In SolidWorks Flow Simulation, what does “Bulk-Average Temperature” refer to? What’s the different between “Bulk-Average” and “Average”?
The Bulk-Average temperature at any given point on a face is dependent on a few things, amongst which are the temperature of the fluid contacting that face, and the flow-rate of the fluid contacting that face. As an example, let’s have hot water flowing through a cold pipe.
If you take samples of the temperature of the water along a cross section as it flows through the pipe, you will find minimum temperatures at the edges of the pipe, and maximum temperatures at the center of the pipe. This is not due only to the area of the hot water that contacts the cold pipe, but it is also due to the flow-rate of the water. At the edges of the pipe, the water flows more slowly, and at the center of the pipe, the water flows more rapidly.
If I ask, “What is the temperature of the water at the cross section?” we cannot say it’s the minimum temperature, nor can we say it’s the maximum temperature. Is it the average of the two temperatures? No, not even the average. You see, the average temperature would take (Tmin+Tmax)/2. That is not accurate. And easy proof of this is: increase the flow rate of the water through the pipe, and take sample temperatures across the same cross section- you can already predict that you will get very different temperature values. Why? The temperature of the water in our experiment hasn’t changed. We’re taking samples at the same locations along the radius of the pipe, so the sample locations haven’t changed. But the flow-rate of the water has changed.
In order to give a correct answer, we must take into account the area over which each temperature sample is constant. This is dependent on the flow-rate of the fluid. so the type of “average” we are looking for is a weighted average of the area * flow-rate. This “Area and flow-rate weighted average” is called the Bulk Average.
