Step 6: Custom Closures
Complete input file for this step: 06_custom_closures.i
Closure System
The closure system is another powerful system provided by THM. It allows users to either use pre-defined closure sets or entirely define their own ones.
Flow channels have the closures parameter, which corresponds to the name of a Closures object. This parameter can be specified globally in [GlobalParams] block or on per-component basis. Note that not all components require the closure parameter (for example heat structures have no concept of closure correlations).
Custom closure correlations are provided via MOOSE material system. Users can use any pre-built material objects that come with MOOSE and provide the required value. The most notable and useful material is ADParsedMaterial which allows users to provide the closure formula on the input file level.
Tip: THM provides convenient materials for computing Reynolds and Prandtl number, named ADReynoldsNumberMaterial and ADPrandtlNumberMaterial, respectively.
The last bit to know is which material properties we need to supply. In single-phase flow, they are Darcy wall friction factor f_D and convective wall heat transfer coefficient Hw.
Add Custom Closure Correlations
Before we add our custom materials it is advisable to define two named parameters. One parameter will be for the wall friction and the other for the wall heat transfer correlation. They will hold the names of components where we will be applying each closure.
Note: Heat transfer is optional, so defining a wall heat transfer coefficient on blocks that do not have wall heat transfer linked to them makes no sense.
This will avoid problems of defining closure correlations on components that do not support it. For example, we would not want to define flow closures on heat structure blocks, etc.
In our tutorial we will define the following two subsets of blocks:
flow_blocks = 'core_chan up_pipe top_pipe hx/pri hx/sec down_pipe bottom_b bottom_a'
ht_blocks = 'core_chan hx/pri hx/sec'
The flow_blocks parameter will hold the names of components where we prescribe the wall friction closure. The ht_blocks parameter will contain the names of components that have the wall heat transfer associated with them.
To use the custom closure set, we create a closures object of the class Closures1PhaseNone, which does not create any of its own Materials:
[Closures]
[no_closures]
type = Closures1PhaseNone
[]
[]
Then the name we gave this closures object (no_closures) is passed to the closures parameter. Again, this can be done on a global level or per-component basis.
Materials
In our model, we will use the Churchill's formula for wall friction closure and Dittus-Boelter formula for convective wall heat transfer closure
The Churchill's formula can be set up by using the ADWallFrictionChurchillMaterial
[f_mat]
type = ADWallFrictionChurchillMaterial
block = ${flow_blocks}
D_h = D_h
f_D = f_D
mu = mu
rho = rho
vel = vel
[]
The Dittus-Boelter formula can be set up by using ADWallHeatTransferCoefficient3EqnDittusBoelterMaterial.
[Hw_mat]
type = ADWallHeatTransferCoefficient3EqnDittusBoelterMaterial
block = ${ht_blocks}
D_h = D_h
rho = rho
vel = vel
T = T
T_wall = T_wall
cp = cp
mu = mu
k = k
[]
Since these are materials, we need to put these two blocks into the top-level block [Materials].