Maximum OH Concentration Research Articles

Effect of relative humidity on SOA formation from aromatic hydrocarbons: Implications from the evolution of gas- and particle-phase species

Relative humidity (RH) plays a significant role in secondary organic aerosol (SOA) formation, but the mechanisms remain uncertain. Using a 30 m3 indoor smog chamber, the influences of RH on SOA formation from two conventional anthropogenic aromatics (toluene and m-xylene) were investigated from the perspective of both the gas- and particle- phases based on the analysis of multi-generation gas-phase products and the chemical composition of SOA, which clearly distinguishes from many previous works mainly focused on the particle-phase. Compared to experiments with RH of 2.0%, SOA yields increased by 11.1%–133.4% and 4.0%–64.5% with higher RH (30.0%–90.0%) for toluene and m-xylene, respectively. The maximum SOA concentration always appeared at 50.0% RH, which is consistent with the change trend of SOA concentration with RH in the summertime field observation. The most plausible reason is that the highest gas-phase OH concentration was observed at 50.0% RH, when the increases in gas-phase OH formation and OH uptake to aerosols and chamber walls with increasing RH reached a balance. The maximum OH concentration was accompanied by a notable decay of second-generation products and formation of third-generation products at 50.0% RH. With further increasing RH, more second-generation products with insufficient oxidation degree will be partitioned into the aerosol phase, and the aqueous-phase oxidation process will also be promoted due to the enhanced uptake of OH. These processes concurrently caused the O/C and oxidation state of carbon (OSc) to first increase and then slightly decrease. This work revealed the complex influence of RH on SOA formation from aromatic VOCs through affecting the OH concentration, partitioning of advanced gas-phase oxidation products as well as aqueous-phase oxidation processes. Quantitative studies to elucidate the role of RH in the partitioning of oxidation products should be conducted to further clarify the mechanism of the influence of RH on SOA formation.

Effects of OH concentration and temperature on NO emission characteristics of turbulent non-premixed CH4/NH3/air flames in a two-stage gas turbine like combustor at high pressure

This study examined the effects of OH concentration and temperature on the NO emission characteristics of turbulent, non-premixed methane (CH4)/ammonia (NH3)/air swirl flames in two-stage combustors at high pressure. Emission data were obtained using large-eddy simulations with a finite-rate chemistry method from model flames based on the energy fraction of NH3 (ENH3) in CH4/NH3 mixtures. Although NO emissions at the combustor exit were found to be significantly higher than those generated by CH4/air and NH3/air flames under both lean and stoichiometric primary zone conditions, these emissions could be lowered to approximately 300 ppm by employing far-rich equivalence ratios (ϕ) of 1.3 to 1.4 in the primary zone. This effect was possibly due to the lower OH concentrations under far-rich conditions. An analysis of local flame characteristics using a newly developed mixture fraction equation for CH4/NH3/air flames indicated that the local temperature and NO and OH concentration distributions with local ϕ were qualitatively similar to those in NH3/air flames. That is, the maximum local NO and OH concentrations appeared at local ϕ of 0.9, although the maximum temperature was observed at local ϕ of 1.0. Both the temperature and OH concentration were found to gradually decrease with the partial replacement of CH4 with NH3. Consequently, NO emissions from CH4/NH3 flames were maximized at ENH3 in the range of 20% to 30%, after which the emissions decreased. Above 2100 K, the NO emissions from CH4/NH3 flames increased exponentially with temperature, which was not observed in NH3/air flames because of the lower flame temperatures in the latter. But, the maximum NO concentration in CH4/NH3 flames was occurred at a temperature slightly below the maximum temperature, just as in NH3/air flames. The apparent exponential increase in NO emissions from CH4/NH3 flames is attributed to a similar trend in the OH concentration at high temperatures.

Photolysis-driven indoor air chemistry following cleaning of hospital wards.

Effective cleaning techniques are essential for the sterilization of rooms in hospitals and industry. No-touch devices (NTDs) that use fumigants such as hydrogen peroxide (H2 O2 ), formaldehyde (HCHO), ozone (O3 ), and chlorine dioxide (OClO) are a recent innovation. This paper reports a previously unconsidered potential consequence of such cleaning technologies: the photochemical formation of high concentrations of hydroxyl radicals (OH), hydroperoxy radicals (HO2 ), organic peroxy radicals (RO2 ), and chlorine radicals (Cl) which can form harmful reaction products when exposed to chemicals commonly found in indoor air. This risk was evaluated by calculating radical production rates and concentrations based on measured indoor photon fluxes and typical fumigant concentrations during and after cleaning events. Sunlight and fluorescent tubes without covers initiated photolysis of all fumigants, and plastic-covered fluorescent tubes initiated photolysis of only some fumigants. Radical formation was often dominated by photolysis of fumigants during and after decontamination processes. Radical concentrations were predicted to be orders of magnitude greater than background levels during and immediately following cleaning events with each fumigant under one or more illumination condition. Maximum predicted radical concentrations (1.3×107 molecule cm-3 OH, 2.4ppb HO2 , 6.8ppb RO2 and 2.2×108 molecule cm-3 Cl) were much higher than baseline concentrations. Maximum OH concentrations occurred with O3 photolysis, HO2 with HCHO photolysis, and RO2 and Cl with OClO photolysis. Elevated concentrations may persist for hours after NTD use, depending on the air change rate and air composition. Products from reactions involving radicals could significantly decrease air quality when disinfectants are used, leading to adverse health effects for occupants.

Fractal Analysis of Fluorescence Images to Assess Robustness of Reference-surface Positioning in Flame Fronts

ABSTRACT Images of laser-induced fluorescence (LIF) of hydroxyl radicals (OH) produced in premixed flames are here analyzed from the perspective of fractal theory applied to flame wrinkling. The results confirm that the signal isosurface showing the best correlation with the position of the maximum fluorescence gradient is in a strong correspondence with the inner layer within which the reference surface of maximum heat release can be found. Nonetheless, fluctuating maximum OH concentration or poor levels of signal-to-noise ratio could lead to a partial failure of either threshold or maximum gradient techniques of front extraction from LIF images. On the other hand, potential deviations from the reference surface can be tolerated provided that the chosen isosurface has a fractal dimension D 2 ranging between 1.10 and 1.26, well below the limit for passive scalars (D 2 = 1.37).

Ozone enhancement due to the photodissociation of nitrous acid in eastern China

Abstract. PM2.5, particulate matter with a diameter of 2.5 µm or less, is one of the major components of air pollution in eastern China. In the past few years, China's government has made strong efforts to reduce PM2.5 pollution. However, another important pollutant (ozone) is becoming a problem in eastern China. Ozone (O3) is produced by photochemistry, which requires solar radiation for the formation of O3. Under heavy PM2.5 pollution, solar radiation is often depressed, and the photochemical production of O3 is prohibited. This study shows that during late spring and early fall in eastern China, under heavy PM2.5 pollution, there was often strong O3 photochemical production, causing a co-occurrence of high PM2.5 and O3 concentrations. This co-occurrence of high PM2.5 and O3 is unusual and is the main focus of this study. Recent measurements show that there were often high HONO surface concentrations in major Chinese megacities, especially during daytime, with maximum concentrations ranging from 0.5 to 2 ppbv. It is also interesting to note that high HONO concentrations occurred during high aerosol concentration periods, suggesting that there were additional HONO surface sources in eastern China. Under high daytime HONO concentrations, HONO can be photodissociated to OH radicals, which enhance the photochemical production of O3. In order to study the above scientific issues, a radiative transfer model (TUV; tropospheric ultraviolet–visible) is used in this study, and a chemical steady-state model is established to calculate OH radical concentrations. The calculations show that by including the OH production of photodissociated HONO, the calculated OH concentrations are significantly higher than the values without including this production. For example, by including HONO production, the maximum OH concentration under high aerosol conditions (AOD = 2.5) is similar to the value under low aerosol conditions (AOD = 0.25) in the no-HONO case. This result suggests that even under high aerosol conditions, the chemical oxidizing process for O3 production can occur, which explains the co-occurrence of high PM2.5 and high O3 in late spring and early fall in eastern China. However, the O3 concentrations were not significantly affected by the appearance of HONO in winter. This study shows that the seasonal variation of solar radiation plays important roles for controlling the OH production in winter. Because solar radiation is at a very low level in winter, adding the photolysis of HONO has a smaller effect in winter than in other seasons, and OH remains at low values by including the HONO production term. This study provides some important scientific insight to better understand O3 pollution in eastern China.

Characterization of OH, H2O, and temperature profiles in industrial flame treatment systems interacting with polymer films

Industrial flame treatment is a common technique for oxidizing polymer films to improve adhesion and wettability. These processes are strongly influenced by flame properties such as temperature and the concentration of key combustion radicals (e.g., OH). In this work, we perform in-situ laser absorption measurements of line-of-sight averaged temperature, H2O and OH for an industrial premixed ribbon burner interacting with a chilled-roller polymer-treatment system. Measurements in the unobstructed flame exhibit profiles typical of premixed flames. The addition of a chilled roller above the flame significantly alters the temperature of the flame throughout its vertical profile and shifts the location of the maximum OH concentration above the burner. Measurements at the surface of the polymer, together with post analysis of the surface oxidation, directly verify previous numerical modeling results and surface characterization studies that suggested that the hydroxyl radical in the flame (OH) is critical to surface oxidation. This study may enable future active control of premixed burner systems to maintain desired wettability and adhesion properties of polymer films.

Numerical Study of Ignition Process in Turbulent Shear-less Methane-air Mixing Layer

In this work, ignition process in a turbulent shear-less methane-air mixing layer is numerically investigated. A compressible large eddy simulation method with Smagorinsky sub-grid scale model is used to solve the flow field. Also, a thickened flame combustion model and DRM-19 reduced mechanism are used to compute species distribution and the heat release. Non-reacting mean and RMS axial velocity profiles and mean mixture fraction are validated against experimental data. Instantaneous mixture fraction contours show that the large bursts penetrate from the fuel stream into that of the oxidizer and vice versa and a random behaviour in the cross-stream direction. Flame kernel initiation, growth and propagation are analysed and compared with the experimental data. The ignition results show that the flame is not stable and blow-off occurs, but a more detailed investigation shows that local and short time flame stabilization exist during blow-off. During these local stabilization, heat release increased at the upstream edge of the flame. Most_upstream flame edge scalar analysis shows that the methane mass fraction has a dominant role in the local flame stabilization. OH, HO2, CH2O and heat release contours demonstration reveal that HO2 and CH2O mass fraction as well as the heat release reach a maximum on the border of the flame, but the maximum OH concentration is located in the middle of flame kernel.

Overview: oxidant and particle photochemical processes above a south-east Asian tropical rainforest (the OP3 project): introduction, rationale, location characteristics and tools

Abstract. In April–July 2008, intensive measurements were made of atmospheric composition and chemistry in Sabah, Malaysia, as part of the "Oxidant and particle photochemical processes above a South-East Asian tropical rainforest" (OP3) project. Fluxes and concentrations of trace gases and particles were made from and above the rainforest canopy at the Bukit Atur Global Atmosphere Watch station and at the nearby Sabahmas oil palm plantation, using both ground-based and airborne measurements. Here, the measurement and modelling strategies used, the characteristics of the sites and an overview of data obtained are described. Composition measurements show that the rainforest site was not significantly impacted by anthropogenic pollution, and this is confirmed by satellite retrievals of NO2 and HCHO. The dominant modulators of atmospheric chemistry at the rainforest site were therefore emissions of BVOCs and soil emissions of reactive nitrogen oxides. At the observed BVOC:NOx volume mixing ratio (~100 pptv/pptv), current chemical models suggest that daytime maximum OH concentrations should be ca. 105 radicals cm−3, but observed OH concentrations were an order of magnitude greater than this. We confirm, therefore, previous measurements that suggest that an unexplained source of OH must exist above tropical rainforest and we continue to interrogate the data to find explanations for this.

Kinetic effects of toluene blending on the extinction limit of n-decane diffusion flames

The impact of toluene addition in n-decane on OH concentrations, maximum heat release rates, and extinction limits were studied experimentally and computationally by using counterflow diffusion flames with laser induced fluorescence imaging. Sensitivity analyses of kinetic path ways and species transport on flame extinction were also conducted. The results showed that the extinction strain rate of n-decane/toluene/nitrogen flames decreased significantly with an increase of toluene addition and depended linearly on the maximum OH concentration. It was revealed that the maximum OH concentration, which depends on the fuel H/C ratio, can be used as an index of the radical pool and chemical heat release rate, since it plays a significant role on the heat production via the reaction with other species, such as CO, H 2, and HCO. Experimental results further demonstrated that toluene addition in n-decane dramatically reduced the peak OH concentration via H abstraction reactions and accelerated flame extinction via kinetic coupling between toluene and n-decane mechanisms. Comparisons between experiments and simulations revealed that the current toluene mechanism significantly over-predicts the radical destruction rate, leading to under-prediction of extinction limits and OH concentrations, especially caused by the uncertainty of the H abstraction reaction from toluene, which rate coefficient has a difference by a factor of 5 in the tested toluene models. In addition, sensitivity analysis of diffusive transport showed that in addition to n-decane and toluene, the transport of OH and H also considerably affects the extinction limit. A reduced linear correlation between the extinction limits of n-decane/toluene blended fuels and the H/C ratio as well as the mean fuel molecular weight was obtained. The results suggest that an explicit prediction of the extinction limits of aromatic and alkane blended fuels can be established by using H/C ratio (or radical index) and the mean fuel molecular weight which represent the rates of radical production and the fuel transport, respectively.

Determination of OH Number Densities Outside of a Platinum Catalyst Using Cavity Ringdown Spectroscopy

It is demonstrated that cavity ringdown spectroscopy (CRDS) can be used to probe reaction intermediates desorbing from the surface during a heterogeneous catalytic reaction and provide information valuable in understanding the reaction kinetics. During water formation from H2 and O2, desorbed OH molecules outside of a polycrystalline platinum catalyst were quantified as a function of the relative hydrogen concentration, alphaH2 using CRDS. The temperature of the catalyst was 1500 K, the total pressure was 26 Pa, and the flow was set to 100 sccm. At a distance of 6.5 mm from the Pt catalyst, the maximum OH concentration was found to be 1.5+/-0.2x10(12) cm(-3) at an alphaH2 value of 10%, and the rotational temperature was determined to be 775+/-24 K. The desorbed OH molecules were also probed using laser-induced fluorescence (LIF), and the alphaH2-dependent OH abundance was compared with the CRDS results. The relative concentration of OH probed with LIF appeared to be lower at alphaH2=30-50% compared to what was determined by CRDS. The observed discrepancy is suggested to be due to electronic quenching, as was indicated by a shorter fluorescence lifetime at alphaH2=30% compared to at alphaH2=10%.

Sulphuric acid closure and contribution to nucleation mode particle growth

Abstract. Sulphuric acid concentrations were measured and calculated based on pseudo steady state model with corresponding measurements of CO, NOx, O3, SO2, methane and non-methane hydrocarbon (NMHC) concentrations as well as solar spectral irradiance and particle number concentrations with size distributions. The measurements were performed as a part of the EU project QUEST (Quantification of Aerosol Nucleation in the European Boundary layer) during an intensive field campaign, which was conducted in Hyytiälä, Finland in March–April 2003. In this paper, the closure between measured and calculated H2SO4 concentrations is investigated. Besides that, also the contribution of sulphuric acid to nucleation mode particle growth rates is studied. Hydroxyl and hydroperoxy radical concentrations were determined using a pseudo steady state box model including photo stationary states. The maximum midday OH concentrations ranged between 4.1×105 to 1.8×106 molecules cm-3 and the corresponding values for HO2 were 1.0×107 to 1.5×108 molecules cm-3. The dominant source term for hydroxyl radicals is the reaction of NO with HO2 (56%) and the reaction of CO with OH covers around 41% of the sinks. The sulphuric acid source term is the reaction SO2 with OH and the sink term is condensation of sulphuric acid. The closure between measured and calculated sulphuric acid concentrations is achieved with a high agreement to the measured values. In sensitivity studies, we used different values for the non-methane hydrocarbons, the peroxy radicals and nitrogen dioxide. The best fits between calculated and measured values were found by decreasing the NO2 concentration when it exceeded values of 1.5 ppb and doubling the non-methane hydrocarbon concentrations. The ratio, standard deviation and correlation coefficient between measured and calculated sulphuric acid concentrations are 0.99, 0.412 and 0.645, respectively. The maximum midday sulphuric acid concentrations varied between 3×105 to 1.9×107 molecules cm-3 for the measurements and 3×105 to 1.4×107 molecules cm-3 for the calculations, respectively. An average participation of sulphuric acid to the nucleation mode particle growth rates is 8.8%. Classifying the days into two groups – ''polluted'' days with air masses originated over Central Europe or UK, and ''cleaner'' days with air masses originated over the Northern Atlantic or the Polar regions – reflects an equal sulphuric acid contribution to the aerosol growth in both air mass classes.

315 液滴火炎構造に及ぼす雰囲気中の水蒸気および燃料蒸気濃度の影響(OS-2 液滴の着火・燃焼)

An experimental study was performed to clarify the flame structure of a burning fuel droplet. OH concentration distributions were measured along diametrical line passing through the front stagnation point of a fuel droplet flame in the gaseous stream. The laser induced fluorescence was applied for the measurement of OH concentration distributions. An Nd:YAG laser-pumped pulsed dye laser emitting monochromatic radiation at a wavelength of 283.62nm was used as the light source for these measurements. The results showed that the maximum of OH concentration was located near the flame front. An increase in the relative humidity in the gaseous stream caused the decrease of OH concentration. The increase in benzene vapor concentration in the gaseous stream around a benzene droplet flame causes the increased maximum OH concentration and the outward extension of the region in which OH is present.

Hohenpeissenberg Photochemical Experiment (HOPE 2000): Measurements and photostationary state calculations of OH and peroxy radicals

Abstract. Measurements of OH, total peroxy radicals, non-methane hydrocarbons (NMHCs) and various other trace gases were made at the Meteorological Observatory Hohenpeissenberg in June 2000. The data from an intensive measurement period characterised by high solar insolation (18-21 June) are analysed. The maximum midday OH concentration ranged between 4.5x106 molecules cm-3 and 7.4x106 molecules cm-3. The maximum total ROx (ROx =OH+RO+HO2+RO2) mixing ratio increased from about 55 pptv on 18 June to nearly 70 pptv on 20 and 21 June. A total of 64 NMHCs, including isoprene and monoterpenes, were measured every 1 to 6 hours. The oxidation rate of the NMHCs by OH was calculated and reached a total of over 14x106 molecules cm-3 s-1 on two days. A simple photostationary state balance model was used to simulate the ambient OH and peroxy radical concentrations with the measured data as input. This approach was able to reproduce the main features of the diurnal profiles of both OH and peroxy radicals. The balance equations were used to test the effect of the assumptions made in this model. The results proved to be most sensitive to assumptions about the impact of unmeasured volatile organic compounds (VOC), e.g. formaldehyde (HCHO), and about the partitioning between HO2 and RO2. The measured OH concentration and peroxy radical mixing ratios were reproduced well by assuming the presence of 3 ppbv HCHO as a proxy for oxygenated hydrocarbons, and a HO2/ RO2 ratio between 1:1 and 1:2. The most important source of OH, and conversely the greatest sink for peroxy radicals, was the recycling of HO2 radicals to OH. This reaction was responsible for the recycling of more than 45x106 molecules cm-3 s-1 on two days. The most important sink for OH, and the largest source of peroxy radicals, was the oxidation of NMHCs, in particular, of isoprene and the monoterpenes.

Measurements of OH and HO2 radical concentrations and photolysis frequencies during BERLIOZ

This paper presents the measurements of OH and HO2 radical concentrations as well as photolysis frequencies of different molecules during the Berliner Ozone (BERLIOZ) field experiment in July/August 1998 at the rural site Pabstthum about 50 km NW of Berlin. Radical concentrations were measured using laser‐induced fluorescence (LIF) spectroscopy, while filter radiometers and a scanning spectroradiometer were used to obtain photolysis frequencies. The radical data set covers the time period from 20 July to 6 August and consists of more than 6000 simultaneous measurements of OH and HO2 with a typical time resolution of about 90 s. The maximum OH and HO2 daytime concentrations were 8 × 106 and 8 × 108 cm−3, respectively. While nighttime values of OH were usually below the detection limit of our instrument (3.5 × 105 cm−3), HO2 did show significant concentrations throughout most of the nights (on average 3 × 107 cm−3). The OH concentration was mainly controlled by solar UV radiation and showed a high linear correlation with J(O1D). A deviation from this general behavior was observed around dawn and dusk, when OH concentrations well above the detection limit were observed, although J(O1D) was essentially zero. A comparison with data sets from previous campaigns revealed that even though the linear correlation is found in other environments as well the slope [OH]/J(O1D) differs significantly. The diurnal cycles of HO2 were less dependent on the solar actinic flux but were predominantly influenced by NO. During episodes of high NO, HO2 remained below the detection limit (1 × 107 cm−3) but started to rise rapidly as soon as NO started to decrease.

The measurement of tropospheric OH radicals by laser‐induced fluorescence spectroscopy during the POPCORN Field Campaign

A highly sensitive OH measurement instrument has been developed. It is based on laser‐induced fluorescence (LIF) detection of OH using the A²Σ+v′ = 0‐X²Π v″ = 0 transition at 308.15 nm at low pressure. The LIF instrument detects OH directly and with high specificity, a fact that was demonstrated by recording laser excitation spectra (Q1(3), Q21(3) and P1(1) lines) of ambient OH. For high time resolution (typ. 60–100 s), the laser wavelength was modulated on‐/off‐ resonance with the P1(1) line. Here, we report some of the OH measurements obtained by this technique during its first application in a tropospheric field campaign (“POPCORN”), which was conducted in August 1994 in a rural environment in the North‐East of Germany. These include diurnal OH concentration profiles with maximum OH concentrations up to 1.4×107cm−3at noon. Minimum OH concentrations were measured in the morning and evening down to the detection limit of (3–6) × 105cm−3(SNR=2, measurement time 1 min.). During the day, OH fluctuations were observed on a time scale of minutes and hours. These were highly correlated to the flux of the solar UV radiation which is responsible for the primary OH production by photolysis.

Flame structure of an axisymmetric hydrogen-air diffusion flame

A detailed model including a full scheme of combustion reactions and the governing equations of fluid mechanics was designed for two-dimensional hydrogen burning systems, and applied to a Burke-Schumann type hydrogen-air diffusion flame to elucidate its flame structure and combustion reaction mechanism. The same flame was also experimentally investigated and radial profiles of OH radical concentration and rotational temperature through the flame were determined by a conveniently improved line-of-sight absorption method. Simulation suggested that a well-developed diffusion flame occurs from heights about 5 mm from the burner mouth, in very good agreement with the experimental results. At each horizontal section of the well-developed diffusion region, the calculated rates of the chemical reactions showed considerable values within an annular zone about 5 mm thick, in which there is a point where H2 and O2 are simultaneously exhausted, and in whose vicinity the temperature becomes maximum. This result confirmed one of the Burke-Schumann's predictions, but radial displacements of about 1 mm between the peeks of the rates of different reactions and also between the maximum OH radical concentration and the maximum temperature were found in both experiments and calculation, showing that the reactions do not occur in an infinitely thin region, contradicting one of the Burke-Schumann's assumptions. At the lowest part of the flame, below the burner tip, the issuing air and the H2 diffusing downward meet and react like premixed gas mixtures; the whole flame was found to be held at the burner rim by large heat release rates at that region. The experimental results, in very good agreement with the simulation, showed that the maximum OH concentration through the flame occurs at a height around 2 mm, confirming the premixed-like intense combustion reactions taking place at the lower part of the flame.

Measurements of superequilibrium hydroxyl concentrations in turbulent nonpremixed flames using staturated fluorescence

The first quantitative, time- and space-resolved measurements have been obtained for probability density functions of OH concentration in nonpremixed flames. Measurements using single-pulse, laser-saturated fluorescence in laminar, transitional and turbulent nonpremixed H 2 -air flames provide unambiguous evidence for substantial OH superequilibrium concentrations, in qualitative agreement with predictions of laminar and turbulent combustion models. The average degree of superequilibrium (¯OH/¯OH AE ) is typically 4–5 near the jet exit and approaches unity far downstream. The maximum instantaneous OH concentration measured in transitional and turbulent H 2 -air flames is ∼6×10 16 molecules/cc in accord with the maximum determined by partial equilibrium thermodynamic calculations and with the maximum OH concentrations measured in premixed H 2 -air flames.

Two-dimensional measurements of the [OH] in a constant volume combustion chamber

The planar distribution of the OH concentration in a methane-air flame in a constant volume combustion chamber has been measured with two-dimensional laser-induced fluorescence. A Nd:YAG-pumped pulsed dye laser with frequency doubling was used to form a sheet of laser light which excited the OH A 2 Σ ( v ′=1)−X 2 Π ( v ″=0) transition. The resulting fluorescence from the OH molecule in a propagating flame was observed with a low-light-level vidicon camera. A map of the OH concentration in the combustion chamber was obtained over a field of view covering 45 mm×60 mm with a spatial resolution of 0.6 mm×0.6 mm. For a single 5 ns laser pulse, the fluorescence signal-to-noise ratio (S/N) is 10 at the maximum OH concentration of 2.4×10 16 cm −3 . The measured OH concentration distribution agrees with the prediction of a laminar flame model to within the experimental accuracy.

Laser Fluorescence Measurements of the OH Concentration in a Combustion Boundary Layer†

Abstract A laser induced fluorescence technique was used to measure the OH radical concentration in the boundary layer combustion of a lean hydrogen-air mixture (π = 0.2) flowing over a heated (1170 K.) platinum plate. The highly nonequilibrium OH concentration profile was observed 5 to 50 mm from the leading edge of the plate and to within 0.25 mm of the surface. The maximum OH concentration occured near the leading edge of the platinum plate and was ~200 times its equilibrium value.