Journal of Engineering Thermophysics最新文献

The article studies electrodeposition in an electrodynamic field used, for example, in electrostatic filters for cleaning smoke from solid particles, as a method for separating micro- and nanoparticles by size. The dependence of the particle charge acquired in the corona discharge of a special electrode system, and hence the Coulomb force on their radius, allows spatially separating particles by size in a gas flow. A theoretically calculated curve of the dependence of the deposition length on the particle size in a channel equipped with corona-forming and precipitation electrodes is shown. By spraying micro- and nanoparticles into an air flow passing through a specially created precipitation channel, the dependences of the deposition length on the particle size were obtained, which are in good agreement with the calculated ones for microparticles. Separation of nanopowders failed, apparently due to the hindrance of proportional deposition of particles of different sizes by Coulomb forces by the ion wind, which has a complex vortex structure between the electrodes.

Spray cooling is a promising method of heat removal and is widely used in various industries and technologies, e.g., cooling of electronic components, metallurgy, aerospace engineering, and power engineering. However, its efficiency depends on many factors, including the characteristics of the spray flow, properties of the working fluid, and parameters of the heat transfer surface. Despite the intensive research in this area, many aspects of the influence of the nozzle geometry, droplet distribution, and heat transfer modes remain understudied. The present work is aimed at filling these gaps, which will enable optimization of heat transfer processes and improvement of the efficiency of cooling systems. Parameters such as the volume flow rate of liquid, droplet diameter and velocity, and their influence on the heat transfer process were studied in this work. The obtained experimental data were compared with known literature correlations for the non-boiling mode. The measured values of the flow velocity and the Sauter diameter of droplets generated by the nozzle were compared with theoretical calculations.

The paper presents findings of experiments on heat-and-mass transfer and measurements of pressure drop in structured packings with different numbers of wipers. Usually, each element of packing has two wipers. The aim of the study is understanding of how the basic parameters of column change when each element of packing has three wipers, except for the first layer from below with four wipers. Several series of experiments were carried out. In the first series, packing with three wipers was not irrigated. In the second series, the column worked in the normal mode with irrigation and had two wipers per layer. The third series was conducted with three wipers per layer and four wipers on the first layer from below. For comparison with experiments with a large column, measurements were made with identical mixture composition at the inlet, the layers of the packing rotated relative to one another by 70({}^{circ}), and the packing irrigated from 18 nozzles. The measurement results are presented below.

This study examines the evaporation of sessile droplets at the final stage, when the contact angle is less than 20({}^{circ|}) and the droplet has become thin. The investigation is conducted on low-reflective porous surfaces, namely robust black silicon with a TiO({}_{2}) coating and a glass plate coated with graphite. The high-speed Schlieren technique with a graded filter was employed to measure thicknesses down to 2 (mu)m. The characteristics of non-volatile water and volatile ethanol thin liquid films were visualized. It was observed that the structure of surfaces and the type of liquid play a crucial role in the dynamics and rupture of thin liquid films.

Various types of passive heat transfer intensifiers are used in channel heat exchangers to increase the heat transfer coefficient. Both insert structures (spirals, twisted tapes, and discrete turbulators) and mechanical or chemical modification of the channel wall (corrugation and deposition of coatings with specified properties) are applied. This paper presents the results of investigation into heat transfer rate at forced circulation of water in a circular channel with spiral intensifiers with hydrophobic coating. The experiments were conducted on a closed circulation circuit at a pressure in the storage vessel of 0.03–0.04 MPa. The test section was a stainless steel tube with length of 2 m, inner diameter of 7.6 mm, and wall thickness of 0.2 mm. Heating was due to electric current flow in the tube wall. The spiral intensifiers have a winding pitch of 4 mm; the fluoroplastic sleeve thickness was 0.9 mm. The experiments were conducted at mass velocities of 35–466 kg/m({}^{2}). The range of heat flux density was (8000<q<32,000) W/m({}^{2}). Measurements of pressure drop across this test section in the regimes of single-phase and two-phase flows were carried out, and the dynamics of pressure drop during formation of two-phase flow at different regime parameters was shown.

In this work, the void fraction based on the Zuber and Findlay model is evaluated at onset of nucleate boiling (ONB) using the Delhaye et al expression of true quality and onset of significant void (OSV) using the Manon expression of true quality. The distribution parameter correlations of Hancox–Nicoll, Nabizadeh, DIX and a data fitting distribution parameter correlation from the DEBORA experiment which was extended for light water were considered. The numerical modeling of the forced convective subcooled boiling water was done at the pressures and mass fluxes of 12.5 MPa and 2800 kg m(^{-2}) s(^{-1}), 15.7 MPa and 3600 kg m(^{-2}) s(^{-1}), and 16.2 MPa and 3836 kg m(^{-2}) s(^{-1}), respectively, corresponding to the operating parameters of pressurized water reactors (PWRs). The result of the calculation indicates that the Dix and Nabizadeh distribution parameters highly deviated from the DEBORA fitted data prediction at the pressure and mass flux range used, and both distribution parameters almost overlapped in their prediction of void fraction at 16.2 MPa. Within the pressure and mass flux range used for this calculation, the Hancox and Nicoll distribution parameter predicted void fraction were below the DEBORA fitted data prediction and deviation increased as pressure and mass flux decreased.

The paper proposes a new algebraic model for describing turbulence in a round pipe. The model relies on the assumption that two hypotheses are sufficient for description of the mean velocity of turbulent motion: the Prandtl mixing length hypothesis and the hypothesis of fractal intermittency near pipe walls. The model was constructed with application of the “maximum simplicity” principle, which made it possible to significantly reduce the empirical constants to two constants that have a clear physical meaning and are universal. It is shown that the mean velocity profile calculated by this model coincides with high accuracy with experimental data in the entire flow region, including both the near-wall region and the region of developed turbulence at the pipe axis. The deviation from the results of known experiments does not exceed the measurement uncertainty for the entire range of Reynolds numbers greater than 20000. The results obtained indicate the possibility of constructing a turbulence model for flow in pipes and ducts without empirical constants.

This study addresses the urgent need to reduce greenhouse gases by investigating high-temperature heat pump systems and binary organic Rankine cycle generators that utilize low-temperature industrial waste heat. Traditionally, R245fa has been used in these systems due to its low working pressure and high critical temperature. However, its high Global Warming Potential necessitates a transition to alternative refrigerants. This research focuses on the heat transfer and pressure drop characteristics of the R245fa/R1234ze(E) refrigerant mixture, a promising alternative with a lower GWP. Experiments were conducted using horizontal micro-fin tubes with copper test tubes of 9.52 mm outer diameter, varying fin heights and numbers, under mass velocities of 100 and 200 kgm(^{-2})s(^{-1}) and mass fractions of 90/10, 80/20, and 65/35 (R245fa/R1234ze(E)) at an average saturation temperature of 60°C. The results showed that the R245fa/R1234ze(E) mixture had lower condensation heat transfer coefficients and frictional pressure drops compared to pure R245fa. The minimum heat transfer coefficient occurred at a 65/35 mass % mass mixture, which is close to the point where the largest temperature glide appeared. Additionally, the frictional pressure drop decreased with increasing mass fractions of R1234ze(E). These findings suggest that the R245fa/R1234ze(E) mixture, despite its lower heat transfer performance compared to pure R245fa, presents a viable lower-GWP alternative for high-temperature heat pump and binary generation systems. This contributes to the development of more efficient and environmentally friendly refrigerant systems, supporting global efforts to reduce greenhouse gas emissions. Further research is needed to optimize the performance of these mixtures in practical applications.

In this study, copper foam with varying filling rates (porosity of 95% and pore density of 20 ppi) was combined with paraffin wax and integrated into a heat sink to investigate temperature fluctuations within the heat sink after charging and discharging of copper foam phase change materials (PCM) and an empty heat sink with varying filling rates. The study focused on three key PCMs: RT-42HC, RT-50HC, and RT-60HC. The PCMs had copper foam filling ratios of (psi=0.18), (psi=0.37), and (psi=0.55), with three heating loads (0.9 kW/m², 1.8 kW/m², and 2.7 kW/m²). The data suggest that after 90 minutes of charging, RT-42HC ((psi=0.37)) can decrease the baseline temperature by 20.29% at 0.9 kW/m² and a maximum of 35.49% with a foam filling ratio of (psi=0.18). Under a heating load of 2.7 kW/m², RT-50HC ((psi=0.18)) can reduce the baseline temperature by up to 35.49%. At the same load, RT-50HC ((psi=0.18)) can reduce the reference temperature by 32.45%.RT-42HC ((psi=0.55)) has a maximum enhancement ratio of 4.38 at SPT = 50° and a heating load of 18 W, whereas RT-50HC ((psi=0.55)) has a maximum enhancement ratio of 4.3 at SPT=60° and a load of 27 W. In the cycle test with an 18 W heating load, RT-42HC ((psi=0.37)) had the most favorable influence, lowering the reference temperature by a maximum of 21.94%.

The problem of melting of an ice layer with inclusions in the form of bubbles with a radiation-absorbing gas has been solved numerically. The problem statement is radiative-conductive heat transfer in a two-phase semi-transparent medium with selective absorption of radiation with a first-order phase transition. The radiative transfer equation was solved by the modified mean flux method with taking into account a wide range of optical properties of the two-phase medium and the radiation source. The temperature fields and the density field of the resulting radiation flux during the melting of the ice layer have been calculated, as well as the melting rate, versus various optical parameters of the medium. The effect of anisotropic scattering by the medium and strong absorption of radiation by the gas on the heating and melting of the ice layer was studied.