13 research outputs found

    Performance of multicell, axial-entry cyclones for industrial gas cleaning

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    Imperial Users onl

    Underexpanded jet development from a rectangular nozzle with aft-deck

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    An experimental study is reported of underexpanded supersonic jet plumes issuing from a high-aspect-ratio convergent rectangular nozzle. Schlieren visualization, Pitot probe, and Laser Doppler Anemometry measurements are carried out to capture the plume development in the near field, and in particular the effect on the plume flow of a finite-length extended shelf or aft-deck attached to the lower nozzle wall. This creates asymmetry in the inviscid shock cell pattern and the entrainment characteristics, both of which influence shear-layer growth and plume trajectory. A net pressure force is induced on the aft-deck wall, which leads to transverse deflection of the jet plume once it leaves the aft-deck, both upward and downward, depending on aft-deck length and nozzle pressure ratio. For sufficiently high nozzle pressure ratio and a sufficiently long aft-deck, separation and reattachment of the plume from the aft-deck is observed. Detailed mapping of both mean velocity and turbulence in the plume near field has been carried out, enabling comparison of flow behavior for a clean nozzle and a nozzle with aft-deck. The data provided are proposed as a suitable benchmark validation test case for computational fluid dynamics studies of rectangular nozzle plumes with aft-deck interaction effects

    Effect of scarfing on rectangular nozzle supersonic jet plume flow characteristics

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    An experimental and computational fluid dynamics study is reported of supersonic jets issuing from a high-aspect-ratio rectangular convergent–divergent nozzle with and without a scarfed exit. Schlieren visualization and laser Doppler anemometry measurements captured near-field aerodynamic development of an unheated jet at overexpanded, design, and underexpanded conditions. Reynolds-averaged Navier–Stokes computational fluid dynamics predictions using an eddy viscosity closure (Spalart–Allmaras model) for clean and scarfed geometries were compared with measurements to examine the ability to capture nozzle scarfing effects. The measured plume shape for a scarfed nozzle was strongly affected at overexpanded conditions (a distorted four-lobe shape was observed), whereas a rectangular shape was retained for underexpanded flow although plume bifurcation occurred. The development of the plume shape and the mixing rate was a consequence of the strong vortices that occur with rectangular nozzles, with extra vortices introduced by scarfing. The nozzle exit static pressure changed dramatically when scarfing was added, influencing plume secondary flows and near-field development. The main features of scarfed jet development were predicted qualitatively correctly; the four-lobe overexpanded shape was reproduced but the strength of pressure-driven secondary velocities was overpredicted. The experimental data provided represent a challenging validation test case for computational fluid dynamics studies of three-dimensional supersonic jet plumes with scarfed interaction effects

    Influence of nozzle exit conditions on the near-field development of high subsonic and underexpanded axisymmetric jets

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    Detailed knowledge of jet plume development in the near-field (the first 10–15 nozzle exit diameters for a round jet) is important in aero-engine propulsion system design, e.g., for jet noise and plume infrared (IR) signature assessment. Nozzle exit Mach numbers are often high subsonic but improperly expanded (e.g., shock-containing) plumes also occur; high Reynolds numbers (O (106)) are typical. The near-field is obviously influenced by nozzle exit conditions (velocity/turbulence profiles) so knowledge of exit boundary layer characteristics is desirable. Therefore, an experimental study was carried out to provide detailed data on nozzle inlet and exit conditions and near-field development for convergent round nozzles operated at Nozzle Pressure Ratios (NPRs) corresponding to high subsonic and supersonic (underexpanded) jet plumes. Both pneumatic probe and Laser Doppler Anemometry (LDA) measurements were made. The data revealed that internal nozzle acceleration led to a dramatic reduction in wall boundary layer thickness and a more laminar-like profile shape. The addition of a parallel wall extension to the end of the nozzle allowed the boundary layer to return to a turbulent state, increasing its thickness, and removing vena contracta effects. Differences in nozzle exit boundary layers exerted a noticeable influence but only in the first few diameters of plume development. The addition of the exit extension removed the vena contracta effects of the convergence only design. At underexpanded NPRs, this change to nozzle geometry modified the shock cell pattern and shortened the potential core length of the jet

    Experimental Studies of Coaxial Jet Flows

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    The role of teachers’ metacognitive awareness on language learners’ knowledge of grammar and critical thinking skills

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    The present study was an attempt to discover the probable effects of EFL teachers’ Metacognitive Awareness (MA) on Iranian EFL learners’ knowledge of grammar and critical thinking ability. The study aimed to see whether or not teachers’ MA can improve language learner’ knowledge of grammar and critical thinking skill. To this end, 100 EFL teachers were selected and completed Metacognitive Awareness Inventory (MAI). Then, 10 teachers with lowest degree of MA and 10 with highest degree of MA were selected to teach 207 intermediate language learners. The learners selected took a grammar tests and a critical thinking ability questionnaire test before and after the treatment. Collecting and analyzing the data, it was revealed that teachers’ MA affected language learners’ knowledge of grammar, while it did not influence the participants’ critical thinking skills. The findings of the present study might be useful for language teachers and teacher trainers

    Underexpanded Jet Development from a Rectangular Nozzle with Aft-Deck

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    Computational fluid dynamics prediction of intake ingestion relevant to short take-off and vertical landing aircraft

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    Intake ingestion can cause several major problems (e.g. compressor surge and stall) for short take-off and vertical landing (STOVL) aircraft operating in ground effect. Numerical predictions of the flowfield associated with a generic twin-jet plus intake model operating under ingestion flow conditions are reported using computational fluid dynamics (CFD) techniques. The results have been compared with laser Doppler anemometry (LDA) validation measurements taken in a specially designed test case configuration. The k-ϵ turbulence model and both first-order and second-order (QUICK) convection discretization schemes were employed. Fine meshes and second-order accurate discretization were found essential to produce solutions close to grid independence. A reasonable prediction of the general flow pattern has been achieved. Several features of the mean velocity field were close to the experimental results; however, the k-ϵ model was shown to produce significant errors in the prediction of the forward penetration distance of the ground sheet flow and in the shape of velocity profiles and turbulence levels near to the intake

    Evaluating an ESP textbook: a case study of English for banking purposes at EDBI

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    An English language textbook for banking purposes had been designed based on the language needs of EDBI staffs. Consequently, an English course for banking purposes was held at EDBI using the designed book. To ensure about the validity and quality of the textbook, it was evaluated by 2 language experts through an interview before the course and 30 bank’s staffs as the learners through a set of questionnaire after the course. Based on the quantitative and qualitative findings of the study, both language experts and EDBI’s staffs had positive attitudes toward the materials, topics, activities and tasks, language skills and physical appearance of the text book and recognized it relevant to their language needs. However, they asked for some modifications such as edition of misspelling and ungrammatical use of some sentences and additions of new materials such as a glossary and business and banking correspondences to the existing ones. The results of the study became a basis to further improvement and correction until the draft would be finalized as a ready use material. Therefore, some modifications and alternations were conducted based on their evaluations to make the product more valid and more practical. The findings of the present research bear significant implications for materials developers and teachers especially in applying appropriate materials in ESP courses and evaluating the textbooks in the related studies
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