This model describes the flow of a gas 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), N-1 temperatures, and either one or N-1 gas composition vectors.

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 problems occur in tubes with very low pressure drops.

Flow reversal is fully supported.

Modelling options

The actual gas used in the component is determined by the replaceable Medium package.In the case of multiple component, variable composition gases, the start composition is given by Xstart, whose default value is Medium.reference_X.

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.

if UniformComposition is true, then a uniform compostion is assumed for the gas through the entire tube length; otherwise, the gas compostion is computed in N equally spaced nodes, as in the case of thermal variables.

The following options are available to specify the friction coefficient:

If QuasiStatic is set to true, the dynamic terms are neglected in the mass, momentum, and energy balances, i.e., quasi-static behaviour is modelled. It is also possible to neglect only the dynamic momentum term by setting DynamicMomentum = false.

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; therefore, the state variables are the outlet pressure and the inlet flowrate. If HydraulicCapacitance = 1 the reverse takes place.

Start values for the pressure and flowrate state variables are specified by pstart, wstart. The start values for the node temperatures are linearly distributed from Tstartin at the inlet to Tstartout at the outlet. The (uniform) start value of the gas composition is specified by Xstart.

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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