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Thermal Convection
Convection (or convective heat transfer) is the transfer of heat from one place to another due to the movement of fluid. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes of conduction (heat diffusion) and advection (heat transfer by bulk fluid flow). Convection is usually the dominant form of heat transfer in liquids and gases. Note that this definition of convection is only applicable in Heat transfer and thermodynamic contexts. It should not to be confused with the dynamic fluid phenomenon of convection, which is typically referred to as ''Natural Convection'' in thermodynamic contexts in order to distinguish the two. Overview Convection can be "forced" by movement of a fluid by means other than buoyancy forces (for example, a water pump in an automobile engine). Thermal expansion of fluids may also force convection. In other cases, natural buoyancy forces alone are entirely responsible for fluid mo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Thermal Conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. For instance, metals typically have high thermal conductivity and are very efficient at conducting heat, while the opposite is true for insulating materials like Rockwool or Styrofoam. Correspondingly, materials of high thermal conductivity are widely used in heat sink applications, and materials of low thermal conductivity are used as thermal insulation. The reciprocal of thermal conductivity is called thermal resistivity. The defining equation for thermal conductivity is \mathbf = - k \nabla T, where \mathbf is the heat flux, k is the thermal conductivity, and \nabla T is the temperature gradient. This is known as Fourier's Law for heat conduction. Although commonly expressed as a scalar, the most general form of th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heisler Chart
Heisler charts are a graphical analysis tool for the evaluation of one-dimensional transient conductive heat transfer in thermal engineering. They are a set of two charts per included geometry introduced in 1947 by M. P. Heisler which were supplemented by a third chart per geometry in 1961 by H. Gröber. Heisler charts permit evaluation of the central temperature for transient heat conduction through an infinitely long plane wall of thickness 2''L'', an infinitely long cylinder of radius ''r''o, and a sphere of radius ''r''o. Each aforementioned geometry can be analyzed by three charts which show the midplane temperature, temperature distribution, and heat transfer. Although Heisler–Gröber charts are a faster and simpler alternative to the exact solutions of these problems, there are some limitations. First, the body must be at uniform temperature initially. Second, the Fourier's number of the analyzed object should be bigger than 0.2. Additionally, the temperature of the surr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heat Transfer Enhancement
Heat transfer enhancement is the process of increasing the effectiveness of heat exchangers. This can be achieved when the heat transfer power of a given device is increased or when the pressure losses generated by the device are reduced. A variety of techniques can be applied to this effect, including generating strong secondary flows or increasing boundary layer turbulence. Principle During the earliest attempts to enhance heat transfer, plain (or smooth) surfaces were used. This surface requires a special surface geometry able to provide higher values per unit surface area in comparison with a plain surface. The ratio of of an enhanced heat transfer surface to the plain surface is called Enhancement Ratio " E_h ". Thus, E_h= The heat transfer rate for a two-fluid counterflow heat exchanger is given by In order to better illustrate the benefits of enhancement, the total length 'L' of the tube is multiplied and divided in the equation Where is the ove ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heat Transfer Coefficient
In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ). It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid. The heat transfer coefficient has SI units in watts per square meter kelvin (W/m2/K). The overall heat transfer rate for combined modes is usually expressed in terms of an overall conductance or heat transfer coefficient, . In that case, the heat transfer rate is: :\dot=hA(T_2-T_1) where (in SI units): *: surface area where the heat transfer takes place (m2) *: temperature of the surrounding fluid (K) *: temperature of the solid surface (K) The general definition of the heat transfer coefficient is: :h = \frac where: *: heat flux (W/m2); i.e., thermal power per unit area, q = d\dot/dA *: difference in temperature bet ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mixed Convection
In fluid thermodynamics, combined forced convection and natural convection, or mixed convection, occurs when natural convection and forced convection mechanisms act together to transfer heat. This is also defined as situations where both pressure forces and buoyant forces interact. How much each form of convection contributes to the heat transfer is largely determined by the flow, temperature, geometry, and orientation. The nature of the fluid is also influential, since the Grashof number increases in a fluid as temperature increases, but is maximized at some point for a gas. Characterization Mixed convection problems are characterized by the Grashof number (for the natural convection) and the Reynolds number (for the forced convection). The relative effect of buoyancy on mixed convection can be expressed through the Richardson number: : \mathrm=\frac The respective length scales for each dimensionless number must be chosen depending on the problem, e.g. a vertical length for ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Conjugate Convective Heat Transfer
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. History The problem of heat transfer in the presence of liquid flowing around the body was first formulated ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heat Transfer Coefficient
In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ). It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid. The heat transfer coefficient has SI units in watts per square meter kelvin (W/m2/K). The overall heat transfer rate for combined modes is usually expressed in terms of an overall conductance or heat transfer coefficient, . In that case, the heat transfer rate is: :\dot=hA(T_2-T_1) where (in SI units): *: surface area where the heat transfer takes place (m2) *: temperature of the surrounding fluid (K) *: temperature of the solid surface (K) The general definition of the heat transfer coefficient is: :h = \frac where: *: heat flux (W/m2); i.e., thermal power per unit area, q = d\dot/dA *: difference in temperature bet ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bulk Temperature
In thermofluids dynamics, the bulk temperature, or the average bulk temperature in the thermal fluid, is a convenient reference point for evaluating properties related to convective heat transfer, particularly in applications related to flow in pipes Pipe(s), PIPE(S) or piping may refer to: Objects * Pipe (fluid conveyance), a hollow cylinder following certain dimension rules ** Piping, the use of pipes in industry * Smoking pipe ** Tobacco pipe * Half-pipe and quarter pipe, semi-circula ... and ducts. The concept of the bulk temperature is that adiabatic mixing of the fluid from a given cross section of the duct will result in some equilibrium temperature that accurately reflects the average temperature of the moving fluid, more so than a simple average like the film temperature. Kreith, Frank, and Bohn, Mark S. ''Principles of Heat Transfer, Sixth Edition.'' Brooks/Cole: Pacific Grove, CA, 2001. References Continuum mechanics Heat transfer Temperature {{ther ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
RWTH Aachen University
RWTH Aachen University (), also known as North Rhine-Westphalia Technical University of Aachen, Rhine-Westphalia Technical University of Aachen, Technical University of Aachen, University of Aachen, or ''Rheinisch-Westfälische Technische Hochschule Aachen'', is a German public research university located in Aachen, North Rhine-Westphalia, Germany. With more than 47,000 students enrolled in 144 study programs, it is the largest technical university in Germany. In 2018, the university was ranked 31st in the world university rankings in the field of engineering and technology, and 36th world-wide in the category of natural sciences.Daten & Fakten – RWTH AACHEN UNIVERSITY – Deutsch Rwth-aachen.de (12 December 2011). Retrieved on 2013-09-18. [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mixed Convection
In fluid thermodynamics, combined forced convection and natural convection, or mixed convection, occurs when natural convection and forced convection mechanisms act together to transfer heat. This is also defined as situations where both pressure forces and buoyant forces interact. How much each form of convection contributes to the heat transfer is largely determined by the flow, temperature, geometry, and orientation. The nature of the fluid is also influential, since the Grashof number increases in a fluid as temperature increases, but is maximized at some point for a gas. Characterization Mixed convection problems are characterized by the Grashof number (for the natural convection) and the Reynolds number (for the forced convection). The relative effect of buoyancy on mixed convection can be expressed through the Richardson number: : \mathrm=\frac The respective length scales for each dimensionless number must be chosen depending on the problem, e.g. a vertical length for ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
