Experimental study of the effectiveness of a water system in blocking fire-induced smoke and heat in reduced-scale tunnel tests

A water system, consisting of several water mist nozzles, has been installed in a reduced-scale tunnel. Its effectiveness in blocking fire-induced smoke and heat is tested, with and without longitudinal ventilation. A total of 14 fire tests have been carried out, with 250 ml methanol in an iron tray (25 cm × 20 cm) as fuel. Temperatures have been measured by 30 thermocouples, located upstream and downstream of the fire location. The aim is to assess the effectiveness of the water system in preventing smoke spread and in reducing the temperature in the tunnel. Interaction of the water with the fire is avoided. The impact of water pressure, ventilation velocity and nozzle arrangement on the effectiveness in smoke blocking and temperature reduction is discussed. The result confirms that the water system effectively reduces the temperatures and prevents smoke spreading in the absence of longitudinal ventilation. However, strong longitudinal ventilation (0.8 m/s ventilation velocity in the reduced-scale tunnel, corresponding to critical velocity in full-scale (1:10) tunnel) reduces the effectiveness in blocking the smoke spreading by the water system, although the temperature reduction downstream the water system remains in place. Higher water pressure makes the cooling effect stronger, because more and smaller water droplets are injected into the tunnel. For a given level of water pressure level, the impact of the nozzle row configuration is small in the tests.     

Interaction between water spray and smoke in a fire event in a confined and mechanically ventilated enclosure

This work deals with the interaction between water droplet flows and smoke in a fire event in a confined and ventilated enclosure. The objective is to identify the specific effect of water spray in the specific environment of a confined and ventilated enclosure. The study is based on 17 large-scale fire tests performed in one room of 165 m³ ventilated at a renewal rate of 15.4 h⁻¹. The fire source is a propane gas burner with a heat release rate of between 140 and 290 kW. The water spray system consists of two Deluge nozzles with a nozzle coefficient of 26 l/min/bar^0.5. The test parameters are the fire heat release rate, the water flow rate, from 50 to 124 l/min, and the activation time. The study focuses on three topics, the interaction of the droplets with the smoke, the droplet evaporation process and the energy transferred to the droplets. The water spray significantly modifies the smoke stratification by mixing and cooling the gas phase. The rate of droplet evaporation has been determined from the water mass balance and is of the same order of magnitude as the rate of water vapor production by the combustion reaction. Heat transfer from the smoke to the droplets has been investigated using the energy balance equation. For a fire scenario in a confined and ventilated enclosure, the energy released by the fire is mainly transferred to the walls and extracted by the ventilation network. In the event of water spray activation, a significant share, up to 65%, is transferred to the droplet flows.     

Model scale tunnel fire tests with automatic sprinkler

The study focuses on the performance of an automatic sprinkler system in a model scale tunnel with longitudinal ventilation. A total of 28 tests were carried out in a 1:15 model scale tunnel using an automatic sprinkler system with glass bulbs. The maximum heat release rate, energy content and failure of the automatic sprinkler system were analysed. The results show that high ventilation rates and low water flow rates result in a failure of the automatic sprinkler system in a longitudinal ventilated tunnel fire. The main reason for the failure under the tested water flow rates was the effect of the longitudinal flow on the fire development and the hot gas flow close to the sprinklers. The fire development and the activation heat release rate of the first activated bulb are intimately related to the ventilation velocity. The fire spread to the neighbouring wood crib was investigated and a presentation of tests conducted using a deluge system are given.     

Modeling of fire suppression by fuel cooling

Fire suppression with water spray was investigated, focusing on cases where fuel cooling is the dominant suppression mechanism, with the aim to add a specific suppression model addressing this mechanism in Fire Dynamics Simulator (FDS), which already involves a suppression model addressing effects related to flame cooling. A series of experiments was selected, involving round pools of either 25 or 35 cm diameter and using both diesel and fuel oil, in a well-ventilated room. The fire suppression system is designed with four nozzles delivering a total flow rate of 25 l/min and injecting droplets with mean Sauter diameter 112 μm. Among the 74 tests conducted in various conditions, 12 cases with early spray activation were especially considered, as suppression was observed to require a longer time to cool the fuel surface below the ignition temperature. This was quantified with fuel surface temperature measurements and flame video recordings in particular. A model was introduced simulating the reduction of the pyrolysis rate during the water spray application, in relation to the decrease of the fuel local temperature. The numerical implementation uses the free-burn step of the fire to identify the relationship between pyrolysis rate and fuel surface temperature, assuming that the same relationship is kept during the fire suppression step. As expected, numerical simulations reproduced a sharp HRR decrease following the spray activation in all tests and the suppression was predicted in all cases where it was observed experimentally. One specific case involving a water flow rate reduced such that it is too weak to allow complete suppression was successfully simulated. Indeed, the simulation showed a reduced HRR but a fire not yet suppressed. However, most of the tests showed an under-estimated duration before fire suppression (discrepancy up to 26 s for a spray activation lasting 73 s), which demonstrates the need for model improvement. In particular the simulation of the surface temperature should require a dedicated attention. Finally, when spray activation occurred in hotter environments, probably requiring a combination of fuel cooling and flame cooling effects, fire suppression was predicted but with an over-estimated duration. These results show the need for further modeling efforts to combine in a satisfactory manner the flame cooling model of FDS and the present suggested model for fuel cooling.     

Studies on buoyancy driven two-directional smoke flow layering length with combination of point extraction and longitudinal ventilation in tunnel fires

This paper investigates the buoyancy-driven smoke flow layering length (both upstream and downstream) beneath the ceiling with combination of point extraction and longitudinal ventilation in tunnel fires. A theoretical model is developed based on previous back-laying model with only longitudinal ventilation, with modified actual heat release rate, as well as modified upstream and downstream opposing longitudinal air flow velocities by the induced flow velocity due to point extraction. Experiments are carried out in a reduced scale model tunnel with dimensionless of 72 m×1.5 m×1.3 m. A LPG porous gas burner is used as fire source. The smoke flow layering length both upstream and downstream are identified based on temperature profiles measured along the ceiling, for different experiment conditions. CFD simulations with FDS are also performed for the same scenarios. Results show that with combination of point extraction and longitudinal ventilation, the smoke flow layering length is not symmetric where it is longer downstream than that upstream. The upstream smoke layering length decreases, while the downstream layering length increases with increase in longitudinal ventilation velocity; and they both decrease with increase in point extraction velocity. The predictions by the proposed theoretical model agree well with the measurements and simulation results.     

Fire development in a 1/3 train carriage mock-up

To study what parameters that control the initial fire spread and the development to local flashover in a metro carriage, a total of six fire tests were conducted in a mock-up of a metro carriage that is about 1/3 of a full wagon length. They were carried out under a large scale calorimeter in a laboratory environment. The focus was on the initial fire development in a corner scenario using different types of ignition source that may lead to a fully developed fire. The ignition sources used were either a wood crib placed on a corner seat or one litre of petrol poured on the corner seat and the neighbouring floor together with a backpack. The amount of luggage and wood cribs in the neighbourhood of the ignition source was continuously increased in order to identify the limits for flashover in the test-setup. The tests showed that the combustible boards on parts of the walls had a significant effect on the fire spread. In the cases where the initial fire did not exceed a range of 400–600 kW no flashover was observed. If the initial fire grew up to 700–900 kW a flashover was observed. The maximum heat release rate during a short flashover period for this test set-up was about 3.5 MW. The time to reach flashover was highly dependent on the ignition type: wood cribs or backpack and petrol. A full developed carriage fire was achieved as a result of intense radiation from the flames and ceiling smoke layer. This was mostly dependent on the amount of fire load nearby the ignition source and how strong the vertical flame spread on the high pressure laminate boards mounted to walls and ceiling above the ignition source was, leading to a ceiling flame. In such cases, the seats alone did not contain sufficient fuel for the fire to spread within the train, and additional fuel (luggage) is required near the seats. For fully developed carriage fires, the fire starting on the seat in the corner spread to the opposite seat on the same side of the aisle, then horizontally spread to seats on the other side of the aisle, and finally a longitudinal flame spread along the carriage was observed. When and where the fire stopped or whether it reached a fully developed stage was mostly dependent on the amount of fire load nearby the ignition source and how strong the vertical flame spread on the high pressure laminate boards mounted to walls and ceiling above the ignition source was.     

Fine-spray (water mist) protection of shipboard engine rooms

Twenty-three fire tests were conducted to determine the ability of current fine-spray (water mist) technologies to extinguish fires specified in the International Maritime Organization (IMO) fire test procedure for engine rooms greater than 3000 m³. The tests were conducted using nozzle installed at a 5 m height and 1·5 m spacing in a large test facility (2800 m² area and 18 m height). Two types of nozzles were tested: a low pressure nozzle operating between 1·2 and 1·5 MPa with flow per nozzle between 12·0 and 13·41/m and a multi-nozzle high-pressure prototype consisting of seven nozzles operating at 6·9 MPa flowing at 5·3 1/m per prototype. These nozzles were selected because they had been shown to extinguish IMO fire tests in enclosures with a protected area of 83 m² and a ceiling height of 4·5 m. The fire tests selected from the IMO procedure included 6 MW diesel spray fires on top of the IMO engine mock-up, a shielded 6 MW diesel spray fire adjacent to the mock-up, a 1 MW shielded diesel spray fire adjacent to the mock-up, and a wood crib within a 2 m² pan filled with heptane. In tests in which no additional enclosure surrounded the nozzles other than the test facility, fires were not significantly affected by the water mist using either nozzle. To further investigate mist-system capabilities, a ceiling was then placed directly over the nozzles at a 5 m height covering an area of 188 m². Using 90 high-pressure prototype nozzles, the test fires were not extinguished. A 940 m³ enclosure was then formed by dropping tarpaulins to the floor from the ceiling. A 4 m² vent was placed in the wall. With the 90 high-pressure prototype nozzles, the 6 MW spray fire on top of the mock-up was extinguished. When the 6 MW fire was shielded beside the mock-up, the fire was not extinguished. With the vent closed, the 6 MW shielded spray fire was extinguished. Under the same test conditions, a 1 MW shielded diesel spray fire and a 0·1 m² heptane pool fire were not extinguished. The fire test results indicated that protection of engine rooms with volumes of about 1000 m³ is possible by optimizing current fine-spray technology while significantly larger volumes will require improved discharge characteristics.     

气力输送水泥粉体装置灭木垛火试验研究

为了研究水泥粉体扑灭木垛火灾的可行性,通过自行设计的气力输送水泥粉体装置开展木垛火的灭火试验研究,利用温度采集装置和摄像机等仪器测试水泥粉体灭火前后木垛火的变化规律.结果表明,气力输送水泥粉体能有效抑制木垛表面明火,显著降低木垛火的燃烧温度,其中水泥粉体主要通过堆积覆盖作用进行灭火,而过大的木垛高度和孔隙率对水泥粉体灭...     

固定式压缩空气泡沫系统单喷头喷洒特性及灭火效能

对5 种典型结构喷头进行了单喷头喷洒压缩空气泡沫和纯水试验,对喷洒介质在底面上的密度分布以及喷洒的泡沫25%析液时间进行比较分析。研究发现,喷头结构类型和流动参数都会影响泡沫析液特性,喷头内部旋芯会使泡沫稳定性降低;下垂式水喷头的底面分布均匀性较好,而扰流喷头和两种七孔喷头的底面分布均匀性相对较差,且下垂式水喷头和扰流喷头在更高的压力和流量条件下的底面分布均匀性降低;同时,压缩空气泡沫系统的喷头选型可以在一定程度上参考相应喷头的喷水特性。选用最优性能参数喷头,建立火灾模型,开展固定式压缩空气泡沫系统全尺寸变压器热油火灾试验,验证喷头与固定式系统的适配性。     

The reaction of a fire plume to a droplet spray

The reaction of a fire plume to an applied water spray is crucial to fire suppression. While considerable research has been devoted toward fire plumes without suppression, and some computer simulations of fire suppression have provided insight, little experimental data exist detailing how the structure of a fire plume changes during suppression. Experiments were performed in which 5, 15 and 50 kW gas burner fires were exposed to a spray from one of three spray sources. Flow rates from the nozzles or fire sprinkler ranged from 6 to 106 L/min. Contours of infrared (IR) intensity of the fire plume show that the plume decreases in height and increases in width with the increasing strength of the applied spray. Based on the height of the maximum fluctuations of IR intensity, the thermal plume height decreases with increasing spray strength, but the overall projected area of the plume changes very little. The plume height depends on the ratio of the drag of the droplets on the air to the momentum of the plume, allowing the results to be generalized to typical fire suppression applications.     

Experimental study of the effect of water spray on the spread of smoldering in Indonesian peat fires

Peatland fires remain a major contributor of environmental problems in Indonesia. Several studies on peat fire suppression have been conducted with multiple methods, such as quarrying, water spray, artificial rain, and foam spray. This research is focused on laboratory scaled experiments of Indonesian peat smoldering fire behaviour and suppression by a water mist system. The peat used in this work was obtained from two different locations, namely Papua and South Sumatra, Indonesia. During the suppression tests, the intensity of the water mist spray was varied by changing the distance between the nozzle and the peat surface. Meanwhile, the time periods of spray were 15 min (short period of suppression) and approximately 2 h for full suppression until the peat fire was extinguished. The peat temperature and the total mass lost during the smoldering reaction were recorded to get the burning rate ratio for each sample. The spread rate of the smoldering process was identified by the changes in the local temperatures of the peat bed. The results show that the spread rate of the smoldering combustion front was affected by particle size and permeability of peat material. The short duration of water suppression failed to extinguish the peat fires. A re-ignition phenomenon was identified due to the persistence of stored heat in the core of the peat. In addition, the total water required to fully suppress both peat fires is about 6 L/kg peat.     

特高压换流变压器泡沫喷雾灭火系统失效分析

模拟不同数量喷头掉落下的冷喷试验,考察泡沫喷雾灭火系统受爆炸冲击、高温炙烤等特殊因素的影响,对特高压换流变压器泡沫喷雾灭火系统进行失效分析。结果表明,喷头工作压力、有效喷雾量和覆盖范围受掉落喷头占比影响显著,当掉落喷头占比超过20%时,喷头工作压力和有效喷雾量均低于标准要求,喷雾射程下降,喷雾范围缩小,无法形成全覆盖;喷头掉落后喷射的水柱有引发火灾扩大的风险;通过提高供液流量进行增强的方案仍会形成灭火盲区,满足不了灭火需求。     

Investigation of the effect of tunnel ventilation on crib fires through small-scale experiments

This paper adopts a series of 1:20 scale tunnel experiments based on a series of large-scale tunnel experiments to study the influence of forced ventilation on fires. The small-scale tunnel has dimensions of 0.365 m (W)×0.26 m (H)×11.9 m (L). Cribs using a wood-based material provide the fuel source and forced ventilation velocities from 0.23 to 1.90 m/s are used. From the study of the measured heat release rate (HRR) and mass loss rate data it is found that the forced air velocity affects the fire spread rate and burning efficiency and further affects peak HRR values at different air velocities. A simple model to describe these influences is proposed. This model is used to reproduce the enhancement of peak HRR for cribs with different porosity factors noted by Ingason [1] and to assess the effects of using different length of cribs on peak HRR. The results from these analyses suggest that different porosity fuels result different involvement of burning surface area and result different changes in peak HRR. However, no significant difference to the enhancement on fire size is found when the burning surface area is similar. It is also found that the trend in the enhancement on fire size by using sufficiently long crib and available ventilation conditions matches the predictions of Carvel and Beard [2] for two-lane tunnel heavy goods vehicle fires.     

Experimental study of water mist fire suppression in tunnels under longitudinal ventilation

This paper studies water mist fire suppression under different longitudinal ventilation velocities in tunnels by small-scale experiments. After a scaling study, two mist nozzles are used for suppressing crib fires under 5 ventilation speeds. The result comes out that fire suppression process can be divided into three stages including flame unitary restraining stage, surface flame extinguishing stage and inside flame suppression stage. Several factors influencing efficiency are investigated. When the interval between mist nozzle and fire source enlarges, the relationship curve between fire suppression time and ventilation velocity shows a ‘V’ figure. The best ventilation speed exists. Following the rules summarized, a coupling system of water mist and ventilation may increase fire suppression efficiency remarkably.     

Critical ventilation velocity for multi-source tunnel fires

Ventilation is an effective method for controlling smoke during a fire. The “critical ventilation velocity” u_cr is defined as the minimum velocity at which smoke is prevented from spreading under longitudinal ventilation flow in tunnel fire situations. All previous studies on this topic have simulated fire scenarios in which only one fire source exists. This study conducted small-scale experiments and numerical simulations to investigate u_cr for cases in which two tunnel fires occur simultaneously. The tunnel was 4 m long, 0.6 m wide and 0.6 m tall. Three cases of two variously separated fires were experimentally explored and six cases were examined numerically. Both the experimental and simulation results indicated that for two identical fires, u_cr declines with separation. When the two fire sources are separate completely, u_cr can be determined by considering only a single fire. When the larger fire is upstream of the smaller downstream fire, u_cr also decreases with the separation. When two such fires sources are completely separate, u_cr can be evaluated by considering only the larger fire. The concurrent ventilation flow and flow of downstream smoke from the larger fire are strong enough to suppress the smoke flow from the smaller fire. However, when the smaller fire is upstream of the larger fire, the decrease in u_cr becomes insignificant as distance increases and the flow at u_cr must overcome the flow from both fires.     

改性干水灭火剂灭木垛火试验研究

新型干水灭火剂因其高含水量和特殊核壳结构,具有良好的灭火效果。为探究干水对木垛火的灭火效果,自行制备磷酸二氢铵改性干水并开展小尺寸木垛火灭火试验。结果表明：干水灭火剂能够扑灭小尺寸木垛火且不发生复燃,改性干水的控火时间仅为8 s,控火过程中火焰高度持续快速下降,而干粉灭火剂的控火时间则为20 s,喷撒结束39 s后发生复燃;改性干水对火焰区及木垛的温度抑制效果均优于干粉灭火剂,控火时间内,改性干水作用下木垛表面的平均温降速率高达17.00℃/s,是干粉灭火剂作用下平均温降速率的1.94倍;改性干水能够有效降低木垛内部温度,在喷撒50 s内,木垛中心的平均温降速率为8.78℃/s,而干粉灭火剂缺乏冷却作用,木垛中心的平均温降速率仅为6.10℃/s,无法有效抑制阴燃。     

Definition and experimental evaluation of the smoke “confinement velocity” in tunnel fires

An experimental study is carried out on a reduced scale tunnel model (scale reduction is 1:20). The main objective is to evaluate the longitudinal velocity induced into a tunnel when a fire plume continuously released is confined and extracted between two exhaust vents located on both sides of the fire source. For the experimental simulations, fire-induced smoke is simulated by an air and helium mix release. Smoke flow is symmetrical as regards the fire location and experiments are realized for an half tunnel with only one vent activated downstream the source. The vent extraction flow rate is step by step increased and the length of the stratified smoke layer downstream the vent as well as the longitudinal fresh air flow induced, are measured. A confinement velocity is then associated to the minimum value of the longitudinal air flow needed to prevent the smoke layer propagation downstream the vent. This velocity is evaluated for several values of the fire heat release rate and finally compared with the corresponding critical velocity obtained for a longitudinal ventilation system.     

Pulsations during large-scale fire tests in the Runehamar tunnel

Four large-scale fire tests have been carried out with heavy goods vehicle (HGV) cargos in the Runehamar tunnel in Norway. During two of the fire tests, large pulsations of the gas flow inside the tunnel were observed. These pulsations were registered only when the measured HRR was higher than 125–135 MW. Two different periods of pulsations were registered, short periods of approximately 4 s and longer periods of approximately 18 s. In the article, the pulsations are presented and explanations are given, using a frequency response analysis based on an impedance approach. Using this approach, the authors were able to find the intrinsic resonances of the system, which were close to the periods of 4 and 18 s found in the experiments. Several factors can affect the pulsations, but the calculations show that the oscillation periods are properties of the system. The analyses show further that at certain frequencies (close to those found during the tests), a small disturbance in the flow can create large amplitudes in the pressure. It is proposed that this phenomenon should be studied in conjunction with future full scale tunnel tests.     

Longitudinal ventilation for smoke control in a tilted tunnel by scale modeling

Longitudinal ventilation systems are commonly installed in new tunnels in large cities of the Far East including Mainland China, Hong Kong and Taiwan. Many tunnels are found in big cities and some of them are inclined at an angle to the horizontal. However, smoke movement in tilted tunnels is not fully understood. In some of the tunnels, the ventilation system was designed based on presumed smoke movement pattern without experimental demonstration. Smoke movement pattern in a tilted tunnel model was studied by using a scaled model. A 1/50 tunnel model of length 2 m with adjustable angle to the horizontal was constructed by transparent acrylic plastics. A small 0.097 kW propanol pool fire was used as the heat source combined with burning pellets generating smoke. A fan placed at the upstream end was used to create longitudinal ventilation. Different ventilation rates were set using a transformer to control or adjust the fan speed. Experiments were performed with the tunnel angle varying up to 30° to the horizontal. Effect of smoke screens was also studied. The observed smoke movement patterns indicated that the shape of the buoyant plume inside the tunnel depends on the tilted angle. Smoke would flow along the tunnel floor due to gravity. The bending angle of the plume depends on the tunnel angle. Tunnel inclined at greater angles to the horizontal would give larger amount of smoke flow. Smoke movement pattern for a tilted tunnel with smoke screens was observed to be very different from some design projects. All results will be reported in this paper.     

Gas temperatures in heavy goods vehicle fires in tunnels

Large-scale fire tests were carried out with heavy goods vehicle (HGV) cargos in the Runehamar tunnel in Norway. The tunnel is a decommissioned, two-way-asphalted road tunnel that is 1600 m long, 6 m high and 9 m wide, with a slope varying between 0.5% uphill and 1% downhill. In total four tests were performed with fire in an HGV set-up and a longitudinal ventilation flow of approximately 3 m/s. In three tests, mixtures of different cellulose and plastic materials were used; in the fourth test a commodity consisting of furniture and fixtures was used. In all tests the mass ratio was approximately 82% cellulose and 18% plastic. A polyester tarpaulin covered the cargo.One purpose of the large-scale tests was to obtain new relevant gas temperature-time data from large-scale HGV fires in tunnels. There is presently a lack of such information for road tunnels. The maximum heat release rates produced by the four different fire loads varied between 66 and 202 MW resulting in maximum gas temperatures at the ceiling ranging between 1281 and 1365 °C. A comparison with literature values shows that the gas temperatures obtained here are uniformly higher than those obtained in other similar large-scale test series conducted using solid materials. A mathematical correlation of a temperature–time curve is given and this is the best representation of the measured temperature and a combination of frequently used temperature curves for tunnels (the HC curve and the RWS curve).