An installation was designed, manufactured and tested for methane pyrolysis and hydrogen production. Methane heating and pyrolysis occurs due to burning a fraction of methane-hydrogen mixture (60
The paper presents a direct numerical simulation of a laminar premixed methane-air flame using a detailed kinetic mechanism. In this study, the numerical solver laminarSMOKE has been extended by a module for conjugate heat transfer between the flame and nozzle walls. Switching the isothermal boundary condition at the nozzle wall to the conjugated heat transfer condition results in a higher temperature of the nozzle exit edge, a reduction in a gas density gradient near the nozzle exit and a reduction in the low-frequency oscillations of the flame caused by the buoyancy force.
This paper presents the linear stability analysis of a swirling flow in a model combustion chamber. The flow features a strongly coherent component associated with vortex core precession. Global stability analysis was performed on the mean field obtained via the large eddy simulations, incorporating turbulent viscosity. The eigenvalue spectrum revealed a mode with a dimensionless frequency of 0.76. Its eigenfunction distributions indicated that this mode corresponds to a precessing vortex core (PVC). These findings confirms that the PVC is a global unstable mode developing in a mean turbulent flow and will serve as a basis for further work on the development of PVC control strategies.
The paper deals with the study of heat transfer intensification during cooling of a surface treated by laser with a dispersed coolant flow. A laser ablation technique using a pulsed laser is described; this treatment leads to the formation of craters of up to 600 µm in diameter on a copper surface with the following roughness parameters: Ra = 1.48 µm, Rz = 9.8 µm. The results demonstrate an increase in the removed heat flux by 35–40
The effect of a compliant coating (thin-film) over a porous surface on the stability of a supersonic (M = 2) boundary layer on a flat plate to disturbances of the first mode of instability under the conditions of experiments in the T-325 wind tunnel was calculated using linear stability theory. The calculations showed that a porous coating with a thin-film membrane on the surface can stabilize the boundary layer at Mach number M = 2 at the most amplifying frequency on an impermeable solid plate. The dependences of the spatial amplification rates of disturbances on a number of factors were established. Nonmonotonic (with minima) dependences of these growth rates on film tension, pore radius, porous coating depth, and porosity were obtained. The spatial growth rates of disturbances decrease with decreasing film thickness and the ratio of the gas pressure inside the pores to the static pressure in the boundary layer. It was found that stabilization of a supersonic (M = 2) boundary layer relative to the case of a smooth impermeable plate can occur only with a film thickness of less than 18 nm.
The supercavities were studied in a slit channel with a hydrofoil inside. The hydrofoil attack angle was 21°. The dynamics of tracing flat cavitation bubbles in a slit channel were studied using a high-speed visualization with algorithmic diagnostics method. The methods of an algorithmic diagnostics of a two-phase flow were used to detect the size of bubbles, their velocity, direction of bubbles movement and spatial distribution. There are three types of supercavity patterns arising in slit channels: a transparent pulsating cavity with periodic vortex formation at the site of its closure; a steam and a steam-water mixture coexisting inside the cavity; a cavity completely filled with liquid with tracing vapor-gas bubbles inside. It is determined that the bubbles move inside supercavity from its boundaries to the center; further they move to the front of the cavity and merge with it. The maximum velocity of bubbles is half the velocity of the oncoming flow.
To create an efficient microchannel reactor for hydrogen production, steam reforming of carbon monoxide in a slotted annular channel was experimentally studied. The microchannel reactor is formed by a submillimeter gap between two cylinders, with a catalyst applied to the outer side of the inner cylinder. Platinum applied to cerium oxide was used as a catalyst. The thermal characteristics of the shift reaction with the formation of carbon dioxide and hydrogen were experimentally studied. Experiments were carried out at a steam-to-carbon monoxide ratio of 3:1 at different mixture flow rates. It was shown that a fourfold increase in the mixture flow rate leads to a significant increase in the temperature difference between the reactor inlet and outlet, caused by the heat release of the reaction (from ΔT ≈ 20 °C at a contact time of 189 ms to 80 °C at a contact time of 46 ms). According to analysis of the composition of the outgoing gas, with an increase in the mixture flow rate, the degree of carbon monoxide conversion decreases significantly.
A multiblock numerical model was used to study the phenomenon of vacancy formation in a chain of dust particles. Self-consistent spatial distributions of the space charge and electric potential around the dust particles for two cases were obtained by means of numerical calculations: a linear chain of five particles and a chain of four particles with a vacancy. The equilibrium charges of particles and the magnitudes of the main forces acting on them (the ion drag force, the Coulomb force of particle repulsion, and the force exerted by an external electric field on charged particles) were calculated. It was shown that vacancy formation, without changing the structural parameters of the chain, occurs under conditions of strong ion trails, with the fourth and fifth particles in the strong ion trail of the second particle.
Experimental data on the nature, thermophysical parameters, and boiling regimes of closed, two-phase, vacuum-sealed, water-filled thermosyphons are presented, and their potential applications are discussed. The high efficiency of thermosyphons in the slug and hyperslug regimes is experimentally confirmed.
The integral method of heat balance was applied for analytical solutions of a problem about forming dynamic and thermal boundary layers in a pyrolysis reactor (thermal decomposition of methane) with the conditions of variable viscosity and thermal diffusivity values (variable within the layers span). It was demonstrated that a layer of quiescent gas at the wall temperature on the inner surface of reactor wall is heated up to a temperature of 1000 °C (this occurs due to a high viscosity and a high thermal diffusivity within these layers). The isotaches and isotherms in this layer are oriented normal to the wall surface. In this case, boundary layers are formed at a certain distance from the wall. Within this layer, the velocity is almost zero, and the layer temperature equals the wall temperature (exceeding the gas temperature beyond the layers). At high temperatures, the nearwall layer of gas at rest is a place for intense production of carbon (the pyrolytic graphite) which deposits on the reactor walls up to a total carbon-based plugging of the reactor cross-section and the halt in pyrolysis process.
A study on transition processes for experimental time dependencies of the pitch angle for a case of free rotation of a body discovered the final intervals with a steady amplitude of oscillations — known as quasi-self-excited oscillations. Statistical analysis demonstrated that the maximum possible difference between the amplitudes of self-excited oscillations (including the case of quasi-self-excited oscillations) at the normalized oscillation frequency of 0.02 was about 1 degree.
The emergence and development of a vortex system during separation of a supersonic airflow on a rib of a dihedral model with the opening angle of 270° was studied in details. The appearance of a separation vortex above the side face was detected for a flow with an angle of attack α = 1°. For a case with α > 2°, boundary-layer separation occurs near the rib of the corner configuration. With an increase in pressure drop between the top and side faces of the model, the separation flow size grows and at an angle α = 3.7° a new vortex is observed (a secondary vortex). For a flow with an angle of attack α = 8°, a third vortex is clearly observed. This vortex is located above the secondary vortex and it has a minor influence on the pressure distribution and the limiting streamlines behavior on the side face. The flow at this angle of attack α results in repeated separations for thin boundary layers (near the configuration rib and beneath the enlarged secondary vortex). Within the tested interval of the angle of attack α = 8° – 24°, the sizes of those local separations remain almost the same. It was shown that analysis of limiting streamlines patterns and the surface pressure distribution cannot fully elucidate the actual structure of a complex vortex system without gaining some additional data.
The dynamics of droplet surface temperature was studied for a sessile water droplet on a structured black silicon substrate that was heated to 90°C. The wetting properties of black silicon were analyzed during substrate heating in the temperature range of 30–90°C. IR thermography revealed that nucleation sites form at temperatures close to the boiling point at the liquid–black silicon interface. The dynamics of surface temperature changes for a thin liquid droplet during the final stages of evaporation was also examined, including the formation and subsequent development of bubble nuclei within the droplet. It is shown that convection and bubble formation causes temperature field nonuniformity.
The paper presents experimental results from a study on supersonic flow across a cylinder. Selected measurements of pressure on the cylinder surface were taken. Different variants of jet blowing were tested with a focus on variation in the pressure distribution on a surface. The difference between variants was in positioning the blowing hole on the cylinder circumference in the mean cross section. The paper describes the influence of the hole position on the pressure distribution for a case of interaction between a supersonic free stream and a jet. New experimental data on a pressure distribution on a circular cylinder surface with at different positions of the blow hole are now available.
This paper describes a new phenomenon of generating a stable double precessing vortex in a highly swirling turbulent flow. It also identifies the range of flow conditions providing the clearest observation of this effect. It also identifies key features of the internal flow structure and the kinematic scheme of the double vortex precessional motion. The geometric parameters of the helical vortex are measured depending on the flow regime. The obtained results are important not only from a practical standpoint for the possibilities of controlling the flow structure in industrial apparatuses, but also from the standpoint of fundamental problems in vortex dynamics. Precession of a helical structure represents a canonical case of self-induced motion of a vortex filament with a helical axis of rotation.
Experiments were conducted on bubble motion in an inclined tube with a cocurrent liquid flow. The study determined the influence of surfactant concentrations, tube inclination angle and flow velocity on the gas bubble sizes and bubble uprising velocity. It was found that a combination of liquid flow and a surfactant suppresses the bubble coalescence. Under the specific conditions, the drag coefficient for a buoyant bubble in the nearwall flow was calculated. This drag coefficient is proportional to the Weber number both for a bubble in pure water and in water with surfactant.
Multi-rotor aerial vehicles have the rather short history of active usage, so many aspects of vehicle aerodynamics remain poorly investigated. One such area is the impact of propeller-generated flows on multi-rotor airframe and fuselage components. The paper studies the effects of propeller on the aerodynamic drag coefficient and the sideforce acting on the fuselage of a meteorology drone (with a spherical shape). Numerical simulations were performed using the RANS approach with a direct account for the rotor rotation. The numerical results are verified on the basis of PIV-measurements. The results indicate that the aerodynamic performance of the quadcopter fuselage depends significantly on the unsteady flow field generated by the propellers.
The development of non-stationary thermal gravity-capillary convection in a layer of ethyl alcohol with a free surface after sudden electric heating of one of the vertical walls of a rectangular cavity was experimentally studied. The effect of a heated liquid flow along the free surface onto the opposite thin metal wall of the cavity was studied. The influence of the liquid layer heights and heat flux densities on the heated wall of the cavity was investigated. Temperature fields on the thin wall and on the free surface of the liquid layer were measured using a FLIR x6530sc thermal imager. Computer processing of thermal imaging films allowed the construction of distributions of temperature and temperature gradients along the wall height depending on time. The amplitude-frequency characteristics of temperature pulsations associated with the occurrence of secondary flows in the heated liquid flow on the wall were determined, and their effect on instantaneous temperature fields on the wall was studied against the background of a monotonic change.
The paper presents theoretical models for aerothermochemical interaction between the streamlined surface and dissociated air for the case of combustion and sublimation near a frontal critical point of a blunt body. The methods and results were generalized for obtaining a solution based on the similarity method for the case of combustion and sublimation. The approach is based on an example of graphite in an arbitrary cross-section of cone, sphere, wedge, cylinder, plate, and a spherical cone for a case of a laminar flow with a high Reynolds number. For a hypersonic flow past bodies with various geometry shapes, calculations for mass flux, heat flux and drag coefficient were presented. The results are linked through universal dependencies expressed through stagnation enthalpy, pressure, velocity gradient, pressure gradient and surface temperature.
