Heat Transfer

• Heat Transfer


  The transfer of heat from one place to another due to a temperature-difference driving force and the heat flows from the high- to low-temperature region

  
• Mechanisms of Heat Transfer

  • Conduction
  • Convection
  • Radiation

  
• Thermodynamics


  Deals with systems in equilibrium and predicts the amount of energy required to change a system from one state to another
• Heat Transfer


  Deals with systems in non-equilibrium and is used to predict how fast the energy is transferred, supplements the 1st and 2nd principles of thermodynamics

  
• Heat is an energy transfer across a system boundary due to the temperature different between a system and its surrounding

  
• Conduction


  1. Heat can be conducted through solids, liquids and gases

  2. Heat is conducted by the transfer of energy of motion between adjacent molecules

  3. Transfer is affected by the ability of the touching objects to conduct heat

  
• Thermal conductivity, k


  A physical property that indicates how fast the heat will flow in a given material, expressed as W m-2 K-1

  
• Conduction also arises from the movement of free electrons in the metals which accounts for the high thermal conductivities
• In fluids, conduction occurs as a result of the kinetic energy transfer between one molecule to another
• Fourier's Law


  1. Heat transfer rate = k * A * (T1 - T2) / L

  2. Heat flows from high temperature to low temperature region

  
• Fourier's Law


  • The driving force of heat transfer by conduction is the temperature gradient

  
• Thermal conductivity, k, is a physical property that indicates how fast the heat will flow in a given material

  
• Thermal conductivity of gases

  If temperature is higher, the molecules have higher velocities and will collide with one another, exchanging energy and momentum

  
• Thermal conductivity of liquids and solids

  Molecules are more closely spaced, exchanging energy through collision process

  
• Convection


  1. Transfer of heat between an object and fluid (surface to fluid)

  2. Transfer of heat by bulk transport and mixing of warmer and cooler portions in a gas or liquid

  
• Convection


  • Transfer is affected by the speed of the fluid and the ability of the object to conduct heat
  • Depends on the viscosity and thermal properties of the fluid

  
• Convective heat transfer


  1. q_cv = h_cv * A * (T_w - T_f)

  2. Heat transfer rate depends on convective heat transfer coefficient, area, and temperature difference between wall and fluid

  
• Types of Convection


  • Forced convection
  • Natural (free) convection
• Shell and tube heat exchanger is an example of heat transfer application in unit operation

  
• q


  Heat transfer rate by conduction
• k


  Thermal conductivity [W m-1 K-1]

  
• A


  Heat transfer area [m2]
• ∂T/∂x


  Temperature gradient
• qcv


  Heat transfer rate by convection
• hcv


  Convective heat transfer coefficient
• Tw


  Wall temperature

  
• Tf


  Fluid temperature
• q


  Heat transfer rate by radiation
• σ


  Stefan-Boltzmann constant
• A1


  Surface area 1
• T1


  Temperature of surface 1
• T2


  Temperature of surface 2
• Heat Balance


  Rate of heat in + Rate of generation heat = Rate of heat out + Rate of accumulation of heat
• Rate of transfer process

  Driving force / Resistance
• Conduction


  Fourier's Law
• T1


  Temperature at one side of slab/wall

  
• T2


  Temperature at other side of slab/wall
• ∆x


  Thickness of slab/wall
• R


  Thermal resistance
• r1


  Inner radius of hollow cylinder