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ME 259Heat TransferLecture Slides I

- Dept. of Mechanical Engineering,

Introduction

- Reading Incropera DeWitt
- Chapter 1

Heat Transfer as a Course

- Has a reputation for being one of the most

challenging courses in ME - Why??
- Physically diverse thermodynamics, material

science, diffusion theory, fluid mechanics,

radiation theory - Higher-level math vector calculus, ODEs, PDEs,

numerical methods - Physically elusive heat is invisible developing

intuition takes time - Appropriate assumptions required to simplify and

solve most problems - However, Heat Transfer is interesting, fun, and

readily applicable to the real world

Relevance of Heat Transfer

- Electric Power Generation
- Alternate Energy Systems
- Combustion/Propulsion Systems
- Building Design
- Heating Cooling Systems
- Domestic Appliances
- Materials/Food Processing
- Electronics Cooling Packaging
- Cryogenics
- Environmental Processes
- Space Vehicle Systems

Definition of Heat Transfer

- Flow of energy due solely to a temperature

difference - all other forms of energy transfer are

categorized as work - from 2nd Law of Thermodynamics, heat flows in

direction of decreasing temperature - heat energy can be transported through a solid,

liquid, gas, or vacuum

Heat Quantities

Relationship Between the Study of Heat Transfer

Thermodynamics

- 1st Law of Thermodynamics for Closed System
- Thermodynamics - allows calculation of total heat

transferred (Q) during a process in which system

goes from one equilibrium state to another (i.e.,

the big picture) - Heat Transfer - provides important physical laws

that allow calculation of instantaneous heat

rate, length of time required for process to

occur, and temperature distribution within

material at any time (i.e., the details

required for design)

Heat Transfer Modes

- Conduction
- transfer of heat due to random molecular or

atomic motion within a material (aka diffusion) - most important in solids
- Convection
- transfer of heat between a solid surface and

fluid due to combined mechanisms of a) diffusion

at surface b) bulk fluid flow within boundary

layer - Radiation
- transfer of heat due to emission of

electromagnetic waves, usually between surfaces

separated by a gas or vacuum

Heat Transfer Modes - Conduction

- Rate equation (Fourier Biot, ?1820) is known as

Fouriers law for 1-D conduction, - where qx heat rate in x-direction (W)
- qx heat flux in x-direction (W/m2)
- T temperature (C or K)
- A area normal to heat flow (m2)
- k thermal conductivity of material
- (W/m-K) see Tables A.1-A.7

Heat Transfer Modes - Conduction

- Steady-state heat conduction through a plane

wall

T1

T2

k

L

q? (T1gtT2)

x

Heat Transfer Modes - Conduction

- Example What thickness of plate glass would

yield the same heat flux as 3.5? of glass-fiber

insulation with the same S-S temperature

difference (T1-T2) ?

Heat Transfer Modes - Conduction

- Insulation R-value
- where 1 W/m-K 0.578 Btu/hr-ft-F

Heat Transfer Modes - Convection

- Rate equation (Newton, ?1700) is known as

Newtons law of cooling - where q heat flux normal to surface
- q heat rate from or to

surface As - Ts surface temperature
- T? freestream fluid

temperature - As surface area exposed to fluid
- h convection heat transfer coefficient
- (W/m2-K)

q?

Fluid flow, T?

Ts (gtT?)

As

Heat Transfer Modes - Convection

- The convection heat transfer coefficient (h)
- is not a material property
- is a complicated function of the many parameters

that influence convection such as fluid velocity,

fluid properties, and surface geometry - is often determined by experiment rather than

theory - will be given in most HW problems until we reach

Chapter 6

Heat Transfer Modes - Convection

- Types of Convection
- Forced convection flow caused by an external

source such as a fan, pump, or atmospheric wind - Free (or natural) convection flow induced by

buoyancy forces such as that from a heated plate - Phase change convection flow and latent heat

exchange associated with boiling or condensation

Heat Transfer Modes - Radiation

- Rate equation is the Stefan-Boltzmann law which

gives the energy flux due to thermal radiation

that is emitted from a surface for a black body - For non-black bodies,
- where E emissive power (W/m2)
- ? Stefan-Boltzmann constant
- 5.67x10-8 W/m2-K4
- ? emissivity (0lt ?lt1) of surface
- Ts surface temperature in absolute
- units (K)

Heat Transfer Modes - Radiation

- Radiation incident upon an object may be

reflected, transmitted, or absorbed - where
- G irradiation (incident radiation)
- ? reflectivity (fraction of G that is

reflected) - ? transmissivity (fraction of G that is

transmitted - ? absorptivity (fraction of G that is

absorbed) - ? emissivity (fraction of black body

emission) - E and the interaction of G with each

participating object determines the net heat

transfer between objects

G

?G

?G

?G

Heat Transfer Modes - Radiation

- Heat transfer between a small object and larger

surroundings (AsltltAsur) - where ? emissivity of small object
- As surface area of small object
- Ts surface temperature of small
- object (K)
- Tsur temperature of surroundings (K)

Tsur

q

Ts

? , As

Conservation of Energy Control Volume

- Control volume energy balance
- from thermodynamics
- Incropera DeWitt text notation

Q

mass out

W

mass in

Conservation of Energy Control Volume

- Energy rates
- where

Conservation of Energy Control Surface

- Surface energy balance
- since a control surface is a special control

volume that contains no volume, energy generation

and storage terms are zero this leaves

Eout

Ein

Summary The Laws Governing Heat Transfer

- Fundamental Laws
- Conservation of mass
- Conservation of momentum
- Conservation of energy
- Heat Rate Laws
- Fouriers law of heat conduction
- Newtons law of convection
- Stefan-Boltzmann law for radiation
- Supplementary Laws
- Second law of thermodynamics
- Equations of state
- ideal gas law
- tabulated thermodynamic properties
- caloric equation (definition of specific heat)

Objectives of a Heat Transfer Calculation

- ANALYSIS
- Calculate T(x,y,z,t) or q for a system undergoing

a specified process - e.g., calculate daily heat loss from a house
- e.g., calculate operating temperature of a

semiconductor chip with heat sink/fan - DESIGN
- Determine a configuration and operating

conditions that yield a specified T(x,y,z,t) or

q - e.g., determine insulation needed to meet a

specified daily heat loss from a house - e.g., determine heat sink and/or fan needed to

keep operating temperature of a semiconductor

chip below a specified value

Classes of Heat Transfer Problems

- Thermal Barriers
- insulation
- radiation shields
- Heat Transfer Enhancement (heat exchangers)
- boilers, evaporators, condensers, etc.
- solar collectors
- finned surfaces
- Temperature Control
- cooling of electronic components
- heat treating quenching of metals
- minimizing thermal stress
- heating appliances (toaster, oven, etc.)