Mikiya Araki

Pressure effects on combustion of methanol and methanol/dodecanol single droplets and droplet pairs in microgravity

Abstract This paper presents the results of an experimental investigation on the combustion of single droplets and two-droplet arrays of pure methanol and methanol/dodecanol mixtures in air under microgravity conditions. The initial droplet diameters, d 0 , were nominally 0.9 mm. The independent experimental variables were the ambient pressure (0.1–9.0 MPa), fuel mixture ratio (methanol/dodecanol: 100/0–15/85), and interdroplet separation distance l ( l / d 0 = 2.3–8.0). For pure methanol, the results show that the droplet lifetime decreases with increasing interdroplet separation distances at low pressures. At higher pressures (3.0 MPa and above) the droplet lifetime was independent of separation distance. The flame extinguished at a finite droplet size only for pure methanol at 0.1 MPa, in qualitative agreement with theoretical predictions. The extinction droplet diameter was nearly independent of the droplet spacing. Methanol/dodecanol–mixture droplets exhibited microexplosion for both single droplets and droplet arrays. The paper presents maps of the disruption regime for both single droplets and droplet pairs. The difference between the disruptive behavior of single droplets and droplet pairs is explained by differences in liquid-phase circulation induced by the gas-phase asymmetry of the droplet pair. The paper also presents results of the dependence of the onset of disruption (in terms of both volume and time) on the pressure and initial fuel mixture ratio.

Thrust Measurement of a Rectangular Hypersonic Nozzle Using an Inclined Baffle Plate

Thrust measurement of a rectangular hypersonic nozzle employed in the precooled turbojet engine under development in the JapanAerospace Exploration Agency is carried out using an inclined baffle plate. A 1.0%-scaled model of the nozzle was manufactured, and its gross thrust was investigated. The hypersonic nozzle has a variable throat and an external ramp to compensate the drastic change in the nozzle pressure ratio through the operation. At takeoff, an intensive jet noise is expected due to the high-speed exhaust jet, and in the present study, the impact of aerodynamic-tab jet noise suppressors on the thrust vector of the nozzle is investigated. Because of the simplicity, a baffle plate, which basically provides an on-axis forcemeasurement, is applied to a simultaneousmeasurement of the magnitude andangle of the thrust vector by giving anangle to the impact surface relative to the jet. It is shown that the magnitude of the thrust vector is kept constant regardless of the aerodynamic-tabmass fraction, which implies that a lossless mixing between the main jet and aerodynamic tab can be assumed. It is also shown that the thrust angle changes with the increase in the aerodynamic-tab mass fraction because of the deformation of the jet cross section.

Feasibility of Aerodynamic-Tab Jet Noise Suppressors in a Hypersonic Nozzle at Takeoff

I N THE Japan Aerospace Exploration Agency (JAXA), a precooled turbojet engine specified for hypersonic propulsion is under development [1,2]. This engine uses a precooling cycle using liquid hydrogen fuel, and it is capable of accelerating a vehicle from takeoff toMach 5. It is planned that the vehicle takes off horizontally using existing runways and that the exhaust velocity is supersonic throughout the operation. Thus, jet noise suppression at takeoff is one of the most important issues in the development. At takeoff, the nozzle pressure ratio (NPR) is limited to 2.7 since no ram pressure can be expected in such an airbreather, while at cruise, NPR reaches 200 due to the increase in flight Mach number and altitude [2]. To compensate such drastic change in NPR, a rectangular hypersonic nozzlewith a variable throat realizing the change in throat area by 2.5 times will be employed [2]. However, it is still insufficient to satisfy all operation conditions of the engine, and the nozzle will inevitably be operating under offdesign conditions (overexpansion mode) at takeoff. Because of the complicated geometry and the offdesign operation, it seems to be challenging to apply jet noise suppressors to such an actual hypersonic nozzle. Tabs attached at the nozzle exit are expected to be a promising device for screech and broadband noise reduction in a supersonic jet, and their noise reduction performance and mechanism have been investigated in detail [3–5]. Recently, a new concept of jet noise suppressors using auxiliary gas injection has been proposed [6–8], which achieves similar effect to tabs aerodynamically. Both in mechanical-tab and aerodynamic-tab (AT) concepts, much research is carried out using round/rectangular nozzles that are farther idealized when compared with actual hypersonic nozzles. Furthermore, in theAT concept, bleed air will be used as its working gas, and the total pressure and temperature available in real engines should be taken into account. In the present study, noise reduction performance and feasibility of AT jet noise suppressors in an actual hypersonic nozzle operating under takeoff conditions are investigated experimentally.

Mean-Flow and Acoustic Characteristics of Cold Jets from a Rectangular Hypersonic Nozzle

Mean-flow and acoustic characteristics of cold-air jets issuing from a rectangular hypersonic nozzle were investigated experimentally. The final goal of this study is (even though qualitative to some extent) to establish a method capable of predicting the acoustic characteristics of the high-temperature and high-velocity jets issuing from a hypersonic engine. As the first step, a high-enthalpy wind-tunnel plant without anechoic treatment was used for the cold-air jet experiments, and the acoustic data obtained were compared with those obtained in a small anechoic chamber. It is shown that, while it is necessary to pay attention to some noted exceptions, the directivity patterns and spectral shapes agree well at a wide range of radiation angles by taking into account the correction of −2.5  dB. The mean-flow measurements revealed that, due to the complicated geometry of the nozzle, the jet has a highly asymmetric velocity profile at the ramp end and that the development of the jet plume is also inclined an...

Characteristics of HCCI Diesel Combustion Operated with a Hollow Cone Spray

Thie paper presents two factors for improving the performance and emissions characteristics in HCCI diesel combustion, one is reducing compression ratio and another is changing the injector position. In a previous study,it was shown that HCCI diesel combustion could be realized by utilizing a hollow-cone spay with normal injection pressure. However there remained two major problems of engine instability and increase in BSFC (decrease in brake thermal efficiency). By reducing the compression ratio from 18.8 to 16.8, the engine stability was much improved to the level of conventional diesel combustion and the increase in BSFC became almost half, which was mainly due to the change of combustion phasing. In addition to this, application of 5 mm inside position of the injector realized almost no penalty of BSFC at higher load condition. At around 15 deg BTDC, small increment of pressure rise was observed, which must be due to the occurrence of cool flame, and the heat released during this stage did not vary with load. Then this cool flame energy is shown to be another important factor for the increase in BSFC at lighter load.

Acoustic Simulation of Hot Jets Issuing from a Rectangular Hypersonic Nozzle

Acoustic simulation of hot jets issuing from a rectangular hypersonic nozzle was carried out. The aim of this study is, even though qualitative to some extent, to establish a method capable of predicting the acoustic characteristics of the high-temperature and high-velocity jets issuing from the realistic hypersonic nozzle having a complicated geometry. Two kinds of test nozzles, 2.4% scaled model with afterburner for hot jet experiments and 1.0% scaled model for cold-air/helium mixture jet experiments, were manufactured and tested. By using a high-enthalpy wind-tunnel plant, acoustic characteristics of the hot jets issuing from the rectangular hypersonic nozzle for several representative but realistic high-temperature and high-velocity jet conditions were obtained. At the same time, by using a small anechoic chamber, the acoustic characteristics of the hot jets were duplicated using cold-air/helium mixture jets having the same velocities. Although we have to pay attention to some noted exceptions, the di...

Experimental Investigation of Jet Noise Sources in a Hypersonic Nozzle at Takeoff

I NVESTIGATING the noise sources in the high-speed exhaust jet is one of themost important technical issues in the development of the next generation’s supersonic/hypersonic engines. In the Japan Aerospace Exploration Agency, a precooled turbojet (PCTJ) engine, which is capable of accelerating the vehicle from takeoff to Mach 5 with a single engine cycle, is under development [1–5]. In the PCTJ engine, a variable hypersonic nozzle with an external ramp is employed to compensate for the drastic change in the nozzle pressure ratio (NPR). The NPR changes from 2 at takeoff to 200 at cruise; at takeoff, the nozzle operates under an offdesign, overexpanded condition [1]. One of the largest concerns in the development of this engine is the intensive jet noise at takeoff. From the viewpoint of the noise suppressor design [3,4], for such a realistic hypersonic nozzle having complicated geometry and operating under offdesign conditions, understanding the complicated behavior of the noise sources is quite important. Panda and Seasholtz [6] and Panda et al. [7] investigated the behavior of jet noise sources in a single round jet. They applied a Rayleigh scattering method to measure the local density fluctuation in the jet and, by directly comparing the data with the pressure fluctuation obtained with far-field microphones, a cross-correlation analysis was carried out. The coherence between these two data is indicated with isocontours, wherein the location of the sources is clearly illustrated. Panda et al. [7] pointed out that the noise source measured by the Rayleigh scattering method exhibits strong directional characteristics, and they offered an explanation for the observed directional variation based on the two-noise source model [8–10]. These works provided essential information in terms of the behavior of the source, and these experimental evidences contributed to the important paper by Tam et al. [11], wherein the behavior of the jet noise sources is clearly illustrated. In the present study, based on the methods proposed in these pioneeringworks, the behavior of the source is investigated. Since the Rayleigh scattering method requires significant cost and complexity, the authors applied a simpler method: a variant of schlieren technique. Such optical deflectometry methods have also been employed byseveral researchers to investigate the behavior of the source [12–14]. To gain a high sensitivity capable of detecting the acousticwave propagation in the ambient, a laser schlieren optical system that realizes a small spot size is used. The behavior of the source in a supersonic jet issuing from a realistic hypersonic nozzle operating at the takeoff condition is investigated through a cross-correlation analysis between the schlieren data and the far-field noise.

Application of Over-Expansion Cycle to a Larger Gasoline Engine With Late-Closing of Intake Valves

This paper presents further investigation into the effect of over-expansion cycle with late-closing of intake valves on the engine performance in gasoline engines. A larger single-cylinder test engine with the stroke volume of 650 cc was used with four kinds of expansion ratio (geometrical compression ratio) from 10 to 25 and four sets of intake valve closure (I.V.C.) timings from 0 to 110 deg C.A. ABDC. Late-closing has an effect of decreasing the pumping work due to the reduction of intake vacuum, althogh higher expansion ratio increases the friction work due to the average cylinder pressure level. Combining the higher expansion ratio with the late-closing determines the mechanical efficiency on the basis of these two contrastive effects. The indicated thermal efficiency is mostly determined by the expansion ratio and little affected by the nominal compression ratio. The value of the indicated thermal efficiency reaches to 48% at most which is almost comparable with the value of diesel engines. The improvement of both indicated and brake thermal efficiency reaches to 16% which is much higher than ever reported by the authors. A simple thermodynamic calculation could successfully explain the behavior of the indicated thermal efficiency. The brake thermal efficiency could also be improved due to the increase in both mechanical and indicated efficiencies.Copyright

Operation Characteristics of a Spark-Ignition Engine for Power Generation Fueled with Real Biogas Produced with Low-Temperature Gasification

This paper presents operation and the characteristics of electric power generation of a spark-ignition engine fuelled with biogas produced through a low-temperature biomass gasification process. The biomass resource was mixture of pig manure and wood chips. Two kinds of load was connected to the generator, one was electric resistance and the other is Ni-MH battery manufactured by Kawasaki Heavy Industry Co. The engine was successfully operated at the maximum BMEP (Brake Mean Effective Pressure) of 0.38 MPa which was 65 % of the original gasoline. The generation efficiency was more than 26 %, which was even higher than that of gasoline. The power could also be charged to the battery, but the BMEP and generation efficiency were 0.19 MPa and 15 %, respectively, which were pretty lower than those of resistance.

Tomogram Visualization of Supersonic Jet Noise Using an Optical CT Method

Supersonic jet noise was visualized using a newly developed optical CT (Computed Tomography) method. From a converging round nozzle whose exit diameter is 8.0 mm, air is injected into the atmosphere. The pressure ratio is set at 2.25 (Mj=1.14), and an under-expanded jet is formed. A sound wave is pressure fluctuation, and the density fluctuates at the same time, Using a kind of schlieren method, the fluctuation of the density gradient is detected. A He-Ne laser beam passes through the supersonic jej, and the beam direction is slightly refracted due to the density gradient inside and around the supersonic jet. The beam refraction is converted to a brightness signal using a knife edge and a photo sensor. The signal obtained with the photo sensor is the integrated value along the laser beam path. Using a modified Abel inversion, the local refraction of the laser beam is calculated and the tomogram is re-constructed. It is shown that, using the optical CT method, non-intrusive acoustic measurement is possible even inside the supersonic jet. It is shown that the sound source of the screech tone exists around the 4th shock cell of the under-expanded jet, and that intensive fluctuation is observed not only in the shear layer but also along the center axis of the jet, implying the existence of the traveling shock wave.