Numerical investigations on combustion and emission characteristics of a novel elliptical jet-stabilized model combustor
Graphical abstract
Introduction
The liquid fuel combustion process in a gas turbine combustion chamber is very complex, involving multi-physics couplings such as flow, atomization, radiation field, and the combustion reaction. At the same time, high combustion efficiency and low pollution emissions are critical indicators for the design of modern gas turbines [[1], [2], [3], [4], [5]]; as such, this has been one of the most important research fields in combustion science. In recent years, many studies have been dedicated to optimizing the combustion process and to achieving low emissions from gas turbine combustors. Experimental study has been regarded as one of the most direct and reasonable tools in design optimization of combustion chambers. Nevertheless, experimental study is both costly and time consuming. In contrast, numerical simulations based on computational fluid dynamics (CFD) have the obvious advantages of low cost and high efficiency, and have been widely used by researchers.
The demand for gas turbine combustors with high efficiency combined with low emissions has prompted researchers to optimize the combustion processes. There are two main approaches to achieving low pollution emissions: external flue gas control and combustion control in the furnace. As for the external flue gas control, selective catalytic reduction and selective non-catalytic reduction are two commonly used methods [6,7]. In these methods, a selective reductant is injected and mixed in a flue gas stream containing NOx. Usually, these post combustion methods for NOx removal will be required when combustion control method alone fail to meet existing NOx emission regulations [8].
The key to achieving low emissions using combustion control is to control the temperature uniformity in the combustor and to decrease the combustion temperature [9]. Therefore, many researchers had focused on reducing NOx emission by using new combustion technologies, such as moderate or intense low oxygen distribution (MILD) combustion [[10], [11], [12], [13]], colorless distributed combustion (CDC) [14,15], and flameless combustion [16]. These combustion technologies all control emissions by promoting a lower combustion temperature and a uniform temperature distribution across the combustor [17]. Flue gas recirculation (FGR) control is an efficient method of reducing NOx emission from combustion. For FGR, the flue gas is usually recirculated into the flame zone or added to the combustion air [18]. Baltasar et al. [19] presented an experimental and numerical study of FGR in a gas-fired laboratory furnace. They found that NOx emissions significantly decreased when flue gas was recycled. Yu et al. [20] investigated and confirmed that the FGR method is an efficient way to achieve high thermal efficiencies and low pollutant emissions. Subsequently, they used air-induced-FGR and fuel-induced-FGR methods to reduce NOx emissions in a non-premixed combustion system [21]. Ling et al. [22] investigated the effect of FGR supply location on the emission performance of a pilot-scale furnace. The results indicated that FGR supplied through the duct between primary air and inner secondary air attained optimal performance for reducing NOx emissions. De Santis et al. [23] employed CFD analysis to investigate the operation of an industrial micro gas turbine combustor under FGR conditions. Numerical analysis revealed that FGR was a viable technique to enhance the efficiency of CO2 capture and attain lower NOx emissions. In addition, FGR had also been extensively applied in diesel engines to reduce NOx emissions [[24], [25], [26]]. Changing the combustor geometry also has an important role in NOx emissions. Watanabe et al. [27,28] numerically investigated the turbulent spray combustion in a jet mixing type combustor. Results showed that employing a suitable baffle plate in the combustor reduced NOx emission by 40%, and smaller air inlet diameter decreased the NO mole fraction. Tu et al. [29] numerically studied the effect of furnace body configuration on combustion performance. They found that increasing the angle between the furnace roof and sidewall caused a stronger recirculation of flow field, leading to a lower combustion temperature peak and lower NOx emissions. Chmielewski et al. [30] used a variable geometry combustion chamber to increase the combustion efficiency and reduce NOx emissions for a miniature gas turbine. Somarathne et al. [31] applied a secondary air injection system to a cylindrical swirl combustor to reduce NOx emissions. Pourhoseini et al. [32] investigated the use of a portion of natural gas as the secondary gas supplied directly into the flame, this reduced NOx emissions owing to an enhanced flame soot radiation. The water spray injection method was also used in a gas turbine combustor to reduce NOx emissions. Farokhipour et al. [33] found that injection at the end of the primary zone achieved the best performance of NOx reduction. Amani et al. [34] provided a multi-objective optimization of water spray injection to achieve low NOx emissions in a gas-turbine combustor. They found that a swirl number of 1.96 with a small injection angle was optimal.
Recently, the combustion and NO emission characteristics in jet-stabilized combustors have been widely investigated. Usually, in a jet-stabilized model combustor, the secondary air is injected to the combustor by four radial wall jets, which plays a critical role in flame stabilization. The flow of this configuration is non-swirling, and it does not require the swirls part for stabilization. On the other hand, jet stabilization provides a wide range of operations [35]. These advantages make this type of combustor attractive for research. Bauer et al. [35] experimentally studied the combustion characteristics and NOx emissions for a jet-stabilized combustor; their results could be used to validate numerical models. Kurreck et al. [36] predicted the flow and temperature structure using the standard k-ε turbulent model. Bazdidi-Tehrani and Zeinivand [37] employed the realizable k-ε turbulent model coupled with the presumed probability density functions (PDF) model to simulate the spray combustion of a jet-stabilized combustor; they also investigated the effects of the number and locations of stabilizer jets on combustion characteristics and NOx emissions [38]. Results showed that an increase in the axial distance of stabilizer jets resulted in a decrease in NO emissions, and increasing the number of jet holes increased the NO emissions. Alemi et al. [39] analyzed NO emission performance controlled by the jet injection direction and the jet diameter; the results indicated that jet injection direction was significant at high jet Reynolds number, and that an injection direction of 20° towards the downstream offered the lowest NO emissions.
In these studies of jet-stabilized combustors, the cross-sectional shape of the jet-stabilized combustor was circular. For circular jet-stabilized combustor, the circular symmetry of the cross-sectional circular shape means that four circumferential perpendicular stabilizing air jets must have the same mass flow rate to maintain uniformity of temperature and flow fields. In contrast, the major axis and minor axis of the elliptical shape are unequal in length, so the stabilizer jet air supplies of elliptical combustors can be more flexibly adjusted to organize the temperature field and to control the emissions. Accordingly, a novel jet-stabilized combustor with an elliptical cross-sectional shape is developed. The main objective of the present study is to analyze the combustion and emission characteristics of the elliptical combustor, and investigate the effects of the semi-major/semi-minor axis ratio of elliptical shape and the jets air regulation method on the developed elliptical combustor performance. The results of work will provide support for the design of low emission gas turbine combustor.
Section snippets
Combustor geometry model
In the current study, the proposed elliptical jet-stabilized combustor configuration is depicted in Fig. 1. The combustor is based on the circular jet-stabilized combustor in the literature [[35], [36], [37], [38], [39]]. The main discrepancy between combustors in the literature and the developed combustor in this paper is the shape of the cross section. As presented in Fig. 1(b), the cross section of the combustor is determined by two parameters: a, the semi-major axis, and b, the semi-minor
Discretization procedure
The finite volume method is used to discretize the transport equations in this numerical model and the SIMPLEC method is chosen for the pressure-velocity coupling algorithm in the CFD software Fluent. The discrete ordinates (DO) radiation model with 5 × 5 angular discretization is employed, resulting in 200 discrete directions. This model is performed in an iterative way with one energy iteration per radiation iteration. The Eulerian-Lagrangian approach is adopted for taking into account the
Verification of numerical model
To confirm the accuracy of the numerical method, the predicted results of the current numerical model are compared with experimental data [35]. The profiles of temperature at various axial locations (z = 98, 182, 224, 266 mm) compared with the experimental data are presented in Fig. 4, where the equivalence ratio is 0.43. At a location of z = 98 mm, although the numerical results of realizable k-ε model are slightly smaller than the experimental data, the trend matches that of the experiment.
Conclusions
The combustion characteristics and emissions of a newly developed elliptical jet-stabilized combustor with various semi-major/semi-minor axis ratios are numerically investigated under various equivalence ratios and are then compared with equivalent values for a circular jet-stabilized combustor. The conclusions can be summarized as follows:
- (1)
Compared with circular jet-stabilized combustor, the stabilizer jet air supply of the elliptical combustor can be more flexibly adjusted to organize the
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 51576054).
References (48)
- et al.
A review of cavity-based trapped vortex, ultra-compact, high-g, inter-turbine combustors
Prog Energy Combust Sci
(2018) - et al.
Combustion performance of an adjustable fuel feeding combustor under off-design conditions for a micro-gas turbine
Appl Energy
(2017) - et al.
A multi-objective CFD optimization of liquid fuel spray injection in dry-low-emission gas-turbine combustors
Appl Energy
(2017) - et al.
Experimental study of slight temperature rise combustion in trapped vortex combustors for gas turbines
Energy
(2015) - et al.
Effect of swirl field on the fuel concentration distribution and combustion characteristics in gas turbine combustor with cavity
Energy
(2018) - et al.
Combustion, performance, and selective catalytic reduction of NOx for a diesel engine operated with combined tri fuel (H2, CH4, and conventional diesel)
Energy
(2017) - et al.
Selective catalytic reduction in a rotary air heater (RAH-SCR)
Energy
(2018) - et al.
Review of modern low emissions combustion technologies for aero gas turbine engines
Prog Aero Sci
(2017) - et al.
Mild combustion
Prog Energy Combust Sci
(2004) - et al.
Emissions of NO and CO from counterflow combustion of CH4 under MILD and oxyfuel conditions
Energy
(2017)