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7.2. Effect of System Parameters on CHF

In a typical CHF experiment with a uniform heat flux on a heater tube, CHF first occurs at the end of the channel. Figure 7.1 shows conceptually the effect of the various system parameters. One notes the following behavior:

  1. For a given pressure (P), fixed mass flux (G), tube length (L) and tube inside diameter ( tex2html_wrap_inline4535 ) the critical heat flux (CHF) increases approximately linearly with the inlet subcooling (i.e., the difference in enthalpy, tex2html_wrap_inline5362 , between saturated liquid and inlet liquid). This relationship occurs over fairly wide ranges, but has no fundamental significance, except to indicate more energy goes into saturating the fluid. If a very wide range of inlet subcooling is used, then departures from linearity are observed.
  2. For fixed P, tex2html_wrap_inline4535 , L and tex2html_wrap_inline5362 , CHF rises approximately linearly with G at low values of G but then rises much less rapidly for high G values; this is discussed later.
  3. For fixed P, tex2html_wrap_inline4535 , G, and tex2html_wrap_inline5362 , critical heat flux decreases with increasing tube length L. However, the power input required for burnout, tex2html_wrap_inline5388 , increases at first rapidly, and then less rapidly, also as shown in the figure. For very long tubes, the critical power may appear to asymptote to a constant value independent of tube length in some cases. Again, this only applies over a limited range of length.
  4. For fixed P, G, tex2html_wrap_inline5362 and L, CHF increases with tube diameter, tex2html_wrap_inline4535 , the rate of increase decreasing as the diameter increases.

Finally, the effect of system pressure on CHF is similar to that encountered in pool boiling. The parametric effects illustrated in Figure 7.1 are typical of those encountered for upflow. Experimental data for downflow shows surprisingly little difference to upflow particularly for large mass velocities.

Critical heat flux in cross flow over a tube bundle was investigated by Lienhard and Eichorn (1976) and their results are illustrated in Figure 7.2. For low cross flow velocities, the vapor departure from the heater surface follows the characteristic 3-dimensional jet form also occurring in pool boiling. The critical heat flux was, in this region, close to the value of pool boiling as shown in Figure 7.2. At high liquid velocities, the pattern of vapor departure changes to a two-dimensional form as shown and the critical heat flux begins to increase with increasing liquid velocity. A wide variety of data has been obtained for burnout in annuli and rod bundles. The two main reasons for the interest in this geometry are:

(1) In the nuclear industry, the main interest is CHF in rod bundle geometries. The annulus might be regarded as a "single rod bundle" or "subchannel."
(2) By making the outer channel wall transparent, it is sometimes possible to view the processes occurring on the inner (heated) surface, illustrating the CHF mechanism.

Most of the data obtained have been for the case where the inner surface is heated. However, more recently, data has appeared where both surfaces are heated and the fraction of the power input to, say, the outer surface is varied. Typical of this latter data is that of Jensen and Mannov (1974), some of which is illustrated in Figure 7.3. For a fixed inlet subcooling, the critical quality (quality at burnout) initially increases as the fraction of power on the outer surface is increased. In this region, CHF occurs first on the inner surface. As the fraction of power on the outer surface is further increased, a maximum burnout quality is reached, and beyond this point burnout begins to occur first on the outer surface and the critical quality decreases with increasing fractional power on that surface.


next up previous contents
Next: Correlation Methods for CHF Up: BURNOUT AND THE CRITICAL Previous: Introduction and Objectives


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