Effects of Inlet Boundary Layer on Pressure Recovery, Energy Conversion and Losses in Conical Diffusers

1957 ◽  
Vol 61 (554) ◽  
pp. 116-124 ◽  
Author(s):  
F. A. L. Winternitz ◽  
W. J. Ramsay
Keyword(s):  
Boundary Layer ◽  
Shape Parameter ◽  
Pressure Coefficient ◽  
Pressure Recovery ◽  
Pressure Coefficients ◽  
Performance Variation ◽  
Inlet Conditions ◽  
Wire Cloth ◽  
Diffuser Angle ◽  
Total Angle

SummaryA study has been made of the effect of inlet conditions on the performance of conical diffusers with 4:1 area ratio and 5 and 10 degrees total angle of expansion. The conditions at entry were varied by using different approach lengths of diffuser inlet diameter, and by means of projecting annular screens of woven wire cloth. With this new technique it was possible to vary the velocity distribution substantially within moderate settling lengths, and to produce velocity profiles with inflections. The suitability of the annular screen method of boundary layer generation for diffuser investigations was confirmed from the comparison with the approach length results.Energy and pressure coefficients, as well as diffuser energy efficiency and the conversion efficiency, were found to depend on the diffuser angle β and the momentum thickness ratio at inlet θ/D0. The latter emerged as one of the chief parameters controlling diffuser performance. Variation in the inlet shape parameter H of the order of 20 per cent did not significantly affect the pressure recovery or the losses in the diffuser. For moderately thick boundary layers, θ/D0 X β<0·1, the diffuser angle could be eliminated as a parameter by plotting the pressure coefficient against θ/D0 X β. The Reynolds number based on diffuser inlet diameter was, for essentially incompressible flow, 2·5 X 105 in all tests.

Effects of Inlet Conditions on the Flow in a Fishtail Curved Diffuser With Strong Curvature

1996 ◽  
Vol 118 (4) ◽  
pp. 772-778 ◽  
Author(s):  
M. I. Yaras
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Large Scale ◽  
Static Pressure ◽  
Pressure Recovery ◽  
Centrifugal Impeller ◽  
Flow Development ◽  
Diffuser Flow ◽  
Inlet Conditions ◽  
Strong Curvature

The paper presents detailed measurements of the incompressible flow at the exit of a large-scale 90-degree curved diffuser with strong curvature and significant stream-wise variation in the cross-section aspect ratio. The diffuser flow path approximates the so-called fish-tail diffuser utilized on small gas turbine engines for the transition between the centrifugal impeller and the combustion chamber. Five variations of the inlet boundary layer are considered. The results provide insight into several aspects of the diffuser flow including: the effect of flow turning on diffusion performance; the dominant structures influencing the flow development in the diffuser; and the effect of the inlet boundary layer integral parameters on the diffusion performance and the exit velocity field. The three-dimensional velocity distribution at the diffuser exit is found to be sensitive to circumferentially uniform alterations to the inlet boundary layer. In contrast, circumferential variations in the inlet boundary layer are observed to have only secondary effects on the velocity field at the diffuser exit. The static pressure recovery is observed to be comparable to the published performance of conical diffusers with equivalent included angle and area ratios. Furthermore, both the static pressure recovery and the total pressure losses are observed to be relatively insensitive to variations in the inlet boundary layer. The physical mechanisms dominating the flow development in the diffuser are discussed in light of these observations.

The Inception of Cavitation on Isolated Surface Irregularities

1960 ◽  
Vol 82 (1) ◽  
pp. 169-183 ◽  
Author(s):  
J. W. Holl
Keyword(s):  
Boundary Layer ◽  
Cross Section ◽  
Shape Parameter ◽  
Pressure Coefficient ◽  
Surface Irregularity ◽  
Relative Height ◽  
Circular Arc ◽  
Actual Surface ◽  
Roughness Elements ◽  
Surface Irregularities

The inception of cavitation on isolated surface irregularities imbedded in a turbulent boundary layer is investigated experimentally and theoretically. Two families of cylindrical roughness elements having constant cross sections are studied. One family has a circular-arc cross section. The other family has a triangular cross section and was selected to simulate the separating flow which is typical of an actual surface irregularity. The theoretical minimum-pressure coefficient for the circular-arc irregularities is determined as a function of the relative height of roughness for several values of the boundary-layer shape parameter. Cavitation tests in the water tunnels of the Ordnance Research Laboratory on roughness elements ranging from 0.002 to 0.5 in. in height indicate that the incipient-cavitation number of an isolated surface irregularity is dependent upon the relative height of roughness, the boundary-layer shape parameter, the velocity, and other variables as yet unknown.

Review: Effects of Inlet Conditions on Conical-Diffuser Performance

1981 ◽  
Vol 103 (2) ◽  
pp. 250-257 ◽  
Author(s):  
A. Klein
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Experimental Evidence ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Shape Parameter ◽  
Conical Diffuser ◽  
Free Discharge ◽  
Inlet Conditions ◽  
Different Sources

The available experimental evidence of the effects of inlet conditions on the performance of conical diffusers with a free discharge is reviewed. The effects of inlet boundary layer thickness blockage, inlet shape parameter, turbulence, and Reynolds number are discussed. It is shown that many of the inconsistencies between different sources of data are the result of nonturbulent approach flows. Graphs are presented as guidelines for diffuser design.

scholarly journals Probability Characteristics, Area Reduction, and Wind Directionality Effects of Extreme Pressure Coefficients of High-Rise Buildings

2021 ◽  
Vol 11 (15) ◽  
pp. 7121
Author(s):  
Shouke Li ◽  
Feipeng Xiao ◽  
Yunfeng Zou ◽  
Shouying Li ◽  
Shucheng Yang ◽  
...  
Keyword(s):  
Probability Distribution ◽  
Pressure Coefficient ◽  
Wind Pressure ◽  
Extreme Pressure ◽  
Distribution Law ◽  
Distribution Fitting ◽  
Pressure Coefficients ◽  
High Rise ◽  
Area Reduction ◽  
The Mean

Wind tunnel tests are carried out for the Commonwealth Advisory Aeronautical Research Council (CAARC) high-rise building with a scale of 1:400 in exposure categories D. The distribution law of extreme pressure coefficients under different conditions is studied. Probability distribution fitting is performed on the measured area-averaged extreme pressure coefficients. The general extreme value (GEV) distribution is preferred for probability distribution fitting of extreme pressure coefficients. From the comparison between the area-averaged coefficients and the value from GB50009-2012, it is indicated that the wind load coefficients from GB50009-2012 may be non-conservative for the CAARC building. The area reduction effect on the extreme wind pressure is smaller than that on the mean wind pressure from the code. The recommended formula of the area reduction factor for the extreme pressure coefficient is proposed in this study. It is found that the mean and the coefficient of variation (COV) for the directionality factors are 0.85 and 0.04, respectively, when the orientation of the building is given. If the uniform distribution is given for the building’s orientation, the mean value of the directionality factors is 0.88, which is close to the directionality factor of 0.90 given in the Chinese specifications.

Influence of Turbulent Flow Characteristics and Coherent Vortices on the Pressure Recovery of Annular Diffusers: Part A — Experimental Results

2015 ◽  
Author(s):  
Marcus Kuschel ◽  
Bastian Drechsel ◽  
David Kluß ◽  
Joerg R. Seume
Keyword(s):  
Boundary Layer ◽  
High Pressure ◽  
Static Pressure ◽  
Turbulent Transport ◽  
Axial Flow ◽  
Pressure Recovery ◽  
Turbulence Structure ◽  
Reynolds Stresses ◽  
Wide Range ◽  
Operating Points

Exhaust diffusers downstream of turbines are used to transform the kinetic energy of the flow into static pressure. The static pressure at the turbine outlet is thus decreased by the diffuser, which in turn increases the technical work as well as the efficiency of the turbine significantly. Consequently, diffuser designs aim to achieve high pressure recovery at a wide range of operating points. Current diffuser design is based on conservative design charts, developed for laminar, uniform, axial flow. However, several previous investigations have shown that the aerodynamic loading and the pressure recovery of diffusers can be increased significantly if the turbine outflow is taken into consideration. Although it is known that the turbine outflow can reduce boundary layer separations in the diffuser, less information is available regarding the physical mechanisms that are responsible for the stabilization of the diffuser flow. An analysis using the Lumley invariance charts shows that high pressure recovery is only achieved for those operating points in which the near-shroud turbulence structure is axi-symmetric with a major radial turbulent transport component. This turbulent transport originates mainly from the wake and the tip vortices of the upstream rotor. These structures energize the boundary layer and thus suppress separation. A logarithmic function is shown that correlates empirically the pressure recovery vs. the relevant Reynolds stresses. The present results suggest that an improved prediction of diffuser performance requires modeling approaches that account for the anisotropy of turbulence.

A Superposition Analysis of the Turbulent Boundary Layer in an Adverse Pressure Gradient

1951 ◽  
Vol 18 (1) ◽  
pp. 95-100
Author(s):  
Donald Ross ◽  
J. M. Robertson
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Turbulent Boundary Layer ◽  
Velocity Profile ◽  
Pressure Gradient ◽  
Adverse Pressure Gradient ◽  
Pressure Recovery ◽  
Stress Coefficient ◽  
Wall Shear

Abstract As an interim solution to the problem of the turbulent boundary layer in an adverse pressure gradient, a super-position method of analysis has been developed. In this method, the velocity profile is considered to be the result of two effects: the wall shear stress and the pressure recovery. These are superimposed, yielding an expression for the velocity profiles which approximate measured distributions. The theory also leads to a more reasonable expression for the wall shear-stress coefficient.

CFD Modeling of Pressure Coefficients for a Natural Ventilation Study in an Experimental Low Rise Building

2013 ◽  
Author(s):  
Robel Kiflemariam ◽  
Cheng-Xian Lin
Keyword(s):  
Natural Ventilation ◽  
Pressure Coefficient ◽  
Wind Pressure ◽  
Analytical Models ◽  
Cfd Modeling ◽  
Engineering Practice ◽  
Pressure Coefficients ◽  
Wind Pressure Coefficient ◽  
Detailed Assessment ◽  
Ventilation Rates

Mean wind pressure coefficient (Cp) is one of the major input data for natural ventilation study using building energy simulation approach. Due to their importance, they need to be accurately determined. In current engineering practice, tables and analytical Cp models only give mostly averaged results for simpler models and configurations. Considering the limitation of tables and analytical models, Computational Fluid Dynamics (CFD) could provide a means for an accurate and detailed assessment of Cp. In this paper, we make use of a relatively high resolution, detailed experiments done at Florida Intentional University to validate a CFD modeling of the pressure coefficients Cp. The results show that existing CFD model has a good agreement with experimental results and gives important information of distribution of Cp values over the surface. The local values of the Cp are investigated. In addition, the CFD derived Cp and discharge coefficient (Cd) values are utilized in semi-analytical ventilation models in order to get a more accurate value of ventilation rates.

Correlation Between Pressure Recovery of Highly Loaded Annular Diffusers and Integral Stage Design Parameters

2017 ◽  
Author(s):  
Dajan Mimic ◽  
Bastian Drechsel ◽  
Florian Herbst
Keyword(s):  
Boundary Layer ◽  
Dynamic Pressure ◽  
Boundary Layer Separation ◽  
Flow Coefficient ◽  
Tip Vortex ◽  
Pressure Recovery ◽  
Design Parameters ◽  
Steam Turbines ◽  
Pressure Losses ◽  
Induced Velocity

Exhaust diffusers significantly enhance the available power output and efficiency of gas and steam turbines by allowing for lower turbine exit pressures. The residual dynamic pressure of the turbine outflow is converted into static pressure, which is referred to as pressure recovery. Since total pressure losses as well as construction costs increase drastically with diffuser length, it is more than favourable to design shorter diffusers with rather steep opening angles. However, those designs are more susceptible to boundary layer separation. In this paper, the stabilising properties of tip leakage vortices generated in the last rotor row and their effect on the boundary layer characteristics are examined. Based on analytical considerations, for the first time a correlation between the pressure recovery of the diffuser and integral rotor parameters of the last stage, namely the loading coefficient, flow coefficient and reduced frequency, is established. Both, experimental data and scale resolving simulations, carried out with the SST-SAS method, show excellent agreement with the correlation. Blade tip vortex strength predominantly depends on the amount of work performed in the rotor, which in turn is described by the non-dimensional loading coefficient. The flow coefficient influences mainly the orientation of the vortex, which affects the interaction between vortex and boundary layer. The induced velocity field accelerates the boundary layer, essentially reducing the thickness of the separated layer or even locally preventing separation.

Effects of Rotating Blade Wakes on Separation and Pressure Recovery in Turbine Exhaust Diffusers

2008 ◽  
Author(s):  
Olaf Sieker ◽  
Joerg R. Seume
Keyword(s):  
Boundary Layer ◽  
High Energy ◽  
Flow Coefficient ◽  
Pressure Recovery ◽  
High Mass ◽  
Scale Model ◽  
Swirl Number ◽  
Annular Diffuser ◽  
Bladed Rotor ◽  
Turbine Exhaust

Highly efficient turbine exhaust diffusers can only be designed by taking into account the unsteady interactions with the last rotating row of the turbine. Therefore, a scale model of a typical gas turbine exhaust diffuser consisting of an annular and a conical diffuser is investigated experimentally. To investigate the influence of rotating wakes, a variable-speed rotating spoke wheel with cylindrical spokes as well as with NACA bladed spokes generates high-energy turbulent wakes simulating turbine rotor wakes. For the rotor with the NACA blades, the drive of the wheel is run in motor as well as in generator mode. Additional measurements in a reference configuration without a spoke wheel allow the detailed analysis of changes in the flow pattern. 3-hole pneumatic probes, static pressure taps, as well as a 2D-Laser-Doppler-Velocimeter (LDV) are used to investigate velocity profiles and turbulent characteristics. Without the wakes generated by a spoke wheel, the annular diffuser (with a 20° half cone opening angle) separates at the shroud for all swirl configurations. Increasing the swirl results in increasing pressure recovery at the shroud whereas the hub boundary is destabilized. For a non-rotating spoke rotor and low swirl numbers, the 20° annular diffuser separates at the shroud. Increasing the swirl number, a strong deceleration of the axial velocity at the shroud is generated without separation and a higher pressure recovery is achieved. The boundary layer at the shroud of the 20° annular diffuser separates for all operating points with the bladed rotor. A partly stabilized 20° annular diffuser can only be achieved for much higher values of the flow coefficient than that for the design point. At this high mass flow, the NACA-bladed rotor operates as a turbine, resulting in the generator mode of the electric drive. Contrary to the numerical design calculations, the flow at the shroud of a 15° annular diffuser does not separate for all swirl configurations in the experiment. Pressure recovery of the 15° annular diffuser can be increased by increasing the inlet swirl whereas the hub boundary layer is destabilized. For the NACA bladed rotor, the flow in the 15° annular diffuser as well as the pressure recovery strongly depend on the flow coefficient. For flow coefficients lower than the design value, the flow partly separates at the shroud whereas large flow coefficients result in increased pressure recovery. The pressure recovery also depends on the direction of swirl and thus the swirl number.

scholarly journals Computational assessment of wind flow field on and around atypical buildings

2020 ◽  
Author(s):  
Rajasekarababu KB
Keyword(s):  
Surface Pressure ◽  
Pressure Coefficient ◽  
Tall Buildings ◽  
Ground Level ◽  
Pressure Variation ◽  
Middle Level ◽  
Computational Results ◽  
Pressure Coefficients ◽  
Building Surfaces ◽  
Side Face

Abstract This article provides an overview of pressure coefficients ( Cp ) on atypical tall buildings with the application of CFD. Various modifications in architectural shapes on tall buildings eventually lead to a reduction in the wind load on building surfaces. The surface pressure on conventional (Square and rectangular) buildings is relatively different in comparison to other tall buildings. This study is to evaluate the surface pressure coefficient over rectangular, taper and setback buildings. The computational results show that the taper building has 7% Cp rise at ground level ( y/H= 0.225) in the windward face, and 34% Cp fall at the middle level ( y/H= 0.475) in the side face when compared with the rectangular building. Whereas for the setback building, Cp at ground level near setback ( y/H= 0.225) has reduced to about 25% and about 6% at the middle level ( y/H= 0.475) in windward than that in the rectangle building. Also, the side faces of the setback showed a 15% drop in Cp than other buildings. In leeward face, Cp is reduced to 56% near setback at the top of the building ( y/H= 0.725). This valuation of the Cp on these buildings shows that the effect of setbacks on building reduces the pressure variation on all faces and the downstream wake vortices.

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