Thermophysics and Aeromechanics最新文献

Under certain conditions, the turbulent transition can produce local turbulent structures knows as puffs. Puffs are the zones of turbulence surrounded by laminar flow; their behavior depends mainly on the Reynolds number (Re). We study the influence of initial flow conditions on puffs generation in circular tubes with different lengths using the direct numerical simulation (DNS) approach at a constant Reynolds number Re ≃ 2200. The literature data show that in the flow with Re ≃ 2000, the singled puffs are generated as metastable structures with alternatives for decay or survival, while at Re > 2300 these puffs can interact and produce more complicated and stable configurations (double and triple puffs). Puffs are classified as stable puffs if the lifetime exceeds considerably the characteristic time in the system (typically defined as the flow area length divided by the value of axial velocity in a laminar flow interval). The literature data evidences an increase in the puffs amount with increasing in the Reynolds number, but the problem of flow reproducibility for flow structure at the same Reynolds remains an open issue. The published data are important for prognosis and control of turbulence in various engineering application.

The results of multi-criteria analysis obtained on the basis of experimental data in the process of studying the main properties and characteristics of thermal conversion of alternative liquid fuels are presented. Kerosene was used as the basic hydrocarbon component. Bioadditives are represented by vegetable oils (rapeseed oil, tall oil, camelina oil, and waste culinary oil) and methyl esters of fatty acids obtained during the processing of these oils. The combustion characteristics of fuel compositions were experimentally determined: ignition delay times, threshold temperatures of combustion initiation, burnout duration, and concentrations of the main gas emissions. Promising fuel compositions are identified taking into account the main energy and environmental indicators. The possibilities of using compositions with methyl esters of fatty acids to reduce specific anthropogenic emissions in the composition of combustion products are substantiated.

A low-parameter equation has been obtained to describe the viscosity coefficient of liquid and gaseous neon at temperatures from 24.6 to 700 K and pressures from 0.044 to 50 MPa. This equation allows data obtaining within the experimental error. It is shown that this equation, proposed for calculating the viscosity coefficient of liquid and gas, allows reliable extrapolation beyond the studied area.

The experimental data on the effect of a pair of weak shock waves on the flow in the boundary layer of swept plates with initial angles χ = 35° and 40° at Mach number M = 2 are presented. The incident flow was disturbed using a weak shock wave (SW) generator made in the form of a two-dimensional sticker on the side wall or on the nozzle surface in the test section of the wind tunnel. For the latter case, shadow visualization of the flow past the models was carried out and the inclination angles of the weak SWs were determined. Measurements with a constant temperature hot-wire anemometer allowed us to record for the first time the effect of a weak SW from the leading edge of the sticker on the flow in the boundary layer of a flat plate with large sweep angles of the leading edge. On the model with χ = 35°, in the vicinity of the maximum effect of a pair of weak SWs, the flow characteristics were measured with a continuous change in the rotation angle of the model. The measurement results suggest that the sweep angle χ ≈ 48° is the critical sweep angle of the blunt leading edge, at which longitudinal vortices are not generated in the boundary layer by a “co-directional or overtaking” weak shock wave. The conclusions of previous studies that with an increase in the sweep angle along the leading edge, there is a decrease in the intensity of the effect of weak shock waves on the flow in the boundary layer, and that flow turbulization occurs for a model sweep angle of 50°, have been confirmed.

The experimental results on heat transfer during the gas coolant flow in a space formed by a dense packing of seven heated tubes are presented. Eight separating inserts with longitudinal displacers were used to fix the tubes and ensure a uniform gas flow field in the internal and external channels of the tube bundle. Gas mixtures with a large difference in the Prandtl number were used as the working fluid: air (Pr = 0.7) and helium-xenon mixture (Pr = 0.23). The experiments were carried out in the range of Reynolds numbers of 2218–12900.

Interaction of an external artificial disturbance with a blunted leading edge of a swept wing is studied under the conditions of a model experiment. Specific features of disturbances of a three-dimensional boundary layer arising due to significant bluntness of the leading edge and transverse flow are identified. It is shown that localized disturbances generated in the incident flow interact with the boundary layer on the swept wing and generate unsteady streaky structures; high-frequency wave packets are formed near these streaky structures. The dynamics of the development of wave packets and localized streaky structures in a three-dimensional boundary layer in a gradient flow is quantified.

The processing of experimental dependences for pitching vs. time curve for free oscillations of a cone with the attached hemisphere rear part gives the pitching moment coefficients, equivalent coefficients for pitch damping, spectral coefficients for pitching angle and pitching moment coefficient for self-induced oscillations flow mode. It was shown that a dependency of pitch damping vs. oscillation amplitude presents a hyperbolic behavior. The frequency spectrum for pitching and pitching moment coefficient demonstrates a domination of one frequency. There is no hysteresis in a dependency of the pitching moment on the attack angle for self-induced oscillations.

The efficiency of a regenerative heat exchanger with an intermediate coolant and drop irrigation under winter operating conditions has been experimentally studied. It was found that the temperature efficiency of the heating and cooling columns increased with an increase in the packing irrigation density from 0.11 to 0.20 kg/(m²·s). The maximum temperature efficiency of 71 % for the heating column was obtained at an irrigation density of 0.21 kg/(m²·s). In the cooling column of the heat exchanger, the air flow from the room was cooled, and moisture was condensed there. In the heating column of the heat exchanger, the opposite process of intense evaporation of moisture and humidification of the air flow entering the room was observed. The maximum humidity efficiency of the heating column was about 80% at a packing irrigation density of 0.17–0.25 kg/(m²·s).

Experiments were performed on formation of phase interface for a case of distilled water boiling on cylindrical surfaces with the diameter of 1 and 3 mm. During the experiment, a bubble boiling regime was ensured, these bubbles are detected with the high-speed PSV (Particle Shadow Velocimetry) technique with 8000 f.p.s. rate. Data processing provides for bubble size distribution and defines the mean detachment diameter and the bubble growth rate as a function of heat flux. The effect of the boiling surface curvature on boiling characteristics was evaluated.

The mass transfer in nanopores is understood respectively from the multiscale flow regime and the nanoscale non-continuum flow regime according to the size of the nanopore. If no interfacial slippage occurs, the mass flow rate through the nanopore is normally far smaller than the classical Hagen–Poiseuille equation calculation especially for small nanopores owing to the fluid-pore wall interaction which results in the significant effects of the viscosity enhancement and the non-continuum property of the very thin adhering layer in the nanopore. If the interfacial slippage occurs, in the multiscale flow regime, the adhering layer-pore wall interfacial slippage is advantageous over the adhering layer-continuum fluid interfacial slippage especially for small nanopores because of generating much greater flow rates through the nanopore. A hydrophobic nanopore wall is thus preferential. In the nanoscale non-continuum flow regime, the wall slippage more easily occurs, and its effect is determined by both the power loss on the nanopore and the intrinsic parameter of the nanopore.