• Energy Research

Abstract In view of the importance of the condensation-induced water hammer in various engineering fields, this study performed an analytical modeling and numerical simulation to obtain the map of the onset of the condensation-induced water hammer. Two linear boundaries were experimentally observed for the transitions between no condensation-induced water hammer and non-periodic condensation-induced water hammer and between non-periodic condensation-induced water hammer and periodic condensation-induced water hammer. The analytical modeling based on the energy balance equation was conducted to derive the onset of the condensation-induced water hammer. The analytical model suggested that the map of the onset of the condensation-induced water hammer could be displayed by two essential non-dimensional parameters, such as modified Jakob number and Froude number. The model indicated that the slope of the boundary of the onset of the condensation-induced water hammer was the product of modified Stanton number, non-dimensional interfacial area concentration, and non-dimensional temperature difference. The model could predict the boundaries between no condensation-induced water hammer and non-periodic condensation-induced water hammer and between non-periodic condensation-induced water hammer and periodic condensation-induced water hammer satisfactorily. The one-dimensional nuclear thermal-hydraulic system analysis code, RELAP5, was used for simulating the onset of the condensation-induced water hammer. The obtained boundary corresponded to the experimentally obtained boundary between non-periodic condensation-induced water hammer and periodic condensation-induced water hammer.

highlights � The performance and flow field inside ejectors are studied numerically and experimentally. � The pressures before the second shock position remain constant during the critical mode. � NXP has an optimal value for entrainment ratio, but no effect on the critical discharged pressure. abstract The present paper performs experimental and numerical investigations on the global performance and internal flow of a supersonic air ejector. The effects of operation parameters and geometrical factor on the air ejector performance have been analyzed. The results show that: the static wall pressure and axisymmetric line static pressure remain constant in critical mode under different discharged pressures, but they both increase in sub-critical mode with the increase of the discharged pressure. The shock position of the mixed fluid moves upstream in critical mode. The second shock position disappears in sub-critical mode. The experimental and numerical results indicate that there exists an optimal nozzle exit position (NXP) corresponding to maximum entrainment ratio, but the critical value of discharged pressure is almost independent of NXP.

Abstract The condensation induced water hammer (CIWH) phenomenon may cause serious damage to the pipes and related system, which often occurs during the steam-water direct contact condensation process. In this paper, an experimental investigation was performed to study CIWH phenomenon caused by steam-water direct contact condensation in a horizontal pipe. The entire CIWH process was captured by a high speed video camera and its pressure fluctuation was synchronously measured. Four typical flow patterns were observed during CIWH process: stratification flow, wave flow, slug flow and bubble collapse. Bubble collapse would generate a high pressure peak. Based on different variations of steam-water phase interface and pressure fluctuation signals, three types of CIWH were defined: non-periodic CIWH, periodic CIWH and no CIWH. A CIWH region map was given considering the effect of steam mass flux and water temperature. In periodic CIWH region, the generation frequency of CIWH was found to range from 0.19 Hz to 0.39 Hz, which decreased with the rise of steam mass flux and water temperature. A dimensionless correlation was obtained to predict Strouhal number of CIWH generation frequency. Predicted values corresponded well to the experimental data with the deviation in the range of −16% to +23%.

Abstract Melt behavior in reactor pressure vessel (RPV) lower head is important for estimating melt leakage to reactor cavity (pedestal), molten core concrete interaction (MCCI) and radioactivity release to the environment. In order to expand universal knowledge of severe accident research on boiling water reactors (BWRs), the melt behavior in BWR lower head and leakage paths during hypothetical loss-of-coolant accident (LOCA) and station blackout (SBO) accident were analyzed with MELCOR 2.1. The instrument guide tube (IGT) failure, a potential lower head failure mechanism, was modeled in MELCOR code. When IGTs failure was not considered the lower head failed through creep rupture, resulting in the discharge of all the debris into the reactor cavity. Otherwise, the debris discharged out through the openings at the IGT locations, but a part of debris still remained in the lower plenum as solid state. The pressure vessel wall did not suffer from creep rupture after IGTs failure. The timing of events in SBO accidents were delayed compared with LOCAs, and the oxidation degree of core debris was higher in SBO accidents. In each case, only SS (stainless steel) and a small amount of Zr (zircaloy) were present in molten state in the lower plenum. Most of the discharged debris was in solid state.

Abstract The ejector refrigeration integrated in the air-conditioning system is a promising technology, because it could be driven by the low grade energy. In the present study, a theoretical calculation based on the real gas property is put forward to estimate the ejector refrigeration system performance under overall modes (critical/sub-critical modes). The experimental data from literature are applied to validate the proposed model. The findings show that the proposed model has higher accuracy compared to the model using the ideal gas law, especially when the ejector operates at sub-critical mode. Then, the performances of the ejector refrigeration circle using different refrigerants are analyzed. R290 and R134a are selected as typical refrigerants by considering the aspects of COP, environmental impact, safety and economy. Finally, the ejector refrigeration performance is investigated under variable operation conditions with R290 and R134a as refrigerants. The results show that the R290 ejector circle has higher COP under critical mode and could operate at low evaporator temperature. However, the performance would decrease rapidly at high condenser temperature. The performance of R134a ejector circle is the opposite, with relatively lower COP, and higher COP at high condenser temperature compared to R290.

The laminar and transition opposing mixed convection was experimentally investigated when the air flow was in the entrance region through a rectangular duct. The pressure drops were measured under different air flowrates, convective heat fluxes and inclination angles, with the maximum value less than 6.0 Pa. The friction factors were calculated and analyzed under different Reynolds numbers, Grashof numbers and inclination angles. The results indicated that the pressure drops increased with the mean air velocity, and the friction factors showed different characteristics when the Reynolds numbers were larger or smaller than 1500. Finally, the correlations of friction factors were fitted considering the influences of Reynolds number, Grashof number, inclination angle and length to diameter ratio, which fitted the experimental data with the deviations less than ±10.0%.

Abstract A steam–water injector (SI), which is a passive jet pump, has been widely used in various industries. In the present work, exergy analysis models of a centered water nozzle SI were developed based on experiments to evaluate the performance of SI. The thermodynamic perfect degree of SI was found to be not so bad, and exergy efficiency was within the range of 18%–45%. In general, SI is used to work as a pump. However, exergy analysis can only evaluate the thermodynamic perfect degree of SI, but cannot evaluate its lifting–pressure performance. Therefore, thermo–mechanical exergy was divided into temperature–based exergy and pressure–based exergy in the present work, and the pressure–based exergy analysis was introduced. Such analysis was determined to be more reasonable than exergy analysis for a passive jet pump and helpful for achieving optimal SI design. Moreover, the effects of physical and geometric parameters on exergy efficiency and pressure–based exergy efficiency were investigated experimentally, and several optimal values for these geometric parameters were found. Finally, the distributions of exergy losses in separate parts of SI, particularly inevitable exergy losses, were evaluated. These analyses will be helpful in eliminating the effects of inevitable factors and in identifying the key factor, thereby considerably improving SI performance.

Abstract The relatively low entrainment ratio is the key problem that confines the ejector development in the industry. In this study, the adjustable-geometry ejectors (with three different spindle positions) and ejectors with bypass (with two different bypass inlet installations) were adopted to promote the entrainment ratio of an ejector. The performance of the adjustable-geometry combined with bypass was also investigated. Results showed that the adjustable-geometry ejector and ejector with bypass could improve the entrainment ratio, and both ejectors reached a peak value. Compared with the original ejector, the maximum increment was 51.47% for the adjustable-geometry ejector and 26.0% for the ejector with bypass, respectively. Meanwhile, the combined ejector increased the entrainment ratio, although the combination was only effective at higher induced pressures than that of the adjustable-geometry ejector without bypass. The maximum increment was up to 74.5% compared to that of the original ejector.