Search Results

In this paper, experimental investigation on spray atomization and droplet dynamics inside a thermostatic expansion valve (TXV), a component commonly used in vehicle refrigeration system, was conducted. A needle and an orifice were copied from a commercial TXV and machined to be mounted inside a chamber with optical access so that the flow inside the TXV is simulated and visualized at the same time. The break-up and atomization of the refrigerant were documented near the downstream of the orifice under different feed conditions for two TXV with different geometry. A Phase Doppler Anemometry (PDA) system was used later to measure the size and velocity of atomized refrigerant droplets. The results showed that the droplet size variation along the radial direction is slightly decreased at near downstream and increased at farther downstream due to the coalescence.

Recently, with the increasing interest on some potential alternative substitutes of petroleum fuels such as biodiesel and butanol, more and more researches are focused on the field of bio-fuels because they are renewable and friendly to the environment and can possibly reduce domestic demand on foreign petroleum. Bio-fuels are generally used in the commercial market by mixing with petroleum-based diesel or gasoline. Since the volatilities and boiling points of ethanol/butanol and diesel/biodiesel fuels are significantly different, micro-explosion can be expected in the blend mixture. In this study, a numerical model of micro-explosion including bubble generation, bubble expansion, and final breakup for multi-component bio-fuel droplets is proposed. From the simulated results of droplet characteristics at the onset of micro-explosion, it is concluded that micro-explosion is possible under engine operation condition for ethanol/butanol-diesel/biodiesel fuel blends.

Biodiesel fuel can be produced from a wide range of source materials that affect the properties of the fuel. The diesel engine has become a highly tuned power source that is sensitive to these properties. The objectives of this research were to measure and predict the key properties of biodiesel produced from a broad range of source materials to be used as inputs for combustion modeling; and second to compare the results of the model with and without the biodiesel fuel definition. Substantial differences in viscosity, surface tension, density and thermal conductivity were obtained relative to reference diesel fuels and among the different source materials. The combustion model revealed differences in the temperature and emissions of biodiesel when compared to reference diesel fuel.

Biodiesel fuels and their blends with diesel are often used to reduce emissions from diesel engines. However, biodiesel has been shown to increase the NOx emissions. Operating a compression ignition engine in low-temperature combustion mode as well as using multiple injections can reduce NOx emissions. Experimental data for biodiesel are compared to those for diesel to show the effect of the biodiesel on the peak pressure, temperature, and emissions. Accurate prediction of biodiesel properties, combined with the KIVA 3V code, is used to investigate the combustion of biodiesel. The volume fraction of the cylinder that has temperatures greater than 2200 K is shown to directly affect the production of oxides of nitrogen. Biodiesel is shown to burn faster during the combustion events, though the ignition delay is often longer for biodiesel compared to diesel.

A modified KIVA code is used to simulate the operation of a small bore HSDI engine with biodiesel from various sources. Genetic modified soybean biodiesel has very similarly combustion characteristics with soybean biodiesel, albeit less NOx. Yellow grease biodiesel burns in a similar manner as regular diesel with less NOx. All types of biodiesel show longer ignition delay but faster combustion, except after TDC injection, where biodiesel exhibits some diffusion combustion. Delaying injection is an effective way to reduce NOx emission. Over 90% of reduction in NOx is observed when fuel injection is delayed from 335° to 363°.

A numerical study of micro-explosion in multi-component droplets is presented. The homogeneous nucleation theory is used in describing the bubble generation process. A modified Rayleigh equation is then used to calculate the bubble growth rate. The breakup criterion is then determined by applying a linear stability analysis on the bubble-droplet system. After the explosion/breakup, the atomization characteristics, including Sauter mean radius and averaged velocity of the secondary droplets, are calculated from conservation equations. Micro-explosion can be enhanced by introducing biodiesel into the fuel blends of ethanol and tetradecane. Micro-explosion is more likely to occur at high ambient pressure. However, increasing the ambient temperature does not have a significant effect on micro-explosion. There exists an optimal composition in the liquid mixture for micro-explosion.

The operation of a small bore high speed direct injection (HSDI) engine with a MVCO injector is simulated by the KIVA 3V code, developed by Los Alamos National Laboratory. The MVCO injector extends the range of injection timings over conventional injectors and it extra flexibility in designing injection schemes. Combustion from very early injection is observed with MVCO injections but not with conventional injection. This improves the fuel economy of the engine in terms of lower ISFC. Even better efficiency can be achieved by using biodiesel, which may be due to extra oxygen in the fuel improving the combustion process. Biodiesel sees a longer ignition delay for the initial injection. It also exhibits a faster burning rate and shorter combustion duration. Biodiesel also lowered both NOx and soot emissions. This is consistent with the general observation for soot emissions.

The KIVA-3V code, developed by Los Alamos National Laboratory, with modifications that improve its capability with biodiesel simulations was used to model the operation of an HSDI engine using blends of soybean biodiesel and diesel. Biodiesel and their blends with diesel are frequently used to reduce emissions from diesel engines, although previous studies showed that biodiesel may increase NOx emission. The paradox may be resolved by running the engine in low temperature combustion mode with biodiesel/diesel blends, as low temperature combustion simultaneously reduced NOx and soot. The modified KIVA code predicts the major combustion characteristics: peak combustion pressure, heat release rate and ignition timing accurately when compared with experimental measurements. It also correctly predicts the trend of NOx emissions. It was observed that the cylinder temperature distribution has a strong effect on emission levels.

Pure diesel and biodiesel were tested inside a constant-volume combustion chamber which simulates the in-cylinder conditions similar to a diesel engine and is more flexible to change the engine operation boundary conditions. The ambient temperature effect on flame lift-off length for both fuels was first investigated with fixed injection pressure, duration, ambient density, and ambient oxygen concentration. This was determined from time-averaged OH chemiluminescence imaging technique. Then, the impacts of the observed lift-off length variations on oxygen ratio upstream of the lift-off location and the soot formation process were also studied. A Forward Illumination Light Extinction (FILE) soot measurement technique was adopted to study the soot formation process. The FILE technique with the capability of two-dimensional time-resolved quantitative soot measurement provides the much-needed information to investigate the soot formation mechanism.

Performance and emissions of a common-rail production diesel engine fueled with soybean-derived biodiesel was investigated. The work was broken down into two categories. First, adjustment of injection timing and EGR ratio was investigated as a means to reduce NOx emissions to levels comparable with those obtained when using pure diesel fuel. Next, simultaneous reduction of NOx and soot emissions was investigated using high rates of EGR combined with late injection timings to approach the low-temperature combustion regime. Results from the first part of the study indicate that optimization of engine control parameters for use with biodiesel can be beneficial to performance and emissions. It was found that adjusting the engine's MAF setpoint table to reflect the difference in stoichiometric air-fuel ratio between diesel and biodiesel brought NOx emissions to comparable or lower levels.

Ternary blends of butanol-biodiesel-diesel with different blending ratios were tested inside a constant volume chamber under various ambient temperatures so as to investigate the spray and combustion characteristics of the fuels. Applying the high speed imaging, a sudden drop in spray penetration was observed at ambient temperature of 800 K and 900 K for fuels with certain blending ratio, but not at 1000 K and 1200 K. When the spray penetration of the butanol-biodiesel-diesel blends was compared to that of the biodiesel-diesel blends under non-combusting environment, a sudden drop in spray penetration length was also observed at 1100 K. The results indicated that for the non-combusting case, the tip of the spray jet erupted into a plume sometime after injection for the butanol-biodiesel-diesel blend at an ambient temperature of 1100 K. Such phenomenon was not seen with the biodiesel-diesel blend, neither with the same fuel but at a lower ambient temperature of 900 K.

A 2.5L, V-6, port-injected, spark-ignition engine was modified for optical access by separating the head from the block and installing a Bowditch extended piston with a fused-silica top and a fused-silica liner in one of the cylinders. Two heads were employed in the study. One produced swirl and permitted modulation of the swirl level, and another produced a tumbling flow in the cylinder. Planar laser-induced exciplex fluorescence, which allows the simultaneous, but separate, imaging of liquid and vapor fuel, was extended to capture components of different volatilities in a model fuel designed to simulate the distillation curve of a typical gasoline. The exciplex fluorescence technique was calibrated in a separate cell where careful control of mixture composition, temperature and pressure was possible. The results show that large-scale motion induced during intake is critical for good mixing during the intake and compression strokes.

Flash boiling has drawn much attention recently for its ability to enhance spray atomization and vaporization, while providing better fuel/air mixing for gasoline direct injection engines. However, the behaviors of flash boiling spray with multi-component fuels have not been fully discovered. In this study, isooctane, ethanol and the mixtures of the two with three blend ratios were chosen as the fuels. Measurements were performed with constant fuel temperature while ambient pressures were varied to adjust the superheated degree. Macroscopic and microscopic characteristics of flash boiling spray were investigated using Diffused Back-Illumination (DBI) imaging and Phase Doppler Anemometry (PDA). Comparisons between flash boiling sprays with single component and binary fuel mixtures were performed to study the effect of fuel properties on spray structure as well as atomization and vaporization processes.

The spray structure and vaporization processes of flash-boiling sprays in a constant volume chamber under a wide range of superheated conditions were experimentally investigated by a high speed imaging technique. The Engine Combustion Network’s Spray G injector was used. Four fuels including gasoline, ethanol, and gasoline-ethanol blends E30 and E50 were investigated. Spray penetration length and spray width were correlated to the degree of the superheated degree, which is the ratio of the ambient pressure to saturated vapor pressure (pa/ps). It is found that parameter pa/ps is critical in describing the spray transformation under flash-boiling conditions. Three distinct stages namely the slight flash-boiling, the transition flash-boiling, and the flare flash-boiling are identified to describe the transformation of spray structures.

The numerical models presented in this study are established based on discrete phase model (DPM) of spray dispersion and evaporation considering the cold wall operating condition of port injection system. All the models were implemented into the CFD software FLUENT. Gas flow and film flow and spray are coupled by mass, momentum and energy transfer due to spray impingement, film evaporation and surface shear stress. Influences of impact parameters including injection height, injection duration and injection angle on the formation and evaporation of wall-film are discussed. The results show that, with the increase of injection height, the maximum film thickness and wall film ratio decrease, and fuel vapor mass ratio increases. The reductions of film thickness and wall film ratio are not obvious as the increasing of injection height. Extending the injection duration could add the maximum film thickness and film area.

HTL is a kind of biodiesel converted from wet biowaste via hydrothermal liquefaction (HTL), which has drawn increasing attention in recent years due to its wide range of raw materials (algae, swine manure, and food processing waste). However, from the previous experiments done in a constant volume chamber, it was observed that the presence of 20% of HTL in the blend produced as much soot as pure diesel at in chamber environment oxygen ratio of 21%, and even more soot at low oxygen ratios. It was also observed that n-butanol addition could reduce the soot emission of diesel significantly under all tested conditions. In this work, the spray and combustion characteristics of HTL and diesel blends with n-butanol added were investigated in a constant volume chamber. The in-chamber temperature and oxygen ranged from 800 to 1200 K and 21% to 13%, respectively, covering both conventional and low-temperature combustion (LTC) regimes.