Buildings.Airflow.Multizone.BaseClasses

Package with base classes for Buildings.Airflow.Multizone

Information

This package contains base classes that are used to construct the models in Buildings.Airflow.Multizone.

Extends from Modelica.Icons.BasesPackage (Icon for packages containing base classes).

Package Content

Name	Description
DoorDiscretized	Door model using discretization along height coordinate
ErrorControl	Interface that defines parameters for error control
PowerLawResistance	Flow resistance that uses the power law
TwoWayFlowElement	Flow resistance that uses the power law
TwoWayFlowElementBuoyancy	Flow resistance that uses the power law
ZonalFlow	Flow across zonal boundaries of a room
powerLaw	Power law used in orifice equations
powerLawFixedM	Power law used in orifice equations when m is constant
windPressureLowRise	Wind pressure coefficient for low-rise buildings
Examples	Collection of models that illustrate model use and test models

Buildings.Airflow.Multizone.BaseClasses.DoorDiscretized

Door model using discretization along height coordinate

Information

This is a partial model for the bi-directional air flow through a door.

To compute the bi-directional flow, the door is discretize along the height coordinate, and uses an orifice equation to compute the flow for each compartment.

The compartment area dA is a variable, which allows using the model for a door that can be open or closed.

Extends from Buildings.Airflow.Multizone.BaseClasses.TwoWayFlowElementBuoyancy (Flow resistance that uses the power law).

Parameters

Connectors

Modelica definition

Buildings.Airflow.Multizone.BaseClasses.ErrorControl

Interface that defines parameters for error control

Information

This is an interface that defines parameters used for error control.

Dymola does error control on state variables, such as temperature, pressure and species concentration. Flow variables such as m_flow are typically not checked during the error control. This can give large errors in flow variables, as long as the error on the volume's state variables that are coupled to the flow variables is small. Obtaining accurate flow variables can be achieved by imposing an error control on the exchanged mass, which can be defined as

  dm/dt = m_flow.

Parameters

Modelica definition

Buildings.Airflow.Multizone.BaseClasses.PowerLawResistance

Flow resistance that uses the power law

Information

This model describes the mass flow rate and pressure difference relation of an orifice in the form

    V_flow = k * dp^m,

The model is used as a base for the interzonal air flow models.

Extends from Buildings.Fluid.Interfaces.PartialTwoPortInterface (Partial model transporting fluid between two ports without storing mass or energy), Buildings.Airflow.Multizone.BaseClasses.ErrorControl (Interface that defines parameters for error control).

Parameters

Connectors

Modelica definition

Buildings.Airflow.Multizone.BaseClasses.TwoWayFlowElement

Flow resistance that uses the power law

Information

This is a partial model for models that describe the bi-directional air flow through large openings.

Models that extend this model need to compute mAB_flow and mBA_flow, or alternatively VAB_flow and VBA_flow, and the face area area. The face area is a variable to allow this partial model to be used for doors that can be open or closed as a function of an input signal.

Extends from Buildings.Fluid.Interfaces.PartialFourPortInterface (Partial model transporting fluid between two ports without storing mass or energy), Buildings.Airflow.Multizone.BaseClasses.ErrorControl (Interface that defines parameters for error control).

Parameters

Connectors

Modelica definition

Buildings.Airflow.Multizone.BaseClasses.TwoWayFlowElementBuoyancy

Flow resistance that uses the power law

Information

This is a partial model for models that describe the bi-directional air flow through large openings.

Models that extend this model need to compute mAB_flow and mBA_flow, or alternatively VAB_flow and VBA_flow, and the face area area. The face area is a variable to allow this partial model to be used for doors that can be open or closed as a function of an input signal.

Extends from Buildings.Airflow.Multizone.BaseClasses.TwoWayFlowElement (Flow resistance that uses the power law).

Parameters

Connectors

Modelica definition

Buildings.Airflow.Multizone.BaseClasses.ZonalFlow

Flow across zonal boundaries of a room

Information

This is a partial model for computing the air exchange between volumes. Models that extend this model need to provide an equation for port_a1.m_flow and port_a2.m_flow.

Extends from Buildings.Fluid.Interfaces.PartialFourPortInterface (Partial model transporting fluid between two ports without storing mass or energy).

Parameters

Connectors

Modelica definition

Buildings.Airflow.Multizone.BaseClasses.powerLaw

Power law used in orifice equations

Information

This model describes the mass flow rate and pressure difference relation of an orifice in the form

V = k sign(Δp) |Δp|^m

where V is the volume flow rate, k > 0 is a flow coefficient Δ p is the pressure drop and m ∈ [0.5, 1] is a flow coefficient. The equation is regularized for |Δp| < Δp_t, where Δp_t is a parameter. For turbulent flow, set m=1 ⁄ 2 and for laminar flow, set m=1.

The model is used for the interzonal air flow models.

Implementation

For |Δp| < Δp_t, the equation is regularized so that it is twice continuously differentiable in Δp, and that it has an infinite number of continuous derivatives in m and in k.

If m is not a function of time, then a, b, c and d can be pre-computed. In this situation, use Buildings.Airflow.Multizone.BaseClasses.powerLawFixedM, which allows to compute these values outside of this function, for example as parameters of a model.

Inputs

Outputs

Modelica definition

Buildings.Airflow.Multizone.BaseClasses.powerLawFixedM

Power law used in orifice equations when m is constant

Information

This model describes the mass flow rate and pressure difference relation of an orifice in the form

V = k sign(Δp) |Δp|^m

where V is the volume flow rate, k > 0 is a flow coefficient Δ p is the pressure drop and m ∈ [0.5, 1] is a flow coefficient. The equation is regularized for |Δp| < Δp_t, where Δp_t is a parameter. For turbulent flow, set m=1 ⁄ 2 and for laminar flow, set m=1.

The model is used for the interzonal air flow models. It is identical to Buildings.Airflow.Multizone.BaseClasses.powerLaw but it requires the polynomial coefficients as an input. This allows a more efficient simulation if m and therefore also a, b, c and d are constant.

Implementation

For |Δp| < Δp_t, the equation is regularized so that it is twice continuously differentiable in Δp, and that it has an infinite number of continuous derivatives in m and in k.

If m, and therefore also a, b, c and d, change with time, then it is more convenient and efficient to use Buildings.Airflow.Multizone.BaseClasses.powerLaw.

Inputs

Outputs

Modelica definition

Buildings.Airflow.Multizone.BaseClasses.windPressureLowRise

Wind pressure coefficient for low-rise buildings

Information

This function computes the wind pressure coefficient for low-rise buildings with rectangular shape. The correlation is the data fit from Swami and Chandra (1987), who fitted a function to various wind pressure coefficients from the literature. The same correlation is also implemented in CONTAM (Persily and Ivy, 2001).

The wind pressure coefficient is computed based on the natural logarithm of the side ratio of the walls, which is defined as

G = ln(x ⁄ y)

where x is the length of the wall that will be connected to this model, and y is the length of the adjacent wall as shown in the figure below.

Based on the wind incidence angle α and the side ratio of the walls, the model computes how much the wind pressure is attenuated compared to the reference wind pressure Cp0. The reference wind pressure Cp0 is a user-defined parameter, and must be equal to the wind pressure at zero wind incidence angle, i.e., α = 0. Swami and Chandra (1987) recommend C_p0 = 0.6 for all low-rise buildings as this represents the average of various values reported in the literature. The attenuation factor is

C_p ⁄ C_p0 = ln(1.248 - 0.703 sin(α ⁄ 2) - 1.175 sin²(α) - 0.131 sin³(2 α G) + 0.769 cos(α ⁄ 2) +0.071 G² * sin²(α ⁄ 2) + 0.717 cos²(α ⁄ 2)),

where C_p is the wind pressure coefficient for the current angle of incidence.

This function is used in Buildings.Fluid.Sources.Outside_CpLowRise which can be used directly with components of this package.

References

• Muthusamy V. Swami and Subrato Chandra. Procedures for Calculating Natural Ventilation Airflow Rates in Buildings. Florida Solar Energy Center, FSEC-CR-163-86. March, 1987. Cape Canaveral, Florida.
• Andrew K. Persily and Elizabeth M. Ivy. Input Data for Multizone Airflow and IAQ Analysis. NIST, NISTIR 6585. January, 2001. Gaithersburg, MD.

Implementation

Symmetry requires that the first derivative of the wind pressure coefficient with respect to the incidence angle is zero for incidence angles of zero and π. However, the correlation of Swami and Chandra has non-zero derivatives at these values. In this implementation, the original function is therefore slightly modified for incidence angles between 0 and 5 degree, and between 175 and 180 degree. This leads to a model that is differentiable in the incidence angle, which generally leads to better numeric performance.

Inputs

Outputs

Modelica definition