This model describes the flow of water or steam in a rigid tube. The basic modelling assumptions are:

The mass, momentum, and energy balance equation are discretised with the finite element method. The state variables are one pressure, one flowrate (optional) and N specific enthalpies.

The turbulent friction factor can be either assumed as a constant, or computed by Colebrook's equation. In the former case, the friction factor can be supplied directly, or given implicitly by a specified operating point. In any case, the multiplicative correction coefficient Kfc can be used to modify the friction coefficient, e.g. to fit experimental data.

A small linear pressure drop is added to avoid numerical singularities at low or zero flowrate. The wnom parameter must be always specified: the additional linear pressure drop is such that it is equal to the turbulent pressure drop when the flowrate is equal to wnf*wnom (the default value is 1% of the nominal flowrate). Increase wnf if numerical instabilities occur in tubes with very low pressure drops.

Flow reversal is fully supported.

Modelling options

Thermal variables (enthalpy, temperature, density) are computed in N equally spaced nodes, including the inlet (node 1) and the outlet (node N); N must be greater or equal than 2.

The dynamic momentum term is included or neglected depending on the DynamicMomentum parameter.

Two parameters are available to tune the numerical method. The stabilisation coefficient alpha varies from 0.0 to 1.0; alpha=0.0 corresponds to a non-stabilised method, which gives rise to non-physical oscillations; the default value of 1.0 corresponds to a stabilised method, with well-damped oscillations. The mass lumping coefficient (ML) allows to use a hybrid finite-element/finite-volume discretisation method for the dynamic matrix; the default value ML=0.0 corresponds to a standard FEM model, ML=1.0 corresponds to a full finite-volume method, with the associated numerical diffusion effects. Intermediate values can be used.

The following options are available to specify the friction coefficient:

If HydraulicCapacitance = 2 (default option) then the mass buildup term depending on the pressure is lumped at the outlet, while the optional momentum buildup term depending on the flowrate is lumped at the inlet. If HydraulicCapacitance = 1 the reverse takes place.

Start values for pressure and flowrate are specified by pstart, wstart. The start values for the node enthalpies are linearly distributed from hstartin at the inlet to hstartout at the outlet.

A bank of Nt identical tubes working in parallel can be modelled by setting Nt > 1. The geometric parameters always refer to a single tube.

This models makes the temperature and external heat flow distributions visible through the wall connector. If other variables (e.g. the heat transfer coefficient) are needed by external components to compute the actual heat flow, the wall connector can be replaced by an extended version of the DHT connector.


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