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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 non-dimensional loss coefficient, which can be specified as a constant or as a polynomial, piecewise-linear, or piecewise-polynomial 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, piecewise-linear, or piecewise-polynomial 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
6.21-3) 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
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= | heat flux (W/m
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= | fluid mass flow rate (kg/s) |
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= | specific heat capacity of fluid (J/kg-K) |
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= | empirical heat transfer coefficient (W/m
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= | external temperature (reference temperature for the liquid) (K) |
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= | temperature downstream from the heat exchanger (K) |
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= | heat exchanger frontal area (m
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Equation 6.21-5 can be rewritten as
where
is the upstream air temperature. The heat transfer coefficient,
, can therefore be computed as
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(6.21-7) |
or, in terms of the fluid velocity,
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(6.21-8) |