Jet Momentum Ratio Research Articles

Velocity field characteristics of the turbulent jet induced by direct contact condensation of steam jet in crossflow of water in a vertical pipe

Direct contact condensation of jets in fluid has been widely applied in many industrial applications owing to the low requirement of driving potential and high efficiency of heat and mass transfer. Here, experiments are carried out to investigate the velocity field characteristics of the turbulent jet induced by direct contact condensation of steam jet in crossflow of water in a vertical pipe. Visual equipment is specially invented to investigate the velocity field characteristics by using Particle Image Velocimetry (PIV) measurement technique. The high intensity laser light reflected by the pure steam region just outside the nozzle-exit brings out severe damage to the CCD camera. To solve this technical problem, a black plate is adopted to shield the pure steam region. According to the contours of the velocity fields and streamlines, the influences of jet momentum ratio, jet Reynolds number and water temperature on the jet flow field are explored. The jet centerline trajectory equations in exponential form are established based on the local maximum mean velocity. By introducing the jet Reynolds number and jet momentum ratio, the correlation for prediction of jet centerline trajectory equations is proposed, and the predicted results are within 30% of the experimental data. The reciprocal of the local maximum mean velocity and the half-width of the jet are proportional to the downstream coordinate along the jet velocity centerline trajectory. The scaled velocity field complies with the self-similarity principle.

A Model for Predicting the Lift-Off Height of Premixed Jets in Vitiated Cross Flow

Lean premixed single-stage combustion is state of the art for low pollution combustion in heavy-duty gas turbines with gaseous fuels. The application of premixed jets in multistage combustion to lower nitric oxide emissions and enhance turn-down ratio is a novel promising approach. At the Lehrstuhl für Thermodynamik, Technische Universität München, a large-scale atmospheric combustion test rig has been set up for studying staged combustion. The understanding of lift-off (LO) behavior is crucial for determining the amount of mixing before ignition and for avoiding flames anchoring at the combustor walls. This experiment studies jet LO depending on jet equivalence ratio (0.58–0.82), jet preheat temperature (288–673 K), cross flow temperature (1634–1821 K), and jet momentum ratio (6–210). The differences to existing LO studies are the high cross flow temperature and applying a premixed jet. The LO height of the jet flame is determined by OH* chemiluminescence images, and subsequently, the data is used to analyze the influence of each parameter and to develop a model that predicts the LO height for similar staged combustion systems. A main outcome of this work is that the LO height in a high temperature cross flow cannot be described by one dimensionless number like Damköhler- or Karlovitz-number. Furthermore, the ignition delay time scale τign also misses part of the LO height mechanism. The presented model uses turbulent time scales, the ignition delay, and a chemical time scale based on the laminar flame speed. An analysis of the model reveals flame stabilization mechanisms and explains the importance of different time scale.

Critical Ignition Criteria for Monomethylhydrazine and Red Fuming Nitric Acid

Understanding the parameters governing ignition is crucial for safe rocket engine design, and while hypergolic propellants have been used for several decades, there is still a lack of data on the early condensed-phase reactions for many systems. The ignition and combustion of monomethylhydrazine (MMH) and red fuming nitric acid (RFNA) have been investigated in an unlike doublet impinging jet apparatus. Variation of the jet diameter and total propellant mass flow rate at constant oxidizer-to-fuel and jet momentum ratio allowed for investigation of critical ignition thresholds. Ignition probability was determined for each condition and classified as good ignition, a transition region, or failed ignition. Conditions resulting in reactive stream separation are compared with literature data obtained with hydrazine/nitrogen tetroxide. Two critical MMH/RFNA ignition thresholds were identified: a residence time of 0.2–0.4 ms and a nondimensional sheet growth rate of . Two power law correlations were identified allowing for prediction of an MMH/RFNA sheet growth rate given a residence time or Reynolds number. Thermal ignition theory was adapted to simulate the experiment, and selection of model parameters resulted in a trend that supports the experimental data and provides insight into the condensed-phase reaction rates. The results from this work are intended to provide an experimental data set for validation of MMH/RFNA combustion models and to guide the selection of injection conditions for impinging jet systems.

Toward ultra-low emission distributed combustion with fuel air dilution

Colorless distributed combustion (CDC) has been shown to offer enhanced combustor performance for stationary gas turbine application with near zero emissions, high combustion intensity and efficiency, thermal field uniformity, and enhanced stability. Mixture preparation to form hot and low oxygen concentration environment paves the path to achieve CDC conditions. In this paper, a new approach of air dilution in partially premixed combustion conditions is employed and the results compared to premixed and non-premixed injection of air and fuel. Portion of the fuel is introduced in the air stream and portion of the air is introduced in the fuel stream such that the local equivalence ratios for each stream is well outside the flammability limit to eliminate flashback and instabilities. The experimental data demonstrated ultra-low emissions with this injection scheme. At equivalence ratio of 0.6, NO emission was 63% lower than non-premixed combustion mode. Also NO emission was similar to the premixed combustion with the advantage of eliminating flashback and flame instabilities that often prevail in premixed combustion conditions. Dilution provided 50% CO reduction as compared to non-premixed combustion. Numerical simulations, validated through Particle Image Velocimetry, were performed to outline the mixing process in each of the three cases. The methane mixture fraction prior to ignition, determined numerically, was found to be one half of that for the non-premixed case and close to that of the premixed case. This enhanced mixture preparation, associated with the new air and fuel dilution technique, resulted in reduced emission. Also the jet momentum ratio (between both streams) is enhanced, mainly due to the air addition to the fuel stream, to result in better mixing and a better reaction distribution for ultra-low emissions. Further reduction of NOx is expected with improved distributed combustion condition.

Emission Measurements and CH* Chemiluminescence of a Staged Combustion Rig for Stationary Gas Turbine Applications

An optically accessible combustion rig was constructed to study the combustion characteristics of a reactive jet in a vitiated crossflow. The rig features two staged combustion zones. The main combustion zone is a swirl stabilized dump combustor. The second combustion zone, which is axially downstream from the main combustion zone, is formed by a transverse jet injecting either fuel or a premixed fuel/air mixture into the vitiated stream. The rig was designed to investigate the transverse jet conditions, equivalence ratio, and momentum ratios that produce low NOx and give an adequate temperature rise before the simulated high pressure turbine. A water-cooled sampling probe extracts exhaust gas downstream for emission measurements. As a baseline, the main combustion zone was fired without the transverse jet and the results compare closely to the work of previous researchers. The emission survey with the transverse jet found several conditions that show a benefit of staging compared to the baseline of firing only the main combustion zone. The flame structure from the transverse jet was captured using high speed CH* chemiluminescence, which shows the extent of the flame front and its penetration depth into the vitiated stream. The chemiluminescence images were averaged and compared to the Holdeman correlation, which showed good agreement for injection with fuel only but poorer agreement when premixed.

Large-eddy simulation of pulsed high-speed subsonic jets in a turbulent crossflow

The jet in crossflow problem has been extensively studied, mainly because of its application to film cooling and injector design. It has been established that in low-speed flow, pulsing the jet significantly enhances mixing and jet penetration. This work investigates the effects of pulsing of high-speed subsonic jets (Ma∼0.47−0.77) on mixing and jet trajectory in turbulent subsonic crossflows by using large-eddy simulation. Jets with different density and momentum ratios are pulsed from an injector into a crossflow for a range of Strouhal numbers. Results are compared with the earlier low-speed cases to determine the sensitivity of jet penetration scaling on the relatively high Reynolds and Mach numbers of the current pulsed jets. It is shown that the classical momentum-ratio-based scaling (which reduces to the velocity ratio scaling in equal density low-speed case) can still be used for these high Reynolds and jet Mach number cases, although the scaling constant is slightly different. However, when the jet momentum ratio is different for the same Strouhal, Reynolds and Mach numbers, there is a significant change in the scaling constant, suggesting a sensitivity that could impact the original scaling relationship itself.

Similarity criteria for estimating gas emission from scale models

Similarity, or the relationship between a model and a prototype, is an important issue when using physical modelling techniques. The objective was to investigate similarity criteria using Reynolds number (Re) and jet momentum ratio (Rm) in two scale models (SM-1 of 1:6 and SM-2 of 1:12.5) of a pig house under isothermal conditions to predict airflow and ammonia emission. Experiments were conducted using two ventilation control strategies (constant inlet opening and constant inlet velocity) in SM-1 and SM-2 by (1) keeping the same inlet Re with constant inlet opening, and (2) keeping the same Rm with constant inlet opening and constant inlet velocity. Aqueous ammonia solutions, with four different total ammoniacal nitrogen (TAN) concentrations and two different pH levels, were used as emission sources inside the scale models. Non-dimensional normalised emission rate, defined as the ratio of the measured emission rate and the reference emission rate at inlet air velocity of 1 m s−1 of the scale models, was proposed for when using Re and Rm as similarity. Rm was found to be a better scaling parameter than Re for predicting the influence of airflow on ammonia emission rate when using the non-dimensional normalised emission rate in scale models with both ventilation control strategies and ammonia solutions of different TAN and pH.

Numerical and Experimental Study on Jet Trajectories and Mixing Behavior of Venturi-Jet Mixer

An investigation of the jet trajectories and mixing behavior of venturi-jet mixers, in which turbulent jet injects fluid at an arbitrary angle to mix incompressible fluids, is described in this paper. Numerical results of an incompressible cross flow-jet mixing in venturi-jet mixers are presented and validated against experimental results. Jet trajectories, concentration decay of tracer jet, spatial unmixedness, and mixing length are computed for a range of injection angles (45 deg≤θo≤135 deg). Twenty-five cases are studied with five different initial injection angles, each with five different jet momentum ratios. As an example of practical insights that can be gained from such detailed analysis, the resulting flow field is used to obtain an equation for trajectory and tracer concentration in the mixer. The penetration of jet scales with the third root of the jet-to-mainstream momentum ratio and that of with square root of downstream distance. The decay of mean concentration scales with the inverse of axial distance and with the negative root of injection angle. The results show a consistency in the experimental data and simulation has provided a good insight into the flow details and has paved the way in optimization of the geometry based on jet injection angle to get a good mixing efficiency.

Separation Control on a Trailing-Edge Flap Using Air Jet Vortex Generators

Nomenclature A = area CD = drag coefficient CL = lift coefficient Cp = pressure coefficient CV = jet velocity ratio Cμ = blowing coefficient (jet momentum ratio) c = chord DLG = flap deflection, lap and gap f = jet orifice diameter L = lift M = Mach number n = number of jets p = static pressure p0 = total pressure q = freestream dynamic pressure S = wing reference area V = speed x/c = nondimensional distance measured in a chordwise sense α = angle of attack, or jet skew angle β = jet pitch angle γ = ratio of specific heats = flap deflection λ = jet orifice spacing ρ = density

Similitude Criteria for a Two-dimensional Wall Jet in an Isothermal Mechanically Ventilated Enclosure

A scale model experiment is an important method to study ventilation airflow pattern inside buildings that affects indoor air quality. The similitude criteria are the decision factors to guarantee using the result of model study to predict actual airflow of prototype. It is important to decide the similitude criteria between scale model and prototype. The aim of this project focuses on deciding the similitude criteria of a two-dimensional wall jet between the model and prototype to establish same airflow pattern in an isothermal mechanically ventilated enclosure. The parameters of the Reynolds number, Euler number, and jet momentum ratio proposed by previous researchers are investigated to find which parameter is the similitude criterion for a two-dimensional wall jet in isothermal condition. Experiments were conducted in two geometrically similar scale models. The similitude criteria were verified by comparing latent parameters with airspeed distribution following the kinematic similitude, whereas the airflow pattern following the dynamic similitude between the model and prototype. Experimental results indicate that the Euler number is not the similitude criterion, but the jet momentum ratio related to inlet airspeed is the similitude criterion of a two-dimensional wall jet in an isothermal mechanically ventilated enclosure. Reynolds number may be used as the similitude criterion only for flows at low Reynolds numbers. Our results are useful for the researcher to decide the parameters and boundary conditions of scale model study for the prediction of prototype performance of a two-dimensional wall jet in an isothermal mechanically ventilated enclosure.

Micro-jets in confined turbulent cross flow

The mixing of sub-millimetre diameter jets issuing into a turbulent cross flow is examined with a combination of laser diagnostic techniques. The cross flow stream is in a confined duct and the micro-jet issue from the sides of injector vanes. A range of cross jet momentum ratios, cross flow temperatures and turbulence intensities are investigated to examine the influence on the jet mixing. Methane, seeded with acetone, was used to measure the concentrations of the jets and the mixing of the jet fluid in the duct. Unlike previous jet in cross flow work, mixing appears to be dominated by the free stream turbulence, rather than the cross jet momentum ratios. Temperature increases in the free stream appear to increase the rate of mixing in the duct, despite the associated decrease in the Reynolds number. The dominance of the free stream turbulence in controlling the mixing is of particular interest in respect of gas turbine injection systems, as the cross jet momentum ratio is insufficient in defining the mixing process.

Stability and emissions control using air injection and H 2 addition in premixed combustion

We experimentally study lean premixed combustion stabilized behind a backward-facing step. For a propane–air mixture, the lean blowout limit is associated with strong pressure fluctuation arising simultaneously with strong flame–vortex interactions, which have been shown to constitute the mechanism of heat release dynamics in this flow. A high-speed air jet, issuing from a small slot and injected perpendicular to the main flow near the step, is used to disrupt this mechanism. For momentum ratio of jet to main flow below unity, the jet dilutes the mixture, further destabilizing the flame or leading to complete blowout. Above unity, the flame becomes more stable, and the pressure oscillations are suppressed. Flow visualization and OH*/CH* chemiluminescence measurements show that a strong jet produces a more compact flame that is less driven by the wake vortex, anchored closer to the step, and deflected upwards away from the lower wall of the channel. This renders the flame less vulnerable to heat loss and strong strains, which improves its stability and extends the flammability limit. Adding hydrogen to the main fuel improves the flame stability over the entire range of the air jet mass flow, with better results for momentum ratio larger than 1; H 2 pulls the flame further upstream, away from the shear zone and the unsteady vortex. NO x emission benefits from the air jet, while, with H 2 addition, NO x concentration is higher in the products as the overall burning temperature rises. However, hydrogen addition enables extending the flammability limit further by increasing air supply in the primary stream, hence achieving lower NO x . The study suggests a simpler, almost passive, multi-objective combustion control technique and indicates that hydrogen addition can be a successful in situ approach for NO x reduction.

Orifice Diameter Ratio Effect on the Mixing Performances for Split Triplet Injectors

The effect of orifice diameter ratio on the mixing qualities of unlike-doublet and split-triplet impinging elements is experimentally studied. The quality of mixing was checked by performing cold-flow tests with nonreacting immiscible simulant liquids. The local volume fractions are determined by direct measurement of the volume of each liquid. The test matrix comprises combinations of the orifice diameter ratios from 1 to 1.5, with the jet momentum ratios (oxidizer/fuel) in the range of 0.5-6. Results show that impinging elements of unequal orifice diameters exhibit substantially different mixing qualities. Maximum mixing efficiency occurs at higher momentum ratios with increasing diameter ratio. Relative jet velocity ratio for optimum mixing ranges from 0.65 to 0.78 and from 0.87 to 0.92 for unlike-doublet and split-triplet elements, respectively. The split-triplet element is superior to the unlike-doublet element in both the mixing efficiency and the mixing-controlled characteristic velocity in the test range of interest. The diameter ratio effect on the mixing of split-triplet element is less significant than that on the unlike-doublet element mixing. The mixing factor of the split-triplet element for maximum mixing efficiency is 0.75.

Effect of Momentum Ratio on the Mixing Performance of Unlike Split Triplet Injectors

Experimental investigation of mixing and mixing-controlled combustion efficiencies for sprays formed by unlike impinging split triplet injector elements was carried out. The quality of mixing was checked by performing cold-flow tests with inert simulant liquids. Measurements of local mass and mixture ratio distributions were made for different injection configurations and different jet momentum ratios. Results show that the quality of macroscopic mixing can be effectively characterized by the jet momentum ratio. The unlike split triplet element with lateral fuel injection is superior in mixing to either unlike doublet or unlike split triplet elements with central fuel injection. Secondary impingement appears to play a significant role in the extent of mixing. Mixing characteristics of the unlike split triplet element and its contribution to the promotion of macroscopic mixing efficiencies are discussed in detail.

AIRFLOW PATTERN SIMILARITY CRITERIA FOR CEILING SLOT-VENTILATED AGRICULTURAL ENCLOSURES UNDER ISOTHERMAL CONDITIONS

The use of a scaled model can be an effective technique to predict ventilation performance of full-scale prototypes. Many criteria have been proposed to predict the behavior of airflow in a scale model and a prototype. Because of inconsistent results of proposed similarity criteria, more validation work is needed to clarify the conflicts. Experiments to study airflow similarity in ceiling slot-ventilated agricultural enclosures using different similarity criteria were conducted on two scale models. The studies focused on the Reynolds number (Re) and inlet jet momentum ratio (Rm) as the similitude criteria for isothermal airflow. The experimental results offer better agreement using Rm as opposed to Re as the similitude criterion for isothermal airflow pattern control.

Equations for a Ventilation Design Derived from Computational Fluid Dynamics

Computational fluid dynamics (CFD) was used in a study of the air flow characteristics in the occupied building zone. Correlation equations between the mean air speeds and the percentage dissatisfied with the macroscopic flow numbers were derived. Ten macroscopic flow numbers including the total ventilation rate, the air change rate, ventilation rate, air diffusion performance index, modified jet momentum number, two new flow numbers and three expressions of jet momentum ratio were investigated. A total number of 25 numerical experiments under isothermal flow conditions with four different geometrical arrangements (labelled A–D) were performed. The correlation equations derived from CFD were compared with those obtained from experimental studies. It is found that the jet momentum ratio R_M2 gives the equation with the best correlation coefficient and so is recommended for ventilation design.

Equations for a Ventilation Design Derived from Computational Fluid Dynamics

Computational fluid dynamics (CFD) was used in a study of the air flow char acteristics in the occupied building zone. Correlation equations between the mean air speeds and the percentage dissatisfied with the macroscopic flow numbers were derived. Ten macroscopic flow numbers including the total ventilation rate, the air change rate, ventilation rate, air diffusion performance index, modified jet momentum number, two new flow numbers and three expressions of jet momentum ratio were investigated. A total number of 25 numerical experiments under isothermal flow conditions with four different geometrical arrangements (labelled A-D) were performed. The correlation equations derived from CFD were compared with those obtained from experi mental studies. It is found that the jet momentum ratio RM2 gives the equation with the best correlation coefficient and so is recommended for ventilation design.

Air diffusion terminal devices: Macroscopic numbers describing jet momentum

Macroscopic flow numbers relating to the jet momentum of air diffusion terminal devices are studied in this paper. Diffuser jet momentum, jet momentum number and jet momentum ratio are reviewed in the literature. New expressions for jet momentum ratios and jet volume numbers are proposed for specifying the mean room air speed in mechanically ventilated enclosures. The proposal is validated against experimental data from seven sites under fifteen operating flow conditions. Two macroscopic flow numbers correlate with the measured mean air speed in the room.

MODELING OF JET-GENERATED RECIRCULATING FLOWS IN A MINIATURE COMBUSTOR

The complicated flow fields in miniature annular combustors are investigated numerically. The flow parameters studied include the jet tube length and momentum ratio of the primary jet. From the numerical results it is found that four different recirculations, namely, the primary, second, third, and dilution recirculations, might be generated in the combustor owing to the interaction between the mainstream and the lateral jets. In all cases studied, the primary recirculation can be observed, although its strength, size, and location are changed with the flow parameters. The jet tube length produces a significant effect on the recirculation flow structure in the primary zone. The increase of the momentum ratio of the primary jet results in the suppression and enhancement of the primary and second recirculations, respectively.

Characteristic Modes and Thermal Structure of Nonpremixed Circular-Disc Stabilized Flames

ABSTRACT The characteristic modes and structure of the nonpremixed circular-disc stabilized flamesare experimentally studied. Using direct and schlieren photography techniques, the flames in the stability domain are systematically classified into seven characteristic modes: recirculated, transition, unsteady detached, laminar ring, developing, split flashing, and lifted. The flame behaviorand the combusting and cold flow patterns in each characteristic mode are comprehensively discussed. Two stability limits which are closely related to the flow structure are found. The flame and lift-off heights are correlated to the central-to-annular jet momentum ratio. The structure and transport mechanism in the recirculation bubble dominate the flame behavior for central-to-annular jet momentum ratio lower than 7.5. For the central-to annular jet monentum ratio higher than 7.5, the combusting flow behaves like a turbulent diffusion flame.The temperature profiles are probed by a fine-wire thermocouple. The thermal structure in the recirculation area is found 10 be complicatedly dominated by the vortex structure in the bubble.