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To retrieve the fuel debris in Fukushima Daiichi Nuclear Power Plants (1F), it is essential to infer the fuel debris distribution. In particular, the molten metal spreading behavior is one of the vital phenomena in nuclear severe accidents because it determines the initial condition for further accident scenarios such as molten core concrete interaction (MCCI). In this study, the fundamental molten metal spreading experiments were performed with different outlet diameters and sample amounts to investigate the effect of the outlet for spreading-solidification behavior. In the numerical analysis, the moving particle full-implicit method (MPFI), which is one of the particle methods, was applied to simulate the spreading experiments. In the MPFI framework, the melting-solidification model including heat transfer, radiation heat loss, phase change, and solid fraction-dependent viscosity was developed and implemented. In addition, the difference in the spreading and solidification behavior due to the outlet diameters was reproduced in the calculation. The simulation results reveal the detailed solidification procedure during the molten metal spreading. It is found that the viscosity change and the solid fraction change during the spreading are key factors for the free surface condition and solidified materials. Overall, it is suggested that the MPFI method has the potential to simulate the actual nuclear melt-down phenomena in the future.

Critical heat flux (CHF) plays an important role as the upper limit of heat dissipation process during the in-vessel reactor external reactor vessel cooling (IVR-ERVC). IVR-ERVC is regarded as a very effective way to release the decay heat after the core melt. This severe accident mitigation countermeasure has already been applied in some advanced PWRs and is considered to be used in some advanced BWRs as well. In this paper, experiments of the macro-fin structure positioned on a slope with 5° and 10° downward inclination angle were carried out. The results of the copper bar experiments were then analyzed and compared with the results obtained from a copper bare block experiments which were conducted previously at the same test facility. It was found that the CHF of the finned structure decreased compared with that of the bare surface. This shows that the current CHF data of the bare surface cannot be used directly to the design of new advanced BWRs. The hot/dry spot theory designated for the copper bar was brought up with an attempt to explain the difference of copper bare block and copper bar. Results show that using this theory can explain the decrease of CHF of the copper bar.

Understanding the effects of property variations on velocity and temperature profiles is important for the accurate prediction of supercritical heat transfer in regenerative cooling channels. In th...

Accepted: 2021-08-23, 資料番号: PA2210027000

A full scale-resolving simulation of cascades flutter is time consuming because of computational inefficiency owing to its low nondimensional frequencies. To improve the efficiency and reliability of the numerical analyses for such flows, we propose an efficient scale-resolving simulation method dedicated to time-periodic flows by extending the harmonic balance approach to a large-eddy simulation. This method combines convergence calculations of the steady-state problem based on the harmonic balance method for periodic components, and the nonlinear time-marching method for small scale turbulent fluctuations. Using the proposed method, deterministic periodic components and stochastic turbulent fluctuations are calculated simultaneously, and the effect of turbulent fluctuations on deterministic periodic components is directly calculated without using turbulence models. In this paper, we present the algorithm of the simulation technique and the progress of validation calculations for channel flow excited in the streamwise direction.

Due to the important role critical heat flux (CHF) plays in the boiling field, it is of great significance to study CHF, especially the mechanism of CHF in the nucleate boiling. In this stu...

The containment spray system is an accident mitigation system used in nuclear reactors that reduces the high temperature and pressure generated inside the containment vessel during reactor accidents by condensing the steam and cooling the gas atmosphere. However, the heat and mass transfer mechanisms of flying spray droplets in a surrounding gas mixture have not yet been fully understood. To investigate these phenomena at a fundamental level, it is critical to obtain detailed temperature and flow distributions inside a water droplet as it descends. Currently, little experimental data has been reported detailing temperature and flow measurements inside water droplets, especially at the beginning of droplet evaporation, which contains highly transient exchange phenomena. In this study, the temperature and flow distributions were measured inside a 2-mm pendant water droplet during the initial period of its evaporation in an upward air stream using temperature-sensitive particles. Measurements were conducted once per 15 ms to determine variations in the droplet internal flow patterns and once per 0.1 s to determine the simultaneous temperature and flow characteristics. The flow distributions inside the droplets were measured using particle image velocimetry, and the temperature distributions inside the water droplets were measured simultaneously using the temperature-dependent phosphorescence decay constant. Droplet internal flow patterns were measured during a flow development period of 90 ms, the droplet internal flow development in a 20 °C air stream and 100 °C air stream were compared, and temperature and flow distributions were measured simultaneously inside evaporating droplets at different air stream temperature conditions within the first 1 s of droplet evaporation. The results indicated that the droplet internal flow development in a hot air stream is different than that in a room temperature air stream due to the Marangoni convection induced by the temperature gradient on the droplet surface under high temperature conditions. Owing to the resulting enhancement of heat and mass exchange within the droplets, the heat and mass transfer between the droplets and the surrounding gas can be influenced.

The spreading-solidification behaviour of the stratified molten metals was investigated. This is important in understanding the practical fuel debris distribution spread and solidified in the primary containment vessel (PCV) of Fukushima Daiichi Nuclear Power Plants (1F NPPs). In this study, the effect of outflow diameter on the material distribution before discharging was studied both experimentally and numerically. The two simulant metals were chosen so that the density ratio could be similar to the practical fuel and structure elements of the plant. They were arranged in a vessel and discharged on a receiving plate. The spreading experiments were performed using various outlet diameters with a density and reverse density stratification arrangement. After the experiment, X-ray analysis was performed to evaluate the material distribution in the solidified material. Moreover, a numerical analysis was performed to reveal the mechanisms that affect the material distribution after solidification. As a result, the low-density metal accumulated at the centre region regardless of the outlet diameters in the density stratification. In contrast, the outlet diameters affected the material distribution in the reverse density stratification because they affected the material outflow order. These findings may help increase our understanding of the fuel debris distribution in 1F NPPs.

Large Eddy Simulation (LES) has already been widely applied for unsteady flow simulations (e.g., axial cascade simulation), showing its superiority in the case of turbulent flow fields. However, there are several obstacles in the practical use of LES as a cascade design tool for gas turbine engines. Several calculation parameters such as the inlet turbulence, spanwise extents, and many other boundary conditions, affect calculation results for LES simulation and can cause reliability problems. This study concentrates on investigating the effects of LES calculation parameters on the prediction of profile loss at high air inlet angle off-design condition. The LES flow field results were found to be sensitive to four parameters (i.e., inlet turbulence, spanwise domain extent, time step size, and outlet domain). The inlet turbulence affects the size of the laminar separation bubble, while the spanwise domain extent affects the velocity fluctuations of the wake in correspondence of the large-scale vortices shedding. The time step size affects the velocity fluctuations of the wake because of the number of times that the low-pass filter is applied. Finally, the outlet domain extent affects the whole calculation domain (from the leading edge to the wake). Overall, the results show that all four calculation parameters can change the flow characteristics, affecting the prediction of profile loss at off-design condition when using LES.

For an adequate understanding of the broadband acoustic phenomena generated by a rocket exhaust jet impinging on a flame deflector, this study experimentally clarifies the factors that cause the di...

In this study, effects of a finned structure on downward-facing flow boiling were investigated. Compared to a bare surface, a heated surface with a finned structure has a higher CHF and heat transfer coefficient. Two high-speed cameras were utilized to observe boiling behaviors from the downward view and side view. Under low heat flux conditions, there existed a front zone and wake zone due to the existence of the extended heating surface, which was confirmed by a numerical simulation. The bubbles tended to escape from two sides of the extended surface. Under high heat flux conditions, two vapor films formed due to the existence of the finned structure, allowing more water to be supplied to the surface. Meanwhile, the existence of the fin enhanced the turbulence and made the flow field velocity distribution non-uniform, thus enhancing heat transfer and CHF. The maximum vapor film thickness of finned surface was larger than that of the bare surface. A larger maximum volume of vapor generating on the surface suggested a larger amount of latent heat consumption and thus, leading to a bigger CHF. These results demonstrated the potential of CHF enhancement using the finned structure design.

Since the 1980s, numerous experiments have been performed to explore the mechanisms of molten core–concrete interaction (MCCI) phenomena. However, previous experimental results show uncertainties pertaining to several aspects such as the dependence of concrete type on the ablation profile (isotropic for limestone-rich concrete and anisotropic for siliceous concrete). To explore the mechanism governing these aspects as well as to predict MCCI behavior in real severe accident events, a number of simulation codes have been developed for process calculation. However, uncertainties exist in the codes because of the use of different empirical models. In this study, a two-dimensional computational code was developed using multi-physics models to simulate MCCI phenomena based on the moving particle semi-implicit (MPS) method. In contrast to previous studies, gas generation was simulated by calculating the force balance of the bubbles in the liquid pool in order to investigate its influence on the ablation behavior. Moreover, chemical reaction was modeled by calculating the heat release from the redox reaction between the corium and melted concrete during the erosion process. CCI-2 and CCI-3 tests were simulated by applying this code to study the aspects that may affect the ablation profile with different concrete types. The simulation result generally matches the experimental results on both axial and radial ablation rates. Application of the gas release and chemical reaction model suggested that one possible reason for the anisotropic ablation profile observed in the siliceous concrete tests is the heat transfer enhancement near the sidewall from the gas release and chemical reaction. In contrast, the test using limestone-rich concrete generated an isotropic ablation profile because axial ablation was also enhanced by the large gas discharge from the bottom concrete. In addition to the ablation profile, the crust formation and concrete–corium interface profile at the bottom in the CCI-3 test were reproduced with the help of the proposed models.

Due to the important role critical heat flux (CHF) plays in the nucleate boiling field, it is of great significance to study CHF, especially the mechanism of boiling crisis occurrence. In this study, an extended numerical model to predict CHF for flow boiling was proposed based on the Monte Carlo method. This model considers of the bubble generation, bubble growth and bubble departure. CHF was assumed based on the percolative nature of bubble interactions. Numerical simulation results showed good agreement with the experimental results, which demonstrated that this model was able to be used to predict CHF. Further works are needed to apply this model to a wider range of experiments and other working conditions.

The difference in the amount of molten corium and outlet diameter influences the corium spreading, which is essential to enhance the safety of nuclear power plants. To investigate this phenomenon, in this work, the influence of the scale relation between the amount of molten metal and the outlet diameter (L/D) on the corium spreading was investigated. Experiments involving the spreading of a molten corium simulant on a stainless-steel plate were performed under various nozzle sizes and sample amounts. The spreading behavior during the experiment was recorded via a video camera. Based on the obtained experimental data, a dimensionless analysis was performed to determine the dominant parameters responsible for the spreading phenomena, and spreading models were developed to infer the solidified area and thickness. The results indicated that the spreading mode may vary at approximately L/D = 6, owing to the change in the force balance represented by the Weber and Reynolds numbers. The developed spreading model considering both the length and thickness values can help understand and predict the spreading of the fuel debris in the first floor.

It is important to clarify the molten material jet breakup process to estimate corium behavior in the lower plenum of a boiling water reactor (BWR). To identify the effect of control rod guide tubes (CRGTs) on the jet breakup behavior, a molten material (U-alloy) breakup experiment considering CRGTs in a BWR lower plenum was conducted under isothermal conditions. The experiment results show that jet breakup fraction for the case with CRGTs (pitch/diameter ratio of 1.37) was only approximately 20 % of that for the case without CRGTs. A coarser pitch/diameter ratio of 2.47 was also tested, but this configuration only slightly reduced the amount of jet breakup. The experiments also indicate that the CRGTs had no significant effect on the fragmentation of droplet diameter. Furthermore, the velocity distribution was measured around the jet with particle image velocimetry. The velocities of the water surrounding the jet for the cases with CRGTs were relatively larger than those in the case without CRGTs.

Thermal stratification in the suppression pool of the Fukushima Daiichi nuclear power plants was experimentally investigated in sub-atmospheric pressure conditions using a 1/20 scale torus shaped setup. The thermal stratification was reproduced in the scaled-down suppression pool and the effect of the steam flow rate on different thermal stratification behaviors was examined for a wide range of steam flow rates. A sparger-type steam injection pipe that emulated Fukushima Daiichi Unit 3 (F1U3) was used. The steam was injected horizontally through 132 holes. The development (formation and disappearance) of thermal stratification was significantly affected by the steam flow rate. Interestingly, the thermal stratification in the suppression pool vanished when subcooling became lower than approximately 5 °C. This occurred because steam bubbles are not well condensed at low subcooling temperatures; therefore, those bubbles generate significant upward momentum, leading to mixing of the water in the suppression pool.

Numerous experiments have been performed to explore the mechanisms of molten core-concrete interaction (MCCI) phenomena since the 1980s. However, previous experimental results show that uncertainties pertaining to several aspects such as the mixing process and crust behavior remain. To explore the mechanism governing such aspects, as well as to predict MCCI behavior in real severe accident events, a number of simulation codes have been developed for process calculations. However, uncertainties exist among the codes because of the use of different empirical models. In this study, a new computational code is developed using multiphysics models to simulate MCCI phenomena based on the moving particle semi-implicit (MPS) method. Momentum and energy equations are used to solve the velocity and temperature fields, and multiphysics models are developed on the basis of the basic MPS method. The CCI-2 experiment is simulated by applying the developed code. With respect to sidewall ablation, good agreement is observed between the simulation and experimental results. However, axial ablation is slower in the simulation, which is probably due to the underestimation of the enhancement effect of heat transfer provided by the moving bubbles at the bottom. In addition, the simulation results confirm the rapid erosion phenomenon observed in the experiment, which in the numerical simulation is explained by solutal convection provided by the liquid concrete at the corium/concrete interface. The results of the comparison of different model combinations show the effect of each multiphysics model on ablation behavior.

New BWR containment designs are considering cavity flooding as an accident management strategy. Unlike the PWR, the BWR has many Control Rod Guide Tube (CRGT) penetrations in the lower head. During a severe accident scenario with core melt in the lower plenum along with cavity flooding, the penetrations may affect the heat transfer on the ex-vessel surface and disrupt fluid flow during the boiling process. A small-scale experiment was performed to investigate the issues existing in downward-facing boiling phenomenon with an extended surface. The results were compared with a bare (flat) surface. The mass flux of 244 kg/m 2 s, 215 kg/m 2 s, and 177 kg/m 2 s were applied in this study. CHF conditions were observed only for the 177 kg/m 2 s case. The boiling curves for both types of surfaces and all flow rates were obtained. The boiling curves for the highest flow rate showed lower surface temperatures for the extended surface experiments when compared to the bare surface. The downstream location on the extended surface yielded the highest surface temperatures as the flow rate was reduced. The bubble accumulation and low velocity in the wake produced by flow around the extended surface was believed to have caused the elevated temperatures in the downstream location. Although an extended surface may enhance the overall heat transfer, a reduction in the local heat transfer was observed in the current experiments

This study aims to suggest a dimensionless number to determine the formation and disappearance of thermal stratification induced by direct contact condensation in a 1/20 scaled-down suppression pool of the Fukushima Daiichi nuclear power plant. The modified Richardson number, which represents the ratio of buoyancy force to inertia of steam, was employed to explain the thermal stratification in the suppression pool. Steam condensation experiments were performed at both sub-atmospheric and atmospheric pressures. A blow-down pipe with an inner diameter of 12.7 mm was used to inject and condense steam in the suppression pool. The effects of steam mass on the thermal stratification behaviors were examined by varying the steam mass flow rate (mass flux) from 0.50 kg/hr (1.10 kg/m2·s) to 2.50 kg/hr (5.48 kg/m2·s). The steam condensation was visualized using a high-speed camera. In the results, thermal stratification was successfully reproduced in this study, and it was observed that the behaviors of thermal stratification (vertical temperature profile) was affected by the steam mass flow rate and the subcooling temperature. The Richardson number was evaluated by measuring the size and frequency of steam bubbles. The time evolution of the Richardson number was analyzed with vertical temperature profiles of water in the suppression pool. The critical Richardson number, which determines the formation and disappearance of thermal stratification, was suggested to be of the order of 1.

Accepted: 2014-11-04, 資料番号: SA1150163000

Flow-induced acoustic resonance in a piping system containing closed tandem side branches was investigated experimentally. Velocity perturbation was induced at the mouth of a cavity by using two pumps and a block. An unusual acoustic mode change from a higher to a lower mode was observed when the flow rate in the main pipe increased. This phenomenon was examined by high time-resolved particle image velocimetry (PIV). It was found that fluid flows at different points in the cavity have some phase differences between them and that they interact with each other. The relationship between the fluid flows was clarified and the phase difference between the acoustic pressure fluctuations at different points in the cavity was obtained. Using these data, the phase delays of oscillations at different points were two-dimensionally mapped. This two-dimensional phase map was helpful in clarifying the feedback mechanism of self-induced vibrations.

Due to complexity of direct contact condensation (DCC), it is difficult to predict the thermal hydraulic phenomenon in a suppression pool (SP) of LWRs. Especially, the momentum, induced at condensation interfaces, depends on several interrelated parameters such as the steam mass flux, subcooling, and the diameter of the injection nozzle. Complicated interaction of those parameters creates difficulties in developing a comprehensive analytical model, which applicable to various conditions. To investigate the criteria of thermal stratification created by DCC, experiments were performed using a downsized suppression pool. Time resolved temperatures were acquired by vertically aligned thermocouples. Additionally, steam bubbles were visualized by a high speed camera in order to examine bubble shapes according to the mass flux and subcooling. Both steam bubble frequency and amplitude were analyzed for different DCC regimes. Finally, Richardson number was chosen as a suitable parameter for the dimensional analysis of experimental results. Corresponding velocity at far field in synthetic jet theory was employed to calculate Richardson number. The criteria for the occurrence of the thermal stratification were clearly determined according to the Richardson number.

Droplet entrainment and deposition are a couple of significant mechanisms for the heat transfer in annular two-phase flows existing in some heat exchange systems. The basic physics include the peeling of droplets from the liquid film due to high friction with the gas phase and the collision of droplets with the liquid film or deposition into the liquid film. Droplet deposition particularly plays a crucial role in the course of film dryout events which might have a vital importance for particular systems. In this study, a new numerical method (named as VOF–MPS hybrid method) based on the moving particle semi-implicit (MPS) method was developed to analyze the droplet deposition onto a stagnant thin liquid film. That proposed method combines the volume of fluid (VOF) solver of the open-source CFD code OpenFOAM with the MPS method. VOF–MPS technique introduces the surface tension force calculation of VOF model into the MPS method. MPS method formerly employed the continuum surface force (CSF) approach based on the particles. Three-dimensional (3D) VOF–MPS simulation with the novel surface tension modeling addition to the MPS-based modeling provides a smoother liquid–gas interface on the crown formed after the impact. Droplet deposition experiments were also carried out for the validation and comparison of two models. VOF–MPS method could predict the crown parameters such as crown thickness relatively better than the MPS method employing the CSF. However, the instabilities formed at the tip of the crown, observed in the experiments, could not be resolved with both methods.

Numerical simulation and experimentation using high time-resolved particle imaging velocimetry were conducted in order to clarify the flow characteristics in a small-scale feedback fluidic oscillator operated under low-Reynolds-number water flow. Flow oscillations were triggered by the Coanda effect and a periodically oscillating jet. Based on spectral analysis of the velocity fluctuations inside the oscillator body, the Strouhal number was not constant and the frequency was not linear with respect to the flow rate for the Reynolds number range from 238 to 714. The flow rate fluctuations of the feedback flows through the two feedback channels were determined quantitatively. By comparing the simulation and experimental results, the mechanism of the small-scale feedback fluidic oscillator was clarified by considering the time for a jet to oscillate from the center of the control region to either of the attachment walls and the time for a jet to oscillate from either of the attachment walls to the center of the control region separately.

To improve internal combustion engines, it is critical to measure the temperature distribution of their walls. However, only a few studies have quantitatively visualized the instantaneous temperature distribution of a wall exposed to flames. Lifetime-based temperature imaging was performed using a simple measurement system consisting of a non-intensified high-speed CMOS camera, a pulsed UV laser, and a phosphor coated quartz glass covered with a thin metal film. The uncertainty in the temperature measurement was ±2.0 °C for measurements of uniform temperature fields in the range 40–250 °C. Single-shot wall temperature measurements were demonstrated for combustion by flames in an engine cylinder. This enabled inhomogeneous temperature distributions in the engine with or without flames to be visualized. Even under motoring conditions, there was a large temperature gradient across the side window at the highest position of the piston. The side window temperature decreased near the intake valve. The intake gas flow had a cooling effect and the intake pressure affected the temperature distribution. The engine speed and the coolant temperature increased the magnitude of temperature but they did not affect the overall temperature distribution, i.e. the spatial distribution and the spatial gradient of the temperature. The instantaneous temperature of the top of the piston was visualized in the transient process after firing had commenced.

Fully resolved large eddy simulation (LES) is applied to two simple geometry flowfields with well-defined boundary conditions. The LES results are compared with simulations based on a Reynolds-averaged Navier-Stokes (RANS) model with turbulence, and pros and cons of using high-resolution LES for turbomachinery flows are discussed. One flow is a linear compressor cascade flow composed of the tip section of GE rotor B at Rec = 4 × 105 with a clearance, and the other is a Mach 1.76 supersonic turbulent boundary layer at Reδ = 5000 that laminerizes through a 12-degree expansion corner. The grids are prepared fine enough to resolve the turbulent boundary layer through a grid sensitivity study. The liner cascade result shows that all the turbulent shear layers and boundary layers including those in the small tip clearance are well resolved with 800 million grid points. The Reynolds stress derived from the LES results are compared directly with those predicted from the Spalart-Allmaras one-equation RANS turbulence model. The two results agreed qualitatively well except for the shear layer surrounding the tip leakage vortex, demonstrating that the RANS model performs well at least for flowfields near the design condition. From the simulation of the turbulent boundary layer experiencing sudden expansion, noticeable decreases of both Reynolds stress and local friction coefficient were observed, showing that the turbulent boundary layer has relaminarized through the sudden expansion. The boundary layer downstream of the expansion exhibits a nonequilibrium condition and was different from the laminar boundary layer.

For an adequate understanding of the broadband acoustic phenomena generated by a rocket exhaust jet impinging on a flame deflector, this study experimentally clarifies the factors that cause the di...

5th European Conference for Aeronautics and Space Sciences, EUCASS 2013 (July 1-5, 2013.), Munich, Germany, "Many of liquid rocket engines used in first stage of launch vehicles operate at a pressure above the critical pressure of oxygen, 5.04 MPa. The cryogenic oxygen injected into a combustion chamber is heated up to several thousand kelvin, therefore the high-density cryogenic oxygen necessarily experiences sudden expansion called “pseudo-boiling”1 at the pseudo-critical temperature. The pseudo-boiling has large impact on the mixing characteristics, hence a jet injected from a temperature below the pseudo-critical temperature must be distinguished from that injected from a temperature above the pseudo-critical temperature. The former is referred to as “transcritical jet” while the latter is simply called supercritical jet. Quantitative measurement data of transcritical jets are indispensable both for the better understanding of mixing phenomena and for the validation of numerical methods, but there are only few quantitative experimental data made public.2 In this report, axial and radial temperature profiles of a single phase cryogenic nitrogen jet under supercritical pressure are measured, and the influence of pseudo-boiling upon the jet mixing will be discussed.", 資料番号: PA1510093000

A visualization study was performed to investigate the flow of an underexpanded nitrogen gas jet injected into water. The stagnation pressure was varied in the range 0.5–8.0 MPa. The gas jet length and expansion angle were obtained from time-averaged images captured using a high-speed camera. The gas jet length and expansion angle increased approximately linearly with increasing stagnation pressure. The entrainment velocity and the velocity of entrained water droplets in the gas jet were obtained by particle image velocimetry.

The phenomenon of flow-induced vibration of bluff bodies has been studied extensively. The vast majority of these studies have concentrated solely on one degree-of-freedom oscillation in the inline or cross-flow directions. Herein, experiments were carried out with a cylinder in a water channel with two degrees-of-freedom. The cylinder was cantilever mounted with a low natural frequency (typically 65 Hz) in the inline and cross-flow directions. The Reynolds number fell in the range 1.17 × 10 3 Re 4 . The oscillating frequency of the cylinder and the surrounding flow were measured simultaneously using high temporal resolution particle image velocimetry (PIV), which is non-intrusive with respect to the flow and has high spatial and temporal resolutions. The vibration of the cylinder was found to be anisotropic. There was a discrepancy between the vibration frequencies in the inline and cross-flow directions, the difference being a function of reduced velocity.

The design of a wave rotor requires an understanding of the pressure wave dynamics in the cells (rotor passages). The present paper describes a two-dimensional numerical simulation and an experimental visualization of the wave rotor compression process. First, a unique experimental apparatus with fixed cells and rotating ports was constructed for the visualization and direct measurements; this arrangement is opposite to the conventional setup. Next, experimental and numerical results were compared to verify the simulation modeling, particularly with regard to the propagation velocity of pressure waves in the cells. Last, the effects of gradually opening the cell to the ports and leakage through the clearance, which are considered to be dominant factors in the wave rotor operation, on the pressure wave dynamics were carefully investigated. The results showed that the gradual passage opening greatly influences the primary shock wave, whereas leakage mostly influences the reflected shock wave. Moreover, it was revealed that the leakage generates an extra pressure wave during the compression process due to the interaction between adjacent cells.

The estimation of pressure waves generated in the cells or rotor passages is a crucial factor in wave rotor design. However, it is difficult to estimate the pressure wave analytically, for example, by the method of characteristics, because the mechanism of pressure wave generation and propagation in the cells is extremely complicated as compared with that in a shock tube. In this study, a simple numerical modeling scheme was developed to facilitate the design procedure. This scheme considers three dominant factors in the loss mechanism (gradual passage opening, wall friction, and leakage) for simulating the pressure waves precisely. The numerical scheme itself is based on the one-dimensional Euler equations with appropriate source terms to reduce the calculation time. The modeling of these factors was verified by comparing the results with those of a two-dimensional numerical simulation; the results were validated by the experimental data in our previous study. With regard to wave rotor miniaturization, the leakage flow effect that involves the interaction between adjacent cells was investigated extensively. A port configuration principle was also examined and analyzed in detail to verify the applicability of the present numerical modeling scheme to the wave rotor design.

Cryogenic single round and coaxial jets were observed under subcritical and transcritical conditions using a high-speed camera to explore the effects of the single-phase-like and two-phase-like properties of transcritical fluids on the unsteady behavior and flow structures of the jets. Cryogenic nitrogen and gaseous nitrogen were used as injectants. Cryogenic nitrogen was clearly visualized in all cases as the dark core. In the cases of single round jets, the large-scale characteristics of the jet behavior observed, such as the width of the dark core and the length scale of wave development in the mixing layers, were similar to those observed under subcritical conditions. Furthermore, dense nitrogen in the mixing layer entrained into the ambient environment and stretched in a manner similar to that observed under subcritical conditions. However, in contrast to the droplet formation observed under subcritical conditions, the entrained dense nitrogen diffused in a manner similar to turbulent mixing in a variable-density single-phase mixing layer. This means that the microscale behaviors that occur under transcritical conditions, which are comparable to turbulent eddies, are similar to those that occur in variable-density single-phase jets. In the cases of coaxial jets, the turbulent-eddy-scale phenomena observed were also similar to those observed for variable-density single-phase jets. However, in contrast to the phenomena observed for single round jets, the larger-scale behavior of the dark core was different from that observed under subcritical conditions. Two significant features were observed only under transcritical conditions. The first feature was the formation of shear-layer instability waves near the injector exit. The second feature was the shedding of dense nitrogen lumps from the end of the dark core at nearly uniform intervals. Because similar behavior was not observed under subcritical conditions, the single-phase-like properties of transcritical fluids are considered to have induced these features. The present correlation between the dark core length and the outer-to-inner jet momentum flux ratio was found to be in good agreement with the empirical correlation for variable-density single-phase coaxial jets., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1610078000

The dynamic (time resolved) PIV (particle imaging velocimetry) measurement technique was applied to high-speed gas flow in a narrow channel with an obstacle. The boundary layer was visualized with a high-speed APX RS camera and an Nd:YLF high repetition double-pulse laser. Nitrogen gas seeded with oil particles using Laskine nozzle flows through a 10 ? 10 mm2 square channel with Reynolds numbers of 11?000 and 34?000. Although a sufficient quantity of images was difficult to capture for the Re = 34?000 flow to visualize the vortex evolution in time for the time resolved analysis of the boundary layer, large scale structures of turbulence at the edge of the thin plate are clearly visualized in the temporal domain. Fluctuation transfer velocities in the boundary layer were measured employing the whole field two-point velocity correlation. It is proposed that dynamic PIV can open a way of measuring the fluctuation transfer velocities in the whole flow target area simultaneously for high-speed turbulent flows even in small scales.

The design of a wave rotor requires an understanding of the pressure wave dynamics in the rotor passages. The present paper describes a two-dimensional numerical simulation and an experimental visualization of the wave rotor compression process. First, a unique experimental apparatus with fixed cells and rotating ports was constructed for visualization and direct measurements; this arrangement is opposite to the conventional setup. Next, experimental and numerical results were compared to verify the simulation modelling, particularly with regard to the propagation velocity of pressure waves in the passages. Lastly, the effects of gradually opening the passage to the ports and leakage through the clearance, which are considered to be dominant factors in wave rotor operation, on the pressure wave dynamics were carefully investigated. The results showed that the gradual passage opening greatly influences the primary shock wave, whereas the leakage mostly influences the secondary (reflected) shock wave. Moreover, it was revealed that the leakage generates an extra pressure wave during the compression process due to the interaction between adjacent passages.