# Numerical Investigations of Micro-Scale Diffusion Combustion: A Brief Review
## 3 Numerical Studies of Micro-Diffusion Flames
The flame shape and structure were also found to behave differently at fuel flows with high, intermediate and low velocities. The impacts of thermal interactions between flames and the solid walls were different depending on the fuel velocity. For example, for higher fuel velocities, the external surface of the tube had a positive effect on the flame stability, while this effect was negative at lower velocities as the walls absorbed heat from the flame. Figure 12 compares these impacts for different fuel flow velocities. Numerical simulation methodology that was adopted in this work was similar to that of [PERSON] and [PERSON] [22], with the addition of the heat interactions between the fluid and solid by coupling the boundary conditions. [PERSON] et al. took advantage of the latest advances in numerical investigation of micro-combustion devices and conducted an interactive experimental and numerical study to optimize a micro-mix hydrogen combustion in a test burner and achieve minimum NOx emissions [24]. In this work, authors studied the implications of the turbulent flow field on the NOx emissions by applying different combustion models and characterizing the turbulent micro-mix flame structures. In order to reduce the NOx emissions, they used a cross-flow mixing of air and hydrogen, where gaseous hydrogen is injected perpendicularly into an air cross-flow and reacts in multiple miniaturized diffusion-type flames. The simulations were performed by the commercial CFD tool STAR-CCM+ [25]. In their numerical simulation approach, they used the k-epsilon turbulent model to predict the flow field. To characterize the hydrogen combustion process, they used and compared the following models: (1)  
Figure 1:4 Experimental and numerical study of the thermal interaction with solid micro tube in non-premixed hydrogen jet flames by [PERSON] et al. [23]. **Left:** OH-PLIF images of hydrogen micro-jet flames at different fuel flow velocities: (**a**) 10 m/s; (**b**) 5 m/s; (**c**) 2.5 m/s; (**d**) 1 m/s; (**e**) 0.35 m/s; (**f**) 0.17 m/s; **Right:** Computational OH mole fractions of hydrogen micro-jet flames at different fuel flow velocities: (**a**) 2.5 m/s; (**b**) 1 m/s; (**c**) 0.35 m/s; (**d**) 0.17 m/s. EDM (Eddy Dissipation) model, (2) ED-FR (Eddy Dissipation-Finite Rate) model, and (3) EDC (Eddy Dissipation Concept) model. The calculation of reaction rates and species consumption/formation rates were obtained by implementing a detailed hydrogen reaction mechanism including 19 reversible reactions and 9 species. In the wall treatment of the RANS model, the local dimensionless wall distance, which depends on the wall distance, the friction velocity and the kinematic viscosity was utilized. The geometric model to perform the numerical simulations was derived from the burner configuration APU 030 ED 6.7 [26]. Comparisons between the findings from these three models and the experimental results illustrated qualitative agreement with the experimental findings (see Figure 13); however, the predictions from the EDC model that was closest to the measured values, had 21.5% error. In general, the numerical simulations were able to capture the micro-mix flame anchoring and structure for the operation parameters considered in this work. However, further investigations will be necessary to increase the precision of the numerical results involving diffusion flames, especially at micro scales. Most recently, [PERSON] et al. investigated the combustion characteristics of micro-jet diffusion flames with a mixture of hydrogen and methane fuels and examined the stabilization mechanisms for different fuel mixture compositions [27]. They reported that flames with hydrogen as the base fuel would always attach to the burner, as opposed to flames with methane as the base fuel. Comparing the dominant intermediate-consumption steps in both methane and hydrogen, those of hydrogen flame were found to have significantly lower activation energies.