ThermofluidStream.FlowControl.SpecificValveType parametrization

[RaphaelGebhart]

ThermofluidStream.FlowControl.SpecificValveType currently allows the user by default to specify a valve via a combination of Kvs and zeta (or alternatively Cvs_US/m_flow_ref_set and zeta). This formulation effectively lets the user define the valve diameter indirectly via:

d = (zeta * c2 * Kv^2)^(1/4)
c2 = 6.22e-10

which is in my opinion very impractical and not common in data sheets.

Quick derivation

Using SI units and the defining relations

V_flow = Av * sqrt(dp / rho)
Av = c1 * Kv,  c1 = 27.7e-6

(see Modelica.Fluid.Valves.BaseClasses.PartialValve and Modelica.Fluid.UsersGuide.ComponentDefinition.ValveCharacteristics), together with

dp = 1/2 * zeta * rho * v^2
v = V_flow / A
A = pi/4 * d^2

one obtains

zeta = d^4 / (c2 * Kv^2)
c2 = c1^2 * 8 / pi^2 = 6.22e-10

(Note: some sources, e.g. https://www.schweizer-fn.de/zeta/armaturen/armaturen.php, report c2 = 626.3, which is (except for rounding errors) equivalent when the diameter is expressed in millimeters.)

Resonable parametrization

The model currently also allows the user (as last option) to explicitly provide d_valve and zeta. Combining the equations in ThermofluidStream.FlowControl.Internal.PartialValve (line 40) and ThermofluidStream.FlowControl.SpecificValveType (lines 36, 83, 92, with line 94 simplified to k_u = k_u_zeta) yields the physically consistent relation

dp = 1/2 * zeta * rho * v^2

which is in my opinion the only practical version of SpecificValveType.

Proposed changes (if we want to keep the model)

• Deprecate all alternative parameterizations (Kvs, Cvs, etc.) in favor of d_valve.
• Reorder parameters so that d_valve is listed first.
• Raise an error if Kvs, Cvs, or similar parameters are used.

Alternative solution

Create a new model where the user can define Kv, Cv, or zeta+d as a function of valve opening. The model could support linear or logarithmic interpolation.

I would prefer creating a new model, because the current one offers too many possibilities for incorrect usage.