Coflow Air Research Articles

Personalized ventilation with embedded air treatment system for simultaneous cooling and sorption-based carbon and humidity capture

This study presents a novel approach to address thermal comfort and air quality challenges in built environments using personalized ventilation (PV). Conventional techniques rely on delivering clean, cool and dehumidified outdoor air occupant microclimate. In contrast, this research extends the use of these conventional PV systems to deliver decarbonized and dehumidified air using a localized indoor air treatment system. The system meets air quality requirements of the occupant while minimizing its size and energy consumption through the innovative PV airflow supply from co-flow air terminal. Hence, a compact multi-functional personalized ventilation (MFPV) device, employing an adsorbent system for simultaneous CO2 and H2O capture from indoor air, is developed and its feasibility is tested. The device utilizes thermoelectric cooling (TEC) to cool the supply air and regenerate the adsorbent. Mathematical models are developed for the system to predict performance an appropriate control strategy. The models are verified experimentally using a constructed prototype of the device and used to optimize the design and operation via a four-way valve and switching the TEC current polarity such that the flow was periodically guided to cooling and regeneration TEC sides. The optimal operation was achieved with a purge-to-supply flowrate of 40 %, adsorbent mass of 125.3 g of Lewatit® VP OC 1065, along with two TEC modules with a power of 7.2.8 Wh and a cycle time of 7.5 min. The MFPV resulted in 22 % and 58 % energy savings compared to conventional co-flow and single flow PV system using treated outdoor air, respectively.

Numerical Study of Velocity and Mixture Fraction Fields in a Turbulent Non-Reacting Propane Jet Flow Issuing into Parallel Co-Flowing Air in Isothermal Condition through OpenFOAM

This research employs computational methods to analyze the velocity and mixture fraction distributions of a non-reacting Propane jet flow that is discharged into parallel co-flowing air under iso-thermal conditions. This study includes a comparison between the numerical results and experimental results obtained from the Sandia Laboratory (USA). The objective is to improve the understanding of flow structure and mixing mechanisms in situations where there is no involvement of chemical reactions or heat transfer. In this experiment, the Realizable k-ε eddy viscosity turbulence model with two equations was utilized to simulate turbulent flow on a nearly 2D plane (specifically, a 5-degree partition of the experimental cylinder domain). This was achieved using OpenFOAM open-source software and swak4Foam utility, with the reactingFoam solver being manipulated carefully. The selection of this turbulence model was based on its superior predictive capability for the spreading rate of both planar and round jets, as compared to other variants of the k-ε models. Numerical axial and radial profiles of different parameters were obtained for a mesh that is independent of the grid (mesh B). These profiles were then compared with experimental data to assess the accuracy of the numerical model. The parameters that are being referred to are mean velocities, turbulence kinetic energy, mean mixture fraction, mixture fraction half radius (Lf), and the mass flux diagram. The validity of the assumption that w߰ = v߰ for the determination of turbulence kinetic energy, k, seems to hold true in situations where experimental data is deficient in w߰. The simulations have successfully obtained the mean mixture fraction and its half radius, Lf, which is a measure of the jet’s width. These values were determined from radial profiles taken at specific locations along the X-axis, including x/D = 0, 4, 15, 30, and 50. The accuracy of the mean vertical velocity fields in the X-direction (Umean) is noticeable, despite being less well-captured. The resolution of mean vertical velocity fields in the Y-direction (Vmean) is comparatively lower. The accuracy of turbulence kinetic energy (k) is moderate when it is within the range of Umean and Vmean. The absence of empirical data for absolute pressure (p) is compensated by the provision of numerical pressure contours.

Effect of preheating air temperature on sooting tendency in laminar co-flow diffusion flame of n-heptane

This work investigates the effect of preheating air temperature on the sooting tendency in the laminar co-flow diffusion flames of n-heptane. A modified co-flow burner was employed to produce steady n-heptane laminar diffusion flames with the preheating temperatures of co-flow air varying from 344 K to 588 K. The planar laser-induced incandescence (LII) calibrated by the line-of-sight attenuation (LOSA) was performed to quantitatively measure the soot volume fraction and the soot yield in flames and an ICCD camera was used to capture the flame luminosity images. The results reveal that the visible flame height is decreased with the preheating co-flow air temperature, and it is hardly affected by the air velocity with a constant air temperature. As the co-flow air temperature increases, the soot inception and oxidation and the peak soot concentration initiate at the lower positions in flames, and the soot loading in flames has a remarkable increasing trend by assessing the evolution of the soot volume fraction and the soot yield. However, the enhancement rate of total soot loading in flames with the co-flow air temperature below the initial fuel temperature is lower than that for the co-flow air temperature above the initial fuel temperature. In addition, the temperature sensitivity of soot loading is assessed by the maximum soot volume fraction, which is in accordance with those of the alkane in previous research. According to the response of the maximum soot yield to the co-flow air temperature and adiabatic flame temperature, the temperature sensitivity of sooting tendency in n-heptane flames is further confirmed.

Mechanism and effect assessment of sub-atmospheric pressure and co-flow air to suppress the flicker of buoyancy-driven methane laminar diffusion flame

To further reduce the combustion instability behavior in high-altitude areas, a synergistic suppression of buoyancy-driven methane laminar diffusion flame flicker was achieved by sub-atmospheric pressure combined with co-flow air, and the suppression mechanism was revealed through the schlieren technique. As the pressure decreases, the fluid density gradient within the shear layer decreases, slowing down the formation of vortices. When the co-flow air increases, the vortex is accelerated to rise until the vortex forms downstream of the flame and the flame flicker is completely suppressed. The suppression of flame flicker by sub-atmospheric pressure and co-flow air was nonlinear. The flicker frequency, oscillation amplitude, and flame mean height of flames were assessed for a pressure of 0.5–1.0 atm and co-flow air of 4–16 L/min. The sub-atmospheric pressure reduces the flicker frequency, which increases with increasing co-flow air. The reduction rate of oscillation amplitude at sub-atmospheric pressure varied among different co-flow air flow rates. The initial and that almost realize flame stabilization co-flow air flow rates are more effective than the intermediate flow rate in reducing the oscillation amplitude. The flame mean height decreases monotonically with decreasing pressure, but there is a small peak in the downward trend with increasing co-flow air.

Effects of varying the liquid fuel type and air co-flow conditions on the microscopic spray characteristics in an atmospheric annular co-flow spray burner

The atomization process is critical for combustion systems since it directly influences emissions and performance. Thus injection system should provide the spray required structure and characteristics, e.g., angle and droplet size distribution. Therefore, this work investigates the effects of varying the fuel type, air co-flow rates, fuel mass flow rate and air co-flow temperature on the spray characteristics (e.g., droplet size distribution and droplet velocity) in an annular co-flow spray burner. These effects were investigated by measuring droplet sizes and velocities at different radial and axial positions of n-Heptane, n-Decane and n-Dodecane sprays under non-reacting conditions at a room pressure of 1 atm and temperature of 298K and using the Microscopic Diffused Back-illumination (MDBI) technique. In addition, the Sauter mean diameter (SMD) for different flow conditions were predicted using three well-known correlations and compared to experimental measurements. The outcomes of this research provided a fair understanding of the influence of varying these parameters on the droplet sizes and velocity through a wide test matrix. Finally, the findings reported here will support future research into the function of phase change in flame stability.

Computational study of hydrogen and air co-flow jets mixing in dual-combustor ramjet in presence of ramp shock generator

Flame stabilization distribution and efficient fuel mixing are highly significant in the performance of ramjet engine. In this paper, numerical studies are done to investigate the supersonic combustion of hydrogen and air co-flow in presence of the ramp edge. In this study, flame maintenance and combustion formation in a dual-combustion ramjet engine are fully investigated. The primary emphasis of this work is to disclose the impact of produced shocks due to the existence of ramp edge on the structure of fuel and air-jet immediately downstream of injector. The shock interactions of two different angles (10 deg and 30 deg) of the ramp edge are investigated. Favre-averaged conservation equations are considered for the simulation of compressible flows inside the combustor. Various mechanisms expressing the flame spreading characteristics are also examined. Depending on the angle of the ramp edge, the produced shock waves influence on size and strength of vortices. Our findings also show that high angles of ramp edge augment the fluctuation of the vortices by the splitter and consequently, fuel mixing enhances in the combustion chamber of ramjet engine. Our findings also confirm that the similar jet pressure would be more effective on the vortex production downstream of the splitter. The application of ramp edge with angle of 30 deg would increase average momentum thickness of the mixing layer up to 55%.

Critical conditions at liftoff limit of a laminar n-butane-air jet diffusion flame

The stability mechanism of laminar coflow jet diffusion flames in normal gravity has been studied computationally and experimentally. N-butane, the heaviest alkane in a gaseous state at ambient temperature and pressure, is used as the fuel since the reaction mechanism is similar to that of higher (liquid) hydrocarbons. The critical mean n-butane jet and coflowing air velocities at flame stability limits are measured using a small fuel tube burner (0.8 mm inner diameter). The time-dependent, axisymmetric numerical code with a detailed reaction mechanism (58 species and 540 reactions), molecular diffusive transport, and a radiation model, reveals a flame structure. A fuel-lean peak reactivity spot (i.e., reaction kernel), possessing the hybrid nature of diffusion-premixed flame structure at a constant temperature of ≈1560 K, is formed at the flame base and controls the flame stability. In a near-quiescent environment, the flame base resides below the fuel tube exit plane and thereby premixing is limited. As the coflowing air velocity is increased incrementally under a fixed fuel jet velocity, the flame base moves slightly above (≈1 mm) the burner exit and vigorous premixed combustion becomes prevailing. The local heat-release rate at the reaction kernel nearly doubles due to the increased convective oxygen flux (i.e., a blowing effect). The local Damköhler number, newly defined as a ratio of the square root of the local heat-release rate and the local velocity, decreases gradually first and drops abruptly at a critical threshold value and the flame base lifts off from the burner rim. The calculated coflow air velocity at liftoff is ≈0.38 m/s at the fuel jet velocity of 2 m/s, which is consistent with an extrapolated measured value of 0.41 m/s. This work has determined the critical Damköhler number at the stability limit quantitatively, for the first time, for laminar jet diffusion flames.

Ultra-Lean Premixed Turbulent Combustion: Challenges of RANS Modelling

The main challenge of improving spark ignition (SI) engines to achieve ever increasing thermal efficiencies and near-zero pollutant emissions today concerns developing turbulent combustion under homogeneous ultra-lean premixed mixtures (HULP). This continuous shift of the lean operation limit entails questions on the applicability limits of the combustion models used to date for SI engine design and optimization. In this work, an assessment of flamelet-based models, widely used in RANS SI engines simulations of premixed turbulent combustion, is carried out using an open-source 3D-CFD platform to clarify the applicability limits on HULP mixtures. Two different consolidated approaches are selected: the Coherent Flame Model (CFM) and the Flame Area Model (FAM). Both methodologies are embedded by the authors into the same numerical structure and compared against measurements over a simplified and controlled flame configuration, which is representative of engine-like conditions. The experimental steady-state flame of type “A” of the Darmstadt Turbulent Stratified Flame (TSF) burner is selected for the assessment. This configuration is characterized by flame measurements over a strong shear and mixing layer between the central high-speed CH4-air jet and the surrounding slow air co-flow, hence, it represents an interesting controlled condition to study turbulent HULP mixtures. A comparison between computed results and experimental data on trends of mean flow velocity, turbulence, temperature and mixture stratification was carried out. This enabled us to assess that the investigated flamelet-based combustion models failed in providing accurate and reliable results when the flame approaches turbulent HULP mixture conditions, demonstrating the urgency to develop models able to fill this gap.

Primary atomization of liquid-fuel jets in confined turbulent pipe-flows of air at elevated temperatures

The primary atomization of evaporating laminar liquid jets of either pure n-pentane or n-heptane injected continuously from a circular nozzle concentrically and axisymmetrically into high-temperature turbulent coflows of air in confined a pipe has been studied experimentally. Fuel-jet bulk velocities up to 2.7 m/s, air coflow bulk velocities up to 37 m/s and temperatures up to 1150 K were investigated within a 25 mm round pipe. In the near-field region of the injector nozzle, liquid fuel jets were formed whose length increased with the fuel injection velocity and decreased with the air velocity. Smaller droplets were formed at both higher fuel and air velocities. Droplet diameters within the range from 200 to 900 µm were measured, indicating the polydispersed nature of the fuel spray. The mean droplet diameter increased non-monotonically with increasing axial distance from the nozzle due to a complex competition between droplet coalescence and evaporation. No significant effect of the air inlet temperature was observed within the investigated range of conditions on the jet length and droplet size. However, smaller droplets were obtained in flows with lower turbulence intensities and longer longitudinal integral lengthscales. Analysis of the data reveals that the mean droplet diameters can be predicted by the empirical expression (dmean/djet) = 34 × 103 We∆1.29Re∆-3, independent of the two perforated plates selected and used in the present work. The resulting data can contribute towards a better understanding of interfacial dynamics, and the development and validation of advanced multiphase-flow and reacting-flow models.

Effect of the Air Coflow Temperature and Spinneret Position on the Solution Blow Spinning Technique

Effect of the Air Coflow Temperature and Spinneret Position on the Solution Blow Spinning Technique

An investigation of thermal radiation from laminar diffusion flames in a tri-coflow burner with central oxygen

In natural gas combustion processes, oxygen-enriched combustion is often used to improve heat transfer and thermal efficiency. In this investigation, a triple-port burner is employed to investigate the effect of oxygen enrichment on thermal radiation when introducing oxygen into the center of a normal coflow diffusion flame, while maintaining an outer coflow of air. Two fuels are investigated: methane, being the primary constituent of natural gas and having a low propensity for soot formation, and ethylene, which acts as a reference fuel with a much higher propensity for soot formation. The flow rates of air in the outer annulus and fuel in the inner annulus are held constant, while the flow rate of oxygen in the central tube is systematically increased. Total radiation, which is converted to radiation loss fraction, is measured using a wide-angle radiometer. Soot volume fraction and soot temperature are measured using two-color ratio pyrometry with a digital camera.For low flow rates of central oxygen when an inner oxygen-fuel flame is apparent, the thermal radiation is increased due to an increase in soot concentration. At these low flow rates, thermal radiation is increased by 20% for methane and 26% for ethylene, as compared to a normal coflow flame without central oxygen. The increase in radiation from these flames results in a decreasing, or relatively constant soot temperature in the soot laden region as the increased soot concentration radiates heat from the flame, even though gas temperatures upstream near the central oxygen flame are quite high. For relatively high flow rates of central oxygen, radiation from the tri-coflow methane flame increases by up to 31% compared to a normal coflow flame, while radiation from the ethylene flame monotonically decreases. For both fuels, when oxygen flow rates are high, soot concentration decreases (due to a change in flame shape and enhanced oxidation rates) and soot temperature increases. Product gases are the dominant emitters of radiation in the methane flames studied. Soot is the dominant radiant emitter in the ethylene flames until high flow rates of central oxygen reduce the soot concentration, such that high temperature product gases take over as the dominant radiant emitters. Overall, low flow rates of central oxygen can increase thermal radiation from the coflow methane flame by 20% due to increased soot concentration, and high flow rates of central oxygen can increase thermal radiation by 31% due to increased gas temperatures. However, the methane flames with central oxygen still emit at least 41% less radiation than the coflow ethylene flame. These results indicate that when considering the effect of oxygen enrichment on thermal radiation, the change in overall flame (gas and soot) emissivity can be just as important as the change in temperature.

Lift-off height of autoignited jet flame in hot air coflow with different O2 contents

This paper aims to understand the effects of coflow oxygen content on the lift-off height of autoignited jet flame and develop a lift-off height prediction model. A jet-in-coflow burner was used to performed autoignition experiments in hot air coflow with different oxygen contents. At a fixed oxygen content, as the fuel jet velocity increases, two types of autoignited flames were observed: (1) nozzle-stabilized flame (NS); (2) lifted flame (LF). As the oxygen content increases, it takes a larger fuel jet velocity for the transition from NS to LF and the transition from LF to blow-out. For a fixed fuel jet velocity, the lift-off height decreases significantly with increasing oxygen content and is more sensitive to the oxygen content at oxygen-lean conditions. Based on the modified large-scale mixing model, a lift-off height model was developed that considers the combined effect of small-scale strain and large-scale mixing.

Ignition measurement of non-premixed propane with varying co-flowing AIR through high-speed schlieren stereoscopic colour imaging

The flame kernel propagation characteristics stemmed from the spark ignition of low-Re, non-premixed propane (C3H8) under varying air co-flow conditions were studied using DFCD (Digital Flame Colour Discrimination) incorporated stereoscopic image processing technique. The current work focused on the viability of obtaining the combustion initiating kernel propagation speed (SP), equivalence ratio (Φ) and kernel depth (z) characteristics simultaneously as these parameters are important in the fundamental combustion theories and highly desired in the field of practical burner monitoring and diagnostics. Through the superimposition of schlieren and DFCD-enhanced colour images, the observed spatial location of kernel spearhead was found to conform to the disturbance front observed from the schlieren visualisation. Flame stretch principle was applied to experimentally determine the un-stretched kernel propagation speeds (SP,O). Concurrently, the estimation of the temporal state of reactant mixture equivalence ratio (Φe) variation was made using DFCD-segregated spatial colour signal characteristics. Further comparison were made against the CHEMKIN simulated results of un-stretched laminar flame speed (SL,O) at different Φ. The experimentally determined SP,O values were found to overlap the SL,O range corresponding to the measured Φe. The steady occurrence of digital colour fidelity observed for all considered cases suggest that the most reactive mixture fraction (ξMR) path followed by the kernel occurred in the fuel-rich premixed Φ condition (approximately Φe = 1.34 to 1.40). Furthermore, stereoscopic reconstruction of DFCD-enhanced flame colour images provided additional dimensional linkage to correlate the dimensional-spectral variation along with changes in combustion flow conditions. The dimensional-spectral property, through the computation of local colour ratio and stereoscopically reconstructed z, illustrated linearity in both the time domain and in temporal cross-correlation with Φe. The observed shift of the cross-correlated cluster towards the fuel-richer Φe condition is coherent to the corresponding increase in Re and exemplifies the observed conformance of the derived SL,O and Φe to properties of premixed combustion along the propagation front potentially denoting the path of ξMR.

Experimental investigation on lift-off, blowout and drop-back in partially premixed LPG open flames in tubular burner

The higher pollutant level in non premixed combustion and safety issues pertaining to premixed combustion can be counteracted by partially-premixed mode of combustion. The partially premixed flames (PPF) exhibit the benefits of both premixed and non premixed flames. PPF enhances complete combustion leading to reduced soot formation and hence lower emission. However, the equivalence ratio plays an important role in the stability of such flames. This paper reports the experimental investigation on the flame characteristics and stability of partially premixed LPG-air flames in tubular burner. The stability curve obtained for the base case without any secondary flow shows that the velocity at lift-off, drop-back and blowout increases with increasing equivalence ratio. In the presence of secondary co-flow air, the lift-off and blow off velocity decreases compared to base case indicating poor stability due to extensive flame stretch leading to aerodynamic quenching. The experimental results show that the velocity of flow at lift off, blow out and drop back are higher in the presence of secondary swirl air than the base case. Co-swirl air increases the stability due to better mixing at the flame base with increased residence time. Flame stability deteriorates with co-flow air as co-flow strains the flame boundary due to flame stretch.

EXPERIMENTAL INVESTIGATIONS ON THE ROLE OF GROUP COMBUSTION MODE IN LIFTED SPRAY FLAMES WITH KEROSENE FUEL

In the present work, detailed experimental investigations on lifted spray flames have been carried out to identify the various combustion modes using group combustion number analysis with kerosene fuel. Solid cone pressure swirl nozzle N1 ((m_f ) ̇ =1.7 kg/hr at ΔPinj = 9 bar) is considered and fuel injection pressure varied from 3 - 48 bar. Detailed droplet size and droplet velocity measurements using shadowgraphy and PIV helped obtain the group combustion number and dimensionless droplet separation distance. A Chiu (1977, 1982) criterion of group combustion number is used along with experimental observations to understand the spray combustion mode in lifted spray flames. Internal group combustion mode is observed at lower injection pressures (3 bar) in which individual droplet combustion is significant. At intermediate pressures, the combustion mode is defined by internal group combustion with a double reaction zone. The inner reaction zone burns with local extinction, and the outer reaction zone burns like a diffusion flame. A group combustion number range of 1 - 103 is investigated for various combustion modes in lifted spray flames without coflow air. For air coflow conditions (Vcoflow = 0.4 m/s), both individual droplet burning mode and double reaction zone flame structure disappear from the flame. At higher injection pressures, small fuel vapor cloud burning with external group combustion mode is observed in the G range of 103 - 106.

Piv Planar Investigation for a Co-Flowing Air Interaction with a Liquid Spray Utilizing Air-Assisted Swirl Atomizer Centralized in a Disk Stabilizer

Piv Planar Investigation for a Co-Flowing Air Interaction with a Liquid Spray Utilizing Air-Assisted Swirl Atomizer Centralized in a Disk Stabilizer

Computational analysis on the stability and characteristics of partially premixed butane-air open flames in tubular burner

Partially premixed combustion is one of the developing areas of combustion research that has the advantages of both premixed and diffusion mode of combustion. The present work involves a computational study on the stability and characteristics of partially premixed butane-air flames. The effect of operating parameters like fuel-air ratio, primary aeration, and the presence of co-flow and co-swirl on the stability and flame characteristics has been studied. The simulation results show that the height of the flame decreases with an increase in primary aeration and also in the presence of a co-swirl stream. It has also been found that the stability of flames increases with co-swirl air but deteriorates with the presence of the co-flow air. The flame temperature increases with primary aeration and it has been observed that the peak flame temperature shifts away from the burner mouth for lower primary aeration. It has been observed that the flame stability improves with co-swirl air which is attributed to the re-circulation zone created due to the swirl motion which acts as a heat source. The poor stability in the presence of co-flow air is attributed to flame stretching and aerodynamic quenching of the stretched flamelets. The lift-off velocity and the stable operating range increases with equivalence ratio and also with co-swirl air.

Effects of Varying the Liquid Fuel Type and Air Co-Flow Conditions on the Microscopic Spray Characteristics in an Atmospheric Annular Co-Flow Spray Burner

Effects of Varying the Liquid Fuel Type and Air Co-Flow Conditions on the Microscopic Spray Characteristics in an Atmospheric Annular Co-Flow Spray Burner

Numerical study on the characteristics of a nano-aluminum dust-air jet flame

Nano-sized aluminum dust is quite attractive in the application scenarios with limited residence time. This study simulated an aluminum nanoparticle-air jet flame in a burner that was proposed to characterize the mixing and burning properties of dust and air in a classical ramjet configuration. The model considers the flow of gas phase by employing three-dimensional multicomponent Navier-Stokes equations, and the aluminum particles via the Lagrange approach. A combustion sub-model was developed to characterize the heat convection, radiation and combustion processes of nano-sized aluminum particles. Thereafter, the model was validated by comparing with experimental results of dust jet flow and aluminum particle combustion. With the present model, the characteristics of flow field, and variations of particle temperature, heat release and statuses etc. were examined in detail, by which the mixing zone and flame structure were defined. The results showed that the nano-aluminum particles in the inner mixing layer were firstly ignited by the hot co-flow air and burned, and these particles in the core jet region were ignited downstream. Once the aluminum particles were ignited, they burned out promptly in this burner. The effects of particle size were evaluated, and the results show that the lengths of mixing layer and reaction zone, as well as the width of reaction zone reduce with decreasing particle size, whereas the width of mixing layer stays the same.

Effect of breathing profiles on nebuliser drug delivery targeting the paranasal sinuses in a post-operative nasal cavity

Chronic rhinosinusitis (CRS) is a common sinonasal condition that debilitates quality of life. Nasal sprays are inefficient for targeting the sinuses because of the highly inertial atomised droplets. This study evaluated sinus drug delivery via a nebuliser in a human nasal cavity using Computational Fluid Dynamics (CFD) simulations of different breathing conditions. The nebuliser was quantified through high-speed imaging of the spray plume and average velocity, while laser diffraction was used for droplet size distributions. A CT scan was acquired of a 75-year-old male with a confirmed diagnosis of CRS, and airway segmentation was performed to extract the nasal cavity and sinuses. Virtual surgery was then performed on the computational model. Two nozzle designs for the nebuliser under normal breathing, slow exhalation, and breath hold were investigated. The results demonstrated that drug delivery with a fully blocked nostril with a breath hold notably improved deposition targeting the maxillary sinuses and ethmoid sinuses, while using a nozzle that allowed co-flow of air was recommended for targeting the frontal and sphenoid sinuses.