An experimental study on stability rating of impinging-jet injectors using air injection in a subscale chamber

Combustion instability rating is investigated experimentally in a subscale chamber with impinging-jet injectors. This study is focused on jet flow of both propellants of fuel and oxidizer without considering a chemical reaction. Air-injection tests using scaling techniques are proposed to predict acoustic instability in an actual full-scale combustion chamber of a rocket engine. In the present approach, a subscale chamber has been designed to have the same natural frequencies as those in a full-scale chamber, and air is injected into the subscale chamber instead of fuel and oxidizer and their flow rates are maintained. From the tests, damping factors are obtained as a function of a hydraulic parameter of jet flow. Based on the data, the instability map is drawn, where three acoustically unstable regions are presented. It is found that they coincide fundamentally with the results from hot-fire tests. Accordingly, as the first approximation, the proposed approach can be applied cost-effectively to the stability rating of jet injectors without losing essential features of acoustic instability when mixing of fuel and oxidizer jets is the dominant process in instability triggering.

     

Experimental study on the icing characteristics of LPLi injectors under various injection conditions

Development of alternative fuels has been increasing to replace conventional fuels because of severe environmental pollution from the exhaust emissions of ground vehicles and the depletion of fossil fuels. LPG (Liquefied petroleum gas) fuel provides clean energy for gasoline-fueled engines and LPG fuel R&D (Research & development) has actively progressed in reaching the application stage. However, the icing phenomenon in liquid-phase LPG injection systems remains a major problem. The purpose of this study was therefore to investigate the temperature distribution on the icing protection tip and icing formation characteristics under various test conditions (i.e. injection pressure, injection frequency, injection duty). The results showed that increasing the injection frequency induced a big the temperature drop, whereas increasing the injection duty decreased the temperature drop under the same test conditions. Increasing the injection pressure caused a small temperature drop because of an increase in the injection velocity, which meant that much of the injected LPG evaporated at a region distant from the nozzle tip. The protection tip with a double-hollow structure had a smaller the temperature drop than the all-in-one type protection tip made of the same material (copper).

     

A scaling method for combustion stability rating of coaxial gas-liquid injectors in a subscale chamber

A scaling method to examine combustion-stability characteristics of a coaxial injector is devised based on the acoustics and combustion dynamics in a chamber. The method is required for a subscale test of stability rating with a model chamber, which is cost-effective compared with an actual full-scale test. First, scaling and similarity rules are considered for stability rating and thereby, three conditions of acoustic, hydrodynamic, and flame-condition similarities are proposed. That is, for acoustic similarity, the natural or resonant frequencies in the actual chamber should be maintained in the model chamber. And, two parameters of density ratio and velocity ratio are derived for the requirement of hydrodynamic and flame-condition similarities between the actual and the model conditions. Next, one example of an actual combustion chamber with high performance is selected and the proposed scaling method is applied to the chamber for understanding of the method. The design operating condition for a model test is presented by mass flow rates of propellants. Stability boundaries can be identified on the coordinate plane of chamber pressure and mixture ratio of fuel and oxidizer by applying the scaling method.     

Combustion characteristics of a swirl chamber type diesel engine

A numerical model that utilizes Computational Fluid Dynamics (CFD) techniques is simulated for the analysis of a swirl chamber type diesel engine. This research also reveals the effects of swirl chamber passage hole geometry on the combustion characteristics of a swirl chamber type diesel engine depending on the shape, angle, and area of the jet passage. Turbulence kinetic energy is generated by compound effects of the pressure, heat release, NOx concentrations, and soot concentrations. Results show that combustion characteristics are affected by the passage hole areas and the passage hole inclination angles.     

Combustion and emission characteristics of HCNG in a constant volume chamber

Finding an alternative fuel and reducing environmental pollution are the main goals for future internal combustion engines. Hydrogenmethane (HCNG) is now considered an alternative fuel due to its low emission and high burning rate. An experimental study was carried out to obtain fundamental data for the combustion and emission characteristics of pre-mixed hydrogen and methane in a constant volume chamber (CVC) with various fractions of hydrogen-methane blends. A pre-mixed chamber was used to obtain a good mixture of these gases. Exhaust emissions were measured using a Horiba exhaust gas analyzer for various fractions of hydrogen-methane blends. The results showed that the rapid combustion duration was shortened, and the rate of heat release elevated as the hydrogen fraction in the fuel blend was increased. Moreover, the maximum mean gas temperature and the maximum rate of pressure rise also increased. These phenomena were attributed to the burning velocity, which increased exponentially with the increased hydrogen fraction in the fuel blend. Exhaust HC and CO₂ concentrations decreased, while NO_X emission increased with an increase in the hydrogen fraction in the fuel blend. Our results could facilitate the application of hydrogen and methane as a fuel in the current fossil hydrocarbon-based economy and the strict emission regulations in internal combustion engines.     

Combustion and heat transfer characteristics inside the combustion chamber of a wood pellet boiler

A grate-firing boiler was developed for wood pellet fuel, and then its combustion characteristics were tested. The flame was stretched to the exit of the combustion chamber, implying insufficient space for complete combustion. As a first step to resolve this problem, a numerical simulation was conducted for the combustion chamber. Turbulent and chemically reacting flow was considered by implementing a homogeneous reaction model. Flow field from the simulation showed strong recirculation flow at the upstream corner of the chamber, along which the flame was stretched to the exit. Based on these results, we suggest possible modification of the combustion chamber to improve combustion characteristics, such as relocating its exit or installing internals like guide vanes.     

Hot-fire injector test for determination of combustion stability boundaries using model chamber

This study realizes the conceptual method to predict combustion instability in actual full-scale combustion chamber of rocket engines by experimental tests with model (sub-scale) chamber. The model chamber was designed based on the methodologies proposed in the previous work regarding geometrical dimensions and operating conditions, and hot-fire test procedures were followed to obtain stability boundaries. From the experimental tests, two instability regions are presented by the parameters of combustion-chamber pressure and mixture (oxidizer/fuel) ratio, which are customary for combustor designers. It is found that instability characteristics in the chamber with the adopted jet injectors can be explained by the correlation between the characteristic burning or mixing time and the characteristic acoustic time. In each instability region, dynamic behaviors of flames are investigated to verify the hydrodynamically-derived characteristic lengths of the jet injectors. Large-amplitude pressure oscillation observed in upper instability region is found to be generated by lifted-off flames.     

Experimental study of acoustic damping induced by gas-liquid scheme injectors in a combustion chamber

In a liquid rocket engine, acoustic damping induced by gas-liquid scheme injectors is studied experimentally for combustion stability by adopting linear acoustic test. In the previous work, it has been found that gas-liquid scheme injector can play a significant role in acoustic damping or absorption when it is tuned finely. Based on this finding, acoustic-damping characteristics of multi-injectors are intensively investigated. From the experimental data, it is found that acoustic oscillations are almost damped out by multi-injectors when they have the tuning length proposed in the previous study. The length corresponds to a half wavelength of the first longitudinal overtone mode traveling inside the injector with the acoustic frequency intended for damping in the chamber. But, new injector-coupled acoustic modes show up in the chamber with the injectors of the tuning length although the target mode is nearly damped out. And, appreciable frequency shift is always observed except for the case of the worst tuned injector. Accordingly, it is proposed that the tuning length is adjusted to have the shorter length than a half wavelength when these phenomena are considered.     

Effects of various baffle designs on acoustic characteristics in combustion chamber of liquid rocket engine

Effects of various baffle designs on acoustic characteristics in combustion chamber are numerically investigated by adopting linear acoustic analysis. A hub-blade configuration with five blades is selected as a candidate baffle and five variants of baffles with various specifications are designed depending on baffle height and hub position. As damping parameters, natural-frequency shift and damping factor are considered and the damping capacity of various baffle designs is evaluated. Increase in baffle height results in more damping capacity and the hub position affects appreciably the damping of the first radial resonant mode. Depending on baffle height, two close resonant modes could be overlapped and thereby the damping factor for one resonant mode is increased exceedingly. The present procedure based on acoustic analysis is expected to be a useful tool to predict acoustic field in combustion chamber and to design the passive control devices such as baffle and acoustic resonator.     

Combustion and heat transfer characteristics in a combustion chamber of a 13-step grate wood pellet firing boiler

A grate-firing boiler was developed for wood pellet fuel, and its combustion and heat transfer characteristics were tested. The first prototype revealed problems with fuel delivery and residence time. To solve these problems, we quadrupled the number of grates and developed the second prototype with a baffle; however, the volume significantly increased in the second prototype. By examining the space usage in the second prototype, we designed the third prototype with inclined grates, which reduced the number of grates by 25 % and reduced the volume by 20 %. The experimental results show that the third prototype had similar combustion performance to the second prototype with the reduced volume. Furthermore, some grates remained unused in the combustion, and a recirculation flow appeared in the CFD results, which suggests the possibility of additional design improvements.     

Investigation of the scuffing characteristics of candidate materials for heavy duty diesel fuel injectors

The objective of this study is to characterize and understand the evolutionary processes that produce changes in the friction and surface damage in materials for possible use as heavy duty diesel fuel injector plungers. This work has involved the development of test methods to impart reciprocating motion to various metals, ceramics, and coated specimens in the presence of diesel fuel-like fluids. Commercial and candidate plunger materials, including 52100 steel, zirconia, cermets (TiC in Ni₃Al matrix), and TiN coatings, were evaluated on a crossed-cylinders-like scuffing test we call the ‘pin-on-twin’ geometry. Contacts were lubricated by on-highway #2 diesel and Jet A aviation fuels. Using friction-based criteria, the material ranking was in good agreement with field experience with actual injectors from the diesel engine industry. Zirconia and cermets exhibited promising scuffing resistance in both fuels. Scuffing generally became more severe in the Jet A fuel. Experimental results indicated that smoother surfaces that are required to sustain higher injection pressures could be more vulnerable to scuffing due to their thinner lubricant films. Material transfer was the major scuffing mechanism of zirconia, cermets, and TiN coatings against steel. Micro-scratches were also observed on the matrix material of cermets.     

Performance characteristics of tilted three-lobe journal bearing configurations

A tilted three-lobe journal bearing is a geometric variation of the usual symmetric three-lobe configuration. This investigation of the performance characteristics of the three-lobe configurations over a wide range of tilt angles shows that, within a specific range of tilt angles, the tilted configurations exhibit superior dynamic performance to the usual symmetric three-lobe bearing.     

Experimental study of acoustic characteristics of expansion chamber with mean flows

For the measurement of transmission loss of the silencers which have mean flow, the noise levels of signals depicted by microphone usually prohibit accurate measurements of the transmission loss. To overcome this difficulty, static pressure compensation technique has been developed to increase the dynamic range of the microphones, resulting accurate measurement of transmission loss in the presence of high noise level. Series of experiments to investigate the acoustic characteristics of expansion chamber with mean flow has been performed using the pressure compensated microphones. Results demonstrate: (1) frequency shift due to the presence of mean flow, (2) effect of length, diameter, offset length and twisting angles to the transmission loss of expansion chamber with and without mean flow (up to 50m/sec), (3) relation between the proposed parameter ‘aspect ratio’ and number of peaks of transmission loss in low frequency region.     

Studying the proportional discharge dynamics in mixtures of various gases using a chamber with preliminary electron multiplication

By using a chamber with preliminary multiplication of ionization electrons and extraction of the electron component of signals, the proportional discharge dynamics was studied in various gas mixtures, including mixtures of noble gases with gases, whose ionization potential is below the excitation energy of noble gas atoms (Penning mixtures). It was revealed that the main signals with durations of several microseconds, which are determined by the collection of electrons from primary avalanches, have “tails” of secondary avalanches with durations of up to tens of microseconds with a total charge comparable or exceeding the charge in the primary avalanches.     

Numerical analysis of flow characteristics of a swirl-stabilized premixed burner with different swirl vane configurations

This study numerically investigates the flow and combustion characteristics of a swirl-premixed burner with a curved vane swirler (i.e., curved type) to reduce the pressure loss of the burner nozzle with a conventional vane swirler (i.e., flat type). For the curved type swirler, the pressure loss is decreased by 35 % due to the inhibition of flow separation at the vane, and NO_X emission is reduced by 69 % because fuel-air mixing is enhanced at the flame front, leading to more uniform fuel distribution toward downstream. In addition, the characteristics of the jet penetration are analyzed because the jet behavior is crucial to fuel-air mixing. The jet trajectory in non-swirling flow can be reasonably predicted by a conventional formula, but it is difficult to predict the jet trajectory in swirling flow by this formula because of the effect of a radial pressure gradient formed by the swirling flow.

     

A comparison of various loading configurations of the proximal femur for the evaluation of reconstructive surgical procedures.

The study was designed to evaluate the effect of different loading configurations on stem and bone stresses in simulated total hip arthroplasty. The traditional experiment design of loading the model through the head of the prosthesis by the resultant joint force was compared with a more realistic model which included an abductor strap to simulate the abductor muscle force. In addition, an alternative experiment design was evaluated in which a loading arm was clamped directly on to the head of the prosthesis. The results show that loading the model by the resultant joint force not only changes the magnitude of the stresses but also the stress distribution compared to the abductor muscle model. The new experiment design closely approximates stresses seen in the abductor muscle model below the lesser trochanter. In the proximal region, the stresses are increased on the medial side and decreased on the lateral side. The advantages of the proposed loading model are: (a) easy and reproducible set-up and alignment is facilitated, (b) different positions of the femur (flexion, extension) can be simulated and (c) a more realistic stress distribution and magnitude is achieved.     

Co-simulation of dynamic systems in parallel and serial model configurations

Recent advancements in simulation software and computation hardware make it realizable to simulate complex dynamic systems comprised of multiple submodels developed in different modeling languages. The so-called co-simulation enables one to study various aspects of a complex dynamic system with heterogeneous submodels in a cost-effective manner. Among several different model configurations for co-simulation, synchronized parallel configuration is regarded to expedite the simulation process by simulating multiple submodels concurrently on a multicore processor. In this paper, computational accuracies as well as computation time are studied for three different co-simulation frameworks: integrated, serial, and parallel. For this purpose, analytical evaluations of the three different methods are made using the explicit Euler method and then they are applied to two-DOF mass-spring systems. The results show that while the parallel simulation configuration produces the same accurate results as the integrated configuration, results of the serial configuration show a slight deviation. It is also shown that the computation time can be reduced by running simulation in the parallel configuration. Therefore, it can be concluded that the synchronized parallel simulation methodology is the best for both simulation accuracy and timeefficiency.     

Combustion characteristics of anthracite in a vertical cyclone combustor

Isothermal, gas combustion and coal tests were undertaken in order to characterise a vertical cyclone combustor for burning anthracite. Inert particles (alumina cement) were used during the first two series of tests, in order to characterise chamber temperatures, material collection efficiencies and size distribution of particles. Mixture ratios from 0.4 up to 1.6 were tested in steps of 0.2. Under isothermal conditions, an optimum penetration length of vortex finder into the combustion chamber was found to be approximately 10% of the chamber length. The highest temperature can be obtained in the lower section of the combustion chamber both in gas and in coal combustion. The collection efficiency of the cyclone combustor in the various modes of operation was found to be excellent. For coal tests, mixed firing with coal and gas was adopted to sustain the flame in the combustor. The mixed fuels investigated here contain 60, 70 and 80% by mass of coal, and the 70% case was found favorable in the context of carbon burnout and collection efficiency.     

Combustion and radiation characteristics of oxygen-enhanced inverse diffusion flame

The characteristics of combustion and radiation heat transfer of an oxygen-enhanced diffusion flame was experimentally analyzed. An infrared radiation heat flux gauge was used to measure the thermal radiation of various types of flames with fuel, air and pure oxygen. And the Laser Induced Incandescence (LII) technique was applied to characterize the soot concentrations which mainly contribute to the continuum radiation from flame. The results show that an oxygen-enhanced inverse diffusion flame is very effective in increasing the thermal radiation compared to normal oxygen diffusion flame. This seems to be caused by overlapped heat release rate of double flame sheets formed in inverse flame and generation of higher intermediate soot in fuel rich zone of oxygen-fuel interface, which is desirable to increase continuum radiation. And the oxygen/methane reaction at slight fuel rich condition (φ=2) in oxygen-enhanced inverse flame was found to be more effective to generate the soot with moderate oxygen availability.     

Numerical and experimental studies on the mixing characteristics of two jets with inclined angles in a rectangular chamber

Mixing characteristics in a rectangular chamber are investigated using experimental and numerical methods. A mixing chamber with an axial inlet (representing fuel inlet) and a side inlet (representing air inlet) is designed. Two jets with different momentum ratios through the two inlets are mixed in the chamber. Computational fluid dynamic simulation is validated by experimental data of particle image velocimetry that measures flow velocity distribution. The momentum ratios of the two jets and the height of the axial inlet significantly influence the penetration depth of axial jet into the mixing chamber and the pressure drops at recirculation zones.