Alejandro M. Briones

Micrometer-Sized Water Droplet Impingement Dynamics and Evaporation on a Flat Dry Surface

A comprehensive numerical and experimental investigation on micrometer-sized water droplet impact dynamics and evaporation on an unheated, flat, dry surface is conducted from the standpoint of spray-cooling technology. The axisymmetric time-dependent governing equations of continuity, momentum, energy, and species are solved. Surface tension, wall adhesion effect, gravitational body force, contact line dynamics, and evaporation are accounted for in the governing equations. The explicit volume of fluid (VOF) model with dynamic meshing and variable-time stepping in serial and parallel processors is used to capture the time-dependent liquid-gas interface motion throughout the computational domain. The numerical model includes temperature- and species-dependent thermodynamic and transport properties. The contact line dynamics and the evaporation rate are predicted using Blakes and Schrages molecular kinetic models, respectively. An extensive grid independence study was conducted. Droplet impingement and evaporation data are acquired with a standard dispensing/imaging system and high-speed photography. The numerical results are compared with measurements reported in the literature for millimeter-size droplets and with current microdroplet experiments in terms of instantaneous droplet shape and temporal spread (R/D(0) or R/R(E)), flatness ratio (H/D(0)), and height (H/H(E)) profiles, as well as temporal volume (inverted A) profile. The Weber numbers (We) for impinging droplets vary from 1.4 to 35.2 at nearly constant Ohnesorge number (Oh) of approximately 0.025-0.029. Both numerical and experimental results show that there is air bubble entrapment due to impingement. Numerical results indicate that Blakes formulation provides better results than the static (SCA) and dynamic contact angle (DCA) approach in terms of temporal evolution of R/D(0) and H/D(0) (especially at the initial stages of spreading) and equilibrium flatness ratio (H(E)/D(0)). Blakes contact line dynamics is dependent on the wetting parameter (K(W)). Both numerical and experimental results suggest that at 4.5 < We < 11.0 the short-time dynamics of microdroplet impingement corresponds to a transition regime between two different spreading regimes (i.e., for We < or = 4.5, impingement is followed by spreading, then contact line pinning and then inertial oscillations, and for We > or = 11.0, impingement is followed by spreading, then recoiling, then contact line pinning and then inertial oscillations). Droplet evaporation can be satisfactorily modeled using the Schrage model, since it predicts both well-defined transient and quasi-steady evaporation stages. The model compares well with measurements in terms of flatness ratio (H/H(E)) before depinning occurs. Toroidal vortices are formed on the droplet surface in the gaseous phase due to buoyancy-induced Rayleigh-Taylor instability that enhances convection.

A numerical investigation of flame liftoff, stabilization, and blowout

The effects of fuel stream dilution on the liftoff, stabilization, and blowout characteristics of laminar nonpremixed flames (NPFs) and partially premixed flames (PPFs) are investigated. Lifted methane-air flames were established in axisymmetric coflowing jets. Because of their flame suppression characteristics, two predominantly inert agents, CO2 and N2, were used as diluents. A time-accurate, implicit algorithm that uses a detailed description of the chemistry and includes radiation effects is used for the simulations. The predictions are validated using measurements of the reaction zone topologies and liftoff heights of both NPF and PPF. While an undiluted PPF is stabilized at the burner rim, characterized by significant radical destruction and heat loss to the burner, the corresponding undiluted NPF is lifted and stabilized in a low-velocity region extending from the wake of the burner. Detailed comparison of diluted NPF with PPF reveals that the base structures of both the flames are similar and exhi...

EFFECT OF FUEL BLENDS ON POLLUTANT EMISSIONS IN FLAMES

ABSTRACT Fuel blending represents a promising approach for reducing both NOx and particulate emissions from flames. This paper reports a fundamental investigation on the effects of blending hydrogen with different fuels (methane and n-heptane) on the structure and emission characteristics of counterflow nonpremixed and partially premixed flames (PPF). The emission behavior is characterized in terms of the concentrations and emission indices of various pollutant species as a function of hydrogen content in the blend. Results indicate that hydrogen blending has a much more favorable effect on emissions in heptane flames than in methane flames. With hydrogen addition in methane/hydrogen blends, the emission index of C2H2 (which is an important soot precursor) is reduced, CO remains unchanged, and NO increases slightly. In heptane/hydrogen blends, however, emission indices of all three species (NO, C2H2, and CO) decrease significantly with hydrogen addition. The behavior is attributed to two factors. First, for the same strain rate, the addition of hydrogen decreases carbon content in the fuel, which reduces the amount of CO and C2H2 formed. Because C2H2 is the major source of CH that leads to the formation of prompt NO, and because prompt NO is the dominant contributor to total NO in heptane flames, hydrogen addition leads to a dramatic decrease in NO emission in these flames. Second, the addition of hydrogen changes combustion chemistry due to the higher reactivity of H2 and the higher concentrations of H and OH radicals, which increase the CO and C2H2 oxidation rates. In addition, this enhances the C1 path for methane oxidation and decomposition of n-heptane to C3–C6 species and, thus, decreases the formation of C2H2. The net result is a decrease in the emission of all three pollutant species, although the effect is significantly higher for n-heptane flames. For PPFs, the addition of hydrogen increases the physical separation between the two reaction zones—namely, the rich premixed and the nonpremixed zones (i.e., the flames become broader). The emission of NO, C2H2, and CO species is also affected favorably due to hydrogen addition, but the effect is less significant for PPFs compared to that for nonpremixed flames.

Flame Stabilization in Small Cavities

This research is motivated by the necessity to improve the performance of ultracompact combustors, which requires flame stabilization in small cavities. An extensive computational investigation on the characteristics of cavity-stabilized flames is presented. A high-fidelity, time-accurate, implicit algorithm that uses a global chemical mechanism for JP8-air combustion and includes detailed thermodynamic and transport properties as well as radiation effects is used for simulation. Calculations are performed using both direct numerical simulation and standard k-e Reynolds-averaged Navier-Stokes model. The flow unsteadiness is first examined in large axisymmetric and small planar cavities with nonreactive flows. As with previous investigations on axisymmetric cavities, multiple flow regimes were obtained by varying cavity length (x/D o ) : wake backflow regime, unsteady cavity vortex regime, steady cavity vortex regime, and compressed cavity vortex regime. However, planar cavities only exhibit steady cavity vortex and compressed cavity vortex regimes. Two opposed nonaligned air jets were positioned in this planar cavity: the outermost air jet in coflow with the mainstream flow (i.e., normal injection). The fuel jet was injected either in coflow, crossflow, or counterflow with respect to the mainstream flow. Flow unsteadiness was observed to be relatively small for coflow- and crossflow-fuel-jet injection. By reversing the air jet positions (i.e., reverse injection), the flow unsteadiness is promoted regardless of fuel jet positioning. Finally, the effect of combustion and cavity equivalence ratio (φ CAV ) on flame unsteadiness is addressed. With normal injection (reverse injection), low and high φ CAV leads to low (high) and high (low) flame unsteadiness, respectively. Based on these results recommendations are provided to designers/engineers to reduce flame unsteadiness in these cavities.

Effect of Trapped Vortex Combustion with Radial Vane Cavity Arrangements on Predicted Inter-Turbine Burner Performance

The complex combustion processes, including chemical reactions, turbulence, unsteady, multiphase flow, evaporation and heat and mass transfer pose great challenges in modern propulsion system design and development. Ultra-short compact, high performance combustion systems are desirable for advanced propulsion systems from the standpoint of lower fuel consumption and increased material durability. AFRL has proposed placing an Ultra-Compact Combustor (UCC) between a high pressure turbine stage and low pressure turbine stage to create an innovative Inter-Turbine Burner (ITB) concept. This paper focuses on ITB combustor technologies that can enable the development of compact, highperformance combustion systems. Compact combustors weigh less and take up less volume in space-limited turbine engine for aero applications. The earlier designs conceived and developed at AFRL/RZTC is based on the idea that the flame speed under turbulent conditions is directly proportional to the square root of gravity and high-g flames offer increased flame speeds, which would aid in the design of shorter combustion systems. This idea led to an ITB with a circumferential cavity in which fuel and air injected at selected points led to rich combustion in the circumferential cavity. This was further followed by lean combustion and flame stabilization with the aid of a radial vane with notch. Even though this concept exhibited good merits through several rig tests and numerical studies carried out over the years at AFRL/RZTC, it does not allow scaling of the geometry and configuration for higher mass flow rates, larger size and increased thrust requirements. This paper presents an alternative concept for the UCC that uses a Trapped Vortex Cavity (TVC) to replace the high swirling circumferential cavity combustion to enhance mixing rates via a double vortex system in the TVC, followed by further mixing of the free stream air through the vane with a notch. Flow field predictions utilizing FLUENT are presented for concept evaluation in a systematic way to understand the flow development and physics, leading to the incremental combustion enhancement, total pressure loss, the entrainment and the calculated exit temperature profile. The analysis supplements the understanding of the design space required for future engine designs that may use this novel, compact combustion systems.

Evaporation Characteristics of Pinned Water Microdroplets

Alejandro M. Briones∗ and Jamie S. Ervin University of Dayton Research Institute, Dayton, Ohio 45469 Larry W. Byrd U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433 Shawn A. Putnam Universal Technology Corporation, Dayton, Ohio 45434 Ashley White University of Dayton, Dayton, Ohio 45469 and John G. Jones∗∗ U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433

Experimental and Computational Studies of an Ultra-Compact Combustor

Reducing the weight and decreasing pressure losses of aviation gas turbine engines improves the thrust-to-weight ratio and improves efficiency. In ultra-compact combustors (UCCs), engine length is reduced and pressure losses are decreased by merging a combustor with adjacent components using a systems engineering approach. High-pressure turbine inlet vanes can be placed in a combustor to form a UCC. Eliminating the compressor outlet guide vanes (OGVs) and maintaining swirl through the diffuser can result in further reduction in engine length and weight. Cycle analysis indicates that a 2.4% improvement in engine weight and a 0.8% increase in thrust-specific fuel consumption are possible when a UCC is used. Experiments and analysis were performed in an effort to understand key physical and chemical processes within a trapped-vortex UCC. Experiments were performed using a combustor operating at pressures in the range of 520–1030 kPa (75–150 psi) and inlet temperature of 480–620 K (865–1120 °R). The primary reaction zone is in a single trapped-vortex cavity where the equivalence ratio was varied from 0.7 to 1.8. Combustion efficiencies and NOx emissions were measured and exit temperature profiles obtained, for various air loadings, cavity equivalence ratios, and configurations with and without turbine inlet vanes. A combined diffuser-flameholder (CDF) was used in configurations without vanes to study the interaction of cavity and core flows. Higher combustion efficiency was achieved when the forward-to-aft momentum ratios of the air jets in the cavity were near unity or higher. Discrete jets of air immediately above the cavity result in the highest combustion efficiency. The air jets reinforce the vortex structure within the cavity, as confirmed through coherent structure velocimetry of high-speed images. A more uniform temperature profile was observed at the combustor exit when a CDF is used instead of vanes. This is the result of increased mass transport along the face of the flame holder. Emission indices of NOx were between 3.5 and 6.5 g/kgfuel for all test conditions. Ultra-compact combustors (with a single cavity) can be run with higher air loadings than those employed in previous testing with a trapped-vortex combustor (two cavities) with similar combustion efficiencies being maintained. The results of this study suggest that the length of combustors and adjacent components can be reduced by employing a systems level approach.Copyright

Effect of Vane Notch and Ramp Design on the Performance of a Rectangular Inter-Turbine Burner

This research is motivated by the need to improve and optimize the performance of AFRL’s Ultra-Compact Combustor (UCC) in terms of greater combustion efficiency, reduced pressure losses and exit temperature profile requirements. The UCC operates as an Inter-turbine Burner (ITB) situated in between the high and low pressure turbine stages. The detailed understanding of the effect of the vane cavities, that are essential for the transport and mixing of the combustion products and incoming air stream in a threedimensional ITB model would be very difficult to optimize and could be experimentally and computationally prohibited. Therefore, a simple representation of somewhat similar burner is used here for optimization of the vane cavities, for improved mixing and reduced losses, using modeling and simulation. The ITB generally contains vanes to redirect the flow direction and to assist the mixing, but in this investigation we model the Trapped Vortex Combustor (TVC) ITB with a single vane with various notch designs and with ramps typically found in high speed combustion applications. In addition, the full configuration is simplified with only two fuel injection sites to get a faster turn around and to get a better understanding of the local flow development. A total of five vane configurations are studied: (1) Vane (V), (2) Vane + Notch (VN), (3) Vane + Altered Notch (VAN), (4) Vane + Extended Altered Notch (VEAN), and (5) Vane + Double Notch (VDN). Two ramp configurations are tested as well: (1) Ramp (R) and (2) Reverse Ramp (RR). FLUENT is used for modeling the three-dimensional ITB using a global eddy dissipation mechanism for C12H23-air combustion with detailed thermodynamic and transport properties. Calculations are performed using the Realizable k-e RANS turbulence model. The combustor efficiency for all ITB configurations is above 99%. Results indicate the major contributor to total drag for all ITB vane and ramp configurations investigated is the pressure drag. The side walls of the combustor do not contribute to drag. The top wall of the ITB is primarily exposed to viscous drag, whereas the bottom wall, which includes the TVC, is primarily exposed to pressure drag with small contributions from viscous drag. The vanes and ramps mainly contribute to drag due to pressure drag. The vanes contribute the most to the overall combustor drag (or pressure loss). The total drag in the combustor decreases with the addition of vane notches (or cavities). Drag decreases in descending order from V to VN, VAN, VEAN, and VDN. Whereas VN and VAN decrease pressure drag (or pressure losses) by only ~3%, VEAN and

Experimental Characterization of the Reaction Zone in an Ultra-Compact Combustor

Significant benefits can be obtained with respect to engine thrust-to-weight ratio and specific fuel consumption if the length, weight, and pressure drop of the combustor can be reduced. The ultra-compact combustor (UCC) has the potential to aid the realization of these benefits by integrating neighboring components such as the compressor exit diffuser and the turbine inlet guide vanes (IGV) within the combustor using a systems-level engineering approach. The UCC presented here utilizes a trapped-vortex cavity. This combustor design has been shown to exhibit larger turn-down ratios, higher flame stability, shorter flame lengths, and acceptable NOx emissions when compared to conventional richburn, quick-quench, lean-burn combustors. The axial distance required to complete combustion within the mainstream dictates a minimum combustor length for obtaining acceptable levels of combustion efficiency. Hence, characterization of the reaction zone within a UCC is required to optimize the length. In this study OH* chemiluminescence imaging is used to assess the characteristics of the reaction zone via windows in the side and top of the combustor. CO, NOx, and total hydrocarbon (THC) emissions indices obtained with gas-sample probes at the exit of the combustor as well as computed combustion efficiencies are provided as a reference for the OH* chemiluminescence. Configurations with no turning vanes (CDF and CDF-2), with standard vanes (CDF-2SV), and with radialvane-cavity (RVC) vanes (CDF-2RV) were used. The first study shows that the CDF-2 configuration has similar combustion efficiencies compared to that of the previously studied CDF configuration between 0.6 1.1 but has a higher peak OH* intensity and higher window exit intensity than that of the CDF configuration. The second study shows that the addition of standard vanes to the UCC decreases the peak and exit OH* intensities and lowers the exit temperature peak to 30% height, while the addition of the RVC vane tends to increase the peak and exit OH* intensities and raise the exit temperature peak to 50% height. Combustion efficiencies are similar for the CDF-2, CDF-2SV, and CDF-2RV configurations up to = 1.1. Combustion efficiency remains above 99% for the CDF2SV configuration up to = 2.0. The third study shows that OH* intensity increases

Heat Release in Turbine Cooling II: Numerical Details of Secondary Combustion Surrounding Shaped Holes

Film cooling plays a critical role in providing effective thermal protection to components in modern gas turbine engines. Most of the previous studies on film cooling were conducted using either cylindrical or shaped coolant holes with nonreactive pure gases in the cross-stream flow. In this paper, the chemically reactive film cooling over a surface with shaped coolant hole is investigated by a Reynolds-averaged Navier–Stokes approach with a shear-stress transport k-!model to simulate the turbulentflow.To take into account the secondary combustion resulting from the unburned fuels in the crossflow, a two-step reaction scheme was used for the combustion of propane. An eddydissipation concept approach was used to account for the turbulence–chemistry interaction. The three-dimensional simulation was performed on an unstructured hybrid grid. The characteristics of reactive thermal flows, jet– crossflow interactions, species transport, and fuel consumption were investigated at different equivalence ratios and blowing ratios. Numerical results provide insight into where reactions take place and how fuel is consumed.