Journal of Engineering Thermophysics最新文献

The flow of a viscous fluid film along the outer surface of a vertical cylinder is considered. For this purpose, a model nonlinear evolution equation for the deviation of the film thickness from the unperturbed level is used. When the region of instability corresponding to a certain azimuthal wave number is narrow enough, a simplified system of equations is obtained from the original equation. It is valid for describing spatially periodic solutions at all wave numbers from the neighborhood of this region. The solutions of this system are presented for several values of the azimuthal wave number.

This study explores the flow of a viscous, incompressible, and electrically conducting fluid between plates, where one is perfectly conducting and the other is non-conducting. It focuses on the influence of thermal and solutal buoyancy forces on heat and mass transfer driven by free convection. Both plates are of infinite length, and a uniform transverse magnetic field is imposed to the flow. The analysis also incorporates the effect of chemical reactions within the fluid. The governing equations for momentum, thermal energy, mass concentration, and generalized Ohm’s law are solved with the Laplace transform method. The study examines the influence of various key parameters, including the Hartmann number, Hall current, Soret number, thermal and solutal Grashof numbers, radiation parameter, Schmidt number, and chemical reaction parameter, on flow characteristics (such as velocity profiles and shear stress), heat transfer (temperature profiles and Nusselt number), mass transfer (concentration profiles and Sherwood number), along with the induced magnetic field and current density. The Lorentz force within the flow suppresses convective activities in the flow domain, leading to a reduction in the viscous drag forces exerted on the plates. Among the governing parameters, the Soret number and thermal buoyancy forces play a particularly significant role in shaping the current flow configuration. The magnetic field strength decreases as we move from the conducting plate toward the non-conducting plate. Additionally, the variation in the density of the induced current (and its velocity) between the plates exhibits a parabolic distribution, with the peak values occurring near the center of the flow. When buoyancy forces are sufficiently large and directed forward or downward, they can alter both the flow direction and the orientation of the induced magnetic field and current density.

To investigate the stationary nanofluid droplets effects ofsubstrate surface temperature and particle concentration on thefreezing time, deformation, and droplet contact angle during thefreezing process, high-speed CCD image observation was used tostudy the morphological changes during the freezing process of(TiO({}_{2})–H({}_{2})O) nanodroplets. Nanoparticle droplets wereprepared in this study using magnetic stirring and ultrasonicmixing. Three substrate surface temperatures (268, 265, and 263 K)and four concentrations of TiO({}_{2}) nanoparticles (5, 10, 30, and50 mg/mL) were considered. The findings demonstrate that theaddition of nanoparticles will result in the droplets appearingsanded, a considerable change in the form of the droplet tip, anda decrease in the release of bubbles upon freezing. The increasein supercooling at high concentrations ((>5) mg/mL) causes thedroplet height to rise, its volume to expand upon freezing, andits shape to shift from ‘‘peach-core’’ to ‘‘cone-like.’’ WhenTiO({}_{2}) nanoparticles were added, the droplets’ longitudinalmorphology changed throughout the freezing process, but lateraldiffusion was unaffected, even though the contact angle (theta)marginally shrank as concentration increased. The droplets withthe lowest concentration of TiO({}_{2}) particles exhibit thehighest longitudinal deformation rate (alpha) during the dropletfreezing process. As the subcooling degree increases, (alpha)also rises and reaches its maximum at 263 K or 22.26(%), but asthe concentration of nanoparticles grows, (alpha) drops, and sodoes the coefficient of segregation, (gamma).

This article describes in detail the design of a cryostat developed by the authors, which has high thermal insulation properties. The development is based on the possibility of performing phase transitions of the working fluid from a gaseous state to a liquid, and then to a solid in a hermetic cryostat jacket. The processes of phase transitions occurring in the jacket should be considered as isochoric, since they occur in a closed volume of the jacket. The increase in thermal insulation properties is ensured by the fact that the hermetic space of the jacket is filled with a working fluid in the form of a heavy monatomic gas with low heat capacity and static thermal conductivity (for example, xenon, krypton or an azeotropic mixture of gases or freons). These gases have a condensation and crystallization temperature higher than the temperature of the cryogenic liquid stored in the inner vessel (e.g., helium, hydrogen, neon, nitrogen, oxygen, argon, methane, liquefied natural gas). The cryostat design is described in detail, calculations are given to justify the rational choice of the working fluid for filling the cryostat jacket. When operating this cryostat, there is no need for vacuum pumps, as well as operating costs for maintaining a vacuum. Partial preservation of thermal insulation properties is also ensured in the event of an emergency depressurization of the jacket from the outside. In addition, the operational safety of storing explosive or toxic cryogenic liquids is increased.

The paper presents relations for calculation of the thermal loads and trajectory of motion and its optimization during the spacecraft reentry with the second space velocity into the Earth atmosphere. The aerodynamic characteristics of the spacecraft have been determined. Relations for designing and optimization of the weight of a two-layer indestructible thermal shield are given. The trajectory of motion during the aerodynamic maneuver has been generated. The convective and radiative heat fluxes have been calculated. Heat transfer analysis for the model descent trajectory has been carried out. Optimal thicknesses of thermal shields made of different materials have been obtained. The results confirm the principal possibility of creating indestructible thermal shield for the considered class of spacecraft.

The article is devoted to the justification of the introduction of a flow thermodynamic potential to assess the maximum energy efficiency of gas systems in which there is no mechanical work, but a certain useful effect is realized: energy separation (vortex tubes), mass transfer and bubbling devices (cyclones, ejectors of various types), flow-through chemical reactors, plasmatrons, acoustic gas systems, Leontiev’s pipe, etc. Based on the results of the limiting energy theorem for gas systems, a relation for a thermodynamic potential of a special type is proposed. The thermodynamic flow potential has two equivalent forms: a potential as a function of the mechanical energy of the gas flow and a function of the internal energy of the gas flow.

Experimental data of the superfluid helium dynamics in a U-shaped cylindrical channel filled with metal balls of equal diameter are presented. The description of the experimental cell and the results of the investigations are given in the form of time dependences of the vapor–liquid interface position on time. The values of oscillations amplitudes and frequencies at different pressures and conditions of experiments are presented for two diameters of backfill particles. The causes of oscillations, the possibility of a stationary state, as well as changes in the character of heat and mass transfer processes at the transition through (lambda)-point are discussed.

The review focuses on the study of heat and mass transfer processes in liquid droplets, which play a critical role in various natural and technological systems. The main mechanisms of heat transfer and mass transport in droplets are considered, including convective and diffusive transport, evaporation, condensation, and interactions between the droplet and its surrounding environment. Unresolved challenges remain in deriving simplified estimates for unsteady, non-isothermal heat transfer in liquid droplets under rapidly changing boundary conditions on free surfaces. The review proposes several approaches to assess the intensity of heat and mass transfer. Factors influencing heat and mass transfer intensity, such as droplet size, temperature, liquid composition, gas properties, and external conditions, are analyzed. Theoretical approaches and models commonly used to describe these processes, as well as experimental research results, are presented. This review highlights the relevance of the problem and outlines prospects for further research into heat and mass transfer in droplets to improve technologies and deepen fundamental knowledge of interfacial processes. Understanding the physical mechanisms and accurately predicting droplet behavior and heat transfer are essential for advancing modern technologies, including materials science, spray cooling of microelectronics, inkjet printing, fuel droplet atomization in internal combustion engines, as well as applications in medicine (disease diagnostics) and biotechnology (micro- and mini-reactors).

The present work describes an investigation of mixed convective nanofluid flow in the presence of a porous medium on a sheet surface under radiation and mass transpiration conditions. Suitable similarity transformations are applied to convert the governing partial differential equations (PDEs) into ordinary differential equations (ODEs). Later, the precise yielding of the domain was confirmed by the resulting ODEs. To improve thermal efficiency, silver nanoparticles are dispersed throughout the fluid flow. The primary focus of the current study is on accurate solutions for two-Dimensional nanofluid flow and the impact of several factors on velocity and temperature profiles, including mass transpiration, thermal radiation, inverse Darcy number, and volume fraction. The result shows a single stretching solution.

This study investigates thermal and fluid behaviors in smooth microchannels under slip flow regime using an alternative numerical technique namely the thermal lattice Boltzmann method (TLBM). This method is based on D2Q9 model with lattice-BGK (Bhatnagar–Gross–Krook) approximations. In this procedure, an internal energy distribution function uses to calculate temperature, and a momentum distribution function to evaluate macroscopic quantities like density, pressure and velocity etc. With these macroscopic quantities, the important physical properties such as the average flow friction, mass flow rate, and the heat transfer rate are investigated and discussed for different governing parameters. The relative ramp heights (0(%)–10(%)) and Knudsen number (Kn) (0.01–0.10) are the most important parameters in this study. The average frictional resistance decrease with increasing Kn but increasing with ramps height, whereas the mass flow rate reduced both for ramps height and Kn. Moreover, the heat transfer rate decreased significantly with Kn and very slowly with ramps height. Another important properties, the combined effect of thermal and hydraulic properties called the coefficient of performance (COP) is studied to compare the efficiency of different microchannels. COP decreases with increasing ramp height as well as Kn. Optimal performance is observed with very low ramp heights. The microchannel with negative ramps perform better than positive ramps case. The COP of sawtooth microchannels is calculated to compare with the friction (pressure drop) and heat transfer of smooth microchannel. Finally, the obtained result is compared, and an excellent agreement is found with published work.