Study on behavior of ephemeral wave in gas‐liquid two‐phase flow: Part 2. Characteristics in flow parameter of ephemeral waves

In order to clarify the behavior of ephemeral waves, flow parameters of ephemeral waves, such as the number and residence zone lengths per unit axial length, mean values and standard deviations of wave velocity, width, and maximum holdup, were determined using a wave‐vein analysis in upward huge wave flow and annular flow. The mean values of wave velocities, widths, and maximum holdups of ephemeral waves are compared with those of liquid lumps having main wave‐veins, and the differences in the parameters between those liquid lumps are discussed. Furthermore, the parameters of both active ephemeral waves and inactive ephemeral waves are determined, and the characteristics in the flow parameters of two types of ephemeral waves are presented. © 1999 Scripta Technica, Heat Trans Asian Res, 29(1): 1–14, 2000     

Wave venation in downward gas-liquid two-phase flow (part I,time-spatial behavior chart of interface and analysis of main wave-vein)

Time-series information on both the cross-sectional mean liquid holdup along a tube axis and the gas-liquid phase distribution along a tube diameter was obtained by means of supermultiple cross-sectional mean liquid holdup probes (S-CHOP) and semi-supermultiple point-electrode probes (SS-PEP) for vertical downward gas-liquid two-phase flow. Typical time-spatial behavior charts of interface and gas-liquid interfacial profiles are presented. Close inspection of these results reveals that a huge wave and a disturbance wave appear in downward two-phase flow as well as upward flow. It was clarified that the huge wave flow region covers a wide range of superficial gas velocities. Wave velocity, wave width and maximum liquid holdup of individual waves were examined by wave-vein analysis. Histograms of these flow parameters were also studied. It was found that there exist distinct differences in wave width between the huge wave and the disturbance wave. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(8): 499–510, 1996     

Effects of liquid viscosity on inception of disturbance waves and droplets in gas–liquid annular two‐phase flow

The structure of gas–liquid two‐phase flow is investigated in order to establish a reliable criterion for the development of disturbance waves and droplets considering the effects of liquid viscosity. The structure of the gas–liquid interface and the flow rate of droplets entrained in gas are measured simultaneously at five kinematic viscosities (1.0, 3.2, 9.9, 30, 70 mm²/s). The time‐series traces of liquid film thickness measured by five holdup probes reveal that the inception of disturbance waves occurs at a liquid Reynolds number of 200 or a non‐dimensional liquid film thickness of 6.5. It is also shown that droplets are generated before the inception of disturbance waves with increasing liquid kinematic viscosity at a liquid velocity of 0.02 to 0.03 m/s. As previously published criteria for the inception of droplets are found to be unsatisfactory, a new critical condition for droplet generation balancing the interfacial shear stress $τi$ with the wave height h and surface tension σ is proposed: $τih/σ=0.025$. This relation describes the action of shear force and surface tension on wave crests, and is notably independent of liquid viscosity. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 529–541, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20176     

A simplified model of gas–liquid two‐phase flow pattern transition

An experiment of upward gas–liquid two‐phase flow was conducted in an air–water isothermal system under atmospheric pressure. The differential pressure was measured at the fully developed section by using a variable reluctance type transducer to classify the flow patterns and their transitions. The flow behavior was observed with a high‐speed video camera. The probability density function (PDF) of the differential pressure signal was employed to identify the flow pattern. A simplified one‐dimensional flow model was proposed to clarify dominant factors affecting the formation and transitions of flow patterns. The model dealt with the gas‐component advection based on the spatiotemporal void fraction behaviors by considering the gas compressibility, the wake, and the liquid phase redistribution mechanism. The simulation results of the model indicated four kinds of the void wave patterns (ripple‐like, rectangular, distorted rectangular, and uniform wave patterns) depending on gas and liquid volumetric fluxes. These void wave patterns corresponded well to the experimentally observed flow patterns. The transitions among void wave patterns agree well with the Mishima–Ishii flow pattern map. The friction loss estimated by the present model coincides fairly well with Chisholm's empirical formula. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(7): 445–461, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20029     

Experimental study on hydrodynamic performance of a wave energy converter within multi‐heaving‐buoys

A compact buoy‐array‐type wave energy converter called multi‐heaving‐buoys (MHB) is introduced in this study. The hydrodynamic performance of MHB under regular wave conditions was first investigated experimentally in a wave tank located in Ocean University of China. It was found that a limited number of heaving buoys had little effect on the wave fields around the device. The small period of the incident waves caused an intense interaction between the waves and the buoys. The phase difference between the buoys in different rows was determined by the distance between the buoys. It was found that the response amplitude operator of the buoys varied from 0.6 to 1.2. Correspondingly, the range of the averaged relative velocity of the heaving buoys was 0.6–1.3. The upper limit of the acceleration of the buoys' motion was 0.2 times that of gravity. All of the experimental results provide valuable information for the future design of the hydraulic pressure power take‐off systems. Copyright © 2017 John Wiley & Sons, Ltd.     

Wave venation in downward gas-liquid two-phase flow (part II,cluster analysis for huge waves and disturbance waves and characteristics of those waves)

Wave velocities and wave widths were determined using wave-vein analysis for a wide range of air and water flow rates. Cluster analysis by a K-mean algorithm was applied to the discrimination between huge and disturbance waves for the present experimental conditions. Individual waves discriminated by cluster analysis reasonably correspond to those recognized from the relation between wave velocity and wave width. Appearance flow conditions for liquid slug, huge and disturbance waves were clarified. The characteristics of wave velocity, wave width, and maximum liquid holdup for huge and disturbance waves are discussed. Comparison between liquid slug, huge wave and disturbance wave flow parameters reveals that there exist distinct differences in wave width of these waves. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25 (8): 511–521, 1996     

Numerical analysis for a falling liquid film with interfacial waves on an inclined plate. Part 2: Effects of interfacial waves on flow dynamics and heat transfer

Flow and temperature fields in falling liquid films with interfacial waves have been obtained by means of a numerical simulation in which the algorithm is based on the HSMAC method and interfacial boundary conditions are treated with newly proposed methods. Small‐amplitude disturbance waves at a low frequency develop into isolated solitary waves which are composed of a roll wave and capillary waves. Waves disturbed at a high frequency interfere with each other and develop into disordered waves. Circulation flow is observed in the roll wave, while there is no circulation flow in the disordered waves. Temperature fields in the wavy film are distorted by convection effects and differ greatly from those in the laminar film. The circulation in the roll wave has an especially strong effect on the temperature fields. The interfacial waves enhance the heat transfer by two kinds of effects: the variation of film thickness and convection in the film. The dominating effect depends on the Prandtl number. © 2000 Scripta Technica, Heat Trans Asian Res, 29(3): 233–248, 2000     

Continuity wave in the elongated bubble region of horizontal slug flow

Visualized observation on the wave feature in a horizontal slug flow was made with a high‐speed digital camera. It was found that the liquid film flow in the elongated bubble region of slug flow behaves as a continuity wave. Theoretical analysis was carried out and it reveals that the liquid film flow is a continuity wave with celerity the same as the translational velocity of the elongated bubble. The control equation for the liquid film height in the elongated bubble region was derived. The results predicted by the equation fit well with the observed data. A new conclusion was obtained that slug flow has continuity wave in it, explaining the stable slug flow wave characterization. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 547–554, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20092     

Numerical study of two‐phase flow of liquid helium in a vertical converging‐diverging nozzle

The fundamental characteristics of the two‐dimensional gas‐liquid two‐phase flow of liquid helium through a vertical converging‐diverging duct near the lambda point are numerically investigated to realize the further development and high performance of new multiphase superfluid cooling systems. First, the governing equations of the two‐phase flow of liquid helium based on the unsteady thermal nonequilibrium multifluid model with generalized curvilinear coordinates system are presented, and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two‐dimensional structure of the gas‐liquid two‐phase flow of liquid helium though vertical converging‐diverging nozzle is shown in detail, and it is also found that the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid‐ to vapor‐phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 432–448, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20071     

Performance of sub‐cooled PEMFCs

Numerical simulations of a proton exchange membrane fuel cell were carried out for various temperatures ranging from well below the freezing temperature of water to a moderate ambient temperature, and also for various inlet temperatures, to investigate its performance. A three‐dimensional serpentine flow field was used to determine the cell behavior temperature conditions. The saturation of liquid water was considered for various ambient temperatures in order to obtain realistic estimates of cell performance, with special emphasis placed on sub‐cooled temperatures. Results show that both the ambient and the inlet temperature have strong influences on cell performance, although the inlet temperature has much more important influence than the ambient temperature. In addition, liquid water saturation is enhanced at higher inlet temperatures. Moreover, for sub‐cooled ambient temperatures the liquid saturation level is higher in the shoulder region near the inlet section than in the outlet section; this trend is reversed for higher ambient temperatures. There is a high probability that operation of the cell at sub‐cooled temperatures and higher inlet temperatures will result in the formation of ice throughout the system, which may further degrade the cell performance. The model was validated by comparison of predicted polarization curves with those found in the literature. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental‐ and numerical‐simulation research on the inner–secondary‐air ratio in a 600‐MWe down‐fired boiler

Using a phase Doppler‐anemometer measurement system, the cold gas/particle‐airflow behavior in a 1:40 scale‐model furnace was assessed to study the influences of adjusting the inner–secondary‐air ratio in a 600‐MW_e multi‐injection and multistaging down‐fired boiler. Numerical simulations were also conducted to verify the results of the modeling trials and to provide heat‐state information. The results demonstrate that reducing the inner–secondary‐air ratio from 19.66% to 7.66% gradually enhances the downward velocity decay of the gas/particle airflow, while the inner secondary‐air downward‐entraining effect on the fuel‐rich flow is weakened. Lowering the inner–secondary‐air ratio greatly inhibits the decay of the near burner–particle volume flux. In addition, the fuel rich–flow ignition distance is reduced, from 1.02 to 0.87 m. A lower inner–secondary‐air ratio is harmful to restrain early NO_x formation. Reducing the ratio also causes the fuel‐rich flow to turn upwards ahead, while the penetration depth of this flow gradually decreases and the maximum temperature in the hopper region falls from 1900 to 1800 K. On the basis of these data, an optimal inner–secondary‐air ratio of 13.66% is recommended.     

Three‐dimensional simulation of water droplet movement in PEM fuel cell flow channels with hydrophilic surfaces

Water management is one of the critical issues in proton exchange membrane fuel cells, and proper water management requires effective removal of liquid water generated in the cathode catalyst layer, typically in the form of droplets through cathode gas stream in the cathode flow channel. It has been reported that a hydrophilic channel sidewall with a hydrophobic membrane electrode assembly (MEA) surface would have less chance for water accumulation on the MEA surface. Therefore, a comprehensive study on the effect of surface wettability properties on water droplet movement in flow channels has been conducted numerically. In this study, the water droplet movements in a straight flow channel with a wide range of hydrophilic surface properties and effects of inlet air velocities are analyzed by using three‐dimensional computational fluid dynamics method coupled with the volume‐of‐fluid (VOF) method for liquid–gas interface tracking. The results show that the water droplet movement is greatly affected by the channel surface wettability and air flow conditions. With low contact angle, droplet motion is slow due to more liquid–wall contact area. With high air flow velocities, increasing the contact angle of the channel surface results in faster liquid water removal due to lesser liquid–wall contact area. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical investigation of liquid water dynamics in wave‐like gas channels of PEMFCs

For the air feed in proton exchange membrane fuel cells (PEMFCs), the wave‐like gas channel (GC) shows obvious advantages over the straight GC because the former enhances collision of secondary flow and diffusion in the gas diffusion layer (GDL). However, it is prone to water flooding, which brings greater pressure drop, larger pressure oscillation, and blocking of reaction area. In the present study, numerical models of the water dynamic processes, including water droplets emerging from micropores on the GDL surface and removing through the GC, are established based on the volume of fluid (VOF) method. Water coverage ratio and pressure drop are calculated to evaluate the water flooding. The effects of the dimensional parameters of wave‐like GC and contact angle of channel walls on the water accumulation are studied. The emergence and removal of liquid water is a quasiperiodic and oscillating process. Multicycle simulations show that channel pressure drop increases linearly with greater growth rate than channel length. The equilibrium position of water droplet is strongly dependent on the relative wettability of the GDL and bipolar plate (BPP) surfaces. And the geometric parameters of GC have a significant impact on the pressure, water removal behavior and detachment time. Smaller bent angle brings bigger pressure drop, and larger cycle length is helpful for relieving the oscillation of pressure.     

Investigation of Ring Waves in Gas–Liquid Two-Phase Flow in a Microchannel

Gas–liquid two-phase flow in minichannels and microchannels displays a unique flow pattern called ring film, in which stable waves of relatively large amplitudes appear at seemingly regular intervals and propagate in the flow direction. In this paper, the behaviors of ring waves, which correspond to ring films that appeared in ring film flow and disturbed ring film flow regions, have been investigated experimentally in gas–liquid two-phase flows of nitrogen-distilled water and nitrogen/30 wt% ethanol–water solution in a 150-μm-diameter silica tube to elucidate their generation mechanism and propagation behavior. In order to clarify the existence region and characteristics of ring waves, the flow patterns observed in a microchannel were investigated and flow pattern maps were made. Furthermore, the velocity of the ring wave was also investigated and compared with the gas slug velocity. In these velocity measurements, high-speed video images were taken at 6,000 frames per second and the formation of ring films and the relationship between the wave amplitude and velocity were determined. The results indicate an interfacial instability leading to the formation and growth of ring waves with both low and high wave amplitudes. The wave velocity is correlated to the wave amplitude, with the large amplitude waves moving much faster than the low amplitude waves. As a result, coalescence of large and low amplitude waves has been observed.     

Numerical investigation on the effects of re-organized shock waves on the flow separation for a highly-loaded transonic compressor cascade

This paper presents a detailed numerical investigation of the influence of re-organized shock waves on the flow separation for a highly-loaded transonic compressor cascade. The boundary layer suction (BLS) was used to control the location and strength of shock waves, with the aspirated slot locating at 49% chord, where is just downstream of the impingement point of shock wave at the leading edge. The numerical simulation is based on NUMECA, a commercial software, where the cell-centered control volume approach with third-order spatial accuracy is used to solve the 3-D Reynolds-averaged Navier-Stokes equations under the Cartesian coordinate system. Several conclusions can be made through the observation of the numerical results. (1) Multiple shock waves in cascade passage leaded the velocity deficits of boundary layer on suction surface downstream of shock wave, resulting in seriously separated flow on the suction side of blade, especially when the front shock wave is much stronger than the rest of the shocks. (2) BLS with small mass flow rate can not effectively improve the boundary layer. When the impingement point of oblique shock wave coming from cascade leading edge is bled to downstream of the passage shock wave by BLS, the boundary layer flow is greatly improved. However, if the BLS mass flow rate exceeds a critical value (1.2%), the boundary layer downstream of shock wave would separate from suction surface. (3) At the blade mid-span, the aerodynamic performance of compressor blade is improved as BLS mass flow rate increases. The optimum BLS is about 1.2%. Compared with the baseline case, the BLS with flow rate of 1.2% increases the total pressure recovery coefficient by 12%, and decreases diffusion factor by 18% and deviation angle to 7 ° while keeping the pressure rise constant. (4) The three dimensional flow structure of the compressor cascade ranged from 25% span to 75% span was improved greatly with the 1.2% BLS flow rate. However it could not control the development of the corner boundary layer effectively.     

Size effect on two‐phase regime for condensation in micro/mini tubes

Analysis was conducted to predict the influence of tube size on two‐phase flow regimes for flow condensing in mini/micro tubes. According to the importance of the interfacial tension compared to the interfacial stress, the regimes were classified into three kinds: axial symmetrical, semisymmetrical, and asymmetrical. The results indicated that the surface tension of the fluid media obviously affects the flow regimes; for tubes with an inner diameter less than 100 to 600 mm, the flow regimes would be axially symmetrical depending on the media. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(1): 65–71, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10076     

Linear temporal and spatial stability formulations of two‐dimensional surface waves on evaporating,isothermal, or condensing liquid films

When a liquid film is under the evaporating or condensing condition, the flow stability is clearly different from that under an isothermal condition due to the thermal non‐equilibrium effect at the interface, especially under a lower Reynolds number. Based on Prandtl's boundary layer theory and complete boundary conditions, the universal temporal and spatial stability formulations are established using the collocation method for two‐dimensional surface waves of the evaporating or condensing and isothermal liquid films draining down an inclined wall. The evolution equations indicate that the flow stability is closely related to the Reynolds number, thermocapillarity, inclination angle, liquid property, and evaporation, isothermal, or condensation actions. The effects of the above factors are investigated with neutral stability curves at different Reynolds numbers, and stability characteristics are fully indicated in theory for evaporating or condensing films. Results show that the evaporation process destabilizes the film flow and condensation process stabilizes the film flow. Thermocapillarity has a stabilizing effect in an evaporation condition and an adverse effect in the condensation condition. For a lower Reynolds number, the vapor recoil and thermocapillary take dominant effects when compared to the inertia force in determining flow stability. At a higher Reynolds number, the flow stability is controlled by the inertia force. Present study indicates that the disturbance increases with an increase of the Reynolds number and inclination angle, and decreases with increase of Ka numbers. Furthermore, the effects of liquid properties and inclination angle are always significant. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 243–257, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20062     

Analytical modelling of transient phase‐change problems

In this paper, an analytical solution for the temporal location of moving solid–liquid interface of a phase‐change process, occurring in parallel plate channels, is presented. The motion of the solid–liquid interface is governed by the convection from the surface of one of the plates, while constant heat supply is assumed to occur on the surface of the other plate. The steady location of the solid–liquid interface is also determined. The variation of the Biot number versus the Fourier number is investigated. The results of this study indicate that simple analytical solutions for transient phase‐change problems with heat flux and convective boundary conditions that are of practical importance to the people working in the field can be obtained. Copyright © 2000 John Wiley & Sons, Ltd.     

Radioluminescent nuclear battery containing CsPbBr3 quantum dots: Application of a novel wave‐shifting agent

Radioluminescent nuclear battery has been widely studied for its miniaturization and long life. In this study, all‐inorganic perovskite quantum dots (CsPbBr₃ QDs) were selected as a novel wave‐shifting agent combined with liquid scintillator PPO (2,5‐diphenyloxazole). The QDs were used to regulate the emission spectrum to match different GaAs devices. The maximum output power of the RL nuclear battery was greatly enhanced by 1.91 to 2.53 times. Perovskite QDs with adjustable emission spectra were used as wave‐shifting agents and exhibited more excellent properties and application prospects than traditional wave‐shifting agents. The Monte Carlo method was used to simulate the energy deposition of fluorescent materials under various radioactive source models. Results verified the advantages of using liquid radioactive sources and liquid fluorescent materials. The application and reference value of perovskite QDs in nuclear detection and nuclear medical imaging were also discussed.     

Numerical simulation of steam–water two‐phase flow under dynamic load

A separated‐phase physical model for steam–water two‐phase flow on a rotating platform was developed. The mesh generation for a horizontal pipe was conducted, and the finite volume method was used to discretize the equations. Equations were solved with the SIMPLE algorithm after setting the initial and boundary conditions. Predicted results were compared with experimental data, and they agreed well with each other. The results showed that the fluid outlet pressure and pressure drop in the test section increased with increasing dynamic load. However, the effective heat transferred to the fluid decreased with the increase of dynamic load. The developed model can be used to simulate the gas–liquid two‐phase flow under different gravity or rotary conditions.