Department of Mechanical Engineering, University Visvesvaraya College of Engineering, Bangalore University, Bangalore, Karnataka, India

Transcription

1 EXPERIMENTAL INVESTIGATION ON EFFECTS OF HEAT FLUX AND DENSITY ON SMOLDERING OF COTTON Ramesh D K *1, Manjunath S O #1, Sanjay R #2, Sai Naveen S #3, Jayantha #4 * Associate professor, # BE Scholar Department of Mechanical Engineering, University Visvesvaraya College of Engineering, Bangalore University, Bangalore, Karnataka, India Abstract- The smoldering of cotton is basically dependent on two characteristics, density and heat flux. The onset of smoldering of cotton is determined under the three different scenarios: A. high heat flux (20.04kW/m 2 ) followed by cooling, B. Medium Heat Flux (12.43kW/m 2 ) followed by cooling, C. Low Heat Flux (4.84kW/m 2 ) followed by cooling. The density has been varied between 14kg/m 3 to 30kg/m 3 and the ignition temperature are noted down depending on the combination of heat flux provided and the density of cotton. The difference of 21 O C was found in the ignition temperature, between high heat flux followed by cooling(scenario A) and the low heat flux followed by cooling (scenario C) with density 30kg/m 3. A temperature difference of 20 o C was found in the ignition temperature between cotton with densities 30kg/m 3 and 15 kg/m 3. Keywords: Smoldering, Ignition Temperature, Cotton, Density, Heat Flux I. INTRODUCTION Smoldering is a slow, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed Phase fuel [4]. Many solid materials can sustain a smoldering reaction, including coal, cellulose, wood, cotton, tobacco and some types of dust. Smoldering is a complex process affected by particle size, permeability, density, initial temperature, ignition source, air moment through and around the material [3]. The fundamental difference between smoldering and flaming combustion is that smoldering occurs on the surface of the solid rather than in the gas phase. Smoldering is a surface phenomenon but can propagate to the interior of a porous fuel if it is permeable to flow. When a layer is susceptible for ignition is based on a key element Ignition Temperature. Cotton will self ignite at roughly 400 o C but can sustain a flame at low temperatures, somewhere around C to C. The ignition temperature of a substance is a direct rating on the measure of the minimum temperature at which the substance ignites, without the presence of an external spark or flame. Because of the fact that the material auto igniters at the temperature range, it is also referred to as the substance auto ignition temperature. Cotton can also spontaneously ignite as a result of water absorption and desorption, which builds up heat and increases temperature. This usually happens in the storage room of cotton and cause fire and huge damages. II. EXPERIMENTAL SETUP 2.1 EXPERIMENTAL SETUP As depicted in the fig. 1.1 it consists of metallic mesh, electric heater, and vertical arrangements of thermocouple. Metallic Mesh: Metallic Mesh of (0.15X.0.15X0.15 m 3 ) is used to hold the cotton above the hot plate. The cotton is packed inside the mesh, such that a required density is obtained throughout the sample. Metallic mesh is insulated from hotplate in order to eliminate heating of cotton at vertical surface of the sample. Electric heater: A metallic plate of width 15cm, breadth 15cm and thickness 1cm is used as Hot Plate. Hot plate is heated by the aluminium foil powered by electric supply (220V-AC) sandwiched between two metal plates, upper metal plate is called Hot Plate which is chosen as ignition source. Supply voltages can be varied from 0V-220V to maintain the required heat flux at the hot plate. Temperature rise of 15 o C to 20 o C per minute is obtained at top surface of hot plate. Thermocouples: Five thermocouples are arranged vertically at the centre of the hot plate, 3cm of spacing is provided between each thermocouples. First thermocouple senses the temperature of hot plate and remaining thermocouples are inserted inside the cotton sample to sense the local temperature in the sample. IJIRAE , All Rights Reserved Page -137

2 Fig 1.1 Test Rig for ignition of smoldering fire. Fig. 1.2 CATIA model of test rig 2.2 PROCEDURE Cotton sample with five thermocouples is packed in metallic mesh such that density of cotton (30kg/m 3 and 15 kg/m 3 ) sample is maintained uniformly throughout the sample. Initially at atmospheric condition initial room temperature, density and humidity of sample is noted down. Power supply to the hot plate is turned on and a heat flux (20.04kW/m 2, 12.43kW/m 2, and 4.84kW/m 2 ) is set by varying the voltage. Once the heat flux is set temperature at the hotplate begins to rise. Observations are made at an interval of 30 seconds and temperature at every thermocouple is noted down. Once the temperature at the hot plate reaches the predetermined temperature, the power is cut off. Hot plate temperature tends to increase even after the power off. Highest temperature recorded at the hot plate is noted as maximum temperature. If the predetermined temperature is sufficient to start the smoldering of cotton, the experiment is turned off when char and ash is cooled below C. If the cut off temperature was not sufficient to start the smoldering, sample is cooled to ambient temperature and again cut-off temperature is increased in the step of C until cut-off temperature results in smoldering. III. HEAT FLUX SCENARIOS It is investigated for three different heat flux scenarios for two different densities to understand how the heat flux affects the ignition temperature which is shown in fig. 3. In real life it can be seen that material contact with the hot body will smolder easily. Scenario A involves fast heating of a sample by high heat flux and subsequent cooling of the hot plate. This scenario happens when an ignition source is in contact with a material and then removed. Scenario B involves slow heating of the hotplate than scenario A and subsequent cooling. It happens when heating period is extended. Scenario C involves slow heating for a longer period followed by cooling. These three different scenarios are observed under two different densities of cotton sample, which is discussed more detailed in section 3.1 and HIGH DENSITY (30KG/M 3 ) SCENARIO A: HIGH HEAT FLUX FOLLOWED BY COOLING 20.04kW/m 2 and resulting in a temperature rise of o C per minute at hot plate as shown in fig. 2A. The power is switched off at a cut off temperature of 257 o C and maximum recorded hot plate temperature was 267 o C which is shown in fig. 3A. The temperature is increased because of the production of smoldering fire. This increased hot plate temperature after cut off temperature is resulted in ignition. IJIRAE , All Rights Reserved Page -138

3 3.1.2 SCENARIO B: MEDIUM HEAT FLUX FOLLOWED BY COOLING 12.43kW/m 2 and resulting in a temperature rise of o C per minute at hot plate as shown in fig. 2B. The power is switched off at a cut off temperature of 268 o C and maximum recorded hot plate temperature was 280 o C which is shown in fig. 3B SCENARIO C: LOW HEAT FLUX FOLLOWED BY COOLING 4.84kW/m 2 and resulting in a temperature rise of 5-10 o C per minute at hot plate as shown in fig. 2C. The power is switched off at a cut off temperature of 278 o C and maximum recorded hot plate temperature was 288 o C which is shown in fig. 3C. 3.2 LOW DENSITY (30KG/M 3 ) SCENARIO A: HIGH HEAT FLUX FOLLOWED BY COOLING 20.04kW/m 2 and resulting in a temperature rise of o C per minute at hot plate as shown in fig. 2D. The power is switched off at a cut off temperature of 238 o C and maximum recorded hot plate temperature was 246 o C which is shown in fig. 3D SCENARIO B: MEDIUM HEAT FLUX FOLLOWED BY COOLING 12.43kW/m 2 and resulting in a temperature rise of o C per minute at hot plate as shown in fig. 2E. The power is switched off at a cut off temperature of 260 o C and maximum recorded hot plate temperature was 268 o C which is shown in fig. 3E SCENARIO C: LOW HEAT FLUX FOLLOWED BY COOLING 4.84kW/m 2 and resulting in a temperature rise of 5-10 o C per minute at hot plate as shown in fig. 2F. The power is switched off at a cut off temperature of 271 o C and maximum recorded hot plate temperature was 285 o C which is shown in fig. 3F. 4.1 RECORDED DATA IV. RESULTS The temperatures were measured at the hot plate and along the centreline of the cotton as shown in fig The fig. 3 shows temperature as a function of time for smoldering scenarios. The temperature for smoldering scenario is more errastic since the smoldering process dominates as heat source. Smoldering also results in high temperature. When sample is exposed to atmospheric air the temperature decreases. When smoldering occurs the temperatures curve in fig. 3 evolve in a way that is dependent on the heat flux scenario. Time to onset of smoldering varies according to scenario. 4.2 DETERMINATION OF IGNITION TEMPERATURE The maximum temperature of the hotplate for different heat flux scenarios is found using the plots as shown in the fig. 3. For these smoldering scenarios the maximum hotplate temperature is found from point where the hotplate temperature levels off or where the hot plate temperature has a significant increase [1]. The ignition starts at different time for each scenario and for each density. The time for ignition start increasing with decrease in heat flux. The Table 1 shows the ignition temperature for different densities and heat flux scenarios. It is reasonable to assume that ignition occurred between upper temperature limit (T high ) and lower temperature limit (T low ). The estimated temperature for onset of smoldering (T avg ) is the average of T low and T high as shown in Table 1. It is noticed that the ignition temperature for scenario A is lower than that for scenario B and scenario C. Also ignition temperature for scenario A with density 30 kg/m 3 is greater than that for scenario A with density 15kg/m 3. V. DISCUSSION The table 1 shows ignition temperature for different for different densities and heat flux scenarios. The ignition temperature increases with increasing density. For scenario A the ignition temperature is increased from 242 o C to 262 o C as the density is increased from 15kg/m 3 to 30kg/m 3. Similarly ignition temperature for scenario B is increased from 264 o C to 274 o C and ignition temperature for scenario C is increased from 278 o C to 283 o C over the same density interval. Density affects both heat transfer and energy production in cotton. Heat transfer in porous materials is a complex combination of conduction, convection and radiation. The increase in conduction due to denser medium can hardly compensate for the reduced heat transfer due to suppressed convection. Thus heat spread within the cotton will slowdown. Also lower densities have higher smoldering velocities, as the density increases the velocity decrease [3]. The table 1 shows that the heating scenario affects the ignition temperature. The effect of heating scenario on ignition temperature should be included when defining new standards for material testing. It must be emphasized that measured ignition temperatures are apparatus dependent: The use of different ignition source, sample sizes or geometries, could result in different ignition temperature [2]. The experimental results presented show that both density and het flux influence the ignition temperature for onset of smoldering in cotton. The effects are significant, thus both density and heat flux should be included as parameters in standard tests for determining ignition temperatures for dusts and other cellulose based materials. IJIRAE , All Rights Reserved Page -139

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6 VI. CONCLUSION A model has been developed to carry out the experiment to determine ignition temperature in smoldering of cotton for different densities and heat flux scenarios. This study reveals that with increase in density, the ignition temperature increases and with increase in heat flux, the ignition temperature decreases. It is also found that repeated heating of the same sample results in a higher ignition temperature. It can be concluded that both density and heat flux are the parameters that affect the ignition temperatures in a systematic way. REFERENCES [1]. B.C. Hagen, V. Frette, G. Kleppe, B.J. Arntzen, Onset of smoldering in cotton: effects of density, Fire Safety Journal 46 (2011) [2]. T.J. Ohlemiller, Cellulosic insulation material. III. Effects of heat- flow geometry on smolder initiation, Combustion Science and Technology 26 (1981) [3]. B.C. Hagen, V. Frette, G. Kleppe, B.J. Arntzen, Effects of heat flux on smoldering in cotton, Fire safety Journal 61 (2013) [4]. OHLEMILLER, T. J., AND LUCCA, D. A. An experimental comparison of forward and reverse smolder propagation in permeable fuel beds. Combustion and Flame 54, 1-3 ( ), [5]. T.J. Ohlemiller, F.E. Rogers, Cellulosic insulation material. II. Effect of additives on some smolder characteristics, Combustion Science and Technology 24 (1980) [6]. T.K. Chan, D.H. Napier, Smouldering and ignition of cotton fibres and dust, Fire Prevention and Technology 4 (1973) [7]. T.J. Ohlemiller, Smoldering combustion, in: P.J. DiNenno (Ed.), SFPE Handbook of Fire Protection Engineering, third ed.,society of Fire Protection Engineers, 2002, pp [8]. J.L. Torero, A.C. Fernandez-Pello, Natural convection smolder of polyurethane foam, upward propagation, Fire Safety Journal 24 (1995) [9]. J.C. Jones, Thermal calculations on the ignition of a cotton bale by accidental contact with a hot particle, Journal of Chemical Technology and Biotechnology 65 (1996) [10]. K.N. Palmer, Smouldering combustion in dusts and fibrous materials, Combustion and Flame 1 (1957) [11]. BADR, O., AND KARIM, G. A. Experimental-study of self-ignition and smoldering of moist cellulosic materials. Journal of Energy Resources Technology- Transactions of the Asme 114, 2 (1992), [12]. CHAN, T. K., AND NAPIER, D. H. Smouldering and ignition of cotton fibres and dust. Fire Prevention and Technology 4 (1973),