THERMODYNAMICS I INTRODUCTION TO NAVAL ENGINEERING.
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Transcript of THERMODYNAMICS I INTRODUCTION TO NAVAL ENGINEERING.
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THERMODYNAMICSI
INTRODUCTION TONAVAL
ENGINEERING
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ENERGY ANDHEAT TRANSFER
INTRODUCTION TONAVAL
ENGINEERING
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THERMODYNAMICS:
The science concerned with the inter-relationship between
thermal energy and mechanical energy
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WHAT ARE THE TYPES OF ENERGY?
Stored– Potential (based on position)– Kinetic (based on velocity)
Transitional– Energy that is in the process of being
transferred from one object or system to another. All energy in transition begins and ends as stored energy
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HOW CAN KINETIC ENERGY BE STORED ENERGY?
Definition of energy– “the ability to produce an effect”
The Bullet example– Has the capability for an effect– But it needs something to hit in order
to transfer that energy (the effect)
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MECHANICAL ENERGY
Potential Energy (PE)– PE = mgh
Kinetic Energy (KE)– KE = (1/2)mV2
Mechanical Energy in TRANSITION:– Called Work– Wk = FD
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THERMAL ENERGY Stored Thermal Energy: Called Internal
– Internal Potential Energy• Associated with the force of attraction that exist
between molecules.
– Internal Kinetic Energy• Associated primarily with the activity of the molecules
Thermal Energy in TRANSITION:– Called Heat– Requires a temperature difference between two
systems
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MEASURING THERMAL ENERGY
In the real world we use CALORIES and JOULES but...
BRITISH THERMAL UNIT– Quantity of Heat required to raise the
temperature of 1 lbm of water from 50.9F to 60.9F
– Please don’t write that down
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MODES OF HEAT TRANSFER
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(1) CONDUCTION
Heat flows from hotter to colder region when there is physical contact between the regions
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CONDUCTION (cont.)
The total quantity of heat passed– is proportional to the cross-sectional
area of the conductor over a given time– is proportional to the time of heat flow– in a given length of time is proportional
to the thermal gradient (temp difference)
– depends on the thermal conductivity of the substances
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GENERAL CONDUCTION EQUATION
Q = kTA (t1-t2)/Lwhere
Q : Quantity of heat (Btu or cal)k : Coefficient of thermal conductivity (Btu/((hr)(F)(ft))T : Time (hr)t1 : Temp at hot end (F)
t2 : Temp at cold end (F)
L : Distance between the two ends (in)A : Cross sectional area (sq ft)
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CONDUCTION EXAMPLE
HOT COLD
LENGTH
AREA
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(2) RADIATION
Mode of heat transfer that does not involve any physical
contact between the emitting and receiving regions
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(3) CONVECTION
The mechanical transportation of a mass of fluid from one place to another– Beyond the molecular level– Movement of fluid within fluid– Transportation, not transfer– Fluid’s thermal energy remains in stored
form unless it is transferred by radiation or conduction
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CONVECTION (cont)
TWO TYPES OF CONVECTION– Natural
• Occurs when there are differences in the density of different parts of the fluid. The difference in density are usually caused by a temperature difference.
– Forced• Occurs when some mechanical device,
such as a pump or a fan, produces movement
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DEFINITIONS Sensible Heat
– When heat added results in the change in temperature (kinetic energy)
Latent Heat– When the heat added results in a physical change of
the substance (potential energy)
Saturation Temperature/Pressure– Psat/Tsat– The point at which liquid and vapor may exist in
equilibrium contact with each other
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DEFINITIONS (cont)
Saturated Liquid/Vapor– A liquid/vapor at a specified pressure which is at
Tsat for the pressure– “Wet vapor”
Subcooled Liquid– A liquid at that specified pressure which is below
the Tsat
Superheated Vapor– A vapor that has been raised above Tsat for a
given pressure
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DEFINITIONS (cont)
Latent Heat of Vaporization– Amount of heat necessary to change a mass
of liquid to vapor without changing the temperature
Latent Heat of Fusion– Amount of heat that must be
added/removed to a unit mass to melt/solidify it
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GENERAL CONDUCTION EQUATION REVISITED
Q = kTA (t1-t2)/L
MAXIMIZE MINIMIZE
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CounterflowHeat
Exchanger
Parallel Heat Exchanger
TYPES OF HEAT EXCHANGERS
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Crossflow Heat Exchanger
TYPES OF HEAT EXCHANGERS
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FACTORS FOR “K”
THE BOUNDARY LAYER SCALE/CHEMICAL DEPOSITS SOOT/DIRT BUILDUP