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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. The reason is that the convective heat transfer coefficient is not differentiable around zero for certain 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

NameDescription
Buildings.HeatTransfer.Convection.Functions.HeatFlux.constantCoefficient constantCoefficient Constant convective heat transfer coefficient
Buildings.HeatTransfer.Convection.Functions.HeatFlux.wall wall Free convection, wall
Buildings.HeatTransfer.Convection.Functions.HeatFlux.floor floor Free convection, floor
Buildings.HeatTransfer.Convection.Functions.HeatFlux.ceiling ceiling Free convection, ceiling
Buildings.HeatTransfer.Convection.Functions.HeatFlux.rayleigh rayleigh Rayleigh number with smooth transition to lower limit
Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses BaseClasses Base classes for convective heat transfer coefficients

Buildings.HeatTransfer.Convection.Functions.HeatFlux.constantCoefficient

Constant convective heat transfer coefficient

Information

This function computes the convective heat transfer coefficient as h=hCon, where hCon=3 is a default input argument. The convective heat flux is q_flow = h * dT, where dT is the solid temperature minus the fluid temperature.

Extends from Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux (Partial function for convective heat flux).

Inputs

TypeNameDefaultDescription
TemperatureDifferencedT Temperature difference solid minus fluid [K]
CoefficientOfHeatTransferhCon3Constant for convective heat transfer coefficient [W/(m2.K)]

Outputs

TypeNameDescription
HeatFluxq_flowConvective heat flux from solid to fluid [W/m2]

Modelica definition

function constantCoefficient 
  "Constant convective heat transfer coefficient"
  extends Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux;
  input Modelica.SIunits.CoefficientOfHeatTransfer hCon = 3 
    "Constant for convective heat transfer coefficient";
algorithm 
  q_flow :=hCon*dT;
end constantCoefficient;

Buildings.HeatTransfer.Convection.Functions.HeatFlux.wall

Free convection, wall

Information

This function computes the buoyancy-driven convective heat transfer coefficient for a wall as h=1.3*|dT|^0.3333, where dT is the solid temperature minus the fluid temperature. The convective heat flux is then q_flow = h * dT.

Extends from Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux (Partial function for convective heat flux).

Inputs

TypeNameDefaultDescription
TemperatureDifferencedT Temperature difference solid minus fluid [K]

Outputs

TypeNameDescription
HeatFluxq_flowConvective heat flux from solid to fluid [W/m2]

Modelica definition

function wall "Free convection, wall"
  extends Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux;
algorithm 
  q_flow := noEvent(smooth(1, if (dT > 0) then 1.3*dT^1.3333 else -1.3*(-dT)^1.3333));

end wall;

Buildings.HeatTransfer.Convection.Functions.HeatFlux.floor

Free convection, floor

Information

This function computes the buoyancy-driven convective heat transfer coefficient for a floor as h=k*|dT|^0.3333, where k=1.51 if the floor is warmer than the fluid, or k=0.76 otherwise, and where dT is the solid temperature minus the fluid temperature. The convective heat flux is then q_flow = h * dT.

Extends from Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux (Partial function for convective heat flux).

Inputs

TypeNameDefaultDescription
TemperatureDifferencedT Temperature difference solid minus fluid [K]

Outputs

TypeNameDescription
HeatFluxq_flowConvective heat flux from solid to fluid [W/m2]

Modelica definition

function floor "Free convection, floor"
  extends Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux;

algorithm 
  q_flow  := noEvent(smooth(1, if (dT>0) then 1.51*dT^1.3333 else -0.76*(-dT)^1.3333));
end floor;

Buildings.HeatTransfer.Convection.Functions.HeatFlux.ceiling

Free convection, ceiling

Information

This function computes the buoyancy-driven convective heat transfer coefficient for a ceiling as h=k*|dT|^0.3333, where k=1.51 if the fluid is warmer than the ceiling, or k=0.76 otherwise, and where dT is the solid temperature minus the fluid temperature. The convective heat flux is then q_flow = h * dT.

Extends from Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux (Partial function for convective heat flux).

Inputs

TypeNameDefaultDescription
TemperatureDifferencedT Temperature difference solid minus fluid [K]

Outputs

TypeNameDescription
HeatFluxq_flowConvective heat flux from solid to fluid [W/m2]

Modelica definition

function ceiling "Free convection, ceiling"
  extends Buildings.HeatTransfer.Convection.Functions.HeatFlux.BaseClasses.PartialHeatFlux;

algorithm 
   q_flow  := noEvent(smooth(1, if (dT>0) then 0.76*dT^1.3333 else -1.51*(-dT)^1.3333));
end ceiling;

Buildings.HeatTransfer.Convection.Functions.HeatFlux.rayleigh

Rayleigh number with smooth transition to lower limit

Information

This function returns the Rayleigh number. The parameter RaMin 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)).

Inputs

TypeNameDefaultDescription
Lengthx Layer thickness [m]
Densityrho Mass density [kg/m3]
SpecificHeatCapacityc_p Specific heat capacity [J/(kg.K)]
DynamicViscositymu Dynamic viscosity [Pa.s]
ThermalConductivityk Thermal conductivity [W/(m.K)]
TemperatureT_a Temperature of surface a [K]
TemperatureT_b Temperature of surface b [K]
RealRa_min Minimum value for Rayleigh number

Outputs

TypeNameDescription
RealRaRayleigh number

Modelica definition

function rayleigh 
  "Rayleigh number with smooth transition to lower limit"
 input Modelica.SIunits.Length x "Layer thickness";
 input Modelica.SIunits.Density rho "Mass density";
 input Modelica.SIunits.SpecificHeatCapacity c_p "Specific heat capacity";
 input Modelica.SIunits.DynamicViscosity mu "Dynamic viscosity";
 input Modelica.SIunits.ThermalConductivity k "Thermal conductivity";
 input Modelica.SIunits.Temperature T_a "Temperature of surface a";
 input Modelica.SIunits.Temperature T_b "Temperature of surface b";
 input Real Ra_min "Minimum value for Rayleigh number";
 output Real Ra "Rayleigh number";
protected 
 Modelica.SIunits.TemperatureDifference dT "Temperature difference";
algorithm 
  dT :=abs(T_a - T_b);
  Ra := rho^2*x^3*Modelica.Constants.g_n*c_p*dT/((T_a+T_b)/2*mu*k);
  Ra := Buildings.Utilities.Math.Functions.smoothMax(x1=Ra, x2=Ra_min, deltaX=Ra_min/10);
end rayleigh;

Automatically generated Thu Oct 24 15:10:21 2013.