Excess Gas Temperature Research Articles

Impact of ventilated tunnels on smoke peculiarities in train carriage fires with multiple lateral openings

This research investigates the dynamics of smoke dispersion in a subway train carriage with multiple lateral openings within a ventilated subway tunnel. Numerous extensive full-scale numerical simulations were conducted to analyze the thermal plume distribution within multiple lateral openings of a train carriage, and the longitudinal and transverse maximum excess ceiling temperature. The results reveal that the heat release rates have a greater impact on the airflow in the carriage than velocities ,even when the longitudinal ventilation velocity reaches 8 m/s or the heat release rate reaches 15 MW. The multiple lateral openings cause significant differences in smoke flow characteristics compared to tunnels with both ends open. Additionally, the transverse maximum ceiling excess temperature inclines towards the lateral opening side due to asymmetrical air entrainment. The analysis of longitudinal maximum ceiling excess temperature identified two regions based on the dimensionless heat release rate, showing a linear increase followed by a constant phase dependent on the heat release rate, with a maximum longitudinal excess gas temperature of 1242 K. Additionally, a relationship between the dimensionless heat release rate and the transverse dimensionless maximum ceiling excess temperature was established. This study can be used as a guide for smoke exhaust mitigation and fire safety measures in the event of subway train carriage fires.

Experimental study on temperature attenuation of smoke flow driven by dual fire sources in a tunnel

A series of reduced-scale tunnel fire tests were carried out to investigate ceiling smoke temperature attenuation in a naturally ventilated tunnel under double fire sources of various sizes and distances. Experimental results showed that the interactions between the two flames affect excess gas temperature attenuation. Under a given fire size, the ceiling smoke temperature gradually decays more slowly when the spacing between the two fire sources is longer and then goes through a steady stage. Moreover, for different smoke spread regions (i.e., the radial ceiling jet region, the shooting flow region and the one-dimensional tranquil flow region), the ceiling smoke temperature attenuation trends are different. Models of non-dimensional excess ceiling smoke temperature attenuation in the radial ceiling jet and shooting flow regions were proposed using the plume radius at the ceiling level as the characteristic length scale. In the one-dimensional tranquil flow region, an exponential model was developed by theoretical analysis to predict non-dimensional ceiling smoke temperature. The lumped attenuation coefficient k was observed to decrease first and then approach a relatively stable value with an increasing fire spacing. This study is of great significance to the design and protection of tunnel structures under multiple fire sources.

Experimental and theoretical studies on the smoke temperature distribution along the tunnel ceiling with natural ventilation by the board-coupled shafts

Experiments were conducted in a 1/20 scale model tunnel to explore the gas temperature distribution along the tunnel ceiling by the natural ventilation, both the traditional shaft and the board-coupled shaft (BCS) were included. The influence of the shaft type and the board location on the gas temperature distribution along the tunnel ceiling was investigated. The results indicate that the variation trends of excess gas temperature distribution in the near fire region and the far fire region are consistent with the exponential attenuation, whether it is a traditional shaft or a BCS shaft. The relational function of the reference temperature between the far fire region and the near fire region was developed. Beyond that, the correlation function between the smoke layer thickness in the near fire region and the far fire region was developed. Furthermore, the mechanisms of the main heat transfer in the near fire region and the far fire region were discussed. And based on the premise that the radiative heat transfer dominates the heat transfer to the tunnel walls in the near fire region and the convective heat transfer dominates the heat transfer in the far fire region, empirical models for predicting the ceiling excess gas temperature distribution in the near fire region and the far fire region were developed. These studies may contribute positive significance for promoting the application of natural ventilation with vertical shafts.

An experimental investigation of stratification characteristic of fire smoke in the corridor under the effect of outdoor wind

An investigation of previously established correlations between gas temperature distribution and smoke stratification in corridors and tunnels has been carried out. The investigated correlations are based on excess gas temperature ratios and Froude number scaling. A set of experiments were carried out in a scale-modelled “room-corridor” to investigate the stratification characteristic of smoke induced by fire and outdoor wind. The smoke flow pattern was visualized and the wind speed, the heat release rate and area of the window were investigated. The dimensionless parameter Froude number (Fr) was used to describe the stratification characteristics. The results show that when Fr ≤ 0.29, the smoke stratification is extremely strong; when 0.29 < Fr ≤ 0.58, the stratification intensity remains high and the smoke stratification is still strong; when 0.58˂Fr˂0.92, the stratification strength decreased significantly; when 0.92 ≤ Fr, the fire smoke mixed completely with the lower air and the smoke stratification failed. This can provide reference for the study of smoke stratification in corridor.

Insights to scaling remote plasma sources sustained in NF3 mixtures

Remote plasma sources (RPSs) are being developed for low damage materials processing during semiconductor fabrication. Plasmas sustained in NF3 are often used as a source of F atoms. NF3 containing gas mixtures such as NF3/O2 and NF3/H2 provide additional opportunities to produce and control desirable reactive species such as F and NO. In this paper, results from computational investigations of RPS sustained in capacitively coupled plasmas are discussed using zero-dimensional global and two-dimensional reactor scale models. A comprehensive reaction mechanism for plasmas sustained in Ar/NF3/O2 was developed using electron impact cross sections for NF2 and NF calculated by ab initio molecular R-matrix methods. For validation of the reaction mechanism, results from the simulations were compared with optical emission spectroscopy measurements of radical densities. Dissociative attachment and dissociative excitation of NFx are the major sources of F radicals. The exothermicity from these Franck–Condon dissociative processes is the dominant gas heating mechanism, producing gas temperatures in excess of 1500 K. The large fractional dissociation of the feedstock gases enables a larger variety of end-products. Reactions between NFx and O atom containing species lead to the formation of NO and N2O through endothermic reactions facilitated by the gas heating, followed by the formation of NO2 and FNO from exothermic reactions. The downstream composition in the flowing afterglow is an ion–ion plasma maintained by, in oxygen containing mixtures, [F−] ≈ [NO+] since NO has the lowest ionization potential and F has the highest electron affinity among the major neutral species.

Effect of tunnel cross section on gas temperatures and heat fluxes in case of large heat release rate

Tests with liquid and solid fuels in model tunnels (1:20) were performed and analysed in order to study the effect of tunnel cross section (width and height) together with ventilation velocity on ceiling gas temperatures and heat fluxes. The model tunnel was 10 m long with varying width (0.3 m, 0.45 m and 0.6 m) and height (0.25 m and 0.4 m). Test results show that the maximum temperature under the ceiling is a weak function of heat release rate (HRR) and ventilation velocity for cases with HRR more than 100 MW at full scale. It clearly varies with the tunnel height and is a weak function of the tunnel width. With a lower tunnel height, the ceiling is closer to the base of continuous flame zone and the temperatures become higher. Overall, the gas temperature beneath the ceiling decreases with the increasing tunnel dimensions, and increases with the increasing longitudinal ventilation velocity. The HRR is also an important factor that influences the decay rate of excess gas temperature, and a dimensionless HRR integrating HRR and other two key parameters, tunnel cross-sectional area and distance between fuel centre and tunnel ceiling, was introduced to account for the effect. An equation for the decay rate of excess gas temperature, considering both the tunnel dimensions and HRR, was developed. Moreover, a larger tunnel cross-sectional area will lead to a smaller heat flux.

Large Eddy Simulation of a Polydisperse Ethanol Spray Flame

Ethanol is identified as an interesting alternative fuel. In this regards, the predictive capability of combustion Large Eddy Simulation approach coupled to Lagrangian droplet dynamic model to retrieve the turbulent droplet dispersion, droplet size distribution, spray evolution and combustion properties is investigated in this paper for an ethanol spray flame. Following the Eulerian-Lagrangian approach with a fully two way coupling, the Favre-filtered low Mach number Navier-Stokes equations are solved on structured grids with dynamic sub-grid scale models to describe the turbulent carrier gas phase. Droplets are injected in polydisperse manner and generated in time dependent boundary conditions. They evaporate to form an air-fuel mixture that yields spray flame. Part of the ethanol droplets evaporates within the prevaporization area before reaching the combustion zone, making the flame to burn in a partially premixed regime. The chemistry is described by a tabulated detailed chemistry based on the flamelet generated manifold approach. The fuel, ethanol, is modeled by a detailed reaction mechanism consisting of 56 species and 351 reversible reactions. The simulation results including excess gas temperature, droplet velocities and corresponding fluctuations, droplet mean diameters and spray volume flux at different distances from the exit plane show good agreement with experimental data. Analysis of combustion spray features allows gaining a deep insight into the two-phase flow process ongoing.

The maximum ceiling gas temperature in a large tunnel fire

Effects of different ventilation systems, ventilation velocities, heat release rates, tunnel geometries and fire sources on the maximum excess gas temperature beneath the ceiling in large tunnel fires are analyzed. Data from numerous model scale tests and most of the large scale tunnel fire tests that have been performed worldwide are used and analyzed. Correlations for the maximum ceiling excess gas temperature in the vicinity of the fire source are proposed for low and high ventilation flows. The temperature data indicate two regions, depending on the dimensionless ventilation velocity. Each can be divided into two sub-regions. The first sub-region exhibits linear increase which transits into a constant period, depending on the fire size, ventilation and the effective tunnel height. The maximum excess gas temperature is found to be 1350 °C.

Temperature stratification in tunnels

An investigation of previously established correlations between gas temperature distribution and smoke stratification in mines has been carried out for tunnel applications. The investigated correlations are based on excess gas temperature ratios and Froude number scaling. This paper describes a comparison between two large scale tests carried out in a road tunnel and two well defined model scale tests. In each of the tests, a longitudinal flow was maintained. The temperature data obtained at different locations and different heights have been used for the comparison. A good correspondence between the experimental data and the correlations has been found when the gas temperature data were used. However, the correspondence between the previously established correlation of gas temperature stratification and Froude number, was not reliable. It is postulated that the main reason for this may be the way the experiments were carried out. New correlations between the temperature stratification and the Froude number are also explored.

The maximum temperature of buoyancy-driven smoke flow beneath the ceiling in tunnel fires

In order to detect a fire and provide adequate fire protection to a tunnel structure, the maximum gas temperature beneath the ceiling to which the structure is exposed needs to be estimated. Theoretical analysis of maximum gas temperature beneath a tunnel ceiling based on a plume theory is given. The heat release rate, longitudinal ventilation velocity and tunnel geometry are taken into account. Two series of model-scale experimental tests were also carried out. The results of both analysis and experiments show that the maximum excess gas temperature beneath the ceiling can be divided into two regions. When the dimensionless ventilation velocity is greater than 0.19, the maximum excess gas temperature beneath the tunnel ceiling increases linearly with the heat release rate and decreases linearly with the longitudinal ventilation velocity. When the dimensionless ventilation velocity is less than 0.19, the maximum excess gas temperature beneath the ceiling varies as the two-thirds power of the dimensionless heat release rate, independent of the longitudinal ventilation velocity. In both regions, the maximum excess gas temperature varies as the −5/3 power of the vertical distance between the fire source bottom and tunnel ceiling. The investigation presented here considers only the cases when the continuous flame region is lower than the ceiling height.

Effects Of Flue Spaces On The Initial In-rack Plume Flow

This paper describes the effects of the vertical flue space (gap) on the convective Heat Release Rate (HRR) and the in-rack plume flow in rack storage fires. Four large-scale tests with Standard Class II commodity, which consists of double triwall corrugated board cartons, were carried out with vertical flue spaces of 75 mm, 150 mm, 225 mm and 300 mm. Measurements of convective HRR per height of the rack storage, in-rack flame height, centreline excess gas temperature, centreline gas velocity and in-rack temperature profiles are presented. The objective of the study was to compare the results of the four full-scale rack storage tests to the results of the 1:3 model-scale tests and one large-scale test carried out in an earlier study

A six-wafer combustion system for a silicon micro gas turbine engine

As part of a program to develop a micro gas turbine engine capable of producing 10-50 W of electrical power in a package less than one cubic centimeter in volume, we present the design, fabrication, packaging, and experimental test results for the 6-wafer combustion system for a silicon microengine. Comprising the main nonrotating functional components of the engine, the device described measures 2.1 cm/spl times/2.1 cm/spl times/0.38 cm and is largely fabricated by deep reactive ion etching through a total thickness of 3800 /spl mu/m. Complete with a set of fuel plenums, pressure ports, fuel injectors, igniters, fluidic interconnects, and compressor and turbine static airfoils, this structure is the first demonstration of the complete hot flow path of a multilevel micro gas turbine engine. The 0.195 cm/sup 3/ combustion chamber is shown to sustain a stable hydrogen flame over a range of operating mass flows and fuel-air mixture ratios and to produce exit gas temperatures in excess of 1600 K. It also serves as the first experimental demonstration of stable hydrocarbon microcombustion within the structural constraints of silicon. Combined with longevity tests at elevated temperatures for tens of hours, these results demonstrate the viability of a silicon-based combustion system for micro heat engine applications.

The Transient Ceiling Flows Of Growing Rack Storage Fires

Transient ceiling flows induced by growing rack storage fire were investigated in the period where the convective heat release rates produced by these fires had a power-law dependence on time to the third power. Empirical correlations for the ,maximum excess gas temperature, maximum gas velocity, temperature profile depth, and velocity profile depth of the transient ceiling flow were established in terms of a normalized convective heat release rate of the fire and a normalized radial distance from the fire axis.

Demonstration of Ceramic Design Methodology for a Ceramic Combustor Liner

Successful application of ceramics in the gas turbine environment involves brittle material design and analysis, unique material properties, and laboratory demonstration of components. Alternative ceramic structures for application in a hybrid ceramic-metal combustion system were considered. Thermal stresses and probabilities of failure were calculated for tube, ring, and stave designs. The stress levels are relatively low for axial and radial temperature gradients in all 6 in. (152 mm) diameter combustors. Scaling to larger sizes is also considered. A hot streak temperature distribution resulted in stresses in the stave and tube which were three times the stresses in the ring design. Bend specimens were cut from one end of a 6 in. (152 mm) diameter, 10 in. (254 mm) long REFEL silicon carbide tube to assure a representative flaw distribution. Strength, strength scatter, and strength degradation were evaluated at 2200°F (1200°C) and were incorporated in the stress analysis. A low probability of failure was predicted. The REFEL tube was tested in the hybrid ceramic-metal combustion system at gas temperatures in excess of 2660°F (1460°C), 16 atm pressure, and 6.9 lb/s (3.1 kg/s) mass flow while burning No. 2 fuel oil. Successful ceramic performance was demonstrated at 2000°F (1090°C) steady state and at 2000°F (1110°C) per minute shutdown cooling rate.

Heating of dense interstellar clouds by magnetic ion slip - A constraint on cloud field strengths

Observations suggest that fragmentation into stars occurs at densities comparable to those of massive interstellar clouds. However, fragmentation cannot occur if the magnetic field strength and density are related by B=B/sub 0/n/sup y/ with y=2/3, as for isotropic contraction with a frozen-in field. A semiempirical upper limit to the parameter y is derived by considering the thermal equilibrium of a cloud heated by magnetic ion slip (ambipolar diffusion) and requiring that the theoretical temperatures do not greatly exceed observed temperatures. Cooling by rotational transitions of CO, H/sub 2/, and HD and by inelastic gas-grain collisions is included in the equilibrium calculations. It is found that a value of y as large as 2/3 would result in gas temperatures in excess of 100 K at densities greater than 10/sup 4/ cm/sup -3/ and can therefore be excluded by the observations. A value of yapproximately-less-than0.55 is required if the scale of the magnetic field gradient is of the same order as the cloud radius. The calculated value of this upper limit is sensitive to the choice of B/sub 0/. The value of y can be decreased by preferential flow of gas along field lines or by magnetic ion slip. The time scalemore » for magnetic ion slip is reexamined.« less

Extrathermal Electrical Conductivity Measurements in a Magnetoplasmadynamic Generator

A DETAILED description of the International Research and Development Co., Ltd., magnetoplasmadynamic (MPD) generator has been given previously1,2. On July 14, 1964, a series of power generation experiments with caesium seeding of the helium were undertaken. This series differed from the previously reported runs3 mainly in the generator and nozzle configuration and materials. To reduce end current leakage, the tantalum nozzle used previously was replaced by a boron nitride nozzle, the exit region of the generator lengthened by 2 in. and the interior of the diffuser inlet spray-coated with alumina. The generator channel was constructed from boron nitride in attempts to reduce the internal current leakage encountered previously with alumina3. Sixteen pairs of narrow (1/16 in. × 1/2 in.) tantalum electrodes replaced the five pairs of square (1/2 in. × 1/2 in.) electrodes used previously; the gap between the electrodes was large (3/16 in.), to reduce insulator and boundary layer electrical breakdown. Five pairs of tantalum pins (1/16 in. diam.) were used to monitor open circuit voltages, two pairs in the nozzle and three pairs in the generator exit region. Four static pressure tappings and three thermocouples were also located in the channel wall. During operation of the loop nozzle inlet gas temperatures in excess of 1,700° C were measured. The maximum channel temperature measured was 1,110° C with a corresponding flow velocity of 1,730 m/sec and a Mach number of 0.79.

TRANSPIRATION COOLING WITH LIQUID METALS

The development of hybrid propulsion concepts at the Naval Ordnance Test Station has created the requirement for a lightweight nozzle system capable of withstanding gas temperatures in excess of 6200°F for burn times longer than 30 sec. A series of 40 nozzle-material test firings illustrated that tungsten nozzle insert systems provided the best operation. The inserts tested included tungsten metal-oxide compacts and tungsten infiltrated with metals selected on the basis of their latent heats of vaporization and boiling points. Temperaturetime profiles taken during firings indicated that heat transfer rates were lower with the tungsten-metal systems than for unmodified tungsten, which indicated the possibility that transpiration or mass transfer cooling was being realized. A laboratory program was initiated to subject infiltrated tungsten samples to a plasma-arc heat source. It was found that, during exposure to high heat fluxes, temperatures were reduced by inducing flow of the molten infiltrant by pressurizing the back side of the specimen. This technique, in combination with ablative sacrificial materials, offers an excellent nozzle system for long-time firing durations in hybrid and solid propellant rockets.