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UNSTEADY-STATE HEAT CONDUCTION

UNSTEADY-STATE HEAT CONDUCTION. P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi. Applications where rate/duration of heating/cooling is a Design Parameter……. Measurement of Gas temperature. T 1. T 2 > T 1. T 2. When to record the Gas temperature?.

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UNSTEADY-STATE HEAT CONDUCTION

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

  2. Measurement of Gas temperature T1 T2 > T1 T2

  3. When to record the Gas temperature?

  4. Welding Process : How to decide the Rate of Welding?

  5. Injection Molding Process They need you to help them estimate the time required for a part in a mold to cool to an acceptable temperature for removal.

  6. Resin Transfer Molding Process (1) Insert fiber preform (2) Close mold (3) Inject mixed resin/catalyst (4) Part solidifies via reaction (5) Open mold (6) Remove part The cycle time for step (4) is approximately the same 25 minutes for parts of all sizes made by AeroForm. All other molding operations (1), (2), (3), (5) and (6) take a total of 5 minutes.

  7. General Conduction Equation • The general form of these equations in multidimensions is: For Rectangular Geometry:

  8. Transient-conduction • Temperature will vary with location within a system and with time. • Temperature and rate heat transfer variation of a system are dependent on its internal resistance and surface resistance. • If we have a ball with initial temperature of T0 and it is left in fluid at Te. • Heat is transferred by convection at the surface. • As the surface temperature decreases, heat is transferred from the center of the ball to the surface, then to the fluid.

  9. Thermal Analysis of Cooling of Ball Rate of Change in energy of metal ball = Rate of Heat transfer Rate of Heat Transfer = Rate of Convection by fluid = Rate of Conduction transfer in the metal ball

  10. At any instant: • Now, if the system itself is copper or the volume is small, the temperature response within the slab is considerably different from that if it is glass or the volume is large. • The response has to do with what is called the internal resistance of the material. • Further, if the convection coefficient is very high, then the surface temperature almost becomes identical to the fluid temperature quickly. • Alternatively, for a low convection coefficient a large temperature difference exists between the surface and the fluid. • The value of the convection coefficient controls what is known as the surface resistance to heat transfer. • Thus, the temperature variation within the system is dependent on the internal and surface resistances. • The larger internal resistance or the smaller surface resistance, the larger temperature variation within the system, and vice versa.

  11. T T0 Rcond Rconv Total thermal resistance of the system: Rtot T T0

  12. A Biot number is defined as:

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

  14. System with negligible internal resistance For this case Bi  1.0.and the temperature profile within the body is quite uniform. The rate of change in internal energy of the body is equal to the rate of heat taken away from the surface by convection:

  15. Rearranging: Integrating to any time interval:

  16. Define, Thermal Time Constant

  17. Thermal Time Constant The total energy transferred in time t

  18. The Fourier number (Fo) or Fourier modulus, named after Joseph Fourier, is a dimensionless number that characterizes transient heat conduction. Conceptually, it is the ratio of the heat conduction rate to the rate of thermal energy storage. It is defined as:

  19. Hot Rolling of Steel Sheets

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