Guangwen Xu

Inverse influence of initial diameter on droplet burning rate in cold and hot ambiences: a thermal action of flame in balance with heat loss

Abstract Isolated droplet burning were conducted in microgravity ambiences of different temperatures to test the initial diameter influence on droplet burning rate that shows a flame scale effect and represents an overall thermal action of flame in balance with heat loss. The coldest ambience examined was room air, which utilized a heater wire to ignite the droplet. All other ambiences hotter than 633 K were acquired through an electrically heated air chamber in a stainless steel can. An inverse influence of initial droplet diameter on burning rate was demonstrated for the cold and hot ambiences. That is, the burning rate respectively decreased and increased in the former and latter cases with raising the initial droplet diameter. The reversion between the two influences appeared gradual. In the hot ambiences the burning rate increase with increasing the initial droplet diameter was larger at higher temperatures. A “net heat” of flame that denotes the difference between “heat gain” by the droplet and “heat loss” to the flame surrounding was suggested responsible for the results. In low-temperature ambiences there is a negative net heat, and it turns gradually positive as the ambience temperature gets higher and the heat loss becomes less. Relating to luminous flame sizes and soot generation of differently sized droplets clarified that the flame radiation, both non-luminous and luminous, is determinative to the net heat in microgravity conditions. In addition, the work identified two peak values of soot generation during burning, which appeared respectively at the room temperature and at about 1000 K. The increase in ambience temperature made also bigger soot shells. The heat contribution of flame by both radiation and conduction was demonstrated hardly over 40% in the total heat required for droplet vaporization during burning in a hot ambience of 773 K.

Burning droplets of heavy oil residual blended with diesel light oil: Distinction of burning phases

This work succeeds a previous report in Combustion Science and Technology (Vol. 174, pp. 115-145, 2002), and aims at distinguishing between the liquid and solid phases of burning heavy fuel oil (HFO) droplets in terms of burning times. The durations of the two burning phases were determined from the previously measured component times for burning steps that prevailed inburning droplets of blend oils between a heavy oil residual (HOR) and adiesel light oil (LO). A hot-air chamber was employed to acquire the high-temperature conditions of droplets occurring in actual burners. The result showed that the duration of the solid phase is generally larger than, and in some particular cases it is comparable to, that of the liquid phase. As for the examined blend oils, the pure HOR exhibited comparable phase durations, while the other blend oils had longer durations for the solid phase than for the liquid phase. Under the tested conditions, the durations of both phases tended to decrease with increasing LO fraction and ambience temperature and decreasing the initial droplet diameter. For typical HFOs, such as those equivalent to the blend oils with LO fractions greater than 26 wt. %, the relative ratio of durations between the solid phase and the liquid phase appeared weakly dependent on oil composition, temperature, and droplet size. This resulted in an average ratio of about 1.3 and a corresponding fluctuation from 1.0 to 1.6. The pure HOR slightly deviated from such values, however, manifesting an average of 1.0 and a fluctuation of 0.8 to 1.3. Furthermore, the HOR amount in the oil droplets determined the duration of the solid phase, whereas the initial droplet diameter was linearly correlated with the duration of the liquid phase. This work finally compared the ember times for burning out soot and coke particles, which further verified the dominance of coke burning over the solid phase.

Burning droplets of heavy oil residual blended with diesel light oil: Characterization of burning steps

Highlighting the burning process of heavy oil droplets resulted in the resolution of the whole process into four burning steps that in turn defined four component times (in succession): the ignition delay, flame lifetime, coke glowing delay, and coke ember time. The present work was devoted to measuring the component times in an atmospheric hot-air chamber, and to further analyzing their constitutional characteristics with respect to oil compositions and burning conditions by using hybrid oils blending a heavy oil residual (HOR) and a diesel light oil (LO) at different fractions to simulate the diversified heavy oils. The four burning steps generally prevailed during burning, but their time durations relative to the total burning time varied with oils, temperatures, and droplet sizes. The increase in the LO fraction considerably reduced the ignition delay and coke ember time but little affected the flame lifetime and coke glowing delay. Notwithstanding, the ratios of component times to total burning times appeared independent of the LO fraction when the fraction was definitely higher, such as greater than 30 wt.%. Increasing the chamber temperature much decreased the ignition delay and coke glowing delay but little changed the flame lifetime. This led the coke ember time and flame lifetime to become dominant in the total burning time at higher temperatures. The initial droplet diameter influenced the component times and their relative values, but the influence appeared secondary to the effects of oil composition and chamber temperature. With these results, the article suggested that the burner designs for different heavy oils should focus on the oil-induced differences in fuel ignition and coke burnout rather than in volatile burn-up.

Kinetics of rapid coal devolatilization measured using a spot heater apparatus

Kinetic analysis of rapid coal devolatilization in a spot heater apparatus was made for Tatong coal. A multiple-stage measuring technique was devised to provide the change of total volatiles with time/temperature during the devolatilization. On the basis of the analyses of the total volatiles yield and measured temperature of coal particle, the kinetic parameters of the rapid devolatilization for Tatong coal were estimated by applying Distributed Activation Energy Model (DAEM).

Interacting droplet combustion under conditions of extinction

This paper presents the results of an experimental investigation into the extinction behavior of single droplets and binary droplet arrays of decane fuel in normal gravity and microgravity. The test ambients were air at reduced pressure for the normal gravity experiments and nitrogen/oxygen and helium/oxygen at reduced pressure for the microgravity experiments. The data show that interaction effects have a stronger influence on the extinction behavior of a binary array than the quasi-steady burning behavior. In normal gravity, the burning rate constant is only a very weak function of the interdroplet spacing at initial nondimensional separation distances greater than 5. The extinction droplet diameter, however, is much smaller (if it exists) for the droplet array at an instantaneous non-dimensional spacing of approximately 20. The microgravity experiments in nitrogen/oxygen (0.15 oxygen mole fraction) ambients exhibited different extinction behavior than the normal gravity tests. In this ambient condition, the flame surrounding the binary array always extinguished at a larger droplet diameter compared with single droplets. The microgravity experiments in the helium/oxygen (0.25 oxygen mole fraction) ambients exhibited behavior opposite of the nitrogen diluted tests with the binary array extinguishing at smaller droplet diameters than a single droplet. For the microgravity experiments, the observed difference between the two ambients is attributed to the importance of radiative loss. The nitrogen/oxygen tests had larger, weaker flames and smaller burning rates and consequently were affected more significantly by radiative loss from the flame zone compared to the helium/oxygen tests. A qualitative explanation of the different extinction behavior is detailed in the paper.