The Prandtl number is a dimensionless number approximating the ratio of momentum diffusivity and thermal diffusivity. It is named after the German physicist Ludwig Prandtl. It is defined as: where: : kinematic viscosity,, : thermal diffusivity,, : dynamic viscosity, : thermal conductivity, : specific heat, : density, .
Convection is the movement of molecules within fluids. It cannot take place in solids, since neither bulk current flows or significant diffusion can take place in solids. Convection is one of the major modes of heat transfer and mass transfer. Convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid.
The Péclet number is a dimensionless number relevant in the study of transport phenomena in fluid flows. It is named after the French physicist Jean Claude Eugène Péclet. It is defined to be the ratio of the rate of advection of a physical quantity by the flow to the rate of diffusion of the same quantity driven by an appropriate gradient. In the context of the transport of heat, the Peclet number is equivalent to the product of the Reynolds number and the Prandtl number.
In heat transfer at a boundary (surface) within a fluid, the Nusselt number is the ratio of convective to conductive heat transfer across (normal to) the boundary. Named after Wilhelm Nusselt, it is a dimensionless number. The conductive component is measured under the same conditions as the heat convection but with a (hypothetically) (or motionless) fluid. A Nusselt number close to unity, namely convection and conduction of similar magnitude, is characteristic of "slug flow" or laminar flow.
The Grashof number is a dimensionless number in fluid dynamics and heat transfer which approximates the ratio of the buoyancy to viscous force acting on a fluid. It frequently arises in the study of situations involving natural convection. It is named after the German engineer Franz Grashof. for vertical flat plates for pipes for bluff bodies where the L and D subscripts indicates the length scale basis for the Grashof Number.
In fluid mechanics, the Rayleigh number for a fluid is a dimensionless number associated with buoyancy driven flow (also known as free convection or natural convection). When the Rayleigh number is below the critical value for that fluid, heat transfer is primarily in the form of conduction; when it exceeds the critical value, heat transfer is primarily in the form of convection.
Advection, in chemistry and engineering, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries these impurities downstream. Another commonly advected property is heat, and here the fluid may be water, air, or any other heat-containing fluid material.
A convection cell is a phenomenon of fluid dynamics that occurs in situations where there are temperature differences within a body of liquid or gas. Fluids are materials that exhibit the property of flow. Both gases and liquids have fluid properties, and in sufficient quantity, even particulate solids such as salt, grain, and gravel show some fluid properties. When a volume of fluid is heated, it expands and becomes less dense and thus more buoyant than the surrounding fluid.
A thermal column (or thermal) is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and an example of convection. The Sun warms the ground, which in turn warms the air directly above it.. Dark earth, urban areas and roadways are good sources of thermals. The warmer air expands, becoming less dense than the surrounding air mass.
Convection ovens or fan ovens or turbo ovens augment a traditional oven by circulating heated air using a fan. The fan motor is in a separate enclosure, to protect it from overheating and melting any plastic components including wire insulation. Food warms faster in a convection oven, because the moving air strips away the thin layer of air which otherwise would surround and insulate the food. This is the same physical phenomenon which causes wind chill.
The Brazil nut effect is the name given to a phenomenon in which the largest particles end up on the surface when a granular material containing a mixture of objects of different sizes is shaken. In a typical container of mixed nuts, the largest will be Brazil nuts. The phenomenon is also known as the muesli effect since it is seen in packets of breakfast cereal containing particles of different sizes but similar density, such as muesli mix.
The traditional East Asian calendars divide a year into 24 solar terms (節氣). Báilù or Hakuro is the 15 solar term. It begins when the Sun reaches the celestial longitude of 165° and ends when it reaches the longitude of 180°. It more often refers in particular to the day when the Sun is exactly at the celestial longitude of 165°. In the Gregorian calendar, it usually begins around September 7 and ends around September 23.