Unit B-4: List of Subjects

Transcription

1 ES312 Energy Transfer Fundamentals Unit B: First Law of Thermodynamics ROAD MAP... B-1: The Concept of Energy B-2: Work Interactions B-3: First Law of Thermodynamics B-4: Heat Transfer Fundamentals Unit B-4: List of Subjects Energy Transfer due to Heat Interaction Conduction Heat Transfer Convection Heat Transfer Radiation Heat Transfer

2 PAGE 1 of 6 Energy Transfer due to Heat Interaction CONDUCTION / RADIATION / CONVECTION HEAT TRANSFER The subject of heat transfer, or more generally the transport of energy, is of importance to all engineers and scientists, for its energy, initially derived from the sun, on which the world runs At one time or another, every engineer is likely to be confronted with a heat transfer problem Design of computer circuits: electrical engineers may be concerned with temperature variations of heat sinks and cooling fans Design of aerospace vehicles: aerospace/mechanical engineers may be concerned with thermalstresses on vehicles MECHANICS OF HEAT TRANSFER (ENERGY TRANSPORT) Heat can be transferred in three different modes: conduction, convection, and radiation The complexity of energy transport mechanism increase: conduction => convection => radiation All of these different modes of heat transfer require the existence of a temperature difference All of these different modes of heat transfer are from the high temperature medium to a lower temperature one

3 PAGE 2 of 6 Conduction Heat Transfer Heat transfer from warm air to cold canned drink through the wall of the aluminum CONDUCTION HEAT TRANSFER Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles Conduction can take place in solids, liquids, or gases: Gases and liquids: conduction is due to the collisions of the molecules during their random motion Solids: conduction is due to the combination of vibrations of molecules in a lattice and the energy transport by free electrons THERMAL CONDUCTIVITY A cold canned drink (COLA) in a warm room, for example, eventually warms up to the room temperature as a result of conduction heat transfer through the aluminum can It is observed that the rate of heat conduction ( Q cond ) through a layer of constant thickness ( x ) is proportional to the temperature difference ( T ) across the layer and the area ( A ) normal to the direction of heat transfer, and is proportional to the thickness of the layer: T Qcond kt A (kt is the constant of proportionality, called the thermal conductivity ) x FOURIER S LAW OF HEAT CONDUCTION dt In the limiting case ( x 0): Qcond kt A (Fourier s Law of heat conduction) dx The temperature gradient (dt/dx) is negative, when temperature decreases in the positive x direction

4 PAGE 3 of 6 Convection Heat Transfer Heat transfer from a hot surface to air by convection The cooling of a boiled egg by forced and natural convection CONVECTION HEAT TRANSFER Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion. The faster the fluid motion, the greater the convection heat transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction Heat transfer processes that involve change of phase of a fluid are also considered to be convection because of the fluid motion induced during the process such as the rise of the vapor bubbles during boiling or the fall of the liquid droplets during condensation. FORCED AND NATURAL CONVECTIONS Convection is called forced convection, if the fluid is forced to flow in a tube or over a surface by external means (such as: fan, pump, and/or wind). Convection is called free (or natural ) convection, if the fluid motion is caused by buoyancy forces induced by density differences due to the variation of temperature in the fluid. NEWTON S LAW OF COOLING The rate of heat transfer by convection ( Q conv ) can be determined by the Newton s law of cooling as: Qconv hats Tf (Newton s law of cooling) h = Convection heat transfer coefficient A = Surface area through which heat transfer takes place Ts = Surface temperature Tf = Bulk fluid temperature away from the surface

5 PAGE 4 of 6 Radiation Heat Transfer Unlike conduction and convection, heat transfer by radiation can occur between two bodies, even when they are separated by a medium colder than both of them RADIATION HEAT TRANSFER Radiation is the energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules. Unlike conduction and convection, the transfer of energy by radiation does not require the presence of an intervening medium. Energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum: this is exactly how the energy of the sun reaches the earth. STEFAN-BOLTZMANN LAW The maximum rate of radiation that can be emitted from a surface at an absolute temperature ( T s ) is given by the Stefan-Boltzmann law: 4 Q AT emit,max s A = Surface area through which heat transfer takes place = W/m 2 K 4 (Stefan-Boltzmann constant) The idealized surface that emits the maximum rate of radiation is called the blackbody. The measure of how closely a surface approximates a blackbody is called emissivity ( ): 4 Q AT (where, 0 1, and 1 for blackbody) emit,max s In the special case of a relatively small surface of emissivity and surface area A at absolute temperature Ts that is completely enclosed by a much larger surface at absolute temperature Tsurr separated by a gas (such as air) that does not intervene with radiation can be given by: 4 4 Q A T T rad s surr

6 PAGE 5 of 6 EXERCISE B-4-1 (Do-It-Yourself) Consider a person standing in a breezy room at 20 C. Determine the total rate of heat transfer from this person (in W ) if the exposed surface area and the average outer surface temperature of the person are 1.6 m 2 and 29 C, respectively, and the convection heat transfer coefficient is 6 W/m 2 C. Solution A person is standing in a breezy room. The total rate of heat loss from the person is to be determined. Assumptions (1) The emissivity and heat transfer coefficient are constant and uniform. (2) Heat conduction through the feet is negligible. (3) Heat loss by evaporation is disregarded. Analysis The heat transfer between the person and the air in the room will be by convection (not conduction). Applying the Newton s law of cooling: 2 o 2 o Qconv hats Tf 6 W/m C1.6 m C 86.4 W The person will also lose heat by radiation to the surrounding wall surfaces. Applying the Stefan-Boltzmann law: Qrad ATs Tsurr W/m K 1.6 m K 81.7 W The total heat transfer from the body is determined by adding these two: Qtotal Qconv Qrad W

7 PAGE 6 of 6 For heat transfer analysis purposes, a standing person can be modeled as a 30-cm-diameter, 170-cm-long vertical cylinder with both top and bottom surfaces insulated and with the side surface at an average temperature of 34 C. For a convection heat transfer coefficient of 15 W/m 2 C, determine the rate of heat loss from this person (in W ) by convection in an environment at 20 C. Assumptions (1) Steady operating conditions exist. (2) Heat transfer by radiation is not considered. (3) The environment is at a uniform temperature. Analysis The heat transfer between the person and the surrounding air will be by convection (not conduction). Applying the Newton s law of cooling: 2 o o Qconv hats Tf hdlts Tf 15 W/m C0.3 m1.7 m34 20 C 336 W