A wide range of applications such as shell tube heat exchangers, steam pipes in power plants electronic cooling and cooler pipes in nuclear reactor use natural convection as the conventional method for cooling. Natural convection is highly attractive when compared with forced convection since it does not require any extra components, thus offering high reliability and low design and maintenance cost. Conversely, the main challenge in natural convection is free convection from a single heated horizontal cylinder submerged in a liquid with a free surface. The crucial operational parameters that influence the presence and motion of plumes mainly include the Prandtl number, Rayleigh number, submersion depth of the cylinder below the vertically confining surface and magnitude of horizontal confinement. Even though the most recent particle-image velocimetry measurements have uncovered the effect of plume motion on the boundary layer, plume formation region and surrounding flow fields, no computational models of fluid dynamics are available in literature that demonstrate the mechanisms of this free convective flow phenomenon.
Inspired by this deficiency, Kuang C. Lin, Yashraj Bhosale and Cun-Yan Zhou Huang at National Sun Yat-Sen University in Taiwan developed a model of 3D computational fluid dynamics that would, for the first time, predict the existing particle-image velocimetry data and measure Nusselt numbers collected from the liquid flow above a heated horizontal cylinder. They aimed at understanding complex heat and fluid flow associated with previous works that measured only the free convectional water flow above a heated horizontal cylinder. Their research work is now published in the peer-reviewed journal, Applied Thermal Engineering.
Professor Kuang C. Lin and colleagues conducted the transient CFD simulation on a non-uniform mesh with nearly one million grids. The research team characterized the plume formation. The effect of the Rayleigh number on velocity fields was also undertaken. Eventually, the variation of ensemble-averaged Nusselt numbers was investigated.
The simulation results were obtained in the free convective water flow around a heated horizontal cylinder with the top surface open to air in the Rayleigh number ranging from 105 to 5 × 106 and a Prandtl number of 5.98. Excellent agreements between the computed and measured similarity solutions for a Rayleigh number of 1.33 × 106 proved the capability of the computational model to simulate flow velocity, boundary layers and Nusselt numbers present in a vertical plane perpendicular to the cylinder axis at different circumferential locations.
In response to the need for understanding complex heat and fluid flow associated with the previous experiments that measured the free convection water flow above a heated horizontal cylinder, a 3D natural convection model has been presented herein. The computational results concerning the periodic swaying motion of the plume and its time of a sway period have been found to be consistent with the experimental observations. Additionally, the computational analysis reveals the correlation among the near-cylinder flow features, boundary-layer thickness and plume formation region. Moreover, the effect of Rayleigh number on the velocity fields and heat transfer characteristics has been identified.
Kuang C. Lin, Yashraj Bhosale, Cun-Yan Zhou Huang. 3D-CFD Investigation into Free Convection Flow Above a Heated Horizontal Cylinder: Comparisons with Experimental Data. Applied Thermal Engineering, volume 120 (2017) pages 277–288.
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