THERMODYNAMICSI

INTRODUCTION TONAVAL

ENGINEERING

ENERGY ANDHEAT TRANSFER

INTRODUCTION TONAVAL

ENGINEERING

THERMODYNAMICS:

The science concerned with the inter-relationship between

thermal energy and mechanical energy

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

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)

MECHANICAL ENERGY

Potential Energy (PE)– PE = mgh

Kinetic Energy (KE)– KE = (1/2)mV2

Mechanical Energy in TRANSITION:– Called Work– Wk = FD

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

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

MODES OF HEAT TRANSFER

(1) CONDUCTION

Heat flows from hotter to colder region when there is physical contact between the regions

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

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)

CONDUCTION EXAMPLE

HOT COLD

LENGTH

AREA

(2) RADIATION

Mode of heat transfer that does not involve any physical

contact between the emitting and receiving regions

(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

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

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

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

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

GENERAL CONDUCTION EQUATION REVISITED

Q = kTA (t1-t2)/L

MAXIMIZE MINIMIZE

CounterflowHeat

Exchanger

Parallel Heat Exchanger

TYPES OF HEAT EXCHANGERS

Crossflow Heat Exchanger

TYPES OF HEAT EXCHANGERS

FACTORS FOR “K”

THE BOUNDARY LAYER SCALE/CHEMICAL DEPOSITS SOOT/DIRT BUILDUP