How is convection current generated


If you blow dry your hair in the morning after showering, you are making use of the convection. How exactly do you do that you will learn in this post.

  • Convection simply explained
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  • General information on heat transfer and convection current
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  • Convection without mass transfer
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  • Convection with mass transfer
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Convection simply explained

The convection, or also Heat flow, is a method of energetic heat transfer from one place to another.

Unlike the Conduction, must with convective heat transfer not just a material carrier be present, which transports the energy. He also has to flowable be. The thermal energy is then transported together with the material. That is why convection is also known as heat flow. Typical fluids capable of flowing are air or water.

The word convection derives from the Latin word"Convehere"from, which means something like "collect" or "mix together".

General information on heat transfer and convection current

With convection takes a flowing fluid warmth either on or gives it up. For this purpose it flows over the surface of another medium whereby it becomes a Alignment the Temperatures comes. The other medium can either be a solid or also a fluid. If both media are fluids, there is an additional one Mass transfer. This results from a simple mixingof the two fluids. Even more important is the question of which one force is responsible for the convection. Here we either speak of freer or more forcedconvection.

And exactly this Distinctions let's take a closer look now!

Convection without mass transfer

A convection without mass transfer occurs when a Fluid at the Overflow the surface of a Solid either Gives off heat or records. This also forms the simplest case of convection from since the Interface of the solid static and the convection is therefore one pure heat exchange represents. When the water flows over the surface, a Boundary layer which depends on the composition of the two substances, for example the concentration, the temperature or the flow rate. This takes place in this thermal boundary layer nonlinear heat transport in the fluid to or from the solid. In case of a convectionbetween one Solid and one Fluid is also the Identification of the boundary layers easier than with convection between two fluids.

Convection with mass transfer

A fluid flows over now a different fluid with different temperature, then both occur Warmth as well a Mass transfer on. In this case, on the one hand there is Alignment the Composition of matter and on the other hand they go Boundary layers flowing into each other. The fluid flows over a Mixture of substances or solid with a lower saturation vapor or sublimation pressure, it also comes to one Mass transferbecause the vapor or sublimation pressure of the solids is exceeded. The fabric diffuses into the fluid. An example of this would be the Drying.

Free convection

Next, let's examine which Forces for the creation of the flow are responsible. Are these not external influences, but for example the Gravity, Differences in temperature or density, then we are talking about the free or natural convection.

Special case: Rayleigh-Bénard convection

A Special case of free convection is the Rayleigh-Bénard convection. she is a natural phenomenon, that in Center of flat, horizontal fluids occurs as soon as a critical temperature is achieved.

For example, imagine one Saucepan with water before that on the stove warmed up becomes. The critical temperature then describes the temperature at which the water in the saucepan due to the temperature difference between the bottom of the saucepan and the surface begins to flow. During the process, the liquid on the warm underside expands and increases due to the lower density in the middle on. It then cools down again on the surface and sinks with increased density at the edge of the pot. So for this type of convection Density differences in the water responsible.

The Marangoni convection

A another special case of free convection is the Marangoni convection. With her, the flow does not arise through differences in density, but through differences in the Surface tension. An easy example for this is one lit candle:

The waxwhich is closer to the flame is located is warmer than the wax on the edge of the candle. The Surface tension generally takes with increasing temperatureso that the surface tension of the wax is higher at the edge than in the middle. Simply put, the surface is then pulled from the center of the candle to the edge, and takes wax near the surface with it. This creates a Circular motion, so one flow.

More examples of free convection

The potential of free convection can be seen very well on one Updraft power plant detect.

Here is a see-through Solar collector the Air heated on the ground during the day and the Pressure difference towards the Outside air used. It arises thermal lift, which draws the warm air from the floor and to the Turbine drive can be used. The mechanical energy is converted into electrical energy. The greater the temperature differences and the higher the tower and the associated pressure differences, the better the output of the updraft power plant.

Forced convection

are external forces responsible for the flow of the fluid, then one speaks of the forced convection. And that's exactly what we're going to take a look at at the end. External forces can, for example Pumps or fans be. They take care of Pressure differences and thus for convective currents in a fluid.

The forced convection almost always occurs parallel to the natural convection on which but in the face of forced in most cases neglected can be.

Example of forced convection

A well-known example of this is the Cooling your computer. As you read this post, a fan blows air through your computer to cool your processor, among other things. This fan creates a flow of air and thus causes convection.

You can also use forced convection in the morning after showering. Do you dry yourself with one hairdryer your hair, then through that Blower in the hair dryer a Airflow generated, which in turn convection evokes.

Very good! Now you know convection and its differentiation possibilities as one Form of heat transport. Have fun with your studying.