OCR Text |
Show Temperature Effects: Temperature finds its influence through the vapor pressure of depositing compounds, the physical phase of aerosols and deposits, and the particle deposition mechanisms. Thermophoretic mechanism of particle transport by virtue of a temperature gradient is dominant in turbines with actively cooled blades (Vermes, 1979). Investigators have observed an increase in combustor wall temperature with a decrease in fuel hydrogen content mainly due to an increase in radiation from flames (Stein et al., 1981; Pillsbury et al., 1978; Sims, 1978 and 1979; Novick,1982). These higher wall temperatures arise from the tendency of coal-derived liquids to produce a large quantity of particulate matter in burning flames resulting in more illuminous flames and higher wall temperatures (Decorso, 1981; Friswell, 1979; Seglem, 1980; Gollahalli, 1982). Higher combustor wall temperatures will necessitate further cooling of the internals of the combustor, and this will increase the importance of thermophoresis as a deposition mechanism due to the increase in the gas/wall temperature di fference. Perhaps the most overwhelming evidence supporting thermophoresis originates from a series of investigations in residual oil combustion (Cohn, 1982). Three trends are present: (1) For a fixed gas temperature, deposition increases for increasing wall temperatures in the 370 C (700 F) to 650 C (1200 F) range; deposition decreases for increasing wall temperatures greater than 650 C (1200 F). (2) For a fixed wall temperature, deposition increases with increasing gas temperature. (3) For wall temperatures less than 370 C (700 F), there is a sharp decrease in deposition for any given gas temperature. 1.5.22 |