Droplet Burning Time Research Articles

Karakteristik Pembakaran Droplet Campuran Metil Oleat – Etanol dengan Penambahan Multi -Walled Carbon Nanotubes

This research intended to investigate the combustion characteristics of methyl oleic – ethanol blend with OH functionalized multi-walled carbon nanotubes (MWCNT-OH) addition. Methyl oleic is an unsaturated fatty acid methyl ester, which is a constituent of various biodiesels. The observed fuel was a mixture of Methyl oleic with 20% vol of ethanol. MWCNT-OH content was varied by 100 ppm, 200 ppm, 300 ppm, 400 ppm, and 500 ppm (wt based). The presence of ethanol in the droplets is intended to promote the microexplosion phenomenon, generating smaller droplets and a shorter burning time. The experimental results show that the MWCNT-OH addition on the droplet of Methyl oleic – ethanol blend decreased the ignition delay and droplet burning time, while the constant burning rate and droplet temperature (and flame temperature) were increased. Reducing ignition delay time due to the higher thermal conductivity of nanofluid droplets results in more effective heat absorption from the environment and better heat transport inside the droplet. Hence, droplet vaporization, flammable mixture formation and ignition occur in a shorter time. Furthermore, this condition encourages a higher burning rate and a lower droplet burning time. The higher droplet and flame temperatures are related to the higher heating value of the methyl oleic – ethanol – MWCNT-OH mixture and higher droplet burning rate, which results in a higher heat release rate.

Dynamics and combustion of single aluminium agglomerate in solid propellant environment

Aluminized composite propellant used in solid rocket motors contain a lot of aluminium particles because high combustion energy is generated and propulsion efficiency increases by burning aluminium particles. The combustion of aluminum occurs in a significant portion of the combustion chamber and produces aluminum oxide smokes and residues that are carried into the flowfield. Agglomerates have non-spherical shape, and consist of aluminium droplet and oxide particle (oxide cap) attached to the droplet. Unlike the liquid droplet ignition, the solid oxide film blocks the liquid aluminum from the penetration of the oxidizer hence prevents the particle from its ignition. Development of robust models of aluminum particle dynamics is essential in the design of advanced propulsion systems. The mathematical model of two-phase flow around a single aluminum droplet with oxide cap is developed. The model solves the continuity, momentum, energy and species continuity equations simultaneously to obtain the species and temperature profiles and the burning time of droplet. The results of numerical simulations are compared with predictions from semi-empirical correlations and computational data.

High vapour pressure nanofuel droplet combustion and heat transfer: Insights into droplet burning time scale, secondary atomisation and coupling of droplet deformations and heat release

Combustion characteristics of ethanol-water (EW) droplets laden with ceria nanoparticles are investigated. The present experimental study focuses on three facets of droplet combustion (i) burning time scale of droplets with and without NPs, (ii) pathways of secondary atomisation due to interface deformations and (iii) coupling of droplet shape deformations and flame heat release. A theoretical vaporisation timescale is advocated which considers natural convection-based evaporation, mass loss due to daughter droplet ejections, and flow through porous media. Droplets seeded with ceria nanoparticles, exhibit arrested surface undulations although internal ebullition is discernibly enhanced as compared to EW droplets without NPs. Deformations and formation of surface craters in EW droplets are traced to the imbalance between local vapour recoil (due to rapid ethanol vaporisation) and surface tension. Such craters collapse and form high-speed ligaments which eventually break at the tip through Rayleigh–Plateau mechanism. This pathway of secondary atomisation of EW droplets has been elucidated using a modified local weber number. On the contrary, for nanofuels, bubble rupture is the mechanism behind the surface crater formation. Proper orthogonal decomposition (POD) technique is utilised for investigating the droplet shape and flame heat release coupling. EW droplet shape and HR are found to be a synced system with a phase lag arising from the flame response timescale. However, a weak coupling is detected for nanofuel droplets.

Intrinsic thermoacoustic feedback loop in turbulent spray flames

This paper investigates low-frequency thermoacoustic instabilities of a turbulent spray flame, a phenomenon known as ‘rumble’. Based on experimental data, a network model analysis is performed, which suggests an intrinsic thermoacoustic (ITA) feedback loop as the root cause of instability. At first, the ITA nature of the observed instability is confirmed by a parametric analysis, which reveals the sensitivity of the instability frequency to the time delay of the flame. Then, we investigate pure acoustic modes and pure ITA modes, which couple to the full system modes such that the origin and the trajectory of each mode is trackable. Finally, we show that the unstable mode frequency scales with the inlet bulk velocity: a feature that clearly separates the observed instability from classical cavity modes. The network model results are corroborated by phasor plots, in which all relevant phase information is compiled. It reveals the phase of the Flame Transfer Function (FTF) contributing to the instability and provides an estimate of the ITA frequency, which agrees well with the dominant peak in the experimental pressure spectrum. Additionally, the obtained flame phase is used to infer the instability frequency by the experimentally measured droplet burning time τB, which reproduces similar trends as experimental and network model results. This theoretical study confirms that (1) ITA feedback loops are important for spray flames, (2) that the ITA instability of the experiment scrutinized is controlled by the droplet dynamics and (3) ITA modes appear also in acoustically closed systems.

Combustion Characteristics of Horizontal Diesel Spray Burner in Low‐Pressure Chamber Based on Multivariate Correlation Analysis

At present, the burning problem of low power burners widely used in emergency and stove heating on the plateau has gradually attracted people’s attention. The fuel injection pressure, inlet air flow, air distribution regulation coefficient, and environmental pressure were considered respectively, and then the four factors and six levels of a uniform experimental investigation were conducted in a low‐pressure chamber with adjustable atmospheric pressure. The flame morphology, flame temperature, and NO emission were investigated, and their correlation with each independent variable was analyzed. In addition, burning of droplet swarm was obtained with the simulate numerical calculation by the ANSYS FLUENT. The results showed that the low‐pressure environment had a negative correlation with the maximum width and position of flame, the characteristics of flame uplifted and NO emission. Air volume determines the length of the flame, and atmospheric pressure and fuel injection pressure determine the maximum width of the flame. The shape of the horizontal jet flame in plateau environment is more dependent on the regulation of air volume. In order to reduce NO emission in plateau environment, it may be necessary to maintain a large air supply volume and reduce fuel injection pressure appropriately. At the microscopic level of droplet combustion, change mechanisms of macro flame shape in different sub‐atmosphere were analyzed. The mechanism of flame morphological change in the plateau environment is the increase in droplet burning time and path. The adjustment of flame under sub‐atmospheric pressure through the change of operating parameters is essential to improve the burning rate of droplets.

Time-resolved detection of diffusion limited temperature gradients inside single isolated burning droplets using Rainbow Refractometry

Temporal and spatial resolved temperature measurements inside vaporizing and combusting droplets are needed to validate and improve numerical and simple approximated models. Elastic light scattering techniques such as Rainbow Refractometry are capable of probing unsuspended droplets below 100µm. The high sensitivity of the rainbow structure to refractive index changes enabled the detection and resolution of time-varying temperature gradients inside combusting droplets. This was accomplished through comparison of different droplet combustion models as well as direct inversion using a Simulated Annealing algorithm. Models incorporating diffusion limited heat transport showed very good agreement with the experimental data. Accordingly, the direct inverted results showed also reasonable agreement and the feasibility of the direct inversion of the temperature. Finally, an analytical solution of the heat conduction equation for a constant droplet size with exponential increasing surface temperature described the process for half of the droplet burning time very well, enabling the incorporation in spray combustion models to effectively account for temperature variation within combusting droplets.

Phenomenology of electrostatically charged droplet combustion in normal gravity

Experimental findings are provided on the effect of electrostatically charging a fuel on single-burning droplet combustion in normal gravity. It was established that significant modification of the flame morphology and the droplet burning time could be achieved, solely by the droplet charge, without the application of external electric fields. Negative charging of the droplets of mixtures of isooctane with either ethanol or a commercially available anti-static additive generated intense motion of the flame and abbreviated the droplet burning time by as much as 40% for certain blend compositions. Positive charging of the droplets generated almost spherical flames, because electrostatic attraction toward the droplets countered the effect of buoyancy. By comparing combustion of droplets of the same conductivity but different compositions, coupling of electrostatics with combustion chemistry was established.

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.

Soot formation characteristics of well-defined spray flames

The effects of fuel composition, interdroplet spacing, and excess oxygen concentration on soot formation in well-defined spray flames have been studied. The spray flames resulted from the combustion of a continuous stream of fuel droplets of uniform size and spacing in a uniform and stable gas environment. A specially-developed electrostatic probe was used to separate and extract samples of liquid droplets and soot particles from the spray flames. To provide requisite insight concerning factors affecting soot formation in spray flames, measurements were made of flame temperature by the Kurlbaum technique and flame position from photographic images. The soot emission index (mg of soot formed per g of fuel burned) was observed to increase substantially with either aromatic hydrocarbon content of the fuel or droplet density. Fuels containing polycyclic aromatic hydrocarbons (naphthalenes) exhibited the greatest propensity to form soot. But the effect of interdroplet spacing on soot formation diminishes with increasing tendency of the fuel to form soot. No perceptible effect of excess oxygen concentration on the soot emission index was observed. A phenomenological correlation was developed to explain the dependence of soot emission index on interdroplet spacing and ambient oxygen concentration by, attendant changes in flame temperature and position as well as fuel flow rate and droplet burning time.