主要区别——对流与辐射

对流和辐射都是传热机制。它们允许热能从一个地方传输到另一个地方。对流和辐射的主要区别在于对流是一种传热机制，涉及到物质的质量流动。另一方面，辐射是利用电磁能进行的热传递。因此，辐射能通过真空传递热量。

什么是对流(convection)？

对流是物质通过物质的质量流进行传热的机制。为了传导热量，材料本身的一部分会移动，也就是说，材料内部有质量的传递。通常，对流发生在流体中。然而，对流效应有时可以在固体中看到，如板块构造的情况。对流主要有两种类型：自然对流和强迫对流。

Convection is a complex process and there is no simple equation that fully describes it. However, we can make use of an approximation for cases where a fluid is heated using a solid surface. For these cases, the heat transfer rate  is given by,

where  is the surface area that heat is transferred through,  is the temperature of the solid,  is the temperature of air.  is known as the convective heat transfer coefficient. This coefficient depends on a number of properties including the density, viscosity, and the flow rate of the fluid.

自然对流

在自然对流中，物质的流动是由密度的差异引起的。例如，让我们考虑一壶水在炉子上加热。当水壶底部的水加热时，它就会膨胀。这意味着水分子现在被放置得更远，这导致底部的水密度降低。现在，水壶底部的水比水壶顶部的水密度小。由于密度的差异，底部较暖的水上升到顶部，而顶部较冷的水下沉到底部。这个过程不断重复，直到顶部和底部温度相同。

上升的热流体不能沿着冷流体下沉的同一条线上升。因此，在下一个循环上升/下降之前，流体需要水平移动。这将在流体中建立对流单元，如下图所示。

Convection Cells

自然对流是造成洋流的主要原因，也是洋流的主要因素之一。

对流也是板块构造的一个重要因素。地幔的内部比外部更热，这就导致了地幔中建立了对流细胞。地幔是固体的，地幔内部物质的运动相当缓慢，大约每年20毫米。

Convection in the Earth’s Mantle

强制对流

当使用外部机制（如使用风扇或泵）移动材料时，会发生强制对流。风扇加热器是强制对流的一个很好的例子。在人体内，心脏也起着泵的作用，负责身体周围的强制对流热。

什么是辐射(radiation)？

辐射通过电磁辐射来描述热的传递。由于动能，组成物体的分子总是在运动。这会导致这些分子中的电荷移动，从而产生电磁波。

物体通过辐射放出热量的速率由Stefan-Boltzmann定律给出：

where is the surface area of the object and  is its absolute temperature.  is the Stefan-Boltzmann c***tant, .

The quantity  is called emissivity. It takes a value between 0 and 1.  is higher for darker objects with darker surfaces, which emit and absorb radiation well. Shiny surfaces absorb and emit much less radiation and they have emissivities closer to 0. An ideal surface that is both a perfect absorber and an emitter of radiation has an emissivity of 1 and is called a blackbody.

As the object is emitting radiation to surroundings, it is also absorbing radiation from surroundings. If the surroundings are at a temperature of , the net rate at which a body radiates heat is given by

If there is a net heat radiation is from the body to the surroundings.

Objects emit some wavelengths of radiation more than others. Typically, the hotter the body, the lower the wavelength that is emitted the most. For instance, hotter stars should have a bluer colour (**aller wavelength) compared to colder, redder ones (larger wavelength). For an ideal blackbody at an absolute temperature , Wien’s law gives the wavelength  which is emitted the most:

在室温下，物体辐射的主波长在红外线范围内。下图显示了黑体在几种不同温度下辐射的给定波长的能量密度。

Radiation – Wien’s Law

热像图利用人体发出的热辐射来筛查疾病，红外线相机用于在黑暗中“看”。来自遥远恒星的辐射也被用来测量地球和恒星之间的距离。

Difference Between Convection and Radiation – Thermogram of an Energy-efficient house in the foreground, radiating much less thermal energy compared to a traditional house radiating much more energy (background)

对流(convection)和辐射(radiation)的区别

起源

对流是物质热膨胀的结果。

辐射是由于分子动能引起的物质中电荷运动的结果。

机制

对流涉及物质的质量传递，通常是流体。

辐射涉及电磁波。物质本身不运动。

中等的

对流需要介质。

辐射不需要介质，可以通过真空传递热量。

温度依赖性

对流导致热流量与温差近似成正比。

辐射产生的热流率取决于物体和周围环境温度的四次方之差。

References

Liu et al., (2007). Small-scale convection in the upper mantle beneath the Chinese Tian Shan Mountains. Physics of the Earth and Planetary Interiors(163), 179-190

Image Courtesy

“center: convection cells in a vessel, top: heat output, bottom: heat input” by Eyrian (Own work) [CC BY-SA 3.0], via Wikimedia Comm***

“Shows how ocean ridges are formed, lithosphere subducted at trenches; good for understanding plate tectonics.” by Surachit (Own work) [CC BY-SA 3.0], via Wikimedia Comm***

“Wien’s law of radiation / Prawo Wiena” by 4C (Own work, based on JPG version) [CC BY-SA 3.0], via Wikimedia Comm***

“Thermogram of a Passivhaus building, with traditional building in background.” by Passivhaus Institut (Copied to Comm*** from http://en.*********.org. Original source Passivhaus Institut, Germany – http://www.passiv.de)  [CC BY-SA 3.0], via Wikimedia Comm***