Flow in a blower casing which has no diffuser part and large tongue clearance was studied, The region where pressure rise occurs and the rate of rise were examined by static pressure measurement. Pressure rise occurred not only in the scroll part, but also in the section between scroll and exit duct. In the latter case, sudden change of pressure was observed within a short distance. Pressure recovery factor C_p on the branching streamline of flow from scroll to exit duct was calculated. C_p=0.83 at the maximum-pressure point and 0.77 at the design point were obtained. It was found that this section between scroll and exit duct acted as a high-performance diffuser. The rate of pressure recovery on the branching streamline was evaluated. At the design point, it was about 40 percent in the scroll part. The remaining 60 percent was recovered in the section between scroll and exit duct.

View full abstract

Flow in a blower casing which has no diffuser part and large tongue clearance was studied. The factors which resulted in good pressure recovery on the branching streamline of main flow from scroll to exit duct were investigated by flow visualization using oil-film and depth-tuft methods. In the main flow, swirling flow which produces centrifugal force pressing the flow against the wall was observed. On the bottom plate, an inward flow pattern resembling that of free vortex flow sinking into its center was observed. This means that low-speed fluid near the wall is drawn into the space between impeller and casing. In the section between scroll and exit duct, a wall branching streamline separating the inward flow and exit flow boundary layes was observed. Thus the boundary layer of the exit flow was thin where flow was decelerated. It is concluded that centrifugal force produced by secondary flow and boundary layer suction are the main causes of high pressure recovery.

View full abstract

The transient process on rotating stall in a centrifugal blower with a backward-leaning blade impeller was measured in the impeller and in the vaneless diffuser by means of the high-response pressure sensor as well as the semiconductor laser 2-focus velocimeter. It is shown that a small intermittent fluctuation, which is called prestall, occurs prior to rotating stall due to the interaction between the impeller passage flow and the diffuser wall flow under the condition of radially inward flow in the vaneless diffuser, and prestall develops into rotating stall with decreasing flow rate. The focus is placed on disturbances of wavelength larger than the blade spacing in the autocorrelation analysis. The autocorrelation coefficient of the pressure fluctuation is proposed as a quantitative precursor scale of rotating stall in the vaneless diffuser in the present paper.

View full abstract

In order to clarify the mechanism of the occurrence of unstable characteristics of a high-specific-speed diagonal flow fan, detailed measurements of the internal flow fields ahead of and behind the rotor were performed using a hot wire probe. The behavior of the flow fields in the low flow range including rotating stall was mainly investigated and the characteristics of the rotating stall, namely, the rotational speed of the stall cell, the number of cells and the cell distribution were analized. The effects of tip clearance on the unstable characteristics and internal flow fields were also discussed based on the measured results.

View full abstract

Rotating stall in the radial vaneless diffuser with large radius is studied experimentally. The static pressure on the wall and the velocity distribution are measured and analyzed, and the critical flow angle calculated theoretically is compared with the experimental one. Measured velocity distributions show that the existence of reverse flow is closely related to the occurrence of stall. In our study, rotating stall occurs at a much larger flow angle than that in conventional studies. Also, in a wide range of flow angle, a compound pressure vibration of two different frequencies is oberved, which shows the steady coexistence of two different types of stall cells. Furthermore, the dimensionless frequency of stall propagations is more strongly dependent on the flow angle than in the conventional cases. These results are due mainly to the large ratio of radius in our diffuser.

View full abstract

In this study, the a posteriori approach was used for the analysis of unsteady flow in a centrifugal compressor at lower flow rate as opposed to the a priori approach used for lumped parameter models. The derivatives of the pressure fluctuations obtained experimentally which were computed by a noise-free numerical differentiation, were used to obtain the phase diagrams. After ordinary differential equations were constructed to model the dynamics of the system behavior, they were numerically analyzed. The phase diagrams by the derivatives showed complex trajectories at first glance. However, by using a band pass filter for the pressure fluctuations, it was found that the phase diagrams consisted of three independent wave forms and formed one of the tori.

View full abstract

A detailed study of the fluid moment on a precessing impeller shroud has been made. Experiments and calculations based on a bulk-flow model were carried out to elucidate the unsteady flow between the impeller shroud and the casing. The fluid moment was estimated from the pressure distribution on the impeller shroud in the precession motion. The results of the experiments were compared with those of the calculations. It was found that : (1) the moment largely depends on the leakage flow rate and the pressure loss at the impeller outer radius ; (2) a bulk-flow model incorporating the measured behavior of the pressure loss can simulate the complete leakage flow distribution fairly well ; (3) The fluid moment is mainly caused by the two-dimensional inviscid unsteady flow due to the precession motion.

View full abstract

Fluid forces acting on a rotating centrifugal impeller in whirling motion are studied experimentally. In the first report, a two-dimensional impeller installed in a parallel-walled vaneless diffuser was used. Here in the second report, fluid forces on a three-dimensional impeller in a vaned diffuser are investigated when it is forced to whirl on a circular orbit with a constant eccentricity. The experimental apparatus is improved to be able to synchronize the whirling phase with the rotating one. Although a strong interaction between rotor vanes and guide vanes is observed at a certain specific whirling ratio, the fluid forces averaged throughout one whirling period, which are dominant for the stability of the rotating system, exert a damping effect on the rotor under most operating conditions.

View full abstract

A numerical method using the pseudocompressibility method for solving 2-dimensional incompressible Navier-Stokes equations, which maintains high accuracy in time advancement, is described in this paper. Since the pseudocompressibility method is fundamentally applied to obtain steady solutions, subiteration in pseudotime is generally introduced for time-accurate solutions. Using subiterations, the system of equations is solved through complicated treatments in pseudotime to satisfy a divergence-free condition in physical time. In this work, a simplified method to solve unsteady flow problems is proposed by setting the subiteration in pseudotime to the physical time advancement. As a numerical example, the flow around a cylinder is calculated to verify numerical accuracy. The result shows good agreement with experimental data. Next, the flow around a 2-dimensional thin arc airfoil, which is taken from the blade of a propeller fan used in outdoor units of air-conditioners, is solved. From the results, leading edge separation and reattachment can be clearly observed.

View full abstract

Effects of incidence angle variation in the vane height direction on the flow pattern through guide vanes were investigatd. The secondary flow along the hub wall turns upstream near the middle spacing between the vanes when the inlet flow forms a negative incidence angle on the hub side. Its flow unites with the through flow from the inlet and rolls up widely in the passage. With increasing incidence angle, the secondary flow approaches the suction surface and eventually forms a strong vortex on the hub wall, without rolling up in the middle spacing. The effects of this strong vortex on the downstream flow, however, are almost the same as those of the rolling up. Moreover, it was discussed that the hydraulic loss of the guide vanes can be predicted accurately from the bending loss obtained from the actual flow turning angle and the shock loss at the inlet. It may be necessary for the case of guide vanes with extremely large camber angle to taken into account the mixing loss based on the flow distortion at the outlet.

View full abstract

A viscous stage calculation method was used for investigating complex three-dimensional flow phenomena, particularly under off-design operating conditions, occurring in a low-specific-speed diffuser pump stage with a centrifugal impeller and a vaned bowl diffuser. The incompressible version of Dawes three-dimensional Navier-Stokes code was extended for the stage calculation by introducing an inter-row mixing process proposed by Denton. The numerical prediction succeeded in predicting major flow patterns obtained by a multicolor oil-film flow visualization technique which featured the use of tri-primary-color fluorescent powder as pigment. The meridional and blade-to-blade secondary flows, including the inpeller inlet recirculation at the shroud side, the diffuser inlet recirculation at the hub side, and the formation of the extensive vortex on the diffuser hub surface, were investigated numerically and experimentally in detail. The flow mechanism leading to the onset of impeller inlet recirculation was also discussed in detail.

View full abstract

In the very low discharge range of a semi-open-type centrifugal impeller, severe self-sustained oscillations were induced when a few percent of air was introduced at the upstream of impeller. In order to elucidate the mechanism of this oscillation, instantaneous measurements of blade-to-blade pressure distribution were performed and a linear stability analysis of gas-liquid flow in the pumping system was carried out. The theoretical results revealed that the present dynamic instability is caused by the unbalance between the pump-head drop rate due to the system resistance and bubble accumulation in the impeller channel which is largely influenced by the size and the behavior of the bubble zone. The measured results revealed that the bubble accumulation zone causes the recirculating flow at an impeller inlet to disappear and then induces a sudden strong reverse flow with strongly swirling bubble flow.

View full abstract

A three-dimensional linear analysis is carried out using a finite-span cascade model to investigate three-dimensional effects of cavitation on rotating cavitation in inducers. It is assumed that the fluctuations of the flow fields consist of 0th- and 1st-order modes in the spanwise direction. It is shown that there are 4 modes of instabilities corresponding to rotating cavitation : two of them are forward-travelling modes and the other two modes are backward-travelling ones. Both the forward- and backward-travelling modes include one mode in which the disturbances are nearly constant in the spanwise direction : the 0th-order radial mode. This corresponds to the mode predicted by a previous 2-D linear analysis, and is found to be only weakly influenced by the three-dimensionality of cavitation. For the other modes in the forward- or backward-travelling modes, the disturbances change significantly in the spanwise direction : the 1st-order radial mode. It is shown that the 1st-order radial modes have a larger unstable region than the 0th-order radial modes.

View full abstract

Single-flow-passage pumps for fish transportation, also called fish pumps, have been used widely in the fisheries industry. However, the pump efficiency is low and the vibration is large because of imbalance of the impeller. Therefore, a diagonal-flow pump with two blades is newly developed. The performance characteristics of the new pump and the flow conditions at the impeller exit are experimentally investigated. This pump shows high efficiency and stable operating conditions over a wide range of flow rate. Moreover, the pump impeller has a large slip factor which is nearly equal to the value calculated from Wiesner's formula.

View full abstract

A new type of centrifugal blood pump which has no rotating shaft or seal has been developed. The impeller is suspended magnetically inside the pump casing without any mechanical contact. Pump characteristic tests using various configurations of impellers and diffusers were carried out in order to increase pump efficiency at low flow rate. Pump losses were analyzed considering the losses due to the flow through the gap between the impeller and the casing, and disk friction loss was found to be large. Hemolysis tests and animal experiments showed that hemolysis caused by this pump is low enough for its clinical use and no thrombus formation was found after 13 days of continuous operation.

View full abstract

Hydraulic energy loss in an elbow draft tube of GAMM Francis turbine is extracted from performance diagrams acquired in a normal model test which adopts a draft tube outlet as a low pressure measuring section, and in a special model test with a draft tube inlet section. An obtained diagram of draft tube energy loss versus discharge coefficient reveals that the loss is minimum at the two specific discharges, where the pertinent positive and negative swirls are supplied from the runner outflow to suppress the flow separation. The draft tube energy loss is maximum at a discharge between the two discharges above mentioned, where the flow at the runner outlet is free from swirl and induces the flow separation.

View full abstract

In order to study the applicability of flow simulations by using the three-dimensional Euler code with pseudo-compressibility, flow simulations of a Francis runner with a high specific speed were carried out and results were compared with those of experiments on the pressure distributions on blade surfaces. As a result, it is concluded that the simulation results obtained using this code indicate a good agreement with the experimental ones, and this code is useful for predicting the flow in a Francis runner.

View full abstract

Jet interference is the most impotant factor inhibiting the increase in specific speed of multinozzle Pelton turbine. Since the jet interference has a strong negative scale effect, it is difficult to predict the intensity of the jet interference in a high head prototype from low head model tests. In this study, we predict the intensity of the jet interference in terms of the relative jet path. The intensity can be defined as a ratio of the residual volume in the first jet when a bucket contacts the second jet to the full volume responsible to the bucket. In a 6-nozzle model turbine, comparing the predicted jet interference intensity with the efficiency deterioration measured by model tests due to the jet interference, the intensity shows good correlation with the jet interference in the operating region of high unit speed and high unit discharge where the efficiency deterioration is extreamly high.

View full abstract

Jet interference brings serious deterioration of efficiency in multinozzle Pelton turbines especially in the operating region of higher unit speed and higher unit discharge. Since the jet interference is intensified by the scale effect of the specific hydraulic energy for a turbine, it is difficult to predict the jet interference of a prototype from is low-head model test results. An alternative approach for predicting prototype interference is therefore urgently required. In our 1st report, a newly proposed intensity for the jet interference based on the relative jet paths reveals a good correlation with the efficiency deterioration due to the jet interference. In the present report, the scale effect of specific energy on the jet interference intensity is investigated. The velocity coefficient of a free jet is reduced with increasing specific hydraulic energy of the turbine, because the entrainment of water droplets in the turbine housing into the free jet increases with the increase of jet flow rate, or of specific energy. Using the relative jet paths considering the decay of jet velocity, the predicted intensity of jet interference shows a good correlation with the increased deterioration of efficiency due to the scale effect of specific energy.

View full abstract

In order to clarify severe pump erosion practically, cavitation aspects are precisely measured at a specific speed N_s=300 (m, m³/min, rpm) under various test conditions, especially at low flow rates, through a clear-lucite front shroud of the test impeller and the lucite casing, by means of instantaneous photography and a high-speed video camera, the maximum framing rate of which is 40500 frames per second. Erosion tests are also carried out using three-dimensional impellers made of aluminum alloy. Severe erosion takes place on several parts of the test impeller, i.e., on the main shroud and on the suction side. At a low flow rate, severe erosion occurs on the pressure side near the leading edge. This results from a vortex cavitation arising in the reverse flow region near the pump inlet.

View full abstract

This paper describes the deformation mechanism, breakup type, and critical breakup condition of liquid bubbles due to air streams. Experiments are conducted using a horizontal air-suction wind tunnel. The breakup processes of liquid bubbles are photographed using two stroboscopes with a short time interval. The breakup pattern of liquid bubbles varies with airstream velocity, equivalent diameter, and dimensionless liquid film thickness of a liquid bubble. The breakup conditions of liquid bubbles are shown using the relationship between the equivalent diameter and the relative velocity ; equivalent diameter and Weber number for a liquid bubble. The critical Weber number for a liquid bubble increases with the decreasing equivalent diameter. The critical breakup pattern of a liquid bubble due to an airstream is the bursting of a thin liquid film facing to the wake area.

View full abstract

Experiments on visualization of high-speed fluid phenomena in metallic ducts were conducted by three kinds of time-resolved neutron radiography techniques. In the direct film method with a pulsed neutron beam, frozen neutron radiographs of air-water flows in a rectangular duct were obtained successfully. The high-frame-rate neutron radiography with pulsed and steady neutron beams allowed visualization of boiling water two-phase flow in a tube at the recording speed of 500 frames/s and air-water flows in a rectangular duct at up to 1000 frames/s, respectively. The quality of the images obtained from the time-resolved neutron radiography was sufficient for observation of the fluid behavior. The limit of the time-resolution was clarified based on the sensitivity and light-decay constant of the converter and the measurement error of neutrons calculated by statistical analysis.

View full abstract

A direct numerical simulation has been conducted for turbulent flow through a square pipe. Turbulent flow solutions are obtained using the 2nd order accurate central difference scheme as well as Leonard's QUICK scheme for discretizing convective terms on three grid systems of different resolution. Comparison between the results has revealed that the QUICK scheme provides more accurate results with less computational effort than the central difference scheme. In the present condition, the solution for the QUICK scheme converges monotonically with increasing number of grid points, while the result for the central difference scheme shows rather complex behavior.

View full abstract

A computational study on developing turbulent flow in a square-sectioned 90 degree bend at Reynolds number of 4×10⁴ is reported. The ratio of bend mean radius of curvature to hydraulic diameter is 2.3 and straight ducts 45 and 30 hydraulic diameters long are attached to the inlet and outlet planes of the bend, respectively. In calculation, an algebraic Reynolds stress model was adopted in order to predict anisotropic turbulence precisely, and a boundary fitted coordinate system was introduced as the method of coordinate transformation. Calculated results were compared with the experimental data measured separately by two research groups. In this resarch, special attention is paid to the comparison of calculated results with the distribution of the Reynolds stresses. Moreover, the production mechanism of streamwise velocity fluctuation is examined by evaluating its production term for calculated results. In consequence of this calculation, it was found that the present method predicted relatively well the characteristic features of Reynolds stresses, although there are a few points of discrepancy. The examination of the production term for streamwise velocity fluctuation suggests that the production term including the streamwise velocity gradient with respect to bend angle plays an important role in its generation near the both side walls.

View full abstract

The flow between eccentric rotating cylinders is studied numerically using a curvilinear coordinate system. The outer cylinder is fixed and the inner cylinder rotates at a constant speed. The three-dimensional Navier-Stokes equations and the equation of continuity are solved. A body-fitted grid system is generated and the basic equations in a physical domain are transformed into a calculational domain and solved by means of a finite difference method. A series of computations is carried out at Re=130 to 500 and the eccentric ratio is 0.1 to 0.5. The flow patterns, velocity vectors and vorticities are obtained. The gap width between two eccentric cylinders changes in a circumferential direction and the maximum Taylor vortex is found in the region where the gap width becomes narrow in the flow direction and the vortex becomes weakest in the region where the gap width is expanding. When the Re number is slightly larger than the critical Re number, the Taylor vortex flow and parallel stable flow coexist in a fluid between two eccentric rotating cylinders.

View full abstract

The purpose of the present study is to develop a new system which classifies submicron particles accurately by applying an almost rigidly rotating through-flow between a rigidly rotating housing and a rotor. We performed a linear analysis of the interior region and the Ekman layer developing between the upper and lower walls with arbitrary axisymmetric configurations, and considered the relationship between the flow structure and the wall configuration. The conclusions are as follows. (1) In the interior region, the azimuthal velocity component increases with the gradients of both walls in the radial direction, and the axial velocity component increases with an increase of the difference between the curvatures of both walls. (2) When the coordinate normal to the wall is used in the Ekman layer, the effect of wall configuration appears implicitly in both the azimuthal velocity component and the velocity component tangent to the wall, but appears explicitly in the velocity component normal to the wall.

View full abstract

We have theoretically considered the condition of a wall configuration necessary for the occurrence of a rigidly rotating region, Hide's prediction for an almost rigidly rotating through-flow between two rigidly rotating concentric spheres, and the relationship between the wall configuration and the flow structure of the interior region and the Ekman layer. The conclusions are as follows. (1) For the occurrence of the rigidly rotating region, it is necessary to make the axial section of the flow field C-shaped. (2) We have supported Hide's prediction theoretically and clarified the details of the flow structure. (3) When either the rotor or the housing walls, or both are disk-shaped, the interior axial velocity component ω_I is proportional to the thickness of the Ekman layer δ on the disk and the interior axial vorticity component ω_I. On the other hand, when both the rotor and the housing walls have curvatures, ω_I depends not only on δ and ω_I but also on the interior azimuthal velocity component and the radial change of δ.

View full abstract

We have experimentally investigated the conditions of occurrence of a steady axisymmetric flow regime and the velocity distributions in the interior region and the Stewartson layer for an almost rigidly rotating. through-flow in a concentric-sphere system and a disk-cylinder system (the rotor and housing walls consist of disk and cylinder surfaces). The conclusions are as follows. (1) The steady axisymmetric flow regime occurs when ε≤2.0E^0.48 in both systems, where ε and E are the Rossby and Ekman numbers, respectively. (2) The experimental results of the velocity distributions in the interior region agree with their linear analytical solutions. (3) The absolute value of the axial velocity component increases downstream and with a decrease of the Ekman number in the Stewartson layer along the rotor in the disk-cylinder system. (4) The reverse flow in the Stewartson layer which occurs in the gap between the rotor and housing walls is prevented by narrowing the gap.

View full abstract

Firstly by applying the method of flow analysis for cavitating-hydrofoils of finite span already proposed in the 2nd report, mutual interferences between the cavitating-line and lifting-line and also aspect ratio effects on induced flows and fluid forces are examined through some numerical examples of the partially-cavitating flat plate hydrofoil of elliptic plan form. Thus notable differences between induced flows and fluid forces of partially- and super-cavitating hydrofoils and essential features of three-dimensionality are synthetically clarified for optimum semi-closed cavity models in respective cavity flows.

View full abstract

In the present work, incompressible high Reynold's number flows around a compressor blade cascade at low speeds (Mach number 0.25) are analyzed by solving the N-S equations using the finite difference method. A generalized coordinate system is used so that a sufficient number of grid points are distributed in the boundary layer and in the wake region. A third-order upwind scheme is used for the convective term, which suppresses the non-linear instability for calculation of the high Reynold's number flow used in the present computation. The experimental-flow Reynold's number defined on the basis of blade chord length and cascade inlet velocity is 7×10⁵. Grid topology is thought to be as important as flow solver, therefore calculations are carried out using H-type and C type grids and the results of the calculations are compared with the experimental data obtained for a controlled diffusion-type axial compressor rotor blade cascade for three different inlet flow conditions, which included design and off-design conditions. The study led to a clear understanding of the importance of the grid topology on the prediction of experimental flow behavior.

View full abstract

Improvement of the generation power in micro-hydraulic turbines is shown in the range of low flow rate and low head using a new inverter generation system. The generation power can be obtained down to very low flow rate and head using the new inverter generation system with variable rotational speed control, where a water turbine connot produce the electric power in a normal constant-speed-control system. Experiments were carried out using a practical micro-hydraulic turbine with 2 kW output and variable rotational speed in a forest on the mountain, Mie University. The total efficiency is kept at a high value even in the very low partial load area. Compared with the constant speed control, the generation power is improved over 20 percent by varying the rotational speed.

View full abstract

In the previous paper, the design and performance of a diaphragmless shock tube which could generate, with a higher degree of repeatability, weak shock waves up to shock Mach number of 1.2 were reported. In the present paper, an experimental improvement of the facility, by which the pressure difference between the driver chamber and the driven channel can be increased to about 4 times higher than the previous system, was achieved so that the attainable shock Mach number was extended to M_s=1.6 at the initial atmospheric pressure. Higher shock Mach number up to 3 can also be readily obtained by decreasing the pressure in the driven channel. A numerical simulation and a theoretical analysis were perfomed in order to clarify the characteristics of this diaphragmess shock tube. Although the numerical simulation revealed that the flow near the rubber membrane is very complicated, however, with the help of simplified one-dimensional model, in which the varying cross-section is taken into account, shock Mach numbers achievable in this shock tube were well predicted as a function of the pressure ratio between the high-pressure chamber and low-pressure channel. Good agreement was obtained between the experimental results and theoretical prediction.

View full abstract

Flow characteristics of sheath flow chambers were examined to improve fluid performance of flow cytometers. The chamber consists of a channel for sheath fluid, a sample nozzle along the channel, and a capillary tube in the downstream. The experimental results suggest that L/d_c>60 or L/d_c<15, where L is the distance between the sample nozzle and the inlet of capillary tube and d_c is the inlet diameter of the capillary tube, allows stable sheath flow geometry at high sample flow rate.

View full abstract

Pure tones are unexpectedly generated from slits such as gaps between a window and a pole or a damper when wind is strong. This type of pure tone is referred to as whistle tone in this series of papers. We already clarified the characteristics of whistle tone generated by an air jet from a rectangular slit. That is, it was found that the frequencies of whistle tone noise coincided with those of velocity fluctuation, and that its scale of turbulence, or the range of the coherent area, was comparable to the exit area of the slit used. In the present study, we used a nonsymmetrical slit which had an edge plate on one of the long sides and investigated the effect of the nonsymmetry on the generation of edge tone. The results are summarized as follows : (1) the structure of the vortex is strongly affected by the edge plate, and the sound pressure level becomes smaller, and (2) the fundamental relationship between the generated sound and the velocity fluctuation near the exit of the slit is the same as that in the case of a symmetrical slit.

View full abstract

When a Shinkansen train runs through a tunnel at a high speed, pressure developed outside the train shows substantial fluctuations which lead to pressure change inside the train. In order to prevent aural discomfort among passengers, it is necessary to estimate these pressure changes for the Shinkansen train which has a ventilation system with continuous draught. In this paper a numerical method for calculating pressure is shown using the method of characteristics. Calculations are performed on running tests of Shinkansen trains Series 100 N and 300 and the calculated results are compared with the measured ones. The results show that the numerical method simulates well the pressure changes outside and inside a Shinkansen train running through a tunnel, and countermeasures to alleviate aural discomfort should be taken in cases of trains traveling at a higher speed.

View full abstract

Mist concentration distributions were measured by 4ch optical CT scanners with a mist particle size counter. The counter system is a kind of Mie scattered particle counter, and can measure particle size at a fixed position. To examine the validity of the system, mist concentration and mist speed were measured using a spray and ultrasonic atomizer. Results showed that the technique was applicable for rapid, high-resolution measurement of mist concentration distributions.

View full abstract

Instability of a boiling natural circulation loop with a chimney at low pressure and low heater power was investigated by linear stability analysis and experiment. A homogeneous and thermodynamic equilibrium model for two-phase flow was used. The effect of flashing induced by pressure drop in the heated channels and the chimney was considered. The effects of coupling between two boiling channels were investigated. It was found that in-phase-mode instability was apt to occur when channel inlet subcooling was large and boiling began in the chimney. In-phase-mode instability easily occurred when channel length became short and the chimney became long. Out-of-phase-mode instability was apt to occur when chimney length became small and boiling began in the channel. It was suggested that in-phase-mode instability was density weve oscillation induced by flashing in the chimney and out-of-phase-mode instability was density wave oscillation induced by boiling in the channels. The analytical results agreed qualitatively with experimental results.

View full abstract

Experiments are are conducted to investigate two-phase flow instabilities in a boiling natural circulation loop with a chimney induced by flashing in the chimney at lower pressure. The type of instability that occurred in the experiments is suggested to be density wave oscillations induced by flashing in the chimney. The differences from other instabilities such as geysering, flow pattern transition instability, and natural circulation oscillations are discussed on the basis of the dynamic characteristics, the oscillation period, and the transient flow resume.

View full abstract

Using a pulsed free expansion jet of gaseous molecules (Ar, CO, CO₂, NO, N₂O), effects of large-size clusters on film condensation were studied experimentally. The cluster size was measured with a quadrupole mass spectrometer and a reflective TOF mass spectrometer by taking into account the fragmentation effects due to the ionization process. Large size clusters were obtained by increasing the source pressure and decreasing the temperature. The condensation rate or sticking probability of clusters first increased and then saturated to the some extent with increasing the clustere size.

View full abstract

A new two-equation heat-transfer model which incorporates some essential features of second-order modeling is proposed. The present model is applicable to heat-transfer problems in both wall and free turbulent flows not unusually deviating from the equilibrium state. Furthermore, by introducing the Kolmogorov velocity scale, the model can appropriately express the low-Reynolds-number effects in the near-wall region, and is also applicable to complex heat-transfer fields with flow separation and reattachment. The proposed model predicts quite successfully heat transfer in both wall and free turbulent flows, i.e., a homogeneous isotropic decaying flow, a homogeneous shear flow, a boundary-layer flow heated from the origin, and a boundary-layer flow subjected to a sudden change in wall thermal conditions, whereas such predictions have been almost impossible with existing two-equation heat-transfer models.

View full abstract

The diverter surface of the ITER Fusion Engineering Reactor is exposed to strong radiation locally up to 20 MW/m². We have proposed a diverter cooling system which consists of concave surfaces cooled by two-dimensional liquid jets. Experiments are conducted to investigate local heat transfer coefficeint and critical heat flux on flat and concave surfaces under various cooling conditions. Based on photographic study, a critical heat flux model was derived by modifying a Haramura's model to take account of the subcooling effect. The proposed correlation based on this model was in good agreement with the experimental data.

View full abstract

An analytical study has been performed of critical heat flux of a two-phase thermosyphon, in which a liquid film and a vapor flow in the manner of countercurrent annular flow. The point which can be determined from the three equations of momentum, of its partial derivative with void fraction, and of mass balance in the thermosyphon, is proven to correspond to a maximum falling liquid rate to the thermosyphon. This point, furthermore, becomes identical to the point at which a generated envelope line intersects the mass balance line. The CHF calculated from the maximum liquid rate is found to be in good agreement with the existing CHF data.

View full abstract

Critical heat flux (CHF) for flow-boiling of water in small-diameter tubes under low-pressure conditions was measured over a wide range of parameters, and the results were compared with several existing CHF correlations and theoretical models. The experiment was conducted with atmospheric-pressure water in tubes with inner diameter D ranging from 1.0 to 6.0 mm, heated length-to-diameter ratio L/D from 1 to 100, inlet water subcooling ΔT_in from 0 to 90 K, and mass velocity G from 0 to 19740 kg/(m²·s). The highest CHF attained in this experiment was 158 MW/m². The results demonstrated the effects of channel geometry and subcooling under low-pressure condition. In, addition, the flow pattern near CHF conditions in a tube with small diameter and short heated length was observed using a high-speed video system. The theoretical model of Celata et al. (Int J. Heat Mass Transf., 1994) for a subcooled condition showed good agreement with the present subcooled data when L/D≥25.

View full abstract

An analytical method of the direct contact melting of a two-dimensional arbitrarily shaped solid on an isothermally heated horizontal plate is proposed. To verify the validity of the analytical method, experimental results are also presented for the melting of trapezoidal-shaped two-dimensional ice inclined on an isothermally heated plate. The present analytical results show good agreement with the experimental data, which implies that the present analysis is applicable to study melting of a two-dimensional arbitrarily shaped solid. Three patterns of melting phenomena are observed depending on the aspect ratio of trapezoidal-shaped ice : "stable", "inclined" and "fall-over" melting. It is shown that rapid melting occurs in the case of the stable and the fall-over pattern, in which a position that the How direction of the liquid layer underneath the ice changes is found near the center of ice width.

View full abstract

Solidification around a horizontal cylinder in a porous medium saturated with NaCl-aqueous solution was studied analytically and experimentally in order to clarify the effects of natural convection caused by temperature and concentration gradients and forced convection based on upward or downward flow of the solution on the solidification process. In the experiment a packed bed of glass beads was used as a porous medium and temperature distribution around the cylinder was measured. A numerical analysis was carried out and the temperature distribution obtained by the analysis agreed well with the experimental one. Growth rates of the volume of solid phase formed around the cylinder with various flow rates of the solution were shown.

View full abstract

In order to predict fog in an air-induced tunnel, heterogeneous nucleation and water-mist development in turbulent duct flows with cold bottom/top surfaces are numerically investigated. The distribution function of fine aerosol, considered as seeds of water-mist, and the Stokes force of rather large particles are taken into consideration regarding the particle diffusivity and the k-ε turbulent model as well as gravity precipitation. For a 2-dimensional duct of 2 m×360 m with air flow rate of 1.0 m/s, mist formation occurs mainly near the upper wall and water-mist develops up to 10μm in radius. In the mist regions of the tunnel deposition flux at the bottom surface is much larger than the vapor flux because of the contribution of precipitation. The effects of the distribution function of seeds and the Reynolds number are also discussed in detail.

View full abstract

Single-droplet evaporation of fuel at high pressures and high temperatures has been studied experimentally under microgravity conditions. Suspended n-heptane droplets were employed in the experiments at pressures in the range of 0.1∼5.0 MPa, and temperatures varying from 400 to 800 K. Temporal variations of the droplet diameter were measured for subcritical environments with a computer-aided image analysis system. Microgravity conditions were used to prevent natural convection that makes phenomena complicated, and realized by using 5 m and 110 m drop towers and parabolic flights. It was observed that dense fuel vapor accumulates around a droplet and that the droplet surface becomes obscure in high-pressure and high-temperature environments. At ambient pressures below the critical pressure of the fuel, the slope of the variations of the squared nondimensional droplet diameter with the corrected time initially increases but later becomes constant. At pressure of 5.0 MPa and temperatures below the critical temperature, it becomes gentler in the latter half of the droplet lifetime.

View full abstract

Heat transfer in laminar flow between two parallel plates with repeated rib-type eddy promoters was studied numerically. It has been found that the heat transfer is enhanced with respect to the flat plate channel flow when the nondimensional clearance between the promoter and the heated duct wall is larger than 1.0. The highest value of the averaged Nusselt number obtained in the present study is 23 which is about 4.3 times the constant value of 5.385 typical of smooth duct flow. The observed heat transfer enhancement is due to 1. Transport of the relatively low-temperature main fluid body into a region close to the heated wall, and 2. Flow mixing which results in a more uniform temperature distribution across the main flow field.

View full abstract