1. ENT 255 HEAT TRANSFER BASICS OF HEAT TRANSFER

2. THERMODYNAMICS & HEAT TRANSFER HEAT => a form of energy that can be transferred from one system to another as a result of temperature difference. THERMODYNAMICS => a science that deals with the amount of heat transfer as a system undergoes a process from one equilibrium state to another. HEAT TRANSFER => a science that deals with the determination of the rate of such energy transfer. This is a main quantity of interest in the design and evaluation of heat transfer equipment.

3. APPLICATION AREAS OF HEAT TRANSFER Heat transfer is commonly encountered in engineering systems and other aspects of life Human body -> constantly rejecting heat to its surroundings, and human comfort is closely tied to the rate of this heat rejection. Household appliances -> the heating and air conditioning system, the refrigerator and freezer, water heater, iron, computer etc. Major role in design of car radiators, solar collectors, various components of power plants, spacecraft etc.

4. ENERGY OF A SYSTEM TOTAL ENERGY => the sum of all forms of energy of a system, includes internal, kinetic, and potential energy INTERNAL ENERGY => represents the molecular energy of a system, consists of sensible, latent, chemical, and nuclear forms. HEAT or THERMAL ENERGY => the sensible and latent forms of internal energy that can be transferred from one medium to another as a result of temperature difference. HEAT TRANSFER => the exchange of heat or thermal energy. HEAT TRANSFER RATE => the amount of heat transferred per unit time HEAT FLUX => the rate of heat transfer per unit area

5. FIRST LAW OF THERMODYNAMICS CLOSED SYSTEM => a system of fixed mass OPEN SYSTEM => a system that involves mass transfer across its boundaries. Also known as control volume FIRST LAW OF THERMODYNAMICS => energy balance for any system undergoing any process can be expressed as: E in – E out = ΔE system

6. When a stationary closed system involves heat transfer only and no work interactions across its boundary, energy balance relation reduces to: Q = mC v ΔT Under steady conditions and in absence of any work interactions, the conservation of energy relation for a control volume with one inlet and one exit with negligible changes in kinetic and potential energies can be express as: Q = mC p ΔT Q = Q/Δt m = m/Δt

7. QUIZ 1 (part A) Using your own words, define the following terms: 1.Energy 2.Thermal conductivity 3.Insulation 4.Transient 5.Finned surface

8. HEAT TRANSFER MECHANICSM Heat can be transferred in three different modes: –Conduction –Convection –Radiation

9. CONDUCTION Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles, and is expressed by Fourier’s Law of Heat Conduction as: Q cond = -k A dT/dx k = thermal conductivity of the material A = area normal to the direction of heat transfer dT/dx = temperature gradient

10. CONVECTION Convection is the mode of heat transfer between a solid surface and the adjacent liquid or gas that is in motion, and involves the combination effects of conduction and fluid motion. The rate of convection heat transfer is expressed by Newton’s Law of Cooling as: Q conv = h A s (T s – T inf ) h = convection heat transfer coefficient A s = surface area through which convection heat transfer takes place T s = surface temperature T inf = temperature of the fluid sufficiently for from the surface.

11. RADIATION Radiation is the energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.

12. QUIZ 1 (part B) 1.2 kg of liquid water initially at 15°C is to be heated to 95°C in a teapot equipped with a 1200 W electric heating element inside. The teapot is 0.5 kg and has an average specific heat of 0.7 kJ/kg.°C. Taking the specific heat of water to be 4.18 kJ/kg.°C and disregarding any heat loss from the teapot, determine how long it will take for the water to be heated. * List your assumptions; state the system properties; use energy balance equation [W = J/s]