Yuyin Zhang

Imaging of droplets and vapor distributions in a Diesel fuel spray by means of a laser absorption–scattering technique

The droplets and vapor distributions in a fuel spray were imaged by a dual-wavelength laser absorption-scattering technique. 1,3-dimethylnaphthalene, which has physical properties similar to those of Diesel fuel, strongly absorbs the ultraviolet light near the fourth harmonic (266 nm) of a Nd:YAG laser but is nearly transparent to the visible light near the second harmonic (532 nm) of a Nd:YAG laser. Therefore, droplets and vapor distributions in a Diesel spray can be visualized by an imaging system that uses a Nd:YAG laser as the incident light and 1,3-dimethylnaphthalene as the test fuel. For a quantitative application consideration, the absorption coefficients of dimethylnapthalene vapor at different temperatures and pressures were examined with an optical spectrometer. The findings of this study suggest that this imaging technique has great promise for simultaneously obtaining quantitative information of droplet density and vapor concentration in Diesel fuel spray.

Spray and mixture properties of evaporating fuel spray injected by hole-type direct injection diesel injector

Abstract The spray and mixture properties are the vital factors that control the fuel—air mixing formation, subsequent combustion, and exhaust emission processes in direct injection (DI) diesel engines. In the current study, by means of the ultraviolet-visible laser absorption-scattering (LAS) technique, the spray and mixture properties of the evaporating fuel spray injected by hole-type DI diesel injector are quantitatively investigated under various injection conditions in terms of spray tip penetration, spray cone angle, equivalence ratio distributions of liquid/vapour phases, mass of ambient gas entrained, and evaporation ratio etc. The principle and imaging procedures of the LAS technique are first summarized and discussed. Then, the effects of injection pressure and nozzle hole diameter on the spray and mixture properties are examined in particular. The results show that — for the injection system and ambient conditions in this study — increasing the injection pressure from 90 MPa to 120 MPa results in approximately 40 per cent more ambient gas being entrained into the spray on average, and the effect of nozzle hole diameter on the mixture properties shows two contrasting trends.

Effect of split injection on stratified charge formation of direct injection spark ignition engines

Abstract The effects of split injection with various dwells and mass ratios on the spray and mixture characteristics in an ambient environment similar to the late stage of compression stroke in direct injection spark ignition (DISI) engines were investigated by using the laser absorption scattering (LAS) technique. Through splitting the fuel injection process with appropriate dwells and mass ratios, some benefits for the stratified charge formation of DISI engines can be achieved. First, the phenomenon of high-density liquid phase fuel piling up at the leading edge of the spray can be circumvented and the subsequent reduction in the spray penetration length for both liquid and vapour phases is seen. Second, the radial width of the ‘combustible mixture’ (equivalence ratio of vapour φv in a range of 0.7 ≤ φv ≤ 1.3) is significantly extended. Finally, the quantity of ‘over lean’ (φv < 0.7) mixture in the spray is significantly reduced. These results are believed to contribute to the stratified lean operation and the reduction in smoke and unburned hydrocarbons (UBHC) emissions of DISI engines. Further, the mechanism behind these effects of the split injection was clarified by analysing the interactions between the two pulsed sprays and the spray-induced ambient air motion using the LIF-PIV technique.

Imaging of vapor/liquid distributions of split-injected diesel sprays in a two-dimensional model combustion chamber

The ultraviolet-visible laser absorption-scattering (LAS) technique was extended to simultaneous imaging of two-dimensional vapor/liquid mass distributions in a nonaxisymmetric evaporating diesel spray such as that injected into a two-dimensional model combustion chamber of direct-injection diesel engines. The main error for vapor measurement is incurred by the temperature dependence of molar absorption coefficients (TDMAC) of the test fuel. Thus, selecting a test fuel of negligible TDMAC can greatly increase the measurement accuracy. By means of this extended LAS technique, the overall mass, as well as the mass distribution of vapor and liquid phase per projection area perpendicular to the laser beam, was obtained, and then the behavior of split-injection diesel spray in a two-dimensional model combustion chamber in a high-temperature high-pressure constant-volume vessel was observed. Finally, the effects of injection mass ratios on the distributions of vapor and droplets and characteristics of fuel/air mixing were examined.

VAPOR DISTRIBUTION MEASUREMENT OF HIGHER AND LOWER VOLATILE COMPONENTS IN AN EVAPORATING FUEL SPRAY VIA LASER ABSORPTION SCATTERING (LAS) TECHNIQUE

Abstract A laser imaging technique has been proposed in this article for measuring vapor distribution of lower and high volatile components in a multicomponent fuel spray, based on the LAS technique. For a two-component fuel spray which contains n-octane (boiling point: 399 K) and n-tridecane (boiling point: 508 K), vapor distribution of the low boiling point (LBP) component, n-octane, has been determined by using p-xylene (boiling point: 411 K) as its substitute, which has the similar physical properties and absorbs the ultraviolet light but not the visible light, according to the requirements of the LAS technique. Likewise, vapor behavior of the higher boiling point (HBP) component, n-tridecane, has been obtained by using α-methylnaphthalene (boiling point: 517 K). It has been found that difference in vaporization rate between the LBP and HBP components is great over the spray plume at 473 K, this difference, however, reduces significantly as the ambient temperature increases to 573 K; The LBP component vaporizes mainly in the outer region, while the HBP mostly in the inner region of the spray.

Vapor/Liquid Behaviors in Split-Injection D.I. Diesel Sprays in a 2-D Model Combustion Chamber

Some experimental investigations have shown that the trade-off curve of NOx vs. particulate of a D.I. diesel engine with split-injection strategies can be shifted closer to the origin than those with a single-pulse injection, thus reducing both particulate and NOx emissions significantly. It is clear that the injection mass ratios and the dwell(s) between injection pulses have significant effects on the combustion and emissions formation processes in the D.I. diesel engine. However, how and why these parameters significantly affect the engine performances remains unexplained. The effects of both injection mass ratios and dwell between injections on vapor/liquid distributions in the split-injection diesel sprays impinging on a flat wall have been examined in our previous work. In this paper, the behaviors of the split-injection diesel sprays in a 2-dimensional model combustion chamber, which was installed in a high-temperature and high-pressure constant volume vessel filled with nitrogen, was observed by use of the ultraviolet-visible laser absorption-scattering (LAS) imaging technique. The effect of the injection mass ratios and the effect of the dwell(s) between injections on the distributions of fuel vapor and droplets were clarified through qualitative imaging of the optical thickness of vapor and the optical thickness of droplets, respectively. The findings give an implication to the potential relation between the vapor/liquid behaviors in the split-injection sprays and the reduction mechanism of NOx and particulate emissions of the D.I. diesel engine.

Characterization of mixture formation processes in DI gasoline engine sprays with split Injection strategy via laser absorption and scattering (LAS) technique

In order to investigate the effect of split injections on mixture formation processes in Direct Injection (DI) gasoline engine sprays, an experimental study was conducted applying the laser absorption and scattering (LAS) technique to the sprays using double pulse injections with various dwells and mass ratios. The effects of various dwells and mass ratios between the pulsed injections on the spatial concentration distributions in the spray, the penetration of vapor and liquid phases, and the mean equivalence ratios of the vapor phase and overall spray, were clarified. It was found that the phenomenon of high concentration liquid spray piling up at the leading edge of the spray is avoided by the double injections with enough dwell or appropriate mass ratio. The maximum penetration length of the spray significantly decreases, especially for the liquid phase with high concentration. Moreover, the mean equivalence ratios including vapor phase and overall spray are significantly affected by the double pulse injections with various dwells and mass ratios.

Characterization of Mixture Formation Processes in D.I. Gasoline Sprays by the Laser Absorption Scattering (LAS) Technique - Effect of Injection Conditions

Mixture formation processes play a vital role on the performance of a D.I. Gasoline engine. Quantitative measurement of liquid and vapor phase concentration distribution in a D.I. gasoline spray is very important in understanding the mixture formation processes. In this paper, an unique laser absorption scattering (LAS) technique was employed to investigate the mixture formation processes of a fuel spray injected by a D.I. gasoline injector into a high pressure and temperature constant volume vessel. P-xylene, which is quite suitable for the application of the LAS technique, was selected as the test fuel. The temporal variations of the concentration distribution of both the liquid and vapor phases in the spray were quantitatively clarified. Then the effects of injection pressure and quantity on the concentration distributions of both the liquid and vapor phases in the spray were analyzed.