Annular Gas Research Articles

Experimental and Computational Investigations of Flow and Mixing in a Single-Annular Combustor Configuration

A complementary experimental and computational study of the flow and mixing in a single annular gas turbine combustor has been carried out. The object of the investigation is a generic mixing chamber model, representing an unfolded segment of a simplified Rich-Quick-Lean (RQL) combustion chamber operating under isothermal, non-reacting conditions at ambient pressure. Two configurations without and with secondary air injection were considered. To provide an appropriate reference database several planar optical measurement techniques (time-resolved flow visualisation, PIV, QLS) were used. The PIV measurements have been performed providing profiles of all velocity and Reynolds-stress components at selected locations within the combustor. Application of a two-layer hybrid LES/RANS (HLR) method coupling a near-wall k − e RANS model with conventional LES in the core flow was the focus of the computational work. In addition to the direct comparison with the experimental results, the HLR performance is comparatively assessed with the results obtained by using conventional LES using the same (coarser) grid as HLR and two eddy-viscosity-based RANS models. The HLR model reproduced all important flow features, in particular with regard to the penetrating behaviour of the secondary air jets, their interaction with the swirled main flow, swirl-induced free recirculation zone evolution and associated precessing-vortex core phenomenon in good agreement with experimental findings.

Massively parallel LES of azimuthal thermo-acoustic instabilities in annular gas turbines

Increasingly stringent regulations and the need to tackle rising fuel prices have placed great emphasis on the design of aeronautical gas turbines, which are unfortunately more and more prone to combustion instabilities. In the particular field of annular combustion chambers, these instabilities often take the form of azimuthal modes. To predict these modes, one must compute the full combustion chamber, which remained out of reach until very recently and the development of massively parallel computers. In this article, full annular Large Eddy Simulations (LES) of two helicopter combustors, which differ only on the swirlers' design, are performed. In both computations, LES captures self-established rotating azimuthal modes. However, the two cases exhibit different thermo-acoustic responses and the resulting limit-cycles are different. With the first design, a self-excited strong instability develops, leading to pulsating flames and local flashback. In the second case, the flames are much less affected by the azimuthal mode and remain stable, allowing an acceptable operation. Hence, this study highlights the potential of LES for discriminating injection system designs. To cite this article: P. Wolf et al., C. R. Mecanique 337 (2009).

Numerical investigation of a perturbed swirling annular two-phase jet

A swirling annular gas–liquid two-phase jet flow system has been investigated by solving the compressible, time-dependent, non-dimensional Navier–Stokes equations using highly accurate numerical methods. The mathematical formulation for the flow system is based on an Eulerian approach with mixed-fluid treatment while an adjusted volume of fluid method is utilised to account for the gas compressibility. Surface tension effects are captured by a continuum surface force model. Swirling motion is applied at the inlet while a small helical perturbation is also applied to initiate the instability. Three-dimensional spatial direct numerical simulation has been performed with parallelisation of the code based on domain decomposition. The results show that the flow is characterised by a geometrical recirculation zone adjacent to the nozzle exit and by a central recirculation zone further downstream. Swirl enhances the flow instability and vorticity and promotes liquid dispersion in the cross-streamwise directions. A dynamic precessing vortex core is developed demonstrating that the growth of such a vortex in annular configurations can be initiated even at low swirl numbers, in agreement with experimental findings. Analysis of the averaged results revealed the existence of a geometrical recirculation zone and a swirl induced central recirculation zone in the flow field.

Primary instabilities of liquid film flow sheared by turbulent gas stream

Evolution of excited waves on a viscous liquid film has been investigated experimentally for the annular gas–liquid flow in a vertical tube. For the first time the dispersion relations are obtained experimentally for linear waves on liquid film surface in the presence of turbulent gas flow. Both cocurrent and countercurrent flow regimes are investigated. As an example of comparison with theory, the experimental data are compared to the results of calculations based on the Benjamin quasi-laminar model for turbulent gas flow. The calculation results are found to be in good agreement with experiments for moderate values of film Reynolds number.

Dynamics of annular gas–liquid two-phase swirling jets

The dynamics of annular gas–liquid two-phase swirling jets have been examined by means of direct numerical simulation and proper orthogonal decomposition. An Eulerian approach with mixed-fluid treatment, combined with an adapted volume of fluid and a continuum surface force model, was used to describe the two-phase flow system. The unsteady, compressible, three-dimensional Navier–Stokes equations have been solved by using highly accurate numerical methods. Two computational cases have been performed to examine the effects of liquid-to-gas density ratio on the flow development. It was found that the higher density ratio case is more vortical with larger spatial distribution of the liquid, in agreement with linear theories. Proper orthogonal decomposition analysis revealed that more modes are of importance at the higher density ratio, indicating a more unstable flow field. In the lower density ratio case, both a central and a geometrical recirculation zone are captured while only one central recirculation zone is evident at the higher density ratio. The results also indicate the formation of a precessing vortex core at the high density ratio, indicating that the precessing vortex core development is dependent on the liquid-to-gas density ratio of the two-phase flow, apart from the swirl number alone.

Application of gentle annular gas veil for electrospinning of polymer solutions and melts

AbstractA novel experimental device for elevated‐temperature electrospinning of highly volatile and quickly crystallizing polymer solutions and melts was developed. The main parameters of gentle annular gas veil in electrospinning process were studied. The influence of electrical conductivity of solvent on the diameter of electrospun fibers from ultra‐high molecular weight polyethylene was experimentally determined. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Reaction mechanism of cyclohexane selective photo-oxidation to benzene on molybdena/titania catalysts

Photo-induced oxidative dehydrogenation of cyclohexane has been investigated in an annular gas–solid continuous flow reactor on molybdenum-based catalysts as a function of the molybdenum loading. The formation of polymolybdate species on the anatase support surface increased with increasing molybdenum load up to that corresponding to monolayer formation, resulting in a high selectivity to benzene formation, while with titania alone the process was 100% selective to forming carbon dioxide. After monolayer formation, segregation of MoO3 crystallites has been observed, with a significant loss in photo-oxidative dehydrogenation activity. A mechanism for the photo-induced oxidative dehydrogenation of cyclohexane, based upon consecutive oxy-dehydrogenation steps occurring on active molybdenum sites in competition with total oxidation on bare titania, has been proposed. This mechanism considers the oxy-dehydrogenation of cyclohexane to cyclohexene, followed by its further oxy-dehydrogenation to benzene occurring on the molybdenum oxide active sites.

Reduction of Wellbore Effects on Gas Inflow Evaluation Under Underbalanced Conditions

Summary Wellbore storage effects have been identified to significantly smear the accuracy of evaluating reservoir productivity through the fluid outflow rate from the annulus during underbalanced drilling. Such effects have continuously introduced considerable errors in characterizing the reservoir during underbalanced drilling. Conceptually, because of the ready volume-changing ability of the gas, wellbore storage becomes a determining factor during underbalanced drilling of a gas reservoir. Wellbore storage could either cause decrease (unloading effects) or increase (loading effects) in the annular gas density, depending on the choke opening procedures. Correspondingly, annular fluid outflow rate is considerably affected. Because it is practically difficult to deduct the fluid-flow rate attributable to the wellbore storage from the total fluid outflow rate, reducing the influence of wellbore effects on the evaluation of gas-reservoir productivity is presented in this study. Volumetric production analysis at the wellbore-sand face is introduced through a mathematical modeling of inflow of gas bubbles into the wellbore. This mathematical modeling utilizes forces such as the viscous force, drilling fluid ejecting forces from the bit nozzles, buoyancy, interfacial tension, and gas-reservoir forces for its analyses. Some analytical results that are overshadowed by wellbore storage are presented and supported by extensive experimental studies.

Investigation of Waves Interaction in Annular Gas–Liquid Flow Using High-Speed Fluorescent Visualization Technique

High-speed fluorescent visualization complex has been developed for quantitative investigation of the process of interaction of waves on liquid film in annular two-phase flow. Evolution of ripples on disturbance waves surface and disturbance waves coalescence are investigated. It is shown that all the ripples in presence of disturbance waves appear at the base of the back front of disturbance waves and then either decelerate, travel on substrate and are overtaken by the following disturbance wave or accelerate, grow and then disappear at the front of disturbance wave. The disappearance happens due to entrainment of liquid into the core of gas stream. Several scenarios of coalescence of disturbance waves were identified. For disturbance waves with close velocities different types of remote interaction were observed.

Low-order modelling of unsteady flame behaviour in a spray combustor

The effect of mass flow rate perturbations on the stability of a liquid-fulled annular aeroengine gas turbine is investigated using a three-dimensional uRANS code. The simulations are performed on a single sector of an idealised annular gas turbine geometry. A Lagrangian fuel spray description is used with a Monte Carlo solution of Williams' spray equation. Broadband mass flow fluctuations are imposed at the combustor inlet and dilution ports, and the effect on heat release is quantifed using flame transfer functions. The excitation of inlet and dilution flows leads to signifcant unsteady combustion and the implications for self-excited oscillations are considered. The numerical results are then compared to a low-order model which is developed to account for the different physical mechanisms responsible for unsteady heat release when the inlet and dilution port air mass flow rates are perturbed. The low-order model is used to predict the possibility of self-excited oscillations as well as to investigate the potential of altering the phase and relative magnitude of the inlet and dilution port excitation as a possible means of controlling thermoacoustic oscillations.

The Impact of Real Gas Properties on Predictions of Static and Rotordynamic Properties of the Annular Gas Seals for Injection Compressors

Predictions are presented for an annular gas seal that is representative of the division-wall seal of a back-to-back compressor or the balance-piston seal of an in-line compressor. A two-control-volume bulk-flow model is used including the axial and circumferential momentum equations and the continuity equations. The basic model uses a constant-temperature prediction (ISOT) and an ideal gas law as an equation of state. Two variations are used: adding the energy equation with an ideal gas law (IDEAL), and adding the energy equation with real gas properties (REAL). The energy equations assume adiabatic flow. The ISOT model has been used for prior calculations. Concerning predictions of static characteristics, the calculated mass leakage rates were, respectively, 9.46 kg∕s, 9.55 kg∕s, and 7.87 kg∕s for ISOT, IDEAL, and REAL. For rotordynamic coefficients, predicted effective stiffness coefficients are comparable for the models at low excitation frequencies. At running speed, REAL predictions are roughly 40% lower than ISOT, which could result in lower predicted critical speeds. Predicted effective damping coefficients are also generally comparable. REAL and IDEAL predictions for the crossover frequency are approximately 20% lower than ISOT. REAL predictions for effective damping are modestly lower in the frequency range of 40–50% of running speed where higher damping values are desired.

Identification of Rotordynamic Forces in a Flexible Rotor System Using Magnetic Bearings

A method is presented for parameter identification of an annular gas seal on a flexible-rotor test rig. Dynamic loads are applied by magnetic bearings (MBs) that support the rotor. MB forces are measured using fiber-optic strain gauges that are bonded to the poles of the MBs. In addition to force and position measurements, a finite element rotor model is required for the identification algorithm. The FE rotor model matches free-free characteristics of the test rotor. The addition of smooth air sealed to the system introduces stiffness and damping terms for identification that are representative of reaction forces in turbomachines. Tests are performed to experimentally determine seal stiffness and damping coefficients for different running speeds and preswirl conditions. Stiffness and damping coefficients are determined using a frequency domain identification method. This method uses an iterative approach to minimize error between theoretical and experimental transfer functions. Test results produce seal coefficients with low uncertainties.

Convergent/divergent segmented exhaust nozzle

A segmented exhaust nozzle for attenuating noise from a turbofan jet engine without adversely impacting the operability or operability limit related performance of the engine. The exhaust nozzle includes spaced apart fan nozzle inner and outer walls which form an annular exhaust gas flow path therebetween. The fan nozzle outer wall is segmented at the downstream end. The outer wall curves inwardly towards the inner wall and then turns back away from the inner wall to form an arcuate protrusion that extends into the exhaust gas flow path forming an aerodynamic throat. Through the segmented portion of the nozzle, the outer wall then continues to curve away from the inner wall before again curving back towards the inner wall at a nozzle exit station. The nozzle exit effective area is approximately equal in cross sectional area to a conventional exhaust nozzle exit area. The inwardly curving and then segmented outwardly curving portion of the exhaust nozzle forms a geometric influction that serves to reduce noise without negatively affecting engine operability or operability limit related performance.

A Design to Improve the Effective Damping Characteristics of Hole-Pattern-Stator Annular Gas Seals

Predictions are presented for hole-pattern-stator seals for which the hole depth is varied axially, showing that various depth patterns can significantly improve damping performance in terms of both increasing the seal’s effective damping and reducing its crossover frequency—the frequency at which effective damping changes from positive to negative. Test results are presented for the seal with the best variable hole depth (VHD) pattern showing an increase in peak damping by a factor of 1.6 over a constant hole depth (CHD) design versus predictions of 2.7. The crossover frequency is reduced by approximately 40%. The VHD seal has a significant negative static stiffness that probably arises from a friction-factor-jump phenomenon, not flow path divergence. The VHD seal leaks less than a comparable CHD design.

Axisymmetric waves in electrohydrodynamic flows

The formation of nonlinear axisymmetric waves on inviscid irrotational liquid jets in the presence of radial electric fields is considered. Gravity is neglected but surface tension is considered. Electrohydrodynamic waves of arbitrary amplitude and wavelength are computed using finite-difference methods. Particular attention is paid to nonlinear traveling waves. In the first class of problems, an electric field generated by placing the liquid jet inside a hollow cylindrical electrode held at constant voltage, its axis coinciding with that of the jet, is studied. The jet is assumed to be a perfect conductor whose free surface is stressed by the electric field acting in the hydrodynamically passive annulus. In the second class of problems, the annular gas is a perfect conductor that transmits a constant voltage onto the liquid/gas surface. The liquid axisymmetrically wets a constant-radius cylindrical rod electrode placed coaxially with respect to the hollow outer electrode, and held at a different constant voltage. The fluid dynamics and electrostatics need to be addressed simultaneously in the inner region. Axisymmetric interfacial waves influenced by surface tension and electrical stresses are computed in both cases. The computations are capable of following highly nonlinear solutions and predict, for certain parameter values, the onset of interface pinching accompanied with the formation of toroidal bubbles. For given wave amplitudes, the results suggest that, for the former case, the electric field delays bubble formation and reduces wave steepness, while for the latter case the electric field promotes bubble formation, all other parameters being equal.

Use of Finite Element Analysis to Engineer the Cement Sheath for Production Operations

Abstract Many successfully cemented wells begin to show annulus pressure buildup, which is often caused by damage to cement sheath integrity due to post-cementing operations. This problem may be temporarily dealt with by releasing the annulus pressure. However, this method of annular gas production is not a long-term solution to the problem. A finite element analysis (FEA) method has been developed to analyze the effects of various well events and operations such as cement hydration, casing pressure testing, completions and production on the integrity of the cement sheath during the life of the well. The three-dimensional FEA modeling considers the sheath's mechanical properties, such as Young's modulus, Poisson's ratio and tensile strength, in addition to confined compressive strength, and helps provide the user with a cement design that can maintain an annular seal over the life of the well. This paper discusses how a cementing design specialist can model the events upon and after cement placement to help provide a long-term seal of the annulus. The required input data is discussed and the output information is shown for example wells. Life of the Well The main purpose of the annular cement is to provide effective zonal isolation for the life of the well so that oil and gas can be produced safely and economically(1). To achieve this objective, the drilling fluid should be removed from both the wide and narrow annulus and the entire annulus should be filled with competent cement. The cement should meet both the short-term and long-term requirements imposed by the operational regime of the well. Traditionally, the industry has concentrated on the short-term properties that are applicable when the cement is still in slurry form. This effort is necessary and important for effective cement slurry mixing and placement. However, the long-term integrity of cement depends on the material/mechanical properties of the cement sheath, such as the Young's modulus, the tensile strength and the resistance to downhole chemical attack. Considering properties of the cement sheath for long-term integrity is critical if the well is subjected to common changes in the pressure and temperature of the cased well. After cement is placed in the annulus, if no fluid immediately migrates to the surface, short-term properties such as density, rate of static gel strength development and fluid loss of the cement may have been designed satisfactorily. However, recent experience has shown that after well operations such as cement hydration, casing pressure testing, completions and production, the cement sheath could lose its zonal isolation capabilities to provide zonal isolation(1). This failure can create a path for formation fluids to enter the annulus, which pressurizes the well's annulus, and could render the well unsafe to operate. The failure can also result in premature water production that can limit the economic life of the well. Failure of the cement sheath is most often caused by pressure or temperature-induced stresses inherent in well operations. Loss of isolation can shorten or destroy the life of the well because of corrosive fluid attack on the casing and cement sheath and/or sustained gas pressure on the casing annulus.

A new method of entrainment fraction measurement in annular gas–liquid flow in a small diameter vertical tube

A new method was introduced to measure liquid entrainment fraction in gas–liquid two-phase upward annular flow in a vertical tube (i.d.=9.525 mm). In this method, a new liquid–gas separator was designed and the chemically-based titration method was used to effectively measure the entrainment fraction in real time. Experiments were conducted at low system pressure (∼1 atm), and relatively low gas and liquid superficial velocities ( V sg = 25.8 –45.5 m/s, and V sl = 0.15 –0.30 m/s). Data analysis shows that the results are repeatable and occupy the range commonly seen in annular flow. The entrainment fraction was found to be under 7% for all the experimental set points. The repeatability of the test results and comparisons with previous entrainment data indicate that the new technique can perform as well as the film removal technique.

Discussion: “The Lomakin Effect in Annular Gas Seals Under Choked Flow Conditions” (Arghir, M., Defaye, C., and Frêne, F., 2007, ASME J. Eng. Gas Turbines Power, 129, pp. 1028–1034)

The authors’ predictions that choked flow in an annular gas seal is likely to produce negative direct stiffness was recently borne out in tests conducted by the discussers. A smooth seal was tested in air with supply pressure 18.3bar and discharge near atmospheric pressure. The seal has dimensions: L=100mm, D=100mm, and radial clearance Cr=0.305mm. The measured direct static stiffness is negative, K=−1.93MN∕m. The model available to the discussers is based on the work of Kleynhans and Childs (1) and would not converge at the full pressure differential. The direct stiffness predicted by this model becomes negative and its magnitude increases as the input supply pressure increases from ∼10bars, but the code stops converging above a δP of ∼13bars. What do the authors predict for this seal and the test conditions provided above? Additional test results are available for the seal in (2).

Closure to “Discussion of ‘The Lomakin Effect in Annular Gas Seals Under Choked Flow Conditions’ ” (2007, ASME J. Eng. Gas Turbines Power, 129, p. 1143)

Closure to “Discussion of ‘The Lomakin Effect in Annular Gas Seals Under Choked Flow Conditions’ ” (2007, ASME J. Eng. Gas Turbines Power, 129, p. 1143)

Rotordynamic Force Coefficients for a New Damper Seal Design

The objective of the following work was to determine frequency-dependent rotordynamic force coefficients for a new annular gas damper seal design. Both rotating and nonrotating experimental tests are presented for inlet pressures at 1000psig(69bar), a frequency excitation range of 20-300Hz, and rotor speeds up to 15,200rpm. Two different testing methods were used for determining coefficients: (1) dynamic pressure response method and (2) mechanical impedance method. The dynamic pressure method required the measurement of internal seal cavity pressure modulations in combination with the vibratory motion, whereas the mechanical impedance method used the measurement of external shaker forces, accelerations, and motion of the mechanical system. In addition to the new fully partitioned damper seal (FPDS) tests, the same experiments were conducted for a conventional pocket damper seal (PDS) design. Results of the frequency-dependent force coefficients and the internal seal dynamics for the two different gas damper seals are compared. The conclusions of the tests show that the FPDS design possesses significantly more positive direct damping and direct stiffness compared to the conventional PDS. The experiments also show the measurement of same-sign cross-coupled (cross-axis) stiffness coefficients for both seals, which indicate that the seals do not produce a destabilizing influence on rotor-bearing systems.