Y.Z. Li

A Mathematical Model on Interaction of Smoke Layer with Sprinkler Spray

Abstract A mathematical model was developed for predicting the downward descending behavior of the buoyant smoke layer under sprinkler spray. The behavior of the smoke layer was determined by considering the interaction between the drag force of the sprinkler spray and the buoyancy force of the hot smoke layer itself in the spray region. The smoke layer may be pulled down with its thickness increased at the center of the spray region due to the cooling and drag effects of the sprinkler spray, thus to form a downward “smoke logging” plume. In the mathematical model developed in this paper, the critical condition under which the smoke layer lost its stability, as a serious concern, was predicted. Additionally, the length of the downward plume, which was rarely investigated before, was also further calculated. Full-scale experiments were carried out to validate the model. Results showed that the predictions, including the critical condition and the length of the plume, by the mathematical model agreed well with that observed and measured in the experiments. The length of the downward plume was shown to increase with the sprinkler operating pressure by an approximately linear correlation.

Experimental Studies on Mechanical Smoke Exhaust System in an Atrium

Mechanical smoke exhaust systems are usually required in big atria of the Far East. However, the design criteria are not clearly demonstrated to be worked out through systematic full-scale tests. There are many arguments on sizing the exhaust fans, and whether a high exhaust rate is required in an atrium is a puzzle. Atrium hot smoke tests might be required to be carried out occasionally to evaluate their performance, especially when the design does not comply with the prescriptive code in having a high-enough exhaust rate. Experimental studies on mechanical smoke exhaust systems are necessary and have been carried out. The results are reported in this paper. The measured results are also compared with the empirical equations used in designs, following NFPA-92B derived by a two-layer picture. A simple zone model reported earlier is applied to justify the results. This model is also developed based on a two-layer picture. There are significant deviations of the measured results from those predicted by the equations with a two-layer pattern. Mass exchange through the two layers in an atrium fire is suggested to be the key point. By taking the estimated mass flow rate across the two layers by experiments to be 30% of the exhaust rate under high ventilation conditions, the two-layer zone model is modified. Slightly better argument with the experiments was found for the self-developed two-layer zone model, but still there are large deviations for the NFPA-92B equations.

Experimental Studies on the Rise-Time of Buoyant Fire Plume Fronts Induced by Pool Fires

Experimental studies were conducted in the PolyU/USTC Atrium to investigate the rise-time of buoyant plume front induced by pool fires. Through a series of full-scale burning tests, three semi-empirical equations were derived to predict the rise-time of buoyant plume front induced by a fire at three positions. These three positions are at the centre, near a wall and in the corner of the atrium floor respectively. The rise-time in dimensionless form and the plume radius equation are analyzed based on the measured data.

Experimental Studies on Stability of Smoke Layer with a Sprinkler Water Spray

Abstract The adverse effects of a sprinkler on smoke exhaust systems used to be debated in designing fire safety provisions. However, very few experimental studies on the interaction of a sprinkler with the smoke layer have been reported; therefore, this study investigates and reports on the interaction of a smoke layer with a sprinkler water spray by a specially designed rig with 25 tests. Smoke spreading from a fire chamber was collected in an adjacent hall. The stability of the smoke layer was then studied by the discharged water spray. The ratio of spray drag to smoke buoyancy is proposed to determine smoke stability.

Experimental Study on the Burning Characteristics of Wood Cribs in a Confined Space

A series of experiments were conducted to study the burning characteristics of wood cribs in a confined space. The results show that burning conditions of wood cribs in a confined space fall into two regimes: the smaller porosity factor burning regime and the larger porosity factor burning regime. The scaled steady mass loss rates of wood cribs burning in the larger porosity factor regime linearly increases with the increase of the porosity factor in a confined space. The concentrations of CO, CO2 and O2 in combustion products, the flame temperature and the mass loss rate exhibit different time-variations or relationships with the porosity factor for wood cribs burning in these two different regimes.

On the Operation Time of Horizontal Ceiling Vent in an Atrium

Smoke control by a horizontal ceiling vent in an atrium is studied in this paper. A bi-directional airflow moving upward and downward across the vent might be observed and this would affect the efficiency of smoke extraction. Whether the flow is bi-directional or uni-directional depends on the pressure difference across the vent upon activation. Before opening the horizontal ceiling vent, the pressure difference should be higher than a critical value. Thickness of the smoke layer formed under the ceiling before opening the vent is found to be important. The required smoke layer thickness for getting such a critical pressure is discussed in this paper. Two situations of no temperature difference between the inside and outside of the atrium; and the outside temperature is higher than the inside temperature, are considered.

Full Scale Experiments on Studying Smoke Spread in a Road Tunnel

Three full scale tests were conducted in a road tunnel to study the smoke spread with different fire sizes and wind speed conditions. The smoke temperature under the ceiling, the smoke layer height distribution and the travel of the smoke front in a 1000 m long domain along the road tunnel were measured. Results showed that wind speed had much influence on the spread of smoke in the tunnel. When wind speed was such low as less than 1 m/s, a smoke layer could form and stabilize all along the tunnel. But when the wind speed was such high as more than 2 m/s, smoke layer could only be maintained in a distance of about 400 m in the downstream. The slowdown of the traveling of the smoke front in the downstream along the tunnel was more obvious when the wind speed was smaller. All these full scale data presented here can be used for further study on the verification or improvement of existing fire models for enhancing their applicability to such long tunnels.

An Empirical Equation to Predict the Growth Coefficient of Burning Rate of Wood Cribs in a Linear Growth Model

A series of bench scale experiments are carried out to study the growth characteristics of cubic wood cribs burning. A comparison among linear fitting, power-law fitting, and exponential fitting shows that linear growth model seems to be the best to describe the growth of wood crib fires. An empirical linear model is also achieved to predict the linear growing burning speeds of wood cribs with different stick spacing conditions based on the experimental data obtained.

Stability of Smoke Layer Under Sprinkler Water Spray

Stability of smoke layer induced by a fire will be studied in this paper. A series of full-scale burning tests were carried out to study the interaction of the hot smoke layer with a sprinkler water spray. A new rig was built to give a smoke layer. The results show that the stability of a smoke layer will be disturbed by the discharged water spray. The operating pressure of the sprinkler system is a key point. When the smoke layer lost its stability, the dragging force acting on the smoke layer was found to be less than the buoyancy.Copyright

Tracking a Ceiling Jet Front for Hot Smoke Tests in Tunnels

There are difficulties in tracking the ceiling jet front while carrying out hot smoke tests in long tunnels with a small fire. In this study, an experimental method using infrared beams is suggested. The arrival time of the hot ceiling jet front to a designated position is measured by the abrupt attenuation of the infrared beam intensity. Field measurements are carried out in an underground tunnel of length 88 m. Diesel pool fires up to 1.5 MW are studied. The results on tracking the ceiling jet front are compared with the measurements by using the thermocouples, thermal resistors, and visual observations. This method is demonstrated to be useful for tracking the ceiling jet front at positions further away from the fire. Therefore, the method is suitable for studying the ceiling jet in hot smoke tests for evaluating smoke management systems in long tunnels.