The Important System-Describing Parameters

The first logical step in analyzing any physical process is to try to ascertain what system parameters are important. The burn-out process involves a large number of important system parameters, and, while some of these can be 

In burn-out experiments, a test section is part of a loop which may be open or closed, and the question arises as to whether or not any of the loop equipment, such as condensers, heaters, pumps, or pipe fittings, have any significant effect on the burn-out flux. This issue came to prominence at the Boulder Heat Transfer Conference in 1961 with a Russian paper by Aladyev (A4) describing burn-out experiments in which a branch pipe, connecting to a small vessel, was fitted close to the test section inlet. The test section itself was a uniformly heated tube 8 mm in diameter and 16 cm long. The results are reproduced in Fig. 9, and show burn-out flux plotted against exit steam quality. Curve (A) was obtained with the branch vessel filled with cold water, 

Many experimenters have adopted the practice of feeding a preformed mixture of steam and water to their test sections, either out of interest in this type of system or else to avoid the power demanded by long channels. The CISE Laboratories in Italy have produced a considerable amount of data of this kind (S4), and a typical example of their results is shown in Fig. 13. The curves have a characteristic swan-neck shape similar to the Russian data for unstable flow conditions shown in Fig. 9, and the burn-out flux values are generally below those for normal steady-flow conditions. 

The conclusion to be drawn from the above examples and many others is that softness in a boiling system, preceding the boiling channel inlet, may cause flow oscillations of low frequency. It is probably the pressure perturbations arising from the explosive nature of nucleate boiling that initiates the oscillation, and the reduced burn-out flux which follows probably corresponds to the trough of the flow oscillation, as a reduction in flow rate always drops the burn-out flux in forced-convection boiling. 

From the point of view of systematic data analysis, it has been found that consistent results can be obtained only with burn-out data produced under stable conditions. The unstable condition leads to considerable scatter, depending on the particular setting of a valve, the size of a compressible volume, the method of preforming a steam and water mixture, and so on. These latter quantities have either been recorded with very low accuracy, or have not been recorded at all. Therefore, the unstable-conditions data appear to be of little value, except for qualitative purposes. In any case, one is usually not interested in instabilities apart from knowing how to avoid them, which is by having a hard inlet. 

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With the important system-describing parameters identified, the next step is to find out if any of them are related in a simple way to the burn-out flux. A simple relationship, provided it is universally valid, helps considerably in... 

Thus far, the burn-out phenomenon has been discussed mainly in terms of the important system-describing parameters. This approach is preferable, since the system parameters are, in fact, the independent variables and they must uniquely and unequivocally determine the heat flux required to produce burn-out. It can be argued, however, that burn-out, being a local phenomenon, may be described entirely by local parameters of this there can be no dispute. The problem is to find a description of these local parameters that works. Our... 

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