DR.PRADIP DUTTA Department of Mechanical Engineering Indian Institute of Science Bangalore

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Transcription:

DR.PRADIP DUTTA Department of Mechanical Engineering Indian Institute of Science Bangalore

What is Heat Transfer? Energy in transit due to temperature difference. Thermodynamics tells us: How much heat is transferred (δq) How much work is done (δw) Final state of the system Heat transfer tells us: How (with what modes) δq is transferred At what rate δq is transferred Temperature distribution inside the body Heat transfer complementary Thermodynamics

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 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).............................................................................. T 1 T 1 >T 2 solid or stationary fluid q T 2 RATE: q(w) or (J/s) (heat flow per unit time)

Conduction (contd ) q T 1 k x T 2 A Rate equations (1D conduction): Differential Form q = - k A dt/dx, W k = Thermal Conductivity, W/mK A = Cross-sectional Area, m 2 T = Temperature, K or o C x = Heat flow path, m Difference Form q = k A (T 1 -T 2 ) / (x 1 -x 2 ) Heat flux: q = q / A = - kdt/dx (W/m 2 ) (negative sign denotes heat transfer in the direction of decreasing temperature)

Conduction (contd ) Example: The wall of an industrial furnace is constructed from 0.15 m thick fireclay brick having a thermal conductivity of 1.7 W/mK. Measurements made during steady state operation reveal temperatures of 1400 and 1150 K at the inner and outer surfaces, respectively. What is the rate of heat loss through a wall which is 0.5 m by 3 m on a side?

moving fluid T s >T T q T s Energy transferred by diffusion + bulk motion of fluid

Rate equation (convection) U y U u(y) q y T T(y) Heat transfer rate q = ha( T s -T ) W T s Heat flux q = h( T s -T ) W / m 2 h=heat transfer co-efficient (W /m 2 K) (not a property) depends on geometry,nature of flow, thermodynamics properties etc.

Convection (contd ) Convection Free or natural convection (induced by buoyancy forces) Forced convection (induced by external means) May occur with phase change (boiling, condensation)

Convection (contd ) Typical values of h (W/m 2 K) Free convection gases: 2-25 liquid: 50-100 Forced convection gases: 25-250 liquid: 50-20,000 Boiling/Condensation 2500-100,000

q 1 q 2 T 1 T 2 RATE: q(w) or (J/s) Heat flow per unit time. Flux : q (W/m 2 )

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=εσT s4 (W/ m 2 ) ε= emissivity (property) σ=stefan-boltzmann constant

Rate equations (Contd.) q r a d. T s u r q co n v. T s Area = A Radiation exchange between a large surface and surrounding Q r a d = εσ(t s4 -T sur4 ) W/ m 2

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 70 mm, and its surface temperature and emissivity are 200 C and 0.8, respectively. If the coefficient associated with free convection heat transfer from the surface to the air is 5 W/m 2 K, what is the rate of heat loss from the surface per unit length of pipe?

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