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# Different Space Discretization Constant per material layer

The Engineering Reference discusses the constant "C" under Basic Finite Difference Solution Approach and specifically says:

> As of version 7, the value of this constant can be controlled by the user with the input field called Space Discretization Constant in the HeatBalanceSettings:ConductionFiniteDifference input object. The discretization method allows CondFD to assign different node spacing or grid size to different material layers in a wall or roof, as building walls and roofs typically consist of several layers of different materials having different thermal properties.

How can this setting be applied to a specific layer? I only could set this for the entire model (all material layers), which extends runtimes significantly. I would like to set this for only one (thinner) layer for project.

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Reading just above and below the Eng Ref section you quoted, I believe this is saying that this Space Discretization Constant is a global setting which EnergyPlus uses to calculate the number of nodes within each material layer -- you don't have direct control over the number of nodes. See highlighted portions from the Eng Ref section below.

In summary, increasing the Space Discretization constant will reduce the number of nodes for material layers depending upon their thermal diffusivity, thereby reducing simulation time.

It does not look like you can apply Conduction Finite Difference (CondFD) to just one specific material layer within a construction assembly. However, you can apply CondFD to individual surfaces or construction assemblies in multiple ways:

The other surfaces and construction assemblies in the model will use the global heat balance algorithm. This may be enough to apply CondFD just to the surfaces with a construction assembly that contains the thinner layer you want to focus on, reducing the simulation time by applying a different heat balance algorithm (likely the default conduction transfer function) to the remaining surfaces.

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I was afraid that this was an unintended mis-phrasing of the specific section that I cited. I noticed the other passages as well where it talked about this being global setting for CondFD. What I didn't know was that I can set/trigger CondFD for individual surfaces/constructions. This may become helpful in other instances but will be less so in my particular case. I have a rather thick material layer with a very low diffusivity next to a thinner material layer with higher diffusivity and wanted get more intervals only for the latter to study it in more detail.

( 2022-11-08 11:28:52 -0600 )edit