Effects of Roughness on Turbulent Flow in Microchannels and Minichannels

2008 ◽  
Author(s):  
Timothy P. Brackbill ◽  
Satish G. Kandlikar
Keyword(s):  
Experimental Study ◽  
Laminar Flow ◽  
Turbulent Flow ◽  
Roughness Parameter ◽  
Reynolds Numbers ◽  
Turbulent Regime ◽  
Roughness Effects ◽  
Relative Roughness ◽  
Roughness Elements ◽  
Moody Diagram

The effect of roughness ranging from smooth to 24% relative roughness on laminar flow has been examined in previous works by the authors. It was shown that using a constricted parameter, εFP, the laminar results were predicted well in the roughened channels ([1],[2],[3]). For the turbulent regime, Kandlikar et al. [1] proposed a modified Moody diagram by using the same set of constricted parameters, and using the modification of the Colebrook equation. A new roughness parameter εFP was shown to accurately portray the roughness effects encountered in laminar flow. In addition, a thorough look at defining surface roughness was given in Young et al. [4]. In this paper, the experimental study has been extended to cover the effects of different roughness features on pressure drop in turbulent flow and to verify the validity of the new parameter set in representing the resulting roughness effects. The range of relative roughness covered is from smooth to 10.38% relative roughness, with Reynolds numbers up to 15,000. It was found that using the same constricted parameters some unique characteristics were noted for turbulent flow over sawtooth roughness elements.

Effects of Low Uniform Relative Roughness on Single-Phase Friction Factors in Microchannels and Minichannels

2007 ◽  
Author(s):  
Timothy P. Brackbill ◽  
Satish G. Kandlikar
Keyword(s):  
Large Diameter ◽  
Reynolds Numbers ◽  
Transition To Turbulence ◽  
Roughness Effects ◽  
Relative Roughness ◽  
Flow Parameters ◽  
Friction Factors ◽  
Turbulent Flow Regime ◽  
Critical Reynolds Numbers ◽  
Large Diameter Pipes

Nikuradse’s [1] work on friction factors focused on the turbulent flow regime in addition to being performed in large diameter pipes. Laminar data was collected by Nikuradse, however only low relative roughness values were examined. A recent review by Kandlikar [2] showed that the uncertainties in the laminar region of Nikuradse’s experiments were very high, and his conclusion regarding no roughness effects in the laminar region is open to question. In order to conclusively resolve this discrepancy, we have experimentally determined the effects of relative roughness ranging from 0–5.18% in micro and minichannels on friction factor and critical Reynolds numbers. Reynolds numbers were varied from 30 to 7000 and hydraulic diameters ranged from 198μm to 1084μm. There is indeed a roughness effect seen in the laminar region, contrary to what is reported by Nikuradse. The resulting friction factors are well predicted using a set of constricted flow parameters. In addition to higher friction factors, transition to turbulence was observed at decreasing Reynolds numbers as relative roughness increased.

scholarly journals An experimental study and modelling of roughness effects on laminar flow in microchannels

2007 ◽  
Vol 594 ◽  
pp. 399-423 ◽  
Author(s):  
G. GAMRAT ◽  
M. FAVRE-MARINET ◽  
S. LE PERSON ◽  
R. BAVIÈRE ◽  
F. AYELA
Keyword(s):  
Laminar Flow ◽  
Numerical Simulations ◽  
Three Dimensional ◽  
Volume Averaging ◽  
Roughness Height ◽  
Experimental Investigations ◽  
Layer Model ◽  
Roughness Effects ◽  
Relative Roughness ◽  
Effective Roughness

Three different approaches were used in the present study to predict the influence of roughness on laminar flow in microchannels. Experimental investigations were conducted with rough microchannels 100 to 300μm in height (H). The pressure drop was measured in test-sections prepared with well-controlled wall roughness (periodically distributed blocks, relative roughness k* =k/0.5H≈0.15) and in test-sections with randomly distributed particles anchored on the channel walls (k* ≈0.04–0.13). Three-dimensional numerical simulations were conducted with the same geometry as in the test-section with periodical roughness (wavelength L). A one-dimensional model (RLM model) was also developed on the basis of a discrete-element approach and the volume-averaging technique. The numerical simulations, the rough layer model and the experiments agree to show that the Poiseuille number Po increases with the relative roughness and is independent of Re in the laminar regime (Re<2000). The increase in Po observed during the experiments is predicted well both by the three-dimensional simulations and the rough layer model. The RLM model shows that the roughness effect may be interpreted by using an effective roughness height keff. keff/k depends on two dimensionless local parameters: the porosity at the bottom wall; and the roughness height normalized with the distance between the rough elements. The RLM model shows that keff/k is independent of the relative roughness k* at given k/L and may be simply approximated by the law: keff/k = 1 − (c(ϵ)/2π)(L/k) for keff/k>0.2, where c decreases with the porosity ϵ.

scholarly journals MAXIMIZATION OF FLIGHT TIME FOR AIRPLANES WITH ELECTRICAL DRIVE BY POWERPLANT OPTIMIZATION

Aviation ◽  
2007 ◽  
Vol 11 (2) ◽  
pp. 37-40
Author(s):  
Sergey Serokhvostov
Keyword(s):  
Power Plant ◽  
Laminar Flow ◽  
Turbulent Flow ◽  
Electrical Power ◽  
Reynolds Numbers ◽  
Flight Time ◽  
Electrical Drive ◽  
Electrical Power Plant

In this paper, the problem of maximizing the flight time of an airplane with an electrical power plant (AEP) by the optimization of the mass of the accumulator in cases of fixed and non‐fixed airframe is considered. Variants of high (turbulent flow) and low (laminar flow) Reynolds numbers are taken into account. Dependence of flight time on airplane parameters is obtained. The behaviour of flight time as a function of accumulator mass near the maximum is also investigated. A comparison between the results obtained and the data for the existing AEP is made. On the basis of the results obtained, the influence of aircraft parameters on flight time is analyzed.

scholarly journals Numerical analysis of a leading edge tubercle hydrofoil in turbulent regime

2019 ◽  
Vol 878 ◽  
pp. 292-305 ◽  
Author(s):  
Blanca Pena ◽  
Ema Muk-Pavic ◽  
Giles Thomas ◽  
Patrick Fitzsimmons
Keyword(s):  
Numerical Analysis ◽  
Turbulent Flow ◽  
Flow Regime ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Main Flow ◽  
Hydrodynamic Performance ◽  
Turbulent Regime ◽  
Transitional Regime ◽  
Turbulent Flow Regime

This paper presents a numerical performance evaluation of the leading edge tubercles hydrofoil with particular focus on a fully turbulent flow regime. Efforts were focused on the setting up of an appropriate numerical approach required for an in-depth analysis of this phenomenon, being able to predict the main flow features and the hydrodynamic performance of the foil when operating at high Reynolds numbers. The numerical analysis was conducted using an improved delayed detached eddy simulation for Reynolds numbers corresponding to the transitional and fully turbulent flow regimes at different angles of attack for the pre-stall and post-stall regimes. The results show that tubercles operating in turbulent flow improve the hydrodynamic performance of the foil when compared to a transitional flow regime. Flow separation was identified behind the tubercle troughs, but was significantly reduced when operating in a turbulent regime and for which we have identified the main flow mechanisms. This finding confirms that the tubercle effect identified in a transitional regime is not lost in a turbulent flow. Furthermore, when the hydrofoil operates in the turbulent flow regime, the transition to a turbulent regime takes place further upstream. This phenomenon suppresses a formation of a laminar separation bubble and therefore the hydrofoil exhibits a superior hydrodynamic performance when compared to the same foil in the transitional regime.

Effect of Triangular Roughness Elements on Pressure Drop and Laminar-Turbulent Transition in Microchannels and Minichannels

2006 ◽  
Author(s):  
Timothy P. Brackbill ◽  
Satish G. Kandlikar
Keyword(s):  
Turbulent Flow ◽  
Reynolds Numbers ◽  
Subject Area ◽  
Internal Flows ◽  
Local Pressure ◽  
Flow Area ◽  
Friction Factors ◽  
Roughness Elements ◽  
Initial Work ◽  
Constricted Flow

Roughness elements affect internal flows in different ways. One effect is a transition from laminar to turbulent flow at a lower Reynolds number than the predicted Re = 2300. Initial work at RIT in the subject area was performed by Schmitt and Kandlikar (2005) and Kandlikar et al. (2005), and this study is an extension of these efforts. The channel used in this study is rectangular, with varying separation between walls that have machined roughness elements. The roughness elements are saw-tooth in structure, with element heights of 107 and 117 μm for two pitches of 405 μm and 815 μm respectively. The resulting hydraulic diameters and Reynolds numbers based on the constricted flow area range from 424 μm to 2016 μm and 210 to 2400 respectively. Pressure measurements are taken at sixteen locations along the flow length of 88.9 mm to determine the local pressure gradients. The results for friction factors and transition to turbulent flow are obtained and compared with the data reported by Schmitt and Kandlikar (2005). The roughness elements cause an early transition to turbulent flow, and the friction factors in the laminar region are predicted accurately using the hydraulic diameter based on the constricted flow area.

The Steady-State and Dynamic Characteristics of a Full Circular Bearing and a Partial Arc Bearing in the Laminar and Turbulent Flow Regimes

1967 ◽  
Vol 89 (2) ◽  
pp. 143-153 ◽  
Author(s):  
F. K. Orcutt ◽  
E. B. Arwas
Keyword(s):  
Steady State ◽  
Laminar Flow ◽  
Turbulent Flow ◽  
Flow Regime ◽  
Dynamic Characteristics ◽  
Reynolds Numbers ◽  
Operating Parameters ◽  
Design Data ◽  
Design Charts ◽  
Laminar And Turbulent Flow

The steady-state and dynamic characteristics of a full circular bearing and a centrally loaded, 100 deg, arc bearing are calculated for a range of eccentricity ratios to 0.95 and of mean Reynolds numbers to 13,300, and presented in design charts. These are compared with the measured performance of these bearings over the same ranges of the operating parameters. There is good correlation between the theoretical and test data, leading to the conclusion that the present turbulent lubrication analysis may be used to obtain general design data for self-acting bearings, operating in the superlaminar flow regime, to supplement that presently existing for laminar flow bearings.

scholarly journals Transition From the Turbulent to the Laminar Regime for Internal Convective Flow With Large Property Variations

1970 ◽  
Vol 92 (3) ◽  
pp. 506-512 ◽  
Author(s):  
C. W. Coon ◽  
H. C. Perkins
Keyword(s):  
Laminar Flow ◽  
Turbulent Flow ◽  
Circular Tube ◽  
Reynolds Numbers ◽  
Bulk Temperature ◽  
Heating Rates ◽  
Nusselt Numbers ◽  
Friction Factors ◽  
High Heating ◽  
Flow Situation

The results of a primarily experimental study of the transition from turbulent flow to laminar flow as a consequence of high heating rates are presented. Results are reported for hydrodynamically fully developed, low Mach number flows of air and helium through a vertical, circular tube. The electrically heated section was 100 diameters in length; entering Reynolds numbers ranged from 1700–40,000, and maximum wall-to-bulk temperature ratios reached 4.4. As a means of predicting the occurrence of a transition from turbulent flow to laminar flow, the experimental results are compared to the acceleration parameter suggested by Moretti and Kays and to a modified form of the parameter that is appropriate to a circular tube. It is suggested that the variable property turbulent flow correlations do not provide acceptable predictions of the Nusselt number and the friction factor if the value 4μq′′G2DTcp≃1.5×10−6 based on bulk properties, is exceeded for an initially turbulent flow situation. It is further suggested that Nusselt numbers and friction factors at locations down-stream from the point xDlaminar≃(2×10−8)(Tinlet)(Reb,inlet)2TwTbmax−1 for bulk temperatures in degrees Rankine may be obtained from the laminar correlation equations even though the flow is initially turbulent.

Turbulent Flow Behaviour in a Pipe Fully Submerged in a Hot Fluid

2017 ◽  
Author(s):  
Benjamin Steen ◽  
Kamran Siddiqui
Keyword(s):  
Heat Transfer ◽  
Laminar Flow ◽  
Turbulent Flow ◽  
Vertical Plane ◽  
Flow Behaviour ◽  
Turbulent Regime ◽  
Fluid Temperature ◽  
Cross Sectional ◽  
Turbulence Transition ◽  
Flow Mixing

We report on an experimental study conducted to investigate the flow behaviour in a heat exchanger pipe submerged in a hot stagnant fluid. Particle Image Velocimetry (PIV) was used to measure the two-dimensional velocity field in the mid-vertical plane of the tube. Fluid temperatures in the cross-sectional plane were also measured using thermocouples. The mode of heat transfer into the pipe was mixed convection where both inertia and buoyancy contributed to the convection. The results show that when the contribution of buoyancy-driven flow (natural convection) was smaller than that of the inertia-driven flow (forced convection), in an originally turbulent flow, the shear-induced turbulence dominated the flow and the turbulent velocity profile was not influenced by the heat input. In an originally laminar flow, the role of buoyancy was primarily limited to the initiation of instabilities in the laminar flow to trigger the turbulence transition. The temperature profiles indicate the presence of stably stratified layer inside the pipe in originally laminar flow regime that suppressed the heat transfer rate. In originally turbulent regime, the fluid temperature field was nearly uniform indicating efficient flow mixing.

Flow Development in a Parallel Plate Channel

1976 ◽  
Vol 98 (1) ◽  
pp. 145-154 ◽  
Author(s):  
A. Z. Szeri ◽  
C. C. Yates ◽  
S. M. Hai
Keyword(s):  
Laminar Flow ◽  
Velocity Profile ◽  
Turbulent Flow ◽  
Parallel Plate ◽  
Reynolds Numbers ◽  
Momentum Equation ◽  
Essential Difference ◽  
Eddy Viscosity Model ◽  
Asymptotic Profile ◽  
Plate Channel

The paper presents a study of the flow that occurs in a finite, parallel plate channel. The experimental work consists of velocity profile measurements upstream of and inside the channel of a belt-type apparatus. Theoretical prediction of velocity profile development is made via numerical methods in both laminar and turbulent situations. In the laminar flow case analytical solution of a linearized form of the momentum equation was also possible. Good agreement is shown between prediction and experimental results for all Reynolds numbers tested; in turbulent flow this occurs particularly when employing Reichardt’s eddy viscosity model. For laminar flow the entrance length is estimated to be 0.008–0.01 times the Reynolds number, while in turbulent flow no essential difference was found between an entrance and the corresponding asymptotic profile. Upstream from the entrance the similar laminar profiles of Sakiadis were observed experimentally.

Characterization of Surface Roughness Effects on Laminar Flow in Microchannels by Using Fractal Cantor Structures

2009 ◽  
Author(s):  
Chengbin Zhang ◽  
Yongping Chen ◽  
Panpan Fu ◽  
Mingheng Shi
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Fractal Dimension ◽  
Pressure Drop ◽  
Laminar Flow ◽  
Flow Direction ◽  
Roughness Effects ◽  
Relative Roughness ◽  
Smooth Channel

The fractal characterization of the topography of rough surfaces by using Cantor set structures is introduced in this paper. Based on the fractal Cantor surface, a model of laminar flow in rough microchannels is developed and numerically analyzed to study the characterization of surface roughness effects on laminar flow. The effects of Reynolds number, relative roughness, and fractal dimension on laminar flow are all discussed. The results indicate that the presence of roughness leads to the form of the detachment, and eddy generation is observed at the shadow of the roughness elements. The pressure drop in the rough channel along the flow direction is no longer in a linear fashion and larger than that in the smooth channel. The fluctuation characteristic of pressure drop along the stream, which is due to the vortex formation at the wall, is found. Differing from the smooth channel, the Poiseuille number for laminar flow in rough microchannels is no longer only dependent on the cross-sectional shape of the channel, but also strongly influenced by the Reynolds number, relative roughness and fractal dimension of the surface.

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