Heat Transport Characteristics of a Non-Premixed Turbulent Flame and nomaly in Heat Transfer : Counter-Gradient Diffusion

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The results have been reported in the following three papers. The abstracts are :(1) "A two-thermocouple probe technique for estimating thermocouple time constants in flows with combustion : in situ parameter-identification of a first-order lag system" : A two-thermocouple probe, composed of two fine-wire thermocouples of unequal diameters, is a novel technique for estimating thermocouple time constants without any dynamic calibration of the thermocouple response. This technique is most suitable for measuring fluctuating temperatures in turbulent combustion. In the present study, the reliability and applicability of this technique are appraised in a turbulent wake of a heated cylinder (without combustion). A fine-wire resistance thermometer (cold wire) of fast response is simultaneously used to provide a reference temperature. A quantitative and detailed comparison between the cold-wire measurement and the compensated thermocouple ones shows that a previous estimation scheme gives ther … More mocouple time constants smaller than appropriate values, unless the noise in the thermocouple signals is negligible and/or the spatial resolution of the two-thermocouple probe is sufficiently high. The scheme has been improved so as to maximize the correlation coefficient between the two compensated- thermocouple outputs. The improved scheme offers better compensation of the thermocouple response. The present approach is generally applicable to in situ parameter-identification of a first-order lag system.(2) "Simultaneous Measurement of Velocity and Temperature in High Temperature Turbulent Flows" : This paper deals with a simple and reliable technique for simultaneous measurement of velocity and temperature in high-temperature turbulent flows, including combustion. The technique is based on the combination of a laser Doppler velocimeter (LDV) and a compensated fine-wire thermocouple. A central issue of this study is to find the applicability and reliability of the technique proposed. For this purpose, a two-thermocouple probe with a fine cold wire (Tagawa, Shimoji and Ohta, 1998), which enables in situ measurement of thermocouple time constants and accurate compensation of the thermocouple response, is combined with the LDV.The technique is tested in a turbulent wake behind a heated cylinder, whose thermal field is of ordinary temperature and fluctuates with relatively small amplitude. This enables critical assessment of the measurement accuracy. The results show that the technique is highly reliable and is effective in investigating heat transport processes in various non-isothermal turbulent flows.(3) "Heat transport characteristics of a turbulent flame formed in a curved rectangular duct" : Heat transport characteristics of a non-premixed turbulent flame formed in a curved rectangular duct (180-degree bend) are investigated experimentally. Key turbulence quantities of velocity and thermal fields such as Reynolds stress components and turbulent heat fluxes are measured using a combined LDV and fine-wire thermocouple technique. These measurements show direct evidence of the occurrence of counter-gradient heat transport, which can be ascribed to the existence of a strong pressure gradient in the radial direction of the curved duct. The shadowgraph technique for visualizing the temperature field reveals totally different behavior of the burned gas parcels between the high- and the low-pressure sides of the flow, and the observations may help us explain the mechanism of the occurrence of the counter-gradient heat transport in turbulent flows with combustion. Less