SEARCH · Engineering Papers

Results for “ducts”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 127 records · Page 7

All sources NASA NTRS DOE OSTI

DOE OSTI2020

Ducted Fuel Injection vs. Free-Spray Injection: A Study of Mixing and Entrainment Effects Using Numerical Modeling

Diesel engines are an important technology for transportation of both people and goods. However, historically they have suffered a significant downside of high soot and nitrogen oxides (NO x ) emissions. Recently, ducted fuel injection (DFI) has been demonstrated to attenuate soot formation in compression-ignition engines and combustion vessels by 50% to 100%. This allows for diesel engines to be run at low-NO x emissions that would have otherwise produced significantly more soot due to the soot/NO x tradeoff. Currently the root causes of this soot attenuation are not well understood. To be able to better optimize DFI for use across a variety of engines and conditions, it is important to understand clearly how it works. This study expands on the current understanding of DFI by using numerical modeling under nonreacting conditions to provide insights about the roles of entrainment and mixing that would have been much more challenging to obtain experimentally. We report this study found that DFI enhances charge gas entrainment upstream of the duct and blocks entrainment inside of the duct. Mixing is enhanced by the duct, which results in lower peak equivalence ratios at the exit of the duct.

42 ENGINEERING↗

NASA NTRS1973

Sound measurements within and in the radiated field of an annular duct with flow.

Recent theory on sound propagation for flow ducts initiated a research program (sponsored by NASA-Langley, Contract NAS1-10472) which has the objective of comparing detailed acoustical measurements with the analytical predictions for sound propagation in a duct with sheared flow. The effects considered experimentally mean flow, shear, wall impedance, and source distribution on sound propagation within and radiation from finite lenght annular ducts. Results reported here are confined to acoustic distribution in hard annular ducts with and without mean flow and radiated field patterns for the same conditions. The acoustic distributions are shown for single and multi-mode excitation. The flow duct facility is also described.

Plumblee, H. E.↗

NASA NTRS1973

A 727 airplane center duct inlet low speed performance confirmation model test for refanned JT8D engines, phase 2

The results from testing of a 0.3 scale model center duct inlet (S duct) for the Pratt and Whitney Aircraft JT8D-100 engines are presented. The objective of this test was to demonstrate that the required airflow of the JT8D-100 engine (480 lb/sec as compared to 334 lb/sec for JT8D-15) can be achieved with minimum modifications to the existing 727 airplane structure at acceptable levels of total pressure recovery and distortion. Steady-state pressure recovery, steady-state pressure distortion, and dynamic pressure measurements were taken at the engine face station. Surface static pressure measurements were taken along the duct. Test results indicated that the required airflow was achieved with acceptable pressure recovery (comparable to the current 727-200 S duct). Inlet inflow angle variation within the 727 airplane operating regime (minus 5 to 5 degrees) had no effect on the inlet performance. Pressure distortion at static and forward speed at takeoff airflow conditions are within P and WA limits for the Phase II duct when equipped with vortex generators. Static crosswind operation between 10 knots and 25 knots appears feasible at full takeoff power.

Kaldschmidt, G.↗

NASA NTRS1975

Generalized wave envelope analysis of sound propagation in ducts with stepped noise source profiles and variable axial impedance

A finite difference formulation is presented for sound propagation in a rectangular two-dimensional duct without steady flow. Before the difference equations are formulated, the governing Helmholtz equation is first transformed to a form whose solution tends not to oscillate along the length of the duct. This transformation reduces the required number of grid points by an order of magnitude. Example solutions indicate that stepped noise source profiles have much higher attenuation than plane waves in a uniform impedance liner. Also, multiple stepped impedance liners are shown to have higher attenuation than uniform ducts if the impedances are chosen properly. For optimum noise reduction with axial variations in impedance, the numerical analysis indicates that for a plane wave input the resistance should be near zero at the entrance of a suppressor duct, while the reactance should be near the optimum value associated with the least-attenuated mode in a uniform duct.

Baumeister, K. J.↗

NASA NTRS1975

The transmission of sound in nonuniform ducts

The method of weighted residuals in the form of a modified Galerkin method with boundary residuals was developed for the study of the transmission of sound in nonuniform ducts carrying a steady, compressible flow. In this development, the steady flow was modeled as essentially one dimensional but with a kinematic modification to force tangency of the flow at the duct walls. Three forms of the computational scheme were developed using for basis functions (1) the no-flow uniform duct modes, (2) positive running uniform duct modes, with flow, and (3) positive and negative running uniform duct modes, with flow. The formulation using the no-flow modes was the most highly developed, and has advantages primarily due to relative computational simplicity. Results using the three methods are shown to converge to known solutions for several special cases, and the most significant check case is against low frequency, one dimensional results over the complete subsonic Mach number range. Development of the method is continuing, with emphasis on assessing the relative accuracy and efficiency of the three implementations.

Eversman, W.↗

NASA NTRS1971

Acoustic radiation from lined, unflanged ducts: Acoustic source distribution program

An acoustic radiation analysis was developed to predict the far-field characteristics of fan noise radiated from an acoustically lined unflanged duct. This analysis is comprised of three modular digital computer programs which together provide a capability of accounting for the impedance mismatch at the duct exit plane. Admissible duct configurations include circular or annular, with or without an extended centerbody. This variation in duct configurations provides a capability of modeling inlet and fan duct noise radiation. The computer programs are described in detail.

Beckemeyer, R. J.↗

NASA NTRS1975

Design and testing of new center inlet and S-duct for B-727 airplane with refanned JT8D engines

The work described in this paper was part of the NASA refan program. The airflow requirement of the refanned JT8D (-100 series) engine increased about 50% above that of the basic JT8D. This required a redesign of the center inlet and S-duct of the Boeing 727 airplane. The paper describes the design constraints for the S-duct and the analytic method used to define the lines of the new duct. Model tests that were conducted at static, angle-of-attack, and crosswind conditions are described with a variety of flow control devices. Test results showed that the new inlet and S-duct have a pressure recovery comparable to that of the existing inlet. By employing corotating vortex generators, less flow distortion was obtained for the core region than the existing duct has with its counter rotating vortex generators.

Ting, C. T.↗

NASA NTRS1975

Wind tunnel blockage tests at Mach 5 of vacuum duct models for two sound radiation shields

Two sound shield models with dummy vacuum exhaust ducts were tested in a Mach 5 pilot quiet tunnel. The first model simulates a new sound shield of 3 in. (7.62 cm) inside diameter and the second model is a shield of 4 in. (10.16 cm) inside diameter. The dummy vacuum exhaust ducts were attached to the external housing of the models. The flow in the first model, which had a by pass mass flow ratio of about 0.6, could not be started except at the two highest test Reynolds numbers where only the central core flow region was started. The flow in the second model with a mass ratio of approximately 0.3 was fully started except at the lowest unit Reynolds number where some unsteadiness and partial flow separation at the wall was observed. Since the external housing and dummy vacuum ducts were the same for both models, these results indicate that the ratio of by pass mass flow to total mass flow for a wind tunnel sound shield of this particular design must be less than about 0.3. Hence, a lower limit is imposed on the inlet diameter of the sound shield in relation to the exit diameter of the wind tunnel nozzle. This lower limit on the inlet diameter may possibly be reduced by improvements in streamlining of the external housing and ducts, by reductions in blockage area, or by the use of external ducting shrouds.

Beckwith, I. E.↗

NASA NTRS1976

A multi-modal solution for the transmission of sound in nonuniform ducts

The method of weighted residuals in the form of a modified Galerkin method with boundary residuals is developed for the study of the transmission of sound in nonuniform ducts carrying a steady compressible flow. In this formulation the steady flow is modeled as essentially one-dimensional but with a kinematic modification to force tangency of the flow at the duct walls. Good agreement has been obtained with known results for transmission and reflection coefficients in hard walled ducts up to near sonic velocities. In ducts with acoustically compliant boundaries good comparisons have been more difficult to achieve except at low Mach numbers. The problem of transmission in a straight, acoustically treated duct with a uniform flow has been formulated and the Galerkin method used with basis functions derived from the case when flow is absent. Results indicate that favorable comparisons with exact computations can be obtained if care is taken in choosing the basis functions.

Eversman, W.↗

NASA NTRS1977

Transmission of sound through nonuniform circular ducts with compressible mean flows

An acoustic theory is developed to determine the sound transmission and attenuation through an infinite, hard-walled or lined, circular duct carrying compressible, sheared, mean flows and having a variable cross section. The theory is applicable to large as well as small axial variations, as long as the mean flow does not separate. Although the theory is described for circular ducts, it is applicable to other duct configurations - annular, two dimensional, and rectangular. The theory is described for the linear problem, but the technique is general and has the advantage of being applicable to the nonlinear case as well as the linear case. The technique is based on solving for the envelopes of the quasi-parallel acoustic modes that exist in the duct instead of solving for the actual wave. A computer program was developed. The mean flow model consists of a one dimensional flow in the core and a quarter-sine profile in the boundary layer. Results are presented for the reflection and transmission coefficients in ducts with varying slopes and carrying different mean flows.

Nayfeh, A. H.↗

NASA NTRS1977

Effect of flow on quasi-one-dimensional acoustic wave propagation in a variable area duct of finite length

The general equation for the velocity potential of quasi-one-dimensional acoustic wave motion in a variable area, finite duct with one-dimensional flow is derived by using a perturbation technique. The nonlinear second-order partial differential equation is linearized and then solved, by either a power series expansion method or the Runge-Kutta fourth-order method, for harmonic time dependence. The boundary condition taken at the duct mouth is that of matching the impedance of the duct sound field to that of the radiation field at the duct opening. Three axial Mach number variations along the duct axis are considered and the results obtained are compared with those for the case of constant Mach number, to determine the influence of the axial velocity gradient on sound propagation. The effect of flow on the radiation impedance is also considered.

Lumsdaine, E.↗

NASA NTRS1978

Sound propagation through a subsonic jet due to a source near the duct exit

Matched asymptotic solutions are constructed for the acoustic potentials of a periodic point source located in a two-dimensional subsonic jet near the exit of the duct with the ratio of the duct thickness to the acoustic wave length as the small parameter. The leading term of the far field solution has the same directionality effect as that for an infinite jet without the duct and that when the plane at the duct exit is considered to be a plane of symmetry. However, the intensity is different because of the wave propagation into the duct and is dependent on the location of the source.

Ting, L.↗

NASA NTRS1979

Modal propagation angles in a cylindrical duct with flow and their relation to sound radiation

The main emphasis is upon the propagation angle with respect to the duct axis and its relation to the far-field acoustic radiation pattern. When the steady flow Mach number is accounted for in the duct, the propagation angle in the duct is shown to be coincident with the angle of the principal lobe of far-field radiation obtained using the Wiener-Hopf technique. Different Mach numbers are allowed within the duct and in the external field. For static tests with a steady flow in an inlet but with no external Mach number the far-field radiation pattern is shifted considerably toward the inlet axis when compared to zero Mach number radiation theory. As the external Mach number is increased the noise radiation pattern is shifted away from the inlet axis. The theory is developed using approximations for sound propagation in circular ducts. An exact analysis using Hankel function solutions for the zero Mach number case is given to provide a check of the simpler approximate theory.

Rice, E. J.↗

NASA NTRS1979

Reciprocity principle in duct acoustics

Various reciprocity relations in duct acoustics have been derived on the basis of the spatial reciprocity principle implied in Green's functions for linear waves. The derivation includes the reciprocity relations between mode conversion coefficients for reflection and transmission in nonuniform ducts, and the relation between the radiation of a mode from an arbitrarily terminated duct and the absorption of an externally incident plane wave by the duct. Such relations are well defined as long as the systems remain linear, regardless of acoustic properties of duct nonuniformities which cause the mode conversions.

Cho, Y. C.↗

NASA NTRS1979

Reciprocity principle in duct acoustics

Cho, Y.-C.↗

NASA NTRS1981

Influence of exit impedance on finite difference solutions of transient acoustic mode propagation in ducts

The time-dependent governing acoustic-difference equations and boundary conditions are developed and solved for sound propagation in an axisymmetric (cylindrical) hard-wall duct without flow and with spinning acoustic modes. The analysis begins with a harmonic sound source radiating into a quiescent duct. This explicit iteration method then calculates stepwise in real time to obtain the steady solutions of the acoustic field. The transient method did not converge to the steady-state solution for cutoff acoustic duct modes. This has implications as to its use in a variable-area duct, where modes may become cutoff in the smal-area portion of the duct. For single cutoff mode propagation the steady-state impedance boundary condition produced acoustic reflections during the initial transient that caused finite instabilities in the numerical calculations. The stability problem is resolved by reformulating the exit boundary condition. Example calculations show good agreement with exact analytical and numerical results for forcing frequencies above, below, and nearly at the cutoff frequency.

Baumeister, K. J.↗

NASA NTRS1981

Note on reflection and transmission coefficients for converging-diverging ducts

Simple formulas for calculating acoustic reflection and transmission coefficients for converging-diverging ducts are derived; they extend the method of Cho and Ingard to arbitrary, slowly varying ducts. These formulas involve two parameters. The first is a function of duct shape and the second is the ratio of the duct radius downstream of the throat to that upstream of the throat to the upstream of the throat. An extension of the method to include mean flow is made for symmetric ducts.

Durbin, P. A.↗

NASA NTRS1983

Sound propagation in segmented exhaust ducts - Theoretical predictions and comparison with measurements

Suppressions due to acoustic treatment in the annular exhaust duct of a model fan have been theoretically predicted and compared with measurements. The predictions are based on the modal analysis of sound propagation in a straight annular flow duct with segmented treatment. Measured values of the mode distribution of the fan noise source (fan-stator interaction source only) and the acoustic impedance of the treatment in the duct were used as input to the prediction program. The predicted suppressions (under the assumption of uniform flow in the duct) compared well with the measured in-duct suppression for all test conditions. The measurements were made in an anechoic chamber and the interaction modes generated in the fan spanned a cut-off ratio range from nearly one to seven.

Joshi, M. C.↗