Fuel droplet vaporization and spray combustion theory

A critical review is presented of modern theoretical developments on problems of droplet vaporization in a high-temperature environment and of spray combustion. Emphasis is placed upon analytical and computational contributions to the theory with some mention of empirical evidence. Four areas of basic phenomena are discussed in some detail: (i) droplet slip and internal circulation, (ii) transient heating of the droplets, (iii) multicomponent fuel vaporization, and (iv) combustion and vaporization of droplet arrays, groups, and sprays. Various relationships amongst these phenomena are analyzed as well. Several other problem areas are given brief mention. Future directions are suggested.     

Flame propagation through concentration gradient

IntroductionIn the matter of internal combustion engine, thesimultaneous reduction of fuel consumption andpollutant emission are required. Pedial premixedcombustion is expected to be a combustion method torealize this. The combustion method has both theadvantage of premixed combustion (low environmentalpollution ) and that of diffusive combustion (highefficiency ). The method is already used in the directinjection gasoline engine and in diesel engine with twostage injection or spray atomizatio…     

Multicomponent fuel droplet vaporization and combustion using spectral theory for a continuous mixture

The Quasi-Steady vaporization and combustion of a multicomponent, spherically symmetrical droplet composed of a thermodynamically ideal mixture of mutually soluble fuels is analyzed theoretically by approximating the discrete mixture by a Continuous Mixture (CM). The CM is described locally by a general Probability Density Function (PDF), which is approximated by a truncated spectral expansion with a number of ‘components’ much smaller than the number of chemical components in the original mixture. Two methods (Galerkin and Orthogonal Collocation OC) are proposed, discussed, and OC is used, to solve the evolution of the spectral governing equations. The present paper generalizes the methods employed in most earlier Continuous Mixture Theory (CMT) studies, in which the PDF describing the mixture is assumed to have a predetermined mathematical form. These methods are illustrated for the practical cases of vaporization and combustion of individual droplets of gasoline, diesel or aviation fuel JP4. The results show that in most cases our spectral OC provides useful results with as few as six spectral pseudocomponents.     

Forced convection droplet evaporation with finite vaporization kinetics and liquid heat transfer

Stagnation point calculations, including the effects of liquid phase heat transfer and finite rate evaporation kinetics, are presented for the case of a high Reynolds number flow over a vaporizing droplet. A correlation is developed to compute the entire droplet vaporization rate from the stagnation point results. Numerical emphasis is placed on high temperature, rapid vaporization processes such as occur in flight vehicle engines, and sufficient calculations are presented to allow estimates for any given case to be made.     

Finite-rate evaporation and droplet drag effects in spherical flame front propagation through a liquid fuel mist

An evolution equation for a laminar flame front propagating into an air and liquid fuel mist cloud is derived for the first time, accounting for both the finite-rate evaporation of the fuel droplets and the slip velocity between them and their host environment. The asymptotic analysis employed in developing the equation exploits the usual inverse large activation energy parameter associated with chemical reaction in the flame and a small drag parameter. It is demonstrated that, in the no-slip velocity case, increasing the vaporization Damköhler number can produce flame extinction, presumably due to the more intense heat loss incurred due to droplet heat absorption necessary for vaporization. Droplet drag can also induce extinction due to the longer residence time of the droplets in any locale (than if there was no slip), leading to more vaporization with greater attendant heat loss. The predicted results for droplet velocity are compared to independent experimental data from the literature with good qualitative agreement.     

Simulation of transient convective burning of an n-octane droplet using a four-step reduced mechanism

The transient burning of an n-octane fuel droplet in a hot gas stream is numerically studied using a four-step reduced mechanism, with considerations of droplet surface regression, deceleration due to the drag of the droplet, internal circulation inside the droplet, variable properties, non-uniform surface temperature, and the effect of surface tension. Two different types of the four-step mechanism are examined and found almost identical. The four-step mechanism has earlier instant of the wake-to-envelope transition than the one-step mechanism at low ambient temperature, but this difference between the two mechanisms diminishes when the ambient temperature is increased. The four-step mechanism has smaller mass burning rate for a wake flame but greater mass burning rate for an envelope flame than the one-step mechanism. The two mechanisms have small differences in the critical initial Damkohler number. Lower ambient temperature yields later wake-to-envelope transition and smaller mass burning rate. Higher ambient pressure has greater overall mass burning rate because of greater gas density and thus greater concentrations of reactants for a major part of the lifetime. Greater ambient mass fraction of oxygen yields faster oxidation kinetics and greater Damkohler number. As the ambient mass fraction of oxygen increases, the instant of wake-to-envelope transition advances for an initial wake flame, and finally the initial flame becomes an envelope flame when the ambient mass fraction of oxygen exceeds some critical value. A correlation is developed for the critical initial Damkohler number in terms of the ambient temperature, ambient pressure, and ambient mass fraction of oxygen.     

The transient response of an evaporator fed through an electronic expansion valve

The primary task of an expansion valve in a refrigerating machine is to control the mass flowrate into the evaporator to obtain optimal operation without ‘hunting’ under given conditions. A stepper-motor-controlled expansion valve and an evaporator have been studied. The tests were carried out on a fixed refrigerating machine of cooling capacity less than 6 kW, with constant condensation conditions and variable evaporating temperature (−20 to +10°C) and compressor speed (1000 to 3000 r.p.m). Two control algorithms (proportional/derivative and qualitative optimal regulation) have been developed for opening and closing the valve with the stepper motor. The control parameters depend on both the expansion valve and the evaporator transfer functions. In steady-state conditions, the system is stable with a superheat equal to the set value. Under transient conditions, with step excitations of 300 and 1000 r.p.m as well as for cold-start of the machine, the control algorithms are adequate for regulating the refrigerant flowrate into the evaporator. © 1997 John Wiley & Sons, Ltd.     

Exploring injected droplet size effects on steady liquid penetration in a Diesel spray with a two-fluid model

An approach to include droplet size effects in a two-fluid model for Diesel sprays when the locally-homogeneous flow (LHF) assumption is employed is developed. The model is then employed to study the effect of droplet sizes on the steady liquid penetration in vaporizing Diesel sprays when several injection and chamber parameters are changed. These parameters include the orifice diameter, injection pressure, ambient temperature and the ambient density. The computed steady liquid penetration is compared with constant volume measurements made under Diesel conditions at the Sandia National Laboratories. It appears that under typical Diesel conditions, the steady liquid penetration is controlled by entrainment and mixing alone. However, at lower ambient densities, droplet sizes may also be important.     

采用优化算法分析燃烧火焰辐射光谱求取火焰温度

通过对优化算法的尝试利用从气体燃烧火焰发出的辐射光谱拟合分析出火焰燃烧温度的方法,简化了传统测温用双色法所要求的火焰绝对辐射强度的标定,同时有助于了解火焰辐射黑度随波长的变化规律。通过和热电偶测量值的比较,证明经该方法得到的计算结果具有一定的精度,使燃烧火焰温度的在线监测成为可能。     

火焰进气预热塞电加热特性分析

在自行建立的火焰进气预热塞试验台上,研究了不同通电电压、空气流速、环境温度下火焰进气预热塞的电加热特性,得出预热塞电功率、预热塞电热棒表面温度随加热时间的变化规律。结果表明:随着时间的推移,预热塞的电功率表现出先期快速下降,后期缓慢下降的特性;预热塞电热棒表面温度表现出先期快速上升,后期缓慢上升的特性。随着通电电压的增大,电功率先期下降的速度越快,电热棒表面温度先期上升的速度越快,加热90 s时,预热塞的电功率和电热棒表面温度随着电压的增加而近似线性增加;随着空气流速的增加或环境温度的下降,电热棒表面温度先期上升的速度变慢,加热90 s时,电热棒表面温度随着空气流速的增加或环境温度的下降而近似线性下降。     

Flame spread over aviation kerosene with an obstacle in liquid phase

The phenomena of flame spread over aviation kerosene with an obstacle in liquid phase are investigated experimentally through surface temperature measurement by using infrared camera,schlieren images of subsurface flow in front of and behind obstacle and residence time of flame obtained from video recording.Experimental results reveal that obstacle has no effect on gas phase controlled flame spread.But for liquid phase controlled flame spread,flame can be stopped by an obstacle with its top edge flush with oil surface,and the residence time decreases with the increase of initial temperature of fuel.That conduction and radiation only play a subsidiary role in flame spread over liquid fuel was proved by schlieren images and surface temperature profiles.     

Transient heating of an evaporating droplet with presumed time evolution of its radius

New solutions to the heat conduction equation, describing transient heating of an evaporating droplet, are suggested, assuming that the time evolution of droplet radius R_d(t) is known. The initial droplet temperature is assumed to be constant or allowed to change with the distance from the droplet centre. Since R_d(t) depends on the time evolution of the droplet temperature, an iterative process is required. Firstly, the time evolution of R_d(t) is obtained using the conventional approach, when it remains constant during the timestep, but changes from one timestep to another. Then these values of R_d(t) are used in the new solutions to obtain updated values of the time evolution of the distribution of temperatures inside the droplet and on its surface. These new values of droplet temperature are used to update the function R_d(t). This process continues until convergence is achieved, which typically takes place after about 15 iterations. The results of these calculations are compared with the results obtained using the previously suggested approach when the droplet radius was assumed to be a linear function of time during individual timesteps for typical Diesel engine-like conditions. For sufficiently small timesteps the time evolutions of droplet temperatures and radii predicted by both approaches coincided. This suggests that both approaches are correct and valid. Similarly to the case when droplet radius is assumed to be a linear function of time during the timestep, the new solutions predict lower droplet temperature and slower evaporation when the effects of the reduction of R_d are taken into account. It is shown that in the case of constant droplet initial temperature, models both taking and not taking into account the changes in the initial droplet temperature with the distance from the droplet centre predict the same results. This indicates that both models are correct.     

Modeling of laminar flame propagation through organic dust cloud with thermal radiation effect

In this research, a mathematical model is performed to analyze the structure of flame propagation through a two-phase mixture consisting of organic fuel particles and air. In contrast to previous analytical studies, thermal radiation effect is taken into consideration, which has not been attempted before. In order to simulate of the dust combustion phenomenon, it is assumed that the flame structure consists of four zones: preheat, vaporization, reaction and post flame (burned). Furthermore, radiative heat transfer equation is employed to describe the thermal radiation exchanged between these zones. The obtained results show that the induced thermal radiation from flame interface into the preheat and vaporization zones plays a significant role in the improvement of vaporization process and burning velocity of organic dust mixture, compared with the case in which the thermal radiation factor is neglected. According to present results, flame structure variables such as the burning velocity, mixture temperature, mass fraction of volatile fuel particles and gaseous fuel mass fraction strongly depend on radiative heat transfer. These predictions have reasonable agreement with published experimental data.     

Numerical simulation of the upward propagation of a flame in a vertical tube filled with a very lean mixture

Upward propagation of a premixed flame in a vertical tube filled with a very lean mixture is simulated numerically using a single irreversible Arrhenius reaction model with infinitely high activation energy. In the absence of heat losses and preferential diffusion effects, a curved flame with stationary shape and velocity close to those of an open bubble ascending in the same tube is found for values of the fuel mass fraction above a certain minimum that increases with the radius of the tube, while the numerical computations cease to converge to a stationary solution below this minimum mass fraction. The vortical flow of the gas behind the flame and in its transport region is described for tubes of different radii. It is argued that this flow may become unstable when the fuel mass fraction is decreased, and that this instability, together with the flame stretch due to the strong curvature of the flame tip in narrow tubes, may be responsible for the minimum fuel mass fraction. Radiation losses and a Lewis number of the fuel slightly above unity decrease the final combustion temperature at the flame tip and increase the minimum fuel mass fraction, while a Lewis number slightly below unity has the opposite effect.     

Gasification characteristics of coke and mixture with coal in an entrained-flow gasifier

To enhance clean energy utilization and reduce greenhouse gases, various gasification technologies have been developed in the world. The gasification characteristics, such as syngas flow rate, compositions, cold gas efficiency and carbon conversion, of petroleum coke and mixture of petroleum coke and lignite were investigated in a 1 T/d entrained-flow gasifier (I.D. 0.2 m × height 1.7 m) with quencher as a syngas cooler. CO concentration was 31–42 vol% and H₂ concentration was almost 22 vol% in the gasification experiments of petroleum coke. In the case of mixture of petroleum coke and lignite, CO concentration was 37–47 vol% and H₂ concentration was almost 25 vol% due to synergy effect. The gasification of mixture resulted in higher syngas heating value and cold gas efficiency because of the higher H₂ and CO composition in syngas.     

Wave propagation in an initially stressed magneto-thermoelastic medium with voids and microtemperatures

Abstract

Present analysis is concerned with the propagation of plane waves in an initially stressed magneto-thermoelastic half-space with microtemperatures and voids. It is noticed that there exist four kinds of coupled dilatational waves in addition to shear and microtemperature waves in such type of medium. Using appropriate boundary conditions, the amplitude and energy ratios for the reflected waves have been presented in closed form when (i) a set of coupled dilatational waves is made incident and (ii) a shear wave is made incident. Amplitude ratios of various reflected waves are presented graphically to explore the effects of magnetic field, initial stress and void parameter. It is verified that there is no loss of energy at the free surface during reflection phenomena.