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Solids are better conductor than liquids and liquids are better conductor than gases.

P6.2 – Convection

Metals are very good conductors of heat, while air is very poor conductor of heat. You experience heat transfer by conduction whenever you touch something that is hotter or colder than your skin e. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continous circulation pattern and heat is transfered to cooler areas.

You see convection when you boil water in a pan. The bubbles of water that rise are the hotter parts of the water rising to the cooler area of water at the top of the pan.

Convection of Liquid Apparatus

You have probably heard the expression "Hot air rises and cool air falls to take its place" - this is a description of convection in our atmosphere. Heat energy is transfered by the circulation of the air. This thermal infrared image shows hot oil boiling in a pan. The oil is transfering heat out of the pan by convection.

Notice the hot yellow centers of rising hot oil and the cooler outlines of the sinking oil.


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The short time duration of the standard 0. In reviewing the above animated GIF, one can easily see the impact of longer test times on the temperature profile. The simulation is based on a MTPS sensor employing a low power pulse in contact with water.

The animation is run over an exagerated test time of 20 seconds to highlight the impact that test time has on convection. The "mushroom" pattern that emerges with the longer test time is caused by convection.

Conduction, convection and radiation

It is important to note that for approximately the first 5 seconds of the measurement the temperature profile lines are parrallel and the impact of convection in this phase of the measurement is negligible. Ultimately, the "proof is in the pudding" and testing results confirm the accuracy of the method in being uniquely suitable for testing liquids. The chart below higlights performance on the testing of distilled water.

Note the high precision and repeatability of the measurement in testing the thermal conductivity of water - high precision is a hallmark of a suitable and appropriate method. He therefore adopted a film theory as an approximation.

The reason for the existence of a film around a hot body may be seen as follows: Consider a horizontal wire maintained at a given temperature in a fluid, the fluid adjacent to the wire will become heated and rise while the cooler fluid of greater density will flow into its place. Thus a convection current is set up by the difference in density between the hot and cold fluid.

Everyday Examples of Convection

This condition is usually referred to as free convection. At the surface of the wire the fluid is stationary due to viscosity.

Convection ( Read ) | Physics | CK Foundation

As we proceed from the surface of the wire the velocity of the convection currents increase until a distance is reached at which the critical velocity conditions in the fluid are exceeded and the stream line flows bursts into turbulent motion. The discontinuity between the stream line and turbulent motion constitutes the outer boundary of the film. At the inner boundary the fluid has the temperature of the hot surface and at the outer boundary the temperature of the ambient fluid.


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The actual configuration of the outer boundary is unknown. As an approximation we might assume that it was an eccentric ellipse or cylinder, etc. For ease of calculation Langmuir adopted the simplest approximation and assumed that the outer boundary of the relatively stagnant film was a cylinder concentric with the wire.