slide1 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
HEAT TRANSFER PowerPoint Presentation
Download Presentation
HEAT TRANSFER

Loading in 2 Seconds...

play fullscreen
1 / 15

HEAT TRANSFER - PowerPoint PPT Presentation


  • 140 Views
  • Uploaded on

HEAT TRANSFER. DR.PRADIP DUTTA Department of Mechanical Engineering Indian Institute of Science Bangalore. Heat transfer. Thermodynamics. complementary. What is Heat Transfer?. “Energy in transit due to temperature difference.” Thermodynamics tells us:

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'HEAT TRANSFER' - damaris


Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

HEAT TRANSFER

DR.PRADIP DUTTA

Department of Mechanical Engineering

Indian Institute of Science

Bangalore

what is heat transfer

Heat transfer

Thermodynamics

complementary

What is Heat Transfer?

“Energy in transit due to temperature difference.”

Thermodynamics tells us:

  • How much heat is transferred (Q)
  • How much work is done (W)
  • Final state of the system

Heat transfer tells us:

  • How (with what modes) Q is transferred
  • At what rate Q is transferred
  • Temperature distribution inside the body
modes
MODES:
  • Conduction

- needs matter

- molecular phenomenon (diffusion process)

- without bulk motion of matter

  • Convection

- heat carried away by bulk motion of fluid

- needs fluid matter

  • Radiation

- does not needs matter

- transmission of energy by electromagnetic waves

applications of heat transfer
APPLICATIONS OF HEAT TRANSFER
  • Energy production and conversion

- steam power plant, solar energy conversion etc.

  • Refrigeration and air-conditioning
  • Domestic applications

- ovens, stoves, toaster

  • Cooling of electronic equipment
  • Manufacturing / materials processing

- welding,casting, soldering, laser machining

  • Automobiles / aircraft design
  • Nature (weather, climate etc)
needs medium temperature gradient

T1>T2

T2

T1

.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . ………….. . ..

q’’

(Needs medium, Temperature gradient)

CONDUCTION

. . . . . .

. . . . .

. . . . . .

. . . . .

. . . . . . .

solid or stationary fluid

RATE:

q(W) or (J/s) (heat flow per unit time)

conduction contd

x

A

T1

q

T2

k

Conduction (contd…)
  • Rate equations (1D conduction):
  • Differential Form
  • q = - k A dT/dx, W
  • k = Thermal Conductivity, W/mK
  • A = Cross-sectional Area, m2
  • T = Temperature, K or oC
  • x = Heat flow path, m
  • Difference Form
  • q = k A (T1 - T2) / (x1 - x2)

Heat flux: q” = q / A = - kdT/dx (W/m2)

(negative sign denotes heat transfer in the direction of decreasing temperature)

conduction contd7
Conduction (contd…)
  • Example:

The wall of an industrial furnace is constructed from 0.2 m thick fireclay brick having a thermal conductivity of 2.0 W/mK. Measurements made during steady state operation reveal temperatures of 1500 and 1250 K at the inner and outer surfaces, respectively. What is the rate of heat loss through a wall which is 0.5 m by 4 m on a side ?

slide8

Ts>T∞

moving fluid

T∞

q”

Ts

CONVECTION

  • Energy transferred by diffusion + bulk motion of fluid
rate equation convection
Rate equation (convection)

U 

T

U 

y

y

u(y)

q”

T(y)

Ts

Heat transfer rate q = hA( Ts-T ) W

Heat flux q” = h( Ts-T ) W / m2

h=heat transfer co-efficient (W /m2K)

(not a property) depends on geometry ,nature of flow, thermodynamics properties etc.

convection contd

Free or natural convection (induced by buoyancy forces)

May occur with phase change (boiling, condensation)

Convection

Forced convection (induced by external means)

Convection (contd…)
convection contd11
Convection (contd…)

Typical values of h (W/m2K)

Free convection gases: 2 - 25

liquid: 50 - 100

Forced convection gases: 25 - 250

liquid: 50 - 20,000

Boiling/Condensation 2500 -100,000

slide12

RATE:

q(W) or (J/s) Heat flow per unit time.

RADIATION

T1

q1”

q2”

T2

Flux : q” (W/m2)

rate equations radiation
Rate equations (Radiation)

RADIATION:

Heat Transfer by electro-magnetic waves or photons( no medium required. )

Emissive power of a surface (energy released per unit area):

E=Ts4 (W/ m2)

= emissivity (property)………

=Stefan-Boltzmann constant

rate equations contd

Ts u r

q”r a d.

q”co n v.

Ts

Area = A

Rate equations (Contd….)

Radiation exchange between a large surface and surrounding

Q”r a d = (Ts4-Tsur4) W/ m2

radiation contd
Radiation (contd…)
  • Example:An uninsulated steam pipe passes through a room in which the air and walls are at 25°C. The outside diameter of pipe is 80 mm, and its surface temperature and emissivity are 180°C and 0.85, respectively. If the free convection coefficient from the surface to the air is 6 W/m2K, what is the rate of heat loss from the surface per unit length of pipe ?