Critical Airflow Velocity Research Articles

The impact of a two-dimensional vibration excitation on the critical incident flow velocity of a sessile droplet

The movement of a sessile droplet can be initiated by different mechanisms. In addition to an incident flow, a vibration or differences in the surface properties can initiate the movement of a droplet. In many cases, the mechanisms occur in combination and their interaction is not well understood. We report on the investigation of superposition of an incident flow and a two-dimensional vibration excitation acting simultaneously. The focus is on the analysis of the critical air flow velocity required for the detachment of a droplet. More precisely, it is investigated how a simultaneous two-dimensional vibration affects the critical incident flow velocity. In particular, the influence of a phase shift of both excitation sources with respect to each other is investigated. For this purpose a rectangular Plexiglas flow channel equipped with two electromagnetic shaker is used. One oscillation source acts vertical to flow direction, while the second shaker operates simultaneously in horizontal direction. Two scenarios are considered for the orientation of the horizontal vibration excitation. Case 1: the horizontal vibration excitation is in the direction of the incident flow. Case 2: the horizontal vibration is transverse to the incident flow. The excitation frequency and acceleration are varied within the series of experiments. Water droplets of different volumes (7.8 – 23.4 μl) are studied. The substrate is polymethylmethacrylate (PMMA), which has a hydrophilic character, at static contact angles of about 74°for a water droplet. The data obtained reveal that the critical velocity for detachment of a sessile droplet can be significantly reduced by superimposing an oscillatory excitation only for certain frequencies. A significant decrease in the critical velocity is observed for an excitation in the range of the first and second eigenfrequencies. The phase offset of the two vibration sources will also affect the critical droplet detachment velocity if the horizontal excitation source is parallel to the incident flow. However, if the excitation is in the horizontal plane perpendicular to the incident flow, a phase shift between the two vibration sources has no effect.

Molecular dynamic simulations of Ti-6Al and Fe-12Cr alloys for their heat transfer and oxygen transport behaviors

The heat transfer and oxygen transport behaviors of Ti-6Al and Fe-12Cr alloys directly affect their aeronautical material application. In order to investigate the flame retardant mechanism in atomic scale, molecular dynamic (MD) simulations were performed to study the heat transfer and oxygen transport performances of Ti-6Al and Fe-12Cr alloys in the linear and uniform heating treatment. Next, the ignition performances of Ti-6Al and Fe-12Cr alloys were further verified by the frictional ignition tests. Therefore, this is a multi-scale research with a novel combination of MD simulations and ignition tests. Those obtained results show that the heat is more concentrated at the heating edge of Ti-6Al alloy while at the same heating rate. This leads to the intensification of atomic motion at the heating edge, and reduces the structural thermal stability of Ti-6Al alloy’s heating edge. Therefore, the heat transfer performance of Fe-12Cr alloy at the same state is better than that of Ti-6Al alloy. When there is an oxide layer on the surface of these two alloys, Fe-12Cr alloy can inhibit the oxygen transfer more effectively than Ti-6Al alloy, and further improve its flame retardant property. In addition, the frictional ignition tests with Ti alloy rotor under the same airflow environment show that the critical airflow velocity of the Fe-12Cr alloy stator is about 114[Formula: see text]m/s higher than that of Ti-6Al-4V alloy stator at 384∘. Generally, according to MD simulations and frictional ignition tests, it could be concluded that the flame retardancy of Fe-12Cr alloy is higher than Ti-6Al alloy.

Nonlinear flutter analysis of functionally graded carbon

In this paper, geometrical nonlinear flutter analysis of functionally graded carbon nanotube-reinforced composite plates is presented. The governing equations of the system are obtained using the Mindlin plate theory and von Karman geometric nonlinearity. The linear piston theory is utilized to determine the aerodynamic pressure. Applying the Hamilton principle, equations of motion of a system are derived in the matrix form, and then the finite element method is used to obtain the discretized equations. Application of Newmark’s time integration and Newton–Raphson method are used to solve the nonlinear oscillation equations for determining the nonlinear flutter response and critical flow velocity. To check the validity of the present formulation, numerical results are compared with the previous data in the literature. The effects of stiffeners, type carbon nanotube (CNT) and angle of attack of airflow, thickness (ratio h/a) of the plate on critical airflow velocity and nonlinear dynamic response of the SFG-CNTRC plates are studied.

Experimental study on the influence of drainage tank structure on separation performance of vertical corrugated plate dryer in PWR

Corrugated plate dryer (CPD) is an important steam-water separation (SWS) devices in the steam generator of Pressurized Water Reactor (PWR) nuclear power plants (NPPs), and the drainage tank (DT) in CPD determines the separation efficiency (SE). This paper designs an experimental bench to study SWS performance of a full-scale CPD commonly used in PWR NPPs. The influence of structure of the DT (including the changes of the height of DT and the number of baffles in the DT) at the lower part of CPD affects the SE, critical airflow velocity (CAV), and pressure drop is studied from the perspective of mechanism. It can be concluded that the increase in the number of baffles inside the DT enhances the CAV and SE of CPD. Meanwhile, increasing the number of baffles has a more pronounced effect on boosting the CAV than that of the height of DT within a specific range, which provides an important reference for the miniaturization design of NPPs.

Experimental study of the formation and diffusion of soap bubbles for wrapping and transporting indoor airborne pollution

A novel soap-bubble-assisted local exhaust ventilation (SBA–LEV) system is proposed in this study, which uses soap bubbles to separate centralized released airborne pollutants from ambient air and prevents mixing and dilution in the pollutant diffusion process. Four SBA-LEV system modes were proposed, and the formation and free diffusion of the soap bubbles in Mode A were studied by using a single-nozzle air-supply experimental setup. The velocity range of the soap bubble formation was determined, and the relationship between the maximum bubble-forming flow velocity and nozzle diameter was obtained. For a given nozzle diameter, the diameters of the generated soap bubbles were essentially the same and independent of the blowing velocity. Moreover, the soap bubble diameter increased linearly with an increase in nozzle diameter. In the soap bubble diffusion process, it showed that the moving trajectory of a soap bubble is similar to a parabola, and the error between the theoretical and experimental values was less than 20%. Moreover, it indicated that the diffusion range of a soap bubble group expanded with an increase in diffusion height. When the blowing velocity was near the minimum critical airflow velocity, the diffusion range of soap bubbles is the smallest and suitable for capture. Finally, a general design and efficiency evaluation method of the SBA–LEV system was discussed. The SBA-LEV system can effectively reduce the difficulty of pollutant capture by exhaust airflow and the required exhaust air volume, and finally improve the pollutant-removal and energy-saving efficiency of LEV system.

Spray characteristics of gel fuel in a gas-centered coaxial swirl injector with variable recess ratios

This paper has studied the spray characteristics of gel in a series of different gas-centered coaxial injectors by experimental observations. Impacts of recess ratio on breakup features, such as the breakup length, spray angle and droplet size distribution, have been also investigated. The gel spray morphology shows that the spray field could be divided into not-fully and fully-atomization zones with the different gaseous flow rates. Then a critical airflow velocity has been proposed to distinguish the different atomization patterns. Experimental results reveal that the larger recess ratio would widen the droplet spatial distribution, increase the spray angle and reduce the Sauter Mean Diameter (SMD) in the spray field. Besides, the contraction impact of inner gaseous flow could also be visualized by these experiments. It could be an effective way to improve its atomization performance of gel solutions based on the present coaxial injectors.

Influence of the structure of drainage tank on the separation performance of corrugated plate dryer in NPPs: A short communication

As an important steam-water separation (SWS) device in the steam generator of the nuclear power plant, a corrugated plate dryer (CPD) determines the steam dryness, and its drainage tank (DT) plays a key role for SWS. The influence of the structure of the DT on the overall performance of the CPD is studied. Besides, DT with partition is designed for the first time. In this paper, an experimental system for study on separation performance (SP) of CPDs is designed and built, and the study is conducted by changing structure of DT. The influence of structure of DT on separation efficiency (SE), critical airflow velocity (CAV) and pressure drop (PD) of CPD are studied. It is found that SE, CAV and PD of the CPD which adopt DT with partition are the largest among the three different DT structures used. In order to improve the separation efficiency of the CPD, it is possible to install the partition in DT instead of increasing the height of DT. This can reduce the volume of the DT, thereby reducing the volume of the CPD, which provides the reference and possibility and for the miniaturization design of NPPs.

Multiphysics Modeling Simulation and Optimization of Aerodynamic Drum Magnetic Separator

Aerodynamic Drum Magnetic Separator (ADMS) uses an adjustable air flow to enhance the separation of magnetic particles from gangue. In order to explore the matching relationship between the magnetic field, the flow field, and the gravity field, as well as the capture and separation behavior of particles under the action of multi-physics, a related simulation model is established using the finite element software COMSOL Multiphysics and the accuracy of the simulation results is verified by measurement, formula calculation, and magnetic separation experiment. The trajectories and capture probabilities of particles in different magnetic fields and flow fields are calculated, as well as the critical airflow velocity corresponding to a specific capture probability. In addition, the magnetic field characteristics and particle capture effect of N-S alternate arrangement and N-N homopolar arrangement are compared by optimizing the permutation of magnetic poles. This model may provide a reference for the accurate control of magnetic separation enhanced by a coupling force field.

Determination of rational process variables for multi-step centrifugal mills

This paper presents the results of experimental studies into milling under multiple impacts in multi-step centrifugal mill designed at the Chersky Institute of Mining. In order to enhance milling capacity, it is proposed to add breakers of the mill with more breaking elements. However, addition of breaking elements sometimes fails to intensify milling and can adversely affect on the milling efficiency to to higher velocity air flows generated in the working space between the top and bottom breakers. The tests have determined the rational number of additional breaking elements, conditioned by the critical air flow velocity of 10 m/s in the breaker chamber of the lab-scale multi-step centrifugal mill. Given these conditions are fulfilled, the fineness number of the multi-step centrifugal mill with additional breaking elements grows considerable as against the base case of the mill without additional elements.

Pyrolysis and piloted ignition of thermally thick PMMA exposed to constant thermal radiation in cross forced airflow

Impact of cross forced airflow on pyrolysis and piloted ignition of PMMA (Poly methyl methacrylate) under constant heat flux is experimentally, analytically and numerically addressed in this study. Experimental setup consisting of a heating and blowing units was employed to provide the target constant heat flux and airflow velocity. Surface temperature and ignition time were measured in tests to examine the combined effect of forced airflow and heat flux. No ignition was observed when the heat flux and air velocity were 11.53 kW/m2 and 1.43 m/s combined, indicating a critical airflow velocity dominates ignition. An analytical model and a solid phase numerical model were developed to predict and validate the experimental measurements. Both surface and in-depth absorption of thermal radiation were considered in the models. The results showed both the critical temperature and ignition time increase with airflow velocity. Radiation reducing was found to be enhanced by increasing airflow velocity. Both the analytically and numerically predicted results employing either pure surface or in-depth absorption of thermal radiation matched the experimental measurements at acceptable levels, but the agreement between the calculations of in-depth absorption and measurements was better.

The Motion of Strawberry Leaves in an Air-Assisted Spray Field and its Influence on Droplet Deposition

HighlightsA visualization method for the motion of strawberry leaves in an air-assisted spray field is proposed.Strawberry leaves showed two motion states in different critical velocity ranges of the sprayer airflow.The airflow instability and the turbulence effect are considered important factors for the leaf vibrations.A strawberry leaf azimuth angle in the range of 90° to 270° can provide good deposition with smaller droplets.Abstract. The reasonable motion of crop plants in an air-assisted spray field can improve droplet deposition. Therefore, this study focuses on the motion of strawberry leaves and the droplet deposition mechanism in an air-assisted spray field. First, this study proposes a descriptive method for strawberry leaf motion in an air-assisted spray field and clarifies the important influence of strawberry leaf motion on droplet deposition. Second, an experiment was performed on the motion and droplet capture of single strawberry leaves in multi-position postures in an air-assisted spray field. The results showed that the leaves had two motion states (i.e., low amplitude with low frequency and high amplitude with high frequency) at different airflow velocities and inclination angles, and the critical airflow velocity corresponding to the two motion states was determined to be 8.7 m s-1. When the azimuth angle of the strawberry leaves is in the range of 90° to 270°, a reasonable inclination angle of the airflow and the high frequency and high amplitude vibration state of the leaves driven by the airflow will provide good deposition and canopy penetration of droplets with smaller diameters. Keywords: Air-assisted spray field, Droplet deposition, Motion, Spray, Strawberry leaves.

Review and Prospect of the Measurement Technology of the Thickness of the Liquid Film on the Wall of the Corrugated Plate Dryer

The corrugated plate dryer is an important steam-water separation device in the steam generator in the secondary loop of a light water reactor (LWR). It is essential to understand its steam-water separation mechanism. Most studies have shown that the separation efficiency of the corrugated plate is related to the thickness of the liquid film on the wall of the corrugated plate. The relationship between the thickness of the liquid film and the critical airflow velocity of the corrugated plate has been studied by us. At present, scholars mostly use image method, ultrasonic method and capacitance method to measure the thickness of thin liquid film. We have analyzed the application of these three methods in liquid film thickness measurement. Some important examples are listed. The results show that the capacitance method is recommended for liquid film thickness measurement experiments whereas its accuracy is not particularly high. To pursue the higher accuracy, the image method is recommended. Planar laser-induced fluorescence (PLIF) technology is the first choice among image methods. Ultrasonic method is applicable in the case that the measurement data is large.

A numerical and experimental study on the divergence instability of an inverted cantilevered plate in wall effect

We present a numerical and experimental study on the static-divergence instability of an inverted cantilevered plate in an uniform axial subsonic airflow. The flow is assumed ideal and is confined by a rigid channel wall. The inverted cantilevered plate, unlike a conventional one, is with a clamped trailing edge and a free leading edge. The equation of the plate motion is solved by the finite-difference method, and the linearized boundary element method is applied for the fluid equations. A new trigonometric form vortex panel model is developed for the solution of fluid force. An equivalent test method is developed for the divergence instability and is applied for the experimental study. The effect of the rigid wall is evaluated for various distances between the plate and the rigid channel wall, and the critical airflow velocity increases with the increase in the distance. The numerical calculations and predication show good agreement with the experimental results. When the distance is large enough, the numerical results are in accordance with the existing theory for an unconfined airflow. Finally, an approximated solution of the critical airflow velocity in terms of the distance is suggested.

Effect of physical property parameters on critical airflow velocity of a corrugated plate dryer

Corrugated plate dryers are important steam-water separation devices in the steam generator of the nuclear power plant. No previous analysis of effect of physical property parameters on critical airflow velocity of corrugated plate dryer has been performed. Therefore, research in this area is innovative and novel. Based on theoretical correlation of the critical airflow velocity proposed in reference (Wang and Tian, 2020); effect of physical property parameters on critical conditions of the liquid film rupture under horizontal shear of airflow is experimentally studied. In the experiment, pure water, 2.5% (mass fraction) butanol, and ethanol are selected as three different liquids, respectively. Thickness of the liquid film on corrugated plate wall is measured by PLIF (Plane Laser Induced Fluorescence) method. Experimental results show that liquid film rupture is related to the stability parameter Kapitza number. The larger the Kapitza number, the better the liquid film stability and the higher the critical airflow velocity.

Study on fracture morphology of water film on the wall of vertical corrugated plate under shear of the air flow

Corrugated plate is a main steam-water separation device, and the water film cracking mechanism has a great influence on the steam-water separation efficiency. Based on high-speed photography method, the rupture morphology of the water film is experimentally studied. Rupture morphology of the water film under the shearing of the air flow is numerically simulated according on the improved moving particle semi-implicit (MPS) method, and the initial distribution of the air flow velocity in the corrugated plate channel is obtained. The relationship between critical air flow velocity when the water film is broken and the water film width is given. Comparison between experimental results and numerical results shows that improved MPS method can simulate location of film rupture, the rupture pattern, the droplet splash direction, and flight speed of the droplet after rupture, respectively. There is a positive correlation between critical air flow velocity and water film width on the wall.

Study on fluctuation feature and breakdown characteristic of water film on the wall of corrugated plate

Corrugated plate dryer is a very important steam-water separation equipment for nuclear engineering. And heat and mass transfer phenomenon is commonly observed in the separation process. It is meaningful to study the flow, fluctuation and breakdown characteristics of the water film on corrugated plate wall. Water film thickness of steady flow is measured based on plane laser induced fluorescence (PLIF) technique and time series of water film thickness are obtained. Besides, amplitude and frequency domain information of water film fluctuations are analyzed by the power spectral density (PSD) method. A two-dimensional model of water film breakdown is established. A model of the relative position of the liquid film rupture is established. Critical airflow velocity when the water film breakdown is measured experimentally. Based on theoretical results, equations for calculating critical airflow velocity of the corrugated plate under different water film thicknesses are fitted in the form of nike function. Fitting equations for calculating critical airflow velocity in the form of nike function agree well with experimental results. The relative position of water film breakdown is merely affected by the water film Reynolds number and the structure parameters of the corrugated plate.

Analysis of fluctuation and breakdown characteristics of liquid film on corrugated plate wall

Corrugated plate dryer is a very vital separation equipment for nuclear engineering and it is meaningful to study the flow, fluctuation and breakdown characteristics of the liquid film on corrugated plate wall. Liquid film thickness of steady flow is measured based on plane laser induced fluorescence (PLIF) technique and time series of liquid film thickness are obtained. Besides, amplitude and frequency domain information of liquid film fluctuations are analyzed by probability density function (PDF), power spectral density (PSD) and wavelet method. A two-dimensional liquid film breakdown model is established. Critical airflow velocity when the water film breakdown is measured experimentally. Based on theoretical results, equations for calculating critical airflow velocity of the corrugated plate under different liquid film thicknesses are fitted in the form of nike function. Fitting equations for calculating critical airflow velocity in the form of nike function agree well with experimental results.

Study on characteristics of water film breakdown on the corrugated plate wall under the horizontal shear of airflow

The corrugated plate (CP) dryer is a very important component for the steam-water separation in the steam generator because the separation efficiency influences the overall thermal efficiency of a nuclear power plant. Carryover and breakdown phenomena of the water film at the corner of the CP have a great impact on the steam-water separation effect. The study of the complicated steam-water separation mechanism is beneficial to improving its efficiency. Breakdown and flow characteristics under horizontal shear of water film falling down from the vertical CP is studied. Water and air are used as the two phases in the study. This paper derives and validates a model for calculating the critical airflow velocity of water film breakdown at the corner of the CP. On the basis of the hydrodynamics theory and applying the image processing method, the relationship between water film thickness and critical airflow velocity of water film breakdown is investigated through an experiment. Additionally, the water film thickness is measured with the application of the Charge Coupled Device (CCD) high-speed camera acquisition system. The Planar Laser Induced Fluorescence (PLIF) method is applied for the water thickness measurement in the experiment. Results reveal that the critical airflow velocity of water film breakdown is related to the CP structural parameters, properties of the two phases and water film thickness. Comparison of the experimental result and the result calculated by the model is performed as well. The results calculated by the developed model are in good agreement with the obtained experimental results as the water film thickness is large in a certain range. The results show that the critical airflow velocity of water film breakdown and the water film thickness are negatively correlated. In order to analyze the factors contributing to the critical airflow velocity of water film breakdown, three novel parameters are introduced firstly. In particular, the results reveal that a larger value of parameters is better to the increase of the critical airflow velocity of water film breakdown. And when the flexural angle of the CP corner is approximately 27 degrees, the steam-water separation performs best. It recommends that the flexural angle of the CP corner should not exceed 43.2 degrees.

Investigation on flow and breakdown characteristics of water film on vertical corrugated plate wall

Steam and water separation effect in the corrugated plate dryer plays a decisive role in the economic indicators of nuclear power plants. Investigation on flow characteristics of water film on vertical corrugated plate wall is beneficial to the research of steam-water separation mechanism and also plays an important role in the analysis of the water film flow characteristics and instability. In this paper, the water film thickness is measured by Planar Laser Induced Fluorescence (PLIF) technique. The relationship between the free falling water film thickness and time and the relationship between the average water film thickness and the water film Reynolds number are obtained, respectively. With probability density function (PDF), power spectral density (PSD) and wavelet analysis methods, the flow fluctuation instability of water film on the corrugated plate wall is analyzed. Characteristic frequency and vibration amplitude of water film flow fluctuation are obtained and the relationship between the water film thickness and the Reynolds number is fitted. Results reveal that the water film thickness has a certain regularity with the change of the Reynolds number and the two have a positive correlation. As the Reynolds number increases, the generation of solitary waves increases the water film volatility. The water film thickness is multi-frequency and has no obvious periodicity. When the water film Reynolds number in the experiment is less than 4055 and the flow is in a certain range of flow resistance, the water film breakdown will not happen owing to its own gravity. It indicates that the gravity have little influence on phenomenon of breakdown of a water film with a stable flow in a vertical corrugated plate. Therefore, the force which the water film is subjected to at the corrugated plate corner is analyzed. The model is established and the theoretical equation of the critical airflow velocity of water film breakdown and the water film thickness is derived as well.

Experimental and fundamental critical analysis of diffusion model of airflow drying

ABSTRACTScientific literature of agromaterial drying present contradictory conclusions in terms of the kinetic effect of airflow velocity. Some authors confirmed that it does not trigger any modification of drying, while some articles tried to establish empirical models of the effective diffusivity Deff versus the airflow velocity, what is fundamentally erroneous. By analyzing internal and external transfer phenomena, this research aimed at recognizing that once air velocity is higher than a critical airflow velocity (CAV), the internal transfers become the limiting phenomenon. CAV depends on the effective diffusivity and the product size. It was calculated in the cases of two studied raw materials (apple and carrot), differently textured by instant controlled pressure drop (DIC). Values of CAV greatly depend on diffusivity of water within the matrix. At temperature T = 40°C, they were 1 m/s for untreated carrot and 2.1 m/s for DIC-textured carrot, whose Deff values were 1.31 and about 3 × 10−10 m2/s, respectively. Also, at temperature T = 40°C, they were 2.1 m/s for untreated apple and 3 m/s for DIC-textured apple, whose Deff were 1.4 and about 10.4 × 10−10 m2/s, respectively.