Ta Hui Lin

Engine performance and pollutant emission of an SI engine using ethanol-gasoline blended fuels

The purpose of this study is to experimentally investigate the engine performance and pollutant emission of a commercial SI engine using ethanol–gasoline blended fuels with various blended rates (0%, 5%, 10%, 20%, 30%). Fuel properties of ethanol–gasoline blended fuels were first examined by the standard ASTM methods. Results showed that with increasing the ethanol content, the heating value of the blended fuels is decreased, while the octane number of the blended fuels increases. It was also found that with increasing the ethanol content, the Reid vapor pressure of the blended fuels initially increases to a maximum at 10% ethanol addition, and then decreases. Results of the engine test indicated that using ethanol–gasoline blended fuels, torque output and fuel consumption of the engine slightly increase; CO and HC emissions decrease dramatically as a result of the leaning effect caused by the ethanol addition; and CO2 emission increases because of the improved combustion. Finally, it was noted that NOx emission depends on the engine operating condition rather than the ethanol content. r 2002 Elsevier Science Ltd. All rights reserved.

Emissions and efficiency of a domestic gas stove burning natural gases with various compositions

The heating value of a fuel, which depends on its composition, strongly affects burner performance. Using the same gas stove to burn natural gas with various heating values is inappropriate and hazardous due to the possible occurrence of incomplete combustion (i.e. a great increase of CO emissions and/or soot formation), liftoff, flashback and inadequate heat input. In this study, we aim to assess the effects of changes in gas composition on burner performance and propose suitable design or operational factors of domestic gas stoves burning natural gas with various heating values. A single gas burner, originally designed for burning natural gas with low heating value, is adopted to investigate the effects of variations in gas composition on the burner performance. The influence of five significant parameters, including gas composition, primary aeration, gas flow rate (heat input), gas supply pressure, and loading height, on the thermal efficiency and CO emissions were reported and discussed. Using natural gas with high heating value instead of natural gas with low heating value results in a decrease in thermal efficiency (due to higher thermal input) and an increase in CO emission (caused by incomplete combustion). These problems can be significantly improved by decreasing the gas pressure to a suitable value, by enlarging the primary aeration to a favorable level, by selecting a proper thermal input, or by adjusting the optimized heating height.

Theory of laminar flame propagation in off-stoichiometric dilute sprays

Abstract The structure and propagation of a steady, one-dimensional planar, low-speed flame in a dilute, monodisperse, sufficiently off-stoichiometric and weakly-heterogeneous spray, with bulk gas-phase burning, upstream droplet vaporization and downstream droplet vaporization/combustion, is analyzed using activation energy asymptotics. A prevaporized mode and a partially prevaporized mode of flame propagation are identified. Results show that lean and rich sprays exhibit qualitatively opposite behavior in response to the extent of mixture heterogeneity; specifically, the burning intensities of lean and rich sprays are respectively reduced and enhanced with increasing liquid fuel loading and increasing initial droplet size. Classification of all possible spray burning modes as a function of the mixture stoichiometry and initial droplet size is also presented.

Combustion of liquid fuel sprays in stagnation-point flow

Abstract Abstract The steady combustion of polydispersed sprays of ethanol and kerosene in the stagnation-point flow of lean methane/ air mixtures is experimentally investigated. Using laser Doppler velocimetry, the axial and radial velocity profiles of the dropletsare measured for water sprays in the presence or absence of lean methane/ air flame. These results are then compared with the velocity profiles obtained for small MgO particles under identical flow conditions. The results show the effects of gas expansion on droplet deceleration (acceleration) in the pre-flame (post-flame) regions. Also, it is found that addition of water spray results in the formation of a distributed region of yellow-orange emission downstream of the lean methane flame. In combustion of ethanol spray, certain critical fuel-concentration/ velocity limits are identified above which the flame becomes acoustically unstable. The implications of the study to the modeling of turbulent spray combustion are discussed.

The interaction between external and internal heat losses on the flame extinction of dilute sprays

Abstract The flame extinction of a dilute spray burning in a steady, one-dimensional, low-speed, sufficiently off-stoichiometric, two-phase flow, is studied using activation energy asymptotics. A completely prevaporized burning and a partially prevaporized burning of spray are identified on the basis of a critical value of the initial droplet radius for completing the vaporization process at the flame. The heat loss mechanism is composed of the external heat loss represeted by the buck heat conduction from the system to the surrounding, and the internal heat loss associated with the droplet gasification process. The interaction between external and internal heat losses on the flame extinction of dilute sprays is generally discussed on the basis of three parameters, namely the external heat loss, the initial droplet radius, and the liquid fuel loading. For the spray burning without external heat loss, there exists a triple-valued extinction curve in a rich spray, but is no extinction occurrence in a lean spray. Introducing the external heat loss to the spray flame, results show that for a lean spray, the flame flux at extinction is first increased, then decreased, and finally approaches exp(−0.5) with increasing the droplet radius; yet the correspondingly asymptotic value of the external heat loss is slightly larger than exp(−1.0). For a rich spray, the flame flux at extinction is monotonically increased with increased initial droplet radius or decreased liquid fuel loading.

Theory of laminar flame propagation in near-stoichiometric dilute sprays

Abstract The structure and propagation of a steady, one-dimensional planar, low-speed flame in a dilute, monodisperse, near-stoichiometric spray, with bulk gas-phase burning, upstream droplet vaporization and downstream droplet vaporization/combustion, are studied through the classification of burning structures such as slightly lean, slightly rich, and transition from lean to rich sprays, and using the matched asymptotic techniques in the limit of large activation energy. Results show that the flame speed of a slightly rich spray may be less than that of a gaseous premixture if the liquid droplets are large enough, but at decreased droplet sizes, the flame speed of the rich sprays may be increased to a value higher than that of the premized one. The optimum flame speed exists at a special condition of prevaporized burning, which has a stoichiometric gas-phase mixing. Furthermore, the flame propagation rate of a slightly lean spray increases with decreasing liquid fuel loading and decreasing initial droplet size, while the opposite holds for a slightly ric spray. The transition between slightly lean and slightly rich sprays is smoothly achieved through the variations of the initial droplet size and the mass fraction of liquid fuel in the study.

The influence of external heat transfer on flame extinction of dilute sprays

Abstract The extinction of a dilute spray flame burning in a steady, one-dimensional, low-speed, sufficiently off-stoichiometric, two-phase flow, and experiencing the external heat transfer from the spray to a tube wall upstream is further analyzed. The external heat transfer results in globally external heat loss, excess enthalpy burning and external heat gain, respectively, to the spray system with increasing the wall temperature. However, the droplet gasification provides the overall internal heat loss and heat gain for rich and lean sprays, respectively. Therefore, the burning and extinction of the dilute spray flame can be fully described by the interaction between external and internal heat transfers in two spray models which were identified to be the completely and partially prevaporized burnings. The C-shaped and S-shaped extinction curves are clearly classified and mapped with parameters of the wall temperature, the overall external heat transfer and the initial droplet size. Variations of the extinction curves under the influence of transition from overall external heat loss to heat gain, and the jump between the completely and partially prevaporized burnings on flame extinction, are reported and discussed for both lean and rich sprays.

Experiment on the dynamics of a compound drop impinging on a hot surface

A stable stream of compound drops which are composed of core fluid, water, encased by a layer of shell fluid, diesel, was utilized to investigate the dynamic behavior of a liquid-liquid compound drop impinging on a hot surface above the Leidenfrost temperature. The core-to-shell mass ratio and the modified normal Weber number, which takes into account the two interfaces involved, were taken to be the controlling parameters. The outcomes of a compound drop impacting on a hot surface consist largely of reflection with or without secondary drops. Based on energy conservation, the dissipated energy was estimated and a criterion for secondary drop formation was presented. The normal velocity after impact is reduced due to viscous dissipation while the tangential component remains almost unaffected. In addition, there is an interesting phenomenon of the core drop escaping from the compound drop. The experimental results show that an increasing core-to-shell mass ratio raises the momentum loss, reduces the number of secondary drops, and promotes core-drop escaping.

Influence of Vorticity on Counterflow Diffusion Flames

Abstract The extinction characteristics of diffusion flames of methane and butane in counterflow jets under opposing rigid-body rotation are investigated. It is found that the critical volumetric concentration of fuels in nitrogen corresponding to flame extinction decrease to a minimum value and increase thereafter as the jet angular velocity increases. This tendency could be described on the basis of variation of the rate of stretch and eventual breakdown of laminar flow caused by the angular velocity. An absolute mimimum fuel concentration corresponding to local extinction for diffusional burning is identified showing favorable agreement with the existing data. It is established that as the angular velocity of the jets increase, the unbalanced pressure gradients result in radially inward secondary flows. This leads to the development of recirculation zones on both sides of the stagnation plane whose thickness increase with angular velocity. Also, when oxygen enriched air is used, the jet vorticity may r...

Extinction Theory of Stretched Premixed Flames by Inert Sprays

ABSTRACT The extinction of a premixed flame propagating in a stagnation-point flow under the influence of an inert spray is studied using activation energy asymptotics. A completely prevaporized and partially prevaporized spray are identified on the basis of a critical value of the initial droplet size for completing the vaporization process at the flame. Parameters for flame extinction consist of the liquid loading and the droplet size of the spray indicating the internal heat loss associated with liquid vaporization, and the flow stretch which strengthens and weakens the burning strength of the Le 1 flame, respectively. The Le 1 flame show that flame extinction is possible for ...