Heat Recirculation Research Articles

Dynamics of a holder-stabilized laminar methane-air premixed flame in a preheated mesoscale combustor at ultra-lean condition

The flame dynamics at ultra-lean condition under the synergistic effect of heat recirculation and flow recirculation is experimentally studied in this work. The previously reported anomalous blow-off of lean H2/CH4/air premixed flame with Lewis number Le < 1.0 is obtained for the lean CH4/air premixed flame at Le ≈ 1.0, and the underlying mechanisms are analyzed in terms of heat recirculation, flow recirculation, preferential diffusion, and stretch effects. An anomalous standoff distance between the stable flame root and flame holder is also observed for the lean CH4/air premixed flame. The blow-off dynamics at large and small values of Reynolds numbers are quite different. The flame with local extinction and reignition appears in the blow-off process at large Reynold numbers (≥100), whereas it is absent at small Reynold numbers (<100). The results attained in this study can help to gain insights into flame dynamics in many practical combustors.

A simplified numerical approach to hydrogen and hydrocarbon combustion in single and double-layer porous burners

Motivated by the fuel hydrogen applications in porous combustors, as well as hydrogen production in syngas porous devices, this work shows a simplified one-dimensional, steady state heat and mass transfer model for stabilized premixed flames in porous inert media. Single-layer and double-layer porous burner are studied. The model has three conservation equations, describing the heat transfer in the solid and fluid phases and the mass transfer in the reacting flow. The model considers a plug flow and is solved numerically by using the finite volume method. The results are compared with benchmark data, depicting the superadiabatic flames and the heat recirculation process. A parametric analysis of the model reveals the effects of the porous media properties and the Lewis and Peclet numbers on the heat and mass transfer processes. Furthermore, the effects of the flame stand-off parameter in double layer porous burner are also analyzed. The results have considered the values of the dimensionless parameters based on reference data for hydrogen/air and methane/air combustion in porous burners built with SiC and Al2O3.

Investigation on premixed methane/air combustion characteristics in heat recirculation micro combustor with separating cylinder

Nowadays, the low combustion efficiency and combustion instability are the two major challenges in the design and development of meso/micro combustor. Heat recirculation and catalytic combustion are one of the best methods to solve those problems. The low combustion rate far from catalytic surfaces in micro combustor is a serious issue in catalytic combustion. To solve this problem, the novel micro combustor with separating cylinder is proposed. The fuel combustion efficiency in HRMCC is higher than that of HRMC at the same inlet velocity. When the inlet velocity is 0.5 m/s, the combustion efficiency gap between HRCC and HRMCC reaches up to the maximum value of 7.4 %. The methane conversion per unit catalyst in HRMCC is higher than that of HRMC, and it also increases as inlet velocity increases. Besides, the location of separating cylinder in HRMCC does not show obvious influence on combustion efficiency and methane conversion per unit catalyst. However, it has a great influence on the size of high temperature region in HRMCC. The high temperature area size becomes larger as the separating cylinder in HRMCC becomes further from outlet.

A non-monotonic blow-off limit of micro-jet methane diffusion flame at different tube-wall thicknesses

In order to provide guideline for choosing a suitable tube-wall thickness (δ) for the micro-jet methane diffusion flame, the effect of tube-wall thickness on the blow-off limit is investigated via numerical simulation in the present work. The results show that the blow-off limit of micro-jet methane diffusion flame firstly increases and then decreases with the increase of tube-wall thickness. Subsequently, the underlying mechanisms responsible for the above non-monotonic blow-off limit are discussed in terms of the flow filed, strain effect and conjugate heat exchange. The analysis indicates that the flow field is insignificant for the non-monotonic blow-off limit. A smaller strain effect can induce the increase of the blow-off limit from δ=0.1 to 0.2 mm, and a worse heat recirculation effect can induce the decrease of the blow-off limit from δ=0.2 to 0.4 mm. The non-monotonic blow-off limit is mainly determined by the heat loss of flame to the tube-wall and the performance of tube-wall on preheating unburned fuel. The smallest heat loss of flame to the tube-wall and the best performance of tube-wall on preheating unburned fuel result in the largest blow-off limit at δ=0.2 mm. Therefore, a moderate tube-wall thickness is more suitable to manufacture the micro-jet burner.

An asymptotic approach to heat recirculation in diffusion flames fueled by organic particles

Owing to their safety, stability and controllability, diffusion flames have found extensive applications in medicine and power generation. Regarding the significance of recirculation impact on micro-combustors, an efficient method should be developed for better analysis of the micro-combustors performance. In this paper, an asymptotic method is developed to model diffusion flames propagation through a biofuel in counterflow configuration with the consideration of heat recirculation effect. The flame structure includes pre-heat, post-vaporization and oxidizer zones. Micron-sized lycopodium particles and air can be regarded as biofuel and oxidizer, respectively. Mass and energy conservation equations are investigated in each zone. For evaluation of the thermal recirculation impact, a specific term is included in the energy conservation equation. Furthermore, the effects of changes in the flame temperature, mass fraction of the gaseous fuel and oxidizer (relative to fuel and oxidizer Lewis numbers), mass particle content, particle radius and equivalence ratio were examined considering and ignoring the thermal recirculation effect. The results indicate that increase of heat recirculation coefficient will rise the flame temperature and shift the flame position to the fuel nozzle side. Also, consideration of thermal heat recirculation will enhance the gaseous fuel production in the pre-heat and post-vaporization zones.

Role of H2/CO Addition to Flame Instabilities and Their Control in a Stepped Microcombustor

ABSTRACT The effect of H2/CO addition on flame dynamics of propane-air mixtures in a stepped microcombustor with heat recirculation was investigated experimentally. Unstable flame propagation regimes and their transition modes were observed for a range of mixture velocities (5–8 m/s) and equivalence ratios (0.6–0.95). Up to 60% H2 addition to lean propane-air mixtures helps improve the flame stability range in the combustor. The suppression of these flame instabilities with H2 addition is attributed to the increase in the overall mixture reactivity and burning velocity. The enhancement in OH intensity with H2 addition indicated a change in the elementary reactions, thereby affecting the flame dynamics. Contrary to H2 addition, CO addition does not show any noticeable effect on the formation of stable flame modes. The change in mixture properties and burning velocity leads to a change in the flame behavior and alters the heat release rate due to H2 addition to propane-air mixtures. A higher percentage of H2 addition led to the reappearance of the unstable (rotating) flames in the microcombustor. Intrinsic flame instabilities appeared at higher H2 percentages due to differential diffusion and increased thermal-wall coupling effects.

Edge flames in mixing layers: Effects of heat recirculation through thermally active splitter plates

A numerical study is carried out to investigate the stabilization and dynamic properties of the edge flame formed in the wake of two merging streams, one containing fuel and the other oxidizer, separated by a splitter plate. Several plates are considered to illustrate the effects of their thermo-physical properties on the edge flame for low- and high-Lewis-number mixtures. The objective is to provide a comprehensive understanding of the effects of the heat recirculation cycle, from the edge flame through the splitter plate and back to the fresh reactants, on the edge flame. A diffusive-thermal model is adopted, with the flow field determined by solving the incompressible Navier–Stokes equations in the vicinity of the plate trailing edge, and the combustion field determined by solving the transport equations with a constant density. Two distinct modes of flame stabilization are identified and examined: a stationary mode, where the edge flame is held stationary at a well-defined distance, whether attached to or lifted from the tip of the plate, and an oscillatory mode where the edge flame undergoes sustained oscillations relative to an (unstable) equilibrium position. To characterize the heat recirculation effect under varying flow/mixture conditions, we introduce the thermal sensing distance, as a heuristic parameter that determines whether substantial thermal interaction between the edge flame and the plate occurs, and the average output heat flux, as a universal measure characterizing the efficiency of the heat recirculation cycle.

Optical to Near-infrared Transmission Spectrum of the Warm Sub-Saturn HAT-P-12b

Abstract We present the transmission spectrum of HAT-P-12b through a joint analysis of data obtained from the Hubble Space Telescope Space Telescope Imaging Spectrograph and Wide Field Camera 3 and Spitzer, covering the wavelength range 0.3–5.0 μm. We detect a muted water vapor absorption feature at 1.4 μm attenuated by clouds, as well as a Rayleigh scattering slope in the optical indicative of small particles. We interpret the transmission spectrum using both the state-of-the-art atmospheric retrieval code SCARLET and the aerosol microphysics model CARMA. These models indicate that the atmosphere of HAT-P-12b is consistent with a broad range of metallicities between several tens to a few hundred times solar, a roughly solar C/O ratio, and moderately efficient vertical mixing. Cloud models that include condensate clouds do not readily generate the submicron particles necessary to reproduce the observed Rayleigh scattering slope, while models that incorporate photochemical hazes composed of soot or tholins are able to match the full transmission spectrum. From a complementary analysis of secondary eclipses by Spitzer, we obtain measured depths of 0.042% ± 0.013% and 0.045% ± 0.018% at 3.6 and 4.5 μm, respectively, which are consistent with a blackbody temperature of 890+60 −70 K and indicate efficient day–night heat recirculation. HAT-P-12b joins the growing number of well-characterized warm planets that underscore the importance of clouds and hazes in our understanding of exoplanet atmospheres.

Smaller than Expected Bright-spot Offsets in Spitzer Phase Curves of the Hot Jupiter Qatar-1b

Abstract We present Spitzer full-orbit thermal phase curves of the hot Jupiter Qatar-1b, a planet with the same equilibrium temperature—and intermediate surface gravity and orbital period—as the well-studied planets HD 209458b and WASP-43b. We measure secondary eclipse depths of 0.21 ± 0.02% at 3.6 μm and 0.30 ± 0.02% at 4.5 μm, corresponding to dayside brightness temperatures of K and K, respectively, consistent with a vertically isothermal dayside. The respective nightside brightness temperatures are and K, in line with a trend that hot Jupiters all have similar nightside temperatures. We infer a Bond albedo of and a moderate day–night heat recirculation efficiency, similar to HD 209458b. General circulation models for HD 209458b and WASP-43b predict that their bright spots should be shifted east of the substellar point by tens of degrees, and these predictions were previously confirmed with Spitzer full-orbit phase curve observations. The phase curves of Qatar-1b are likewise expected to exhibit eastward offsets. Instead, the observed phase curves are consistent with no offset: 11° ± 7° at 3.6 μm and −4° ± 7° at 4.5 μm. The discrepancy in circulation patterns between these three otherwise similar planets points to the importance of secondary parameters like rotation rate and surface gravity, and the presence or absence of clouds, in determining atmospheric conditions on hot Jupiters.

Effect of thermal condition of solid wall on the stabilization of a preheated and holder-stabilized laminar premixed flame

A novel combustor with a flame holder and preheating channels was developed to investigate the synergistic effect of heat and flow recirculation on combustion characteristics. This paper mainly focuses on the thermal condition of flame holder and combustion chamber wall (CCW) and their interaction on flame stabilization. Based on the distribution characteristics of temperature and flow fields, it can be deduced that the interaction between the flame holder and CCW can further increase the incoming fresh mixture temperature and decrease the height of recirculation zone, which subsequently affect the anchoring locations and temperatures of the flame root and top. To be more specific, the flame holder and CCW synergistically improve the preheating temperature of fresh mixture, making it higher than simple linear superposition. Moreover, the anchoring location of the flame root is mainly determined by the heat exchange of flame holder, but CCW matters more significantly for the flame top. Additionally, the anchoring temperature of the flame root is higher than that of the flame top. This study can help us to gain insights into the effect of conjugate heat exchange on flame stability when the effect of heat recirculation and flow recirculation works synergistically in practical combustors (like aero-engine).

Effects of bluff-body on the thermal performance of micro thermophotovoltaic system based on porous media combustion

To improve thermophotovoltaic system efficiency, a porous media combustor with a bluff-body as the flame holder was proposed. A two-dimensional numerical simulation of premixed hydrogen/air combustion inside a planar porous media channel with the local thermal non-equilibrium model was conducted. Effects of the bluff-body flame holder on mean wall temperature and system efficiency were examined. By inserting the bluff-body, the blowout limits of the planar porous media combustor for ϕ = 0.6, 0.8, 1.0, 1.2 are increased by 33%, 19%, 12%, 20%, respectively. Numerical results show that the flame stabilization is dominated by flow and heat recirculation. Further analysis reveals the mechanism of heat transfer on flame stabilization due to three heat recirculation paths. Considering the view factor and the external quantum efficiency, the system efficiency with the bluff-body is increased by 14.72% as compared to the one without the bluff-body at ϕ = 0.8, vin = 4 m/s.

Parametric analysis on the combustion and thermal performance of a swirl micro-combustor for micro thermophotovoltaic system

A swirl micro-combustor which has strong capability to stabilize the flame has been developed and is promising to be used in micro thermophotovoltaic power generators in this work. Heat recirculation plays a requisite role in flame stability and thermal performance of micro combustors. Thus, the effects of three parameters (center inlet radius w1, swirler width w2, and step height w3) that determine the size and location of the recirculation zones in a premixed hydrogen/air flame are investigated numerically. The results show that inner recirculation zone and corner recirculation zone generated by a swirler are critical for stabilizing the flame. The parameter w1 has an insignificant influence on the thermal performance. Enlarging the inner recirculation zone by decreasing w2 provokes enhanced heat transfer, and increases the temperature level and temperature non-uniformity of combustor walls. The size of corner recirculation zone is increased by increasing w3, which raises the outer wall temperature uniformity. Based on the parametric analysis, the newly designed micro combustor possesses a high wall temperature level. Additionally, the energy output increment is between 9.1% and 14.2% at hydrogen mass flow rate of 0.0006 g/s when stoichiometric hydrogen/air ratio ranges from 0.6 to 1.4.

Direct combustion of waste oil in domestic stove by an internal heat re-circulation atomization technology: Emission and performance analysis

Direct use of waste oil as fuel to meet the residential energy demands, is very attractive due to its potentials to decrease fossil fuel consumption, reduce pollution and increase sustainability. This paper uses a domestic stove with an internal heat re-circulation and self-atomization technology to burn yellow waste cooking oil (WCO-1), brown waste cooking oil (WCO-2) and waste lubricant oil (WLO). Emission factors (EFs), energy efficiency and modified combustion efficiency (MCE) of this combined fuel/stove system were determined under space-heating and cooking modes. The results showed that EFs of CO, PM2.5, total 16 PAHs and corresponding toxic equivalent quantity (TEQ) values ranged from 2.18 × 103 to 4.90 × 103 mg/MJnet, 16.36–69.40 mg/MJnet, 2.39–12.93 μg/MJnet and 0.16–0.92 μg of TEQ/MJnet. WCO-1 was verified to be the cleanest fuel with the highest energy efficiency (85.3 ± 3.3% and 90.4 ± 2.2%) and lowest emission levels, such as NO (53.75 ± 2.62 and 37.09 ± 5.41 mg/MJnet), NO2 (82.40 ± 3.96 and 56.87 ± 8.29 mg/MJnet) and PM2.5 (20.94 ± 6.55 and 16.35 ± 5.06 mg/MJnet) compared to WCO-2 and WLO. The estimated total cost of using waste oil for each household in winter was much cheaper than some current available clean energy means, including only USD$ 400 of stove price and USD$ 250/ton of fuel per year. It is a promising candidate choice for replacing low-quality solid fuels in rural China and 2.62 million rural households would achieve environmental and economic benefits if promoting direct combustion of waste oil for daily heating and cooking.

Towards the development of a high power density, high efficiency, micro power generator

An innovative method to use combustion-based source for portable power generation for Microelectromechanical systems with significant improvement in power density and conversion efficiency has been reported in this study. A triple microcombustor configuration with backward facing steps and associated provision for efficient heat recirculation is proposed for flame stabilization. The presence of three parallel combustion zones increases the combustion intensity compared to a single combustor configuration and enhances the overall thermal performance. A novel technique to extract maximum heat from the hot combustion products for thermoelectric power generation is implemented. The thermoelectric modules are mounted to facilitate direct contact of the module surface with the hot combustion products. A significant improvement in heat transfer rate from the hot combustion gases to the thermoelectric modules (~16 times) is achieved. An electric power output of 4.9 W with overall conversion efficieny of 5.09% at a mixture velocity of 10 m/s and a mixture equivalence ratio of 0.9 is achieved. This signifies an improvement of ~10–20% in overall conversion efficiency as compared to previously reported systems.

Investigation of microcombustion reforming of ethane/air and micro-Tubular Solid Oxide Fuel Cells

Thermal partial oxidation, or fuel-rich combustion, is a non-catalytic option for reforming hydrocarbons to synthesis gas for direct conversion in micro-Tubular Solid Oxide Fuel Cells (SOFCs). A number of studies have demonstrated the potential of using heat recirculation to sustain combustion at high equivalence ratios, where the concentration of synthesis gas can be maximized, but few have connected the fuel-rich combustion reforming to SOFCs to understand how the reforming effects the electrochemical reactions. This study investigates microcombustion of ethane/air at equivalence ratios from 1.0 to 5.0, flow rates of 10–250 mL min−1 and maximum wall temperatures of 800 °C, 900 °C and 1000 °C. The weak flame, flame with repetitive extinction and ignition (FREI) and normal flame regimes are characterized along with the exhaust composition at each condition. Micro-Tubular SOFCs (mT-SOFCs) open circuit voltage, polarization and power density are found to be effected by FREI. High fuel utilization of ~64% is achieved. Long term testing and comparison with a H2 baseline is reported.

Numerical investigation of the local extinction and re-ignition mechanisms of premixed flame in a micro combustor with a flame holder and preheating channels

A novel combustor with a flame holder and preheating channels, which takes advantage of heat and flow recirculation, was developed to improve the stability of the micro flame in our previous work. The results demonstrated that the lean methane/air premixed flame could be stabilized within a wide operating range. Certainly, the flame blows off when the equivalence ratio of the fuel mixture becomes too small or too big or the Reynolds number is too large. The present work reports and analyzes the blow-off dynamics of a methane/air laminar premixed flame at a relatively large Reynolds number under the synergistic effect of heat and flow recirculation. Interestingly, a flame with local extinction and re-ignition appears, which is first found in the micro combustor. The underlying mechanisms of this interesting phenomenon are systematically discussed in terms of the flow recirculation effect, stretch effect, conjugate heat exchange, and preferential transport effect. We find that the counter flow in the recirculation zone and stretch effect induce the local extinction of the flame. The smaller heat loss from the residual flame to the flame holder and the better preferential transport effect contribute to the re-ignition of the fuel mixture. The preheating effect on the unburned fuel mixture is not expected to be insignificant for the local extinction and re-ignition phenomenon. These results mean that the generated mechanism of the flame with local extinction and re-ignition in the micro combustor is different from that in the conventional combustors.

The role of dilution level and canonical configuration in the modeling of MILD combustion systems with internal recirculation

MILD combustion regime emerged as one of the most promising technology to ensure low pollutant emissions and large fuel flexibility.Large-scale use of such technology requires important developments of its numerical modelling. The difficulty of this task lies in the recirculation of mass and heat that results in coupling chemistry and turbulence phenomena.In this work, the effect of the internal exhaust gas recirculation on the performances of a tabulated chemistry model was investigated by carrying out an experimental and numerical study on a cyclonic burner. FGM (Flamelet Generated Manifold) model was adopted. It makes two main assumptions: a turbulent flame can be considered as an ensemble of 1D laminar flames (Flamelets), and the space of compositions can be represented by a lower-dimensional manifold by defining few controlling variables. In this work, two different 1D flame configurations and two controlling variables (a mixture fraction and a progress variable) were used. The comparisons between model and experiments were reported for different dilution conditions at inlet and two diluent species.Results showed that the role of the internal EGR is crucial in the establishment of MILD regimes. In fact, for air conditions the model is not able to predict the low and more distributed reactivity of the system. On the other hand, inlet diluted conditions show a good agreement with the experiments due to burned product effects. CO2-dilution instead of N2, improves the model results due to the lower reaction rates. Finally, the Flamelet configuration does not strongly affect the simulation results.

Characteristics of liquid fuel combustion in a novel miniature vortex combustor

Miniature liquid fuel combustor in power generation requires confining combustion in the combustor chamber. However, this process faces substantial challenges that include higher heat loss, poor fuel vaporization rate, and low mixing residence time. In response to this challenge, a new method is proposed by introducing novel miniature combustor utilizing the advantage of the vortex flow motion that consists of two parts, a double chamber (inner tube and outer tube) at the top and a vortex-trapped chamber that is attached to anchor the flame at the bottom. The present combustor was operated by using n-heptane liquid fuel at fuel flow rate of 1.0–2.5 mL min−1 and airflow rate of 7.0–20.0 L min−1. The results showed that the heat transfer and recirculation mechanism sustain the combustion in the combustion chamber at fuel-lean regime. Confinement of the combustion inside the combustion chamber is attributed to recirculation phenomenon that was demonstrated through the enhancement of residence time and mixing rate. It is observed that the annulus enhanced the fuel vaporization rate, which is verified by measuring the temperature distribution at the combustor surfaces. The results of the thermal analysis also showed that the heat transfer enhancement is attributed to the vortex flow motion.

The role of heat recirculation and flame stabilization in the formation of NOX in a thermo-photovoltaic micro-combustor step wall

The health and durability of micro thermophotovoltaic systems are contingent upon the level of gaseous emissions of micro combustors regarding their small size, thickness, and compactness. In small combustion devices, the flame stabilization is achieved via conjugated heat transfer from the stabilized flame to the fresh reactant via the step of the micro-combustors. The step could also create a recirculation of products, and a stagnation zone for the fluid, as a result leading to the accumulation of pollutants. In turbulent H2 flame, the main attention is given to the NOX as no other noxious emission, especially carbon emission (CO, CO2, PAH, and VOC), form during the combustion of hydrogen. The existence of NOX in the presence of water, as in the combustion of hydrogen is prevalent, could lead to corrosion in combustor interior walls and other detrimental impacts for the ecosystem. In the presented work, micro-combustion of H2 flame in a cylinder with a step is simulated and the formation of nitrogen oxides is analyzed. The influence of different combustor specifications (equivalence ratio, solid materials) NOX species are discussed and evaluated. Results revealed nitrogen oxides form and accumulate in the vertical step of the microchannel and that the microchannel walls are more prone to the high concentrations of nitrogen oxides. The application of cavity promotes the two-dimensionality of flow, resulting in effective heat transfer from the hot gas to the cavity walls. This not only leads to flame anchoring to the cavity walls but also results in significant NOX.

Performance Evaluation and parametric optimization of a Heat Sink for Cooling of Electronic Devices with Entropy Generation Minimization

Thermal management is essential for electronics products. Experimental and CFD analysis of a heat sink for thermal management are represented. After analysis, compared with the experimental and computation analysis. Also, Entropy generation model, fluid flow model and heat transfer model are described. Total entropy generation calculated Sgen and entropy generation minimization are done for best utilization of heat sink for cost, size and better power management. Optimized heat sink by two method one Parametric Optimization and other Multi-variable optimization for minimizing entropy generation. From the results behaviour of mass flow rates with temperature, velocity, pressure drops and other parameters are analysed. The maximum temperature is located at heated base surface of heat sink, below the channel outlet, due to the low velocity of the fluid flow and resulting high concentration of heat flux. The combined conduction–convection heat transfer in the heat sink produces very complex three-dimensional heat flow pattern with large, longitudinal, upstream directed heat recirculation zones in the highly conducting aluminium materials, where the fluid and solid are in direct contact. A detailed description of the average heat transfer coefficient, temperature, heat flux and Nusselt number was obtained. The goal of this research is to find the cooling of heat sink at minimum entropy generation for saving energy and sustainable development.