Experimental Studies of Nucleation Phenomena Within Thermal Boundary Layers—influence on Chemical Vapor Deposition Rate Processes

Abstract

In carrying out partial vapor condensations using actively cooled surfaces it is known that 'mist' formation can occur within thermal boundary layers (Rosner and Epstein, 1968), dramatically modifying total deposition fluxes. Using a combination of flash-evaporation (Rosner and Liang, 1986) and laser probing techniques, we report new experimental results on binary alkali salt (K 2SO. + Na,SO.) deposition from combustion gases showing that the deposition rate of potassium sulfate first increases with the addition of sodium sulfate until the concentration of Na2S0. reaches a (target surface temperature dependent) 'threshold' value. Further increases in the concentration of Na2S0. dramatically decrease the total deposition rate of K 2SO .., implying that potassium sulfate-containing microdroplets are formed within the thermal boundary layer, which, despite their thermophoretic drilt toward the target, are not collected as effectively as the 'parent' K 2SO.-vapor species. Laser light scattering measurements clearly reveal that suspended particles exist near the deposition surface under these conditions, OUf experimental results on mass transfer rate and light scattering are consistent with those predicted using laminar boundar)' layer theory (Castillo and Rosner, 1989b) coupling both binary salt vapor deposition with particle vapor scavenging and deposition. Comparisons of our observed mist onset conditions (implying critical supersaturations near unity) with those expected using homogeneous nucleation theory suggest that the binary alkali sulfate mist nucleation mechanism is, instead, heterogeneous, even in our relatively 'clean' combustion products. Because of the well-known vapor pressure reduction phenomenon associated here with the formation of non-ideal solutions, binary systems are shown to provide convenient 'vehicles' to investigate BL mist formatio n onset conditions and CVD-rate consequences without requiring the more extreme surface coolings characteristic of unary condensible vapor systems. An understanding of this dramatic phenomenon, obtained via such laboratory experiments and calculations, will allow its inclusion in future deposition rate calculations of engineering importance.

Laboratory studies of binary salt CVD in combustion gas environments

“…Such nucleation would reduce the net potassium sulfate deposition rate due to the smaller capture efficiency for the newly formed particles. Based on this physical picture, confirmed by recent experiments in which we detected the light scattered by the mist particles (Liang et al, 1987), a theoretical model has also been developed to quantitatively predict deposition processes in binary systems beyond the threshold for vapor phase condensation (Castillo and Rosner, 1987d). Deposition under such conditions is the subject of a separate paper (Liang et al, 1987).…”

“…This is because the effective vapor pressure approach is equivalent to one based on chemical element concentrations, and since chemical elements are conserved in ordinary chemical reactions, transport laws cast in terms of local element concentrations (irrespective of the species distribution) become insensitive to the vapor phase chemistry . This is especially true when all species that transport each element have comparable Fick (and Soret) diffusivities, as is nearly the case in these alkali sulfate salt-containing combustion product gas examples, provided mist formation does not occur (Liang et al, 1987). Predicted Results and Comparison with Experimental Observations.…”

“…The nebulizer itself consists of a par- Laser shown used to probe BL (Rosner and Liang, 1987) 1938 December 1987 Vol. 33, No.…”

“…Negligible phase change (mist formation) occurs within the vapor diffusion boundary layer; that is, phase change takes place only a t the macroscopic vapor/condensate interface (station w ) . [The subject of boundary layer phase change processes, introduced previously, is beyond the scope of this paper; it is dealt with by Castillo and Rosner (1987a+) and Liang et al, (1987).] Thus, we can symbolize the deposition process as:…”

“…The final section offers important conclusions concerning binary alkali sulfate solution nonideality and vapor phase decomposition/chemical reactions. Results under more complex conditions (including binary vapor condensation within the boundary layer) and a corresponding theoretical analysis are presented in separate papers (Liang, 1987;Liang et al, 1987; Castillo and Rosner, 1986a-d). …”