Bypass Stream Research Articles

Performance characteristics and optimisation of a geared intercooled reversed flow core engine

Intercooled turbofan cycles allow higher overall pressure ratios to be reached, which gives rise to improved thermal efficiency. In addition, intercooling allows for the size, weight and exhaust jet velocity of the core to be reduced. For an optimum jet velocity ratio and fixed thrust, the fan pressure ratio and specific thrust are also reduced, which benefits propulsive efficiency. A new intercooled core concept is proposed in this paper, which promises to alleviate limitations identified in previous intercooled turbofan designs. This concept facilitates the installation of the intercooler and reduces core losses at high overall pressure ratios. This engine concept takes advantage of intercooling and the arrangement of the high pressure spool to reach and exceed overall pressure ratios of 80. In addition, given the reduction in core size, bypass ratios beyond 14 have been considered. In order to identify efficiency gains and performance characteristics which are due to the novel arrangement alone, the geared intercooled reversed flow core engine has been compared with a geared intercooled engine with a more conventional core. Finally an optimisation exercise has been carried out to identify the best configuration for both the geared intercooled reversed flow core concept and the conventional core concept. In this paper, it is demonstrated that the geared intercooled reversed flow core concept allows for a 2.3% reduction in block fuel burn. The reductions are due to the improved core efficiency, higher overall pressure ratio as well as efficiency gains from the use of a mixed exhaust. The sensitivity analysis shows that the improvements are highly dependent on pressure losses in the core and bypass stream and that careful design of the mixer chutes and intercooler headers to achieve low losses is essential if the concept gains are to be realised.

A systematic approach for synthesizing combined mass and heat exchange networks

Mass and heat are very important resources in the process industry. Numerous approaches have been proposed for the optimization of either mass or heat exchange networks. Since the process usage of mass and heat is typically intertwined, it is important to account for such interactions. The objective of this paper is to introduce a systematic method for the simultaneous synthesis of combined mass- and heat-exchange networks (CMAHENs). The proposed method is based on a novel approach that incorporates the mass pinch technology (MPT) for mass exchange networks (MENs) synthesis and the pseudo-T-H diagram approach (PTHDA) for the heat exchange networks (HENs) synthesis. New bypass streams are included in the structural representation of the problem to expand the search space. A combined optimization approach is applied to minimize the total annualized cost of the CMAHEN. Finally, two cases are solved to illustrate the application of the proposed method.

CFD simulation of single- and multi-phase flows through submerged membrane units with irregular fiber arrangement

The performance of submerged membrane bioreactors (sMBRs) treating municipal or industrial wastewater is significantly affected by hydrodynamic conditions in the membrane unit. These hydraulics are induced by air bubbles injected into the stagnant suspension to remove the cake layer. The aeration sequence consumes the biggest proportion of the overall energy input of sMBRs. Therefore, the objective of this study was to investigate the impact of irregular fiber arrangement on the aeration efficiency. For this purpose, a geometry model based on X-ray computer tomography (CT) scans was developed to map the instantaneous displacement of fibers. The scanned images were processed and implemented into the CFD code as porosity and friction factor matrices. Single-phase simulations show the impact of local fiber arrangement, superficial inlet velocity and solids concentration on the distribution of liquid velocity and turbulent viscosity. Bypass streams were found to reduce the expected cake removal averaged over the corresponding cross-section of the hollow-fiber bundle. Initial multi-phase simulations suggest higher mixture velocities in cross-sectional regions without fibers compared to fiber segments. These results serve as a proof of principle and indicate the potential of this novel CFD approach in module design optimization to reduce aeration requirements.

A brine evaporative cooler/concentrator for autonomous thermal desalination units

In recent years growing attention has been paid to the problem of brine disposal due to the raising awareness of significant environmental issues related to the use of desalination processes for fresh water production. This is particularly relevant when desalination units are located in remote sites, characterised by major complexity in the construction and management of intake and outfall structures. In the present work a novel device, named brine evaporative cooler/concentrator (BECC, patent pending), has been developed for coupling with small-scale thermal desalination plants in order to reduce the problem of brine disposal. Such device fulfils two different functions: i) cooling of the recirculating brine (which is often mixed with cold seawater to feed the unit, acting first as a cooling medium for the condensation of the vapour) and ii) concentration of the brine to de disposed. The BECC device is based on the principle of evaporative cooling, i.e. a primary liquid stream is cooled by means of a secondary liquid stream (by-pass stream), which in turn evaporates in contact with atmospheric air, thus being cooled naturally. The two streams are separated by a heat-conductive surface, through which heat is transferred from the primary stream to the cooling-evaporating by-pass stream. A lab scale BECC pilot unit has been designed, constructed and tested. Geometrical and operating features have been studied in order to allow the operation with concentrated brines, to minimise problems of corrosion, scaling and fouling. The first results have demonstrated the feasibility of the technology and a larger scale prototype unit has been designed for the installation within a 5 m3/d solar membrane distillation unit to be constructed in Pantelleria Island, Italy.

Eddy covariance flux measurements of ammonia by high temperature chemical ionisation mass spectrometry

Abstract. A system for fast ammonia (NH3) measurements with chemical ionisation mass spectrometry (CIMS) based on a commercial Proton Transfer Reaction-Mass Spectrometer (PTR-MS) is presented. It uses electron transfer reaction as ionisation pathway and features a drift tube of polyetheretherketone (PEEK) and silica-coated steel. Heating the instrumental inlet and the drift tube to 180 °C enabled an effective time resolution of ~1 s and made it possible to apply the instrument for eddy covariance (EC) measurements. EC fluxes of NH3 were measured over two agricultural fields in Oensingen, Switzerland, following fertilisations with cattle slurry. Air was aspirated close to a sonic anemometer at a flow of 100 STP L min−1 and was directed through a 23 m long 1/2" PFA tube heated to 150 °C to an air-conditioned trailer where the gas was sub-sampled from the large bypass stream. This setup minimised damping of fast NH3 concentration changes between the sampling point and the actual measurement. High-frequency attenuation loss of the NH3 fluxes of 20 to 40% was quantified and corrected for using an empirical ogive method. The instrumental NH3 background signal showed a minor interference with H2O which was characterised in the laboratory. The resulting correction of the NH3 flux after slurry spreading was less than 1‰. The flux detection limit of the EC system was about 5 ng m−2 s−1 while the accuracy of individual flux measurements was estimated 16% for the high-flux regime during these experiments. The NH3 emissions after broad spreading of the slurry showed an initial maximum of 150 μg m−2 s−1 with a fast decline in the following hours.

Distributed control of plantwide chemical processes

Complex process plants increasingly appear in modern chemical industry. The wide use of material recycles and heat integration (with recycle and bypass streams) profoundly alters plantwide process dynamics and further increases their complexity. The interactions between process units may lead to poor performance of decentralized control systems. On the other hand, the complexity of plantwide systems prohibits the use of centralized controllers that reply on the complex model of the entire plantwide process. This paper addresses the plantwide chemical process control problem from a network perspective. The entire chemical plant is modeled as a network of process units linked by physical mass and energy flow and controlled by controllers that communicate with each other (i.e., distributed controllers). A two-port linear time-invariant representation is proposed to describe the dynamics of each process unit and its corresponding distributed controller. A two-step plantwide linear control design approach is developed. By using the dissipativity theory, the plantwide stability and control performance is translated into the closed-loop dissipativity condition that each distributed controller has to achieve. This allows the distributed controllers to be designed independently and to operate autonomously. The proposed approach is illustrated by a case study of a process network that consists of a reactor and a distillation column.

Synthesis of Distillation Column Sequences for Nonsharp Separations

In this paper an algorithm to generate all possible distillation column sequences for a nonsharp separation of c components to m multicomponent as well as pure desired products, based on the product specifications, is presented. Each desired product is defined by the minimum and maximum acceptable values of product rates as well as the concentration of each component. The possible sequences are generated by the introduction of a separation matrix. In the proposed algorithm each sequence can include between 1 and c − 1 simple and/or complex distillation columns and bypass streams. The sequences with less than c − 1 distillation columns can be generated systematically by the proposed algorithm. Also, a design strategy has been proposed for the optimization of the sequences. This algorithm has been used for a nonsharp separation example. A sequence with less than c − 1 number of distillation columns and bypass streams is selected as the optimum structure for this example. This optimum sequence can cause sign...

Linear stability of a compressible coaxial jet with continuous velocity and temperature profiles

This paper describes an investigation of the stability of a jet with velocity and temperature profiles characteristic of the exit region for a turbofan engine. Because the bypass stream mixes with both the exhaust and the ambient air, these profiles contain thin layers in which the velocity and temperature may vary rapidly. As a consequence, multiple instability modes are possible. In accordance with Rayleigh’s theorem for axisymmetric incompressible shear flows, it turns out that there are three possible modes, only two of which are unstable. We consider the effect on spatial growth rates of varying the diameter and velocity ratios, compressibility, and azimuthal wavenumber. Radiating modes, that are possible when the primary jet is heated, are also studied.

Aerodynamics of Fan Flow Deflectors for Jet Noise Suppression

The present work describes a three-dimensional RANS investigation of the flow around deflector vanes for noise suppression in separate-flow turbofan engines. The vanes are installed in the bypass duct and deflect the bypass plume downward relative to the core plume. This paper considers a single pair of vanes, with NACA0012 airfoil section, installed in a realistically shaped nozzle operating at static conditions. The axial and transverse forces created by the vanes are computed for various vane angles of attack. It it shown that the thrust loss of the bypass stream ranges from 0.04% with the vanes at zero angle of attack to 0.10% for vanes at 8 ◦ angle of attack. For an entire engine with bypass ratio of 5, the corresponding losses are approximately 0.03% and 0.08%. The vanes have an impact of less than 0.025% on the nozzle flow coefficient.

Operability Analysis for Process Systems with Recycle and Bypass Streams

Recycle and bypass streams exist in chemical plants in various configurations. They may lead to plantwide operability problems due to the strong interaction between process units. Based on the concept of dissipative systems, this paper provides a new approach to plantwide operability analysis for processes with by-pass and recycle streams, particularly on plantwide stability, stabilizability, and effects of disturbance. By using the topology of process interconnections, this analysis approach is developed from a network perspective and can be used for large scale process systems.

Full-scale modelling of a food industry wastewater treatment plant in view of process upgrade

A mathematical model for the aerobic part of a food industry wastewater treatment plant (WWTP) was developed and used to assess possible upgrade options. This aerobic part of the WWTP treats two streams. The direct stream is treated in an anaerobic reactor after pre-treatment. The bypass stream is sent directly from the production facility to the aerobic part of the WWTP, without anaerobic treatment. The plant upgrade consists of installing additional volume for nitrification and denitrification, a so-called post-denitrification. An influent characterization translated the available influent measurements into data useful for modelling. It was shown by simulations with the developed model that the physical plant upgrade will result in a 99% decrease in effluent ammonium concentration. In addition a 5% decrease in COD concentration was obtained. However, the effluent nitrate concentration and total nitrogen increased drastically because of the upgrade. Additional control actions, more specifically the increase of the bypass flow rate, were necessary for decreasing this effluent total nitrogen concentration. This was also demonstrated with the developed model.

Theoretical model for sound radiation from annular jet pipes: far- and near-field solutions

An analytical model is presented for sound radiation from a semi-infinite unflanged annular duct. The duct carries a jet which issues into a uniform mean flow while an inner cylindrical centre body extends downstream from the duct exit. This geometrical arrangement forms an idealized representation of a turbofan exhaust where noise propagates along the annular bypass duct, refracts through the external bypass stream and radiates to the far field. The instability wave of the vortex sheet and its interaction with the acoustic field are accounted for in an exact way in the current solution. Efficient numerical procedures are presented for evaluating near-field and far-field solutions, and these are used as the basis for a parametric study to illustrate the effect of varying the hub–tip ratio, and the ratio of jet velocity to external flow velocity. Since the ‘Kutta’ condition can be turned on and off in the current solution, this capability is used to assess the effect of vortex shedding on noise radiation. Far-field directivity patterns are presented for single modes and also for a multi-mode ‘broadband’ source model in which all cut-on modes are assumed to be present with equal modal power. Good agreement is found between analytical solutions and experimental data. Near-field pressure maps of the acoustic and instability portions of the solution are generated for selected tones.

Long-term analysis of a full-scale activated sludge wastewater treatment system exhibiting seasonal biological foaming

The seasonal accumulation of biological foam on the activated sludge system of the Urbana-Champaign Sanitary District Northeast (UCSD-NE) wastewater treatment plant was investigated over an 8-year period by statistical analyses including path analysis, multivariate regression, and principal component analysis. Results of these analyses suggested that variation in the activated sludge reactor temperature and the use of a stream bypassing the primary clarifier were the two main factors determining the observed temporal foam profile. Characterization of the primary clarifier influent and effluent suggested the involvement of high lipid loading rates from the bypass stream in foam accumulation. In light of these results, it is hypothesized that increasing temperatures and lipid loading rates are responsible for foam formation through the same mechanism: the foam-forming microbial population is specialized in consuming lipids, substrates classified as slowly degradable. When the temperature increases, the rate of lipid hydrolysis becomes sufficiently high for this population to become abundant, accumulate on the surfaces of the aeration basins, and cause biological foaming.

A Provisional Heat Transfer Correlation for High-finned Tubes in Inline Arrangement

A heat transfer correlation was proposed for inline-arranged high-finned tubes. This correlation was based on the Bell-model, where two stream assumption is used, the primary stream and the bypass stream. In the proposed correlation, the actual heat transfer coefficient between fin and the primary stream was assumed to be predicted by the generally accepted correlation for staggered arrangements, and a correction factor was introduced, which corrects the bypass flow effect. Comparing existing experimental data, the correction factor was determined as a function of the ratio of fin spacing to fin height and of the Reynolds number. The proposed correlation showed good agreements with experimental data of plain and also serrated fins.

Ureteroarterial Fistula with Ruptured Anastomotic Pseudoaneurysm: Successful Management with Vascular Exclusion, Extra-anatomic Bypass and Nephrectomy

Ureteroarterial fistula is a rare but life-threatening condition most often arising as a consequence of combined vascular and urologic pathology. Only about 70 cases are reported in the English literature. Principles of repair include complete vascular isolation, extra-anatomic bypass, and urinary stream diversion away from major vascular conduits. The case presented herein is only the second reported instance of fistulization to an anastomotic pseudoaneurysm of an iliopopliteal bypass.

New Method for Jet Noise Reduction in Turbofan Engines

A new method for reducing large-scale mixing noise from dual-stream jets is presented. The principle is reduction of the convective Mach number of turbulent eddies that produce intense downward sound radiation. In a jet representing the coaxial exhaust of a turbofan engine, this is achieved by tilting downward, by a few degrees, the bypass (secondary) plume relative to the core (primary) plume. The misalignment of the two flows creates a thick low-speed secondary core on the underside of the high-speed primary flow. The secondary core reduces the convective Mach number of primary eddies, thus hindering their ability to generate sound that travels to the downward acoustic far field. Tilting of the bypass stream is possible by means of fixed or variable vanes installed near the exit of the bypass duct. Subscale aeroacoustic experiments simulated the exhaust flow of a turbofan engine with bypass ratio 6.0. Deflection of the bypass stream resulted in suppression of the peak overall sound pressure level by 4.5 dB and the effective perceived noise level by 2.8 dB. For the nozzle configuration used, the thrust loss is estimated at around 0.5% with the vanes activated and 0.15% with the vanes deactivated.

Engine Cycle and Exhaust Configuration for Quiet Supersonic Propulsion

The thermodynamics and acoustics of a fixed-cycle, bypass ratio 3 supersonic engine with an innovative noise suppression scheme is explored. The silencing method entails installation of variable turning vanes in the bypass exhaust of a separate-flow turbofan engine. During noise-sensitive segments of flight, the vanes impart a slight downward tilt to the bypass plume relative to the core plume, thus thickening the bypass stream on the underside of the jet. This results in a reduction of the convective Mach number of instability waves that produce intense downward sound radiation. Subscale experiments show that, relative to the mixed-flow exhaust, the coaxial separate-flow exhaust with vanes reduces the peak overall sound pressure level by 8 dB and the effective perceived noise level by 7 dB. The noise-equivalent specific thrust on takeoff is reduced from 490 to 390 m/s. Compared to a current-generation low-bypass turbofan engine, the bypass ratio 3 engine is estimated to be 13 dB quieter with the mixed-flow exhaust and 20 dB quieter with the aforementioned suppression scheme. The vane configuration of this study is estimated to cause a thrust loss of 1% at takeoff and 0.25% at supersonic cruise.

Optimum Mixing of Core and Bypass Streams in High-Bypass Civil Turbofan

In a mixed-stream turbofan, except for fan pressure ratio, the mission matched optimum selection of other basic cycle variables like bypass ratio, overall pressure ratio, throttle ratio, and turbine entry temperature depends upon engine-airframe interactions over the prescribed mission and the available technology level. The fan pressure ratio is decided by the mixing conditions that are internal to the engine. This paper, by utilizing a multidisciplinary cycle optimization software, performs a number of numerical experiments to determine a criteria for mixing of bypass and core streams, which results in an optimum fan pressure ratio. The use of optimum fan pressure ratio will aid the other basic cycle parameters in enhancing performance gains caused by mixing, thereby improving the engine-airframe-mission compatibility. The penalty ofnon- or suboptimal mixing, and savings in mission fuel consumption for a mixed-stream turbofan with respect to a separate exhaust turbofan, both at optimum fan pressure ratio are also quantified.

Aerothermal Model for Real-Time Digital Simulation of a Mixed-Flow Turbofan Engine

Thispaperpresents awayto simplifyanexplicitly time-integrated,aerothermodynamictransientmodelofa twinspool,mixed- ow turbofan engine based on state variables and control volumesapproach such that computational performance becomes suitable for real-time digital simulation. The key simpliŽ cation lies in the modeling of main mixer, in which the mixing of core and bypass streams takes place. The number of state variables in an existing thermodynamicmodel has been reduced to six from nine by assuming that, instead of a uniform rate of static pressure rise, the static pressure is uniform across the main mixer. Computational time decreases primarily because the model is now less sensitive to pressure dynamics, thus allowing a larger integration time step. The accuracy of transient simulationsis also less affected by artiŽ cial increases in actual physical size of controlvolumes, permitting further increase in integration time step. Numerical experiments show that simpliŽ ed mixer modeling together with an increase in control volumes size by a factor of three resulted in an integration time step of 1.2 ms. It enables the model to run in real time on a stateof-the-art personal computer type of digital platform, without loosing the accuracy and numerical stability of the analysis. The formulation,validation,and real-time capabilities of the proposed methodology are discussed.

Designing a fast‐igniting catalytic converter system

AbstractThrough analysis and numerical simulation, the late lightoff (∼ 3 min) of the current automobile catalytic converter is attributed to a downstream ignition phenomenon that is unstable to flow‐rate fluctuations. A design is proposed which incorporates a stable and thermally efficient leading‐edge ignition. The key feature involves diverting a small portion of exhaust gas through a bypass stream, which contains an electric preheater and a preigniter, during startup. This allows the preheater to utilize a low power input using the existing battery and yet raise the gas temperature beyond a precise minimum that rapidly lights off the entire preigniter. The hot preigniter is then used to light off the main catalytic converter. Removal of the bypass after complete ignition preserves the preigniter catalyst, a major advantage. The optimal design is shown to lightoff within 10 s and reduce pollution by almost 90% over current designs and thus meet ultra low emission vehicle (ULEV) standards.