Abstract
This work provides the quantitative analyses of droplet evaporation and ignition. Many droplet evaporation and ignition experiments are performed in a furnace and the droplet is supported by a fiber to avoid the experimental difficulties associated with free-falling droplets. In such arrangements, the droplet obtains additional heat transfer from the support fiber and radiation from the furnace wall. First, we investigate the effect of support fiber conduction on droplet evaporation in a weakly convective flow experimentally and theoretically. Experimentally, n-heptane or n-hexadecane droplet with initial diameter of 700 mm or 1000 mm was suspended at the tip of a horizontal or vertical quartz fiber (diameter of 50 mm, 150 mm, or 300 mm) to evaporate in an upward hot gas flow (temperature of 490 K or 750 K) generated by a flat-flame burner or an electrical heater. For all the tests, the droplet Reynolds number ranged from 5 to 17. Theoretically, a simple one-dimensional transient conduction model is formulated in combination with a transient droplet evaporation model which accounts for the heat and mass transfer between the droplet and the ambience with the film theory. The calculations agree well with experiments for all the droplet diameter histories measured, with the effect of fiber conduction on the evaporation rate correctly predicted. In general, fiber conduction leads to enhancement of evaporation, with stronger effect for a lower gas temperature and a larger fiber. However, the total heat inputs are attenuated for fiber diameter of 300 mm. Fiber orientation effect appears negligible. Also found is that the evaporation rate is enhanced in an oxygen-containing gas flow due to the additional oxidation heating around the droplet. Secondly, fiber conduction and liquid-phase radiative absorption are considered in the comprehensive simulation of droplet evaporation under microgravity for pressures up to 20atm. For droplet size variation and evaporation rate constants, good agreement is found between our calculations and the experimental data of Nomura et al. (1996). Radiative absorption and fiber conduction enhance the evaporation rate significantly. Our results indicate that the discrepancy between current theoretical and experimental results is because these models ignored the conduction into the droplet through the fiber and the liquid-phase absorption of the radiation from the furnace wall. These effects existed since all the experiments for microgravity droplet evaporation have been conducted in a hot furnace with the droplet suspended by a fiber. At a low temperature of 470K, the discrepancies are mainly due to the additional fiber conduction, while at a high temperature of 750K, the liquid-phase radiative absorption becomes mainly responsible. Thirdly, we investigate autoignition of n-heptane droplets under microgravity numerically. Our model includes the transience in both the gas and liquid phases, non-ideal thermophysical properties, and the 116-step heptane reaction mechanism of Griffiths. Two-stage ignition manifests for ambient temperature less than 900 K at elevated pressures of 0.5 and 1.0 MPa. The predicted first delays and total delays agree well with the experimental data in the literature. The second delay decreases greatly with increasing pressure because the cool flame shifts closer to the droplet and the evaporation strengthens to yield a stronger Stefan flow supplying more fuel vapor for reaction. The Stefan flow effect, in combination with the inhomogeneous temperature and fuel vapor distributions, explains why the NTC present in homogeneous mixtures is not observed in droplet ignition experiments. For a droplet smaller than the minimum ignition diameter, only first ignition with cool flame is reached. The absence of ZTC in our simulations may be attributed to the weaker inverse temperature dependence of the reaction mechanism adopted.