Discipline: Technology & Engineering
Subcategory: STEM Research
- University of the District of Columbia
Co-Author(s): James McLaurin, Cyree Beckett and Robert Stephenson, University of the District of Columbia, Washington, D.C.
Effective thermal management in various engineering systems is a critical issue, in which utilizing nucleate boiling to enhance heat transfer has attracted particular attention because its capability to remove high heat flux. 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. So 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 current study, in which 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 to track bubble nucleation and growth. Data gathered included measurements of bubble size and shape vs. time, bubble departure frequency, wait and growth times, as well as 2D temperature history of the heater surface and velocity distribution within the liquid surrounding the bubbles. Our findings demonstrate a significant increase in heat transfer coefficient and critical heat flux of nanoemulsion compared to conventional heat transfer fluid. It is also observed here that the bubbles occurred inside the nanoemulsion appear to be more uniform and larger in size. Using the HSV and IR data, we were able to characterize the growth rate and interfacial temperature distribution of the bubbles inside nanoemulsion: the growth rate of the bubbles inside conventional fluid agrees well with classic Rayleigh-Plesset equation with a coefficient of ½, which however, drops to be 1/4 for nanoemulsion. Future research involves more data on the effect of different phase changeable droplets, interfacial material and structures may help explain the unique nucleation process of nanoemulsion
Funder Acknowledgement(s): Current work is financially supported by National Science Foundation HBCU-UP RIA grant under grant No. HRD-1601156; Research performed in part at the NIST Center for Nanoscale Science and Technology in Gaithersburg, MD; Dr. Lei Chen and Dr. Gerard Henein
Faculty Advisor: None Listed,