Mist Flow Research Articles

A triboelectric nanogenerator design for harvesting environmental mechanical energy from water mist

Abstract Mechanical energy is abundant in nature and its potential is yet to be fully explored. Despite intensive research, triboelectric nanogenerators (TENGs) can only collect the energy stored in bulk water; reports on harvesting environmental mechanical energy from small scale water sources, including water mist, are still lacking. In this research, which is inspired by how tapered micro-pillars collect water from the air, a solid-liquid TENG is proposed for harvesting mechanical energy from water mist. The contact area of a water droplet with a dielectric layer in the TENG was found to be the key factor influencing the performance of the TENG, and higher current and voltage were achieved using a tapered electrode with a large diameter or by setting multiple tapered electrodes in parallel. An increase in local ambient humidity was observed to cause a significant increase in the saturation voltage and reduce the voltage saturation time of the TENG. However, an increase in ambient temperature and the amount of unpurified water mist degraded the performance of the TENG. By optimizing the parameters, the TENG can produce a maximum open-circuit voltage of 9.5 V and a short-circuit current of 250 nA simultaneously. Seven commercial light-emitting diodes (LEDs) were easily lit by the TENG, and their brightness was sensitive to varied mist flow rates. These results could inform the design and fabrication of new self-powered humidity sensors and TENGs for collecting energy from air.

A new wellbore fluid load diagnosing model based on the energy conservation law

Liquid load in the borehole would affect the rate stability of gas well adversely. However, the existing liquid load detection techniques are somewhat limited by practical application. For the purpose of clarifying the liquid-carrying mechanism of two-phase flow in the wellbore with a higher liquid rate, it is necessary to accurately diagnose the presence of liquid loading in the wellbore and reasonably formulate the production measures of the gas well. On the basis of previous studies, this paper established a new model for diagnosing the liquid-carrying conditions of gas–liquid two-phase vertical pipe flow according to the law of energy conservation. Then, by comparing with field practice statistics and indoor experimental data, the accuracy of the new model was verified. Finally, the new model was applied to analyze the liquid-carrying condition of one certain liquid-producing gas well. And the following research results were obtained. First, when the liquid production rate is lower, the critical liquid-carrying gas rate calculated by the new model is much lower than the calculation result of the Turner model. Second, with the increase of the liquid production rate, the critical liquid-carrying gas rate calculated by the new model increases gradually. And the higher the pressure is, the more obvious the increase of the critical liquid-carrying gas rate is. Third, from the perspective of flow pattern, the gas–liquid two-phase vertical pipe flow can be divided into 5 kinds, including bubble flow, slug flow, transition flow, wavy flow and annular mist flow. When the two-phase flow is transition flow, wavy flow or annular mist flow, there is no liquid loading in the wellbore. In conclusion, the calculation result of the new model is basically accordant with field practice statistics and indoor experimental data, and its diagnostic conclusion conforms to the actual situations. Obviously, this model is of universality and provides a theoretical support for the diagnosis of liquid-carrying condition and the prevention of fluid loading in liquid-producing gas wells.

Experimental Investigation of the Micro-hardness of EN-31 Die Steel in a Powder-Mixed Near-Dry Electric Discharge Machining Method

The Powder-Mixed Near-Dry Electric Discharge Machining (PMND-EDM) methodology has proven to be efficient in terms of machining rate, surface morphology, and environmental friendliness, unlike traditional EDM. In this study, the presence of a conductive metallic powder (zinc) in the dielectric medium was responsible for changing the topography of the workpiece (EN-31) and resulted in a higher micro-hardness value of the machined component. In this research, an approach has been made to optimize the significant process parameters by using a Taguchi L9 orthogonal array (OA) to obtain machined components with higher values of micro-hardness, which was measured in terms of Vickers hardness HV. The selected process parameters were tool diameter, mist flow rate, metallic powder concentration, and dielectric mist pressure. By introducing foreign particles (metallic powder), the topography of the machined products has been improved, and the micro-hardness value was found to be enhanced. The confirmation experiment was performed for optimal process parameter settings, and the enhanced microhardness value was found to be 506.63 HV in the machined EN-31 die steel.

Critical Heat Flux Model for Vertical Annular Mist Flow Conditions in a Rod Bundle

As part of its design certification effort, NuScale Power has completed a series of low mass flux [<1000 kg/(m2‧s)] critical heat flux (CHF) tests for a wide range of pressures at Stern Laboratories in Canada. Earlier studies have demonstrated that under annular flow conditions, disturbance waves with circulating vortices traverse the rod surface. The disturbance wave slides over and significantly influences energy transport in the co-current vapor-liquid sublayer at the heater interface. This paper describes the mechanisms leading to the onset of CHF in a vertical rod bundle experiencing annular mist flow conditions. The paper presents a new CHF model that implements a local disturbance wave velocity. A comparison of the model to the U1 CHF data set from Stern Laboratories shows excellent agreement over the full range of annular mist flows, pressures, and subcooled conditions for the specific spacer grids implemented in the study.

Characterizing Mist Distribution in Through-Tool Minimum Quantity Lubrication Drills

Abstract The mist distribution is a critical factor in through-tool minimum quantity lubrication (MQL) drilling since a small amount of lubricant is used. However, it has rarely been discussed because of the difficulty in measuring the mist flow experimentally. In this paper, an optical approach is developed to approximate the mist distribution by using high-speed images from multiple angles. Drill bits with two through-tool channel shapes (circle and triangle) and three helix angles (0 deg, 30 deg, and 45 deg) are 3D printed for mist distribution analysis. Furthermore, computational fluid dynamics (CFD) is conducted to investigate the underlying physics behind mist flow variations. The results show that, in the circular channel, the mist is concentrated near the periphery; the low concentration region shifts away from the chisel point as the helix angle increases. For the triangular channel, the mist is concentrated near three vertices but is less affected by the helix angle. Furthermore, based on the CFD solution, high mist concentration tends to be in low-velocity regions and vice versa. This study confirms a noticeable difference of mist flow distribution in different through-tool channel designs.

Heat transfer intensification in emergency cooling heat exchanger and dry cooling towers on nuclear power plant using air-water mist flow

Advanced nuclear power plants are equipped with passive emergency heat removal systems (PEHRS) for removing the decay heat from reactor equipment in accidents accompanied by primary circuit leakage to the final heat absorber (ambient air). Herein, the intensity of heat dissipation to air from the outer surface of the heat exchanger achieved by buoyancy induced natural convection is extremely low, which need to a large heat exchanger surface area, apply different types of heat transfer intensification including (grooves, ribs and extended surfaces, positioning at higher altitudes, etc.). The intensity of heat removal is also strongly dependent on the ambient air temperature (disposable temperature head). Construction of nuclear power plants in countries with high ambient temperatures (Iran, Bangladesh, Egypt, Saudi Arabia, and others) which are characterized by a high level of ambient temperature imposes additional requirements on the increase of the heat exchange surfaces. The experimental investigation results of heat transfer intensification by a low energy ultrasonic which supply a fine liquid droplet (size ~3 µm) in the cooling air are presented in the present paper. In such case, the heat transfer between the surface and cooling flow involves the following three physical effects: convection, conductive heat transfer, and evaporation of water droplets. The last two effects weakly depend on the ambient air temperature and provide an active heat removal in any situation. The investigation was performed using a high-precision calorimeter with a controlled rate of heat supply (between 7800 and 12831 W/m2) imitating heated surface within the range of Reynolds numbers from 2500 to 55000 and liquid (water) flow rates from 23.39 to 111.68 kg·m-2·h-1. The studies demonstrated that the presence of finely dispersed water results in a significant increase in heat transfer compared with the case of using purely air-cooling. With a fixed heat flux, the energy efficiency increases with increasing water concentration, reaching the values over 600 W·m-2·C-1 at 111.68 kg·m-2·h-1, which is 2.8 times higher than for air cooling. With further development of research in order to clarify the optimal areas of intensification, it is possible to use this technology to intensify heat transfer to the air in dry cooling towers of nuclear power plants and thermal power plants used in hot and extreme continental climates.

The effect of droplets thermophysical properties on turbulent heat transfer in a swirling separated mist flow

The numerical investigation of the effect of the swirl number and the thermophysical properties of water, ethanol, and acetone droplets on the particle scattering, turbulence modification and heat transfer in a droplet-laden flow is carried out. The set of 3D steady-state Reynolds-averaged Navier–Stokes (RANS) equations for the two-phase flow is utilized. The dispersed phase is modeled by the Eulerian approach. The flow swirling significantly shortens the length of the recirculation zone (up to two times in comparison with non-swirling flow at swirl number S = 0.5) for all types of the studied liquids. The highest value of heat transfer rate is obtained for the ethanol droplets (up to 10%), and the lowest one is obtained for the acetone droplets (up to 50%), both in comparison with water droplets. The swirling of the turbulent mist flow causes heat transfer enhancement (more than 1.5 times in comparison with the non-swirling droplet-laden flow). In the swirling flow, the value of the volume fraction of the dispersed phase has a maximum in the axial region of the pipe, and further in the direction of the wall its value is very small. The effect of heat transfer enhancement weakens with increasing Reynolds number of the flow. A region without droplets appears in the wall zone of the swirling two-phase flow, and the region is largest for acetone droplets.

Analysis of the Impact of Selected Parameters of the Hybrid Extinguishing System on the Fire Environment in a Closed Room

The main purpose of this study was to analyze the impact of some parameters (water mist flow rate and type of gas used) of the hybrid extinguishing system on the fire environment (temperature as well as carbon monoxide and oxygen concentrations) in a closed room. Hybrid fire-extinguishing systems in which water mist is driven by inert gas combine the advantages of typical fog systems and fixed gas extinguishing devices. They have been developed in the last years but are now being used more and more often and the preparation of standards for them is planned for 2020. For this purpose, many fire tests with this system should be conducted. Some of them are discussed in this paper. Two different flow rates of water mist (1.5 or 3 dm3/min) and inert gas (nitrogen or air) were used during hybrid system testing. Some parameters of the fire environment in the compartment such as temperature measured by thermocouples as well as carbon monoxide and oxygen concentrations measured by electrochemical gas sensors are presented here. The characteristic values of the extinguishing process are also included. The assumed times of ensuring safe conditions in the room have been confirmed.

Prediction of pressure drop in Venturi based on drift-flux model and boundary layer theory

Venturi, as the primary flow measurement sensor, is widely used in various industrial fields of oil and natural gas. Pressure drop of the Venturi is a crucial factor in the process design of exploitation and transportation of natural gas. Based on the drift-flux model and boundary layer theory, a pressure drop prediction model is established. Except for divergent section, a uniform void fraction model is established basing on drift-flux model. The thickness of boundary layer grows rapidly due to the existence of adverse pressure gradient in the divergent section, which results in an increase of the irrecoverable pressure drop. Considering the influence of slip between gas and liquid, weight coefficient is used to adjust the proportion of displacement thickness in the cross section of the Venturi. Compared to experiment, the theoretical model is applied to stratified wavy flow and annular mist flow. For different diameter, the relative deviations of experiment points are within ±15%.

Numerical analysis of pre-dryout sequences through the route of interfacial evolution in annular gas-liquid two-phase flow with phase change

Numerical modeling of two-phase annular flow boiling has been carried out inside a tube using volume of fluid based computational multifluid dynamics. The proposed numerical methodology has been validated with available experimental results from literature. The stages of bubble nucleation, growth, merging, bursting, droplet entrainment, film rewetting, and dryout has been clearly visualized for diabatic annular mist flow. Plots of liquid phase fraction and average heat transfer coefficient were plotted which shows decreasing trends with axial space. Effect of different degrees of superheats and gas-liquid velocities at a higher working pressure of 40 bar have been studied, which can be commonly observed in boiling water reactor (BWR) conditions. Dryout is found to happen early in the axial length if the degree of superheat and flow velocities increases. The chaotic interfacial behavior has been analyzed with mean, standard deviation and attractor plot of space time-varying values of liquid phase fraction.

Evaluation method of transit time difference for clamp-on ultrasonic flowmeters in two-phase flows

To achieve efficient energy management in industrial plants, it is crucial to measure the steam flow rates at the various points of consumption. Clamp-on time-of-flight ultrasonic flowmeters are useful devices to measure the steam flow rates in existing pipes. However, it is difficult to accurately measure the steam flow rates because of the large acoustic impedance difference between the pipe material and fluid, strong signal attenuation in the fluid, and high temperature. In addition, the steam wetness increases with heat losses and the presence of liquid film and droplets that creates noise in the detected ultrasonic signals, making it difficult to distinguish the target signals from the noisy ones. Hence, a new signal processing method is proposed to determine the transit time difference, particularly at lower signal-to-noise ratios. Two ultrasonic transducers were used to measure the ultrasonic time-of-flight, which varies depending on the flow rate. The transmitted ultrasonic signals were time-dependent due to the generation of guided waves in the pipe wall. The standard deviations of the target signals increased when the flow regime transitioned from stratified to annular mist flow. The guided waves significantly influence the success ratio of determining the transit time difference between the ultrasonic signals transmitted from the upstream and downstream transducers. Based on the results, the use of the standard deviation of the target signals is proposed for estimating the transit time difference. Further, as the standard deviation varied significantly depending on the flow regime, it can be used to identify the flow regime as well.

Investigation on surface finish in powder mixed near dry electric discharge machining method

In this research, an approach has been made to optimize the significant process parameters by using Taguchi L9 orthogonal array (OA). The selected machining parameters for the present research work are tool diameter, mist flow rate, metallic powder concentration and mist pressure while the level set for each parameter was three. It was observed that tool type at level 2 (3 mm), flow rate at level 2 (10 ml/min), powder concentration at level 3 (8 g/l) and mist pressure at level 2 (0.5 MPa) were most influential in obtaining maximum surface finish of 0.303 µm in the machined EN-31 sample by PMND-EDM.

Enhancement of Mist Flow Cooling by Using V-Shaped Broken Ribs

Substantial heat transfer enhancement can be achieved by cooling with air/water mist flow because of droplet impingement and liquid film/fragment evaporation on the heated surface, which leads to a high heat-removal rate. An experimental investigation was conducted in a square channel with continuous and broken V-shaped ribs. To generate a mist flow, micro droplets were introduced into the gas stream. The rib angle of attack was 45°, and the rib spacing-to-height ratios were 10 and 20. The air Reynolds number ranged from 7900 to 24,000, and the water-to-air volume flow ratio was less than 0.1%. The net heat inputs ranged from 1.1–3.1 W/cm2 and 3.4–9.4 W/cm2 for the air and mist flow cases, respectively. Because the deposited liquid fragments produced uneven temperature distribution on the heated surface, steady-state infrared thermography was used to visualize the heat transfer distribution. Two to seven times higher heat transfer was attained for the broken ribs when using the mist flow than when using air flow. This increase was mainly attributed to the broken structure, which facilitated liquid transport and enhanced liquid coverage. In addition, the broken ribs produced a smaller friction factor than continuous ribs. The broken structures were beneficial for higher thermal performance in the mist flow.

Analysis of parallel mini-channels’ complex flow boiling and dryout dynamics based on the pressure drop signals

Based on a heat exchange experiment, the authors used R141b as the working medium, and analyzed the differential pressure signal of parallel mini-channels (N = 9, L = 250 mm, Wc = 2 mm, H = 2 mm). Different flow patterns (bubble flow, slug flow, annular flow, dryout, mist flow) have different pressure drops. Flow boiling characteristics were analyzed by the adaptive optimal kernel time–frequency representation (AOK-TFR), autoregression (AR) power spectrum and recurrence plot (RP). Then, the dryout kinetics of working fluids were revealed. Finally, through the combination of three recursive characteristic parameters, the dryout prediction based on the differential pressure signal was proposed, which provides the theoretical basis for the dynamic characteristics of boiling heat transfer. At the same time, it gives a solution for prevention of the second type of heat transfer deterioration in a project.

Online Measurement of Wet Gas by Modeling Frequency and Amplitude Characteristics of a Vortex Flowmeter

In this article, an accurate online measurement of gas flowrate in wet gas was developed by modeling the frequency and amplitude characteristics of a vortex flowmeter in annular mist flow. The low-frequency modulation behaviors of the vortex signals were analyzed by the ridge method based on continuous wavelet transform (CWT). It indicates that the instability of frequency and amplitude of the signals both enhance with the increase of liquid loading; and therefore, a feature extraction method based on ridge-averaged was proposed and validated by the tests. A film extraction device was developed to measure the flowrate of the liquid film and further obtain the droplet mass loading. Then, a uniform linear equation for the overreading was obtained experimentally on the basis of the scaling parameter analysis in particle-laden flow. A uniform exponential equation for the dimensionless signal amplitude of the sensor output was developed with droplet mass loading. Finally, a wet gas measurement model with the simultaneous equations from these two correlations was established and the iterative solution algorithm was given. In the present cases, the percentage errors (PEs) of the gas flowrate predicted by the modeling are within ±1.5% and the mean absolute PE is 0.37%. The method proposed in this article can achieve accurate measurement of gas flowrate by using only one vortex flowmeter and, hence, creates a simple inexpensive metering system for wet gas flow.

Интенсификация теплообмена воздушных теплообменников аварийного расхолаживания и сухих градирен АЭС с использованием водовоздушного аэрозоля (тумана)

Интенсификация теплообмена воздушных теплообменников аварийного расхолаживания и сухих градирен АЭС с использованием водовоздушного аэрозоля (тумана)

Effect of evaporating droplets on flow structure and heat transfer in an axisymmetrical separated turbulent flow

The results of a numerical investigation of the effect of droplets’ thermophysical properties on the flow structure, turbulence, and heat transfer in a two-phase mist flow in a sudden pipe expansion are presented. Predictions are carried out for droplets of water, ethanol, and acetone for initial droplet diameters varying in the range d1 = 1–100 μm. The RANS approach is used to simulate a gaseous phase, and the motion and heat transfer of a dispersed phase are computed using the two-fluid Eulerian model. Gas phase turbulence is predicted using the model of Reynolds stress transport modified for the case of particle presence. It is shown that the addition of droplets leads to a significant increase in heat transfer (up to 50% at ML1 = 0.05) in comparison with a single-phase separated flow. Heat transfer enhancement with the use of ethanol droplets is slightly higher than the corresponding value for water droplets (by approximately 10–20%). The length of the region of intensified heat transfer is maximal for water droplets, while it is minimal for acetone droplets. The effect of heat transfer intensification and suppression of carrier gas phase turbulence is minimal for the acetone droplets.

Sustainable drilling process of 1045 steel plates regarding minimum energy consumption and desired work quality

Green manufacturing technologies comprising low energy processes with minimum environmental pollution are progressive issues that considered by the researchers and various industries during last decades. The aim of current work is to study effect of longitudinal ultrasonic vibration and minimum quantity lubrication on drilling force, surface roughness and burr height. For this purpose series of experiment are designed based on response surface methodology to analyze effect of vibration amplitude, mist flow rate, cutting speed and feed rate on machining power, exit burr height and surface roughness. Analysis of variances was also performed to identify which factor has great impact on process quality characteristic. Results indicated that the most influential factor for cutting force, surface roughness and burr height are feed rate, vibration amplitude and spindle speed, respectively. Also, setting of 10 μm vibration amplitude, 190 mL/h flow rate and 910 rpm spindle speed and 80 mm/min feed rate causes 98% desirability in multi response optimization.

A Study on the Effect of Fluid Properties and Watercut on Liquid Carry-Over in Gas-Liquid Cylindrical Cyclone Compact Separators

Prediction of the operational envelope (OE-limited range of gas and liquid velocities) for liquid carry-over is essential for the optimized performance of gas-liquid cylindrical cyclone (GLCC©) compact separators. This study presents for the first time the operational envelop for three-phase gas-oil-water flow incorporating pressure and level control configurations. A series of experiments were conducted to evaluate the performance of a 3 in. diameter GLCC in terms of OE for liquid carry-over. Experiments were carried out at different watercuts ranging from 0% to 100% utilizing water and two different types of mineral oils namely: light oil and heavy oil with specific gravities of 0.859 and 0.937, respectively. The liquid level was controlled at 6 in. below the GLCC inlet for all the experimental flow conditions. The experimental results indicate that OE for liquid carry-over for three-phase flow is very sensitive to watercut. As the watercut reduces, the OE for liquid carry-over reduces monotonically. Also, the OE for heavy oil (indicated by higher viscosity) reduces as compared to light oil. The superficial gas velocity required to create an annular mist flow in the upper part of the GLCC increases with the increase of watercut and viscosity.

CFD simulation on the flow and heat transfer characteristics of mist flow in wire-wrapped rod bundle

The heat transfer characteristics of post-CHF (critical heat flux) is very important and complicated, which has aroused wide attention in the topic of nuclear reactor safety. In this paper, a phase change model of liquid droplet during mist flow is proposed to consider the heat transfer of mist flow. Couple with this phase change model, three dimensional CFD simulation has been carried out to study the flow and heat transfer characteristics of mist flow in a wire-wrapped rod bundle. The discussion and analysis focus on the turbulent flow behavior and the heat transfer performance of post-CHF mist flow in the rod bundle. The effect of vapor properties such as density, heat capacity, thermal conductivity and viscosity on heat transfer are considered in the CFD simulation, and those properties are dependent on temperature. According to the CFD simulation result, the pressure distribution, temperature distribution, velocity distribution and phase distribution in the wire-wrapped rod bundle are analyzed in detail.