**Abstract**

Oxy-flames from burners with separated jets present attractive perspectives because the separation of reactants generates a better thermal efficiency and reduction of pollutant emissions. The principal idea is to confine the fuel jet by oxygen jets to favor the mixing in order to improve the flame stability. This chapter concerns the effect of equivalence ratio on characteristics of a non-premixed oxy-methane flame from a burner with separated jets. The burner of 25 kW power is composed with three aligned jets, one central methane jet surrounded by two oxygen jets. The numerical simulation is carried out using Reynolds Average Navier-Stokes (RANS) technique with k-ε as a turbulence closure model. The eddy dissipation model is applied to take into account the turbulence-reaction interactions. The study is performed with different global equivalence ratios (0.7, 0.8 and 1). The validation of the numerical tools is done by comparison with experimental data of the stoichiometric regime (Ф = 1). The two lean regimes of Ф = 0.7 and 0.8 are investigated only by calculations. The velocity fields with different equivalence ratio are presented. It yields to increase of longitudinal and transverse velocity, promotes the fluctuation in interaction zone between fuel and oxygen also a better mixing quality and a decrease of the size of the recirculation zone.

**Keywords:** oxy-flame, turbulent, separated-jets, equivalence ratio, flame stability

## **1. Introduction**

This chapter concerns the numerical simulation and the PIV measurements on oxy-fuel burners with three separated jets. The mixing, dynamic and the temperature fields for both reacting and non-reacting flows are investigated.

Industrial systems with combustion phenomena such as burners, aeronautical engines and gas turbines are subject to increasingly important constraints, both economically (cost reduction, improved performance, etc.) than on the environmental level (reduction of pollutant emissions), requiring the development of new techniques to respond effectively to these industrial constraints. The development passes by new method of combustion in order to reduce pollutant emissions and fuel consumption, as well as by the improvement of flame stability [1, 2]. In previous studies, significant reductions of nitric oxide emissions have been successfully achieved by using low NOx technologies or oxy-combustion systems [3].

In air combustion, nitrogen leads to high fuel consumption and low combustion efficiency because nitrogen in the air acts as energy ballast. The substitution of air with pure oxygen leads to an increase in the laminar combustion rate up to 1300%, improves the thermal efficiency, increases the adiabatic flame temperature (2200 K for CH4-Air, 3090 K in oxy-combustion) reduce fuel consumption by 50% and, from an environmental point of view, reduce the formation of nitrogen oxides by up to 95% [4].

**2. Burner configuration and experimental set-up**

*A New Combustion Method in a Burner with Three Separate Jets*

the combustion products before the mixing of the reagents.

The configuration of the burner illustrated in **Figure 1** consists in separating the fuel and oxygen per injection in order to increase the dilution of the reactants with

This burner consists of three non-ventilated jets, one central with internal diameter dg equal 6 mm that contains the fuel and two side jets with internal diameter dox equal 6 mm contain pure oxygen. Boushaki et al. [22] have studied this three-jet burner configuration. The separation distance between the jets (S) used is 12 mm. The gas density equal to 0.83 kg m<sup>3</sup> and the oxygen is supplied by liquid air with a purity of 99.5% with a density of 1.354 kg m<sup>3</sup> (at 1 atm and at 15° C). The thermal power (P) of the burner is equal to 25 kW, therefore the flow rate and the output speed of the natural gas are respectively mng = 0.556 g s<sup>1</sup> and

The first study in this document is the control technique, consists in inclining the side oxygen jets towards the natural gas jet as shown in **Figure 1**. The angle of oxygen jets (ϴ) compared to the vertical direction varies from 0 to 30° (0, 10, 20, and 30°), however, we will present the effect of angle of the side oxygen jets on the

The combustion is carried out inside a square chamber of 60 60 cm2 section and

a height of 1 m. The side walls are water cooled and refractory lined inside the combustion chamber. a converging 20 cm high and a final section of 12 12 cm is placed at the end of the chamber to limit the entry of air from above. In order to allow optical access to all flame zones, six windows are provided in each face of the chamber. The Particle Image Velocimetry (PIV) was used as a measurement technique to characterize the experimental dynamic field. The PIV technique requires a laser sheet that clarifies the flow area studied a CCD camera, control equipment and an acquisition PC. The laser used is the Nd-YAG Bi-pulse with frequency 10 Hz and wavelength of 532 nm. The laser chain used is composed by a first divergent cylindrical lens and then by a second convergent spherical lens. The Mie signal emitted by the particles is collected by CCD camera of type a Lavision FlowMaster

**2.1 Basic configuration of the burner**

*DOI: http://dx.doi.org/10.5772/intechopen.90571*

Ung = 27.1 ms <sup>1</sup>

**Figure 1.**

**19**

*Schematic view of the burner.*

.

dynamic fluid, for many detail you can see [12].

(12-bit dynamic and resolution 1280 1024 pixels).

The flames from multiple jets aligned have used in many industrial installation. Several studies have been published on the dynamic properties of non-reacting multiple jets [5–9]. Lee et al. [10] have studied the geometry parameters of diffusion flames and giving a number of variables such as the number of jets and the distance between the jets. Lenze et al. [11] have studied the influence of three and five non-premixed flames, with town gas and natural gas burners. Their measurements concern flame width, flame length and concentrations in confined and free multiple flames.

A new generation of highly separated fuel and oxidant injection burners is of great interest to industrialists. The idea of this burner consists of separating combustible and oxidant to dilute the reactants with combustion products before the mixing of the reactants [12–14].

For this new combustion in a burner with three separated jets, the separation of jets provides a high dilution of reactants by combustion products in the combustion chamber. Consequently, this dilution decreases the flame temperature and decline in NOx production. In the literature, it has been proven that the separation of reactants are capable to change the flow structure, the flame characteristics, generates a better thermal efficiency and as well as reduction of pollutant emissions [15–18].

Salentey et al. [16] was interested in the characterization the flames from multiple jets aligned through dynamic properties (speed of the jets and distance injectors) and the flame topology (stability, length, blow ...). Lesieur et al. [14] has studied numerically the characteristics of a burner with three jets, focusing on the mixing of the jets, their dynamics and the pollutant emissions. Boushaki et al. [12] was interested on two main areas for flow, passive control with changing the diameter of the burner in order to affecting the dynamics flow; and active control requiring external energy intake through actuators while retaining the geometry of the combustion chamber.

The present chapter reports the results of a numerical and experimental investigation of the dynamic field on a burner with 25 kW power composed of three jets, one central jet of natural gas and two side jets of pure oxygen [19, 20]. One control systems, passive, is added to the basic burner to ameliore the combustion process to ensure the stabilization of flame and as well as pollutant reductions. The passive control is based on the inclined of side oxygen jets towards the central natural gas jet in burner with three separated jets.

Few works, are investigated the effect of equivalence ratios (in lean regime) on characteristics of non-premixed oxy-methane flames from burner with separated jets. However, the aim of this contribution is to investigate numerically the effect of different equivalence ratio on the combustion characteristics of a diffusion methane oxy-flame in a stabilized separated burner.

The mixture of hydrogen and natural gas is a new mixed fuel. The use of a mixed mixture of fuel and hydrogen has the advantage of modifying very effectively the properties of the fuel while preserving the distribution facility. Due to this, the high molecular diffusivity of hydrogen, the extended flammability limits, the high laminar flame speed and the low ignition energy, the addition of hydrogen in the fuel makes it possible to work in a combustion poor. Increasing flammability limits in the presence of hydrogen offset the adverse effects of poor combustion such as local extinctions, radiation energy losses, and flame stretching [21].
