packageFlowModels

Flow models for pressure drop calculations

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

Information

This package contains basic flow models that are 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 power-law relation is replaced by a function that has a 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 four main functions are

These functions compute the mass flow rate or the pressure drop, respectively. The first two functions assume that the flow resistance is quadratic in the mass flow rate, and the other two functions allow for a flow exponent between 1 and 2. All these functions are two times continuously differentiable. First and second order derivatives are provided in the function that have the suffix _der and _der2.

For the powerLaw functions, the coefficients that are an argument to the functions can be computed using the function Buildings.Fluid.BaseClasses.FlowModels.powerLawData. This computation is done outside the above functions because the arguments generally are all parameters, and hence precomputing them avoid repetitive evaluation.

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 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
powerLawData	Function that computes the coefficients used by powerLaw_dp and powerLaw_m_flow
powerLaw_dp	Power law used in pressure drop equations when the flow exponent is constant and may be different from 2
powerLaw_dp_der	1st derivative of function that computes mass flow rate for given pressure drop
powerLaw_dp_der2	2nd derivative of function that computes mass flow rate for given pressure drop
powerLaw_m_flow	Inverse of power law used in pressure drop equations when the flow exponent is constant and may be different from 2
powerLaw_m_flow_der	1st derivative of function that computes pressure drop for given mass flow rate
powerLaw_m_flow_der2	2nd derivative of function that computes pressure drop for given mass flow rate
Validation	Collection of validation models