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Convection Introduction Agronomy 541 : Lesson 12a Convection Introduction Developed by D. Todey and E. Taylor It is suggested that you watch Video 12A and complete the exercise in the video before continuing with the lesson. Podcast Version Full Podcast List On a sunny afternoon, a breeze blows through the trees creating a gentle rustling sound, familiar to everyone. This pleasing sound has a more scientific association than creating an attractive afternoon sound. It helps to transfer excess heat away from the leaves on the trees. It serves to reduce the temperature difference between the leaves and the surrounding air. This heat transfer, called convection will be discussed in this lesson. What You Will Learn in This Lesson: Definitions of convection. About flow around an object. How surfaces affect the flow near them. Agronomy 541 : Lesson 12a Convection Convection Atmospheric convection is simply the effect of wind. Wind is important, not just because it blows things away, breaks things, and is associated with severe storms. Transport of heat and moisture by the wind is called convective transfer. Heat, as it relates to leaves, is the main item that we will look at. Large objects are not as likely to be at air temperature as small objects. That is a generalization, of course, but there are plenty of examples. Large pipes go from the automobile engine to the radiator. The radiator is just a series of small pipes or tubes with cooling fins attached to them. Radiators in an automobile do not cool by radiation. They are designed to cool by convection. Air is forced through the fins of the radiator, either by the motion of the automobile, or by the fan next to the automobile radiator. The movement of the air carries the heat away from the small pipes and the cooling fins in the radiator. There is no convection in outer space. Lack of air disallows heat transfer via convection. Having no convection present, things cool by radiation. Spaceships cool by radiation. An astronaut on a space walk outside a spaceship cools by radiation. Also, if the sun is shining on the astronaut, they heat by radiation. But there is no air temperature effect, no wind effect, because those things do not exist there. A radiator designed to cool a spaceship is quite different from a "radiator" that we see on an automobile engine. Agronomy 541 : Lesson 12a Convection Convection Thermal Boundary Layer The temperature of small objects is strongly influenced by air temperature. The temperature of the air is strongly influenced by the temperature of large surfaces. Figure 12.1 shows a car. The top of the car is a large surface. Cut a tiny piece out of the top of a car, maybe an inch or two wide and six inches long, and set that piece near the top of the automobile. The piece is sitting next to the top of the car. It will also be flat and at the same orientation as the top of the car. Fig. 12.1 A car is parked in the sunlight. A small piece of metal (a), similar in composition and orientation to the sun, is placed above the car top (b). Study Question 12.1 In the sun of the afternoon, which will be warmer: the top of the car or the little piece of metal? Top of the car Piece of metal Check Answer Agronomy 541 : Lesson 12a Convection Convection Thermal Boundary Layer (cont.) If it is parked in the sun, the top of the car will be warmer than the piece of metal. It would be difficult for air to move over such a large object as the car and carry away the heat, whereas air will move very freely around the small piece of metal. A very small object is tightly coupled to the atmospheric conditions. A large item is de-coupled from the atmosphere. A large item can have a different temperature than the air of its environment. The top of the car can heat up to a much warmer temperature than the air around it. The asphalt of a sidewalk can become much warmer than the air a foot above the sidewalk. A small piece of asphalt, broken off and suspended in the air, will be close to the air temperature. It will not heat up greatly if it is suspended in the air. Again, it is a case of size influencing the effectiveness of convection. A thin object, such as a pencil lead, suspended in the air, must of necessity be very close to the air temperature. Again that is because the air can move freely around it. Even though the lead may be a dark color, absorbing a lot of sunlight, the coupling to the atmosphere and freedom of air moving around it will keep it close to air temperature. Convection will quickly carry away heat that the item gains from the radiation impinging on it. The molecules of air near the hood of the car (Figure 12.2) are at the same temperature as the metal. A few molecules away, the temperature will still be close to the surface temperature, but somewhere in between that and the temperature of free air. At some distance from the car, the air temperature will be the same as the environmental air. The atmosphere will no longer be influenced by the surface. The layer where the temperature of the surface directly influences the atmosphere is known as the boundary layer. It is an unstirred layer adjacent to a surface. Fig. 12.2. When air (a) does not move or moves slowly across a car, a boundary layer (b) forms where air is stagnant, immediately adjacent to the car's surface. If the car begins to move the boundary layer will be wiped away at a speed of 20 mph or so. There is a temperature gradient across the boundary layer. If the temperature of the car were the same as the temperature of the atmosphere, there would be no thermal boundary layer. Only where there is a temperature difference next to a surface will there be a boundary layer. This layer of stagnant air essentially insulates the object from the atmospheric environment around it. Boundary layers can exist in any fluid. Air and waters are fluids. Air is a gas, water is a liquid, but are both fluids. Wherever there is a fluid, there can be a boundary layer. Temperature changes across a solid material are referred to as a thermal gradient, or when you are referring to temperature changes across a uniform substance not in contact with a boundary. But in a fluid in contact with an object, a boundary layer can be formed. The boundary layer will vary in thickness depending on the temperature difference and other factors including air flow. The boundary layer over the automobile may be an inch or two thick when the air is still. When the car is moving, wind will likely wipe away this boundary layer. The air striking the front of the car will essentially remove the boundary layer. Perhaps where the air is not striking directly, there will still be a thin layer, but often the car surface will cool to the air temperature of the environment once it is moving. Stagnant air insulates an object from the environment, and boundary layers exist in all fluids. These are important facts to remember. If there is wind, the boundary layer becomes thin and may disappear altogether. Because air can move freely around small objects, small objects always have thin boundary layers whether or not there is wind. There is a temperature gradient across the thermal boundary layer. Agronomy 541 : Lesson 12a Convection Convection Moisture Boundary Layer There is also a moisture boundary layer. If a wet object is suspended in the air, the moisture evaporating (water molecules breaking free from the surface) will saturate the air next to this object. Moving away from the evaporating surface, the amount of water vapor in the air will decrease until the moisture content would be the same as the background humidity of the surrounding atmosphere. Any surface which has water that can evaporate will set up a moisture boundary layer. The depth and strength of the boundary layer are influenced by the moisture gradient and wind flow. In this lesson, the thermal boundary layer is emphasized first. Boundary layers also apply to the surface of the Earth in regard to the rest of the atmosphere. The temperature structure near the surface is shown in Fig. 12.3. Fig. 12.3 Temperature structure (a) and moisture structure (b) over and some distance away from a grass field. Different surfaces react differently at different times of day in regard to heat and moisture transfer at the surface. But some distance away from the surface all the profiles become similar to the background temperature and humidity becoming affected by the surface. An example of a moisture boundary layer was displayed in Lesson 7 discussing dew. Water evaporating from a moist surface helps saturate the air near it. Air can mix away more of the moisture in windy conditions. The saturated and near-saturated air are blown away by the wind. So the evaporation rate continues strongly. Discussion Topic 12.1 How would these profiles change during the day? Study Question 12.2 When does a plant use most water? windy day calm day Check Answer FYI : Surface Heat Heat and moisture from the surface warm and evaporate water into a layer 1-2 km thick above the surface. Above this point, the atmosphere is essentially insulated or unaffected by changes in the surface.
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