Abstract
Droplet evaporation characterization, although of great significance, is still challenging. The recently developed phase rainbow refractometry (PRR) is proposed as an approach to measuring the droplet temperature, size as well as evaporation rate simultaneously, and is applied to a single flowing n-heptane droplet produced by a droplet-on-demand generator. The changes of droplet temperature and evaporation rate after a transient spark heating are reflected in the time-resolved PRR image. Results show that droplet evaporation rate increases with temperature, from −1.28×10−8 m2/s at atmospheric 293 K to a range of (−1.5, −8)×10−8 m2/s when heated to (294, 315) K, agreeing well with the Maxwell and Stefan–Fuchs model predictions. Uncertainty analysis suggests that the main source is the indeterminate gradient inside droplet, resulting in an underestimation of droplet temperature and evaporation rate. With the demonstration on simultaneous measurements of droplet refractive index as well as droplet transient and local evaporation rate in this work, PRR is a promising tool to investigate single droplet evaporation in real engine conditions.
Highlights
phase rainbow refractometry (PRR) was applied to droplet streams with a low sampling frequency, but impracticalDroplet heating and evaporation are coupled heat and to monitor the evaporation rate of a single droplet.mass transfer processes accompanying droplet combus- 55 Most of measurements of droplet evaporation rate tion, which is widely applied in liquid fuel powered de- were conducted under a constant droplet temperature.5 vices, such as gas turbines and internal combustion en- While in real spray combustion applications, droplets gines, etc
Mass transfer processes accompanying droplet combus- 55 Most of measurements of droplet evaporation rate tion, which is widely applied in liquid fuel powered de- were conducted under a constant droplet temperature
In ejection into a hot gas medium until its surface tempera- some extreme cases, droplets are subject to a transient ture approaches boiling temperature, and the 60 heat, i.e., spark heating in spark ignition (SI) engines droplet evaporation rate changes dramatically be- or flame heating when passing through a flame fron
Summary
PRR was applied to droplet streams with a low sampling frequency (less than twenty Hz), but impracticalDroplet heating and evaporation are coupled heat and to monitor the evaporation rate of a single droplet.mass transfer processes accompanying droplet combus- 55 Most of measurements of droplet evaporation rate tion, which is widely applied in liquid fuel powered de- were conducted under a constant droplet temperature.5 vices, such as gas turbines and internal combustion en- While in real spray combustion applications, droplets gines, etc. Droplet heating and evaporation are coupled heat and to monitor the evaporation rate of a single droplet. Mass transfer processes accompanying droplet combus- 55 Most of measurements of droplet evaporation rate tion, which is widely applied in liquid fuel powered de- were conducted under a constant droplet temperature. The droplet temperature increases upon its are heated up with the evaporation rate increasing. In ejection into a hot gas medium until its surface tempera- some extreme cases, droplets are subject to a transient ture approaches boiling temperature, and the 60 heat, i.e., spark heating in spark ignition (SI) engines droplet evaporation rate changes dramatically be- or flame heating when passing through a flame fron-. The evaporation rate influences droplet life-time, local rarely measured experimentally, due to the lack of propmixing and air-fuel ratio and eventually combustion [1– er tools. Based on this component [13, 14] to multicomponent [15,16,17,18,19] at s- 70 technique, the change of droplet temperature and trantandard conditions as well as elevated temperature and sient evaporation rate of a single isolated droplet under
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