Large Eddy Simulation of Strongly Heated Internal Gas Flows

Abstract

This work is concerned with the modeling of a strongly heated, low Mach number, gas flowing upward within a vertical tube with constant heat flux boundary conditions. Four llarge eddy subgrid models are compared to two different RANS turbulence models in their ability to predict the temperature distribution in the heated pipe. All LES models predict the mean velocity profile reasonably well. The RMS values of the Smagorinsky-Lilly model and the WALE model show the same shape but different magnitude while the standard Smagorinsky model shows the maximum at a different location. The mean temperature profiles along the wall in the section with the prescribed heat flux are underpredicted by all LES models. This work is concerned with the modeling of a strongly heated, low Mach number, gas flowing upward within a vertical tube with (nearly) constant heat flux boundary conditions, in which forced convection is dominant. The heating rate is sufficiently high such that the fluid properties vary significantly in both the radial and axial directions. Since the flow is continually adjusting to the changing properties, fully developed velocity and temperature profiles never occur. Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) provide means for determining detailed information about these heated flows that may be difficult to obtain experimentally. The motivation for this problem stems from considerations that gas may be effectively employed as a coolant for advanced power reactors. In this case, it is desirable to achieve high thermal efficiencies which require high gas exit temperatures. To achieve high temperatures the mean velocity of the gas may be low enough that the relevant Reynolds number of the flow is less than 10,000. Experimental data for this class of flows is sparse. Perkins 1 obtained mean temperature distributions for dominant forced convection flow through a circular cylinder with significant gas property variations. Using essentially the same experimental apparatus, Shehata 2 obtained mean velocity distributions under some of the same flow conditions. They employed three different constant wall heat flux boundary conditions: “low” and “intermediate” heating rates at inlet Reynolds numbers of approximately 6000, and a “high” heating rate case at an inlet Reynolds number of approximately 4200. Perkins 1 characterized these three cases as turbulent, “subturbulent,” and laminarizing, respectively. The results of Perkins 1 and Shehata 2 have since been reported in detail by Shehata and McEligot. 3,4 Consequently, subsequent references to the Shehata/McEligot data refer to the data as presented in the PhD. thesis of Shehata 3 and the report of Shehata and McEligot, 4 derived from the results of Perkins 1 and Shehata. 2 These experimental data provide an opportunity to verify turbulence and subgrid scale models under the influence of strong heating and, consequently, large variations in fluid properties.