Xinchun Chen

A USM turbulence-chemistry model for simulating NOx formation in turbulent combustion

A unified second-order moment (USM) turbulence-chemistry model for simulating NOx formation in turbulent combustion is proposed. All the correlations, including the correlation of the reaction-rate coefficient fluctuation with the concentration fluctuation, are closed by the transport equations in the same form. This model abandons the series expansion approximation of the exponential term or the approximation of using a product of two single-variable PDFs instead of a joint PDF. The proposed model is used to simulate methane–air jet diffusion combustion and NOx formation. The combustion prediction results are compared with those using the EBU-Arrhenius model and other two versions of the second-order moment model. The NOx prediction results are compared with those using the pure presumed PDF model. Validation of predictions using the experimental data given by the Sandia National Laboratory, USA indicates that the proposed model gives better results than other models, and it is much economical than other refined models.

Second-order moment turbulence-chemistry models for simulating NOx formation in gas combustion

Abstract For simulating NOx formation in combustion presently used turbulence-chemistry models either cannot or are inconvenient to simulate finite reaction rate, or need extremely large computation time and storage, difficult to be used in engineering complex flows. Hence, different second-order moment (SOM) turbulence-chemistry models are proposed. The SOM models can well simulate CH4–O2 turbulent combustion and NOx formation, verified by experiments. The results indicate that the SOM models may be economic and reasonable for predicting NOx formation in combustion.

Origin of Superlubricity in a-C:H:Si Films: A Relation to Film Bonding Structure and Environmental Molecular Characteristic

Superlubricity of Si-containing hydrogenated amorphous carbon (a-C:H:Si) films has been systematically investigated in relation to the film bonding structure and the environmental atmosphere. Structural diversity induced by hydrogen incorporation (i.e., 17.3-36.7 at. % H), namely sp(2)-bonded a-C, diamond-like or polymer-like, and tribointeractions activated by the participation of environmental gaseous molecules mainly determine the frictional behaviors of a-C:H:Si films. A suitable control of hydrogen content in the film (i.e., the inherent hydrogen coverage) is obligate to obtain durable superlubricity in a distinct gaseous atmosphere such as dry N2, reactive H2 or humid air. Rapid buildup of running-in-induced antifriction tribolayers at the contact interface, which is more feasible in self-mated sliding, is crucial for achieving a superlubric state. Superior tribological performances have been observed for the polymer-like a-C:H:Si (31.9 at. % H) film, as this hydrogen-rich sample can exhibit superlow friction in various atmospheres including dry inert N2 (μ ∼ 0.001), Ar (μ ∼ 0.012), reactive H2 (μ ∼ 0.003) and humid air (μ ∼ 0.004), and can maintain ultralow friction in corrosive O2 (μ ∼ 0.084). Hydrogen is highlighted for its decisive role in obtaining superlow friction. The occurrence of superlubricity in a-C:H:Si films is generally attributed to a synergistic effect of phase transformation, surface passivation and shear localization, for instance, the near-frictionless state occurred in dry N2. The contribution of each mechanism to the friction reduction depends on the specific intrafilm and interfilm interactions along with the atmospheric effects. These antifriction a-C:H:Si films are promising for industrial applications as lubricants.

Studies on the effect of swirl on no formation in methane/air turbulent combustion

Most present studies on pollutant formation concentrate on chemical reaction kinetics. To understand the interaction between turbulence and chemistry in NO formation, the effect of swirl number on NO formation in methane/air turbulent combustion is studied by experiments, in which a small amount of ammonia is added to the fuel to simulate fuel nitrogen, and simultaneously by numerical simulation, using a second-order-moment PDF turbulence-chemistry model. The predicted results are in overall agreement with the measured results. Both predictions and experiments show that as the swirl number increases from 0 to 1, the thermal NO at first increases and then decreases. In contrast, the fuel NO at first decreases and then increases. The studies also show that the increase in swirl number first leads to a rapid decrease and then a slower increase in turbulence intensity, and first an increase and then a slight decrease of temperature near the exit. As the activation energy of thermal NO formation is much larger than that of fuel NO formation, these results imply that the thermal NO is predominantly affected by temperature, whereas the fuel NO is predominantly affected by species mixing via turbulence. The research results are expected to be used for developing low-NO x burners.

Structural and environmental dependence of superlow friction in ion vapour-deposited a-C?:?H?:?Si films for solid lubrication application

Understanding the tribochemical interaction of water molecules in humid environment with carbonaceous film surfaces, especially hydrophilic surface, is fundamental for applications in tribology and solid lubrication. This paper highlights some experimental evidence to elucidate the structural and environmental dependence of ultralow or even superlow friction in ion vapour-deposited a-C?:?H?:?Si films. The results indicate that both surface density of silicon hydroxyl group (Si?OH) and humidity level (RH) determine the frictional performance of a-C?:?H?:?Si films. Ultralow friction coefficient ? (?0.01?0.055) is feasible in a wide range of RH. The dissociative formation of hydrophilic Si?OH surface and the following nanostructure of interfacial water molecules under contact pressure are the origin of ultralow friction for a-C?:?H?:?Si films in humid environment. The correlation between contact pressure and friction coefficient derived from Hertzian contact model is not valid in the present case. Under this nanoscale boundary lubrication, the friction coefficient tends to increase as the contact pressure increases. There even exists a contact pressure threshold for the transition from ultralow to superlow friction (????0.007). In comparison, when tribotested in dry N2, the observed superlow friction (????0.004) in the absence of water is correlated with the formation of a low shear strength tribolayer by wear-induced phase transformation.

Self-Assembled Graphene Film as Low Friction Solid Lubricant in Macroscale Contact

Promoted by the demand for solid lubricants, graphene has been proved to be a promising material for potential applications in reducing friction and wear. Here, a novel lubricating system where graphene sliding against graphene is developed to achieve low friction in macroscale contact. And the large area graphene film used here were prepared by a unique self-assembly technique based on Marangoni effect. Low friction coefficient of about 0.05 is obtained, and it is demonstrated that the film thickness, applied normal load and annealing process all have important influences on the tribological properties of graphene. The expedient fabrication procedure of large-area graphene film with excellent transferability and high-performance friction-reducing behaviors of the developed lubricating system both have a promising perspective in engineering applications.

Growth mechanism and composition of ultrasmooth a-C:H:Si films grown from energetic ions for superlubricity

Growth mechanism and ion energy dependence of composition of ultrasmooth a-C:H:Si films grown from ionization of tetramethylsilane (TMS) and toluene mixture at a fixed gas ratio have been investigated by varying the applied bias voltage. The dynamic scaling theory is employed to evaluate the roughness evolution of a-C:H:Si films, and to extract roughness and growth exponents of α ∼ 0.51 and β ∼ 0, respectively. The atomically smooth surface of a-C:H:Si films with Ra ∼ 0.1 nm is thermally activated by the energetic ion-impact induced subsurface “polishing” process for ion dominated deposition. The ion energy (bias voltage) plays a paramount role in determining the hydrogen incorporation, bonding structure and final stoichiometry of a-C:H:Si films. The hydrogen content in the films measured by ERDA gradually decreases from 36.7 to 17.3 at. % with increasing the bias voltage from 0.25 to 3.5 kV, while the carbon content in the films increases correspondingly from 52.5 to 70.1 at. %. The Si content is kept al...

AFM Studies on Liquid Superlubricity between Silica Surfaces Achieved with Surfactant Micelles.

By using atomic force microscopy (AFM), we showed that the liquid superlubricity with a superlow friction coefficient of 0.0007 can be achieved between two silica surfaces lubricated by hexadecyltrimethylammonium bromide (C16TAB) solution. There exists a critical load that the lubrication state translates from superlow friction to high friction reversibly. To analyze the superlow friction mechanism and the factors influencing the critical load, we used AFM to measure the structure of adsorbed C16TAB molecules and the normal force between two silica surfaces. Experimental results indicate that the C16TAB molecules are firmly adsorbed on the two silica surfaces by electrostatic interaction, forming cylinder-like micelles. Meanwhile, the positively charged headgroups exposed to solution produce the hydration and double layer repulsion to bear the applied load. By controlling the concentration of C16TAB solution, it is confirmed that the critical load of superlow friction is determined by the maximal normal force produced by the hydration layer. Finally, the superlow friction mechanism was proposed that the adsorbed micellar layer forms the hydration layer, making the two friction surfaces be in the repulsive region and meanwhile providing excellent fluidity without adhesion between micelles.

Evolution of tribo-induced interfacial nanostructures governing superlubricity in a-C:H and a-C:H:Si films

Hydrogenated amorphous carbon (a-C:H) is capable of providing a near-frictionless lubrication state when rubbed in dry sliding contacts. Nevertheless, the mechanisms governing superlubricity in a-C:H are still not well comprehended, mainly due to the lack of spatially resolved structural information of the buried contact surface. Here, we present structural analysis of the carbonaceous sliding interfaces at the atomic scale in two superlubricious solid lubricants, a-C:H and Si-doped a-C:H (a-C:H:Si), by probing the contact area using state-of-the-art scanning electron transmission microscopy and electron energy-loss spectroscopy. The results emphasize the diversity of superlubricity mechanisms in a-C:Hs. They suggest that the occurrence of a superlubricious state is generally dependent on the formation of interfacial nanostructures, mainly a tribolayer, by different carbon rehybridization pathways. The evolution of such anti-friction nanostructures highly depends on the contact mechanics and the counterpart material. These findings enable a more effective manipulation of superlubricity and developments of new carbon lubricants with robust lubrication properties.Hydrogenated amorphous carbon is a promising solid lubricant, but the underlying mechanisms surrounding its superlubricity remain unclear. Here the authors reveal that the attainment of a superlubricious state is dependent on the in-situin-situ formation of a nanostructured tribolayer through different carbon rehybridization pathways.