Combustion characteristics and thermal performance of premixed hydrogen-air in a two-rearward-step micro tube
Graphical abstract
Introduction
Nowadays, the energy crisis [1], [2] and emission issues [3], [4] are more and more serious accompanied by global economic development and manufacturing production. The urgent need for compact, reliable, light weight, instantly rechargeable, efficient and powerful energy source [5], [6], has accelerated the development of micro-mechanical devices [7], [8] and thermo-photovoltaic (TPV) application for the past decade [9]. Micro-power devices [10] such as micro thermo photovoltaic systems (micro-TPV system) are the candidates with powerful energy source [11], [12].
Combustion characteristics [13] among the methane fueled combustion [14], methane mixture fueled combustion [15], liquid ethanol fueled combustion [16], [17], methane fueled catalytic combustion [18], [19] and hydrogen fueled combustion [20] show that the hydrogen fueled combustion with high energy density and low pollution also attracts researchers’ interests [21], [22]. However, the combustion residence time is limited by the small volume. The combustion stabilization is challenged by the high surface-area-to-volume ratio and the large heat loss ratio [23], [24]. Various parameters such as the combustor size, fuel property, and gas temperature affect the heat loss [25], [26]. Hence, for the application of micro-TPV system, the combustion stability comes the first. Some commonly methods such as the addition of bluff-body [27] or porous media [28] in combustion chamber can also enhance combustion stability in micro combustor [29]. Lee et al. [30] and Hosseini et al. [31] observed that the premixed conventional combustion and the flameless micro-combustion are more stable in the combustor with bluff-body. Li et al. [32] found that the porous medium in micro-combustor can promote the thermal transport and flame stability. Additionally, the cavity in the combustion chamber helps the flame to be anchored in a short distance from the inlet [33], [34].
A rational utilization of the heat generation by the chemical reaction can not only enhance the flame stability and thermal performance, but also improve the efficiency of the system. One direct approach is to choose a proper combustor material with an effective wall thermal conductivity [35] since a higher wall thermal conductivity plays an important role in the heterogeneous reactions [36]. The combustor materials such as silicon carbide, quartz glass, alumina ceramic, copper and steel also greatly affect the thermal performance of the combustor [37]. The appropriate thermal conductivity of combustor is preferred to obtain a higher radiation efficiency and a high power output [38], [39]. However, a lower external heat transfer coefficient goes against the flame stability [40], since the transferred heat from the hot wall to the unburned gas contributes to the ignition [41], [42] and improves the stability by preheating the incoming fuel and enhancing the reactivity inside the burner [43], [44]. Besides, Wan et al. [37] revealed that the warm external wind helps flame stability and inhibits extinction, which also causes the reaction to be faster [45]. Another effective method is to use the combustor designed with heat recirculation [46], [47]. Heat recirculation is regarded as the most important factor affecting the combustion process in micro-combustors [48], [49], because the quenching diameter can be reduced in the micro combustor with heat recirculation [50]. Bagheri and Hosseini [44] also observed that the inner reactor heat recirculation can improve the thermal quenching limit, while the outer reactor heat recirculation presents a higher range of emitter efficiency, and the increase of recirculation zone improves combustion efficiency [51].
Due to the small size of combustion chamber and high ratio of surface-area-to-volume, the thermal conditions and combustion chamber constructions have greatly effects on the combustion [52]. Firstly, the mass transfer and radical quenching significantly affect flame stability in micro reactors [53], which can be modeled as a discontinuity in species concentration at the wall with wall heat flux [54] or proper mass conservation [55]. Secondly, the combination of fluid dynamics and heat transfer is more pronounced for the micro combustion, which becomes a critical factor in the design of combustor [24]. Researches with various combustor designs found that the optimization of fluid field in the combustion chamber enhances flame stability [56], [57] and improves thermal performance [58]. In addition, Wan et al. [59] improved the combustion stability by coupling the effects of flame-wall and extending flow recirculation zone in a divergent combustor with a cylindrical flame holder. Also, Yang et al. [60] found that the mean outer wall temperature of the micro combustor with a heat recuperator is 123 K higher than that without a heat recuperator. Jiang et al. [61] developed a high-temperature and high-uniformity micro planar by fabricating a heat-recirculation chamber. Furthermore, Pan et al. [62] and Su et al. [63] indicated that the radiation efficiency of outer wall in the heat-recirculation combustor is significantly higher than that without heat recirculation designed. The electrical power generation of the micro-TPV system depends on the radiation energy of the emitter [64], as the outer thermal performance of the combustor plays the most important role in this system [65], [66].
The application of the micro combustion or micro-power devices are limited by weak flame stability, narrow operational range, non-uniform wall temperature and very low efficiency. In this work, the combustors are equipped with two rearward-steps and various inlet shapes to address the flame instability, in addition, the combustors with a larger size are also compared to improve the efficiency of the micro-TPV system by experimentally observing thermal performances in the combustors. The combustion characteristics, flame front shapes, flame stretch ratio and flame stability in the combustors under different step length and flow rates are also compared to expend the limit of combustion in micro combustor and improve the system power output. Working performances of these combustors for the micro-TPV system with InGaAsSb PV cells are conducted with power output and system efficiency.
Section snippets
Physical and numerical model
The micro-TPV system consists of an emitter, filter and PV cells [34], [59]. As shown in Fig. 1 and Table 1, the micro combustion acts as the emitter. The radiation heat from the micro combustor can be selectively absorbed by the InGaAsSb PV cells, with a cut-off wavelength 2345 nm [67]. The efficiency of micro-TPV system is finally determined by the efficiencies of the micro combustor, the filter and the PV cell as follow:
For the InGaAsSb PV cells, the efficiency can
Validation and verification
To evaluate the boundary conditions, thermal parameters and accuracy of the numerical model in Section 2.1. The numerical results with experimental data of the outer wall temperature in the micro combustor are compared, which is conducted under the same geometrical structure of combustor, mass flow rate and equivalence ratio. Fig. 3 presents the pictures and outer wall temperature profiles of the premixed hydrogen-air combustion in the micro combustors with OD = 4 mm and varied first step
Conclusions
Premixed hydrogen-air combustion in two different inner diameter micro-combustors are conducted and compared. A detailed hydrogen-air reaction mechanism is adopted to investigate the combustion characteristics, flame stability, thermal performance and system efficiency in this work. The following conclusions are obtained.
- (1)
The first rearward-step in the combustor contributes to the flame stability, where the fluid strain and fluid converge will become negative. The heat-recirculation zone behind
Conflict of interests
The authors declare that they have no conflict of interests regarding the publication of this paper.
Acknowledgements
This work is supported by the National Natural Science Foundation of China under the research grant of 51676066, the China Scholarship Council under the research grant of 201706130116 and Hunan Provincial Innovation Foundation for Postgraduate CX2017B080.
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