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 volume method. The state variables are one pressure, one flowrate (optional) and N-1 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.

The model assumes that the mass flow rate is always from the inlet to the outlet. Small reverse flow is allowed (e.g. when closing a valve at the outlet), but the model will not account for it explicitly.

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 than or equal to 2.

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

The density is computed assuming a linear distribution of the specific enthalpy between the nodes; this requires the availability of the time derivative of the inlet enthalpy. If this is not available, it is possible to set avoidInletEnthalpyDerivative to true, which will cause the mean density of the first volume to be approximated as its outlet density, thus avoiding the need of the inlet enthalpy derivative.

The following options are available to specify the friction coefficient:

If HydraulicCapacitance = 2 (default option) then the mass storage term depending on the pressure is lumped at the outlet, while the optional momentum storage 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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