Armfield Engineering Education
HEAT TRANSFER AND THERMODYNAMICS
HT14 Combined Convection and Radiation - Issue 6
A hot surface loses heat (heat is transferred) to its surroundings
by the combined modes of convection and radiation. In practice these modes
are difficult to isolate and therefore an analysis of the combined effects
at varying surface temperature and air velocity past the surface provides
a meaningful teaching exercise. The heated surface studied is a horizontal
cylinder which can be operated in free convection or forced convection when
located in the stream of moving air. Measurement of the surface temperature
of the uniformly heated cylinder and the electrical power supplied to it
allows the combined effects of radiation and convection to be compared with
theoretical values. The dominance of convection at lower surface temperatures
and the dominance of radiation at higher surface temperatures can be demonstrated
as can the increase in heat transfer due to forced convection.
Ordering Specification
• A small scale accessory designed to introduce students to the principles
of combined convection (free and forced) with radiation from a horizontal
heated cylinder.
• Comprises a heated cylinder mounted in a vertical air duct, with
a fan at the base of the duct which can be used to provide air flow over
the cylinder.
• Heater rating is 100 Watts at 24V DC.
• The air flow is manually adjustable over the range 0-7m/s.
• The air flow is measured by a vane type anemometer in the outlet
duct.
• The accessory is mounted on a PVC baseplate which is designed to
stand on the bench top and connect to the Heat Transfer Service Unit without
the need for tools.
• A comprehensive instruction manual is supplied.
Technical Details
The equipment consists of a centrifugal fan with vertical outlet duct at the top of which is mounted a heated cylinder. The heated cylinder has an outside diameter of 10mm, a heated length of 70mm and is internally heated throughout its length by an electric heating element which is operated at low voltage for increased operator safety.
The heating element is rated to produce 100 Watts nominally at 24V DC into the cylinder. The power supplied to the heated cylinder can be varied and measured on the HT10X. Electrical connections to the cylinder incorporate temperature resistant insulation with plug connection to the variable 24V DC supply socket on HT10X. The mounting arrangement for the cylinder in the duct is designed to minimise loss of heat by conduction to the wall of the duct. The surface of the cylinder is coated with heat resistant paint which provides a consistent emissivity close to unity. A K-type thermocouple attached to the wall of the cylinder, at mid position, allows the surface temperature to be measured under the varying operating conditions. The thermocouple is fitted with a standard plug for direct connection to the service unit HT10X. A variable throttle plate at the inlet to the fan allows the velocity of the air through the outlet duct to be varied and a vane type anemometer within the fan outlet duct allows the air velocity in the duct to be measured over the range 0-7 metres/sec. The inside diameter of the outlet duct is 70mm. A K-type thermocouple located in the outlet duct allows the ambient air temperature to be measured upstream of the heated cylinder. The thermocouple is fitted with a miniature plug for direct connection to the service unit HT10X. The accessory is designed to be used in conjunction with the HT10X Heat Transfer Service Unit and is assembled on a mounting plate to allow it to stand on the bench alongside the HT10X.
Experimental Capabilities
• Determining the combined heat transfer (Q radiation + Q convection
) from a horizontal cylinder in natural convection over a wide range of
power inputs and corresponding surface temperatures.
• Measuring 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
• Determining the effect of forced convection on the heat transfer
from the cylinder at varying air velocities
Essential Armfield Equipment
HT10X Heat Transfer Service Unit
Optional Armfield Accessories
HT10X-304 Educational Software with USB interface
Requirements
HT14-A: 220/240V/1ph/50Hz @ 1.5A
HT14-B: 120V/1ph/60Hz @ 3A
HT14-G: 220V/1ph/60Hz @ 1.5A
Shipping Specification
Volume: 0.1m³
Gross Weight: 9kg
Overall dimensions
Height: 1.20m
Width: 0.35m
Depth: 0.30m
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