Group Combustion Number Research Articles

Unsteady combustion and vaporization of a spherical fuel droplet group

An unsteady model has been developed to study the behaviour of a spherical group of fuel droplets. A numerical method has been employed to solve the governing equations. The results show that the steady-state model is no longer acceptable for a spherical fuel droplet group. A new and more reasonble parameter named the distance to diameter ration of droplet Sr, Sr=n−1/3/rto was introduced in place of the group combustion number G to characterize the behaviours of a spherical fuel droplet group. The results also show that the d2 law is still obeyed by the droplets in a vaporizing spherical fuel droplet group which is not very dense and the droplets in a burning spherical fuel droplet group which is just dilute. The combustion time of a spherical fuel droplet group increases with the increase of droplet diameter and decreases with the increase of droplet number density. The vaporization time of a spherical fuel droplet group increases with the increase of droplet diameter and the droplet number density.

Group combustion of char/carbon particles

Extensive literature exists for the experimental data on coal/char ignition and combustion. While most of the experiments are performed with a cloud or stream of particles, the theoretical modeling used to compare and interpret the experimental data is based on the individual particle combustion (IPC) model. As opposed to individual particle modeling, a group combustion (GC) theory is proposed for the combustion modeling of char/carbon particles. For a cloud of liquid drops, the group behavior implies the formation of a flame (group flame) around a large number of drops rather than a flame around each drop. More generally, the group behavior for a cloud of particles represents the change in the burning characteristics due to collective behavior of particles with or without a group flame. To gain a basic understanding of the group behavior, a model such as the analysis of a spherically symmetric cloud of particles burning in quiescent air is presented here. Each particle within the cloud produces CO, due to both the oxidation of C to CO and the reduction of CO 2 to CO which subsequently oxidizes to CO 2 in the homogeneous gas phase. Generalized results for the burning rate and the flame structure are given as a function of group combustion number (G). Predicted results show unexpected results including the independence of the burning rate of CO kinetics. Quantitative results for both the cases of frozen and fast CO kinetics are given. There is a group flame for the case of fast CO kinetics. It is shown that the group flame occurs at G > 5 while for a cloud of liquid drops, the group flame occurs at G > 0.1. The higher critical group combustion number is attributed to the lower burning rate of particle inside the cloud compared to the burning rate of liquid drops inside the cloud. The results show that there exists mainly three modes of combustion: (i) Individual Particle Combustion (IPC, low G), (ii) Group Combustion (GC, intermediate G) and (iii) Sheath Combustion (SC, high G). Criteria are given for identifying the mode of combustion from the experimental conditions. The criteria and the establishment of modes of combustion are independent of the extent of CO kinetics. It is found that the experimental data, obtained with a stream of particles and commonly interpreted with the IPC model, indicate the combustion modes to vary from IPC to SC modes. These data are now reinterpreted with the group theory.

Internal group combustion of liquid droplets

A quasi-steady internal group combustion model of a spherical droplet cloud has been developed to assess the effects of the collective behavior of fuel droplets on combustion characteristics and cloud structures. Analytical solutions are obtained for the predictions of the distributions of the temperature, concentrations of fuel vapor, and oxidizer in strongly and weakly interacting zones. Overall burning rate under internal group combustion mode at various flame penetration is also obtained. Numerical analysis is carried out to determine the combustion behavior of droplet clouds at some selected group combustion numbers. It was found that for a cloud of n-butylbenzene droplets, the group envelope flame is stabilized on the boundary of the droplet cloud for a group combustion number of 1.36. As the group combustion number decreases the envelope flame penetrates into the droplet cloud and divides the cloud into two zones; a strongly interacting zone located inside the group envelope flame and a weakly interacting zone established between the envelope flame and the boundary of the cloud. In the strongly interacting zone, the droplet vaporizes and the vapor produced is consumed at the group envelope flame. The droplets in a weakly interacting zone burn with an envelope flame surrounding each droplet. When the group combustion number decreases to 0.1, the group envelope flame degenerates into a point flame at the center of the droplet cloud.

Group Combustion of Liquid Droplets

Abstract Quasi steady “external group combustion” of droplet cloud with non-uniform droplet size distributions has been developed to assess the group burning behavior of fuel sprays in industrial and jet engine combustors. The exact solutions for the perturbation equation for power law droplet size distributions are obtained. Numerical analysis pertaining to the initial phase of group combustion with mean droplet size approximation is also presented. The study reveals that current spray burners are operating at high group combustion number regime, wherein the burning mechanism coincides with external group burning mode as previously predicted by Suzuki and Chiu. A spray with a higher group combustion number is also featured by a “sheath vaporization” which is localized in a thin layer located in the outermost region of the cloud. Sprays at lower group combustion number regime cease to burn with external combustion modes when the group combustion number is less than the critical value which is determined by the fuel's properties and combustion environment. Droplet size distribution is also identified as a major parameter that affects the total burning rate of a spray.