Normalized parameter values, dissipative

As the triggering process of CHF in flow boiling is very complex, CHF predictions rely heavily on empirical correlations based on dimensionless numbers derived from experimental CHF databases. These dimensionless numbers include parameters that influence the CHF values measured, such as mass velocity, subcooling at the channel inlet, fluid properties, heated length and the diameter of the channel. Due to differences in the apparent mechanisms, correlations are normally separately proposed for CHF under subcooled and under saturated conditions. Most of these correlations were developed based on databases for water and macroscale charmels because the majority of studies on CHF focused mainly on nuclear applications. Recently, this scenario has been changing because of the necessity to dissipate high power densities in microprocessors and power electronics and data for a wider variety of fluids are now appearing in the literature. 

The knowledge of the distribution of in the volume of the tank enables us to identify the zones where the energy dissipation is the most intense. Moreover, this parameter indicates which points in the tank are most at risk of local heating, if the heat transfer to outside is not sufficiently rapid. Figure 7d represents the distributions of ( ) (< ) = J(2n N D/T) ) in the same vertical plane as previously (0 = 3°). The maximum value of the viscous dissipation function is located at the impeller tip. It is clear that the distribution of < ) in the whole tank is more or less similar to the one of the normal and shear stresses x and x. Thus, only these stresses have to be considered in the case of a paddle agitator. 

By making certain simplifying assumptions, it is possible to estimate typical values of peak temperature [3], However, the precision of such calculations is limited because the temperature spikes depend not only on sliding velocity and normal force, but also on parameters whose effects are difficult to quantify, such as surface topography, asperity deformation, oxidation kinetics and transient heat dissipation. 

A stable starting point for the kinds of flows enconntered in a stirred tank is the k-8 model. This model assumes that the normal stresses are roughly equal and are adequately represented by k. Two differential eqnations are used to model the production, distribution, and dissipation of tmbulent kinetic energy the k-equation, and the s-equation. These equations were developed for free shear flows, and experimentally determined constants are established for the model parameters. One of these constants is nsed to relate local values of k and e to an estimate of (uv) using a modified tmbulent viscosity approach ... 

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