Convection

Convection is the heat transfer by the macroscopic movement of a fluid, such as a car's engine kept cool by the water in the cooling system.

Learning Objective

Illustrate the mechanisms of convection with phase change

Key Points

Convection is driven by the large scale flow of matter in fluids. Solids cannot transport heat through convection.
Natural convection is driven by buoyant forces: hot air rises because density decreases as temperature increases. This principle applies equally with any fluid.
Convection can transport heat much more efficiently than conduction. Air is a poor conductor and a good insulator if the space is small enough to prevent convection.
Convection often accompanies phase changes, such as when sweat evaporates from your body. This mass flow during convection allows humans to cool off even if the surrounding air's temperature exceeds the body temperature.

Terms

natural convection
A method for heat transport. A fluid surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid then moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current.
positive feedback
a feedback loop in which the output of a system is amplified with a net positive gain each cycle.

Example

Example: Calculating Heat Transfer by Convection: Convection of Air Through the Walls of a House. Most houses are not airtight: air goes in and out around doors and windows, through cracks and crevices, following wiring to switches and outlets, and so on. The air in a typical house is completely replaced in less than an hour. Suppose that a moderately-sized house has inside dimensions 12.0m×18.0m×3.00m high, and that all air is replaced in 30.0 min. Calculate the heat transfer per unit time in watts needed to warm the incoming cold air by 10.0 ºC, thus replacing the heat transferred by convection alone. Strategy Heat is used to raise the temperature of air so that Q=mcΔT. The rate of heat transfer is then Q/t, where t is the time for air turnover. We are given that ΔT is 10.0ºC, but we must still find values for the mass of air and its specific heat before we can calculate Q. The specific heat of air is a weighted average of the specific heats of nitrogen and oxygen, which is c=cp≅1000 J/kg⋅C (note that the specific heat at constant pressure must be used for this process). Solution (1) Determine the mass of air from its density and the given volume of the house. The density is given from the density ρ and the volume $m=\rho V=(1.29 \ kg/m^{3})(12.0m\times 18.0m\times 3.00m)=836 \ kg$ (2) Calculate the heat transferred from the change in air temperature: Q=mcΔT so that $Q=(836 \ kg)(1000 \ J/kg \cdot ^{\circ}C)(10 \ ^{\circ}C)=8.36\times 10^{6} \ J$ (3) Calculate the heat transfer from the heat Q and the turnover time t. Since air is turned over in $t=0.500h=1800s$, the heat transferred per unit time is $frac{Q}{t}=\frac{8.36\times 10^{6} \ J}{1800 \ s}=4.64 \ kW$This rate of heat transfer is equal to the power consumed by about forty-six 100-W light bulbs. Newly constructed homes are designed for a turnover time of 2 hours or more, rather than 30 minutes for the house of this example. Weather stripping, caulking, and improved window seals are commonly employed. More extreme measures are sometimes taken in very cold (or hot) climates to achieve a tight standard of more than 6 hours for one air turnover. Still longer turnover times are unhealthy, because a minimum amount of fresh air is necessary to supply oxygen for breathing and to dilute household pollutants. The term used for the process by which outside air leaks into the house from cracks around windows, doors, and the foundation is called "air infiltration."

Full Text

Convection

Convection (illustrated in ) is the concerted, collective movement of ensembles of molecules within fluids (e.g., liquids, gases). Convection of mass cannot take place in solids, since neither bulk current flows nor significant diffusion can occur in solids. Instead heat diffusion in solids is called heat conduction, which we've just reviewed.

Convection Cells

Convection cells in a gravity field.

Convection is driven by large-scale flow of matter. In the case of Earth, the atmospheric circulation is caused by the flow of hot air from the tropics to the poles, and the flow of cold air from the poles toward the tropics. (Note that Earth's rotation causes changes in the direction of airflow depending on latitude.). An example of convection is a car engine kept cool by the flow of water in the cooling system, with the water pump maintaining a flow of cool water to the pistons.

While convection is usually more complicated than conduction, we can describe convection and perform some straightforward, realistic calculations of its effects. Natural convection is driven by buoyant forces: hot air rises because density decreases as temperature increases. This principle applies equally with any fluid. For example, the pot of water on the stove in is kept warm in this manner; ocean currents and large-scale atmospheric circulation transfer energy from one part of the globe to another.

Convection in a Pot of Water

Convection plays an important role in heat transfer inside this pot of water. Once conducted to the inside, heat transfer to other parts of the pot is mostly by convection. The hotter water expands, decreases in density, and rises to transfer heat to other regions of the water, while colder water sinks to the bottom. This process keeps repeating.

Convection and Insulation

Although air can transfer heat rapidly by convection, it is a poor conductor and thus a good insulator. The amount of available space for airflow determines whether air acts as an insulator or conductor. The space between the inside and outside walls of a house, for example, is about 9 cm (3.5 in)—large enough for convection to work effectively. The addition of wall insulation prevents airflow, so heat loss (or gain) is decreased. Similarly, the gap between the two panes of a double-paned window is about 1 cm, which prevents convection and takes advantage of air's low conductivity to prevent greater loss. Fur, fiber and fiberglass also take advantage of the low conductivity of air by trapping it in spaces too small to support convection. In animals, fur and feathers are lightweight and thus ideal for their protection.

Convection and Phase Changes

Some interesting phenomena happen when convection is accompanied by a phase change. It allows us to cool off by sweating, even if the temperature of the surrounding air exceeds body temperature. Heat from the skin is required in order for sweat to evaporate from the skin, but without air flow the air becomes saturated and evaporation stops. Air flow caused by convection replaces the saturated air by dry air and thus evaporation continues.

Another important example of the combination of phase change and convection occurs when water evaporates from the ocean. Heat is removed from the ocean when water evaporates. If the water vapor condenses in liquid droplets as clouds form, heat is released in the atmosphere (this heat release is latent heat) . Thus, an overall transfer of heat from the ocean to the atmosphere occurs. This process is the driving power behind thunderheads—great cumulus clouds that rise as much as 20.0 km into the stratosphere. Water vapor carried in by convection condenses, releasing tremendous amounts of energy, and this energy allows air to become more buoyant (warmer than its surroundings) and rise. As the air continues to rise, more condensation occurs, which in turn drives the cloud even higher. Such a mechanism is called positive feedback, since the process reinforces and accelerates itself. These systems sometimes produce violent storms with lightning and hail, and constitute the mechanism that drives hurricanes.

http://en.wikipedia.org/wiki/Cumulus_clouds

Cumulus clouds are caused by water vapor that rises because of convection. The rise of clouds is driven by a positive feedback mechanism.

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