Mist Flow Research Articles

Prediction of turbulent flow characteristics and heat transfer in a dilute droplet-laden flow over a backward-facing step

We present numerical investigation of the influence of the thermophysical properties of droplets on turbulence modification, scattering of droplets, and heat transfer augmentation in a two-phase mist flow over a backward-facing step. Predictions are made for droplets of water, ethanol, acetone, and glycerol, with droplet diameters in the range d 1 =1–100 μm and mass fractions in the range ML 1=0.01–0.1 at the inlet. The RANS approach is used to simulate the gaseous phase, and the motion and heat transfer of the dispersed phase are computed using Eulerian model. Carrier phase turbulence is predicted using a second moment closure model. The effect of attenuation of the gas phase turbulent kinetic energy is shown to be minimal for acetone droplets (more than 7%), whereas the largest effects are seen for glycerol and water droplets (up to 15%). Heat transfer enhancement is shown to reach a maximal value with the use of ethanol (more than twice as high as for single-phase flow over the backward-facing step), while minimal intensification is obtained for the evaporation of acetone droplets (up to 25%).

Modelling the Condensation Process of Low-Pressure Refrigerants in Mini-Channels.

The measure of the energy efficiency of the non-adiabatic two-phase condensation process of refrigerants in mini-channels is both the value of the heat transfer coefficient α and the flow resistance expressing the external energy input required to realize the flow. The modelling of this very complex process is effective if the condensation mechanism in mini-channels is correctly identified. It has been proven that the effects of changes in the condensation mechanism are the different structures of the two-phase flow resulting from process interactions both in the channel cross-section and along the flow path. The research aimed to connect the value of the heat transfer coefficient with the flow structures occurring during condensation. Thermal and visualization studies of the condensation process of low-pressure refrigerants were carried out: Novec649, HFE7100 and HFE7000 in tubular mini-channels with diameters dh = 0.5; 0.8; 1.2; 2.0 mm. Based on visualization studies, flow structures were proposed to be divided into 3 main groups: dispersive, stratified and intermittent. Based on this, a computational correlation was derived for determining the heat transfer coefficient and frictional resistance depending on the type of flow structure. The research shows that the highest values of the heat transfer coefficient occur during the mist flow and the lowest during the bubble flow.

Film growth mechanism of mist-chemical-vapor-deposited magnetite

Crystal growth techniques using mist precursors, such as mist chemical vapor deposition (mist CVD), are useful for growing epitaxial thin films of various functional oxides under non-vacuum conditions and at relatively low temperatures. Their growth mechanism remains elusive, however, so we developed a home-made reaction chamber for mist CVD and investigated the epitaxial growth mechanism of magnetite Fe3O4 using alcoholic mists of iron acetylacetonate precursors. We show here that grown epitaxial films’ structural and physical properties depend on mist flow rates (or N2 carrier gas flow rates). Increasing the mist flow and promoting mists’ reactions on substrate surfaces produce Fe3O4 epitaxial films having larger magnetizations and lower electrical resistivities and exhibiting the Verwey transition. We also show that films’ properties are modified by changing either water additive concentrations in alcoholic mist solutions or the distance (the joint pipe length) between the mist bottle and the reaction chamber. Our results highlight the significance of the mist-flow-induced reactions for mist CVD epitaxial growth, providing new insight into the mechanism of epitaxial growth by mist CVD.

Numerical study of single-loop pulsating heat pipe with porous wicking layer

Pulsating heat pipe (PHP) has attracted much attention due to its simple structure and high thermal performance. Incorporating a wicking layer in the PHP is considered to be a promising approach to enhance the PHP thermal performance, but its heat transfer mechanism is still unclear. In this study, we built a numerical model of WPHP (viz. single loop PHP with a wicking layer) to explore the effects of the wicking layer property and the inclination angle on the heat pipe thermal performance under different filling ratios of 30%, 50%, and 70%. This model regarded the fluid circulation within the heat pipe as compressible flow and considered the variations of the saturated temperature and latent heat. We found that the working fluid performed as a mist flow in the WPHP and exhibited a sudden liquid condensation within the WPHP channel at the filling ratios of 50% and 70%, which could enhance the heat pipe thermal performance by 57% and 76%, respectively. However, the WPHP worked as a traditional heat pipe at the low filling ratio of 30%, with the performance even worse than SPHP (viz. single loop PHP with smooth wall). This study is significant for the thermal performance enhancement of pulsating heat pipes incorporating wicking layers.

On the evolutions of induction zone structure in wedge-stabilized oblique detonation with water mist flows

Two-dimensional wedge-stabilized oblique detonations in stoichiometric and fuel-lean H2/O2/Ar mixtures with water mists are studied using Eulerian-Lagrangian method. The effects of water droplet mass flow rate on flow and chemical structures in the induction zone, as well as physical / chemical roles of water vapor, are investigated. The results show that the oblique detonation wave (ODW) can stand in a range of water mass flow rates for both stoichiometric and fuel-lean mixtures. With increased droplet mass flow rate, the deflagration front in the induction zone is distorted and becomes zigzagged, but the transition mode from oblique shock wave (OSW) to ODW does not change. Moreover, the initiation and transition locations monotonically increase, and the OSW and ODW angles decrease, due to droplet evaporation and water vapor dilution in the induction region. For fuel-lean mixtures, the sensitivity of characteristic locations to the droplet loading variations is mild, which signifies better intrinsic stability and resilience to the oncoming water droplets. The chemical explosiveness of the gaseous mixture between the lead shock and reaction front is studied with the chemical explosive mode analysis. The smooth transition is caused by the highly enhanced reactivity of the gas immediately behind the curved shock, intensified by the compression waves. Nonetheless, the abrupt transition results from the intersection between the beforehand generated detonation wave in the induction zone and OSW. Beside, the degree to which the gas chemical reactivity in the induction zone for fuel-lean mixtures is reduced by evaporating droplets is generally lower than that for stoichiometric gas. Also, physical and chemical effects of water vapor from liquid droplets result in significant differences in ODW initiation and morphology. When abrupt transition occurs, both physical and chemical effects of water vapor influence transition location and ODW angle, but only physical one is important for smooth transition.

Fluctuant pressure coefficient of the wake behind a bluff body in mist flow

Fluctuant pressure coefficient is presented as an index of vortex energy of the wake behind a bluff body in mist flow. Calculation for vortex energy is obtained from inter-phase force analysis, based on Basset-Boussinesq-Oseen equation and conservation of mechanical energy. Vortex energy is weakened by liquid disturbance, the relationship between the fluctuant pressure coefficient and liquid velocity is set up on theory analysis. An existence criterion for vortex streets is proposed using the relationship. A new algorithm for prediction of liquid velocity is put forward and compared with the method in Higham's patent. The predicted results fit the experimental results well.

Simulation of Hydrate Particle Deposition in Horizontal Annular Mist Flow

Summary In the prediction of hydrate deposition, few studies have considered the hydrate particles generated from droplets in the gas core, which makes it difficult to calculate the hydrate particle deposition accurately. Previous studies have introduced the effective deposition ratio (EDR) to predict the hydrate particle deposition from the gas core quantitatively, which simplifies the prediction process. However, a quantitative description of the EDR has not been studied. The current work has established a prediction model of hydrate particle deposition on a pipe wall and developed a method for solving the EDR based on the force analysis and removal mechanism (lifting, rolling, or slipping) of hydrate particles on the horizontal pipe wall in annular mist flow. The effects of gas velocity, pipe diameter, particle size, and other factors on the EDR of hydrate particles were analyzed using the optimized discrete particle model (DPM) in Fluent. The results show that the EDR is inversely proportional to the gas velocity under the comprehensive impacts of the turbulent kinetic energy and gas shear force. Under the influence of the liquid film "wrapping," the EDR rapidly grows and then stabilizes with the increase in particle size. Under the joint influence of the liquid film distribution and forces acting on the particles, the EDR first rapidly declines and then grows with the increase in pipe diameter, and finally tends to be stable. Through regression fitting of more than 700 sets of data, the empirical expression of the EDR of hydrate particles was established for the first time, and the error was within the allowable range in engineering. This work lays a foundation for the accurate calculation of hydrate particle deposition generated from droplets in a gas core.

Microstructural Gradational Properties of Sn-Doped Gallium Oxide Heteroepitaxial Layers Grown Using Mist Chemical Vapor Deposition.

This study examined the microstructural gradation in Sn-doped, n-type Ga2O3 epitaxial layers grown on a two-inch sapphire substrate using horizontal hot-wall mist chemical vapor deposition (mist CVD). The results revealed that, compared to a single Ga2O3 layer grown using a conventional single-step growth, the double Ga2O3 layers grown using a two-step growth process exhibited excellent thickness uniformity, surface roughness, and crystal quality. In addition, the spatial gradient of carrier concentration in the upper layer of the double layers was significantly affected by the mist flow velocity at the surface, regardless of the dopant concentration distribution of the underlying layer. Furthermore, the electrical properties of the single Ga2O3 layer could be attributed to various scattering mechanisms, whereas the carrier mobility of the double Ga2O3 layers could be attributed to Coulomb scattering owing to the heavily doped condition. It strongly suggests the two-step-grown, lightly-Sn-doped Ga2O3 layer is feasible for high power electronic devices.

Simulating Leakage Impact on Steel Industrial System Functionality

Steels exhibiting ultralow sulfur content tend to gain increasing usage or application in manufacturing pipes used in oil transportation, as well as offshore platform construction. For these systems or platforms, it is worth indicating that they call for high resistance and impact strength relative to the formation of lamellar cracks. In this study, it was documented that when a refinery is compromised, the resultant fluids that are released tend to cause a loss in the refinery’s transportation capability. This outcome suggests that there is likely to be a decrease in the efficiency of transmission. Notably, the study examined the extent to which a leak presence affects flow behavior in the context of annular mist flow regime’s deep water production flow line. Another context that was investigated involved slug and stratified flow regimes. From the results, given the slug and annular flow regimes, any leakage could alter the pattern of material flow.

Spray cooling heat transfer during glass tempering process and influencing factors on the quality of tempered glass

Compared with traditional physical tempering methods, spray cooling method has a series of advantages, such as smaller energy consumption and better tempering quality. However, this technology has not reached a mature level. Present study experimentally investigated the spray cooling for glass tempering at high water/air mass flow ratios for the first time, which verified the feasibility of the spray cooling method. Based on the Eulerian-Lagrange method, a two-phase flow model was established to simulate the spraying cooling on heated glass surface. Both wall-jet and Lagrangian wall-film models were adopted to realize the conversion of several modes of heat transfer. A good agreement between the simulated and experimental temperature and wall film distributions supported the reliability of the established model. Subsequently, the spray cooling heat transfer was numerically studied, and the effects of spray distance, mist flow rate and glass thickness on the tempering quality and heat transfer performance were discussed. The results showed that a decrease in spray distance and an increase in mist flow rate and glass thickness obviously enhanced the temperature difference and quality of the tempered glass. Within the scope of the experiment, the optimum spray distance and mist flow rate were 272.5 mm and 6.94 × 10−4 kg/s, respectively.

Experimental Study of Horizontal Flow Boiling Heat Transfer Coefficient and Pressure Drop of R134a from Subcooled Liquid Region to Superheated Vapor Region

For the past few years, research in the field of flow boiling heat transfer has gained immense popularity for unravelling the dominant mechanism responsible for controlling heat transfer and identifying a parametric trend for understanding the characteristics of flow boiling heat transfer. This has led to several assumptions and models for predicting heat transfer during flow boiling without any known generalized mechanism. This study therefore seeks to experimentally study the characteristics of heat transfer during flow boiling over a wide range but small increase in vapor quality from a single-phase subcooled region through to a two-phase superheated vapor region. The study was performed with an R134a refrigerant in a single horizontal circular stainless-steel smooth tube that had an internal diameter of 5 mm. In this experimental study, local heat transfer coefficients and frictional pressure drop were measured for low heat fluxes of 4.6–8.5 kW/m2, mass fluxes of 200–300 kg/(m2s), vapor quality from −0.1 to 1.2 and a low constant saturation pressure of 460 kPa. Flow patterns observed during the study were recorded with a high-speed camera at 2000 fps. In covering a wide range of vapor quality, a peak of heat transfer coefficient near a vapor quality of zero and a local minimum observed in the low vapor quality region were observed, and both were sensitive to heat flux and mildly sensitive to mass flux. Generally, at low vapor quality, the heat transfer coefficient deteriorated with vapor quality and this was sensitive to heat flux but insensitive to mass flux and vapor quality, indicating nucleate boiling dominance in low vapor quality regions. In high vapor quality regions, the heat transfer coefficient was sensitive to mass flux and insensitive to heat flux. This indicates the dominance of convective boiling. In the low vapor quality regions, the flow patterns observed were slug and intermittent, while in the high vapor quality region, annular and mist flow patterns were observed. Generally, frictional pressure drop increased with increasing mass flux and vapor quality in the two-phase region.

Online Measurement of Gas and Liquid Flow Rates in Wet Gas Using Vortex Flowmeter Coupled With Conductance Ring Sensor

Wet gas flow commonly exists in several industries. The online measurement of gas and liquid flow rate is beneficial for economic efficiency. A combination of vortex flowmeter and liquid film sensor was proposed for two-phase flow measurement of wet gas. The specific form is as follows: the dual-modality sensing system was designed and calibrated by an annular mist flow loop. The fluid materials are pure air and water liquid. The ranges of operating pressure, the superficial gas velocity, and superficial liquid velocity are 1.5 bar–3.5 bar, 18.86–37.73 m/s, and 0.0028–0.028 m/s. Then the function relations of liquid film thickness and vortex frequency, with gas and liquid Weber numbers were built, respectively. Finally, a novel model for calculating gas and liquid flow rates in wet gas measurement was established, and solved by the Newton iterative algorithm. It showed that the percentage error (PE) of gas flow rate predicted by the model is within ±1.5%, and the mean absolute percentage error (MAPE) is 0.55%. The full-scale PE of liquid flow rate is within ±5% and the MAPE is 3.48%. The data show that this solution for gas and liquid flow rates measurement of wet gas by combining a vortex flowmeter with a conductance ring sensor is feasible.

Flow Pattern Recognition Based on the Received Waveforms of a Clamp-on Ultrasonic Flowmeter for Wet Steam Flows

Clamp-on ultrasonic flowmeter is a powerful tool for measuring flow rate in an existing pipe. However, measuring flow rate of wet steam flow has not been established yet. It is important to investigate the various error factors caused by the wetness fraction. Based on our previous investigations, it was clarified that the error of the flow rates tends to increase with the wetness fraction because of the changes in the velocity distribution in the wet steam and the effect of the liquid holdup. Therefore, it is important to estimate the flow regime of the wet-steam flow in the pipe. This study proposed a flow pattern recognition method using a machine learning based on the guided wave that is the propagated ultrasound through the pipe wall. The guided wave changes depending on the flow regime due to the fluctuations at the liquid-gas interface. Using the received ultrasonic signals of the guided wave in stratified flow, annular mist flow, and transition between them were used as the teaching data, and the other conditions were tested for the flow pattern recognition. Flow patterns were accurately predicted in adiabatic air-liquid two-phase and wet-steam flows in horizontal pipes. Furthermore, it was shown that the selection of the guided wave region is important to predict the flow pattern in various pressure conditions using the same teaching data.

Multi objective study in powder mixed near dry electric discharge machining by using utility concept

Powder mixed near dry electric discharge machining is an advanced hybrid form of electric discharge machining method which makes minimum use of dielectric oil and is quite sustainable in machining terms. In this article an approach has been made to optimize the variable parameters such as tool type, metallic powder concentration, dielectric mist pressure, and mist flow rate in order to study their effect along with their range in order to enhancing the machining performance in terms of increased material removal rate (MRR), improved surface finish, improved micro-hardness, reduction of residual stress and tool wear rate. Analysis of variance along with Taguchi L9 methodology was utilized for conducting experiments and it was revealed by the confirmation experiments that by this technique, the optimized results for improved material removal rate, improvement of surface finish, enhanced micro-hardness, reduced residual stress and tool wear rate were 0.63 (mg/s), 0.79 (µm), 249 (HV), 286 (MPa) and 1.07 (mg/min) respectively. The optimized values were obtained at Tool diameter (3mm), flow rate of mist (15 ml/min), concentration of metallic powder (8 gm/l) and mist pressure (0.5 MPa) of dielectric medium.

Experimental Investigation on Mist Flow and Heat Transfer in a Uniformly Heated Vertical Cylinder

The steady, internal, and ultrasonic-generated water mist cooling in a uniformly heated vertical cylinder with evaporating subcooled water film has been investigated experimentally. The goal is to elucidate the effects of different operating parameters, viz. water mist concentration, carrier gas velocity, loaded heat flux, and flow orientation on the heat transfer rate and pressure drop characteristics. The experiments were performed in a turbulent flow regime over a range of Reynolds number (from 6000 to 18000) using air as the carrier fluid. Compared with the cylinder without water mist, the results showed that using water mist as a coolant increases the Nusselt numbers along the cylinder, high Nusselt numbers are obtained by suspending the high-water mist and it was about 224% for heat flux q=1.56 kW/m2. The results also showed that the local Nusselt number rapidly decreases as the heat flux increases, this decreasing in the Nusselt number was the most at high heat flux q=4.81 kW/m2, in which the water film early breakdown and the breakdown point gets closer to the entrance region. In addition, a new empirical correlation of the heat transfer rate was proposed to estimate the real benefits in using water mist of the enhanced cylinder.

High pressure saturated flow boiling of CO2 at the micro scale

Saturated flow boiling heat transfer of carbon dioxide (CO2) at the microscale was studied with pressures ranging from 2.4 MPa to 6 MPa (i.e., reduced pressure ranging from 0.3 to 0.81, respectively), mass fluxes ranging from 1199 kg/m2s to 2428 kg/m2s, mass qualities up to 0.94, heat fluxes up to 270 W/cm2, and ΔTsat up to 150 K. A maximum heat transfer coefficient (HTC) of about 150 kW/m2K, and a maximum critical heat flux (CHF) of about 200 W/cm2 was recorded. Comparison to established correlations were made and discussed based on a flow map of saturated flow boiling of carbon dioxide by Cheng et al. The map classified the flow into different patterns (i.e., bubbly, annular, dry-out, and mist flows), and provided insight about the observed HTC trends based on the mass quality. The Shah and the Cheng et al. correlations predicted the HTC reasonably well for mass qualities of up to 0.45 with mean average errors of about 23%, but for a mass quality of 0.94, the Kandlikar, the Shah, and the Cheng et al. correlations underpredicted experiments with mean average errors of 80%, 24%, and 98%, respectively. The critical heat flux condition was predicted well with different models, and the Katto et al. correlation provided the lowest mean average error of 23.0%.

Experimental investigation on the thermo-hydraulic performance of air–water two-phase flow inside a horizontal circumferentially corrugated tube

In this study, flow visualization, pressure drop and thermal characteristics of the corrugated tube with non-boiling air–water two-phase flow were experimentally studied. Glass tubes were used to visualize the flow patterns of two-phase flow. Furthermore, the thermal behavior of two phase flow was studied in the copper made tubes which was under constant heat flux. The total of 740 W thermal energy was applied on tubes. Superficial velocity of liquid was within the range of 0.42 m/s to 1.69 m/s. Also, the superficial velocity of air was within the range of 0.21 m/s to 1.06 m/s. The results revealed that the corrugations significantly affect the flow patterns which results in earlier transition. The flow patterns of bubbly, churn, wavy slug, wavy plug, wavy stratified and mist flow were observed inside corrugated tube. Also, it was found that the maximum Nusselt number increment in corrugated tube was 3%. Besides, formation of the mist flow in corrugated tube reduced the Nusselt number. Also, the pressure drop was significantly affected by the air–water two-phase flow presenting around 165 % in the pressure drop compared to the single phase flow.

Experimental study of gas-water two-phase flow patterns in fracture: Implication for enhancing coalbed methane production

It has been long considered that the gas and water in coal seam fractures are produced in the form of stratified flow; however, recent researches and production practices have proved that two-phase flow in fractures shows various flow patterns. In this paper, visualized experiments were conducted to investigate the diversity of two-phase flow pattern in fracture and their controlling factors. The results show that the flow pattern can be bubbly, stratified, slug, wavy, annular-mist and mist flow under different combinations of gas and liquid flow rates. Among them, the slug flow is characterized by strong disturbance, which may easily cause serious reservoir damage during the drainage process of coalbed methane (CBM) wells. It has been observed that the variations of flow rate, fracture surface roughness, aperture, tortuosity as well as liquid viscosity and surface tension not only determine the proportions of each flow pattern but affect the intensity of slug flow greatly. Based on the observations, it is found that slug flow could be mitigated during the processes of reservoir stimulation and drainage. During the reservoir stimulation process, low-damage fracturing fluid, multi-graded proppant and surrounding rock fracture network stimulation are conducive to the prevention of slug flow. Meanwhile, during the drainage process, a two-phase flow pattern identification template can be used to predict flow pattern distribution, and a continuous but slow drainage strategy could be adopted to prevent slug flow and promote CBM production. This study can guide the establishment of CBM development strategy. • Multiple two-phase flow patterns have been observed in fractures through visualized experiments. • Flow pattern distribution and slug flow strength are impacted by flow rate, fracture and fracturing fluid properties. • Optimizations of stimulation technology and drainage schedule are crucial for inhibiting slug flow during CBM development.

Study on parameter’s optimization for the minimizing of the residual stress in powder mixed near-dry electric discharge machining

There are some undesired effects left in the machined products by electric discharge machining (EDM) and the most prominent among those is the residual stress. These induced residual stresses influence fatigue behavior, dimensional stability, and stress corrosion which is responsible for the failure of the machined product. It was observed that the products obtained by powder mixed near dry- EDM or (PMND-EDM) have a considerably low value of residual stress (RS). The objective of this research is to minimize the residual stresses induced in the machined workpiece (EN-31) by optimizing the process parameters which affects the machining characteristics significantly. Selected parameters were tool diameter, mist flow rate, metallic powder concentration, and mist pressure while the workpiece selected was EN-31 due to its desirable mechanical properties and immense applications in the manufacturing industry. The minimum value of residual stress at optimized values of process parameters was found to be 106.32 MPa.

Flow boiling heat transfer and dryout characteristics of ammonia in a horizontal smooth mini-tube

A boiling two-phase flow heat transfer experimental system for ammonia in a horizontal smooth tube with an inner diameter of 3 mm was established. The purpose is to explore the characteristics of flow boiling and dryout phenomenon. The experimental conditions: heat flux density is 10–30 kW·m−2, the mass flux is 40–200 kg·m−2·s−1, the saturation temperature is −10–10 °C, and the range of vapor quality is 0.1–1. The results show that the dominant heat transfer in different flow patterns is different, intermittent flow with nucleate boiling, annular flow and mist flow with convective boiling; the increase of mass flux can reinforce the convective boiling, the heat flux density can enhance the nucleate boiling, the increase of saturation temperature has no significant effect on the nucleate boiling, but it can weaken the convective boiling. The increase of mass flux and heat flux density will increase the friction pressure drop, and the increase of saturation temperature can decrease the friction pressure drop. The dryout visualization results and attenuation of heat transfer show that the increase of saturation temperature, heat flux density and mass flux can decrease the initial vapor quality of dryout, which is consistent with the results of Mori et al. model. Compared with the experimental data, the existing two-phase flow correlations are not suitable for the post-dryout prediction for ammonia, and the Wojtan et al. model and Mori et al. model can provide a reference for the dryout prediction.