Buildings.Fluid.BaseClasses.FlowModels

Flow models for pressure drop calculations

Information

This package contains a basic flow model that is used by the various models that compute pressure drop. Because the density does not change signficantly in heating, ventilation and air conditioning systems for buildings, this model computes the pressure drop based on the mass flow rate and not the volume flow rate. This typically leads to simpler equations because it does not require the mass density, which changes when the flow is reversed. Although, for conceptual design of building energy system, there is in general not enough information available that would warrant a more detailed pressure drop calculation. If a more detailed computation of the flow resistance is needed, then a user can use models from the Modelica.Fluid library.

Package Content

Name Description

basicFlowFunction_dp Basic class for flow models

basicFlowFunction_m_flow Basic class for flow models

Examples Collection of models that illustrate model use and test models

Name	Description
basicFlowFunction_dp	Basic class for flow models
basicFlowFunction_m_flow	Basic class for flow models
Examples	Collection of models that illustrate model use and test models

Buildings.Fluid.BaseClasses.FlowModels.basicFlowFunction_dp

Basic class for flow models

Information

Function that computes the pressure drop of flow elements as

  m_flow = sign(dp) * k * sqrt(|dp|),

with regularization near the origin. The variable m_flow_turbulent determines the location of the regularization.

Inputs

Type Name Default Description

Pressure dp Pressure difference between port_a and port_b (= port_a.p - port_b.p) [Pa]

Real k Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2)

MassFlowRate m_flow_turbulent Mass flow rate [kg/s]

Boolean linearized false = true, use linear relation between m_flow and dp for any flow rate

Type	Name	Default	Description
Pressure	dp		Pressure difference between port_a and port_b (= port_a.p - port_b.p) [Pa]
Real	k		Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2)
MassFlowRate	m_flow_turbulent		Mass flow rate [kg/s]
Boolean	linearized	false	= true, use linear relation between m_flow and dp for any flow rate

Outputs

Type Name Description

MassFlowRate m_flow Mass flow rate in design flow direction [kg/s]

Type	Name	Description
MassFlowRate	m_flow	Mass flow rate in design flow direction [kg/s]

Modelica definition

function basicFlowFunction_dp "Basic class for flow models"


  input Modelica.SIunits.Pressure dp(displayUnit="Pa") 
    "Pressure difference between port_a and port_b (= port_a.p - port_b.p)";
  input Real k(unit="") 
    "Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2)";
  input Modelica.SIunits.MassFlowRate m_flow_turbulent(min=0) "Mass flow rate";
  input Boolean linearized = false 
    "= true, use linear relation between m_flow and dp for any flow rate";
   output Modelica.SIunits.MassFlowRate m_flow 
    "Mass flow rate in design flow direction";
protected 
  Modelica.SIunits.Pressure dp_turbulent(displayUnit="Pa") 
    "Turbulent flow if |dp| >= dp_small, not a parameter because k can be a function of time";
protected 
 Real kSqu(unit="kg.m") "Flow coefficient, kSqu=k^2=m_flow^2/|dp|";
 constant Real conv(unit="m.s2/kg") = 1 "Factor, needed to satisfy unit check";
 constant Real conv2 = sqrt(conv) "Factor, needed to satisfy unit check";
algorithm 
  kSqu:=k*k;
  dp_turbulent :=m_flow_turbulent^2/kSqu;
  if linearized then
     m_flow :=k*dp*conv2;
  else
     m_flow :=Modelica.Fluid.Utilities.regRoot2(x=dp, x_small=dp_turbulent, k1=kSqu, k2=kSqu);
  end if;
end basicFlowFunction_dp;

Buildings.Fluid.BaseClasses.FlowModels.basicFlowFunction_m_flow

Basic class for flow models

Information

Function that computes the pressure drop of flow elements as

  dp = 1/k^2 * sign(m_flow) m_flow^2

with regularization near the origin. The variable m_flow_turbulent determines the location of the regularization.

Inputs

Type Name Default Description

MassFlowRate m_flow Mass flow rate in design flow direction [kg/s]

Real k Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2)

MassFlowRate m_flow_turbulent Mass flow rate [kg/s]

Boolean linearized false = true, use linear relation between m_flow and dp for any flow rate

Type	Name	Default	Description
MassFlowRate	m_flow		Mass flow rate in design flow direction [kg/s]
Real	k		Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2)
MassFlowRate	m_flow_turbulent		Mass flow rate [kg/s]
Boolean	linearized	false	= true, use linear relation between m_flow and dp for any flow rate

Outputs

Type Name Description

Pressure dp Pressure difference between port_a and port_b (= port_a.p - port_b.p) [Pa]

Type	Name	Description
Pressure	dp	Pressure difference between port_a and port_b (= port_a.p - port_b.p) [Pa]

Modelica definition

function basicFlowFunction_m_flow "Basic class for flow models"
  input Modelica.SIunits.MassFlowRate m_flow 
    "Mass flow rate in design flow direction";
  input Real k(unit="") 
    "Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2)";
  input Modelica.SIunits.MassFlowRate m_flow_turbulent(min=0) "Mass flow rate";
  input Boolean linearized = false 
    "= true, use linear relation between m_flow and dp for any flow rate";
  output Modelica.SIunits.Pressure dp(displayUnit="Pa") 
    "Pressure difference between port_a and port_b (= port_a.p - port_b.p)";
protected 
 Real kSquInv(unit="1/(kg.m)") "Flow coefficient";
 constant Real conv(unit="m.s2/kg") = 1 "Factor, needed to satisfy unit check";
 constant Real conv2 = sqrt(conv) "Factor, needed to satisfy unit check";
algorithm 
  kSquInv:=1/k^2;
  if linearized then
     dp := m_flow/k/conv2;
  else
     dp :=Modelica.Fluid.Utilities.regSquare2(x=m_flow, x_small=m_flow_turbulent, k1=kSquInv, k2=kSquInv);
  end if;
end basicFlowFunction_m_flow;

HTML-documentation generated by Dymola Tue Sep 29 08:09:28 2009.