Convection currents are the result of differential heating. It is this movement that creates circulation patterns known as convection currents in the atmosphere, in water, and in the mantle of Earth. In the atmosphere, as air warms it rises, allowing cooler air to flow in underneath.

Convection currents transfer thermal energy through many fluids, not just hot water in a pot. For example, convection currents transfer thermal energy through molten rock below Earth’s surface, through water in the oceans, and through air in the atmosphere. Convection currents in the atmosphere create winds.

Convection happens because warm air is less dense than the cold air around it, so it is lighter and rises or goes up in the atmosphere. There is a constant balancing act going on all the time in our atmosphere as moist, warm air goes upward and cooler, denser air moves down.

Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy. Heat energy is transferred from hot places to cooler places by convection. In this way, convection currents that transfer heat from place to place are set up.

Convection carries air containing water vapour upwards, so the air just above the surface does not become ‘saturated’ (Section 1.2. Continued rise and further cooling then results in the condensation of water vapour onto aerosols in the air: clouds form and latent heat is released to the atmosphere.

Convection is the circular motion that happens when warmer air or liquid — which has faster moving molecules, making it less dense — rises, while the cooler air or liquid drops down. Convection is a major factor in weather. That current can result in wind, clouds, or other weather.

There are two types of convection: natural convection and forced convection.

Forced Convection Heat Transfer In natural convection, any fluid motion is caused by natural means such as the buoyancy effect, i.e. the rise of warmer fluid and fall the cooler fluid. Whereas in forced convection, the fluid is forced to flow over a surface or in a tube by external means such as a pump or fan.

The parameter of importance in forced convection is the Péclet number, which is the ratio of advection (movement by currents) and diffusion (movement from high to low concentrations) of heat. When the Peclet number is much greater than unity (1), advection dominates diffusion.

Forced convection creates a more uniform and therefore comfortable temperature throughout the entire home. This reduces cold spots in the house, reducing the need to crank the thermostat to a higher temperature, or putting on sweaters.

Reynolds number

An effectiveness of indicates that the fin actually acts as insulation, slowing down the heat transfer from the surface. This situation can occur when fins made of low thermal conductivity materials are used. An effectiveness of indicates that the fins are enhancing heat transfer from the surface, as they should.

10.1.2 Forced Convection In forced convection, the Nusselt number depends on the rate of heat transfer through a boundary layer from a surface hotter or cooler than the air passing over it, a process analogous to the transfer of momentum by skin friction.

So, the Nusselt number may be viewed as the ratio of convection to conduction for a layer of fluid. If Nu=1, we have pure conduction. Higher values of Nusselt mean that the heat transfer is enhanced by convection.

Dimensional Analysis for Forced Convection

Group B

• Ratio of buoyancy to viscous force. Q: Grashof number.
• Ratio of inertia force to viscous force. R: Prandtl number.
• Ratio of momentum to thermal diffusivities. S: Reynolds number.
• Ratio of internal thermal resistance to boundary layer thermal resistance.

Common units used to measure the convective heat transfer coefficient are:

1. 1 W/(m2 K) = 0.85984 kcal/(h m2 ° C) = 0.1761 Btu/(ft2 h ° F)
2. 1 kcal/(h m2 ° C) = 1.163 W/(m2 K) = 0

In convection, heat supplied is absorbed by the molecules and the material expands. As this happens, density decreases and heated part of the material begins to move upwards. During the process, heat is transferred by actual motion of the molecules from one place to another. Was this answer helpful?

Convection. Convective heat loss is the transfer of heat from a body to moving molecules such as air or liquid. The thin air layer adjacent to the skin is heated by conduction from the body but carries the heat away from the body in the ambient air currents. This leads to a convective heat loss.

Spontaneous convection is driven by buoyancy for the most part and surface tension to a lesser extent.

• exposed surface area.
• viscosity.
• density.
• conductivity (when conductivity is high, there is no need for convection)
• acceleration due to gravity.

The volumetric specific heat of carrying medium.

Natural convection on a surface depends on the geometry of the surface as well as its orientation. It also depends on the variation of temperature on the surface and the thermophysical properties of the fluid. The velocity and temperature distribution for natural convection over a hot vertical plate are shown in Fig.

The function itself depends on the flow conditions (laminar or turbulent) and geometry of flow direct and geometry of the solid surface. Natural convection: In simple words, h is complex function of fluid property (not material property), flow conditions, geometry of the flow…

So, though colloquially it can be said that in the case of convection, heat rises, what really happens is that the air is heated by conduction. Colder and denser air falls to the bottom, which has the effect of causing the hotter and less dense air to rise, and the heat is carried away.

Convection is heat transfer due to a density differential within a fluid. As water’s temperature increases in the presence of a heat source, it will become less dense and rise. As it moves up and away from the heat source, it cools and becomes more dense and sinks.

The heat convection coefficient, the most important parameter for the convection process, is dependent on not only the wind velocity (the speed of air convection near the pavement surface) but also the pavement surface characteristics (roughness) [189,190][189][190].