Combustion Characteristics Of Droplets Research Articles

03/01636 Combustion characteristics of droplets composed of light cycle oil and diesel light oil in a hot-air chamber: Xu, G. et al. Fuel, 2003, 82, (3), 319–330

03/01636 Combustion characteristics of droplets composed of light cycle oil and diesel light oil in a hot-air chamber: Xu, G. et al. Fuel, 2003, 82, (3), 319–330

Combustion characteristics of droplets composed of light cycle oil and diesel light oil in a hot-air chamber☆

Development of gas turbines fueled with light cycle oil (LCO) and oil mixture of LCO and diesel light oil (LO) requires an understanding of the droplet burning and vaporization characteristics of those oils. The present study is devoted to comparing the burning characteristics of isolated fuel droplets composed of an LCO and an LO. The tests were conducted in an atmospheric hot-air chamber preset at 1173 K, and the examined LCO had a lower cetane number but higher volatility and aromatics content compared to LO. It was demonstrated that the burning of the LCO droplet was sootier, while that of the LO droplet was more disruptive. At the tested temperature, coke formation was indistinct for both the oils, whereas slightly higher ignition delay time was shown for the LO droplet. The microexplosive burning more or less complicated the time-series droplet size d, an explicit burning rate constant, however, was still definable according to the d 2-law to show the overall regression speed of the droplet surface area d 2 with burning time t. The rate constant exhibited little difference for smaller LCO and LO droplets but was greater for LO when the droplet was larger. The rate constant also gradually increased with increasing the initial droplet diameter d 0, which caused the relative size d/ d 0 to be unified (normalized) into a single curve by a burning time t/ d 0 n (1.0< n<2.0). Analysis revealed that this unification resulted from the respective overlaps of the unsteady and quasi-steady burning phases for differently sized droplets. Further, it was clarified that the unification and analysis are generally valid to isolated liquid fuel droplet burning in hot ambiences.

On the vaporization and thermal oxidation of chlorinated hydrocarbon/alcohol sprays

We report species concentration data obtained during the vaporization and thermal decomposition of polydisperse multicomponent chlorinated hydrocarbon (CHC)/alcohol sprays. Mixtures of 1,1,1-trichloroethane (C2H3Cl3) (TCA) and isopropanol (C3H7OH) are atomized in the postflame environment of a methane/air (Φ=0.95) flame. Species concentrations, determined by extractive Fourier transform infrared analysis, reveal perturbations in TCA destruction and by-product species formation. The effect of isopropanol addition on by-product species concentrations is similar to previous results involving the thermal oxidation of TCA/alkane mixtures. TCA destruction is incomplete at injection temperatures less than 1000 K. Under these conditions, TCA/isopropanol sprays produce lower residual TCA concentrations than pure TCA sprays. Review of physical property data, numerical modeling (to be published elsewhere), and comparisons with previous experimental results for TCA/heptane and TCA/hexadecane together suggest two primary findings. First, the conventional approach of parameterizing fuel volatility by the fuel saturation temperature at 101.3 kPa does not correlate with the trends evident in our data. For these fuel mixture and conditions, the latent heat of vaporization (ΔHv) provides a better correlation. Second, the data also suggest that, for incineration-resistant multicomponent mixtures, the molecular weights of mixture constituents may influence droplet combustion characteristics and should be considered when formulating waste-blending strategies.

Droplet Combustion, Microexplosion, and Sooting Characteristics of Several Energetic Liquid Propellants

The combustion characteristics of droplets of four newly synthesized energetic fuels, namely, 1 ) 1,4dimethyl ester cubane (which is a solid slightly soluble in conventional liquid fuels ), 2) dihydrobenzvalene, 3) phenyl azide, and 4 ) carborane (which are all liquids ), were experimentally identie ed and quantie ed. Results show that the gradual concentration and eventual condensation of dimethyl ester cubane dissolved in benzene can lead to strong droplet microexplosion; that the three liquid fuels all soot profusely, with the sooting propensity, when compared with other fuels, being in the following order: JP-10 < benzene ’ dihydrobenzvalene < phenyl azide < carborane; and that phenyl azide additionally exhibits fast gasie cation rates, advanced microexplosion events, and sensitivity to halide addition. It is emphasized that a high-energy-density fuel does not automatically imply that it is fast burning, and that it must also possess desirable combustion characteristics, especially minimal sooting formation, before it can be considered for use as a jet fuel or jet fuel additive.

単一油滴による重質燃料油の燃焼特性評価

This paper describes the fundamental study about the evaporation and combustion characteristics of heavy fuel oil. Firstly, the difference of thermogravimetric curves between heavy fuel oil and marine diesel oil was examined using a thermobalance. Secondly, the combustion characteristics of a single droplet of both fuels were observed in detail using a high-speed video camera. Furhermore, the four types of hydrocarbons, saturated hydrocarbons“Sa”, aromatic hydrocarbons“Ar”, resin“Re”and asphaltene“Asp”were extracted from the fuels and evaporation and single droplet combustion characteristics of each were examined. The results obtained from the examinations are as follows: (1) Ar are surely one origin of soot formation. In case of marine diesel oil combustion, it is possible that combustion of Sa improves the combustion of Ar. Howevver, for heavy fuel oil which includes much heavier components, further investigation in combustion characteristics of Ar, Re and Asp is necessary.; (2) Asp is the heaviest component to evaporate and easy to form cinder after the single droplet combustion. Further, in one experiment all Asp was extracted from heavy fuel oil. This fuel then showed better evaporation and combustion characteristics (i.e. less cinder formation) than normal heavy fuel oil.

An experimental investigation of the burning characteristics of water-oil emulsions

An experimental investigation was conducted on the combustion characteristics of droplets of n-heptane, n-decane, n-dodecane, n-hexadecane and iso-octane emulsified with various amount of water and freely falling in a furnace of controlled temperature. Results demonstrate the intricate influences of water emulsification on the ignition, extinction and micro-explosion of the droplet response, and that the droplet burning time can be significantly reduced through judicious fuel blending so as to minimize the ignition delay and advance the onset of micro-explosion.

The effect of liquid mass transport on the combustion and extinction of bicomponent droplets of methanol and water

The time-dependent combustion of isolated, bicomponent liquid droplets of methanol and water is simulated using a spherosymmetric, finite-element, chemically reacting flow model. The computations consider multi-component molecular transport and detailed chemical kinetics (19 species, 89 forward reactions) in the gas phase; semiempirically formulated vapor-liquid equilibrium with water vaporization/condensation at the liquid surface; and liquid species mass transfer and energy conservation in the liquid phase. The results are compared with previously reported data from microgravity drop tower, freely falling isolated droplet, and suspended droplet combustion experiments, all of which show significant departure from d 2-law burning behavior and/or substantial water accumulation in the liquid phase. The liquid mass Peclet number (defined as the ratio of the droplet surface regression velocity to the effective diffusion velocity of water within the liquid droplet) is greater than 20 for methanol burning in air. The numerical model predicts little or no water accumulation for such circumstances. However, numerical results are consistent with experiments when it is speculated that sufficient internal liquid phase motion is present to reduce the effective liquid mass Peclet number to the order of one. Such internal motion has been noted in droplet combustion experiments and most likely arises from droplet generation/deployment techniques and/or surface tension gradients. Calculations show that the droplet combustion characteristics of methanol/water mixtures of 0%–50% initial water content are extremely sensitive to the liquid mass transport effects and might be used to better determine the magnitude of liquid mass transport present in microgravity experiments.

Studies of influences of liquid-phase species diffusion on spherically symmetric combustion of miscible binary droplets

Spherically symmetric combustion of binary miscible droplets is studied for the case where liquid-phase species transport is slow relative to droplet surface regression rates. Attention is focused upon later periods of combustion, following decay of initial transients, when droplet species profiles change slowly relative to droplet size changes, and d 2 law combustion (the square of the droplet diameter decreases linearly with time) closely holds. The gas phase is assumed to remain quasi-steady. It is demonstrated that when droplet species profiles change slowly relative to droplet size changes, the mass-flux fraction of a given species off the droplet surface is approximately equal to the volume-averaged mass fraction of that species within the droplet. The ratio of the liquid-phase species diffusion coefficient to the burning-rate constant is treated as a small parameter, and asymptotic solutions for droplet species profiles are developed in terms of this parameter. A concentration boundary layer, where species profiles change sharply in the radial coordinate, is shown to be present at the droplet surface. It is shown that accounting for the boundary-layer structure may be important when predictions of droplet combustion characteristics are made. Sample solutions for heptane-dodecane mixtures and water-methanol mixtures indicate that the variables most affected by accounting for the concentration boundary-layer structure are droplet temperatures and surface compositions.

Experimental Study on Evaporation and Combustion of Single Coal-Slurry Droplets (Behavior on a Hot Surface in an Elevated-Temperature Environment)

The combustion characteristics of single CWM and COM droplets on a hot surface were investigated. The surrounding temperature was carefully adjusted to the temperature of the hot surface. The vaporization and combustion behavior of a single slurry droplet under a uniform temperature condition could then be observed. The ignition delay time of the COM droplet was shorter than that of the CWM droplets. When the initial diameter of the droplet decreased, the total burning time decreased proportionately with the square of the initial diameter. The effect of the size distribution of the pulverized coal appeared on the lower temperature limit of the surface reaction process of the fixed carbon. Ten kinds of pulverized coal were compared to obtain the relation between the fuel ratio of the coal and the apparent diffusion coefficient that controlled the combustion of the slurry droplet.

Single-droplet combustion of coal slurry fuels

The combustion characteristics of single droplets of coal slurry fuels were experimentally investigated using both spontaneous and forced ignition. Results showed that the combustion is a sequential two-staged process, consisting of gas-phase combusion of the volatiles followed by combusion of the solid residue, which is mostly carbon. A splashing combustion phenomenon, which corresponds to the outgassing of the thermally cracked fuel gas from the droplet interior, occurred during the later period of gas-phase combustion in the same way as for residual heavy fuel oils. However, no disintegration or disruption of the droplet was observed. The duration of combustion of the solid residue agglomerate was several times longer than that of the volatiles, and therefore constituted a major part of the total burning time. The gas-phase burning time as well as the total burning time were found to vary linearly with the square of the initial droplet diameter, thus obeying the “square law of initial droplet diameter.” The overall burning rate coefficients of coal slurry fuels were about 1 2 to 1 4 times those of residual heavy fuel oils because of the long solid combustion time. The overall burning rate coefficient was found to depend mainly on the residual carbon content of the coal slurry fuel.

Initial Observations on the Free Droplet Combustion Characteristics of Water-In-Fuel Emulsions†

A new mechanical technique for the production of well characterized small diameter isolated free droplets of multi-component and emulsified liquids is described. The technique is employed in development of an experimental facility to generate, inject, and combust free droplets of liquid fuels in a well defined, hot convective atmosphere. Initial observations of the burning characteristics of isolated free droplets of water in pure n-paraffin emulsions are reported. The existence of free droplet secondary atomization and the importance of fuel physical characteristics, water content, and internal phase structure to optimizing this phenomenon are confirmed. Observations are found to be in agreement with earlier predictions of the required internal phase super heat (Avedisian and Andres, 1978) and results suggest previously published suspended droplet combustion data on emulsified fuels to be of limited quantitative value. Observations are compared with the prediction of and physical models used in recent an...

Combustion characteristics of droplets of coal/oil and coal/oil/water mixtures

The utilization of coal/oil mixtures in furnaces and boilers appears to be a viable short-to-medium range solution to partially alleviate the demand on petroleum supply with minimum combustor modifications. The present investigation first demonstrates that the combustion characteristics of the coal/oil mixture droplets depend intimately on the intensity of internal circulation. Rapid internal motion favors batch-distillation-type vaporization which causes the coal particles to agglomerate. On the other hand, slow internal motion traps the volatile components in the inner core of the rapidly-heated droplet and may eventually lead to internal boiling and subsequently fragmentation of the droplet. Experimental results reveal a mixed behavior, although the degree of agglomeration is significant even at moderate levels of coal loading. It is subsequently shown that the addition of small quantities of water significantly enhances the potential and intensity of droplet explosion. Practical implications of the present results are also discussed.