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Lab Manuals for Thermal Engineering- II - TE-II By Smart Brain

  • Thermal Engineering- II - TE-II
  • Practical
  • Anna University -
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Date: Exp No: STEFAN BOLTZMANN APPARATUS AIM: To determine the value of Stefan Boltzmann constant for radiation heat transfer. APPARATUS: Hemisphere, Heater, Temperature indicator, Stopwatch. THEORY: Stefan Boltzmann law states that the total emissive power of a perfect black body is proportional to fourth power of the absolute temperature of black body surface. Eb = σT4 Where σ = Stefan Boltzmann constant = 5.6697 x 10-8 W/(m² K4) DESCRIPTION: The apparatus consists of a flanged copper hemisphere fixed on a flat nonconducting plate. A test disc made of copper is fixed to the plate. Thus the test disc is completely enclosed by the hemisphere. The outer surface of the hemisphere is enclosed in a vertical water jacket used to heat the hemisphere to a suitable constant temperature. Three Cr-Al thermocouples are attached at three strategic places on the surface of the hemisphere to obtain the temperatures. The disc is mounted on an ebonite rod which is fitted in a hole drilled at the center of the base plate. Another Cr-Al thermocouple is fixed to the disc to record its temperature. Fill the water in the SS water container with immersion heater kept on top of the panel. SPECIFICATIONS: Specimen material : Copper Size of the disc :  20mm x 0.5mm thickness Base Plate :  250mm x 12mm thickness (hylam) 52

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Heater : 1.5 kW capacity, immersion type Copper Bowl :  200mm Digital temperature indicator : 0 -199.9° C Thermocouples used : 3 nos. on hemisphere Stop Watch : Digital type Overhead Tank : SS, approx. 12 liter capacity Water Jacket :  230 mm, SS Mass of specimen, ‘m’ : 5 gm Specific heat of the disc Cp : 0.38 kJ/kg K PROCEDURE: 1. Remove the test disc before starting the experiment. 2. Allow water to flow through the hemisphere, Switch on the heater and allow the hemisphere to reach a steady state temperature. 3. Note down the temperatures T1,T2 & T3. The average of these temperatures is the hemisphere temperature Th . 4. Insert the test disc at the bottom of the hemisphere and lock it. Start the stop clock simultaneously. 5. Note down the temperature of the test disc at an interval of about 15 sec for about 15 to 20 minutes. 53

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OBSERVATION TABLE: Let Td = Temperature of the disc before inserting into the plate in K Thermocouple Temperature of the copper hemisphere ° C T1 T2 T3 Th Average of T1 , T2 and T3 = Temperature – time response of test disc: Time ‘t’ sec Temper Time Temper ature Td ‘t’ ature Td °C sec °C CALCULATIONS: 1. Plot the graph of temperature of the disc v/s time to obtain the slope (dT/dt) of the line, which passes through/nearer to all points. 2. Average temperature of the hemisphere Th  T1  T2  T3  273.15 3 54

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3. Td = Temperature of the disc before inserting to Test chamber º K (ambient) 4. Rate of change of heat capacity of the disc = mC p dT dt Net energy radiated on the disc = σ Ad (T4h – T4d) Where  d2 = area of the disc = d Cp = = 20 mm specific heat of copper = 0.38 kJ/kg–K 4 in m2 Ad Rate of change of heat capacity of the disc = Net energy radiated on the disc mC p dT = σ Ad (T4avg – T4d) dt Thus ‘σ’ can be evaluated as shown dT dt   4 4 A d (T avg - Td ) mC p Result: The experiment on Stefan Boltzmann apparatus has been conducted and the value of Stefan Boltzmann constant is determined. 55

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