HEAT TRANSFER

• Heat transfer


  The movement of energy due to a temperature difference
• Heat transfer


  • Can only occur in the presence of a temperature difference
  • Occurs through convection, conduction, radiation or a combination
• Steady-state heat transfer


  All parameters are stabilized with respect to time, temperatures are constant at all locations
• Unsteady-state (transient) heat transfer


  Temperature changes with time are important
• One dimensional conduction heat transfer


  1. Depends on temperature difference

  2. Material thickness

  3. Area through which heat flows

  4. Resistance of material to heat flow
• Convection heat transfer


  Transfer of energy due to the movement of a heated (or cooled) fluid
• Natural convection


  Heated fluid moves due to change in fluid density
• Forced convection


  Fluid is moved by other methods (pumps, fans, etc.)
• Convective heat transfer coefficient (h)

  Measure of the rate of heat transfer between a fluid and a surface
• Surface resistance (Rs)


  Inverse of the convective heat transfer coefficient (1/h)
• Bulk temperature (tb)


  Temperature of the fluid away from the surface
• Surface temperature (ts)


  Temperature of the surface
• Convection


  • Natural convection occurs when a heated fluid moves due to the change in fluid density
  • Forced convection occurs when the fluid is moved by other methods (pumps, fans, etc.)
• Convective heat transfer coefficient


  Used to describe convection heat transfer between a fluid and a surface
• Radiation heat transfer


  Transfer of heat from one object directly to another without the use of an intervening medium
• Thermal radiation


  Part of the electromagnetic spectrum which ranges from radio waves to gamma rays
• Thermal radiation


  • Striking a surface may be reflected, absorbed, or transmitted
  • Polished aluminum reflects most radiant energy
  • Flat black surfaces absorb most radiant energy
  • Clear glass transmits most radiation from high temperature sources, but reflects radiation from low temperature sources
• In the practical world, there are few heat transfer applications that can be called pure conduction, convection, or radiation. Usually, all three methods are involved to some extent.
• Radiation becomes very important at higher temperatures, but is usually insignificant at temperatures near ambient.
• Heat transfer through walls

  Combination of conduction, convection, and radiation, although conduction and convection are of primary importance
• Thermal resistance (RT)


  Sum of the resistance of each wall component and the surface resistances on each side of the wall
• Thermal conductivity (k) and conductance (C) values


  • Provided in Table 5.02
• Heat transfer through pipe walls


  Radial heat transfer, where the cross sectional area varies from the inside to the outside of the pipe
• Heat transfer through composite pipes and cylinders

  Includes convection heat transfer coefficients and multiple layers of materials
• Heat Transfer
• Heat flow through composite cylinders


  1. Fluid to inner surface

  2. Through material a

  3. Through material b

  4. Through material c

  5. Outside to surroundings
• Reynolds number


  A dimensionless parameter indicating the degree of turbulence of fluid flow
• Laminar flow


  Smooth flow with Re < 2000
• Turbulent flow


  Agitated flow with Re > 2000
• Prandtl number


  A dimensionless parameter relating the relative thickness of hydrodynamic and thermal boundary layers
• Dimensionless ratios commonly used in heat transfer

  • Biot
  • Fourier
  • Graetz
  • Grashof
  • Nusselt
  • Peclet
  • Prandtl
  • Reynolds
  • Thermal diffusivity
• Convective (surface) heat transfer coefficient

  Affects the rate of heat transfer to/from a flowing fluid
• Heat exchangers are used extensively in many industrial applications to transfer heat from one fluid to another
• Note that the two equations give somewhat different solutions. This is not unexpected since the equations are not exact. They are simply two relationships approximating a very complex heat transfer process. Thus, while the results should be reasonably close, we should expect differences.
• Heat exchangers


  Used extensively in many industrial applications, especially in processing applications where many heating and cooling operations are involved. The basic function is to transfer heat from one fluid to another.
• Heat exchangers


  • Fluids may be mixed, as when steam is added to water
  • Fluids must be physically separated by a plate, pipe wall, or other good heat conductor
• Types of heat exchangers


  • Double-pipe heat exchanger
  • Shell-and-tube heat exchanger
  • Plate-type heat exchanger
• Double-pipe heat exchanger


  One fluid flows through the inner pipe, the other fluid flows through the outer pipe
• Shell-and-tube heat exchanger


  1. One fluid flows through the tubes, the other fluid is in the shell surrounding the tubes

  2. Several tube-and-shell flow patterns are possible with different baffle and tube arrangements
• Plate heat exchanger


  1. Made with multiple plates shaped to provide flow channels between adjacent plates

  2. Hot and cold fluids must alternate between plates along the length of the heat exchanger

  3. Plates can be added or removed to adjust the total heat transfer area