High-temperature Gas Medium Research Articles

Influence of different factors on gap breakdown process with hot electrode and high temperature gas medium in low voltage circuit breaker chamber based on particle-in-cell/Monte-Carlo collision simulation

Different factors such as gas composition inside the low voltage circuit breaker (LVCB) chamber and the residual plasma in the post-arc stage affect the breakdown process, which in turn affects the breaking capacity of LVCBs. In this paper, the effects of non-parallel electrode structure, gas temperature and pressure, electrode temperature, and gap distance on gap breakdown of hot electrode under high temperature gas conditions were studied, for which a particle-in-cell/Monte-Carlo collision simulation model has been established, which takes into account the effects of high-temperature gas components, cathode electron thermal emission, electron collision ionization and other effects, and simulation studies have been conducted. The simulation results show that the increase in gap gas temperature, the decrease in air pressure, and the increase in electrode temperature will lead to the gap breakdown more easily. With the increase in the gap length, the breakdown voltage increases, but the average electric field intensity required for breakdown decreases. In the non-parallel electrode structure, the breakdown occurs first at the position with the shortest gap distance, then the cathode sheath forms and extends along the electrode surface to other areas, and finally, the entire gap breaks down.

Particle-in-cell/Monte Carlo collision simulation on gap breakdown characteristics of under the conditions of hot-electrode and high-temperature gas medium in low-voltage circuit breaker chamber

The gas composition inside the low-voltage circuit breaker (LVCB) chamber and the residual plasma in the post-arc stage affect the breakdown process, which in turn affects the breaking capacity of LVCBs. In this paper, the back-arc breakdown and post-arc re-breakdown phenomena occurring inside the LVCB chamber are categorized as the breakdown in the case of high-temperature gas gap of hot electrodes, for which a particle-in-cell/Monte Carlo collision simulation model has been established, which takes into account the effects of high-temperature gas components, cathode electron thermal emission, electron collision ionization, and other effects, and simulation studies have been conducted. The simulation results show that the gap breakdown is mainly caused by the high-temperature hot free background gas and the cathode thermal electron emission. A plasma sheath layer is formed at the cathode during the breakdown process, and the electric field strength in the sheath layer is higher than that in other regions. With the development of the streamer to the cathode, the thickness of the sheath layer becomes narrower and the electric field strength increases, and finally, a discharge plasma channel is formed in the gap.

A high pressure, high temperature gas medium apparatus to measure acoustic velocities during deformation of rock.

A new setup to measure acoustic wave velocities through deforming rock samples at high pressures (up to 1000 MPa), temperatures (up to 700 °C), and differential stresses (up to 1500 MPa) has been developed in a recently refurbished gas medium triaxial deformation apparatus. The conditions span a wide range of geological environments and allow us to accurately measure differential stress and strains at conditions that are typically only accessible in solid medium apparatus. Calibrations of our newly constructed internal furnace up to 1000 MPa confining pressure and temperatures of up to 400 °C demonstrate that the hot zone is displaced downward with increasing confining pressure, resulting in temperature gradients that are minimized by adequately adjusting the sample position. Ultrasonic velocity measurements are conducted in the direction of compression by the pulse-transmission method. Arrival times are corrected for delays resulting from the geometry of the sample assembly, and high-precision relative measurements are obtained by cross correlation. Delays for waves reflected at the interface between the loading piston and sample are nearly linearly dependent on differential applied load due to the load dependence of interface stiffness. Measurements of such delays can be used to infer sample load internally. We illustrate the working of the apparatus by conducting experiments on limestone at 200 MPa confining pressure and room temperature and 400 °C. Ultrasonic data clearly show that deformation is dominated by microcracking at low temperature and by intracrystalline plasticity at high temperature.

Mathematical Simulation of the Heat and Mass Transfer in the Movement of Liquid Droplets in a Gas Medium Under the Conditions of their Intense Phase Transformations

A physical and mathematical model of the movement of a liquid droplet in a high-temperature air flow has been formulated with regard for the main factors of this movement: the inertia of the droplet, its viscous friction, the surface tension of the liquid in it, the air drag, the gravity, lift, and Magnus forces acting on the droplet, its turbophoresis and thermophoresis, the convective heat flows inside the droplet, the turbulence and compressibility of the carrying medium, the concentration of the dispersed phase in it, the nonstationarity of the movement of the droplet, its phase transformations, the collisions of the droplet with the neighboring liquid droplets in the gas flow, the partial fragmentation of the droplet, and its breakdown. Numerical and experimental investigations of the influence of the indicated factors on the velocity of movement, the heating, and the intensity of evaporation of a liquid droplet in a high-temperature gas medium have been performed. The results of these investigations were generalized for determining the possible applications of the model developed.

Assessment of operation of safety channel signal cables at nuclear power plants under fire conditions

Introduction. Signal cables of safety systems, installed at nuclear power plants (NPPs), retain the ability to conduct modulated signals during the time period needed to switch the reactor facility to a safe mode. However, the ability of signal cables to transmit signals correctly in the high temperature gas medium, which is typical for the early stage of a room fi re, has not been exposed to research.Aims and objectives. The co-authors offer a theoretical assessment of the ability of NPP safety system cables to correctly transmit modulated electric signals if exposed to fi re and current loads. The theoretical research into the temperature of the conductor of a signal cable at the initial stage of fi re has been performed towards this end.Theoretical background. The steady state heat conduction equation, describing heat transmission from the cable core to the environment through the cylinder-shaped insulation layer, is used to measure the temperature of the cable strand.Results and discussion. Temperature dependences describing the relation between the temperature of the conductor of a single - strand and single-wire cable KNEPng(А)-HF on the gas medium temperature are obtained. Relations between the temperature of the gas medium in the room on fi re and the current intensity in the electric cable (if the cable is laid vertically) are presented with account taken of the dependence between the specifi c resistance of the wire and the temperature if the maximal permissible operating temperature of cable strands is 70 °С, the maximal permissible operating temperature of cable strands in the overload operation mode is 80 °С, and the maximal cable strand heating temperature is equal to 160 °С when the short-circuit failure occurs. Maximal current intensity values are obtained for various operating modes in the condition of temperatures typical for the initial stage of an indoor fire, they allow to correctly conduct modulated signals within the time period needed to switch the reactor facility to a safe mode.Conclusions. The developed mathematical model and results of numerical experiments allow to assess the infl uence of the temperature in the room of a nuclear power plant in case of fi re on the ability of a signal cable of the safety system to transfer undistorted modulated signals depending on current loads and signal cable laying patterns (whether it is laid vertically or horizontally), and also to expand the range of the room temperature dependence on the current load provided in Electrical Installations Code (EIC).

Microexplosive Fragmentation of a Group of Inhomogeneous Fuel Droplets

We have experimentally studied the process of microexplosive fragmentation of two-component (diesel fuel–water) liquid droplets heated in a high-temperature gas medium. The experiment involved a group of 10–30 droplets falling down in a tubular muffle furnace. The delay times of microexplosion were studied and it was established that integral characteristics of the process significantly depend on the relative arrangement of droplets in the group. Limiting distances between droplets (within an average initial droplet radius of 8–10) are established for which the microexplosion characteristics are close to those observed in experiments with single droplets.

Cyclic Detonation of the Ternary Gas Mixture Propane–Oxygen–Steam for Producing Highly Superheated Steam

Systematic experiments were performed on an innovative pulse detonation steam superheater (PDSS) for determining the concentration limits of detonation of the ternary mixtures propane–oxygen–steam at normal atmospheric pressure. The experiments were carried out while varying the fuel–oxygen equivalence ratio (from 0.3 to 1.7) and the volume fraction of steam (from 0 to 0.7). Cyclic detonation of ternary mixtures was shown to be able to create a high-temperature gas medium that has a temperature above 2250 K and contains to 80% highly superheated steam and to 20% CO2, and also CO, O2, and H2 at low concentrations at atmospheric pressure. Such medium was proposed to be used for advanced treatment of household and industrial organic waste to produce a gas mixture of CO and H2. Because the filling of PDSS with the “cold” ternary gas mixture is periodic, the increase in the temperature of its walls and internals is insignificant, and PDSS can be built from conventional (not necessarily refractory) construction materials.

Gas-Vapor Mixture Temperature in the Near-Surface Layer of a Rapidly-Evaporating Water Droplet.

Mathematical modeling of the heat and mass transfer processes in the evaporating droplet–high-temperature gas medium system is difficult due to the need to describe the dynamics of the formation of the quasi-steady temperature field of evaporating droplets, as well as of a gas-vapor buffer layer around them and in their trace during evaporation in high-temperature gas flows. We used planar laser-induced fluorescence (PLIF) and laser-induced phosphorescence (LIP). The experiments were conducted with water droplets (initial radius 1–2 mm) heated in a hot air flow (temperature 20–500 °C, velocity 0.5–6 m/s). Unsteady temperature fields of water droplets and the gas-vapor mixture around them were recorded. High inhomogeneity of temperature fields under study has been validated. To determine the temperature in the so called dead zones, we solved the problem of heat transfer, in which the temperature in boundary conditions was set on the basis of experimental values.

Electrical conductivity of Icelandic deep geothermal reservoirs up to supercritical conditions: Insight from laboratory experiments

Electrical resistivity is extensively used in geothermal systems to accurately determine the existing conditions of the reservoirs at depth. Up to this point, technical challenges related to pore fluid confinement made difficult to measure electrical conductivity at temperatures and pressures representative of very deep geothermal reservoirs. In this study, we are overcoming these limitations thanks to a new electrical resistivity cell designed to fit into a high temperature gas medium apparatus. This allows us to perform resistivity measurement at temperatures up to 700 °C and at effective pressures up to 100 MPa (i.e. a confining pressure of 130 MPa and an equilibrium pore pressure of 30 MPa) using cm-scale plugs. Rock samples originate from five boreholes located in the Icelandic geothermal fields of Reykjanes (RN-17B/Hyaloclastite, RN-19/RN-30/dolerites) and Hengill (NJ-17/basalt and NJ-17B/Hyaloclastite). These samples were selected for their high degree of hydrothermal alteration in the epidote and amphibole facies (i.e. temperature of 250 °C and 400 °C respectively), and their wide range of porosities (from 3% to 20%). To determine the effects of surface, mineral and electrolytic conductions on bulk electrical conduction, experiments were performed under dry and saturated conditions using three different fluid salinities.At temperatures ranging from 25 to ~350 °C, electrical conductivity in all our experiments increases as a result of both increasing surface and electrolytic conduction. Then, under supercritical conditions, i.e. temperature from 374 °C to 600 °C, electrical conductivity strongly decreases due to the evolution of water density and dielectric constant that affect both surface and electrolyte conduction. At higher temperatures (500 °C–700 °C), the rock conductivities lie within the range of dry rock electrical conductivity values, suggesting that mineral conduction controls the bulk conductivity with ferromagnesian minerals acting as principal contributors of mineral conduction. Amphibole-rich samples show an irreversible increase in conductivity at temperature above 500 °C–600 °C, which can be attributed to amphibole dehydration. Comparison of these laboratory data to magnetotelluric soundings and downhole temperatures obtained beneath several geothermal areas indicate a good agreement between laboratory and large-scale surveys. Our results provide a general trend that helps interpreting electrical conductivity surveys in the Icelandic crust.

Ignition of Particles of Wet Woody Biomass under Convective Diffusion of Water Vapor in the Near-Wall Region

This paper presents the results of an experimental and theoretical study of heat and mass transfer during ignition of wet wood particles in a high-temperature gas medium. Experiments were carried out in a setup which provides conditions similar to the combustion spaces of boiler units. The main heat transfer parameters (ambient temperature) and integrated ignition characteristics (ignition delay) were measured. The measurement error of these parameters did not exceed 18%. The convective transfer of water vapor formed during evaporation of pore moisture and pyrolysis products were found to have an insignificant effect on the ignition characteristics and conditions. From the results of the experiments, a mathematical model of the ignition process was developed which describes the simultaneous occurrence of the main processes of thermal preparation under conditions of intense phase (evaporation of water) and thermochemical transformations (thermal decomposition of the organic part of the fuel, thermochemical interaction between water vapor and carbon coke, ignition of volatiles) taking into account the convective diffusion of water vapor and pyrolysis products in the near-wall gas area during the induction period. The theoretical ignition delay is in satisfactory (within the confidence interval) agreement with the experimental value. The numerical model of the diffusion flame adequately (good agreement between experimental and theoretical ignition delays) describes the ignition of a wet wood particle.

Experimental Determination of Conditions for the Explosive Fragmentation of a Heterogeneous Water Droplet in Heating in a High-Temperature Gas Medium

An experimental study has been made of the processes of warming up, evaporation, and boiling of a heterogeneous (with a solid graphite particle) droplet of water in a flow of high-temperature (500–800 K) combustion products. The mechanism of explosive fragmentation (breakup) of the heterogeneous droplet with enhancement of vaporization on internal interfaces (during the nucleation of vapour bubbles at the graphite-particle surface) has been established. The characteristic times of heating of the heterogeneous droplet (2.5–4 mm in size) with an inclusion (in the shape of a cylindrical disk of diameter and height 2 mm) to explosive-fragmentation conditions have been determined. The times of complete evaporation of a liquid film from the inclusion during the implementation of the process of vaporization from the free exterior surface of the droplet have also been given. A significant difference of the conditions and characteristics of the investigated processes from the "graphite substrate–water droplet" system has been shown. Conditions under which the controlling share of heat to internal interfaces of the heterogeneous droplet is supplied through the liquid film or the inclusion have been determined.

On the Mechanism of Interaction of Two Water Droplets Moving Successively at a Small Distance from Each Other in a High-Temperature Gas Medium

An experimental study has been made of the macroscopic regularities of travel of two water droplets with initial diameters of 1 to 3 mm with a velocity to 3 m/s through a high-temperature (~1100 K) gas counterflow propagating with a velocity no higher than 1.5 m/s. Consideration has been given to successive motion of the two droplets in the gas medium with variation of the initial distance between them in the range 5–30 mm. The hypothesis that the rate of evaporation of the second droplet in the gas counterflow is much lower than that of the first droplet has been substantiated experimentally. The physical model of the processes of approach of the droplets and their subsequent coagulation has been formulated

Influence of the "Self-Radiation" of Combustion Products on the Intensity of Evaporation of an Inhomogeneous Water Droplet in the Flame

The processes of heat transfer during the heating, evaporation, and boiling of an inhomogeneous (with a solid inclusion) droplet of a liquid (water) in a high-temperature (800–1500 K) gas medium have been modeled numerically. The inclusion (carbonaceous particle) in the shape of a disk of height and diameter 2 mm has been considered. The volume of the water enveloping the inclusion ranged within 5–20 μL. It has been shown that the ″self-radiation″ of triatomic gases in combustion products (using commercial alcohol as an example) significantly intensifies (compared to the air heated to the same temperatures) the heating of the inhomogeneous liquid droplet. A comparative analysis of the influence of the temperature of the gas medium and of the thickness in the liquid film enveloping the inclusion on the basic characteristic of the process under study, i.e., the time of existence (complete evaporation) of the droplet, has been made. The reliability of the results of theoretical investigations and the legitimacy of the conclusions drawn have been assessed experimentally.

Evaporation of a water drop with a solid opaque inclusion moving through a high-temperature gaseous medium

The process of evaporation of an inhomogeneous (containing a graphite particle) water drop moving through a high-temperature (about 1100 K) gas medium has been experimentally studied using highspeed (no less than 105 fps) video recording tools, the PIV scanning optical method, and Tema Automotive software. The influences of the ratio of water and inclusion masses, shape of inclusion (by the example of cylindrical disk, cube, and parallelepiped), and its surface area on the integral characteristics of liquid evaporation when heterogeneous drops are passed through a channel (length 1 m, inner diameter 0.2 m) with high-temperature gases are established.

Peculiarities of heat transfer in water droplets with a solid inclusion during heating in a high-temperature gas medium

A physical model and a mathematical model of heat transfer in the conditions of inhomogeneous (with a solid inclusion—a carbon particle) liquid droplet evaporation while moving through high-temperature (800–1500 K) gases are formulated. Numerical investigations were performed using, as an example, a spherical inhomogeneous water droplet during heating in the air medium. The most probable mechanism of phase transitions in a water–carbon particle–heated air system is considered (the initial droplet size, radius, varied in the range from 0.5 mm to 1.5 mm, the inclusion radius was 0.1–1 mm). It has been found that in certain conditions, besides water evaporation from the outer (free) droplet surface, intensive vaporization is possible at the liquid–solid inclusion interface. Conditions of realization of these phase transitions in inhomogeneous water droplet–high-temperature gas medium systems are identified.

Investigation of the Extinguishing Features for Liquid Fuels and Organic Flammable Liquids Atomized by a Water Flow

The processes of heat and mass transfer were investigated experimentally while moving and evaporating the atomized water flow in high-temperature combustion products of typical liquid fuels and organic flammable liquids: gasoline, kerosene, acetone, crude oil, industrial alcohol. We determined typical periods of liquid extinguishing by an atomized water flow of various dispersability. Data of the discharge of extinguishing medium corresponding to various parameters of atomization and duration of using the atomization devices was presented. It is shown that U m ≈3.5 m/s is a minimal outflow velocity of droplets during moving while passing the distance of 1m in the high-temperature gas medium to stop the combustion of organic liquids.

Evaporation of Water in the Process of Movement of its Large Masses Through a High-Temperature Gas Medium

A numerical investigation of the heat transfer in large monolithic water masses and water masses with different spaces filled with water vapor and gases, moving through high-temperature gases at temperatures higher than 1000 K under the conditions of phase transformations, has been performed. The completeness of evaporation of the water from a water mass moving in a high-temperature gas medium was numerically estimated with account for the degree of inhomogeneity of the water mass.

Influence of the form of a solid inclusion in an inhomogeneous liquid droplet on the conditions of its “explosive” destruction under intense heat exchange

The boiling conditions at the interface in a water droplet (initial conventional diameter of 3–4 mm) with a solid nontransparent inclusion (characteristic sizes up to 2 mm) in a high-temperature (higher than 600 K) gas medium are distinguished by the results of experimental investigations with the use of highspeed (no less than 105 frames per second) video registration. The main stages of the process, such as liquid heating, evaporation from a free droplet surface, bubble boiling at internal interfaces between the media, bubble growth, bubble motion through the liquid film, and “explosive” decomposition of the droplet into a group of smaller droplets. The characteristic times of these stages and the influence of the inclusion form on them are established by the example of graphite particles (sphere, disc, cone, parallelepiped, and irregularly shaped polygon).

Effect of the content of salt admixtures on integral characteristics of evaporation of water drops moving through high-temperature gas media

Experimental research on the influence of the content of typical salt admixtures in water on integral characteristics of its evaporation while moving through a high-temperature gas medium was carried out using NaCl nanopowder. Experiments were conducted with single large (more than 1 mm) drops and an aggregate of (more than one hundred) small (under 0.3 mm) water droplets moving through a channel filled with high-temperature combustion products. Conditions were determined under which the content of salt admixtures in water has a strong effect (the decrease of mass changes several times) on the intensity of evaporation of the latter in the region of hightemperature gases.

Explosive Breakup of a Water Droplet with a Nontransparent Solid Inclusion Heated in a High-Temperature Gaseous Medium

This paper investigates the evaporation of a droplet with a comparably sized solid nontransparent in a high-temperature (500-800 K) gas medium. Water evaporates from the free surface of the inclusion. During this process, intensive vapor formation occurs on the inner interface water droplet - solid inclusion with the subsequent explosive decay of the droplet. Experiments have been conducted using high- speed (up to 10 5 fps) video cameras and software Phantom Camera Control. The conditions of the explosive vapor formation of the heterogeneous droplet were found. The typical phase change mechanisms of the heterogeneous droplet under the conditions of intensive heat exchange were determined.