Micro-thermophotovoltaic System Research Articles

Reacting flow characteristics based on the axis-switching phenomenon in a baffled micro combustor with rotated noncircular holes for micro-thermophotovoltaic system

• The geometric features related to the axis-switching phenomenon are numerically investigated for a multihole baffled micro combustor. • In the combustor of noncircular holes, secondary flows are more developed. • The flow mixing and combustion efficiency are improved by noncircular air holes. • The best thermal field for a combustion-based TPV system is obtained for baffles with 30°-rotated square holes and 15°-rotated triangular holes. • The emitter efficiency for TPV applications is better than the swirling flow combustor. For a thermophotovoltaic (TPV) generator, a combustor design for strengthening the axis-switching phenomenon is explored. Various simulations based on a Reynolds stress model and detailed chemical mechanism of GRI 3.0 are performed for methane (CH 4 )–air flames in a multihole baffled combustor. The reacting flow fields are investigated with the baffle plates of rotated square and triangular holes. The spreading and merging of multiple air jets past the baffle plate are significantly affected by rotating noncircular air holes. Considering the crossover location changes of the jet half-width, the air jets have a flow structure that quickly generates the axis-switching phenomenon. Compared with the baffled combustor of circular air holes, the center recirculation zone in the combustor of noncircular holes is enlarged by approximately 200% with a hole rotation. As the hole rotation increases, secondary flows become more complicated and the merging of air jets is quickly finished. Based on these flow structures, the region with the stoichiometric condition is observed in various forms of hexagonal, star, circular, and flower shapes. As a result, the flammable region and combustion efficiency are considerably increased. The flame lengths are roughly evaluated as three times the mixing lengths. From the viewpoint of a heat emitter for the TPV application, the heat transfer on the combustion chamber wall is discussed. Compared with the combustor with circular air holes, the combustor with noncircular air holes shows a 5% increase in flammable region and a 5.9% improvement in combustion efficiency. For a combustion-based TPV system, the best uniformity of wall temperature is observed for baffles with 30°-rotated square holes and 15°-rotated triangular holes and the highest mean wall temperatures are obtained when the flame length is approximately 1/3 of the combustor length.

Dilution effect on thermal performances and NO emission characteristics of an ammonia-oxygen micro-thermophotovoltaic system

• Thermal performances, emission characteristics and flammability behaviors of a diluted ammonia/oxygen micro-system are numerically investigated. • The dilution could reduce the wall temperature to protect the combustor. • Increasing the N 2 dilution rate β N 2 leads to a decreased outer wall temperature non-uniformity and NO emissions. • The flammability limit is sensitive to the dilute gas type. Ammonia (NH 3 ) is considered to be an attractive carbon-free fuel. However, the low flame speeds of ammonia/air hinder its applications. Using oxy-fuel combustion technology is an efficient method to address these issues. Nevertheless, dilution gases are still needed in oxy-combustion flames to protect the combustor and to reduce emissions. In the present work, the effects of 1) the N 2 dilution rate β N 2 and 2) the dilute gas types (nitrogen, argon, or carbon dioxide) on the thermal performances, NO x emissions, and the flammability behaviors of an ammonia/oxygen micro-combustor are evaluated. Numerical results indicate that the N 2 dilution rate plays a critical role in determining the thermal performances and NO emissions of the micro-combustor. It is found that increasing β N 2 is shown to be accompanied with a lower mean outer wall temperature and a higher outer wall temperature uniformity. However, a high dilution rate would lead to a sharp increase in the mean non-uniformity of the outer wall temperature R ¯ Tw and result in the flame blow-off at high flow rate conditions. The dilution rate corresponding to the maximum radiation efficiency decreases with the increase of the inlet velocity. Furthermore, increasing β N 2 contributes to the reduction of NO emissions. Finally, the effects of three types of diluting gases are evaluated by comparing the results of using 1) nitrogen, 2) argon, and 3) carbon dioxide. The present work provides a preliminary design of the diluted ammonia-oxygen micro-thermophotovoltaic system.

Effects analysis on the catalytic combustion and heat transfer performance enhancement of a non-premixed hydrogen/air micro combustor

In this work, a type of non-premixed hydrogen/air micro combustor is designed for the application of micro-thermophotovoltaic (MTPV) systems. The micro combustor is presented with a backward-facing step and unique inlet shape to enhance the mixing performance and flame stabilization. The combustion, flow, and heat transfer characteristics of non-premixed hydrogen/air combustion in micro combustors with/without catalyst segment are numerically investigated. The results indicate that the homogeneous reaction is obviously weakened in the catalytic combustor, but a higher and more uniform outer wall temperature is obtained, and the outer wall temperature difference can be decreased by up to 28%. The flow characteristics of gaseous mixture in the catalytic combustor are better than those in the non-catalytic combustor. When the inlet velocity is 10 m/s, the average flow velocity and pressure loss of the catalytic combustor are decreased by up to 5.6% and 250 Pa, respectively. Furthermore, the heat of reaction and total heat flux at the gas–solid interface of the catalytic combustor are obviously higher and lower than those of the non-catalytic combustor, and the outer wall heat loss ratio is increased by up to 0.59% when the inlet velocity is 10 m/s. All in all, it can be concluded that adopting catalytic combustion to the micro combustor is extremely suitable for the application of MTPV systems.

Beyond the combustion chamber: Heat transfer and its impact on micro-thermophotovoltaic systems performance

Conversion of thermal radiation into electrical current in thermophotovoltaic devices is directly influenced by the heat transfer, since the cell efficiency depends on the temperature. For micro-scale devices, heat management is yet more important due to the proximity between the components. A detailed study in these systems needs to considers the presence of wavelength-selective optical filters, used to reflect undesired radiation. In this study, we propose a model where the dependence of the physical parameter with the radiation wavelength is taken into account through a discretization of the radiation spectrum into several bands and estimation of average values. By coupling this procedure with a computational fluid dynamics solver, the cell efficiency as a function of geometrical and operational parameters can be estimated. The results indicate the existence of an optimal emitter/cell distance, depending on the emitter temperature and the heat transfer coefficient between the cell and the external medium. A secondary cooling mechanism, the air flow between the emitter and the filter, can also help to reduce the cell temperature. While the emitter average temperature has a strong impact on the cell efficiency, the temperature distribution in the emitter surface has no significant influence, allowing the use of average values.

Influence of hole size and number on pressure drop and energy output of the micro-cylindrical combustor inserting with an internal spiral fin with holes

Targeting at optimizing the energy output and thermal performance of the micro combustor in the application of micro-thermophotovoltaic (MTPV) systems, but the introduction of spiral fins brings higher pressure loss. Thus, a novel design of the micro combustor with the spiral fin opening is developed. The influence of inlet velocities, the hole size and hole number of the spiral fin on the pressure drop and thermal characteristic and energy characteristic are numerically investigated. It's illustrated that the spiral fin opening is conductive to decrease the pressure loss and optimize the outer wall temperature distribution, but has a negative influence on increasing the mean outer wall temperature of the micro combustor and energy output in the MTPV systems. With the increase of the hole size and hole number of the spiral fin, the pressure loss decreases and the outer wall temperature uniformity increases significantly, while the mean outer wall temperature drops and total energy output decreases. The better performance obtains when the micro combustor with spiral fin with four 0.5 mm holes.

Comparative investigation of combustion and thermal characteristics of a conventional micro combustor and micro combustor with internal straight/spiral fins for thermophotovoltaic system

The energy output and energy conversion efficiency of MTPV system are relatively low due to the energy loss. In order to improve the energy output of micro-thermophotovoltaic (MTPV) system, the internal straight and spiral fins are introduced into the micro combustor. The impact of hydrogen mass flow rate, equivalence ratio, and materials on the thermal performance are investigated. The increase of hydrogen mass flow rate brings higher average outer wall temperature, but the temperature difference also increases and the temperature uniformity becomes worse. The equivalence ratio of 1 is suggested to obtain higher average outer wall temperature and better temperature uniformity. The materials with higher thermal conductivity can obtain better thermal performance. Meanwhile, the higher thermal conductivity can also reduce the impact of introduction of internal fins.

Heat transfer enhancement and exergy efficiency improvement of a micro combustor with internal spiral fins for thermophotovoltaic systems

• A novel micro combustor with internal spiral fins was proposed. • Studied the influence of internal spiral fin number on thermal performance. • Investigated the impact of spiral fin length on heat transfer characteristics. • Studied the impact of internal spiral fin pitch distance on thermal performance. Targeting at addressing the low exergy efficiency of micro thermophotovoltaic (MTPV) system, a novel micro combustor with internal spiral fins is proposed. The geometrical parameters, including internal spiral fin number, length and pitch distance, on the thermal performance and exergy efficiency are numerically investigated. The introduction of internal spiral fins can effectively improve the exergy efficiency and energy output of MTPV system, but with higher pressure loss. The results show that the micro combustor with 8 spiral fins exhibits the highest exergy efficiency of 66.9%, which is 6.9% higher than that of the conventional one at the inlet velocity of 5 m/s. Moreover, the micro combustor with 8 spiral fins obtains the highest energy output is 5.01 W at inlet velocity of 8 m/s. Taking the pressure loss into consideration, the optimal internal spiral fins length of 10 mm is suggested.

Effects of propane addition and burner scale on the combustion characteristics and working performance

In order to investigate the micro combustion characteristic, thermal performance, as well as boost the energy efficiency and extend its application, the effects of burner size and fuel properties on combustion and working performance are studied. The results indicate that 5% propane added to premixed H2/air combustion contributes to the flame relocation and flame stretch, affecting the thermal performance of burner. It also enhances the heat transfer of gas-wall and increases the radiation power at a lower fuel flow rate. Moreover, the chamber diameter and heat recirculation zone also effectively alter the working performance of the burning reactions and extend the limit of flame blow-off, and a more uniform temperature profile along the outer wall can be achieved in the tube with suitable step height or larger size. It is also found that the small tube size boosts the radiation temperature and limits the chemical input, while the heat release from burning is more difficult to be transferred to the wall in the large chamber. Subsequently, a comparison study indicated that the micro-thermophotovoltaic system with InGaAsSb photovoltaic cells and tube T5-2 is more suitable for the application and generates an electrical power output of 1.74 W when the hydrogen flow rate is 876 mL/min and the equivalence ratio is 1.0.

Beyond the Combustion Chamber: Heat Transfer and its Impact on Micro-Thermophotovoltaic Systems Performance

Beyond the Combustion Chamber: Heat Transfer and its Impact on Micro-Thermophotovoltaic Systems Performance

Numerical investigation on a multi-channel micro combustor fueled with hydrogen for a micro-thermophotovoltaic system

Targeting at improving the energy output of the micro-thermophotovoltaic (MTPV) system, the heat transfer performance and energy output of the conventional micro combustor (CMC) and the multi-channel micro combustor (MCMC) with various channel shapes in the MTPV system are compared. The mean outer wall temperature of the MCMC with diamond channel is improved by 86.5 K than that of the CMC at the inlet hydrogen mass flow rate (m˙in,hydrogen) of 1.64 × 10-6 kg/s. Furthermore, four arrangements of MCMC are compared in this work. The double-layer MCMC with layers counterflow exhibits the highest total energy conversion efficiency of 13.2% when the m˙in,hydrogen is 8.10 × 10−7 kg/s. The equivalence ratio (Φ) of premixed hydrogen/air plays a crucial role on the thermal performance of DMCMC-layer counter flow. The outer wall temperature and its uniformity becomes better as the equivalence ratio increases. The MPTV system with a DMCMC-layers counterflow can obtain an energy output of 19.3 W when the m˙in,hydrogen is 1.64 × 10-6 kg/s and Φ is equal to 1. Namely, the multi-channel micro combustor has the potential to be a novel energy resource for the MTPV system.

Non-premixed combustion characteristics and thermal performance of a catalytic combustor for micro-thermophotovoltaic systems

Micro-combustors with reliable flame stability and high surface temperature are crucial for micro-thermophotovoltaic devices. Recently, we verified the feasibility of catalytic combustion for non-premixed CH4/air in an asymmetrical planar micro-combustor. The upper limit of operational average velocity is 1.2 m/s and the maximum combustion efficiency is 52.4%. In the present work, a symmetrical configuration was proposed to improve the performance of the prototype combustor. Numerically investigation shows that the symmetrical combustor can be operated at an average velocity above 9 m/s, and the maximum combustion efficiency and surface radiation efficiency are 98.5% and 25.9%, respectively. In addition, as the average velocity is increased, the combustion efficiency decreases monotonically while the radiation efficiency and radiant energy output increase first and then decrease. Moreover, the combustion efficiency achieves its peak value under stoichiometric ratio. The accumulation of O2 near the Pt surface under fuel lean condition causes a sharp increase in O(s) coverage and a decrease in heat release rate. Furthermore, a larger wall thermal conductivity leads to more uniform wall temperature distribution and better heat recirculation effect on the incoming gases. However, a medium thermal conductivity of 1–10 W/(m·K) favors a larger radiant energy output.

Thermodynamics and entropy generation studies of a T-shaped micro-combustor: Effects of porous medium and ring-shaped ribs

In order to achieve a better energy conversion efficiency, a T-shaped micro-combustor with porous medium and ring-shaped ribs is proposed and studied in this work. For this, 3D model of a premixed H2/air-fuelled combustor is developed with k-ε turbulence and EDC chemical reaction models applied. The model is first validated with the experimental data available in the literature and then used to evaluate the effects of (1) the rib, (2) inlet flow velocity of the fuel–air mixture, (3) the axial location, (4) the porosity of porous medium, and (5) its size. The temperature difference is observed to be reduced by 60.73 K than that of conventional I-shaped combustor with the same surface area as the inlet velocity is 5 m/s. In addition, the size and location of porous medium are found to affect temperature distribution. To gain insights on the energy losses and thermodynamic efficiency, entropy generation analysis is conducted. It is revealed that the thermodynamic second law efficiencies of the combustors with porous medium are found to exceed 60%. This study proposed a novel design of a micro-combustor with porous medium implemented for micro-thermophotovoltaic system which can achieve a more uniform outer wall temperature and a higher energy conversion efficiency.

Numerical investigations on the liftoff velocity of H2-air premixed combustion in a micro-cylindrical combustor with gradually changed section area

Sustaining and stabilizing flames are crucial issues in micro-combustion. In some micro-electro-mechanical systems such as the micro-thermophotovoltaic system, the flame should be formed in the combustion chamber, not outside it (combustion without liftoff). So, study of the liftoff phenomenon is important and vital in these systems. The aim of this study is to evaluate effect of changing combustor section area on the critical liftoff velocity in a micro-cylindrical combustor. For this purpose, the critical liftoff velocities are numerically identified for four combustor configurations (convergent, divergent, convergent-divergent and divergent-convergent combustion chamber). Premixed mixture of hydrogen-air has been used as reactants for the current investigation. Turbulence model implemented in this paper is RNG k-epsilon and combustion reaction was modeled with 10 species and 21 steps scheme using Eddy Dissipation Concept model. Two non-dimensional numbers d1/d2 (inlet to outlet diameter ratio) and d1/d3 (inlet to throat diameter ratio) are defined. For d1/d2 > 1.0, the combustion chamber is convergent, otherwise it is divergent. When d1/d3 > 1.0, the micro combustor is convergent-divergent and for d1/d3 < 1.0, the micro combustor is divergent-convergent. The results indicate that with increasing d1/d2, the liftoff occurs in a lower inlet flow velocity. With varying d1/d3, from 0.71 (2.0/2.8) to 1.0 (2.0/2.0), the liftoff velocity is reduced. Based on the numerical results, it can be said that the use of convergent and convergent-divergent combustion chamber decreases liftoff velocity. Meanwhile, the combustor with diverging and diverging-converging structure can enhance liftoff velocity. In the same condition, critical liftoff velocity of divergent-convergent micro combustor is the highest among all cases and this configuration is appropriate for Micro Electro-Mechanical Systems that work with high inlet velocity.

Numerical investigations on thermal performance enhancement of hydrogen-fueled micro planar combustors with injectors for micro-thermophotovoltaic applications

In order to address the issue about the low output power of the micro-thermophotovoltaic system, in this work, the inlets of the traditional micro planar combustor are modified into injectors for better thermal performance and higher output power of the micro-thermophotovoltaic system. The thermal performance of traditional and modified micro planar combustors is numerically investigated and compared. It is found that the thermal performance of the modified micro planar combustor is greatly enhanced with the help of the injectors. This is due to that the reduction of the step diameter leads to the rise of the velocity of the inlet flow, resulting in the chemical reaction zone is closer to the center of the combustion chamber in the micro planar combustor. Furthermore, the positive effects and negative effects on thermal performance enhancement brought by the reduction of the step diameter of injectors are vividly discussed under different hydrogen mass flow rate, hydrogen/air equivalence ratio and solid wall material. Finally, some guidelines are proposed for the applications of the micro planar combustors with injectors in the micro-thermophotovoltaic system. This work provides us a simple and practical way to address the issue about the low output power of the micro-thermophotovoltaic system.

Comparative study of combustion and thermal performance in a swirling micro combustor under premixed and non-premixed modes

A swirling micro combustor fueled by hydrogen/air has been proposed based on the concept of swirling flow stabilization. The effects of premixed and non-premixed combustion modes on the combustion efficiency, flame stability, and thermal performance of the micro combustor at various hydrogen/air mass flow rates and equivalence ratios are discussed and compared. The results show that premixed mode has a lower flame location than non-premixed mode, and the flame location of non-premixed mode is more susceptible to changes of hydrogen/air mass flow rate and equivalence ratio than premixed mode. When the hydrogen mass flow rate is lower, the non-premixed mode is more suitable for micro thermophotovoltaic (MTPV) system because the wall temperature is more uniform. However, the premixed mode has a higher and more uniform wall temperature distribution at high hydrogen mass flow rate. Moreover, the hydrogen/air equivalence ratio has a significant effect on the thermal performance in the swirling micro combustor, and the advantage of a high and uniform wall temperature distribution of premixed mode is more obvious at small equivalence ratio. This work provides a reference for the choice of micro combustor combustion modes under different hydrogen/air mass flow rates and equivalence ratios in the MTPV system.

A novel combined baffle-cavity micro-combustor configuration for Micro-Thermo-Photo-Voltaic applications

The major issues of Micro-Thermo-Photo-Voltaic (MTPV) micro-combustors are flame instabilities, which narrow the operational range, and non-uniform wall temperature, which lowers the overall efficiency. The purpose of the present research is to propose a novel micro-combustor with combined baffle and cavity configuration to address these issues. For this aim, a numerical modeling approach is validated and used. The performance of the improved combustor is compared with another recent baffle-bluff configuration. It is shown that the novel design improves the average wall temperature by 36.4 K and mitigates its standard deviation by 13.4 K. Moreover, using the optimal baffle thickness, these enhancements can be augmented by 4% raise of average wall temperature, 62% increase of temperature uniformity, and 20% reduction in overall emission. The baffle length of 0.6 times the combustor length and thickness of 0.0625 times the baffle spacing result in the optimal operation due to the flame lift-off in the upstream direction. According to the sensitivity analysis, the most effective geometrical parameters are the baffle length and thickness. It is expected that using this novel micro-combustor with optimized design parameters improves the overall efficiency of MTPV systems.

Experimental and numerical investigation of a micro-thermophotovoltaic system with different backward-facing steps and wall thicknesses

The combustion characteristics and thermal performance of a premixed H2/air combustion in the micro-combustor with a backward-facing step is studied through experimental tests and numerical simulations. The effects of step length and wall thickness on the flame stability and thermal performance of the micro-combustor are presented and compared. The results show that the outer wall temperature can be improved by the reduction of wall thickness and modified through the relocation of the high temperature zone. With the decrease of the step length, the flame location is reduced in the combustion chamber, while the same location of peak temperature is obtained in the combustors with varied wall thicknesses. The efficiency of the micro thermophotovoltaic (micro-TPV) system with GaSb and InGaAsSb PV cells are calculated and compared. The micro-TPV system with GaInAsSb PV cells and the micro-combustor with step length 5 mm and outer diameter 3.6 mm produced the highest energy output and efficiency.

Enhancement of thermal performance by converging-diverging channel in a micro tube combustor fueled by premixed hydrogen/air

As the core component of the micro thermophotovoltaic (MTPV) system, the micro combustor with a high and uniform wall temperature distribution is beneficial to improve the energy conversion efficiency. In this paper, a micro tube combustor with converging-diverging channel is proposed and the thermal performance is numerically investigated, compared with that of the micro combustor with cylindrical channel. The effects of inlet velocity of H2/air mixture, dimensionless position and diameter of throat, and solid material on the thermal performance are widely analyzed. Results show that the outer wall temperature and emitter efficiency of the micro combustor with converging-diverging channel are higher than that of the micro combustor with cylindrical channel, and the converging-diverging channel has more uniform temperature distribution. The converging-diverging micro combustor with dimensionless throat position l = 0.375 and dimensionless throat diameter β = 0.4 is more suitable for the application of MTPV system. When H2/air inlet velocity is 11 m/s and H2/air equivalence ratio is 1.0, the mean wall temperature is increased by 82.39 K and the emitter efficiency is increased by 6.59%, while the normalized temperature standard deviation is reduced by 65.85%. Additionally, the use of SiC as wall material can improve the thermal performance of the micro combustor. It is worth noting that this work will offer us significant guidelines for the optimized work of micro tube combustor.

The effect of embedded high thermal conductivity material on combustion performance of catalytic micro combustor

A novel micro combustor embedded with high thermal conductivity material has been proposed for micro-thermophotovoltaic system application. The effect of the materials, which is graphite and nickel with widths 1 mm and 3 mm on the thermal and radiant performance of the new micro combustor, was experimentally and numerically investigated. It was found that high and uniform temperature distribution along the walls of the combustor was obtained by embedding graphite with width of 1 mm because of the enhanced heat transfer, which is desirable for the micro-thermophotovoltaic system. The available radiation energy increased from 4.6 W for the conventional combustor to 5.2 W for the novel combustor when the inlet velocity was 1.0 m/s and the hydrogen/oxygen equivalence ratio was 0.5 under the experimental conditions. The available radiation efficiency increased from 8.9% to 9.8% and the effect of the embedded material could be enhanced by increasing the chemical energy input. Based on the combustor and fuel in this study, the conditions for good thermal performance are graphite with width of 1 mm, inlet velocity of 1.0 m/s and 0.5 equivalence ratio of hydrogen/oxygen by considering the reliability and durability of the combustor.

Numerical study on energy conversion performance of micro-thermophotovoltaic system adopting a heat recirculation micro-combustor

In order to evaluate the energy conversion performance of micro-thermophotovoltaic system (MTPV) effectively, a novel calculation method combining Fluent with MATLAB program is proposed, which has considered the non-uniformity of temperature distribution of radiant surface, the angular coefficient between the emitter and the PV cell and its cooling efficiency. In this paper, numerical study on hydrogen/air premixed combustion in micro-combustors integrated with MTPV system to produce electricity has been adopted. Results show that the overall efficiency of heat recirculation combstor is almost twice as large as obtained in straight channel combustor. Furthermore, dimensionless baffle length, a key parameter in designing micro-combustor with heat recirculation, has been investigated. It is concluded that the subsystem efficiencies of MTPV system are increasing first and then descending with an increase in dimensionless baffle length. However, as the dimensionless baffle length reaches a certain point, the working performance of subsystems changes little. Meanwhile, it is demonstrated that increasing the dimensionless channel gap reasonably will improve the combustion process. Finally, the energy conversion performances as a function of hydrogen volume flow rate are calculated and compared. Results indicate that choosing a proper inlet velocity is of great pivotal to gain a better energy conversion performance of MTPV system. Of all the investigated cases, the maximum power generated with the MTPV is 1.4014 W and the overall efficiency is 0.7092%.