Heat Transfer - Lecture 1

8/13/2019 Heat Transfer - Lecture 1

1/39

AME 331 Heat Transfer

Lecture #1

Tuesday, 14 January 2014

1


2/39

Lecture, Discussion, Office Hours

2

Office Hours:

Rick Martin, BHE-315

Tu 9:00 am to 10:30 am

Tu 2:30 pm to 4:00 pm

Manny Dekermenjian,BHE-315 Th 10:00 am to 11:00 am

Th 2:30 to 4:30 pm

TA, Runhua Zhao, RRB-207

M 3-5 pm

W 4-6 pm

TA, DJ Lee, ???-???

W 10 am-noon ???

Lecture, Discussion:

Section #28761

GFS-118, Lecture

Tu, Th 11:00 to 12:20

Section #28762

GFS-118, Lecture

Tu, Th 12:30 to 1:50

Loc TBD, Discussion #1

Tu 4-5 pm???

Loc TBD, Discussion #2

W 3-4 pm


3/39

About the Instructor

Manny Dekermenjian

Principal, ENVIRON International, Inc.

PhD, BS University of California, Los Angeles

Licensed Professional Engineer in CA, TN

Certified Fire/Explosion Investigator

Editorial Board, International Society of EnvironmentalForensics

Expertise: Applying engineering principles to theinvestigation of real world disasters

3


4/39

About the Instructor

Rick Martin

Principal, Martin Thermal Engineering

PhD University of California, Berkeley

BS, MS Stanford University

Licensed Professional Engineer in CA, NV, AZ, CO, KY

Certified Fire Investigator

Certified Fire Protection Specialist

Secretary, NFPA 86 Ovens & Furnaces, NFPA 87 Fluid Heaters

Extracurricular Interest: Philosophy of Faith and Science

4


5/39

The Charge

The practice of science and engineering involve thepursuit and application of truth.

Excellence in engineering is paramount becauseanything less can lead to technology failures, whichmay cause personal injury and property damage.

Engineering students should seek to augment orimprove their: Knowledge of the way humans interact with the natural

world;

Habits of observing, investigating, analyzing.

Desire to apply engineering fundamentals to the service ofhumankind.

Engineering is a nobleprofessionadmire and respectit!

5


6/39

Syllabus

Textbook: Heat & Mass Transfer Fundamentals and Applications Authors: Yunus Cengel, Afshin Ghajar

Chapters 1 to 9, 11 to 13

Supplementary Textbook: Heat Transfer 2ndEdition (Schaums Outline)

Announcements See Blackboard

Dr. Martin Routine emails: [email protected]

Urgent emails: [email protected]

Dr. Dekermenjian Routine emails: [email protected]

Urgent emails: [email protected]

6
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


7/39

Homework: See Class Schedule (posted on Blackboard) for assigned

problems.

Due most Thursdays

Late if not received by instructor at the end of class 25% penalty if 1 to 24 hourslate 50% penalty if 25 to 48 hours late

75% penalty if 49 or more hours late

Solutions posted on Blackboard after 2ndday

Late HW may be submitted by PDF via email. Regular HW must be submitted as hardcopy, in class.

Late credit will be given for homework submitted up to thelast day of class (Thursday, May 1, 2014)

7

Homework


8/39

Course Schedule

8


9/39

24% Homework (12 assignments, almost weekly)

10% Quiz (4 quizzes, biweekly except for midterm

weeks)

30% Midterm (2 midterms)

30% Final

6% Design Project (presentation at course end)

0-4% Extra credit (Optional = 2 during term)

9

Grading


10/39

Heat Transfer

is a difficult subject.

Dont fall behind.

Do ALL the homework. You can earn a good grade, but you will have

to work hard for it.

If you dont understand something, ask theInstructor or TA right awaydont wait.

10


11/39

Presentation of Homework Solutions

Name

Class Section (1 or 2)

Assignment #, Problem #

Problem Statement Brief summary in your own words

Sketch Set up the problem visually (as needed for clarity)

Nomenclature Define variables (as needed for clarity)

Assumptions e.g., steady, 1-D, properties constant, etc. (very important!) Governing Equations mass, momentum, energy

Boundary Conditions heat flux, temperature, convection

Solution Show pertinent steps to obtaining solution

Illustration Graphs, tables (as needed for clarity)

Discussion Interpretation of results (general interest)

Final Answer For the decision maker

As a Professional Engineer, you must have someone check your work. Communicateclearly so they can follow it.

11


12/39

Significant Figures

Accuracy Precision An answer with too many/few significant figures is a partially

incorrect answer (and potentially embarrassing)

Homework problems willbe checked for correct presentation ofsignificant figures.

Quizzes and Exams will be docked for gross misuse of significantfigures.

If an input has low precision, answer should also have lowprecision Some inputs (e.g., 10C) may have

higher precision (e.g., 10.0C)

To avoid round-off error, carry extrasignificant figures through intermediatecomputations, but truncate at the end.

12


13/39

CHAPTER 1

Introduction to Heat Transfer

13


14/39

Video Clip

http://www.youtube.com/watch?v=T-SZYZLfZ7E

14
http://www.youtube.com/watch?v=T-SZYZLfZ7Ehttp://www.youtube.com/watch?v=T-SZYZLfZ7Ehttp://www.youtube.com/watch?v=T-SZYZLfZ7Ehttp://www.youtube.com/watch?v=T-SZYZLfZ7E


15/39

Observations?

What are the three methods of heat transferdepicted?

At what temperature do you think meltingoccurred?

What can you say about the geometry of thethree different experiments?

What can you say about the time scale of thethree different experiments?

How much electric power do you think thesethree appliances deliver?

15


16/39

Chapter 1 - Introduction

Heat Transfer A Predictive Science

Non-equilibrium process

Rate of energy transfer

Conduction

Fouriers Law

Thermal Conductivity

Material properties

16


17/39

Steady (Not Transient) System

17

( ) 0 Steady StatevM C T =


18/39

Review of Thermodynamics

First Law + = + Production + Input = Output + Storage

Energy is neither created nor destroyed = 0 In this class, conversionof energy from chemical, nuclear, or

electrical to thermal constitutes Production

Energy flowsin and out of system due to:

Work across system boundary Heat across system boundary

Mass across system boundary Storage is change of internal energy (temperature)

18


19/39

Review of Thermodynamics

What is the driving forcefor heat transfer?

Second Law (Entropy)

Consider a blacksmithcooling a red-hothorseshoe in a bath of coolwater.

Thermodynamicspredictsthe final temperature

Heat Transfer predicts howmuch time is required

19


20/39

Enthalpy vs Internal Energy

For a controlvolume (moving): H = enthalpy

(total)

h = enthalpy (perunit mass)

For a control mass(stationary):

U = internal energy(total)

u = internal energy(per unit mass)

20


21/39

Specific Heat

21

Specific Heat:

How much heat (kJ)

can be stored in a

substance per unitmass (kg) for a unit

rise of temperature

(C).

Specific heat varies

with temperature.


22/39

Molecular Processes

Conduction: Molecules of higher energy (temperature) transfer

energy by collisions/vibration with adjacentmolecules of lower energy (temperature).

Convection: Conduction augmented by a moving fluid.

Radiation: Transfer of energy by photons, without contact

between bodies.

Phase Change: Boiling/condensation; melting/freezing. [Not

studied here] Mass Transfer:

Mass and energy exchange in moving fluids. [Notstudied here]

22


23/39

Conduction

Gradient

Spatial variation of a scalar or vector property

= P

Heat Conduction in a given direction is

proportional to that temperature gradient

23


24/39

Variation in Space, Time

Temperature Field

May vary in all three dimensions

May vary in time

, , , Partial Derivative

Rate of change with respect to one variable

Other variables held constant ,, ,

24


25/39

Illustration

For a given y, z, and t, Heat flow in x direction isproportional to temperature gradient in x direction

If gradient is negative(T declining) in x-direction, heatflow ispositivein x-direction.

Proportionality requires a negative sign. k is thermal conductivity.

25

xQ T

kA x

=


26/39

Think Stop

Can a temperature field be variable intime but not in space?

Can a temperature field be variable inspace but not in time?

Can a temperature field be invariant inboth space and time?

What is temperature? [Not studiedhere] For the curious

1

,

26


27/39

Problem Solving Pathway

Objective:

Determine howtemperature varies in abody.

Differential Equation:

Fouriers Law

Boundary Conditions:

Temperature at hot surface Temperature at cold

surface

27


28/39

Terminology

Heat Flux = Flow of heat

through an area:

= =2

The symbol = meanshas units of

28


29/39

Terminology

Temperature Gradient =

Change in temperature

over a distance in a

particular direction

= lim0 =

29


30/39

Terminology


= the property of a

substance that

governs the quantityof heat flow by

molecular motion in

a given direction.

30

Wk

m K=


31/39

Terminology

Density = mass of a substance in a given volume

=

=

3

Specific Heat = heat energy required to raise a

unit mass of a substance by a unit temperature

rise = = 31


32/39

Terminology

Heat Generation = Heat energy released into

the interior volume of a substance, typically

through chemical reaction or electrical

resistance

=

=

3

32


33/39

Fouriers Law

In one dimension, Fouriers Law of Conduction

is an Ordinary Differential Equation:

33

Heat Flux Temperature GradientThermal Conductivity

xx

Q dTq k

A dx= =

2

xQ W

A m=

Wk

m K=

dT K

dx m=


34/39

First Law in One-Dimension

First Law:

Production:

Heat Generation Heat Inflow:

Fourier Conduction

Heat Outflow:

Fourier Conduction Energy Storage

Heat Capacity

34

E in out EE E + = +

DifferentialVolumetric

Volume ElementHeat Generation

E gene V =

0

in

x

dTQ k A

dx =

=

out

x x

dTQ k A

dx +

=

VolumeVolumetricHeat Capacity Time Rate

Of Change

E

Tc V

t

=


35/39


35


36/39


36


37/39

Example Problem

Does the answer seem like a largeor smallrate of heattransfer?

Compare to a light bulb.

37


38/39

Collegium Practicum

Human Body Heat Transfer

Prevention of Overheating

Perspiration

Exhaling

Blood vessel dilation

Prevention of Overcooling

Shivering

Blood vessel contraction

Subcutaneous fat layer

38


39/39

Collegium Practicum

Personal Conduction Technologies

Skiing Hand and Foot Warmers:

Chemical heat release.

4 Fe(s) + 3 O2(g) 2 Fe2O3(s) First Aid Ice Pack:

Phase change heat absorption.

Water-based gel from freezer.

Electric Blanket: Sleeping comfort; Energy savings.

Low-voltage models are easier to safeguard.

39

Documents

Heat Transfer - Lecture 1