Numerical study of fuel temperature influence on single gas jet combustion in highly preheated and oxygen deficient air

Study of gas fuel temperature influence on combustion performance by means of numerical modeling is a main objective of this work. Combustion of a single jet of propane in a cross-flowing stream of preheated and oxygen deficient air is analyzed. Both Eddy-Break-Up and PDF/mixture fraction combustion models coupled with RNG k-ε turbulent model were applied and compared. Thermal and prompt NOx models were employed to calculate NO formation. Flame volume ratio and gas temperature uniformity ratio were used to describe the flame features. Results of numerical study show that Eddy-Break-Up model is more suitable for predicting of temperature field and flame shape. It was showed that flame during High Temperature Air Combustion (HiTAC) is spread over much larger volume than during conventional combustion. For temperature of the preheated and oxygen deficient air equal to (1041 K–1273 K) the flame volume was found almost constant at fixed oxygen concentration and fuel inlet temperature. Mean residence time of gas parcels inside the flame volume increases with reduction of oxygen concentration as well as with decrease of fuel inlet temperature and slightly increases with decrease of the preheated air temperature. Increase of the fuel inlet temperature results in smaller flame, shorter mean residence time, smaller temperature peaks and lower formation of NO. Keyword: gas combustion, fuel preheating, air preheating, numerical modeling Introduction High Temperature Air Combustion (HiTAC) applied to industrial furnaces offers energy savings, reduction of pollutants emissions and high quality of the product at increased production rate. Although this technology has been developed for more than 10 years and applied commercially in different types of furnace as reported by Yasuda et al. [1], the basic chemicalphysical phenomena is still needed to be better explained. Apart from a semi-industrial tests both the experimental and numerical methods were employed to study properties of single fuel jet behaviour during High Temperature Air Combustion. Analysing the literature of the subject it is well seen that study of a single fuel jet allowed explaining and measuring many unique features of the HiTAC. For example experimental studies of single fuel jet were performed by Hasegawa et al. [2], Gupta et al. [3], Bolz et al. [4], Kitagawa et al. [5], Lille et al. [6] and Blasiak,et al. [7]. Numerical studies focused on studies of the structure of flame and different mathematical models using well-known general-purpose codes as FLUENT [8,9] and CFX [10]. In all the cases a flame jet of propane or methane in a cross flow of preheated and diluted combustion air was subject of numerical modelling. Several modelling approaches were employed, for example, a) Eddy-Break-Up model with a two (or three) steps chemical equilibrium and b) PDF/mixture fraction model with equilibrium. The turbulent models employed were k-ε model, RNGkε , RSM and LES. These studies were performed for different oxygen content in the combustion air from 2% (vol. %) up to 21% as well as for various preheat air temperature. It was concluded that the latter combustion model is suitable for predicting of temperature and NOx emission. Advanced turbulent models, like large eddy simulation (LES) and RSM, give a small differences in the near field predicting of the flow, in contrary to the empirical constants as for example Cs in LES model which has significant influence on the predictions [8]. Main features of HiTAC flame (uniform temperature distribution, chemical species profiles, flow pattern and NOx emissions) were obtained. In all works the fuel gas inlet temperature was assumed to be equal to a room temperature. However in a furnace always the forepart of the fuel injection nozzle is surrounded by hot flue gas. Since the nozzle is not cooled the fuel temperature is elevated. Temperature of fuel jet as high as 500K was measured in the experimental single flame furnace, which is described, for example by Lille et al. [6] and Blasiak et al. [7]. Therefore, influence of the fuel elevated temperature on results of mathematical modelling should be verified. On the other hand, preheating of fuel gas can be of interest in new applications of HiTAC or when burning low calorific gas fuels. For these reasons, the numerical study of influence of the fuel gas temperature on combustion performance was undertaken. In this work, high temperature air combustion of a single propane jet at