Thermophysics and Aeromechanics最新文献

The paper presents experimental data on the effect of morphology of the black silicon superhydrophilic surface structure fabricated by plasma-chemical etching on heat transfer during pool boiling of water. Silicon surfaces with homogeneous and hybrid microstructures are investigated. Heat transfer experiments were carried out on pre-selected microstructured surfaces with the best characteristics of capillary wicking. It is shown that the critical heat flux (CHF) for a surface with a hybrid structure is approximately three times higher than the CHF for a smooth silicon surface (660 kW/m²), reaching a value of 1914 kW/m², while the CHF for a surface with a homogeneous structure exceeds the CHF for a smooth surface by the factor of 2.4, reaching a value of 1568 kW/m². At that, the maximum recorded heat transfer coefficient (HTC) of the surface with a homogeneous capillary structure, on the contrary, is the highest (77 kW/(m²K)), almost twice exceeding the heat transfer coefficients for the unmodified surface in the region of moderate heat fluxes. The surface with a hybrid structure demonstrates a delay in boiling incipience when compared with the results for a smooth surface, but with a further increase in the heat flux it significantly exceeds the HTC for the smooth reference surface, ultimately reaching a maximum value of 45 kW/(m²K) in the pre-crisis region.

The results of experimental and computational studies for the coolant flows mixing at different temperatures for standard operational parameters of a nuclear power unit (NPU) are presented in this paper. Experiments were performed using an upgraded version of measurement model. This study defines a temperature filed in the mixing zone and offers the optimal set of operation parameters calculated from the condition of maximum temperature pulsation within the mixing zone. Simulation was performed using a software kit ANSYS Fluent. The grid model is based on the block structure in ANSYS ICEM code. Numerical simulation was performed for unsteady problem statement based on the LES WALE turbulence model. Analysis of simulation and experimental fields for flow velocity and temperature confirmed the validity of the developed numerical method. A qualitative compliance for the probability density distribution and the spectral-correlation characteristics for temperature signals in the mixing zone was observed. The spectral power density for calculated temperature and velocity distributions fits the case of 1D energy spectrum of developed isotropic turbulence.

This paper presents the results of numerical simulation of a submerged jet leaving an orifice of diameter D with velocity U, when the nozzle is subjected to mechanical oscillations (vibrations) of frequency f. Vibrational excitation leads to jet bifurcation at Reynolds number Re = DU/v > 50 in wide ranges of oscillation amplitude Z₀, Strouhal number St = fD/U, and it is similar to acoustic forcing. The obtained estimates of the characteristic jet thickness and its expansion angle α performed to study the influence of Z₀, Re, and St parameters allow identification of the optimal values St_opt of the Strouhal number, at which the splitting process is most pronounced. The previously noted effect of an increase in angle α with an increase in the excitation amplitude and saturation after some threshold values of amplitude Z₀ are confirmed. These results also indicate that with a decrease in Re, the threshold amplitude (above which saturation in α occurs) increases, and the maximum possible values of α drop significantly. The influence of Re on the optimal Strouhal number was found: with a decrease in Re from 3000 to 100, the St_opt values drop significantly.

On the basis of a complete system of acoustic equations, the evolution of acoustic disturbances in a high-temperature reactive mixture of carbon dioxide, carbon monoxide, and oxygen CO₂/CO/O in the dissociation-recombination reaction, as well as in the nonequilibrium endothermic reaction of CO₂ dissociation and exothermic reaction of afterburning of carbon monoxide in oxygen, is investigated. A realistic single-temperature Arrhenius model of chemical kinetics is used. The values of thermodynamic parameters on the trajectory of hypersonic flight in the Martian atmosphere are taken as stationary environmental conditions. For the dissociation–recombination reaction with a negative thermal effect near chemical equilibrium in the asymptotic high-frequency limit, attenuation of acoustic disturbances is shown. Under the same conditions, acoustic disturbances amplify in the nonequilibrium exothermic reaction and attenuate in the nonequilibrium endothermic reaction.

Numerical modeling of the local flow structure and mixing process during injection of a gas-droplet wall jet into a turbulent heated air flow has been performed. The numerical solution is based on a system of axisymmetric Reynolds-averaged Navier–Stokes equations (RANS) taking into account the two-phase character of the flow. To describe the dynamics and heat and mass transfer in the dispersed phase, the Euler two-fluid approach is applied. A significant effect of the liquid mass concentration on the distribution of parameters over the channel cross section and thermal efficiency is shown. An increase in thermal efficiency reaches 30 % in the initial part of the channel and 200 % in its end part in comparison with a single-phase wall air jet.

This paper presents the results of experiments and simulation on the regularities of a high speed gas-particle flow impinging a complex target under conditions of cold spray deposition. The target consists of a substrate and an installed in front partition with conical converging/diverging perforation. Numerical simulation was performed using the ANSYS Fluent in the mode of single particles motion (without the influence of particles on the gas flow parameters). Also, the results obtained were compared with previously investigated penetration of gas-particle flow through a mask with a cylindrical perforation.

The results of an experimental study of the flow around a 10 ° sharp cone in two large-scale TsAGI wind tunnels are presented. The Mach and Reynolds numbers of the free-stream flow varied in a wide range of M = 0.2–6 and Re = 3–34 million. The drag, lift, and side force coefficients were measured. The behavior of the pitching, rolling, and yawing moment coefficients was studied. The distribution of static pressure on the cone surface and pressure fluctuations were analyzed. A physical model of the cone streamlining was constructed. An ordered system of data was prepared to contain both the results of experiments and test calculations, intended for the validation of computer programs and models.

The paper presents the experimental results on capillary liquid spreading over superhydrophilic black silicon surfaces of various morphologies created by cryogenic plasma-chemical etching. Five samples of capillary surfaces with different microstructure characteristics were made, including two hybrid surfaces combining microstructuring of different heights. The morphology of surfaces and characteristics of their wetting with water were studied, including the Wi number, which characterizes the capillary absorption of liquid. It was found that a greater height of microstructures ensures better liquid spreading. The maximum values of the Wi number are achieved on hybrid surfaces with a greater density of “high” microstructures. The prospects of using hybrid capillary surfaces to increase the critical heat flux during water boiling are shown.

The processes of heat transfer and phase transformations in a freshwater reservoir during the cold season are considered. A non-stationary computational model applied for determining the thermal behavior of a reservoir and the ice cover dynamics is based on the numerical solution of the Stefan problem in a generalized formulation. Climatic conditions are considered in the form of dependences of the average daily values of air temperature, wind velocity, humidity and solar radiation flux. The influence of heat fluxes and snow cover height on the reservoir temperature and ice growth dynamics is analyzed.

It is demonstrated in the paper that the range of an equilibrium SF₆ jet normalized to the nozzle diameter is independent of the dimensional stagnation temperature if the values of four dimensionless parameters (jet pressure ratio, Reynolds number based on the nozzle diameter, and two temperature factors) remain unchanged. In contrast to the equilibrium case, the range of a vibrationally excited SF₆ microjet is an essentially nonmonotonic function of the dimensional stagnation temperature.