The aim of this study is to understand the thermal behaviour at the inlet of a hydrothermal oxidation reactor. This reactor was designed to eliminate charged waste water and operating in turbulent flow to limit corrosion at the injection points. Three numerical modelling were carried out to simulate the thermal behaviour of the reactor. The first program simulates the reactor operating in turbulent flow. The second one takes into account the problem of injection and the last one simulates a laminar flow with injection. Only, the program in laminar flow gives a good estimation of the thermal behaviour in the first meters of the reactor, in spite of the calculated Reynolds number which is higher than the critical one. However, in these operating conditions the flow has to be in a intermediate state. It seems that viscosity is larger than the one of pure water and the flow is laminar. (c) 2007 Elsevier B.V. All fights reserved.