Vol 27, No 4 (2018)

A three-dimensional numerical model of calculating in the Euler approach is developed to calculate a two-phase turbulent near-wall flow; simulation of thermal efficiency of a gas-droplet shielding injected into a transverse trench through inclined cylindrical holes is fulfilled. The influence of the main thermo-gas-dynamic characteristics of the two-phase flow on thermal efficiency is analyzed. Significant increase in thermal efficiency was obtained by adding droplets in the nearwall coolant flow (up to 2 times in comparison with a single-phase flow). A particular advantage of this method of coolant injection is achieved at high injection parameters. It is shown that the use of two-phase gas-droplet near-wall shielding is promising for protection of surfaces against thermal influence of the heated gas flows.

The features of propene oxidation in high-density mixtures of C₃H₆/O₂ ([C₃H₆]₀ = 0.23–0.25 mol/dm³, [O₂]₀ = 0.76–0.82 mol/dm³), diluted with argon, carbon dioxide and water vapor at uniform heating (1 K/min) to T ≤ 620 K are investigated for the first time. From the time dependences of reaction mixtures temperature it is found that propene self-ignition occurs at 465 K and does not depend on the nature of the diluent. Using mass spectrometry analysis it is demonstrated that in the composition of the products of propene oxidation in the Ar and CO₂ medium predominate methanol, acetaldehyde, acetone, acetic acid and formaldehyde; in the oxidation in the H₂O medium, only small amof O₂ in the oxidation of propene increases in ounts of these substances were registered. Degree of consumption the following order: CO₂ ≪ Ar < H₂O, which is a consequence of the involvement of CO₂ and H₂O molecules in chemical transformations. Mechanisms of the observed processes are discussed.

Exergy is a useful tool for quantifying the locations, types, and magnitudes of wastes and losses caused by unavoidable irreversibilities in real processes. However, for the typical and popular turbulent piloted non-premixed methane–air jet flame, the Sandia flame D, no detailed exergy losses have been reported. This study reports the local exergy losses of the Sandia flame D for the first time by demonstrating the local exergy losses from heat transfer, chemical reaction, gas diffusion, and viscous dissipation. The results show that the local exergy losses from heat transfer, chemical reaction, gas diffusion, and viscous dissipation are in the ranges of 0–23210.17, 0–10796.30, 0–6.79, and 0–3.39 kW/m³ in the computational domain, respectively. These make the total local exergy loss of the Sandia flame D vary in the range of 0–23282.45 kW/m³, and it is mainly contributed by heat transfer (71.42%) and chemical reaction (28.56%), followed by gas diffusion (0.01%) and viscous dissipation (0.01%). The results obtained from this study illustrate well what the local exergy losses of the Sandia flame D are as well as how they are caused and contributed.

The effects of temperature-dependent viscosity and thermal conductivity on heat transfer and frictional flow characteristics of water flowing through a microchannel are numerically investigated in this work. The hydrodynamically and thermally developing flow with no-slip, notemperature jump, and constant wall heat flux boundary condition is numerically studied using 2D continuum-based conservation equations. A significant deviation in Nusselt number from conventional theory is observed due to flattening of axial velocity profile due to temperaturedependent viscosity variation. The Nusselt number shows a significant deviation from conventional theory due to flattening of the radial temperature profile due to temperature-dependent thermal conductivity variation. It is noted that the deviation in Nusselt number from conventional theory is maximum for combined temperature-dependent viscosity and thermal conductivity variations. The effects of temperature-dependent viscosity and thermal conductivity on the Fanning friction factor are also investigated. Additionally, the effects of variable fluid properties on Poiseuille number, Prandtl number, and Peclet number are also investigated.

The paper presents a review of research dealing with the limit energy theorem for fast gas flow systems (FGFS’s) [19, 21, 23, 24, 38] where no mechanical work is performed. FGFS’s include gas-discharge lasers and plasmatrons, chemical gas reactors, vortex tubes, acoustic systems, various gas mixing devices, astrophysical objects, etc. The analysis shows that these flow systems do not belong to the Carnot class. In contrast to the Carnot cycle efficiency, the efficiency of energy conversion in FGFS’s depends on the properties of the working medium, and the conventional FGFS cycle is essentially irreversible. For the chosen class of FGFS’s, the theorem yields a more strictly formulated second law of thermodynamics. Implications of the theorem for various fields of physics and engineering are discussed.

Conventional fluids have poor heat transfer properties, but their vast applications in power generation, chemical processes, heating and cooling processes, electronics and other microsized applications make the reprocessing of those thermofluids to have better heat transfer properties quite essential. Recently, it has been shown that the addition of solid nanoparticles to various fluids can increase the thermal conductivity and can influence the viscosity of the suspensions by tens of percent. Thermophysical properties of nanofluids were shown dependent on the particle material, shape, size, concentration, the type of the base fluid, and other additives. In spite of some inconsistency in the reported results and insufficient understanding of the mechanism of the heat transfer in nanofluids, it has been emerged as a promising heat transfer fluid. In the continuation of nanofluids research, the researchers have also tried to use hybrid nanofluid recently, which is engineered by suspending dissimilar nanoparticles either in mixture or composite form. The idea of using hybrid nanofluids is to further improve the heat transfer and pressure drop characteristics by trade-off between advantages and disadvantages of individual suspension, attributed to good aspect ratio, better thermal network and synergistic effect of nanomaterials. As a conclusion, the hybrid nanofluids containing composite nanoparticles yield significant enhancement of thermal conductivity. However, the long-term stability, production process, selection of suitable nanomaterials combination to get synergistic effect and cost of nanofluids may be major challenges behind the practical applications.

Improved second-order mathematical models are developed for description of the processes of vertical turbulent exchange in stably stratified water body. The models are based on algebraic representations of Reynolds stresses and fluxes and the use of a differential transport equation of dispersion of fluctuations of the vertical velocity component. A numerical model of the vertical turbulent exchange under simultaneous stratification in temperature and salinity is developed. In the case of stable stratification defined by only a variation in salinity, role ofWeinstock modification of the relaxation time scale of a scalar field is evaluated.

For the first time the limit conditional cycles in P-V and T-S coordinates of the device implementing the stratification effect (TL) for subsonic and supersonic flow regimes have been presented. The analysis of energy efficiency of heat exchange processes in TL on the basis of the developed limit cycles is carried out. As one of the integrative criteria of the device functioning the indicator of power exchange efficiency of the flow machine is offered.

Results of analysis of thermodynamic cycle efficiency with consideration of nonisothermal nature of phase transition indicate that when using the mixture of R32/R152a (30/70%) in the vapor compression heat pumps, the coefficient of performance is up to 4% higher in comparison with R32/R134a (30/70%). The study shows that in cycles with two-stage compression the coefficient of performance increases by 6.5% as compared to the single-stage process. The value of exergic efficiency of the single-stage cycle is equal to 0.47 for zeotropic mixture of R32/R152a (30/70%), and it is equal to 0.45 for the R32/R134a (30/70%) mixture. Data obtained for the zeotropic mixtures are as effective as those obtained for refrigerant R134a.

The article presents the rationale for production of dried fruits and vegetables using a solar drying unit. To intensify the drying process, convection of drying agent flow in the proposed drying chamber is studied using Navier–Stokes equations. Numerical methods are used for solving equations describing the process of convective heat transfer. As a result, graphical interpretations of isolines of drying agent flow are obtained and location of passive zones in the dryer chamber are identified. Uniformity of the temperature zones in the chamber is ensured by supplying additional drying agent into the passive zones. Temperature values at various levels of the drying chamber are experimentally obtained. Results for drying cut-up mass of vegetables and fruits are presented.