CFD as applied to high temperature air combustion in industrial furnaces
Authors: Weihong Yang, Blasiak Wlodzimierz
Combustion in a high temperature and oxygen deficient atmosphere shows different characteristics compared to combustion in a normal atmosphere. This is known as high temperature air combustion or HiTAC. The existing mathematical models have proven not to be suitable for simulation of HiTAC. It is a challenge of numerical simulation to be able to reflect the characteristics of HiTAC. The objective of this study is to develop and experimentally verify a mathematical model. We also expect to develop some parameters to classify the characteristics of HiTAC, which are different from normal combustion. In this work, the available mathematical models were investigated and developed. The numerical simulations undertaken here include numerical calculation of a single fuel jet in HiTAC conditions (including both cross-flow and co-flow of the fuel jet and air flow) and the modelling of a HiTAC test furnace with two different High Cycle Regenerative Systems (one flame and two flame systems).
The results show that the combustion model used for simulation of HiTAC must be capable of expressing precise reaction rates in a high-temperature and low oxygen partial pressure atmosphere. Concepts including the oxidation mixture ratio, furnace-gas-temperature-uniformity-ratio, the furnace flame occupation coefficient and the flame entrainment ratio were defined to describe the characteristics of HiTAC, which provides help for optimal design of a HiTAC furnace and burner. Additionally, the benefits of HiTAC technology are quantitatively demonstrated by mathematical models. These benefits are: lower peak temperature, larger flame volume, more uniform thermal field, lower local firing rate, higher heat transfer, higher energy utilizing efficiency and lower combustion noise. The operating parameters, including the oxygen concentration and the temperature of the preheated combustion air, the fuel temperature, the fuel flow rate, the excess air ratio and flame locations are shown to have stronger influences on combustion and NO emission in the HiTAC furnace. The optimum combination of these parameters should be considered. NO emissions formed by N2O-intermediate mechanism are very important during HiTAC operation. The approximate percentage of NO production by nitrous oxide according to the Zeldovich and prompt mechanism varies from 5:95 at 10% oxygen concentration to 95:5 at 5% oxygen concentration. The critical diameter and the length of the furnace fitted with HiTAC technology are proposed for an optimum design for HiTAC operation.
The numerical simulation results and are very encouraging and can be used as an analytical or a design tool of an industrial furnace.
Download PDFYang, W. and Wlodzimierz, B. (2006) CFD as applied to high temperature air combustion in industrial furnaces.