Effect of Diluents on Rapid Phase Transition of Water Induced by Combustion Almerinda Di Benedetto, Francesco Cammarota, Valeria Di Sarli, and Ernesto Salzano Istituto di Ricerche sulla Combustione, CNR, 80124 Napoli, Italy Gennaro Russo Dipt. di Ingegneria Chimica, Universita ` degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy DOI 10.1002/aic.12778 Published online November 7, 2011 in Wiley Online Library (wileyonlinelibrary.com). When exploding CH 4 /O 2 /N 2 mixtures with high oxygen contents in a nonadiabatic vessel, the pressure–time histories display oscillations of different frequencies and very high pressure peaks (hundreds of bars). We have attributed this anomalous behavior (combustion-induced rapid phase transition, cRPT) to the occurrence of cycles of condensation/ evaporation of water at the vessel walls, followed by superheating of the liquid film due to radiative heat transfer from the flame, which culminates in the water rapid phase transition. We now report a detailed analysis of the role played by the addition of a diluent (CO 2 ,N 2 , He, and Ar) on the anomalous behavior. The limit values of the diluent concentration at which the cRPT phenomenon is suppressed have been found and correlated to the kinematic viscosity and the thermal diffusivity through the Prandtl number. The less effective diluent has been found to be Ar followed by He, N 2 , and CO 2 in the order listed. V V C 2011 American Institute of Chemical Engineers AIChE J, 58: 2810–2819, 2012 Keywords: rapid phase transition, methane combustion, water explosion, diluent, Prandtl number Introduction Oxidation or partial oxidation reactions involving hydro- carbons and/or hydrogen are present in most of the chemical processes. Typical processes are syngas production (steam reforming and partial oxidation), ethylene and ethylene oxide production, combustion, and oxy combustion. Such processes all utilize air or oxygen as the oxidant. Nevertheless, oxidation is one of the most hazardous chemical processes, owing to its potential to undergo uncon- trolled combustion such as deflagration and detonation. Deflagration causes a fast increase of pressure up to the thermodynamic value (8 bar), eventually leading to the rupture of the reactor. Under certain conditions, combustion may transit to a det- onation mode giving rise to pressure higher than the thermo- dynamic value (40 bar). To avoid the dramatic consequences of deflagration and also detonation, a great effort is continuously devoted to the identification of ranges of operating conditions that lead to such combustion modes. Recently, we have discovered a combustion mode, which has been named combustion-induced rapid phase transition (cRPT). 1 More precisely, we found that CH 4 /O 2 /N 2 mixtures may exhibit exceptional and anomalous behavior, when exploding at extreme concentration conditions in a no adia- batic vessel. Explosion tests were performed in a 5-L vessel for CH 4 /O 2 /N 2 mixtures with stoichiometric CH 4 /O 2 ratio, changing the oxygen air enrichment factor, E ¼ O 2 /(O 2 þ N 2 ), from 0.21 (air) up to 1 (pure oxygen). We have found that at E  0.4, the pressure temporal trend starts oscillating, eventually culminating in very high peaks (up to 400 bar) largely exceeding the adiabatic values. We have attributed the oscillating behavior to the occurrence of cycles of con- densation and vaporization at the vessel walls of the water produced by combustion. Such cycles culminate in the superheating and then in the explosive vaporization (i.e., the rapid phase transition) of the water, with the formation of shock waves that lead to overadiabatic pressure peaks. Explosive vaporization occurs when a liquid reaches its boiling point in the absence of nucleation sites. 2 In such con- ditions, the boiling process may be delayed and the liquid starts superheating without boiling. When a limit temperature is reached (superheating temperature), homogeneous nuclea- tion occurs spontaneously and the liquid starts boiling in an explosive manner. Such explosion is a physical explosion that is termed rapid phase transition. 2–10 The consequent rapid production of high-pressure vapor exerts sudden pres- sure on the surrounding fluid, thus leading to the formation of strong shock waves. 11,12 Superheated liquid explosions have been observed in the concomitance to vessel ruptures, in nuclear or foundry accidents, and to volcano erup- tions. 11,13–16 The cRPT is a phenomenon localized at the vessel walls that spreads over the entire vessel, leading to strong over- pressures. Indeed, a significant temperature gradient is estab- lished across the boundary layer between the wall tempera- ture (10  C) and the bulk gas temperature (2500  C). This temperature gradient drives the condensation of the water Correspondence concerning this article should be addressed to A. Di Benedetto at dibenede@irc.cnr.it. V V C 2011 American Institute of Chemical Engineers 2810 AIChE Journal September 2012 Vol. 58, No. 9