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Which Best Describes How Air Moves During Convection
Convection is the process by which heat is transferred by the movement of a heated fluid, such as air or water.
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Natural convection results from the tendency of most liquids to expand when heated, i.e. becomes less dense and increases due to increased buoyancy. The circulation caused by this effect causes uniform heating of water in a boiler or air-heated room: the heated molecules expand the space in which they move towards each other at a higher speed, rise, then cool and approach each other again. An increase in density and consequent sinking.
Forced convection refers to the transport of fluid by means other than density resulting from changes in temperature. Air movement by a fan or a water pump are examples of forced convection.
Atmospheric convection currents can result from local heating effects such as solar radiation (heating and upwelling) or contact with cold surface masses (cooling and sinking). Such convection currents usually move vertically and are responsible for many atmospheric phenomena such as clouds and thunderstorms. Even with disturbances like air fronts and hurricanes, air moves through our planet’s atmosphere. This pattern, called atmospheric circulation, occurs because the Sun heats the Earth more at the equator than at the poles. It is also affected by the Earth’s rotation.
In the tropics, near the equator, warm air rises. When it is about 10 to 15 km (6 to 9 miles) above the Earth’s surface, it begins to flow away from the equator and toward the poles. Air that rises north of the equator flows north. Air that rises south of the equator flows south. The cooled air falls back to the earth, flows towards the equator and warms up again. Now the heated air rises again and the pattern repeats itself. This pattern, known as convection, occurs on a global scale. It also occurs to a lesser extent in individual storms.
Conduction, Convection, And Radiation
But because the Earth rotates, the air moving north and south of the equator also rotates with the Earth’s rotation. Air moving north turns to the right. Air moving south turns to the left. The force of the Earth’s rotation turning the flowing air is known as the Coriolis effect. If the Earth did not rotate, there would be only one large convection cell between the equator and the North Pole and one large convection cell between the equator and the South Pole. However, because the Earth rotates, the convection is divided into three cells north of the equator and three cells south of the equator. We may not realize it, but heat transfer in our homes keeps the HVAC industry running. Without air conditioning, our buildings would remain stuffy, miserable places in the summer. This is because heat can come from outside, from internal objects (such as cooking appliances) and from our bodies. Thus, this article covers the three main types of heat transfer: conduction, convection, and radiation. It also provides some examples of these processes in our homes. We’ll also look at R-value and emissivity as they relate to conductivity and emissivity.
Conduction is perhaps the simplest of the three methods of heat transfer. This happens when two objects are in direct contact with each other. The atoms vibrate against each other and the energetic particles of the warm object transfer some of the heat energy to the surrounding cold body.
You can feel the conductivity when you sit in front of the fireplace and briefly place a metal poker in the fire. The handle will eventually get hot. This is because the heat from the fire was transferred to the poker, which transferred the heat to your hands through direct contact.
Conductivity is one of the reasons we build walls with insulation. Conductive heat transfer occurs by contact, so hot objects and gases inside the walls will touch the insulation before touching the wall surface. Insulation is made of several different materials, making it difficult for heat to transfer from inside the walls to our rooms by conduction.
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Also called the R-factor, the R-value represents how well a barrier resists conductive heat transfer. It is a tool that people use mostly in the construction industry to select the right insulation materials. Higher R-values indicate greater resistance to conductive heat transfer, allowing heat to travel longer through walls or ceilings.
The R-value is a measure of heat flow from one side of the wall or ceiling to the other. Different thicknesses of insulating materials will have different R-values. For example, a 1.5-inch-thick fiberglass barrier on a wall can nearly double the R-value of a previously uninsulated 2-by-4-inch wood frame from 4.6 to 9 (Kennedy State University).
Instead of transferring heat to contact, convection does not require any direct contact. Convection occurs when a fluid (vapor or liquid) moves and the thermal energy of the fluid moves with it. Thus, if heat energy in the outside air enters a building through a leaky window, this is an example of heat transfer by convection.
Density is partly responsible for the convective “heat rise” phenomenon. But the heat doesn’t actually increase; Warm liquids contain more heat and are less dense than cold liquids, so warm liquids float and cold liquids sink. Convection and density also partially explain why the pool bottom is cool at the surface. Water is liquid, and warmer molecules rise to the top because they are less dense than cooler ones.
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Remember when we said that conduction goes through walls? Imagine if the wall had a crack. there will be no obstacles preventing warm gases from entering the room from inside the walls; Hot gases don’t have to touch anything to transfer heat. Convection occurs when a fluid moves heat, rather than through direct contact. In this case, this liquid is simply ordinary air in the house.
Hot air rises, causing warmer gas molecules to accumulate at the top of rooms, so we use ceiling fans to manipulate convection in a process called “forced convection.” According to the University of Calgary Energy Education, the clockwise rotation of the ceiling fan blades creates a forced updraft that pulls in cooler air and forces it to mix with hot air already near the ceiling. Overall, you end up with a warmer room because forced convection disrupts the natural separation of warm and cool air. In contrast, a counter-clockwise rotation creates the effect of a descending wind chill, making us more comfortable in the summer.
Radiation does not use direct contact or liquids to transfer heat. Instead, radiation heat transfer occurs via electromagnetic waves in air or vacuum.
All objects emit some form of radiation. For example, our bodies emit infrared radiation that we cannot see. However, some devices can receive infrared radiation and display it as a thermal image. These devices help us “see” in poor visibility, such as when our military needs to find people at night or when firefighters need to “see” through thick smoke.
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A more obvious example is solar radiation. The sun emits ultraviolet (UV) waves that can enter our homes through windows. Windows reflect some of the waves, but most of them pass through the glass and heat our homes.
Emissivity is the ability of an object to radiate energy in the form of thermal radiation. It exists on a scale of 0 to 1, with 0 being a perfect reflector that neither emits nor absorbs radiation, and 1 being a “perfect blackbody” that absorbs and emits energy at high rates.
Of course, no real object has a value of 0 or 1. They have some value in between. Shiny, reflective objects have an emissivity closer to 0, while dark surfaces have an emissivity closer to 1.
Let’s say you want to apply aluminum foil to a black asphalt surface early in the morning. Returning after sunset, the black surface would be much hotter. This is because asphalt absorbs much more heat than gloss foil and has a higher emissivity.
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As you can see, conduction, convection and radiation determine the movement and amount of heat energy in our home. We also perform heat in the HVAC industry and our goal is to create comfort through natural processes that facilitate heat transfer.
When we say that there is “cooled gas” at a certain point in the system, it can be bad or good, depending on where it originates. Flash gas is another cooking term. he
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