Experimental research on using CO₂-Ar microjets to control liquid fuel combustion instability and pollutant emission

Highlights

• The high flow rate CO₂-Ar jets reduced the pulsation pressure amplitude by 90%.

• High flow rate jets reduced NOx emissions but caused CO emissions to increase.

• Jets with a high CO₂ volume ratio lead to an increase in CO emissions.

Abstract

Combustion instability has become an obstacle to the development of modern gas turbines. In this study, four categories of jet tubes inject CO₂-Ar mixture to the flame root at different total injection volume flow rates and mixing ratios. These jets do not change the dominant combustion instability frequency, yet the large flow rate jets can significantly weaken the pressure and the heat addition oscillations. The tube type or the CO₂-Ar volume fraction in the jets has no apparent influence on the control efficacy. When the total jet volume flowrate reaches 10 L/min, the maximum amplitude reduction ratio of pulsating pressure and CH∗ chemiluminescence intensity is 90% and 89%, respectively. Meanwhile, the jets caused a significant drop in the flame temperature, and the NOx emission was reduced by at least 35%. However, the side effect of the jets is that the flame is on the verge of blowing out, and the CO emission increases sharply. The higher the volume fraction of CO₂ in the jets, the more likely it is to generate CO. Designers need to compromise the impact of jets on the safety and cleanliness of the combustors.

Introduction

Modern gas turbines must comply with current environmental protection regulations and reduce fuel consumption and pollutant emissions simultaneously. Lean combustion technology significantly affects the combustion process and reduces NOx formation by mixing fuel and air [1]. However, lean combustion may bring undesired combustion instabilities due to the positive feedback between heat addition rate and sound pressure [[2], [3], [4]]. Combustion instability may cause severe vibrations on combustion devices and even the entire system. Excessive heat and structural vibrations can cause severe damage to the combustion system. Reducing the phase difference between pressure oscillations and heat release fluctuations can enhance the coupling of acoustics and combustion [5]. Many factors are involved in liquid fuel combustion instability, e.g., the fuel category, flow, atomization, evaporation, mixing, and chemical reactions [[6], [7], [8], [9]].

One of the widely used passive control methods is stabilizing the pressure, such as installing acoustic damping devices on the combustion chamber sidewalls or the intake end [10,11]. Another method is to disturb the chemical reaction process, breaking the coupling between heat addition rate and pressure, such as injecting mixed gas into the flame center [12]. Ruan et al. [13] found that the unstable combustion transition process of liquid RP-3 jet fuel is related to the periodic interaction between the vortex and the inner shear layer. One study also reported that the high momentum jets could affect the reaction kinetics and produce many vortices. The acoustic wave and the heat addition rate are no longer in phase, which leads to a decrease in combustion instability growth rate [14]. Dynamic characteristics of non-premixed flames under acoustic forcing were studied by Kypraiou et al. [15]. Compared with radial fuel injection, axial fuel injection is more sensitive to sound amplitude, and flame stability decreases as the sound intensity increases.

A stable jet control strategy was used by Umm et al. [16], which reduced the low-frequency acoustic pressure oscillation amplitude in a liquid-fueled combustor from 5 kPa to 3 kPa. The CH∗ chemiluminescence amplitude peak decreased by 56%. In addition, low frequency modulated air jets can more successfully reduce acoustic wave perturbations [17]. LaBry et al. [18] successfully controlled the propane-air premixed flame instability by using counter-rotating jets. Injecting air in cross-stream direction has proved beneficial to suppress unstable vortex shedding [19]. Deshmukh et al. [20] found that it is beneficial to suppress thermoacoustic instability when the jet plane is close to the combustion center. The jet effect on liquid-fueled turbulent flames is much more complicated than that on gas combustion. The jets will strengthen the local entrainment of droplets, thus improving the fuel and air mixing [21]. Krishnan et al. [22] used the secondary air jets to weaken the pressure oscillation. Furthermore, by constructing a time-varying weighted spatial turbulence network, the vortex interactions and intermittent thermoacoustic instability were studied.

The influence of jet species has been considered in some studies. In a pioneering research, hydrogen jets quickly broke the dependence between sound waves and heat addition and reduced the pulsating pressure amplitude within 50 ms, but the jets also increased CO emissions [23]. The N₂/O₂ jets and CO₂/O₂ jets control effects on the unstable CH₄/air flame and NOx emissions had been experimentally studied by Zhou et al. [24]. The pulsating pressure amplitude damping ratio reached 80% because of the inert effect of CO₂. The CO₂ molecules did not participate in the combustion but reduced the reaction zone temperature. Therefore, the thermal NOx emission was reduced. Oztarlik et al. [25] indicated that the hydrogen jets along the flow direction changed the flame transfer function (FTF) of CH₄/air premixed flame. Although hydrogen jets can reduce combustion instability, the jets also cause a rapid increase in NOx emission. Yi et al. [26] used optical diagnostic methods to study the influence of the mixed fuel characteristics on the spray flame under the oxygen-enriched condition. The flame shape is affected by oxygen concentration. As the oxygen concentration increased, the flame lift-off height first dropped rapidly and then remained. Zheng et al. [27] studied the mixed fuel effect on the combustion and emissions for a diesel engine under a high EGR rate. With the increase of the pilot-main interval, the NOx emissions first decreased and then increased, and the peak heat release rate decreased. At the same time, CO and THC emissions increased.

This study tried to use the inert CO₂-Ar blending jets to stabilize the ethanol spray combustion and investigate the effect of the mixture jets on NOx and CO emissions. CO₂-Ar mixed gas with different blending ratios and flow rates were injected into the flame root along the cross-flow direction through four types of jet tubes, respectively. Our previous research showed that the flame root is the leading site for droplet rupture, secondary atomization, coalescence, and dispersion [12]. Therefore, the jet plane is set at the flame root. As far as the authors know, it is the first time to study the influence of the CO₂-Ar jets and the microjet tube shape on the attenuation effect to the swirling spray combustion oscillation. Meanwhile, the effect of CO₂-Ar microjets with different volumetric flow rates and mixing ratios on NOx and CO emissions during the unstable spray combustion was studied for the first time.

This paper is organized as follows. First, the details of the experimental equipment and the diagnostic techniques used are introduced in Sec. 2. Then, four different shapes of jet tubes are used to generate CO₂-Ar microjets, and the effects of blending ratios and flow rates on combustion stability control, NOx and CO emissions are analyzed in Sec. 3.