Entropy Analyses of Droplet Combustion in Convective Environment with Small Reynolds Number

This paper analyzes the entropy generation rate of simple pure droplet combustion in a tempera-ture-elevated air convective environment based on the solutions of flow, and heat and mass transfer between the two phases. The flow-field calculations are carried out by solving the respective conservation equations for each phase, accounting for the droplet deformation with the axisymmetric model. The effects of the temperature, velocity and oxygen fraction of the free stream air on the total entropy generation rate in the process of the droplet combustion are investigated. Special attention is given to analyze the quantitative effects of droplet deformation. The results re-veal that the entropy generation rate due to chemical reaction occupies a large fraction of the total entropy generated, as a result of the large areas covered by the flame. Although, the magnitude of the entropy generation rate per volume due to heat transfer and combined mass and heat transfer has a magnitude of one order greater than that due to chemical reaction, they cover a very limited area, leading to a small fraction of the total entropy generated. The en-tropy generation rate due to mass transfer is negligible. High temperature and high velocity of the free stream are advantageous to increase the exergy efficiency in the range of small Reynolds number (<1) from the viewpoint of the second-law analysis over the droplet lifetime. The effect of droplet deformation on the total entropy generation is the modest.

Analytical and numerical studies on a single-droplet evaporation and combustion under forced convection

Existing droplet evaporation/combustion mod- els in computational fluid dynamics （CFD） simulation of spray combustion are based on simplified 1-D models. Both these models and recently developed 3-D models of single- droplet combustion do not give the conditions for the different existing droplet combustion modes. In this paper, droplet evaporation and combustion are studied both analytically and numerically. In the analytical solution, a 2-D axisymmetric flow surrounding an evaporating and combusting droplet was considered. The governing equations were solved using an integral method, similar to the Karman-Pohlhausen method for solving boundary-layer flows with pressure gradient. The results give a local evaporation rate and flame radius in agree- ment with experimental results. In numerical simulation, 3-D combusting gas flows surrounding an ethanol droplet were studied. The prediction results show three modes of droplet combustion under different relative velocities, explaining the change in the evaporation constant with an increase in relative velocity observed in experiments. This implies that different droplet combustion models should be developed in simu- lating spray combustion. The predicted local evaporation rate and flame radius by numerical simulation are in agree- ment with the analytical solution in the range of azimuthal angles 0° 〈 θ 〈 90°. The numerical results indicate that the drag force of an evaporating and combusting droplet is much smaller than that of a cold solid particle, and thus the currently used drag models should be modified.

Studies on theory and modeling of droplet and spray combustion in China: a review

Combustion phenomena were discovered still in far ancient time of China.From the 50’s of the last century,owing to the fast development of energy and power,aeronautical and astronautical,chemical and metallurgical engineering,combustion theory started to be studied in China.The Chinese scientists studied the theory of ignition,laminar fame propagation,droplet combustion,and spray combustion.Later,from the 80’s of the last century,numerical modeling of combustion started to be studied in China,including turbulence modeling,turbulent combustion modeling,two-phase turbulence modeling and two-phase combustion modeling,in the approaches of Reynolds Navier–Stokes(RANS)modeling,large-eddy simulation(LES),and direct numerical simulation(DNS)of combustion.Due to the limitation of a paper size,this paper gives only a review of studies on theory and modeling of droplet and spray combustion in China.

An Experimental Study on the Phenomena inside the Burning Aviation Kerosene Droplet

This paper conducted an experimental study of droplet behavior characteristics occurring within a burning aviation kerosene droplet and on the droplet interface.The droplet combustion was performed in a quiescent high-temperature environment by means of the droplet suspension technique.The soot particles were used as tracers to visualize the fluid motion in the droplet.The shape variation of droplet,internal flow pattern in the burning droplet,and instabilities at gas-liquid phase interface were recorded by a high speed camera.The internal fluid flow in the droplet was revealed to be caused by the Marangoni effect,which was generated because of the surface tension gradient induced by the temperature gradient on the droplet surface.The studies showed that the Marangoni effect has a significant impact on the internal flow.The amplitude of the capillary waves on the surface of the droplet was non-uniform and presented the amplitude characteristics of large-small-some.The wavelength of the capillary waves for burning aviation kerosene droplet ranged from 110μm to 120μm.