What is Convection?
Convection is the process of heat transfer in which transfer of heat energy occurs by the mass movement of molecules of the fluids like gases and liquids. Gases and liquids are not a good conductor of heat under normal conditions, but they can easily transfer heat.
Heat transfer through convection occurs other through diffusion or advection or both. Convection does not take place in solids, because no movement of its constituent particles occurs. The diffusion of heat occurs in solids, and it is called thermal conduction.
The process in which heat transfer occurs between a surface and a liquid or a fluid that is in contact with the surface is called convection heat transfer. Convection plays a major role while transferring heat from one liquid to another liquid through a barrier. Heat transfer by convection either occurs due to thermal diffusion (motion of fluid molecule) or advection, in which heat is transferred by the bulk motion of heat currents in the fluid.
Newton's Law of Cooling
Under normal conditions, the heat transfer due to convection is directly proportional to the temperature difference between the parts. Newton's law of cooling has expressed this phenomenon, and the law states that:
The heat transfer rate of a body due to convection is directly proportional to the temperature difference between the body and its surroundings. The temperature difference should be small, and the nature of the radiating surface remains the same. This is the convection heat transfer equation:
P = \[\frac{{dq }}{dt}\] = hA(T - T0)
Where
P = \[\frac{{dq }}{dt}\] is the rate of heat transfer
h is the coefficient of the convection heat transfer
A is the surface area of the body
T is the temperature of the object
T0 is the temperature of the fluid or liquid which is subjected to convection
Heat-transfer coefficient h depends on the following factors:
Density, Thermal conductivity, Viscosity, coefficient of convective heat transfer, and specific heat capacity
The constant of proportionality mentioned in the above formula is an important parameter and is called a coefficient of convective heat transfer 'h'. It is defined as:
The rate at which heat transfer occurs between a solid and a fluid part per unit surface area and for the unit temperature difference:
𝒉 = \[\frac{{q}}{ΔT}\]
Where
q is the local heat flux density [W.m-2]
H is the heat transfer coefficient [W.m-2.K]
ΔT is the temperature difference [K]
FAQs on Heat Transfer By Convection
1. What is heat transfer by convection?
Heat transfer by convection is a process where heat is transferred from one place to another through the actual movement of fluid particles (liquids or gases). When a part of the fluid is heated, it becomes less dense and rises. The cooler, denser fluid from the surroundings then moves in to take its place, gets heated, and also rises. This continuous circulation of fluid, known as a convection current, effectively transfers heat.
2. What are the two main types of convection?
The two primary types of convection are:
- Natural Convection: This occurs naturally due to density differences in the fluid caused by a temperature gradient. For example, the formation of sea and land breezes is driven by the natural heating and cooling of air over land and water.
- Forced Convection: This occurs when an external device, such as a fan, pump, or stirrer, is used to move the fluid and accelerate the heat transfer process. A common example is using a fan to cool down a hot computer processor or a convection oven circulating hot air to cook food faster.
3. Can you give some common examples of heat transfer by convection in daily life?
Certainly. Heat transfer by convection is observed in many everyday situations. Some key examples include:
- Boiling Water: When water is heated in a pot, the water at the bottom gets hot, rises, and is replaced by cooler water from the top, creating a circulating current.
- Room Heaters: A heater warms the air near it. This warm air rises, and cooler air from the rest of the room flows in to be heated, gradually warming the entire room.
- Sea and Land Breezes: The differential heating of land and sea creates pressure differences that drive the movement of air.
- Air Conditioners: AC units are typically placed high on a wall because they release cool, dense air that sinks, displacing the warmer air in the room upwards towards the unit to be cooled.
- Blood Circulation: The human body uses blood circulation (a form of forced convection) to transfer heat and maintain a stable body temperature.
4. How is forced convection different from natural convection?
The key difference lies in the driving force behind the fluid's movement. In natural convection, the fluid moves on its own due to buoyancy forces arising from density changes when it's heated. This process is passive. In contrast, forced convection relies on an external mechanical force—like a fan or a pump—to circulate the fluid. This makes the heat transfer process much faster and more controlled than its natural counterpart.
5. Why does convection only occur in fluids and not in solids?
Convection fundamentally requires the bulk movement of particles to transfer heat. In fluids (liquids and gases), the atoms and molecules are free to move around, allowing them to form convection currents. When heated, they can physically travel from a hotter region to a cooler one. In solids, particles are held in fixed positions within a crystal lattice structure. They can only vibrate in place, which allows them to transfer heat through conduction, but they cannot flow to create convection currents.
6. How does convection compare to the other modes of heat transfer, conduction and radiation?
The three modes of heat transfer are distinct in their mechanism and medium requirements:
- Convection: Transfers heat via the bulk movement of a fluid (liquid or gas). It requires a medium with mobile particles.
- Conduction: Transfers heat through direct molecular collisions without the bulk movement of the medium itself. It is the primary mode of heat transfer in solids.
- Radiation: Transfers heat through electromagnetic waves (like infrared radiation) and does not require any medium at all. This is how the Sun's heat reaches Earth through the vacuum of space.
In simple terms, conduction is transfer by 'touching', convection is transfer by 'flowing', and radiation is transfer by 'waves'.
7. What does the convection heat transfer coefficient (h) represent in physics?
The convection heat transfer coefficient (h) is a crucial parameter in Newton's Law of Cooling. It quantifies the rate of heat transfer between a solid surface and a moving fluid. A higher value of 'h' indicates a more effective transfer of heat. It is not a property of the material itself but depends on several factors, including:
- The fluid's properties (like viscosity and thermal conductivity).
- The velocity of the fluid flow.
- The geometry and orientation of the surface.
For example, fast-moving air (forced convection) has a much higher 'h' value than still air (natural convection), which is why blowing on hot food cools it down quicker.
8. What are some large-scale examples of natural convection that affect the Earth?
Beyond local weather like sea breezes, natural convection drives massive systems on a planetary scale. Two prominent examples are:
- Atmospheric Circulation: The Sun heats the Earth's equator more than the poles. This causes hot air at the equator to rise and flow towards the poles, while cold, dense polar air sinks and flows towards the equator. These large-scale convection cells, combined with the Earth's rotation, create global wind patterns.
- Mantle Convection: Inside the Earth, the semi-molten rock in the mantle is heated by the core. This hot rock slowly rises, cools near the crust, and sinks back down, creating enormous convection currents. This process is the driving force behind the movement of tectonic plates, causing earthquakes and volcanic activity.
