Study On Combustion Characteristics And Flame Morphologies Of Gas Jets At Subatmospheric Pressures

Fires at subatmospheric pressures bring mankind new problems and challenges with more activities in plateau and sky. The fire prevention technology at high altitudes is becoming increasingly important, therefore, studying fire burning characteristics under low pressures is of important practical significance. According to previous studies, the condition of the low pressure environment distinguishing normal atmospheric environment is the reduction of air density and partial pressure of oxygen. Buoyancy effect and soot production are largely influenced by air density and partial pressure of oxygen respectively. Buoyancy is closely related with flame flickering and flame morphology and Froude number is generally used to characterize the role of buoyancy, while soot relates to combustion efficiency, flame shape, and smoky tendency. Therefore, buoyancy and soot were chosen as the paths when studying effects of subatmospheric pressure on combustion. Besides, gaseous fuels were selected as research objects since their mass loss rates are equal under different pressures.Natural environment at high altitude and artificial environment in hypobaric chamber are the two main methods to achieve subatmospheric pressures. Natural environment are suitable for conducting large scale fire experiments although it is expensive and difficult. Comparative experiments were implemented in each cone calorimeter in Lhasa and Hefei. Gas analyzer, thermocouple tree, radiometer, and kapnometer were employed to measure combustion products, fire plume temperature, radiation flux, and extinction coefficient respectively. A simplified thermochemistry based on the measured concentrations of O2and CO2was used when calculating heat release rates. In view of distinct macroscopical phenomena at two different altitudes, we continued experiments in a hypobaric chamber for studying combustion mechanism under multilevel low pressures. A combined normal video and the CH filtering technique was employed in the flame image recording to facilitate determining the flame shape and smoke point. The flame images of flickering flames were recorded by a480fps high-speed camera, and Fast Fourier Transform were carried out to measure flickering frequency.Fire behaviors such as heat release rate and radiation flux of three gaseous hydrocarbon fuels were experimentally measured in each cone calorimeter at two different altitudes in Lhasa and Hefei. The effects of subatmospheric pressure on combustion efficiency and radiation fraction were further revealed when summarizing results of methane, acetylene, and propane fires. The heat release rates, fire plume temperatures, and smoke transmittances at the lower pressure are higher while the radiation fluxes are lower. Hence, the combustion efficiencies at the lower pressure are higher and the radiation fractions and smoke production rates are lower.The flickering behavior was studied for methane, ethylene, and propane flames at eight subatmospheric pressure levels. The relationship between flickering frequency and pressure and the influence of exit velocity on frequency were highlighted. Generally, the flickering behaviors were observed falling into three regimes, i.e. tip flickering, intermittent flickering, and continuous flickering. The continuous flickering appears when the fuel flow rate or pressure is increased above a particular value. The flickering frequency is found insensitive to fuel type or flow rate. The observed frequencies generally vary from about8Hz at0.3atm to12Hz at1.0atm. The logarithmic relationship between the measured frequency and pressure is close to the previous theoretical analysis. In addition, an important phenomenon of multiple frequencies was observed in the tests.The luminous shapes were presented to examine the scaling of flame heights and widths of laminar stable flames burning methane, ethylene, and propane in a subatmospheric pressure chamber. The linear Reynolds scaling was generally suitable for hydrocarbon fuels although slopes were changing under the combined action of buoyant and soot. The buoyant was unimportant in determining flame height when under the largest Fr or at0.3atm. Oppositely, at the smallest Fr, buoyancy dominant tendency was reached. In Froude scaling of width, the unchanged slopes between normalized flame width and Fr indicated a good linearity relationship.The luminous shapes were measured for laminar stable non-smoky flames burning methane, ethylene, and propane under ambient pressures of0.02-0.1MPa. Diverse changing trends happen under various fuel flow rates and different pressure ranges by accounting micro flames at subatmospheric pressures and results over smoke points in references. First, under conditions of relatively higher fuel flow rates and pressure ranges, flame heights follow an increasing-decreasing trend. Second, a decreasing-increasing trend can be concluded under relatively lower fuel flow rates and pressure ranges. Third, a decreasing-increasing-decreasing trend will happen under proper fuel flow rate and pressure range.Experimental study on smoke point of acetylene laminar jet diffusion flame was performed in a chamber with subatmospheric pressures of0.03-0.1MPa. The relationships between smoke point flame height, fuel mass flow rate, residence time and pressure are summarized as follows:The smoke point flame height was observed to vary with pressure in the same pattern as ethylene and methane flames at elevated pressures. The scaling of the smoke point fuel mass flow rate with the pressure to a power law with a negative exponent is qualitatively consistent with results found in most earlier works. The residence time at smoke point has been found to increase with decreasing pressure over a wide pressure range.