.ThermoPower.Water.Flow1D

Information

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

• The fluid state is always one-phase (i.e. subcooled liquid or superheated steam).
• Uniform velocity is assumed on the cross section, leading to a 1-D distributed parameter model.
• Turbulent friction is always assumed; a small linear term is added to avoid numerical singularities at zero flowrate. The friction effects are not accurately computed in the laminar and transitional flow regimes, which however should not be an issue in most applications using water or steam as a working fluid.
• The model is based on dynamic mass, momentum, and energy balances. The dynamic momentum term can be switched off, to avoid the fast oscillations that can arise from its coupling with the mass balance (sound wave dynamics).
• The longitudinal heat diffusion term is neglected.
• The energy balance equation is written by assuming a uniform pressure distribution; the compressibility effects are lumped at the inlet, at the outlet, or at the middle of the pipe.
• The fluid flow can exchange thermal power through the lateral surface, which is represented by the wall connector. The actual heat flux must be computed by a connected component (heat transfer computation module).

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 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 than or equal to 2.

The following options are available to specify the friction coefficient:

• FFtype = FFtypes.Kfnom: the hydraulic friction coefficient Kf is set directly to Kfnom.
• FFtype = FFtypes.OpPoint: the hydraulic friction coefficient is specified by a nominal operating point (wnom,dpnom, rhonom).
• FFtype = FFtypes.Cfnom: the friction coefficient is computed by giving the (constant) value of the Fanning friction factor Cfnom.
• FFtype = FFtypes.Colebrook: the Fanning friction factor is computed by Colebrook's equation (assuming Re > 2100, e.g. turbulent flow).
• FFtype = FFtypes.NoFriction: no friction is assumed across the pipe.

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.

Revisions

• 16 Sep 2005 by Francesco Casella:
  Option to lump compressibility at the middle added.
• 30 May 2005 by Francesco Casella:
  Initialisation support added.
• 24 Mar 2005 by Francesco Casella:
  FFtypes package and NoFriction option added.
• 16 Dec 2004 by Francesco Casella:
  Standard medium definition added.
• 8 Oct 2004 by Francesco Casella:
  Model now based on Flow1DBase.
• 24 Sep 2004 by Francesco Casella:
  Removed wstart, pstart. Added pstartin, pstartout.
• 22 Jun 2004 by Francesco Casella:
  Adapted to Modelica.Media.
• 15 Jan 2004 by Francesco Casella:
  Computation of fluid velocity u added. Improved treatment of geometric parameters
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• 1 Oct 2003 by Francesco Casella:
  First release.