Heat Transfer, Fluid Dynamics, Fire Plume and Enclosure Fire - Assignment Example

Assignment 2a Hеаt Transfer, Fluid Dynamics, Fire Рlumе & Еnсlоsurе Fire Student Name: Student Number: Date: Part One [a] Calculation of the heat flux rate Heat transfer () = Where: – Thermal conductivity of gypsum board = 0.17 W/ (mK) – Surface area of the wall = 10 m2 – The temperature of the hotter surface – The temperature of the colder surface (ambient temperature) – Thickness of the board = Heat flux () = [b] Calculation of the temperature of the two faces of the wall (i) Heat flux) Where: - Heat transfer coefficient (20 W/m2.oC) – Temperature of the fire – Temperature of the surface directly exposed to fire (This is the temperature of the wall surface facing the fire source) (ii) Heat transfer through the board by conduction = = ) = = 826.8oC (This is the temperature of the second wall surface that is not directly facing the fire compartment). Part Two We can use Heskestad’s flame height model equation below to determine the height of the fire pool. Where: – Flame height (m) – Fire size or energy release rate (kW) = 900 kW D – Pool fire diameter (m) = 0.6m Therefore, However, when the fire pool is idealized as a radiant panel, we obtain the width and depth by the calculations as shown in the excel sheet shown below.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     INPUT DATA             RESULTS                                                 width a = 5   m       emitted radiation = 134.01   kW/m²                                         depth b = 5   m       configuration factor = 0.07                                             distance x = 10   m       received radiation at surface = 9.85   kW/m²                                         temperature Tr = 1000   °C                                                         emissivity e = 0.9                                                                                             Width = 5m Depth = 5m Part Three i. Introduction Enclosed compartments in a building occur in various shapes and sizes. However, most of these rooms roughly form a cube shape with a width (W), height (H) and depth (D). In most cases, the spaces are wide, deep, or wider and deep compared to the height. If the rooms are ventilated from one side, they end up having a very high depth-to-height ratio. Understanding the behavior of fires in deep enclosures is crucial in developing appropriate severity correlations of time, heat release rate, and position to add some input to various fire models (Krasny, et al., 2008). HRR is the most important significant parameter in a ventilation-controlled fire. It shows the severity of the fire. A fire becomes fully developed when HRR reaches its maximum. A ventilation-controlled fire grows in three main stages; growth stage, fully developed stage, and finally, decay. At full development, a fire is mainly controlled by the ventilation available in an enclosure (Lilley, 2010). The experiment was designed to demonstrate how fire develops in a long enclosure. The purpose of this demonstration was to understand the characteristics of a flame front as it moves across the fuel package located in a deep enclosure. The enclosure provides a prototype with conditions similar to those found in a real building. All the three processes involved in the heat transfer to the structural members exposed to fire, i.e. Convection, conduction, and radiation, are also demonstrated. ii. Description of the Test Apparatus The enclosure used is made of sheet steel with the following dimensions; length = 1.5 m, width = 0.3 m, and height = 0.3 m. To enable air flow into the enclosure, a 0.3m X 0.3m opening is made on one of its sides. One side of the 1.5m X 0.3m face is covered using a fire-resistant glass to aid in safe observations of fire behavior. Five trays made of sheet steel, each 0.3m X 0.25m, containing 500 ml of methylated spirit are placed inside the enclosure. Above the center of the trays, 25 mm below the ceiling of the enclosure, type K thermocouples have been placed (T/C1 - T/C5) to monitor the gas temperatures. Tray number 3 has another thermocouple (T/C11) located 170 mm below the ceiling enclosure, close to the glass wall. In addition, five thermocouples are spot-welded on the roof of the enclosure and above each of the trays (T/C6 – T/C10) to measure the surface temperature of the steel. Four thermocouples are spot-welded onto a 6 mm diameter steel rod passing through the center of the longitudinal side of the enclosure, about 150 mm above the floor of the enclosure. Of these four thermocouples, one is spot-welded within the enclosure (T/C12) while the other 3 are spot-welded outside the enclosure (T/C13 – T/C15). Two thin plates made of steel are placed midway of the longitudinal distance, 25 mm apart and 100 mm outside the glazed side of the enclosure, opposite to T/C11. The two plates are directly exposed to the radiation from the burning fuel. One of the plates is insulated at the back, while the other plate is left uninsulated. On each of these plates, a thermocouple is welded, T/C16 and T/C17 on the insulated plate and uninsulated plate respectively. The ambient temperature is measured using another thermocouple, T/C18, located outside the enclosure. All the temperature readings of the thermocouples were recorded using a data-logger at an interval of 5 seconds. The diagram below shows the locations of the thermocouples and other details of the apparatus that may be of interest. Figure 1: Set-up of the long-enclosure apparatus to demonstrate the development of fire. iii. Test Observations A few seconds after ignition, the temperature of the air/combustion gas in the enclosure begins to increase, with the thermocouples located above the trays recording the highest temperatures. Generally, on igniting the fuel on the rear tray, the flame was observed to move quickly to the front, from left to right. AT first, fire was established in tray 1 and quickly moved to the subsequent nearby trays – and propagated rapidly from right to the left. Once the fuel in all the trays was ignited, the flames were observed to move rapidly towards a ventilation opening at the front of the enclosure (Thomas, et al., 2005). The flame front continued to burn the fuel in the front trays, until it was exhausted, before moving to the front of the rearward trays until fuel in the rearmost tray was exhausted. The flame was observed to stay at the ventilation even after exhausting fuel in the front trays. The ambient temperature during this experiment was 25oC. At this temperature, a flame front would rapidly establish itself once the fuel is ignited. The burning duration is considered to end when the T/C5 recorded a temperature below 500oC. The results are presented in the temperature-time profiles shown in figure 2. It can be observed that T/C1 records the slowest increase in temperature. Another observation made was that there was no much variation in the duration of burning in trays 1-4, and tray 5 had a shorter burning duration. This can be explained by the fact that burning gases from trays 1-4 rise and form an inward ceiling jet that develops behind the flame front. These ceiling jet dropped down on reaching the cooler back wall. As burning continues, the cooler air enters the enclosure from the open side, warms up, and rises. This sets up convectional currents in the enclosure, drawing fuel from back to front (Moinuddin & Thomas, 2009). During the experiment, three phases of burning could be identified: Phase One: Rapid burning up to the front of the enclosure in the frond trays (tray 4 & 5). Burning in the front trays occurred across the width of the ventilation, and not across the width of the enclosure. As the fuel in the front trays was exhausted, the flame front moved to the rear trays and extended across the width of the enclosure. Burning was maintained at full width as the flame front moved back to the rear trays. It was observed that the flame remained at the opening of the enclosure despite exhausting the fuel in the front trays (Moinuddin & Thomas, 2009). Phase Two: Here, there was a relatively stable burning up to the rear most raw. The flow of both the smoke and fire flames in this phase was in 2-dimensional. Initially, HRR was fairly stable and constant, but over time, it decreased as burning continued in the rear trays. Phase Three: These was the final phase of burning. The phase was characterized by a more vigorous burning as the flame front moved to the rear tray. iv. Test Measurements The temperature readings of all the thermocouples were recorded against time to monitor the temperatures in different regions in the enclosure. A plot of temperature-time profiles for T/C1 – T/C5 is represented in figure 2. Figure 2: Air/combustion gas temperature-time profiles recorded by thermocouples T/C1 – T/C5 located above the trays. It can be seen that in the first few seconds of burning, TC1 records the highest temperature (769.7oC), and T/C5 records the highest temperature (841.7oC) just before the decay stage. This behavior is due to the development of an inward jet as burning continues, which has been explained in the previous section. T/C1 is located on top of the front tray, while T/C5 is located on top of the rear tray. This test observation shows that the temperature of the ceiling above the front tray where T/C1 is located records a higher temperature during the test duration, compared to the temperature of the ceiling above the rear tray where T/C5 is located. This observation indicates that structural members located in the rear position will experience severe burning compared to structural members located at the front (Moinuddin, 2013). Another important graphical representation is the plot of the temperature-time profiles for the steel surface of the enclosure recorded by T/C6 – T/C10. These temperatures are below 400oC and lower than those recorded by T/C1 – T/C5. Temperatures below 600oC cannot cause severe structural weakness as it would be in the case of temperatures above 600oC. Figure 3 below shows the temperature-time profile for these thermocouples. Figure 3: Temperature-time profile recorded by thermocouples T/C6 – T/C10 located on the steel surface of the enclosure. For the thermocouples that were welded onto the steel rod, it was observed that T/C12 that was spot-welded inside the enclosure recorded higher temperatures at any instance during the burning period in comparison to the other thermocouples (T/C13 – T/C15) that were located outside the enclosure. The graphical representation of the thermocouple measurements are shown in figure 4. Figure 4: Graphical representation of thermocouple measurements welded on the steel rod (T/C12 – T/C15). Inside the enclosure, heat continues to build up as burning continues, thus, increasing the temperatures inside the compartment compared to the outside, where heat can only be transferred by means of conduction through the enclosure material. Inside the enclosure there is rapid transfer of heat by both radiation and convection. The highest temperatures recorded on all the thermocouples welded on steel varied from 473 – 190oC. Even with gas temperatures reaching as high as 800oC, the temperature of steel did not exceed 474oC at which steel would still retail significant structural strength. The lower temperature is because of excessive loss of heat to the surrounding, and also high ratio of exposed area to the mass. This also explains why the temperatures recorded by the thermocouples decrease further away from the surface of the enclosure, from T/C13 to T/C15. For example, T/C15 records temperatures below 200oC in the duration of the experiment. The temperature-time profile in figure 5 below compares the temperature readings of thermocouple T/C11 and T/C16 and T/C17. T/C11 was located on top of tray 3, 170 mm below the ceiling enclosure and close to the glass wall, T/C16 on an insulated plate, and T/C17 on un-insulated plate as seen in figure 1. T/C1 records the highest temperature (692.2oC) of the three thermocouples throughout the experiment because it is directly exposed to fire from the burning gases in the enclosure. Heat travels by both conventional and radiation means. The temperatures recorded by the thermocouples T/C16 and T/C17 are much lower (< 200oC). This is because the steel plate is a good conductor and loses heat faster, thus, recording lower temperatures. At first, it is observed that the un-insulated plate records a slightly higher temperature compared to that recorded on an insulated plate. However, as burning continues, the temperature of the insulated plate rises faster than that of the un-insulated plate. Figure 5: Graphical representation of thermocouple measurements of steel target plates (T/C16 & T/C17) and the inside gas temperature (T/C11). Uninsulated plate conducts heat faster and loses it faster compared to an insulated plate. Insulation minimizes heat transfer by reducing the effects of conduction, radiation, and convection (Ufuah & Bailey, 2011). Thus, the plate tends to accumulate the heat from the source, which increases its temperature. v. Conclusions Fire in an enclosure will move towards the means of ventilation. The results reported in this experiment show that the severity of fire in a compartment can be well characterized by its size and duration (HRR and Q). The test results also show that fires are not uniform within a compartment and tend to move towards a ventilated area. This means that structural members are variably exposed to severe heat conditions depending on their relative location to the source of fire. The experiment demonstrates the practical mechanisms through which heat transfer occurs in real fire scenarios, and how they can be analyzed, both quantitatively and qualitatively. Conducting this flame propagation experiment provides a better understanding of behavior of compartment fires and safety considerations that can be incorporated in building design. Part Four [a] Fresh air flows into the enclosure through the lower part of the opening, while hot gases flow out of the enclosure through the upper part of the opening. This is because fresh air is denser than the hot gases. Thus, the cooler and denser fresh air flows in to replace the warm rising less dense air. [b] See the supplied spread sheet below: [c] The insulated thermocouple (T/C6) is hotter than the uninsulated thermocouple (T/C17) and this was expected. The highest temperature recorded by T/C16 is 152.7oC while the highest temperature recorded by T/C17 is 19.2oC. The range between the two temperatures is 133.5oC (87%). This difference in temperature can be explained by the heat transfer process that occur in the two plates. The insulating material reduces the transfer of heat through conduction, radiation, and convection. As more heat is generated, the insulating material absorbs the incoming heat energy, only losing a fraction of it to the surrounding. As a result, there is heat accumulation in the material, which makes the temperature of the material to rise (Thomas, et al., 2005). On the other hand, the uninsulated plate absorbs heat and transfers it to the surrounding environment through the effects of heat transfer processes, thus, recording lower temperatures. [d] Maximum temperature recorded by thermocouple T/C19 (uninsulated) = 386.8oC Maximum temperature recorded by thermocouple T/C20 (insulated) = 389.8oC The temperature recorded by thermocouple T/C20 (insulated) is slightly higher than that recorded by T/C19 (insulated). This difference in temperatures is attributed to the effects of insulation, which tend to retain the heat absorbed without losing it to the surrounding. For thermocouple T/C19: (Equation 1) For thermocouple T/20: (Equation 2) Equation 1 & 2 above were used to demonstrate the basic heat transfer calculations presented in the excel sheets below. T/C19 (uninsulated)                 Temp. of steel (deg. Cel) =       386.8       Ambient temperature (deg. Cel) =     25       Temp. of fire (deg. Cel) =       900       Convective heat transfer coefficient of steel (W/m^2degC) = 20       Convective heat transfer coefficient of air (W/m^2degC) = 14       Thermal conductivity (k) of steel =     46       L (m) =           1.5       W (m) =           0.3       H (m) =           0.3       Area (m^2) =         1.44       dT =           361.8       Stefan Boltzmann constant (W/m2degK4) =     5.67E-08       Emissivity =         0.9       Convection from steel to ambient =       10419.84     Radiation from steel to ambient =       2.65862E-05     Convection from steel to ambient + Radiation from steel to ambient = 10419.84003 Watt/m^2K^4 Convection from fire to steel =         10346.112     Radiation from fire to steel =         3.77116E-05     Convection from fire to steel + Radiation from fire to steel =   10346.11204 Watt/m^2K^4 T/C20 (insulated)                 Temp. (deg. Cel) =         389.8       Ambient temperature (deg. Cel) =     25       Temp. of fire (deg. Cel) =       900       Convective heat transfer coefficient of steel (W/m^2degC) = 20       Convective heat transfer coefficient of air (W/m^2degC) = 20       Thermal conductivity (k) of steel =     46       L (m) =           1.5       W (m) =           0.3       H (m) =           0.3       Area (m^2) =         1.44       dT =           510.2       Stefan Boltzmann constant (W/m2degK4) =     5.67E-08       Emissivity =         0.9       Convection from fire to steel =         14693.76     Radiation from fire to steel =         3.74911E-05     Convection from fire to steel + Radiation from fire to steel =   14693.76004 Watt/m^2K^4 Convection from insulator to ambient =       10506.24     Radiation from insulator to ambient =       2.68067E-05     Convection from insulator to ambient + Radiation from insulator to ambient = 10506.24003 Watt/m^2K^4 [e] Fire load per unit area = Where: – Heat of combustion of the fuel (MJ/Kg) D – Enclosure depth (0.3m) W – Enclosure width (0.3m) Therefore, Fire load per unit area = Mass of fuel = 1.18kg Mass flow rate = Where: – Empirical constant (0.5 kg.s-1.m -5/2) A0 – Area of the opening (m2) H0 – Height of the opening (m) Therefore, Rate of burning = =0.0123 kg/sec. [f] Duration of burning = = [g] The experimental HRR is higher than that obtained theoretically. This may be attributed to unsteady fire growth and decay periods experienced during the experiment. The conditions of burning and type of fuel also plays a role in determining the HRR parameter of fire in an enclosure. References Read More