Viscous Scaling Phenomena in Miniature Centrifugal Flow Cooling Fans: Theory, Experiments and Correlation

2010 ◽  
Vol 132 (2) ◽  
Author(s):  
Patrick A. Walsh ◽  
Edmond J. Walsh ◽  
Ronan Grimes
Keyword(s):  
Reynolds Number ◽  
Scale Effects ◽  
Pressure Coefficient ◽  
Pressure Rise ◽  
Chord Length ◽  
Flow Coefficient ◽  
Power Coefficient ◽  
Small Scale ◽  
Cooling Fans ◽  
Novel Theory

This paper analyzes the scale effects that occur in miniature centrifugal flow fans and investigates the possibility of optimizing blade geometry so that performance can be enhanced. Such fans are typically employed in small scale heat sinks such as those used for processor cooling applications or in portable electronics. The specific design parameter varied is the blade chord length, and the resulting fan performance is gauged by examining the flow rate, pressure rise, and power consumption characteristics. The former two are measured using a BS 848 fan characterization rig and the latter, by directly measuring the power consumed. These characteristics are studied for three sets of scaled fans with diameters of 15 mm, 24 mm, and 30 mm, and each set considers six individual blade chord lengths. A novel theory is put forward to explain the anticipated effect of changing this parameter, and the results are analyzed in terms of the relevant dimensionless parameters: Reynolds number, chord length to diameter of fan ratio, flow coefficient, pressure coefficient, and power coefficient. When these characteristic parameters are plotted against the Reynolds number, similar trends are observed as the chord length is varied in all sets of scaled fans. The results show that the flow coefficient for all the miniature fans degrade at low Re values, but the onset of this degradation was observed at higher Re values for longer blade chord designs. Conversely, it was found that the pressure coefficient is elevated at low Re, and the onset Re for this phenomenon correlates well with the drop off in flow coefficient. Finally, the trend in power coefficient data is similar to that for the flow coefficient. The derived theory is used to correlate this data for which all data points fall within 6% of the correlation. Overall, the findings reported herein provide a good understanding of how changing the blade chord length affects the performance of miniature centrifugal fans; hence, providing fan designers with guidelines to aid in developing optimum blade designs, which minimize adverse scaling phenomena.

On the Performance of Miniature Centrifugal Fans With Varying Blade Cord Length

2008 ◽  
Author(s):  
Patrick A. Walsh ◽  
Edmond Walsh ◽  
Ronan Grimes
Keyword(s):  
Reynolds Number ◽  
Pressure Coefficient ◽  
Pressure Rise ◽  
Chord Length ◽  
Flow Coefficient ◽  
Power Coefficient ◽  
Portable Power ◽  
Novel Theory ◽  
Centrifugal Fans ◽  
Data Points

This paper focuses on optimizing the blade design of miniature centrifugal flow fans for application in processor cooling solutions of portable power electronics. The design parameter varied is the blade chord length and the resulting fan performance is gauged by examining flow rate, pressure rise and power consumption characteristics. The former two of these are measured using a BS848 fan characterization rig and the latter by directly measuring the power consumed. These characteristics are studied for three sets of scaled fans with diameters of 15mm, 24mm and 30mm, and each set considers six individual blade chord lengths. A novel theory is put forward to explain the anticipated effect of changing this parameter and the results are analyzed in terms of the relevant dimensionless parameters: Reynolds number; chord length to diameter of fan ratio; flow coefficient; pressure coefficient and power coefficient. When these characteristic parameters are plotted against Reynolds number, similar trends are observed as the chord length is varied in all sets of scaled fans. The results show that the flow coefficient for all the miniature fans degrade at low Re values but the onset of this degradation was observed at higher Re values for longer blade chord designs. Conversely, it was found that the pressure coefficient is elevated at low Re and the onset Re for this phenomenon correlates well with the drop off in flow coefficient. Finally, the trend in power coefficient data appears to be identical to that for the flow coefficient. The derived theory is used to correlate this data for which all data points fall within 6% of the correlation. Overall, the findings reported herein provide a good understanding of how changing the blade chord length affects the performance of miniature centrifugal fans and provides guidelines for designers to aid in selecting the optimum fan design for a specific application.

On Reliable Performance Prediction of Axial Turbomachines

2010 ◽  
Author(s):  
Michael Heß ◽  
Peter F. Pelz
Keyword(s):  
Performance Prediction ◽  
Pressure Coefficient ◽  
Scale Up ◽  
Pressure Rise ◽  
Reynolds Numbers ◽  
Flow Coefficient ◽  
Factor V ◽  
Relative Roughness ◽  
Reliable Performance ◽  
Reliable Scale

There is a need to reliably predict the performance (efficiency and total pressure rise) of axial turbomachines from model tests for different load ranges. The commonly used scale-up formulas are not able to reliably predict the performance, especially beyond the design point. Furthermore these formulas do not regard changes in relative roughness as they usually occur in practice. An improved scale-up formula is proposed which achieves not only the reliable scale-up of efficiency, but also the scale-up of the pressure coefficient. It is motivated from measurements on two geometric similar axial model fans with a diameter of 1000 mm respectively 250 mm at different rotational speeds, hence Reynolds numbers. By bonding grains of sand to the impeller the influence of relative roughness was investigated. For applying the formula to different load ranges a factor V is introduced that depends on the quotient of effective flow coefficient to optimal flow coefficient.

scholarly journals Low Reynolds Number Scaling Eeffects For Small-Scale Rotors

2021 ◽  
Author(s):  
Ammar Jessa
Keyword(s):  
Reynolds Number ◽  
Scaling Laws ◽  
Full Range ◽  
Reynolds Numbers ◽  
Power Coefficient ◽  
Small Scale ◽  
Force Coefficient ◽  
Thrust Coefficient ◽  
Rolling Moment ◽  
Moment Coefficient

<div>Three T-Motor rotors with different diameters but otherwise identical relative geometries were tested in fully edgewise flow at different advance ratios and Reynolds numbers. The objective was to verify whether the existing scaling relationships between rotor size and the aerodynamic forces are applicable to small scale rotors that operate at relatively low chord-Reynolds numbers. The rotors were mounted onto a test stand housed inside a closed loop wind-tunnel where the air speed of the tunnel was varied to achieve different advance ratios. The chord-Reynolds umber at 75% of the radius of each blade were matched for ranges from 39,000 to 117,000. The experimental data was also compared to computational results from a blade element momentum theory-based method. The results showed that the existing coefficient based scaling laws can be used to predict the performance parameters for the thrust coefficient, power coefficient, longitudinal force coefficient, side force coefficient and, rolling moment coefficient for the full range of Reynolds numbers tested. Although for the pitching moment coefficient, a coefficient approach became less applicable for chord-Reynolds number of less than 100,000.</div>

Influence of Viscous Effects on Impact Tubes

1953 ◽  
Vol 20 (2) ◽  
pp. 253-256
Author(s):  
C. W. Hurd ◽  
K. P. Chesky ◽  
A. H. Shapiro
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Pressure Coefficient ◽  
Pressure Rise ◽  
Experimental Investigations ◽  
Stream Velocity ◽  
Fluid Density ◽  
Viscous Effects ◽  
Point Pressure ◽  
The Impact

Abstract Experiments were conducted to determine the effect of viscosity on the pressure rise recorded by a blunt-nosed impact tube in incompressible flow. The results are presented in terms of the pressure coefficient (Cp ≡ 2Δp/ρV∞2) as a function of Reynolds number (Rey ≡ V∞α/ν), where Δp is the excess of stagnation-point pressure over free-stream static pressure, V∞ is the free-stream velocity, α is the radius of the impact tube, ρ is the fluid density, and ν is the kinematic viscosity of the fluid. Above Reynolds numbers of 1000, there is no effect of viscosity, and Cp is equal to unity. Between Rey ≌ 50 and Rey ≌ 1000, Cp is slightly less than unity, but has a minimum value of 0.99. For values of Rey less than 50, Cp is always greater than unity. When the Reynolds number is below unity, the pressure rise is independent of the fluid density, and the data may be represented approximately by the formula Cp ≌ 5.6/Rey. The results are compared with the experimental investigations of Barker and of Homann, and with the theoretical studies of Stokes and of Homann.

Wavelet Analysis of Reynolds Number Effect on 3-D Vorticity in a Turbulent Near Wake

2003 ◽  
Author(s):  
M. W. Yiu ◽  
H. Li ◽  
Y. Zhou
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Lift Coefficient ◽  
Pressure Coefficient ◽  
Vortex Formation ◽  
Small Scale ◽  
Stream Velocity ◽  
Near Wake ◽  
Scale Structures ◽  
Three Components

When Reynolds number, Re (≡U∞d/v, where U∞ is the free stream velocity, d is the cylinder diameter and v is the kinematic viscosity of the fluid), is in the range of 103 to 104, there is a large variation in the near-wake formation region in terms of the base pressure coefficient, the fluctuating lift coefficient, the vortex formation length, which have previously been connected to the generation of small-scale Kelvin-Helmholtz vortices. This work aims to investigate how this Re variation affects the three components of vorticity in terms of time-averaged and small-scale structures and also to provide a relatively complete set of 3-D vorticity data. All three components of vorticity data were simultaneously measured in the intermediate region of the turbulent wake using a multi-wire vorticity probe. It is observed that the root-mean-square (rms) values of the three vorticity components increase with Re, especially the streamwise component, which shows a large jump from Re = 5×103 to 104. At the central frequencies of f0 and 2f0, the contributions from the large-scale and intermediate-scale structures of ωzi2/(ωz2)max decreases 13% and 16% respectively as the Re. increases. However, at the central frequency of 16f0, the contribution of the small-scale structure of ωzi2/(ωz2)max dramatic suddenly 7% increase at Re = 5×103 to 104. The result suggest the generation of small-scale Kelvin-Helmholtz vortices in the spanwise structure. The effect of Re on vorticity signals, spectra, contributions from the wavelet components to the vorticity variances are also examined.

A Scaling Law for Cavitation Inception in Circular Jet Flows

2007 ◽  
Author(s):  
Brian A. Edge ◽  
Eric G. Paterson ◽  
Mario F. Trujillo
Keyword(s):  
Reynolds Number ◽  
Numerical Simulations ◽  
Scale Effects ◽  
Scaling Law ◽  
Pressure Coefficient ◽  
Cavitation Number ◽  
Scaling Relations ◽  
Length Scale ◽  
Cavitation Inception ◽  
Dynamic Head

The historical data for circular jets indicates that the incipient cavitation number increases with the diameter of the jet. This trend is not explained by the classic cavitation theory which expects incipient cavitation number to remain constant regardless of the jet diameter, flow parameters, or water quality. This paper explores the origins of cavitation scale effects and explains the correlation between the incipient cavitation number, jet diameter, and nuclei size. This is accomplished through turbulence-resolving CFD simulations of the jet flow field at three length scales and Rayleigh-Plesset bubble dynamics for three nuclei sizes. The numerical simulations show that incipient cavitation number (σi) changes significantly as the size of the jet is altered while the Reynolds number and the value of the minimum pressure coefficient are held constant. Larger nuclei bubbles (100μm) exhibit an increase in σi with jet diameter, while moderate (50μm) and small (10μm) nuclei bubble exhibit a decrease in σi as jet diameter increases. The value of σi associated with a small jet was similar for all nuclei sizes. As the jet increased in size, the disparity between the values of σi associated with each nuclei size was found to increase substantially. The equilibrium form of the Rayleigh-Plesset equation was used to derive a correction to the classic theory of cavitation inception. This correction is a function of initial nuclei size and the dynamic head of the flow. As either the nuclei properties or dynamic head of the fluid change, the magnitude of the correction term will also change. This correction to the classic cavitation theory was used to make predictions of how σi will change as length scale and Reynolds number are altered. These equilibrium predictions were found to be in good agreement with the numerical simulations of cavitation inception for large and moderate (100μm and 50μm) nuclei bubbles. Comparisons with the small (10μm) nuclei bubbles indicate that the inertial terms are quite significant for these bubbles, resulting in large discrepancies between the full numerical solution and the equilibrium predictions. In general, the equilibrium scaling relations show that as the length scale of a flow is held constant and the Reynolds number is increased, σi will converge to −CPmin. The scaling relations also show that when Reynolds number is held constant and the length scale of a flow is increased, σi will depart from −CPmin.

scholarly journals A CFD Simulation on the Performance of Slotted Propeller Design for Various Airfoil Configurations

CFD letters ◽  
2021 ◽  
Vol 13 (3) ◽  
pp. 43-57
Author(s):  
Wan Mazlina Wan Mohamed ◽  
Nirresh Prabu Ravindran ◽  
Parvathy Rajendran
Keyword(s):  
Reynolds Number ◽  
Power Efficiency ◽  
High Reynolds Number ◽  
Power Coefficient ◽  
Small Scale ◽  
Power Ratio ◽  
Thrust Coefficient ◽  
High Lift ◽  
Propeller Design ◽  
High Reynolds

The usage of slots has gained renewed interest in aerospace, particularly on propeller design. Most of the works have focused on improving the aerodynamic performance and efficiency. Modern research on propeller design aims to design propellers with high thrust performance under low torque conditions without any weight penalty. Although research on slotted design has been done before, none has been done to understand its impact on different airfoils on the propeller blade. Thus, this study aims to provide extensive research on slotted propeller design with various airfoil of different properties such as high Reynolds number, low Reynolds number, symmetrical, asymmetrical high lift, and low drag. This work has been investigated using computational fluid dynamics method to predict propeller performance for a small-scale propeller. The slotted blade designs' performance is presented in terms of thrust coefficient, power coefficient, efficiency, and thrust to power ratio. Here, the slotted APC Slow Flyer propeller blade's performance has been investigated for diverse types of airfoils with the shape and position of the slot is fixed which is a square-shaped at 62.5% of the chord length. The flow simulations are performed through three-dimensional computational fluid dynamic software (ANSYS Fluent) to determine the thrust coefficient, power coefficient, efficiency, and thrust to power ratio measured in advancing flow conditions. Findings show that the slotted propeller design composed of symmetrical, high Reynolds number, high lift airfoils can benefit the most with slots' implementation. These improvements were 19.49%, 69.13%, 53.57% and 111.06% in terms of thrust, power, efficiency and trust to power ratio respectively.

Extending Classical Friction Loss Modeling to Predict the Viscous Performance of Pumping Devices

2019 ◽  
Vol 141 (10) ◽  
Author(s):  
Abhay Patil ◽  
Wenjie Yin ◽  
Rahul Agarwal ◽  
Adolfo Delgado ◽  
Gerald Morrison
Keyword(s):  
Fluid Dynamics ◽  
Reynolds Number ◽  
Power Input ◽  
Flow Coefficient ◽  
Power Coefficient ◽  
Viscosity Effects ◽  
Computational Fluid Dynamics Cfd ◽  
Specific Speed ◽  
Using Data ◽  
Efficiency Data

The affinity law modified for viscosity effects is further extended to include the power input and efficiency. The power input and efficiency data generated using computational fluid dynamics (CFD) are utilized to represent dimensionless power coefficient and efficiency for the pump under consideration. The goal of modifying the affinity laws for power input is achieved by developing a new relationship where the power coefficient is modified by multiplying it by rotational Reynolds number raised to a power Π*Rew−Pat. This new relationship is then represented as a function of a modified flow coefficient ф*Rew−Mo. All the data collapse onto a single curve for varying values of the exponents Morrison number (Mo) and Patil number (Pat). Pat is further characterized as a function of flow regime and specific speed. The method also holds true for efficiency prediction, however, with different values of Mo and Pat. The proposed method is validated by using data collected from published literature.

scholarly journals SCALE EFFECTS ON STABILITY AND WAVE REFLECTION REGARDING ARMOR UNITS

1986 ◽  
Vol 1 (20) ◽  
pp. 165 ◽  
Author(s):  
Atsuyuki Shimada ◽  
Toshimi Fujimoto ◽  
Syozo Saito ◽  
Tsutomu Sakakiyama ◽  
Hiromaru Hirakuchi
Keyword(s):  
Reynolds Number ◽  
Scale Effect ◽  
Critical Reynolds Number ◽  
Scale Effects ◽  
Minimum Weight ◽  
Small Scale ◽  
Large Wave ◽  
Wave Flume ◽  
Scale Test ◽  
The Stability

In the studies on stability of the armor units and reflection from those, there are some indications on scale effects which are included in the results of small scale experiments. In this study, the fact has been confirmed with large wave flume test, and estimated the critical Reynolds Number where was no scale effect. And by this result on the stability of urmor units, we can evaluated the results in small and middle scale test, and can correct the minimum weight of armor units. So we can design the breakwaters and seawalls rationaly and economicaliy. However, it has not been confirmed the critical Reynolds Number where the influence of scale effect on reflection became negligible.

A Theory of Peripheral Jets in Proximity to the Ground With Application to Ground-Effect Machines

1964 ◽  
Vol 8 (02) ◽  
pp. 16-20
Author(s):  
Quentin Wald
Keyword(s):  
Incompressible Flow ◽  
Additional Assumption ◽  
Close Agreement ◽  
Velocity Ratio ◽  
Lift Coefficient ◽  
Pressure Coefficient ◽  
Ground Effect ◽  
Flow Coefficient ◽  
Power Coefficient ◽  
Height Ratio

A theory of peripheral jets in proximity to the ground has been developed which yields results in a simple closed form and is in close agreement with the exact inviscid solution. By the introduction of one additional assumption beyond the usual assumption of inviscid incompressible flow, the pressure relations can be determined by a momentum integration. The additional assumption is the general form of the velocity distribution across the jet. The base pressure coefficient, lift coefficient, mass-flow coefficient and power coefficient are all obtained as simple functions of a velocity-ratio parameter which is in turn related to the thickness/height ratio and jet angle. Application to ground-effect machines is considered briefly.

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