Terminology and Mechanisms of Convection
– Convection has different usages in scientific and engineering contexts.
– In fluid mechanics, convection refers to fluid motion driven by density differences.
– In thermodynamics, convection refers to heat transfer by convection.
– Some phenomena may be referred to as a form of convection, such as thermo-capillary convection and granular convection.
– Convection can occur in fluids at all scales larger than a few atoms.
– Convection arises due to body forces acting within the fluid, such as gravity.
– Different types of convection occur in different circumstances.
– Convection can be driven by density differences, temperature variations, or concentration gradients.
Natural Convection
– Natural convection is the flow of a fluid driven by gravity and density differences.
– It occurs when there are hot and cold regions of air or water.
– Natural convection is present in nature and engineering applications.
– It plays a role in weather systems, plate tectonics, and fluid flows.
– Natural convection can be observed in the cooling of molten metals and free air cooling in industrial applications.
– Gravitational convection is a type of natural convection induced by buoyancy variations.
– It is caused by material properties other than temperature, such as variable composition or concentration gradients.
– Gravitational convection can be seen in the diffusion of substances in different mediums.
Importance and Applications of Convection
– Natural convection has attracted attention from researchers due to its presence in nature and engineering.
– It plays a role in various phenomena such as weather systems, plate tectonics, and heat dissipation.
– Natural convection is utilized in the formation of microstructures and cooling processes.
– It is commonly observed in the cooling of molten metals and fluid flows around heat-dissipation fins.
– Natural convection can be harnessed for free air cooling in industrial applications.
– Natural circulation systems include tornadoes, ocean currents, and household ventilation.
– Some solar water heaters utilize natural circulation.
– The Gulf Stream circulates due to the evaporation of water and increasing salinity and density.
– Convection occurs on a large scale in atmospheres, oceans, planetary mantles, and stars.
– Gas and dust convection is thought to occur in the accretion disks of black holes.
Specific Examples and Applications
– Solid-state convection occurs in a mixture of nitrogen ice and carbon monoxide ice.
– Thermomagnetic convection happens when a magnetic field is imposed on a ferrofluid with varying magnetic susceptibility.
– Combustion in a zero-gravity environment relies on thermal expansion and chemical reactions for ventilation.
– Natural convection is demonstrated through various experiments using liquids and gases.
– Oceanic circulation is influenced by solar radiation, surface wind conditions, and density changes.
– Mantle convection drives tectonic plate movement and occurs at rates of centimeters per year.
– The stack effect is the movement of air in buildings due to temperature and moisture differences.
– Convection within stars is driven by thermal gradients and contributes to heat in the Earth’s core.
– Water convection at freezing temperatures is influenced by nonlinear density variations.
– Natural circulation is important in the design and operation of nuclear reactors.
Mathematical Models and Onset of Convection
– Archimedes number, Grashof number, Richardson number, and Rayleigh number describe and predict convection.
– Mixed convection involves both natural and forced convection.
– The magnitude of the Grashof number and the square of the Reynolds number determine dominant convection.
– The onset of natural convection is determined by the Rayleigh number.
– Rapid diffusion and high viscosity inhibit natural convection. Source: https://en.wikipedia.org/wiki/Convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convection is unspecified, convection due to the effects of thermal expansion and buoyancy can be assumed. Convection may also take place in soft solids or mixtures where particles can flow.
Convective flow may be transient (such as when a multiphase mixture of oil and water separates) or steady state (see Convection cell). The convection may be due to gravitational, electromagnetic or fictitious body forces. Heat transfer by natural convection plays a role in the structure of Earth's atmosphere, its oceans, and its mantle. Discrete convective cells in the atmosphere can be identified by clouds, with stronger convection resulting in thunderstorms. Natural convection also plays a role in stellar physics. Convection is often categorised or described by the main effect causing the convective flow, e.g. Thermal convection.
Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place. Granular convection is a similar phenomenon in granular material instead of fluids. Advection is fluid motion created by velocity instead of thermal gradients. Convective heat transfer is the intentional use of convection as a method for heat transfer. Convection is a process in which heat is carried from place to place by the bulk movement of a fluid and gases