Heat Transfer
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Heat Transfer

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  • Modes of Heat Transfer

Heat Transfer Heat Transfer Presentation Transcript

  • Heat Transfer It is that area of mechanical engineering that deals with the different principles and mechanisms involved in transferring heat from one point to another. Heat Transfer
    • Modes of Heat Transfer
    • Conduction: Is the transfer of heat from one point to another point within a body or from one body to another body when they are physical contact with each other.
    • Convection: Is the transfer of heat from one point to another within a fluid.
    • a. Natural or Free convection – motion of the fluid is due to the
    • difference in density because of a difference in temperature.
    • b. Force Convection – motion of fluid is accomplished by mechanical
    • means, such as a fan or a blower.
    • 3. Radiation: It the flow of heat from one body to another body separated by a distance due to electromagnetic waves.
  • Conduction fire Metal rod t 1 Hotter body t 2 Colder body
  • Convection 2  t 2 t 1 1  Fluid surface
  • Radiation Hot body Cold body
  • Conduction L 1 2 k A Q t 1 t 2 Where: L – thickness, meters A – surface area, m 2 k – thermal conductivity, Q – conductive heat flow, Watts
  • Thermal Circuit Diagram 1 2 R Q
  • Conduction through a Composite Plane Wall L 1 L 2 L 3 k 1 k 2 k 3 1 A 2 3 4 Q
  • Thermal Circuit Diagram 1 2 R 1 Q 4 3 R 2 R 3
  • A furnace is constructed with 20 cm of firebrick, k = 1.36 W/m-  K, 10 cm of insulating brick, k = 0.26 W/m-  K, and 20 cm of building brick, k = 0.69 W/m-  K. The inside surface temperature is 650  C. The heat loss from the furnace wall is 56 W/m 2 . Determine a. the interface temperature and the outside wall temperature,  C b. the total resistance R t, for 1 m 2 Given: L 1 =0.20 m ; L 2 = 0.10 m ; L 3 = 0.20 m k 1 = 1.46 ; k 2 = 0.26 ; k 3 = 0.69 t 1 = 650  C Q/A = 56 W/m 2 L 1 L 2 L 3 1 2 3 4 Q A 1 2 R 1 Q 4 3 R 2 R 3
  • At 1 to 2 At 1 to 3 At 1 to 4
  • Convection Where: Q – convective heat flow, Watts A – surface area in contact with the fluid, m 2 h – convective coefficient, W/m 2 -  C or W/m 2 -K t 1 , t 2 – temperature,  C Fluid A 1  2  Q t 2 t 1 h
  • Conduction from Fluid to Fluid separated by a composite plane wall L 1 L 2 L 3 k 1 k 2 k 3 1 A 2 3 4 Q i  h i t i o  h o , t o
  • Thermal Circuit Diagram 1 2 R 1 Q 4 3 R 2 R 3 i o R i R o
  • Overall Coefficient of Heat Transfer Where: U – overall coefficient of heat transfer, W/m 2 -  C or W/m 2 -K
  • CONDUCTION THROUGH CYLINDRICAL COORDINATES Where: r 1 – inside radius, m r 2 – outside radius, m L – length of pipe, m k – thermal conductivity of material, W/m-  C r 1 r 2 1 2 t 1 t 2 Q k
  • For composite cylindrical pipes (Insulated pipe) r 1 r 2 1 2 t 1 t 2 Q k 1 3 r 3 t 3 k 2
  • Heat Flow from fluid to fluid separated by a composite cylindrical wall r 1 r 2 1 2 t 1 t 2 Q k 1 3 r 3 t 3 k 2 i h i t i o h o t o
  • Overall Coefficient of Heat Transfer
  • Heat Exchangers Types of Heat Exchangers 1. Direct Contact Type: The same fluid at different states are mixed. 2. Shell and Tube Type: One fluid flows inside the tubes and the other fluid on the outside. Direct Contact m 1 , h 1 m 2 , h 2 m 3 , h 3
  •  
  • Shell and Tube Type m c m c m h m h 1 2 A B t wA t wB h 1 h 2
  • By energy balance Heat rejected by the hot fluid = Heat absorbed by the cold fluid Where: m c – mass flow rate of cold fluid, kg/sec m h – mass flow rate of hot fluid, kg/sec h – enthalpy, kj/kg t – temperature,  C C pc – specific heat of the cold fluid, KJ/kg-  C Q – heat transfer, KW h, c – refers to hot and cold, respectively 1, 2 – refers to entering and leaving conditions of hot fluid A, B – refers to entering and leaving conditions of cold fluid
  • Heat Transfer in terms of OVERALL COEFFICIENT Of HEAT TRANSFER U
  • Log Mean Temperature Difference (LMTD) Where:  1 – small terminal temperature difference,  C  2 – large terminal temperature diffrence,  C
  •