#jsDisabledContent { display:none; } My Account |  Register |  Help

# Combined forced and natural convection

Article Id: WHEBN0039124477
Reproduction Date:

 Title: Combined forced and natural convection Author: World Heritage Encyclopedia Language: English Subject: Collection: Publisher: World Heritage Encyclopedia Publication Date:

### Combined forced and natural convection

Combined forced convection and natural convection, or mixed convection, occurs when natural convection and forced convection mechanisms act together to transfer heat. This is also defined as situations where both pressure forces and buoyant forces interact.[1] How much each form of convection contributes to the heat transfer is largely determined by the flow, temperature, geometry, and orientation. The nature of the fluid is also influential, since the Grashof constant increases in a fluid as temperature increases, but is maximized at some point for a gas.[2]

## Cases

Because of the wide range of variables, hundreds of papers have been published for experiments involving various types of fluids and geometries. This variety makes a comprehensive correlation difficult to obtain, and when it is, it is usually for very limited cases.[2] Combined forced and natural convection, however, can be generally described in one of three ways.

### First case

The first case is when natural convection aids forced convection. This is seen when the buoyant motion is in the same direction as the forced motion, thus enhancing the heat transfer.[3] An example of this would be a fan blowing upward on a hot plate. Since heat naturally rises, the air being forced upward over the plate adds to the heat transfer.

### Second case

The second case is when natural convection acts in the opposite way of the forced convection. Consider a fan forcing air upward over a cold plate.[3] In this case, the buoyancy force of the cold air naturally causes it to fall, but the air being forced upward opposes this natural motion, keeping the cool air hovering around the cold plate. This, in turn, diminishes the amount of heat transfer.

### Third case

The third case is referred to as transverse flow. This occurs when the buoyant motion acts perpendicular to the forced motion. This enhances fluid mixing, and enhances the heat transfer.[3] An example of this is air flowing horizontally over a hot or cold pipe. This can encourage phase changes, which often creates a very high heat transfer coefficient. For example, steam leaving a boiler can pass through a pipe that has a fan blowing over it, cooling the steam back to a saturated liquidthat all

## Calculation of total heat transfer

While it may seem like it is possible to simply add or subtract the heat transfer values to or from each other, this will yield inaccurate results. In order to determine the total heat transfer, experimental data has suggested that Nucombined=(Nuforcedn ± Nunaturaln)(1/n) where the plus sign is for cases where heat transfer is assisted (i.e. case one and three) and the minus sign is for when it is hindered (i.e. case two). The value of n ranges between 3 and 4 as the geometry aligns from vertically to horizontally respectively.

## Applications

Combined forced and natural convection is often seen in very-high-power-output devices where the forced convection is not enough to dissipate all of the heat necessary. At this point, combining natural convection with forced convection will often deliver the desired results. Examples of these processes are nuclear reactor technology and some aspects of electronic cooling.[2]

## References

1. ^
2. ^ a b c
3. ^ a b c
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.

Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.