Experimental data from a tubular reactor and from a transpiring wall reactor (TWR) are used to analyze the scaling up of SCWO reactors operating with a hydrothermal flame as a heat source. Results obtained with the tubular reactor show that fluid velocity inside the reactor determines the minimum injection temperature at which a stable hydrothermal flame is formed. When the fluid velocity inside of the reactor is lower, the extinction temperature of the hydrothermal flame in that reactor is also lower. Using this reactor, extinction temperatures are always near or above the critical temperature of water. Total TOC removals are possible working with isopropyl-alcohol at temperatures between 650 and 700 degrees C and residence times of 0.5 s. Results of the TWR show that steady operation with a hydrothermal flame inside is possible even when reagents are injected at subcritical conditions as low as 170 degrees C. Temperature measurements show that reaction is not initiated in the injector but in the reaction chamber, where fluid velocity is lower than 0.1 s. This was explained by estimating that the flame front velocity of a hydrothermal flame is of the order of 0.1 m/s. Thus, it is expected that the flame is stabilized in the reaction chamber and not in the injector, where fluid velocities are higher than 2 m/s. A previously developed model of the TWR was modified in order to describe the ignition in the reaction chamber and not in the injector. The model reproduces satisfactorily experimental data and it was used to propose the design of scaled up reactors for SCWO with a hydrothermal flame inside. (C) 2010 Elsevier B.V. All rights reserved.