Peter L. Kelly-Zion, PhD , Trinity University
Christopher J. Pursell, PhD , Trinity University
The overall goal of this project is to experimentally investigate the thermal and mass transport processes and film instability of evaporating hydrocarbon films. Film evaporation involves a complex coupling between thermal and mass diffusion and the bulk flow (convection) of both the liquid and vapor phases. Film instabilities may induce internal flows in the film and increase the interfacial surface area available for evaporation. Through systematic experimentation during which the relative influences of the transport processes and fluid motion are controlled, this project will provide fundamentally new understanding of the transport behavior involved in the evaporation process and provide much needed experimental data for evaluating numerical and analytical models.
We were engaged in three main investigations during the 2010-2011 reporting period. All three investigations focused on the vapor transport from the surface of evaporating hydrocarbon drops.
One investigation analyzed the effect of drop size on the evaporation rates of sessile drops and compared measured evaporation rates to rates predicted by published models of diffusion-limited evaporation. According to diffusion-limited models of sessile drop evaporation, the evaporation rate is proportional to the radius of the three-phase contact line. In contrast, our measurements of the evaporation rates of four hydrocarbons having a wide range of volatility and molar mass indicate that the evaporation rate is enhanced by a second term that is proportional to the radius to the power of 1.6. Based on some of our previous research, we attribute the higher evaporation rate to the effect of natural convection of the vapor. We developed an empirical correlation for the evaporation rate of sessile drops under conditions of combined diffusive and convective transport of the vapor.
To better understand the influence of natural convection of the vapor on the evaporation of sessile drops, we conducted a series of experiments for which the magnitude of the natural convection was moderated by controlling the ambient gas density. Natural convection is driven by a difference in densities. In the case of an evaporating drop, it is the difference between the density of the vapor-air mixture at the surface of the drop and the ambient air that is important. The experiments were conducted in a pressure chamber and the air density was controlled by controlling the pressure over the range from 1 to 6 atm. At high air densities, the relative density difference is low and for these conditions our measurements suggest a reduced level of natural convection occurs, in accordance with our hypothesis regarding the role of natural convection.
The third major investigation involved the measurement of the vapor concentration distribution above an evaporating hexane drop. Our goal is to obtain data that will enable us to compute the rate of vapor transport by diffusion, which is proportional to the gradient of the concentration. Once the rate of diffusion is computed, the balance between that rate and the measured evaporation rate can be attributed to convection. In this way, the roles of diffusion and convection can be determined quantitatively. The measurements of the vapor concentration are being obtained using infrared spectroscopy combined with computed tomography. Figure 1 shows the distribution of hexane vapor above an evaporating drop of radius 6.5 mm. Due to axial symmetry, the distribution over half of the drop is presented. The measurements indicate that the geometry of the vapor layer that surrounds the drop is approximately 5 mm thick and extends to a radius of over 20 mm. The fact that the vapors extend more prominently in the radial direction than in the vertical direction suggests that the vapors are being convected radially away from the drop in addition to diffusing away.
Figure 1. Measured concentration distribution of hexane vapor above an evaporating drop.
Impact of the Project
This research program continues to have a very large impact on the participating students. During the last year, four undergraduate students participated in the research and learned valuable lessons not only about the physical phenomena we are studying but, more importantly, about how to go about conducting research, i.e. how to think about problems, how to design and conduct experiments, and how to analyze measurement results. Two of the students devoted 10 weeks during the summer to the research and thereby gained an intensive research experience.
For the principal investigator, this PRF supported research is his primary research focus and, therefore, is very important for his continuous development as a researcher and academician.
In addition to the direct impact that the project has on students and the principle investigator, the project is very beneficial to Trinity University's Engineering Science Department, which is small and completely undergraduate. The research activity supported by PRF has had the great effect of helping to raise the Department's scholarly profile.