What is the heat flow sign convention for a Simscape Temperature Source?

I have a Simscape thermal simulation of heat conduction from a 100 C room, through a wall, to a 0C room. There are 100 Watts transferred from left to right in the following picture. I have listed the heat flow values for each of the three thermal elements in the problem.

The conduction element shows a positive 100 W flowing from its A port to its B port; that establishes the heat flow in the problem as positive going from left to right.

My questions relate to the temperature elements.

1) Why are the signs on the two Temperature Source elements opposite the arrow direction in their element graphics?

2) Said another way, if the 100C Temperature Source is adding heat to the system, why isn't its heat flow sign positive?

3) If the 0C Temperature Source is removing heat from the system, why isn't its heat flow sign negative?

Thank you.

Answer 1

[Cris LaPierre]

First a caution. Do not assume direction based on block orientation. You would get the same results no matter the orientation of the blocks. There is no coordinate system applied based on model layout. Any directionality is inferred from the underlying physics.

If you are asking why the arrow doesn't change direction based on heat flow, it is because the icon for this component is fixed. Some componenets may change slightly based on parameters you set, but I can't think of any that change based on simulation values.

A temperature source sets the temperature of an infinte source/sink. That means that, no matter how much heat you add or remove, the source/sink temperature stays the same. Sources/sinks don't add heat to a system. They are just so large (infinite) that, no matter how much heat you remove/add to them, the temperature stays the same.

Of course, that has to be converted to equations that can be simulated. In this case, temperature is assumed to remain constant no matter the flow rate.

[Timothy]

Thanks Cris.