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Numerical Study of Isothermal Flow and Mixing Characteristics in a Cyclone-Jet Hybrid Combustor Cheol-Hong Hwang† and Gwon Hyun Ko*,‡ School of Mechanical Engineering, Inha UniVersity, 253, Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea and School of Mechanical Engineering, Chung-Ang UniVersity, 221, HeukSuk-dong, DongJak-gu, Seoul 156-756, South Korea
The isothermal flow and mixing characteristics of a cyclone-jet hybrid combustor using a combination of cyclone premixed and jet diffusion flames were numerically investigated. The computational fluid dynamics model was validated with experimental results for a cyclone-jet premixed combustor. Good agreement was obtained between the simulated and experimental results. As a result, in the diffusion combustion (DC) mode, the cyclone flow velocity played an important role on the development of swirling flow patterns, but the improvement of fuel-air mixing was limited due to low interaction between axial jet and cyclone flows. On the other hand, the fuel-air mixing characteristics in the hybrid combustion (HC) mode were significantly better than those for the DC mode. Through the modification of axial nozzle geometry, it was found that both the distributed fuel (DF) and multihole fuel (MF) nozzles provided improved fuel-air mixing compared to the single-fuel (SF) nozzle. Finally, it was confirmed with previous experimental results that the detailed understanding of complex flow structure and mixing phenomenon conducted in the present study was very useful for the advanced combustor design in terms of flame stability and pollutant emissions. 1. Introduction Premixed and diffusion combustion modes, which can be classified by the supply method of fuel and air introduced into the combustion chamber, have conflicting features in terms of flame stability and pollutant emissions. That is, a diffusion flame has the advantage of higher flame stability, while a premixed flame has clean combustion characteristics with lower NOx and soot emissions. Most industrial combustors have adopted the diffusion combustion mode because the flame stability has been favored as the most important design factor in a practical combustor. Recently, to meet the increasingly stringent emission regulations, application of the premixed combustion mode has been gradually increased with many advantages such as higher thermal efficiency, compact design of a combustor using shorter flame length, as well as lower pollutant emissions compared to the diffusion combustion mode.1 However, the premixed combustion mode has restricted flammability limits such as flash back and lifting limits in terms of flame instability. There are many flow techniques to enhance the stability of the premixed flame using cyclone flow motion, various swirl generators and pilot flames. In particular, the cyclone flow brings about a swirl flow generated by the tangential inlets of air and/ or fuel and thus improves the fuel-air mixing and the flame stability with recirculation of heat and active chemical species.2,3 Furthermore, the cyclone combustor has a much simpler geometry and easier adjustment of swirl intensity compared to a conventional swirl combustor using a complex swirl generator. Thus, the cyclone combustor has been used satisfactorily in various systems with clean and efficient burning.4-6 As one successful application for the cyclone premixed flame, Ishizuka and co-workers7-9 studied experimentally the tubular premixed flame which was formed by injecting the fresh mixture tangentially from slits on the tube wall into a long tube. They * To whom correspondence should be addressed. Tel.: +82-2-8110932. Fax: +82-2-813-3669. E-mail:
[email protected]. † Inha University. ‡ Chung-Ang University.
found that the tubular flame was thermally stable because of the symmetric structure of the thermal and chemical environments and aerodynamically stable because of appropriate distributions of acoustic pressure and heat release for damping the combustion instabilities. However, additional improvements of combustor design such as the slit length and/or slit width of tubular combustor were required to burn completely unburned gases at high velocities (above 50 m/s) due to the short residence time inside the combustor. As another approach for utilizing the cyclone flame, Onuma et al.10 developed a cyclone-jet combustor to meet the conflicting requirements of low pollution and high flame stability between premixed and diffusion modes. A premixed gas was supplied into nozzles installed tangentially on the sidewall of the combustor as a flame holder to stabilize the main jet diffusion flame. These authors reported that NOx emission could be nearly reduced to that of premixed combustion by an increase in mixing rate. This hybrid mode, utilizing simultaneously the combination of premixed and diffusion modes, provides the 2-fold advantages in terms of flame stability and pollutant emissions. First, since this combustor adopts the diffusion mode as a main flame, achievement of high flame stability can be possible. Second, the swirling premixed flames induced by the cyclone flow motion can further enhance the flame stability as well as the fuel-air mixing, and consequently, lower NOx and CO emissions can be achieved. Hwang et al.11 also studied experimentally the flame stability and pollutant emissions in the cyclonejet hybrid combustor. Through modifying the geometry of the fuel nozzle for the jet diffusion flame, it was known that an increase in fuel-air mixing using a multihole fuel nozzle provided a stable flame region approximately twice as wide as that of the fuel nozzle using a single hole. In addition, an optimized combustor using the hybrid mode (i.e., combination of premixed and diffusion mode) yielded a NOx reduction of 55% compared to that of the pure diffusion mode. These studies provide a potential of cyclone-jet hybrid combustor for efficient and clean burning. However, there has been little detailed
10.1021/ie901797c 2010 American Chemical Society Published on Web 01/26/2010
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Figure 2. Configurations of three types of axial fuel-air nozzles.11
Figure 1. Schematic of a cyclone-jet hybrid combustor and fuel-air supply methods for the diffusion combustion (DC) mode and the hybrid combustion (HC) mode.11
information regarding the phenomena on turbulent flow structure and fuel-air mixing characteristics inside the cyclone-jet hybrid combustor. The current study was motivated to enhance the understanding of flow and mixing characteristics inside the cyclone-jet hybrid combustor. To achieve this, numerical simulations were performed for isothermal turbulent flows. The quantitative characteristics of flow structure and mixing phenomena may be considerably affected by heat release in reacting flows.12 Unfortunately, turbulent combustion models have been not maturated yet for simulating simultaneously the premixed and diffusion flames. In addition, considering that similar behaviors of the recirculation zone were observed between the isothermal and the reacting flows,13 the understanding of flow structure and mixing characteristics for the isothermal turbulent flow will provide useful information on the optimized combustor geometry and operating conditions. The numerical model used was first validated by comparing the predicted velocity profiles in a cyclone-jet premixed combustor with published experimental data by Yamamoto et al.14 Then, detailed flow structure and fuel-air mixing characteristics were compared between the diffusion mode and the hybrid mode. The effects of the axial jet nozzle geometry on flow structure and fuel-air mixing were also discussed under an identical condition of a previous experimental study.11 2. Review of the Cyclone-Jet Hybrid Combustor Figure 1 shows the schematics of a cyclone-jet hybrid combustor and fuel-air supply methods for the diffusion combustion (DC) mode and the hybrid combustion (HC) mode.11 The inner diameter and height of the cyclone-jet hybrid combustor were 40 and 35 mm, respectively. The inner diameter of the combustor exit was reduced by 25 mm to promote the recirculating flow using a stagnation plane. A confined duct was installed at the combustor exit. In the cyclone-jet hybrid combustor, reactants were supplied from two directions: one was in the upward axial direction from the axial nozzle, located at the center of the combustor bottom, and the other was in the tangential direction to two nozzles, located near the bottom of the combustor. In the DC mode, fuel was supplied through the axial fuel nozzle while part of the air was supplied through the axial air nozzle and the rest of air was supplied through cyclone
tangential nozzles. Then the swirling motion was generated by supplying air flow from cyclone nozzles. On the other hand, in the HC mode, parts of fuel and air were supplied through axial fuel and air nozzles like in the DC mode but the rest of the fuel and air were supplied through cyclone nozzles by the premixed mixture. In the cyclone-jet HC mode, since the flame tip formed at each nozzle plays an important role as a stabilizer of the next flame through the supply of heat and radicals, swirling premixed flames are stabilized considerably with complementary cooperation. This swirling premixed flame also affects the stabilization of the jet diffusion flame. First, a recirculating flow induced by the swirling flame can stabilize the diffusion jet flame by accelerating the fuel-air mixing and preheating the reactants. Second, the premixed flame can act as a flame holder of the diffusion jet flame, and thus, the flame can be stabilized even over the normal range of the blowout limit of a jet diffusion flame. In particular, even though the lift-off phenomenon of the jet diffusion flame takes place in the vicinity of a fuel nozzle rim, the supplied fuel and air will be mixed in advance of burning and the premixed reactants may be ignited with the aid of the toroidal swirling premixed flames.15 Therefore, most of the reactant supplied separately by the diffusion flame mode can be burned with premixed (or partially premixed) flame characteristics in the cyclone-jet hybrid combustor. With respect to pollutant emissions, thus, the cyclone-jet hybrid combustor can be contributed to lower soot and NOx emissions compared to the pure diffusion combustor. In particular, adequately adjusting the cyclone flow velocity can achieve low NOx emissions by decreasing the flame temperature and the residence time due to the stretched flames16,17 as well as by increasing the turbulence intensity.18 The axial fuel-air nozzle is a very important design factor to achieve high flame stability and low pollutant emissions through an improvement in fuel-air mixing. In the previous study,11 three types of axial fuel nozzles, as shown in Figure 2, were tested in the HC mode to examine the effect of the local premixing of axial fuel-air on flame stability and pollutant emissions. Nozzle 1 included the fuel nozzle using a single hole (single-fuel type, SF), and nozzle 2 consisted of distributed fuel nozzles to realize the ideal local premixing of fuel-air (distributed fuel type, DF), as suggested by Onuma et al.10 Nozzle 3 included a multihole single-fuel nozzle and was designed for practical application (multihole fuel type, MF). An effective opening area of each fuel nozzle was fixed identically to that of the SF nozzle type with an inner diameter of 4 mm. Table 1 shows the detailed design conditions of the axial fuel-air nozzle for the three types tested. As a major result,
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Table 1. Design Conditions of the Axial Fuel-Air Nozzle fuel nozzle (hole) nozzle type nozzle 1 (single fuel, SF) nozzle 2 (distributed fuel, DF) nozzle 3 (multihole fuel, MF)a a
air nozzle
din (mm) area (mm2) din (mm) area (mm2) 4.00 1.79 1.63
12.57 12.57 12.57
20.0 20.0 20.0
294.52 113.10 263.90
Inlet diameter of the axial fuel nozzle for nozzle 3 is 6 mm.
the nozzle types of DF and MF yielded a much wider stable flame region and lower CO and NOx emissions compared to those of the SF nozzle type. In particular, the MF nozzle type showed the highest flame stability and the lowest pollutant emissions among three types of fuel nozzle. This result was inferred by increased interactions between the swirling premixed and jet diffusion flames. However, this estimation is not clear because there is insufficient data on flow structure and fuel-air mixing in the experimental approach. Therefore, the current study focuses on the flow structure and fuel-air mixing characteristics in the cyclone-jet hybrid combustor using a reliable numerical approach. 3. Description of Numerical Simulations To investigate the turbulent flow and fuel-air mixing in the cyclone-jet hybrid combustor, the commercial code Fluent 6.319 was used. The three-dimensional steady-state continuity, momentum, species, and gas-phase energy equations were discretized using the QUICK interpolation scheme, and the discretized equations were solved using the SIMPLE algorithm. The realizable k - ε model20 was used as the turbulence model because this model provided superior performance for complex flows involving rotation, boundary layers under strong adverse pressure gradients, separation, and recirculation compared to those of other k - ε versions such as the standard k - ε and the RNG k - ε models. For near-wall turbulence modeling, the enhanced wall treatment model which combined a two-layer model with enhanced wall functions was selected to improve the prediction of wall-bounded turbulent flows inside a combustor.19 The solution was considered converged results when the normalized residual of the energy equation was
