introduction of heat transfer
TRANSCRIPT
Fundamentals of Heat Transfer and Mass Transfer
In all things, success depends on previous preparation. And without such preparation there is sure to be failure.Confucius, Analects.
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What is heat transfer
Heat transfer (or Heat) is energy in transit due to a temperature difference.
T1
T2
Overview
Units of HeatObjectives are to:• define and distinguish between various units of heat• define the mechanical equivalent of heat • discuss everyday examples to illustrate these concepts
Units of Heat
Units of Heat• Heat is energy in transit, and is measured in energy units.• The SI unit is the joule (J), or Newton-metre (Nm).• Historically, heat was measured in terms of the ability to
raise the temperature of water.• The kilocalorie (kcal), or Calorie (Cal), or “big calorie”:
amount of heat needed to raise the temperature of 1 kilogramme of water by 1 C0 (from 14.50C to 15.50C)
• The calorie, or “little calorie”: amount of heat needed to raise the temperature of 1 gramme of water by 1 C0 (from 14.50C to 15.50C)
• In industry, the British thermal unit (Btu) is still used: amount of heat needed to raise the temperature of 1 lb of water by 1 F0 (from 630F to 640F)
Mechanical Equivalent of Heat
Joule demonstrated that water can be heated by doing (mechanical) work, and showed that for every 4186 J of work done, the temperature of water rose by 1C0 per kg.
Mechanical Equivalent of Heat
• Conversion between different units of heat:
1 cal = 10-3 kcal = 3.969 x 10-3 Btu = 4.186 J1 Cal = 1 kcal=4186 J
Sensible HeatObjectives are to:• describe what is meant by 'sensible heat‘• define specific heat• explain how the specific heat capacities of materials are
obtained using calorimetry
Specific Heat Capacity
• Sensible heat is associated with a temperature change (can be “sensed”)
• Different substances have different molecular configurations and bonding temperature change not generally the same for equal amounts of heat
• Specific heat capacity, c: amount of energy needed to raise the temperature of 1 kg of a substance by 1K
Calorimeters
Calorimeters (contd.)
Calorimetry: An Exercise in Bookkeeping
Calorimetry: Finding Specific Heats
Calorimetry: Specific Heat
Water: Specific Heat Capacities and Latent Heats
Calorimetry: Mixtures
Water: Warming Curve
Water: Example Problem
Latent Heat
Objectives are to:• Describe what is meant by ‘latent heat‘• Compare and contrast the 3 phases of matter• Relate latent heat to phase changes
Phases of Matter• Heat required for phase changes:
– Vaporization: liquid vapour– Melting: liquid solid– Sublimation: solid vapour
• Heat released by phase changes:– Condensation: vapour liquid– Fusion: liquid solid– Deposition: vapour solid
Phases of Matter
Latent Heat
Methods of Heat TransferObjectives are to:• describe the three methods of heat transfer• Give practical/environmental examples of each
Mechanisms of Heat Transfer
• Conduction• When a temperature gardient exists in a stationary
medium, which may be solid or fluid
• Convection• Will occur between a surface and a moving fluid when
they are at different temperatures
• Radiation• Energy transfer by electromagnetic waves
Conduction
• Concepts of atomic and molecular activity• Transfer of energy from the more energetic
to the less energetic particles of a substance due to interaction between particles
Thermal Conduction
Q TkAt d
Conduction
Fourier‘s Law for Heat Conduction
q is the heat flux (W/m2) or heat transfer rateT is the temperature gradientk is thermal conductivity (W/m.K)
Tkq
Convection
• Energy transfer due to random molecular motion (diffusion)
• Energy is also transferred by bulk, or macroscopic, motion of fluid.
• Free Convection (the flow is induced by bouyancy forces which arise from density differences caused by temperature variations in the fluid)
• Forced Convection ( the flow is caused by external means, such as by a fan, pump, or atmospheric winds.
Convection
Convection
Pool Boiling
Convection
• q = Convective heat flux (W/m2)• h = Convection heat transfer coefficient
(W/m2. K)• Ts = Surface temperature
• T = Fluid temperature
)( TThq s
Radiation
• The energy of the radiation fields is transported by electromagnetic waves (or photons). While the transfer of energy by conduction and convection requires the presence of material medium, radiation does not. In fact, radiation transfer occurs most efficiently in a vacuum.
Radiation
• Heat transfer by electromagnetic waves• Does not need a material medium• Black body: perfect absorber perfect
emitter (at all wavelengths)
4 4
4 4
r a env
net a r env
P A T P A T
P P P A T T
Radiation
)( 44surs TTq
= Stefan Boltzmann constant =5.67x10-8 W/m2K4
= emissivity
Radiation
Convection
Convectionat
Home
Greenhouse Effect
Heat Transfer
Heat Flux Vector
Example
The wall of an industrial furnace is constructed from 15 cm thick fireclay brick having a thermal conductivity of 1.7 W/m.K. Measurements made during steady state operation reveal temperatures of 1400 and 1150 K at the inner and outer surface, respectively. What is the rate of heat loss through a wall that is 50 cm by 3 m on a side ?
Analysis of Heat Transfer(Problems : methodology)
• Known• Find• Schematic• Assumptions• Properties• Analysis• Comments
Relevance of Heat Transfer
• Play an important role in many industrial and environmental problems.
• Energy production and conservation• In solar energy, water heating, design of
incinerators, cryogenic storage equipment, in the cooling of electronic equipment, etc.
SI Units and Prefixes(a) SI unitsQuantity Unit SI symbol FormulaSI base unitsLengthMassTimeTemperatureSI supplementary unitPlane angleSI derived unitsEnergyForcePowerPressureWork
meterkilogramsecondkelvin
radian
joulenewton
wattpascaljoule
mkgsK
rad
JNWPaJ
----
-
N-mkg-m/s2
J/sN/m2
N-m
(b) SI prefixes SI symbolMultiplication factor Prefix for prefix1 000 000 000 000 = 1012
1 000 000 000 = 109
1 000 000 = 106
1 000 = 103
100 = 102
10=101
0.1=10-1
0.01=10-2
0.001=10-3
0.000 001 = 10-6
0.000 000 001 = 10-9
0.000 000 000 001= 10-12
teragigamegakilo
hectodekadecicentimilli
micronanopico
TGMkhdadcmµnp
Conversion Factors and Definitions
(a) Fundamental convers ion factorsEnglish unit Exact SI value Approximate SI
valueLengthMassTemperature
1 in1 lbm
1 deg R
0.0254 m0.453 592 37 kg
5/9 K
-0.4536 kg
-
(b) DefinitionsAcceleration of gravityEnergy
1g=9.8066 m/s2 (32.174 ft/s2)Btu (British thermal unit)amount of energy required toraise 1 lbm of water 1 deg F (1 Btu = 778.2 ft-lbf)kilocalorie amount of energy required to raise 1 kg ofwater 1 K (1 kcal=4187 J)
Length 1 mile=5280 ft; 1 nautical mile = 6076.1 ft.Power 1 horsepower = 550 ft-lbf/sPressure 1 bar 105 PaTemperature degree Fahrenheit tF=9/5tC+32 (where tC is degrees)
(Celsius)degree Rankine tR=tF+459.67Kelvin tK=TC+275.15 (exact)
Kinematic viscosity 1 poise 0.1 kg/m-s1 stoke 0.0001 m2/s
Volume 1 cubic foot = 7.48 gal
(c) Useful convers ion factors1 ft = 0.3048 m1 lbf = 4.448 N1 lbf = 386.1 lbm-in/s2
1 kgf = 9.807 N1 lbf/in2 = 6895 Pa1 ksi = 6.895 Mpa1 Btu = 1055 J1 ft-lbf = 1.356 J1 hp = 746 W = 2545 Btu/hr1 kW = 3413 Btu/hr1 quart = 0.000946 m3 = 0.946 liter1 kcal = 3.968 Btu