LAB_12 - Convection - Lab 12 Convection"Recently when we...

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Lab 12 133 Convection Summary Natural and Forced Convection If a hot solid object is immersed in a fluid (a liquid or a gas) at a lower temperature, it will begin to cool by transferring thermal energy to the fluid. If for example, the object discharges heat to the sur- rounding air and causes the air to circulate as a result of its temperature change, we call this process natu- ral convection . If, on the other hand, air is blown over the object using a fan or some other device, we call this process, forced convection . In natural convection, as air (and other fluids) warm they usually become less dense and tend to rise. Thus, the air flows over the object without any mechanical blowing. The word "convection" refers to the motion of fluid and the subsequent transport of thermal energy away from (or towards) an object by virtue of this fluid motion. Forced convection is usually more efficient and thus can be used to cool the object or heat the fluid faster. This process is used in a wide variety of engineering applications including space heating and air conditioning and a variety of internal combustion engines. This experiment investigates both natural and forced convection. Educational Objectives After performing this experiment, students should be able to: 1. Define and measure heat transfer coefficients. 2. Describe the dependence of heat transfer coefficients on fluid velocity. 3. Understand wind-chill factors. Background Information Heat flow is expressed in dimensions of energy per unit time; typical units are Cal/s, BTU/h, Joules/sec and watts. Most engineers use the symbol q for heat flow. When this heat flow is divided by the area A , perpendicular to the direction of heat flow, the resulting expression q / A is called heat flux. Thus the heat flux across a large window may be the same as that for a smaller window, although the heat flow depends greatly on the area of the windows. "Recently, when we sent the first astronaut to the moon, the press hailed it as a great scientific achievement. Of course it was nothing of the sort; it was a mar- velously skillful engineering job. Now that there is a national Academy of Engineering, perhaps the title of engineer will recover its recognition." - Vannevar Bush with his Differential Analyzer
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Lab 12 134 If a surface is hotter than the surrounding fluid, so that: then heat will flow from the hot surface to the cooler fluid. The heat flux is usually proportional to the temperature difference Τ between the surface and the fluid, where The constant of proportionality between this heat flow per unit area (heat flux or q / A ) and the temperature difference is called the convective heat transfer coefficient h . The convective heat transfer coefficient h depends on properties of the fluid (air), such as its ther- mal conductivity and density, but usually it does not depend on characteristics of the surface. We can rearrange the above equation to write the heat flow q in the form: q = h A T Wind Chill Factor -
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This note was uploaded on 12/16/2011 for the course ENGR 102 taught by Professor Cattell during the Fall '10 term at Community College of Philadelphia.

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LAB_12 - Convection - Lab 12 Convection"Recently when we...

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