DISEÑO DE UN EXPERIMENTO MODELO PARA ESTUDIAR LA IGNICIÓN ESPONTANEA EN VEGETACIÓN TIPICA DE CHILE

Abstract

This paper develops the design of a experimental model to study the spontaneous ignition of typical chilean vegetation. This in the context of the spread of forest fires, where the spontaneous ignition plays an important role in the “Spotting” process of the spread, creating new fire source at great distances from the initial fire source. The above is based on the three main Mechanisms of Heat Transfer; Conduction, Convection and Radiation, with main focus on the radiation, and a thermal model which explains the heat transfer within the vegetation, which takes into consideration the depth (δ) to which penetrates the heat into the sample of vegetation. The design is based on both input factors of the experiment (Incident Radiation, Bulk Density, Radio of the Heater, Radio of the Cylinder free of vegetation, Moisture fraction of the Sample, Heater’s Temperature) and the Response Variables to be studied (Time of Ignition, Mass loss rate, Temperature inside the layer). The experimental setup consists of a Power supply device, a system of data acquisition, digital Multimeter, Thermocouples type K, an Analytical Balance, a Ceramic Heater (which is responsible for irradiating the vegetation sample) and a structure built with Thorlabs (brand) pieces, allowing the disposition of the devices in order to be able to do the registration of the measurements. Measurements are made on a representative wildland litter contained in a glass container of 125 mm diameter and 50 mm height, leaving a cylinder shaped space free of vegetation in the center with 40 mm diameter, where the heater is located. The characterization of the vegetation under study is done by calculating the Density, Bulk Density, Porosity and Surface Volume Ratio. It is defined the methodology to understand the Heater behavior, to standardize the measurements of the response variables and to This paper develops the design of a experimental model to study the spontaneous ignition of typical chilean vegetation. This in the context of the spread of forest fires, where the spontaneous ignition plays an important role in the “Spotting” process of the spread, creating new fire source at great distances from the initial fire source. The above is based on the three main Mechanisms of Heat Transfer; Conduction, Convection and Radiation, with main focus on the radiation, and a thermal model which explains the heat transfer within the vegetation, which takes into consideration the depth (δ) to which penetrates the heat into the sample of vegetation. The design is based on both input factors of the experiment (Incident Radiation, Bulk Density, Radio of the Heater, Radio of the Cylinder free of vegetation, Moisture fraction of the Sample, Heater’s Temperature) and the Response Variables to be studied (Time of Ignition, Mass loss rate, Temperature inside the layer). The experimental setup consists of a Power supply device, a system of data acquisition, digital Multimeter, Thermocouples type K, an Analytical Balance, a Ceramic Heater (which is responsible for irradiating the vegetation sample) and a structure built with Thorlabs (brand) pieces, allowing the disposition of the devices in order to be able to do the registration of the measurements. Measurements are made on a representative wildland litter contained in a glass container of 125 mm diameter and 50 mm height, leaving a cylinder shaped space free of vegetation in the center with 40 mm diameter, where the heater is located. The characterization of the vegetation under study is done by calculating the Density, Bulk Density, Porosity and Surface Volume Ratio. It is defined the methodology to understand the Heater behavior, to standardize the measurements of the response variables and to VIII standardize the measurements of the studied factors. Finally the corresponding measurements are performed to validate the experimental model with samples of Radiata pine needles (density = 614,64 [kg/m3], bulk density = 37,23 [kg/m3], porosity = 0,939, surface-volume ratio = 3,68 [mm-1]), with maximum intensity of input voltage that can be delivered to the heater. It is obtained a value of emitted radiation of 29,69 [W], a value of incident radiation on vegetation of 23,89 [W] and spontaneous ignition times close to 290 seconds. Plots of the behavior of mass loss rate and temperature inside the layer during the process are shown.En este trabajo se lleva a cabo el diseño de un experimento modelo que permita estudiar la ignición espontánea de vegetación típica de Chile dentro del contexto de los incendios forestales, en donde la ignición espontánea juega un rol importante en la etapa de “spotting” de la propagación de incendios generando nuevos focos de fuego a grandes distancias de los focos de incendio. Todo esto basado en los tres principales mecanismos de transferencia de calor; conducción, convección y radiación, centrando el interés en la radiación, y en un modelo térmico que explica la transferencia de calor dentro de la vegetación, el cual toma en consideración la profundidad (δ) hasta la cual penetra el calor dentro de muestras de vegetación. El diseño se basa tanto en los factores de entrada del experimento (Radiación incidente, Densidad Aparente, Radio del Calentador, Radio del espacio libre de vegetación, humedad de la muestra, temperatura del calentador) y las variables de respuesta que se van a estudiar (Tiempo de ignición, Pérdida de masa y Temperatura al interior del estrato). El arreglo experimental consta de un potenciómetro, un sistema de adquisición de datos, un multímetro digital, termocuplas tipo K, una balanza analítica, un calentador cerámico de nitrato de silicio (que se encarga de irradiar la muestra de vegetación) y una estructura construida con piezas de marca Thorlabs la cual permite hacer la disposición de los aparatos de manera de poder llevar a cabo el registro de las mediciones. Las mediciones se realizan en una representación de un estrato vegetal contenida en un recipiente de vidrio de 125 milímetros de diámetro y 50 milímetros de altura dejando un cilindro libre de vegetación en el centro con un diámetro de 40 milímetros en donde se ubica el calentador. La caracterización de la vegetación que se quiere estudiar se hace a través del cálculo de la densidad, densidad aparente, porosidad y relación superficie volumen. Se define la metodología para conocer el comportamiento del calentador, para estandarizar las mediciones de las variables de respuesta y estandarizar las mediciones de los factores estudiados. Finalmente se realizan un par de mediciones, para validar el experimento modelo, con muestras de agujas de pino tipo radiata (densidad= 614,64 [kg/m3] , densidad aparente= 37,23 [kg/m3] , porosidad= 0,939, relación superficie volumen = 3,68 [mm-1]), con la intensidad máxima de voltaje de entrada que se le puede entregar al calentador, se tiene un valor de radiación emitida de 29,69 [W], un valor de radiación incidente en la vegetación de 23,89 [W] y tiempos de ignición espontánea cercanos a los 290 segundos. Se muestran también los gráficos del comportamiento de pérdida de masa y temperatura al interior del estrato durante el proceso.