Thermophysics and Aeromechanics最新文献

The effect of capillary-porous coatings obtained by the method of directed plasma spraying on the dynamics of evaporation and heat transfer during nitrogen boiling under conditions of steady-state heat release on copper tubular heaters with a diameter of 16 mm was experimentally studied. It is shown that the presence of coatings leads to an increase in the critical heat flux relative to a smooth heater and heat transfer intensification by the factor of up to 3.5 in the region of low heat fluxes. According to the analysis of high-speed video filming, intensification of heat transfer at low heat fluxes is associated with a significant activation of stably operating nucleation sites. With a subsequent increase in the heat load, intensification relates to a significant contribution of high-intensity heat transfer in the macro-layer zone in local areas between the ridges of structured coatings. It is shown that there is a decrease in the slope of boiling curves of the modified heaters in the pre-crisis regimes, determined by the pulsating behavior of interfaces and accompanied by significant fluctuations in the surface temperature.

A pioneering experimental study of the influence of distributed suction of a supersonic boundary layer on a flat plate on its stability to controlled (artificial) disturbances at the freestream Mach number M_∞ = 2 is performed. Experimental results are compared to numerical predictions, and good quantitative agreement is observed. The conclusions of the linear stability theory that suction of the type considered in the study stabilizes the flow in a supersonic boundary layer are experimentally validated; moreover, the stabilizing effect of suction is more pronounced than the destabilizing effect of surface porosity.

The paper presents the results of the study of effective turbulent heat transfer along a rod bundle at its transverse streamlining, performed using the CONV-3D code based on the DNS approach. To determine the effective heat conductivity coefficient, the mixing of two plane-parallel coolant flows, moving at the same velocities and having different inlet temperatures in the working area was simulated. Comparison of the calculation results with experimental data for water has confirmed the usability of numerical modeling instead of real experiments. Such a replacement of a real experiment with its computational analogue is relevant for investigation of liquid metal coolants.

The structure of a gas-liquid flow and interfacial mass transfer during the transition from the slug to the bubbly flow of ethanol-CO₂ mixture in a horizontal straight microchannel were experimentally studied. The experiments were performed for a channel with a rectangular cross section of 141×385 µm in the range of superficial gas and liquid velocities J_G = 0.16–0.8 m/s and J_L = 0.22–0.5 m/s. To determine a change in the volume of the elongated bubble along the microchannel length due to CO₂ absorption, the method of high-speed visualization with subsequent image processing was applied. The bubble frequency, velocity, size, and change in the volume of gas slugs and bubbles along the channel were measured, and volumetric coefficient of mass transfer from liquid k_La was calculated. The measured volumetric mass transfer coefficient was compared with the known correlations and a new correlation was proposed for the transition from the slug to the bubbly flow due to interfacial mass transfer.

The paper presents results of a theoretical study for parameters of a compressible boundary layer for the case of a re-entry space vehicle flying in atmosphere with the free flow at Mach 6 ≤ M ≤ 10 with sublimation of the carbon coating (graphite). Since a high flight velocity results in a higher wall temperature and a higher mass loss rate, the effect of wall material evaporation decreases the wall temperature as compared with the case of zero-sublimation flow. All that increases the gas mixture density in the sublimation vapor cloud nearby the wall; this is beneficial for stability of the high-speed boundary layer in the response to the first-mode disturbance. As for the second mode disturbances, the lower values of spatial amplification rate with increasing Mach number is observed due to the surface material sublimation. The position of laminar-turbulent transition was evaluated using the e^N method. Our computations demonstrated that (for a flow with M = 6) the surface sublimation has no influence for laminar-turbulent transition; this transition is governed by the growth of three-dimensional (3D) first mode disturbances. At higher Mach numbers (for M = 8 and higher) we observe that the disturbance amplification rate in the downstream direction becomes smaller. The transition is driven by a 2D second mode. The graphite coating sublimation has destabilizing influence for the second mode; that accelerates the boundary layer transition to turbulence.

A modified method of transfer units is developed for a countercurrent film cooling tower with a structured tubular packing with surface intensifiers in order to determine the thermal efficiency of the gas and liquid phases and the temperature of the cooled water at the output. The approach of presenting the number of transfer units, taking into account additional terms with reverse mixing coefficients is applied to indirectly consider the hydrodynamic structure of flows and a decrease in the heat and mass transfer efficiency, compared with the ideal displacement model. An experimental installation with a layout (column) of a Plexiglass cooling tower with a diameter of 200 mm and a height of 2 m is described. Experimental data for water cooling in a structured packing block in the form of a vertical bundle of tightly packed polyethylene pipes with a diameter of 0.05 m with an annular discretely structured surface roughness are presented. Generalized calculated empirical expressions for the drag of dry and irrigated pipes, as well as the dependence of the volumetric mass transfer coefficient on air velocity at different irrigation densities, are obtained. The parameters of expression of the modified number of transfer units are identified based on experimental data on thermal efficiency in the gas phase. As a result, the dependence of the thermal efficiency in the gas phase on the pressure and design characteristics of the structured packing is obtained taking into account the reverse mixing of the flows. Reverse mixing is shown to reduce thermal efficiency by 8 – 15 %, which must be taken into account in the calculations of film cooling towers. The calculation results for the SK-400 industrial cooling tower using the presented expressions are provided and the agreement of the thermal efficiency of the cooling tower with the calculation according to the proposed method is shown.

The results of thermodynamic calculations and experimental studies on hydrocarbon gas pyrolysis at atmospheric pressure in a combined plasma-chemical reactor with the production of hydrogen and carbon black with nanocarbon structures are presented. The plasma pyrolysis technology consists of heating hydrocarbon gas in an electric arc combined reactor to a temperature that ensures its dissociation into hydrogen and carbon black in a single technological process.

In this paper, the analysis of the 2nd law of thermodynamics characteristics of assisted mixed convection heat transfer coupled with fluid flow within a vertical convergent isothermal channel is numerically carried out. The numerical model is undertaken for 2D, laminar and steady flow using finite volume approach and air as working fluid. The analysis focused on two pertinent geometric situations namely the variation of the minimal distance (D_min) of the convergent channel while keeping the convergence angle fixed, and the change of the latter (2°, 10°, and 20°) while D_min kept unchanged. Whereas, D_max was used as the characteristic length. The measurements are performed for the following buoyancy-assisted cases: upward/downward flow within hot/cold walls. Several buoyancy parameters are considered ranging from 0.1 to 10 at Reynolds number equal to 100. The effect the aforementioned geometric parameters on entropy production and its distribution is investigated. The results revealed that the energy degradation is dominated by heat transfer irreversibility. Also, this latter is more influenced by D_min variations.

The article presents the results of pulse heating measurements for the thermal resistances of contacts of liquid lead with Al₂O₃ ceramics and steel in gaps, modeling a heat conducting liquid-metal sublayer in the developed fuel rods of fast reactors of a new generation. The method of obtaining and processing experimental data is described, the results of estimating the measurement error are presented, and the dependence of thermal resistances of liquid lead contacts in model gaps on temperature and number of melting and crystallization of lead is investigated. Based on the experimental results, the thermal resistance of a heat conducting liquid-metal sublayer in fuel rods is evaluated.

Tests of a streamlined cone with a rear hemispherical part were carried out in a supersonic wind tunnel using an installation of free oscillations along the pitch angle at Mach number M = 1.75 and for several values of the body inertia moment (calculated relative to the axis of rotation). In all tests, after the completion of the transient process, undamped oscillations of the cone were recorded with an amplitude depending on the reduced frequency. The dependence for the of undamped oscillation amplitude on the reduced frequency exhibits a pronounced resonant behavior.