Fire Effect on Natural Ventilation in Compartments - Term Paper Example

FIRE EFFECT ON NATURAL VENTILATION IN COMPARTMENTS By Student’s name Course code and name Professor’s name University name City, State Date of submission Introduction Natural ventilation designs are becoming increasingly utilised especially in green buildings which require energy usage to be kept at a minimum low. In coming up with proper space ventilation, full-scale experiments as well as FDS simulations have been carried out in a bid to determine the ventilation variables and smoke layer descent under various defined conditions. The approach of utilising an N-percentage rule in coming up with a proper estimate for the smoke layer height is also investigated within this study in a bid to maximise efforts to achieve fire safety in compartments. Simulations carried out using CFAST under a controlled temperatures showed a consistent outcome with respect to experimental values. Inasmuch as the thermocouples being used in the physical experiment were being affected by direct radiation and direct burning, the CFAST simulations were used to compensate for lack of experimental values. The fire scenarios that were observed in this study varied from closed vent with open door, to close door but vent open for room number 1, 2 and 3 depending on the required configuration. It was noted that allowing natural ventilation to a compartment causes unstable smoke layer temperature that is worth being justified. The number of vertical ventilations also affect this relationship depending on the number of horizontal interconnecting spaces. This has brought about the debate on whether passive fire construction designs achieve active fire safety management. Striking a balance between manageable fire safety designs and Green Building has led to ventilation investigations since too much ventilation does not achieve safety in the long run. For safety to be achieved, more fire dampers and other mitigation measures shall be required in such a case which shall in turn affect natural ventilation designs. The main objective of study was to establish fire effect on natural ventilation in compartments through experimentation and verifications through CFAST simulations. Research Methodology Figure 1: A schematic presentation of compartments used for experiment. The two storey building design above was subjected to investigation with the main aim of exploring the effects of natural ventilation on flow of fire smoke. The first two rooms (room 2 and 3) were geometrically alike and interconnected by a natural ventilation shaft while the last combination was of room 1 and 3 were open to natural ventilation via direct doors. Room 1 and two represented the first floor while room 3 presented the high floor. The interior of room 1 measured, room 2 measured and lastly room 3 measured same as room 2. The natural ventilation shaft was designed to match the requirements of Building Design and Construction Code. The size of the ventilation shaft was in accordance to these standards while the cross sectional areas for the vents were maintained at the same dimensions and mounted below the ceiling for this full scale experiment. A single indoor airflow route was designed with door 1 as the inlet for natural ventilation and an interconnecting ventilation shaft in a bid to achieve natural performance. Since door 2 was open and door 3 closed, the overall ventilation path was. Thermocouples were installed in each of the spaces to provide a way of measuring the room temperature. Each of these sets consisted of K-type thermocouples that were tied vertically to an iron chain with calibrated distances and vertically erected at the side of the natural ventilation. A 26 cm diameter pan with unleaded gasoline (with a known heat release rate of 50kW), was placed inside the 1st room to act as the fuel and at the same time for purposes of dense smoke production. The number of scenarios under investigation were two i.e. fire in Room 1 with closed vent but open door and fire in Room 2 with closed vent but open door. Fire was initiated on fuel located at Room 1 with closed vent but open door and observations made with consideration of scenario 1. Observations were followed closely while ensuring that the data logger was on for purposes of recording data to be used for analysis purposes. This experiment was repeated for scenario 2 which required fire to be started in Room 2 with closed vent but open door. CFAST simulation was also set up to give an impression of the room shown in figure 1 above. The external and interior layout was simulated to achieve the same values as in the experimental parameters. The fire source was placed in the middle of room 1 and 2 in accordance to the experiment in order to achieve various scenarios that were in experimentation. This form of numerical simulation has been proven to give good results when the physical parameters are inputted accordingly. The rate of heat release was set at 50 kW as in the full scale experiment for purposes of achieving standardised results. The grid size for this simulation was set toin order to provide for sufficient accuracy levels and computational purpose as in Lai et al. (2013). The openings were also set to the acceptable real life experimental situation so as to ensure an output that was of similar nature. The interior wall surfaces were set to brick since the full scale experiment left no evidence of interior surface burns. The experiment was then set rolling with the full scale scenarios under investigation in mind. The wind velocities were set to 0m/s because the experiment was investigating effects of natural ventilation. The results of this simulation were documented alongside those obtained from the experiment for comparison purposes. Results and Analysis Case No. 1 Figure 2: A comparison graph of temperature against time for scenario 1 experimental data and CFAST simulation. This case was concerned with fire in Room 1 with closed vent but open doors and the main task which involved a comparison between CFAST simulation and experiment fire. The results obtained were documented and the graphs plotted as shown in figure 2 above. Once fire was ignited, the fuel started to generate large amounts of smoke which began to ascend up the wall. At this point, the smoke started descending down the ground and eventually to room number 2. The smoke layer in room 1 descended to a 1.5m level by 220 seconds with a similar match in room 2 at 335 seconds. Continued heating in Room 1 led to a decrease in smoke height to approximately 1.3m at 565 seconds. The difference exhibited between simulated and experimental temperature data was high due to a slight difference in thermocouple positioning. Carrying out a comparison between the simulated and experiment data for the above scenario comes in to explain the deviations that were observed. The extreme temperature increments noted in the simulations were due to the situation of a thermocouple right above the flame contrary to the situation of thermocouples in the full scale experiment. The influence of radiation was therefore affected by the distant flame although radiant temperature was much higher than the local gas temperature. According to experiments carried out in the past by Lai et al. (2013) smoke layer descent cannot be associated to the inconsistent behaviour that the latter possesses. It was however noted that the smoke spread in Room 2 was a bit consistent due to low thermal effect from direct heat. Case No. 2 Figure 3: A comparison graph of temperature against time for scenario 2 experimental data and CFAST simulation. When the fire source was placed in Room 2, the results achieved were as shown in the graph in figure 3 above. While the strategic experimental thermocouples meant for observation may have been subjected to smoke heat, the CFAST simulation for this experiment was quiet consistent from the graph above. Lower measuring points were however noted to have be affected by thermal radiation and direct heat. Comparing the smoke layer between the experimental and simulated data there was a difference with the former being on the higher but acceptable level. Case No. 3 Figure 4: Temperature in room 3. Figure 5: Smoke layer height against time. The temperature in Room 3 changes drastically during the first approximated 100 seconds as shown in figure 4. It however reaches an optimum of 70ºC from the 150th second and remains at that temperature until the experiment is terminated during the 600th second. It is also important to note that while temperature increases the pressure decreases as shown in figure 6. This is due to reduced volume as volume of gas increases with temperature thereby reducing as per Boyle’s law which states that pressure is inversely proportional to volume. Figure 6: Pressure in room 3 Figure 7: Concentration of carbon monoxide gas against time. The concentration of toxic gases (CO and CO2) also forms the same type of graph as temperature. It is noted that increase in temperature leads to increased combustion and subsequent increase in amount of toxic gases. This however attains an optimum level after some time as shown in graph 7 and 8. It is also notable from graph 5 that as the as the experiment goes on, the level of smoke continues to get low making the rescue exercise to become hard – this is because the level of tenability continues to reduce. Figure 7: Concentration of carbon dioxide gases against time. Smoke effect to the building occupancy Despite the weather conditions, the fire intensity has been held responsible of smoke movement in high rise buildings. This is because the intensity of the fire determines the extent to which the contents in the building undergo combustion. The heat energy rises up the structure until the temperature reaches an equilibrium during which smoke and toxic gases stratify as in lower level and upper level. This stratification endangers the occupants of a building who enter the smoke free stairways only to discover smoke storeys below. In the case under consideration, the height of the smoke was noted to drop as the experiment time was prolonged. Further on, it has been established that accidental deaths are caused by the presence of toxic substances in smoke. From the experimental observations, these gases mainly carbon monoxide reach a certain level of equilibrium. It is this level that causes reduced tenability – in short, the availability of the toxic gases are a major attribute towards loss of occupancy. These attributes of combustion have been used to design buildings in order to ensure safe occupancy through the ASET (Available Safe Egress Time) and RSET (Required Safe Egress Time) principles. Buildings should be designed putting in mind ability of occupants to vacate without necessarily seeking external assistance. Conclusion This study successfully depicts the achievable smoke conditions and their effects on tenability of a two storey building. In as much as the data obtained for CFAST simulation and full scale experiment are not the same due to positioning of the principle thermocouples, the similarity aids in the comparison exercise. The pressure is also noted to fall with respect to temperature of the room since volume of air increases due to expansion. Similar experiments are necessary given the scenarios in study for the purpose of designing buildings in which occupants can be able to vacate prior to increase in toxicity to an untenable extent as seen in ASET and RSET. List of References Lai, C.-m.et al., 2013. Determinations of the fire smoke layer height in a naturally ventilated room. Fire Safety Journal, Volume 58, pp. 1-14. Read More