대류 열전달 실험 레포트

대류 열전달 실험 레포트

대류 열전달 실험 레포트.docx

본문 1. Heat transfer modes Heat transfer is the transport of energy due to a temperature difference between different amounts of matter. We know that an ice cube taken out of the freezer will melt when it is placed in a warmer environment such as a glass of liquid water or on a plate with room air around it. Energy in thse modes can be transmitted to the nearby molecules by interactions or by exchange of molecules such that energy is emitted by molecules that have more on average to those that have less on average. This energy exchange between molecules is heat transfer by conduction, and it increases with the temperature difference and the ability of the substance to make the transfer. This is expressed in Fourier몶s law of conduction, Giving the rate of heat transfer as proportional to the conductivity, k, the total area, A, and the temperature gradient. The minus sign indicates the direction of the heat transfer from a higher-temperature to a lower-temperature region. Often the gradient is evaluated as a temperature difference divided by a distance when an estimate has to be made if a mathematical or numerical solution is not available. Values of conductivity, k, are on the order of 100 W/m K for metals, 1 to 10 for nonmetallic solids as glass, ice, and rock, 0.1 to 10 for liquids, around 0.1 for insulation materials, and 0.1 down to less than 0.01 for gases. A different mode of heat transfer takes place when a medium is flowing, called convective heat transfer. In this mode the bulk motion of a substance moves matter with a certain energy level over or near a surface with a different temperature. Now the heat transfer by conduction is dominated by the manner in which the bulk motion brings the two substances in contact or close proximity. Examples are the wind blowing over a building or flow through heat exchangers, which can be air flowing over/through a radiator with water flowing inside the radiator piping. The overall heat transfer is typically correlated with Newton몶s low of cooling as where the transfer properties are lumped into the heat transfer coefficient, h, which then becomes a function of the media properties, the flow and geometry. The final mode of heat transfer is radiation, which transmit energy as electromagnetic waves in space. The transfer can happen in empty space and does not require any matter, but the emission of the radiation and the absorption do require a substance to be present. Surface emission is usually written as a fraction, emissivity , of a perfect black body emission as with the surface temperature, , and the Stefan-Boltzmann constant, . Typical values of emissivity range from 0.92 for nonmetallic surfaces to 0.6 to 0.9 for nonpolished metallic surfaces to less than 0.1 for highly polished metallic surfaces. Radiation is distributed over a range of wavelengths and it is emitted and absorbed differently for different surfaces. 2. The heat transfer coefficient (film coefficient) It is in thermodynamics and in mechanics is the proportionality coefficient between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, 봓T): q: heat flux, thermal power per unit area. 하고 싶은 말 좀 더 업그레이드하여 자료를 보완하여, 과제물을 꼼꼼하게 정성을 들어 작성했습니다. 위 자료 요약정리 잘되어 있으니 잘 참고하시어 학업에 나날이 발전이 있기를 기원합니다 ^^ 구입자 분의 앞날에 항상 무궁한 발전과 행복과 행운이 깃들기를 홧팅 키워드 대류, 실험, 레포트, 열전달