1. Introduction to Heat Transfer

2. TOPICS TO BE DISCUSSED Introduction
Difference between Thermodynamics and Heat Transfer
Heat Transfer Applications
Modes of Heat Transfer
Laws associated with conduction,convection and radiation
Conservation of Energy principle in heat transfer

4. Relationship of Heat Transfer to Thermodynamics
We learnt in thermodynamics that energy can be transferred by interactions between a system and its surroundings
Heat and work are the only form of interactions between a system and its surroundings
Thermodynamics is concerned with equilibrium end states and processes
But does not provide information on the nature of the process or the rate at which energy is transferred
Energy transfer rates are important in engineering process design and development

5. Heat Transfer Applications
Boilers
Nuclear reactors
Heat Exchangers
Condensers and cooling towers
Electronic equipment cooling
Cooling of Gas Turbine blades
Jet Propulsion
Combustion process

6. Modes of Heat Transfer There are three basic modes of heat transfer
Conduction
Convection
Radiation
We will first examine all three modes briefly and then examine them in more detail later

7. Conduction
Conduction heat transfer is due to random molecular and atomic vibrational, rotational and translational motions
High temperature and more energetic molecules vibrate more and transfer energy to less energetic particles as a result of molecular collisions or interactions
The heat flux (a vector) qx´´ (W / m2)
is characterized by a transport property know as the
Thermal Conductivity, k (W / m · K)
W = watts m = Meters K = temperature in Kelvin

8. Fourier Law of Heat conduction

9. Consider the heat flux through a slab of thickness, L
X=L x L
qx´´ T2 T1
For the one-dimensional plane, the heat flux or heat transfer rate is Fourier’s Law:
The total heat transfer through a given cross-sectional area, A, is given by:
where

10. Convection
Convection heat transfer involves both energy transfer due to random molecular motions and by bulk motion of the fluid
Convection heat transfer includes both forced convection and natural convection
In convection heat transfer, the transfer of heat is between a surface and a moving fluid (liquid or gas), when they are at different temperatures. The rate of transfer is given by Newton’s Law of Cooling.
Moving fluid T∞ Ts
q’’
Ts > T∞

11. Different types of Convection and Phase change process

13. Typical values of convection heat transfer coefficient
Process
h (W / m2 K)
Free Convection
Gases
2 -25
Liquids
Forced Convection
50 -20,000
with Phase Change
Boiling or Condensation
,000

14. Radiation
All surfaces of finite temperature emit energy in the form of electromagnetic waves
In the absence of an intervening medium, there is a heat transfer by radiation between two surfaces at different temperatures
The maximum flux, E (W / m2), at which radiation may be emitted from a blackbody surface is given by:
Stefan Boltzmann Law
where
Eb or E = Surface emissive power (W / m2)
T = absolute temperature (K)
σ = Stefan-Boltzmann constant = 5.67 x (W / m2 ּ K4) Eb Ts

15. Conservation of Energy for a Control Volume or System
Consider a control volume (C. V.)
shown here:
Energy Conservation or First Law of Thermodynamics requires that for the C.V.
The rate of energy inflow (Ein) and rate of energy generation (Eg) must be balance by the rate of energy outflow (Eout) and energy storage (Est), Hence,
For a short time interval Δt,

16. Conservation of Energy for a Control Surface
For a surface illustrated below, there is no mass or volume, and consequently, Eg = 0, and Est = 0.
For conservation of energy for the control surface under steady state or transient conditions:
Surface
Surroundings
qcond”
qrad” T1
Fluid
qconv”
Tsur U T T2 T T x

17. THANK YOU