Discipline: Technology and Engineering
Subcategory: Materials Science
Robert Stephenson - University of the District of Columbia
Co-Author(s): Jiajun Xu, University of the District of Columbia, Washington, DC
Effective thermal management in various engineering systems is a critical issue, in which utilizing nucleate boiling to enhance heat transfer has attracted particular attention. Nanoemulsion is of specific interest because of its high heat flux control capability. However, nucleate boiling is a complex process that still requires more understanding. On one side, researchers have been relying on speculative hypotheses for decades to understand nucleate boiling heat transfer, which is a generally highly empirical and over simplified practice. On the other side, there is still disagreement on fundamental questions like: how nucleation occurs at the liquid–vapor interface for fluids with very low contact angles, and what are the physical mechanisms triggering critical heat flux etc. There is an urgent need to collect data that enables detailed measurements of the phase, temperature, and velocity distribution during nucleation. In this study, a combination of synchronized high-speed video (HSV) and infrared (IR) thermography was used to characterize the nucleation, growth and detachment of bubbles generated during nucleate boiling. In addition, nanoemulsion was used in the current study, where nanosized phase changeable droplets were formed inside the nanoemulsion and served as the boiling nuclei. With this unique combination, it allows controlled nucleation, time-resolved temperature distribution data for the boiling surface and direct visualization of the bubble cycle. Cycle data gathered included measurements of the change in bubble geometry vs. time, bubble departure frequency, along with bubble wait and growth times. 2D temperature history of heater surface and velocity distribution within the liquid surrounding the bubbles were also recorded utilizing the synchronized HSV / IR method. Findings demonstrate a significant increase in the heat transfer coefficient and critical heat flux of nanoemulsion compared to conventional heat transfer fluids. It is also observed that the bubbles occurring inside the nanoemulsion appear to be larger than average size and uniform in shape. Using the HSV and IR data, characterization of the growth rate and interfacial temperature distribution of the bubbles inside the nanoemulsion were possible. the growth rate of the bubbles inside conventional fluids agree well with the classical Rayleigh-Plesset equation resulting in a coefficient of ½, however, the coefficient drops to be 1/4 for the nanoemulsion. Future research involves more data on the effect of different phase changeable droplets, interfacial material and structures that may help explain the unique nucleation process of nanoemulsion.
Funder Acknowledgement(s): Funding was provided by an NSF/HBCU-UP RIA
Faculty Advisor: Jiajun Xu, firstname.lastname@example.org
Role: I handled procurement of critical equipment needed to conduct observations such as custom fabricated view-port for mounting ITO boiling surface, and custom fabricated flange for electrical inputs and outputs. After the procurement process I assembled the complete observation chamber along with installation of measurement, power, and heating instruments. After assembly and testing for operability, I rewired and synchronized the high speed and Infrared cameras to ensure accurate recording. Along with camera synchronization, I edited the video capture software script to activate all cameras recording simultaneously. I analyzed videos to extract bubble cycle measurements and created a database for measurement data storage. I assisted in data analyses and characterization of the bubble cycle.