.ThermofluidStream.HeatExchangers.Internal.PartialNTU

Information

This is the partial parent class for all heat exchangers based on the the effectiveness-NTU method.

For stream dominated applications the following assumptions are made for mass flow regularization close to zero:

• if the mass flow on both sides of the heat exchanger is zero, no heat is transferred

Heat exchanger time constant

The model approximates the transient behavior of the heat exchanger with a first-order ordinary differential equation.
Based on the conservation of energy one can derive that its time constant TC can be approximated by the ratio of thermal inertia (wall + fluid) dU / dT to the enthalpy flow rate "inertia" dH_flow / dT:

TC ~ (m_HEX * c_HEX + m_FluidA * c_FluidA + m_FluidB * c_FluidB) / (m_flow_FluidA * c_FluidA + m_flow_FluidB * c_FluidB) ,

where m is the mass, m_flow is the mass flow rate, c is the specific heat capacity, U is the internal energy, T is the temperature and H_flow = m_flow*h is the enthalpy flow rate.

The default time constant TC = 0.01 is not realistic and will be updated in the next major release. For example a heat exchanger with a mass of 10 kg and mass flow rates of 0.5 kg/s of air on both sides has a time constant in the magnitude of about 10 s. The default time constant can also lead to a stiff system, and thereby increase simulation time.

Note that the time constant also avoids algebraic loops and may also be beneficial from a numerical point of view.

Contents

Name	Description
MediumA	Medium model A
MediumB	Medium model B