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COMBINED CONVECTION AND RADIATION
YEDITEPE UNIVERSITY DEPARTMENT OF
MECHANICAL ENGINEERING
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YEDITEPE UNIVERSITY ENGINEERING FACULTY
MECHANICAL ENGINEERING LABORATORY
Combined Convection and Radiation
1. Objective:
&
To determine the combined heat transfer (Qradiation + Qconvection) from a
horizontal cylinder in natural convection over a wide range of power inputs and
corresponding surface temperatures.To demonstrate the relationship between power
input and surface temperature in free convection.
&
To compare the contribution of heat transfer by convection with heat transfer by
radiation and from the measurements to show the domination of the convective heat
transfer coefficient Hc at low surface temperatures and the domination of the radiation
heat transfer coefficient Hr at high surface temperatures.
&
To determine the effect of forced convection on heat transfer from the surface of a
cylinder at varying air velocities and surface temperatures.To demonstrate the
relationship between air velocity and surface temperature for a cylinder subjected to
forced convection.
2. Equipment:
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The 'Combined Convection and Radiation' accessory
A centrifugal fan
with a vertical outlet duct
at the top of which is mounted a heated,
horizontal cylinder.
&
One K type Thermocouple is fitted in the wall of the duct and one K type
thermocouple attached to the heated cylinder.
&
HT10X Heat Transfer Service Unit
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IFD5 PC Interface Console
&
IBM Compatible PC
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3. Theory:
3.1 Natural Convection and Radiation
If a surface, at a temperature above that of its surroundings, is located in stationary air at the same
temperature as the surroundings then heat will be transferred from the surface to the air and
surroundings. This transfer of heat will be a combination of natural convection to the air (air heated
by contact with the surface becomes less dense and rises) and radiation to the surroundings. A
horizontal cylinder is used in this exercise to provide a simple shape from which the heat transfer can
be calculated.
Note: Heat loss due to conduction is minimised by the design of the equipment and measurements
mid way along the heated section of the cylinder can be assumed to be unaffected by conduction at
the ends of the cylinder. Heat loss by conduction would normally be included in the analysis of a real
application.
In the case of natural (free) convection the Nusselt number Nu depends on the Grashof and Prandtl
numbers and the heat transfer correlation can be expressed in the form:
Nu = f(Gr, Pr) and the Rayleigh number Ra = (Gr Pr)
The following theoretical analysis uses an empirical relationship for the heat transfer due to natural
convection proposed by VT Morgan in the paper "The Overall Convective Heat Transfer from
Smooth Circular Cylinders" published in TF Irvine and JP Hartnett (eds.), Advances in Heat Transfer
vol. 16, Academic, New York, 1975, pp 199269.
Ts = Surface temperature of cylinder (K)
D = Diameter of cylinder
(m)
L = Heated length of cylinder
Ta = Ambient temperature of air
(K)
Heat transfer area (surface area)
As = (pDL)
m²
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This note was uploaded on 11/30/2010 for the course UNKNOWN a taught by Professor A during the Spring '10 term at Boston Conservatory.
 Spring '10
 a

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