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Show Condensation G (t, r) and Chemical Reaction R (t, r) Condensation is the mechanism by which vapors are collected on cooled heat transfer surfaces. All vapors that enter the thermal boundary layer around a heat transfer surface and subsequently are deposited on the surface are treated within the condensation term of the model. At least three mechanisms for this process are available: (1) vapors may traverse the boundary layer and heterogeneously condense on the heat transfer surface; (2) vapors may homogeneously nucleate to form a fume and subsequently deposit by thermophoresis on the surface; and (3) vapors may heterogeneously condense on other particles in the boundary layer and arrive at the heat transfer surface by thermophoresis Q1-26). Inorganic-Vap·o;i c:::> Tacky Deposit An illustration of Condensation deposition by condensation on a tube in cross flow is presented in Figure 4. Condensation deposits have no granularity and are more uniforml y deposited on the tube than either thermophoretically or inertially deposited material. The deposits are tacky and have a strong influence on the surface capture efficiency. Condensation deposits also contact the metal surface more efficien tly than inertially impacted deposits, increasing the tenacity with which the deposit clings to the surface. Figure 4. Schematic illustration of condensation on a tube in cross flow. The condensation flux is described by the following equation G;= 'i e km(Xi.b - xs) + X~b L Gi (3) where ~i is the condensation efficiency, e is a blowing factor (which will be very near unity for this application), km is a mass transfer coefficient with a value that depends in known ways on geometry, Reynolds number, and fluid properties, and x represents a mole fraction of species i in the bulk gas (subscript b) and at the tube surface (subscript s). The second term on the right side of the equation represents convective transport to the surface. Chemical reactions, such as sulfation of alkali species and combustion of residual carbon in the ash, are similar to condensation in their mathematical treatment (H). Both condensation and chemical reactions are strongly temperature dependent and give rise to spatial variation in ash deposit composition. Chemical reaction terms are not fully incorporated in the model and were not used in the predictions discussed later in this report. 7 |
