Thermodynamics II: 1st Law of Thermodynamics

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27 Οκτ 2013 (πριν από 3 χρόνια και 9 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
-
vapor: radiator in home steam
-
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


Should be made of materials that readily
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 & steady
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

Steady Flow Process


Process in which the working substance
flows steadily and uniformly through
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?