Mist Jet Research Articles

Flow dynamics and heat transfer characteristics of air/water mist jet impingement using Eulerian-Eulerian approach

The present numerical study focuses on a two-phase Eulerian-Eulerian approach to model the flow structure and heat transfer characteristics of an air/water mist jet emanating from a coaxial jet nozzle and impinging on an isothermal heated flat plate. A detailed parametric numerical study has been performed to investigate the effect of various operating parameters like air Reynolds number (2400-9142), mist Reynolds number(1.87–8.68), mist loading fraction (0 % - 0.96 %), droplet diameter (1 μm – 60 μm), and non-dimensional nozzle to plate spacing (10–42) on the flow structure and heat transfer characteristics of air/water mist jet. The present numerical results are in agreement with the experimental results, within 16 % accuracy. The heat transfer increases with an increase in the air Reynolds number or mist loading fraction. The increase in air Reynolds number provides better heat transfer enhancement as high as 94.5 % compared to 27.6 % in the case of an increment in the mist loading fraction. Additionally, the effect of droplet diameter on the flow structure and heat transfer characteristics of air/water mist jet is analyzed at depth. The increase in nozzle-to-plate distance from 10 to 42 leads to a decrease in stagnation point Nusselt number of the order of 31 %. Further, correlations have been proposed to predict the Nusselt number at the stagnation point, stagnation region, and average Nusselt number for the target plate.

Grooving of CFRP with water mist jet–assisted nanosecond pulsed laser: effect of process parameters on removal mechanisms and HAZ formation

Grooving of CFRP with water mist jet–assisted nanosecond pulsed laser: effect of process parameters on removal mechanisms and HAZ formation

Experimental and numerical study on the heat transfer of a flat plate impinged by air-water mist jet

Experimental and numerical study on the heat transfer of a flat plate impinged by air-water mist jet

Investigation of Thermal-Flow Behavior and Droplet Dynamics of Mist Sweeping Impinging Jet Cooling

Abstract This paper presents a 2-D numerical study to investigate the fluid flow behavior and cooling characteristics caused by injecting tiny water droplets into the sweeping air jet through a fluidic oscillator. An unsteady Reynolds-averaged Navier–Stokes (URANS) simulation accompanied with the k–ω SST turbulence model is used in this study. The movement and evaporation of the mist are simulated by using the discrete phase model (DPM). The study has been conducted for a target wall with a constant heat flux of 3,000 W/m2, jet-to-wall distance of 4D, ReD = 2,500, and a mist/air mass ratio of 5% with a droplet size of 5 microns. A comparison between the cooling performance of steady and sweeping jets is presented for two impingement schemes (i.e., confined and unconfined impingement). The approach of using a slip upper wall boundary condition as an alternative to the unconfined impingement scheme is investigated as well. The results show that adding mist provided a temperature reduction of 5–10% on the target wall in all cases when compared to the air cases. Mist mostly follows the air jet behavior in both steady and sweeping jets in both impingement schemes. The liquid droplet coalescence phenomenon prevails in the sweeping jet case, while it is not as significant in the steady jet case. For the confined impingement, both mist jets provided the similar average temperature reduction. However, the steady mist jet introduced a 58% more target wall shear compared to the sweeping mist jet. For the unconfined impingement, the steady mist jet achieved a better average cooling performance compared to that of the sweeping mist jet. However, the steady mist jet introduced a 72% more target wall shear compared to the sweeping mist jet. Using a slip upper wall boundary condition to reduce the computational time resulted in similar average heat transfer distribution on the target wall to the unconfined case. However, the flow pattern, vortical structures, and droplet dynamics were very different.

Computational Analysis of Air/Mist Film Cooling Using A Sweeping Jet Fluidic Oscillator–Part II: Sweeping Mist Jets

Abstract An ongoing research effort has been conducted for years to investigate the feasibility of mixing the air with mist (micron-level water droplets) to enhance heat transfer. It becomes very interesting to investigate how these tiny droplets behave in conjunction with the periodic sweeping jets. Will they move synchronously with the fluid or asynchronously with a phase lag? How would the interaction between the droplets and the sweeping jets affect the film cooling effectiveness? To answer these questions, Part II of this paper specifically focuses on investigating the droplet dynamics and thermoflow behavior of droplet evaporation on film cooling effectiveness in a sweeping jet. An unsteady Reynolds Averaged Navier–Stokes simulation accompanied by the k–ω shear stress transport turbulence model is used in this study. The multiphase computational fluid dynamics model employs an Eulerian–Lagrangian approach. The Eulerian method is used for the continuous phase including air and water vapor and the Lagrangian method in terms of the discrete phase model is used to simulate the dispersed phase (e.g., liquid droplets) in a continuous phase (e.g., air). A mist ratio of 10% with a droplet size of 10 μm was used in this study. The results show that, for a blowing ratio of 1, using mist provides better film cooling performance with an average enhancement of 50–90% in comparison to that without mist. Using a mist ratio of 10% could save approximately 50% of the cooling air. It is observed that the liquid droplets mostly follow the main sweeping flow and its vortical structure and horseshoe vortices, but with a phase lag between the droplets and the main sweeping jet. This phase lag further enhances both temporal and spatial film cooling surface protection during the sweeping motion.

Mixture Between Fire and Mist Jets Characteristics for Flame Extinguish

The extinguish process using the water mist jet is one of the most efficient system for hydrocarbon flame accidentally developed. The environmentally friendly of the system is an important challenge. Both free jets characteristics are analysed in aim to have a quick suppression of the flame. The paper is focused on the geometrical shape of the cone realised by every jet (flame and water mist). On this basis the relative position of the jets is decisive in aim to obtain the desired effect. Experimental measurements were made, and the results are presented in the paper including the infrared temperature field. The experimental analysis allows to find the adequate angle between jets for the flame extinction. So, the optimum position of the water mist jet applied on the fire was found. A thermal flame extinguish efficiency evaluation is presented, too. The system is non pollutant and the damages are reduced.

Experimental and numerical study of air-water mist jet impingement cooling on a cylinder

The paper presents the study of experimental as well as the numerical study of air-water mist jet impingement cooling over a heated cylinder in the non-boiling region. The studies are conducted for various mist loading fraction, f = 0.0025, 0.0050, 0.0075 and 0.01; Reynolds number, Remix.=8000, 10,500, 13,000 and 15,500; and surface-to-nozzle spacing, H/d = 30, 40, 50 and 60. Enhancement in the heat transfer (η) for mist jet impingement as compared to air jet impingement is obtained from experimental and numerical analysis. The numerical study also helped in understanding the distribution and tracking of droplets in the computational domain. High enhancement in the heat transfer is observed with high mist loading fraction. Also, the lowest Reynolds number yielded the highest enhancement in heat transfer with comparison to the higher Reynolds number. At various surface-to-nozzle spacing, the enhancement in heat transfer is high for low surface-to-nozzle spacing near the stagnation zone to a certain point, after which point the enhancement reverses as higher enhancement in heat transfer is observed for high surface-to-nozzle spacing. As high as 408% and 775% enhancement in the heat transfer at the stagnation point is observed for f = 0.01, Remix.= 8000and H/d = 30 during experimental and numerical analysis respectively. The correlation has been proposed to estimate the enhancement in heat transfer at the stagnation point.

Realization of micro EDM drilling with high machining speed and accuracy by using mist deionized water jet

In electrical discharge machining, both the gas and liquid dielectric have unique advantages and inevitable disadvantages. The mist medium with a special property between gas with liquid is expected to have a better performance in EDM. In this paper, the performance of mist deionized water jet has been studied based on the disadvantages of deionized water in deep micro hole drilling: electrolysis action, electrophoresis action and gap-contamination by debris. Based on the analyzation on the advantages of mist jet, three projects of experiments have been done to verify its performance on the suppressing of electrolysis action and improving the debris exhaust. The results show that the micro EDM drilling with high machining speed and accuracy can be realized with mist jet. This method is particularly effective in deep micro hole drilling, since deeper holes can be obtained at faster machining speed with mist jet due to its significant improvement in debris exhaust.

Experimental Investigation of Air–Water Mist Jet Impingement Cooling Over a Heated Cylinder

Experimental investigation on heat transfer mechanism of air–water mist jet impingement cooling on a heated cylinder is presented. The target cylinder was electrically heated and was maintained under the boiling temperature of water. Parametric studies were carried out for four different values of mist loading fractions, Reynolds numbers, and nozzle-to-surface spacings. Reynolds number, Rehyd, defined based on the hydraulic diameter, was varied from 8820 to 17,106; mist loading fraction, f ranges from 0.25% to 1.0%; and nozzle-to-surface spacing, H/d was varied from 30 to 60. The increment in the heat transfer coefficient with respect to air-jet impingement is presented along with variation in the heat transfer coefficient along the axial and circumferential direction. It is observed that the increase in mist loading greatly increases the heat transfer rate. Increment in the heat transfer coefficient at the stagnation point is found to be 185%, 234%, 272%, and 312% for mist loading fraction 0.25%, 0.50%, 0.75%, and 1.0%, respectively. Experimental study shows identical increment in stagnation point heat transfer coefficient with increasing Reynolds number, with lowest Reynolds number yielding highest increment. Stagnation point heat transfer coefficient increased 263%, 259%, 241%, and 241% as compared to air-jet impingement for Reynolds number 8820, 11,493, 14,166, and 17,106, respectively. The increment in the heat transfer coefficient is observed with a decrease in nozzle-to-surface spacing. Stagnation point heat transfer coefficient increased 282%, 248%, 239%, and 232% as compared to air-jet impingement for nozzle-to-surface spacing of 30, 40, 50, and 60, respectively, is obtained from the experimental analysis. Based on the experimental results, a correlation for stagnation point heat transfer coefficient increment is also proposed.

A new tunnel fire detection and suppression system based on camera image processing and water mist jet fans

Several tunnel fire detection and fighting systems are currently available in the market, each with its own pros and cons. Although no single system is perfect, the water mist system is one of the top-performing conventional tunnel fire suppression systems available. The problem is that such system is expensive. More affordable equipment with similar performance would be a breakthrough in the field of tunnel safety. Accordingly, this study develop a new water mist system, in which ventilation jet fans are utilized in such a way to achieve economical feasibility. The system features monitoring cameras use to determine fire coordinates and mist-generating jet fans employed to suppress fire. The front of the ventilation jet fans is equipped with nozzles, which spray water frontward through the fans, thereby creating mist and propelling it toward the location of a fire. The mist that shoots out of the fans reduces ambient temperature, flushes oxygen, cools the surface of inflammable materials, and weakens radiative and convective effects. The simulation of the proposed system shows a low heat release rate, smoke and toxic fumes reduction, tunnel ventilation speed increases, and improved visibility. These improvements enhance tunnel safety and make tunnel conditions during a fire less threatening to human health. The cost of the system put forward in this work can be further reduced by optimizing the materials that constitute the pipes and fittings and removing the firefighting pumps.

Cooling Effects of Cold Mist Jet with Transient Heat Transfer on High-Speed Cutting of Titanium Alloy

High cutting temperature will lead to a short tool life and a poor machining quality during titanium alloy machining. High efficiency cooling methods are always required to reduce the cutting temperature. In this paper, heat transfer experimental platform was set up and one-dimension heat transfer model was established to get critical heat flux (CHF) used to evaluate the cooling capacity of the cooling methods. The effect of cold mist jet (CMJ) was evaluated by transient heat transfer tests and titanium alloys cutting experiments, comparing with cold air jet, flood cooling and minimum quantity lubricant. The experimental results revealed that CMJ with a high CHF has a stronger cooling capacity than other cooling methods and has greatly reduced the cutting temperature in high-speed cutting of titanium alloys. And tool life and surface quality have been both improved with CMJ. This paper offered a way for evaluating the cooling effects of different cooling methods with heat transfer experiments.

An experimental study on heat transfer and rewetting behavior of hot horizontal downward facing hot surface by mist jet impingement

Heat transfer characteristics and rewetting behavior of 0.15 mm thick hot horizontal stainless-steel foil (SS-304) have been reported in the present paper during the mist jet impingement cooling from the bottom side through experimental investigation. The pneumatic internal mixing full cone mist nozzle (Lechler 136.134.xx.A2) with a spray angle 20° is used to develop a mist jet with a varied range of mist loading fraction (0.48–1.18). The initial surface temperature of the plate is maintained at 500 ± 10 °C with the help of an AC transformer. The present study uses an infrared thermal imaging camera (A655sc, FLIR system) to record the temperature of the hot foil. Various heat rewetting parameters namely, rewetting time, rewetting temperature and Peclet number are estimated from temperature data. The value of surface heat flux is found to be maximum at the stagnation point and found to gradually decrease in the radial direction away from the stagnation point, for different mist loading fractions. The non-dimensional rewetting velocity (Peclet number)) is found to decrease with the increase in the mist loading fraction. Correlations have been proposed to estimate maximum surface heat flux, rewetting temperature and Peclet number as a function of mist loading fraction and non-dimensional radial distance.

Numerical analysis on the rapid fire suppression using a water mist nozzle in a fire compartment with a door opening

Abstract Fire suppression using a water mist nozzle directly above an n-Heptane pool in a fire compartment with a door opening was numerically investigated using the Fire Dynamics Simulator (FDS) for the purpose of application in nuclear power plants. Input parameters for the numerical simulation were determined by experimental measurements. Water mist was activated 10 s after the fire began. The sensitivity analysis was conducted for three input parameters: total number of cubic cells of 6032–2,926,400, droplets per second of 1000–500,000, and extinguishing coefficient of 0–100. In a new simple calibration method of this study, the extinguishing coefficient yielding the fire suppression time closest to that measured by experiments was found for use as the FDS simulation input value. When the water mist jet flow made contact with the developed fire, the heat release rate instantaneously increased, and then rapidly decreased. This phenomenon occurred with a displacement of the flame near the liquid fuel pool. Changing the configuration of the door opening with different aspect ratios and opening ratios had impact on the maximum value of the heat release rate due to the flame displacement.

Mist flow visualization for round jets in Aerosol Jet®printing

With the microdroplets of water serving as light scattering particles, the mist flow patterns of round micro-jets can be visualized using the Aerosol Jet(R) direct-write system. The visualization images show that the laminar mist jet appears to extend to more than 20 times the diameter of nozzle orifice, D, for jet Reynolds number Re < 600, especially with D = 0.3 mm and less. For smaller jets (e.g., with D = 0.15 mm), laminar collimated mist flow might be retained to 40xD for Re < 600 and for Re ~ 1500 within 20xD from the nozzle. The laminar part of mist flow associated with larger jets (e.g., with D = 1.0 mm for Re < 600) tends to exhibit noticeable gradual widening due to viscous diffusion. For free jets, their breakdown length--the distance from nozzle where transition from laminar to turbulent mist flow takes place as signaled by the inception of a rapid widening of mist stream--is shown to decrease with increasing Re. The presence of impingement wall tends to prevent turbulence development, even when the wall is placed further downstream of the free-jet breakdown length for a given Re . The critical Re for an impinging jet to develop turbulence increases as the standoff S is reduced. The mist flow of impinging jet of D = 1.0 mm seems to remain laminar even for Re > 4000 at S = 12 mm.

Realization of High-speed Micro EDM for High-aspect-Ratio Micro Hole with Mist Nozzle

Micro EDM technology is widely used in micro hole machining for its small force and high precision. However, the slow machining speed and large tool wear are the two major problems. In this research, a mist nozzle, whose central axis is coaxial with the tool axis, was proposed to evacuate debris from the narrow gap area in micro-EDM for high-aspect-ratio holes and to improve the machining speed and accuracy. In order to verify the effectiveness of the mist nozzle, experiments by supplying mist jet and water jet, and by changing the flow rate of deionized water were carried out. By comparing the machining results of micro hole drilling with mist jet and water jet, the effectiveness of the mist nozzle was confirmed.

Experimental investigation of in-process mitigation of welding distortion for stainless steel plate using air-atomized mist cooling

In this study, welding-induced out-of-plane distortion of stainless steel sheets is examined in the case of applying air-atomized mist jet on the rear side of the weld pool. An internally mixed type air atomizing spray nozzle is attached to the welding torch and both are moved at the same speed and direction onto the welding centerline. The test specimens are 2 mm thick grade 304 stainless steel plates. Bead-on-plate welds are made on the plates using a DC Gas tungsten arc welding (GTAW) process. The measured welding distortion and residual stresses are compared with those of a conventional GTAW welding process. The experimental results show that the welds performed with a trailing heat sink using air-atomized mist have almost zero out-of-plane distortion. In addition, a slight welding residual stress reduction is achieved.

Biodegradation and hard machining performance comparison of eco-friendly cutting fluid and mineral oil using flood cooling and minimum quantity cutting fluid techniques

As manufacturing advances, there is a growing demand for harder materials in various fields, such as aerospace and defence. Hard materials are difficult to produce, and hard machining causes high cutting temperatures, which increase cutting force and affect the surface finish of the product. To lower the cutting forces and obtain better surface finishes on the workpiece, metal cutting fluids are generally preferred. However, the use of these fluids has been questioned by many researchers recently. Mineral-oil-based conventional cutting fluids are essentially crude petroleum derivatives that have low biodegradability. Therefore, these fluids are associated with a high risk of environmental pollution (air, soil, water). In this study, the characteristics and biodegradable potential of bio-cutting fluid (BCF) and commercially available mineral oil (MO)-based cutting fluid are compared, in addition to their hard machining performance during the machining of hardened AISI H-13 steel. To minimise the usage of cutting fluid during hard machining, an indigenously designed and fabricated minimum quantity cutting fluid (MQCF) application system is used. In the MQCF system, pressurised air is mixed internally with the cutting fluid, forming a uniform mist at the nozzle exit. The same is delivered to the machining region. The influence of flood cooling and MQCF techniques using both BCF and MO on the cutting force, feed force, coefficient of friction, and workpiece surface roughness, which were studied during hard machining with industrial feed-speed combinations. The pressurised MQCF mist jet reduces the cutting zone contact length, because of its optimised parameters and the better lubricating characteristics of BCF. The experimental results show a significant reduction in cutting force, feed force, coefficient of friction, and surface roughness for MQCF.

Heat transfer behaviour of a dilute impinging air-water mist jet at low wall temperatures

This paper presents experimental work on the heat transfer characteristics of a dilute impinging air-water mist jet. Time-averaged heat transfer measurements are reported, as well as simultaneously obtained high-speed shadowgraph imaging flow visualisation and heat transfer measurements; images of the mist jet impingement and the liquid film that forms on the impingement surface are also included. It is determined that the surface liquid morphology varies with mist loading fraction, and that the heat transfer behaviour of the mist jet reflects these changes. Intermittent thermal disturbance caused by the impingement of the mist jet droplets is proposed as the dominant heat transfer mechanism when the mist loading fraction is very low; as the mist loading fraction increases, evaporation from the surface liquid film and convective cooling due to the film flow become more significant.

A Computational Study of High-Speed Microdroplet Impact onto a Smooth Solid Surface

Numerical solutions of high-speed microdroplet impact onto a smooth solid surface are computed, using the interFoam VoF solver of the OpenFOAM CFD package. Toward the solid surface, the liquid microdroplet is moving with an impinging gas flow, simulating the situation of ink droplets being deposited onto substrate with a collimated mist jet in the Optomec Aerosol Jet printing process. The computed values of maximum spread factor, for the range of parameters of practical interest to Aerosol Jet printing, were found in very good agreement with some of the correlation formulas proposed by previous authors in the literature. Combining formulas selected from different authors with appropriate modifications yields a maximum spread factor formula that can be used for first-order evaluations of deposited in droplet size during the Aerosol Jet technology development. The computational results also illustrate droplet impact dynamics with lamella shape evolution throughout the spreading, receding-relaxation, and wetting equilibrium phases, consistent with that observed and described by many previous authors. This suggests a scale-invariant nature of the basic droplet impact behavior such that experiments with larger droplets at the same nondimensional parameter values may be considered for studying microdroplet impact dynamics. Significant free surface oscillations can be observed when the droplet viscosity is relatively low. The border line between periodic free surface oscillations and aperiodic creeping to capillary equilibrium free surface shape appears at the value of Ohnesorge number around 0.25. Droplet bouncing after receding is prompted with large contact angles at solid surface (as consistent with findings reported in the literature), but can be suppressed by increasing the droplet viscosity.

Effects of Cutting Atmosphere on High-Speed End Milling Process for Titanium Alloy Ti6Al4V

As titanium alloys have a high strength-to-weight ratio and superior corrosion resistance, they are widely used in the aerospace, biomedical, and automotive industries. However, these alloys exhibit very poor machinability, which results in problems such as short tool life. This study investigates the effect of the cutting atmosphere on tool wear during high-speed end milling of the titanium alloy Ti6Al4V. Dry cutting, cold air jet cutting, cutting fluid mist jet cutting, and cutting fluid flush cutting were considered in order to determine the optimum cutting atmosphere and conditions. For down-cutting speeds of 200−300 m/min, the cutting atmosphere and cutting speed were adopted as experimental parameters. Down-cutting was performed in order to measure the width of the tool flank wear land as the cutting length was increased. The results indicated that the optimum cutting method was cold air jet cutting. For a cutting length of 500 mm, this method produced a narrower flank wear land than dry cutting. In addition, for longer cutting lengths of up to 4000 mm, the wear rate for cold air jet cutting was less than or equal to that for dry cutting, and no chipping or excessive wear was observed.