{"id":20871,"date":"2019-01-21T17:18:08","date_gmt":"2019-01-21T17:18:08","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=20871"},"modified":"2023-02-18T08:36:09","modified_gmt":"2023-02-18T08:36:09","slug":"boiling-crisis-critical-heat-flux","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/heat-transfer\/boiling-and-condensation\/boiling-crisis-critical-heat-flux\/","title":{"rendered":"Boiling Crisis – Critical Heat Flux"},"content":{"rendered":"
This chapter will study flow boiling<\/strong> in a vertical heated channel. The boiling and heat flux curve regimes are similar (not the same) as those in pool boiling<\/strong><\/a>. The process also occurs in modern high-pressure forced circulation boilers.<\/p>\n <\/a>The pioneering work on pool boiling was done in 1934 by\u00a0S. Nukiyama<\/strong>, who used electrically heated nichrome and platinum wires immersed in liquids in his experiments. Nukiyama was the first to identify different regimes of pool boiling<\/strong> using his apparatus. He noticed that boiling takes different forms, depending on the value of the wall superheat temperature\u00a0\u0394T<\/strong>sat<\/sub><\/strong> (also known as the excess temperature), <\/strong>defined as the difference between the wall temperature,\u00a0Twall<\/sub><\/strong>, and the saturation temperature,\u00a0Tsat<\/sub><\/strong>.<\/p>\n Four different boiling regimes \u00a0of pool boiling (based on the excess temperature) are observed:<\/p>\n These regimes are illustrated on the\u00a0boiling curve<\/strong>\u00a0in the figure, which is a plot of\u00a0heat flux\u00a0<\/a>versus the excess temperature. Although the boiling curve given in this figure is for water, the general shape of the boiling curve remains the same for different coolants. Note that the specific shape of the curve also depends on the system parameters, such as the pressure<\/a> and coolant flow rate. Still, it is practically independent of the geometry of the heating surface. The curve will be different for flow boiling as in the fuel channel, but major results will be similar.<\/p>\n In\u00a0<\/strong>BWRs<\/strong><\/a>, coolant boiling occurs at\u00a0normal operation,<\/strong> and it is a very desired phenomenon. Typical\u00a0flow qualities<\/strong>\u00a0in\u00a0BWR cores<\/strong><\/a>\u00a0are on the order of 10 to 20 %.<\/p>\n Although the earliest core designs assumed that surface boiling could not be allowed in\u00a0PWRs<\/strong><\/a>, this assumption was soon rejected. Two-phase heat transfer is now one of the normal operation heat transfer mechanisms in PWRs.<\/p>\n In both designs, the\u00a0nucleate boiling heat flux<\/strong> cannot be increased indefinitely. We call it the \u201ccritical heat flux<\/strong>\u201d (CHF<\/strong>) at some value. The steam produced can form an insulating layer over the surface, deteriorating the heat transfer coefficient. Dynamic changes of boiling regime associated with exceeding the critical heat flux are widely known as \u201cboiling crisis\u201d.<\/p>\n The\u00a0boiling crisis<\/strong>\u00a0can be classified as:<\/p>\n But the\u00a0critical heat flux<\/strong>\u00a0is used for both regimes.<\/p>\n Note that the opposite phenomenon to DNB is known as return to nucleate boiling (RNB)<\/strong> and occurs at point D, known as the Leidenfrost point<\/a>.<\/p>\n <\/a>As was written, in nuclear reactors<\/a>, limitations of the local heat flux<\/strong> are of the highest importance for reactor safety. There are thermal-hydraulic phenomena for pressurized water reactors<\/a> and boiling water reactors<\/a>, which cause a sudden decrease in heat transfer efficiency<\/strong>\u00a0(more precisely in the heat transfer coefficient<\/strong>). These phenomena occur at a certain value of heat flux, known as the \u201ccritical heat flux<\/strong>\u201d. The phenomena that cause heat transfer deterioration are different for PWRs and BWRs.<\/p>\n In both types of reactors, the problem is more or less associated with departure from nucleate boiling. The nucleate boiling heat flux cannot be increased indefinitely, and we call it the \u201ccritical heat flux<\/strong>\u201d (CHF<\/strong>) at some value. The steam produced can form an insulating layer over the surface, deteriorating the heat transfer coefficient. Immediately after the critical heat flux has been reached, boiling becomes unstable, and film boiling occurs. The transition from nucleate boiling to film boiling is known as the \u201cboiling crisis<\/strong>\u201d. As was written, the phenomena that cause heat transfer deterioration are different for PWRs and BWRs.<\/p>\n <\/a>In the case of PWRs<\/a>, the critical safety issue is named DNB<\/strong> (departure from nucleate boiling<\/strong>), which causes the formation of a local vapor layer<\/strong>, causing a dramatic reduction in heat transfer capability. This phenomenon occurs in the subcooled or low-quality region. The behavior of the boiling crisis depends on many flow conditions (pressure, temperature, flow rate). Still, the boiling crisis occurs at relatively high heat fluxes and appears to be associated with the cloud of bubbles adjacent to the surface. These bubbles or films of vapor reduce the amount of incoming water. Since this phenomenon deteriorates the heat transfer coefficient and the heat flux remains, heat accumulates <\/strong>in the fuel rod, causing the dramatic rise<\/strong> of cladding and fuel temperature<\/strong>. Simply, a very high-temperature difference is required to transfer the critical heat flux being produced from the surface of the fuel rod to the reactor coolant (through the vapor layer).<\/p>\n In the case of PWRs, the critical flow is inverted annular flow<\/strong>, while in BWRs, the critical flow is usually annular flow. The difference in flow regime between post-dry-out flow and post-DNB flow is depicted in the figure. In PWRs<\/strong> at normal operation,<\/strong> the flow is considered to be single-phase. But a great deal of study has been performed on the nature of two-phase flow<\/strong> in case of transients and accidents<\/strong> (such as the loss-of-coolant accident \u2013 LOCA or trip of RCPs<\/strong>), which are of importance in reactor safety and in must be proved and declared in the Safety Analysis Report<\/strong> (SAR).<\/p>\n One of the key safety requirements of pressurized water reactors is that a departure from nucleate boiling (DNB) will not occur during steady-state operation, normal operational transients, and anticipated operational occurrences (AOOs). Fuel cladding integrity will be maintained if the minimum DNBR remains above the 95\/95 DNBR limit for PWRs ( a 95% probability at a 95% confidence level). DNB criterion is one of the acceptance criteria in safety analyses as well as it constitutes one of the safety limits in technical specifications.<\/p>\n An important duty of the plant operator is to control plant parameters such that a safe margin to DNB<\/strong> (or distance from DNB on the heat transfer curve) is maintained. Any sudden, large change in the following plant parameters\/directions will decrease the margin to DNB:<\/p>\n Therefore, the function of the operators and the plant design is to prevent a sudden, large change in these plant parameters.<\/p>\n\n
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Critical Heat Flux<\/h2>\n
Departure From Nucleate Boiling – DNB<\/h2>\n
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