Journal of Engineering Thermophysics最新文献

The paper presents an experimental study of the heat transfer during condensation of modeling freon R21 in downward flow conditions in an element of a plate-fin heat exchanger with inclined-texture perforated fins. The experiments were carried out for mass velocity of 20 to 50 kg/m²s and wall subcooling of 0.8 to 1.1 K with a heat exchanger with fin density of 850 fins per meter. The texture on the surface of the perforated fins of the heat exchanger was at angle of 45 degrees to the flow direction and made it possible to significantly enhance the heat transfer in comparison with plain fins. It has been found that the heat transfer coefficient depends on the vapor quality, and at a mass velocity of 20 kg/m²s, it exceeds the corresponding value at a velocity of 50 kg/m²s because of a thinner condensate film at the top of the texture.

The wettability, evaporation droplets, and corrosion were experimentally studied for textured copper samples with graphene and smooth copper samples with fluorographene. With increase in the fluorination time, the wettability on fluorographene surfaces grew. Textured graphene surfaces have shown the maximum starting contact angle of 93°. Unlike smooth surfaces with graphene, the textures had a more stable contact line. With the fluorination time increase from 20 to 240 hours, inhomogeneous structures were formed on the surface of fluorographene, which led to roughness increase 2.6–2.7 times. With longer fluorination of graphene, the corrosion current became higher, which is associated with the defectiveness and high hydrophilicity of the surface. With longer fluorination time, the corrosion current became 1.6 times higher. As (EDS) analysis showed, corrosion led to about 10 to 15-fold decrease in the amount of fluorine on fluorographene. The highest anti-corrosion properties were demonstrated by a copper sample subjected to laser texturing, on which several layers of graphene were synthesized.

The paper presents the results of a study of heat transfer in laminar-wave liquid films falling down the outer surface of a vertical cylinder. As intensifiers of heat transfer during boiling, single-layer micromesh coatings with different geometric characteristics and a two-layer gradient mesh coating were used. The working liquid was mixture of refrigerants R114-R21 with the initial concentration of the low-boiling component (R114) equal to 12%. The film Reynolds number at the entrance to the test section varied from 400 to 1300; the heat flux density varied in the range of 0–6 W/cm². The results of measuring the heat transfer coefficients in the regimes of evaporation and nucleate boiling of the film are presented. A comparison is made of the results obtained on the single-layer and two-layer mesh coatings, as well as with previously obtained data for a combined coating (created by the method of deformational cutting in combination with a mesh coating). It has been shown that, in comparison with the smooth surface, the heat transfer coefficient during boiling of the falling film can be increased up to 2 times with the single-layer mesh coating and up to 1.7 times with the two-layer gradient mesh coating.

This paper presents the results of an experimental study of combustion of diesel fuel at atomization by a steam jet and fuel presence in the mixing and gas generation zone at different temperatures of the air supplied to the burner chamber. The research aim is to develop a method of burning liquid hydrocarbon fuel atomized by a jet of superheated steam as a promising approach to increasing the combustion efficiency and reducing the content of harmful substances in combustion products. When the temperature of the primary air was increased to 200°, the flame temperature was found to grow, which led to reduction of the concentration of CO in the combustion products and increase in the NO(_{rm x}) content. After this value, no changes in the content of harmful substances were observed, which is probably due to the complete evaporation of the fuel in the gas generation chamber due to the heat of the supplied air.

Carbon nanotubes (CNT) are used in a variety of applications, including energy storage, device modelling, automotive parts, water filters, thin-film electronics, coating etc. The present article investigates the steady three-dimensional (3D) magnetohydrodynamics (MHD) rotating flow of water based nanofluid containing CNT namely SWCNT and MWCNT past a stretching/shrinking sheet in occurrence of magnetic field and homogeneous-heterogeneous chemical reactions. Further, the heat transfer phenomena is analyzed in presence of thermal radiation and heat source/sink. The governing partial differential equations (PDEs) are converted to non-linear ordinary differential equations (ODEs) by using suitable similarity transformations, and then solved numerically using bvp4c code of MATLAB software. The impacts of magnetic field, rotational, suction/injection, thermal radiation, heat source/sink parameters, nanoparticle (NP) volume fraction, and homogeneous and heterogeneous reactions on the velocity, temperature, and concentration profiles as well as the skin friction coefficient and Nusselt number are presented in graphs and tables. It is found that The primary velocity decreases while the second velocity increases for higher values of rotational parameter and the flow dominates in MWCNTs the SWCNTs.

Pressurization systems for propellant tanks of launch vehicles (LVs) with a liquid rocket engine are complex LV systems to provide cavitation-free operation of pumps and constant pressure in the gas cushion of the fuel tanks at constant consumption of liquid components of rocket propellant. There were already studies on the influence of the pressurant type on the heat and mass transfer in the LV propellant tank. The simulation was performed for a ground experiment with liquid nitrogen with helium gas and nitrogen gas used as the pressurant. In this work, experimental studies were performed on the dynamics of evaporation and condensation, changes in the pressure in the gas cushion, as well as changes in the mass of liquid nitrogen with the vessel pressurized with helium, nitrogen, and their mixtures with molar content of nitrogen of 23.2 mol. % and 52.2 mol. % and filled with liquid nitrogen to 30–70%. The experiments were conducted in a cylindrical vessel with height of 650 mm and internal diameter of 213 mm, pressurized to pressure of 0.3 MPa. The experiments resulted in dependences of variation of the pressure in the vessel and the mass of liquid nitrogen in the vessel at the stage of pressurization with a nitrogen-helium mixture of various concentrations at different levels of initial filling of the vessel with liquid nitrogen. The dependences of temperature changes in the liquid and vapor phases at eight different levels in the vessel height (50–600 mm) were obtained. During condensation, at the pressurization stage, and immediately after the end of pressurization, the surface layer of the liquid (about 10 mm) was observed to warm to a temperature close to the saturation point corresponding to the pressure in the vessel. The formation of this layer leads to cessation of condensation, cessation of pressure drop in the vessel, and beginning of evaporation due to external heat inflows. When the vessel is half filled with the liquid, loss of stability of the heated near-surface layer of liquid is observed because of the formation of large-scale convective flows. The destruction of the heated surface layer and its cooling to the temperature of the core of liquid, which is significantly subcooled, results in abrupt intensification of the condensation of the nitrogen vapor and significant decrease in the pressure in the vessel to a pressure equilibrium with the temperature of the liquid core.

The experimental results on heat transfer and pressure drop during the gas coolant flow into a space formed by a dense packing of 7 heated tubes are presented. To fix the tubes rigidly, 8 spacer grids, evenly distributed along the tube lengths, are used together with longitudinal displacers, which ensure a uniform gas flow field in the internal and external channels of the tube bundle. As a working fluid, gas mixtures with a large difference in the Prandtl number were used: air (Pr = 0.7) and helium-xenon mixture (Pr = 0.23). The experiments were carried out in the range of Reynolds numbers of 1926–11200. The wall temperature distributions of the central and peripheral tubes along the length are measured in detail. Particular attention is paid to the areas of gas flow restructuring near the spacer grid. The heat transfer coefficients and friction factors are determined, and the obtained correlations are compared with the known correlations for round channels. The effect of spacer grids, fixing the heated tubes, on local and average heat transfer and friction factors has been analyzed.

A model has been built for unsteady motion of gas in a pipe with a permeable wall clogged by sand. Solutions to the resulting system of equations are given. Analytical formulas have been obtained for determination of the pressure field in the pipe and the fluid flow rate in any cross section of the pipe. Numerical calculations have been carried out for real-life values of the system parameters.

Closed form solution of the transient thermal analysis in functionally graded cylindrical and spherical bodies under different thermal loads are presented. It is assumed that the material properties change according to the different inhomogeneity parameters according to the power-law in the radial direction. In the partial differential equation obtained under these conditions, the time dependence is eluted with the Laplace transform and transformed into Bessel differential equation in Laplacian space. This equation, which is solved with Bessel functions in Laplacian space, is transformed into physical space with the modified inverse Durbin method. Behaviors of cylindrical and spherical bodies under constant, convective and time-dependent boundary conditions are investigated. Besides, it is aimed to obtain benchmark solutions for these structures to the literature.

Ensuring the necessary accuracy of measurement of unsteady temperature of gas in an aircraft gas turbine engine (GTE) is a topical problem. The use of thermocouples in the gas temperature control loop of the automatic control system (ACS) of a GTE is complicated by the need of reducing the thermocouple inertia, which varies significantly in dependence on the GTE operation regimes. The existing methods and means for compensating the inertia of aircraft thermocouples in the gas temperature control loop of the GTE ACS are based solely on the use of a mathematical model of thermocouple in the form of a first-order inertia element. This mathematical description of avionics thermocouples with wire sensors is very approximate. An avionics thermocouple is described more accurately with a second-order mathematical model and in some cases with a third-order one in accordance with OST 1 00334-79 “Temperature Sensors. Dynamic characteristics.” The difficulty with the use of second- and third-order thermocouple models is associated with the need to establish the dependence of all time constants of a selected model on the changing operating regimes of GTE. No such dependencies have been determined yet for practical use. The purpose of this work is to find out the functional dependence of all time constants occurring in mathematical models up to the third-order inclusive on actual operating parameters of GTE. The time constants calculated from the established dependencies can be used for continuous correction of the gas temperature control loop of the GTE ACS to ensure optimal correction of the dynamic characteristics of thermocouples.