Digital holographic interferometry investigation of liquid hydrocarbon vapor cloud above a circular well

The current study investigates evaporation of liquid hydrocarbons from a circular well cavity of small depth. Gravimetric analysis is performed to measure the evaporation rate and digital holographic interferometry is used for the measurement of normalized mole fraction profile inside the vapor cloud above the well. Phase unwrapping has been implemented to obtain continuous phase distribution in the image plane. The Fourier-Hankel tomographic inversion algorithm is implemented to obtain the refractive index change distribution inside the object plane, i.e., vapor cloud. Four liquid hydrocarbons, i.e., pentane, hexane, cyclohexane, and heptane, are studied. The radius of circular well cavities is varied in the range of 1.5 to 12.5 mm. Results using a quasi-steady, diffusion-controlled model are compared with the experimental evaporation rate. Measured evaporation rates are higher than the diffusion-limited model calculation for all working fluids and well sizes. This difference is attributed to natural convection occurring inside the vapor cloud due to the density difference between the gas-vapor mixture and the surrounding air. Holographic analysis confirms the presence of natural convection by revealing the formation of a flat disk-shaped vapor cloud above the well surface. Experimentally obtained vapor cloud shape is different from the hemispherical vapor cloud obtained using the pure diffusion-limited evaporation model. The gradient of vapor mole fraction at the liquid-vapor interface is higher compared to that of the diffusion-limited model because of the additional transport mechanism due to natural convection. Transient analysis of the vapor cloud reveals time invariant overall shape of the vapor cloud with a reduction in average magnitude of vapor concentration inside the vapor cloud during evaporation. The existing correlation for sessile droplet cannot successfully predict the evaporation rate from a liquid well. A new correlation is proposed for evaporation rate prediction, which can predict the evaporation rate within a root mean square error of 5.6% for a broad size range of well cavity.