.AixLib.Fluid.BaseClasses.FlowModels

Information

This package contains a basic flow model that is used by the various models that compute pressure drop.

Assumption and limitations

Because the density does not change signficantly in heating, ventilation and air conditioning systems for buildings, the flow models compute 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.

All functions have an argument m_flow_turbulent that determines where the flow transitions to fully turbulent flow. For smaller mass flow rates, the quadratic relation is replaced by a function that has finite slope near zero pressure drop. This is done for numerical reasons, and to approximate laminar flow, although the implementation does not use a linear function.

Implementation

The two main functions are AixLib.Fluid.BaseClasses.FlowModels.basicFlowFunction_dp and AixLib.Fluid.BaseClasses.FlowModels.basicFlowFunction_m_flow that compute the mass flow rate or the pressure drop, respectively. Both functions are two times continuously differentiable. First and second order derivatives are provided in the function that have the suffix _der and _der2.

Contents

Name	Description
basicFlowFunction_dp	Function that computes mass flow rate for given pressure drop
basicFlowFunction_dp_der	1st derivative of function that computes mass flow rate for given pressure drop
basicFlowFunction_dp_der2	2nd derivative of flow function2nd derivative of function that computes mass flow rate for given pressure drop
basicFlowFunction_m_flow	Function that computes pressure drop for given mass flow rate
basicFlowFunction_m_flow_der	1st derivative of function that computes pressure drop for given mass flow rate
basicFlowFunction_m_flow_der2	2nd derivative of function that computes pressure drop for given mass flow rate
Validation	Collection of validation models

Revisions

• April 10, 2009 by Michael Wetter:
  First implementation.