| Soot causes environmental pollution and inhalable particles seriously endanger human health.In the combustion process,soot has an important influence on the flame shape and heat transfer characteristics.Buoyanct turbulent combustion is a common combustion form.Exploring the relationship between the soot characteristics,combustion characteristics and fuel properties has important guiding significance for understanding the combustion process and soot emission control.In this paper,propane/nitrogen blends are employed as fuel and fuel properties are changed by varying propane fraction.The soot characteristics are explored by using laser-induced incandescence developed in recent years.The dependence of soot property and combustion characteristics on fuel property is revealed,and the corresponding prediction model is established based on theoretical analysis.1.Study on soot sheet characteristics of buoyant propane/nitrogen turbulent diffusion flameA laser-induced incandescence system was set up.The incandescence signal calibration experiment was carried out.The result was applied to the study of soot sheet characteristics of propane/nitrogen buoyant turbulent diffusion flame.The quantity,shape and spatial distribution characteristics of soot sheet were studied by equivalent treatment of soot sheet.The results show that the number,area,location and soot soot volume fraction sheet are all related to propane fraction.With the decrease of propane fraction,the number of soot sheet in flame decreases and its distribution changes.Meanwhile,the average values of the long and short axis of the equivalent ellipse decrease with the decrease of propane fraction at lower altitudes,while the values at higher positions show the opposite trend.The soot sheet moves away from the flame axis with the decrease of propane fraction as a whole,and the soot volume fraction in the soot sheet decreases gradually with the decrease of propane fraction.In the flame,the probability density function of long-axis of equivalent ellipse presents a double-peak shape,and that of the short-axis presents a single-peak shape.This indicates that the soot sheet mainly exists in two states,i.e.,small scale and long but narrow scale.The propane fraction has little effect on the shape of the probability density function of soot sheet but affects the value of the function.2.Study on soot volume fraction characteristics of buoyant propane/nitrogen turbulent diffusion flameSoot volume fraction characteristics of propane/nitrogen buoyant turbulent diffusion flame were studied using laser-induced incandescence.The results show that propane fraction nearly has no effect on the distribution of the parameters of soot volume fraction,but only affects the values of some characteristic parameters.The intermittency of turbulent flame makes the mean soot volume fraction lower than the instantaneous value,and both values decrease with the decrease of propane fraction.The mean soot volume fraction has the same distribution pattern as the root mean square.The propane fraction can affect the values near the axis,but has little effect on the distant position.The intermittency of soot increases with the decrease of propane fraction,which greatly affects the mean value and root mean square of soot volume fraction,indicating that nitrogen reduces the probability of soot occurrence.The fluctuation intensity of soot volume fraction is independent of propane fraction,and has no obvious relationship with the mean and root mean square of soot volume fraction.The propane fraction has little effect on the distribution of the probability density function of soot volume fraction,but only on the value of the function.The normalization probability is only related to the soot volume fraction,but not to the position and the propane fraction,and conforms to the e-exponential decay model.The total soot volume in flame decreases with the decrease of propane fraction.Based on the theory of soot formation,a dimensionless model for the soot volume fraction and fuel properties is derived.3.Study on flame features and heat transfer characteristics of buoyant propane/nitrogen turbulent diffusion flameThe relationship between propane fraction and flame appearance,flame pulsation,temperature and flame radiation fraction of propane/nitrogen flame was studied.The results show that when the heat release rate is constant,the total flame height and flame diameter have no obvious relationship with propane fraction,while the blue flame height increases with the decrease of propane fraction.The soot free length fraction is introduced to quantitatively analyze the blue flame height.On the basis of the soot volume fraction model,a dimensionless prediction model of the soot free length fraction is established.Flame pulsation frequency is influenced by propane fraction.An empirical formula of flame pulsation frequency is obtained by fitting the experimental data based on two dimensionless numbers.The formula is also compared with previous research.With the decrease of propane fraction,the flame temperature at the lower part of the flame decreases,while the temperature at the upper part of the flame and the plume increases.This is due to the two competing effects of nitrogen on the temperature.With the decrease of propane fraction,the soot production decreases resulting in the decrease of flame radiation fraction.Based on the theory of soot formation and radiation heat transfer,a dimensionless prediction model of flame radiation fraction was established. |
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