Illustrate Advection using RadTherm
RadTherm’s automatic convection calcuations allow users to quickly set-up models with advection links. This example illustrates the results of three simulations:

1. Radiation Only
2. One fluid node
3. Two fluid nodes circulating

The model is a simple cube 10cm X 10cm X 10 cm, resulting in a 1 liter volume.

Inside is a sphere with diameter of 2.8 cm, located at the centroid of the enclosure.

In each case, a 25 Watt heat load is applied to the sphere, resulting in a rough model of a light bulb or other heat source. 

The box was modeled as two cube-halves constructed of 1.5mm thick steel and no seams - all nodes conduct throughout the cube. Two halves were used to accomodate a part-level assignment of fluid node connections to each surface. If three fluid nodes were used, then the box elements would need to distributed into three separate (though conducting) parts. Any number of fluid nodes can be employed to reach higher levels of simulation accuracy. Fluid temperature and convection coefficient data can also be imported from computational fluid dynamics CFD software at an element-level basis.

Case 1: No Convection
In this case we see the effects of radiation only - equal transfer to all surfaces due to the 3-D symmetry of the geometry. This is equivalent to a perfect vacuum in the box.

Case 2: One Fluid Node

With a single fluid node, we are one step closer to an airfilled box. The fluid node has a volume of one liter and is connected to the sphere outer surface and the box interior surface. RadTherm automatically calculates the convection coefficient of each surface node. The convection rate is based on fluid movement due to temperature differences between the fluid and wall (no forced convection was used but is available in RadTherm). The fluid properties used are based on the fluid film
temperature (Tw+Tf)/2.

Case 3: Two Fluid Nodes
In this case the fluid in the box is represented with two fluid nodes of 0.5 liter volume each, one connected to the sphere and the top half of the box; the other connected to the bottom half of the box only. An estimated circulation rate between the fluid nodes of one fluid turnover in 5 minutes was used (0.2 liter/min). This flow was not used to calculate the convection rate, only the mixing rate of the two fluids. The same convection method as Step 2 was used. Two fluid nodes resulted in a less homogeneous and more realistic  representation of naturally induced advection. For the highest level of accuracy, a combined CFD / RadTherm approach can be used. The image below shows the a cross sectional pane of fluid temperature results from CFD for a model of similar geometry (only the bottom sphere was heated and flow patterns developed around the upper sphere and within the enclosure). See Technical Bulletins 330-350 for details on this type of approach.