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Effects of blade angle on combustion characteristics in a micro combustor with a swirler of micro fan type

  • Won Hyun Kim
  • Tae Seon ParkEmail author
Letter
  • 38 Downloads

Abstract

In a micro combustor, various vortices can enhance the wall heat transfer rate and get the best combustion. For an efficient combustion, the swirler of micro fan type is introduced for a micro combustor. In general, the swirling flows can change significantly the wall heat fluxes and this feature is sensitively varied by the fan configuration. So, to examine the geometric effect of fan swirler on the combustion characteristics, reacting flows depending on six blade angles are numerically investigated. The highest efficiency and heat loss related to vortical flows are observed for the specific blade angles.

Keywords

Micro combustor Blade angle Fan swirler Heat loss Combustion efficiency Swirl flow 

List of symbols

Ai

Surface area of combustion chamber, m2

D

Combustor diameter, m

Df

Fuel hole diameter, m

Do

Outer wall diameter, m

qi

Wall heat flux, W/m2

\(\dot{m}_{{CH_{4} }}^{inlet}\)

CH4 mass flow rate of inlet, kg/s

LHV

Lower heating value of methane, kJ/kg

T

Temperature

U

Streamwise velocity, m/s

Uc

Streamwise velocity at center axis, m/s

Z

Mixture fraction

Zst

Stoichiometric mixture fraction

νt

Kinematic viscosity

ϕG

Global equivalence ratio

θ

Blade angle, degree

1 Introduction

In the development of micro combustor, the combustor configuration is dependent on the device application. If the micro combustor is used for the energy conversion such as micro thermo-photovoltaic (TPV) system, the high wall heat flux should be obtained [1, 2]. But, if the purpose of the micro combustor is power generation, the design is tried to get the efficient combustion with reducing the wall heat flux. It is difficult to combine two design directions into a system, because each combustor is designed to maximize one kind of advantage.

Up to now, many efforts have been devoted to get an efficient micro combustor. The popular one is to make vortices induced by bluff bodies and geometrical variations in the micro combustor [3], because the vortical flows improve the combustion efficiency and stable flame structure. Such vortices can be generated by a fan swirler like the large-scale engine [4]. Despite the various usefulness, but such study has been scarcely attempted for the application to a micro combustor.

The vortices inside a micro combustor [4, 5, 6, 7, 8, 9, 10] is divided into two patterns near the combustor center and the chamber wall. In many combustors, the recirculations near the chamber wall contribute to the increase the wall heat transfer. Due to the flame anchoring effect, the high temperature zone is formed near the chamber wall. As a result, the heat loss related to the increased heat flux is increased. This is a bad factor for the micro combustor for energy generation. Kim and Park [9] detailedly discussed the heat transfer characteristics based on the variations of vortical flows in the micro combustor with a multi-hole baffle for MGT-SOFC (solid oxide fuel cell) hybrid system. In their combustor, the interaction between a center vortex and wall vortices creates a peculiar flame structure increasing the wall heat transfer. From the results of various conditions, they showed that the wall heat flux increases as the wavy pattern of flame structure is severe [10]. On the other hand, the center vortex is good to enhance the fuel–air mixing and prolong the time scales relevant to the chemical reaction and combustible mixture residence. It allows more efficient and stable combustion. According to Kim and Park [9], in the baffled micro combustor, large center vortex gives short flame length and high combustion efficiency, but the combustor displays the increased wall heat transfer. This is what needs to be improved for the best micro combustor.

In the present study, the compromise design of a micro combustor is explored by a fan swirler. So, a micro fan is introduced to make swirl flows. It is expected that the wall recirculations are reduced with a moderate center recirculation. That is, it takes the benefit of the baffled combustor and weakens the wall heat transfer. Figure 1 shows the schematic view of the present micro combustor. The expected patterns of recirculating flows are depicted in Fig. 1a. Unlike the multi-hole baffled combustor, the axial stream after the fan can reduce wall vortices. By this background, various fans are tested for the micro combustor and combustion characteristics are discussed.
Fig. 1

Schematic figures and computation domain

2 Numerical procedure

2.1 Governing equations

For three-dimensional incompressible turbulent flows, the governing equations of continuity, momentum, Reynolds stresses, energy, and species are adopted.
$$\frac{{\partial \left( {\rho U_{i} } \right)}}{{\partial x_{i} }} = 0$$
(1)
$$\frac{{\partial \left( {\rho U_{i} U_{j} } \right)}}{{\partial x_{j} }} = - \frac{\partial P}{{\partial x_{i} }} + \frac{\partial }{{\partial x_{j} }}\left( {\mu \frac{{\partial U_{i} }}{{\partial x_{j} }} - \rho \overline{{u_{i}^{'} u_{j}^{'} }} } \right)$$
(2)
$$\frac{\partial }{{\partial x_{k} }}\left( {\rho U_{k} \overline{{u_{{_{i} }}^{'} u_{{_{j} }}^{'} }} } \right) = D_{ij} + P_{ij} + \phi_{ij} + \varepsilon_{ij} + F_{ij}$$
(3)
$$\frac{{\partial \left( {\rho U_{j} h} \right)}}{{\partial x_{j} }} = \sum\limits_{j} {\left[ {\frac{\partial }{{\partial x_{i} }}\left( {\rho h_{j} D_{j,m} \frac{{\partial Y_{j} }}{{\partial x_{i} }}} \right)} \right]} + \frac{\partial }{{\partial x_{i} }}\left( {k_{f} \frac{\partial T}{{\partial x_{i} }}} \right)$$
(4)
$$\frac{\partial }{{\partial x_{j} }}\left( {U_{j} Y_{i} } \right) = \frac{\partial }{{\partial x_{j} }}\left( {D_{i,m} + \frac{{\nu_{t} }}{{Sc_{t} }}} \right)\nabla Y_{i}$$
(5)
where Ui, P, μ, ρ, \(\overline{{u_{i}^{'} u_{j}^{'} }}\), h, T, kf, hi, Dij, Pij, ϕij, εij, Yi, Di,m and Sct represent average velocity, pressure, viscosity, density and Reynolds stress, enthalpy, temperature, thermal conductivity, enthalpy of i species, diffusive term, production, pressure-strain, dissipation rate, mass fraction, diffusion coefficient of i species, and turbulent Schmidt number, respectively. To realize the methane (CH4)-air flame of a moderate global equivalence ratio condition (ϕG= 0.5), the Reynolds stress model and a detailed chemistry of GRI Mech-3.0 are used [11].

2.2 Computational domain and numerical method

The computational domain of present micro combustor and various fan swirler designs are plotted in Fig. 1b. To consider the effect of fan configurations, six blade angles (θ) are selected based on the preliminary study. A special flame is observed for θ ≈ 10°. So, the interval of 5° is selected for 0°–20° and the angle interval is set to be large for the other range. Uniform mass flow rates of fuel and air side are assigned to the inlets and the inlet temperatures are 300 K. The Reynolds number based on the fuel hole diameter (Df) is 3000. The outlet sets to a constant pressure. At the inlet, the turbulent kinetic energy is 1.5 (0.04Uc)2 and the Reynolds stresses are obtained from the isotropic relation. The conjugated heat transfer is considered for the fan swirler and combustor wall. The wall material is treated as the stainless steel [9, 10]. The assumption of the no-slip boundary and zero fluxes of species are adopted for the inner wall of the combustor. In particular, the convective and radiative heat transfer via the outer wall are used to measure the heat loss.

The SIMPLEC algorithm for pressure–velocity coupling and the second-order upwind scheme for convection terms are used [11]. All convergence criterions for the governing equations are set to be 10−6.

2.3 Validation and grid dependency

For validation of numerical method, due to no available data for a currently proposed combustor, the present results are compared with the experimental results of Al-Abdeli and Masri [12] for the bluff body swirl burner. Figure 2a shows the streamwise velocity profile along the centerline. As can be seen, the present result is in good agreement with the experimental data.
Fig. 2

Comparison of centerline velocity and temperature profiles

To check the grid dependence for the present micro combustor, three control volumes (CVs) of 300,000, 600,000, 900,000 are selected. Figure 2b shows the centerline temperature profile for fan blade angle of θ ≈ 10º. As the grid becomes dense, the temperature profile gets closer to a unique distribution. Based on such results, the final grid is set to the resolution of 900,000 CVs for further simulations.

3 Result and discussion

Figure 3a shows profiles of the centerline temperature and fuel mass fraction depending on the blade angle of the fan swirler. Depending on the blade angle, the variations of temperature and CH4 mass fraction are clear. With increasing blade angle, the rapid temperature rise is observed, but the temperature rise is delayed for θ > 15º. It is very interesting that the fastest temperature rise is shown when θ ≈ 10º. This informs that the fuel–air mixing by the fan swirler for θ ≤ 15º becomes more fast and intense than those of θ > 15º. For such conditions, the flame anchoring and active reaction zone are formed further upstream. This can be readily observed in Fig. 3b of the OH radical contours of x–y plane. The reaction zone and high heat release region with an intense OH radical move upstream and then downstream, as the blade angle increases. These changes in reaction zone and temperature profile are closely linked to the change of vortical flows.
Fig. 3

Centerline distributions of temperature and fuel mixture faction with OH radical contours

So, to explore the vortical structure strongly coupled to the thermal field, the flow structures and thermal field are analyzed together. Figure 4a shows the iso-surfaces of Z = Zst overlaid the temperature contours and streamlines for several blade angles. The present combustor has a feature of the combustor with swirling inlet. Accordingly, like other swirl combustor, the vortical flows are significantly modified by the blade angle. In the figure, the variations of fan configuration change the recirculations and the resulting flame structures are clearly varied. As the blade angle gets larger, the flame length decreases. The shortest flame is observed at θ ≈ 10º. After the angle, it increases again with a radially contracted flame. Considering the blade angle of the present fan swirler, the tangential velocity is maximized at θ ≈ 45º. This is well reflected on the variations of streamtraces, but the flame structure varies apart from it. To check this difference, the streamlines of x–y plane near the fan swirler are plotted in Fig. 4b. Since the tangential velocity of angular momentum is directly influenced by the blade angle, the creation of vortical flow structure affecting the fuel–air mixing is significantly varied by the fan blade angle. And, there is no wall vortices unlike the baffled combustor of Kim and Park [9, 10]. However, a pair of two vortices is generated in the center region near the fan swirler. The far vortex from the center is mainly driven by the air stream and the other vortex is induced by the fuel stream. So the interaction between vortices becomes significant in terms of fuel–air mixing for stoichiometric mixture. The present combustor has the higher momentum of fuel stream than that of air stream. The vortical structure plays a role in supplying the fuel to the air stream. As can be seen in Fig. 4a, for the case of two active vortices, the reacting flow is well developed in the center region. As the swirling flow strength gets stronger, i.e. the blade angle increases, two-vortex structure is weakened and the center vortex disappears at θ > 45º. Therefore, for θ = 45º, 75º, more incomplete mixing state of fuel–air is obtained and the flame is stretched to the streamwise direction. Also, this feature diminishes the flame temperature. From this result, we know that the fan swirler becomes an effective tool to change the recirculating flow patterns in the micro combustor.
Fig. 4

Iso-surface of Z = 0.055 overlapping the streamlines and temperature contours with streamlines of x–y plane

In a non-premixed combustion, even if the equivalence ratio of inlet is feeding to off-stoichiometric condition (ϕG< 1.0), the stoichiometric condition can be locally realized by the fuel–air mixing. That is, conditions of fuel-lean and fuel-rich are created regionally. So, this mixing state is measured by using the local equivalence ratio (ϕlocal) which is defined as ϕlocal= (ZZZst)/(ZstZstZ). Figure 5 represents the combustible region in the range of ϕlocal= 0.45–1.6 (based on the flammability of premixed methane-air combustion [13]) with temperature contours. In the figure, the solid lines are ϕlocal= 1 of Z = 0.055. This represents a flame zone. For a non-premixed micro combustor, the local equivalence ratio becomes ϕlocal= ϕinlet when the fuel and air is becoming perfectly mixed. However, it is very difficult for most of small scale combustors because of the inefficient mixing problem [1, 2, 3, 4]. The high temperature flame zone is observed locally at the region of ϕlocal≈ ϕG= 1.0. The circumferential development is very sensitive to the variation of fan geometry. For instance, for θ ≤ 15º, the flammability region is created more spatially broad than θ > 15º at the same axial location. As shown in Fig. 4, two vortices are developed near the fan swirler. For low blade angle, the wall side vortex is located toward the combustion chamber wall. As the blade angle increases, the vortex moves to the center region. Considering this feature, the ϕlocal≈ ϕG region is changed with the related to the movement of the vortex.
Fig. 5

Comparison of local equivalence ratio (left) and temperature (right) for several axial locations

Figure 6 represents the maximum temperature (Tmax) and combustion efficiency (η) which is defined as 1−\(\dot{m}_{{CH_{4} }}^{inlet} /\dot{m}_{{CH_{4} }}^{outlet}\). As the bladed angle increases, Tmax and η are in the range of 2005–2140 K, 0.74–0.98, respectively. The case of θ ≈ 10º gives the best performance. This is improved about 7% for temperature and 32% for efficiency than that of θ ≈ 75º which gives the lowest performance. From a closer inspection of Figs. 4, 5 and 6, the best combustor has the configuration with an equally sized two vortices.
Fig. 6

Comparison of Tmax and combustion efficiency

To see the wall heat transfer characteristics, Fig. 7 shows the averaged wall temperature and the heat loss ratio for different blade angles. And inner wall temperature contours are added. Herein, the heat loss ratio (qloss) is defined as \(\sum {q_{i} A_{i} } /\dot{m}_{{CH_{4} }}^{inlet} LHV\). As seen, the heated inner wall region is well matched to the variation of flame characteristics in Fig. 4. That is, a radially broad flame structure for θ < 45º gives more heat loss via combustor wall than θ ≥ 45º. In particular, the highest heat loss is observed for θ ≈ 15º. In comparison with θ ≈ 10º, such case yields more strong vortices in the combustor. Due to the development of strong vortices, the flame zone is more stretched to downstream. The interaction between the high temperature zone and the combustion chamber wall increases the heat loss. Therefore, even though the difference in Tmax between θ ≈ 10º and θ ≈ 15º is minor, the difference between combustion efficiencies becomes 7.5%.
Fig. 7

Volume-averaged wall temperature with inner wall temperature contours and heat loss ratio

4 Conclusions

Combustion characteristics of the micro combustor with a swirler of micro fan type are numerically investigated. To see the geometrical effect of the micro fan on the reacting flows, six blade angles are examined.

As the blade angle increases, the rapid temperature rise at the centerline is obtained for θ ≤ 15º, but the temperature rise is reduced for θ > 15º. It is because the reaction zone is anchored more upstream and the flame length is largely reduced, as the blade angle approaches to θ = 15º. The variation of the blade angle makes the two-vortex structure in the micro combustor. As the blade angle approaches to θ = 15º, the vortex region near the chamber wall is reduced and the center vortex is activated. After all, two vortices are developed clearly. After the angle, the center vortex is reduced and one-vortex structure is obtained. According to this feature, the configuration producing moderate two vortices is considered as a best micro combustor. On the other hand, the maximum heat loss is observed at θ = 15º. This is related to the enhanced mixing due to the enlargement of the far vortex from the combustor center, because the vortex stream is originated from the air flow. Finally, various usability of the fan swirler for the micro combustor has been confirmed.

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Copyright information

© The Korean Society of Mechanical Engineers 2019

Authors and Affiliations

  1. 1.School of Mechanical EngineeringKyungpook National UniversityDaeguSouth Korea

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