Journal of Engineering Thermophysics最新文献

Experimental dependences (pressure-temperature) of phase equilibrium of hydrofluorocarbon R134a hydrate have been obtained in aqueous solution of the NaCl salt. The measurements were carried out at a salt concentration in water of 3.25 wt. % in the pressure range from 0.5 to 2.5 bar at temperatures varying from −3 to 10°C. In contrast with the pure water system, the presence of salt in the solution had an inhibitory effect. The equilibrium temperature on the equilibrium curve of R134a hydrate at a set pressure decreased by 12–25%, unlike the temperature for the system with pure water. Combined washing process followed by centrifugation decreased the mass concentration of the salt in the solution by 92% compared to the initial salt concentration.

An experimental study of gas liquid flow in an inclined circular pipe was performed. Experiments were performed for two values of superficial liquid velocities and different gas flow rates. The angle of pipe inclination varies from 10° to 80° with respect to horizontal position. Wall shear stress was measured in the apex point of the pipe using an electrodiffusional technique. A strong effect of pipe inclination on wall shear stress was shown. The highest values of wall shear stress corresponded to the angles of 40° to 60°. A significant increase of wall shear stress at low gas flow rates was detected.

Laser Doppler anemometry (LDA) is one of the main non-contact methods for measuring the velocity characteristics of flows and pressure pulsations. Determining the quantitative patterns of the flow of multiphase liquids in small-sized channels is known to be a difficult task. The authors demonstrate the eventual use of LDA to study cavitation flows in slits with a height of 1.2 mm. The cavitation flow is created by means of the NACA0012 hydrofoil. The experimental equipment is positioned using a micro-displacement device. The distortion of the luminous flux through the channel walls is taken into account. This paper presents an algorithm for searching the central region of the slit. A method for filtering noise effects is described. The results of measuring the average flow velocity near the cavitating hydrofoil in the slit are provided in the article.

The possibility of reducing the intensity of shock waves from a body going supersonic in the atmosphere was investigated. Parameters of near- and far-field shock waves from a thin body with a disk located ahead of the body across the incoming flow at local heating of the incoming flow were calculated. The Mach number of the incoming cold air flow was 2. The calculations were performed by the combined “phantom body” method. The calculation results show that simultaneous use of an aerodynamic surface ahead of the body and heating of the incoming flow makes it possible to reduce the intensity of far-field shock waves by approximately 30%.

A homogeneous eutectic alloy of the magnesium-lithium-calcium ternary system containing 72.5 at.% magnesium, 19.1 at.% lithium, and 8.4 at.% calcium was prepared. Experimental studies of the relative elongation, density, thermal expansion, heat capacity, thermal diffusivity and thermal conductivity of this ultralight alloy were performed in the temperature range of 120 K to 710 K. The behavior of density and thermal expansion in the melting-crystallization region and in the liquid state was also studied. A comparison with previously obtained results on the thermophysical properties of pure magnesium and magnesium-lithium, magnesium-calcium eutectic alloys was made. The temperature dependences were constructed and the table of the investigated properties recommended values were presented. It was found that in the Mg–Li–Ca alloy a phase transition occurs in the range of 225 K to 245 K, where the heat capacity and the linear thermal expansion coefficient change abruptly. This kind of transition was found earlier in the study of the Mg–Li eutectic alloy and is presumably related to the martensitic transformation of the lithium-rich bcc magnesium-lithium alloys to the hcp structure.

A study of heat transfer at evaporation and boiling in liquid film flowing down the horizontal tube bundle in tube-and-shell evaporator is reported. This bundle includes duralumin tubes with micro-arc oxidizing (MAO) treatment, which heat transfer characteristics are the subject of conducted tests. Refrigerant R21 at the pressure of (sim)3 bar of absolute pressure is used as working liquid. Hot water pumped through bundle tubes used as a heat carrier. The boiling curves obtained for the tested tubes with MAO treatment showed considerable heat transfer enhancement in comparison with plain tube; however, hysteresis phenomena due to a porous structure of micro-arc oxidizing coatings are revealed. These phenomena should be taken into account at operating heat transfer equipment.

The results of mathematical modeling taking into account the phase transition occurring at filtration of air flow through a tubular heat exchanger are presented. The heat exchanger is made of thin-walled plastic tubes (3.2 mm in diameter), filled with water and sealed at both ends. The calculation results qualitatively coincide with the results of experimental studies carried out under natural conditions. The model with consideration of the “water–ice” and “ice–water” phase transitions makes it possible to describe the experimentally observed intersection of time dependences of air temperature in cross sections. This intersection could not be obtained without account of the phase transition in the model. The quantitative difference between the calculation results and the experiment is explained by the choice of insufficiently small value of the relative temperature difference; this parameter needs clarification in further research.

The raising concern about climate change and global warming provides the searching for more environmental friendly refrigerants for chiller applications. In this study, an investigation for low GWP refrigerants to replace R123 has been conducted for evaluation for chiller systems. R1233zd(E), R1234ze(E), R1234yf, R290, and R600 have been compared with R123 under different effecting factors. R1233zd(E) and R600 display a close COP performance when compared with the baseline R123, while other R123 alternatives present a low COP level (more than 5% COP decrease from baseline R123). R290 displays the highest value of the compressor impeller speed while R123 the lowest. In general, R1233zd(E) is preferred for drop-in option to replace R123 due to close COP and unit capacity by volume. R1234ze(E), R1234yf, and R290 are preferred for chiller down-size options. Increasing evaporating temperature from 1°C to 5°C can display more than 15% COP enhancement and it does not have the significant change for the mass flow rate and the compressor impeller speed. The condensing temperature increase from 30°C to 40°C has significant influence on the system COP drop, with (sim30)% COP decrease there. Increasing the compressor isentropic efficiency from 0.6 to 0.8 can make the COP increased by more than 30%, and make the compressor power consumption to be reduced by more than 24%. From the single-stage cycle chiller system to the two-stage cycle chiller system, the COP can be improved by 3.0% for R123, 3.1% for R1233zd(E), 4.4% for R1234ze(E), 5.1% for R1234yf, 4.0% for R290, and 3.5% for R600, respectively. In general, R1234yf is more sensitive for the cycle improvement.

The work investigates the relationship between the energy separation in a Ranque–Hilsch tube and flow crisis manifestation in a translational swirling flow. The laser Doppler anemometry was used for the measurement of the transverse profiles of circumferential and longitudinal velocities in the mid-section along the entire length of the working channel in a vortex tube with a square cross-section. Analysis of the experimental data revealed signs of a series of hydraulic jumps that realize structural transitions from supercritical flow regimes of a near-wall swirl flow with longitudinal velocity exceeding the critical one to subcritical regimes, the longitudinal velocity in which is less than the critical one. The identified features suggest that there may be an increase in temperature in the near-wall flow due to the conversion of excess kinetic energy released during hydraulic jumps into heat, while conserving the momentum flow. A number of phenomena associated with the Ranque effect are discussed within the concept of crisis of a translational swirling flow and the more general concept of vortex breakdown.

The results of an experimental study on the gas-liquid flow structure and interphase mass transfer are presented for a periodic slug flow of ethanol-CO₂ mixture in a straight microchannel and a microchannel with a sudden change in the cross-section. The experiments were carried out for a microchannel with successive contractions and expansions along its length and average cross-section of (132times 387) (mu)m². For determination of the change in the volume of the gas slug along the microchannel length because of CO₂ absorption, the method of high-speed flow visualization was used, followed by digital image processing. The frequency of generation, speed, size, and volume variation of the gas bubbles were measured along the length of the channel. From the data obtained, the volumetric mass transfer coefficient from the liquid side was determined. A comparison was made of the obtained volumetric mass transfer coefficient for the straight channel and for the channel with sudden enlargement of the cross-section: slight enhancement of the mass transfer in the channel with the sudden change in the cross-section, resulting from the pulsating shape of the bubbles, has been shown.