Journal of Engineering Thermophysics最新文献

The paper presents 3D numerical modeling of spreading dynamics of R21 (mol. fraction: 0.9) and R114 refrigerant mixture film. We considered an outer flow along a round vertical cylinder at Reynolds number 104 and contact angles of 10°, 30°, 50°, 70°, and 90°. The simulation was performed in OpenFOAM software on the basis of the volume of fluid (VOF) method. Simulated results show an occurrence of the liquid jets flowing from the upper continuous falling film in the range of contact angles 30°–90°, while the liquid flow at contact angle of 10° keeps the only continuous film till the bottom boundary of the cylinder. We defined two scenarios for wetting of the cylinder sidewall at different contact angles: liquid near the contact line moves down and towards to the cylinder surface at (theta = 10^circ), 30°, and 50°, while only vertical velocity differs from zero at (theta = 70^circ) and 90°, excluding jets at (theta = 70^circ). The increase of the contact angle leads to complication of the structure of the refrigerant mixture falling films, arising of jets and redistribution of flow rate between them.

Turbulent natural convection at isothermal vertical plate is studied experimentally for Prandtl number Pr = 13.5–15. The results of an experimental investigation of time-averaged temperature and velocity fields as well as of shear stress and heat transfer rate in turbulent boundary layer are presented. The thickness of viscous and conduction sublayers is determined. The dependence of these thicknesses on the process parameters for Pr = 13.5–15 is found. The statistical characteristics of the turbulent boundary layer (left( {left( {overline {{u}'^2} } right)^{1/2},left( {overline {{T}'^2} } right)^{1/2}} right)) are obtained. According to the experimental data, the law of variation of the viscous sublayer thickness is found. The laws of variation of the thickness of dynamic and thermal boundary layers are established. The wall shear stress and the shear stress variation across the boundary layer are determined. It has been found experimentally that the flow in the outer part of the boundary layer has a jet character.

Industrial shell and tube heat exchangers require often pipes of considerable length. The surface treatment of such tubes to enhance heat transfer parameters of the equipment must be cheap and simple in manufacturing. Two kinds of tube surface treatment are compared: sandblasting and machining. The tubes with OD = 10 mm, ID = 6 mm, and 625 mm long were tested within a bundle of horizontal tubes installed in a rectangular evaporator shell. Hot water pumped through bundle tubes used as a heat carrier. Refrigerant R21 at the pressure of (sim)3 bara is used as working liquid. Tests have been performed at (Re) numbers from 500 to 1500. The micro-rough tubes (machined duralumin tube, sandblasted copper tube) provide considerable heat transfer enhancement at transition to bubble boiling. At low heat fluxes (evaporation regime) heat transfer on these tubes are close to plain tubes.

In pool boiling applications, appropriate surface properties of the heating surface are crucial for improving the heat transfer. In this study, two different Al₂O₃ nano-structured surfaces were fabricated using dip-coating method by controlling coating thickness, and adopted to conduct pool boiling experiments with R-141b. Prepared Al₂O₃ nano-structured surfaces were characterized by scanning electron microscopy (SEM), elemental dispersive spectroscopy (EDS), ellipsometer, contact angle meter, and 2D profilometer to investigate the surface morphology, elemental composition, thickness, angle of contact, and surface roughness, respectively. Based on the surface characterization and boiling curves, the effects of coating thickness (300 nm and 400 nm) on the pool boiling heat transfer were examined. The heat transfer coefficient (HTC) of Al₂O₃ nano-structured surface (S2) was the highest, followed by those of Al₂O₃nano-structured surface (S1), and plain copper surface. The outstanding heat transfer performance of Al₂O₃ nano-structured surfaces is mainly associated with enhanced surface wettability and increased active nucleation site density.

It is known that nonuniform irrigation leads to a significant reduction in the productivity of column and a noticeable decrease in the quality of separation of mixtures. This work studies different ways of artificially-created nonuniform irrigation of packing. The results of these experiments are compared with the data for uniform irrigation of packing, all other things being equal.

This work presents a study of a range of carbon dioxide concentrations at which gas hydrate growth is observed in a homogeneous nonequilibrium solution. The study was done by the method of molecular dynamics. Calculation of the potential energy, tetrahedral order parameter F₃ and the number of hydrate cavities has shown that a steady growth occurs at carbon dioxide concentrations of 4.94–12.20 mol. %. At lower concentrations, there are no significant changes in the structure of the solution over the considered time interval. At higher concentrations of carbon dioxide in a supersaturated solution, separation into the gas phase and saturated solution is observed.

Correlations for calculation of heat loads during a return of spacecraft at the second cosmic velocity are given. Analysis of the heat transfer for a model descent trajectory has been carried out. The convective and radiative heat fluxes, the relative heat transfer coefficient, and the radiative-equilibrium surface temperature have been calculated. The results obtained are a basis for design and optimization of the heat shield of spacecraft.

An experimental study of the heat transfer coefficients of Mg–Ca eutectic alloys with a calcium content of 10.5 and 73.0 at % is carried out. These alloys are considered as promising biodegradable and ultralight construction materials. In the course of the study, new reliable experimental data on thermal conductivity ((lambda)) and thermal diffusivity ((a)) are obtained in the temperature range 300–(571ldots 695) K of the solid state. The estimated errors of the obtained data are 3.0–3.4% and 2.0–2.4% for (lambda) and (a), respectively. Reference tables and recommended temperature dependences of the studied properties are developed, which can be used for various scientific and practical applications.

Shell and tube heat exchangers are used in factories for heat exchangers. Major issues of these heat exchangers are low heat, low frequency and high-pressure drop. Numerical study of the difference between gap and angle of barrier in volume firth and outlet move medium is determined for variation or movement of pipes to accept flow medium. Each baffle system is considered under the variation of flow medium controlled in the path and heat exchanger. It defines the variation of surface and heat exchanger with the flow of hot and cold fluid in surface and heat exchanger and the number of baffle structures. The number of turn units (NTU), overall heat exchanger coefficient (U) and efficiency ((varepsilon)) are evaluated based on different Reynolds numbers (Rec) on the shell side (17693–30331). On the surface edge of the proposed method, the pressure drop is evaluated as the loss of strength from the system. In addition, the impact of inlet cooling fluid temperature, baffle spacing and baffle cutting ratio (BCR) is analyzed and discussed. As the results in all cases, the proposed configuration significantly increases the efficiency of heat exchangers compared to the other two configurations. The proposed method, reducing the pressure drop by 12.40%–11.22%, increases U, (varepsilon) and NTU by 4.51%, 2.87%–4.51% and 5.78%–8.17% respectively.

Entropy and exergy analysis of a thermal system are the most powerful tools that can be employed to specify the optimum operating conditions of that system and utilization rate from the system. In the experimental thermal system in this work, entropy generation and exergy analyzes of GO–DW nanofluids have been carried out in straight copper pipes with constant heat load and 8 mm and 16 mm inner diameters. While the heat loads applied to the pipes are 250 W and 350 W, the range of fluid flow rate values in the pipes is 0.9 l/min–1.8 l/min. GO–DW nanofluids with 0.01% and 0.02% volumetric concentrations and DW have been used as working fluids in the pipes. The outcomes acquired from this work have been matched with the studies using different nanofluids in the literature and it has been noticed that the outcomes are reasonable and consistent. The results of the study have been presented at different GO–DW nanofluid concentrations in pipes with 8 mm and 16 mm inner diameters as thermal, friction and total entropy production, output exergy ratio and 2nd law efficiency. The obtained outcomes have exhibited that the lowest total entropy generation has been obtained for the 8 mm diameter pipe and the nanofluid with 0.02% GO–DW concentration. Besides, 2nd law efficiency is 12% higher for the 8 mm diameter pipe than 16 mm at flow rate of 1.2 l/min and 0.02% GO–DW nanofluid, and 350 W heat load.