• Journal of ILASS-Korea
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• v.28 no.2
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• pp.75-88
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• 2023

In this paper, the atomization characteristics of bi-swirl coaxial injectors for a 420 N-class bipropellant thruster were investigated. Three types of injectors, namely closed-type, open-type, and screw-type, were manufactured and designed to have the same spray angle and injection pressure drop. Water was used as a simulant, and cold-flow tests were conducted under ambient temperature and pressure conditions. Since the inner and outer injectors were designed to be the same type, only the inner fuel injectors that were easy to measure were used. Using a phase doppler particle analyzer, the velocity and diameter of atomized droplets were measured. Closed-type swirl injector exhibited droplet distributions with relatively high velocities and small SMD compared to the other two injectors. Open-type swirl injector formed droplets with reverse velocities in the center region and had a large recirculation zone. Screw-type swirl injector showed a sharp decrease in droplet velocity and size with radial distance from the liquid film breakup point. For the same design requirements, the closed-type swirl injector has superior atomization performance.

• Fire Science and Engineering
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• v.30 no.6
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• pp.111-117
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• 2016

The present study examined the free surface flow of a liquid sheet near a sprinkler head using a Computational Fluid Dynamics (CFD) model and considered the feasibility of the empirical model for predicting the initial spray characteristics of the sprinkler head through a comparison of the CFD results. The CFD calculation for a simplified sprinkler geometry considering the nozzle and deflector were performed using the commercially available CFD package, CFX 14.0 with the standard $k-{\varepsilon}$ turbulence model and theVolume of Fluid (VOF) method. The predicted velocity of the empirical model at the edge of deflector were in good agreement with that of the CFD model for the flat plate region but there was a certain discrepancy between the two models for the complex geometry region. The mean droplet diameter predicted by the empirical model differed significantly from the measured value of the real sprinkler head. On the other hand, the empirical model can be used to understand the mechanism of droplet formation near the sprinkler head and predict the initial spray characteristics for cases without experimental data.

• Fire Science and Engineering
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• v.34 no.4
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• pp.7-12
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• 2020

A numerical study was conducted to investigate the effects of the spray characteristics of water mist on the attenuation of thermal radiation. The attenuation process of the thermal radiation, generated from a hot surface panel, passing through the water mist was calculated via Fire Dynamics Simulator (FDS), and the effects of the flow rate, droplet mean diameter, and spray injecting angle of the water mist were analyzed. The results indicated that the increase in flowrate and decrease in droplet size led to an increase in the attenuation of thermal radiation. As the thermal radiation passed through the spray droplets, the effect of the spatial distribution of spray droplets was verified by calculating the thermal radiation attenuation at different spray injecting angles. The results indicated that the radiation attenuation increases as the spray angle increases. This implies that a wider distribution of spray droplets, irrespective of the droplet size and flowrate, increases the attenuation effect on thermal radiation.

• Fire Science and Engineering
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• v.33 no.5
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• pp.13-18
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• 2019

In this study, the soot formation characteristics of Jet-A1 liquid fuel droplet flames were investigated by measuring the soot concentration under atmospheric conditions similar to the working environment of the Korea Space Launch Vehicle (KSLV) To obtain the desired atmospheric conditions, the oxygen concentration in the combustion chamber was maintained at 30% and the pressure was varied between 0.1 and 0.06 MPa. The full-field light extinction technique was used to measure the concentration of soot particles generated by applying the identical to 2-mm-diameter Jet-A1 fuel droplets. The soot concentration of the Jet-A1 droplet flames was the highest in the nitrogen-substituted atmosphere and the lowest in the carbon dioxide-substituted atmosphere, despite the pressure. the pressure was decreased the measured soot concentrations reduced as a function of Pⁿ.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1999.10a
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• pp.27-27
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• 1999

To illustrate the global variation of the droplet mean diameters and the turbulent flow characteristics in counterflowing internal mixing pneumatic nozzle, the experimental measurements at five axial downstream locations(i.e., at Z=30, 50, 80, 120, and 170mm) were made using a PDPA(Phase Doppler Particle Analyzer) under the different air injection pressures ranging from 40 ㎪ to 120 ㎪. A nozzle with axi-symmetric tangential-drilled four holes at an angle of 15$^{\circ}$ has been designed and manufactured. The distributions of velocities, turbulence intensities, turbulence kinetic energy, turbulent correlation coefficients, spray angle, droplet mean diameters, volume flux, number density are quantitatively analyzed. It is possible to discern the effects of increasing air pressure. It indicates that the strong axial momentum in spite of more or less disparity between the velocity components means more reluctant to disperse radially, and that axial fluctuating velocities are substantially higher than those of radial and tangential ones, suggesting that the disintegration process is enhanced under higher air assist. The larger droplets are detected in the spray centerline at the near stations and smaller ones are generated due to further subsequent breakup at farther axial locations are attributed to the internal mixing type nozzle characteristics. Despite of the strong axial momentum, the poor atomization around the centre close to the nozzle exit is attributed to the lower rates of spherical particles which are not subject to instantaneous breakup. As it goes downstream, however, substantial increases in SMD(Sauter Mean Diameter) from the central part toward spray periphery are understandable because the droplet relative velocity is too low to bring about any subsequent disintegration.

• Journal of the Korean Society of Propulsion Engineers
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• v.6 no.2
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• pp.37-44
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• 2002

A theoretical study of spray and combustion characteristics due to coaxial twin-fluid injection is conducted to investigate the effects of liquid jet property, droplet size, contact length and liquid jet velocity. Model is properly validated with measurements and shows good agreement. Prediction of jet contact length, droplet size, liquid jet velocity reflects genuine features of coaxial injection in physical and practical aspects. Both the jet contact length and tile droplet size are reduced in a linear manner with an increase of injector diameter. Cross sectional area of liquid intact core is reduced with augmented jet splitting rate, thus the jet is accelerated to maintain the mass continuity and with an assistant of momentum diffusion by burnt gas.

• Particle and aerosol research
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• v.17 no.3
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• pp.43-54
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• 2021

Size distributions of atmospheric particulate matter (PM) and its water-soluble organic and inorganic components were measured between January and February 2021 at an urban site in Gwangju in order to identify the major factors that determine their size distributions. Their size distributions during the study period were mainly divided into two groups. In the first group, PM, NO₃^-, SO₄^2-, NH₄⁺ and water-soluble organic carbon (WSOC) exhibited bi-modal size distributions with a dominant condensation mode at a particle size of 0.32 ㎛. This group was dominated by local production of secondary water-soluble components under atmospheric stagnation and low relative humidity (RH) conditions, rather than long-range transportation of aerosol particles from China. On the other hand, in the second group, they showed tri-modal size distributions with a very pronounced droplet mode at a diameter of 1.0 ㎛. These size distributions were attributable to the local generation and accumulation of secondary aerosol particles under atmospheric conditions such as atmospheric stagnation and high RH, and an increase in the influx of atmospheric aerosol particles by long-distance transportation abroad. Contributions of droplet mode NO₃^-, SO₄^2-, NH₄⁺ and WSOC to fine particles in the second group were significantly higher than those in the first group period. However, their condensation mode contributions were about two-fold higher in the first group than in the second group. The significant difference in the size distribution of the accumulation mode of the WSOC and secondary ionic components between the two groups was due to the influx of aerosol particles with a long residence time by long-distance transport from China and local weather conditions (e.g., RH).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.7
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• pp.567-578
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• 2015

Single droplet-wall collision heat transfer characteristics on a heated plate above Leidenfrost temperature were experimentally investigated considering the effects of impact velocity. The collision characteristics of the droplet impinged on the heated wall and the changes in temperature distribution were simultaneously measured using synchronized high-speed video and infrared cameras. The surface heat flux distribution was obtained by solving the three-dimensional transient heat conduction equation for the heated substrate using the measured surface temperature data as the boundary condition for the collision surface. As the normal impact velocity increased, heat transfer effectiveness increased because of an increase in the maximum spreading diameter and a decrease in the vapor film thickness between the droplet and heated wall. For We < 30, droplets stably rebounded from a heated wall without breakup. However, the droplets broke up into small droplets for We > 30. The tendency of the heat transfer to increase with increasing impact velocity was degraded by the transition from the rebounding region to the breakup region; this was resulted from the reduction in the effective heat transfer area enlargement due to the breakup phenomenon.

• Fire Science and Engineering
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• v.30 no.3
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• pp.86-93
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• 2016

The effects of the shape of a drencher head on the flow characteristics and radiation attenuation of a water curtain, in order to prevent a fire spread, were experimentally studied. The distance (h) between the orifice exit and deflector and the diameter of deflector (D) were varied as the shape factors of the head, with the same orifice diameter (d). It was found that an increase in h leads to an increase in the water flow rate and spray angle. However, the change in the spray angle decreases with increasing D. Increasing D brings about a subtle increase in the water flow rate and a significant decrease in the spray angle. A larger value of D makes it possible to produce a flatter pattern of the water curtain, but reduces the uniformity of the droplets inside the spray angle. The mean droplet diameter decreases significantly as the operating pressure increases. However, the variation in the shape of the drencher head does not significantly affect the change in the mean diameter at the same operating pressure. Finally, it was found that the radiation attenuation afforded by the water curtain at the same operating pressure was affected by water flow rate and droplet uniformity, which were determined by h and D, respectively.

• Journal of ILASS-Korea
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• v.13 no.3
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• pp.134-142
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• 2008

In the case of heavy duty diesel engines, the Urea-SCR system is currently considered to reduce the NOx emission as a proved technology, and it is widely studied to get the high performance and durability. However, the nozzles to inject the urea-water solution into the exhaust pipe occur some problems, including the nozzle clogging, deposition of urea-water solution on the inner wall of the exhaust pipe, resulting in the production of urea salt. In this study, a swirl-type twin-fluid nozzle to produce more fine droplets was used as a method to solve the problems. The effect of the nozzle cap geometry, including the length to diameter ratio ($l_o/d_o$) and chamfer, on the spray characteristics were investigated experimentally. The length to diameter ratio of nozzle cap were varied from 0.25 to 1.125. The chamfer angle of the nozzle cap was constant at 90o. The mean velocity and droplet size distributions of the spray were measured using a 2-D PDA (phase Doppler analyzer) system, and the spray half-width, AMD (arithmetic mean diameter) and SMD (Sauter mean diameter) were analyzed. At result, The larger length to diameter ratio of nozzle cap were more small SMD and AMD. The effect of the chamfer did increase the radial velocity, while it did not affect the atomization effect.