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1 PHYS1001 Physics 1 REGULAR Module 2 Thermal Physics METHODS OF HEAT TRANSFER CONDUCTION CONVECTION RADIATION ptD_transfer.ppt.

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Presentation on theme: "1 PHYS1001 Physics 1 REGULAR Module 2 Thermal Physics METHODS OF HEAT TRANSFER CONDUCTION CONVECTION RADIATION ptD_transfer.ppt."— Presentation transcript:

1 1 PHYS1001 Physics 1 REGULAR Module 2 Thermal Physics METHODS OF HEAT TRANSFER CONDUCTION CONVECTION RADIATION ptD_transfer.ppt

2 2 Overview of Thermal Physics Module: 1.Thermodynamic Systems: Work, Heat, Internal Energy 0 th, 1 st and 2 nd Law of Thermodynamics 2.Thermal Expansion 3.Heat Capacity, Latent Heat 4.Methods of Heat Transfer: Conduction, Convection, Radiation 5.Ideal Gases, Kinetic Theory Model 6.Second Law of Thermodynamics Entropy and Disorder 7.Heat Engines, Refrigerators

3 3  METHODS OF HEAT TRANSFER energy transfer (heat, Q) due to a temperature difference,  T CONDUCTION CONVECTION RADIATION §17.7 p591 system, T Q net Environment, T E European heat wave, 2003  ~ deaths in France References: University Physics 12 th ed Young & Freedman

4 4 Live sheep trade Sunday, October 26, 2003 Sheep to shore... finally The Labor Opposition will pursue the Government over the cost of the "ship of death" saga, which ended on Friday when 50,000 Australian sheep at sea for three months began being unloading in Eritrea.

5 5 Heat Conduction Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole.heat transfer If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones. THTH TCTC conduction though glass Q Q

6 6 For conduction between two plane surfaces (eg heat loss through the wall of a house) the rate of heat transfer is energy transferred through slab Q THTH TCTC L Thermal conductivity k (W.m -1. K -1 ) steady-state heat current H = dQ/dt QQ A

7 7 steady-state Thermal Conduction through a uniform slab 0 x L TCTC THTH

8 8 MaterialThermal conductivity k (W.m -1.K -1 ) diamond 2450 Cu 385 Al 205 Brick 0.2 Glass 0.8 Body fat 0.2 Water 0.6 Wood 0.2 Styrofoam 0.01 Air Thermal conductivity, k property of the material k diamond very high: perfect heat sink, e.g. for high power laser diodes k human low: core temp relatively constant (37 o C) k air very low: good insulator * home insulation * woolen clothing * windows double glazing Metals – good conductors: electrons transfer energy from hot to cold

9 9 Heat Convection Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense and rises (natural or free). Convection assisted by breeze, pump or fan – forced convection. Hot water is likewise less dense than cold water and rises, causing convection currents which transport energy. Convection coefficient, h  T between surface and air way from surface

10 10

11 11 Sea & Land Breezes, Monsoons 35 o C20 o C17 o C11 o C What is the role of heat capacity, c of water and soil?

12 12 RADIATION Energy transferred by electromagnetic waves All materials radiate thermal energy in amounts determined by their temperature, where the energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum. Thermal radiation wavelength ranges: IR ~  m Visible ~  m 800 – 400 nm UV ~  m For exam: more detail than in the textbook

13 13 Ludwig Boltzmann ( ) All objects above absolute zero emit radiant energy and the rate of emission increases and the peak wavelength decreases as the temperature of object increases

14 14 Thermography

15 15 Power absorbed by surface of an object A, a Q abs TsTs Absorption & Stefan-Boltzmann Law Incident radiation (INTENSITY I - energy passing through a square metre every second I inc = P / A I abs = a I inc Surface Area, A Absorption coefficient, a = 0 to 1 Stefan-Boltzmann constant σ = 5.67 x W.m -2.K -4

16 16 Power radiated from the surface of an object A, e, T Q rad Emission & Stefan-Boltzmann Law P net > 0  net heat transfer out of system Surface Area, A Emissivity, e = 0 to 1 Stefan-Boltzmann constant σ = 5.67 x W.m -2.K -4

17 17 A blackbody absorbs all the radiation incident upon it and emits the max possible radiation at all wavelengths ( e = a = 1 ) A graybody is a surface that absorbs a certain proportion of the energy of a blackbody, the constant being constant over the entire band of wavelengths (0  e = a < 1) emissivity e absorption coefficient (absorptivity) a Stefan-Boltzmann constant  = 5.67  W.m -2.K -4

18 18 Wien’s Displacement Law Wien constant b =  m.K Blackbody: absorbs ALL the EMR radiation falling on it & emits the max possible energy over all wavelengths

19 19  Wien’s Displacement Law Wien constant: b =  m.K Blackbody radiation curves show different peak wavelengths at various temperatures

20 20 Stefan-Boltzmann constant,  = 5.67 x W.m -2.K -4 * emissivity, e = 0 to 1 Blackbody, e = 1 * Absorption coefficient, a = 0 to 1 * At a temperature T a = e all wavelengths * T > 700 o C visible radiation (dull red ~ 800 o C white ~ 2000 o C) * Black surface ( e ~ 1) – good emitter / absorber * Polished surface ( e ~ 0.01) – poor emitter / absorber, good reflector * Hot stars – blue * Cool stars - red Water ( e ~ 0.96) Earth ( e ~ 0.3)

21 21 Sun (6000 K - hot!)Earth (300 K - cold!) Visible radiationInfrared radiation

22 22 Sun & Photosynthesis T Sun ~ 6000 K peak ~ 480 nm (blue/green) UV (ionization of molecules) ~ 9% Visible (excite molecules) ~ 49 % IR (warm) ~ 42% 0.1% of radiant energy captured by chlorophyll of plants What about life on Earth if the Sun was hotter or colder ?

23 23 Emissivity, e – the nature of the surface e ~ 0.8 e ~ 0.4 Summer clothing: white reflects radiant energy better than black. Wrap an ice-cube in black cloth and another in aluminium foil and place both in the sunshine. What will happen? Why is the pupil of the eye black?

24 24 Problem D1: Estimate the Sun’s temperature Assume e = 1 Distance from Sun to Earth: R SE = 1.5 x m Radius of the Sun: R S = 6.9 x 10 8 m Solar radiation at Earth’s surface : I = 10 3 W.m -2  = 5.67 x W.m -2.K -4 Solution Power radiated by Sun P rad = I A = I 4  R SE 2 = (10 3 )(4  )(1.5x10 11 ) 2 W = 2.83 x W Surface area of the Sun, A Sun = 4  R S 2 = 5.98 x m 2 T 4 = P rad / (A Sun e  )  T = 5.4 x 10 3 K R SE RSRS

25 25 Problem D.2 Estimate the Earth’s surface Temperature T E (assume NO atmosphere) Solar constant I o = 1360 W.m -2 A E = 4  R E 2 P abs = P rad  T E = 255 K = -18 o C e = 1 Earth Earth albedo (reflectivity)   = 0.3 A disk =  R E 2 What is natural greenhouse effect? Power absorbed by Earth: P abs = (1-   ) A disk I o Power radiated by Earth: P rad = A E  T E 4 R SE radiation emitted from the surface of a sphere

26 26 Earth’s albedo (reflectivity)   = 0.3 Greenhouse Effect

27 27 American Journal of Physics: Feb 1983

28 28 Selective surfaces Emissivity, e – the nature of the surface Value of e is temperature and wavelength dependent 0 1 e short long Selective surface used in solar collectors Good emitter / absorber at short wavelengths Poor emitter / absorber at long wavelengths

29 e 0 1 e Black paint White paint Black skin White skin Black skin absorbs heat more readily than white skin but also radiates heat more readily - the heat balance favours black skin in the tropics short long

30 30 Problem D.3 Steel plates are placed on a non-conducting opaque surface, normal to incident solar radiation (direct + diffuse) of 750 W.m -2. Neglecting convection, calculate the equilibrium temperature T for a polished steel plate ( e = 0.07) and a dull steel plate ( e = 0.8). Assume graybody behaviour. Calculate the effect of coating a steel plate with a selective surface e =  m short long

31 31 Solution Identify / Setup P abs = a I A I = 750 W.m -2 e = a Equilibrium: P abs = P rad Execute The equilibrium temperature is the same for both surfaces – the equilibrium temperature is unaffected by the area and nature of the surface if the emissivity (and absorptivity) remain constant over the range of temperature & wavelength.

32 32 Assume Sun a blackbody at 5800 K Absorbed: virtually all the incident radiation lies at < 2  m thus P abs = e I A = (0.92)(750) A = 690  Radiated: most of the radiation will be at wavelengths > 2 µm P rad = e  A T 4 = 0.1  A T 4 Equil  P abs = P rad  T = 591 K = 317 o C 250 o C hotter e 2  m short long

33 33 Problem D.4 Semester 1, 2007 Examination An igloo is a hemispherical enclosure built of ice. Elmo’s igloo has an inner radius of 2.55 mm and the thickness of the ice is 0.30 m. This thickness can be considered small compared to the radius. Heat leaks out of the igloo at a rate determined by the thermal conductivity of ice, k ice = 1.67 W.m -1.K -1. At what rate must thermal energy be generated inside the igloo to maintain a steady air temperature inside the igloo at 6.5 o C when the outside temperture is -40 o C? Ignore all thermal energy losses by conduction through the ground, or any heat transfer by radiation or convection or leaks.

34 34 Solution Identify / Setup thickness  t = 0.30 m radius r = 2.55 m k ice = 1.67 W.m -1.K -1 The rate of energy production must be equal to the rate of loss of thermal energy by conduction through the hemispherical ice wall. Rate of energy transfer by conduction 6.5 o C -40 o C

35 35 dQ/dt = ? W k = 1.67 W.m -1.K -1 dT = {6.5 – (– 40)} o C = 46.5 o C thickness of ice, dx = 0.30 m area, A = surface area of hemisphere = (4  R 2 ) / 2 = 2  R 2 Because the thickness of the ice is much smaller than either the inside or outside radius, it does not matter which radius is used – taking the average radius R = ( ) m = 2.70 m

36 36 Execute dQ/dt = – (1.67)(2  )(2.70) 2 (46.5)/0.30 W dQ/dt = – 1.2  10 4 W Evaluate sensible value units significant figures did I answer the question ?

37 37 Problem D.5 Suppose a human could live for 120 min unclothed in air at 8 o C. How long could they live in water at 8 o C?

38 38 Problem D.5 Suppose a human could live for 120 min unclothed in air at 8 o C. How long could they live in water at 8 o C? Solution Identify / Setup Thermal conductivities k air = W.m -1.K -1 k water = 0.6 W.m -1.K -1 Execute Evaluate

39 39 Why do droplets of water dance over the very hot pan ? Problem D.6

40 40 Why do droplets of water dance over the very hot pan ? Water at the bottom of the drops is evaporated and provides insulation against further evaporation.

41 41 Problem D.7 An aluminium pot contains water that is kept steadily boiling (100 ºC). The bottom surface of the pot, which is 12 mm thick and 1.5  10 4 mm 2 in area, is maintained at a temperature of 102 °C by an electric heating unit. Find the rate at which heat is transferred through the bottom surface. Compare this with a copper based pot. The thermal conductivities for aluminium and copper are k Al = 235 W.m -1.K -1 and k Cu = 401 W.m -1.K -1

42 42 Solution Identify / Setup T H = 102 o C T C = 100 o C Base area A = 1.5x10 4 mm 2 = 1.5x10 -2 m 2 Base thickness L = 12 mm = 12x10 -3 m k Al = 235 W.m -1.K -1 k Cu = 401 W.m -1.K -1 dT/dx = (T H – T C ) / L dQ/dt = ? W Execute Al, dQ/dt = 5.9x10 2 W Cu, dQ/dt = 1.0x10 3 W Cu pots ~ 2 times more efficient

43 43 “Body Heat” Heat lost by convection is very important for humans For a naked person, h ~ 7.1 W.m -2.K -1 Assume the person’s surface area is 1.5 m 2, skin temperature of 33 o C and surrounding temperature 29 o C. A = 1.5 m 2 T = 33 o C T E = 29 o C dQ/dt convection = h A  T = (7.1)(1.5)(4) W = 43 W If there is a breeze, convection losses are greater – wind chill factor Viscosity of fluids slows natural convection near a stationary surface by producing a boundary layer which has about the same insulating values of 10 mm plywood.

44 44 For the naked person, estimate the net rate of energy radiated.  = 1.5 m 2 T = 33 o C = ( ) K = 306 K T E = ( ) K = 302 K assume e = 1  = 5.67x10 -8 W.m -2.K -4 P radiation = e  A ( T 4 – T E 4 ) = 39 W dQ/dt loss = dQ/dt radiation + dQ/dt convection = 39 W + 43 W = 82 W Ans. similar to the rate at which heat is generated by the body when resting

45 45 Why not heat the water at the top? CONVECTION

46 46 Warm air rises above the ground Forced convection CONVECTION

47 47 Why do we get this pollution haze? Temperature inversion prevents air rising and the dispersing the pollution CONVECTION

48 48 Why are the cooling coils at the top ? In bathrooms, the heater is often near the ceiling. Problem ? CONVECTION

49 49 Wien constant b f = 2.83 k B /h s -1.K RADIATION

50 50 Why are pipes in solar panels painted black ? RADIATION

51 51 Highly reflecting metal foil keeps inside temperature low. RADIATION

52 52 Lemur at the right is active during the day; it points its belly toward the sun on cold mornings. Lemur at left is nocturnal, so the dark fur poses no disadvantage in absorbing excessive sunlight. RADIATION

53 53 Think about * Two different materials at the same temperature have different emissivities. Which one glows the brightest? * Why are fireplace pokers made of iron and not copper? * Some animals have hair which is composed of solid tubular strands, while others have hollow, air-filled tubes. Where would one more likely find the latter animal: In cold climates, or warm? * Steel reinforcement bars add stability to concrete walls. Do they also enhance the insulating value of concrete? * Wind chill factor * Drapes hung close to a cold window

54 54 * Clothing * Should you lower the blinds and draw the curtains on a hot day? * When one steps from a shower on a cold morning, why does the tile floor seem so much colder than the air? * Place a wooden spoon and a metal spoon in the freezer. Which will cool faster? After several hours, what would they feel like? * Why do people become "flushed" when overheated?


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