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.

Flow reversal is not supported by this model; if you need flow reversal, please consider using the Flow1DFEM model.

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 following options are available to specify the friction coefficient:

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

If HydraulicCapacitance = HCtypes.Downstream (default option) then the compressibility effect depending on the pressure derivative is lumped at the outlet, while the optional dynamic momentum term depending on the flowrate is lumped at the inlet; therefore, the state variables are the outlet pressure and the inlet flowrate. If HydraulicCapacitance = HCtypes.Upstream the reverse takes place. If HydraulicCapacitance = HCtypes.Middle, the compressibility effect is lumped at the middle of the pipe; to use this option, an odd number of nodes N is required.

Start values for the pressure and flowrate state variables 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 available to connected components 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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