[en] In the present work, after an overview of the main analytical results obtained by the authors on a two-region rewetting model a lot of experimental data relevant to the rewetting of metallic tubes by various coolant is considered in order to correlate theoretical and experimental results

[en] In boiling heat transfer, many vapors are generated on the heating surface in high heat flux region. If many vapors are stayed on the heating surface, they are collaborated and developed to a vapor film. As the result, the heating surface is covered with the vapor film and the critical heat flux would be decreased. On the rough heating surface, boiling bubbles do not detach completely from the surface. In this case, the fluid state on the surface is considered as a kind of non-equilibrium two phase flow. Authors investigated experimentally the effect of surface roughness on the boiling heat transfer. The heating surfaces with lattice grooves which had different depth and width were employed as typical examples of surface roughness, and the grooving angle was approximately 60 degrees at the top. The authors obtained experimental results on water boiling for the rough heating surface that the critical heat flux decreased below a half value of the usual critical heat flux and the heat flux increased monotonously with the increasing of superheat of the heating surface under some condition of heating surface. The roughness of the heating surface in boiling heat transfer gives considerably an effect on the critical heat flux and the characteristics of heat transfer. The experimental results are different from the common knowledge about boiling heat transfer for the surface roughness. According to the present research, authors need some additional treatment to the surface of nuclear fuel elements and the design of electronic circuits etc

[en] Calculations are described in which new models for deposition and entrainment in annual flow are applied to a variety of conditions in annular and related flows. These include steady adiabatic flow, flow with evaporation, flow with dryout and post-dryout regions, flow with condensation and transient flows. The transient cases considered included both excursions with a flow decrease (transient dryout) and excursions with a flow increase (transient rewetting). Wherever possible, the predictions were compared with experimental data; this showed that the new models gave improved predictions in every case. Although it was not possible to make comparisons with experimental data in the case of flow transients leading to rewetting, several new aspects of this process emerged from the calculations. Specifically, it was found that the rewetting rate could be governed either by the progress of a front at which the film flowrate is zero (the reverse of a transient dryout situation) or by a front whose velocity is limited by the sputtering phenomenon

[en] A recently developed, simple two-fluid model was applied to correlate the steady-state heat transfer data for inverted annular flow film boiling of water in a vertically mounted circular tube at pressure from 0.1 to 2.0 MPa. Within the ranges of mass flux from 50 to 530 Kg/(m²s), in let subcooling from 2 to 31 K, and Weber number of vapor less than 15, the average deviation of the predictions for the overall heat transfer coefficient, if compared to the 2829 available data points, was found to be 12.2%. As other applicable heat transfer correlations predict the same data base with three-fold larger average deviations, the present analytical model may be considered quite effective despite of the obvious simplicity of constitutive relations assumed. For further improvement, extension of the empirical data base to higher inlet subcoolings and mass fluxes in the pressure range from about 0.2 to 1.8 MPa would be most welcome

[en] Formation mechanism of the vapor mushroom generated from a boiling wall with a high heating power is studied. Its heat transfer mechanism is also investigated by studying the evaporation of the liquid macrolayer between the vapor mass and the heating wall. It is demonstrated that heat conduction across the macrolayer and evaporation at the free surface is not a mode efficient enough to account for the major portion of heat transfer. Alternatively, it is proposed that macrolayer is primarily consumed through evaporation of much thinner microlayers at the bottoms of vapor stems penetrating the macrolayer. Analysis based upon the proposed mechanism shows the macrolayer does not totally dry out in a vapor-mass cycle before boiling crisis. Other mechanisms accountable for the consumption of macrolayer are also discussed

[en] Mechanical non-equilibrium in the cross section of a two phase flow (boiling flow) has been suggested. This approach allows the explanation of the superheated thermodynamic state of the wall layer. The pressure difference can be simply calculated using the pressure of saturation corresponding to the know temperature of the heated surface. The authors gain valuable analytic equations for heat transfer DNB. It has been demostrated physically that DNB is caused by a sonic limit for the evaporation

[en] The present paper deals with the results of an experimental investigation on critical heat flux in forced convective flow boiling during transients caused by simultaneous variations of pressure, flow rate and/or thermal power. The three parameters were varied according to an exponential law for the flow rate and the pressure decrease, and to a ramp and a step law for the input power increase. Experiments were carried out employing a tubolar test section which was electrically and uniformly heated. Test parameters included the flow rate half-flow decay time, several values of the initial power (before the transient) and the final power (at the end of the transient) in the case of step transients, and the slope of the ramp in the case of ramp transients, and the depressurization rate. An analysis of the experimental data was performed using the local conditions approach, and applying the quasi-steady-state method. The effect of the simultaneous variation of two or three main parameters on the time-to-crisis was also analysed for transients in which only one of the parameters was varied

[en] In this work the CATHARE V1.3 code was applied to two Numerical Benchmark Exercises proposed in the aim to test the behaviour of thermalhydraulic codes against some spurious numerical effects. In the analysis of the first exercise, which is related to a two-phase flow along a convergent-divergent nozzle, it was seen that the CATHARE code well predicts the expected physical trends in good agreement with the majority of other codes. The application of the code to the second exercise, concerning the water packing phenomenon, showed the presence of this numerical effect. From a sensitivity analysis on time step and mesh size, it was seen that the amplitude of the pressure spikes can be reduced increasing time step and decreasing the space increments

[en] The mechanisms of nucleation have been analysed. Starting from the assumption that the activation of micro-cavities in the wall surfaces is the most probable nucleation mechanism in practical flashing system, the authors study in detail the nucleation in a micro-cavity. A three step nucleation criterion is proposed, namely: trapping cavity, activable cavity and active cavity. Then, a new nucleation model is presented. The output of the model is the prediction of the bubble departure frequency versus the thermodynamic state of the liquid and the geometry of the cavity. The model can also predict the nucleation site density if the nature of the wall and the surface roughness are know. The prediction have been successfully compared with some preliminary experimental results. By combining the present model with Jones'theory, the flashing inception is correctly predicted. The use of this nucleation model for the complete modelling of a flashing non-equilibrium flow is in progress

[en] The possibility of describing physical phenomena in the subcooled boiling region is of considerable importance to nuclear reactor technology and to the chemical process industry. In liquid cooled heat exchangers where high heat fluxes can be expected in a superheated boundary near the heated surface, bubbles beging to grow, although the mean enthalpy of the liquid has not reached the corresponding saturation temperature. They detach from the wall and move in to the liquid core, condense there in the surrounding subcooled water, contributing to the heating of the liquid. This process is characterized by a thermodynamic non-equilibrium