工程热力学与传热学热工学第12章RadiationHeatTransfer(免费阅读).pptVIP

主要内容 12-1 热辐射的基本概念 12-2 热辐射的基本定律 12-3 辐射换热计算 12-4 气体辐射 12-1 热辐射的基本概念 辐射强度Radiant intensity 辐射力Emissive power 辐射力 在同温度的前提下，物体的辐射力越大，其吸收率也越大。 第三节 辐射换热计算 物体的表面尺寸、形状及所处的相对位置，称为空间热阻； 黑体间的辐射换热 第四节 气体辐射 第四节 气体辐射 例题：保温瓶的夹层玻璃表面为什么镀一层反射率很高的材料？夹层抽真空的目的是什么？ 答：（1）保温瓶的夹层玻璃表面镀一层反射率很高的材料，能把辐射出去的能量反射回来，以减少辐射传热。 ?? （2）夹层抽真空的目的是减少导热与对流传热。 例题：大气中的CO2含量增加，导致地球温度升高，为什么？ 答：CO2气体对热辐射波长是有选择性辐射和吸收的。其光带都处于红外区段，对太阳辐射呈现透明体性质，但对地球表面辐射的红外波长具有吸收作用，阻碍了地球热辐射进入太空，起到了“温室效应”的作用，导致地球气温升高。 Greenhouse effect Your company slogan Chapter 12Radiation 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. In fact, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum. This is how the energy of the sun reaches the earth. In heat transfer studies we are interested in thermal radiation, which is the form of radiation emitted by bodies because of their temperature. It differs from other forms of electromagnetic radiation such as x-rays, gamma rays, microwaves, radio waves, and television waves that are not related to temperature. All bodies at a temperature above absolute zero emit thermal radiation. Radiation is a volumetric phenomenon, and all solids, liquids, and gases emit, absorb, or transmit radiation to varying degrees. However, radiation is usually considered to be a surface phenomenon for solids that are opaque to thermal radiation such as metals, wood, and rocks since the radiation emitted by the interior regions of such material can never reach the surface, and the radiation incident on such bodies is usually absorbed within a few microns from the surface. 意义和目的 意义： 热辐射是热量传递的三种基本方式之一，在许多领域中具有重要应用，特别是航天器热控制中，热辐射更是必须考虑的. 日常生活中也随处可见热辐射的例子，如：