Measuring the Mass Loss of the Material Polymethyl Methacrylate - Lab Report Example

Aim of the Experiment The aim of the experiment is to measure the mass loss of the material called (Polymethyl Methacrylate) (PMMA). With the duration of time, follow the process of combustion of materials in the enclosed space with the effect of different ventilation size, from the beginning of the ignition period, growth period, flashover period, fully developed period and end by decay of the fire. Apparatus Apparatus that used in this experiment included the firebox, thermocouples, PMMA (Polymethyl Methacrylate), mass balance, protective apparatus, as well as a stopwatch. The equipment use in experience was a small- scale compartment firebox with external dimensions of (0.65m long by 0.34m wide by 0.38m high), the walls of the firebox, roof and floor constructed from material Monolux- 500 non-combustible board 0.025m thick ( see Figure 1). Two full open ventilation (0.23m high by 0.155m wide) and the second ventilation open size(0.105m high by 0.155m wide) to allow the student to see clearly the fire inside the enclose. A twelve thermocouples type k nickel- chromium and nickel- aluminum with stainless steel sheeth used to measure the temperature inside the firebox. There were two fuel sample used in the experiment was Polymethyl Methacrylate (PMMA) with which sit in a rectangle steel tray used for small-scale fire experiments. An electronic scale was used to measure the weight of the burned sample fuel, PMMA, during the experiment from beginning up to end, it was installed via the bottom firebox with apillar holding up the steel tray. Figure 1: Firebox front view Methodology There were two experiments with two different burning material in Wight in the laboratory of combustion with different size ventilation open at each time for the same firebox. Because of that, students can observe the impact of open ventilation, which contribute the combustion of burning material (PMMA) and their interaction. From the beginning of the test until the end, was calculating the time it takes to process of all the stages have been writing time, where the time it takes to test number one was 43 min, each 30 second there would be record time and mass for PMMA. The second test was take 27 minutes and each 30 second were record time (see Tables of Appendix A). In addition, there was record for the air temperature on the rear wall of the compartment during the experiment through twelve thermocouples. To speed up the process of ignition used powder at the beginning of the experiment. Measurements and Accuracy Measurement of temperature was done using thirteen 12 K Type thermocouples with 0.1C accuracy made of Inconel connected on one side of the firebox for recording the data with high accuracy. In addition, temperature was measured at a minimum of two seconds using Squirrel Data Logger log at 0.04 second. The 100mm x 100mm PMMA for first test and The 200mm x 200mm PMMA for second test. Data obtained/explanation and Analysis of Data / Time line / personal observation Compartment Fire Development Fire observation entails identifying stages of fire development. It is important to note that fire conditions can differ significantly throughout in a building where one compartment can have a fully developed fire while the adjacent compartment can have the fire at the growth stage. It is important to recognize the stages of fire development and probable progression in order to predict what will occur next. Fire development consists of four stages namely; incipient, growth, fully developed, and decay (Smith, 2014). The figure below demonstrates one of the ways of determining fire developmental stages. Figure 2: thermocouples temperature in unit of time for the large sample 1 2, 5, 9, 10, 6 Figure 3: thermocouples temperature in unit of time for the small sample Temperatures Timeline in the Firebox Thermocouples 1 2, 5, 6, 9, and 10 showed similar temperatures that ranged between 0-4500C. This can be attributed to the location of the thermocouples in the firebox since all these were located lower than the fuel bed. Thermocouple 1 had the lowest temperature since it was located at the farthest location of the firebox while thermocouple 5 had the second lowest temperature since it was located near the ventilation door where air entered most in the firebox. In this group, thermocouple 6 showed the highest temperature because it was located nearest to the fuel bed. On the other hand, thermocouples 4, 7, 8, 11 and 12 had the highest temperatures. This is because these thermocouples were located at the top of the firebox where a lot of heat was produced. In this group, thermocouple 1 had the highest temperature and this is because it was located near the vent. The highest temperature was at 800oC. Incipient Stage Fire ignition requires heat, fuel, along with oxygen. After the beginning of combustion, incipient stage mostly depends on the characteristics and configuration of the fuel. Air within the compartment avails enough oxygen for the fire to ignite the fire. In this experiment, splinter was used for igniting PMMA that was the fuel. Growth Stage Growth stage develops after incipient stage. If there is, enough oxygen in the compartment the fuel will ignite more and heat release rate from the fire increases. As gases in the compartment are heated, the gases expand and this increases the pressure. Increased pressures pushes the fire down in the compartment and out via available openings. In the large sample the growth stage began approximately 6 minutes after ignition where the temperature oscillated between 57-500 CO. This indicates that only about 4 minutes were available for individuals to survive before temperatures exceeded 60 CO. On the other hand, in the small sample the growth stage began approximately 14 minutes after ignition and this shows that people had around 14 minutes to survive. Flashover Stage Flashover is the transition of the fire from the growth stage to fully developed fire. When flashover takes place, there is fast change to state of total surface involvement of all materials that are ignitable in the compartment. Generally, flashover stage normally requires temperature of 500- 600 CO in the compartment. In this experiment, the small sample did not reach the flashover stage since the temperature of 500- 600 CO was never reached. However, in the large sample, flashover stage reached after only 6 minutes. Fully Development Fire Stage In this stage the greatest energy is released though ventilation limits the stage. Un-burnt gases accrue and they burn frequently as they leave the compartment and this leads to formation of flames that can be seen. A fully developed fire has temperature range of between 700o C – 1200o C. in this experiment, the fire never fully developed. In the large sample, the fire was fully developed after approximately 12 minutes. At this stage the fire produces significant heat, there are visible flames and smoke darkens to darker gray. It is vital for fire fighters to be knowledgeable of this stage because it poses high risk to their lives. Decay Stage The fire reaches this stage after the available fuel gets finished and due limited availability of oxygen. The fire thus starts to decrease. In this experiment, the large sample reached this stage after 24 minutes where the fire started decreasing. This was indicated by the decrease in temperature after 24 minutes. Heat Release Rate (HRR) When a material is burnt, it releases a specific amount of energy per unit time. The rate at which the energy is released changes with time in most materials. Figure 4: mass loss rate for both samples Figure 5: HRR for both samples The above figure shows illustrate HRR for both samples. The HRR for both samples began at 0 and increased progressively. The HRR for the large sample was about 430kW while the HRR for the small sample was about 320. The differences in HRR can be attributed to the vent for large fire being bigger when compared to the small fire and hence there was higher supply of oxygen which increased rate of burning for the large sample. Another reason is that the size of the burning material is directly proportional to the energy that is released. Evaluation of the Experiment Predicting flashover Prediction of the flashover stage is very important because it represents the transition stage between developing fire and fully developed fire (McCaffery, Quintiere & Harkleroad) MQH is used in predicting the hot gas layer of the compartment. According to MQH, for flashover to occur, the upper of gas should be 600oC. The following formula is used to calculate thermal penetration time of the wall: However, in this experiment the input time is greater than the thermal penetration time (550s) and hence the effective heat transfer coefficient can be calculated from: h = k/∂ = 8.75 The hot gas layer temperature MQH correlation is given by:  (HRR obtained from the graph) HRR has been calculated per unit of time by multiplying mass loss rate with complete heat of combustion Hc which is: Large sample: T = 430 + 6.85 (62/ (0.040 × 0.2770.5 × 0.00875 × 1.17)1/3 = 807 and therefore Q =34.14 kW Figure 6: Predicting flashover using various methods From the above figure, the minimum energy needed for flashover to take place is 12 kW. In this experiment, the large sample released 34.14 kW and hence reached the flashover stage. The small sample did not reach flashover stage as aforementioned. As the experiment indicates, the main factors that affect the rate of fire development include the size and amount of fuel (combustible materials), supply of air in particular oxygen as well as mass loss rate of fuel and heat release rate. The temperature increases with the development of fire and hence in this experiment thermocouples was used to examine fire development stages in a fire box. The experiment shows that flashover is only reached when the fire is large. It is important to understand how the fire develops and different fire development stages to know what has actually occurred in an event of fire breakout in order to respond appropriately and avoid any misunderstanding during fire response. Further experiments and research should be done to determine how an interface develops during fire growth to be able to predict the fire behaviour appropriately. Read More