LAB 3 Radiation Heat Trasfer

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M A LofAEngineering Y S I A Faculty

EPF 3105 Food Process Engineering Laboratory 2

EXPERIMENT 3 RADIATION HEAT TRANSFER SESSION TIME: THURSDAY (2.00PM - 5.00PM) GROUP: 1

GROUP MEMBERS: YONG KAI SIANG SURIANI BT JUMALI SITI MARIAM BT MOHD ZAHIRUDDIN SYAHRUL ANIS HAZWANI BT MOHD BAROYI SITI NUR FAZLIANA BT ABDULLAH

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LECTURER NAME : DR. ROSELIZA BINTI KADIR BASHA

EXPERIMENT 3: RADIATION HEAT TRANSFER Introduction: Radiation heat transfer is concerned with the exchange of thermal radiation energy between two or more bodies. Thermal radiation is defined as electromagnetic radiation in the wavelength range of 0.1 to 100 microns (which encompasses the visible light regime), and arises as a result of a temperature difference between two bodies. No medium need exist between two bodies for heat transfer to take place. Rather, the intermediates are photons which travel at the speed of light. All bodies radiate energy in the form of photons moving in a random direction, with random phase and frequency. When radiated photons reach another surface, they may be absorbed, reflected or transmitted. The heat transferred into or out of an object by thermal radiation is a function of several components. These include its surface reflectivity, emissivity, surface area, temperature and geometric orientation with respect to other thermally participating objects. In turn, an object‟s surface reflectivity and emissivity is a function of its surface conditions (roughness, finish, etc.) and composition. In this experiment, we conducted three experiments related to radiation heat transfer which are inverse square law of heat, Stefan-Boltzmann law and emissivity. Inverse Square Law for Heat Inverse square law is a relationship that states that electromagnetic radiation is inversely proportional to the square of the distance from a point source. A point source of gamma rays emits in all directions about the source. It follows that the intensity of the gamma rays decreases with distance from the source because the rays are spread over greater area as the distance increases. As light radiates from a point source, the intensity of light (I) is inversely proportional to the square of the distance(x) from the source. I = (1/x2) As intensity is the power per unit area (W/m2), it naturally decreases with the square of the distance as the size of the radiative spherical wave front increases with distance. Inverse square law is applied in radiation protection and patient dose calculations. This is because, if the radiation strength (intensity) is known at a specific

point, then intensity at any distance from that source may be calculated. According to Nave (2012), any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given radius r is the source strength divided by the area of the sphere. Being strictly geometric in its origin, the inverse square law applies to diverse phenomena. Point sources of gravitational force, electric field, light, sound or radiation obey the inverse square law.

Figure: Illustration of intensity and the distance. Stefan-Boltzmann Law The thermal energy radiated by a blackbody radiator per second per unit area is proportional to the forth power of the absolute temperature and is given by = 𝛔T4 j/m2s Stefan-Boltzmann Law 𝛔 = 5.6703x 10-8 watt/m2 K4 For hot objects other than ideal radiators, the law is expressed in the form: = ε𝛔T4 Where ε is the emissivity of the object (ε = 1 for ideal radiator/black body). If the hot object is radiating energy to its cooler surroundings at temperature Tc, the net radiation loss rate takes the form

= ε (T4 – Tc4) A black body is defined as a body that absorbs all radiation that falls on its surface. A black body is a hypothetic body that completely absorbs all wavelengths of thermal radiation incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self-luminous. All blackbodies heated to a given temperature emit thermal radiation. Emissive of different surface polished silver anodized matt black Emissivity is a measure of the efficiency in which a surface emits thermal energy.it is defined as the ratio of energy being emitted related to that emitted by a thermally black surface (a black body). A black body is a perfect emitter of heat energy and has an emissivity value of 1. A material with an emissivity value of 0 would be considered a perfect thermal mirror. The emissivity coefficient, ԑ indicate the radiation of the heat from a „grey body‟ according the Stefan-Boltzmann Law, compared with the radiation of heat from a ideal „black body‟ with the emissivity coefficient = 1. For a grey body reactor, The Stefan-Boltzmann Law can be expanded to give qg = ԑ σ (Ts4 – Ta4). Where the radiating surface for a black body ԑ=1, and for a grey body, ԑ