Meso-scale Combustor Research Articles

Thermodynamic and entropy generation analyses of Telsa-valve structured meso-scale combustors fuelled with hydrogen for thermophotovoltaic applications

The present work numerically examed the effects of hydrogen volume flow rates (Qv) and equivalence ratios (Φ) on thermal performances of meso-scale combustors structured with Tesla-vale type flow channels. Four distinct Tesla-valve configurations are implemented and then subsequently compared. The reverse-flow Tesla valve with counter-flow combustor (RC) structure shows a remarkable improvement of 72.6 % to the combustor wall temperature at Qv = 100 mL/min. The diodicity (Di) of the Tesla valve is found to be increased with a higher Qv, and a lower Φ contributes to a higher Di. Besides, Di is decreased when Φ goes up, stabilizing at Φ = 0.9. The reverse-flow Tesla valve exhibits a more uniform pressure distribution and entropy production than the forward-flow Tesla valve. At Φ = 0.9, the hydrogen-to-air ratio maximized heat release, producing the highest entropy. Tesla-valve structured combustors demonstrate near complete combustion before Φ reaching 0.9, the combustion efficiency (ηcombustion) is gradually decreased after Φ getting to 1.0. The RC construction achieved maximum combustion efficiency under conditions of insufficient oxygen. This present study demonstrates the feasibility of enhancing thermal and overall performances of meso-scale combustors structured with Tesla-valve channels, and revealing the key parameters effects on the combustion characteristics in these combustors.

The Effect of Insulation Thickness on Heat Transfer Characteristics and Flammability in Tube Mesoscale Combustors

Micropower generator is a micro-scale energy source that has two main components, namely a micro/mesoscale combustor and thermophotovoltaics (TPV). The micro-scale combustor is one part that functions as a combustion chamber that produces heat in micropower plants. Heptane is used as fuel, while the combustor combustion chamber with a diameter of 3.5 mm is made from duraluminium-quart glass tube. Combustion stability in the combustion chamber is influenced by several factors, such as temperature, geometry, and combustion chamber design. In order to maintain flame stability, mesh is added to the combustion chamber. One way to minimize heat loss in the combustion chamber is to add an insulating layer to the combustion chamber. This research aims to prove the role of adding an insulating layer in flame stability in mesoscale burners. It is necessary to add an appropriate insulating layer to minimize heat loss so that it remains stable in the mesoscale burner. This experimental test shows that the temperature distribution when adding an insulation layer with a thickness of 3 mm has a higher temperature on the outside compared to a thickness of 6 mm. Meanwhile, the temperature inside the combustor chamber with a thickness of 6 mm is superior to that with a thickness of 3 mm. The flame limit of the combustor with a mesh distance of 5 mm for liquid heptane fuel was successfully stable at an equivalent ratio of ɸ0.97 – 1.5 with a maximum speed of 31.7.

Characterization of Combustion in Cylindrical Meso-Scale Combustor with Wire Mesh Flame Holder as Initiation of Energy Source for Future Vehicles

The research aims to analyze and reveal combustion characteristics in a Cylindrical Meso Scale (CMS) Combustor with a wire mesh flame holder as a reference for designing a compact, efficient, and high-density energy source for future vehicles. This experiment analyzes the combustion ’s of a butane gas (C4H10)-air mixture in a cylindrical meso-scale (CMS) combustor with the addition of wire mesh flame holder on the stability of the combustion flame, as initiation of future vehicle energy source. The diameter of the CMS combustor with wire mesh flame holder is varied to give an idea of the effect of heat loss on the combustion flame's characteristics. The results show that the wire mesh as a flame holder is essential in the combustion stabilization mechanism. A stable flame can be stabilized in a CMS combustor with wire mesh. Variations in the diameter of the CMS combustor will result in variations in the surface-to-volume ratio, heat loss, and contact area of the wire mesh flame holder. At a large diameter, it produces the characteristics of a combustion flame with a more stable flame stability limit than a smaller diameter, a dimmer flame visualization than a smaller diameter at the same air and fuel discharge, a more distributed flame mode map area than the smaller diameter, lower flame temperature and combustor wall temperature than the smaller diameter, and relatively higher energy output than the smaller diameter.

Comparative study of combustion and thermal performance of a meso-scale combustor under co- and counter-rotating fuel and oxidizer swirling flows for micro power generators

Direct energy conversion systems, such as thermophotovoltaic and thermoelectric generators, have received increasing attention in micro power generation. Micro and meso-scale combustors are one of the most core components in these systems. So, developing combustion stabilization technologies for micro or meso-scale combustors is of great importance. In these systems, a hydrocarbon fuel with high energy density is burned in a micro or meso-scale combustor. Many studies have been conducted to explore various combustion stabilization techniques, but as a novelty, in this work, we study the combustion and thermal performance of a meso-scale micro power generator powered by a swirling fuel jet discharging into a co- or counter-rotating air coflow. To do so, we used 3D-printed axial swirlers with double annulus to form the swirling co- and counter-rotating fuel (methane) jet-air coflow configurations at various air and fuel flow rates. Blow-out limit, flame characteristics, combustor mean outer wall temperature, pollutant emissions, emitter efficiency, and normalized temperature standard deviation are investigated in this study. The results show that swirl addition enhances the blow-out limit significantly and co-rotating swirling flows generally enhances the flame blow-out limit when compared with the counter-rotating swirling flows mode at high fuel flow rates. Moreover, the combustor with co-rotating swirling flows has shorter lift-off height and longer flame length. The sensitivity of the flame lift-off height to an increment of the fuel mass flow rate is smaller in co-rotating than in counter-rotating swirling flows (more than 40%). Furthermore, it is observed that under the same operating conditions, co-rotating swirling flows exhibit lower values of CO and NOx in the flue gas and higher values of mean outer wall temperature, combustion efficiency, emitter efficiency, and normalized temperature standard deviation. The enhancement of the emitter efficiency by implementing co-swirl configuration is about 35%, 26%, and 8% for the methane flow rates of 0.050, 0.100, and 0.150slpm, respectively when compared with the counter-swirl mode. The results of this work can provide useful information to choose between co- and counter-rotating flows for combustors of combustion-based micro power generators.

Ultra-lean premixed combustion of a holder-stabilized 80 %hydrogen–20 %methane–air mixture in a preheated mesoscale combustor

To achieve cleaner emissions, the ultra-lean premixed flames of 80 %H2–20 %CH4–air with the coupling effect of flow and heat recirculation were experimentally investigated. The flame behaviors for various Re values are observed and discussed in detail. The conventional stable flame, stable wedge-shaped residual flame, and wedge-shaped residual flame with upward and downward oscillations appeared in sequence as the equivalence ratio decreased. For a stable wedge-shaped residual flame with a decreasing Re value, the operating range broadens first and then narrows. By contrast, the operating range of the wedge-shaped residual flame with oscillations narrowed monotonically and slightly. With a decreasing Re value, the resident location of the conventional stable flame base remained nearly the same. When the equivalence ratio was reduced further, a stable wedge-shaped residual flame appeared. Subsequently, a wedge-shaped residual flame with oscillation occurred when the equivalence ratio was further reduced. Finally, the blow-off mechanisms were revealed.

Numerical and Experimental Reconstruction of Temperature Distribution and Soot Concentration Field for Thin-Slice Flames Using a Charge Coupled Device Camera

This work proposed a novel model to simultaneously reconstruct temperature distribution and soot volume fraction field for two-dimensional thin-slice flames using the knowledge of monochromatic radiation intensities at two wavelengths using a CCD camera. The deduction process and numerical analysis of the model were described. Effects of wavelength combinations and measurement errors on reconstruction accuracy were considered in detail. Numerical results have proven the model’s accuracy and showed that the temperature and soot volume fraction fields can be reconstructed well even with noisy input data from flame radiation. In addition, a series of experiments were conducted on a mesoscale combustor to obtain the real thin-slice flames for further experimental reconstruction via the model. The experimental results indicated that the proposed model can successfully reconstruct the flame temperature distribution and soot volume fraction field and the main features of thin-slice flames also can be reasonably reproduced.

Characteristics of Butane Combustion in Mesoscale–Combustor Due to Stainless Steel Flame Holder Type

Mesoscale–combustor is one component of the micropower generator. The mesoscale–combustor function is to produce heat energy from the combustion of chemical hydrocarbon compounds. The small dimensions of the meso-combustor cause large losses and short residence times for the reactants. These two factors lead to narrow flame flammability limits. This study aims to increase flammability limits by lengthening the reactant residence time and increasing reactant mixing with inserts of flow retardants called flame holders. There are two kinds of flame holders used in this study, namely holes and lines. Lines type produces flames at an equivalent ratio of Φ0.83–1.76 and speeds of 17.01 cm/s to 72.93 cm/s, while holes type produces flames at an equivalent ratio of Φ0.91–1.71 and a flow rate of 24.94 cm/s to 54.95 cm/s. This phenomenon shows that the line type flame holder is able to inhibit flow better and form vortices better, thereby increasing heat recirculation, flame stability, and widening flammability limits.

A Review on Flame Stabilization Technologies for UAV Engine Micro-Meso Scale Combustors: Progress and Challenges

Unmanned aerial vehicles (UAV)s have unique requirements that demand engines with high power-to-weight ratios, fuel efficiency, and reliability. As such, combustion engines used in UAVs are specialized to meet these requirements. There are several types of combustion engines used in UAVs, including reciprocating engines, turbine engines, and Wankel engines. Recent advancements in engine design, such as the use of ceramic materials and microscale combustion, have the potential to enhance engine performance and durability. This article explores the potential use of combustion-based engines, particularly microjet engines, as an alternative to electrically powered unmanned aerial vehicle (UAV) systems. It provides a review of recent developments in UAV engines and micro combustors, as well as studies on flame stabilization techniques aimed at enhancing engine performance. Heat recirculation methods have been proposed to minimize heat loss to the combustor walls. It has been demonstrated that employing both bluff-body stabilization and heat recirculation methods in narrow channels can significantly improve combustion efficiency. The combination of flame stabilization and heat recirculation methods has been observed to significantly improve the performance of micro and mesoscale combustors. As a result, these technologies hold great promise for enhancing the performance of UAV engines.

Optimizing thermal performances and NOx emission in a premixed ammonia-hydrogen blended meso-scale combustor for thermophotovoltaic applications

As a carbon-free energy carrier, ammonia has attracted significant interest in the combustion field as a potential substitute for fossil fuels. However, the focus has been given to the application at meso-scale conditions, particularly with regard to thermal performance and NOx emissions. Therefore, the present study numerically investigates a 3-dimensional time-domain premixed ammonia/oxygen meso-scale combustor to optimize its' thermal performance and NOx emission for power generation applications. The numerical model is firstly validated by using experimental data available in the literature. Then, the effects of 1) the inlet pressure (Pin), 2) the equivalence ratio, and 3) the hydrogen blended ratio on the temperature uniformity, the combustor outer wall mean temperature (OWMT), NO emission, and exergy efficiency are examined. The results indicate that increasing Pin intensifies the mixing process of the mixture gases, thus reducing the residence time for the high-temperature flame in the combustion chamber. The optimized OWMT and NO emissions are up to 26% and 40.3% respectively, with only 9% compensation of the standard deviation achieved, when the inlet velocity is set to 0.5 m/s and Pin is 3.0 bar. Furthermore, varying the equivalence ratio in the range of 0.95–1.1 has a minor influence on improving thermal performances, but a significant impact on mitigating the NOx emission performance. Additionally, blending less than 15% hydrogen has a significant reduction in the maximum NOx emission (up to 53%); however, the influence on the OWMT can be neglected. Further exergy analysis reveals that elevating Pin results in a decrease in the exergy efficiency due to the increased inlet exergy. In general, this work provides a preliminary method for improving the thermal performance and NOx emission of an ammonia/hydrogen-oxygen-fueled meso-scale combustor for power generation purpose.

A numerical study on heat transfer characteristics of a mesoscale combustor partially filled with wire meshes

Porous media are often inserted into micro-/meso-scale combustors for flame stabilization. The heat transfer processes involved are very complicated which still need to be comprehensively investigated. In the present study, the coupled heat transfer processes between gas phases and both the porous media and tube wall of a meso-scale combustor partially filled with wire meshes were numerically analyzed. First, it is found that there exists a heat loss segment (i.e., heat is transferred from near-wall gas to wall) on the inner wall of the porous zone when the inlet mixture velocity is not high enough (≤0.4 m/s), but this phenomenon vanishes as the inlet velocity is further increased. Meanwhile, the porous zone can be divided into two sub-regions, i.e., a heat recirculation zone and a heat loss zone. Interestingly, the boundary shape of these two zones will be inversed with the increase of inlet velocity. Furthermore, the heat recirculation efficiency of inner wall changes non-monotonically with an increasing inlet velocity, which differs from the variation trend of the scenario with a full insertion of porous media reported in the literature. However, the heat recirculation efficiency of the porous media decreases monotonically versus the inlet velocity. In summary, this study unraveled that the flame location and heat release rate vary with the inlet velocity, which affects the temperature distributions of both porous media and tube wall, and this in turn acts on the two pathways of heat recirculation of the combustion system.

The Influence of Heat Recirculation Segment of Meso-Scale Combustor o Gas and Liquid Fuel Inside Microscale Combustor

This study aims to determine the effect of heat recirculation segments configuration on the stability of the flame. Mesoscale combustor used is made from duralumin-quart glass tube, and Ø 3.5 mm diameter combustion chamber. The heat recirculation segments part is made of duralumin as a segment of evaporation and air-fuel mixing. Combustor A has heat recirculation segments length of 7 mm, while combustor B has a heat recirculation segments length of 10 mm. The fuel used is liquid hexane and butane gas fuel. The results showed, the stability of hexane flame in combustor A between the equivalent ratio of ɸ 0.73-1.28 temperature can reach 803.8oC. Combustor B is stable at an equivalent ratio of ɸ 0.76-0.98 to a temperature of 580.9oC. Furthermore, for butane flames; combustor A, stable in the range of ɸ 0.68-1.31 temperature to 931.3oC. Combustor B has a stability range of ɸ 0.79-1.41 temperature reaches 857.2oC. Combustor A with a 7 mm heat recirculation segments length is proven to produce wider flame stability, using both liquid fuel and gas. So, combustor A can be recommended to be applied to micro power generators. It should be noted that butane flames appear to have more flame stability than hexane flames. This is because gaseous fuels are more homogeneous (gaseous) so they are more quickly mixed with air.

Influence of jet velocity and heat recuperation on the flame stabilization in a non-premixed mesoscale combustor: An exergetic approach

An experimental and numerical model to determine the exergy balance based on flow availability and availability transfer in the process of liquefied petroleum gas (LPG)/air combustion in mesoscale gas turbine combustor is developed to elucidate the second law efficiency and total thermodynamic irreversibility. In terms of developing an energy and exergy-efficient combustor design, the present work highlights the influence of vortex shedding and recirculation in the volumetric entropy production and the exergy efficiency. It is performed in a heat recuperative high-intensity LPG-fueled mesoscale combustor for mini-gas turbine applications. The combustor is operated at different thermal inputs ranging from 0.2 to 1.0 kW under range of equivalence ratios of ϕ = 0.4–1.23. The Favre-averaged governing equations are solved by using finite volume-based approach. The standard k–ε turbulence model with modified empirical constant, Cɛ1=1.6, is considered to model the turbulence quantities. The volumetric reaction-based eddy-dissipation concept model and a reduced skeletal model (50 species and 373 reactions) are used for turbulence–chemistry interaction. The design methodology, total volumetric entropy generation, destructive exergy due to thermodynamic irreversibility, exergy efficiency, flow recirculation, and mixing characteristics (reacting and non-reacting) are reported. The entropy generation rate due to thermal conduction is approximately 50% of the total entropy generation, while its contribution percentage due to chemical reaction is the smallest. The exergy efficiency reaches its peak with ηII = 79.41% at 1.0 kW under fuel-rich condition, while its minimum value of 41.49% is obtained at 0.2 kW under fuel-lean (ϕ = 0.8) condition.

Ultra-lean dynamics of holder-stabilized hydrogen-enriched flames in a preheated mesoscale combustor near the laminar critical limit

This work experimentally investigates the ultra-lean dynamics of a 40% H2–60% CH4 flame near the laminar critical limit in a preheated mesoscale combustor with a flame holder. These experiments are conducted to verify a conjecture we proposed in a previous publication and reveal the ultra-lean flame dynamics under the synergistic effects of heat and flow recirculation. Notably, not only is our conjecture confirmed, but also some novel flame behaviors are found. As the equivalence ratio ϕ is decreased from 0.500 to 0.320, the conventional stable flame, stable residual flame, periodic residual flame with repetitive local extinction and re-ignition (periodic RFRER), and periodic oscillating residual flame are observed in sequence. For the stable residual flame (0.370 ≥ ϕ ≥ 0.355), the left and right flame roots reside directly behind the flame holder, and the flame tip stays near the combustor exit. For the periodic RFRER (0.350 ≥ ϕ ≥ 0.340), observed experimentally for the first time, the flame roots reside at almost the same location, but the flame tip oscillates up and down over time with pinch-off events. For the periodic oscillating residual flame (0.335 ≥ ϕ ≥ 0.320), found for the first time, the stable flame roots also reside at almost the same location, but the residual flame tip oscillates up and down over time without a pinch-off event. When ϕ decreases to 0.315, the oscillating residual flame extinguishes, and its blow-off dynamics are revealed in detail.

Ultra-lean dynamics of a holder-stabilized hydrogen enriched flames in a preheated mesoscale combustor

To provide the theoretical basis to suppress the unstable flames under the coupling effect of flow and heat recirculation, the present work experimentally studies the ultra-lean dynamics of a holder stabilized 40%H2–60%CH4–air premixed flame in a preheated mesoscale combustor. The regime diagram of the flame behaviors at various operating conditions is obtained. It is observed that the blow-off limit first increases slightly and then decreases sharply (the anomalous blow-off limit) with the decreased Re value. Three types of the flame behaviors (i.e., the conventional stable flame, the stable residual flame, and the periodic oscillating residual flame) are found before the flame blow-off. In addition, with the decreased Reynolds number, the operating range for the stable residual flame broadens first and then narrows, but that of the periodic oscillating residual flame decreases monotonically, which are observed for the first time. The results show that, with the decreased Reynolds number, the flame root of the conventional stable flame anchors almost at the same location right behind the holder, while the flame tips obviously shift upstream. With the decreased equivalence ratio, the left and right flame tips in the downstream channel shift toward each other and finally merge into a single flame tip, which results in the formation of the stable residual flame. When the equivalence ratio decreases further, the periodic oscillating residual flame occurs. The flame tip periodically oscillates up and down over time. In the end, the blow-off dynamics of the stable residual flame and periodic oscillating residual flame are revealed.

Experimental Study on the Influence of Gas-Solid Heat Transfer in a Mesoscale Counterflow Combustor

ABSTRACT Combustion at small scales is inhibited by the large amount of external surface area leading to excessive heat losses and extinguishment. This limitation is diminished by transferring heat from the products to the reactants through solid surfaces leading to the development of heat recirculating reactors. A promising design is the counterflow configuration in which reactants in one channel are preheated by energy transferred through the wall from the products in the adjacent channel. In the lean equivalence ratio limit of the reactor, this arrangement encourages the stabilization of the flame. In the high equivalence ratio limit, the enhanced heat transfer may encourage blow-off. Therefore, it is important to understand the relationship between heat transfer and the operating range of the reactor. In this paper, the importance of the channel surface area-to-volume ratio is investigated analytically and experimentally in a mesoscale combustor. Reactors with different channel shapes were fabricated via additive manufacturing and studied. The change in heat transfer area affected the stable operating range, maximum temperature, and location of flame stabilization. The emission measurements showed low CO and NOx emissions. For all reactors, the stable range was limited by flashback when the burning rate exceeded the flow velocity and by blow-off when the burning rate was insufficient. This work quantifies the importance of heat transfer surface area to the operation of the counterflow reactor and guides the optimization of reactor design.

Investigation of heat transfer characteristics of hydrogen combustion process in cylindrical combustors from microscale to mesoscale

In the field of micro and mesoscale combustion, the feature of flame-wall thermal coupling is of great significance because of its small scale nature. Thus, this work provides a comprehensive heat transfer analysis in cylindrical combustors from the perspective of numerical simulation. The combustor has a fixed length-to-diameter aspect ratio of 10, and the channel diameter is scaling up from 1 mm to 11 mm to explore the influence of chamber dimension on heat transfer and flame structure. The distribution of convective and radiative heat flux on inner surface, contribution of thermal radiation are given. Moreover, the role of radiation in flame structure is analyzed, and the convective and radiative heat losses are quantitatively analyzed. We find that radiative heat flux is smaller compared to convective heat flux, and the proportion of radiative heat flux becomes larger with an increasing diameter. Thermal radiation does not change the flame structure when the diameter is less than 3 mm. When the diameter is greater than 5 mm, thermal radiation changes the location of flame front. The heat loss becomes larger at a smaller diameter, and heat loss ratio can reach approximately 73.6% in the combustor with diameter of 1 mm.

Optimizing thermal performance and exergy efficiency in hydrogen-fueled meso-combustors by applying a bluff-body

Recognizing the importance of a bluff-body on the flow separation and its role in enhancing the fuel mixing, the effect of applying a bluff-body on the thermal performance and exergy efficiency in hydrogen-fueled meso-scale combustors are discussed using 3D numerical simulations involving a detailed kinetics mechanism. Key parameters including the bluff-body dimensionless axial location l and combustor configuration are evaluated to obtain the optimal geometry. It is found that the bottom wall mean temperature (BWMT) and its uniformity are significantly enhanced with a bluff-body applied, generally accompanied with a high average Nusselt number. This is mainly due to the fact that the bluff-body can create longitudinal vortices and disrupts the growth of the thermal boundary layer, leading to intensified heat transfer. There is a non-monotonic relationship between BWMT and l with the optimum position being l = 2/5. Further, the proposed Combustor A is involved with a higher and more uniform BWMT. Finally, the exergy efficiency in the bluff-body combustor is found to be higher compared to the conventional combustor, although the elevated pressure loss. This work demonstrates the feasibility of implementing a bluff-body to enhance the thermal performance, and some of the interesting findings can be also applied to power and propulsion systems utilizing the combustor wall temperature.

Anomalous blow-off behavior of a holder-stabilized premixed flame in a preheated mesoscale combustor

The present study investigates the blow-off limits of a laminar methane–air premixed flame under the synergistic effect of heat and flow recirculation by means of experimentation and three-dimensional numerical simulation. An anomalous blow-off phenomenon (the blow-off limit decreases with the decrease in Reynolds number (Re)) is experimentally observed and reproduced via simulation. First, the flame base structures for various Re values are studied. Subsequently, the underlying mechanisms are revealed in terms of flow field, stretch effect, preferential transport effect, and conjugate heat transfer. The main causes of the present anomalous blow-off phenomenon are the flow recirculation effect, preferential transport effect, and conjugate heat transfer, rather than the stretch effect. Specifically, a weaker flow recirculation effect, a weaker preferential transport effect, a larger heat loss ratio from the combustor to the ambient environment, and a lower preheated temperature of the fresh mixture altogether lead to a smaller blow-off limit at a smaller Re. In addition, the sharp reductions in both the local equivalence ratio and preheated temperature of the fresh fuel-oxidizer mixture explain why the blow-off limit sharply decreases from Re = 80 to 40. This work offers insights into flame blow-off behaviors under the synergistic effect of heat and flow recirculation in many practical combustors.

Ultra-lean blow-off dynamics of a holder-stabilized premixed flame in a preheated mesoscale combustor near laminar critical condition

The present work experimentally illustrates and analyzes the ultra-lean flame blow-off dynamics near laminar critical condition under the synergistic effect of heat and flow recirculation which exists in many practical combustors (such as aero-engine). The results indicate that the flame blow-off process can be divided into three stages at Reynolds number Re = 240. In the first stage, the flame initially becomes thinner over time. In the second stage, the residual flame with repetitive local extinction and re-ignition (RFRER) appears (the second stage), which is experimentally found for the first time for the methane/air premixed mixture of Lewis number Le = 1.0. The analysis indicates that the local extinction of the flame is mainly caused by the stretch effect on two sides. In the final stage, the residual flame with oscillation appears. The oscillating residual flame is finally carried convectively upstream and extinguishes within the recirculation zone as the present Lewis number effect is small. The analysis also indicates that the residual flame of hydrocarbon fuel cannot be maintained, but this issue can be solved via adding enough fast-diffusing species (such as hydrogen). This mechanism for suppressing or preventing the local extinction of the residual flame may be also suitable for the turbulent flames in many practical combustors. The present study provides the theoretical basis for predicting the flame status and suppressing the unstable flames at extreme operating conditions.

Acoustic parametric instability, its suppression and a beating instability in a mesoscale combustion tube

The present work reports thermo-acoustic instability in a mesoscale tube of diameter 8 mm (> quenching diameter) and length 702 mm. Mixtures with Lewis number, Le~ 0.8 (rich C2H4/O2/CO2), 1.05 (lean C2H4/O2/CO2) and 1.34 (lean C2H4/O2/N2) are used. Several new flame responses are observed. For lower burning velocity mixtures, flame extinction is observed due to heat loss when primary instability transforms to secondary instability for all Le mixtures. However, parametric cellular structures which are characteristic of parametric instability are observed only for Le of 0.8. It is proposed based on calculations and experiments that parametric structures will be observed only when diameter of tube is two times larger than the characteristic wavelength of parametric instability. If the tube diameter doesn't allow formation of parametric structure whirling and counter-rotating flames are observed instead of parametric structures. Suppression of acoustic parametric instability is observed for higher burning velocity mixtures with Le>1 and its mechanism is discussed. For a range of SL, a beating instability is observed for CO2 diluted mixtures of Le>1, where pressure oscillation and flame motion show beating oscillations of frequency around 15 Hz. This beating instability is believed to be caused by non-linear interaction of acoustic instability with pulsating instability of flame front which is caused due to combined radiative and convective heat loss. Due to CO2 dilution, the radiative heat losses could play a significant role in inducing pulsating instability.