Buildings.HeatTransfer.Convection.Functions.HeatFlux
Correlations for convective heat flux
Information
This package contains functions for the convective heat transfer. Input into the functions is the temperature difference between the solid and the fluid. The functions compute the convective heat flux, rather than the convective heat transfer coefficient, because the convective heat transfer coefficient is not differentiable around zero for some flow configurations, such as buoyancy driven flow at a horizontal surface. However, the product of convective heat transfer coefficient times temperature difference is differentiable around zero.
Package Content
| Name | Description |
|---|---|
 ceiling
|
Free convection, ceiling |
| constantCoefficient
|
Constant convective heat transfer coefficient |
| floor
|
Free convection, floor |
| rayleigh
|
Rayleigh number with smooth transition to lower limit |
| wall
|
Free convection, wall |
 BaseClasses
|
Base classes for convective heat transfer coefficients |
Buildings.HeatTransfer.Convection.Functions.HeatFlux.ceiling
Free convection, ceiling
Information
This function computes the buoyancy-driven convective heat flux as
q̇ = h ΔT,
where ΔT is the solid temperature minus the fluid temperature and h is the convective heat transfer coefficient for a ceiling, computed as
h=k |ΔT|0.3333,
where k=1.51 if the fluid is warmer than the ceiling, or k=0.76 otherwise.
Extends from Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux (Partial function for convective heat flux).
Inputs
| Type | Name | Default | Description |
|---|---|---|---|
| TemperatureDifference | dT | Temperature difference solid minus fluid [K] |
Outputs
| Type | Name | Description |
|---|---|---|
| HeatFlux | q_flow | Convective heat flux from solid to fluid [W/m2] |
Modelica definition
Buildings.HeatTransfer.Convection.Functions.HeatFlux.constantCoefficient
Constant convective heat transfer coefficient
Information
This function computes the buoyancy-driven convective heat flux as
q̇ = h ΔT,
where ΔT is the solid temperature minus the fluid temperature and h is the convective heat transfer coefficient.
Extends from Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux (Partial function for convective heat flux).
Inputs
| Type | Name | Default | Description |
|---|---|---|---|
| TemperatureDifference | dT | Temperature difference solid minus fluid [K] | |
| CoefficientOfHeatTransfer | hCon | 3 | Constant for convective heat transfer coefficient [W/(m2.K)] |
Outputs
| Type | Name | Description |
|---|---|---|
| HeatFlux | q_flow | Convective heat flux from solid to fluid [W/m2] |
Modelica definition
Buildings.HeatTransfer.Convection.Functions.HeatFlux.floor
Free convection, floor
Information
This function computes the buoyancy-driven convective heat flux as
q̇ = h ΔT,
where ΔT is the solid temperature minus the fluid temperature and h is the convective heat transfer coefficient for a floor, computed as
h=k |ΔT|0.3333,
where k=0.76 if the fluid is warmer than the floor, or k=1.51 otherwise.
Extends from Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux (Partial function for convective heat flux).
Inputs
| Type | Name | Default | Description |
|---|---|---|---|
| TemperatureDifference | dT | Temperature difference solid minus fluid [K] |
Outputs
| Type | Name | Description |
|---|---|---|
| HeatFlux | q_flow | Convective heat flux from solid to fluid [W/m2] |
Modelica definition
Buildings.HeatTransfer.Convection.Functions.HeatFlux.rayleigh
Rayleigh number with smooth transition to lower limit
Information
This function returns the Rayleigh number. The parameterRaMin is used to transition
to a lower limit for the Rayleigh number.
This is helpful to avoid a Rayleigh number of zero or
to avoid an expression for a convection coefficient that
has an infinite derivative near zero, i.e., if h=f(Ra(1/2)).
Extends from Modelica.Icons.Function (Icon for functions).
Inputs
| Type | Name | Default | Description |
|---|---|---|---|
| Length | x | Layer thickness [m] | |
| Density | rho | Mass density [kg/m3] | |
| SpecificHeatCapacity | c_p | Specific heat capacity [J/(kg.K)] | |
| DynamicViscosity | mu | Dynamic viscosity [Pa.s] | |
| ThermalConductivity | k | Thermal conductivity [W/(m.K)] | |
| Temperature | T_a | Temperature of surface a [K] | |
| Temperature | T_b | Temperature of surface b [K] | |
| Real | Ra_min | Minimum value for Rayleigh number |
Outputs
| Type | Name | Description |
|---|---|---|
| Real | Ra | Rayleigh number |
Modelica definition
Buildings.HeatTransfer.Convection.Functions.HeatFlux.wall
Free convection, wall
Information
This function computes the buoyancy-driven convective heat flux as
q̇ = h ΔT,
where ΔT is the solid temperature minus the fluid temperature and h is the convective heat transfer coefficient for a wall, computed as
h=1.3 |ΔT|0.3333.
Extends from Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux (Partial function for convective heat flux).
Inputs
| Type | Name | Default | Description |
|---|---|---|---|
| TemperatureDifference | dT | Temperature difference solid minus fluid [K] |
Outputs
| Type | Name | Description |
|---|---|---|
| HeatFlux | q_flow | Convective heat flux from solid to fluid [W/m2] |