Youngbin Yoon

Effect of geometric parameters on the liquid film thickness and air core formation in a swirl injector

Many theoretical and experimental studies have been conducted to investigate elements of swirl injector hydrodynamics, such as variations in liquid film thickness or air core diameter. From these studies, some theoretical relationships have been established through an approximate analytical solution of flow hydrodynamics in a swirl nozzle. However, experimental studies on elements such as the measurement of liquid film thickness have not produced conclusive results. In a swirl injector, the atomization process is significantly influenced by the liquid film thickness. Thus, it is possible to investigate the effects of various geometric parameters on spray characteristics through the measurement of liquid film thickness. We used a specially designed injector based on the electrical conductance method to measure the liquid film thickness accurately. The liquid film thickness was measured through precise calibration, and the accuracy of this measurement was demonstrated in comparison with previous theories and experiments. From these results, we present an empirical relation for the liquid film thickness by adding orifice length to an existing analytical equation. The variations and stability of the air core were also examined by visualizing the formation of the air core in the swirl chamber with a high-speed camera system. This study confirms that air core shape and liquid film thickness are directly related. Thus, study of the fluctuations of liquid film thickness under various geometric conditions can be applied to the analysis of internal flow.

Numerical Study of Scram Accelerator Starting Characteristics

A numerical study is carried out to investigate the ignition and the detonation initiation process in a scram accelerator operating at a superdetonative mode. To simulate the scram accelerator launching process, a conical projectile is considered, injected with an initial velocity of 2500 m/s from the 1 atm air into a 25 atm 2H 2 + O 2 + mN 2 mixture. As a dilution gas, nitrogen is selected and assumed to be inert. The time accurate solutions of Reynolds averaged Navier-Stokes equations for chemically reacting flows are obtained by using a point-implicit method and an upwind-biased third-order scheme with a steady-state solution for airflow as an initial condition. To examine combustion characteristics and ram-accelerator operation limits, mixture compositions are varied from 2H 2 + O 2 + 3.76N 2 to 2H 2 + O 2 + 9N 2 by changing the amount of N 2 . The flowfield results show the detailed ignition mechanism, the initiation process of the oblique detonation, and the staring characteristics of the scram accelerator. The results also identify clearly the combustion characteristics of the operational failures at lower and upper dilution limits that have been observed in experiments

Numerical Study of Mixing Enhancement by Shock Waves in Model Scramjet Engine

Anumerical study hasbeenconductedto investigatetheeffectofshockwaveson thesupersonichydrogen ‐airjet e ame stabilized in a Mach 2.5 circularcross-section combustor. Thenumerical model utilizes multispecies Navier ‐ Stokes equationswith detailed chemical reaction modelsand employsa k‐! shear stresstransport model. A wedge is mounted on the side wall of the combustor in order to e nd the interaction of the oblique shock waves with the hydrogen‐air jetlike e ame. The interaction between the shock waves and the mixing layer is classie ed according to the increasing tendency of the growth rate of the mixing layer downstream of the shock waves. It is found that the shock wavescreatea radially inward/outward aire owto thee ame and elongatea e ame-holding recirculation zone, and thus fuel ‐air mixing is enhanced signie cantly, resulting in improved combustion efe ciency. Also, the overall performance is investigated by changing the shock position and considering the mixing/combustion efe ciency and total pressure loss in a model scramjet combustor. Because there exists a tradeoff between the enhanced mixing/combustion efe ciency and the decreased total pressure recovery, it is suggested that the optimized shock position needs to be determined in order to obtain the maximum overall combustor performance using the overall performance index.

Effect of Ambient Gas Density on Spray Characteristics of Swirling Liquid Sheets

The spray and breakup characteristics of a swirling liquid sheet were investigated by measuring the spray angle and breakup length as the axial Weber number We l was increased up to 1554 and the ambient gas pressure up to 4.0 MPa. As the We l and ambient gas density p increased, the disturbances on the annular liquid sheet surface were amplified by the increase of the aerodynamic forces, and thus the liquid sheet disintegrated from the injector exit. The measured spray angles according to the ambient gas density differed before and after the sheet broke up. Before the liquid sheet broke up, the spray angle was almost constant; however, once the liquid sheet started to break up, the spray angle decreased. As the ambient gas density and We l increased, the increasing aerodynamic force caused the breakup length to decrease. Finally, the measured breakup lengths according to the ambient gas density and We l were compared with the results of the linear instability theory. Considering the attenuation of sheet thickness in the linear instability theory, the corrected breakup length relation agreed well with our experimental results.

Comparative Study of Spray Characteristics of Gas-Centered and Liquid-Centered Swirl Coaxial Injectors

This study investigated the spray and Sauter mean diameter characteristics of gas-liquid swirl coaxial injectors by measuring and analyzing spray angles and mean drop sizes. Two different types of gas-liquid swirl coaxial injectors were designed and tested for this experiment: 1) a gas-centered swirl coaxial injector and 2) a liquid-centered swirl coaxial injector. The spray patterns were obtained for both types of injectors. The spray angles of these liquidcentered swirl coaxial injectors showed a decreasing trend as a function of the momentum flux ratio over the conditions studied. Good agreement was generally achieved between the predicted and measured spray angles. On the other hand, with increasing momentum flux ratio, the spray angle of the gas-centered swirl coaxial injector initially decreased when the momentum flux ratios were relatively low, but the trend reversed over the high momentum flux ratio range studied. The Sauter mean diameter data of these two types of injectors were also obtained using an image-processing method. Results showed that the Sauter mean diameter distribution of the liquid-centered swirl coaxial injector had a solid cone shape, whereas the gas-centered swirl coaxial injector exhibited two distinct Sauter mean diameter distribution regimes. The gas-centered swirl coaxial injector had a solid cone shape along the centerline, and it had a hollow cone shape at the periphery.

Nitrogen oxides emissions in turbulent hydrogen jet non-premixed flames: Effects of coaxial air and flame radiation

An experimental study was performed to investigate the effects of fuel-air mixing on NO x emissions in hydrogen non-premixed flames with coaxial air. The major parameters used to modify the fuel-air mixing were fuel jet velocity and coaxial air velocity. Measurements of NO x emission, flame length, and volume were made to investigate the relationship between flame residence time, global strain rate, and NO x emission scaling. Global strain rate and flame residence time from measured flame length and flame volume were used as parameters for the analysis of the experimental data to identify the relevant parameters that lead to the observed NO x scaling law in coaxial air flames. The overall half-power scaling was observed in coaxial air flames, irrespective of coaxial air conditions, but the degree of deviation from the half-slope curve differed in each case. Comparison of the results of pure hydrogen flames with those of helium-diluted hydrogen flames showed that flame radiation played a significant role in 100% hydrogen flames with coaxial air, and the deviation from half-power scaling was due to the difference in the flame radiation. Global strain rate rather than flame residence time is a more appropriate scaling parameter to predict the emission index of NO x /τ B in the turbulent hydrogen non-premixed flames with and without coaxial air. The half-power scaling law previously observed in hydrogen non-premixed jet flames with no coaxial air is applicable to NO x emission in hydrogen non-premixed jet flames with coaxial air, if flame radiation is taken into consideration.

Computational investigation of shock-enhanced mixing and combustion

A computational investigation of shock-enhanced mixing and combustion is presented. To understand the influences of the mixing process on the combustion process, the mixing characteristics of the reacting case are compared with those of the nonreacting case. Parametric studies varying the conditions of fuel injection are conducted to find the trends of the mixing and combustion processes. Three-dimensional Navier-Stokes equations with a chemical reaction model and κ-ω turbulence model are used. The upwind method of Roes flux difference splitting scheme is adopted. It is shown that the mixing process has a strong influence on the combustion process, whereas the combustion process does not have any significant effect on the mixing process. The combustion process is divided into two mixing regimes: a convection-dominated regime, where the burning rate increases with distance from the injection plane, and a diffusion-dominated regime as one moves downstream, where burning rate is constant. In the parametric studies, varying the fuel pressure with the fuel density held fixed makes little difference, whereas varying the fuel density makes a significant difference in mixing rate and burning rate. A prediction of minimum combustor length for complete combustion is made.

Effect of Recess on the Spray Characteristics of Liquid-Liquid Swirl Coaxial Injectors

The effects of recess in a liquid-liquid swirl coaxial injector on the spray characteristics have been investigated by measuring the spray angle and breakup length in eight cases of recess length. The orifice recess, a geometric parameter, has a strong influence on the spray and mixing characteristics by varying the interaction point between two liquid sheets. The variation of the recess length resulted in three different injection regimes: external, tip, and internal mixing injection. The coaxial spray characteristics in the external mixing injection regime are found to be mainly governed by the merging phenomenon and momentum balance between two liquid sheets. In the internal mixing injection regime, an impact wave is generated by the impingement of the inner spray on the outer liquid film inside the injector. The amplitude of the impact wave is gradually attenuated due to the fluid viscosity, flowing along the outer injector wall after impingement. This attenuation of the impact wave according to the recess length could affect the stability of a liquid sheet; therefore, the breakup length is found to increase with the increase in recess number.

Assessment of planar liquid-laser-induced fluorescence measurements for spray mass distributions of like-doublet injectors

The planar liquid-laser-induced fluorescence (PLLIF) technique has been known to be a useful tool for the measurement of the spray mass distributions for various spray injectors because it can obtain two-dimensional images with high spatial resolutions without any intrusion on the spray field. In the cases of dense sprays, however, it has been known that the extinctions of the incident laser beam or fluorescence signal and the secondary emission can cause errors in quantifying the spray mass distributions. Since a like-doublet injector, which is commonly used in liquid rocket engines, has a locally concentrated spray zone at the spray centre, we investigated the applicability of the PLLIF technique for this injector. From the experimental results, we found out that the extinctions of the incident laser beam and fluorescence signal are not significant because the concentrated spray zone is narrow. Also, we found out an optimal incident laser power which can avoid a nonlinear increase of fluorescence signal at the spray centre as well as obtain a high signal-to-noise ratio, and we measured the spray mass concentration of the like-doublet injector spray using the optimal laser power. In order to assess the accuracy of the PLLIF data, we converted the spray mass concentration into the mass flux distribution and compared it with the data obtained by a mechanical patternator and phase Doppler particle analyser. From the result that the PLLIF data showed good agreement with those of the mechanical patternator, we concluded that the PLLIF technique can be successfully applied to measuring the mass distributions of the like-doublet injectors.

Effects of Orifice Internal Flow on Breakup Characteristics of like-Doublet Injectors

Under cold-flow and atmospheric ambient pressure conditions, the breakup characteristics of liquid sheets formed by a like-doublet injector were investigated. The sheet breakup wavelength, which induces the sheet to be broken into ligaments, and the sheet breakup length, which is important for finding the flame location, were measured using stroboscopic light. After sheet breakup, liquid ligaments are formed intermittently, and their wavelengths are believed to be related to the combustion instability of liquid-rocket engine. Therefore, the wavelength and the breakup length of ligaments broken into fine drops were also measured. Because these spray characteristics are affected by the flow characteristics of the two liquid jets before they impinge on each other, the focus was on the effects of orifice internal flow, such as the cavitation phenomenon that occurs inside a sharp-edged orifice. From the experimental results, it was found that liquid jet turbulence delays sheet breakup and shortens the wavelengths of both sheets and ligaments. Because the turbulence strength of a sharp-edged orifice is stronger than that of a round-edged orifice, the shape of orifice entrance gives large differences in the spray characteristics. With these results, empirical models of the spray characteristics of a like-doublet injector were proposed, and these models can provide useful and practical data for use in designing liquid-rocket combustors.