Thermodynamics II: 1st Law of Thermodynamics

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27 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

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Thermodynamics II: 1st
Law of Thermodynamics

Objectives

Comprehend the principles of operation
of various heat exchangers

Understand boundary layers

Comprehend the First Law of Thermo

Comprehend the basic principles of
open/closed thermo systems

Comprehend thermo processes

Heat Exchangers

Def’n: device used to transfer thermal
energy from one substance to another

Direction of Flow

-
> Parallel: not used by Navy

-
> Counter: more efficient; used by Navy

-
> Cross: used extensively

Number of passes (single or multiple)

Heat Exchangers

Type of Contact

Direct: mixing of substances; pour hot into
cold

Indirect/surface: no direct contact; some
thin barrier used

Phases of Working Substance

liquid
-
liquid: PLO cooler

liquid
-
vapor: condenser

vapor
-
-
heat

Heat Exchangers

Boundary layer/film: w/in pipes or
channels of fluid flow, the fluid adjacent
to the wall is stagnant

-
> local temp increases

-
>
D
T me瑡l de捲eases

-
> amount of heat transfer decreases

-
> reduced efficiency & possible damage

Try to minimize film by adjusting flow or
increasing turbulence

Heat Exchangers

conduct heat & have minimal corrosion

Maximize surface area for heat transfer

Minimize scale, soot, dirt, & fouling
-
>
reduces heat transfer, efficiency, &
causes damage

First Law of

Thermodynamics

First Law of Thermodynamics

First Law of Thermodynamics

Principle of Conservation of Energy:

energy can neither be created nor destroyed,
only transformed (generic)

energy may be transformed from one form to
another, but the total energy of any body or
system of bodies is a quantity that can be
neither increased nor diminished (thermo)

First Law of Thermodynamics

General Energy Equation

Energy In = Energy Out, OR

U
2
-

U
1

= Q
-

W (or u
2

-

u
1

= q
-

w)

Where:

U
1

= internal energy of system @ start

U
2

= internal energy of system @ end

Q = net thermal energy flowing into system
during process

W = net work done
by

the system

Thermodynamic System

Def’n: a bounded region that contains
matter (which may be in gas, liquid, or
solid phase)

Requires a working substance to receive,
store, transport, or deliver energy

May be open (mass can flow in/out) or
closed (no flow of mass out of
boundaries)

Thermodynamic Processes

Def’n: any physical occurrence during
which an effect is produced by the
transformation or redistribution of
energy

Describes what happens within a system

Two classifications: non
-
flow

Non
-
Flow Process

Process in which the working
fluid does not flow into or out
of its container in the course of
the process (closed system)

Energy In = Energy Out

Q
-

W = U
2

-

U
1

Example: Piston being
compressed

Process in which the working substance
some device (i.e., a turbine) (open
system)

Assumptions (at any cross section):

Properties of fluid remain constant

Average velocity of fluid remains constant

System is always filled so vol
in

= vol
out

Net rate of heat xfer & work performed is
constant

Processes
-

Flow Work

Def’n: mechanical energy necessary to
maintain the flow of fluid in a system

Although some energy has been
expended to create this form of energy, it
still represents a stored (kinetic) energy
which can be used

Flow work = pressure x volume (PV)

Processes
-

Enthalpy

Enthalpy: the total energy of the fluid
due to both internal energy & flow
energies

Represents the “heat content” or “total
heat”

Enthalpy (H)

H = U + PV (in ft
-
lb, BTU, or Joules)

h = u + Pv (divide by lbm)

Questions?