主要區別——對流與輻射

對流和輻射都是傳熱機制。它們允許熱能從一個地方傳輸到另一個地方。對流和輻射的主要區別在於對流是一種傳熱機制，涉及到物質的質量流動。另一方面，輻射是利用電磁能進行的熱傳遞。因此，輻射能透過真空傳遞熱量。

什麼是對流(convection)？

對流是物質透過物質的質量流進行傳熱的機制。為了傳導熱量，材料本身的一部分會移動，也就是說，材料內部有質量的傳遞。通常，對流發生在流體中。然而，對流效應有時可以在固體中看到，如板塊構造的情況。對流主要有兩種型別：自然對流和強迫對流。

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.

自然對流

在自然對流中，物質的流動是由密度的差異引起的。例如，讓我們考慮一壺水在爐子上加熱。當水壺底部的水加熱時，它就會膨脹。這意味著水分子現在被放置得更遠，這導致底部的水密度降低。現在，水壺底部的水比水壺頂部的水密度小。由於密度的差異，底部較暖的水上升到頂部，而頂部較冷的水下沉到底部。這個過程不斷重覆，直到頂部和底部溫度相同。

上升的熱流體不能沿著冷流體下沉的同一條線上升。因此，在下一個迴圈上升/下降之前，流體需要水平移動。這將在流體中建立對流單元，如下圖所示。

Convection Cells

自然對流是造成洋流的主要原因，也是洋流的主要因素之一。

對流也是板塊構造的一個重要因素。地幔的內部比外部更熱，這就導致了地幔中建立了對流細胞。地幔是固體的，地幔內部物質的運動相當緩慢，大約每年20毫米。

Convection in the Earth’s Mantle

強制對流

當使用外部機制（如使用風扇或泵）移動材料時，會發生強制對流。風扇加熱器是強制對流的一個很好的例子。在人體內，心臟也起著泵的作用，負責身體周圍的強制對流熱。

什麼是輻射(radiation)？

輻射透過電磁輻射來描述熱的傳遞。由於動能，組成物體的分子總是在運動。這會導致這些分子中的電荷移動，從而產生電磁波。

物體透過輻射放出熱量的速率由Stefan-Boltzmann定律給出：

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:

在室溫下，物體輻射的主波長在紅外線範圍內。下圖顯示了黑體在幾種不同溫度下輻射的給定波長的能量密度。

Radiation – Wien’s Law

熱像圖利用人體發出的熱輻射來篩查疾病，紅外線相機用於在黑暗中“看”。來自遙遠恆星的輻射也被用來測量地球和恆星之間的距離。

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)

對流(convection)和輻射(radiation)的區別

起源

對流是物質熱膨脹的結果。

輻射是由於分子動能引起的物質中電荷運動的結果。

機制

對流涉及物質的質量傳遞，通常是流體。

輻射涉及電磁波。物質本身不運動。

中等的

對流需要介質。

輻射不需要介質，可以透過真空傳遞熱量。

溫度依賴性

對流導致熱流量與溫差近似成正比。

輻射產生的熱流率取決於物體和周圍環境溫度的四次方之差。

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***