Local quality

However, the experimental evidence collected during recent years, concerning mostly the nickel-copper alloy systems, complicated this almost currently accepted interpretation of the alloy catalytic behavior (45). Chemisorptive and subsequent catalytic phenomena appeared to require a different approach for elucidation. The surface reactivity had to be treated as a localized quality of the atoms at the interface, influenced by their neighbors in the crystal lattice (78-80). A detailed general discussion of catalysis on alloys is beyond the scope of this review. In the monograph by Anderson (81) and in the review by Moss and Whalley (82), recently published, a broad survey of the catalytic reactivity of alloys may be found. 

Each laboratory in the OPMBS was required to send a copy of each workbook and the associated raw data, to study management soon after completion of the analysis. This information was provided after internal technical review but before local quality assurance review. Study management used a team of experienced residue chemists to review the results and the raw data and to ensure that the information reported was fully supportable. The laboratory remained at all times responsible for the data reported, however, and the results of the external review were formally considered to be advisory. 

Because the components of the analytical expression for C are not sufficiently known to permit an analytical evaluation, C is determined empirically as a function of the local quality at the point of DNB, XDNB, (under nonuniform heat flux conditions) and the bulk mass flux, G. The empirically determined expression for C is... 

Axial heat flux parameter Y The parameter Y, which replaces the heat flux shape factor in the CHF correlation, is not only a measure of the nonuniformity of the axial heat flux profile but also a means of converting from the inlet subcooling (AHin) to the local quality, X, form of the correlation via the heat balance equation. It is defined as... 

Local quality method Dryout occurs when the local nonuniform heat flux equals the uniform heat flux dryout value at the same local conditions (quality, etc.). 

For nonuniform heat flux the first postulate states that the local quality, X, is the same, and the CHF may be written, via the heat balance containing Y, as... 

Examination of data for subchannels and other geometries suggests that the local quality postulate tends to be more accurate at high mass fluxes, while the total power postulate is more accurate at low mass fluxes. The above two expressions can be made one if C in the equation is multiplied by a function,/(T), where... 

Local quality X, 0.0-0.70 Subchannel type corner channels only... 

For the CHF condition for two-phase crossflow on the shell side of horizontal tube bundles, few investigations have been conducted. Katto et al. (1987) reported CHF data on a uniformly heated cylinder in a crossflow of saturated liquid over a wide range of vapor-to-liquid density ratios. Recently, Dykas and Jensen (1992) and Leroux and Jensen (1992) obtained the CHF condition on individual tubes in a 5 X 27 bundle with known mass flux and quality. At qualities greater than zero, they found that the CHF data are a complex function of mass flux, local quality, pressure level, and bundle geometry. 

Local quality factors such as qfiv, Vj) provide a detailed view of the offset dependence of the efficiency of Hartmann-Hahn transfer. For the optimization of Hartmann-Hahn sequences, the offset-dependent local quality factors must be condensed into a single global quality factor. For example, for a constant rf amplitude, the global quality factor can be defined as the minimum of the local quality factor Vj) in a predefined offset range and (Glaser and... 

Figures 4 and 5 show the local heat transfer coefficient as a function of local quality for Dj, = 2 mm and Di, = 0.77 mm. Two general trends are observed. On figure 4 a strong decrease in the heat transfer coefficient with vapour quality is visible when the vapour quality is greater than a "critical quality whereas on figure 5 the heat transfer coefficient starts decreasing before increasing with vapour quality.

Figure 4 Local heat transfer coefficient versus local quality.

Figure 6 shows the local heat transfer coefficient as a function of local quality, for a given heat flux and mass velocity, for Z>h = 2 mm and f)h = 0.77 mm. It is clear that the heat transfer coefficient increases when the hydrauhc diameter decreases. Thus the local heat transfer coefficient is increased by 74 % 26 % when the hydrauhc diameter is decreased by 62 %. 

Figure 9 Wall temperature and its uncertainty versus local quality (Dh = 2 mm).

Saturated flow boiling begins downstream of the location where the liquid becomes saturated (Zsai). In these experiments, only a few data points were obtained for saturated flow boiling. Figure 8 shows the variation of htp as a function of the local quality (x), for saturated flow boiling. Downstream of Zsai, X is given as. 

Jensen and Tang also give relationships for in-line bundles. Application of Eqs. 15.195-15.202 requires a knowledge of local quality within the bundle. If the mass flux is known, then this can be obtained very simply from a heat balance, but if the mass flux is unknown and has to be calculated (as in kettle reboilers), then recourse must be had to methodologies of the type described by Brisbane et al. [213] and Whalley and Butterworth [214]. As in the case of heat transfer coefficient, simple methods have been developed for prediction of critical heat flux in tube bundles by Palen and coworkers (Palen [215], Palen and Small [224]), who relate the single tube critical heat flux (calculated by the correlations given previously on p. 15.63-15.65) by a simple bundle correction factor O/, as follows ... 

It is probable that all of the above mechanisms play a role, their influence depending on the flow and thermodynamic conditions within the channel. Semeria and Hewitt [300] represented the regions of operation of the various mechanisms in terms of the conceptual diagram reproduced in Fig. 15.122. The regions are plotted in terms of mass flux and local quality. As will be seen, the most important mechanism for tubes of reasonable length (where higher qualities will be generated) is that of annular flow dryout. 

A severe test of annular flow prediction models is provided by some data obtained by Bennett et al. [320], the results of which are illustrated in Fig. 15.130. In these experiments, film flow rate was measured as a function of distance along the channel and, knowing the local quality, the entrained flow rate could be calculated and is plotted. The results show two... 

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