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BACKGROUND
Heat transfer coefficient is used as a characteristic to find the convective heat transfer between a fluid medium (viscous or non-viscous) and any surface flowing over the fluid. The complicated theory of heat transfer is used to find the heat transmitting capacity of a material as a result of temperature difference. Natural convection is the process of heat transfer through a fluid in the presence of bulk fluid motion by natural methods. Such form of convection is also known as ‘free convection’ which is caused due to a change in density between the fluid particles near the hot objects and the surrounding fluid.1 It is a continuous process, and the relative motion of hot and cold fluid particles causes the heat transfer. Heat transfer coefficient is given by:
H= (q-qa)/{As * (Ts – Ta)}
The apparatus used in this experiment contains brass tube which is vertically fitted in a rectangular tube. The opening of the duct is from two sides in which one is made up of Perspex for better visualisation. Seven thermocouples are used to measure the temperature of the vertical tube due to the effect of the heating element placed in it.2
The main objective of this experiment is to determine the heat transfer coefficient in a natural convection process.
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REQUIREMENTS
Materials: Brass tube (Dia. =38mm, Length= 500mm)
Natural convection Apparatus (Duct size= 200mm*200mm*800mm)
Thermocouple (Chromel/Alumel)
Ammeter (0-2 A)
Voltmeter (0-200 V)
Dimmer Stat (2A, 240V)
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PROCEDURE
First of all, switch ON the main supply and adjust the dimmer stat according to the required heat input of 40W, 60W, 70W, etc. Wait for the steady position in thermocouple in all the readings. Now, record the surface temperature from T1 to T7, and the ambient temperature, T8. Repeat the experiment to get the more accurate result by calculating the average temperature difference at different heat input. Now, calculate the heat transfer coefficient by using the formula illustrated in the background section.
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CONCLUSION
The method of determining the heat transfer coefficient in natural convection process has been studied. Natural convection process is a continuous process, and it does not require any force to start.3
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REFERENCES
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Bejan, Convection Heat Transfer, Wiley.
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R. J. G. T. H. Kuehn, “An experimental and theoretical study of natural convection in the annulus between horizontal concentric cylinders,” Journal of Fluid Mechanics, vol. 74, no. 4, pp. 695-719, 1976.
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T. Yuge, “Experiments on Heat Transfer From Spheres Including Combined Natural and Forced Convection,” Journal of heat Transfer, vol. 82, no. 3, pp. 210-220, 1960.
