
Modeling the Pressure Loss Through a Radiator
A radiator is considered to be infinitely thin, and the pressure drop through the radiator is assumed to be proportional to the dynamic head of the fluid, with an empirically determined loss coefficient which you supply. That is, the pressure drop, , varies with the normal component of velocity through the radiator, , as follows:
where is the fluid density, and is the nondimensional loss coefficient, which can be specified as a constant or as a polynomial, piecewiselinear, or piecewisepolynomial function.
In the case of a polynomial, the relationship is of the form
where are polynomial coefficients and is the magnitude of the local fluid velocity normal to the radiator.
Modeling the Heat Transfer Through a Radiator
The heat flux from the radiator to the surrounding fluid is given as
where is the heat flux, is the temperature downstream of the heat exchanger (radiator), and is the reference temperature for the liquid. The convective heat transfer coefficient, , can be specified as a constant or as a polynomial, piecewiselinear, or piecewisepolynomial function.
For a polynomial, the relationship is of the form
where are polynomial coefficients and is the magnitude of the local fluid velocity normal to the radiator in m/s.
Either the actual heat flux or the heat transfer coefficient and radiator temperature may be specified. (either the entered value or the value calculated using Equation 7.213) is integrated over the radiator surface area.
Calculating the Heat Transfer Coefficient
To model the thermal behavior of the radiator, you must supply an expression for the heat transfer coefficient, , as a function of the fluid velocity through the radiator, . To obtain this expression, consider the heat balance equation:
where
=  heat flux (W/m )  
=  fluid mass flow rate (kg/s)  
=  specific heat capacity of fluid (J/kgK)  
=  empirical heat transfer coefficient (W/m K)  
=  external temperature (reference temperature for the liquid) (K)  
=  temperature downstream from the heat exchanger (K)  
=  heat exchanger frontal area (m ) 
Equation 7.215 can be rewritten as
where is the upstream air temperature. The heat transfer coefficient, , can therefore be computed as
(7.217) 
or, in terms of the fluid velocity,
(7.218) 