Heat dissipation factors influencing

In addition, it was concluded that the liquid-phase diffusion coefficient is the major factor influencing the value of the mass-transfer coefficient per unit area. Inasmuch as agitators operate poorly in gas-liquid dispersions, it is impractical to induce turbulence by mechanical means that exceeds gravitational forces. They conclude, therefore, that heat- and mass-transfer coefficients per unit area in gas dispersions are almost completely unaffected by the mechanical power dissipated in the system. Consequently, the total mass-transfer rate in agitated gas-liquid contacting is changed almost entirely in accordance with the interfacial area—a function of the power input. 

The intense heat dissipated by viscous flow near the walls of a tubular reactor leads to an increase in local temperature and acceleration of the chemical reaction, which also promotes an increase in temperature the local situation then propagates to the axis of the tubular reactor. This effect, which was discovered theoretically, may occur in practice in the flow of a highly viscous liquid with relatively weak dependence of viscosity on degree of conversion. However, it is questionable whether this approach could be applied to the flow of ethylene in a tubular reactor as was proposed in the original publication.199 In turbulent flow of a monomer, the near-wall zone is not physically distinct in a hydrodynamic sense, while for a laminar flow the growth of viscosity leads to a directly opposite tendency - a slowing-down of the flow near the walls. In addition, the nature of the viscosity-versus-conversion dependence rj(P) also influences the results of theoretical calculations. For example, although this factor was included in the calculations in Ref.,200 it did not affect the flow patterns because of the rather weak q(P) dependence for the system that was analyzed. 

Heating and cooling often take place while the polymer melt flows, making viscous dissipation an influencing factor during the process. However, since most plastic parts are thin, the conduction often occurs only across the thickness and the viscous heating is a result of shear within the narrow gap of a die or mold cavity. For such cases, the equations reduce to,... 

Other errors, which could influence the results obtained, are, for example, wall effects ( slipping ), the dissipation of heat, and the increase in temperature due to shear. In a tube, the viscosity of a flowing medium is less near the tube walls compared to the center. This is due to the occurrence of shear stress and wall friction and has to be minimized by the correct choice of the tube diameter. In most cases, an increase in tube diameter reduces the influence of wall slip on the flow rate measured, but for Newtonian materials of low viscosity, a large tube diameter could be the cause of turbulent flow. ° When investigating suspensions with tube viscometers, constrictions can lead to inhomogeneous particle distributions and blockage. Due to the influence of temperature on viscosity (see Section Influence Factors on the Viscosity ), heat dissipated must be removed instantaneously, and temperature increase due to shear must be prevented under all circumstances. This is mainly a constructional problem of rheometers. Technically, the problem is easier to control in tube rheometers than in rotating instruments, in particular, the concentric cylinder viscometers. ... 

Factors Influencing Heat Dissipation 18.3.2.1 Type of Cooling System... 

Taking a value of Bq of 10 gauss, for example, it is clear that theoretical reductions in the mean resistivity of about 100 in the case of copper, and more than 1000 in the case of sodium, seem possible. Since these factors appear directly in the rates of heat dissipation per unit volume in the body of the coil, they will have a profoundly favorable influence on the problems of cooling and heat transfer within the coil. This is one of the most important features of a large cryogenic coil. 

Acrylate and methacrylate polymerizations are accompanied by the Hberation of a considerable amount of heat and a substantial decrease in volume. Both of these factors strongly influence most manufacturing processes. Excess heat must be dissipated to avoid uncontrolled exothermic polymerizations. In general, the percentage of shrinkage decreases as the size of the alcohol substituent increases on a molar basis, the shrinkage is relatively constant (77). 

An important difference between apparatus of type (a), (b), (e) or (f) and (c) or (d) is that in the former case the test piece is continuously and totally in contact with the abradant and there is no chance for the very considerable heat generated at the contact surface to be dissipated. The actual rate of slip will influence the rate of wear because, as the speed is increased, heat buildup will rise. Temperature rise during test is one of the important factors in obtaining correlation between laboratory and service. 

This is not necessarily the case at the molecular level where the efficiency of permanent chain extension can be influenced by factors like chain relaxation, topological restraints, chain size and distribution, dissipation of frictional heat, chain repeating structure, and chain slippage. An expression that accounts for chain slippage and provides a measure of true molecular extension has been proposed by Porter et al. ... 

Increasing the barrel diameter will increase the shear rate—other factors being constant. This will increase viscous dissipation and melt temperatures as discussed earlier. Another problem with larger barrel diameters is that the heat transfer surface area increases with the diameter squared, while the channel volume increases with the diameter cubed. As a result, the heat transfer becomes less effective with larger diameter extruders. It is well known in the extrusion industry that the ability to influence melt temperature by changes in barrel temperature is very limited for large extruders. 

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