A CRITERION BASED ON INDEPENDENT PARAMETERS FOR DISTINGUISHING DEPARTURE FROM NUCLEATE BOILING AND DRYOUT IN WATER-COOLED SYSTEMS

The critical heat flux is an important parameter for the design of ste am generators in conventional steam boilers and in water cooled nuclea r reactors; and reliable predictive correlations or models are require d for calculation and design purposes. Notwithstanding the large numbe r of studies which have been carried out in the past, at the authors' knowledge no criterion has been developed to forecast the occurrence o f a departure from nucleate boiling (DNB) or dryout type thermal crisi s on the basis of the fluid inlet parameters and the channel geometry. Therefore thermal-hydraulic designers have to rely on their judgement or on the knowledge of the nominal conditions of the system. The main aim of this work is to demonstrate that there exists the possibility of discriminating between DNB and dryout on the basis of independent p arameters. As a first-level approach, two CHF characters were defined, corresponding to the critical thermodynamic qualify achieved at the t hermal crisis x(cr): at quality lower than zero the crisis was assumed to be of DNB type, while al quality higher than zero the crisis was a ssumed to be of dryout type. According to the wide experimental eviden ce, the transition from DNB to dryout is a continuous one and depends on several parameters. In practical terms, the thermal crisis may be o f DNB type even with critical quality slightly higher than zero. There fore, a second approach was adopted which is based on the physically w ell justified assumption that when the critical thermodynamic qualify is lower than zero the thermal crisis is (almost certainly) of DNB typ e, while when the critical thermodynamic volumetric quality x(vol,cr) is higher than 50 % (i.e. the vapour phase is predominant over the liq uid phase) it seems reasonable to assume that the thermal crisis is of dryout type. This latter approach implies that a certain number of da ta cannot be classified, i.e. those data having x(cr) > 0 and x(vol,cr ) < 50 %. In this study, a comprehensive data base of 16 844 data poin ts was analysed with a synergetic approach based on a neural network m ethodology coupled with a physical thermal-hydraulic one. First a netw ork was trained based on all the available information, i.e. the fluid inlet conditions and the geometric and physical parameters (a total o f 11 input variables). Then the network structure was modified by redu cing or grouping the input parameters based on the thermal-hydraulic e xpertise and a new network was trained. The iteration of this process led to the identification of three parameters which seem to drive the physical phenomenon, i.e. the mass flux G, the inlet quality x(in), an d the ratio between the duct length and diameter L/D. The product G x( in) was considered as representative of the 'specific degree of subcoo ling' introduced in the duct. Starting from this consideration, an ana lytical criterion was found which is based only on two parameters and is able to classify the data points on the basis of the ratio (-G x(in )/G(0))/(L/D), where is G(0) a constant having the dimensions of a mas s flux. In the case of the first approach, this simple criterion allow ed us to correctly classify 97.1 % of data for the overall database (D B-I), and 99.3 % of data when a sub-group of consistent and accurately verified data was considered (DB-2). In the case of the second approa ch, the percentage of correct classifications was 98.4 and 99.1 % for DB-1 and DB-2 data, respectively. These results therefore seem to demo nstrate the possibility of distinguishing DNB from dryout on the basis of inlet parameters. The proposed criterion can be modified on the ba sis of a more detailed knowledge of the thermal crisis mechanism, but nevertheless the procedure adopted here can be entirely applied.