Heat Transfer Lecture 2, MSA University

10/10/2022 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Heat Transfer It is important to understand the rate at which thermal energy is transferred between a system and its surrounding. There are 3 different mechanisms of heat transf...

10/10/2022 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Heat Transfer It is important to understand the rate at which thermal energy is transferred between a system and its surrounding. There are 3 different mechanisms of heat transfer which are: 1- Conduction. 2- Convection. 3- Radiation. 27 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Heat Conduction The most familiar form of heat exchange is conduction, which is the flow of heat directly through a physical material. It is the easiest thermal energy transfer process which can be described quantitatively. In this process the thermal energy can be viewed on an atomic scale as an exchange of kinetic energy between molecules, where the less energetic particles gain energy by colliding with the more energetic particles. 28 1  10/10/2022 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Experiments show that the amount of heat Q that flows through this rod: Increases in proportion to the rod’s cross-sectional area, A; Increases steadily with time, t; Increases in proportion to the temperature difference, ΔT = T2-T1 Decreases with the length of the rod, L.  T  Q  kA  t  L  The constant k is referred to as the thermal conductivity of the rod. It varies from material to material 29 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Application of heat conduction: An application of thermal conductivity in series can be found in the insulated window. Most homes today have insulated windows as a means of increasing their energy efficiency. As a final example of conduction, we note that many biological systems transfer heat by a mechanism known as countercurrent exchange. 30 2  10/10/2022 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Heat Convection Convection occurs when a fluid is unevenly heated. As with the room heater, the warm portions of the fluid rise because of their lower density and the cool portions sink because of their higher density. Thus, in convection, temperature differences result in a flow of fluid. It is this physical flow of matter that carries heat throughout the system. When the movement results from differences in density, as an example of air around a fire, it is referred to as Natural convection. When the heated substance is forced to move by a fan or pump as in some hot-air and hot-water heating systems the process is called forced convection. 31 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Application of heat convection: The common seashore occurrence of sea breezes during the day and land breezes in the evening is one such example. 32 3  10/10/2022 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Heat Radiation All objects give off energy as a result of radiation. The energy radiated by an object is in the form of electromagnetic waves, which include visible light as well as infrared and ultraviolet radiation. Thus, unlike convection and conduction, radiation has no need for a physical material to mediate the energy transfer, since electromagnetic waves can propagate through empty space—that is, through a vacuum. Therefore, the heat you feel radiated from the Sun reaches the Earth across 150 million kilometers of vacuum. 33 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Stefan–Boltzmann law: P  eAT 4 P: the power of radiated body in watts σ: constant,   5.6696  10 W / m.K 8 2 4 A: the surface area of the object e: constant called emissivity, 0 to 1 T: Temperature in Kelvin The energy radiated per time by an object—that is, the radiated power, P—is proportional to the surface area, A, over which the radiation occurs. It also depends on the temperature of the object. 34 4  10/10/2022 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY The emissivity is a dimensionless number between 0 and 1 that indicates how effective the object is in radiating energy. A value of 1 means that the object is a perfect radiator. In general, a dark-colored object will have an emissivity near 1, and a light-colored object will have an emissivity closer to 0. Wear black in the desert? It actually helps, by radiating heat away from the wearer more efficiently. 35 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Note that the same emissivity e applies to both the emission and absorption of energy. Thus, a perfect emitter ( e = 1) is also a perfect absorber. Such an object is referred to as a blackbody. The opposite of a blackbody is an ideal reflector, which absorbs no radiation ( e = 0 ). It follows that an ideal reflector also radiates no energy. This is why the inside of a Thermos bottle is highly reflective. 36 5  10/10/2022 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Experiments show that objects absorb radiation from their surroundings according to the same law, the Stefan–Boltzmann law, by which they emit radiation. Thus, if the temperature of an object is T, and its surroundings are at the temperature Ts , the net power radiated by the object is;  Pnet  eA T 4  Ts4  If the object’s temperature is greater than its surroundings, it radiates more energy than it absorbs and Pnet is positive. On the other hand, if its temperature is less than the surroundings, it absorbs more energy than it radiates and Pnet is negative. When the object has the same temperature as its surroundings, it is in equilibrium and the net power is zero. 37 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Latent Heat (L) The amount of thermal energy (heat) required to change the phase of a given mass, m, of a pure substance from one phase to another with no change in temperature. Q L m Lf Solid Latent heat of fusion Liquid Latent heat of vaporization Lv Liquid Gas Latent heat of sublimation LS Solid Gas 38 6  10/10/2022 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Application of latent heat: A pleasant application of latent heat is found in the making of homemade ice cream. As you may know, it is necessary to add salt to the ice–water mixture surrounding the container holding the ingredients for the ice cream. The dissolved salt molecules interact with water molecules in the liquid, impairing their ability to interact with one another and freeze. This means that ice and water are no longer in equilibrium at 0 ºC; a lower temperature is required. The result is that ice begins to melt in the ice–water mixture, and in the process of melting it draws the required latent heat from its surroundings—which include the ice cream. Thus, the salt together with the ice produces a temperature lower than the melting temperature of ice alone. 39 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Force in the Direction of Displacement: W  Fd The SI units is N.m. = Joule (J). Definition of the joule, J 1 joule = 1 J = 1 N.m. = 1 (kg.m/s2).m = 1 kg.m2/s2. 40 7  10/10/2022 OCTOBER UNIVERSITY FOR MODERN SCIENCES & ARTS MSA UNIVERSITY Force at an Angle to the Displacement: W  ( F cos )d Work is positive if the force has an angle (-90º < θ