Journal of Engineering Thermophysics最新文献

The present numerical work investigates by means of Constructal Design the influence of the geometry of an inclined passive wall solar chimney on the ventilation performance of an attached room. The main purpose is to maximize the mass flow rate of air in the chimney/attached room. The problem is subjected to two constraints: the chimney and room areas. Three degrees of freedom are investigated: the ratio between the exit and inferior bases widths of the chimney ((W_{e}/W_{g})), the ratio between the width of the chimney inferior basis and the absorber wall height ((W_{g}/H_{a})), and the ratio between the opening that connects chimney and room and the absorber wall height ((H_{i}/H_{a})). It is considered unsteady, incompressible, free convective, turbulent flows in a two-dimensional domain. The finite volume method is used to solve the time-averaged equations of continuity, momentum and conservation of energy. For closure of turbulence, it is employed the (k)-(varepsilon) model. Results showed that the best geometric configuration led to a mass flow rate 5.7 times superior than the worst configuration, showing the importance of solar chimney desing in this problem. Moreover, a strong sensibility of the investigated ratios on the mass flow rate was noticed.

Magnetohydrodynamic boundary layer flow and heat transmission processes with a hybrid nanofluid film over a steady stretched sheet are taken into consideration. The impressions of an angled magnetic field, tangent hyperbolic flow, and viscous dissipation upon the momentum and thermal boundary layer are investigated. The leading equations are PDEs transfigured into nonlinear, ordinary ones that apply a non-dimensional transformation. Spectral relaxation methods are exploited for numerical solutions to non-dimensional governing equations with no-slip boundary conditions. This simulation was constructed with the cooperation of the application MATLAB. Present outcomes are matched with literature in the limiting cases and are an excellent agreement. To analyze the flow behavior, thermal physical characteristics, and the nature of the hybrid nanofluid particles’ transport properties, we look at various kinds of hybrid nanofluid particles with the base fluid ethylene-glycol ((EG)), which are Ferro–Copper, ((Fe_{3}O_{4})–Cu) and Single walled carbon nanotubes–Copper Oxide, (SWCNT{-}CuO). The consequences of emerging parameters such as Magnetic parameter, Prandtl number, Brinkman number, Power law index, Weissenberg number, and Angle of inclination are explored through graphs The local skin friction and Nusselt number are also graphically displayed with respect to the above parameters.

Condensers represent an indispensable part of equipment of any power, chemical-technological, cryogenic, refrigeration and other installations used in industry. Reducing the weight, dimensions and cost of devices is always an urgent task. The process of condensation in real devices is a very complex phenomenon. The intensity of energy transfer from vapor to a solid cooled wall is determined, other things being equal, by three interrelated factors: (i) variable irrigation density and change in film flow hydrodynamics as the irrigation density changes, (ii) variable vapor velocity affecting a condensate film in the varying film and vapor flow regimes, and (iii) effect of the diffusion process on heat transfer during condensation of vapor with non-condensable impurities. The authors consider that they have to describe the issues that are poorly covered in the literature, although these issues are of fundamental importance for understanding the process under study. In this paper, the main factors that determine heat transfer during stationary vapor condensation on horizontal tube bundles are considered. An algorithm for calculating a condenser at film condensation of stationary vapor without non-condensable impurities is proposed. A critical analysis of modern experimental studies on heat transfer during condensation has been carried out.

The impinging jet technique is a high-performance cooling technology for microchips which are basic elements of electronic systems and having high heat generation rates in small volumes. In this study, the improvement of heat transfer of the microchips used in all technological products today by air impinging jet has been examined. For this purpose, numerical research has been carried out on the cooling of copper plate surfaces with two different patterns, reverse triangle and reverse semi-circle shaped having 1000 W/m² constant heat flux in rectangular cross-section ducts with adiabatic surfaces, by one and double air jets with distances of D(_{h}) and 2D(_{h}) between them. Numerical computation has been performed for energy and Navier–Stokes equations as steady and three-dimensional by employing the Ansys-Fluent computer program with the k-(varepsilon) turbulence model. The obtained results have been compared with the numerical and experimental results of the study in the literature and it has been seen that they are compatible with each other. The results have been presented as the mean Nu number and the variation of surface temperature for each of both patterned surfaces in single and double jet channels with different distances. Streamline and temperature contour distributions of the jet flow along the channel for different H/D(_{h}) ratios and jet numbers have been evaluated for both patterned surfaces. In double-jet and 2D(_{h}) distance channels compared to D(_{h}), at H/D(_{h}) = 12 and Re = 11,000, the Nu number increases of 67% and 65.9% have been observed on the first-row reverse triangle and semi-circular patterned surfaces, respectively.

The prime findings of a numerical investigation into conduction-convection-radiation heat transfer from an electronic gadget, modeled as a discretely and non-identically heated L-corner, are elucidated. In total, four heaters of different heights are assumed to be embedded in the gadget [three in the left and one in the bottom wall]. The partial differential equations describing temperature variation in the computational domain are deduced by balancing the heat generated with that transported by three possible modes of heat transfer. Air [assumed to be radiatively non-participating] is the medium used for cooling the gadget. Finite difference method is used to enable the governing equations acquire an algebraic form. Consequent equations are solved through the Gauss-Seidel algorithm. Full relaxation is imposed to update the local temperature as it gets iterated, and the iterations are made to halt when the maximum residue goes below 10⁻⁸. The effects of all the pertinent independent properties on different prominent results are rigorously probed into. The explicit role enacted by radiation in the work taken up here has been underlined through certain precisely executed results.

In this work, the process of dissociation of methane hydrate obtained under laboratory conditions in the form of a loose shapeless mass and in a pressed granular form, placed on a warm surface and on a surface covered with a thin layer of water, including the initiation of combustion of the gas released during the dissociation process, was experimentally studied. The rate of dissociation, the characteristic time of complete decomposition, and the proportion of water evaporating over a given time were determined for all cases considered. Based on the analysis of the dynamics of temperature ranges, as well as data illustrating the rate of mass loss by the sample, it was assumed that the process of dissociation of granular hydrate, both in the case without combustion and in the case of combustion, is accompanied by the phenomenon of self-preservation, which is not observed for loose hydrate.

An effective model for describing the relative motion of phases is the drift model, which uses simplified momentum equations that do not take into account inertial forces. For this model, in this paper, we study solutions for which various physical flow patterns are realized. The propagation velocity of the volume concentration of phases is analyzed, which has the most obvious physical meaning at zero phase volume velocity. Solutions with piecewise linear distributions are investigated. The evolution of the state in which at the initial moment of time the volume concentration of phase 1 on the left and right is constant and equal to (0.5+Delta) and (0.5-Delta), respectively, and in the transition zone with a width L linearly varies from the values on the left to the values on the right is studied. Two qualitatively different development scenarios are found. A problem is considered with a continuous distribution of phase volume concentrations at the initial moment of time at which a shock wave is formed (the graph is reversed): the propagation velocity of perturbations from the rear particles turns out to be greater than the velocity of propagation of perturbations from the front particles. A transition from a continuous distribution of volume concentrations of phases to a discontinuous distribution is constructed. The transition of the volume concentration profile of the first phase in the vicinity of the shock wave to a continuous distribution is analyzed taking into account diffusion terms proportional to the second derivative with respect to the coordinate. For this case, the volume concentration profile was studied. The main classes of solutions are found.

Adsorption heat pumps are an alternative way of heat and cold generation. Due to the high adsorption capacity and high specific energy storage capacity, the SWS-1L adsorbent can be efficient in adsorption heat pumps. The physicochemical properties of various adsorbents and kinetics of adsorption and desorption of vapors of working fluids based on them have been studied quite well, and methods have been developed to optimize the efficiency of cycles with consideration of a large number of determining parameters: properties of adsorbent, temperature, pressure, and geometric parameters of the adsorbing heat exchanger. However, there is little experimental data on the change with time in the temperature of the free surface of a layer of adsorbent granules. It is important to know how this temperature varies for accurate calculation of the heat transfer parameters. The work shows that at the beginning of heating, the surface temperature of the heat exchanger (metal) without an adsorbent increases to a quasi-stable value within 40–45 s. In the presence of the adsorbent, this time almost doubles and corresponds to 70–75 s. Increase in the thermal fluid flow from 0.65 l/min to 2.7 l/min (4.1 times) leads to a 3.8-fold decrease in the heating time of the heat exchanger with the adsorbent (also approximately 4 times). The characteristic time of thermal inertia (along the thickness of the heat exchanger wall and along the height of the adsorbent layer) without and with the adsorbent is 0.5–1 s and 4–6 s, respectively. The growth of the thermal fluid velocity leads to a significant reduction in the heating time of the heat exchanger and adsorbent and can also reduce the desorption time in a heat pump.

The development of methods for intensifying heat transfer is a priority task in various technological processes in the energy sector and aerospace engineering. One of the effective ways to enhance heat transfer is to install mutually intersecting ribs on opposite walls of the channels (vortex matrices or latticework). The use of such channels leads to formation of a complex three-dimensional turbulent flow, which contributes to a significant enhancement of heat transfer. Most of available literature publications deal with the study of the integral characteristics of hydraulic losses and the degree of heat transfer enhancement depending on a large number of defining parameters. At that, the local flow structure and heat transfer have not been fully investigated. In particular, this conclusion relates to understanding the mechanism of the flow from subchannels formed by parallel ribs on opposite walls and interaction of these flows with the lateral bounding walls of the latticework. In this work, the main attention is paid to the study of the flow processes without the influence of the side walls of the channel. The results of numerical calculations of separated turbulent flow in a latticework obtained using the RANS and LES methods and the OpenFOAM package are presented here. Calculations were performed for the angles of rib crossing (2beta=60div120) on opposite heat transfer surfaces and the Reynolds number Re = (5,000div15,000), determined from the average flow rate and channel height. Data on the flow structure in a cell of a latticework were obtained. It is shown how the angle of crossing affects the interaction of flows in the lower and upper subchannels. The distribution of local heat transfer on the channel wall and the dependence of the average Nusselt number on the angle of crossing and the Reynolds number were obtained.

The use of mixtures as refrigerants and heat carriers in various power systems has become widespread. The thermophysical properties of mixtures differ from the properties of their components. This paper presents the results of a study of the intensity of heat transfer to a non-azeotropic alcohol-water mixture with weight concentration of the volatile component of 20% during forced circulation in a heated smooth circular channel, as well as in a channel with spiral intensifiers with a hydrophobic coating. The experiments were carried out on a closed circulation circuit at a pressure in the storage vessel of 0.04–0.055 MPa. The test section was a stainless steel tube with inner diameter of 7.6 mm and wall thickness of 0.2 mm. The heating was provided due to the electric current passed in the tube wall. The spiral intensifiers had winding pitch of 4 mm; the thickness of the PTFE sleeve was 0.9 mm. The experiments were carried out at mass flux rates of 44–46 kg/m² and 316 kg/m². The heat flux density varied in the range (1200<q<15200) W/m². The use of the spiral intensifiers with the hydrophobic coating during circulation of the non-azeotropic alcohol-water mixture (20%) in the circular channel led to the formation of a significant amount of the vapor-gas phase in the flow at channel wall temperatures below the saturation point of this mixture. The heat transfer coefficient in the channel with the intensifiers grew 2–5 times compared with those in a smooth channel.