The stability of a horizontal layer of Maxwellian fluid heated from below is considered. Critical Rayleigh numbers, wave-numbers, and frequencies for overstability are determined for both free and rigid boundaries. Elasticity is found to destabilize the fluid, and the presence of rigid boundaries is found to be slightly stabilizing.

The stability of natural convection of a viscous fluid in a vertical slot having isothermal side walls of different temperatures is investigated analytically. Both the conduction and boundary-layer régimes are found to be unstable with respect to stationary disturbances in the form of multicellular secondary flows. Theoretical predictions of the critical Rayleigh number and of the form of the secondary flow are verified by experimental measurements.

An experimental investigation of turbulent natural convection in air is described. The results of this study show good agreement with early investigations and remarkable agreement with the analytical correlation of Bayley [4]: Nu = 0.10 Raf for Rayleigh numbers up to 10r2. Extensive measurements of the temperature field indicate a good similarity in the temperature profiles when compared on the basis of the natural coordinatey. The use of power law temperature protiles is shown

An analytical attempt is made to understand the non-equilibrium interaction between thermal radiation and laminar free convection in terms of a heated vertical plate in a stagnant radiating gas. The effect of radiation is taken into account in the integral formulation of the problem as a one-dimensional heat flux, evaluated by including the absorption in thin gas approximation and the wall elfect in thick gas approximation. The local Nusselt numbers thus obtained help to interpret the gas domains from transparent to opaque and from cold to hot. The present thick gas model approximates the radiant flux as qR=-E[-(I-?)exp(-$aygT'$ whose limit for large a and small but non-zero y is the Rosseland gas, qgs =-(16a/3a) T3(cW/ay), and that for y = 0 and large but finite a is 4", =-c,(8a/3a

Kernel growth from a spark in propane-air mixtures at atmospheric pressure is studied in a constant volume bomb with a high-speed laser schlieren system. The spark current and voltage waveforms of an inductive ignition source are simultaneously recorded with the photographic recordings. The temporal growth of the measured equivalent radii at conditions near the minimum ignition energy shows the existence of a critical radius and the influence of the critical radius on kernel development. In addition, it is shown that the net spark power for ignition can be estimated using data from minimum ignition energy, electrode fall energy losses, and spark calorimetry experiments. These results are used in Part II to develop a model for kernel growth.

Combustion initiation is studied in atmospheric pressure propane-air mixtures in a constant volume bomb with a high speed (10,000 fps) laser schlieren system. The spark current and voltage waveforms are simultaneously recorded for later model input. A phenomenological model for early flame kernel development is presented which accounts for the initial, breakdown generated, spark kernel and its subsequent growth. The kernel growth is initially controlled by the breakdown process and the subsequent electrical power input. A new, spark power induced, mass entrainment term is shown to model this initially rapid volume increase adequately while later growth is mainly dominated by diffusion. Results and model comparisons are presented for the effects of power input, spark energy, and equivalence ratio.

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Heat transfer rates in pulse combustor tail pipes and in other reversing, oscillating, turbulent flows have been found to be much higher than those of steady turbulent flow. To elucidate the mechanisms of the enhancement, the temperature and velocity fields, measured with two-line atomic fluorescence (TLAF) and laser Doppler velocimetry (LDV). respectively, are compared. Time-resolved wall heat fluxes and Nusselt numbers are also presented and discussed. Possible causes for the heat transfer enhancement in oscillating flows are reviewed and discussed in view of the data presented in this paper and the recent literature.