Smoke Flow Research Articles

The behavior of two adjacent vortex rings produced by synthetic jets

Synthetic jets are expected as an effective method of active boundary layer control. A synthetic jet device generates jets by vibrating the diaphragm of a speaker or a piezoelectric actuator by using a method of suction and blowing. The actuators are low power, zero-net-mass-flux and compact devices. Mixing enhancement is one of the key technologies in boundary layer control and it is necessary to control the behavior of vortex rings to promote mixing the crossflow and the boundary layer. The purpose of this study is to investigate the effect of vortex ring interaction due to differences in orifice distance (S=1.5D, 1.7D, 2.0D) on the behavior of vortex rings. In this experiment, the smoke flow visualization was used to describe the interaction process by changing the distance between adjacent orifices. It was confirmed that the height where two vortex rings collapse that in the case of S=2.0D was lower that in the case of S=1.5D.

Research on fire source localization in confined space based on the fire characteristic physical quantity information

Fire source localization is of great significance to the firefighting and evacuation. In order to resolve the problem of precise localization of fire source in confined space, a method based on the diffusion law of hot smoke flow in the early stage of fire is proposed in this paper. According to the fire characteristic physical quantity information collected by the sensor array, the relative variability correlation degree is used to obtain the signal time delays between the sensor units. Then, the direction angle of the sensor units and fire source can be determined through the geometric relationship, and the angular localization principle is used to obtain the fire source localization results. Finally, according to the fire source localization results obtained in different sets of time delays, the localization estimation area is selected based on the dynamic clustering, and the center of this area is output as the comprehensive localization result. The testing results show that this method performs well and achieves a high localization accuracy.

Study of the effectiveness of water mist curtain in blocking fire-induced smoke in tunnel by means of CFD simulations

Tunnel fire smoke control is an important issue in the area of tunnel fire safety. Inspired by fire compartmentation in buildings, it is worth investigating whether a tunnel can be partitioned by a water mist system into a fire zone and safety zones. If so, people can move from the fire zone into a safe zone through the water mist system. Obviously, an essential question is to examine to what extent the fire-induced heat and smoke can be blocked by the water mist system. Thus, in present paper, CFD simulations were conducted to investigated the effectiveness of water mist systems on blocking fire-induced heat and smoke in a full-scale tunnel. We mainly focus on the impact of nozzle combination on smoke blocking effect. Simulation results show that the entrainment caused by the water mist system plays the main role on blocking fire-induced smoke, and the nozzle combination has small impact on heat blocking effect. Based on the momentum balance of the water mist system and ceiling jet smoke flow, a correlation of total water flow rate of the water mist system and fire heat release rate was proposed.

Large eddy simulation of smoke blocking by water sprays in a tunnel fire

This study presents a large eddy simulation (LES) of smoke blocking by water sprays in tunnel fires to investigate the effect of water sprays on the characteristics of the smoke flow. This study uses an LES solver – FireFOAM. In addition, fire experiments using a 1/10 scale model tunnel with two water spray nozzles are conducted to validate the LES results. The length of the model tunnel is 12 m, the height and width are 0.5 m, respectively, and the heat release rate of the fire is 7.38 kW (2.33 MW in full scale). Each spray nozzle operates at a pressure of 0.3 MPa, and the volume median diameter of spray droplets is 226 μm. First, a grid sensitivity analysis on the gas temperatures under free-burn conditions is performed. A 1.25 cm uniform mesh is chosen in the tunnel domain for all subsequent simulations. Then, FireFOAM predictions are validated against experimental data on the distribution of water spray mass flux under conditions without the fire and the gas temperatures measured under conditions with and without the water sprays. The water spray mass flux and gas temperatures predicted by FireFOAM show good agreement with the measurements. Next, the characteristics of the smoke flow are investigated in the presence of the water sprays. It is found that the water sprays block part of the smoke flowing under the ceiling and force it to descend to the ground. The descended smoke flows back in the lower part of the fire zone, and a vitiated air layer is formed. The vitiated air layer is composed of the smoke flowing back and fresh air drawn from the portal. On the other hand, some smoke leaks through the water spray zone and flows along the ceiling on the portal side. It is also found that the spray cooling delays the propagation of the leaked smoke toward the tunnel portal. Furthermore, it is quantitatively shown that the buoyant flow stratification destroyed by the water sprays becomes thermally and chemically stabilized again on the portal side, for the specific case under this study. Last, the smoke blocking performance is quantified based on the mass flow rates of the smoke flowing under the ceiling in the fire zone and the smoke flowing back toward the fire. A comparison of the smoke blocking rates between the predictions (time dependent: 40–50%) and measurements (steady: 39.4%) shows good agreement.

Experimental and numerical simulation on smoke control effect and key parameters of Push-pull air curtain in tunnel fire

A new type of push–pull air curtain was designed, and the smoke control effect was verified by carrying out a small-scale smoke control experiment in a model tunnel. The experimental results show that when the air volume of the system is the same, the smoke control effect of the push–pull air curtain is better than that of exhausting smoke from the side wall of the tunnel. The greater the velocity of the push air, the better the smoke control effect of the air curtain, and the effect of the jet velocity is greater than the cross-sectional area of the air curtain. FDS was used to numerically simulate the influence of factors such as air curtain spacing, air curtain width, push–pull airflow velocity in a tunnel of actual size on the effect of smoke control. The calculation results show that the smoke control ability of air curtain depends on the integrity of the air flow composed of incoming smoke and push–pull air flow. The smoke control efficiency E increases with the increase of momentum ratio R between push air flow of the curtain and incoming smoke flow. For the fire scene given in the paper, when the air curtain spacing is 80 m, the width of the push outlet and pull inlet are 0.6 m and 1.2 m, and the speed of push and pull air speed are 16 m/s and 23 m/s respectively; the momentum ratio R is greater than 6.5, and the smoke control efficiency E of the air curtain can exceed 90%.

Flow and Heat Transfer Characteristics of a Swirling Impinging Jet Issuing from a Threaded Nozzle of 45 Degrees

In order to achieve uniform and effective impingement cooling, a swirling jet with a swirling angle of 45° (SIJ 45°) is put forward in this paper. Namely, there are four 45° spiral grooves equipped on the inner wall of the circular hole. The difference in the flow field and heat transfer characteristics between the conventional impinging jet (CIJ) and SIJ 45° is compared and analyzed. The spiral channels can increase the heat transfer rate and cooling uniformity because of the action of superimposed airflow. In addition, the thread nozzle brings lower pressure loss, which can reduce the airflow friction while effectively ensuring high heat transfer in the center area of the jet. An experimental system is built to investigate the heat transfer and flow characteristics of the impingement surface. Smoke flow visualization technology is used to explore the complex flow field of the CIJ and SIJ 45°, and the heat transfer rate of the target surface is analyzed based on thermocouple data. When 6000≤Re≤30,000, and 1≤h/dj≤8, the averaged Nusselt number (Nu) correlation for SIJ 45° is established, which is in good agreement with the experimental results. SIJ 45° is an effective measure to replace the CIJ, and the research herein provides some reference for designing the structure of new jets.

Design and analysis of smoke flow visualization apparatus for wind tunnel

In the present study, the design and analysis of smoke generator are done for the low-speed wind tunnel. The wind tunnel fan is fitted with the Variable Frequency Drive to produce the wind speed in the range of 3 to 32 m/s with fan speed of 150 to 1500 rpm. The design of smoke generator was done according to Preston Sweeting mist generator principle corresponding to the free stream velocity of 3 m/s. A controlled smoke generator consisting of kerosene reservoir, controlled heater, blower, liquid column height adjustment mechanism, valves etc. was designed and fabricated. The smoke generator produced the smoke at the rate of 154 cm3/s which was close to the design flow rate of 149 cm3/s. To supply the required quantity of smoke in the wind tunnel, the smoke rake of NACA 0010 profile was developed and installed in the rapid contraction section of the wind tunnel to achieve the streamlined flow. The parametric studies were done on the smoke generator at different power inputs and its effects were studied on smoke temperature, smoke discharge and boiling time of the kerosene. The flow visualization was carried out on NACA 0015 airfoil model and the images were captured to examine the flow physics around them under different operating conditions.

Flow analysis of a front opening unified pod (FOUP) subjected to different fan filter unit (FFU) coverage area in mini-environment

Moisture contamination control has been adopted in many wafer handling and storage processes. In portable wafer carriers called front opening unified pods (FOUPs), the current industry-accepted methods of moisture control are gas purging and laminar air curtain (LAC). Both typically use purified clean dry air (CDA). In this study, we investigated five different methods of FOUP moisture control: conventional and diffuser-based purging, LAC, and combinations of each purging method with LAC. Two different scenarios of 100% and 75% FFU coverage area were examined. The FOUP was empty and its internal volume was exposed to a mock-up mini-environment (ME)–a process confinement device used in semiconductor fabs. Also, the LAC was mounted inside the ME above the FOUP opening. Smoke flow visualization, particle image velocimetry (PIV) and humidity measurement were performed in the FOUP median plane. Streamline flow patterns were obtained from the PIV results that can significantly enhance the evaluation of the moisture removal performance of different case studies. The findings showed that the combination of diffuser purge and LAC was the most resistant against external lateral moist air flow wherein it yielded the lowest average humidity. The findings could be beneficial for reducing waste and increasing product quality in semiconductor manufacturing. This can lead to significant reductions in energy usage and drastically reduce manufacturing costs.

Experimental study on the effect of canyon cross wind on temperature distribution of buoyancy-induced smoke layer in tunnel fires

Abstract Experiments were conducted in a 1:20 arced tunnel model to investigate the effect of canyon cross wind on buoyancy-induced smoke flow characteristics of pool fires, involving smoke movement behaviour and longitudinal temperature distribution of smoke layer. The canyon wind speed, longitudinal fire location and fire size were varied. Results show that there are two special smoke behaviours with the fire source positioned at different flow field zones. When the fire source is positioned at the negative pressure zone, with increasing canyon wind speed, the smoke always exists upstream mainly due to the vortex, and the smoke temperature near the fire source increases first and then decreases. However, when the fire source is located in the transition zone and the unidirectional flow zone, there is no smoke appearing upstream with a certain canyon wind speed. Meanwhile, the smoke temperature near the fire sources are decreases with increasing canyon wind speed. The dimensionless temperature rise of the smoke layer ΔTs* along the longitudinal direction of the tunnel follows a good exponential decay. As the canyon wind speed increases, the longitudinal decay rate of ΔTs* decreases. The longitudinal decay rate of ΔTs* downstream of the fire is related to the fire location and canyon wind speed, and independent of the fire size. The empirical correlations for predicting the longitudinal decay of ΔTs* downstream of the fire are established. For a relatively large-scale fire, the longitudinal decay rate of ΔTs* upstream of the fire increases as the distance between the fire source and the upstream portal increases, especially for larger canyon wind speeds.

Numerical Investigation on the length of the near-field region of smoke flow in tunnel fires

In a tunnel fire, smoke flow is often divided into two regions: the near-field and the far-field. The former is a three-dimensional radial flow region close to the fire source and the latter is a one-dimensional steady flow region generally formed a certain distance away from the fire. Although the efficiency of the smoke exhaust system is influenced by the region, limited research has been done to determine the demarcation point between the two regions. In this study, a total of 27 tunnel fire computer simulations were conducted to examine the effect of heat release rate (HRR) and the tunnel's cross-sectional dimensions on the location of the demarcation point. Rather than relying on the tunnel's height, which has been commonly used in previous tunnel fire studies to determine smoke flow characteristics, a hydraulic diameter of the tunnel's cross-section was employed in this study. An improved correlation for the location of the demarcation point is proposed by comparing a theoretical dimensionless model to numerical simulation results.

Flow visualization and force measurement of the clapping effect in bio-inspired flying robots

In this paper, we perform experimental investigations of the aerodynamic characteristics due to wing clapping in bio-inspired flying robots; i.e., micro-air-vehicles (MAVs) that fly by flapping their wings. For this purpose, four flapping MAV models with different levels of clapping (from no clapping at all to full clapping) are developed. The aerodynamic performance of each model is then tested in terms of the average thrust and power consumption at various flapping frequencies. The results show that clapping enhance both thrust and efficiency. To gain some physical insight into the underlying physics behind this clapping-thrust-enhancement, we perform a smoke flow visualization over the wings of the four models at different instants during the flapping cycle.

Realization of People Density and Smoke Flow in Buildings during Fire Accidents Using Raspberry and OpenCV

Fire accidents in residential, commercial, and industrial environments are a major concern since they cause considerable infrastructure and human life damage. On other hand, the risk of fires is growing in conjunction with the growth of urban buildings. The existing techniques for detecting fire through smoke sensors are difficult in large regions. Furthermore, during fire accidents, the visibility of the evacuation path is occupied with smoke and, thus, causes challenges for people evacuating individuals from the building. To overcome this challenge, we have recommended a vision-based fire detection system. A vision-based fire detection system is implemented to identify fire events as well as to count the number people inside the building. In this study, deep neural network (DNN) models, i.e., MobileNet SSD and ResNet101, are embedded in the vision node along with the Kinect sensor in order to detect fire accidents and further count the number of people inside the building. A web application is developed and integrated with the vision node through a local server for visualizing the real-time events in the building related to the fire and people counting. Finally, a real-time experiment is performed to check the accuracy of the proposed system for smoke detection and people density.

Numerical Simulations on the Extinguishing Effect of Water Mist System with Different Parameters of Longitudinal Ventilation in Curve Tunnel Fire

Once a fire occurs in a long curve tunnel, the mixing of hot smoke flow and cold air leads to turbulence due to the curvature’s impact. This phenomenon results in a greater thermal pressure difference at the fire source and a substantially greater temperature field than in the straight tunnel. The longitudinal air flowing along the wall loses a lot of velocity in the curve tunnel due to the massive wall friction. Under the same fire extinguishing conditions, the curve tunnel and straight tunnel have different requirements for longitudinal ventilation. Factors such as tunnel curvature, longitudinal ventilation operation time, and ventilation velocity were all evaluated in order to investigate the influence of longitudinal ventilation parameters on the fire extinguishing effect of water mist in the curve tunnel. The fire extinguishing effect of water mist coupling with longitudinal ventilation in the curve tunnel is studied by numerical simulation, and the recommended values of ventilation operation time and ventilation velocity in the curve tunnel with the participation of the water mist system are given. The results show that (1) the fire extinguishing effect of water mist decreases with the increase of curvature under longitudinal ventilation and (2) fire prevention effect is best when water mist and longitudinal ventilation are used in the curved tunnel, and the ventilation velocity should be greater than 2 m/s.

Qualitative study of airflow structure across wooden louvred window panels for natural ventilation applications

Window ventilation usually used in energy-efficient buildings as an effective natural ventilation system to provide an adequate opening for fresh air to flow into the interior space. It could help to reduce the indoor air temperature and limit the contaminants in buildings. This study aimed to visualise the airflow structure across a wooden louvred window panel using the smoke flow visualisation technique at different airflow speeds of 0.5, 1.0, 2.0, 3.0 and 5.0 m/s in a closed-loop wind tunnel. Two Sony 1920 × 1080i cameras captured the airflow structure, which took the side view, rear view, front view and back view images. The wooden louvred window panel promotes optimum outdoor airflow and facilitates continuous air exchange to replace the indoor air. Results showed that smoke filled the space quickly at the highest airflow speed of 5 m/s. This study used an inclination angle of 75° for the wooden louvred window panel to avoid rain splatter. Besides, wood material could be used as a facade shading device. Therefore, installing the wooden louvred window panels could enhance natural ventilation, ensure indoor thermal comfort and reduce indoor air contaminants.

Effects of Nanosecond Repetitively Pulsed Discharges Timing for Aeroengines Ignition at Low Temperature Conditions by Needle-Ring Plasma Actuator

These days, various national and international research organizations are working on the development of low NOx combustors. The present work describes the experimental and numerical characterization of flow dynamics and combustion characteristics in a rectangular burner. A ring-needle type plasma actuator was developed and driven by a high voltage nanosecond pulsed generator under atmospheric conditions. Smoke flow visualizations and Proper Orthogonal Decomposition (POD) were carried out to identify the relevant flow structures. Electrical characterization of the non-reactive flow was carried out to predict the electrical power and the optimum value of the reduced electric field (EN), which is useful for the implementation of a numerical model for the study of plasma-assisted ignition. A detailed plasma kinetic mechanism integrated with all excited species was considered and validated with experimental studies. Numerical modeling of plasma ignition has been performed by coupling ZDPlasKin with CHEMKIN. Energy and power consumption for methane/air plasma actuation is higher than the air plasma actuation. This could be due to the excitation and ionization of methane that required more energy deposition and power. The mole fraction of O atoms and ozone was higher in the air than the methane/air actuation. However, O atoms were produced in a very short time interval of 10−7 to 10−6 s; in contrast, the concentration of ozone was gradually increased with the time interval and the peak was observed around 10−1 s. Plasma discharges on the methane/air mixture also produced radicals that played a key role to enhance the combustion process. It was noticed that the concentration of H species was high among all radicals with a concentration of nearly 10−1. The concentration peak of CH3 and OH was almost the same in the order of 10−2. Finally, the mixture ignition characteristics under different low inlet temperatures were analyzed for both air and methane/air plasma actuation in the presence of different plasma discharges pulses numbers. Results showed that it is possible to reach flame ignition at inlet temperature lower than the minimum required in the absence of plasma actuation, which means ignition is possible in cold flow, which could be essential to address the re-ignition problems of aeroengines at high altitudes. At Ti = 700 K, the ignition was reached only with plasma discharges; ignition time was in the order of 0.01 s for plasma discharges on methane/air, lower than in case of plasma in air, which permitted ignition at 0.018 s. Besides this, in the methane/air case, 12 pulses were required to achieve successful ignition; however, in air, 19 pulses were needed to ignite.

Influences of inclined tunnel ceiling on plug‐holing phenomenon and mechanical smoke exhaust efficiency in tunnel fires

SummaryIn this paper, the effect of inclined tunnel ceiling on the characteristic of smoke extraction and the efficiency of mechanical smoke exhaust is studied. The parameters of different ceiling inclination angles, exhaust velocities, and heat release rates in the tunnel fire are taken into account. The results demonstrate that for the given smoke exhaust velocity and heat release rate, as the ceiling inclination angle increases, the smoke flow tends to accumulate toward the centerline of the tunnel ceiling, which increases the temperature of the smoke layer and relieves the phenomenon of plug‐holing beneath the shaft. The applicability of previous Fr criterion for predicting whether the plug‐holing phenomenon occurs is also verified. It is shown that the Fr criterion of Fr = 1.5 can also be applicable to the tunnel with different inclination angles. In addition, a predicted correlation for the mechanical smoke exhaust efficiency is established by taking the effect of inclined tunnel ceiling into account, and the predicted value is in good agreement with the results in this paper. This work provides an improved understanding of mechanical smoke extraction in tunnel with inclined ceiling. It may also serve as a reference for the design of a mechanical smoke exhaust system.

Experimental and numerical evaluation of wind-driven natural ventilation of a curved roof for various wind angles

The paper experimentally and numerically investigates the capability of the semi-cylindrical curved roofs in providing natural ventilation. Extensive measurements around a 1:17 scale model of a typical room with a semi-cylindrical roof have been conducted to determine the pressure and velocity field for gaining the discharge coefficients of apertures. Besides, smoke flow visualization and three-dimensional RANS simulations have been performed to correlate the ventilation characteristics, the induced volumetric airflow rate, and the flow trajectories. The prior studies were carried out on the domed roofs, but in the present study, the focus is on the semi-cylindrical curved roofs. It is found that the natural ventilation performance of the curved roof is profoundly sensitive to the wind angle (i.e. α) so that the extreme ventilation takes place at α = 0°, while the α = 75° - 90° gives the lowest airflow rate. The dependence of the airflow rate on α is attributed to variation in the pressure difference between openings (known as the main driving force) caused by flow acceleration and flow separation. Further increase of α slightly ameliorates the airflow rate, although still not comparable with that of α = 0°. Flow visualization results reveal that the height of the curved roof is a key factor in the enhancement of recirculation flow inside the building. Finally, a comparison discloses that the semi-cylindrical curved roof is prone to enhance the natural ventilation inside buildings as much as the wind-catchers, although presumably is cheaper in terms of the structural costs.

Experimental and numerical study on the smoke and velocity distribution in an extra-long railway tunnel fire

In order to fully understand the change of the escape environment in the process of an on-fire train moving to the rescue station at a constant speed, a 1:50 small-scale model experiment and full-scale numerical model were carried out to study the smoke control and flow field in a railway rescue station. The results show that after the train has been stopped for 6 min, the rescue station is full of smoke, and all cross passages except for 1# near the entrance of the rescue station have smoke in them. After the train has been stopped for 6 min, the piston wind speed at the entrance of the rescue station decreases to 1.26 m/s, the maximum velocity in the 1# and 2# cross passages is −1.7 m/s, and the velocity in the other cross passages is smaller, all within 0.5 m/s. The high temperature range (more than 328 K) at human eye-level upstream and downstream of the fire source expands to 230 m and 900 m in a full-scale tunnel. The maximum temperature at human eye-level near the fire source increases to 550 K. A smoke exhaust rate of 0.0049 m3/s in the shaft and air supply volume of 0.0098 m3/s in the parallel pilot can ensure that there is no toxic smoke in the cross passage.

Modulating flow and mixing characteristics of an inclined jet in crossflow at a large backward inclination angle by acoustic excitation

Previous investigations revealed that the backward-inclined jet in crossflow exhibits seemingly lower transverse velocity and turbulence intensity at inclination angles higher than a critical value about 25°, thus resulting in significantly low jet fluid dispersion properties. In this article, the acoustic excitations were applied to a backward-inclined jet in crossflow at an inclination angle higher than the critical value to examine experimentally whether the flow and dispersion characteristics could be improved or not. The flow in the test section of an open-loop wind tunnel was used as the crossflow. A profiled nozzle assembly with a loudspeaker installed in the lower part of it was used to generate the pulsating jet. The instantaneous and time-averaged smoke flow patterns were obtained by the laser-light-sheet assisted flow visualization method. The binary edge detection method was employed to the long-exposure smoke flow images to measure the jet spread width. The turbulence intensities, as well as the Lagrangian integral turbulence time and length scales, were obtained using a hotwire anemometer. The jet-fluid dispersion characteristics were examined using the tracer-gas detection technique. The results revealed that in the domain of jet pulsation intensity and excitation Strouhal number, the pulsed backward-inclined jet in crossflow presented three characteristic flow modes: synchronized oscillating jet, transition, and synchronized shear-layer vortices. The synchronized oscillating jet exhibited violent transverse oscillations due to the high axial jet pulsations induced by the acoustic excitation. The transverse jet width and jet penetration height were enlarged. The turbulence intensities were significantly increased, and the turbulence time and length scales were decreased, therefore led to a significant enhancement in the jet-fluid dispersions and mixing capabilities.

Flow Analysis and Relative Humidity (RH) Measurement in the Horizontal Plane of a Front Opening Unified Pod (FOUP)

Moisture removal techniques such as purge have been developed for the front opening unified pod (FOUP) in the semiconductor industry. The FOUP purge can significantly increase the quality of integrated circuit (IC) products. An experimental model of a simulated clean room, was designed at Taipei Tech. A mini environment (ME) and a FOUP were installed inside the model. Smoke flow visualization technique was performed with the assistance of an innovative laser light sheet generation system. A homemade smoke generator simulates the moist air flow pattern into the FOUP. The FOUP top view flow pattern was recorded by a Hamamatsu digital camera. The particle image velocimetry (PIV) in the FOUP horizontal plane was analyzed. Moreover, the relative humidity (RH) was monitored inside the FOUP. The results show that a purge flow rate of 200 LPM is the optimum purge flow rate for moisture removal performance inside the FOUP. The findings of the present study could significantly increase the quality of chip products in semiconductor manufacturing, decreasing product defect rates and significantly reducing energy consumption.