MECH-352-LAB4 (1).docx - MECH 352 HEAT TRANSFER 1 EXPERIMENT 4 Unsteady State Heat Transfer Submitted on 4th November 2019 OBJECTIVES This experiment is

MECH-352-LAB4 (1).docx - MECH 352 HEAT TRANSFER 1...

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MECH 352: HEAT TRANSFER 1 EXPERIMENT 4: Unsteady State Heat Transfer Submitted on 4 th November 2019
OBJECTIVES This experiment is to exhibit the unsteady state heat conduction with multiple specimens made of steel and brass. These specimens are dunked in a hot bath of constant temperature. INTRODUCTION For a given specimen of room temperature, when it is immersed in hot water of temperature t, the specimen will have its temperature increases, until the whole volume (including the core) reaches the ambient temperature of the hot water. However, it took some time gaining that thermal energy. This period is known as unsteady state heat transfer. In that state, there exists a temperature gradient from the center of the specimen to the surface of the shape and to the ambient room temperature. Unsteady state heat transfer will remain, until the temperature of the core is the same as the ambient temperature from outside, i.e. thermal equilibrium with the hot water. Figure 1 – Heisler Chart for Plane of Thickness 2L [1] 1
Figure 2 – Heisler Chart for Cylinder of Radius r 0 [1] Figure 3 – Heisler Chart for Sphere of Radius r 0 [1] In order to use the Heisler charts, the following dimensionless parameters must be calculated: 2
Dimensionless Temperature: θ = T ( r ,t ) T T i T Biot Number: Bi = hb k for cylinder/sphere or Bi = hL k Fourier Number: τ = αt b 2 for cylinder/sphere of τ = αt L 2 Where: T(r,t) is Temperature at Center [ ], R is Radial Position (at x-axis, r=0) [m], t is Time since step change [s], T_∞ is Temperature of water bath [ ], T i is Initial temperature [ ], h is Heat transfer coefficient [W/m2- ], k is Thermal conductivity [W/m- ], b is Radius of Sphere/Cylinder [m], L is Half thickness of slab [m], α is Thermal diffusivity [m 2 /s] PROCEDURE 1. The apparatus was set as instructed and the water was hot. 2. The thermostat on the water heater was set to 5, and the voltage supplied to the circulating pump was set to 12 volts. 3. Give some time about 15 minutes for the temperature of the water bath left to stabilize at a temperature between 80 and 90 . 4. Lock each specimen using the given set screw (one at a time), and the thermocouple for each sphere was attached to T 3 on the HT10X. 5. When the temperature of the specimen had stabilized at room temperature. 6. The specimen was then immersed into the hot water bath and data was recorded until the temperature T 3 is the same for the hot water for two datasets without changing. 7. The specimen was removed from the shape holder and start a new sheet to record the new dataset.

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