The combustion characteristics of dimethyl ether (DME) in a partially premixed tubular flame burner have been investigated numerically. The species distribution, stability limit of tubular flame, wall temperature and burn-off rate have been studied at different equivalence ratios and inlet velocities. Meanwhile, the corresponding combustion characteristics in the rapidly mixed tubular flame burner have been simulated and compared. The results show that the partially premixed burner forms a unique species distribution and the fuel and the oxidant are not mixed uniformly. This unique species distribution will promote the chemical enthalpy supply to the flame front by recirculation reverse flow in the center, and thus strengthen the combustion process. Based on this unique species distribution, the partially premixed burner has larger tubular flame stability limit, lower heat loss, and higher burn-off rate, especially for lean operating conditions. Comparing the formation mechanisms of the tubular flames in two types of burners, it is found that the partially premixed burner forms partially premixed tubular flame, which is no need to create a uniform premixed gas zone, thereby suppressing the influence of velocity difference between fuel and oxidant present in the rapidly mixed burner.

A three-dimensional numerical simulation of a localized stratified tubular flame burner is carried out by using propane as fuel. The characteristics of the flow field, species distribution, flame structure, and stability limit are investigated. Results show that in the cold flow field, a specific distribution of species is formed due to the unique inlet structure and the effects of vortex breakdown, in which propane is mainly distributed near the wall, while oxygen is uniformly distributed in the burner. At the burning state, propane and oxygen are mixed at the inlet section of the burner and produce a stratified mixture from fuel lean to fuel rich, thus forming a flame that is similar to a triple flame, which plays an important role in promoting the combustion stability. Based on the localized stratification property and the effect of precessing vortex core, the main reaction takes place in the fuel-rich region (Lewis number <1 for C₃H₈ as fuel). Hence, the tubular flame easily forms and is hardly extinguished. In addition, the flow field promotes the chemical enthalpy supply to the reaction zone, thereby enhancing combustion. Essentially, this type of burner does not require a uniform premixed gas to establish the tubular flame which differs from the typical tubular flame burner. The stability limit of the localized stratified tubular flame is greatly improved compared with that of the rapidly mixed tubular flame, especially under lean conditions.

Experimental and computational results on the combustion performances and their formation mechanisms were presented and compared in two types of tubular flame burners by using methane as fuel. Results corroborate that the localized stratified tubular flame burner (LSSTFB) can achieve a large stability limit as a consequence of the edge-like flame structure, which is analogous to that of the edge flame. The edge-like flame structure in the LSSTFB is crucial for stabilizing the combustion and differs from that of the rapidly mixed burner. The global equivalence ratio can decrease to 0.18, which is considerably lower than that in the rapidly mixed burner. Additionally, the considerable stability limit can enable the occurrence of low temperature under ultra-lean conditions, whilst the edge-like flame structure can also reduce the NO_x emission through the Fenimore mechanism. On the premise of the ensuring of the burn-off rate, the NO_x emission in the LSSTFB is lower than that in the rapidly mixed burner.

The paper revealed the in-depth stabilization mechanisms of a novel vortex-tube combustion technique by using ethanol as fuel, which is implemented by a stratified vortex-tube combustor (SVC). The stability properties of the SVC are investigated, showing that the SVC has a wide stability limit and low-pressure fluctuation amplitudes with a uniform flame front. The equivalence ratio at the lean flammability limit is always below 0.2, and the amplitude of pressure fluctuation is less than 2000 Pa, indicating a highly steady combustion process. The non-premixed flame structure guarantees high mass concentrations near the reaction zone, while the vortex flow also decreases the local flow velocity, inhibiting flame blow-out, thus providing good self-adjusting capacity under various global equivalence ratios. The vortex–flame interaction transports the interior high-enthalpy burnt gas to the exterior unburnt gas region, thereby promoting ignition. The exterior unburnt gas is also transported to the flame front where it promotes reaction and yields an intensified combustion. The large tangential velocity and density gradient result in the large values of Richardson number, which suggests that laminarization of the flow occurs and results in good aero-dynamic and thermo-dynamic stabilities. The small values of the Rayleigh number indicate good flame-dynamic stability. Therefore, the resultant good self-adjusting capacity and three types of dynamic stabilities are the intrinsic causes of the ultra-steady combustion process in this combustor.