Thermophysics and Aeromechanics最新文献

The problem of hydrodynamic stability of a boundary layer with diffusion combustion is formulated in the Dan–Lin–Alekseev approximation and at constant Prandtl and Schmidt numbers; it is reduced to solving a system of the tenth-order ordinary differential equations with homogeneous boundary conditions. With Lewis numbers equal to unity, it may be lowered to the eighth order. In the inviscid approximation, the stability problem is reduced to the integration of a single second-order differential equation.

Based on the obtained stability equations and calculations of stationary flow parameters, the stability of a supersonic boundary layer with diffusive combustion on a permeable plate with hydrogen supply through its pores is studied for the first time by direct numerical modeling. With the Mach number M = 2, the possibility of flame flow stabilization is established using calculations. It is shown that within the framework of the inviscid theory of stability, it is possible to obtain quite reliable data on the maximum degrees of the growth of disturbances.

The paper presents the results of numerical simulations of high-velocity turbulent air flows in a plane channel with a variable cross section exhibiting sudden expansion with allowance for coupled heat transfer with copper plates modeling the sensitive elements of heat flux sensors. The simulations are performed for conditions of a high-enthalpy short-duration wind tunnel whose specific features are the short duration of the test regime and unsteady “falling” conditions at the model channel entrance. The wave structure of the supersonic flow, which affects the heat fluxes at the walls, is analyzed for various Mach numbers at the model channel entrance. The numerical algorithm is validated on the basis of experimental data on heating of the sensitive elements of heat flux sensors for unsteady input conditions at the channel entrance. The influence of the Mach number, static parameters, and stagnation parameters on the rate of heating of the sensitive elements located at various points in the channel is studied numerically. The heat fluxes calculated under constant and “falling” conditions at the channel entrance are compared. It is shown that the accuracy of heat flux modeling can be increased by taking into account the intensity of oscillations of the flow parameters and their changes along the channel.

The paper studies the quality of water spraying by system with a compressed-air atomizer designed for water injection into the intake manifold of an internal combustion engine. The system consists of a two-cylinder piston compressor providing compression of water-air mixture in the working chambers. The piston compressor has connection to the atomizer through long channels. The droplet sizes were measured through automatic image processing by the Shadow Photography method. The droplet velocity field was measured by 2D-PIV method. Experimental results demonstrated that the injection system offers a high quality of atomization for the air/water mass ratio higher than 0.46. The Sauter mean diameter was no more than 31.1 µm.

Thermal conductivity of a Novec 7100 fluid sample was measured by the steady-state method of coaxial cylinders. Experiments were performed in the temperature range 350–385 K and pressure range 0.12–0.21 MPa. The error for experimental data on thermal conductivity is about 1.5–2.5 %. The error in measuring temperature and pressure was less than 0.05 K and 4 kPa, correspondingly. The general equation for calculating the thermal conductivity as a function of pressure and temperature was formulated. Thermal conductivity was defined for the ideal gas state. A previously developed approach was tested in application for a single-measurement prediction of thermal conductivity.

The volumetric properties of liquid indium-lead alloys containing 20 and 33 at. % Pb have been measured using gamma-ray attenuation technique at temperatures from the liquidus line to 880 K. The density changes of these alloys during solid–liquid phase transition have been calculated. The obtained experimental values of the molar volume and the volumetric thermal expansion coefficient of melts and the results of calculations according to the laws for an ideal solution and data of other authors have been compared.

The morphology of carbon material formed in an arc discharge in a mixture of i-butane, n-butane, and propane when spraying a graphite-nickel electrode was studied. The experiments were carried out with changing the gas medium pressure. Carbon globules, graphene structures, and carbon nanotubes have been discovered. It was found that at pressures of 75 and 400 torr, carbon globules predominate in the resulting materials. At gas pressures of 200 torr, the material collected from the cold screen surface contains both graphene-like structures and significant amounts of carbon nanotubes. The physical reasons influencing the observed phenomena are discussed.

Results of studying the process of air blowing through a perforated section of the surface on an axisymmetric body with an aspect ratio of 25.3 in an incompressible flow with the Reynolds number Re_L = 4.36·10⁶ are reported. The blowing coefficient C_b is varied in the interval from zero to 0.00885. It is shown that distributed blowing through a perforated wall with improved geometry ensures a significant gain in friction drag as compared to that for the base configuration. Beginning from the frontal boundary of this section and further downstream, stable reduction of local friction is observed, which reaches 72 % directly in the region of blowing with the maximum intensity. In view of the energy expenses on the blowing process, the degree of energy saving can reach 1.4 to 6.1 % for the blowing region being located on the cylindrical part of the model. The efficiency of this method of boundary layer control can be refined by a more accurate determination of the contribution of the drag component induced by the pressure and friction forces on the frontal part of the body. The importance of estimating the possibility of using the proposed approach for the case of air blowing through a surface section on the frontal part of the body is noted.

The paper presents the calculation estimates for efficiency of regenerative cooling for a model cylinder-shaped flow duct using a suspension of heat-conductive metal nanoparticles in n-decane as fuel/coolant. We adapted a standard mathematical model of conjugated heat transfer that accounts for thermophysical properties of the metal nanoparticle suspension and n-decane. The data are presented for heating up the nanosuspension and the model duct walls for the cases of different content of metal nanoparticles in nanosuspension. There exists a range beneficial for heat transfer from n-decane.

With the aim to model the self-excited oscillations of a body, a hypothesis is proposed for the formation of periodic bottom-wake vortex structures whose frequency coincides with the natural frequency of oscillations of the body, and the force effect of the oscillations on the body is mathematically described with a harmonic function of time. Analytical formulas for aerodynamic derivatives and equivalent aerodynamic derivatives are obtained. It is shown that the mathematical model satisfactorily describes the dependence of the pitch angle on time and the dependence of the equivalent aerodynamic derivatives on the amplitude of oscillations for two moments of inertia of the body. The mathematical model predicts a hyperbolic law for the dependence of the amplitude of self-excited oscillations on the reduced frequency.

An experimental study of the flow of air from a container through throttling tubes of various configurations with characteristic times from 0.6 to 9 s was carried out. The equivalent container-outlet area was determined based on the ratio of the length of the throttling tube to its nominal diameter. It was found that during the outflow, the gas temperature inside the container decreased by 10–15 %, which value differs substantially from the theoretical estimate of 60 % obtained assuming that the process was adiabatic. Based on the results of measuring the gas pressure and temperature in the container, a method is proposed for calculating the heat fluxes on the walls.