17 research outputs found
The bakony growth study. By Ćva BodzsĆr. 210 pp. Budapest: Humanbiologia Budapestinensis. 1991
No Abstract.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38554/1/1310050318_ftp.pd
Maternal short stature does not predict their children's fatness indicators in a nutritional dual-burden sample of urban Mexican Maya
The co-existence of very short stature due to poor chronic environment in early life and obesity is becoming a public health concern in rapidly transitioning populations with high levels of poverty. Individuals who have very short stature seem to be at an increased risk of obesity in times of relative caloric abundance. Increasing evidence shows that an individual is influenced by exposures in previous generations. This study assesses whether maternal poor early life environment predicts her child's adiposity using cross sectional design on Maya schoolchildren aged 7ā9 and their mothers (n = 57 pairs). We compared maternal chronic early life environment (stature) with her child's adiposity (body mass index [BMI] z-score, waist circumference z-score, and percentage body fat) using multiple linear regression, controlling for the child's own environmental exposures (household sanitation and maternal parity). The research was performed in the south of Merida, Yucatan, Mexico, a low socioeconomic urban area in an upper middle income country. The Maya mothers were very short, with a mean stature of 147 cm. The children had fairly high adiposity levels, with BMI and waist circumference z-scores above the reference median. Maternal stature did not significantly predict any child adiposity indicator. There does not appear to be an intergenerational component of maternal early life chronic under-nutrition on her child's obesity risk within this free living population living in poverty. These results suggest that the co-existence of very short stature and obesity appears to be primarily due to exposures and experiences within a generation rather than across generations
Modeling the Fuel Spray and Combustion Process of the Ignition Quality Tester with KIVA-3V Modeling the Fuel Spray and Combustion Process of the Ignition Quality Tester with KIVA-3V
Development of advanced compression ignition and low-temperature combustion engines is increasingly dependent on chemical kinetic ignition models. However, rigorous experimental validation of kinetic models has been limited as a result of several factors. For example, shock tubes and rapid compression machines are often limited to premixed gas-phase studies, precluding the use of more realistic, low-volatility diesel or biodiesel surrogates. The Ignition Quality Tester (IQT) constant-volume spray combustion system measures ignition delay of low-volatility fuels; therefore, the IQT has the potential to validate ignition models experimentally. However, a better understanding of the IQT's fuel spray and combustion processes is necessary to facilitate chemical kinetic studies. KIVA-3V is utilized in developing a three-dimensional computational fluid dynamics (CFD) model that accurately and efficiently reproduces ignition behavior and temporally resolves temperature and equivalence ratio regions inside the IQT. The model's fuel spray characteristics (e.g., velocity, cone-angle, oscillations) are experimentally validated; n-heptane is initially studied because of the simplicity of its chemical kinetics and use as IQT calibration fuel. Reduced/skeletal n-heptane chemical mechanisms (60, 42, and 33 species) and one-step chemistry are employed. The CFD results indicate combustion is governed by autoignition kinetics, and perturbations/oscillations in the fuel spray have significant effects on the combustion process, as verified experimentally. The CFD model provides insight into the complex interaction between the fuel spray and combustion processes, which is vital to expanding the fuel research capabilities of the IQT
Experiments and Computational Fluid Dynamics Modeling Analysis of Large <i>n</i>āAlkane Ignition Kinetics in the Ignition Quality Tester
This paper presents experimental
measurements of ignition delays
from low- to high-volatility <i>n</i>-alkanes representative
of diesel and jet fuel compounds that are supplemented with a computational
fluid dynamics (CFD) analysis. The ignition quality tester (IQT) is
shown to be effective for studying ignition of low-volatility fuels,
such as <i>n</i>-hexadecane, which are typically difficult
to measure. Ignition delays, both experimental and modeled, are presented
using an eight-point experimental design matrix (1.5 and 3.0 MPa,
823 and 723 K, and 15 and 21% O<sub>2</sub>). A detailed <i>n</i>-alkane mechanism (C<sub>8</sub>āC<sub>16</sub> with a total
of 2115 species) was reduced to a skeletal 237 species <i>n</i>-hexadecane mechanism using a targeted search algorithm. A CFD model
of the IQT (developed using KIVA-3V) coupled with skeletal mechanisms
predicted ignition delays of <i>n</i>-heptane and <i>n</i>-hexadecane with reasonable accuracy over the eight-point
matrix, with the exception of the highest temperature, lowest pressure,
and oxygen concentration conditions. Temperature sweeps across a range
of pressures (0.1ā1.0 MPa) and temperatures (673ā973
K) were performed for <i>n</i>-heptane, <i>n</i>-decane, <i>n</i>-dodecane, and <i>n</i>-hexadecane.
The negative temperature coefficient (NTC) region was observed experimentally
for the first time for <i>n</i>-hexadecane. The NTC region
for <i>n</i>-dodecane and <i>n</i>-decane has
previously been observed in shock tubes and rapid compression machines
and is reported here for the first time in the IQT. The IQT is thus
capable of capturing NTC behavior for large alkanes and can serve
as an additional experimental validation tool for chemical kinetic
mechanisms of low-volatility surrogates for diesel and jet fuels
Investigation of Iso-octane Ignition and Validation of a Multizone Modeling Method in an Ignition Quality Tester
An ignition quality
tester was used to characterize the autoignition
delay times of iso-octane. The experimental data were characterized
between temperatures of 653 and 996 K, pressures of 1.0 and 1.5 MPa,
and global equivalence ratios of 0.7 and 1.05. A clear negative temperature
coefficient behavior was seen at both pressures in the experimental
data. These data were used to characterize the effectiveness of three
modeling methods: a single-zone homogeneous batch reactor, a multizone
engine model, and a three-dimensional computational fluid dynamics
(CFD) model. A detailed 874 species iso-octane ignition mechanism
(Mehl, M.; Curran, H. J.; Pitz, W. J.; Westbrook, C. K. Chemical kinetic
modeling of component mixtures relevant to gasoline. Proceedings of the European Combustion Meeting; Vienna, Austria, April 14ā17, 2009) was reduced to 89 species for use in these models, and the predictions
of the reduced mechanism were consistent with ignition delay times
predicted by the detailed chemical mechanism across a broad range
of temperatures, pressures, and equivalence ratios. The CFD model
was also run without chemistry to characterize the extent of mixing
of fuel and air in the chamber. The calculations predicted that the
main part of the combustion chamber was fairly well-mixed at longer
times (>āÆā¼30 ms), suggesting that the simpler models
might be applicable in this quasi-homogeneous region. The multizone
predictions, where the combustion chamber was divided into 20 zones
of temperature and equivalence ratio, were quite close to the coupled
CFDākinetics results, but the calculation time was ā¼11
times faster than the coupled CFDākinetics model. Although
the coupled CFDākinetics model captured the observed negative
temperature coefficient behavior and pressure dependence, discrepancies
remain between the predictions and the observed ignition time delays,
suggesting improvements are still needed in the kinetic mechanism
and/or the CFD model. This approach suggests a combined modeling approach,
wherein the CFD calculations (without chemistry) can be used to examine
the sensitivity of various model inputs to in-cylinder temperature
and equivalence ratios. These values can be used as inputs to the
multizone model to examine the impact on ignition delay. The speed
of the multizone model also makes it feasible to quickly test more
detailed kinetic mechanisms for comparison to experimental data and
sensitivity analysis