Session: 11-08-03: Fundamentals of Convection - Natural and Mixed Convection

Paper Number: 70696

Start Time: Monday, 05:50 PM

70696 - Direct Numerical Simulation of Poiseuille-Rayleigh-Benard Flow of Water in the Neighborhood of Its Density Inversion Point 

Poiseuille-Rayleigh-Benard flow is a typical flow in many industrial applications. It has first been studied for atmospheric flows, and then the subject of further work especially in the chemical processes for depositing vapor or cooling electronic components. In the Poiseuille-Rayleigh-Benard configuration, the flow results from superposition of two convective sources: horizontal pressure gradient that drives the main flow within the duct and a vertical temperature gradient which is the cause of thermoconvective structures formation. In this work, a series of direct numerical simulations were conducted by means of the finite volume method in order to understand the characteristic of Poiseuille-Rayleigh-Benard flow of cold water. The duct was filled with the cold water with the Prandtl number of 11.573. The upper and lower walls maintain low and high temperatures, respectively, and there is a horizontal incoming flow at the entrance. Results show that the density inversion plays a key role in the Poiseuille-Rayleigh-Benard flow. The critical Rayleigh number for the onset of Rayleigh-Benard convection is larger than that of the common fluid, and it increases with the increase of the density inversion parameter. The flow will go through three states in turn as the Rayleigh number increases, including horizontal flow, localized traveling wave and periodic traveling wave. The Poiseuille-Rayleigh-Benard flow state depends on the combination of horizontal flow and Rayleigh-Benard convection, in other words, the relative strength of Poiseuille and Rayleigh-Benard flow. When Rayleigh-Benard flow is weak, the horizontal flow relative to the transverse convection is dominant. The disturbance of Rayleigh-Benard convection cannot affect the horizontal flow any more. Therefore, the flow keeps horizontal and almost there is no waves. With the increase of Rayleigh number, the strength of transverse convection can compared with horizontal flow. Although the horizontal flow has always existed, but the Rayleigh-Benard flow has gradually been enhanced, which results in that more rolls appears. Therefore, the entrance section continues to shorten, eventually the periodic traveling waves occur. This kind of the flow state is named as local traveling wave. When the Rayleigh number is sufficiently high, the horizontal flow has no obvious advantage over the transverse thermal convection, even the thermal convection relative to the horizontal flow is dominant. The disturbance of horizontal flow cannot affect the thermal convection any more. By observing the whole process of the flow development, the mechanisms of periodic traveling wave and localized traveling wave are revealed. In addition, the calculation results also show that as the Reynolds number increases, the critical Rayleigh number corresponding to the occurrence of rolling cells also increases while other parameters are fixed. The critical Rayleigh number for the onset of Rayleigh-Benard convection increases with the increase of Reynolds number, which indicates that Rayleigh-Benard convection will be more difficult to happen at a large Reynolds number.

Presenting Author: Yue Huang Chongqing University

Authors:

Ke Li Chongqing University
Yue Huang Chongqing University
You-Rong Li Chongqing University