Elsevier

Combustion and Flame

Volume 210, December 2019, Pages 374-388
Combustion and Flame

Exhaust CO emissions of a laminar premixed propane–air flame interacting with cold gas jets

https://doi.org/10.1016/j.combustflame.2019.09.001Get rights and content

Abstract

This study investigates a laminar premixed flame interacting with cold gas jets, at different cooling jet mass flow fractions (m˙jet*) and diluent types, namely air and N2. A novel burner and wall configuration is used to experimentally induce flame-cooling-air interaction (FCAI). Flame chemiluminescence imaging, exhaust temperature (Texh) and exhaust CO emissions ([CO]exh) measurements are conducted to characterise the flame shape and [CO]exh response to the cooling jets. Flame imaging reveals that the cooling jets greatly affect the flame shape. Measurements of [CO]exh demonstrate a direct correlation with Texh, as decreasing Texh is observed to occur with decreasing [CO]exh. Additionally, the air diluent case shows consistently lower [CO]exh values, relative to the N2 diluent case.

Using a novel modelling approach, the cooling jets are simulated using one-dimensional (1D) fully resolved simulations (FRS). The effect of jet dilution, jet cooling and exhaust gas cooling are independently and jointly investigated in these simulations. The FRS results support the experimentally observed behaviour, and show that exhaust gas cooling and exhaust gas oxygenation produce decreased CO concentrations. Using a chemical reactor network (CRN), the jet mixing process is modelled by a perfectly stirred reactor (PSR), while the exhaust gas cooling process is modelled by a plug flow reactor (PFR). The CRN modelling shows that the jet mass flow rates dictated by m˙jet*, the dilution time (tdil) assumed for cooling jet mixing, and the exhaust gas cooling residence time (tcool), play an important role in determining the [CO]exh. An equilibrium analysis illustrates that the relationship between [CO]exh, Texh and exhaust O2, is due to the thermodynamically favoured equilibrium states. Timescale analyses demonstrate that appropriate modelling of jet mixing, and accounting for the rate of exhaust gas cooling, are important for estimations of [CO]exh.

Introduction

Dry low emission gas turbines typically feature increased power densities to achieve higher thermal efficiencies for reduced fuel consumption [1]. This increases the likelihood of the flame interacting with the combustor liner, which can have potential consequences on engine exhaust emissions. While there have been some studies on pure flame-wall interaction (FWI) of back-side cooled walls [2], [3], relatively few studies have explored how the flame is affected by effusion cooling [4], a common combustor liner cooling method. In effusion-cooled walls, holes in the combustor liner allow the cooling air to form a protective layer between the liner and the flame, including the hot exhaust [5]. The mass flow rate of this cooling air has been previously correlated with gas turbine exhaust CO emissions ([CO]exh) [6]. However, it is unclear whether the measured [CO]exh comes from flame-cooling-air interaction (FCAI), or diversion of primary combustor air. This paper aims to address this by focusing on FCAI.

Due to the difficulty of experimentally and numerically investigating FCAI, few studies have been done regarding this phenomenon. Early effusion cooling studies used turbulent boundary layer models to explore their cooling potential [7], [8]. Subsequent studies have focused on heat transfer effectiveness [9], with few investigating how the jets interact with the flame. Bizzari et al. [10] studied the evolution of the jets in a wind-tunnel type configuration, and demonstrated the difficulty involved in simulating the complex flow field induced by this phenomenon. Moreover, the addition of swirl [11] and combustion [12] into the problem further increases the challenges associated with studying this phenomenon.

One study has used a single-sector model gas turbine combustor, with an effusion-cooled wall, to experimentally characterise the flow field under different operating conditions [13]. It was illustrated that, under specific operating conditions, heavy impingement of the flame onto the effusion-cooled wall occurs, leading to local penetration of the cooling jets into the flame brush. As shown by Yahagi and Makino [14], this can lead to local extinction of the flame around the jet. Flame extinction can potentially lead to incomplete combustion, and could have consequences for [CO]exh [15]. However to the authors’ knowledge, no study has attempted to directly link this behaviour with [CO]exh. Understanding this link would aid in building models to mitigate the potential impact of FCAI on emissions.

Therefore, this work investigates flames interacting with cold gas jets. Flame chemiluminescence (FCL) imaging was conducted to qualitatively determine the response of the flame shape to increasing jet mass flow rates (m˙jet) and diluent types, namely air and N2. Exhaust temperature (Texh) and exhaust CO emissions ([CO]exh) measurements were then conducted to determine the individual CO response to the cooling and diluting effect of the jets. The fundamental mechanisms that affect the observed behaviour was further investigated using one-dimensional (1D) fully resolved simulations (FRS) of the flame under the influence of the cooling jets and wall heat transfer. Source terms were added to the Navier–Stokes equations solved by the FRS, to individually induce the cooling and dilution effect of the jets, as well as the exhaust gas cooling occurring due to wall heat transfer. The FRS results were compared to the experimentally observed behaviour to confirm the direct contribution of the cooling jets and wall heat transfer on CO oxidation, and to reveal the primary controlling parameters. The conclusions of the study were extended to greater practical relevance by exploring the impact of residence time on CO oxidation under these conditions. This was conducted using chemical reactor networks (CRN), which provided scope to investigate residence times relevant to gas turbine combustors. These observations provide a step towards understanding the mechanisms governing the impact of FCAI on [CO]exh.

Section snippets

Burner configuration

The burner used in these experiments is the same as that used by Rivera et al. [16] to study FWI, except for the jet cooled wall section, and is hence only briefly discussed here. The burner is a laminar fully premixed propane–air burner, with the test section shown in Fig. 1. The reactants are metered by two MKS thermal flow controllers, M1559A and M100B for air and fuel, respectively, and are mixed 3 m upstream of the plenum. The plenum contains several layers of steel meshes, a honeycomb

Governing equations

To separate some of the physical processes occurring in the experiment, fully resolved simulations (FRS) were conducted in 1D, to allow the simulation of a larger parameter space using detailed chemical kinetic models. The code used to conduct the 1D FRS in this study is NTMIX-CHEMKIN, an accurate high-order fully parallelised flow solver designed to perform direct numerical simulations (DNS) of flames with detailed chemical kinetic models [21], [22]. This code has been previously used in DNS

Laminar flame-cooling-air interaction

In this section, flame imaging was conducted to determine the response of the flame shape to the cooling jets, under increasing m˙jet and to different diluent types. Measurements of Texh and [CO]exh were then conducted under these conditions to determine how they respond to the cooling effect of the jets, as well as to the dilution they impose on the flow.

The experimental results presented in this section consists of several cases, summarised in Table 1. The three jet configurations consists of

Isolation of primary flame-cooling-air interaction phenomena

In this section, 1D FRS results are presented to confirm the direct contribution of cooling jets and wall heat transfer, to the experimentally observed behaviour. The individual impacts of the cooling and dilution imposed by the cooling jets, as well as of exhaust gas cooling, on [CO]exh are shown in this section. The mechanisms responsible for their behaviour are then investigated using calculations of chemical equilibria under these conditions.

It is clear from Fig. 6, Fig. 7, Fig. 8, that the

Decoupling of cooling jet dilution and exhaust gas cooling residence times

As shown in the previous sections, decreasing exhaust temperatures and increasing exhaust O2 concentrations lead to decreasing CO concentrations due to thermodynamic favouring of CO oxidation. However, given the infinite-time nature of chemical equilibria, systems with short residence times may limit full CO oxidation. To explore the limitations of this behaviour, CRN simulations were conducted to determine the characteristic timescales of CO oxidation under the influence of cooling jets and

Reaction timescale analysis

As shown in the previous section, CO oxidation can be limited by residence time, and vary with dilution timescale, jet mass flow fraction, and exhaust cooling rate. In this section, the dynamics of CO oxidation are explored further by considering timescales relevant to practical combustion systems. For the cooling jets, the combined effect of tdil and m˙jet* variations on CO oxidation are explored in the context of timescales relevant to gas turbine combustors. Meanwhile, the sensitivity of CO

Conclusion

This study investigated a laminar, premixed flame interacting with cold gas jets. A novel burner and wall configuration were developed to experimentally induce flame-cooling-air interaction (FCAI). Measurements of flame shape, exhaust temperature (Texh) and exhaust CO emissions ([CO]exh) were conducted to determine their response to different cooling jet mass flow fractions (m˙jet*) and diluent types, namely air and N2. The conditions that the cooling jets imposed on the flame were then

Acknowledgments

This research was supported by the University of Melbourne through the Research Training Program Scholarship. The research benefited from the computational resources provided through the Energy & Resources Scheme using the Pawsey Supercomputing Centre, and through the National Computational Merit Allocation Scheme using the Australian NCI National Facility, all supported by the Australian Government. The authors acknowledge the generous support of the European Centre for Research and Advanced

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