Study of Single Levitated Fuel Droplet Vaporization Process under Monochromatic Irradiation using IR-Thermography and High speed imaging

Liquid fuels are still the primary power source due to its higher energy density and ease of operation compared to gas fuels. Thus a detailed understanding of the spray vaporization and combustion characteristics for various liquid fuels are very important, down to a single droplet. Owing to difficulties in performing droplet level investigation in an actual spray, the spray models are mostly developed and validated by single pendant droplet experiment. However, the pendant-droplet technique has problems including heat conduction loss through the wire which leads to considerable experimental errors. In order to eliminate the wall-droplet effects, current work uses an acoustic levitator to suspend a single droplet. To understand the importance of the thermo-physical properties of fuel droplet on its vaporization process, a CO2 laser (irradiating at 10.6 μm with a beam diameter of 2mm) has been used as a radiative heating source in current experimental study. The CO2 laser is synchronized with an IR camera (FLIR) and a high speed camera (phantom V12). An IR camera is used to measure the droplet surface temperature variation with time, while a high speed phantom camera is used to accurately capture the droplet size variation with time. A number of different liquid droplets are tested, including water, ethanol, kerosene and diesel. A Parametric study has been done on the effect of heating rate, while the initial droplet size was maintained to be 500μm (±30μm). Results show that ethanol vaporizes faster compared to other liquids due to its high vapor pressure. Theory on vaporization shows that the vaporization rate is highly dependent on the vapor pressure. But the temperature of the droplet also plays an important role as the vapor pressure of a fuel changes with the temperature. Total amount of absorbed heating energy can be divided into two parts. The part responsible for phase change depends on the vaporization rate and value of latent heat of vaporization. The remaining part dictates the rate of temperature rise of the droplet. This shows that the vaporization rate and temperature change of the droplets are inter-related and heavily dependent on vapor pressure, latent heat of vaporization and specific heat. We found that at relatively high laser power heating, kerosene and diesel exhibit interesting phenomenon of stretching and shattering due to their small surface tensions. [email protected]