Enhanced critical heat flux of flow boiling using post-columnar wick
Borumand, Mohammad. 2019. Enhanced critical heat flux of flow boiling using post-columnar wick -- In Proceedings: 15th Annual Symposium on Graduate Research and Scholarly Projects. Wichita, KS: Wichita State University
INTRODUCTION: Flow boiling has attracted significant interest due to its capability to dissipate high heat flux encountered in a wide range of industrial applications such as thermal management of electronics, petroleum transportation and various types of chemical reactors. A technical challenge lies in a poor Critical Heat Flux (CHF). The CHF is a heat transfer limitation in liquid-vapor phase-change heat transfer process, as the liquid supply is chocked by the excessive vapor near the heated surface. Thus, the heat transfer coefficient will significantly decrease when the heat flux exceeds the CHF, which in turn results in a sudden temperature rise of the heated surface and system burn out. The poor CHF is related to the hydrodynamic-instability near the surface. In this study, we examine enhanced CHF by tailoring the hydrodynamic-instability, i.e., increased liquid supply to the heated surface for effective evaporation. Here, we introduce post-columnar wick, i.e. high effective thermal-conductivity and capillary pressure monolayer wick with high permeability post columnar wick for efficient liquid spreading and evaporation. PURPOSE: We aim to develop a mechanistic model to predict enhanced flow boiling CHF of surfaces with columnar post-columnar wick structures. METHODS: Applying the conservation laws and considering the post-columnar wick hindrance for vapor layer axial flow, a separated flow model is developed which provides the phase velocities and average vapor layer thickness. Classical hydrodynamic instability theory that accounts for the increased pressure difference across the interface is then utilized to yield the critical wavelength. Finally, considering interfacial lift-off as the CHF trigger mechanism, the energy balance equation is developed over the entire heated surface to find the CHF. RESULTS: The developed model predicts the enhanced CHF by two times using post-columnar wicks compared to conventional plain surfaces. The model predictions for the plain surface agree the existing experimental data with reasonable accuracy over a wide range of inlet flow velocities. Also, the results for hydrodynamic instability wavelength provide insights into possible future work, aiming at enhancing flow boiling CHF by tailoring hydrodynamic instability wavelength. CONCLUSION: It is concluded that incorporation of post-columnar wick greatly enhances flow boiling CHF by controlling the hydrodynamic instability wavelength and enhancement of liquid supply to the heated surface by delaying surface dry-out.
Presented to the 15th Annual Symposium on Graduate Research and Scholarly Projects (GRASP) held at the Rhatigan Student Center, Wichita State University, April 26, 2019.
Research completed in the Department of Mechanical Engineering, College of Engineering