Turbulent Heat and Mass Transfer, and Combustion Control of a Non-Premixed Turbulent Flame Associated with Counter Gradient Diffusion Phenomena

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The research results are summarized as follows:(1) "Heat transfer characteristics of a non-premised turbulent flame formed in a curved rectangular duct": Heat transfer characteristics of a non-premixed turbulent flame formed in a curved rectangular duct (180° bend) were investigated experimentally. Key turbulence quantities of velocity and thermal fields such as Reynolds stress components and turbulent heat fluxes were measured using a combined LDV and fine-wire thermocouple technique. These measurements provided direct evidence of the occurrence of the anomalous phenomenon of counter-gradient heat transfer, which can be ascribed to the presence of a strong pressure-gradient in the radial direction of the curved duct. The experiment also revealed that the Onset region of this "counter-gradient" diffusion was adjacent to the strong "gradient" diffusion region. The quantitative appraisal of the production terms for the turbulent heat flux showed that the pressure gradient promoted gradie … More nt diffusion on the inner-wall (low-pressure) side of the curved-duct flame and caused counter-gradient diffusion on the outer-wall (high-pressure) side. The schlieren photography for visualizing the density field showed a totally different behavior of the burned gas parcels between the high- and low-pressure sides of the flame. The essential mechanism causing the counter-gradient diffusion can be explained by the unique motion of the high-temperature (low-density) gas parcel on the high-pressure side of the flame.High-temperature fluid motions tend to be preferentially damped by the pressure gradient imposed on the flow field. The occurrence of the counter-gradient diffusion phenomenon will of course lead to the collapse of the "gradient-diffusion hypothesis," on which most conventional turbulence models rely. In such a field, the analogy between heat and mass transfer processes, which holds almost always in normal turbulent passive-scalar transport, can disappear.(2) "Turbulence statistics of a non-premixed flame formed in a curved channel": Statistical characteristics of a non-premixed turbulent flame formed in a curved rectangular channel were investigated experimentally. Two types of flame one is formed in the center of the channel (Flame 1) and the other in the vicinity of the inner-wall (Flame 2) were measured using a simultaneous measurement technique of velocity and temperature. In both flames, counter-gradient diffusion (CGD) emerged in heat transfer at the outer-wall side of the flames. Flame 2 showed more distinct CGD than Flame 1, and its shape was very elongated. Unlike Flame 1, Flame 2 was not associated with strong "gradient-diffusion" heat transfer at the inner-wall side of the flame, and this may strengthen the CGD of Flame 2. Statistical analysis of the turbulent heat-flux revealed the internal structure of the CGD, where high-temperature fluid parcels moving toward the outer-wall side are strongly decelerated (Flame 1), or return with little diffusion (Flame 2). These are the essential features characterizing the turbulent flames under the strong pressure-gradient. Less