definition of natural and forced convection?

Question

able to explain the natural and forced convection phenomenon with the aid of diagram

Answer 1

*Like conduction and unlike radiation, convection requires a medium. However, in conduction the heat is transferred from one molecule to another, whereas in convection the heated fluid itself is actually moving. As it does, it removes or displaces cold air in its path. The flow of heated fluid in this situation is called a convection current.

Convection is of two types: natural and forced. Heated air rising is an example of natural convection. Hot air has a lower density than that of the cooler air in the atmosphere above it and therefore is buoyant; as it rises, however, it loses energy and cools. This cooled air, now denser than the air around it, sinks again, creating a repeating cycle that generates wind.

Forced convection occurs when a pump or other mechanism moves the heated fluid. Examples of forced-convection apparatuses include some types of ovens and even refrigerators or air conditioners. As noted earlier, it is possible to transfer heat only from a high-temperature reservoir to a low-temperature one, and thus these cooling machines work by removing hot air. The refrigerator pulls heat from its compartment and expels it to the surrounding room, while an air conditioner pulls heat from a room or building and releases it to the outside.

Forced convection does not necessarily involve man-made machines: the human heart is a pump, and blood carries excess heat generated by the body to the skin. The heat passes through the skin by means of conduction, and at the surface of the skin it is removed from the body in a number of ways, primarily by the cooling evaporation of perspiration.

*When air is blown across a heated surface, forced convection results. Although the natural convection forces are still present in this latter case, they are clearly negligible compared with the imposed forces. The process of energy transfer from the heated surface to the air is not, however, different from that described for natural convection. The major distinguishing feature is that the maximum fluid velocity is at the outer edge of the boundary layer. This difference in velocity profile and the higher velocities provide more fluid near the surface to carry along the heat conducted normal to the surface. Consequently, boundary layers are very thin.
Differential heating of fluids may itself cause convection in a gravity field, due to variations in density due to a transfer of heat and subsequent fluid expansion, combined with differential buoyancy forces on the different parts of fluid which have differing densities. This type of purely heat-driven convection in gravity fields is sometimes referred to as "natural heat convection," in order to distinguish it from various types of forced heat convection (i.e., heat advection not due to buoyancy of heating). In forced heat convection, transfer of heat is due to movement in the fluid from forces other than heat, such as pumps or natural and artificially-driven mechanically-pushed fluid flows (i.e., flow from external motive forces, such as occurs in rivers or pumps). In addition, buoyancy forces in gravity fields which result from sources of density variations in fluids other than those produced by heat, such as variable composition (for example, salinity), are frequent convection causes.


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Answer 2

Natural Convection Definition

Answer 3

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In this particular problem (let's call it level-1 heat transfer analysis), you really don't need to know the difference between free convection (aka natural convection) and forced convection (though you will in the future, just wait a month until you study level-2 and level-3 heat transfer analysis). All you need to do is know how to read your table of SAMPLE heat transfer coefficients, and know the definition of heat transfer coefficient and know how to algebraically reverse it. It is all the same formula (a definition of "heat transfer coefficient") for the level-1 heat transfer analysis. Not until we study the details of why the convection occurs do we care about different formulas. The idea of "heat transfer coefficient" is that it enables us to "black box" the analysis of the heat transfer problem. It enables us to keep our big picture analysis (which is what you are doing right now) separate from our granular details of fluid mechanics (which someone has already done for you, when they prepared that table of sample values). The table of sample heat transfer coefficients will also serve a purpose later on in the semester, when you actually enter the granular details of fluid mechanics to calculate the heat transfer coefficients. It enables you to check your work against the rough idea of what it should be. It lets you see when your results are completely out of whack. Also, it lets you have a bit of a get-a-way... in case you are lost with your level-2 or level-3 analysis...you can always revert the problem back to the level-1 heat transfer problem you are solving today. ------------------------------------- If you must know though, you can. The main distinction is whether or not the flow of the fluid of interest is pre-existing or initially quiescent. The word "quiescent" just means calm and "quiet". By "pre-existing", what I mean is, as if the interacting solid object WERE NOT of different temperature than the fluid, would the fluid still be flowing? If the fluid flow is pre-existing, whether that be due to wind originating elsewhere in the world or due to a laboratory fan...that is what we call forced convection. The flow would still be there, regardless if your plate were hot, cold or room temperature. If the fluid were initially quiescent and wouldn't be flowing if there weren't a temperature difference...then we call that either free convection or natural convection. I prefer the term "natural convection", such that you can distinguish from its counterpart with the initial letter. In natural convection, the fluid does EVENTUALLY flow...and the reason that it does so, is due to buoyancy and a background gravitational field. The temperature difference itself drives the flow of the fluid, which then enhances the heat transfer. Not very much, because compared to other modes, natural convection is a pretty "poor" mode of heat transfer.

Answer 4

Below is a link with a good diagram of natural convection.

Here is a summary in a few quick bullets:
- When things are heated, they expand (water/ice is one exception)
- Warmer objects are less dense
- Hot air rises
- Cooler air sinks
- Forced convection involves a fan to force the air movement.


A lava lamp is an example of natural convection. The lightbulb in the bottom heats the wax. The warm bubbles of wax expand and become more bouyant and float up through the liquid. At the top they are farther from the heat source (lightbulb) and the wax cools. As the wax cools, it becomes less bouyant and it sinks back down to the bottom.

Answer 5

Heres the easiest way to explain.

Forced convection - uses a fan or equivalent device to move heated gases.

Natural convection - have you ever heard that hot air rises? This is because its less dense than cold air. In natural convection, gases flow (and carry heat) because they are heated and their densities change. If you built a fire and put say 30 feet of duct work above the fire going straight up, you would eventually get a flow of hot air out the top. Thats because the fire heats the air at the bottom of the duct and the hot air starts to rise to the top of the duct. This creates a slight pressure differential which in turn pulls new air in from the bottom and repeats the process.

Check out the little spinning devices on top of your roof during the summer.

Answer 6

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Answer 7

What Jamie said.