Tunnel Velocity Research Articles

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.

Analytical solution for vibrations of a curved tunnel on viscoelastic foundation excited by arbitrary dynamic loads

The paper proposes a new analytical solution for the dynamic response of a curved tunnel resting on a viscoelastic foundation subjected to dynamic loads in arbitrary forms. For the derivation, the governing differential equations and boundary conditions of the problem are established based on the Hamilton principle and the dynamic equilibrium theory. The modal superposition method is employed to obtain the explicit formulations of tunnel response, including deflection, velocity, acceleration, bending moment and shear force. The proposed solution is verified by providing comparisons between its results and those from the Finite Element program ABAQUS. Further parametric analysis explore the influence of radius of curvature and soil-structure relative stiffness ratio on the dynamic response of the tunnel. The proposed solution can be used by practitioners for the design of curved tunnels or pipelines.

Study on Time Factors in the Smoke Control Process of Highway Tunnel Fires

Fire is the most deadly risk during tunnel operation. Early rapid response and a reasonable smoke control plan are very important to evaluate tunnel fire performance. In order to study the relevant time factors for smoke management in a highway tunnel, firstly, the logical sequence and time of the fire alarm system (FAS) startup are investigated and analyzed. Then, according to the one-dimensional fluid mechanics model, the time rule of adjusting the airflow field in the tunnel from the normal operation stage to the emergency ventilation state is analyzed theoretically. Finally, the abovementioned theoretical formulas are verified through the employment of model experiments. The analysis shows that the time that passes from the start of the fire to when the exhaust fan is activated is close to 3 minutes. The time required to form a stable critical wind speed, however, is close to 7 minutes, which is longer than the 5 minutes it takes for the fire to reach its maximum temperature. Due to inertia, it takes about 0.5 to 2 minutes for the air velocity in tunnels of different lengths to drop from the traffic piston wind speed to the critical wind speed. If reverse smoke extraction is required, however, the duration is between 3 and 8 minutes. The conclusion is of guiding significance for the preparation of the emergency linkage control scheme for tunnels, as well as for the setting of initial boundary conditions for CFD fire simulations.

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.

A numerical study on critical velocity and back-layering length with trains’ blockage in longitudinally ventilated tunnel fires

Longitudinal ventilation is commonly adopted to prevent toxic smoke from spreading in tunnel fires. The understanding of the parameters, such as the critical velocity and the smoke back-layering, should be of significant for the ventilation system design in tunnels. In this paper, numerical simulations are carried out to investigate the critical velocity and back-layering length in an arched tunnel with trains’ blockage. A total of 51 simulated cases with varied heat release rates and longitudinal ventilated velocities are considered. The result shows that the smoke layering can be divided into two parts according to the velocity direction of the smoke movement. Based on previous theories, a formula taking the effect of tunnel cross-section and blockage into account for predicting the critical velocity is proposed, which shows good consistency with these results. According to the distribution of longitudinal velocity, it is inferred that different HRR have little effect on the reduction rate of velocity. It is also observed that the variation of dimensionless back-layering length can be divided into three regions as the dimensionless control velocity varies. Finally, the correlation between dimensionless smoke back-layering length and dimensionless control velocity in longitudinally ventilated tunnels is also proposed.

Numerical analysis of heat transfer between air inside and outside the tunnel caused by piston action

Piston action can affect the thermal environment inside the tunnel by changing the air pressure and velocity. This is extremely unfavorable for tunnels in cold regions because it causes frost damage in cold season. Understanding the influence of piston action on the air temperature distribution inside the tunnel can help us determine the length and location of frost damage. In this paper, a heat transfer numerical model considering piston action is developed. And then, a high-speed railway tunnel in the northeast of China is taken as an example to explore the temperature distribution laws via computational fluid dynamic (CFD). Afterwards, the effects of air temperature outside the tunnel and train velocity on temperature distribution are analyzed. The results show that the piston action can accelerate the heat transfer between cold air outside and inside the tunnel, the anti-freezing length near tunnel entrance and exit must be extended based on the calculation result of existed empirical formula. Before the train enters the tunnel, the freezing length near tunnel entrance is 1158 m. After the train passing through the tunnel, the freezing length near tunnel entrance is between 1495 m and 1497 m when the air temperature outside the tunnel decreases from −1 °C to −15 °C. Under different train velocities (80 km/h, 130 km/h, 160 km/h, 200 km/h and 250 km/h), the freezing lengths near tunnel entrance are 1458 m, 1469 m, 1472 m, 1497 m, 1498 m and its near tunnel exit are 330 m, 342 m, 349 m, 356 m, 357 m, respectively. In the section of 50 m away from tunnel entrance, the air temperature near the train changes suddenly, while the air temperature far away from the train changes gently. After the train passing through the tunnel, the air temperature inside the tunnel is consistent to that outside the tunnel within a specific distance from entrance. This study provides a guideline and references for the structure design and the construction of anti-freezing measurements for the tunnels in cold regions.

Kinerja Thermoelectric pada Kotak Pendingin Berdasarkan Rangkaian Thermoelectric dan Putaran Fan Wind Tunnel

<p class="Abstract">A thermoelectric cooler (TEC ) is a component of an electric solid-state cooler that works as a heat pump in the cooling process. Utilizing the peltier effect that is when an electric current is flowed heat absorption occurs on both sides of the thermoelectric and the release of heat on the other side. The use of thermoelectric as a coolant in cool box systems has been widely studied. The purpose of this study was to determine the performance of the cooler using thermoelectric based on the circuit and air velocity in the wind tunnel. The studied performance includes temperature difference, heat absorption, and coefficient of performance. The method used in this study is a real experimental method, with research variables namely series and parallel thermoelectric series and wind tunnel air velocity. Data were taken in the form of cool room temperature cold side temperature, hot side temperature, heatsink temperature, and ambient temperature. Thermoelectric connectivity electrically uses series and parallel circuits and is thermally arranged in series. The results showed that the thermoelectric circuit and wind tunnel air velocity affect the performance of the thermoelectric. Thermoelectric with series circuit produces the highest COP absorption compared to the parallel circuit. The highest performance thermoelectric series is obtained at an airspeed of 9.8 m/s. in these conditions the temperature difference (AT) produced reaches 19<sup>o</sup>C. the heat absorption (q<sub>c</sub>) reaches 34.26 watts and the COP reaches 0.89</p>

Effect of shelf squealer tip configuration on aerothermal performance of gas turbine blade

The tip leakage flow between rotating turbine blades and the stationary turbine casing causes aerodynamic losses and high thermal loads. Numerous novel tip geometries have been proposed for reducing both of these effects, one of them is a shelf squealer tip. The objective of this study is to investigate the aerothermal performance of three squealer blade tip configurations: conventional, vertical shelf and inclined shelf. Heat transfer was measured to analyze heat transfer features on the blade tip using transient IR thermography method. Also, computational fluid dynamics simulation and particle image velocimetry were conducted to visualize the flow characteristics around the blade tip. The experiment was conducted under conditions of a Reynolds number of 140,000 which was based on the inlet velocity in a wind tunnel. Thermal loads with vertical and inclined shelf squealer tip configurations are lower than with the conventional squealer tip. Due to the recessed pressure side rim of the vertical shelf squealer tip, the tip leakage flow was less reattached comparing to conventional squealer tips. This resulted in reduced thermal load on the tip surface and a larger tip leakage vortex around the blade suction side, which increased aerodynamic losses. Especially in the inclined shelf squealer tip, a separation bubble was generated on its inclined pressure side rim, which reduced the tip leakage flow due to the vena contracta effect. Consequentially, the reduced tip leakage flow decreased both heat loads and aerodynamic losses for the inclined shelf squealer tip. Therefore, the inclined shelf squealer tip demonstrated a good performance with respect to both aerodynamic loss and heat transfer.

Study on Spray Evaporation Treatment of Desulfurization Wastewater

Limestone-gypsum wet flue gas desulfurization (WFGD) often produces a certain amount of wastewater with complex water quality and heavy metal pollution which should be treated properly before release. Spaying the desulfurization wastewater into flue duct and using exhausted flue gas heat for evaporation is a promising and economical technology for achieving zero wastewater discharge in thermal power plant. To enable a more in-depth understanding on evaporation of FGD wastewater spray, a visual wind tunnel test rig based on the atomized droplet laser measuring system was built to reveal the impact factors on droplet thermal-fluid behavior. The dominant impact factors such as compressed air pressure and flow rate in air-blast spray nozzle, hot air temperature and velocity in the evaporation tunnel were analyzed to discuss the droplet size distribution and evaporation performance through alternating operate condition. A discrete mathematical model that combines both Eulerian and Lagrangian framework was established to validate the experiment result. It is concluded that introducing high pressure compressed air into the nozzle can contribute to the dispersion of droplets and enhance the evaporation rate. Proper flow rate in spray nozzle is required to avoid incomplete droplets evaporation. Air temperature and velocity in the evaporation tunnel apply positive impact on droplet size distribution and evaporation performance. Numerical simulation results of both dominant factors impact on evaporation behavior and total evaporation rate showed consistency with the experimental outcome.

Sensitivity Investigation on the Pressure Coefficients Non-Dimensionalization

The scaling of large structures to investigate their aerodynamics in wind tunnels is a common and robust procedure to estimate important magnitudes, including pressure coefficients. Different aspects can affect the estimation of pressure coefficients; four examples are the non-dimensionalization, blockage, non-stationarity, and non-Gaussianity of the wind tunnel velocity. This paper shows the variability of pressure coefficients due to these four aspects for the case study of a closed box section of a suspended bridge. It was estimated that the pressure coefficients of similar pressure taps vary significantly due to different sets of wind velocity time history used to non-dimensionalize the wind tunnel pressures. In addition, the stationarity of the wind velocity process was not confirmed for all wind velocity sets and the non-Gaussianity of the wind velocity time history was confirmed.

Effect of lateral concentrated smoke extraction on the smoke back-layering length and critical velocity in a longitudinal ventilation tunnel

In this study, the thermal smoke back-layering behavior and critical ventilation velocity of tunnel fires were investigated experimentally under the combined effect of lateral concentrated smoke extraction and longitudinal ventilation. The smoke back-layering length, critical ventilation velocity, and confinement velocity were analyzed and discussed. The results indicate that the smoke back-layering length decreases with increasing lateral concentrated smoke extraction velocity when the longitudinal ventilation velocity is less than the confinement velocity. However, above the confinement velocity, the smoke back-layering length increases with increasing lateral concentrated smoke extraction velocity. The critical velocity also shows an increasing trend with an increase in the lateral concentrated smoke extraction velocity. Finally, a correlation equation for the critical velocity of tunnel fires under the combined effect of lateral concentrated smoke extraction and longitudinal ventilation was proposed.

Pollutant concentration measurement and emission factor analysis of highway tunnel with mainly HGVs in mountainous area

In recent years, with the rapid growth of traffic flow in highway tunnels, the in-tunnel environmental pollution problem has aroused widespread concern in China. Previous studies mainly focused on the emission factors in tunnels dominated by passenger cars (PCs) and light commercial vehicles (LCVs). However, this study concentrated on the characteristics of traffic flow dominated by heavy-goods vehicles (HGVs), environmental monitoring and assessment in highway tunnels in mountainous areas. To explore the emission characteristics of vehicle pollutants in highway tunnels in such regions, a field measurement was conducted in Qinling No. 3 Tunnel in China, focusing on the traffic volume, air velocity, and pollutant concentration. Measurement results show that the average daily traffic volume in Qinling No. 3 Tunnel is 22,218 pcu/d, with the HGVs comprising more than 65%. The average air velocity in case tunnel reaches 5.09 m/s. The average concentrations of NH3, SO2, CO, and NOx in the tunnel are 4.54 ppm, 0.06 ppm, 6.22 ppm, and 4.39 ppm, respectively. The single vehicle emission rates of measured pollutants are calculated based on collected data, among which, the emission factor of NOx is the largest (average: 9.37 g/(km·veh)), while that of SO2 is the lowest (0.09 g/(km·veh)). The results show that the dilution of NO2 in the tunnel should be considered in the ventilation design of highway tunnels.

Design of Fire Source for Railway Vehicles and Measurement of Critical Velocity in Reduced-Scale Tunnels

In this study, the authors designed a reduced-scale railway vehicle fire, which was necessary for evaluating the fire safety of railway tunnels using a reduced model. To overcome the shortcomings of the methods used in conventional reduced-scale railway tunnel tests, the authors simulated the fire source of a railway vehicle using a methanol fire source for fire buoyancy, and a smoke cartridge for smoke visualization. Therefore, the heat release mass consumption rates of various methane trays were measured using a cone calorimeter (ISO 5660). The critical ventilation velocity in the railway tunnels was obtained using the designed fire source of the railway vehicle, which was evaluated by the measured temperature at the top of the tunnel as well as laser visualization.

Numerical studies on the critical velocity in bending tunnel

Critical velocity is the minimum air velocity required to suppress the smoke spreading against the longitudinal ventilation flow during tunnel fire situations. Former studies show that the critical velocity is concerned with the fire power, fire position and the tunnel geometry etc. Theoretical analysis and CFD simulation are performed in this paper to estimate the critical velocity for the bending tunnel with the different fire places. Results by CFD simulation show that when the fire originates near wall in bending tunnel, it needs larger critical velocity to arrest upwind movement of hot smoke. In research of bending tunnel, because of the route loss, the local loss of tunnel and with the effect of centripetal force, the critical velocity is concerned with radian. The critical velocity increases with the increasing of radian between 0° and 90°when the fire originates both at central and near the wall, while the critical velocity decreases with the increasing of radian between 90° and 180°.

Critical ventilation velocity under the blockage of different metro train in a long metro tunnel

SummaryTo address the effect of metro train blockage on the critical ventilation velocity in a long tunnel, a series of scenarios were conducted numerically through this study, including different fire sizes (5‐10 MW), metro train lengths (80‐120 m), and blockage ratios (φ, 0.50, and 0.57). It is known from the numerical results that the metro train length shows a limited effect on the critical ventilation velocity, which is because the longitudinal ventilation has become stable before reaching the fire source to prevent smoke back‐layering, and increasing the metro train length only increases the distance of stabilizing the longitudinal ventilation. The blockage ratio shows an obvious influence on the critical ventilation velocity, which is because the presence of the metro train can obviously reduce the flow cross‐sectional area of the tunnel. An empirical model is developed as well, while it is known that the critical ventilation velocity increases with the one‐third power of dimensionless heat release rate and (1‐φ). The research outcomes of this study provide a technical guide for the design of the metro tunnel and the relevant emergency management of fire rescue under fire conditions.

Experimental and numerical study on critical ventilation velocity for confining fire smoke in metro connected tunnel

Extensive studies have been conducted on critical velocity in ordinary single tunnels while still lacking the direct guidance for smoke confinement in connected metro tunnels, which is noteworthy as metro networks have expanded in recent years. Focusing on this issue, this study aimed to predict the critical velocity for confining fire smoke in connected metro tunnels. Model experiments and numerical simulations were carried out while considering the inclination of connected tunnels and the incorporation of longitudinal ventilation in a main tunnel, which represent the typical structure and ventilation characteristics of metro connected tunnels. The results show that, under a node-area fire scenario, the critical velocity for confining upstream smoke in a connected tunnel was lower than that for an ordinary single tunnel, caused by the discrepancy in smoke density and heat allocation. Additionally, due to the stack effect driving and hindering smoke diffusion in upward and downward connected tunnels, the dimensionless critical velocity increased and decreased with the increase in upward and downward slopes, respectively. Meanwhile, smoke diffusion was reduced by incorporated longitudinal ventilation in the main tunnel, resulting from a decrease in smoke temperature. A prediction model of the critical velocity for confining smoke in metro connected tunnels was developed, which achieved an improvement on previous model and could support the smoke control design in such metro systems.

An experimental study on piezoelectric energy harvesting from wind and ambient structural vibrations for wireless structural health monitoring

Energy harvesting is an emerging technology holding promise of sustainability amid the alarming rate at which the human community is depleting the natural resources to cater its needs. There are several ways of harvesting energy in a renewable fashion such as through solar, wind, hydro-electric, geothermal, and artificial photosynthesis. This study focuses on energy harvesting from wind vibrations and ambient structural vibrations (such as from rail and road bridges) through piezo transducers using the direct piezoelectric effect. First, the potential of the piezoelectric energy harvesting from ambient wind vibrations has been investigated and presented here. Lead zirconate titanate patches have been attached at the fixed end of aluminum rectangular and trapezoidal cantilevers, which have been exposed to varying wind velocity in a lab-size wind tunnel. The effect of perforations and twisting (distortion) on the power generated by the patches under varying wind velocity has also been studied. It has been observed that the power is comparatively higher in rectangular-shaped cantilever than the trapezoidal one. Perforations and shape distortion showed promising result in terms of higher yield. The laboratory experiments have also been extended to the real-life field condition to measure the actual power generated by the lead zirconate titanate patches under the ambient wind vibrations. Next, energy harvesting from the ambient structural vibrations has been done both experimentally and numerically. Four different prototypes have been considered. The power has been measured across the lead zirconate titanate patches individually and in parallel combination. A maximum power output for Prototype 1 to Prototype 4 has been found to be 4.3428, 11.844, 25.97, and 43.12 µW, respectively. Numerical study has also been carried out in ANSYS 14.5 to perform the parametric study to examine the effect of addition of mass at the free end of cantilever. In a nutshell, this article provides a comprehensive study on the effect of various factors on the amount of energy generated by piezoelectric patches under wind and structural vibrations. The energy generated is sufficient for driving low-power-consuming electronics that can further be used for other applications like wireless structural health monitoring, and so on.

Influence of the water spray flow rate and angle on the critical velocity in tunnels with longitudinal ventilation

Recently, the water spray system has been used in important tunnels to weaken the heat release rate of fires and cool the fire smoke, which has a significant impact on the critical velocity that is the minimum longitudinal ventilation required to prevent the fire smoke from spreading upstream of a tunnel. Therefore, a series of tunnel fires with eight heat release rates responding to from a car fire to a bus fire set as 5 MW, 10 MW, 15 MW, 20 MW, 30 MW, 40 MW, 50 MW and 60 MW were simulated to investigate the influence of the water spray flow rate (0 L/min, 200 L/min, 300 L/min, 400 L/min, 500 L/min and 600 L/min) and spray angle (30°, 60°, 90°, 120° and 150°) on the critical velocity by Fire Dynamics Simulator (FDS). The simulation data of the case without a water spray system were compared with previous studies and agreed well with previous model. However, previous model overestimates the critical velocity in the cases with a water spray system. The critical velocity decreases with the increase of water spray flow rate, while it increases firstly and then decreases with the increase of the spray angle. The change of the critical velocity is illustrated with the flow field and is caused by both the cooling effect of the water spray and inertial force of the longitudinal flow. Furthermore, a new correlation was proposed to predict the critical velocity in the tunnel with water spray system by introducing a modified coefficient β representing the water spray flow rate effect.

Power coefficient measurements of a novel vertical axis wind turbine

AbstractWind tunnel experiments were performed to evaluate the power coefficients of three vertical axis wind turbines (VAWTs), namely, (a) conventional VAWT (two‐bladed troposkien shape), (b) novel 50% shifted troposkien shape‐vertical axis wind turbine (50% STS‐VAWT), and (c) novel 100% STS‐VAWT. All turbines had the same height, swept area, and NACA0015 airfoil with the same chord length. During the power coefficient measurements for each one of the turbines, the wind tunnel velocity was varied while the turbine rotation was kept constant through rheostatic load adjustment. Two different turbine rotations were used (600 rpm and 700 rpm). The resulting tip speed ratios (TSRs) ranged from approximately 2 to 6 in the current investigation. Both the power coefficients and the tip speed ratios were corrected against wind tunnel blockage effects. The 50% STS‐VAWT model showed an overall better aerodynamic performance (both corrected and uncorrected power coefficients) when compared against the other two configurations, likely due to a combination of (a) power generation increased due to blade‐wake interaction (BWI) reductions and (b) power generation capacity related to the blade length of each turbine.

A Novel Empirical Model for CMOS Schottky Diodes up to 67 GHz

A novel empirical model capable of predicting dc current and RF capacitance characteristics for CMOS Schottky diodes is first proposed. The model is developed upon a physical-based equivalent circuit and a set of closed-form formulas. Different current transport mechanisms including thermionic emission, tunneling, and carrier velocity saturation are counted by nonlinear resistances in the intrinsic model, while stray and substrate parasitic effects are also included in the parasitic model. A systematic procedure is built up to directly extract all the model parameters from measurements. The model is validated in 65- and 130-nm CMOS technology, and the modeled results show an excellent agreement with the measured data up to 67 GHz, which is the highest modeling frequency reported up till now.