Session: Rising Stars of Mechanical Engineering Celebration & Showcase

Paper Number: 150461

150461 - Using Combined 3d Velocimetry and Lattice Boltzmann Method to Investigate the Wicking Flow on Micro-Engineered Surfaces 

Computers and electronic devices have become an indispensable part of everyday life. Increasingly high computing power is needed to satisfy our needs related to daily activities, such as online shopping, communicating, autonomous driving and more. Unfortunately, one of the major bottlenecks that limit the performance of many electronic devices is the high heat fluxes that are produced at the heart of those devices. Therefore, effective thermal management technologies are needed to keep the electronics cool at a safe temperature while sustaining high performance. Thin film evaporation, where very thin films of coolant is used to cool heated solid surfaces by continuous evaporation, has emerged as a promising cooling strategy. Compared with other alternatives, thin film evaporation offers numerous advantages including simple design, high cooling capacity and high stability. Moreover, by incorporating micro/nanostructures such as micro-pillars and nanoscale pores, its cooling capacity can be further improved but also eliminates the need for external pumping. However, currently thin film evaporation is far from being optimized. This in a large part is due to a lack of comprehensive understanding of the contact line dynamics at the evaporating surfaces, which hinders our ability to provide reliable prediction and design guidelines. It is also widely accepted that the lack of detailed flow and heat characterizations of the evaporating surfaces at both microscopic and nanoscopic scales has become a major limitation in further improving thin film evaporation.

 In that regard, this work aims to perform a fundamental study of the multiphase flow dynamics during the flow wicking process on micro-engineered surfaces using combined experimental and numerical techniques. A series of micro-engineered wicking structures will be designed and fabricated using microfabrication techniques. The 3D flow fields will be captured employing an advanced velocimetry technique, called astigmatism particle tracking velocimetry (APTV). In addition to in-plane tracking of tracer particles like other techniques, APTV also leverages astigmatism, an optical aberration induced by a cylindrical lens inserted between the camera and the microscope, to determine the z coordinate by the shape of a particle image. With our current setup, particles can be located with an accuracy of 300nm in all three coordinates, which is suited to measurements even within sub-micron films. Moreover, the flow will be numerically modeled using the lattice Boltzmann method (LBM), to explore a larger parameter space that is not easily accessible with experiments, as well as test the effect of various physical processes. The flow field for isothermal, incompressible multiphase fluid systems will be solved using a phase-field LBM which consists of two main parts. The first is a lattice Boltzmann equation (LBE) for tracking the interface between two different fluids according to the so-called conservative phase-field equation. The second part of the method is a pressure-evolution LBE for recovering the macroscopic equations, including conservation of mass and conservation of momentum (Navier-Stokes). Good agreement has been achieved with the literature and available analytical solutions for benchmark problems including the capillary rise problem and the rising bubble problem, which validates our methods. Our preliminary results reinforce the crucial importance of the 3D velocity profile due to no-slip boundary condition at the bottom and side walls to accurately model the wickability. Moreover, the study also highlights the crucial role played by the leading corner film flows during the wicking process. These results will provide valuable insight into the underlying physics, provide validation for current models and improve our ability to develop new designs for thin film evaporation.

Presenting Author: Yaofa Li University of California, Riverside

Presenting Author Biography: Dr. Yaofa Li is an Assistant Professor in the Department of Mechanical Engineering at the University of California, Riverside (UCR). Prior to joining UCR, he was an Assistant Professor at Montana State University. He obtained his Ph.D. from Georgia Tech in 2015 and completed his postdoctoral training at the University of Notre Dame, where his research was jointly supported by Notre Dame and the energy frontier research center, GSCO2. Dr. Li’s research interest focuses on developing experimental, numerical, and theoretical studies of thermal-fluid problems primarily at microscopic scales with applications in multiphase flow, biological flow, and phase change heat transfer. He is a recent recipient of the Faculty Excellence Award (MSU 2021, 2023), the Doctoral New Investigator Award (ACS, 2021), the NASA EPSCoR R3 Award (2022) and the NSF Career Award (2022).

Authors:

Zeeshan Ahmad Khan University of California, Riverside
Arpan Ghimire Bohara University of California, Riverside
Mahedi Hassan Montana State University
Yaofa Li University of California, Riverside