Trasferimento di calore

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1 Trasferimento di calore
Scambio di calore. Effetti combinati del trasferimento di calore modalità/materiali.

2 Trasferimento di calore
Come precedentemente menzionato, il calore viene trasferito tra una regione ad alta temperatura verso una a bassa temperatura. Ciò può avvenire attraverso 3 meccanismi. - Conduzione (trasferimento conduttivo )‏ - Convezione (trasferimento convettivo)‏ - Irraggiamento (trasferimento radiativo)‏

3 Modalità di trasferimento del calore
On the left hand side of the pictures, it is a painted steel door. On the right hand side of the pictures, it is a rendered concrete block wall. Materials have different thermal properties. On the associated movie, we clearly see that the time behaviours of the two materials differ. Heat transfer from the hands to the materials are equivalent, however the thermal signatures of both hands do not dwindle at the same speed. This is due to differences in thermal diffusivity. Thermal diffusivity is the ratio between thermal conductivity and volumetric heat capacity. It represents how fast a material goes back to a volumetric equilibrium after it has gained thermal energy. a = k/ρc [m² s-1] Thermal diffusivity is equivalent to cinematic viscosity in fluid mechanics. Some approximate values at 20 °C: Air x 10-7 Water 1,4 x 10-7 Aluminium x 10-7 Copper 1115 x 10-7 Stainless steel 40 x 10-7 Concrete 6,5 x 10-7 Glass 4,3 x 10-7 PVC 0,8 x 10-7 Wood 21,5 x 10-7 Mineral insulation materials 4,5 x 10-7 Thermograms and sequence credit: Karl Grimnes – Termografi og Måleteknik, Rafael Royo - University of Valencia & Raphaël Danjoux - ITC. Il trasferimento di calore avviene per conduzione dal palmo della mano sulla parete

4 Modalità di trasferimento del calore
Convezione The ocean is rather cold, 13 °C in summer, and solar load is important. The ground temperature is high. Water is evaporating at a high rate. Because of pressure and temperature differences between see level, and top of mountains, natural convection occurs. In meteorology, the term convection is used specifically to describe vertical transport of heat and moisture in the atmosphere, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cbs, towering cumulus clouds, and ACCAS (AltoCumulus CAStelanus) clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection. Image credit Raphaël Danjoux - ITC. Atmospheric turbulences over Big Sur. Hwy 1. July La differenza di temperatura tra l’oceano e la terra induce una forte evaporazione. Le nuvole si muovono a causa di correnti convettive

5 Viene prodotta inoltre luce nel visibile.
Modalità di trasferimento del calore Radiazione Quando si è di fronte ad un fuoco come questo, ci si cucina di fronte e si congela dietro! Il calore viene liberato dalla combustione dei gas e può essere percepito a distanza Viene prodotta inoltre luce nel visibile. In order for combustion to occur, you need fuel and oxidant. The chemical reactions between the two produce heat, water vapour, carbon dioxide and residues (unburned materials). What we call a flame is actually the visible part of this reaction, limited by a three-dimensional fast moving envelope. Flame colour depends on a lot of parameters, the first one being blackbody radiation and spectral band emission. For wood fire (and other materials as well) the red part of a flame is the coolest part, then it is orange, yellow, white. Blue colour appears when soot proportionally diminishes. Some examples of flame temperatures: Candle ca °C Bunsen burner ca °C Acetylene in air ca °C Acetylene in oxygen ca °C Methane, Propane, Butane Natural gas in air ca °C to 1980 °C Besides temperature, other substances have also an influence on the colour: - Sodium > bright yellow - Calcium chloride > yellowish red - Sodium chloride > bright yellow orange - Copper > bright green - Manganese chloride > pale yellow green Image credit Raphaël Danjoux – ITC.

6 Conduzione – definizione
La conduzione avviene tra materiali continui dove l’energia termica viene trasferita direttamente tra le molecole che collidono, senza movimento di massa dei materiali. The only parameter that defines the direction of conductive transfer is the temperature difference. In the drawing below, although the warm (red) and cold (blue) bodies are of different sizes and apparent strengths, the heat exchange will always be in a same direction. The warm pot will progressively become colder, and the cold pot will heat up. Considering the respective volumes, the final temperature will likely be more in favour of the big pot. Really sad!

7 Conduzione Trasferimento di energia di movimento (energia cinetica) tra molecole. Può avvenire nei solidi, liquidi e gas. E’ l’unica che avviene in pratica nei solidi! Molto importante da capire per gli operatori termografici. - Radiative transfer exists in semi-transparent and/or diffusive materials. Facing such a material, a thermography camera would receive radiation not only from the surface but also from the inside. An “equivalent emissivity” can be expressed if the target remains isothermal, or if the internal temperature profile is known. Otherwise, the notion of emissivity, as seen in the Level 1 course, looses its meaning. - In case the material is opaque, the only internal mode of heat transfer is conduction.

8 Conduzione termica / Conduzione elettrica
Il “fluido di elettroni” di un materiale conduttivo conduce attraverso il solido la corrente di calore. Gli elettroni conducono inoltre la corrente elettrica attraverso solidi conduttivi, e le conducibilità termiche ed elettriche di molti materiali hanno grosso modo lo stesso rapporto. The correlation between thermal and electrical conductivities is very strong for metals. These have high conductivities. Un buon conduttore elettrico è normalmente un buon conduttore termico, ma questo non può essere generalizzato. Unfortunately, this cannot be really generalized to absolutely all materials. A lot of ceramics for instance are good electrical resistors and good thermal conductors at the same time.

9 Conduzione Quando il fornello elettrico si scalda, esso trasferisce fisicamente il calore alla pentola. La pentola trasferirà poi il suo calore nell’acqua all’interno, causando il riscaldamento della stessa quindi l’ebollizione. On the resistance, the atoms and molecules are hot, rapidly moving or vibrating and interacting with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring atoms. In solids and liquids, the molecules are closer to each other than in a gas. Denser substances are usually better conductors; metals are excellent conductors. In a gas, conduction is almost zero. Credit for the animation FLIR Airborne training Center. fornello pentola acqua

10 Conduzione Legge di Fourier mono dimensionale. T1 (caldo)‏
T2 (freddo)‏ Altezza Watt spessore Larghezza This is an is an empirical macroscopical law based on observation by the French physicist Joseph Fourier (March 21, 1768 – May 16, 1830). Below is a 1822 extract of the “Discours préliminaire” of “Théorie analytique de la chaleur” (Preliminary discourse. Analytical theory of heat). This document was presented to the “Académie des Sciences”. “Primary causes are unknown to us; but are subject to simple and constant laws, which may be discovered by observation, the study of them being the subject of natural philosophy. Heat, like gravity, penetrates every substance of the universe, its ray occupy all parts of space. The object of our work is to set forth the mathematical laws which this element obeys. The theory of heat will hereafter form one of the most important branches of general physics.” Then, after a discussion of rational mechanics he continues with: “But whatever may be the range of mechanical theories, they do not apply to the effects of heat. These make up a special order of phenomena, which cannot be explained by the principles of motion and equilibria” . Joseph Fourier was also a mathematician, and is best known for initiating the investigation of Fourier series. The Fourier transform is also named in his honour. What we present is a reduction of the general equation to one dimension. Q/t è espresso in watt Q = trasferimento di calore conduttivo (J)‏ t = tempo (s)‏ k = Conducibilità termica (viene usata anche λ) (W m-1 K-1)‏ A = Area = Larghezza x altezza (m²)‏ T1 – T2 = ΔT = (delta T) (K or °C )‏ L = Lunghezza del percorso conduttivo o spessore (m)

11 Conduzione I professionisti dell’edilizia hanno spesso più familiarità con “q”, in watt per m², piuttosto che “Q/t” in watt.

12 Conduzione / Edilizia RESISTENZA TERMICA CONDUTTIVA Rcond per un singolo strato (K m2 W-1). Ohm’s law writes U=RI. ΔT is ”equivalent” to a voltage difference in volts (V). q is ”equivalent” to a current in amperes (A). In the figure below, each of the four layers has its own thermal conduction resistance R=L/k. Resistances are additive when in series. Conduction conductances (reciprocal of resistances) are additive when in parallel. La resistenza termica è l’omologo della resistenza elettrica. L1 L2 L3 L4 R1 R2 R3 R4 R1 R2 R3 R4 R = R1 + R2 + R3 + R4 1/R = 1/R1 + 1/R2 + 1/R3 + 1/R4

13 RESISTENZA TERMICA CONDUTTIVA Rcond per materiali isolanti.
Conduzione / Edilizia RESISTENZA TERMICA CONDUTTIVA Rcond per materiali isolanti. 3,5 Polyurethane Spray 1,4 Vermiculite 3,5 to 4 Polyurethane estruso 2,9 to 3,3 Polystyrene estruso 2,2 to 2,3 Fibra di vetro / lana di vetro 2,5 to 3,3 Polystyrene espanso Lana di roccia 2,3 to 2,6 Cellulosa (organica)‏ R valore per 10 cm Materiale isolante Note that the R value is here expressed in SI units. In the US, Japan, Australia and other countries, units are different, yielding to a R value expressed per inch. Assuming the same thickness, the corresponding factor is: R (SI units) = R (American units) x 0,1761, or R (American units) = R (SI units) x 5,678. Radiative and convective transfers at interfaces also play significant roles. When considering all modes of heat transfer, the term thermal transmittance (U-value) is used. Insulation materials have low thermal conductivities because their purpose is to trap gases, which have low conductivities by nature. There exist materials marketed as ”reflective insulation”. Typically they consist in a sandwich of several layers of conductive insulation materials separated by aluminium coated plastic sheets. External sheets are in plastic or aluminium coated plastic. Two main types are found - Layers are made of low density fibres (layers are sewn together), Layers are made of bubble wrap or (bubble wrap + foam). Thermal properties quoted for such materials are very controversial. There is no exclusive standard for such products, so some marketing and sales organisations say they have developed “ad-hoc” measurement procedures. However, they are obviously not well documented, nor properly validated. What most national labs agree on is that Intrinsic insulating capabilities are usually 5 to 20 times lower than what is required for a new building. A still air layer between the surface and the outer board improves performance, but such realization is rather complicated, as an air passage wider than 0.5 mm annihilates the effect. A poor workmanship may lead to severe condensation. A study carried out in France in winter 2006 (CSTB. 13 June 2007 Report. Mesure de la consommation d’énergie de deux cellules placées en environnement extérieur. Essais in situ) on standardized building cells concluded that, in the best case, the energy consumption with such materials doubles that of traditional fibrous insulation.

14 Conduzione - esempio Calcolo – esempio su una parete:
Materiale standard da isolamento, con una conducibilità termica di: k = 0,04 W m-1 K-1 Su di un area di: A = 10 m2 Temperatura esterna = 0 °C (273,15 K)‏ Temperatura interna = 20 °C (293,15 K)‏ ΔT = 20 K Spessore dell’isolamento L = 0,2 m Significa che dobbiamo fornire costantemente il sistema con 40 W per mantenere la temperatura di 20°C su di una lato quando abbiamo 0°C sull’altro.

15 Conduzione 20 °C 10 °C 0 °C -10°C -10 °C -10 °C -10 °C -10 °C calore Q/t calore Q/t calore Q/t Nessun flusso elevato medio basso Il flusso di calore condotto è proporzionale alla differenza di temperatura

16 Conducibilità termica valori SI
Da buoni a eccellenti conduttori termici ed elettrici (W m-1 K-1) Metalli argento rame oro Alluminio Ottone 80 Ferro 80 Acciaio Materiali isolanti Fibra (Lana minerale, Lana di roccia) 0,04 Sughero 0,05 Gomma 0,17 PVC 0,16 Acqua 0,6 Aria secca 0,0245 Xenon 0,0051 This data is approximate, as material properties depend on composition and structure, operating temperature range, processing dynamics, and other factors. There exist a lot of standards dealing with thermal conductivity. Below is a list. IEEE Standard for the Preparation of Test Procedures for the Thermal Evaluation of Solid Electrical Insulating Materials ASTM D Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials ASTM E Standard Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique ASTM D Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique ASTM D Standard Test Method for Thermal Conductivity of Liquids

17 Conducibilità termica valori SI
(W m-1 K-1) Metalli argento rame oro Alluminio Ottone 80 Ferro 80 Acciaio Materiali isolanti Fibra (Lana minerale, Lana di roccia) 0,04 Sughero 0,05 Gomma 0,17 PVC 0,16 Acqua 0,6 Aria secca 0,0245 Xenon 0,0051 Cattivi conduttori termici ed elettrici This data is approximate, as material properties depend on composition and structure, operating temperature range, processing dynamics, and other factors. There exist a lot of standards dealing with thermal conductivity. Below is a list. IEEE Standard for the Preparation of Test Procedures for the Thermal Evaluation of Solid Electrical Insulating Materials ASTM D Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials ASTM E Standard Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique ASTM D Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique ASTM D Standard Test Method for Thermal Conductivity of Liquids

18 Conducibilità termica valori SI
(W m-1 K-1) Metalli argento rame oro Alluminio Ottone 80 Ferro 80 Acciaio Materiali isolanti Fibra (Lana minerale, Lana di roccia) 0,04 Sughero 0,05 Gomma 0,17 PVC 0,16 Acqua 0,6 Aria secca 0,0245 Xenon 0,0051 E i materiali con una conducibilità termica ed elettrica intermedia? This data is approximate, as material properties depend on composition and structure, operating temperature range, processing dynamics, and other factors. There exist a lot of standards dealing with thermal conductivity. Below is a list. IEEE Standard for the Preparation of Test Procedures for the Thermal Evaluation of Solid Electrical Insulating Materials ASTM D Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials ASTM E Standard Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique ASTM D Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique ASTM D Standard Test Method for Thermal Conductivity of Liquids

19 Conducibilità termica valori SI
Materiali con conducibilità intermedia (W m-1 K-1) Vetro 0,8 Ghiaccio 1,6 Marmo 2 to 3 Mercurio 8,3 Inconel 10 This data is approximate, as material properties depend on composition and structure, operating temperature range, processing dynamics, and other factors There is not many materials with an intermediate conductivity.

20 Convezione - definizione
La convezione è una modalità di trasferimento di calore dove il fluido è portato in movimento dalla gravità o da un’ altra forza, pertanto si ha trasferimento di calore da un posto ad un altro.

21 Convezione, alcuni fatti
La convezione è normalmente la forma dominante di trasferimento di calore nei liquidi e nei gas. Viene usata per caratterizzare l’effetto combinato della conduzione e il flusso del fluido. Il principio base della convezione naturale è che la materia riscaldata diventa meno densa e “galleggia”, mentre quella più fredda “affonda”.

22 Legge del raffreddamento di Newton
Newton affermò che la velocità di perdita di calore di un corpo è proporzionale alla differenza di temperatura tra il corpo e l’ambiente che lo circonda. E’ una variazione quasi istantanea della velocità di cambiamento della temperatura. La soluzione matematica consiste nel risolvere un’equazione differenziale. Newton’s law of cooling is only an approximation. It is NOT an universal solution. Full details can be found and discussed during the lab.

23 Convezione, formula conduttiva convettiva convettiva T = 22 °C T
Per ogni lato della parete convettiva convettiva T = 22 °C a Ir Q/t = flusso termico, W h = coefficiente di trasferimento termico convettivo, W m- 2 K-1 A = area della superficie, m2 Ti , To = temperatura interna ed esterna, ºC (o K)‏ inside T = 17 °C w all inside T = 0 ˚C Wall T = 0° outside w all, Be careful! Newton’s law of cooling applies to all modes of heat transfer. Convection is only part of it. T air = -3 °C outside Flusso di calore Q/t

24 Convezione, formula Similarità tra convezione e conduzione
We can define a convective resistance, the same way we expressed the conductive resistance at the beginning of the chapter. Rconv = 1 / h In building science, when doing theoretical calculations, suggested values for outside and inside convective resistances are found in ISO 6946:2007, Building components and building elements -- Thermal resistance and thermal transmittance -- Calculation method. h < > k/L 0,04 Outside (Re)‏ Downwards +/- 30° from horizontal Upwards Heat flow direction Surface 0,17 0,13 0,10 Inside (Ri)‏ Downwards +/- 30° from horizontal Upwards Heat flow direction Surface

25 Convezione, coefficiente h
Alcuni valori possibili del coefficiente di trasferimento termico convettivo h in W m-2 K-1. 10000 to 12000 Vapore condensante 3500 to 5800 Acqua bollente su una superficie metallica 4700 to 7000 Acqua bollente in tubi 2300 to 4700 Acqua in una pentola, agitata 580 to 2300 Acqua in una pentola 60 to 300 Aria in movimento veloce 25 to 70 Aria in leggero movimento 3,5 to 35 Aria ferma

26 Fattori influenzanti la convezione
La velocità del trasferimento di calore per convezione dipende da: Natura del fluido. Differenza di temperatura tra superficie e fluido. Velocità del fluido. Caratteristiche della superficie, area e orientamento.

27 Convezione, effetto vento
Il raffreddamento è funzione del tempo e della velocità del vento Temperatura in °C Curva di raffreddamento teorica di ½ m² di lastra di ferro del peso di 1 kg. Temperatura aria 10 °C. 0 m s 1 m s 2 m s 3 m s 4 m s 5 m s Curve credit Bernd Schönbach – SIS. Tempo in s

28 Trasferimento di calore per irraggiamento - definizione
Il trasferimento di calore per emissione ed assorbimento di radiazione termica è chiamato trasferimento di calore per irraggiamento.

29 Irraggiamento Il sole emette radiazione che è visibile all’ occhio umano, nella banda compresa tra 0,4-e-0,8 µm. L’irraggiamento è più efficiente se siamo nel vuoto (nello spazio). Photo shows the earth from the moon. Visible radiation is emitted from the sun, and then reflected in the direction of the moon by the surface of the earth. Atmospheric perturbations play a role in the apparent blue colour.

30 Trasferimento di calore per irraggiamento
La radiazione termica è una radiazione elettromagnetica. Tutti gli oggetti con temperatura al di sopra dello zero assoluto, –273,15 ºC, emettono radiazione termica. Nessun mezzo (materia) è necessario. Passa facilmente tra i gas. Passa con difficoltà, o viene bloccato, nei liquidi o nei solidi.

31 Onde Elettromagnetiche
Le onde elettromagnetiche sono normalmente caratterizzate dalla loro lunghezza d’onda,  (lambda). L’unità più comune nell’ IR è il m, micrometro, un milionesimo di metro, comunemente indicato come 1 m = 10-6 m = 1/1000 mm.  Velocità della luce The relation between frequency f and wavelength λ is: λ = c / f with c being the speed of light in the vacuum. ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

32 Bande d’onda + ENERGY - - WAVELENGTH + g X Visible Microwaves UV IR
Radio 10nm Hz 100nm 1µm 10µm 100µm 1mm 300 GHz 10mm 100mm 1m 300 MHz 10m 100m 1km 300 kHz Consider the radio station you listen to every morning in your car while going to work. You turn on your car radio, and listen to the news on your favourite channel. How does it work? Somebody speaks in a microphone. Pressure wave is converted into an electric signal, then amplified. Signal is put on a wave carrier, and sent over the air via an antenna. This device emits some watts in all directions, but at a given wavelength. Let’s say 101 MHz. Your car is equipped with an antenna, which purpose is to absorb a certain quantity of watts. To listen to your favourite program, you need to tune your auto-radio to 101 MHz. Otherwise the conversion does not work. So as to say you do not receive enough watts at the correct frequency (wavelength) to convert it into sound and music. It is here clear that absorption and emission are two sides of the same phenomena, should the frequency be the same. If the car is too far from the emitter, you do not receive enough power for the conversion into music to be possible. You either receive another program, still at the same frequency, or a brrrrr called noise. Sometimes, you may stop receiving anything, for example if there is a mountain between your car and the emitter. Mountain does not let the emitted wave go through. It does not transmit easily. To finish with, you may sometimes also hear another program, superimposing to yours, by reflection on mountains, buildings. WAVELENGTH + ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

33 Herschel La temperatura sale
La scoperta della radiazione infrarosso è comunemente attribuita a William Herschel, un astronomo, all’inizio del 19° secolo. Radiazione visibile da un prisma di vetro a 45° Termometro con bulbo annerito Less than 200 years ago the existence of the infrared portion of the electromagnetic spectrum wasn't even suspected. The original significance of the infrared spectrum, or simply ‘the infrared’ as it is often called, as a form of heat radiation is perhaps less obvious today than it was at the time of its discovery by Herschel in 1800. The discovery was made accidentally during the search for a new optical material. Sir William Herschel – Royal Astronomer to King George III of England, and already famous for his discovery of the planet Uranus – was searching for an optical filter material to reduce the brightness of the sun’s image in telescopes during solar observations. While testing different samples of coloured glass which gave similar reductions in brightness he was intrigued to find that some of the samples passed very little of the sun’s heat, while others passed so much heat that he risked eye damage after only a few seconds’ observation. Herschel was soon convinced of the necessity of setting up a systematic experiment, with the objective of finding a single material that would give the desired reduction in brightness as well as the maximum reduction in heat. He began the experiment by actually repeating Newton’s prism experiment, but looking for the heating effect rather than the visual distribution of intensity in the spectrum. He first blackened the bulb of a sensitive mercury-in-glass thermometer with ink, and with this as his radiation detector he proceeded to test the heating effect of the various colours of the spectrum formed on the top of a table by passing sunlight through a glass prism. Other thermometers, placed outside the sun’s rays, served as controls. La temperatura sale ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

34 Herschel T2 > T1 T3 > T2 > T1 T3 T2 T1
As the blackened thermometer was moved slowly along the colours of the spectrum, the temperature readings showed a steady increase from the violet end to the red end. This was not entirely unexpected, since the Italian researcher, Landriani, in a similar experiment in 1777 had observed much the same effect. It was Herschel, however, who was the first to recognize that there must be a point where the heating effect reaches a maximum, and those measurements confined to the visible portion of the spectrum failed to locate this point. Moving the thermometer into the dark region beyond the red end of the spectrum, Herschel confirmed that the heating continued to increase. The maximum point, when he found it, lay well beyond the red end – in what is known today as the infrared region. When Herschel revealed his discovery, he referred to this new portion of the electromagnetic spectrum as the ‘thermometrical spectrum’. The radiation itself he sometimes referred to as ‘dark heat’, or simply ‘the invisible rays’. Ironically, and contrary to popular opinion, it wasn't Herschel who originated the term ‘infrared’. The word only began to appear in print around 75 years later, and it is still unclear who should receive credit as the originator. ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

35 Herschel Luce rifratta Arcobaleno 3 termometri Prisma in vetro
Herschel’s use of glass in the prism of his original experiment led to some early controversies with his contemporaries about the actual existence of the infrared wavelengths. Different investigators, in attempting to confirm his work, used various types of glass indiscriminately, having different transparencies in the infrared. Through his later experiments, Herschel was aware of the limited transparency of glass to the newly-discovered thermal radiation, and he was forced to conclude that optics for the infrared would probably be doomed to the use of reflective elements exclusively (i.e. plane and curved mirrors). Fortunately, this proved to be true only until 1830, when the Italian investigator, Melloni, made his great discovery that naturally occurring rock salt (NaCl) – which was available in large enough natural crystals to be made into lenses and prisms – is remarkably transparent to the infrared. The result was that rock salt became the principal infrared optical material, and remained so for the next hundred years, until the art of synthetic crystal growing was mastered in the 1930’s. dopo 10 minuti ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

36 Trasferimento di calore per irraggiamento
Il calore è trasferito per emissione ed assorbimento. Gli oggetti emettono e assorbono la radiazione. Il trasferimento di calore netto è la differenza. Più caldo Più freddo

37 Legge di Planck c = Velocità della luce nel vuoto = 3 x 108 m s-1
h = Costante di Planck = 6,6 x J s k = costante di Boltzmann = 1,4 x J K-1 T = Temperatura in K λ = Lunghezza d’onda in metri This law is valid for infinitesimal solid angle. It expresses radiance per unit wavelength interval. Max Planck originally produced it in 1900 in an attempt to improve upon an expression proposed by Wilhelm Wien which fit the experimental data at short wavelengths but deviated from it at long wavelengths. Max Planck received the 1918 Nobel Prize for his work (ceremony was delayed until 1919 because of the 1st world war). The official terminology for I is “black body monochromatic radiance” or “black body monochromatic luminance”. In case the unit is [W m-² µm-1], a 10-6 factor is applied. ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

38 Legge di Planck La formula è più semplice di quanto sembra!
E’ per lo più fatta di costanti. La lunghezza d’onda e la temperatura sono le sole variabili. ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

39 Planck - curve La legge di Planck ha due input.
Per creare una rappresentazione grafica Si sceglie una temperatura (kelvin), Si varia la lunghezza d’onda, Si prende una nuova temperatura, ...ed un’altra, ed un’altra... Il risultato è una famiglia di curve, ognuna delle quali è valida per una specifica temperatura. ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

40 Radianza Monocromatica in W m-3
Planck - curve Radianza Monocromatica in W m-3 Temperatura di corpo nero = 300 K µm ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

41 Radianza Monocromatica in W m-3
Planck - curves Radianza Monocromatica in W m-3 Temperature di corpo nero = 600 K µm ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

42 Radianza Monocromatica in W m-3
Planck - curve Radianza Monocromatica in W m-3 Temperature di corpo nero = 300 K e 600 K µm ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

43 Planck - curve Notare che le curve hanno una forma simile.
Comunque, l’ordine di ampiezza della scala Y è totalmente differente. 300 K - Max = 2,6 x 107 W m-3 600 K - Max = 1 x 109 W m-3 Conseguentemente, una scala lineare non è la scelta migliore per mostrare più curve allo stesso tempo. Una scala Logaritmica-logaritmica è la più comune presentazione. ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

44 Radianza Monocromatica in W m-3
Planck - curve Radianza Monocromatica in W m-3 7 temperature di corpo nero da -75 °C a 4000 °C Note that curves do not cross. µm ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

45 Legge di Stefan-Boltzmann per i corpi grigi
Cosa succede se non è un corpo nero, ma un corpo grigio? La potenza emessa sarà minore di quella di un corpo nero! Ma quanto meno? >>> L’emissività grigia! This formula was given as so in the L1, and emissivity was explained as a ratio between Stefan-Boltzmann of a black body, and the emission of a grey body. ε è l’emissività grigia T è espressa in K La costante di Stefan-Boltzmann,  = 5,67 x W m-2 K-4 ITC Level 2 course. Infrared Science. © 2008 Publ. No T560312_B-en-GB

46 Domande Come si attua la conduzione?
Spiegare la legge monodimensionale di Fourier? Quali sono i parametri che influiscono? In che modo la conducibilità termica e l’emissività sono associate? Per quali condizioni è valida la legge di Fourier? Spiegare come avviene la convezione? Esprimere la formula della convezione. Come si rapporta con la formula della conduzione? Quali sono i fattori che influenzano la convezione? Cos’è l’effetto camino? Che cos’è uno strato limite? Che modalità di trasferimento di calore possono esistere nel vuoto?

47 Scambio termico in una tubazione