air combustion in porous medium combustor for the micro thermophotovoltaic application

Applied Energy 260 (2020) 114352

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Investigation on premixed H2/C3H8/air combustion in porous medium combustor for the micro thermophotovoltaic application

T

Qingguo Penga,b, Wenming Yangb, , Jiaqiang Ea, , Hongpeng Xub, Zhenwei Lib, Kunlin Tayb, Guang Zengb, Wenbin Yub ⁎

a b

⁎

College of Mechanical and Vehicle Engineering, Hunan University, Changsha 410082, China Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore

HIGHLIGHTS

H /C H /air combustion with and without porous medium are investigated. • Premixed kinds of porous medium are studied to boost the combustor thermal performance. • Ten of propane blended ratio and porous medium parameters are analyzed. • Effects • The electrical power and efficiency of the thermophotovoltaic system are compared. 2

3

8

ARTICLE INFO

ABSTRACT

Keywords: Combustion Porous medium Propane addition Thermal performance Micro thermophotovoltaic

The study of premixed H2/C3H8/air combustion in the combustor inserted with porous medium (PM), which involved ten kinds of PM and varied with porosity, wire diameter and pores per inch (PPI), is experimentally and numerically conducted. Moreover, in order to boost the electrical power output and efficiency of the micro thermophotovoltaic (TPV) system, the effects of flow rate and propane blended ratio of the fuel on combustion and the thermal performance are investigated. The results indicate that the flame stability and heat transfer of premixed H2/C3H8/air combustion can be significantly enhanced in PM. The 10% propane blended fuel effectively changes the flame location and temperature, as well as the thermal performance of the combustor and the outer wall temperature distribution, while the thermal properties of the combustor are less decreased with 15% propane added. It is also found that the combustor inserted with PM, which has an areal density of 0.0524–0.0551 g/cm2 (PM #30-0.2, #60-0.14 and #80-0.12) and a porosity of 0.9, achieves the highest mean wall temperature and provides a high and uniform wall temperature at different inlet flow rates. The combustor inserted with PM #30-0.2 achieves the electrical power output 2.02 W and efficiency 1.6% for the micro TPV system under the condition of fuel flow rate 400 mL/min, propane blended ratio 12% and equivalence ratio 0.8.

1. Introduction The development of micro/mesoscale combustion has attracted widespread support for the power and heat generation devices [1,2], because of the light weight, reliable and high energy density and the urgent demand for efficient and clean combustion technology [3,4]. However, the high heat loss ratio and small size of the combustion chamber are detrimental to the flame stability [5,6]. Many efforts have been dedicated to the fundamental and applied researches to enhance the combustion stabilization and improve thermal efficiency in micro device, one of which is the thermophotovoltaic (TPV) power generator, converting heat radiation, from the combustion of hydrocarbon fuels in ⁎

a combustor, into electricity [7,8]. In order to obtain a high and uniform radiation temperature for the micro TPV system, researchers have tested combustion in micro combustors with special design, such as setting multi-channel [9,10], adding bluff body [11,12] and inserting block [13] in the combustion chamber. Furthermore, thermal and flow field controlling methods [14], including heat loss control [15], heat recirculation [16,17] and combustion in porous medium (PM) [18], are excellent ways to maintain the flame stabilization and improve the thermal efficiency [19,20]. In particular, combustion in PM can yield promising results, offering an efficient pathway in gaining high energy density and power output [21,22]. Because the solid matrix of PM offers a higher heat capacity,

Corresponding authors. E-mail addresses: [email protected] (W. Yang), [email protected] (J. E).

https://doi.org/10.1016/j.apenergy.2019.114352 Received 31 July 2019; Received in revised form 4 December 2019; Accepted 9 December 2019 Available online 17 December 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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(a) Non-contact Thermometer Controller and readout Flow Controller

Flow Controller

Height Gauge

Plenum Flow Controller

PC display Propane

Hydrogen

Compressor

(b) Fig. 1. (a) Structure diagram of the combustor and (b) schematic of combustion experimental setup.

mixtures through the PM [40]. Moreover, the combustion efficiency in PM is nearly complete because of the strong heat recuperation, and the flame dynamics and combustion completeness are essentially influenced by structural heat conduction [41]. The fuel properties and heat recirculation mechanism also affect the flame propagation characteristics in PM [27]. Hydrogen is the most attractive fuel for micro combustion with a wide flammability range, high energy density and low emission. However, a high wall temperature gradient can be achieved in the hydrogen fueled combustion, because of the burning region near the inlet and the weak heat capacity of reactants [42,43]. The addition of other fuel in hydrogen is one useful approach in utilization for high efficiency and clean combustion [44]. Law and Kwon [45] indicated that a small amount of propane addition could remarkably reduce the burning velocities and would suppress the propensity of onset of both diffusional-thermal and hydrodynamic cellular instabilities in hydrogen fueled combustion. Norton and Vlachos [46] found that a stabilized flame of premixed propane/air can be gained in narrow channels with necessary settings. The insert of porous matrix in combustion chamber serves to extend the flame stabilization and combustion limits as well as allowing the adoption of fuel with low energy content. For the application of micro TPV system, the sustainable combustion, narrow operational range, low power output and efficiency are major issues. The combustor inserted with PM can address each of these concerns with high heat capacity, conductivity and emissivity. In order to obtain further insight into the combustion in PM and efficiently explore the PM properties, the combustion of premixed hydrogen-propane-air (H2/C3H8/air) in a combustor inserted with varied PM is experimentally investigated. A three-dimensional CFD model with detailed chemical mechanisms and transport is employed to numerically

higher conductivity and higher emissivity than gas, the combustion in PM can be significantly changed [23,24]. The plentiful pores or paths in solid matrix heat up the fresh reactants as they pass through the PM, and the heat recirculation mechanisms from the burning region to the fresh reactants gas increase the flame velocity, thereby generating a high heat release rate at the combustion chamber [25]. The heat transfer of combustor wall from the downstream to upstream also preheats the incoming cold gas and strongly enhances the flame stability [26]. As a result, combustion in PM can contribute to flammability and excess enthalpy combustion and gain higher chemical energy conversion efficiency than that of free flame [27,28]. Besides, the flame thickness and flame width can be changed in PM [29,30]. The improvements of the heat recirculation and transfer in PM assist the reaction at near the extinction condition [31], thus extending the lean flammability limits [32,33]. Furthermore, the material, porosity, PPI (pores per inch) of porous medium significantly affect the flame regime, thermal performance and combustion efficiency [34,35]. Improving the wall temperature is an effective way to elevate the system efficiency [36,37]. The insert of PM in combustor can conspicuously augment the wall temperature of the combustor, subsequently increasing the useful radiation energy of micro TPV system [23,38]. Combustion in PM improves the temperature of reactants to change the burning progress by the heat recirculation mechanism, which is based on the porosity and conductivity of PM solid matrix and the chemical energy input [27]. Besides, the high conductivity and emissivity of the PM also improve the heat transfer from the hot gas to the inner wall and lead to a higher and more uniform wall temperature. In the presence of PM, species concentration as well as temperature distribution becomes more uniform [39]. This is partly because the heat conduction and radiation in PM also contribute to the dispersion of 2

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study the combustion characteristics. Furthermore, the effects of porosity, wire diameter, PPI of PM and propane fraction are investigated, after which the thermal and the working performance of the combustor and micro TPV system are carefully tested and analyzed under various fuel flow rates.

steady model are adopted for the simulation [50]. A UDF is employed to calculate the heat transfer in PM solid matrix. The mixtures are regarded as continuum fluid in the combustion chamber and described by Navier–Stokes equations. Due to the low flow rate, the effects of the fluid volume force and the dissipative force are neglected. The radiation of the solid matrix is considered as a homogeneous gray body, and the effects of solid catalysis are ignored. The basic governing equations of continuity, momentum, species and energy conservation in the gas phase can be given as follows [24,36]:

2. Experiment setting and model 2.1. Physical model and experiment setting

g

The combustor, which works as an emitter, filter and InGaAsSb PV cells are united to form a micro TPV system [8,47]. As shown in Fig. 1, the combustion of H2/C3H8/air in a cylindrical combustor with a step and porous medium (PM) is experimentally and numerically investigated. The combustion chamber is divided into two sections: an inlet section and a PM section, which has a length Lp of 20 mm and is fully inserted with PM. The step length Ls is 7 mm and the step height δ is 3 mm. The inlet diameter din and outlet diameter dout of the combustor is 2 mm and 8 mm, respectively. The wall thickness w is 0.5 mm and the total combustor length Lc is 27 mm. For the experimental settings, propane and hydrogen are provided by the propane tank (99.95% purity) and the hydrogen tank (99.9995% purity), respectively, while an air compressor is adopted to supply oxidant for the combustion. Three Brooks Mass Flow Controllers are employed to regulate the mass flow rate and equivalence ratio of these reactants, which are mixed in the tube and the plenum. Then the mixed gas is ignited by an electric spark generator, and burning in the combustor. A non-contact thermometer, which is a RAYTEK Infrared thermometer with high-performance (Model MA2SSCF, with an accuracy of ± (0.3%Tþ1) K), is utilized to measure the outer wall temperature of the combustor with PM. It is made of the rolled-up stainless steel wire mesh, and the solid skeleton of wires of the stainless steel meshes act as the PM solid matrix. As presented in Table 1, ten kinds of stainless steel meshes are employed to fabricate PM, which have various PPI 30, 60 and 80 and wire mesh diameter. The porous medium porosity is set as 0.85 and 0.9, which can be calculated by [48]:

=1

Vs =1 V

s

(

× Lp ×

t

t

t

g Yl )

+

(

gH)

+

=

c=

Table 1 Properties and parameters of the porous medium.

ui

Label

Porosity

1 2 3 4 5 6 7 8 9 10

30 30 30 30 60 60 60 80 80 80

0.15 0.2 0.25 0.3 0.11 0.13 0.14 0.08 0.1 0.12

0.02985 0.05514 0.07748 0.1126 0.02992 0.04464 0.05240 0.02138 0.03352 0.05369

#30-0.15 #30-0.2 #30-0.25 #30-0.3 #60-0.11 #60-0.13 #60-0.14 #80-0.08 #80-0.1 #80-0.12

0.85,0.9 0.85,0.9 0.85,0.9 0.85,0.9 0.9 0.9 0.9 0.9 0.9 0.9

xi

xi

µ

p + Si xj

=

(

g ui Yl )

=

(

g ui H )

=

1 c 2

+

de2 150 (1

3.5 (1 de

mc =

Areal density (g/cm2)

xi

(2)

xj

xj

(3)

g Dm, l

e

Tg xj

Yl + xj

+ Sh

Rl

(4) (5)

g

|u i | u j

(6)

3

)2

) 3

(7) (8)

where de is the average diameter of particles. For the boundary conditions of this numerical work, the combustor inlet is set as mass flow inlet with an ambient condition 300 K and 0 Pa, and the pressure outlet is adopted. The coupled thermal condition is considered for the interface of gas-wall. Conduction and radiation of gas and solid phases and convection between the gas and solid are included [33]. Radiation and convection are both considered for the heat transfer from the combustor to the environment with an emissivity 0.78 and convective heat transfer coefficient 15 W/(m2·K). Convective heat transfer of the combustor’s contact part to the connection tube is very high with a convective heat transfer coefficient 50 W/ (m2·K). The propane blended ratio (mc) is presented as:

A 3D numerical simulation model of premixed H2/C3H8/air combustion in PM is calculated by ANSYS-FLUENT 19.0. Based on the San Diego Chemical-Kinetic Mechanism, a mechanism with 33 species and 124 reactions from Lin et al. [49] is employed to simulate the details of combustion. The standard k-ε model, DO radiation model and the

Diameter of wire (mm)

=0

where α is the permeability, c is the inertial resistance factor, α and c are calculated by semi-empirical formula Ergun equation [24], μ is the dynamic viscosity.

2.2. Simulation and validation

PPI

g ui )

( ui uj )

+

(

Si =

where Vs is the PM volume, V is the volume of PM section of combustion chamber, ρs is the areal density of the stainless steel wire mesh, ρw is the density of stainless steel, rc is the radius of combustion chamber, Lp and Lm is the length and width of the SS mesh, respectively.

No.

(

where ε is the porosity of porous medium, ρg is the gas density, τij is the stress tensor, Si is the source term of pressure loss based on Darcy Law, Rl is the net production rate of species l and Dm,l is the diffusion coefficient of species l, λe is the effective thermal conductivity (the porous zone being represented by a lumped parameter) [39], Sh is the combination of every heat resource [11]. For the porous medium zone, Si can be described as:

(1)

× rc 2

xi

g ui )

× ms × L m × L p

w

+

t

Vc × 100% Vc + Vh

(9)

where Vc is the mole fraction of propane, Vh is the mole fraction of hydrogen. The fuel can be presented as mc% C3H8+ (1−mc%) H2 + air. In addition, the lower heating value (LHV) of propane and hydrogen is 46.5 MJ/kg and 119.9 MJ/kg, respectively. The fuel-air equivalence ratio Φ plays an important role in the combustion characteristics and thermal performance of the combustion in PM. Our previous investigations showed that a good thermal performance of the combustor with PM is obtained at Φ = 0.85, while the highest radiation efficiency of the micro combustor can be achieved at Φ = 1.0. As shown in Fig. 2, the outer wall temperature distribution is 3

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1300 1200

Wall temperature/K

to the connection tube through the contact part [51]. In addition, the experimental error and the higher temperature of exhaust take much heat away from the combustor in the experimental test. Thus, the numerical model settings and conditions are reasonable and can be employed in this work.

Equivalence ratio 1.0 Equivalence ratio 0.9 Equivalence ratio 0.8 Equivalence ratio 0.7

1100

3. Results and discussion

1000

3.1. Combustion characteristics

900

With the increase of combustion chamber size, the thermal performance of the combustor is inadequate for the application, due to the weak heat convection of hot gas (flame) to the inner wall [52,53]. The heat transfer in the PM section of the combustion chamber is significantly enhanced because of the high conductivity and emissivity of the PM solid matrix [25]. It also preheats the unburnt gas and alters the initial temperature of combustion, which affects the flame front, and stretch ratio and the combustion stability [54]. Hence, the inserted PM really has a major and significant impact on the combustion of premixed H2/C3H8/air. Based on the available researches, the OH radical and CHO radical can correlate well with the flame location and flame front [55,56]. Fig. 4 demonstrates the temperature contour, mole fraction distribution of OH radical and CHO radical in the combustor with (PM #30-0.15 with porosity of 0.9) and without PM, where Vf = 300 mL/min with mc = 10% and Φ = 0.8. There is a significant difference in the reaction constituent and gas temperature distributions in the combustion chamber with and without PM. For the free flame (without PM), the distributions of OH radical and CHO radical are continuous, and the high mole fraction zones of them are located in the central region of the combustion chamber. In addition, the high gas temperature zone is matched well with the high mole fraction regions of OH radical and CHO radical, which have an excellent correlation with flame heat release rate [57]. For the combustion in PM, the concentrated OH radical and CHO radical zone are both divided into two regions: the peak zone at the inlet section and the second highest concentration zone behind the step. This is because of the discontinuity of combustion under micro-scale conditions and the high flame speed with a high gas flow rate. Furthermore, the flame location is significantly lowered in PM, while the front of free flame is near the outlet. The reason is that the inserted PM slows down the flow velocity in the center part of the combustion chamber and makes the reaction species have a more even distribution, thus reducing the flame location. Besides, the preheating of fresh gas and the initial temperature of reaction are both improved in PM, making it possible for the combustion reaction to occur at the inlet section. However, the preheating energy is

800 700 600

0

3

6

9

12

15

18

21

24

27

Distance from inlet/mm Fig. 2. Outer wall temperature profiles of the combustor inserted with PM #300.15 (porosity ε = 0.9) under the condition of Vf = 300 mL/min and various equivalence ratios.

varied with the reduction of Φ. It is obvious that the condition Φ = 0.8 obtains the highest mean wall temperature 1064 K, followed by 1057 K, 1046 K and 1013 K with Φ = 0.7, 0.9 and 1.0, that is, a higher radiation power of the combustor. It indicates that the combustion of premixed H2/C3H8/air needs more oxidant to get better thermal performance of the combustor for the micro TPV application. This can be attributed to the insufficient mixing of fuel and air in such a short time, especially at very high equivalence ratio conditions, thus resulting in incomplete combustion. Hence, the combustion of premixed H2/C3H8/air in varied PM are experimentally and numerically investigated under the condition of Φ = 0.8 with variable fuel bending ratios and flow rates. To evaluate the boundary conditions, thermal parameters and accuracy of the numerical model, the comparison of simulation and experimental results is conducted. Fig. 3 presents the premixed H2/C3H8/ air combustion in the combustor with PM #30–0.15 (porosity ε = 0.9) under the condition of fuel flow rate Vf = 300 mL/min with propane blended ratio mc = 10% (90% H2–10% C3H8) and Φ = 0.8. The numerical simulation results are generally in agreement with the experimental data, although it is a little lower than that of the numerical profile. The biggest temperature difference 47 K (less than 4.2%) between the experimental and numerical results appears in the inlet section, which is due to the preheating of mixtures and a high heat loss

Experimentat data Numerical results

Wall temperature/K

1200 1100 1000 Porous medium 900 800 700 600

0

2

4

6

8 10 12 14 16 18 20 22 24 26

Distance from inlet/mm

Fig. 3. Comparison of simulation and experimental results. 4

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Free flame Porous medium 0.22

h2 h2

c3h8 c3h8

o2 o2

h2o h2o

Reaction zone

0.20

1400

0.16 0.14

1200

0.12

1000

0.10 0.08

800

0.06 0.04

600

0.02

400

0.00

0

3

6

9

12

15

18

21

24

Gas temperature/K

1600

0.18 Mole fraction

co2 co2 1800

27

Distance from inlet/mm Fig. 5. Species and gas temperature profiles of premixed H2/C3H8/air combustion in the central axis under the condition of Vf = 300 mL/min with mc = 10% and Φ = 0.8.

3.2. Effects of porous medium property

(a) without PM

3.2.1. Effects of porosity and wire diameter The output power of micro TPV is proportional to the heat radiation from the surface of the combustor. Fig. 6 and Fig. 7 present the outer wall temperature profiles and mean values of the combustion in the combustor inserted with varied wire diameter of porous medium (Wd) at Vf = 300 mL/min and Φ = 0.8. As shown in Fig. 6(a), with the increasing distance from inlet, the temperature curves are firstly increased and then decreased, where the porosity and PPI of PM is 0.85 and 30, respectively. It also demonstrates that the change of Wd (0.15–0.30 mm) affects the temperature distribution. At the upstream (inlet section) of the combustor, the combustor inserted with PM #300.15 obtains the lowest value, and the temperature difference between PM #30-0.15 and PM #30-0.30 is small. Moreover, the highest temperature occurs in the combustor with PM #30-0.25. The peak temperature of these profiles are gained at z = 7–11 mm, and the combustor with PM #30-0.25 achieves the highest wall temperature of 1141 K with mc = 10%. In addition, the combustor with PM #30–0.3 obtains the highest temperature at the downstream. Fig. 6(b) depicts that the combustor with PM #30-0.25 also gains the highest mean outer wall temperature 1046 K, which is 2 K higher than that of the combustor with PM #30-0.30, followed by PM #300.20 and PM #30-0.15. With the increase of propane mole fraction of the blended fuel at the same fuel flow rate, the input chemical energy is increased per unit time, due to the high molar LHV of propane, which is 22 times the molecular mass of hydrogen. Fig. 6 (a) also shows that the temperature difference between the combustors inserted with varied PM Wd is increased at the upstream of the combustion chamber. This is caused by the impacts of the increase of flow rate (inlet velocity) on the heat transfer of the inlet section. In particular, the combustor inserted with PM #30–0.25 achieves the peak wall temperature of 1161 K and the maximum mean temperature value of 1063 K at Vf = 300 mL/min and mc = 12%, which is 22 K higher than that of the combustor with PM #30-0.15. In general, the mean temperature of the combustor increases first and then decreases with the decrease of Wd. Porosity ε of PM has significant impacts on the flow field and thermal performance of the combustor [32,58]. For the PM with a PPI 30, according to Eq. (1), the increase of porosity decreases the weight and surface area of the PM solid skeleton. It is important to the improvement of heat transfer and preheating, and it also affects the flow of reactants and the combustion efficiency [36,59]. Fig. 7 shows the wall temperature profiles and mean values of premixed H2/C3H8/air

(b) with PM

Fig. 4. Distributions of OH radical (left) and CHO (right) radical with overlaid gas temperature of premixed H2/C3H8/air combustion in the combustor (a) without PM (Free flame) and (b) with PM #30-0.15 with the porosity 0.9 under the condition of Vf = 300 mL/min with mc = 10% and Φ = 0.8.

insufficient for the chain and dissociation reactions for all fresh reactants. Furthermore, the high thermal conductivity and emissivity of PM skeleton enhance the heat transfer of gas-gas, gas-PM and gas-wall, thereby significantly intensifying and improving the combustion and heat transfer. As shown in Fig. 4, the PM in combustion chamber also reduces the radial temperature gradient, that is, the enhanced heat transfer significantly affects the temperature and concentration fields in combustion chamber. It should be pointed out that the temperature and species profile of the combustion with and without PM are different at the central axis of the combustor. Fig. 5 indicates that the axial temperature distribution and gradient are different in the combustion chamber with and without PM. The inserted PM can effectively modify the gas temperature distribution, enhance the combustion stability and provide a more uniform wall temperature profile. As shown in Fig. 5, the free flame locates at about z = 17 mm, and the peak gas temperature is higher than 1700 K. The combustion in PM, on the other hand, locates at about z = 8 mm with a peak gas temperature of 1500 K under the operation condition. Besides, the exhaust gas temperature of the combustion in PM is 600 K lower than that of the free flame. This illustrates that the PM solid matrix effectively promotes the heat transfer to the combustor wall, thereby reducing the exhaust gas temperature and increasing the wall temperature. Moreover, by comparing the slopes of the gas temperature and species curves, it can be inferred that the combustor with PM has a smaller combustion reaction zone and a higher reaction intensity than those of free flame. Hence, the flame stability of premixed H2/C3H8/air combustion is enhanced in PM.

5

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(b)1100

(a) 1200

Mean wall temperature/K

1150

Wall temperature/K

1100 1050 1000 #30-0.30, 300 mL/min-10% #30-0.30, 300 mL/min-12% #30-0.25, 300 mL/min-10% #30-0.25, 300 mL/min-12% #30-0.20, 300 mL/min-10% #30-0.20, 300 mL/min-12% #30-0.15, 300 mL/min-10% #30-0.15, 300 mL/min-12%

950 900 850 800 750 700

0

3

6

9

12

15

18

21

24

1080

#30-0.25 #30-0.15

1060

1040

1020

1000

27

#30-0.30 #30-0.20

12%

10%

Blended fuel

Distance from inlet/mm

Fig. 6. (a) Outer wall temperature distributions and (b) mean outer wall temperature values of the combustor inserted with various wire diameter of PM and the porosity is kept at ε = 0.85.

(a) 1250

(b)

1200

Mean wall temperature/K

Wall temperature/K

1150 1100 1050 1000

#30-0.30, 300 mL/min-10% #30-0.30, 300 mL/min-12% #30-0.25, 300 mL/min-10% #30-0.25, 300 mL/min-12% #30-0.20, 300 mL/min-10% #30-0.20, 300 mL/min-12% #30-0.15, 300 mL/min-10% #30-0.15, 300 mL/min-12%

950 900 850 800 750 700

0

3

6

9

12

15

18

21

24

1140 1120

Distance from inlet/mm

#30-0.25 #30-0.15

1100 1080 1060 1040 1020 1000

27

#30-0.30 #30-0.20

10%

12%

Blended fuel

Fig. 7. (a) Outer wall temperature distributions and (b) mean outer wall temperature values of the combustor inserted with various wire diameter of PM and the porosity is kept at ε = 0.9.

combustion in the combustor inserted with varied PM and porosity of 0.9. As can be seen, the highest temperature value is obtained at the inlet section of the combustor with PM #30-0.25, the center part of the combustor with PM #30-0.2 (the peak value 1192 K), and the outlet section of the combustor with PM #30-0.3, respectively. With the increase of mc, the temperature of the outer wall of the combustor increases. The peak temperature of the combustor with PM #30-0.2 reaches 1215 K, and the mean wall temperature increases from 1100 K to 1179 K. Comparing the temperature distributions and mean values in Fig. 6 and Fig. 7, it can be concluded that the influence of PM Wd is extended with the increase of porosity. Furthermore, a high wall temperature distribution can be obtained in the combustor with ε = 0.9, while the combustor with ε = 0.85 obtains a more uniform temperature distribution and the temperature differences are smaller than that of the ε = 0.9. As presented in Fig. 7, the combustors with PM and ε = 0.9 obtain higher outer wall temperature distributions and mean values, which means a higher radiation power and electrical power output of a micro TPV system [36,60]. Moreover, the porosity ε affects the flow field distribution in the combustion chamber as well as the convective and radiative heat transfer between the fluid and solid matrix. Fig. 8 describes the temperature distribution and the flow field of the combustor with varied Wd and ε = 0.9 under the condition of Vf = 300 mL/min with mc = 10% and Φ = 0.8. According to Eq. (1), the volume and weight of the PM solid skeleton are the same with a fixed chamber size, porosity and solid skeleton density. However, the surface-to-volume

ratio of the solid matrix is changed in the combustor with varied Wd. With the increase of Wd, the surface area of the solid skeleton is reduced and the distance of solid skeleton is increased, which affects the heat transfer in the combustion chamber by reducing the heat exchange area, thereby changing the flow field and affecting the heat transfer between the fluid and the solid. In addition, the flow resistance time is also affected by the change of flow field. It can be observed in Fig. 8 that the temperature distribution and the streamline in the combustion chamber are different in the combustor with varied Wd, especially in the vicinity of the step. The combustion in the combustor with PM #300.25 achieves the highest gas temperature, with a smaller high temperature zone compared to the combustor with #30-0.2. Thus, it obtains the highest average wall temperature (see Fig. 7). 3.2.2. Effects of PPI and propane fraction The fuel properties have significant impacts on the flame propagation characteristics and thermal performance [27,44]. Fig. 9 demonstrates the effects of propane addition ratio on the combustion characteristics and thermal performance. For a fixed chemical energy 87.4 W and fuel-air equivalence ratio, with the increase of propane addition ratio, the distributions of OH radicals and gas temperature in the combustion chamber are significantly changed. The region of high OH radical and gas temperature of the combustion with Vf = 245 mL/ min and mc = 15% is larger than that of the combustion of Vf = 300 mL/min and mc = 10%, which also has a higher flame location in PM. This phenomenon can be explained by the reasons that 6

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#30-0.15

#30-0.20

#30-0.25

#30-0.30

Fig. 8. The computed results of temperature isograms and color nephogram (left) and velocity field (right) with overlaid streamlines of premixed H2/C3H8/air combustion in the combustor with PM PPI 30 and ε = 0.9.

the combustion reactions of propane need more preheating energy and the location of propane-fueled flame is higher than that of hydrogen fueled flame [43]. Fig. 9 also shows that the combustion species distributions and gas temperature profile change in the same trend for the PM combustion with varied mc and fixed energy input. It can be seen that the flame locates at z = 8 mm under the condition of Vf = 300 mL/min and mc = 10%, while the combustion of Vf = 245 mL/min and mc = 15% mainly occurs at z = 8.3 mm, indicating that the flame with a higher mc has a weak stability [61,28]. In addition, the increase of mc with a fixed energy input improves the gas temperature, extends the reaction region, and modifies the outer wall temperature distribution. As mentioned in Fig. 9, the propane addition affects the flame location and thermal performance of the combustion in PM. Fig. 10 shows the outer wall temperature profiles of the combustion in the combustor with varied Wd, where mc = 10% or 15% with the same chemical energy input 87.4 W. Our previous researches [52,59] demonstrated that the inlet connection of combustor and the plenum cause a significant amounts of heat loss, which cut down the energy output of the combustor. The propane addition in hydrogen fueled combustion can effectively change the outer wall temperature distribution. As shown in Fig. 10, the wall temperature of the mc = 10% combustion is higher than that of mc = 15% at the inlet section, which reduces the heat loss from the inlet section to the connection part. However, with the increase of mc, the peak wall temperature is decreased, and the location is shifted towards the combustor downstream. Furthermore, the combustion with mc = 10% gains a higher mean wall temperature than that of mc = 15% with PPI 30 and varied Wd. This is because of a higher flame location and lower energy density of propane [27,43]. In addition, the temperature difference of the two conditions is reduced with the increase of Wd, and the combustor with PM #30-0.2 still achieves the highest mean wall temperature. The PPI is also an important parameter of the PM, affecting the heat transfer in PM and combustion intensity [35]. Six other PM with varied PPI and Wd are involved in this work, as shown in Table 1. Fig. 11

reveals the effects of PPI and mc on the outer wall temperature distributions and mean values of the combustor with PPI 60 and 80. For the PM with PPI 60, with the increase of Wd, the peak temperature value and mean wall temperature are raised. As shown in Fig. 11(a), the biggest temperature difference of the combustion in PM #60-0.11, PM #60-0.13 and PM #60-0.14 occurs at the central parts of the combustor, where locates the peak value of each curve. The maximum temperature difference of the combustion of mc = 10% and mc = 15% in PM #60-0.11 reaches 31 K, and the propane blending ratio mainly affects the wall temperature distribution at the inlet section. Moreover, the combustor with PM #60-0.14 obtains the highest mean wall temperature 1086 K and peak value 1200 K at Vf = 300 mL/min and mc = 10%. For the PM with PPI 80, the combustor with PM #80-0.12 obtains the highest mean temperature of 1089 K. The effect of Wd on the mean wall temperature is similar to the PM with PPI 60 (see Fig. 11), while the effect of Wd on wall temperature distribution for the PM with PPI 80 is greater than that of the PM with PPI 60. Furthermore, with the increase of mc, the mean outer wall temperature is reduced by 6–17 K, while the maximum temperature difference for the combustion of mc = 10% or 15% in PM #80–0.1 can reach 70 K at the combustor inlet section. It can be seen from Figs. 9–11 that the combustion of premixed H2/ C3H8/air in the combustor with PM #30-0.2, #60-0.14 and #80-0.12 obtain a higher mean wall temperature. Fig. 12 compares the wall temperature distribution of the combustor filled with the three kinds of PM under the condition of fuel chemical energy 87.4 W and Φ = 0.8. With the increase of distance from inlet, these temperature profiles have a similar tendency: increases first and then decreases, and the peak value of each curve is located behind the step. For an increased mc, the peak wall temperature is reduced and the biggest temperature difference occurs at the inlet section. The combustion with mc = 10% achieves a higher mean wall temperature and radiation power, which is more suitable for the micro TPV system with a higher wall temperature. Fig. 12 also indicates that the peak temperature location shifts to the combustor downstream with the increase of PPI, while the wall 7

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Fig. 9. Effects of propane blended ratio on combustion in PM #30–0.15 with ε = 0.9. (a) Comparisons of the distributions of mole fraction of OH radical and gas temperature (left:10% propane, right:15% propane), and computed flame structures of premixed H2/C3H8/air with (b) Vf = 300 mL/min with mc = 10% and (c) Vf = 245 mL/min with mc = 15%.

temperature distribution [22,62]. Hence, the mean wall temperature difference for the combustor with PM #30-0.2, #60-0.14 and #80-0.12 are less than 10 K.

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3.3. Thermal and working performance

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As the core component of the micro TPV system, the high radiation temperature of the emitter/combustor is very favorable for the micro TPV system to produce a high power density [37,63]. Fig. 10 and Fig. 11 demonstrate that the combustor inserted with PM #30-0.2, #600.14 and #80-0.12 at ε = 0.9 achieve a higher wall temperature. The combustion with a higher chemical energy input is investigated and compared in Fig. 13, where Vf = 400 mL/min with a mc = 10% or mc = 12%. The values of peak temperature and mean wall temperature are increased with a higher fuel flow rate, and the location of peak value shifts to the downstream due to a larger energy release per unit time and a high flame speed. Furthermore, a higher heat transfer feedback rate occurs at high flow velocity because of the high conductivity and convection in the PM section, resulting in the extension of reaction zone and an increase in temperature [4,51]. When the propane ratio increases from 10% to 12% with Vf = 400 mL/min and Φ = 0.8, the temperature gradient becomes smaller at the inlet section because of a reduced heat loss ratio of the combustor connection part, while the wall temperature is highly improved in the middle part and the outlet section. Thus, the mean wall temperature is increased in Fig. 13(b). Moreover, the highest mean outer wall temperature is obtained at the combustor with different PM under the condition of varied Vf and mc.

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Distance from inlet/mm Fig. 10. Outer wall temperature distributions and mean values of premixed H2/ C3H8/air combustion in the combustor inserted with various wire diameters PM.

temperature is reduced at the inlet section because of the lower the viscous resistance and inertial resistance of PM with a higher PPI. However, with the increase of PPI, the outer wall temperature has increased marginally at the outlet section, that is, the effects of PM on 8

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Fig. 11. Outer wall temperature distributions and mean values of premixed H2/C3H8/air combustion in the combustor inserted with various wire diameters PM. (a) PPI = 60, (b) PPI = 80.

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Vf = 300 mL/min and mc = 10%, the combustor with PM #80-0.12 achieves the highest mean value of 1089 K. At Vf = 400 mL/min, the combustors with PM #60-0.14 and PM #30-0.2 obtain the highest mean value 1153 K and 1170 K with mc = 10% and mc = 12%, respectively. This phenomenon also demonstrates that the PPI and Wd of PM with varied viscous resistance and inertial resistance affect the outer wall temperature distribution and the mean values. Furthermore, the PM with a smaller PPI obtains a higher value under the larger fuel flow rate. Fig. 14 shows the radiation efficiency of the combustor with PM #30-0.2, #60-0.14 and #80-0.12 under three conditions of varied inlet flow rate. The radiation power of the combustor is proportional to the chemical energy input of the combustion [28,64], but the radiation efficiency is reduced as depicted in Fig. 14. It can be explained by the increased exhaust temperature with a larger heat released in the combustion chamber, and the high temperature of the inlet section also improves the heat loss through the connection part. Furthermore, with the increase of inlet flow rate, the gas velocity is significantly increased in PM, hence the chemical residence time is reduced and the ratio of incomplete combustion is increased. Comparing the results shown in Fig. 13 and Fig. 14, it can be found that under the same fuel flow rate, the highest mean wall temperature and the highest radiation efficiency are obtained in the same combustor. For the micro TPV system, the electrical power output and the efficiency are two important parameters to the application [52,65]. Fig. 15 demonstrates the effects of chemical energy input on electrical

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Distance from inlet/mm Fig. 12. Comparisons of outer wall temperature profiles of the combustor inserted with varied PM and mc at the same fuel chemical energy inlet.

The maximum mean temperature difference of the combustion in PM #80-0.12, #60-0.14 and #30-0.2 is 10 K, 4.4 K and 5 K at Vf = 300 mL/ min and mc = 10%, Vf = 400 mL/min and mc = 10% and Vf = 400 mL/min and mc = 12%, respectively. For the combustion of

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Fig. 13. Effects of fuel flow rate on (a) wall temperature distribution and (b) mean value. 9

#80-0.12

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60

Radiation efficiency/%

acquired and summarized as follows:

300 mL/min-10% 400 mL/min-10% 400 mL/min-12%

58 56

(1) The heat transfer and the flame stability of the premixed H2/C3H8/ air combustion in PM can be significantly enhanced, thereby improving the outer wall temperature of the combustor. (2) The addition of propane for the hydrogen fueled combustion strongly affects the flame location and temperature in the combustion chamber and alters the outer wall temperature distribution of the combustor with PM. However, the blended fuel with a high propane ratio reduces the mean outer wall temperature of the combustor. (3) PPI and Wd of PM with varied viscous resistance and inertial resistance affect the outer wall temperature distribution and the mean values. For the PM with varied PPI and Wd, which has a minor discrepancy of PM areal density (0.05514, 0.05240 and 0.05369 g/cm2), a higher mean wall temperature can be obtained. Furthermore, the PM with a smaller PPI obtains a higher value under the larger fuel flow rate. (4) With the increase of fuel flow rate, the combustor with PM provides high electrical power output and energy conversion efficiency for the micro TPV system.

54 52 50 48 46 4 2 0

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#80-0.12

Fig. 14. Effects of fuel flow rate on radiation efficiency of the combustor inserted with varied porous medium.

power output Ee and efficiency η of the micro TPV system with the combustor inserted with PM and InGaAsSb (0.55 eV) PV cells. Ee of the micro TPV system depends on the bandgap of the InGaAsSb (0.55 eV) PV cells and radiation temperature. Only the photon of radiation wavelength less than 2345 nm is able to invoke free electrons on the InGaAsSb PV cells and produce electrical current [6,7]. Furthermore, the maximum power Em of the InGaAsSb PV cells is very low under a lower radiation temperature because of a lower radiation wavelength, for instance, Em = 0.064 W/cm2 at 1000 K. But Em will be significantly improved with the increase of radiation temperature. Hence, the Ee and η of the micro TPV system are increased with the increase of chemical energy input due to a high radiation temperature, as shown in Fig. 15. Furthermore, the combustor with PM #30-0.2 obtains the highest Ee = 2.02 W and η = 1.60% under the condition of Vf = 400 mL/min, mc = 12% and Φ = 0.8.

CRediT authorship contribution statement Qingguo Peng: Software, Data curation, Writing - original draft, Writing - review & editing. Wenming Yang: Supervision, Resources, Project administration, Methodology, Writing - review & editing. Jiaqiang E: Supervision, Funding acquisition, Conceptualization, Methodology, Writing - review & editing. Hongpeng Xu: Software, Formal analysis. Zhenwei Li: Formal analysis, Writing - review & editing. Kunlin Tay: Writing - review & editing. Guang Zeng: Writing review & editing. Wenbin Yu: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

4. Conclusions

Acknowledgements

This work presents both the experimental and numerical investigation on premixed H2/C3H8/air combustion in a combustor inserted with porous medium (PM). Ten kinds of PM with varied PPI and Wd are carefully experimentally studied under the condition of different fuel blend ratio and flow rate. Some fundamental findings can be

(a) 2.4 2.0

(b) 2.0

300 mL/min-10% 400 mL/min-10% 400 mL/min-12%

1.8 1.6

1.8

System efficiency/%

Output electrical power/W

2.2

This work was financially supported by the National Natural Science Foundation of China (Contract NO. 51976054, 51676066), the China Scholarship Council under the research grant of 201706130116.

1.6 1.4 1.2 1.0 0.8 0.6

1.4 1.2 1.0 0.8 0.6 0.4

0.4

0.2

0.2 0.0

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#30-0.2

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0.0

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#60-0.14

Porous medium

#80-0.12

Fig. 15. Effects of fuel flow rate on (a) electrical power output and (b) efficiency of the micro TPV system with a combustor inserted with PM and InGaAsSb PV cells. 10

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References

j.ijhydene.2014.07.156. [28] Meng L, Li J, Li Q. A miniaturized power generation system cascade utilizing thermal energy of a micro-combustor: Design and modelling. Int J Hydrogen Energy 2017;42:17275–83. https://doi.org/10.1016/j.ijhydene.2017.05.147. [29] Voss S, Mendes MAA, Pereira JMC, Ray S, Pereira JCF, Trimis D. Investigation on the thermal flame thickness for lean premixed combustion of low calorific H2/CO mixtures within porous inert media. Proc Combust Inst 2013;34:3335–42. https:// doi.org/10.1016/j.proci.2012.06.044. [30] Shi JR, Li BW, Xia YF, Chen PF, Xu YN, Liu HS. Numerical study of diffusion filtration combustion characteristics in a plane-parallel packed bed. Fuel 2015;158:361–71. https://doi.org/10.1016/j.fuel.2015.05.055. [31] Gao J, Hossain A, Matsuoka T, Nakamura Y. A numerical study on heat-recirculation assisted combustion for small scale jet diffusion flames at near-extinction condition. Combust Flame 2017;178:182–94. https://doi.org/10.1016/j. combustflame.2016.12.028. [32] Li Q, Li J, Shi J, Guo Z. Effects of heat transfer on flame stability limits in a planar micro-combustor partially filled with porous medium. Proc Combust Inst 2019;37:5645–54. https://doi.org/10.1016/j.proci.2018.06.023. [33] Mujeebu MA, Zulki M, Mohamad AA, Bakar MZA. Trends in modeling of porous media combustion. Progr Energy Combust Sci 2010;36. https://doi.org/10.1016/j. pecs.2010.02.002. [34] Li J, Wang Y, Chen J, Shi J, Liu X. Experimental study on standing wave regimes of premixed H2-air combustion in planar micro-combustors partially filled with porous medium. Fuel 2016;167:98–105. https://doi.org/10.1016/j.fuel.2015.11. 033. [35] Liu H, Wu D, Xie M, Liu H, Xu Z. Experimental and numerical study on the lean premixed filtration combustion of propane/air in porous medium. Appl Therm Eng 2019;150:445–55. https://doi.org/10.1016/j.applthermaleng.2018.12.155. [36] Pan JF, Wu D, Liu YX, Zhang HF, Tang AK, Xue H. Hydrogen/oxygen premixed combustion characteristics in micro porous media combustor. Appl Energy 2015;160:802–7. https://doi.org/10.1016/j.apenergy.2014.12.049. [37] Li J, Chou SK, Li ZW, Yang WM. Characterization of wall temperature and radiation power through cylindrical dump micro-combustors. Combust Flame 2009;156:1587–93. https://doi.org/10.1016/j.combustflame.2009.05.003. [38] Li J, Chou SK, Huang G, Yang WM, Li ZW. Study on premixed combustion in cylindrical micro combustors : Transient flame behavior and wall heat flux. Exp Therm Fluid Sci 2009;33:764–73. https://doi.org/10.1016/j.expthermflusci.2009. 01.012. [39] Li J, Li Q, Wang Y, Guo Z, Liu X. Fundamental flame characteristics of premixed H 2 – air combustion in a planar porous micro-combustor. Chem Eng J 2016;283:1187–96. https://doi.org/10.1016/j.cej.2015.08.056. [40] Xu K, Liu M, Zhao P. Chemical Engineering and Processing : Process Intensification Stability of lean combustion in mini-scale porous media combustor with heat recuperation. Chem Eng Process Process Intensif 2011;50:608–13. https://doi.org/10. 1016/j.cep.2011.03.001. [41] Liu Y, Fan A, Yao H, Liu W. Numerical investigation of filtration gas combustion in a mesoscale combustor filled with inert fibrous porous medium. Int J Heat Mass Transf 2015;91:18–26. https://doi.org/10.1016/j.ijheatmasstransfer.2015.07.100. [42] Mohammadi A, Shioji M, Nakai Y, Ishikura W, Tabo E. Performance and combustion characteristics of a direct injection SI hydrogen engine. Int J Hydrogen Energy 2007;32:296–304. https://doi.org/10.1016/j.ijhydene.2006.06.005. [43] Tang A, Xu Y, Shan C, Pan J, Liu Y. A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor. Int J Hydrogen Energy 2015;40:16587–96. https://doi.org/10.1016/j.ijhydene.2015.09. 101. [44] Tang C, He J, Huang Z, Jin C, Wang J, Wang X, et al. Measurements of laminar burning velocities and Markstein lengths of propane – hydrogen – air mixtures at elevated pressures and temperatures. Int J Hydrogen Energy 2008;33:7274–85. https://doi.org/10.1016/j.ijhydene.2008.08.053. [45] Law CK, Kwon OC. Effects of hydrocarbon substitution on atmospheric hydrogen-air ame propagation. Int J Hydrogen Energy 2004;29:867–79. https://doi.org/10. 1016/j.ijhydene.2003.09.012. [46] Norton DG, Vlachos DG. A CFD study of propane/air microflame stability. Combust Flame 2004;138:97–107. https://doi.org/10.1016/j.combustflame.2004.04.004. [47] Lee SI, Um DH, Kwon OC. Performance of a micro-thermophotovoltaic power system using an ammonia-hydrogen blend-fueled. Int J Hydrogen Energy 2013;8. https://doi.org/10.1016/j.ijhydene.2013.05.010. [48] Li J, Wang Y, Shi J, Liu X. Dynamic behaviors of premixed hydrogen-air flames in a planar micro-combustor filled with porous medium. Fuel 2015;145:70–8. https:// doi.org/10.1016/j.fuel.2014.12.070. [49] Lin KC, Te Chiu C. A compact skeletal mechanism of propane towards applications from NTC-affected ignition predictions to CFD-modeled diffusion flames: Comparisons with experiments. Fuel 2017;203:102–12. https://doi.org/10.1016/j. fuel.2017.04.064. [50] Ghose P, Patra J, Datta A, Mukhopadhyay A. Effect of air flow distribution on soot formation and radiative heat transfer in a model liquid fuel spray combustor firing kerosene. Int J Heat Mass Transf 2014;74:143–55. https://doi.org/10.1016/j. ijheatmasstransfer.2014.03.001. [51] Barra AJ, Ellzey JL. Heat recirculation and heat transfer in porous burners. Combust Flame 2004;137:230–41. https://doi.org/10.1016/j.combustflame.2004.02.007. [52] Peng Q, Wu Y, Jiaqiang E, Yang W, Xu H, Li Z. Combustion characteristics and thermal performance of premixed hydrogen-air in a two-rearward-step micro tube. Appl Energy 2019;242:424–38. https://doi.org/10.1016/j.apenergy.2019.03.137. [53] Tang A, Cai T, Deng J, Xu Y, Pan J. Experimental investigation on combustion characteristics of premixed propane/air in a micro-planar heat recirculation combustor. Energy Convers Manage 2017;152:65–71. https://doi.org/10.1016/j.

[1] Chia LC, Feng B. The development of a micropower (micro-thermophotovoltaic) device. J Power Sources 2007;165:455–80. https://doi.org/10.1016/j.jpowsour. 2006.12.006. [2] Ju Y, Maruta K. Microscale combustion : Technology development and fundamental research. Prog Energy Combust Sci 2011;37:669–715. https://doi.org/10.1016/j. pecs.2011.03.001. [3] Daneshvar H, Prinja R, Kherani NP. Thermophotovoltaics : Fundamentals, challenges and prospects. Appl Energy 2015;159:560–75. https://doi.org/10.1016/j. apenergy.2015.08.064. [4] Ferrari C, Melino F, Pinelli M, Ruggero P. Thermophotovoltaic energy conversion : Analytical aspects, prototypes and experiences. Appl Energy 2014;113:1717–30. https://doi.org/10.1016/j.apenergy.2013.08.064. [5] Norton DG, Vlachos DG. Combustion characteristics and flame stability at the microscale: A CFD study of premixed methane/air mixtures. Chem Eng Sci 2003;58:4871–82. https://doi.org/10.1016/j.ces.2002.12.005. [6] Peng QG, Jiaqiang E, Yang WM, Xu HP, Chen JW. Experimental and numerical investigation of a micro-thermophotovoltaic system with different backward-facing steps and wall thicknesses. Energy 2019;173:540–7. https://doi.org/10.1016/j. energy.2019.02.093. [7] Jiang D, Yang W, Tang A. Development of a high-temperature and high-uniformity micro planar combustor for thermophotovoltaics application. Energy Convers Manage 2015;103:359–65. https://doi.org/10.1016/j.enconman.2015.06.083. [8] Yang WM, Chou SK, Shu C, Li ZW, Xue H. Combustion in micro-cylindrical combustors with and without a backward facing step. Appl Therm Eng 2002;22:1777–87. https://doi.org/10.1016/S1359-4311(02)00113-8. [9] Tang A, Pan J, Yang W, Xu Y. ScienceDirect Numerical study of premixed hydrogen/ air combustion in a micro planar combustor with parallel separating plates. Int J Hydrogen Energy 2015;40:2396–403. https://doi.org/10.1016/j.ijhydene.2014.12. 018. [10] Shirsat V, Gupta AK. A review of progress in heat recirculating meso-scale combustors. Appl Energy 2011;88:4294–309. https://doi.org/10.1016/j.apenergy. 2011.07.021. [11] Hosseini SE, Wahid MA. Investigation of bluff-body micro-flameless combustion. Energy Convers Manage 2014;88:120–8. https://doi.org/10.1016/j.enconman. 2014.08.023. [12] Lee BJ, Yoo CS, Im HG. Dynamics of bluff-body-stabilized premixed hydrogen/air flames in a narrow channel. Combust Flame 2015;162:2602–9. https://doi.org/10. 1016/j.combustflame.2015.03.009. [13] Wenming Y, Dongyue J, Kenny CKY, Dan Z, Jianfeng P. Combustion process and entropy generation in a novel microcombustor with a block insert. Chem Eng J 2015;274:231–7. https://doi.org/10.1016/j.cej.2015.04.034. [14] Zhao D, Gutmark E, de Goey P. A review of cavity-based trapped vortex, ultracompact, high-g, inter-turbine combustors. Prog Energy Combust Sci 2018;66:42–82. https://doi.org/10.1016/j.pecs.2017.12.001. [15] Wang Y, Zhou Z, Yang W, Zhou J, Liu J, Wang Z, et al. Instability of flame in microcombustor under different external thermal environment. Exp Therm Fluid Sci 2011;35:1451–7. https://doi.org/10.1016/j.expthermflusci.2011.06.003. [16] Shirsat V, Gupta AK. Performance characteristics of methanol and kerosene fuelled meso-scale heat-recirculating combustors. Appl Energy 2011;88:5069–82. https:// doi.org/10.1016/j.apenergy.2011.07.019. [17] Vijayan V, Gupta AK. Combustion and heat transfer at meso-scale with thermal recuperation. Appl Energy 2010;87:2628–39. https://doi.org/10.1016/j.apenergy. 2010.03.011. [18] Giovannoni V, Sharma RN, Raine RR. Premixed combustion of methane – air mixture stabilized over porous medium : A 2D numerical study. Chem Eng Sci 2016;152:591–605. https://doi.org/10.1016/j.ces.2016.06.039. [19] Jiaqiang E, Peng Q, Zhao X, Zuo W, Zhang Z, Pham M. Numerical investigation on the combustion characteristics of non-premixed hydrogen-air in a novel microcombustor. Appl Therm Eng 2017;110:665–77. https://doi.org/10.1016/j. applthermaleng.2016.08.210. [20] Wan J, Fan A, Yao H. Effect of the length of a plate flame holder on flame blowout limit in a micro-combustor with preheating channels. Combust Flame 2016;170:53–62. https://doi.org/10.1016/j.combustflame.2016.05.015. [21] Meng L, Li J, Li Q. Flame stabilization in a planar microcombustor partially filled with anisotropic porous medium. AIChE J 2018;64:153–60. https://doi.org/10. 1002/aic.15862. [22] Li J, Chou SK, Li ZW, Yang WM. Experimental investigation of porous media combustion in a planar micro-combustor. Fuel 2010;89:708–15. https://doi.org/10. 1016/j.fuel.2009.06.026. [23] Yang WM, Chou SK, Chua KJ, Li J, Zhao X. Research on modular micro combustorradiator with and without porous media. Chem Eng J 2011;168:799–802. https:// doi.org/10.1016/j.cej.2010.12.083. [24] Peng QG, Jiaqiang E, Chen JW, Zuo W, Zhao XH, Zhang ZQ. Investigation on the effects of wall thickness and porous media on the thermal performance of a nonpremixed hydrogen fueled cylindrical micro combustor. Energy Convers Manage 2018;155:276–86. https://doi.org/10.1016/j.enconman.2017.10.095. [25] Chua KJ, Yang WM, Ong WJ. Fundamental experiment and numerical analysis of a modular microcombustor with silicon carbide porous medium: 2012. [26] Federici JA, Vlachos DG. A computational fluid dynamics study of propane/air microflame stability in a heat recirculation reactor. Combust Flame 2008;153:258–69. https://doi.org/10.1016/j.combustflame.2007.09.009. [27] Yang SI, Wu MS. ScienceDirect Properties of premixed hydrogen/propane/air flame in ceramic granular beds. Int J Hydrogen Energy 2014;9. https://doi.org/10.1016/

11

Applied Energy 260 (2020) 114352

Q. Peng, et al. enconman.2017.09.011. [54] Hashemi SM, Hashemi SA. Flame stability analysis of the premixed methane-air combustion in a two-layer porous media burner by numerical simulation. Fuel 2017;202:56–65. https://doi.org/10.1016/j.fuel.2017.04.008. [55] Li J, Richecoeur F, Schuller T. Determination of heat release rate disturbances in unconfined flames based on fluctuations in the travel time of ultrasonic waves. Combust Sci Technol 2012;184:533–55. https://doi.org/10.1080/00102202.2011. 649323. [56] Zhao D, Guan Y, Reinecke A. Characterizing hydrogen-fuelled pulsating combustion on thermodynamic properties of a combustor. Commun Phys 2019;2:1–10. https:// doi.org/10.1038/s42005-019-0142-8. [57] Najm HN, Paul PH, Mueller CJ, Wyckoff PS. On the adequacy of certain experimental observables as measurements of flame burning rate. Combust Flame 2002;113:312–32. https://doi.org/10.1016/s0010-2180(97)00209-5. [58] Yang H, Minaev S, Geynce E, Nakamura H, Maruta K. Filtration combustion of methane in high-porosity micro-fibrous media. Combust Sci Technol 2009;181:654–69. https://doi.org/10.1080/00102200802646748. [59] Peng Q, Yang W, Jiaqiang E, Xu H, Li Z, Yu W, et al. Experimental investigation on premixed hydrogen / air combustion in varied size combustors inserted with porous medium for thermophotovoltaic system applications. Energy Convers Manage 2019;200:112086. https://doi.org/10.1016/j.enconman.2019.112086.

[60] Bani S, Pan J, Tang A, Lu Q, Zhang Y. Micro combustion in a porous media for thermophotovoltaic power generation. Appl Therm Eng 2018;129:596–605. https://doi.org/10.1016/j.applthermaleng.2017.10.024. [61] Chen L, Xia YF, Li BW, Shi JR. Flame front inclination instability in the porous media combustion with inhomogeneous preheating temperature distribution. Appl Therm Eng 2018;128:1520–30. https://doi.org/10.1016/j.applthermaleng.2017. 09.085. [62] Ning D, Liu Y, Xiang Y, Fan A. Experimental investigation on non-premixed methane/air combustion in Y-shaped meso-scale combustors with/without fibrous porous media. Energy Convers Manage 2017;138:22–9. https://doi.org/10.1016/j. enconman.2017.01.065. [63] Jiang D, Yang W. Refractory material based frequency selective emitters/absorbers for high efficiency and thermal stable thermophotovoltaics. Sol Energy Mater Sol Cells 2017;163:98–104. https://doi.org/10.1016/j.solmat.2017.01.022. [64] Lei Y, Chen W, Lei J. Combustion and direct energy conversion inside a microcombustor. Appl Therm Eng 2016;100:348–55. https://doi.org/10.1016/j. applthermaleng.2016.01.162. [65] Pan JF, Huang J, Li DT, Yang WM, Tang WX, Xue H. Effects of major parameters on micro-combustion for thermophotovoltaic energy conversion. Appl Therm Eng 2007;27:1089–95. https://doi.org/10.1016/j.applthermaleng.2006.07.038.

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