Mean OH Concentration Research Articles

Prediction and measurement of a non-equilibrium turbulent diffusion flame

Superequilibrium radical concentrations in a turbulent CO/H 2 /N 2 jet diffusion flame are computed using a two-scalar pdf model and directly measured using single pulse laser saturated OH fluorescence. The model is based on the averaged Navier-Stokes equations and the k∈ l turbulence model. Non-equilibrium chemistry is accounted for by including CO in the partially equilibrated oxyhydrogen radical pool. Two scalars (mixture fraction and eaction progress suffice to describe the thermochemical system. Laser saturated fluorescence is used to directly measure the mean and fluctuating components of OH concentrations and thus the radical pool. Measurements and model both find mean OH concentrations which are four to six times larger than equilibrium with rms values of OH concentration also reasonably predicted. Superequilibrium effects are predicted to lower the mean temperature by as much as 250 K in agreement with experiments. Evidence of the breakdown of partial equilibrium was found in cool fuel-rich zones where predictions of temperature and OH concentration were too high. Extensions of the model to predict thermal NO formation and CO burnout are discussed.

Distribution of aromatic hydrocarbons in the ambient air

Aromatic hydrocarbons were measured in twelve United States cities during 1979–1984 with the help of an instrumented mobile laboratory. Approximately 100 measurements were made at each site over a 1–2 week period on a round-the-clock basis. Measurements at three sites were repeated to obtain seasonal differences. Although variabilities exist in these measurements, the average distribution of aromatic hydrocarbons in urban air is benzene 21%, toluene 36%, ethylbenzene 9%, m/ p-xylene 15%, o-xylene 7%, 3 4 - ethyl toluene 4%,1,2,4-trimethylbenzene 6 % and 1,3,5-lrimethylbenzene 2 %. Average concentrations in the range of 1–9 ppb benzene, 1–17 ppb toluene, 1–5 ppb ethylbenzene, 0.6–10 ppb m/ p-xylene, 0.3–4 ppb o-xylene, 0.2–3ppb 3 4 - ethyltoluene , 0.4–4ppb 1,2,4 trimethylbenzene and 0.1–2ppb 1,3,4-trimethylbenzene have been measured. Maximum concentrations at each site are typically less than 10 times the mean value. For both chemical and meteorological reasons, concentrations of aromatic hydrocarbons are at their highest during night and early morning hours with minimum occurring during late morning and early afternoon. Relative diurnal behavior of aromatic hydrocarbons shows that most are depleted at a rate 5–50 times faster than benzene. Based on data in southern California (site 13), it is estimated that a mean OH concentration of at least 2.6 (±0.6) × 10 6 moleccm −3 must prevail during 0730–1330h even in February. A long-term examination of benzene and toluene data from southern California air suggests that their levels may have declined by a factor of 5–10 over the last two decades. In remote atmospheres benzene is present at a concentration of 0.1–0.2 ppb, and is 1–3 times more abundant than toluene. It is estimated that such air masses are only 2–7 days away from their urban source.

The calculation of mean radical concentrations in turbulent diffusion flames

A prediction model for turbulent diffusion flames is presented and applied to H 2air-diffusion flames and in particular to the calculation of mean radical concentrations. The fast shuffle reactions in the H 2air reaction system are assumed to be in partial equilibrium, whereas the three-body recombination reactions are treated kinetically. These assumptions lead to a two variable formalism which allows the calculation of nonequilibrium values of radicals. It is shown that these values differ considerably from calculations obtained with equilibrium assumptions. The results of the calculations compare favourably with measurements of mean OH concentrations by El-Dib [9].