Electronics forced convection heating

In conduction, heat is conducted by the transfer of energy of motion between adjacent molecules in a liquid, gas, or solid. In a gas, atoms transfer energy to one another through molecular collisions. In metallic solids, the process of energy transfer via free electrons is also important. In convection, heat is transferred by bulk transport and mixing of macroscopic fluid elements. Recall that there can be forced convection, where the fluid is forced to flow via mechanical means, or natural (free) convection, where density differences cause fluid elements to flow. Since convection is found only in fluids, we will deal with it on only a limited basis. Radiation differs from conduction and convection in that no medium is needed for its propagation. As a result, the form of Eq. (4.1) is inappropriate for describing radiative heat transfer. Radiation is... 

Finned surfaces of various shapes, called heat sinks, are frequently used in the cooling of electronic devices. Energy dissipated by these devices is transferred to the heat sinks by conduction and from the heat sinks to tlie ambient air by natural or forced convection, depending on the power dissipation requirements. Natural convection is the preferred mode of heat tiansfer since it involves no moving parts, like the electronic components themselves. However, in (he natural convection mode, the components are more likely to run at a higher temperature and thus undennine reliability. A properly selected heat sink may considerably lower the operation temperature of the components and thus reduce the risk of failure. 

A question that frequently arises in the cooling of heat-generating equipment such as electronic components is whether to use a fan (or a pump if the cooling medium is a liquid)—that is, whether to utilize natural or forced convection in the cooling of the equipment, The answer depends on the maximum allowable operating temperature. Recall that the convection heat transfer rate from a surface at temperature in a medium at is given by... 

On the other hand, the heat fransfer literatiue of the last decade has demonstrated a vivid and growing interest in thermal analysis of flows in micro-channels, botii tiirough experimental and analytical approaches, in connection with cooling techniques of micro-electronics and witii tiie development of micro-electromechanical sensors and actuators (MEMS), as also pointed out in recent reviews [12-16]. Since tiie available analytical information on heat fransfer in ducts could not be directly extended to flows witiiin microch mels with wall slip, a number of contributions have been recentiy directed towards the analysis of internal forced convection in the micro-scale. In the paper by Barron et al. 

In order to identify EPHs of the cell or electrode reactions from the experimental information, there had been two principal approaches of treatments. One was based on the heat balance under the steady state or quasi-stationary conditions [6,11, 31]. This treatment considered all heat effects including the characteristic Peltier heat and the heat dissipation due to polarization or irreversibility of electrode processes such as the so-call heats of transfer of ions and electron, the Joule heat, the heat conductivity and the convection. Another was to apply the irreversible thermodynamics and the Onsager s reciprocal relations [8, 32, 33], on which the heat flux due to temperature gradient, the component fluxes due to concentration gradient and the electric current density due to potential gradient and some active components transfer are simply assumed to be directly proportional to these driving forces. Of course, there also were other methods, for instance, the numerical simulation with a finite element program for the complex heat and mass flow at the heated electrode was also used [34]. 

Film Heat Transfer Coefficient Value Most of the experimental data for liquid metals in forced convection have been obtained for round tubes. Since a large fraction of heat transfer to liquid metals in forced convection is by molecular and electronic conduction, the velocity and temperature distribution of the fluid in the channel is expected to have a noticeable effect. Until data are obtained for the reference channel, however, the data for round tubes is used with the equivalent diameter of the channel replacing the diameter of the tube. Most of the round tube data fall below the L.yon-Martinelli theoretical prediction, and therefore 85% of the Lyon-Martinelli Nusselt Number is used as the best average value in the range of Peclet Number of interest (500-1000). The factor shown in Table X represents the expected accuracy of experimental data. 

The heat transfer mechanisms utilized for the mass reflow soldering of electronic assemblies include convection, radiation (infrared sources), and condensation (vapor phase). The most popular solder reflow method is based on forced convection. 

REP, a rod of metal or alloy, referred to as a consumable electrode, is rotated at high speed about its longitudinal axis. Simultaneously, it is melted gradually at one of its ends by a heat source, such as an arc, a plasma, or an electron beam, etc. A thin film of the molten metal is detached from the rod end and ejected from the periphery of the rod by centrifugal force, forming spherical droplets. The atomization is conducted in an inert atmosphere, usually argon. Helium may be used to increase arc stability and convective cooling efficiency of droplets. 

Such combined convective flows are normally associated with low forced velocities. They can occur, for example, in some electronic cooling situations and in some heat exchangers. 

The second term on the left is the heat transport VQ with Q = f d3v yw2n/, due to convection, viscous heat transport and conduction, respectively. On the right-hand side one has the heating due to the work done by the electric field, the collisional frictional force due to the flow relative to the other species (here electrons) and from the collisional heating due to collisions with other species (electrons) ... 

The components of an electronic system dissipating 180 W are located in a 1.2-m-long horizontal duct whose cross section is 15 cm X 15 cm. Ibe components in llie duct are cooled by forced air, wliich enters at 30°C at a rate of 0.62 m /min and leaves al 38°C. The surfaces of the sheet metal duct are not painted, and thus radiation heat transfer from the outer surfaces is negligible. If the ambient air temperature is 27°C, determine (a) the heat tran.sfer from the outer surfaces of the duct to the ambient air by natural convection and (6) the average temperature of the duct. 

The typical stack construction for PEM (and other) fuel cells is a series-connection of cells with bipolar plates interposed between adjacent cells (or membrane-electrode assemblies). The bipolar plate must provide for electronic conduction from one cell to the next isolation of fuel on one side from oxidant (air) on the other side and distribution of fuel and air reactants to the respective adjacent anode and cathode. In an edge-cooled stack of the type described above, the bipolar plate also contributes to heat rejection by conducting heat laterally to its tinned edges, where forced-air convection is employed. 

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