Dry Air Mixture Research Articles

Long-Term Partial Discharge Behavior of Protrusion Defect in HVDC GIS

This article investigates the partial discharge (PD) behavior of protrusion defects in real-size high-voltage direct current gas insulated switchgear (HVDC GIS) for SF6 and SF6 alternative gases including fluoronitrile–CO2 mixture (10%) and fluoroketone–dry air mixture (6.6%). The evolution of PD apparent charge and PD repetition rate for all investigated gases is presented and discussed. The measurement results indicate that PD behavior changes as a function of time. The PD apparent charge increases and the PD repetition rate decreases with the increase of voltage application time. This evolution can be related to the change in protrusion tip radius due to electrochemical etching: radius of the protrusion’s tip being enlarged. In addition, the pulse sequence analysis (PSA) plots of the PDs caused by this defect are presented. It is observed that the PSA plots change over time. Therefore, for the development of a robust PD monitoring and defect-recognition tool, as well as for the assessment of risks in the operation of HVDC GIS, it is crucial to take these changes into account.

Influence of SF 6 gas mixture ratio in SF 6 /Dry air mixtures on detection properties of PD-emitted lectromagnetic wave with UHF and VHF band antennas

Influence of SF ₆ gas mixture ratio in SF ₆ /Dry air mixtures on detection properties of PD-emitted lectromagnetic wave with UHF and VHF band antennas

A linear stability analysis of two-layer moist convection with a saturation interface

The linear convective instability of a mixture of dry air, water vapour and liquid water, with a stable unsaturated layer residing on an unstable saturated layer, is studied. It may serve as a prototype model for understanding the instability that causes mixing at the top of stratocumulus cloud or fog. Such a cloud-clear air interface is modelled as an infinitely thin saturation interface where radiative and evaporative cooling take place. The interface position is determined by the Clausius–Clapeyron equation, and can undulate with the evolution of moisture and temperature. In the small-amplitude regime two physical mechanisms are revealed. First, the interface undulation leads to the undulation of the cooling source, which destabilizes the system by superposing a vertical dipole heating anomaly on the convective cell. Second, the evolution of the moisture field induces non-uniform evaporation at the interface, which stabilizes the system by introducing a stronger evaporative cooling in the ascending region andvice versain the descending region. These two mechanisms are competing, and their relative contribution to the instability is quantified by theoretically estimating their relative contribution to buoyancy flux tendency. When there is only evaporative cooling, the two mechanisms break even, and the marginal stability curve remains the same as the classic two-layer Rayleigh–Bénard convection with a fixed cooling source.

Investigation of laser‐induced grating spectroscopy of O2 for accurate temperature measurements towards applications in harsh environments

AbstractWe present an in‐depth investigation of laser‐induced grating spectroscopy (LIGS) for temperature measurements in practical applications using a narrow‐band dye laser with 760 nm wavelength and a pulse duration of 8 ns as the source for the pump beams creating the laser‐induced grating. The pump laser wavelength was set to be either resonant with the transition from the band of O2 for generation of thermal LIGS or nonresonant for generation of purely electrostrictive LIGS. Signals were generated in ambient air as well as in high‐pressure or high‐temperature dry air mixtures. Pump laser irradiances up to 11 GW/cm2 were used, which resulted in strong electrostrictive contribution to the overall LIGS signals at atmospheric pressure, with a low thermal contribution due to the weak absorption by the singlet O2 . The advantage and disadvantage of thermal or electrostrictive LIGS for temperature measurements are discussed, as well as potential applications in high‐pressure environments. Furthermore, the precision of the temperature measurement is discussed by comparing different analysis methods.

A linear cascade tunnel for flow investigations of steam turbine rotor tip blades in subsonic nucleating flows

The paper presents details of a unique experimental facility along with necessary accessories and instrumentation for testing steam turbine cascade blades in wet and nucleating steam. A steam turbine rotor tip cascade is chosen for flow investigations. Cascade inlet flow measurements show uniform conditions with dry air and steam and dry air mixture of different ratios. Exit flow surveys indicate that excellent flow periodicity is obtained. Blade surface static pressure and exit total pressure distributions are also presented with dry air and with steam and dry air mixture of different ratios as the working medium at an exit Mach number of 0.52.

Investigation of humid air physical properties influence on the heat and mass transfer processes calculation accuracy

Humid air is a mixture of dry air and water vapor and is widely used in various power and technical devices. Most theoretical studies of the properties of humid air are based on Mendeleev-Clapeyron ideal gas equa-tion, which doesn’t take into account the association, intermolecular interaction, and particularities of the thermodynamic behavior of humid air as a mixture of gases. The use of this equation leads to inaccuracy in determining the parameters of the state of humid air, which can reach up to 55 %. Therefore, the issues con-cerning the actual properties of humid air determining remain poorly understood. The ideal gas model is based on Mendeleev-Clapeyron equation. To study the actual parameters of the humid air state and its properties, Vukalovich-Novikov’s and Molie’s real gas state equations have been used.Several equations of the state of real gas, taking into account the interaction of water vapor molecules have been considered. The actual physical properties of humid air are determined, and their influence on the coupled heat and mass transfer processes for various conditions is evaluated. The actual values of the thermophysical properties of humid air have been derived. The thermodynamic limits for humid air have been determined using Mendeleev-Clapeyron’s, Vukalovich-Novikov’s and Molie’s state equations. It has been proved Vukalovich-Novikov’s state equation to be the best to represent the quantitative relations between the pressure, specific volume, and the temperature of saturated water vapor. Its application reduces the error of calculations by 39–50 %. The results obtained allow to improve the accuracy of calculating heat and mass transfer when designing contact heat exchangers, convective drying units and hygroscopic desalination plants, compression processes with water or water vapor injection, as well as the processes of mixing flue gases with water vapor in the combustion chambers of gas-steam installations.

Oxidation behaviour of borided Incoloy 909

ABSTRACT The oxidation behaviour of Incoloy 909 superalloy subjected to diffusion boronizing treatments was studied at 750 and 850°C in atmospheres of dry air and a mixture of dry air + water vapour for times up to 10 h. The oxidation behaviour of the coated samples was described by the parabolic rate law with activation energies of 26 and 21 kJ mol−1 when oxidation was conducted in dry air or in the mixture dry air + water vapour respectively. Oxidation kinetics of the boronized coatings were sensitive to temperature and the oxidation environment promoting the development of a thick SiO2 layer on top of the coatings which also included minor additions of other oxides that acted as a barrier that reduced oxidation. However, in the conditions that included water vapour boron-rich oxides were also formed and a different oxidation behaviour was experienced.

Graphene Langmuir-Schaefer films Decorated by Pd Nanoparticles for NO2 and H2 Gas Sensors

Abstract NO2 and H2 gas sensing by few-layer graphene (FLG) were studied in dependence on the annealing and decoration of graphene by palladium nanoparticles (NPs). Graphene was deposited onto SiO2 (500 nm)/Si substrates by a modified Langmuir-Schaefer technique. A solution of FLG flakes in 1-methyl-2-pyrrolidone was obtained by a mild sonication of the expanded milled graphite. FLG films were characterized by atomic force microscopy, X-ray diffraction, Raman spectroscopy, and the Brunnauer-Emmett-Teller method. Average FLG flake thickness and lateral dimension were 5 nm and 300 nm, respectively. Drop casting of Pd NP (6–7 nm) solution onto FLG film was applied to decorate graphene by Pd. The room temperature (RT) resistance of the samples was stabilized at 15 kΩ by vacuum annealing. Heating cycles of FLG film revealed its semiconducting character. The gas sensing was tested in the mixtures of dry air with H2 gas (10 to 10 000 ppm) and NO2 gas (2 to 200 ppm) between RT and 200 °C. The response of 26 % to H2 was achieved by FLG with Pd decoration at 70 °C and 10 000 ppm of H2 in the mixture. Pure FLG film did not show any response to H2. The response of FLG with Pd to 6 ppm of NO2 at RT was ≥ 23 %. It is 2 times larger than that of the pure FLG sample. Long term stability of sensors was studied.

Impact of spray droplets on momentum and heat transport in a turbulent marine atmospheric boundary layer

Droplet-laden turbulent airflow (i.e. the mixture of dry air and water vapor) in the marine atmospheric boundary layer is described by an open channel flow configuration in direct numerical simulation (DNS). The dispersed phase, the spray droplets are tracked in a Lagrangian framework, and their impact on the carrier airflow is modeled with the two-way coupling between the two phases. A wide-range droplet size is typically found near the air–sea interface according to the sea spray concentration function (SSCF). The interactions of the droplets with turbulent airflow including mass, momentum, and energy exchange are investigated here. We found a balancing mechanism exists in the droplet effects on the turbulent drag coefficient, since spray droplets lead to a decreased vertical turbulent momentum transport, but also lead to an increased droplet contribution to total drag coefficient. For the heat transfer, as droplet mass loading increasing, the total Nusselt number decreases due to the depression of turbulent heat flux and enhanced negative droplet convective flux.

Forms of the thermodynamic energy equation for moist air

The thermodynamic energy equation is derived in a self‐consistent manner for a mixture of dry air, water vapour and non‐precipitating liquid water. Gaseous components are assumed to be ideal gases, liquid water incompressible, and the mixture to be in thermodynamic equilibrium. Latent energy is treated as a component of internal energy. Various algebraically equivalent, but superficially different, forms are obtained and contrasted. Contrary to suggestions of an earlier study, a widely used form of the thermodynamic energy equation is shown to be legitimate. It neglects only the minute fractional volume occupied by liquid water, but is otherwise exact.

Measurement of the adiabatic wall temperature in a supersonic air-drop flow

The results of the adiabatic wall temperature measurements of a flat surface placed in a supersonic flow with Mach number M=2.95 are presented. Two flow regimes were investigated. The first is a single-phase flow of dry air, the second is an air-drop stream consisting of a mixture of dry air and finely dispersed water droplets. The presence of droplets in the flow led to the formation of ice sheets on the measured surface, which in turn became the source of shock waves, which caused a strong non-uniformity in the variation of the static pressure and temperature on the measured surface. At this stage, the first data on the rate of cooling of the surface of the adiabatic plate with two flow regimes were obtained.

Real-time analysis of toluene removal in dry air by a dielectric barrier discharge using proton transfer reaction mass spectrometry

The proton transfer reaction mass spectrometry (PTR-MS) is used to follow the toluene concentration at the exit of a dielectric barrier discharge, for an inlet gas mixture of dry air with organic molecules at a concentration in the range 10–100 ppm, at atmospheric pressure and room temperature. The precursor ion is H3O+, and toluene is detected at mass 93 u, corresponding to the (C6H5CH3)–H+ ion. The PTR-MS is used together with optical absorption spectroscopies to follow ozone and carbon dioxide molecules. Two different discharge regimes are studied: without or with significant self-warming effects. The PTR-MS diagnostic appears as a very useful tool for precise monitoring of the concentration of this organic molecule, and to get real-time information about the energy efficiency for the removal of the pollutant during the discharge operation, as function of the discharge regime.

Measurement of Moisture Sorption Isotherm by DVS Hydrosorb

Artificial rice made from corn flour, sago, glycerol monostearate, vegetable oil, water and jelly powder was developed by extrusion method through the process stages of material mixing, extrusion, drying, packaging and storage. Sorption isotherm pattern information on food ingredients used to design and optimize the drying process, packaging, storage. Sorption isotherm of water of artificial rice was measured using humidity generating method with Dynamic Vapor Sorption device that has an advantage of equilibration time is about 10 to 100 times faster than saturated salt slurry method. Relative humidity modification technique are controlled automatically by adjusting the proportion of mixture of dry air and water saturated air. This paper aims to develop moisture sorption isotherm using the Hydrosorb 1000 Water Vapor Sorption Analyzer. Sample preparation was conducted by degassing sample in a heating mantle of 65°C. Analysis parameters need to be fulfilled were determination of Po, sample data, selection of water activity points, and equilibrium conditions. The selected analytical temperatures were 30°C and 45°C. Analysis lasted for 45 hours and curves of adsorption and desorption were obtained. Selected bottom point of water activity 0.05 at 30°C and 45°C yielded adsorbed mass of 0.1466 mg/g and 0.3455 mg/g, respectively, whereas selected top water activity point 0.95 at 30°C and 45°C yielded adsorbed mass of 190.8734 mg/g and 242.4161mg/g, respectively. Moisture sorption isotherm measurements of articial rice made from corn flour at temperature of 30°C and 45°C using Hydrosorb showed that the moisture sorption curve approximates sigmoid-shaped type II curve commonly found in corn-based foodstuffs (high- carbohydrate).

Low-current interruption in SF6-alternatives

SF6 is used as an insulation and interruption medium in medium-voltage (MV) gas-insulated switchgear (GIS), but has a very high global warming potential. In recent years, several environmentally friendly (eco-efficient) alternatives have been explored, focusing on dielectric and thermal properties. The interruption of low currents in MV application is emerging as an important topic for equipment manufacturers and users. The reduced arc-quenching properties of the most prominent eco-efficient alternatives may require the use of vacuum interrupters for simple load current interruptions. However, this may not be a cost-effective solution and simpler interruption principles are desirable. In this study, low-current interruption in AirPlusTM, a mixture of dry air and the C5F10O fluoroketone as well as mixtures of CO2 and C5-FK was explored. The authors find that it is possible to achieve the E3 electrical endurance class (100 c/o) with a switch based on the puffer principle in AirPlus with a condensation temperature of −25°C, suitable for secondary distribution MV GIS. The chemical analysis of gas samples taken from the switchgear after 100 successful interruptions indicate only trace amounts of fluoroketone decomposition products.

Investigation of an equilibrium state of water and two-component gas mixture of dry air and water vapor at the separation surface of phases in the conditions of phase transition Part I

In this part of the publication from the point of view the macroscopically and microscopically description of the properties of phase transition the conditions of stationary thermodynamic state (equilibrium) at the separation surface between the liquid and gas phases under uniform heating or cooling from the side of the thermal reservoir (thermostat) are reviewed and formulated. The basic methods of investigations of the equilibrium state of a two-component gas mixture (dry air and water vapor) in contact with a liquid phase under conditions of phase transformation are described. In the framework of the quasi-classical approximation, the macroscopic equations of the balance of pressures at the interfacial phase surface are defined, according to which the effective molar characteristics of the gas (binary) mixture in the conditions of fluctuation mixing with the ideal liquid into the superfacial layer are determined under certain diffusion approximations. According to macroscopic (mechanical) and microscopic (probabilistic) considerations, an equation of state of a nonideal gas mixture in the case of evaporation or condensation during phase transition under stationary conditions is obtained.

Speed-of-Sound Measurements in Compressed Nitrogen and Dry Air

This paper reports accurate measurements of the speed of sound in compressed pure nitrogen and a synthetic ternary mixture of nitrogen, oxygen, and argon with a composition close to that of natural dry air. The data have been measured by a double-path-length pulse-echo technique and cover the temperature ranges between 275 and 400 K for nitrogen and between 240 and 420 K for the dry air mixture with pressures up to 100 MPa. The expanded uncertainties (at the 0.95 confidence level) amount to 2.1 mK for temperature, 50 ppm for pressure, 80 ppm for speed of sound in nitrogen, and 110 ppm for speed of sound in the dry air mixture.

Few-layer Graphene Langmuir-schaefer Nanofilms for H2 Gas Sensing

Graphene nanofilms were deposited onto oxidized Si by modified Langmuir-Schaefer technique with the submerged substrate from the solution of few-layer graphene (FLG) which was obtained by sonication of expanded milled graphite. The films were characterized by SEM, AFM, HRTEM, XRD and Raman spectroscopy. The average FLG flake thickness and lateral dimension are 5 and 300nm, respectively. The intensity of D vs. G Raman bands suggests a low defect density in our samples. The films were decorated by Pd nanoparticles (6 – 7nm) using drop casting and centrifugation. The sensors were contacted by silver paste. The gas sensing was tested in mixtures of dry air with 10 – 10 000ppm of H2. The sensor response at room temperature (RT) was 4% at 10ppm and 13.5% at 1000ppm of H2. The optimum working temperature was 70 ̊C. Here the sensor response was doubled comparing with RT. The FLG film conductivity was of p-type.

Effect of droplet interaction on droplet-laden turbulent channel flow

We present results of direct numerical simulation of heat transfer and droplet concentration in turbulent flow of a mixture of dry air, water vapor, and water droplets in a differentially heated channel. In particular, we study the effects of droplet collisions by comparing results of simulations with and without droplet collision model for several overall droplet volume fractions. The results show that droplet collisions have a large influence on droplet concentration. Maximum local concentrations, which occur close to the walls of the channel, are reduced by almost an order of magnitude for the case with the highest overall volume fraction. In addition, the positive skewness of the local volume fraction is reduced by a factor of two near the walls. These findings show the importance of including four-way coupling, even in cases where the overall droplet volume fraction is only on the order of 10−4 and the Stokes number in wall units is only about 10. In spite of this large effect of droplet collisions on droplet concentration, the effect on the overall heat transfer between the walls of the channel is not more than approximately 17%. That the effect on the overall heat transfer is relatively small can be explained by the lower heat exchange area between droplets and gas in the near-wall areas, which results in a higher temperature difference between droplets and surrounding gas.

Quasi One-Dimensional Model of Natural Draft Wet-Cooling Tower Flow, Heat and Mass Transfer

The article deals with the development of CFD (Computational Fluid Dynamics) model of natural draft wet-cooling tower flow, heat and mass transfer. The moist air flow is described by the system of conservation laws along with additional equations. Moist air is assumed to be homogeneous mixture of dry air and water vapour. Liquid phase in the fill zone is described by the system of ordinary differential equations. Boundary value problem for the system of conservation laws is discretized in space using Kurganov-Tadmor central scheme and in time using strong stability preserving Runge-Kutta scheme. Initial value problems in the fill zone is solved by using standard fourth order Runge-Kutta scheme. The interaction between liquid water and moist air is done by source terms in governing equations.

DNS of turbulent droplet-laden heated channel flow with phase transition at different initial relative humidities

In this paper a turbulent channel flow of a mixture of dry air and water vapor with water droplets is examined. Direct numerical simulation is used to quantify the importance of variations in the initial relative humidity. We focus on the droplet behavior along with the thermal properties of the system, such as the Nusselt number. During the initial stages of the simulations droplets evaporate more if the initial relative humidity is lower in order to reach the saturation condition. The difference in the Nusselt number between the cases of the lowest initial relative humidity and the saturation initial condition is on the order of 10% and this is connected with the different total heat capacity of the system. At the same time, we confront compressible and incompressible formulations comparing the results for both phases. A lower initial relative humidity leads to a larger difference in the mean gas mass density between the two formulations because of larger heat and mass transfer. Moreover, we find a larger relative difference in the Nusselt number between the two formulations in case of a lower initial relative humidity. These findings motivate the need to adopt the complete compressible flow model.