Longitudinal Air Research Articles

Modelling the Smoke Flow Characteristics of a Comprehensive Pipe Gallery Fire with Rectangular Section

In this study, a numerical model of the cable cabin of a comprehensive pipe gallery was established to study the smoke flow diffusion behaviour of a comprehensive pipe gallery fire under a rectangular cross-section. The effects of fire source power (Q = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 MW) and fire source location (D = 10, 20, 40, 50, 60, 80, 100 m) on the smoke flow characteristics—such as smoke layer height and thickness, longitudinal airflow velocity, and ceiling temperature distribution—were analysed, and the corresponding prediction model was fitted. The results show the following: (1) The height of the smoke layer decreases with increasing fire power, and the predictive model of the smoke layer thickness obtained from the fitting is proportional to the smoke mass flow rate and inversely proportional to the aspect ratio of the pipe gallery. (2) Longitudinal air velocity prediction models of D < 50 m and D ≥ 50 m are fitted, and the average error between them and the numerical simulation values is 9.611%. (3) The temperature decay gradient of the smoke decreases gradually with increasing distance from the fire source, while there is a significant temperature difference between the two sides of the fire source. The average relative errors of the dimensionless temperature rise models fitted upstream and downstream of the fire source in the form of ΔTT0=AeBD−XH+C exponentials with respect to the numerical simulations were 11.688% and 7.296%, respectively. The results of the study can provide a reference for smoke flow and fire prevention and control in comprehensive pipe galleries.

Full-scale experimental, numerical, and theoretical research on the spatio-temporal evolution of the flow structure in longitudinal and semi-transversal ventilated tunnels

This paper presents full-scale experimental, numerical, and theoretical research on the spatial and temporal structure of airflow for different modes of ventilation system operation in a road tunnel. The tunnel under investigation is equipped with a semi-transverse ventilation system. There are 19 pairs of axial jet fans in each tunnel tube and 164 air supply vents on both sides of a tube located just above the road surface. A set of seven stand poles with mounted vane anemometers were arranged along the tunnel tube axis. Different configurations of axial jet fans and air supply vents were employed, and changes in airflow velocity were recorded. Since the main objective of the study is to achieve a deeper understanding of the examined phenomena, a numerical model of the flows in the tunnel was built, and the acquired data was used to validate it. Some theoretical considerations are provided and also validated. All of this together reveals a very uneven distribution of longitudinal air velocity along the tunnel axis. Then, time constants when switching the fans on and off are determined. It is shown that the time needed to achieve a stable flow after the fans are turned on is in the order of five minutes, and the decay time after the fans are turned off is much longer; it takes about 15 min to calm down the flow. Next, it is determined that the transverse air supply significantly changes the airflow structure: the average longitudinal air velocity at the inlet portal was up to 25% lower than without it, while values at the outlet portal were similar. Finally, it is indicated that axial fans that operate in the suction mode are similarly efficient to those in the pumping mode.

Natural convection in vertical solar heat collector nanotextured with reduced graphene oxide and silver nanowires

In this study, a vertical shaft is coated using silver nanowires (AgNWs) together with reduced graphene oxide (rGO) via the supersonic spray-coating technique to enhance the absorption of solar radiative heat for use in solar air heaters. The rGO/AgNW-coated surface induces multiple light reflections, similar to that in a perfect black body inside a Helmholtz jar, thus enhancing the collection of solar radiation. Air fed into the vertical heated shaft ascends owing to buoyancy. The air temperature difference, ΔT, between the outlet and inlet is measured for various mass flow rates to quantify the heat transferred to the air from the solar-heated vertical shaft. The longitudinal air velocity and temperature distributions inside the shaft are numerically simulated using the fire dynamics simulator. Both two- and three-dimensional simulations are performed, and the results are compared with experimental data for various mass flow rates. The results confirm that the trends observed in both the experiments and simulations agree well for all cases. Multiple metal meshes are installed inside the vertical shaft to induce turbulence, which enhances the heat transfer intensity. This turbulence enhancement is confirmed via smoke visualization and infrared images.

Development of simplified air drag models including crosswinds for commercial heavy vehicle combinations

Accurate range prediction requires good knowledge of the prevailing wind conditions and how they affect the energy consumption of the ego vehicle. A few different simplified vehicle air drag models that explicitly include the effect from crosswinds are presented and compared through some objective criteria. The models are developed from the normal air drag equation where the effect from wind is implicit and therefore often forgotten or neglected. The purpose is to find a low-complexity model complementing CFD models and wind tunnel tests, that can be used for range estimation and predictive energy management algorithms. To simplify online estimation, a requirement is that the air drag models only contain a few tuning parameters. The models are validated against CFD calculations for a few vehicle combinations and the best models show good accuracy for air attack angles up to at least 60 degrees. It is shown that the parameters of the simplified models can loosely be connected to some basic geometrical attributes of a vehicle combination so it should be possible to give at least a rough estimate of the parameters of a simplified model based on these geometrical attributes. This is useful for making a first estimate of the aerodynamic properties of a vehicle combination after major changes in the exterior, e.g. when adding a trailer. It also highlights that the size and the shape of the vehicle side may be mainly responsible for the high longitudinal air drag sensitivity to crosswinds for large vehicle combinations.

Study on the Air Inlet Velocity and Temperature Distribution in an Inclined Tunnel with Single Shaft under Natural Ventilation

The emergence of inclined tunnels under natural ventilation has brought many new fire safety issues. The smoke movement in the tunnel is affected by the chimney effect induced by the shaft and the downstream tunnel. The characteristics of temperature distribution in inclined tunnels are different from horizontal tunnels, which is worthy of further study. A series of conditions were carried out in an inclined model tunnel with a single shaft to investigate the temperature distribution characteristics. In this study, the longitudinal air inlet velocity is used to replace the longitudinal ventilation wind velocity. Results showed that the variation of fire source location Lf,, shaft height Ls, and the tunnel slope φ have obvious effect on the air inlet velocity. Based on the previous theories and the non-dimension analysis, the formulas of the dimensionless longitudinal inlet air velocity and the distribution of the maximum smoke temperature under the ceiling are proposed, which show good consistency with the simulation results. The reduced-scale experiments were conducted to validate the results of numerical simulation. The error range between the theoretical results and the simulation results is less than 20%.

Study on control of upstream smoke propagation in road tunnels by using a side-wall sprinkler system

Side-wall sprinkler systems used for fire suppression in road tunnels have been found to redistribute the longitudinal air flow within the tunnel, with the most notable feature being an acceleration of the airflow near the ceiling. The present work is driven by the motivation of utilizing the acceleration effect to block the upstream smoke propagation in cases when the ventilation rate is sub-critical. A nozzle prototype that has practical applications in road tunnels was selected for a side-wall water spray system in a 300 m road tunnel, and a total of 60 cases were modelled using the CFD code FDS (version of 6.7.3) to investigate its smoke blocking effect. Experiments and correlations in the literature have validated the accuracy of the FDS method. The confinement velocity reduction rate, which is determined as the reduction of the ventilation rate required to stop the smoke front point at a given position, was found to be positively affected by both the water spray rate and the ventilation rate and was successfully correlated using dimensionless momentums of these two factors. By comparing the confinement velocity reduction rates with the increases in the ceiling velocity in the absence of tunnel fire, utilization efficiencies of the spray-induced accelerations were further obtained.

ФИЗИЧЕСКИЕ ПРИНЦИПЫ И МАТЕМАТИЧЕСКОЕ ОПИСАНИЕ ФОРМИРОВАНИЯ ПОЛЕЙ СКОРОСТИ ВОЗДУХА И КОНЦЕНТРАЦИЙ ГАЗОВЫХ ПРИМЕСЕЙ В ГОРНЫХ ВЫРАБОТКАХ УГОЛЬНЫХ ШАХТ

The air composition of the mine atmosphere in coal mines; Safety Rules regulate heat-humidity and speed conditions created by the ventilation system. At the same time, physic-chemical, aero-gas-dynamic and thermos-physical processes occur, which affect the level of aerological safety. A system of equations recommended that allows one to calculate longitudinal air velocity profiles at any point in the mine ventilation network. These recommended describing the transfer of gas impurities in the mine atmosphere by the equation of turbulent-convective diffusion.

Efficiency of solar air heaters

The authors of the article show the possibility of drying transformer windings using solar energy and the results of increasing the efficiency of a flat air collector by installing metal shavings on the absorber across the longitudinal air flow flowing through the collector. A solar drying installation has been developed, consisting of a solar air collector and a drying chamber. Experimental studies of a solar drying installation with a solar air collector including a conventional flat absorber and with a solar air collector with improved heat removal performance due to transversely installed metal shavings showed a significant difference between the air temperatures in the drying chamber, namely a more efficient use of the solar air collector.

Modeling and adaptive control of air vehicles with partial nonlinear parametrization

This paper studies the control of a generic longitudinal air vehicle dynamic model during the cruise phase with nonlinearly parametrized uncertain aerodynamic functions. Verified by the improved coefficient precision of the aerodynamic forces and moments, using a curve fitting technique on available flight data, it is demonstrated that a nonlinearly parametrized model can more accurately capture the vehicle dynamics than a linearly parametrized model. For control design, the air vehicle model is decomposed into a flight-path angle subsystem with rotational dynamics and a speed subsystem with engine actuation dynamics. An adaptive control scheme is developed using the combination of a function bounding technique and a modified backstepping control method for two interconnected subsystems. The designed adaptive control laws ensure both global stabilization and adaptive non-zero equilibrium point regulation of the closed-loop control system. A thorough simulation study is presented to show the effectiveness of the developed adaptive control scheme for a nonlinearly parametrized air vehicle system model.

Influence of longitudinal wind on mass burning rate of rectangular heptane pool fire with different aspect ratios

Fire is one of the primary disasters that threaten the safe operation of tunnel and underground space. Mass burning rate is a significant parameter in characterizing the behavior of pool fire which is related to thermal feedback, pool shape and ambient wind. Five convection-controlled heptane pool fires are examined inside a longitudinal wind tunnel with aspect ratio of 1, 2, 4, 8, and 16. The variation of burning rate with longitudinal wind is divided into increasing regime at v < 0.96 m/s, decreasing regime at 0.96 ≤ v < 1.98 m/s, and slightly-fluctuating regime at 1.98 ≤ v < 2.49 m/s. The effect of longitudinal wind on burning rate is attributed to heat feedback, flow disturbance and oxidizer supply around the flame. The increment of burning rate with wind speed is proved by the liquid evaporation theory. The heat transfer analysis proves that the enlargement rate of burning rate is linearly proportional to a coupling factor of longitudinal air flow speed and aspect ratio in a unified mathematical correlation Δṁ″/k′=(2n+1n)v which is fully consistent with measurements. The results provide guidance for tunnel fire protection design and prediction of fire development under the condition of vehicle tank leakage inside a tunnel.

Air permeability of thermally modified hemlock wood

Western hemlock (Tsuga heterophylla) is a prevalent coastal species in British Columbia (BC). Its wood has a high potential for thermal modification, a process that can affect numerous physical properties, including air permeability. The current study investigates the longitudinal air permeability of hemlock wood modified at three temperature levels, 170 °C, 212 °C, and 230 °C, and a two-hour treatment length. Permeability values obtained using Darcy’s law and the water-falling volume displacement method were positively correlated with treatment temperature up to 212 °C, after which the permeability decreased slightly. ANOVA followed by the Duncan test revealed that thermal treatment at 212 °C and 230 °C significantly increased air permeability, whereas it was insignificant at 170 °C.

A new-proposed multi-inclined-cylinder radiation model of rectangular pool fire of heptane in longitudinal winds

A rectangular pool fire may be formed which is essentially affected by ambient wind in open space. In this work, the experimental investigations of flame radiation of heptane fire in longitudinal wind are conducted using five rectangular containers with a constant surface area, and variable length-width ratios of 1, 2, 4, 8, 16. The longitudinal air speeds are 0, 0.71, 0.96, 1.22, 1.47, 1.73, 1.98, 2.24 and 2.49 m/s respectively. Results show that as air speed increases, the incident heat flux increases initially and then decreases afterwards, and the threshold value of air flow is approximately at v = 0.96 m/s. The heat flux is primarily determined by burning rate at v ≤ 0.96 m/s whereas it is determined by the combination of flame morphology and burning rate at v > 0.96 m/s. The theoretical formula of radiation emissivity of rectangular pan is deduced using mass burning rate and aspect ratio. The multi-inclined-cylinder model incorporating aspect ratio, flame height, tilt angle and mass burning rate is proposed to evaluate the rectangular flame radiation in air flows. The theoretical values of heat fluxes are acceptably consistent with measurements in longitudinal wind-blown condition.

Long-Term Trends in Secondhand Smoke Exposure in High-Rise Housing Serving Low-Income Residents in New York City: Three-Year Evaluation of a Federal Smoking Ban in Public Housing, 2018-2021.

In July 2018, the U.S. Department of Housing and Urban Development passed a rule requiring public housing authorities to implement smoke-free housing (SFH) policies. We measured secondhand smoke (SHS) exposure immediately before, and repeatedly up to 36 months post-SFH policy implementation in a purposeful sample of 21 New York City (NYC) high-rise buildings (>15 floors): 10 NYC Housing Authority (NYCHA) buildings subject to the policy and 11 privately managed buildings in which most residents received housing vouchers (herein "Section 8"). We invited participants from nonsmoking households (NYCHA n = 157, Section-8 n = 118) to enroll in a longitudinal air monitoring study, measuring (1) nicotine concentration with passive, bisulfate-coated filters, and (2) particulate matter (PM2.5) with low-cost particle sensors. We also measured nicotine concentrations and counted cigarette butts in common areas (n = 91 stairwells and hallways). We repeated air monitoring sessions in households and common areas every 6 months, totaling six post-policy sessions. After 3 years, we observed larger declines in nicotine concentration in NYCHA hallways than in Section-8, [difference-in-difference (DID) = -1.92 µg/m3 (95% CI -2.98, -0.87), p = .001]. In stairwells, nicotine concentration declines were larger in NYCHA buildings, but the differences were not statistically significant [DID= -1.10 µg/m3 (95% CI -2.40, 0.18), p = .089]. In households, there was no differential change in nicotine concentration (p = .093) or in PM2.5 levels (p = .385). Nicotine concentration reductions in NYCHA common areas over 3 years may be attributable to the SFH policy, reflecting its gradual implementation over this time. Continued air monitoring over multiple years has demonstrated that SHS exposure may be declining more rapidly in NYCHA common areas as a result of SFH policy adherence. This may have positive implications for improved health outcomes among those living in public housing, but additional tracking of air quality and studies of health outcomes are needed. Ongoing efforts by NYCHA to integrate the SFH policy into wider healthier-homes initiatives may increase policy compliance.

Noise distribution law of air-conditioning ducts for metro vehicles

The passenger comforts are directly affected by the air supply uniformity and noise of air-conditioning duct inside the metro vehicle. In order to solve the integrated design problem of air supply uniformity and noise control of air-conditioning ducts, a theoretical model is established to analyze the air supply uniformity, noise attenuation, and airflow regeneration noise of the air-conditioning duct. The distribution law of the sound pressure level along the longitudinal air outlet was calculated, and the calculated results were compared with test ones. For a typical air supply duct with a constant cross section: (1) In order to ensure air supply uniformity at the outlet of the static pressure chamber, the area of the side air hole should be gradually increased in an exponential form along the direction of the main flow, and cannot be designed in the equal area form. (2) The airflow speed of each side hole decreased in sequence along the air duct length. In order to ensure the uniformity air supply of each side hole, the area of each side hole must be gradually increased. (3) The noise in the air conditioner duct was mainly concentrated at the front end of the air duct, and the noise at the subsequent air outlet can fully meet the current requirements of subway vehicles for indoor noise. Therefore, the front end of the air duct is the most important area for noise control.

A Simulation Study on the Smoke Control Effect with Different Smoke Exhaust Patterns and Longitudinal Air Supply for Ultra-Wide Tunnels

This study was motivated by the lack of understanding of the smoke control effect on an ultra-wide tunnel fire, with different smoke exhaust patterns (sidewall and top exhaust patterns) and longitudinal air supply volume (0, 30%, 50%, 70%, and 90%). A full-scale ultra-wide tunnel model was constructed based on the FDS and the fire parameters were analyzed, such as the longitudinal spread distance of smoke, the smoke layer height and the temperature at safe height. In addition, the smoke exhaust efficiency was calculated based on the mass flux of CO2, and the smoke control effect with different smoke exhaust patterns and air supply volumes was compared. Results show that the smoke exhaust patterns and air supply ratios have a great influence on smoke spread distance and exhaust efficiency. The smoke spread distance is shortened by increasing the longitudinal air supply volume, and when the ratio of air supply volume to smoke exhaust volume is less than 50%, the top exhaust pattern can control the spread of smoke better with a smaller smoke spread distance. In addition, the height of the smoke layer is controlled above the safe height of 2 m under the top smoke exhaust, and the temperature at both ends of the tunnel (25 °C) is lower than that under the sidewall exhaust pattern (35 °C). The smoke exhaust efficiency was calculated based on the mass flow rate of CO2, and the exhaust efficiency of the top exhaust pattern (~70%) is significantly higher than that of the sidewall exhaust pattern (~55%). However, as the air supply volume increases, there is a reduced increase in the exhaust efficiency. Therefore, taking the economic cost into account, the air supply ratios of 30% and 50% are the best for top and sidewall exhaust patterns, respectively. The results of this work provide important information about smoke distribution characteristics in an ultra-wide tunnel fire and may guide its design of smoke exhaust.

Comfort related woven fabric transmission properties made of cotton and nylon

The interaction of water and air i.e. moisture regain, water vapor transmission, wicking and air permeability with woven textiles are tested to investigate the comfort of woven fabrics made of nylon, cotton, and cotton–nylon mixtures with different yarn counts. The fabrics porosity (based on equation 1 and 2), woven fabric sett (ASTM D3775-03), fabric thickness (ASTM D1777-96), fabric weight (ASTM D3776M - 20), fabric vapour transmission (ASTM E96-00), transfer wicking, and longitudinal wicking (DIN 53924), moisture regain (ASTM D2495-07), and air permeability (ASTM D737) were examined for investigations based on the standards in the brackets. The experimental results showed that fabric transfer wicking, longitudinal wicking, moisture regain and air permeability properties increase as the yarn goes to coarser for all woven fabric samples but the water vapor transmission property decrease. Additionally, fabric transfer and longitudinal wicking capabilities improved with increased nylon fiber blend ratios within fiber conformation. However, the nylon fiber composition has negative impact on air permeability, water vapor transfer rate, wicking and moisture capabilities of the woven fabrics. Generally, it can be concluded that the existence of nylon fiber, and yarn count coarseness improved wicking properties of the woven fabrics and decreased the air permeability property of woven fabrics.

Effect of longitudinal slope on the smoke propagation and ceiling temperature characterization in sloping tunnel fires under natural ventilation

In order to reduce risk and increase resilience of transportation tunnels, the early-phase detection and protection of fire are of great significance for the safety assessment of tunnel. In this work, numerical simulations are conducted to characterize the thermal plume propagation and ceiling temperature profile in a tunnel with longitudinal slope increasing from 0% to 15%. For the sloping tunnel, as there exists elevation difference between the tunnel ends, the longitudinal air flow will be induced from the lower end. Under such circumstances, the stack effect occurs and it becomes the primary driving force for the smoke movement in tunnel. Results show that the thermal plume behaviors in the sloping tunnel, including the thermal flow field and the temperature profile, show apparently different from those in the horizontal one. The induced air flow rate that directly related to the strength of stack effect is quantified, and on this basis, the predictive correlation of the temperature profile under the ceiling is proposed by considering the influence of the stack effect for different tunnel slopes. Besides, the accuracy and applicability of the obtained correlation are further verified by comparing to a total of four series of previous experimental data with a wider range of tunnel slopes, dimensions, and heat release rates.

Gas-liquid hydrodynamics and vortex motion of flame spread over jet fuel in longitudinal air stream

The gas-liquid hydrodynamic characteristics and pulsation behaviors of flame spread over jet fuel are investigated in various longitudinal air streams. The experiments are conducted in concurrent and opposed air streams by igniting fuels at opposite extremes of the tray. Six magnitudes of air stream speeds are used for the tests, namely 0.45, 0.88, 1.3, 1.73, 2.15 and 2.57 m/s. The liquid convection flow is monitored using a rainbow schlieren deflectometry which confirms that the liquid flows in vertical and horizontal direction act together to form a clockwise vortex. Based on non-dimensional parameter group, the effect of longitudinal air stream on liquid hydrodynamics is revealed. The movement of liquid convective flow is primarily driven by thermocapillary force, while the effect of buoyancy is relatively small, regardless of direction and magnitude of air stream speeds. Combined with the boundary layer theory, the flow velocities driven by expansion of hot gas product and opposed thermal buoyancy and local air stream are quantified. The threshold value of air flow speed for establishing gas-phase recirculation cell is determined, and the transformation mechanism of flame pulsation mode is analyzed.

The characteristics of ventilated pool fires of Daegok-Sosa subway lines using Model-Scale tunnels and stations

The characteristics of ventilated pool fires for Daegok-Sosa subway lines with a length of 15.12 km in Seoul, South Korea were studied. The model-scale tunnels and stations were built in scale 1:50 and temperature distribution, smoke (Carbon Monoxide (CO)) spreading, back-layering phenomenon and longitudinal ventilated flow velocity were systematically investigated. Present subscale model was based on the Froude number similitude and a heptane pool fire was used as the heat source. Heat release rate of pool fire in model-scale tunnel was 1.13 kW corresponding to an equivalent 20 MW in full scale. The occurrence of smoke back-layering phenomenon could be observed by whether the upstream temperature rose at a certain position during the whole process of the fire test. In the single line parallel tunnel, the smoke was restricted to downstream of fire source, however, in the double track tunnel the smoke back layering slightly occurred in upstream region of fire source and then it soon disappeared. The smoke propagated far downstream side and the tunnel safety measures should be taken for passenger’s safe evacuation from the CO toxic gas. The longitudinal air velocities in upstream side near the fire source were accelerated and were about 11% higher than the downstream velocities in single line parallel tunnel and more than 20% higher in double track tunnel caused by the blocking effect of fire source. A generalized quasi one-dimensional flow network solver was employed to calculate the longitudinal air velocity in tunnels using the finite volume Ghost Junction Method (GJM) which were compared with the present experimental data. The required minimum longitudinal air velocities (2.0 m/sec for single line parallel tunnel and 2.5 m/sec for double track tunnel) specified in the Korean railway facility technical standards were well satisfied in this experiment.

Mine conveyor belt fire classification

This article presents a conveyor belt fire classification model that allows for the determination of the most effective firefighting strategy. In addition, the effect of belt design parameters on the fire classification was determined. A methodology that involves the use of numerical simulations and artificial neural networks was implemented. An approach previously proposed for modeling fires over conveyor belts was used. With the objective of obtaining some required modeling input parameter and verifying the capacity of this approach to get realistic results, computational fluid dynamics model calibration and validation were carried out using experimental test results available in the literature. Results indicated that scenarios with belt positions closer to the mine roof and greater tunnel heights require a higher longitudinal air velocity to be attacked directly. Furthermore, the belt fire classification model provided by the artificial neural network had an accuracy around 95% when test scenarios were classified.