Direct Numerical Simulations Of Supercritical Hydrothermal Combustion

Supercritical hydrothermal combustion refers to the fast oxidation process with luminous flames that occur in supercritical water.This combustion technology can achieve high conversion efficiencies for the oxidation of many organics in very short residence time,and is mainly applied to the organic waste water treatment,clean conversion and utilization of coal and biomass fuel,upgradation of heavy oil,etc.Due to the high pressure and corrosive environments,it is very challenging to carry out relevant experimental researches.Previous experimental studies mainly focus on the phenomenological demonstrations of the combustion characteristics,such as ignition temperature,conversion efficiencies,etc.For this unconventional flame,fundamental studies on the microscopic combustion mechanisms and on the differences with conventional flames,are very rare.Therefore,in the present work,a direct numerical simulation platform,which is applicable to supercritical hydrothermal combustion and with the capability of massively parallel computing,is developed.Based on this simulation platform,a series of high-fidelity numerical studies have been conducted to systematically investigate the fundamental combustion characteristics,including auto-ignition features,flame structure,flame/turbulence interaction,etc.,leading to a more comprehensive understanding on this unconventional flame.A series of 0D homogeneous auto-ignition calculations are conducted to fundamentally investigate the auto-ignition characteristics of supercritical hydrothermal combustion.Results indicate that the correlation of the ignition delay time with the mixture fraction in the current study is quite different from those reported in canonical conditions.In the fuel-rich side,both the ignition delay time and the strength of heat release decrease steeply with increasing mixture fraction.There is no well-defined“most reactive”mixture fraction.Then,a series of controlled simulations are conducted to investigate the effects of real-fluid properties,reaction mechanism and the chemical participation of H₂O.Parametric studies regarding fuel concentration and oxidant temperature are also conducted to contribute to the series of zero-dimensional study.A series of 2D laminar simulations on the supercritical hydrothermal flames are conducted.A compact bi-branchial flame structure is observed,which lacks a rich premixed branch in contrast with the classic triple flame structure.Combining a detailed investigation on the flame structure and zero-dimensional ignition characteristics,the reason for the flame structure is explained.Transport budget analyses are conducted on these flames.Results show that auto-ignition is the dominant flame stabilization mechanism.With the increasing of fuel concentration,the contribution of flame propagation to the flame stabilization increases.A 2D DNS of turbulent non-premixed hydrothermal flame is conducted to investigate the ignition process,including the forming and evolution of ignition kernels,flame propagation,etc.Statistical analyses suggest that ignition kernels mainly form in a certain range of mixture fraction,which corresponds to the peak location of 0D heat release rate and the valley location of 0D ignition delay time,as described in the 0D auto-ignition calculations.Finally,a DNS of 3D jet flame with Re=5276 is conducted.The DNS database is estabilished.The flame structure,combustion mode,species transport process and flame stabilization mechanism are systematically investigated based on this DNS.Results indicate that,in contrast with the laminar flames,ignition kernels occur in much more fuel-lean side,due to the effects of shear layer and strong turbulence.