modelPartialNTU
Extends from ThermofluidStream.Utilities.DropOfCommonsPlus.
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
considerInertance
The parameter considerInertance is currently not included in the model; see
Idealized.UsersGuide.InertanceNeglect.
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.
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| SI.Area | A | Heat transfer area | |
| SI.CoefficientOfHeatTransfer | k_NTU | 50 | Overall heat transfer coefficient |
| SI.Time | TC | 0.01 | Heat exchanger time constant (increase as recommended in the documentation) |
| Boolean | d1A | displayParameters and displayArea | displayArea at position 1 |
| Boolean | d1kNTU | displayParameters and displaykNTU and not d1A | displaykNTU at position 1 |
| Boolean | d2kNTU | displayParameters and displaykNTU and not d1kNTU | displaykNTU at position 2 |
| Advanced | |||
| Utilities.Units.Inertance | L | dropOfCommons.L | Inertance |
| Advanced › Regularization parameters | |||
| SI.MassFlowRate | m_flow_reg | dropOfCommons.m_flow_reg | Nominal mass flow rate for regularization |
| Layout › Display parameters | |||
| Boolean | displayArea | true | = true, if the heat transfer area A is displayed |
| Boolean | displaykNTU | true | = true, if the overall heat transfer coefficient k_NTU is displayed |
Components
| Type | Name | Default | Description |
|---|---|---|---|
| SI.TemperatureDifference | Delta_T_max | Maximum temperature difference | |
| MediumA.SpecificEnthalpy | dh_A | Specific enthalpy difference medium A | |
| MediumB.SpecificEnthalpy | dh_B | Specific enthalpy difference medium B | |
| SI.HeatFlowRate | q_flow | Heat flow rate A to B (Q_flow) | |
| Real | effectiveness | Heat exchanger efficiency | |
| Real | NTU | Number of transfer units | |
| MediumA.Temperature | T_in_MediumA | Inlet temperature of medium A | |
| MediumB.Temperature | T_in_MediumB | Inlet temperature of medium B | |
| ThermofluidStream.HeatExchangers.Internal.DiscretizedHEXSummary | summary | Summary record of quantities |
Contents
| Name | Description |
|---|---|
| MediumA | Medium model A |
| MediumB | Medium model B |