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The rate constants for the reactions of the ethynyl radical (C2H) with H2,02, C2H2, and NO have been measured by following the time decay of an C2H infrared transient absorption line originating from the ground vibronic state using color center laser spectroscopy. For the H2, 02, and NO reactions, the C2H was produced by excimer laser flash photolysis (ArF, 193 nm) of CF3C2H. In the case of the C2H2 reaction C2H was produced by flash photolysis of acetylene again using the 193-nm ArF excimer line. Excited states of C2H, which are abundant with 193-nm photolysis, were relaxed by buffering the photolysis cell with ~20 Torr of He buffer and ~160 mTorr of SF6. Rate constants of 4.2 X 10_11, 4.8 X 10"13, 1.5 X 10"10, and 3.5 X 10~u cm3 molecule"1 s"1 were obtained for the reactions of C2H with 02, H2, C2H2, and NO, respectively.

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Computational models are developed to predict the natural convection heat transfer and buoyancy for a Montgolfiere under conditions relevant to the Titan atmosphere. Idealized single-and double-walled balloon geometries are simulated using algorithms suitable for both laminar and (averaged) turbulent convection. Steadystate performance results are compared with existing heat transfer coefficient correlations. The laminar results, in particular, are used to test the validity of the correlations in the absence of uncertainties associated with turbulence modeling. Some discrepancies are observed, which appear to be primarily associated with temperature nonuniformity on the balloon surface. The predicted buoyancy for both the single-and double-walled balloons in the turbulent convection regime, predicted with standard two-equation turbulence models, showed trends similar to those with the empirical correlations. There was also good agreement with recently conducted experiments in a cryogenic facility designed to simulate the Titan atmosphere.