UNSTEADY-STATE HEAT CONDUCTION - II

1. UNSTEADY-STATE HEAT CONDUCTION - II P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi Applications where rate/duration of heating/cooling is a Design Parameter……

2. Relationship between the Biot number and the temperature profile.

3. Hot Rolling of Steel Sheets

4. One Dimensional Transient Conduction Governing Differential Equation: Initial Condition: T(x,o)= T0 Boundary Conditions: T(L,t)=TsT

5. Non-dimensionalization of GDE Define a Non dimensional variable for the x-coordinate Define a Non dimensional variable for the temperature: Substitute Dimensionless variables into GDE:

6. Define thermal diffusivity: Define non dimensional variable for time

7. A Pure Dimensionless GDE Initial condition: q(h,0)= 1 Boundary conditions: q(1,z)= 0 At any time Temperature profile will be symmetric about x-axis. Solution in positive or negative direction of x is sufficient.

8. Simplified Problem 1 Initial condition: q(h,0)= 1 Boundary conditions: q(1,z)= 0 0 1 h

11. The boundary conditions are which requires B = 0 Then apply the boundary conditions at the other end which requires cosλ = 0

12. The solution corresponding to the n-th eigenvalue is The general solution is the sum over all n’s The constants Anare determined from the initial conditions

16. Relationship between the Biot number and the temperature profile.

17. Systems with Negligible Surface Resistance • Homeotherm is an organism, such as a mammal or bird, having a body temperature that is constant and largely independent of the temperature of its surroundings.

18. Biot Number of Small Birds

19. Biot Number of Big Birds

20. Very Large Characteristic Dimension

21. Very Large Characteristic Dimension The United States detonated an atomic bomb over Nagasaki on August 9, 1945. The bombings of Nagasaki and Hiroshima immediately killed between 100,000 and 200,000 people and the only instances nuclear weapons have been used in war.

22. The semi-infinite solid Governing Differential Equation: Boundary conditions x = 0 :T = Ts As x → ∞ :T → T0 Initial condition t = 0 :T = T0

23. Notice that there is no natural length-scale in the problem. Indeed, the only variables are T, x, t, and α.

24. Transform the derivatives :

29. h

31. One-dimensional Transient Conduction • One-dimensional transient conduction refers to a case where the temperature varies temporally and in one spatial direction. • For example, temperature varies with x and time. • Three cases of 1-D conduction are commonly studied: conduction through a plate, in a cylinder, and in a sphere. • In all three cases, the surface of the solid is exposed to convection. • The exact analytical solutions to the three cases are very complicated. • However, an approximate solution can be obtained by using graphical tools. • The graphs allow you to find the centerline temperature at any given time, and the temperature at any location based on the centerline temperature.