Soret Effects in Combustion For Power Production or Chemical Synthesis DANIEL E. ROSNER* Chemical Engineering Department , Yale University, New Haven CT USA ABSTRACT For the past 25 years I have been fascinated by the phenomenon of thermally-driven mass transport and its engineering consequences----especially in combustion systems. By about 1980(1) I had already become rather suspicious of the many 'footnotes' I had seen, especially in ChE textbooks, about the supposed dominance of Fickian (Brownian) mass transport and the negligibility of Soret(Thermophoretic)mass transport. Moreover, in my consulting and universitybased research I began encountering dramatic examples in which Soret transport and/or its particulate analogue: 'thermophoresis', were far from 'higher-order' effects [reaching 3.5 decades in some of our experiments on micron-size particle deposition from flames(4)]! The present talk will survey some of these applications, emphasizing our development of rational engineering methods to predict/ correlate such effects(1-8). The applications considered here fall conveniently into two general classes. In the first class are Soret effects in combustion systems for power production or chemical propulsion(2,3). More recently I have been concerned with combustion for the purpose of materials synthesis or processing----a still emerging area of combustion in which one necessarily makes use of a wider variety of 'fuels' and 'oxidizers'(8,1012). In most combustion examples we find the simultaneous conditions which ensure significant Soret effects---viz., the co-existence of large temperature gradients (often 10^6 K/m) with large molecular weight disparities. Moreover, even when the fuel itself is a simple hydrocarbon vapor like CH4(g), higher C-number intermediates or reaction products are often produced locally-----a dramatic example being multi-ring molecules (PAHs) and 'soot' nano-spherule formation in fuelrich regions of pre-mixed- or (especially) diffusion-flames(7). When the fuel itself is light (H2) or heavy (>C12H24, as in diesel fuel vapors), then even such a fundamental property as the adiabatic diffusion flame temperature is appreciably Soret-shifted (7,9), often with important implications for NOx production and radiative heat transfer. Several recent studies of gaseous diffusion flames did explicitly consider the consequences of the fuel vapor Fick diffusivity being significantly different from the local gas thermal diffusivity (ie., non-unity 'Lewis number') but ignored an important implicit consequence of ideal gas kinetic theory---viz., that appreciable fuel vapor Soret effects and non-unity Lewis number effects will necessarily 'go-together'(3,7). These and related fundamental developments will be discussed in the context of the abovementioned combustion engineering applications(3, 10-12). ____________________________________ L.W. Jones Prof. Chemical Engineering; Director Yale Univ. High Temperature Chemical Reaction Engineering Laboratory; Revised/Submitted 12/21/05 for Int. Mtg. on Thermodiffusion #7, San Sebastian, Spain , May 29-June 3, 2006(1) Rosner, D.E., Physicochemical Hydrodynamics (Pergamon), vol.1, 159-185 (1980) (2) Gokoglu, S.A and Rosner, D.E., Int. J. Heat Mass Transfer, vol 27, 639-645(1984); see, also: Rosner, D.E. and Fernandez de la Mora, ASME,J Engrg. for Power, vol 104, 885-894(October 1982) (3) Rosner, D.E., Transport Processes in Chemically Reacting Flow Systems, DOVER Publications, New York(2000); See Supplement 3 (4) Rosner, D.E. and Kim, S.S., Chemical Engineering J. (Elsevier) vol 29(3) 147-157 (1984) (5) Park, H.M. and Rosner, D.E., Chem. Eng. Sci. , vol 43(10)2689-2704(1988) (6) Garcia-Ybarra, P. and Rosner, D.E., AIChE J., vol. 35(1) 139-147(1989) (7) Rosner, D.E., Israel, R.A. and La Mantia, B., Comb. & Flame, vol. 123, 547-560(2000) (8) Rosner , D.E. and Pyykonen, J.J., AIChE J , vol 48(3) 476-491(2002) (9).Rosner, D.E., Arias-Zugasti, M., and La Mantia, B., AIChE J, vol 51(10), 2811-2824 (2005); see, also: Comb.& Flame, vol 135, 271-284(2003) (10) Rosner, D.E., Ind. & Eng. Chem.-Res(ACS), vol 44(8); pp 6045-6055 (2005) (11) Rosner, D.E., Chemical Engineering Education (ASEE), Fall 1997, pp 228-235; see, also: Winter '98 issue (12) Rosner, D.E. , Combustion For The Synthesis or Processing of Valuable Materials, (Treatise-Text in prep.(2005))