Journal of Engineering Thermophysics最新文献

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.

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.

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.

Porous metallic substrate for metal-supported solid oxide fuel cells was developed utilizing stainless-steel powder and triethanolamine as a new binder. Starch was added as an additional agent to increase porosity and gas permeability of the samples. The structure and functional properties of the obtained substrates as the function of the additives content and the processing conditions were investigated. The optimal parameters have been determined. When the combined percentage of the binder and pore-former was raised up to 5%, the porosity and permeability increased up to 46.2% and 3.1 d respectively. As the sintering time of the substrate increased to 6h, the hardness grew up to 311 HRC. A thermal expansion coefficient value of (sim 14times 10^{-6}) has been obtained. The results demonstrate how the preparation process affects all of the major parameters, including porosity, permeability, hardness and roughness. Both the content of additives and processing conditions may vary in relatively broad range to attain particular required properties of the substrates. Better to similar properties compared to literature data have been obtained.

Single-phase heat transfer from a vertical titanium plate with area of 140 mm² to a water droplet flow (spray) normal to the surface of the heat exchanger was studied experimentally. Two models of commercial hydraulic full-cone nozzles with different characteristics of flow and spray pattern were used, the average mass flow rate being 2.4 kg/m²s to 6.46 kg/m²s. The maximum heat flux density of the heater was 204 kW/m². The area-averaged heat transfer coefficient was found to be highly dependent on the mass flow rate of the coolant. The average droplet diameter and the outflow velocity from nozzle openings appear to have secondary influence on single-phase heat transfer.