Flame response temperature profiles

A variety of optical techniques have been used to measure gas temperatures in combustion applications, particularly in flames. There are potentially some important advantages of optical techniques compared to contact techniques such as suction pyrometers (see Figure 5.7). Optical measurement techniques do not disturb the flow, where thermocouples may have a significant impact on the fluid dynamics. Optical techniques can potentially measure higher temperatures as there are not the materials issues compared to thermocouples. For some optical techniques, temperature profiles can be measured at one point in time without the need to make multiple individual measurements over some length of time. Optical techniques often have a much faster response time compared to contact methods. This is particularly important in turbulent and transient flows. 

Fine wire thermocouples are of low cost and easy to use. They have relatively short response times and good spatial resolution. However, they are somewhat intrusive to the flame. Corrections are also required due to heat transfer effects at the junction bead. These include radiation from the bead to the environment and the heat conduction along the thermocouple wires. Often the junction must be coated to prevent catalytic reactions. There are material limitations, due to high temperature oxidizing and reducing conditions. They can only make so many point measurements over a span of time to get a temperature profile. 

Neoprene, or polychloroprene rubber (CR) was one of the very first synthetic rubbers produced. It was a material of choice for exterior applications such as profiles used in vehicles, building seals, and cables. Many more marketable products have benefited from this plastic. Except for SBR and IR, neoprene (CR) elastomers are perhaps the most rubberlike of all materials, particularly with regard to its dynamic response (Table 2.6). CRs are a family of elastomers with a property profile that approaches that of NRs (natural rubbers) but has better resistance to oils, ozone, oxidation, and flame. CRs age better and do not soften up on exposure to heat, although their high-temperature tensile strength may be lower than that of NRs. They are suitable for service at 250C (480F). 

After introduction into the flame, the cavity temperature increases from ambient to a maximum value, which depends on material and conditions. During the heat-up period of the cavity in the flame, a series of physical and chemical changes occur which are accompanied by generation of the characteristic molecular emission. A typical example of recorded response from thiourea, which generates the 2 emission, is shown in Figure 5. The cavity remains in the flame as long as required for recording the emission profile (emission intensity as a function of time). It is then removed and cooled before repetition of the process. 

Previous evidence for the mechanism of formation of CH comes from two experiments. A measurement [6] of spatial profiles in low-pressure C2H2/O2 flames showed the ratio [CH ]/[C2][OH] to be remarkably constant with variation in many different flame parameters. Only the A A state was observed, however. In a more recent study [7] in a low pressure discharge flow at room temperature, emission spatial profiles, measured downstream from mixing of 0 + C2H2, were compared with the results of a computer calculation, varying many discharge parameters. Here it was concluded that CH was produced from the reaction O + C2H, not the reaction C2 + OH deduced from the flame study. The present profiles suggest that neither mechanism is solely responsible for the formation of electronically excited CH. 

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