H E A T T R A N S F E R. LECTURER: MOHSIN MOHD SIES C H A P T E R

Transcription

1 M 4463 LU: MOI MOD I

2 Introduction Basic of eat ransfer

3 Introduction hermodynamics: nergy can be transferred between a system and its surroundings. system interacts with its surroundings by exchanging work and heat Deals with equilibrium states Does not give information about: ates at which energy is transferred Mechanisms through with energy is transferred In this chapter we will learn What is heat transfer ow is heat transferred elevance and importance

4 hermodynamics is about: hermodynamics Interaction of energy with system and surroundings. W Q system surroundings boundary nergy can move in and out of a system in two forms Work (W) and eat (Q)

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8 XML onsider a can of drinks which you want to cool down you would put it in a refrigerator. 20 o urrounding ir = 4 o We know from experience that if we leave it in the fridge ultimately it will reach equilibrium with its surroundings BU OW LOG? answer that. hermodynamics can not

9 Where is heat transfer falls at? here are three principle laws upon which ngineering studies are derived onservation of Momentum (luid Mechanics, Mass ransfer) onservation of nergy (hermodynamics, eat ransfer) onservation of Mass (ontinuity, Mass ransfer) In this course we are primarily interested in the onservation of nergy in eat ransfer

10 he topic of eat ransfer is about understanding, determining and predicting flows of heat ll of eat ransfer study is about answering the question: What is the heat flow rate from to B?

11 eat ransfer roblems wo types. ating problems 2. izing problems

12 What is temperature? hermal energy: atomic/molecular/electronic kinetic energy Measure to determine how hot/cold a material is (intensity of thermal energy) riterion to determine the direction of thermalenergy transport rom a microscopic view, temperature represents atomic or molecular kinetic energy (translation / vibration / rotation)

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14 Importance of heat transfer in engineering ower igh turbine inlet temperatures desired for efficiency. eat transfer from gas or steam to turbine blades (convection, radiation) blades may fail. redict/control temperature of blades. ooling strategies internal cool air passages, cool air bleed through perforated blade surface.

15 urbine blade cooling aculty of Mechanical ngineering Universiti eknologi Malaysia 830 kudai, Johor, Malaysia

16 Biomedical hermal cancer treatments electromagnetic radiation (laser, radio), ultrasonic waves, etc used to heat tumor. ecessary to predict tumor temperature and understand heat transfer to surrounding tissue (conduction, convection). ometimes whole body temperature needs to be raised, lowered, maintained water and air blanket devices (convection and conduction), I lamps (radiation).

17 Building eat is transferred through walls (conduction) to outside air (convection), through windows (radiation, convection, conduction), open doors/windows (convection) eat loss (or gain) determines heating (air-conditioning) requirements.

18 eat exchangers devices designed specifically to promote heat transfer between two fluids car radiators, boilers, condensers, chip cooling, equipment cooling and so on

19 uel cells

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21 otation..... Q - otation used in this course a quantity of heat transfer (same as in thermo) Q - heat transfer rate (per unit time), [J/s = W] q = Q/ - heat flux (per unit time, per unit area), [W/m 2 ] G - heat generation, [W] g = G / V - heat generation per unit volume, [W/m 3 ] LWY Y LO IO O YOU UI

22 ymbols and units hermal energy: =[J] (thermal energy or heat has the same unit as work (=force displacement) emperature: =[ o ] or [K] ( o )=(K) ote: When o or K unit is in the denominator, unit change doesn't affect the numerical value, e.g., specific heat p J/kg.o = J/kg. K, thermal conductivity W/m.o = W/m. K

23 Methods of eat ransfer Objectives are to: describe the three methods of heat transfer give practical/environmental examples of each

24 Modes of eat ransfer here are three principle mode of heat transfer onduction onvection (forced or free) adiation

25 ODUIO

26 ODUIO traightforward transmission of heat within a stationary medium olid, liquid, or gas (usually most important in solids) -Usually in solid(s), maybe liquids -arely gases (negligible to convection) Mechanisms are on molecular/atomic level: molecular vibrations, motion of free electrons an often come up with exact mathematical solutions eed a temperature gradient

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28 onduction is simply: ransfer of energy from more energetic to less energetic particles of a substance due to interactions between particles rom empirical observations (experiments) ourier s Law Q.

29 . Q cond = k. Q: heat transfer rate : cross-sectional area L: length k: thermal conductivity Δ: temperature difference across conductor L

30 onvection

31 onvection

32 onvection at ome

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34 onvection s he convection heat transfer mode is comprised two mechanisms:. nergy transfer due to random molecular motion (diffusion) 2. nergy transfer due to bulk (or macroscopic) motion of the fluid (called advection) If both transport of energy is present, the term OVIO is generally used. If transport of energy due only to bulk motion of the fluid, the term DVIO is used.

35 onvection onvection is what happens when the motion of a heat conducting fluid increases the rate of heat transfer. In other words, the convective air currents increase the rate of heat transfer by improving the conduction at the surface.

36 onvection heat transfer normally takes place in a moving liquid or gas onduction still takes place Usually interested in cooling or heating of a solid object by a fluid stream e.g. pipes in a boiler, cooling fin on an engine xact mathematical analysis usually impossible usually rely on empirical correlations

37 onvection We are interested mainly in cases where there is heat transfer between a fluid in motion and a bounding surface. a. Velocity boundary layer b. hermal boundary layer here are two types of convection: orced convection - flow caused by external means ree convection - caused by buoyancy forces

38 ewton s Law of ooling:.. Q conv = h ( s ) Q is the convective heat transfer rate (W), and is proportional to the difference between surface and fluid temps. s h (W/m 2 K) is convective heat transfer coefficient - depends on conditions in boundary layer, surface geometry, nature of fluid motion, and fluid thermo and transport properties.

39 onvection

40 DIIO adiation is energy emitted by matter that is at a finite temperature. he emission is due to changes in electron configurations of constituent atoms or molecules. ransported by electromagnetic radiation. Does not require a material medium, occurs most efficiently in vacuum.

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44 Ideal adiator tefan-boltzmann Law for Blackbody (Ideal adiator): QȦ rad = Ideal radiator or Blackbody Maximum flux at which radiation may be emitted from a surface, where, s is the absolute temp (K) of the surface σ is the tefan Boltzmann constant (5.67 x 0-8 W/m 2 K 4 )

45 eat flux emitted by a real object (less than that of a blackbody). Q rad 4 s s : emissivity, a radiative property of surface, how efficient radiation emission is compared to blackbody 0 Determination of the net rate at which radiation is exchanged between surfaces is complicated Most often, we only need to know the net exchange between a small surface and the surroundings. or

46 mall surface and large surroundings he net rate of radiation heat exchange between a small surface and a large surroundings per a unit area of the small surface ε: emissivity. Q rad q su r Maximum ε =.00, black charcoal surface, Minimum ε = 0.0, shiny gold surface σ: tefan-boltzmann constant, 5.67 x 0-8 W/m 2 K 4 s ( ) 4 4 = εσ U su r 0

47 . revious equation can also be written in the following form, Q = h r ( s sur ) Where h r is the radiation heat transfer coefficient h 2 r = εσ( s + sur ) ( 2 s + sur2 ) where we have linearized the equation shown earlier.

48 Greenhouse ffect

49 Greenhouse ffect

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51 onservation of energy

52 in = d st /dt =

53 he surface energy balance

54 urface energy balance

55 nalysis of h.t problem Mesti buat seperti ini!!!

56 IMO!!! Q.