Pollutants Emissions of CO and Soot from Flame-wall Interactions in Fundamental and Practical Energy Conversion Systems: A Review

1. Introduction

The utilization of energy via various combustion devices played the pivotal role in the industrial and economical development. However, the growing depletion of fossil fuels combined with the severe environmental issues such as the global warming, particulate matter emission and air pollution urged people to pursue for the advanced combustion technology with the higher efficiency and the lower pollutants emissions. The profound understanding of the complicated combustion processes occurring in practical combustion devices was useful for researchers and engineers to design and optimize the devices with excellent combustion and emission performance. Flame-wall interactions (FWI) as one kind of fundamental combustion phenomenon were universally found in many technical combustors such as internal combustion engines, gas turbines and industrial furnaces and it could significantly affect the energy conversion efficiency and pollutants formation of combustors.¹

FWI referred to the complex phenomenon involving physical and chemical influences when the flame directly contacted with walls.^2,3 The interactions between flame and walls could be distinguished into fluid-mechanism, thermal and chemical aspects based on different research interests.^4-6 Firstly, the flow structure would be disrupted by the wall and it fed back to flame characteristics such as the flame velocity which tended to be more notable in turbulence-dependent flames. Meanwhile, large heat fluxes were observed when the flame propagated toward the walls as the surfaces were prevalently much cooler than impinging flames. The significant heat losses to walls not only slowed the chemical reaction rates due to the lower flame temperatures but also affected the service life of device walls. In addition, the incomplete combustion occurred near the walls due to the lower reaction rates and the radical-radical recombination rates were promoted with the presence of walls. Consequently, the increasing emission levels of unburned hydrocarbons (UHC) and CO could be found. Especially, the formation and deposition of soot were found on the surface of walls in fuel-rich conditions. The deposition of soot exhibited the crucial influence on wall surface morphology and the flow and heat characteristics between the fluid and walls.⁷ These facts explicitly depicted the influences of FWI on combustion and energy conversion efficiency and pollutants emissions. In order to optimize the combustion devices with better combustion and emission performance and develop the advanced combustion technology, the deeper insight and understanding of FWI are essential.

Nowadays, the investigation of FWI has been an attracting research field. It was not only the crucial subjects of research in safety technology,^8-11 enclosure fires,^12-17 catalytically assisted combustion,^18-22 materials synthesis,^23-26 and micro-combustion technology,^27-32 but also the topical concern of combustion chamber design of internal combustion engines,^33-36 gas turbines³⁷ and rocket engines.³⁸ Moreover, both of the development of downsizing of engines in automotive and aircrafts which increased the surface-to-volume ratios³⁹ and the design of the innovative low-NOx gas turbine combustors^40,41 further emphasized the importance of FWI in combustion devices. In the past decades, considerable studies involving FWI were actively carried out. Most of the previous literatures were mainly focused on the flow field variations and heat transfer characteristics of FWI.^42-50 The experimental investigations of the heat transfer mechanism in FWI of laminar and turbulent flames with different parameters such as fuel types,^51-53 equivalence ratios,^54-56 Reynolds numbers,^57-59 nozzle to plate distances^60-62 and combustors configurations^63-65 have been extensively conducted to gain the comprehensive understanding of FWI thermal characteristics. Several numerical simulations were also carried out to establish empirical formulas and unveil the underlying thermal mechanism of FWI.^66,67 The relevant reviews of literatures on FWI heat transfer characteristics were also provided by Viskanta,⁶⁸ Baukal and Gebhart^69,70 and Chander and Ray.⁷¹

Despite the wide range of literatures on heat transfer properties of FWI, there were few publications concerning the influences of FWI on pollutants emissions. The formation of NOx, unburned hydrocarbons, CO and soot due to the incomplete combustion in FWI significantly reduced the combustion efficiency and caused the serious environmental and health issues. The soot production exacerbated the particulate matter emissions and the deposition of soot intensified the mechanical abrasion. Facing to the more stringent emission regulations, the pursuit of the higher efficiency and the lower harmful emissions needed the increased understanding of pollutants emissions in FWI. Meanwhile, the comprehensive information of pollutants emission characteristics in FWI would also be a valuable assistance to industrial engineers and researchers in this field. To the best of our knowledge, there has been little work to summarize the pollutants emissions in FWI. The objective of this review is to systemically analyze various pollutant emission characteristics such as CO, NOx and soot in FWI from fundamental and practical energy conversion systems.

2. CO emissions in flame-wall interactions

Carbon monoxide as one kind of significant gaseous emissions was produced due to the scarce of oxygen in the core reaction zones. The release of CO from flames could be a crucial criterion for many combustion system designs. In case of the domestic cooking, the level of CO emission from the FWI in stove-pot system was important to householders due to the poisoning of CO. Quantitative measurements of CO emissions in FWI of practical combustion systems, such as internal engines, were very difficult due to the complicated combustion processes, uncontrolled inflow and boundary conditions, and the small characteristic scales of space and time. The generic experimental setups which consisted of an impinging plate uprightly above burners were adopted to study the relevant physical and chemical properties of FWI in the laboratory flames. The different characteristics of CO emissions in FWI investigated by researchers were summarized in Table 1.

2.1 Variations of types fuel, oxidizer and flame in investigations of CO emissions from FWI

The adoption of fuel types, oxidizer and other relevant parameters significantly affected the CO emissions in FWI. In studies of CO emissions of FWI, the categories of fuels were various. It ranged from the simple hydrocarbon fuel of methane to complicated fuel mixtures of liquid petroleum gas (LPG). The blends of H₂ and biogas which composed of CO₂ and CH₄ in different proportions were also considered. The oxidizer adopted in most research was compression air due to the prevalent utilization of ambient air in practical combustion systems. The numerical study which aimed to analyze the effects of different wall temperatures on major and intermediate species formation was conducted via the laminar stoichiometric methane flames.⁷³ The extensive experimental data and the detailed chemical mechanism of methane flames contributed to studying and verifying species concentrations. The variations of CO and NOx emissions in premixed impinging flames fueled with methane and ethylene were comparably studied to show the influence of fuel types on emission characteristics from FWI.⁷⁷ In the evaluation of combustion performance of practical cooking burners with different novel configurations, CO emissions as the most important criteria were tested. The LPG was usually chosen as the basic fuel due to its widespread usage in the domestic cooking.^76,83,86 To investigate the effects of fuel composition on pollutants performance of premixed impinging flames, the biogas containing CH₄ and CO₂ with the volume ratio of 1:1 was adopted by Zhen et al..⁸⁸ Meanwhile, considering the better performance of hydrogen in improving the combustion behaviors of the biogas fuel, up to 50% of hydrogen were also added in the mixing fuels.

Most experiments in above studies chose premixed flames instead of diffusion flames to alleviate the disturbance of soot in the measurements of CO. Meanwhile, some other flame types such as partial premixed and inverse diffusion flames (IDF) were also used. The partially premixed natural gas impinging flames which was achieved by the radial jet reattachment combustion (RJRC) nozzle was adopted by Mohr et al.⁷² to investigate the influence of increasing fuel/air mixing ratio on CO production from FWI. The combustion species and heat transfer characteristics of the inverse diffusion impinging flames burning butane were presented to study the detailed relationship between the heat transfer and pollutants emissions.⁷⁵ The work by Zhen et al.⁸⁰ comparably analyzed the effects of swirling and non-swirling flows on the overall CO and NOx emission characteristics of LPG impinging inverse diffusion flames.

  In the investigations of CO emission properties in FWI, either methane or LPG was universally adopted as the test fuel and the premixed flame was also the conventional combustion mode used in these experiments. For impinging flames, these arrangements could produce the large amount of CO due to the incomplete combustion and avoid the disturbance of soot formed in luminous flames. However, the CO emissions of more complicated fuels from diffusion flames which were more prevalently found in practical combustors have not been systemically tested. The pollutants emission characteristics from diffusion flames with more complicated fuel composition which were similar to practical fuels should be studied.

2.2 Effects of equivalence ratios on CO emissions in FWI

The majority of experiments which adopted premixed flames analyzed the pollutants emission characteristics from FWI under different equivalence ratios conditions. The equivalences ratios ranged from fuel-lean (Φ=0.4) to fuel-rich (Φ=4.0) conditions. The pollutants emission performance of flames could be directly affected by variations of equivalence ratios because the dissociation of fuels was controlled by the flame temperature. The flame exhibited the highest flame temperature at nearly stoichiometric condition due to the complete combustion.

A summary of the effects of equivalence ratios on CO emissions which were studied by different researchers was given in Table 1. The discrepancy of CO concentration between the RJRC and Co-Ax nozzles tended to be larger at the stoichiometric condition than that at fuel-lean conditions.⁷² The average concentrations of CO and NO from the impinging premixed flames were presented when equivalence ratios increased from 1.2 to 4 respectively.⁷⁴ Results showed that the emission level of CO increased while the NO level decreased when the equivalence ratios shifted to richer part due to the decrease of flame temperature. Li et al.^78,79 separately analyzed the variations of CO concentrations and the emission index of CO (EICO) from FWI of different burner configurations at equivalence ratios of 0.9 and 1.0. The noticeable augment of CO concentrations and EICO were observed with the increase of equivalence ratios. In the research of Zhen et al.,⁸⁰ the swirling IDF had the minimum value of EICO at equivalence ratio of 1.4 which located at the near stoichiometric condition. The production of CO turned to be larger when the equivalence ratio continued to increase. The phenomenon could be attributed to the quenching effects when the flame touched the plate. Zhen et al.⁸⁸ also made an experimental investigation of the influence of equivalence ratio on the variations of EICO and EINOx (emission index of NOx) from the impinging premixed flame. The tested equivalence ratios ranged from 0.8 to 1.6 with an interval of 0.2 including fuel-lean and fuel-rich situations. It showed that the production of CO decreased when equivalence ratio increased from 0.8 to the stoichiometric condition because of the more complete combustion. The minimum value of CO emission located at equivalence ratio of 1.2 because the higher flame temperature and the higher laminar burning velocity of the slightly fuel-rich condition accelerated CO oxidation. When the equivalence ratio persisted increasing, the augment of CO emission was observed because of the intensive quenching effect of the plate.

It was seen from the above literature that equivalence ratios as one kind of the important experimental parameters exhibited the significant influence on CO production in FWI. The variations of CO emissions from FWI of premixed impinging flames with different equivalence ratios could be divided into two parts. The CO concentration reduced when the equivalence ratio shifted from the fuel-lean to stoichiometric conditions due to the more complete combustion and the higher flame temperature which promoted the CO oxidation. An augment of CO emission was found as the equivalence ratio shifted to the fuel-rich condition because of the quenching effects of plate. Whereas, there are many possible equivalence ratios have not been studied and the detailed variations of CO distributions in FWI are scarce. Moreover, most of the industrial flames prevalently operated with diffusion flames. Very few studies analyzed the CO emission in FWI via diffusion flames.

2.3 Effects of Reynolds numbers on CO emissions in FWI

The Reynold number which affected the mixture flow rates was also the crucial experimental factor in combustion. The effects of the broad range of Reynolds numbers including laminar and turbulent conditions on CO emission properties in FWI have been extensively investigated and summarized in Table 1. In the study of CO variations from methane and ethylene partially premixed impinging flames, Reynolds numbers ranged from 310 to 515 to alleviate the soot formation and the decrease of CO was found with the increase of Reynolds numbers.⁷⁷ The increase of CO concentration from premixed impinging flames was observed by Li et al.⁷⁸ when Re was increase from 500 to 1500. This phenomenon was attributed to the promotion of intermediate combustion species and the enhancement of the interaction between the flame and walls. Ref⁸⁰ offered the variations of CO emission index from the impinging swirling and non-swirling IDFs when Re increased from 2000 to 10000. For non-swirling IDFs, the value of EICO was observed to be ascended with Re due to the increase of the fuel supply. Whereas, the reverse trend of CO variation was found for swirling IDF with increasing Re. This phenomenon was because of the stronger fuel/air mixing and the higher turbulence level caused by the augment of Re. In the numerical investigation of CO and NOx emissions from radial jet reattachment combustion flames with various Reynolds numbers (Re = 1000 to 30000), the higher concentrations of CO and NOx were observed at the larger Re.⁸⁵ Zhen et al.⁸⁸ analyzed the pollutants emissions from impinging 60% BG₅₀–40 % H₂ mixture premixed flames with different Reynolds number of 400, 600 and 800. The results showed that the decline of the value of EICO was found against the increase of Reynolds numbers. The increase of Reynolds numbers enhanced the conversion rate of CO to CO₂.

The variations of Reynolds numbers could significantly affect the CO formation properties in FWI and multiple literatures had been carried out on this phenomenon with different burner configurations. Meanwhile, the variations of other exhaust gases such as NOx and CO₂ were also included in these studies. The influences of Reynolds numbers on CO emission characteristics could be summarized into two aspects.

Firstly, the flow rates at the nozzle exit which increased with Re promoted the entrainment of ambient air. It subsequently impacted the formation of intermediate species of flames.

Secondly, the flame length also extended with the augment of Reynolds numbers. The increase of flame length could promote the interaction between the flame and walls which led to the formation of unburned products. It also induced the flame quenching at the plate surface.

The research above mainly studied the local or overall variations of CO emission with Reynolds numbers. More detailed work is needed to analyze the substantial effects of Re on CO formation in FWI.

2.4 Effects of nozzle to plate distances on CO emissions in FWI

The nozzle to plate distance which referred to the separation distance between the impingement plate and the burner nozzle was one sort of the important experimental parameters that can significantly affect the flame structures and emission performances in FWI.

The variations of pollutants emissions in impinging flames with different nozzle to plate distances had been included in many studies.^{74,77,79-82,88,89} In the analysis of the emission characteristics from the LPG premixed impinging flames with different separation distance between the flame jet and the plate (H/d), the emission level of CO was closer to each other for H/d = 10 and 12 and an obvious augment of CO concentration was observed when H/d increased to 14.⁷⁴ The study of Saha et al.⁷⁷ showed that both methane and ethylene flames produced more CO when H/d decreased because of the incomplete combustion in wall jet region. The concentration of NOx ascended with the H/d as the oxygen content and the gas residence time increased. The value of emission index of CO for single-, twin- and triple-nozzle flames which were measured by Li et al.⁷⁹ tended to be larger with the decrease of nozzle to plate distances. Zhen et al.⁸⁰ illustrated the EICO and EINOx variations with different heights for swirling and non-swirling IDFs at fixed Reynolds numbers and equivalence ratio. The variations of EICO value with nozzle to plate distances were different for swirling and non-swirling IDFs due to the different flame structures. Higher value of EICO was found when the inner reaction zone impinged and quenched on the plate surface. The influence of nozzle to plate distance on NOx and CO emissions from impinging IDFs was investigated by Choy et al..⁸¹ The EICO persisted to decrease when H increased from 80 to 180 mm. It was attributed to the extended residence time for CO oxidation. The increase of nozzle to plate distance declined the CO concentration from LPG premixed flames.⁸²

It was seen from the previous research that the value of CO emission from the majority of premixed impinging flames tended to be larger at the lower nozzle to plate distance. The decrease of distance between the nozzle and the plate increased the contact area between the flame and the plate which enhanced the flame quenching effect and promoted the formation of CO. When the plate was ascended to the higher height, the free flame length turned to be longer and the conversion rate of CO to CO₂ increased due to the more air entrainment. The concentration of CO subsequently declined with the increase of nozzle to plate distance. The variations of nozzle to plate distance exhibited the significant influence on pollutants formation process in FWI.

2.5 Sampling and analyzing methods of CO emissions from FWI

In the studies of emission performance in FWI, the sampling method was crucial to acquire the accurate and explicit information of pollutants formation as the interaction between flame and walls occurred in the very tiny space comparing to the conventional combustion. It also determined the accurate and elaborate distributions of pollutant gases such as CO, CO₂ and NOx. The sampling gases were generally tested via various gas analyzers in these studies. Fig. 1 exhibited that the sampling methods used in these literatures could be universally categorized into sampling probe,^72,74,77 micro-hole^75,78,79 and hood^{76,82,83,87-90} methods.

Fig. 1 The diagram of sampling and analyzing methods adopted in CO measurements from FWI.

In early days, the sampling probe method which set the probe close to the plate surface was utilized by researchers to collect and analyze gas specimen in the impinging flame layers. In order to study the exhaust gases from the RJRC nozzle, a quartz gas sampling probe was adopted by Mohr et al.⁷² and it was placed in contact with the impinging plate to collect the exhaust gases. The probe was iso-kinetic to avoid the chemical reactions of intermediate species. It was located at the fixed radial distance to sample the gases flowing along the plate surface. Then the gases were analyzed through CO, CO₂, O₂, NO and NOx gas analyzers. The accuracy of gas sample was ± 0.5% full scale. Mishra et al.⁷⁴ set a water cooled stainless steel probe with 3.1 mm diameter close to the impingement plate surface to sample gases. The probe was located at the radial position of r/d = 25 where the outer edge of the flame. Meanwhile, the other three circumferential locations at the same distance were also measured to acquire the average values. The similar method was used in the work of Saha et al.⁷⁷ to analyze the emission characteristics after flame impingement. The stainless steel probe was arranged at the outer edge of the plate to measure CO and NOx concentrations when methane and ethylene flames impinged on the flat plate respectively.

Even though the probing method could acquire local in-flame pollutant emission data of the impinging flames, the dimension of probe might disturb the interplay between flames and walls and limited the profound investigation of pollutant gases distributions in flames. In order to overcome these limitations, Sze et al.⁷⁵ drilled a hole of 1 mm in diameter on the copper impinging plate and sampled combustion products. The burner was moved in the radial direction from 0 to 50 mm. The CO, CO₂ and O₂ distributions along the radial direction up to 50 mm with an interval of 1 mm were acquired. Li et al.⁷⁸ studied the emission characteristics in FWI via a premixed LPG flames uprightly impinged on a flat plate. The exhaust gases in impinging flames were collected through a circular hole with 1 mm diameter on the plate. These gas samples were extracted through a micro pump with the flow rate of 0.5 l/min and then analyzed through gas analyzers. Several positions along the radial direction were tested to obtain gases distributions in the impinging flame layer. The same group⁷⁹ also collected the relevant gases samples through the micro hole on the plate at the fixed location to evaluate the influences of nozzle diameter and arrangements on emission performance of impinging premixed flames.

Collecting gases samples through the micro-hole on the impingement surface provided the insight understanding of CO and NOx distributions in the flame layer close to the plate surface. Some researchers also proposed another method of comparing the emission performance from impinging and free flames to highlight the effects of FWI on pollutant formations. A sampling method via a hood device was used here. A standard hood method set a hood which was large enough to enclose the whole combustors above the burner to collect the flue gases. In this way, the impinging flame and the plate were considered as an entire system and exhaust gases from it could be totally collected by the hood. The collecting pollutants which were well-mixed mixtures were measured by various gas analyzers.⁸⁸ Makmool et al.⁷⁶ conducted CO emission test of four kinds of cook-top burners. The exhaust gases was collected via the hood device and analyzed through the CO analyzer. Makmool et al.⁸² continued to systematically study the effects of height and inter-port spacing on heat transfer and emission performances of laminar impinging multiple premixed LPG flames. The CO emission in experiments was collected and measured by the hood method. In the investigation of Muthukumar et al.,⁸³ they chose the hood appliance combined with gas analyzers to examine CO and NOx emissions from novel porous radiant burners.

Collecting exhaust gases via the hood appliance was used to quantify the overall pollutants emissions from impinging flames. Some researchers proposed the combination of the hood and micro-hole methods to comparably investigate pollutants formation processes in FWI. Zhen et al.⁸⁰ analyzed variations of CO and NOx concentrations from impinging swirling and non-swirling inverse diffusion flames from two aspects. The local pollutants concentrations in flame layers were measured through the small hole on the impingement plate. Meanwhile, the sampling probe of gas analyzers was connected to the outlet of the hood which was arranged over the burner to acquire the overall emission index of CO and NOx from impinging swirling and non-swirling IDFs. In the experimental study of pollutants emission from impinging and open inverse diffusion flames carried out by Choy et al.,⁸¹ they used two different sampling methods to collect exhaust gases from two kinds of combustors separately. For impinging IDFs, the flue gases from the flame layer close to the impingement plate was sampled from the micro-hole with 1 mm diameter on the plate. For open IDFs, a hood which was set over the flame tip collected exhaust gases. Then a steel probe was inserted on the top of hood to sample gases. Both of sampled gases were measured by gas analyzers. The comparison between them revealed that impinging IDFs generated more NOx. 

These sampling methods allowed researchers to quantitatively investigate the pollutant emissions in FWI. The micro-hole sampling method could offer the local pollutants distributions in impinging flame layers along the radial directions and the overall emission index of pollutants were obtained through the hood method. The combined measurements exhibited the explicit knowledge of CO and NOx formation process in FWI.

2.6 Variations of measured species in FWI investigations

In the studies of pollutants emissions in FWI, CO and NOx were the most conventional species which were concerned and investigated by most researchers.^{76,78,83-86,90} The concentrations of CO and NOx were measured by the relevant gas analyzers. Li et al.⁷⁸ presented the distributions of CO and NOx concentrations on the radial direction in impinging flames with variations of equivalence ratios, Reynolds numbers and cooling water temperatures. The concentrations of CO and NOx were chosen by Muthukumar and Shyamkumar⁸³ as the testing standards to evaluate the performance of different porous radiant burners (PRB). Mishra et al.⁸⁶ exhibited the CO and NOx concentrations from the novel PRB with variations of input power and equivalence ratios via portable flue gas analyzers.

Meanwhile, the emission index which represented the grams of pollutant emitted per kilogram of fuel burned was also introduced in some research to illustrate the variations of CO and NOx emission properties in FWI.^79-81,88 Li et al.⁷⁹ adopted the EICO to characterize the CO emission from the premixed LPG/air impinging flames with different operation conditions and found that EICO increased with the decrease of the nozzle diameter in the single-nozzle flames. The values of EICO and EINOx of impinging premixed flames were offered by Zhen et al.⁸⁸ to show the influence of different separation distances and hydrogen addition on pollutants characteristics in FWI respectively.

To further analyze the formation processes of CO and NOx in FWI, the concentrations of O₂ and CO₂ were measured as the supplement as well.^72,74,75,85 In the work of Mohr et al.,⁷² it was found that the Co-Ax nozzle produced more CO than that from RJRC nozzle. The O₂ concentration variations showed that RJRC nozzle entrained the more air than the Co-Ax nozzle and the conversion of CO to CO₂ were more completed in RJRC nozzle. Tajik et al.⁸⁵ showed that the higher production of CO at the higher temperature was due to the enhanced CO₂ dissociation. The concentrations of several radicals and intermediate species from flames were also presented to gain deep understanding.^73,82 The concentrations of intermediate species and free radicals were provided by Popp et al.⁷³ in the numerical study of the unburned hydrocarbons formation during the head-on quenching of the laminar methane flames. The distributions of OH radical at different separation distances in impinging flames were offered to show the intensive oxidation regions.⁸²

The concentrations of CO and NOx were universally chosen as the emission standards to evaluate the pollutants emission performance in FWI. The concentration of O₂ and CO₂ sometimes were also measured as the supplement of emission characteristics and it helped to analyze the formation processes of CO and NOx. Whereas, the measurements of distributions of more major and intermediate species in the interaction between flame and walls were scarce except for several numerical simulations limited to the sampling and analyzing methods. The information of more species distributions in FWI under various operation conditions is needed to gain the deep understanding of pollutants characteristics in FWI.  

Overall, in the experimental studies of pollutants emissions in FWI, the concentrations of CO and NOx were universally measured to evaluate the emission characteristics in FWI. The impinging flame device was generally adopted in the analysis of interaction between flame and walls. Researchers adopted the gaseous and light hydrocarbon fuels and premixed combustion mode to avoid the disturbance of soot. The effects of different experimental parameters including equivalence ratios, Reynolds numbers, nozzle to plate distances and etc. on CO formation process were investigated respectively (shown in Fig. 2). Meanwhile, the in-flame and overall pollutants emission properties from FWI were also acquired through the sampling probe, micro-hole and hood method separately. The information above could help to obtain the CO and NOx formation in FWI. Further investigation of CO distributions in the impinging flame layer near the wall surface and more species concentrations will be needed in the future work.    

Fig. 2 The sketch of traditional analysis of CO emissions from flame-wall interactions.

3. CO emissions measured via laser diagnostics techniques

In the usual studies of the influence of FWI on pollutants emission characteristics mentioned above, the concentrations of various combustion products such as CO, NOx and CO₂ were generally acquired via the gas analyzers. However, considering that the boundary layer where the FWI occurred was just in the scale of hundreds of microns, the limited resolution and the spatial dimension of conventional gas analyzers did not satisfy the standards of refined measurements. Nowadays, the advanced laser combustion diagnostics methods which had high spatial and temporal resolution were introduced to study the complex interactions between the flame and walls. The relevant review of the utilization of diverse laser techniques to diagnose complex combustion processes in laboratory and practical combutors had been presented by Dreizler et al..¹ It was seen that the laser-based techniques which possessed advantages including non-intrusive and in situ nature, temporal resolution and multi-parameter measurements were able to obtain the accurate and precise measurements in complicate combustion environments. A summary of the application of laser diagnostics in FWI such as planar laser induced fluorescence of the OH-radical (OH-LIF), Raman scattering and coherent anti-Stokes Raman spectroscopy (CARS) with various operating conditions was presented in Table 2.

Table 2 Comparisons of types of fuel, flame, plate location and other operating conditions in various literature involving laser diagnostics techniques.

3.1 Arrangements of fuel types, flame types and the plate position in FWI laser diagnostics studies

In the most studies which applied the laser diagnostics methods, the methane/air premixed flames were chosen as the fundamental flame because it had the simple chemical mechanism and less soot formation which might interfere the measurements of laser devices.^{92-99,102,103} Several researchers also adopted some other complex gas fuels in the measurements of FWI via laser diagnostics techniques.^91,100,101 Mokhov et al.⁹¹ measured the combustion products distributions along the plate surface in propane-air premixed flames. Except for the conventional methane/air premixed flames, Häber and Suntz¹⁰⁰ also conducted the experiments of the influence of wall materials on pollutants emissions in FWI of propane/air flames with a wide range of equivalence ratios as the supplement for the previous studies. In the investigation of Kosaka et al.,¹⁰¹ they comparably analyzed the influence of wall temperatures on CO formation in stoichiometric methane and dimethyl ether (DME) flames separately. The utilization of DME which was a more complex fuel extended the previous research.

Most of the mentioned studies which investigated the CO emission from FWI via two-photon CO-LIF were under premixed or partially premixed flame conditions. In the work of Chien et al.,⁹⁵ they examined the CO emissions characteristics through methane diffusion impinging flames. Even though they used the methane diffusion flames, the Reynolds number of the flow rate was so small that few soot particles were found in the flames.  

Fig. 3 showed that the arrangements of the impingement plate in experiments could be classified into head-on quenching (HOQ)^93-95,99 and side-wall quenching (SWQ).^{91,92,96-98,100-103} Both of the HOQ and SWQ were two kinds of FWI which were usually found in practical combustors. The situation of HOQ presented the flame perpendicularly propagated toward the cold walls. It was achieved by the plate which was vertically set above the burner to form the stationary wall-stabilized flames. Unlike to the HOQ condition mentioned in section 2, researchers chose the convex wall instead of the flat plate to develop the laser measurement environment. Singh et al.⁹³ set a slight convex water-cooled stainless-steel wall above the burner. The convex wall which was part of a sphere offered the space for the implement of focused point–wise laser diagnostics to measure the CO concentrations in the near wall boundaries. The similar HOQ device was also used to investigate the behaviors of CO formation from FWI in stationary and propagating impinging flames respectively.⁹⁴ The distributions of temperature and CO mole fractions in steady-state and transient processes of FWI were presented respectively. In the investigation of the effect of the pressure on nitric oxide formation in premixed jet-wall stagnation flames, the experimental device of HOQ which could offer the compact and stable flames at the high pressure conditions was adopted.⁹⁹

Fig. 3 Classification of flame-wall interactions.

In the configuration of SWQ, the plate was set parallel to the flow rate direction which allowed the flame to propagate along the surface. The arrangement of SWQ could provide the persistent and stationary FWI in both laminar and turbulent flows in comparison to that of HOQ. In the early study of Mokhov et al.,⁹¹ they directly placed the flat steel plate into the combustion products flows above the burner to form flame-wall interactions. The edge of the plate was sharpened to reduce the disturbance of flows. Fuyuto et al.⁹² developed a traditional SWQ device by putting a water-cooled stainless steel wall parallel to the main flow rate from a premixed methane/air flat flame burner. The flat flame front was divided into two parts and the FWI was found at the front wall surface. To further improve and control the interplay between the flame and walls in SWQ, the team of Dreizler^{97,98,101,102} set a tiny circular ceramic rod above the burner to form a premixed V-shaped flame. One branch of the flame interacted with the vertically oriented stainless steel wall and developed the stable FWI. The interactions could be conveniently controlled by the adjustment of the location of the rod.

The relevant simulation models of SWQ were established to investigate the latent mechanism of pollutants formation in FWI. The two-dimensional (2D) laminar simulation model was developed by Ganter et al.⁹⁶ to numerically analyze the FWI in the premixed stoichiometric SWQ configuration. Various reaction mechanisms and diffusion treatments were comparably studied in the simulations. Heinrich et al.¹⁰³ continued to expand the calculation model and carry out a three-dimensional (3D) numerical simulation to analyze the FWI in SWQ configuration. The computational results including flow structures were also compared with the previous experimental data and two-dimensional simulations. 

In the laser diagnostics of pollutants emission properties in FWI, the methane/air premixed flames which formed few soot particles and provided the steady flame were used as the fundamental flame. The configurations of HOQ and SWQ which were universally found in practical combustion appliances were detailedly analyzed. The HOQ device offered the way to study the emission characteristics in steady and transient FWI. Meanwhile, the SWQ device combined with the V-shaped flame provided the more stable and persistent FWI for laminar and turbulent conditions.  

3.2 Effects of different operating conditions considered in FWI laser diagnostics

Except for developing the relevant laser diagnostics measurements in FWI in some studies, the effects of several experimental parameters including equivalence ratios, laminar and turbulent flow conditions and plate temperatures on pollutants emission characteristics were considered in the investigation of FWI through laser-based diagnostics techniques.^{91-95,97-99,101}

The distributions of CO in impinging flame layers with different equivalence ratios which presented the significant influence on the overall CO emission characteristics from FWI were explicitly analyzed. In the study of OH and CO behaviors in the boundary layer near wall surface, the influence of different experimental factors including equivalence ratios which ranged from 0.83 to 1.0 and plate temperatures (cooled and uncooled) was considered.⁹¹ It was found that the production of CO obviously decreased with the decrease of equivalence ratio at the ranging of near stoichiometric. The augment of the plate surface temperature also inhibited the CO formation in the boundary layer. Jainski et al.⁹⁷ combined the advanced laser diagnostic methods and the side-wall quenching device to study the FWI in atmospheric premixed laminar flame. The performance of the quenching distance which was an important quantity of FWI was evaluated at equivalence ratio of 0.83, 1.0 and 1.2. They also comparably analyzed the influence the laboratory-fixed and the flame-fixed coordinate systems on thermochemical states which presented more details in FWI through the CO/T-state diagrams. In the study of Bohlin et al.,⁹⁸ the temperature distributions in the near-wall region with different equivalence ratios were investigated by the one-dimensional coherent anti-Stokes Raman spectroscopy.

Reynolds numbers as another important experimental factor were also investigated by many researchers. The distributions of flame temperatures and species of the near-wall layer in a quenched FWI were reported by Fuyuto et al.⁹² via the planar LIF with gas flow rates of 7.5 and 15 l/min separately. Kosaka et al.¹⁰¹ investigated the characteristics of CO formation and oxidation processes in FWI of a side-wall quenching device under laminar and turbulent conditions. The Reynolds numbers were set 5000 for both the laminar and turbulent parameters. The turbulent flame was achieved via an inserted turbulence grid. The results showed that the branch of CO formation and oxidation shifted to the lower temperature region due to the impact of turbulence.

Several researchers simultaneously investigated pollutants emission characteristics in FWI with the combination of equivalence ratios and Reynolds number variations. The variations of temperature and CO distributions in FWI with different equivalence ratios and Reynolds numbers were reported by Singh et al..⁹³ It was observed that the profiles of CO mole fraction were similar at equivalence ratio of 0.83 and 1.0 and the reduction of CO concentration mainly located in the hot flame region. In case of the fuel rich flame, the mole fraction of CO almost kept constant in the post-flame region. The peak value of CO mole fraction declined and the position of the flame front shifted to the wall surface with the increase of Re. Mann et al.⁹⁴ presented variations of CO mole fraction in FWI from the atmospheric methane jet impinging flames for different equivalence ratios which ranged from 0.83 to 1.2 and two turbulence intensities. The conditional statistics analysis method was used to acquire the difference of thermos-chemical states between the stationary impinging flames and the flames during head-on quenching from 200 individual experimental data. The higher CO concentration was found at the lower temperature region due to the inhibition of CO oxidation of enthalpy losses. This phenomenon was more significant in fuel lean flames.

Except for changes of equivalence ratios and Reynolds numbers, effects of other relevant experimental parameters on pollutants formation in FWI were also considered. The distributions of CO released from the non-premixed methane impinging flames with different nozzle to plate distances were analyzed by Chien et al..⁹⁵ The relationship between the reaction zone and the heat release zone was established. Versailles et al.⁹⁹ examined the formation processes of NO in the methane/air jet-wall premixed flames and the influence of the operating pressure was also studied. The very limited and negative effect of pressure on the production of nitric oxide was found in fuel-lean flames.

The effects of various experimental parameters including equivalence ratios and Reynolds numbers which were similar to those in section 2 on pollutants emission characteristics from FWI were analyzed via the HOQ and SWQ devices. The utilization of the advanced laser diagnostics methods helped to gain the well spatially resolved measurements of temperature and intermediate species within the subtle boundary layer near the wall surface. These results also corresponded with the consequences acquired by gas analyzers in some cases.

3.3 Development of various laser diagnostics methods in FWI

The progressive development of laser diagnostics techniques helped researchers to gain the better understanding of combustion science and technology. The role and evolvement of laser diagnostics in FWI were summarized and presented in Fig. 4.

Fig. 4 The illustration of various laser diagnostics techniques used in FWI.

The laser-induced fluorescence technique was universally adopted in the measurements of OH and CO distributions from FWI. In the work of Mokhov et al.,⁹¹ they measured the number density of OH in the boundary layer near the wall region through the absorption and laser-induced fluorescence. The spatial resolution for the boundary layer was 0.25 mm and the random error was about 10%. The concentration of CO was acquired by the two-photon LIF and the accuracy of the measured boundary layer was 0.1 and 0.2 mm for the uncooled and cooled plate separately. The gas-phase temperature distributions in the boundary layer at atmospheric pressure were measured by Fuyuto et al.⁹² with the help of multi-line NO-LIF thermometry in the investigation of FWI. Moreover, more intermediate and major species containing OH, CH₂O and CO were measured in flames through single-photon (OH, CH₂O) and two-photon (CO) excitation LIF. The imaging system had the well spatial line-pair resolution of 22 μm and the accuracy of the temperature measurement in the near-wall region was within 50 K. The measurement range of the applied laser diagnostics devices could be as close to the wall surface as 200 μm. In the study of Chien et al,⁹⁵ they illustrated the predominant factors that affected the CO formation and emission from an impinging flame. The relevant distributions of OH and CO in flames were acquired with the utilization of OH-PLIF and CO two-photon PLIF.

Considering the importance of flame temperature distributions in combustion processes, the development of CARS offered researchers an access to measure the instantaneous flame temperatures in FWI. In the analysis of the FWI by laser based techniques conducted by Singh et al.,⁹³ the combination of two-photon laser-induced fluorescence and coherent anti-Stokes Raman spectroscopy could simultaneously offer the distributions of gas phase temperatures and CO. The data of temperature was used to correct the CO concentration acquired from the CO-LIF. In the examination of temperature distributions and CO variations in the transient FWI, the instantaneous temperatures were gained by the nanosecond CARS device.⁹⁴ Jainski et al.⁹⁷ also measured the CO concentrations and temperatures by CO-LIF and CARS. The velocity fields and flame structures were obtained by two-component particle image velocimetry (PIV) and OH-PLIF. The CO/T-state diagrams were also established to analyze thermochemical properties.

Except for the measurements of temperatures and CO mole fractions, more combustion products including N₂, O₂, H₂, CO₂ and CH₄ were detected in the one-dimensional-CARS imaging configuration.⁹⁸ The resolution of the imaging was better than 40 μm and the boundary layer was measured to be approximately 30 μm. Versailles et al.⁹⁹ also studied the NO formation process in FWI and the concentrations of NO and N₂O were detected by NO-LIF. To investigate the FWI in SWQ configuration, Häber et al.¹⁰⁰ evaluated the quenching distances through chemi-luminescence images of OH* and CH* radicals.

The adoption of advanced laser diagnostics techniques in investigations of FWI offered the more accurate and precise spatial resolution of flame species near the wall region. Meanwhile, the scope of the boundary layer which contacted to the wall surface could be much thinner to capture more details in the interplay between the flame and walls. The combination of CO-LIF and CARS also offered the opportunity to detect the carbon monoxide and temperature at the same time and the concentration of CO could be timely modified according to the temperature data. Moreover, the variations of species and temperature distribution from persistent and transient FWI could be acquired with the help of laser diagnostics. The information of other combustion products measured in FWI also contributed to the deep insight of FWI processes.

The investigation of several combustion parameters in FWI via the combination of the advanced laser diagnostics techniques and HOQ/SWQ devices was the extension and supplement of the previous studies of FWI through gas analyzers. It also broke through the limitations of measurements of overall pollutants emissions in FWI and innovatively offered the useful qualitative or semi-quantitative information of species distributions in the subtle boundary layer which was in hundreds micron order near the wall surface. The influence of different universal experimental factors including equivalence ratios, Reynolds numbers and etc. on emission characteristics in FWI were analyzed. The mole fractions of major and intermediate species were acquired. Both of these information mentioned above facilitated the better understanding of FWI.

Despite these advantages of the utilization of advanced laser diagnostics in FWI mentioned above, there are several limitation and challenges to be resolved for present experimental investigations. For example, the measurements of combustion species are merely centered on the simple and major species such as CO, CO₂ and etc. The gas-phase temperatures which located near the wall surface are much higher than the wall surface temperatures due to the limited spatial resolution and the refraction effects of the LIF imaging system. Meanwhile, the useful results of species distributions are obtained at the distance of hundreds of micron scale from the wall instead of the substantial surface. Most of the experimental studies are carried out at the laboratory scale.

In the future work, the detection of combustion species through laser based diagnostics in FWI should contain more major and intermediate species even the large macromolecule polycyclic aromatic hydrocarbons (PAH). Much smaller distance and more precise measurements with higher spatially resolution should be acquired with the advances of laser devices. Several other external experimental parameters such as pressure and surface properties are also required to study to expand the database of FWI and the utilization of relevant advanced diagnostic strategies in more complex practical combustion systems is expected.

4. Soot formation and emission in flame-wall interactions

The previous researchers explicitly investigated the pollutants emission performance in FWI via various laboratory flames. Both of the overall emission index and the spatial distributions of pollutants in the boundary layer near the wall surface were accurately obtained with the aid of various gas analyzers and the advanced laser based diagnostics techniques. The knowledge of pollutants formation processes helped researchers to understand the mechanism of FWI and develop strategies to improve energy utilization. Whereas, most of these studies were carried out in premixed combustion mode fueled with various simple gaseous hydrocarbon fuels. The investigation of the characteristics of pollutants from FWI in diffusion flames which universally appeared in different practical combustion appliances was scarce.

Moreover, the combination of heat losses to the wall and the free radicals quenching on the wall surface in FWI intensified incomplete combustion when the flame propagated toward combustor walls. More unburned hydrocarbons were formed in this process and it reduced the combustion and emission performance. Especially for soot formation which occupied the large percentage of particulate matter emissions not only did harm to human health and environmental issues but also induced the abrasion of combustor walls and decreased the energy utilization efficiency. The phenomenon was more serious in enclosed combustion systems which adopted diffusion flames such as internal combustion engines and industrial combustion furnaces.

Facing to the rigid emission regulations and the development of advanced combustion technology with higher system efficiency and lower pollutant emissions, it is urgent and worthwhile to investigate characteristics of soot formation and evolution in FWI. It also facilitated to the optimization geometry design of various engines.

4.1 Soot characteristics in FWI of internal combustion engines

The production of unburned hydrocarbons was prevalently found in internal combustion engines due to the widespread existence of combustion chamber crevices, oil films absorption and desorption, flame quenching and etc. Among them, the formation of soot from FWI accounted for the dominant sources of UHC because the flame impingement was widely found in the confined volume combustors. Researchers were very interested in characteristics of soot in FWI and the relevant strategies to reduce soot formation in engines.^104-112 The analysis of these studies were shown in Table 3.

Table 3 A summary of previous studies on soot behaviors from flame-wall interactions in practical combustion systems.