Experiment - 09 PDF

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Name of lab experiment:Newton's Law of Cooling...

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Subject: Heat Transfer Lab -------------------------------------------------------------------------------------------------------------------

Experiment-09 Name of the experiment: Lee's Disc Apparatus

Aim of the experiment: To determine the coefficient of thermal conductivity of a bad conductor using Lee's disc apparatus.

Apparatus: Lee's disc apparatus consist of a metallic disc resting on a 5 cm deep hollow cylinder (steam chamber ) of same diameter. It has inlet and outlet tubes for steam. In addition, it has radial holes to insert thermometers. Thermal conductivity is the property of a material . It indicates the ability of a material to conduct heat. When steam is passed through the cylindrical vessel a steady state is reached soon. At the steady state, heat conducted through the bad conductor is equal to heat radiated from the Lees disc.

Theory: The Lee’s Disc experiment determines an approximate value for the thermal conductivity k of a poor conductor like glass, cardboard, etc. The procedure is to place a disc made of the poor conductor, radius r and thickness x, between a steam chamber and two good conductivity metal discs (of the same metal) and allow the setup to come to equilibrium, so that the heat lost by the lower disc to convection is the same as the heat flow through the poorly conducting disc. The upper disc temperature T2 and the lower disc temperature T1 are recorded. The poor conductor is removed and the lower metal disc is allowed to heat up to the upper disc temperature T2. Finally, the steam chamber and upper disc are removed and replaced by a disc made of a good insulator. The metal disc is then allowed to cool through T1 < T2 and toward room temperature T0. The temperature of the metal disc is recorded as it cools so a cooling curve can be plotted. Then the slope s1 =ΔT/Δt of the cooling curve is measured graphically where the curve passes through temperature T1.

Procedure for Simulation: 1.Select the disc material from the drop down combo box. 2.Adjust disc's mass, height and radius by using the sliders. 3.Select the insulator from the drop down combo box. 4.Adjust the thickness of the insulator. 5.Place the insulator by clicking the 'Place Insulator' button. 6.After placing the insulator, start the steamer by clicking 'Start Steam' button. 7.After some time the heat chamber temperature(T1) and the Lee's disc temperature(T2) will become steady. 8.Then remove the object from the Lee's disc by clicking on the 'Remove Insulator' button (the

'Place Insulator' button will change to 'Remove Insulator' after step 5). 9.The temperature of T2 will rise again; stop the heater when the reading of T2 reaches 10 degrees above the steady temperature. The 'Stop Steam' button can be used to do this. 10.Now the temperature at the Lee's disc will start to drop. Start the stop watch, and note down the temperature at T2 from 5 degrees above the steady temperature to 5 degrees below that at each 30 seconds. Using these points a graph has to be plotted and dT/dt can be obtained from it and the Κ value can be calculated. 11.The user can view the graph by clicking on the 'show graph' check box.

Procedure for Real Lab: 1.Determine the mean thickness of metal disc and bad conductor with a screw gauge. 2.Determine the diameter of metal disc and bad conductor with vernier calipers. 3.Find the mass M of the metal disc by a balance. 4.Keep the bad conductor between metal disc and steam chamber. 5.Introduce thermometers through holes in the steam chamber and in the metal disc. 6.Pass steam through the chamber until the temperature indicated by thermometers become steady and note the steady temperature. 7.Remove the bad conductor. 8.Remove the steam chamber when the temperature of the metal disc is 10 0 C above it's steady temperature. 9.Start a stop clock and take time-temperature observation till the temperature of the disc is 10 0 C below it's steady state. 10.Plot a time-temperature graph....