Air-water Mixture Research Articles

Numerical Study on the Performance of the Thermoelectric Generator with Different Air-Water Mixtures

Numerical Study on the Performance of the Thermoelectric Generator with Different Air-Water Mixtures

ПРИМЕНЕНИЕ АЭРОЗОЛЬНОГО МЕЛКОДИСПЕРСНОГО СПРЕЯ ДЛЯ ПОВЫШЕНИЯ ЭФФЕКТИВНОСТИ ОХЛАЖДЕНИЯ ОРЕБРЕННЫХ ТРУБ

The article presents an overview of works on the experimental and computational study of the processes of heat removal from heated surfaces using a water-air mixture. A sharp increase in the heat transfer coefficient is shown even when adding water with a mass content of 0.1 % to the air flow. The factors that determine the efficiency of the heat exchange process are listed: the shape of the water spray jet, the distance from the outlet point of the water-air flow to the heat exchange surface, the characteristics of the heat exchange surface, the method of jet formation, the size of droplets and the location of the outlet points of the gas-droplet flow. The weak applicability of the data available in the literature is shown for calculating the cooling of finned tubes of industrial heat exchangers using a water-air mixture. It is substantiated that in order to establish the most optimal cooling modes for each for a separate heat exchange surface, it is necessary to conduct experimental studies, due to the complexity of heat transfer processes and the presence of a large number of influencing factors. It is shown that the finned tubes of heat exchangers of the passive heat removal system of WWER-1200 have a number of important features that affect the efficiency of their cooling using a water-air mixture. An experimental setup has been developed and a technique has been proposed for performing experiments to study these processes as applied to heat exchangers of a passive WWER safety system.

ЗНЕЗАРАЖЕННЯ ПИТНОЇ ВОДИ ЗА ДОПОМОГОЮ ПЛАЗМИ ЕЛЕКТРИЧНОГО РОЗРЯДУ

For today, taking into account the low efficiency of the disinfection technologies using chlorine-containing compounds and the growing number of chlorine-resistant microorganisms, the question of introducing alternative methods of disinfection in the field of water supply and drainage is relevant. One such alternative method is the treatment of liquids with plasma of electric discharge. This article describes our experiments on the treatment of different types of water from surface sources in Kyiv and water intentionally infected with Escherichia coli (E. coli). The experiments were performed on a laboratory installation with a circulating pump and an ejector-type reactor with integrated electrodes. In the reactor a water-air mixture is formed, in which an electric discharge takes place. The disinfection reactor and at the same time the device for the formation of a heterogeneous water-air environment is an ejector with electrodes inserted into it, which receive pulses of electric current. Discharges initiate a range of different physicochemical phenomena, such as a strong electric field, intense ultraviolet radiation, shock waves of excess pressure and, in particular, the formation of various highly active chemicals such as radicals (OH •, H •, O •) and molecules ( H2O2, H2, O2, O3). All these physicochemical processes occur during the discharge itself, for example, the formation and action of short-lived radicals, as well as the action of relatively long-lived oxidants. The effect of the concentration of microorganisms on the rate and completeness of water disinfection was studied in technical (tap) water with the introduction into the reaction tank of the washoff from two test tubes. This provided an initial concentration of E. coli of 3.4 ∙ 106 CFU / cm3. Water treatment for 30 seconds reduced the concentration of microorganisms by two orders of magnitude (up to 5.4 ∙ 104). After 1 minute of treatment, this figure decreased to 1.7 ∙ 102, and after 3 minutes to 5.2 CFU / cm3, i.e., the treated water corresponded to norms for almost pure water. Experiments have proven the effectiveness of plasma disinfection, even for liquids with a concentration of microorganisms significantly exceeding the maximum allowable concentrations. The content of microorganisms before and after treatment was determined by the method of limiting dilution of samples with subsequent sowing on a nutrient medium in Petri dishes

Перенос “темного водорода” атомарным веществом. Методы диагностики “темного водорода”

This work experimentally shown that traces found on track detectors during the study of low-energy nuclear reactions are also formed in the course of many widely used technical processes (combustion of hydrocarbons, operation of internal combustion engines, physicochemical processes accompanying the process of charging smartphone batteries). This coincidence of the track pattern allows us to consider low-energy nuclear reactions as a significant environmental factor, and indicates the important role of “dark hydrogen” in nature. The paper shows the convective transfer of “dark hydrogen” from the discharge zone along the path of the air-water mixture. Using the theoretical model of “dark hydrogen”, fundamentally new, less laborious, in comparison with track, methods of its registration have been developed and described: 1) measurement of the charge of a copper box with its irradiation with “dark hydrogen”, 2) measurement of pressure in a closed volume when irradiated with “dark hydrogen”, 3) the use of a torsion balance with a nickel plate with magnets when irradiated with “dark hydrogen”.

Theoretical study of spilling wave attenuation by air bubbles, turbulence, and bottom friction

An efficient energy dissipation model with breaking capability is important for accurate prediction of wave evolution, especially when the wave approaches shallower water. So, energy dissipation models based on spilling breaking waves, bottom friction, air bubbles, and turbulence effects have been reformulated to calculate wave height and wave setup in the surf zone. Reductions in wave height predicted by different models have been investigated previously, considering free parameters among others. However, the present study does not incorporate any free parameters. Due to the air-water mixture in shallow water and the nonlinearity of the model equations, we have used an iterative method, which brings consistency to the measured wave decay and wave setup. Results of our simulation are compared with laboratory experimental data obtained by several researchers. The wave height and wave setup are obtained using a new formulation that improves results in comparison with the experimental data. The measured time-averaged void fractions are also compared with the time-averaged turbulent intensity, although a long spatial lag is observed in the present study. The findings imply the potential impacts of disturbed breaking waves, bottom friction, air bubbles, and turbulence effects in wave damping. The results could be useful for coastal engineers in making strategic plans and designing infrastructure to protect coastal communities against coastal hazards.

An Experimental Study on the Force Coefficient and the Discharge Coefficient of a Safety Valve in Air-water Mixture Flow

Due to the low number of experimental investigations on the sizing of safety valves in multiphase flow, a novel set of measurement data of an air-water mixture is reported. This paper presents an experimental study on three different geometries of safety valves, a poppet valve with jet angle θ = 120°, and two-disc valves with deflection angles θ = 0° and θ = 90°, respectively. Our test rig comprises a pipeline with 42.5 mm inner diameter, spray nozzles to supply the added water quality (water mass fraction) to the pressurized airflow up to 40 % mass fraction and an inlet pressure up to 6.6 bar(g). The time histories of force, valve lift, and pressures were recorded. We present correlation data for the force coefficient and the discharge coefficient. The widely used omega technique for the Homogenous Equilibrium Model (HEM) is employed to predict the theoretical mass flux. The results show that the poppet valve experiences less momentum force and lower mass flow rates compared to disc valves, while the disc valve with deflection angle θ = 90° presents the highest discharged flow rates among the tested geometries. Our most important finding is that up to 60 % relative valve lift and 40 % mass fraction, neither the force nor the discharge coefficient changes significantly compared to the pure-air case. Finally, we propose a new correlation with a single equation for the resultant force and the discharge coefficient as a function of the relative valve lift for all tested water mass fractions.

Coupling effect of key parameters of heat recovery components on the HAT cycle performance

Humid Air Turbine (HAT) cycle that uses a water-air mixture as a working fluid has a significant advantage in the distributed generation technologies and combined heat and power. The feature of the HAT cycle is that air humidification is carried out at a variable temperature. This cycle is considered as one of the optimal humidified gas turbine cycles to achieve the energy cascade utilization. However, most studies typically use the method of a single variable to investigate the coupled process of humidification and heat recovery, which limits the further performance improvement of the HAT cycle. Herein, we study the interactions among air humidification, heat recovery and system performance to elaborate the effects of interactive variables on the HAT cycle performance. Considering the complexity of the interaction of water-air ratio, aftercooler effectiveness and recuperator effectiveness, a thorough numerical analysis was carried out from the following perspectives: the thermodynamic correlation between components and systems was investigated by controlling a single parameter; the interaction of multiple variables was performed with identical turbine inlet temperature (TIT) and pressure ratio (PR) conditions; and the coupling relationship between interactive variables with a variation of TIT and PR conditions. A thorough analysis revealed that the variables that significantly impact the HAT cycle efficiency are successively RE, W/A, and AE. The analysis of the coupling effects of interactive variables under different TITs is consistent with the above results. As the PR increases, the W/A and AE have a more significant effect on HAT cycle efficiency and the effect of RE on the HAT cycle efficiency greatly diminishes. The HAT cycle efficiency, specific work output, and heat recovery achieved 43.9%, 556 kJ/kg, 5505.33 kW, where its corresponding TIT, PR, W/A, AE, RE values are 1280 °C, 8, 1.5,0.75, 0.9, respectively. Overall, a thorough analysis of interactive variables is of great significance to fully understand the effect of heat recovery on the efficiency enhancement and specific work output improvement from the thermodynamic perspective.

Geometric Analysis through the Constructal Design of a Sea Wave Energy Converter with Several Coupled Hydropneumatic Chambers Considering the Oscillating Water Column Operating Principle

The work presents a numerical study of a wave energy converter (WEC) device based on the oscillating water column (OWC) operating principle with a variation of one to five coupled chambers. The main objective is to evaluate the influence of the geometry and the number of coupled chambers to maximize the available hydropneumatic power converted in the energy extraction process. The results were analyzed using the data obtained for hydropneumatic power, pressure, mass flow rate, and the calculated performance indicator’s hydropneumatic power. The Constructal Design method associated with the Exhaustive Search optimization method was used to maximize the performance indicator and determine the optimized geometric configurations. The degrees of freedom analyzed were the ratios between the height and length of the hydropneumatic chambers. A wave tank represents the computational domain. The OWC device is positioned inside it, subject to the regular incident waves. Conservation equations of mass and momentum and one equation for the transport of the water volume fraction are solved with the finite volume method (FVM). The multiphase model volume of fluid (VOF) is used to tackle the water–air mixture. The analysis of the results took place by evaluating the performance indicator in each chamber separately and determining the accumulated power, which represents the sum of all the powers calculated in all chambers. The turbine was ignored, i.e., only the duct without it was analyzed. It was found that, among the cases examined, the device with five coupled chambers converts more energy than others and that there is an inflection point in the performance indicator, hydropneumatic power, as the value of the degree of freedom increases, characterizing a decrease in the value of the performance indicator. With the results of the hydropneumatic power, pressure, and mass flow rate, it was possible to determine a range of geometry values that maximizes the energy conversion, taking into account the cases of one to five coupled chambers and the individual influence of each one.

Using an ejection system for supplying water and reagent in combined flotation apparatus

The paper highlights the problem of rational use of water resources, based on the increasing need for compact combined devices based on the processes of coagulation, deposition, and flotation. To improve the quality of water treatment, an original design of the ejection system for water and reagent supply has been developed. The main geometric parameters and features that allow solving the problem of “dead zones” are analysed. The use of such a system increases the efficiency of cleaning the fat-containing wastewater by supplying a water-air mixture and forming easily floated “particle bubble” complexes.

Soft-ANN based correlation for air-water two-phase flow pressure drop estimation in a vertical mini-channel

In this paper, an Artificial Neural Network soft matrix correlation to estimate the pressure drop of air-water two-phase flow is developed. The applicability of the model is extended by using dimensionless physical numbers as inputs (Air-Reynolds number, Water-Reynolds number, and the ratio of Air Inertial Forces to Water Inertial Forces), so the model can be implemented for vertical pipes with the proper combination of diameter-velocity-density-viscosity allowing estimations of dimensional numbers within the range of: Air-Reynolds numbers (430–6100), Water-Reynolds number (2400–7200), and Air-Water-Inertial forces ratio (1.6–1834), including the diameter range from 3 to 28 mm. Experimental measurements of frictional pressure drop of water-air mixtures are determined at different conditions. A search of the most suitable density, viscosity, and friction models was conducted and used in the model. The performance of the proposed ANN correlation is compared against published expressions showing good approximation to experimental data; results indicate that the most used correlations are within a mean relative error ( mre) of 23.9–30.7%, while the proposed ANN has a mre = 0.9%. Two additional features are discussed: i) the applicability and generality of the ANN using untrained data, ii) the applicability in laminar, transitional, and turbulent flow regimen. To take the approach beyond a robust performance mapping, the methodology to translate the ANN into a programmable equation is presented.

Ground water-induced changes in velocities of P and S waves (Vp and Vs) measured using an accurately controlled seismic source

We analyze temporal variations in the travel times of both P and S waves (Vp and Vs) for 14 months at Toyohashi (central Japan) with a continuously operating vibration source that could efficiently produce both P and S waves. Seismic waves produced by the source, which is called the accurately controlled routinely operated signal system (ACROSS), are recorded by three nearby seismic stations, and the travel time variation at each station is estimated using the transfer function calculated from the recorded data. Long-term variations in Vp and Vs are observed and can be interpreted by the change in fluid saturation and crack density of the subsurface rocks. The variation in fluid saturation and crack density are consistent with that in the groundwater level, which is measured at the station nearest to the ACROSS. Short-term responses to rainfalls are observed at the station nearest to the ACROSS system; the interpretation of the changes in crack density and saturation is inconsistent with the ground water observation, partly owing to the initial response to rainfall. This can be interpreted as an air–water mixture within pores or cracks on a fine scale.

Experimental Study on the Transition Characteristics and Criterion From Wall-Peak to Core-Peak Phase Distribution in Vertical Rod Bundles

Aiming at understanding the phase distribution characteristics and developing the transition criterion from wall-peak to core-peak phase distribution in a rod bundle channel, air–water two-phase flow experiments were conducted in 5 × 5 rod bundles in the Interfacial Evolution Research Facility at Chongqing University (IERFC). Based on the experimental data, the influences of gas velocity, liquid velocity, mixing vane spacer grid (MVSG), and geometrical size on phase distribution have been analyzed in detail. With the increasing superficial gas velocity and decreasing liquid velocity, the wall-peak phase distribution turned to core-peak. The wall-peak phase distribution was enhanced by an MVSG, and the transition from the transitional phase distribution to the wall-peak phase distribution appeared when the air–water mixture flowed through the MVSG. The gap size was the key factor for the transition of phase distribution in rod bundles. Moreover, the transition criterion from wall-peak to core-peak phase distribution was developed based on present experimental data and the data in the literature, which was also verified based on the limited data. More experiments were recommended to focus on the detailed phase distribution in the rod bundle channel with different geometrical sizes.

Hydraulic Jump: A Brief History and Research Challenges

This paper presents a brief history of the hydraulic jump and a literature review on hydraulic jumps’ experimental and numerical studies. Leonardo da Vinci noticed this phenomenon early on, but it was only later studied by Bidone in 1820. Since the beginning of the 20th century, the hydraulic jump has received a lot of attention following the development of energy dissipater designs and stilling basins. The late 1920s and early 1930s saw many experimental studies researching the surface roller profile and energy dissipation. The study of internal flow features started in the late 1950s. Starting in the 70s, it was believed that the flow of a jump must be analyzed in its actual configuration of air–water mixture, an aspect that cannot be overlooked. Several experimental studies in the late 1980s and 1990s highlighted the existence of oscillating phenomena under specific flow conditions and particularly, a cyclic variation of jump types over long-lasting experiments. The early 2000s saw many experimental studies researching the complex structure of the separated region in very large channels downstream of the lateral shockwaves. Whereas most of the experiments provide measurements at a point or on a plane, the complete flow field supplied by CFD simulations enables us to have a deeper understanding of the dynamics of coherent structures that are responsible for free-surface fluctuations and aeration in hydraulic jumps. Therefore, in recent years, the computational fluid dynamics (CFD) method, through turbulence models, has become a useful tool to study this complex environmental fluid mechanic problem.

Hydraulic Design Challenges of High Head 880 MW Gouvães Pumped-Storage Power Plant

The 880 MW pumped-storage power plant of Gouvães, which is part of the Alto Tâmega hydro power scheme from Iberdrola is currently under construction in the north of Portugal. The power plant is equipped with 4 reversible Francis pump-turbines with a gross head of 660 m. The power plant features several challenges with respect to the hydraulic design such as: i) ultra-high-head single stage reversible Francis pump-turbine of very low specific speed leading to severe hydraulic transients, ii) a long high-pressure penstock, iii) an headrace differential surge tank linked to a 4700 meters long headrace tunnel and iv) a complex tailrace differential surge tank linked to a 700 meters long tailrace tunnel. To perform the hydraulic design of the pumped-storage power plant, a stepwise approach presented in the paper was undertaken by Iberdrola. A detailed 1D hydraulic transient analysis was performed to optimize the headrace and tailrace surge tank dimensions. Then a combined CFD and reduced scale physical transient tests of both surge tanks was conducted. This enabled to address the complex 3D flow physics that develop in surge tanks but are not captured by 1D transient simulations such as free surface, upsurges against walls, verification of throttle singular head losses and effects not captured by 3D CFD such as air-water mixture and detection of air entrainment risk.

Development of short-range nozzle model for sprinkler machines

The issue of land reclamation modern technologies and technical means development is considered in the article, in particular, the improvement of the structures of sprinkle-forming devices in order to increase their use efficiency, which is a priority direction of scientific and applied research. A short-range deflector nozzle of one-piece design sector action was developed, it includes a replaceable jet nozzle, due to which the pressure at the outlet increases, thereby creating a greater vacuum and better dispersion of the water-air mixture. The jet, upon reaching the vertical splitter of the flow part, is divided into two equal flows, the upper one - passes by the vertical part of the spreader and directed to the deflector central part, and the lower one, in turn, is divided into two more flows by the vertical part of the spreader and directed to the deflector edges. This technical solution allows to improve manufacturability and reduce the product cost, as well as to increase the uniformity of the spray, the range of sprinkle drops flow and the sprinkle gripping area, which provides fine-drop sprinkling, the average diameter of the drops is 0.5 … 0.7 mm, sprinkle drops from deflector nozzles flow range is 4 … 6 m/s. With such sprinkle, the energy impact on the soil and agricultural plants decreases, the bulk density in the upper soil layer and the mass of the sprayed soil decrease, which leads to the increase in crop yields by 6.0-20.0%.

Non-equilibrium conditions in a luminescent medium during hydrodynamic luminescence of water

This work presents the results of determining the temperature of plasma electrons formed in conditions of hydrodynamic luminescence – the light emission of a fluid during a flow in a narrow dielectric channel. Argon was added to the water-air mixture to measure the electron temperature of the medium, and the analysis of temperature was provided by measuring the intensity of argon lines.The obtained data on the population of excited levels of argon is interpreted in terms of various theoretical notions. The cause of the abnormal population of 3s23p5(2Po1/2)4p argon levels in argon-hydrogen plasma is discussed.

Oxidation of Aqueous Toluene by Gas-Phase Pulsed Corona Discharge in Air-Water Mixtures Followed by Photocatalytic Exhaust Air Cleaning

The treatment of wastewaters containing hazardous volatile organic compounds (VOCs) requires the simultaneous treatment of both water and air. Refractory toluene, extensively studied for its removal, provides a basis for the comparison of its abatement methods. The oxidation of aqueous toluene by gas-phase pulsed corona discharge (PCD) in combination with the subsequent photocatalytic treatment of exhaust air was studied. The PCD treatment showed unequalled energy efficiencies in aqueous and gaseous toluene oxidation, reaching, respectively, up to 10.5 and 29.6 g·kW−1·h−1. The PCD exhaust air contained toluene residues and ozone in concentrations not exceeding 0.1 and 0.6 mg·L−1, respectively. As a result of the subsequent photocatalytic treatment, both airborne residues were eliminated within a contact time with TiO2 as short as 12 s. The results contribute to the possible application of the studied approach in closed-loop energy-saving ventilation systems.

РЕЗУЛЬТАТЫ ИССЛЕДОВАНИЯ ПО СОВЕРШЕНСТВОВАНИЮ МЕТОДИКИ ОБОСНОВАНИЯ ПАРАМЕТРОВ ПНЕВМОВАКУУМНОЙ (ЭРЛИФТНОЙ) НАСОСНОЙ УСТАНОВКИ

The article discusses the methodology for substantiating the parameters and determining the technological and technical parameters of a pneumatic vacuum (air-lift) pump unit. The technology of water lifting from wells using a pneumatic vacuum (airlift) pumping unit for water supply of peasant and farm households has been substantiated. According to the results of theoretical and experimental studies, the optimal technological parameters are given: overpressure of compressed air created by the compressor at optimal heights of water lifting; specific gravity of the lifted water-air mixture; pump unit discharge Q; power consumption and efficiency of the pumping system and pump unit, water lifting height Н, diametrical dimension of the pump part DНЧ, permissible salinity and content of solid particles (sand) in it Мд, ambient air temperature for normal operation of the pump unit ТНР.

Computational analysis of gas–liquid flow in an idealised CANDU reactor header under conditions that are relevant to small-break loss-of-coolant and loss-of-flow accidents

Extensive numerical simulations of isothermal air/water flow in an idealised model of the header/feeder system of a CANDU nuclear reactor were performed under conditions that are relevant to postulated loss-of-flow accidents (LOFA) and small-break loss-of-coolant accidents (SBLOCA). Following our recent work, in which the present CFD methodology was validated against in–house experimental results, we used the volume of fluid method to solve the system of multi-phase flow governing equations and the detached eddy simulation model to close this system in turbulent flow. We computed the steady-state liquid flow distribution to the different feeders for different combinations of inlet water and air mass flow rates and different mass extraction rates, namely, values of the ratio of the leak liquid mass flow rate and the inlet liquid mass flow rate, and found that, in general, this distribution tends to become more uniform as the inlet flow rate of either the liquid or the gas increases. A “free surface” separating air above from an air–water mixture below was observed in many cases, generally moving downward with decreasing inlet flow rates and increasing mass extraction ratio. The liquid distribution to non-leaking feeders was not affected significantly by a change in the location of the leak. Time histories of flow properties following a step change in boundary conditions showed that the change of flow rate through each feeder can be represented by a first-order model with a time constant of roughly 1 s. As part of the validation of the present CFD analysis, the computed local pressure loss coefficient for a leaking feeder was found to be in excellent agreement with previously published experimental results. The static pressure loss across the header, estimated from available semi-empirical two-phase flows models was, in most cases, found to be within 10% of the CFD solution.

An alternative empirical function to predict air–water mixture bulk modulus for numerical modeling of liquefaction behavior of induced partially saturated sands

During the past decades, various soil improvement techniques have been developed to mitigate liquefaction hazards. One of the most newfound techniques is induced partial saturation (IPS). The capability of this method in liquefaction mitigation has been demonstrated through experimental and analytical research studies. This technique's main objective is to reduce the degree of saturation to convert fully saturated liquefiable sands into a partially saturated state and consequently reduce the potential of liquefaction occurrence. For numerical modeling of partially saturated sands' liquefaction behavior, the sand's main parameter that should be carefully determined is air–water mixture bulk modulus. A numerical model of a cyclic simple shear liquefaction box (CSSLB) on a shaking table was built to study the liquefaction response of induced partially saturated sands in the presented research. Primarily, a conventional equation was employed to predict air–water mixture bulk modulus of induced partially saturated sands, and the results of numerical liquefaction analysis were not much in accordance with those obtained from shaking table experiments. Then, experimental test results obtained from CSSLB were used to propose an empirical function to predict air–water mixture bulk modulus of induced partially saturated sands. It was shown that the new empirical function could predict the effect of entrapped air in excess pore water pressure generated in induced partially saturated sands during cyclic loading. Finally, this empirical function was validated with experimental results obtained from a laminar box on a shaking table and a centrifuge test. The results showed that the proposed empirical function could be considered as an alternative function in the numerical analysis of liquefaction response of induced partially saturated sands.