How to Couple Steady Fluid Flow and Steady-State Heat Transfer for Natural Convection (Buoyancy)

The ability to predict how a buoyant flow will affect the temperature distribution in fluid and solid components is useful when analyzing applications that involve the heating or cooling of fluid, such as emissions systems, electronics cooling or heating and ventilation systems. Natural convection (or buoyancy) effects can be simulated for these multiphysics applications by coupling steady fluid flow analysis with steady-state heat transfer analysis. The analysis procedure is automated within ALGOR software as illustrated by the following example.

Consider two concentric cylinders with fluid in the annulus between them. The inner cylinder is heated to a constant temperature that is hotter than both the outer cylinder and the fluid. The temperature difference induces buoyant flow in the fluid.

To simulate this multiphysics system, first specify the analysis type as "Steady Coupled Fluid Flow and Thermal". Then, construct a finite element model that consists of at least one part that is considered in both the fluid and thermal analyses and any number of additional solid parts that undergo purely heat transfer phenomena. In this example, the Superdraw environment was used to create a 2-D model of the concentric cylinders system as shown in Figure 1. Finite element information was added in the FEA Editor environment including:

• an internal heat generation loading of 100 in*lbf/(in³*s) on the inner cylinder
• a convection load with an ambient temperature of 0°F on the outer surface of the outer cylinder
• surface prescribed velocities of 0 in/s on the inner and outer surfaces of the fluid part

Figure 1: A 2-D model of the concentric cylinders system includes the solid cylinder parts (yellow and red) as well as a fluid part (green).

Next, specify analysis parameters for this multiphysics simulation using the "Analysis Parameters" dialog as shown in Figure 2.

Figure 2: The "General" tab of the "Analysis Parameters" dialog for "Steady Coupled Fluid Flow and Thermal Analysis" allows for the definition of buoyancy forces and specification of controls for the coupling of analyses including the buoyancy load curve data.

Finally, run the multiphysics simulation. An iterative process is automatically performed between a steady fluid flow analysis and a steady-state heat transfer analysis in order to converge on an accurate solution. When the analysis is completed, the model will be displayed in the Superview IV Results environment, where distributions of both temperature and fluid flow velocity magnitude can be examined as shown in Figures 3 and 4.

Figure 3: A Superview IV Results environment display shows the temperature distribution in both the solid and fluid parts. Warmer temperatures are found in the upper half of the model while cooler temperatures occur in the bottom half due to heat rising from the buoyancy-driven flow.

Figure 4: Vectors of fluid flow velocity magnitude are shown for the fluid part only. Two eddies can be seen where the heated fluid near the inner cylinder rises and the cooler fluid circulates downward near the outer cylinder until the flow cycle repeats.

The capability for natural convection (buoyancy) can be used for many applications and includes support for simulating flows surrounded by complex, 3-D, multi-part assemblies.

For more information about the natural convection (buoyancy) capability, click here or see the ALGOR User’s Guide.