Thermophysics and Aeromechanics最新文献

This paper presents the results of development and experiments with a loop thermosyphon with a microstructured heat transfer surface (3D printed) for the case of a flat evaporator and ethanol coolant. The thermal parameters of this device were tested for the temperature in the range from 20 to 128 °C and for the heat flux ranging from 25 to 530 W and for device filling level of 40 ml (100 % of the evaporator volume). It was found that the heat exchanger with pillar-array surface provides the total thermal resistance about 0.18 K/W (corresponding to the top input heat flux equal to 530 W). The developed micropillar array structure for the inner surface of heat exchanger improves the loop thermosyphon efficiency.

Analytical and numerical solutions for the heating of a composite “hydrocarbon–water” drop with a water micro-droplet located in the center of a spherical drop of hydrocarbon are compared. The conjugation conditions are fulfilled at the interface: continuity of temperature and heat flux. At the outer boundary of the droplet, a condition of heat exchange with a hot gas streamlining the droplet is set. The analytical formula is based on the decomposition of the solution in a series of eigenfunctions of the Sturm–Liouville problem. An original conservative difference scheme for the numerical integration of the equations of thermal conductivity inside a composite droplet is constructed to take into account the abrupt change in thermophysical properties at the interface of hydrocarbon–water media. The calculation results obtained using the analytical formula and the numerical integration method are consistent with each other. The numerical scheme includes radiative heat transfer and the effect of evaporation on the heat transfer coefficient. A comparison of the simulation results with experimental data is presented.

The paper investigates the possibility to control the two-phase flow of immiscible liquids with a high viscosity ratio of the carrier and dispersed phases in a T-junction microchannel, with the dispersed phase being a ferromagnetic liquid, by means of a constant magnetic field. The effects of separation of slugs and microdroplets of a ferrofluid, as well as the controllability of the structure of the parallel flow of immiscible liquids, have been found. The obtained results may be used to design microfluidic systems in order to efficiently sort particles, as well as intensify heat and mass transfer processes.

The paper presents the results of numerical simulation of a round turbulent jet propagation in confined space: the jet at Re = 5.4×10⁴ is supplied into a rectangular cavity with a height to width ratio of 0.16. The URANS and LES calculations reproduce the self-oscillating regime, registered previously in the experiments by Lawson et al. (2005). A significant rearrangement of the flow structure and pressure field occurring at a low-flow jet supply from the narrow side wall allows controlling self-oscillations up to their complete suppression. A map of flow regimes has been obtained for three various positions of the opening, supplying the control jet, depending on the control and primary jets momentum ratio. The calculated data provide a quantitative assessment of the flow controllability by injecting a low-flow jet into the zone of the primary jet propagation perpendicular to its axis.

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.