MECH352-Lab-3 (1).docx - MECH 352 HEAT TRANSFER 1 EXPERIMENT 3 Combined Convection and Radiation Performed on 07 October 2019 Submitted on 21 October

MECH352-Lab-3 (1).docx - MECH 352 HEAT TRANSFER 1...

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MECH 352: HEAT TRANSFER 1 EXPERIMENT 3: Combined Convection and Radiation Performed on 07 October 2019 Submitted on 21 October 2019
OBJECTIVE This experiment is to show the effects of natural and forced convection on the rate of heat transfer and how it differs to generate each type of convections on a simple cylinder. INTRODUCTION Exercise A : NATURAL CONVECTION Heat is transferred by means of two mechanisms, natural convection and forced convection, keeping in mind that radiation occurs simultaneously in both cases. Natural convection occurs when the surrounding air is relatively stagnant and there exists a temperature difference with the surface area of the exposed body (cylinder) and the air. The natural convection then transfers heat from the hot body to the surrounding air, which results in the rise of heated air due to decreased density, also heat is transferred via radiation. Fig 1. Apparatus that natural and forced convections will occur Natural convection makes the rate of heat transfer limited by slight movements of air, because of the air that gained thermal energy and the replacement of colder air at the hot surface, and so on. 1
Some important equations that will be used in this Exercise are the followings: Total heat loss: Q tot = Q c + Q r The average heat transfer coefficient h cm can be obtained using the below relationships: T film = T s + T a 2 β = 1 T film Gr D = ( T s T a ) D 3 ν 2 Since Ra D = Gr D * Pr → Ra D = ( T s T a ) D 3 ν 2 Pr Mean Nusselt number: Nu m = c ( Ra D ) n Average heat transfer coefficient for natural convection: h cm = k Nu m D Average heat transfer coefficient for radiation: T ( ¿¿ s 4 T a 4 ) T s T a ¿ h rm = σ ξ F ¿ Exercise B : FORCED CONVECTION In forced convection condition, the air movement can be greatly increased by an external means such as a fan, compressor, etc. Now, air will be moving at a much faster rate, which results in a higher rate of heat transfer. Thus, the exposed body to forced convection will cool the body quicker and more efficient than natural convection. As the air flows faster, the quicker the hot body cools off, thanks to the increased rate of heat transfer. Important equations for forced convection: Average heat transfer coefficient for forced convection: h fm = k Nu m D Mean Nusselt number: Nu m = 0.3 + 0.62 0.5 Pr 0.33 [ 1 + ( 0.4 Pr ) 0.56 ] 0.25 [ 1 + ( 282000 ) 0.5 ] Reynolds number: ℜ= U c D ν 2
PROCEDURE: Exercise A 1. Make sure the voltage control potentiometer was adjusted to 5 Volts while the selector switch was set to MANUAL. 2. Give 15 mins for each change of the knob for the temperature to stabilize at the desired change. 3. V, I, T9 and T10 values were then recorded. 4. Steps 1 to 3 were then repeated for 8, 12, 15, and 20 Volts. 5. Record the data on the data sheet.

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