Evaporation and ignition of droplets in combustion chambers modeling and simulation

Computer simulation of liquid fuel jet injection into heated atmosphere of combustion chamber, mixture formation, ignition and combustion need adequate modeling of evaporation, which is extremely important for the curved surfaces in the presence of strong heat and mass diffusion fluxes. Combustion of most widely spread hydrocarbon fuels takes place in a gas-phase regime. Thus, evaporation of fuel from the surface of droplets turns to be one of the limiting factors of the process as well. The problems of fuel droplets atomization, evaporation being the key factors for heterogeneous reacting mixtures, the non-equilibrium effects in droplets atomization and phase transitions will be taken into account in describing thermal and mechanical interaction of droplets with streaming flows. In the present paper processes of non-equilibrium evaporation of small droplets will be discussed. As it was shown before, accounting for non-equilibrium effects in evaporation for many types of widely used liquids is crucial for droplet diameters less than 100 μm, while the surface tension effects essentially manifest only for droplets below 0.1 μm. Investigating the behavior of individual droplets in a heated air flow allowed to distinguish two scenarios for droplet heating and evaporation. Small droplets undergo successively heating, then cooling due to heat losses for evaporation, and then rapid heating till the end of their lifetime. Larger droplets could directly be heated up to a critical temperature and then evaporate rapidly. Droplet atomization interferes the heating, evaporation and combustion scenario. The scenario of fuel spray injection and self-ignition in a heated air inside combustion chamber has three characteristic stages. At first stage of jet injection droplets evaporate very rapidly thus cooling the gas at injection point, the liquid jet is very short and changes for a vapor jet. At second stage liquid jet is becoming longer, because evaporation rate decreases due to decrease of temperature. But combustion of fuel vapor begins which brings to increase of heat flux to droplets and accelerates evaporation. The length of the liquid jet decreases again and remains constant slightly oscillating.     

Lowering the Systematic Error in Measurements of Local Heat Transfer Coefficient by Electric Heating of a Plane Wall

A technique that allows simultaneous heating of the heat-exchange surface and estimation of its local temperature from the corresponding electrical resistance of the heating element is presented. The systematic error in measurements of the local heat transfer coefficient has been analyzed. A relation that takes possible heat loss into account and minimizes the measurement errors has been obtained.     

Pulse detonation engines: Technical approaches

The paper contains analysis of the problems preventing from wide use of pulse detonation engines (PDE), and provides suggestions to overcome those problems. In particular, the results of theoretical investigations of basic operating cycle in PDE—deflagration-to-detonation transition (DDT) processes in combustible gaseous mixtures and transmission of detonation into large chambers—are presented. The paper investigates the effect of implosion shock waves on the onset of detonation in gases, and suggests the scheme of detonation transmission from a narrow gap into a wide chamber, which makes it possible to reduce the predetonation length thus shortening the necessary length of the engine.     

A technique for simulation of a fluid flow in branched channels

In this paper, a technique for simulation of the space-time flow parameters in branched channels under transient boundary conditions is presented. The technique is based on the solution of one-dimensional nonstationary equations of gas dynamics. We suggest the conditions of channel junction with regard for distinctive features of a fluid flow in the adjacent region and compare the data on the flow simulation in the channel with a lateral branch by using one-dimensional and three-dimensional models. Comparing the data obtained with the experimental results, we show that the technique being suggested adequately simulates the real processes in the branched channels.     

Seepage flows instability in porous media

The goal of the present study is to investigate analytically, numerically and experimentally the instability of the displacement of viscous fluid by a less viscous one in two- and three-dimensional channels, and to determine characteristic size of entrapment zones. Experiments on miscible displacement of fluids in Hele–Shaw cells were conducted under microgravity conditions. Extensive direct numerical simulations allowed investigating the sensitivity of the displacement process to variation of values of the main governing parameters. The influence of three-dimensional effects (aspect ratio) on displacement instability was studied. One-dimensional model to simulate mixing flux due to frontal displacement instability was developed for engineering applications.