Thermophysics and Aeromechanics最新文献

This article presents the results of numerical simulation of heat transfer between a laminar axisymmetric methane/air flame and a cold flat wall. The simulation is performed for different distances between the nozzle exit and the surface, namely, for one, two, and three burner nozzle diameters. The flow evolution is calculated by the direct numerical simulation method with a detailed kinetic mechanism GRI-MECH 3.0. At a distance of three diameters, there is a significant reduction of heat flux close to the stagnation point due to a local recirculation zone formation between the flame cone and the wall. This phenomenon explains the observed decrease in local heat transfer. The obtained numerical results well agree with the previous flame study by planar optical methods.

Velocity fields and air flow structures have been experimentally established in a local working area with a horizontal panel heated by a gas infrared emitter in the modes of free and mixed (during operation of the air exchange system) convection using the optical SIV (Smoke Image Velocimetry) method. The analysis of the influence of the structure of the air flow, resulting from the heating of surfaces located in the local working area, on the formation of air velocity fields in the operating conditions of a gas infrared emitter was carried out. The extent of the influence of the convection regime on the aeromechanics of the local working area heated by a gas infrared emitter has been established.

A method and results of computing a laminar-turbulent flow past a swept wing under a control action generated by a source of blowing or suction from the model surface are presented. Pioneering results on the influence of three-dimensional blowing and suction sources on stability of the boundary layer on a swept wing and distributions of N-factors of various mechanisms of the laminar-turbulent transition aimed at changing its position are obtained by using the LOTRAN 3 software package.

The paper presents the experimental results on transient nucleate boiling on the heater surface with rapidly increasing surface temperature. According to the results of high-speed video recording with a frequency of 180 000 frames per second and a spatial resolution of 5.5 urn per pixel, the input data for existing models of heat transfer during nucleate boiling must be refined to take into account the existence of cluster and pulsating bubbles. It has been established that bubbles, interacting through the exchange of momentum, heat and vapor mass, accelerate activation of neighboring vaporization sites, so the clusters of bubbles can form at the initial stage of covering the heater surface with vapor. The main characteristics of single, cluster and pulsating bubbles have been studied for the wall superheating from 0 to 14 K above the temperature of nucleation beginning and flow subcooling from 23 to 103 K.

The influence of a thin layer of phase change material (PCM) on the thermal characteristics of the outer wall of a building made of lightweight thermal insulation material was studied numerically. Changes in temperature and heat flux density were analyzed for various locations of a PCM layer in the wall. It is shown that the use of a thin paraffin layer 4 mm thick in a wall made of foamed polyurethane 100 mm thick can reduce the amplitude of heat flux fluctuations on the inner surface of the wall in the summer from 2 to 13 times, depending on the PCM location. The greatest reduction is achieved when installing the PCM in the central area of the wall. Calculations show that when using a PCM in the walls of buildings made of light thermal insulation materials, a positive effect, associated with a 6–8 hour delay in the maximum heat flux entering the room relative to the maximum daily value of the outside air temperature, is observed in the summer.

The paper describes the results of an experimental study of the influence of control surfaces and fuselage on the structure of a separated flow around a model of a small-size unmanned flying vehicle with a straight leading edge of the wing. The use of oil-soot visualization and hot-wire anemometry shows that the separation region location depends on the attitude of control surfaces, while the presence of a fuselage leads to reduction of the critical angle of attack.

Numerical computation of aiding mixed convection and heat transfer characteristics in a channel with a baffled heated wall is carried out in this work. The equations of mass, momentum and energy, alongside the boundary conditions, are solved by the finite volume formulation using the open source OpenFOAM® code. Simulations are accomplished under different parameter combinations, including the Reynolds number, Grashof number, and baffle dimension. The results are presented in terms of streamlines, isotherm contours, Nusselt number, and friction factor. The results obtained revealed that the flow translates from steady to unsteady state at a relatively low value of Reynolds number. The unsteady flow behaviour contributes to improve heat transfer by disturbing the near-wall region. The augmentation of velocity and baffle dimension leads to a notable heat transfer enhancement; however, this enhancement is less sensitive to the heating intensity augmentation.

The paper presents the results of experimental studies on the effect of multi-walled carbon nanotubes (MWCNTs) additives on the viscosity and rheological characteristics of drilling emulsions based on mineral oil. The formulations of typical drilling fluids containing 65% hydrocarbon phase were modified with nanotubes. The mass concentration of nanotubes in emulsion varied from 0.1 to 0.5%. The formulation and method of preparing stable drilling emulsions with MWCNT additives have been proposed. The rheology of drilling emulsions modified with MWCNTs was studied. The dependency of rheological characteristics on the nanotubes concentration was obtained. In general, MWCNT additives can significantly alter the rheological characteristics of drilling hydrocarbon emulsions at lower concentrations compared to the additives in the form of spherical nanoparticles. This is very important for their practical use in industry. The optimal concentration of MWCNTs for controlling the rheological properties of drilling emulsions is about 0.25 wt. %.

The study is aimed at clarifying and revealing the basic principles of the effect of superheated steam and its parameters on the content of solid carbon particles (soot) in intermediate and final combustion products when burning liquid hydrocarbon fuel. Using a laboratory atmospheric atomizing burner, it was determined that there is a significant amount of solid carbon particles at the base of the burner flame. When heated air is used instead of steam, an increase in soot content by ∼75 % is observed. The analysis of the flame glow intensity in a narrow ultraviolet band also showed that in air the glow values are ∼75 % higher than when using superheated steam. At the same time, it has been established that the soot content in the final combustion products is affected only by the parameter of the dynamic effect of a jet, which determines air ejection from the environment both into the gas generation chamber and into the external flame region.

Constructing models of thermophysical characteristics of polymer composite materials in the curing process is a highly pressing task that is substantiated in this paper. A mathematical model of hot curing of polymer composites based on a thermosetting resin in a mold is presented. Based on experimental temperature dependences of thermophysical characteristics measured under different conditions, modeling dependences of the volume heat capacity and thermal conductivity of fiber, fabric and granular polymer composites on the degree of cure and resin fraction during curing are obtained, with their distinctive feature being the replacement of the properties of a porous reinforcing filler with those of the cured resin in the model. The results of experiments and calculations are presented. The proposed models of thermophysical characteristics increase the accuracy of process modeling and calculation of optimal temperature-time curing cycles.