Thermophysics and Aeromechanics最新文献

Investigation results on the airflow in a convergent channel with a hot bottom wall at a set constant heat flux or at a set constant wall temperature are presented. The numerical simulation was carried out in the OpenFOAM software using the k-ω-SST turbulence model. The verification of simulation results demonstrated a good agreement between the calculated data and the experimental velocity profiles and the thermal Stanton number. The study showed that an increase in the temperature of the convergent channel wall leads to the phenomenon of the streamwise velocity overshoot, suppression of turbulence and a decrease in the skin-friction coefficient and thermal Stanton number. In contrast to a zero pressure gradient flow, the type of thermal boundary conditions has a noticeable effect on the skin-friction and heat transfer in the convergent channel.

Numerical simulations of an underexpanded supersonic jet exhausting from a circular nozzle are reported. The study is performed in a three-dimensional formulation using two different approaches: Navier–Stokes equations and Direct Simulation Monte Carlo method. In both cases, a reverse flow zone is formed behind the Mach disk in the first shock cell. Thus, this phenomenon, which was previously observed in axisymmetric simulations, cannot be attributed to inaccuracies of approximation of these equations near the axis of symmetry.

The experimental results on heat transfer when a pulsed multi-nozzle spray flows onto a vertical surface are presented. The behavior of the effective heat transfer coefficient averaged over time and over the entire heat transfer surface has been studied. The experiments were carried out in the regime of evaporative cooling at a constant temperature of the heat transfer surface T_w = 70°C. The duration of pulses for supplying the liquid phase of the spray τ and their repetition frequency F were varied in the experiments within wide limits: τ = 1–10 ms and F = 0.25–50 Hz. In addition, the effect of droplet phase flow rate on heat transfer was studied by changing the pressure in front of the nozzles (ΔP_L = 0.05–0.6 MPa). Preliminary studies have shown that heat transfer during spray impingement onto a surface can be strongly influenced by the co-supply of air due to turbulization of the wall layer and the return of droplets reflected from the surface. It has been established that the main factor determining the intensity of heat transfer when the spray flows onto the surface is the time-averaged mass velocity of the liquid phase. Using this value, generalization of experimental data on the heat transfer coefficient and the thermal efficiency parameter of a pulsed spray was achieved.

The paper demonstrates a possibility of applying the known class of analytical self-similar solutions in the form of the traveling heat wave for a system of nonlinear integral-differential equations describing radiative transfer for non-stationary, quasi-stationary, and regular modes of solution behavior. The solutions are constructed for a kinetic model in the Cartesian geometry under the assumption of local thermodynamic equilibrium with specially chosen absorption and scattering coefficients. A test problem for different solution modes is provided.

The paper describes an experimental study of the influence of multiple cyclic pressure loading on the service life and sorption performance of a composite sorbent whose granules consist of selectively permeable (to helium) microspheres as a filler and pseudoboehmite as a porous binder. A test bench is specially designed and fabricated for the study, which makes it possible to model various operation regimes of gas-separation plants in the pressure range up to 10 MPa. Cyclic tests of pressure loading of the granulated composite sorbent are performed, and the sorption capacity of the sorbent with respect to helium is measured. It is found that the composite sorbent retains its integrity and sorption performance under cyclic loading of 1000 cycles and more at pressures up to 10 MPa.

Using an isothermal drop calorimeter, the enthalpy increment of the Na₁₅Pb₄ and Na₅₀Pb₅₀ alloys was measured and the heat capacity was determined in the temperature range of 420–1075 K, including the solid and liquid states. It has been established that the values of the heat capacity of melts significantly exceed the calculations of this value according to the laws for an ideal solution, and this difference decreases with increasing temperature. The obtained results confirm the currently existing ideas that various complexes with a partially ionic character of the interatomic interaction are formed in the melts of alkali metals with lead systems.

This work examines the transition from kinetic to diffusion combustion using optical diagnostic methods. Experimental data were obtained on the temperature fields, composition and velocity of gas near the leading edge of a hydrogen flame flowing from a 2×20 mm slit into the air. The distribution of the rate of combustion product formation, the intensity of heat release and pressure was obtained using the method of balances in equations of energy, momentum, and mass transfer. It is shown that during the transition to diffusion combustion, heat release along the flame length decreases more slowly than the rate of water formation.

Rarefied gas flow into a vacuum through short linearly diverging and converging channels has been examined with the direct simulation Monte Carlo method. Solution to the problem has been suggested using complete geometric setup with quite large areas on inlet and outlet of a model channel in examined geometry. A mass flow rate through the channel and flow field both inside the channel and upstream and downstream have been calculated in a wide range of gas rarefaction. These calculation results are comparable to corresponding data for the channel with constant cross section. A strong impact of channel geometry and gas rarefaction has been proved.

Using an ultrasonic interferometer in the temperature range from 293 to 393 K at pressures from 0.13 to 1.5–2.8 MPa, the speed of sound (U) was measured in helium-xenon gas mixtures with a helium content of 60.34; 71.72 and 85.32 at. %. The measurement errors of temperature, pressure, and speed of sound were ±20 mK, ±4 kPa, and ± (0.15–0.30) %, respectively. By approximating the experimental data for each composition, equations were obtained to describe changes of the speed of sound as a function of pressure and temperature over the entire measurement range. The existing reference and experimental data on the speed of sound in inert gases and He-Xe mixtures were analyzed. A method for calculating U of mixtures with a helium content above 71.7 at. % He to a temperature of 1500 K and a pressure of up to 7 MPa was developed.

Reflections of hydraulic jumps on shallow water are studied. Theoretical criteria of the transition between the regular and Mach reflections are derived, and it is shown that there is a domain of angles of incidence where both types of reflection are possible. Numerical simulations reveal a hysteresis of this transition, which is consistent with theoretical predictions. It is shown that the hysteresis can be obtained by smoothly varying either the angle of the wedge generating the hydraulic jump or the freestream Froude number.