PERTEMUAN KE-2 PERPINDAHAN PANAS

Prinsip Dasar Perpindahan Panas

Heat conduction

7

dx

dTkAQRate of heat transfer:

where T = temperature at any point in the wall

x = thickness of the wall

A=area of the wall

k= thermal conductivity (Btu/hr.ft.ºF)

-dT/dx = temperature gradient

Thermal Conductivity

8

Silver 410 W/m. °C 237 Btu/h.ft.°F

Copper 385 W/m. °C 223 Btu/h.ft.°F

Diamond 2300 W/m. °C 1329 Btu/h.ft.°F

Quartz 41.6 W/m. °C 24 Btu/h.ft.°F

Sandstone 0.78 W/m. °C 0.45 Btu/h.ft.°F

Ammonia 0.147 W/m. °C 0.085 Btu/h.ft.°F

Water 0.556 W/m. °C 0.327 Btu/h.ft.°F

Hydrogen 0.175 W/m. °C 0.101 Btu/h.ft.°F

Air 0.0206 W/m. °C 0.0119 Btu/h.ft.°F

Metals

Nonmetallic

solids

Liquids

Gases

9

Thermal conductivity is strongly

temperature-dependent.

Heat convection

12

Hot water rises,

cools, and falls

Heated air rises,

cools, then falls. Air

near heater is

replaced by cooler

air, and the cycle

repeats.

hAdTQ

where

h = heat transfer coefficient

Rate of heat transfer:

Heat convection

13

Natural or Free Heat

Convection

Force Heat

Convection

Convection heat-transfer coefficients

14

Free Convection

Forced Convection

Boiling water

Condensation of water vapor

Vertical plate 0.3m high in air 4.5 W/m2. °C 0.79 Btu/h.ft2.°F

Horizontal cylinder, 2-cm diameter, in air 6.5 W/m. °C 1.14Btu/h.ft.°F

Airflow at 2m/s over 0.2-m square plate 12 W/m2. °C 2.1 Btu/h.ft2.°F

Airflow at 2m/s over 0.2-m square plate 75 W/m. °C 13.2 Btu/h.ft.°F

In a pool or container 2,500-35,000 W/m2. °C 400-600 Btu/h.ft2.°F

Vertical surfaces 4,000-11,300W/m2. °C 700-2,000 Btu/h.ft2.°F

15

Convection heat transfer from a plate

Flow

q

Tw

T∞

wall

Since the velocity of the fluid layer at the wall will be zero, the

heat must be transferred only by conduction at that point.

)( TThAQ wNewton’s law of cooling:

Heat radiation

17

)( 4

2

4

1 TTAQ

σ = Stefan-Boltzmann constant

(5.669 x 10-8 W/m2.K4)

ε= emissivity

T= absolute temperature

Rate of heat transfer:

where

Radiation is heat from the sun that

travels in the form of waves.

Quantity Text Notation SI Unit English Unit heat Q Joule (J) Btu (heat transfer) heat rate q Watt (W) Btu/hr (heat transfer rate) (heat energy rate) (rate of heat flow) heat flux q” W/m2 Btu/hr-ft2 (heat rate per unit area) heat rate per unit length q’ W/m Btu/hr-ft volumetric heat generation q W/m3 Btu/hr-ft3 (rate of heat production per unit volume) Conversions: 1 Btu = 1054 J 1 kcal = 4184 J

Practice in class

20

1) 1nm = ? mm

2) 1ft = ? mm (1ft = 0.3048m)

3) 900 ºR = ? ºK

ºF = 9/5ºC+32

ºR = ºF+459.69

ºK = ºC+273.16

ºR = 9/5ºK

4) 1 BTU/hr.ft2 = ? W/m2 (1BTU = 1.05504 kJ; W=J/s)