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National Key Research and Development Plan of China (No. 2016YFE0127500).

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Shoujun Ren
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The oxygen-deficient combustion characteristics of methane in a localized stratified vortex-tube combustor (LSVC) are studied by diluting combustion air with nitrogen. The influences of oxygen mole fraction (0.13 ~ 0.21) on flame configuration, combustion stability, combustion efficiency, and NOx emission characteristics are experimental investigated at the inlet temperature of 300 K. Combined with the numerical simulation method, the NOx generation, and emission mechanisms are analyzed in this combustor. Results show that the LSVC can achieve a wide stability limit, in which the global equivalence ratio can be as low as 0.22 at the lowest oxygen mole fraction (β) of 0.13. To ensure high combustion efficiency, the β should be kept above 0.16 since the oxygen-deficient condition reduces the reaction rate and flame temperature. The combustor can achieve ultra-low NOx emission of below 10 ppm (@ 15 vol.% O2) due to low oxygen concentration and flame temperature. Furthermore, part of NOx entrained into the fuel-rich reduction zone by the swirl flow field is reduced by the reductive species (i.e., CO and H2) to further lowering NOx emissions. The results of this paper can guide the development of the LSVC in the high-efficiency and low-emission combustion fields.
Shoujun Ren
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The combustion of AC 10 H 20 droplets was investigated to explore the combustion performance of liquid fuels in localized stratified vortex-tube combustor (LSVC). The evaporation ratio, flame structure, stability limit, heat loss, and combustion efficiency in the LSVC were investigated under various equivalence ratios and fuel mass fluxes numerically. Results corroborate that the LSVC exhibits a large heat release with uniform flame front, large stability limit, low heat loss, high evaporation rate, and good combustion efficiency under lean operating conditions, indicating good potential to deal with liquid fuels directly. Then, the evaporation and stabilization mechanisms are analyzed. As for the former, the evaporation ratio in LSVC increases sharply along the axial direction toward the outlet, indicating a high evaporation rate, which is optimized through the heat produced by itself efficiently. Viz., the vortex currents can entrain the AC 10 H 20 droplets to interior high-temperature region and then promote the evaporation of liquid fuels. As for the latter, the localized stratified distribution of species in the LSVC results in an edge flame structure, which differs from that in the traditional vortex-tube combustors. The local equivalence ratio increases along the radial direction toward the center. The increased local equivalence ratio of the interior is crucial for stabilization and the decreased local equivalence ratio of exterior enables the heat loss to be reduced. In the end, the edge flame structure and the low heat loss yields a large heat release in the LSVC, which can increase the flame speed, thereby ensuring the stabilization and the high burn-off rate. ARTICLE HISTORY
Shoujun Ren
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A novel vortex-tube combustor with axial fuel injection for NOx reduction was proposed to overcome the instability of ultra-lean combustion. The stability limit, flame configuration, NOx and CO emissions, and flame structure were investigated experimentally under various global equivalence ratios and fuel flow rates. The mechanisms of NOx emission reduction were analyzed by numerical simulation. Results show that the limit of global equivalence ratio can be as low as 0.01 and the amplitude of pressure fluctuation is always less than 1300 Pa, indicating fairly good performance in combustion stabilization of the combustor. In the operation range, there is a trade-off region with low NOx and CO simultaneously. The diffusion-like flame structure in this combustor can enhance the local equivalence ratio, whilst the flow field structure can also promote the transport of chemical enthalpy to the flame front, thus facilitating the stabilization. The enhanced stabilization enables the ultra-lean combustion and the ensued small area of the high-temperature zone to conduct, as well as the low NOx formation through reducing thermal NO. The local fuel-rich region and the flow field structure can promote the NOx to be reduced further via the Fenimore mechanism.
Shoujun Ren
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In order to overcome the instability of ultra-lean combustion, a novel vortex-tube combustor with axial fuel injection was proposed. The stability limit, flame configuration, NOx emissions, and burn-off rate of the combustor were investigated experimentally under various global equivalence ratios (φg) and fuel flow rates (qf). Results show that the combustor exhibits a large stability limit with φg and qf as low as 0.01 and 6×10⁻⁵ m³/s respectively. Complete combustion and ultra-low NOx emissions of less than 3 ppm can be achieved at φg of 0.3 and qf of more than 30 ×10⁻⁵ m³/s, indicating that the combustor has a good potential for ultra-lean combustion and low NOx emission. The pressure fluctuation amplitude is always less than 1300 Pa during the entire experiments. The vortex-induced flame has a diffusion-like flame structure, which provides a suitable equivalence ratio zone under ultra-lean conditions, whilst the high peak combustion temperature indicates an intensified combustion, which is responsible for the large stability limit and low-pressure fluctuation amplitude. Subsequently, the enhanced stabilization can enable the ultra-lean combustion and the ensued low temperature to conduct, whilst the vortex-flow can decrease the local flow velocity and enable the turbulent diffusion velocity of NO to be dominant, which can make the NO emission reduced further. The decreased area of high-temperature region and the appearance of the negative reaction rate region of NO is the essential reason for the decrease of the NO emission at lean cases.
Shoujun Ren
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The stabilization performance and mechanisms in a diffusion‐like vortex‐tube combustor is investigated for oxygen‐enriched combustion. The stability limit, flame configuration, and pressure fluctuation are investigated under various conditions. Results show that a diffusion‐like flame structure is established in the combustor and the nonpremixed peculiarity becomes more prominent with the increase of oxygen mole fraction. The steady combustion can be achieved in the range of global equivalence ratio 0.01 to 1.0 with a low‐pressure fluctuation amplitude always less than 1300 Pa, indicating a good combustion stability of this combustor. Additionally, the stabilization mechanism is discussed from the time matching and velocity matching. Based on the axial fuel entry method, the Damköhler number (Da) is always less than 1.0 as a whole, which is the principal reason for the tubular flame shape and the steady combustion procedure in this vortex‐tube combustor. The intensified combustion under oxygen‐enriched combustion can increase the flame speed, and subsequently reduce the mixing quality and make the yellow flame more visible. Besides, the temperature distribution and the flow field structure can explain the corrugation and deformation of the flame front under oxygen‐enriched conditions.
Shoujun Ren
added 2 research items
The stabilization performance and mechanisms in a diffusion-like vortex-tube combustor is investigated for oxygen-enriched combustion. The stability limit, flame configuration, and pressure fluctuation are investigated under various conditions. Results show that a diffusion-like flame structure is established in the combustor and the non-premixed peculiarity becomes more prominent with the increase of oxygen mole fraction. The steady combustion can be achieved in the range of global equivalence ratio 0.01 ~ 1.0 with a low-pressure fluctuation amplitude always less than 1,300 Pa, indicating a good combustion stability of this combustor. Additionally, the stabilization mechanism is discussed from the time matching and velocity matching. Based on the axial fuel entry method, the Damköhler number (Da) is always less than 1.0 as a whole, which is the principal reason for the tubular flame shape and the steady combustion procedure in this vortex-tube combustor. The intensified combustion under oxygen-eriched combustion can increase the flame speed, and subsequently reduce the mixing quality and make the yellow flame more visible. Besides, the temperature distribution and the flow field structure can explain the corrugation and deformation of the flame front under oxygen-enriched conditions.
The operating limit and pressure fluctuations of a localized stratified vortex-tube combustor (LSVC) are investigated experimentally by taking methane as fuel at the inlet temperature of 300 K and the combustor pressure of 1 atm over a range of equivalence ratios (0.1-1.0). The combustion modes are distinguished according to the properties of pressure fluctuations and the driving mechanisms of each combustion mode are explored in combination with numerical simulation. The results show that the LSVC can realize stable combustion with pressure fluctuation amplitudes always less than 4 kPa in a large operating range, and the flame front is always continuous. The operating flammability limit experimental range can be divided into five regimes with different combustion modes. The first and fifth combustion modes are steady combustion with pressure fluctuation amplitudes less than 1 kPa, and the second to fourth ones are quasi-steady combustion with pressure fluctuation amplitudes between 1 ~ 4 kPa. The spectrum analyses of pressure fluctuations show that there are one low-frequency peak around 300 Hz and one high-frequency peak around 1500 Hz, which are dominated by the first and third axial natural acoustic modes of resonant oscillation combustion, respectively. In the first and fifth combustion modes, the resonances are both weak due to the influences of low flow rate and laminarization respectively. In the second mode, the thermo-acoustic coupling oscillation and the resonance are excited simultaneously, yielding the highest pressure fluctuation amplitude in the entire operating range. The high-frequency resonance causes the high-frequency pressure fluctuation of the third mode. Both the unsteady heat release and flow field affect the pressure fluctuation in the fourth mode. The former produces the low-frequency fluctuation, which can resonate with the natural frequency and excite a weak thermal-acoustic coupling.
Shoujun Ren
added 2 research items
Experimental and numerical (via ANSYS FLUENT) studies have been conducted on the combustion stability and stabilization mechanisms in a localized stratified vortex-tube combustor (LSVC) under lean conditions. The stability limit and flame configuration were obtained under different combustion conditions. Combined with the flow field distribution, the formation mechanisms of the local stratification of species and the resultant flame configuration were analyzed. Results show that the local stratification peculiarity is responsible for the dual flame appearance. On the basis of the local stratification of species, the local equivalence ratio is close to stoichiometry in the vicinity of the flame front, while it is above 1.0 in the interior, enabling the achievement of stable combustion at a global equivalence ratio as low as 0.12 in the LSVC. The flow field can help the transport of the reactive species and yields an intensified combustion and a large density gradient. The peak heat release rate of 0.5 W/mm3 in the LSVC is much higher than that of 0.1 W/mm3 in the rapidly mixed vortex-tube combustor (RMVC) at the global equivalence ratio of 0.6 and the maximum tangential velocity of 26.44 m/s. The flame-vortex interaction theory provides a new perspective to interpret the rapid flame propagation in vortex-tube combustors. Based on the pressure jump theory, the flame speed was obtained via a specific formula closely related to the density gradient and the injection velocity. It turns out that the flame speed in the LSVC is remarkably higher than that in the RMVC at a certain same combustion condition. Moreover, the decrease of local flow velocity resulted from the strong swirl provides a favorable guarantee for the balance with the local flame speed.
The combustion characteristics of the localized stratified swirling tubular flame burner (LSSTFB) were investigated and compared with that in the rapidly mixed burner under lean conditions experimentally and numerically. In order to clarify the stabilization mechanisms of tubular flame in LSSTFB, the appearance of flame, species distribution, stability limit, heat release rate, and the flow field structure were investigated. Results show that the tubular flame stability limit of LSSTFB is much greater than that of the rapidly mixed burner. A specific distribution of species that the unburnt species distribute two sides of the reaction zone is formed in LSSTFB. It leads to the tubular flame with a binary structure processes both premixed and non-premixed flame characteristics. The inner recirculation in LSSTFB produces a lower local flow velocity and preheats the incomplete combustion gases, thereby increasing the balance between the flow field velocity and the flame propagation speed. Thus, in the localized stratified swirling tubular flame burner, there is a wider local equivalence ratio and flow velocity range suitable for flame stabilization.
Shoujun Ren
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Intergovernmental International Scientific and Technological Innovation Cooperation
 
Shoujun Ren
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The combustion characteristics in a localized stratified tubular flame burner (LSTFB) were experimentally studied under lean conditions. The stability limit and pressure fluctuation were obtained under various global equivalence ratios and fuel flow rates. The mechanisms driving the combustion instability were analyzed with the combination of numerical simulation. Results show the lean stability limit of the burner can be as low as 0.12 of the global equivalence ratio. On the basis of the local stratification of species, a kind of unique binary tubular flame is formed with both the premixed and non-premixed flame properties. Meanwhile, the internal recirculation heats the incomplete combustion gases distributed inside the tubular flame, thereby guaranteeing the balance between the flow velocity and the local flame speed. The amplitudes of pressure fluctuation are less than 4 kPa in the entire experimental range because of the flow laminarization caused by the large body force and density gradient. The pressure fluctuation characteristics depend on the relative magnitude of viscous force, inertial force, body force, and density gradient. The Taylor number Ta and stratification parameters Ri* are key parameters in evaluating the flame stability in the LSTFB.