimental setup with a magnetohydrodynamic accelerator after its updating. The feature of this research facility is that more than half of the electric energy supplied from electric circuit is converted directly into the kinetic energy of the flow. Preliminary analytical and experimental studies of the MHD accelerator with a water-cooled channel allow one to conclude that a hypersonic flow with duration up to 10 s is formed at the SMGDU path. The total enthalpy in the core of the flow is close to 20 MJ/kg, while a required high level of total pressure is being maintained. The facility with such flow parameters becomes unique and allows one to significantly expand the range of ground-based testing, for example, testing of thermal protection systems, and testing of novel contactless flow control methods for hypersonic flight conditions.

Flow Modellium software package for calculating high-speed compressible gas flow

Flow Modellium software package for calculating high-speed compressible gas flow

Conceptual Protection Model for Especially Heat-Proof Materials in Hypersonic Oxidizing Gas Flows

This article is a continuation of the publication cycle of the authors on the subject matter “Multifunctional Protective Coatings for Especially Heat-Loaded Constructional Elements of Hypersonic Systems.” A conceptual physicochemical operation model of the protective coating in a high-speed high-enthalpy oxidizing gas flow taking into account and leveling main surface fracture sources by the gas flow is proposed. The model is successfully implemented when developing a whole series of alloys of the Si–TiSi2–MoSi2–B–Y system intended to form thin-layer coatings from them by any method of the stratified deposition providing the reproduction of the structure, phase composition, and morphological features of the deposited material in the coating. During the deposition, the formation of a microcomposition layer is provided. This layer is a refractory silicide framework with the cells filled by a low-melting (relative to the melting point of framework-forming phases) eutectic structural component. This layer transforms into a multilayer system with a series of functional layers (anticatalytic, reradiative, antierosion, heat-proof, and barrier-compensation layers) of micron and submicron thicknesses during high-temperature interaction with oxygen-containing media (the synergetic effect). The protection ability is provided by the formation of self-restoring oxide vitreous film based on alloyed silica. The self-restoring effect consists of rapid filling of random defects with a viscoplastic eutectic component and protective film formation accelerated when compared with known coatings. The high resistance to the erosion carryover is provided by the presence of a branched dendritic-cellular refractory framework. Coatings MAI D5 and MAI D5U, designed in the scope of the proposed concept, are successfully approved in high-speed high-enthalpy oxygen-containing gas flows affecting the samples and constructional elements made of especially heat-proof material of various classes (niobium alloys, carbon–carbon and carbon–ceramic composite materials, and graphitized carbon materials). The protective ability of coatings of 80–100 μm in thickness in flows with the Mach number of 5–7 and enthalpy of 30–40 MJ/kg is no shorter than 600 s at Tw = 1800°C, 200 s at 1900°C, and 60 s at 2000°C, including the constructive elements with sharp edges.

Measurement of heat flux to model surface in the wind tunnel SMGDU

The results of studies of heat fluxes that can occur in the gas supersonic flow created in the wind tunnel with a magnetogasdynamic accelerator are presented. For two operation modes of the installation, measurements of heat fluxes to the surface of a calorimetric model were performed. The dependences of these parameters on induction of magnetic field on axis of the MHD accelerator were obtained for the first time. It was found that their values reached a level of 6 MW/m2. The experimental data were compared with the results of a numerical simulation of a longitudinal flow past cylinder with a flat face in the real conditions of the wind tunnel, that allowed to calculate other characteristics of the gas flow. As a result, it is shown that this setup is enabled to create high-speed gas flows with high enthalpy that can be used in the development of prospective hypersonic aircrafts.

Effects of SiC nanowire decorated with carbon nanosheet on mechanical, heat-dissipation and anti-ablation properties of carbon/carbon composites

Carbon/carbon (C/C) composites containing SiC nanowires (SiC NW) decorated with carbon nanosheet were prepared with the pyrolytic carbon (PyC) as the matrix, where the nanowires and carbon nanosheet were introduced via electrophoretic deposition and chemical vapor deposition respectively. Their flexural strength, heat-dissipation capacity, and anti-ablation properties were investigated, with C/C composites and carbon nanosheet-C/C composites as comparison. Flexural strength of the SiC NW-Carbon nanosheet-C/C(SiC NW-CN-C/C) composites increased by 40.97% and 65.69% separately due to improvement of the interfacial bonding and bonding between PyC sublayers via SiC NW and carbon nanosheet. Their heat-dissipation performance behaved better due to in-situ grown carbon nanosheet, with lower surface temperature when heated under the same condition. Consequently, the anti-ablation property of such composites was promoted with mass ablation rate decreased by 40.74% and 27.27% respectively, which was attributed to the enhanced bonding, the reduced thermal damage benefitted from better thermal performance, and the formation of SiC NW-Carbon nanosheet-PyC (SiC NW-CN-PyC) micro-rods to prevent debonding under impact of the high-speed gas flow and protect carbon fibers against mechanical ablation.

Recent Progress on Supercomputer Modelling of High-Speed Rarefied Gas Flows Using Kinetic Equations

Numerical solution of the Boltzmann equation for stationary high-speed flows around complex three-dimensional bodies is an extremely difficult computational problem. This is because of high dimension of the equation and lack of efficient implicit methods for the calculation of the collision integral on arbitrary non-uniform velocity grids. Therefore, the use of the so-called model (approximate) kinetic equations appears to be more appropriate and attractive. This article uses the numerical methodology recently developed by the second author which includes an implicit method for solving the approximating kinetic equation of E.M. Shakhov (S-model) on arbitrary unstructured grids in both velocity and physical spaces. Since most of model equations have a well-known drawback associated with the velocityindependent collision frequency it is important to determine the deviations of solutions of these equations from the solution of the complete Boltzmann equation or DSMC for high-speed gas flows. Our recent comparison of the DSMC and S-model solutions for monatomic gases with a soft interaction potential shows good agreement of surface coefficients of the pressure, heat transfer and friction, which are most important for industrial applications. In this paper, we compare the solution of model equations and the Boltzmann equation for the problem of supersonic gas flow around a cylinder when molecules interact according to the law of hard spheres. Since this law of molecular interaction is the most rigid, the difference in solutions can show the maximum error that can be obtained by using model equations instead of the exact Boltzmann equation in such problems. Our high-fidelity computations show that the use of model kinetic equations with adaptation in phase space is very promising for industrial applications.