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In many real-life applications (e.g. heat losses at solar central receivers or cooling of photovoltaic panels), natural and forced convection occur at the same time (mixed convection). [4] Internal and external flow can also classify convection. Internal flow occurs when a fluid is enclosed by a solid boundary such as when flowing through a pipe.
However, all types of buoyant convection, including natural convection, do not occur in microgravity environments. All require the presence of an environment which experiences g-force (proper acceleration). The difference of density in the fluid is the key driving mechanism. If the differences of density are caused by heat, this force is called ...
Heat convection occurs when the bulk flow of a fluid (gas or liquid) carries its heat through the fluid. All convective processes also move heat partly by diffusion, as well. The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example ...
Forced convection can occur in both laminar and turbulent flow. In the situation of laminar flow in circular tubes, several dimensionless numbers are used such as Nusselt number, Reynolds number, and Prandtl number. The commonly used equation is =. Natural or free convection is a function of Grashof and Prandtl numbers. The complexities of free ...
Conduction is the main mode of heat transfer for solid materials because the strong inter-molecular forces allow the vibrations of particles to be easily transmitted, in comparison to liquids and gases. Liquids have weaker inter-molecular forces and more space between the particles, which makes the vibrations of particles harder to transmit.
Convection is caused by yeast releasing CO2. In fluid dynamics, a convection cell is the phenomenon that occurs when density differences exist within a body of liquid or gas. These density differences result in rising and/or falling convection currents, which are the key characteristics of a convection cell. When a volume of fluid is heated, it ...
Its transport properties are represented by the phonon conductivity tensor K p (W/m-K, from the Fourier law q k,p = -K p ⋅∇ T) for bulk materials, and the phonon boundary resistance AR p,b [K/(W/m 2)] for solid interfaces, where A is the interface area. The phonon specific heat capacity c v,p (J/kg-K) includes the quantum effect.
Molecular diffusion occurs in gases, liquids, and solids. Diffusion is a result of thermal motion of molecules. Usually, convection occurs as a result of the diffusion process. The rate at which diffusion occurs depends on the state of the molecules: it occurs at a high rate in gases, a slower rate in liquids, and an even slower rate in solids.