Journal of Engineering Thermophysics最新文献

The results of experimental investigation of thermal pulsations in nucleate and film modes of water boiling under Joule heating of a wire heater and a porous cylindrical rod are presented. Pulsation power spectra have been determined from experimental data. It has been shown that in the transition modes of boiling from nucleate to film boiling the frequency dependence of the power spectra acquires a characteristic (1/f) form. Such frequency dependence of the spectra indicates the possibility of large-scale low-frequency emissions. The pulsation power spectra found from experimental data can be used to diagnose the transition to the crisis mode of heat transfer.

The hybrid nanofluid is extensively used in manufacturing for industrial uses because of its exceptional property of enhancing the heat transfer process. The purpose of the present work is to find novel explanations for the behavior of thermal radiation and inclined magnetohydrodynamics effects on the convective viscoelastic flow of water Al₂O₃–Cu hybrid nanofluids over an accelerating permeable surface with mass transpiration. The viscoelastic liquid concept is postulated with the benefit of hybrid nanofluids employing conventional flow patterns that are impacted by the magnetic field. Thermophysical properties of Al₂O₃–Cu and water are employed. Nonlinear PDE for momentum, temperature, and concentration are converted into non-dimensional ODE by employing the proper similarity transformations. The current study is reported to be in very good accordance with earlier research. The velocity field and energy distributions were depicted graphically to show the influence and typical behaviors of physical factors such as the viscoelastic parameter, the Richardson number, the radiation number, etc. In industrial applications, the temperature distribution influenced by radiation is quite important, specifically in accelerated plates where cooling the liquid is necessary to achieve the desired outcome.

The work presents development of a technique for measurement of the local parameters of a near-wall liquid film (film thickness, leading edge velocity, and wave velocity on the film surface) under conditions of a co-current supersonic gradient gas flow. The parameters of an ethanol film with a co-current air flow inside a supersonic conical nozzle with the Mach number M = 2.75 were measured. The decisive role of the gas flow on the parameters of the near-wall film has been shown. It has been established that phase transitions appear in the liquid film within the nozzle because the static pressure in the gas flow over the film drops below the saturated vapor pressure of the liquid.

A simplified model of the heat transfer coefficient in a turbulent film flow of liquid flowing down the spiral tubes of an LNG column is developed. The weight of the liquid and the friction force due to the vapour flow are taken into account. The heat transfer coefficients calculated from the model are compared with the experimental data obtained in Fredheim’s thesis. A good agreement between the calculated and experimental data is obtained.

Mixtures are widely used as refrigerants and coolants in various energy systems. The thermophysical properties of a mixture differ from the properties of its individual components. This paper presents the results of a study of the intensity of heat transfer to a non-azeotropic alcohol-water mixture with a highly volatile component with mass concentration of 30% during forced circulation in a circular channel with spiral intensifiers with a hydrophobic coating. The experiments were carried out in a closed circulation circuit at a pressure of 0.03–0.04 MPa in the storage vessel. The test section was a stainless steel tube 2 m long with internal diameter of 7.6 mm and wall thickness of 0.2 mm. The heating was result of electric current flow in the tube wall. The spiral intensifiers had a winding pitch of 4 mm, and the thickness of the fluoroplastic coating was 0.9 mm. The experiments were carried out at mass flow rates of 36–450 kg/m². The heat flux density range was (8000 < q < 32000) W/m². The pressure drop in this test section was measured in single-phase and two-phase flow regimes, and the dynamics of the pressure drop during the formation of a two-phase flow under various operating parameters was shown. The use of the spiral intensifiers with a hydrophobic coating during circulation of the non-azeotropic alcohol-water mixture (30%) in the circular channel at channel wall temperatures below the saturation temperature of this mixture has led to the formation of a significant amount of the vapor-gas phase in the flow. The appearance of the vapor phase in the flow reduced the pressure drop in the heat-release section with the spiral intensifiers. At almost complete transition of the flow into the vapor phase at the outlet from the section, the pressure drop increased tenfold compared to the pressure drop in the liquid phase flow at the same mass velocity of the flow.

Thermal protection of spacecraft experiences significant thermal loads and requires optimal designing, in terms of both technological and mass characteristics. Carbon aerogels are great interest for development of light high-temperature thermal insulation materials. Introducing them into the structure of composites enables reducing the radiative component of thermal conductivity at high temperatures due to the high extinction coefficient of carbon aerogels in the infrared range. As reinforcing fillers in such materials, highly porous cellular materials can be used, which give the composite sufficient mechanical strength. The physical properties of composites depend strongly on the microstructure of the reinforcing fillers. Therefore, multilayer thermal shield can be designed with choosing, along with the layer thicknesses, the material structure parameters that are optimal for the specific operating conditions of the spacecraft under development. The article presents an algorithm for optimally designing multilayer thermal insulation based on a carbon cellular material filled with aerogel subject to the dependence of the thermophysical properties on the microstructure of the cellular material. Practical application is illustrated with a problem of designing a three-layer thermal shield for a solar probe.

The article presents the results of a numerical and analytical study of the development of a quantum vortex structure in superfluid helium under the influence of a random Langevin force that simulates thermal excitation. The study focuses on issues related to the density of the vortex tangle and distribution of vortex loops by their sizes, as well as the frequency of reconnections. The analytical part presents two methods to solve the problem: continuous and discrete. Numerical simulation is an important tool for solving the stochastic dynamics of quantum vortex filaments subjected to a random force, which is complex task. A comparison of the respective results is carried out.

In the proposed study, experiments were conducted to investigate heat transfer enhancement during evaporation and boiling of R114-R21 refrigerant mixture film flowing down a vertical surface. To improve heat transfer, a dual-scale coating with macroscale longitudinal ribbing and a microscale porous internal structure of sintered bronze particles was printed by combined SLS/SLM (Selective Laser Sintering/Selective Laser Melting) on a flat rectangular substrate ((70times80) mm). The film Reynolds number ranged from 400 to 1300, indicating a change in the film flow regime from the laminar wave to the undeveloped turbulent one. Heat flux density varied from zero to pre-crisis values. The results showed that the presence of the modulated capillary-porous coating can increase heat transfer at nucleate boiling of the falling film by up to four times as compared to a smooth surface. To evaluate the obtained results, the authors compared them with experimental data previously gathered for a flat 3D-printed capillary-porous coating and a microstructured surface created by deformational cutting. The microcharacteristics of the obtained coating were also compared with the active centre size ranges predicted by models of Hsu and Liu et al.

The production of cement clinker faces many management challenges, particularly in terms of consistently high product quality, efficient energy usage, and stable furnace operation. In this study, a machine learning model based on gradient boosting was developed for the efficient operation modes of the kiln (required quality and low energy consumption). The influence of process parameters on the efficiency of the clinker kiln was investigated. As a result, it was shown that stable kiln feeding improves the quality of the final product. High feeding variation leads to an increase in the dispersion of the entire setup and attempts to maintain it in a stable state by changing the volume of burned gas. When there is high feeder operation variation, the lime saturation factor has a significant impact on the outcome. The obtained results can be used to create a digital assistant for the kiln operator.

With application of three-dimensional parabolized system of differential equations including averaged equations of motion in the Oberbeck–Boussinesq approximation and equations for transfer of Reynolds stresses and dissipation rates, a numerical model of the dynamics of a momentumless turbulent wake behind a sphere in a turbulized stratified medium (degenerating external turbulence) was constructed. The components of the mass flow vector and the dispersion of density fluctuations were found from algebraic representations of a locally equilibrium approximation. Numerical simulation of the dynamics of a momentumless turbulent wake behind a sphere and internal waves generated by it in a turbulized linearly stratified medium was performed. The calculation results demonstrate a significant influence of background turbulence on the wake dynamics and internal waves generated by the wake.