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Combined c-44-50

This document describes an experiment to determine the combined convective and radiative heat transfer from a cylindrical surface at varying air velocities. The experimental setup consists of an electrically heated cylinder mounted in a duct, with air velocity controlled by a butterfly valve and measured by a hot wire anemometer. Temperature measurements are taken on the cylinder surface and in the free stream air. Heat input is varied, and the system is allowed to reach stability before recording readings. Heat transfer rates are calculated using equations that account for both radiation and convection. Results are tabulated and used to determine the combined heat transfer at different air velocities and compare to theoretical values.

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Vidhya Nair
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40 views7 pages

Combined c-44-50

This document describes an experiment to determine the combined convective and radiative heat transfer from a cylindrical surface at varying air velocities. The experimental setup consists of an electrically heated cylinder mounted in a duct, with air velocity controlled by a butterfly valve and measured by a hot wire anemometer. Temperature measurements are taken on the cylinder surface and in the free stream air. Heat input is varied, and the system is allowed to reach stability before recording readings. Heat transfer rates are calculated using equations that account for both radiation and convection. Results are tabulated and used to determine the combined heat transfer at different air velocities and compare to theoretical values.

Uploaded by

Vidhya Nair
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Experiment

Combined Convection and Radiation Heat Transfer


Introduction
The combined convection and radiation system has been designed to demonstrate heat transfer
from a solid surface to its surroundings. A hot surface loses heat to its surroundings by the
combined heat transfer modes of convection and radiation. In practice, these modes are difficult
to isolate and, therefore, the analysis of the combined effects provides a meaningful result. The
heated surface studied here is a horizontal cylinder, which can be operated in free convection or
forced convection at variable air velocities. Measurements of the surface temperature and the
electrical power supplied to it allow the combined effects of radiation and convection to be
compared with theoretical values.

Aim
To determine the combined convective and radiative heat transfer from a cylindrical surface at
varying air flow velocities.

Experimental set up
The experimental set up consists of a cylindrical duct mounted on the discharge of a base mounted
centrifugal blower. In the middle of the duct is a hot wire anemometer that allows the air velocity
within the duct to be measured. At the top of the duct is mounted an electrically heated cylinder.
The mounting arrangement of the cylinder ensures that the heat loss by conduction to the wall of
the duct is minimized. A thermocouple attached close to the surface of the cylinder provides
temperature measurement. The surface of the cylinder is coated with a matt black finish which
gives an emissivity close to 1.0. The heater in the cylinder is rated at approximately 100 Watts at
240 volts AC. The air velocity is controlled by the use of an intake butterfly valve. The free stream
air temperature is measured by a duct mounted thermocouple. The cylinder diameter is 10 mm and
is 70 mm long.

Experimental procedure
● Switch ON the mains of the electrical supply.
● Rotate the voltage control clockwise to increase the voltage.
● Turn ON the main switch and the fan switch for forced convection mode.
● Adjust the butterfly valve (throttle valve) to achieve different air flow velocity.
● Allow the system to reach stability and record the readings.
● Repeat experiment for different heater input.
● Once the experimental procedure is complete, turn OFF the power and leave the fan
running for a short duration till the heated cylinder is cooled.

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Heat Transfer Lab
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Heat Transfer Lab
Working Formulae
Heat Input 𝑄𝐼 = 𝑉𝐼 Watts

Heat lost by radiation 𝑄𝑅 = ℎ𝑅 𝐴𝑠 (𝑇2 − 𝑇1 )


Where, AS = πDL
𝑇 4 2 −𝑇 4 1
ℎ𝑅 = ∈ 𝜎
𝑇2 −𝑇1

Heat lost by convection 𝑄𝐹 = ℎ𝐹 𝐴𝑠 (𝑇2 − 𝑇1 )

𝑘
ℎ𝐹 = 𝑁𝑢
𝑟

0.62𝑅𝑒 0.5 𝑃𝑟 0.33 𝑅𝑒 0.5


𝑁𝑢 = 0.3 + × (1 + ( ) )
0.4 .660.25 28200
(1 + ( 𝑃𝑟 ) )
𝑈𝑒𝐷
where, 𝑅𝑒 = 𝑣

𝑄𝑇 = 𝑄𝑅 + 𝑄𝐹

Where, hR = Radiative heat transfer coeffieient W/m2 K


AS = Surface area of the heating cylinder m2
D = Diameter of cylinder = 10 mm
L = Length of cylinder = 70 mm
s = emissivity of cylinder surface = 0.95
σ = Stefan Boltzmann constant = 5.67 × 10−8W/m2 K4
hF = Convective heat transfer coeffieient W/m2 K
k = Thermal conductivity of air at T1 W/m K
Nu = Nusselt number
Pr = Prandtl number at T1
Re = Reynold number
ν = Kinematic viscosity of air at T1 m2/s
Ue = Effective velocity of air = Ua × 1.22 m/s

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Heat Transfer Lab
Observations Tabulations and Calculations

Sl Volts Amps T1◦C T2◦C


No V I

Sample Calculations

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Heat Transfer Lab
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Heat Transfer Lab
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Heat Transfer Lab
Result
The combined convective and radiative heat transfer of air at forced convective conditions were
determined as follows.

Inference

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Heat Transfer Lab

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