Effect of wall heat transfer on shock-tube test temperature at long times |
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Authors: | C Frazier M Lamnaouer E Divo A Kassab E Petersen |
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Institution: | (1) Dept. Mechanical & Space Engineering, Hokkaido University, Kita 13, Nishi 8, Sapporo 060-8628, Japan;(2) Dept. Mechanical Engineering, Osaka University, Yamada-oka 2-1, Suita 565-0871, Japan |
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Abstract: | When performing chemical kinetics experiments behind reflected shock waves at conditions of lower temperature (<1,000 K),
longer test times on the order of 10–20 ms may be required. The integrity of the test temperature during such experiments
may be in question, because heat loss to the tube walls may play a larger role than is generally seen in shock-tube kinetics
experiments that are over within a millisecond or two. A series of detailed calculations was performed to estimate the effect
of longer test times on the temperature uniformity of the post-shock test gas. Assuming the main mode of heat transfer is
conduction between the high-temperature gas and the colder shock-tube walls, a comprehensive set of calculations covering
a range of conditions including test temperatures between 800 and 1,800 K, pressures between 1 and 50 atm, driven-tube inner
diameters between 3 and 16.2 cm, and test gases of N2 and Ar was performed. Based on the results, heat loss to the tube walls does not significantly reduce the area-averaged temperature
behind the reflected shock wave for test conditions that are likely to be used in shock-tube studies for test times up to
20 ms (and higher), provided the shock-tube inner diameter is sufficiently large (>8cm). Smaller diameters on the order of
3 cm or less can experience significant temperature loss near the reflected-shock region. Although the area-averaged gas temperature
decreases due to the heat loss, the main core region remains spatially uniform so that the zone of temperature change is limited
to only the thermal layer adjacent to the walls. Although the heat conduction model assumes the gas and wall to behave as
solid bodies, resulting in a core gas temperature that remains constant at the initial temperature, a two-zone gas model that
accounts for density loss from the core to the colder thermal layer indicates that the core temperature and gas pressure both
decrease slightly with time. A full CFD solution of the shock-tube flow field and heat transfer at long test times was also
performed for one typical condition (800 K, 1 atm, Ar), the results of which indicate that the simpler analytical conduction
model is realistic but somewhat conservative in that it over predicts the mean temperature loss by a few Kelvins. This paper
presents the first comprehensive study on the effects of long test times on the average test gas temperature behind the reflected
shock wave for conditions representative of chemical kinetics experiments. |
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