BACHELOR'S THESIS. Ignition Temperatures of MDF-board and PMMA. Measurements and Numerical Calculations. Gustav Hultberg 2014

Transkript

1 BACHELOR'S THESIS Ignition Temperatures of MDF-board and PMMA Measurements and Numerical Calculations Gustav Hultberg 214 Fire Protection Engineering Fire Protection Engineer Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering

2 Ignition Temperatures of MDF-board and PMMA - Measurements and Numerical Calculations Gustav Hultberg 214 Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering

3 Preface This thesis is about the ignition temperatures of materials. It is the final work for me to receive a Bachelor of Science in Fire Protection Engineering at Luleå University of technology. The work has developed my understanding of the ignition phenomena and given an insight in the difficulties to predict a materials behavior in a fire. I would like to gratefully thank my supervisor for this work, Professor Ulf Wickström, who have guided me through the procedure and work with this thesis. I would also like to thank Alexandra Byström for all the help during the experiments that were done. Jönköping, August 214 Gustav Hultberg

4 Abstract The ignition temperature for a material is not clearly defined throughout the fire-research community. To search for the ignition temperature of for example wood would give you different answers from different sources. This is due to different methods of calculations and measurements. In this report experimental test, computer calculations, and by hand direct calculations of the ignition temperature of MDF-board and PMMA have been conducted. MDFboard is a wood fiber based building material that is produced through pressing wood fibers and adhesives together. PMMA, or Polymethylmetacrylat is (more commonly known as Plexiglas) a plastic material with properties similar to glass, but not as fragile. The computer calculations using the TASEF code and the hand-based direct calculations using equations from Wickström (214) in this thesis gave similar surface temperatures at time of ignition as found in literature. Comparisons with experiments indicate that the surface temperature at the time of ignition depends on the level of heat flux affecting the material. A high level of heat flux will give a higher surface temperature than a low level of heat flux. Further investigations at a small depth under the materials surfaces gave interesting temperatures. At a specific, small depth, the temperatures of the materials subjected to incident radiation heat are quite stable and not dependent on the magnitude of the radiation. This could mean that the temperature at a small depth into the material gives a more exact and usable ignition temperature for these two materials. i

5 Sammanfattning Antändningstemperaturen för ett material är inte entydigt bestämt i forskningen. Att slå upp antändningstemperatur för exempelvis trä ger olika svar i olika källor och detta beror mycket på olika metoder för beräkningar och mätningar. I denna rapport har experimentella försök med konkalorimeter, datorberäkningar och handberäkningar utförts för att undersöka antändningstemperaturen hos två olika material, MDF-skiva och PMMA. MDF-skiva är ett träfiberbaserat byggnadsmaterial som produceras genom att pressa samman träfibrer ihop med ett bindemedel som härdar och gör att fibrerna sedan sitter ihop. PMMA, eller Polymetylmetakrylat är det som i dagligt tal benäms som Plexiglas, ett plastmaterial som kan liknas vid glas, men inte är lika skört och ömtåligt. De dator- och handberäknade temperaturerna gav liknande yttemperaturer vid tid för antändning som finns att söka i litteraturen och varierar med värmen som påverkar materialet. En hög värmepåverkan ger en högre antändningstemperatur vid ytan än en låg värmepåverkan. Närmare undersökningar gjordes en liten bit in i materialet och gav intressanta temperaturer som är lägre än från tidigare forskning och relativt lika för de olika försöken med olika stor värmestrålning mot materialen. Detta tyder på att temperaturen en liten bit in i material är ett mer exakt och användbart mått på ett materials antändningsegenskaper. Temperaturen vid ytan enligt beräkningar är vid antändningstillfället betydligt högre för material som utsätts för en hög strålningsnivå. Däremot om man tittar på temperaturen storleksordningen en millimeter från ytan så visar denna studie att temperaturen där är ungefär densamma för olika infallande strålningsnivåer. ii

6 Table of Contents 1 INTRODUCTION BACKGROUND PURPOSE OBJECTIVE 1 2 LITERATURE REVIEW HEATING MECHANISMS TESTING METHODS THE IGNITION PROCESS IGNITION TEMPERATURES OF MATERIALS MATERIALS 3 3 METHOD EXPERIMENT THE SPECIMENS PREPARATION OF THE EQUIPMENT THE EXPERIMENTS TASEF-MODELING INPUT MATERIAL PROPERTIES BOUNDARY CONDITIONS SPECIMEN GEOMETRY TIME CONTROL SENSITIVITY ANALYSIS DIFFERENT GRID SIZES DIFFERENT MATERIAL DATA DIRECT CALCULATION METHOD 14 4 RESULTS EXPERIMENTAL RESULTS DRY MDF-BOARDS MOIST MDF-BOARDS PMMA RESULTS FROM TASEF CALCULATIONS DRY MDF-BOARDS MOIST MDF-BOARDS PMMA RESULTS FROM DIRECT CALCULATION METHOD 26 5 ANALYSIS TEMPERATURES AT IGNITION DRY MDF-BOARD MOIST MDF-BOARD PMMA COMPARISON WITH LITERATURE 31 6 DISCUSSION AND RECOMMENDATIONS 32 7 REFERENCES 33 iii

7 Nomenclature List of symbols Latin T Temperature in [ C] or [K] q Heat in [J] h Heat transfer coefficient [W/m 2 K] t Time [s] k Conductivity [W/mK] c Specific heat capacity [J/kgK] e Enthalpy [J/m 3 ] Greek σ Stefan-Boltzmanns constant ( [W/m 2 K 4 ]) ε Emissivity [-] β Convection factor [W/m 2 K] γ Convection power ρ Density [kg/m 3 ] Superscripts Rate Per area unit Subscripts AST inc c g r i ig cr tot w Adiabatic Surface Temperature Incident Radiation Convection Gas Radiation Initial Ignition Critical Total Water iv

8 1 Introduction 1.1 Background The process where a material ignites is a fundamental part of research in the science of fire and fire protection. The fire always starts with an ignition and therefore understanding of the ignition process is important for studying the whole fire phenomena. Different materials will react to heat exposure differently and the ignition temperatures of materials are not so well specified in literature. Babrauskas (23) defines the ignition temperature as the surface temperature just prior to the time of ignition, which makes the ignition temperature dependent on the magnitude of the thermal exposure. 1.2 Purpose The purpose of this thesis is to gain deeper understanding to the ignition process and investigate if there is an actual ignition temperature of materials and whether it is possible to define a general ignition temperature for a material which is independent of the intensity of the thermal exposure. When a material is subjected to a high incident heat flux, the (calculated) surface temperature will become much higher at time of ignition than if subjected to a low incident heat flux. But is it possible that at some point, a small depth into the material, the temperature is the same at ignition, independent of what level of incident heat flux the material is subjected to? Pelo (213) suggests that the temperature at 1 mm depth might be an approximation to predict the time for ignition of moist wood. Is it applicable for other materials as well? 1.3 Objective The objective of this thesis is to find, evaluate, and define the ignition temperatures of three materials: dry and moist MDF-board (medium density fiberboard) and PMMA (polymethylmetacrylat). 1

9 2 Literature Review The ignition of solid materials is a complex process that according to Babrauskas (23) and Janssens (1991a) can occur due to external heating, self-heating or a combination of both. The self-heating process is very complex and usually very slow process with time-ranges of hours, days, or even years. Most materials that self-heats and ignites are organic and the heating is caused by oxidation of the material, which takes a long time according to Babrauskas (23). Ignition through external heating is a more understandable process where a heat source provides the energy to ignite the material. (Babrauskas, 23). When a material is heated through an external heat source, the temperature of the material starts to rise. If the net flux to the material is sufficiently high, as Janssens (1991a) describes, the material will start the pyrolysis process. 2.1 Heating mechanisms All the three modes of heat transfer should be considered in ignition according to Babrauskas (23): conduction, convection and radiation. Ignition by conduction occurs when there is a direct contact with a hot body that conducts heat to another body. Ignition by convection occurs when ambient gases (often air) transfer heat to a material. The last, ignition by radiation, is the most important heat transfer mechanism according to Babrauskas (23) and is often the dominant heat transfer mode in flames and in fire scenarios. In a real fire situation convection and radiation often are combined as the heating source, with the hot gases and the radiation from the flame heating the materials subjected to the fire (Quintiere, 26). The interaction of the radiation and the convection is described by Wickström (214) according to Equation 1: = σε(t 4 r T 4 s ) + h c (T g T s ) (1) q tot This equation shows the importance of the radiation and the big influence radiation has in a fire scenario. The radiation temperature in this equation is in power of four, which at high temperatures has a lot of impact on the total heat flux compared to the gas temperature, which only has the power one. There are of course as well situations where the radiation temperature is low and the gas temperature is very high. The gas temperature is then dominating the total heat flux to the surface. This can be in for example a situation where the heat source is a small flame where the emitting of radiation is low and the gas temperature can be relatively high Testing methods There are lots of different testing methods and equipment used for igniting materials. There is for example gas-driven burners that can vary in size from very small to enormous, providing heat from only a few watts to many megawatts (Babrauskas, 23). There are also ovens, electrical or gas-heated in which specimens are placed for heating. In this thesis the cone calorimeter is used, where an electrical heated cone is located above the specimen and provides incident radiation heat flux to the specimen surface. This can be used with or without a piloting spark or electrical wire for piloted- or auto-ignition (Quintiere, 26). 2.2 The ignition process According to Quintiere (26), ignition of a solid material is explained by three steps: heating, air-mixing, and chemical reaction. 2

10 The heating raises the temperature and starts the pyrolysis process where gaseous fuel is produced (Quintiere, 26). When gaseous fuel is released from the material it starts to mix with the ambient air that contains oxygen, which is essential for the combustion. When the mixture of pyrolysis gas and oxygen is flammable its temperature must reach a certain level for it to ignite. There are two types of ignition; piloted ignition or auto-ignition. Piloted ignition is according to Karlsson and Quintiere (2) that a small flame or an electrical spark provides the temperature needed to start the combustion. When auto-ignited, the flammable mixture of pyrolysis gases and air is hot enough to ignite (Karlsson & Quintiere, 2). When flammable gases reach the pilot or have accumulated enough energy to auto-ignite, a chemical exothermic reaction occurs and the gases ignite according to Quintiere (26) and Karlsson & Quintiere (2). 2.3 Ignition temperatures of materials Babrauskas (23) means that there are two definitions of the term ignition temperature: 1. The minimum temperature that the air must be heated to, in order that a specimen placed in the heated air environment would ignite; 2. The surface temperature of the specimen just prior to the point of ignition The first definition was commonly used until second half of 2 th century. It is nowadays not used, because it is not very usable for fire situations since a fire often exceeds this temperature, and in most cases radiation is the greater and most relevant heat source (Babrauskas, 23). The second definition is according to Babrauskas (23) a better way of describing the process in the material but much harder to measure, and the method for measuring will be very important to the results. The surface temperature at the time of ignition will, however, vary with the magnitude of the heat source, which would give one single material several different ignition temperatures. This thesis is evaluating this second definition by Babrauskas (23) and compares it with the temperatures, not only at the surface, but also below surface of the specimens. 2.4 Materials The materials tested in this thesis are MDF-board (Medium Density Fiberboard) and PMMA (Polymethylmetacrylat). MDF-board is a common wood-based building material where woodfibers are mixed with a binder and pressed together, resulting in a uniform material. PMMA is sometimes called acrylic glass, or as it is generally called: Plexiglas. It is often used as an alternative to glass with the advantage that it is much lighter and also impact-proof. Both materials homogenous and their behavior in fire are not affected of for example in which direction the materials face the heat. As an example, regular wood with the fibers in a specific direction will react different if heated across the fibers, or in the end of the fibers (Babrauskas, 23). 3

11 3 Method 3.1 Experiment The specimens The MDF-board was bought from a local building-supply warehouse in Luleå and was produced by Norboard. One batch of the MDF board was placed in an oven with a temperature of 11 ºC to dry and release its moisture content. The drying process was going for 12 days and the specimens were controlled daily to make sure that they had reached a steady weight and released all their moisture content. Dimensions and weight of the board were measured before and after drying to approximately 1x1x12 mm and 79 grams before and 73 grams after drying. The other batch of MDF-board was placed in a room with a controlled relative humidity of 55-6%. This batch was also treated for 12 days and controlled every day to see that the weight stabilized and the moisture content was stable. This batch had the same dimensions as the dried, 1x1x12 mm and weight approximately 79 grams before treatment and 78 grams after. The PMMA was not treated in any way before testing since there are no moist in the material. It was measured to a length and height of 1 mm and a thickness of 1 mm. The weight was approximately 114 grams Preparation of the equipment In the experiments the cone calorimeter is used. This apparatus exposes the specimens to radiant heat flux from a cone located above the specimens. The choice of using the cone calorimeter is because of that the radiation is the primary heat source and the gas temperature is low. This gives a stable incident radiant heat flux to the material that is constant over time. The cone calorimeter is equipped with a pilot spark, which gives a piloted ignition. The advantage of using a spark as pilot instead of for example a small flame is that a small flame would contribute with some heat as well, and the cone would not be the only source of heat. The cone calorimeter with the pilot spark will give conditions that are predictable and steady for the tests. The tests were conducted in the Complab at Luleå University of technology. The cone calorimeter was calibrated to provide the samples with an incident heat flux of 25 kw/m 2, 35 kw/m 2 and 5 kw/m 2 as the three radiation incident heat flux levels for testing. To calibrate the cone calorimeter to the different incident heat flux levels, Equation 2 was used, which according to Wickström (214) gives the incident heat flux when gas temperature (T g) and the adiabatic surface temperature (T AST) is known. The adiabatic surface temperature is defined by Wickström (214) as the temperature of a surface that cannot absorb any heat. Then the equation q inc 4 = σt AST h c (T ε g T AST ) (2) can be derived. The thermal inertia and the heat losses from underneath the surface must then be negligible for the relationship to be valid. During this calibration process, the gas temperature was recorded with a fine thermo-couple right at the sample-holder and the adiabatic surface temperature was recorded with a plate thermometer that gives a good approximation of the adiabatic surface temperature (Wickström, 214). With the plate thermometer it is assumed that the losses to the insulated backside of the thermometer are negligible. The calibration was run 4

12 until a steady-state condition was achieved for the plate thermometer at the specified incident heat fluxes The experiments The specimens were placed in a metal holder. They were insulated from underneath and the upper side was exposed to the radiation from the heater of the cone calorimeter. When the protective lid on the cone calorimeter was removed the specimen got exposed to a constant incident radiation as specified. When ignited the time was recorded with a stopwatch. The fire got extinguished and the sample holder was removed and cooled down to room temperature before testing the next sample. 3.2 TASEF-modeling TASEF is a computer program that uses finite element method for calculation of temperatures in materials (Wickström & Virdi, 213). A 1-D model finite element model was made of the specimens and heating boundary conditions were input to the program, which then calculated the temperature of the specimens. In the following Sections, , the model and input data is described Input material properties When modeling in TASEF, the relevant input data for the PMMA and MDF-board is the temperature-conductivity points (W/mK) and temperature-specific volumetric enthalpy (Ws/m 3 ). The temperature-specific volumetric enthalpy is according to Wickström (214) given by Equation 3. T e(t) = ρ(t)c(t)dt + l i T where: T is the reference temperature (assumed zero C in TASEF) l i is the latent heat due to for example moisture content and evaporation of water (J/m 3 ) (Wickström, 214) If the material contains moisture, the sensitive heat of the water is added to the enthalpy up to a lower temperature limit, T l,. Then it starts evaporating linearly, adding the heat of evaporation and the sensitive heat to upper limit, T u, where all moist is gone. The latent heat due to evaporation of water is according to Wickström (214) calculated as: l w = uρ dry a w (4) where: u is the water content per unit mass of the dry material ρ dry is the density of the dry material a w is the heat of evaporation of water, 226 kj/kg (Ekbom, Lillieborg & Bergström, 1984) This gives the enthalpy at the lower temperature limit, T l, to: e(t l ) = (ρ dry c dry + uρ dry c w )(T l T ) (5) where c w is the specific heat capacity of water, 4181 J/kgK (Wickström, 214) At the upper temperature limit, T u: (3) 5

13 Specific volumetric enthalpy [MJ/m3] e(t u ) = e(t l ) + (ρ dry c dry +.5uρ dry c w )(T u T l ) + uρ dry a w (6) And above the upper temperature limit to: e(t) = e(t u ) + ρ dry c dry (T T u ) (7) Between T and T l and between T l and T u the enthalpy is assumed varying linearly with the temperature. An example for a typical wooden material with dry density 5 kg/m 3, specific heat capacity of 28 J/kgK, moisture content of 15 % by mass that assumes to evaporate between 1 C and 11 C in comparison with dry wood is presented in Table 1. The moist concent of 15 % is typical for indoor-stored wood. Table 1 Numerical example of enthalpy calculation of a moist and a dry wooden material Temperature, T [ C] Specific Volumetric Enthalpy, e, wood 15 % moist by mass [J/m 3 ] 1 (T l) (T u) Specific Volumetric Enthalpy, e, dry wood[j/m 3 ] In Figure 1 this comparison is illustrated graphically. The difference in enthalpy between dry and moist wood is, as seen, significant. The waters sensitive heat up to the point where it starts to evaporate makes the temperature development a bit slower in the moist material. Over that point, the process of evaporating water takes a lot of energy which makes the temperature development significantly slower until the all moist is evaporated. When all moist is evaporated the enthalpy rises at the same rate as for dry wood Temperature [ C] Wood, 15 % moist by mass Dry wood Figure 1 Comparison of numerical example of enthalpy of a moist and a dry wooden material 6

14 Volumetric heat capacity [MJ/m 3 K] Another method, which is used for the input data in TASEF is plotting the volumetric heat capacity (ρ c) versus temperature as in Figure 2, the integral of Equation 3 (the enthalpy) is the area under the line. 2,5 2 1,5 1 PMMA MDF, Temperature [ C] Figure 2 Volumetric heat capacity for PMMA and oven-dried MDF In this way, by plotting the volumetric heat capacity versus temperature, the enthalpy and Equation 3 was solved graphically for input data in TASEF. Calculating the area under one temperature-interval and adding the area from all intervals before gives the enthalpy for the upper limit of the interval. For example, the enthalpy for PMMA at 23 C is calculated by multiplying the volumetric heat capacity for the temperature-interval -23 C (1.845 MJ/m 3 K) with the length of the interval, 23. For the next temperature-step, 53 C, the enthalpy is the sum of the enthalpy at 23 C and the area under the temperature-interval between 23 C and 53 C. The enthalpy is calculated for PMMA and dry MDF-board. For the moist MDF-board, enthalpy was set as dry MDF-board, the volumetric percentage of moist was specified to 4.63 and TASEF automatically generated the enthalpy with moist included. TASEF adds the latent heat of water between the last specified temperature step before 1 C (in this case 75 C) and the next specified temperature step (Wickström & Virdi, 213), in this case 15 C. However, TASEF does not add the sensitive heat of the water. This means that the enthalpy for the moist MDF is without the sensitive heat of water, and the heat of evaporation of water is adding between 75 and 15 C. This will have some impact on the results and should have been done more carefully to give more accurate results of the moist MDF. The difference in enthalpy between dry and moist MDF can be seen in Figure 3. 7

15 Enthalpy [MJ/m 3 ] Moist MDF-board Dry MDF-board Temperature [ C] Figure 3 Enthalpy for dry and moist (4.65 %) MDF-board not considering the sensitive heat of the moisture. PMMA and MDF input was treated temperature-dependent with material properties from Jansson (24). However, since initial temperature is 2 C and the data from Jansson (24) starts at 23 C it is assumed that the material properties are constant between 2 C and 23 C. It is also assumed that the material properties are constant at higher temperatures than Jansson (24) have recorded. The input and source data for PMMA is presented in Table 2 and for MDF in Table 3. Table 2 PMMA input data Temperature [ C] Conductivity [W/mK] (Jansson, 24) TASEF input - conductivity points Volumetric heat capacity [MJ/m 3 K] (Jansson, 24).212 TASEF input - specific volumetric enthalpy points

16 Table 3 MDF input data Temperature [ C] Conductivity [W/mK] (Jansson, 24) TASEF input - conductivi ty points Volu-metric heat capacity [MJ/m 3 K] (Jansson, 24) TASEF input - specific volumetric enthalpy points (dry MDF).16 TASEF input - specific volumetric enthalpy points (4.63 volumetric % moist MDF)

17 3.2.2 Boundary conditions The thermal boundary conditions at the experiment were input to TASEF as different fire-curves with temperatures as presented in Table 4. They were set with constant temperatures throughout the whole simulation. The radiation temperature and gas temperature of the tests were added separately. The gas temperatures were recorded with a fine thermocouple during the tests and the radiation temperatures were calculated from the known incident heat flux. To calculate the radiation temperature when the incident heat flux is known Equation 8 that shows the relation between the incident heat and the radiations temperature is used (Wickström, 214). 4 T r = q inc σ (8) Table 4 Radiation- and gas temperatures for the different heat fluxes, measured and calculated from experiment Incident heat flux, T r [ C] T g [ C] q inc [kw/m 2 ] The thermal exposures were assigned to the exposed surface of the specimen in TASEF. The coefficients related to radiation and convection, such as emissivity (ε), convection factor (β) and convection power (γ) were set according to Table 5 where the input data is shown. Equation 9 shows the heat transfer mechanism with the three coefficients (Wickström, 214). q tot = ε(q inc σt 4 s ) + β(t g T s ) γ (9) The emissivity prescribes how the surface is affected by the radiation, the convection factor how it responds to convection, and is the same as the heat transfer coefficient (h). The convection power is whether the convection is natural or forced and according to Wickström (214), forced convection has the power one and natural convection is greater than one. The convection factor was set to zero in the radiation boundary condition, and the emissivity was set to zero in the convectional boundary condition. This is to separate the gas- and radiation temperature influence on the specimen. 1

18 Table 5 Boundary conditions input in TASEF Boundary ε β γ 25 kw/m 2 radiation 25 kw/m 2 gas 35 kw/m 2 radiation 35 kw/m 2 gas 5 kw/m 2 radiation 5 kw/m 2 gas Specimen geometry One-dimensional finite element models of the MDF-board and the PMMA were created in TASEF. The MDF-board had a thickness of 12 mm and the PMMA 1 mm. Four different grid sizes were tested for the MDF-board at 25 kw/m 2 as a sensitivity analysis and one was used for further simulations. See Appendix A for all the different sets of grid sizes Time control TASEF was set with the default time-increment factor of.1 and in the print-out plot temperatures every third second up to 1 s in the printout-file. 3.3 Sensitivity analysis Different grid size and temperature dependent or independent material data is tested in the following sections to determine what impact grid size and material data has in simulations and which to be preferable in further simulations. 11

19 Temperature [ C ] Temperature [ C ] Different grid sizes To determine the appropriate grid size for model in TASEF, four different grid sizes sets were tested, presented in Appendix A. The tests show that with a general grid size of 2 mm temperatures start to differ from tests with smaller grid sizes. The used grid size for further experiments and grid size 2 gives as seen in the figures almost identical results. Temperatures from the tests are shown in Figure 4 and Figure Used grid size, Surface temperature Grid size 2, surface temperature Grid size 3, Surface temperature Grid size 4, Surface temperature Time [s] Figure 4 Surface temperatures from TASEF for MDF at 25 kw/m 2 with four different grid size sets Time [s] Used grid size, 2 mm depth Grid size 2, 2 mm depth Grid size 3, 2 mm depth Grid size 4, 2 mm depth Figure 5 Temperatures at 2 mm depth from TASEF for MDF at 25 kw/m 2 with four different grid size sets From Figure 4 and Figure 5 it is clear that the differences between the used grid sizes, set 2 and 3 are very small. Grid size 4 differs a lot with the other ones, and to use such big grid size would affect the results. 12

20 Temperature [ C] Different material data As described in Section the material input was treated temperature-dependent with data from Jansson (24). This data are compared with temperature-independent approximative data from Quintiere (1998) for the MDF-board. According to Quintiere (1998) the conductivity is.14 W/mK and the specific heat capacity 28 J/kgK (1.666 MJ/m 3 K). The comparison is done in 25 kw/m 2 incident heat flux and is seen in Figure 6. The surface temperature is almost the same, but deeper in the material temperatures differ, mostly due to the difference in conductivity. As Jansson (24) measured, the conductivity and heat capacity changes with the temperature of the material. His data are therefore likely to give more accurate results Surface - constant material data Time [s] 2 mm depth - constant material data Surface - temperature dependent material data 2 mm depth - temperature dependent material data Figure 6 Calculated temperatures of MDF-board with different material data at 25 kw/m 2 13

21 3.4 Direct calculation method Wickström (214) presents a method of estimating time to ignition of a semi-infinite material by a simple direct calculation method (closed form solution). In the calculations the material assumes to ignite when the surface temperature reach the ignition temperature. Equation 1 gives the time for ignition and Equation 11 the critical incident heat flux. t ig = π(kρc) 4 where [ 2 (T ig T i ) ] ε(q inc.8q inc,cr) q inc,cr = σt 4 ig + h (T ε ig T g ) (11) (1) By inserting Equation 11 into Equation 1 the time to ignition can be calculated for various material properties represented by their (constant) thermal inertia kρc and ignition temperature T ig. To give results comparable to the TASEF calculated temperature results, the material data should be equal in this calculation method and TASEF simulations. ε was set to.8 as in TASEF and h to 12 W/m 2 K. The data from Jansson (24) in Table 2 and Table 3 was temperature-dependent, but this method requires constant material data. Therefore the material data for these calculations has been estimated constant as presented in Table 6. Table 6 Constant material data for numerical calculations Material Conductivity, k [W/mK] Volumetric specific heat capacity, ρc [MJ/m 3 K] MDF-board PMMA

22 4 Results In this chapter results from the experiments, calculations in TASEF and numerical calculations as described in Section 3.4 will be presented. 4.1 Experimental results Dry MDF-boards The results from the MDF-boards tested in the cone-calorimeter are presented in Table 7: Table 7 Ignition time in cone calorimeter test for the dry MDF-board Heat flux: Sample 1 Sample 2 Sample 3 Mean ignition time s 67 s 61 s 65 s kw/m s 39 s 4 s 39 s kw/m s 24 s 22 s 23 s kw/m Moist MDF-boards The results from the MDF-boards tested in the cone-calorimeter are presented in Table 8: Table 8 Ignition time in cone calorimeter test for the moist MDF-board Heat flux: Sample 1 Sample 2 Sample 3 Mean ignition time s 94 s 96 s 99 s kw/m s 48 s 63 s 53 s kw/m s 27 s 28 s 29 s kw/m 2 15

23 4.1.3 PMMA The results from the PMMA-samples tested in the cone-calorimeter are presented in Table 9: Table 9 Ignition time in cone calorimeter test for the PMMA Heat flux: Sample 1 Sample 2 Sample 3 Mean ignition time s 97 s 95 s 96 s kw/m s 52 s 55 s 53 s kw/m s 28 s 25 s 26 s kw/m 2 16

24 Temperature [ C] 4.2 Results from TASEF calculations In the following sections the temperature distributions calculated in TASEF are presented Dry MDF-boards At 25 kw/m 2 The temperatures inside the dry MDF-board are presented in Figure Time [s] 12 mm 9 mm 7 mm 5 mm 4 mm 3 mm 2,5 mm 2 mm 1,5 mm 1 mm,5 mm Surface Figure 7 Calculated temperature at various depths for the dry MDF-board in 25 kw/m 2 from TASEF At the mean ignition time for the experiment, 65 s, the temperatures distribution for the surface and at depths up to 3 mm into the specimen are as presented in Table 1: Table 1 TASEF-generated temperature distribution at time of ignition for the dry MDF-board from surface to 3 mm depth, 25 kw/m 2 Distance from surface [mm] Temperature [ C] (Surface)

25 Temperature [ C] At 35 kw/m 2 The temperatures inside the dry MDF-board are presented in Figure Time [s] 12 mm 9 mm 7 mm 5 mm 4 mm 3 mm 2,5 mm 2 mm 1,5 mm 1 mm,5 mm Surface Figure 8 Calculated temperature at various depths for the dry MDF-board in 35 kw/m 2 from TASEF At the mean ignition time for the experiment, 39 s, the temperatures distribution for the surface and at depths up to 3 mm into the specimen are as presented in Table 11: Table 11 TASEF-generated temperature distribution at time of ignition for the dry MDF-board from surface to 3 mm depth, 35 kw/m 2 Distance from surface [mm] Temperature [ C] (Surface)

26 Temperature [ C] At 5 kw/m 2 The temperatures inside the dry MDF-board are presented in Figure mm 9 mm 7 mm 5 mm 4 mm 3 mm 2,5 mm 2 mm 1,5 mm 1 mm Time [s],5 mm Surface Figure 9 Calculated temperature at various depths for the dry MDF-board in 5 kw/m 2 from TASEF At the mean ignition time for the experiment, 23 s, the temperatures distribution for the surface and at depths up to 3 mm into the specimen are as presented in Table 12: Table 12 TASEF-generated temperature distribution at time of ignition for the dry MDF-board from surface to 3 mm depth, 5 kw/m 2 Distance from surface [mm] (Surface) Temperature [ C]

27 Temperature [ C] Moist MDF-boards At 25 kw/m 2 The temperatures inside the moist MDF-board are presented in Figure mm 9 mm 7 mm 5 mm 4 mm 3 mm 2,5 mm 2 mm 1,5 mm 1 mm Time [s],5 mm Surface Figure 1 Calculated temperature at various depths for the moist MDF-board in 25 kw/m 2 from TASEF At the mean ignition time for the experiment, 99 s, the temperatures distribution for the surface and at depths up to 3 mm into the specimen are as presented in Table 13: Table 13 TASEF-generated temperature distribution at time of ignition for the moist MDF-board from surface to 3 mm depth, 25 kw/m 2 Distance from surface [mm] Temperature [ C] (Surface)

28 Temperature [ C] At 35 kw/m 2 The temperatures inside the moist MDF-board are presented in Figure mm 9 mm 7 mm 5 mm 4 mm 3 mm 2,5 mm 2 mm 1,5 mm Time [s] 1 mm,5 mm Surface Figure 11 Calculated temperature at various depths for the moist MDF-board in 35 kw/m 2 from TASEF At the mean ignition time for the experiment, 53 s, the temperatures distribution for the surface and at depths up to 3 mm into the specimen are as presented in Table 14: Table 14 TASEF-generated temperature distribution at time of ignition for the moist MDF-board from surface to 3 mm depth, 35 kw/m 2 Distance from surface [mm] Temperature [ C] (Surface)

29 Temperature [ C] At 5 kw/m 2 The temperatures inside the moist MDF-board are presented in Figure mm 9 mm 7 mm 5 mm 4 mm 3 mm 2,5 mm 2 mm 1,5 mm 1 mm Time [s],5 mm Surface Figure 12 Calculated temperature at various depths for the moist MDF-board in 5 kw/m 2 from TASEF At the mean ignition time for the experiment, 29 s, the temperatures distribution for the surface and at depths up to 3 mm into the specimen are as presented in Table 15: Table 15 TASEF-generated temperature distribution at time of ignition for the moist MDF-board from surface to 3 mm depth, 5 kw/m 2 Distance from surface [mm] Temperature [ C] (Surface)

30 Temperature [ C] PMMA At 25 kw/m 2 The temperatures inside the PMMA-specimen are presented in Figure Time [s] 1 mm 8mm 6 mm 5 mm 4 mm 3 mm 2,5 mm 2 mm 1,5 mm 1 mm,5 mm Surface Figure 13 Calculated temperature at various depths for the PMMA in 25 kw/m 2 from TASEF At the mean ignition time for the experiment, 96 s, the temperatures distribution for the surface and at depths up to 3 mm into the specimen are as presented in Table 16: Table 16 TASEF-generated temperature distribution at time of ignition for the PMMA from surface to 3 mm deoth, 25 kw/m 2 Distance from surface [mm] Temperature [ C] (Surface)

31 Temperature [ C] At 35 kw/m 2 The temperatures inside the PMMA-specimen are presented in Figure Time [s] 1 mm 8mm 6 mm 5 mm 4 mm 3 mm 2,5 mm 2 mm 1,5 mm 1 mm,5 mm Surface Figure 14 Calculated temperature at various depths for the PMMA in 35 kw/m 2 from TASEF At the mean ignition time for the experiment, 53 s, the temperatures distribution for the surface and at depths up to 3 mm into the specimen are as presented in Table 17: Table 17 TASEF-generated temperature distribution at time of ignition for the PMMA from surface to 3 mm depth, 35 kw/m 2 Distance from surface [mm] Temperature [ C] (Surface)

32 Temperature [ C] At 5 kw/m 2 The temperatures inside the PMMA-specimen are presented in Figure Time [s] 1 mm 8mm 6 mm 5 mm 4 mm 3 mm 2,5 mm 2 mm 1,5 mm 1 mm,5 mm Surface Figure 15 Calculated temperature at various depths for the PMMA in 5 kw/m 2 from TASEF At the mean ignition time for the experiment, 26 s, the temperatures distribution for the surface and at depths up to 3 mm into the specimen are as presented in Table 18: Table 18 TASEF-generated temperature distribution at time of ignition for the PMMA from surface to 3 mm depth, 5 kw/m 2 Distance from surface [mm] Temperature [ C] (Surface)

33 4.3 Results from direct calculation method The surface temperatures at time for ignition calculated by direct calculation method where Equation 1 and 11 gives the time for ignition t ig = π(kρc) 4 where q inc,cr [ 2 (T ig T i ) ] ε(q inc.8q inc,cr) = σt ig 4 + h ε (T ig T g ) (11) (1) as described in Section 3.4. The results from this method are presented in Table 19. Table 19 Surface temperatures at ignition calculated by direct calculation method for dry MDF-board and PMMA exposed to different incident heat fluxes Material Incident heat flux [kw/m 2 ] T ig [ C] Dry MDF-board 25 3 Dry MDF-board Dry MDF-board PMMA PMMA PMMA

34 5 Analysis 5.1 Temperatures at ignition All three materials studied show a great variety of surface temperatures at experimental time for ignition, both calculated by direct calculation method from Section Fel! Hittar inte referenskälla. and obtained from TASEF. The surface temperature seems dependent of the magnitude of the incident heat and rises with a higher heat flux. The ignition of the material might not only be dependent on the surface temperature. When the temperature rises in the material and the material starts releasing pyrolysis gases. The gases that are produced by the materials surface only, are not enough to actually ignite the material and start a flame that will keep burning. For ignition, the material needs to emit a steady flow of pyrolysis gases to ignite a flame and keep burning. Inside the materials though, the temperatures at a certain depth is almost the same at ignition for the same material and all the different incident heat flux levels according to results from TASEF in Section. This depth and the temperatures differ between the dry and wet MDF, as well as for the PMMA. To examine the temperatures at some depth inside the material might be a better approximation for the actual ignition temperature of the material, and in Sections 5.1.1, and below this is analyzed. 27

35 Temperature [ C] Dry MDF-board The dry MDF-board tested show that the temperature at surface differs at the time of ignition between the different levels of incident radiation, both in results from TASEF and the direct calculation method. As seen in Table 2 the span is from 3 to 384 C. When looking deeper into the material, it is seen as suggested by Pelo (213) that the temperatures at one millimeters depth are almost identical at all the different heat fluxes. Table 2 Calculated temperatures at time of ignition for dry MDF-board at the different incident heat fluxes Incident heat flux Time ignition [s] to Surface temperature (TASEFcalculated) [ C] Surface temperature (direct calculations) [ C] 25 kw/m kw/m kw/m Temperature at 1 mm depth (TASEFcalculated) [ C] To examine this further, a simulation in TASEF was conducted for all the different heat fluxes with a grid size of only.1 millimeters from surface to 1.5 mm depth to determine a mutual ignition temperature. The results shows that at a depth between.9 and 1 millimeter the three different incident heat fluxes give the same temperature, approximately 22 C as seen in Figure kw/m2 35 kw/m2 5 kw/m ,6,8 1 1,2 1,4 Depth from surface [mm] Figure 16 Calculated temperature profiles inside the dry MDF-boards at time of ignition for various incident radiation levels. 28

36 Temperature [ C] Moist MDF-board The surface temperatures for the moist MDF-board are as seen in Table 21 almost identical to the surface temperatures for the dry MDF-board from Table 2. The temperatures at one millimeters depth differ a bit from the dry board. Table 21 Calculated temperatures at time of ignition for moist MDF-board at the different incident heat fluxes Incident flux heat Time to ignition [s] Surface temperature (TASEFcalculated) [ C] 25 kw/m kw/m kw/m Temperature at 1 mm depth (TASEFcalculated) [ C] To examine the moist MDF-board further, a test with a finer grid size for all three levels of incident heat flux was conducted in TASEF. In Figure 17 this test shows that the moist MDFboard has a point, although not as clear as for dry MDF-board, where the temperature is almost the same for all three different levels of incident heat flux. The point is estimated as a mean value of the curves intersections to.8 mm depth and the temperature to about 245 C ,6,7,8,9 1 Depth from surface [mm] 25 kw/m2 35 kw/m2 5 kw/m2 Figure 17 Calculated temperature profiles inside the moist MDF-boards at time of ignition for various incident radiation levels. 29

37 Temperature [ C] PMMA Also for the PMMA, the surface temperature results from TASEF and direct calculation method at time for ignition differs a lot between the different incident heat fluxes. As seen in Table 22, surface temperatures span from 276 to 329 C. Table 22 Calculated temperatures at time of ignition for PMMA at the different incident heat fluxes Incident heat flux Time to ignition [s] Surface temperature (TASEFcalculated) [ C] Surface temperature (direct calculations) [ C] Temperature at.5 mm depth (TASEFcalculated) [ C] 25 kw/m kw/m kw/m Temperature at 1 mm depth (TASEFcalculated) [ C] Just as for the MDF-board further test with a smaller grid size were conducted in TASEF, and the temperature distribution for the different heat fluxes at ignition time is shown in Figure 18. The point where the three incident heat fluxes have almost the same temperature is estimated as a mean value of the curves intersections to.7 mm depth and the temperature to about 24 C ,2,4,6,8 1 1,2 Depth from surface [mm] 25 kw/m2 35 kw/m2 5 kw/m2 Figure 18 Calculated temperature profiles inside the PMMA at ignition time for various incident radiation levels. 3

38 5.2 Comparison with literature Babrauskas (23) summaries in Ignition Handbook Table 211, several other experiments regarding ignition temperature of wooden materials. Some of the experiments are as shown in Table 23. Table 23 Summary of ignition temperature results for wood. Source: Babrauskas (23) Source Specimen size [mm] Ignition temperature [ C] Comments Janssens (1991b) 1x1x Surface temp. measured; fluxes 25 to 35 kw/m 2 Li & Drysdale (1992) Moghtaderi et al (1997) 64x64x Temp. measured below surface; flux >2 kw/m 2 1x1x Temp. measured below surface; 2 kw/m Temp. measured below surface; 6 kw/m 2 These temperatures are very similar to the surface temperatures shown in the results in this thesis (3-384 C). However, some of them are measured below the surface of the specimens, and results from TASEF shows that the temperatures below surface are significantly lower, only between C at just one millimeters depth. For PMMA, Babrauskas (23) measured the ignition temperature to 266 C at a heat flux of 2 kw/m 2, which is quite close to the results at.5 mm depth from TASEF ( C). Thomson (1988) has measured the surface temperature at ignition for PMMA exposed to a heat flux of 2 kw/m 2 to be 312 C. This measured value is as well as the measured for MDF-board in the span of the calculated surface temperatures of this thesis, which was C for PMMA depending on the incident heat flux. 31

39 6 Discussion and Recommendations As Section 5.2 shows, the surface temperatures found in literature are similar to the calculated surface temperatures of this thesis. But the temperatures found in literature that was measured below the surface in experiments are, however, very similar to the surface temperatures. This does not correspond with results of this thesis. The results from TASEF-calculations shows that the temperatures of the materials are significantly lower at just one millimeters depth compared to at surface. The difference can be explained by the material properties that were used in TASEF, or the measurement method of the experiments found in literature. In section 5.1 the temperatures at 1 mm for the dry MDF-board,.7 mm for the moist MDFboard respectively.5 millimeters for the PMMA are almost identical for all the different levels of incident radiation. The temperatures at this depth might be a good approximation for the actual ignition temperature, or describe the lowest temperature the material will have to reach for the pyrolysis process to start and ignition to follow. These results propose that when the surface heats up and starts the pyrolysis process, the pyrolysis gases might not be enough to actually ignite the material. The material must be heated enough to produce pyrolysis gases into some depth to get enough gases to form a flammable mixture and ignite. As stated in Section 2.3, Babrauskas (23) second, modern definition of ignition temperatures of different materials were with respect of the surface temperature. This is studied in this thesis and seems to differ a lot with respect of the ambient conditions of the material exposed to heat. The ignition temperature, as suggested in this thesis, is not dependent of the magnitude of the thermal exposure, but the material properties conductivity and specific heat capacity, and the flow of pyrolysis gases out of the material. A large thermal exposure will quickly start the pyrolysis process, but the flow of gases will not be high enough for ignition until the temperature some depth into the material is high enough. The pyrolysis process is then an essential matter for the ignition of a material, and should be taken in consideration in the definition of ignition temperature A suggestion for a new definition of ignition is therefore: The temperature of the material where the pyrolysis process begins This is a definition based on the chemical properties of the material rather than the physical thermal properties like conductivity and specific heat. An interesting topic for further research might be if the depth of which the temperature is constant for different heat fluxes at time for ignition for a material can be calculated in some way. Is the depth depending on material constants such as density or conductivity? Is it depending on the thermal heat exposure level? Other suggestions would be to conduct more tests with a wider range of materials to see if the hypothesis about the temperature at a small depth in the material is usable for all flammable materials, or to compare this method with chemical methods like TGA, Thermo Gravimetric Analysis. 32

40 7 References Babrauskas, V. (23). Ignition Handbook. Issaquah: Fire Science Publishers. Buchanan, A. H. (22). Structural Design for Fire Safety. Canterbury: John Wiley & Sons Ltd. Ekbom, L., Lillieborg, S. & Bergström, L. (1984) Tabeller och formler. Stockholm: Esselte Studium AB Janssens, M. L. (1991b). Fundamental Thermophysical Characteristiscs of Wood and Their Role in Enclosure Fire Growth. Gent: Universiteit Gent. Janssens, M. L. (1991a). Piloted Ignition of Wood: A Review. Fire and Materials, 15, Jansson, R. (24). Measurments of thermal properties at elevated temperatures. SP Swedish national testing and research institute, Fire technology. Borås: SP report. Karlsson, B., & Quintiere, J. G. (2). Enclosure Fire Dynamics. Boca Raton: CRC Press. Li, Y., & Drysdale, D. (1992). Measurments of the Ignition Temperature of Wood. Beijing: Academic Publishers. Moghtaderi, B., Novozhilov, V., Fletcher, D. F., & Kent, J. H. (1997). A New Correlation for Bench-scale Piloted Ignition Data of Wood. Fire Safety (29), Pelo, C. (213). Time to Ignition of Moist Wood, Measurments and Numerical Predictions. Luleå: Luleå University of Technology. Quintiere, J. G. (26). Fundamentals of Fire Phenomena. Chichester: John Wiley & Sons Ltd. Quintiere, J. G. (1998). Principles of Fire Behavior. Albany: Delmar. Thomson, H. E. (1988). Ignition Characteristics of Plastics. Edinburgh: University of Edinburgh. Wickström, U. (214). Heat transfer in fire technology. SP Fire technology and Luleå Technical University. Luleå: Unpublished draft. Wickström, U., & Virdi, K. (213). TASEFplus, a program for thermal analysis of sections exposed to fire. User manual. 33

41 Appendix A This table shows the position of the grid lines of the different models in TASEF in sensitivity analysis. The model is 12 mm deep and TASEF automatically positions a grid line at the top (12 mm) and bottom ( mm). The values in table are vertical position of the grid lines, with distance in millimeters from the bottom of the model. The figure shows the geometry from grid size set 4 in TASEF. Table of different sets of grid sizes in TASEF Grid size set used for experiment Grid size set 2 Grid size set 3 Grid size set Figure from TASEF, geometry of grid size set 4