Counterflow Configuration Research Articles

Mathematical modeling of the production of magnetic nanoparticles through counter-flow non-premixed combustion for biomedical applications

PurposeThe widespread usage of magnetic nanoparticles (MNPs) requires their efficient synthesis during combustion process. This study aims to present a mathematical model for the oxidation of MNPs in a counter-flow non-premixed combustion system to produce MNPs, where the key sub-processes during the oxidation reaction are involved.Design/methodology/approachTo accurately describe structure of flame and determine distributions of temperature and mass fractions of both reactants and products, equations of energy and mass conservations were solved based on the prevailing assumptions that three regions, i.e. preheating, reaction and oxidizer zones exist.FindingsThe numerical simulation was first validated against experimental data and characteristics of the combustion process are discussed. Eventually, the influences of crucial parameters such as reactant Lewis numbers, strain rate ratio, particle size, inert gas and thermophoretic force on structure of flame and combustion behavior were examined. The results show that maximum flame temperature can achieve 2,205 K. Replacing nitrogen with argon and helium as carrier gases can increase flame temperature by about 27% and 34%, respectively. Additionally, maximum absolute thermophoretic force was found at approximately 9.6 × 10–8 N.Originality/valueTo the best of authors’ knowledge, this is the first time to numerically model the preparation of MNPs in a counter-flow non-premixed combustion configuration, which can guide large-scale experimental work in a more effective way.

A flat-plate spiral-channeled membrane heat exchanger for methane dehumidification: Comparison of kraft paper and thin-film composite membrane

Membrane heat exchangers were used for moisture and heat recovery from a hot and humid natural gas stream in indirect contact with an air stream. A plate membrane module with spiral channels on each side was designed to configure gas and air streams in co-current and counter-current flow. Two types of membranes, a commercial kraft paper, and a laboratory-made thin-film composite membrane, were used as the module core. The thermal efficiency and the water recovery from the humid gas were calculated and compared under different operating conditions. At a unit flow ratio of air/gas, the greatest effectiveness was achieved at a methane flow of 100 cm3/min, but water recovery increased at higher gas flows. For the counterflow configuration, an air/methane flow ratio of 2 was sufficient to achieve the highest total effectiveness and water recovery; however, larger air/gas ratios were beneficial to improve heat and moisture transfer for the co-current configuration. The highest water recovery by the kraft paper and composite membrane was achieved (approximately 15 mgH2O/cm2.h) for the input gas with the highest humidity. The dehumidification performance of kraft paper as a porous and hydrophilic membrane was slightly better than that of the composite membrane containing a dense hydrophilic polymer thin film in most cases. A relatively low heat transfer coefficient was found for the module, which was increased by using higher gas flow rates at higher humidity conditions.

Effect of dielectric barrier discharge plasma actuator on the dynamic moment behavior of pitching airfoil at low Reynolds number

The purpose of this paper is to investigate the feasibility of using dielectric barrier discharge plasma actuation for the pitch stability control of a National Advisory Committee for Aeronautics 0012 pitch oscillating airfoil at low Reynolds number. The effects of the plasma actuator on the flow are incorporated into Navier–Stokes equations as a body force source term. The plasma body force is modeled by a phenomenological approach. Solutions are obtained by two-dimensional numerical simulations using the γ−Reθ transition model. The flow control effects of plasma actuator locations, strengths, force directions, and laminar separation flutter (LSF) conditions are investigated. The energy extracted by the airfoil from the flow over one oscillation cycle is defined to evaluate the control performance. The LSF analysis based on energy maps is also implemented. The control effect of co-flow plasma configuration on pitch instability is better than the counterflow configuration. The co-flow plasma actuator located at 0.6c reaches the best control performance and the energy extracted by the airfoil from the flow over one oscillation cycle reduces by 726% relative to the baseline case. The flow feature inducing the pitch instability and the flow control mechanism of co-flow and counterflow plasma actuator is analyzed in terms of the flow structure and pressure distribution, respectively. The results show that the plasma flow control is effective to mitigate the pitch instability across a wide range of Reynolds numbers at which laminar separation flutter occurs without control.

Stationary ethylene–air edge flames in a wedge-shaped region at low and high strain rates

ABSTRACT Edge flames are a canonical two-dimensional flame structure appearing in lifted jet flames and in the growth and repair of flame holes in nonpremixed turbulent combustion. Computational studies of edge flames with hydrodynamic-coupling at high strains are difficult owing to difficulties defining a stationary state. A wedge-shaped counterflow configuration is here used to provide control over the position of the edge flame, and allowing access to stationary, hydrodynamically-coupled retreating flames (at low and high strains). ethylene–air edge flames are established in the resulting non-uniformly strained counterflow, with combustion modeled using a skeletal reduction of the USC Mech II. The details of the ethylene–air edge flame are discussed, and comparisons are made between stoichiometric, fuel-lean, and fuel-rich compositions. Mixture-fraction-based coordinates local to the flame front are developed and analysed for a range of edge-flame speeds and compositions. A strain-rate parameter is described that admits a speed-strain relationship that is insensitive to the mixture stoichiometry, and a model for retreating flame speeds at high strain rates is discussed.

Bistability analysis of opposed jet combustor under cold and reacting flow conditions

Opposed Jet Combustor (OJC) is a widely used layout to study and measure flame properties. The fundamental assumption of the counterflow configuration is that the flow is steady and quasi-one-dimensional. In principle, OJC stagnation planes are generated when two streams of opposing follow impinge against the other. The characteristic of such formed stagnation zone is a function of momentum balancing. But a detailed analysis shows that the flow regime developed in OJC is much more complex. It is seen that for the same set of boundary conditions, the stagnation plane can exist at more than one axial location, thereby showcasing the phenomenon of bistability. In this work, a conical opposed jet combustor producing a flat nozzle exit velocity profile was employed to study this phenomenon of bistability under isothermal and reacting flow conditions. The flow field impingement region was analyzed in search of instabilities and asymmetries. Phase Doppler particle analyzer, camera, and thermocouple measurements were employed to map this asymmetric behavior. The result showcases that the axisymmetric cold flow exhibits three flow regimes, i.e., (1) single symmetric, (2) steady-state multiplicity, and (3) time-dependent unsteady state. More importantly, the bistability has been conclusively witnessed even during combustion experiments. This bistable branching observed during combustion was different from that of cold flow. With the reacting flow, the bistable planes showcase different degrees of stability, with only one plane being stable. The results observed here showcase the need to consider the possibility of bistability even during combustion experiments.

Experimental investigation of heat transfer in a triple tube heat exchanger with coiled spring inserts

AbstractExperiments have been performed to investigate the effect of coiled spring inserts on heat transfer, pressure drop, and performance parameters of a triple tube heat exchanger (TTHX). Three different spring inserts having a pitch of 5, 10, and 15 mm are used and the diameter of the spring wire is taken as 1 mm. The experiments were carried out under a turbulent flow regime, with water as a working medium in parallel and counter flow configurations. The variation in different performance characteristics like heat transfer coefficient, Nusselt number, and effectiveness have been compared at various Reynolds numbers ranging between 4000 and 16,000 in the considered flow patterns. The Nusselt number of TTHX with the lowest pitch spring is found to be higher than that of the plain TTHX by 57.27% at Re = 4000 for the counter flow configuration. Both the thermal performance factor and effectiveness increased as the pitch of the spring insert was decreased. The effectiveness of TTHX with the lowest pitch spring insert is found higher than that of the plain TTHX by 43.84% in the counter flow pattern.

Analysing the transport phenomena of novel dew-point evaporative coolers with different flow configurations considering condensation

This paper entails a study on the transport phenomena of a novel dew-point evaporative cooler with counter-flow closed-loop configuration, a potential alternative to the conventional mechanical vapor compression system. In contrast to the conventional indirect evaporative cooler and the regenerative indirect evaporative cooler, the proposed novel dew-point evaporative cooler is capable of achieving the simultaneous goals of pre-cooling, energy recovery and dehumidification. The novel dew-point evaporative cooler is classified as either type 1 or type 2 based on the relative flow direction between the water and the supply air. A two-dimensional computational fluid dynamics model is judiciously developed and employed to conduct a comparative study which evaluates the performance of the conventional indirect evaporative cooler vis-a-vis both novel type 1 and novel type 2 configurations. Experimental data is then employed to validate the predictive accuracy of the mathematical model. The distributions of both temperature and humidity ratio are plotted for a typical case of the novel cooler incorporating both flow configurations and considering the effects of moisture condensation. Additionally, the impacts made by key parameters on the cooling performance under condensation state are analyzed by using three evaluation indexes, namely, latent efficiency, wet-bulb efficiency, and enlargement coefficient. By regulating several operating parameters, the variations of the three evaluation indexes are plotted via the proposed numerical model. Key results from this study revealed that the novel type 1 flow configuration is capable of achieving the highest latent and wet-bulb efficiencies, and enlargement coefficient. It is also observed that the relative flow direction between the water and the supply air exerts a stronger influence on the cooler performance compared to the changes of the water mass flow rate.

Numerical and experimental investigation of alumina-based nanofluid effects on double-pipe heat exchanger thermal performances

In this study, we investigate the thermal behaviour of nanofluids in a double-pipe heat exchanger. It is about a counterflow configuration, designed to cool a lubrication unit of a thermoelectric power plant. The subject of this work is to evaluate the thermal performances of the exchanger by using a nanofluid based on alumina suspension comparing with deionized water. In order to evaluate the thermal performance of the studied configuration, we carried out numerical experiments in an application developed on COMSOL Multiphysics environment, these experiments are utilized to show the feasibility of this application. As result, we found that the nanofluid with an increase in its volume fraction leads to an increase in the overall exchange coefficient, the convective heat transfer coefficient, as well as the efficiency and the power of the exchanger. It is noted that an increase of 1% in volume fraction, can enhance the overall exchange coefficient, the power and the effectiveness of the exchanger by 17.62%, 1.473% and 10.80% respectively. Besides, it is noted that the increase in the concentration of nanofluids leads to a narrowing of the pinch points of the inlet and outlet temperatures, which means that nanofluids are more efficient in cooling temperatures than conventional fluids.

A generalised spray-flamelet formulation by means of a monotonic variable

The external structure of the spray-flamelet can be described using the Schvab–Zel'dovich–Liñan formulation. The gaseous mixture-fraction variable as function of the physical space, , typically employed for the description of gaseous diffusion flames leads to non-monotonicity behaviour for spray flames due to the extra fuel supplied by vaporisation of droplets distributed into the flow. As a result, the overall properties of spray flames depend not only on Z and the scalar dissipation rate, χ, but also on the spray source term, . We propose a new general coordinate variable which takes into account the spatial information about the entire mixture fraction due to the gaseous phase and droplet vaporisation. This coordinate variable is based on the cumulative value of the gaseous mixture fraction and is shown to be monotonic. For pure gaseous flow, the new cumulative function, , yields the well-established flamelet structure in Z-space. In the present manuscript, the spray-flamelet structure and the new equations for temperature and mass fractions in terms of are derived and then applied to the canonical counterflow configuration with potential flow. Numerical results are obtained for ethanol and methanol sprays, and the effect of Lewis and Stokes numbers on the spray-flamelet structure is analysed. The proposed formulation agrees well when mapping the structure back to physical space thereby confirming our integration methodology.

Computational analysis of spray type counter flow liquid desiccant dehumidifier

The liquid desiccant air conditioning system (LDAC) is considered as the substitute for the conventional air conditioning system mainly because of its energy-saving characteristics. The dehumidifier is one of the most important components in LDAC, therefore, it is chosen as the system for analysis. A 3D computational model of the spray type dehumidifier with counter flow configuration was created and analysed its mass transfer characteristics for two different cases. 2D CFD models studied by various researchers failed to elaborate on the actual flow process. In order to fill this gap 3D model was created and the analysis was conducted to investigate the influence of flow rate of air and desiccant on the mass transfer performance of the dehumidifier. Air and desiccant flow rate have direct influence on the mass transfer performance of the dehumidifier. In this study, the dehumidification performance is evaluated through the simulation by increasing the desiccant flow rate and simultaneously reducing the airflow rate and the results are observed. ANSYS-FLUENT® software is used for the current analysis. The increase in desiccant flow rate and decrease in air flow rate tends to improve the change in humidity ratio (specific moisture removal) from 4.2 to 6.3 g/kg of dry air. The temperature variation of solution keeps increasing as the moisture transfer increases. The rise in temperature of the solution is higher than the air during moisture transfer due to exothermic process. Similarly the equilibrium humidity ratio of solution increased due to moisture absorption during dehumidification, which indicates the decrease in solution concentration during dehumidification. The heating of solution and air during dehumidification will affect the moisture transfer rate, which can be avoided by adding a cooling unit to the liquid desiccant system.

Diffusion-controlled premixed flames

Steady laminar flow structures with mixing, chemical reaction, and normal strain qualitatively representative of turbulent combustion at the small scales are analysed. A contrived counterflow configuration is examined to focus more easily on the premixed flames that are known to occur in multiple-flame structures; the premixed flame can be driven by a controlled heat source which is a surrogate for the diffusion flame in the practical multi-flame structure. Reduction to a one-dimensional similar form is obtained with density and properties variations. It is shown that premixed flames at fuel-rich or fuel-lean conditions sufficiently far from the stoichiometric condition are diffusion-controlled with a propagation velocity very weakly dependent on chemical kinetic details. In particular, heat diffused from a downstream source (i.e. the surrogate for a diffusion flame) is needed to sustain the flame. Effects of normal-strain rate, pressure level, and transport properties, combustible mixture composition, and heat source strength are determined through a parameter study with variations in Damköhler number, Prandtl number, upstream mass fractions of oxygen and propane fuel, and wall temperature. The premixed flame becomes less dependent on the downstream source as strain rate decreases and pressure increases. Small changes in heat and mass diffusivities have stronger effects than an order-of-magnitude change in Damköhler number.

Experimental and numerical investigation of energy separation in counterflow and uniflow vortex tubes

There are mainly two types of vortex tube – counterflow and uniflow. Many papers mention that energy separation in uniflow configuration is inferior to that in counterflow configuration. However, an in-depth analysis to find the reason behind this phenomenon is not found in the literature. In this paper experimental investigation is conducted for comparing energy separation in counterflow and uniflow vortex tubes for same geometrical and operating parameters. Also, 3-D CFD analysis is conducted to find the cause of difference in energy separation in these types of vortex tube. Flow field inside both tubes are analyzed and the rates of work and heat transferred between core region and peripheral region are calculated. Tangential shear work transfer from the core region to the peripheral region is identified as the reason of energy separation, while heat transfer from peripheral zone to axial zone reduces energy separation in both counterflow and uniflow vortex tubes. However, transfer of significantly greater sensible heat from peripheral region to core region in uniflow vortex tube causes more reduction of energy separation by decreasing the hot exit temperature and increasing the cold exit temperature.

Experimental and computational investigation of extinction and autoignition of propane and n-heptane in nonpremixed flows

An experimental and computational investigation is carried out to characterize the influence of reactants on critical conditions for extinction and for autoignition of propane and n-heptane in nonpremixed counterflow configurations. Propane or vaporized n-heptane mixed with nitrogen is transported in one stream while the other stream is made up of air mixed with nitrogen. Measurements of the oxidizer stream temperature needed for autoignition are made at fixed values of the strain rate, either with the fuel mass fraction varied at a fixed oxygen mass fraction or with the oxygen mass fraction varied at a fixed fuel mass fraction. Extinction strain rates for propane are measured as a function of the oxygen mass fraction with room-temperature feed streams and the fuel mass fraction fixed and for n-heptane as a function of the fuel mass fraction with the oxygen mass fraction and feed-stream temperatures fixed. Predictions of critical conditions for extinction and autoignition are made employing detailed kinetic mechanisms. Predictions of critical conditions for extinction are in reasonable agreement with measurements, but there are significant discrepancies for autoignition. Measurements show that increasing the mass fraction of either fuel or oxygen increases the overall reactivity thereby reducing the autoignition temperature. The kinetic models predict the increase in reactivity of the mixing layer with increasing mass fraction of fuel but predict very little change in reactivity of the mixing layer with increasing mass fraction of oxygen, thus failing to predict the influence of oxygen on autoignition. It is concluded that there may exist kinetic pathways responsible for this disagreement that are yet to be discovered, and paths that fail to explain the results are identified.

Analytical modeling of lycopodium-propane dual-fuel combustion system in premixed mode in counter-flow configuration

In this paper, an analytical study was conducted on two possible cases of dual-fuel combustion systems operating in premixed mode in counter-flow configuration fueled simultaneously with lycopodium particles/air and propane/air. Combustion behaviors of two distinct cases were discussed whose difference is whether there is an overlap between the reaction zone formed by combustion of propane and that formed by combustion of lycopodium particles. Non-dimensional forms of mass and energy conservation equations were formulated and analytically solved using Matlab and Mathematica. The derived analytical solution was first validated against experimental data and good agreement was achieved. Then, the profiles of temperature and species distributions for the two cases were discussed to evaluate the effects of overlap between reaction zones. It was found that in both cases, dual fuel mode can be considered as an effective technique for improving the combustion characteristics of lycopodium particles. However, the flame temperature in case 1 is higher than that in case 2. Eventually, influences of fuel and oxidizer Lewis numbers and particle size on the flame temperature and position were parametrically analyzed. Furthermore, flame temperature and position inversely varied with Lewis number of reactants and particle size.

Design and Construction of a Double Pipe Heat Exchanger for Laboratory Application

Engineering education is incomplete without laboratory practices. One of such laboratory equipment necessary for all engineering students to have hands-on in the course of their undergraduate studies is the heat exchanger. This work presents the detailed design and construction of a laboratory type double pipe heat exchanger that can be used both in the parallel and counter flow configuration. The heat exchanger was constructed using galvanized steel for both the tube and shell. Experiments were designed and carried out to test the performance of the heat exchangers. The heat exchanger performance characteristics (logarithm mean temperature difference (LMTD), heat transfer rate, effectiveness, and overall heat transfer coefficient) were obtained and compared for the two configurations. The LMTD tends to be relatively constant as the flow rate was increased for both the parallel and counter-flow configuration but with a higher value for the parallel flow configuration. The heat exchanger has a higher heat transfer rate, effectiveness, and overall heat transfer coefficient and therefore has more performance capability for the counter-flow configuration. The overall heat transfer coefficient increased as the flow rate increased for both configurations. Importantly, as a result of this project, Mechanical Engineering students can now have hands-on laboratory experience on how the double pipe heat exchanger works.

Design and Construction of a Double Pipe Heat Exchanger for Laboratory Application

Engineering education is incomplete without laboratory practices. One of such laboratory equipment necessary for all engineering students to have hands-on in the course of their undergraduate studies is the heat exchanger. This work presents the detailed design and construction of a laboratory type double pipe heat exchanger that can be used both in the parallel and counter flow configuration. The heat exchanger was constructed using galvanized steel for both the tube and shell. Experiments were designed and carried out to test the performance of the heat exchangers. The heat exchanger performance characteristics (logarithm mean temperature difference (LMTD), heat transfer rate, effectiveness, and overall heat transfer coefficient) were obtained and compared for the two configurations. The LMTD tends to be relatively constant as the flow rate was increased for both the parallel and counter-flow configuration but with a higher value for the parallel flow configuration. The heat exchanger has a higher heat transfer rate, effectiveness, and overall heat transfer coefficient and therefore has more performance capability for the counter-flow configuration. The overall heat transfer coefficient increased as the flow rate increased for both configurations. Importantly, as a result of this project, Mechanical Engineering students can now have hands-on laboratory experience on how the double pipe heat exchanger works.

Structures of Ethanol Spray Flames under CO2 Dilution of the Oxidizer in the Counterflow Configuration under MILD Combustion Conditions

Structures of both gaseous and liquid ethanol flames in different oxidizing gas environments in the axisymmetric counterflow configuration at atmospheric pressure are studied. Initially, ethanol/air gas flames are considered where pure ethanol is directed against air at initial temperatures of 400 K, and N2 is successively removed to obtain structures of ethanol/O2 gas flames. Furthermore, the addition of CO2 to the oxidizer side is carried out. Then, an ethanol spray is carried by air and directed against an air stream, and the same procedure is performed as described for the gas flames. The gas strain rate at the fuel side of the configuration is increased from low values of 55/s up to extinction, and the initial droplet diameter is varied. For the combustion of gaseous ethanol in air and in pure oxygen, the nitrogen removal results in an increase in the maximum flame temperature from 2010 K to 2920 K at a gas strain rate of 55/s on the fuel side of the configuration, and the extinction strain rates are 630/s and 26,000/s, respectively. It is confirmed that ethanol spray flames in air show two reaction zones at low strain whereas the lean ethanol spray flames in pure oxygen exhibit a single reaction zone in all situations studied. For increased liquid fuel mass flow rate to a global equivalence ratio of unity, two reaction zones are retrieved. An analysis regarding the addition of CO2 in both the ethanol/oxygen gas and spray flames is also discussed and is found that CO2 dilution of the carrier gas the spray is much more efficient than diluting the opposed gas stream in the counterflow configuration for the generation of MILD combustion conditions in oxy-fuel flames.

Analysis of a Fluidized-Bed Particle/Supercritical-CO2 Heat Exchanger in a Concentrating Solar Power System

Abstract Concentrating solar power (CSP) development has focused on increasing the energy conversion efficiency and lowering the capital cost. To improve performance, CSP research is moving to high-temperature and high-efficiency designs. One technology approach is to use inexpensive, high-temperature heat transfer fluids and storage, integrated with a high-efficiency power cycle such as the supercritical carbon dioxide (sCO2) Brayton power cycle. The sCO2 Brayton power cycle has strong potential to achieve performance targets of 50% thermal-to-electric efficiency and dry cooling at an ambient temperature of up to 40 °C and to reduce the cost of power generation. Solid particles have been proposed as a possible high-temperature heat transfer or storage medium that is inexpensive and stable at high temperatures above 1000 °C. The particle/sCO2 heat exchanger (HX) provides a connection between the particles and sCO2 fluid in emerging sCO2 power cycles. This article presents heat transfer modeling to analyze the particle/sCO2 HX design and assess design tradeoffs including the HX cost. The heat transfer process was modeled based on a particle/sCO2 counterflow configuration, and empirical heat transfer correlations for the fluidized bed and sCO2 were used to calculate heat transfer area and estimate the HX cost. A computational fluid dynamics simulation was applied to characterize particle distribution and fluidization. This article shows a path to achieve the cost and performance objectives for a particle/sCO2 HX design by using fluidized-bed technology.

Studies of autoignition-assisted nonpremixed cool flames

Autoignition-assisted nonpremixed cool flames of diethyl ether (DEE) are investigated in both laminar counterflow and turbulent jet flame configurations. First, the ignition and extinction limits of laminar nonpremixed cool flames of diluted DEE are measured and simulated using detailed kinetic models. The laminar flame measurements are used to validate the kinetic models and guide the turbulent flame measurements. The results show that, below a critical mixture condition, for elevated temperature and dilute mixtures, the cool flame extinction limit and the low-temperature ignition limit merge, leading to autoignition-assisted cool flame stabilization without hysteresis. Based on the findings from the laminar flame experiments, autoignition-assisted turbulent lifted cool flames are established using a Co-flow Axisymmetric Reactor-Assisted Turbulent (CARAT) burner. The lift-off heights of the turbulent cool flames are quantified using formaldehyde planar laser-induced fluorescence. Based on an analogy with autoignition-assisted lifted hot flames, a correlation is proposed such that the autoignition-assisted cool flame lift-off height scales with the product of the flow velocity and the square of the first-stage ignition delay time. Using this scaling, we demonstrate that the kinetic mechanism that most accurately predicts the laminar flame ignition and extinction limits also best predicts the turbulent cool flame lift-off height.

Mutually interacting SNG-air premixed flames

Experimental and numerical studies were conducted to grasp the flame characteristics for mutually interacting SNG-air premixed flames with a counterflow configuration. Five different reaction mechanisms (GRI 3.0, USC 2.0, Aramco 2.0, UCSD, and CRECK) were tested to give the priority in tracing measured lean extinction boundaries. UCSD mechanism best traced the measured data. The flame stability map was presented with a functional dependency on methane mole fractions in the reactant streams issuing from the lower and upper nozzles, by varying the global strain rate.With increasing global strain rate, lean and rich extinction boundaries were gradually slanted, forming an island of flammable region and finally only one flammable condition at 740 s−1 via a continuous shrinkage of flammable region. For asymmetric lean-lean and rich-rich flames, the weaker flame was survived via thermally and chemically symbiotic relation of it to the stronger flame. For lean-rich flames, triple flame is changed into bi-brachial flames via a merging of diffusion flame lean flame at high strain rates. This behavior could be explained well by introducing a specific heat release rate. Extinction mechanism could be reasonably explained by the relation of specific heat release rate (as a reactivity of flame) to conductive heat loss from the strong flame to unburned mixture.