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. 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.
In thermal fluid dynamics,the Nusselt number is the ratio of total heat transfer to conductive heat transfer at a boundary in a fluid. Total heat transfer combines conduction and convection. Convection includes both advection and diffusion (conduction). The conductive component is measured under the same conditions as the convective but for a hypothetically motionless fluid. It is a dimensionless number,closely related to the fluid's Rayleigh number.
In fluid mechanics,the Rayleigh number (Ra,after Lord Rayleigh) for a fluid is a dimensionless number associated with buoyancy-driven flow,also known as free (or natural) convection. It characterises the fluid's flow regime:a value in a certain lower range denotes laminar flow;a value in a higher range,turbulent flow. Below a certain critical value,there is no fluid motion and heat transfer is by conduction rather than convection. For most engineering purposes,the Rayleigh number is large,somewhere around 106 to 108.
A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating,refrigeration,air conditioning,power stations,chemical plants,petrochemical plants,petroleum refineries,natural-gas processing,and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils,which cools the coolant and heats the incoming air. Another example is the heat sink,which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium,often air or a liquid coolant.
Heat transfer is a discipline of thermal engineering that concerns the generation,use,conversion,and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms,such as thermal conduction,thermal convection,thermal radiation,and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species,either cold or hot,to achieve heat transfer. While these mechanisms have distinct characteristics,they often occur simultaneously in the same system.
A heat sink is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium,often air or a liquid coolant,where it is dissipated away from the device,thereby allowing regulation of the device's temperature. In computers,heat sinks are used to cool CPUs,GPUs,and some chipsets and RAM modules. Heat sinks are used with other high-power semiconductor devices such as power transistors and optoelectronics such as lasers and light-emitting diodes (LEDs),where the heat dissipation ability of the component itself is insufficient to moderate its temperature.
Adrian Bejan is a Romanian-American professor who has made contributions to modern thermodynamics and developed his constructal law. He is J. A. Jones Distinguished Professor of Mechanical Engineering at Duke University and author of the books Design in Nature,The Physics of Life,Freedom and Evolution and Time And Beauty. He is a Fellow of the American Society of Mechanical Engineers and was awarded the Benjamin Franklin Medal.
In fluid dynamics,the Schmidt number of a fluid is a dimensionless number defined as the ratio of momentum diffusivity and mass diffusivity,and it is used to characterize fluid flows in which there are simultaneous momentum and mass diffusion convection processes. It was named after German engineer Ernst Heinrich Wilhelm Schmidt (1892–1975).
In the study of heat transfer,critical heat flux (CHF) is the heat flux at which boiling ceases to be an effective form of transferring heat from a solid surface to a liquid.
Micro heat exchangers,Micro-scale heat exchangers, or microstructured heat exchangers are heat exchangers in which fluid flows in lateral confinements with typical dimensions below 1 mm. The most typical such confinement are microchannels,which are channels with a hydraulic diameter below 1 mm. Microchannel heat exchangers can be made from metal or ceramic.
Ferrofluids can be used to transfer heat,since heat and mass transport in such magnetic fluids can be controlled using an external magnetic field.
In fluid dynamics,a synthetic jet flow —is a type of jet flow,which is made up of the surrounding fluid. Synthetic jets are produced by periodic ejection and suction of fluid from an opening. This oscillatory motion may be driven by a piston or diaphragm inside a cavity among other ways.
Yuwen Zhang is a Chinese American professor of mechanical engineering who is well known for his contributions to phase change heat transfer. He is presently a Curators' Distinguished Professor and Huber and Helen Croft Chair in Engineering in the Department of Mechanical and Aerospace Engineering at the University of Missouri in Columbia,Missouri.
Double diffusive convection is a fluid dynamics phenomenon that describes a form of convection driven by two different density gradients,which have different rates of diffusion.
A nanofluid is a fluid containing nanometer-sized particles,called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. The nanoparticles used in nanofluids are typically made of metals,oxides,carbides,or carbon nanotubes. Common base fluids include water,ethylene glycol and oil.
Zeng-Yuan Guo,born 20 February 1936 in Jiangsu Province,is a professor in the Department of Engineering Mechanics in Tsinghua University,Beijing,China. He is a member of the Chinese Academy of Sciences (1997) and a ASME fellow (1998).
Nanofluid-based direct solar collectors are solar thermal collectors where nanoparticles in a liquid medium can scatter and absorb solar radiation. They have recently received interest to efficiently distribute solar energy. Nanofluid-based solar collector have the potential to harness solar radiant energy more efficiently compared to conventional solar collectors. Nanofluids have recently found relevance in applications requiring quick and effective heat transfer such as industrial applications,cooling of microchips,microscopic fluidic applications,etc. Moreover,in contrast to conventional heat transfer like water,ethylene glycol,and molten salts,nanofluids are not transparent to solar radiant energy;instead,they absorb and scatter significantly the solar irradiance passing through them. Typical solar collectors use a black-surface absorber to collect the sun's heat energy which is then transferred to a fluid running in tubes embedded within. Various limitations have been discovered with these configuration and alternative concepts have been addressed. Among these,the use of nanoparticles suspended in a liquid is the subject of research. Nanoparticle materials including aluminium,copper,carbon nanotubes and carbon-nanohorns have been added to different base fluids and characterized in terms of their performance for improving heat transfer efficiency.
The contemporary conjugate convective heat transfer model was developed after computers came into wide use in order to substitute the empirical relation of proportionality of heat flux to temperature difference with heat transfer coefficient which was the only tool in theoretical heat convection since the times of Newton. This model,based on a strictly mathematically stated problem,describes the heat transfer between a body and a fluid flowing over or inside it as a result of the interaction of two objects. The physical processes and solutions of the governing equations are considered separately for each object in two subdomains. Matching conditions for these solutions at the interface provide the distributions of temperature and heat flux along the body–flow interface,eliminating the need for a heat transfer coefficient. Moreover,it may be calculated using these data.
Pillow-plate heat exchangers are a class of fully welded heat exchanger design,which exhibit a wavy,“pillow-shaped”surface formed by an inflation process. Compared to more conventional equipment,such as shell and tube and plate and frame heat exchangers,pillow plates are a quite young technology. Due to their geometric flexibility,they are used as well as “plate-type”heat exchangers and as jackets for cooling or heating of vessels. Pillow plate equipment is currently experiencing increased attention and implementation in process industry.
David S-K Ting is a Canadian academic,author and researcher. He is a professor of mechanical,automotive &materials engineering at the University of Windsor. He is the founder of the Turbulence &Energy Laboratory.
