Introduction W Q. Energy can move in and out of a system in two forms Work (W) and Heat (Q) C H A P T E R. Thermodynamics is about:

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1 M 4792 IUO: O O D MZL BDUL WID X: eat ransfer ractical pproach by Yunus engel Mc Graw ill Introduction Basic of eat ransfer Dr Mazlan bdul Wahid aculty of Mechanical ngineering Universiti eknologi Malaysia wwwfkmutmmy/~mazlan Mazlan 203 Mazlan 203 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 hermodynamics is about: Interaction of energy with system and surroundings W Q hermodynamics system surroundings boundary In this chapter we will learn What is heat transfer ow is heat transferred elevance and importance nergy can move in and out of a system in two forms Work (W) and eat (Q) Mazlan 203 Mazlan 203

2 2 Mazlan 203 Definitions eat transfer is thermal energy transfer that is induced by a temperature difference (or gradient) Modes of heat transfer onduction heat transfer: Occurs when a temperature gradient exists through a solid or a stationary fluid (liquid or gas) onvection heat transfer: Occurs within a moving fluid, or between a solid surface and a moving fluid, when they are at different temperatures hermal radiation: eat transfer between two surfaces (that are not in contact), often in the absence of an intervening medium Mazlan 203 Mazlan 203 eat ransfer Overview Mazlan 203

3 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 Mazlan 203 Mazlan 203 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 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? Mazlan 203 Mazlan 203 3

4 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) Mazlan 203 Mazlan 203 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 urbine blade cooling redict/control temperature of blades ooling strategies internal cool air passages, cool air bleed through perforated blade surface Mazlan 203 aculty of Mechanical ngineering Universiti eknologi Malaysia 830 kudai, Johor, Malaysia Mazlan 203 4

5 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) 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 ometimes whole body temperature needs to be raised, lowered, maintained water and air blanket devices (convection and conduction), I lamps (radiation) Mazlan 203 Mazlan 203 eat exchangers devices designed specifically to promote heat transfer between two fluids car radiators, boilers, condensers, chip cooling, equipment cooling and so on uel cells Mazlan 203 Mazlan 203 5

6 xample: Design of a container closed container filled with hot coffee is in a room whose air and walls are at a fixed temperature Identify all heat transfer processes that contribute to cooling of the coffee omment on features that would contribute to a superior container design Mazlan 203 Mazlan 203 otation otation used in this course Q - a quantity of heat transfer (same as in thermo) Q - heat transfer rate (per unit time), [J/s = W] LWY Y LO IO O YOU UI Mazlan 203 ymbols and units hermal energy: =[J] (thermal energy or heat has the same unit as work (=force displacement) emperature: =[ o ] or [K] ( o )=(K)+2735 ote: When o or K unit is in the denominator, unit q = Q/ - heat flux (per unit time, per unit area), [W/m 2 ] change doesn't affect the numerical value, eg, specific G - heat generation, [W] heat p J/kg = J/kg K, thermal conductivity g = G / V - heat generation per unit volume, [W/m 3 ] W/m o = W/m K Mazlan 203 6

7 Methods of eat ransfer Objectives are to: describe the three methods of heat transfer give practical/environmental examples of each Modes of eat ransfer here are three principle mode of heat transfer onduction onvection (forced or free) adiation Mazlan 203 Mazlan 203 ODUIO 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 Mazlan 203 Mazlan 203 7

8 onduction ransfer of energy from the more energetic to less energetic particles of a substance by collisions between atoms and/or molecules tomic and molecular activity random molecular motion (diffusion) > 2 x o x 2 q x 2 Mazlan 203 Mazlan 203 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 Q cond = k L Q: heat transfer rate : cross-sectional area L: length k: thermal conductivity : temperature difference across conductor Mazlan 203 Mazlan 203 8

9 onduction onsider a brick wall, of thickness L=03 m which in a cold winter day is exposed to a constant inside temperature, =20 and a constant outside temperature, 2 =-20 =20 x q x L=03 m Wall rea, 2 = -20 Under steady-state conditions the temperature varies linearly as a function of x he rate of conductive heat transfer in the x-direction depends on L " 2 q x Mazlan 203 onduction he proportionality constant is a transport property, known as thermal conductivity k (units W/mK) k L " 2 q x = = k L or the brick wall, k=072 W/mK (assumed constant), therefore q x = 96 W/m 2 ow would this value change if instead of the brick wall we had a piece of polyurethane insulating foam of the same dimensions? (k=0026 W/mK) q x is the heat flux (units W/m 2 or (J/s)/m 2 ), which is the heat transfer rate in the x-direction per unit area perpendicular to the direction of transfer he heat rate, q x (units W=J/s) through a plane wall of area is the product of the flux and the area: q x = q x Mazlan 203 onduction onvection In the general case the rate of heat transfer in the x-direction is expressed in terms of the ourier law: q x = k d dx " (high) q x Minus sign because heat flows from high to low or a linear profile d ( 2 ) = < 0 dx ( x2 x) 2 (low) x x 2 x Mazlan 203 Mazlan 203 9

10 onvection onvection at ome Mazlan 203 Mazlan 203 onvection nergy transfer by random molecular motion (as in conduction) plus bulk (macroscopic) motion of the fluid onvection: transport by random motion of molecules and by bulk motion of fluid dvection: transport due solely to bulk fluid motion orced convection: aused by external means atural (free) convection: flow induced by buoyancy forces, arising from density differences arising from temperature variations in the fluid he above cases involve sensible heat (internal energy) of the fluid Latent heat exchange is associated with phase changes boiling and condensation Mazlan 203 Mazlan 203 0

11 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 Mazlan 203 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 Mazlan 203 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 eg pipes in a boiler, cooling fin on an engine xact mathematical analysis usually impossible usually rely on empirical correlations Mazlan 203 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 Mazlan 203

12 ewton s Law of ooling: Q conv = hs( s ) Q is the convective heat transfer rate (W), and is proportional to the difference between surface and fluid temps onvection 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 Mazlan 203 Mazlan 203 onvection onvection ir at 20 blows over a hot plate, which is mainta ined at a temperature s =300 and has dimensions 20x40 cm ir q o = 20 he convective heat flux is proportional to " x q o = 300 he proportionality constant is the convection heat transfer coefficient, h (W/m 2 K) " x q = h( ) ewton s law of ooling or air h=25 W/m 2 K, therefore the heat flux is q x = 7,000 W/m 2 ow would this value change if instead of blowing air we had still air (h=5 W/m 2 K) or flowing water (h=50 W/m 2 K) he heat rate, is q x = q x = q x (02 x 04) = 560 W he heat transfer coefficient depends on surface geometry, nature of the fluid motion, as well as fluid properties or typical ranges of values, see able textbook In this solution we assumed that heat flux is positive when heat is transferred from the surface to the fluid Mazlan 203 Mazlan 203 2

13 3 Mazlan 203 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 Mazlan 203 Mazlan 203 Mazlan 203

14 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 (567 x 0-8 W/m 2 K 4 ) eat flux emitted by a real object (less than that of a blackbody) Q rad s s 4 : 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 Mazlan 203 Mazlan 203 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 Q su r s ε: emissivity Maximum ε = 00, black charcoal surface, Minimum ε = 00, shiny gold surface σ: tefan-boltzmann constant, 567 x 0-8 W/m 2 K ( ) q = εσ U rad su r 0 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 r = εσ( s + sur ) ( 2 s + sur2 ) where we have linearized the equation shown earlier Mazlan 203 Mazlan 203 4

15 Greenhouse ffect Greenhouse ffect Mazlan 203 Mazlan 203 adiation hermal radiation is energy emitted by matter nergy is transported by electromagnetic waves (or photons) an occur from solid surfaces, liquids and gases Dos not require presence of a medium urroundings at sur " q incident = G urface at s " q emitted = missive power is the radiation emitted by the surface Irradiation G is the rate of incident radiation per unit area of the surface, originating from its surroundings Mazlan 203 Mazlan 203 5

16 adiation or an ideal radiator, or blackbody: q emitted = = σ tefan-boltzmann law where s is the absolute temperature of the surface (K) and σ is the tefan- Boltzmann constant, (σ = 567x0-8 W/m 2 K 4 ) or a real surface: q b = = εσ 4 s 4 emitted s ε is the emissivity 0 ε adiation he net radiation heat transfer from the surface, per unit area is q rad = εσ ( 4 s 4 sur he net radiation heat exchange can be also expressed in the form: q rad = h ) where h = εσ( + )( + ) r ( s sur ) r s sur 2 s 2 sur he irradiation G, originating from the surroundings is: q = G = ασ 4 incident sur α is the absorptivity or a grey surface, α=ε 0 a Mazlan 203 Mazlan 203 xample xample atellites and spacecrafts are exposed to extremely high radiant energy from the sun ropose a method to dissipate the heat, so that the surface temperature of a spacecraft in orbit can be maintained to 300 K On a hot summer day you place a black, metal disk on the ground ssuming that the bottom surface of the disk is insulated by the ground, calculate the temperature of the plate when the heat absorbed equals the heat lost Mazlan 203 Mazlan 203 6

17 Quiz: eat ransfer rocesses Quiz: eat ransfer rocesses Identify the heat transfer processes that determine the temperature of an asphalt pavement on a summer day Identify the heat transfer processes that occur on your forearm, when you are wearing a short-sleeved shirt, while you are sitting in a room uppose you maintain the thermostat of your home at 5 throughout the winter months You are able to tolerate this if the outside air temperature exceedes 0, but feel co ld if the temperature becomes lower re you imagining things? Mazlan 203 Mazlan 203 onservation of nergy teady-low nergy quation urroundings, ontrol Volume (V) Boundary, B (ontrol urface, ) ddition through inlet & in nergy conservation on a rate basis: Units W=J/s -ccumulation (torage) & st -Generation & g st & in + & g & out = = Inflow and outflow are surface phenomena Generation and accumulation are volumetric phenomena Loss through outlet & out d dt & st Mazlan 203 or an open system mass flow provides for the transport of internal, kinetic and potential energy into and out of the system he work term is devided in two contributions: low work, associated to pressure forces (=pυ, where υ is the specific volume) and work done by the system 2 V m& i u + pυ + + g z 2 m& i + q W& = 0 ecall: m& = ρvc m & & = Vc = ρ out 2 V u + pυ + + g z 2 Mass flow rate (kg/s) Volumetric flow rate (m 3 /s) out + Mazlan 203 7

18 xample In an orbiting space station, an electronic package is housed in a compartment having a surface area s = m 2, which is exposed to space Under normal operating conditions, the electronics dissipate kw, all of which must be transferred from the exposed surface to space (a) If the surface emissivity is 0 and the surface is not exposed to the sun, what is its steady-state temperature? (b) If the surface is exposed to a solar flux of 750 W/m 2 and its absorptivity to solar radiation is 025, what is its steady-state temperature? or a control surface: q cond urface nergy Balance q rad q conv & & or " cond q in out q = 0 " conv q " rad = 0 x 2 Mazlan 203 Mazlan 203 xample he roof of a car in a parking lot absorbs a solar radiant flux of 800 W/m 2, while the underside is perfectly insulated he convection coefficient between the rool and the ambient air is 2 W/m 2 K a) eglecting radiation exchange with the surroundings, calculate the temperature of the roof under steady-state conditions, if the ambient air temperature is 20 b) or the same ambient air temperature, calculate the temperature of the roof it its surface emissivity is 08 onservation of energy Mazlan 203 Mazlan 203 8

19 9 Mazlan 203 in = d st /dt = Mazlan 203 he surface energy balance Mazlan 203 urface energy balance Mazlan 203 nalysis of ht problem Mesti buat seperti ini!!!

20 Q Q Q IMO!!! Mazlan