scholarly journals Discussion: “Slip-Flow Pressure Drop in Microchannels of General Cross Section”

2012 ◽  
Vol 134 (5) ◽  
Author(s):  
C. Y. Wang
Keyword(s):  
Exact Solution ◽  
Fluid Flow ◽  
Pressure Drop ◽  
Cross Section ◽  
Dynamic Properties ◽  
Fluid Dynamic ◽  
Slip Flow ◽  
Partial Slip ◽  
Boundary Slip ◽  
Flow Pressure

Partial slip occurs in a variety of important fluid flow situations. Recently several sources used the constant boundary slip assumption for the flow in a tube. By comparing with the exact solution for the slip flow in a triangular duct, we show the constant slip assumption invokes substantial errors in both local and global fluid dynamic properties.

Hydrogen Combustion Within a Gas Turbine Reheat Combustor

2012 ◽  
Author(s):  
Madhavan Poyyapakkam ◽  
John Wood ◽  
Steven Mayers ◽  
Andrea Ciani ◽  
Felix Guethe ◽  
...  
Keyword(s):  
Pressure Drop ◽  
Natural Gas ◽  
Gas Turbine ◽  
Dynamic Properties ◽  
Fluid Dynamic ◽  
Fold Increase ◽  
Inlet Temperature ◽  
Turbine Performance ◽  
Auto Ignition ◽  
Operating Window

This paper describes a novel lean premixed reheat burner technology suitable for Hydrogen-rich fuels. The inlet temperature for such a combustor is very high and reaction of the fuel/oxidant mixture is initiated through auto-ignition, the delay time for which reduces significantly for Hydrogen-rich fuels in comparison to natural gases. Therefore the residence time available for premixing within the burner is reduced. The new reheat burner concept has been optimized to allow rapid fuel/oxidant mixing, to have a high flashback margin and to limit the pressure drop penalty. The performance of the burner is described, initially in terms of its fluid dynamic properties and then its combustion characteristics. The latter are based upon full-scale high-pressure tests, where results are shown for two variants of the concept, one with a pressure drop comparable to today’s natural gas burners, and the other with a two-fold increase in pressure drop. Both burners indicated that Low NOx emissions, comparable to today’s natural gas burners, were feasible at reheat engine conditions (ca. 20 Bars and ca. 1000C inlet temperature). The higher pressure drop variant allowed a wider operating window. However the achievement of the lower pressure drop burner shows that the targeted Hydrogen-rich fuel (70/30 H2/N2 by volume) can be used within a reheat combustor without any penalty on gas turbine performance.

Measurement of Steady-Flow Instability and Turbulence Levels in Dacron Vascular Grafts

1992 ◽  
Vol 114 (4) ◽  
pp. 521-526 ◽  
Author(s):  
D. G. Shombert
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Steady Flow ◽  
Flow Instability ◽  
Dynamic Properties ◽  
Fluid Dynamic ◽  
Vascular Grafts ◽  
Turbulent Regime ◽  
Number Range ◽  
Measured Transition

Fluid dynamic properties of Dacron vascular grafts were studied under controlled steady-flow conditions over a Reynolds number range of 800 to 4500. Knitted and woven grafts having nominal diameters of 6 mm and 10 mm were studied. Thermal anemometry was used to measure centerline velocity at the downstream end of the graft; pressure drop across the graft was also measured. Transition from laminar flow to turbulent flow was observed, and turbulence intensity and turbulent stresses (Reynolds normal stresses) were measured in the turbulent regime. Knitted grafts were found to have greater pressure drop than the woven grafts, and one sample was found to have a critical Reynolds number (Rc) of less than one-half the value of Rc for a smooth-walled tube.

A Critical Examination of Correlation Methodology Widely Used in Heat Transfer and Fluid Flow

Volume 1 ◽  
2004 ◽  
Author(s):  
Eugene F. Adiutori
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Pressure Drop ◽  
Laminar Flow ◽  
Turbulent Flow ◽  
Power Laws ◽  
Critical Examination ◽  
Highly Nonlinear ◽  
Flow Pressure ◽  
The Impact

The correlation methodology widely used in heat transfer and fluid flow is based on fitting power laws to data. Because all power laws of positive exponent include the point (0,0), this methodology includes the tacit assumption that phenomena are best described by correlations that include the point (0,0). • If a phenomenon occurs near (0,0), the assumption is obviously valid. For example, laminar flow occurs near (0,0), and therefore the assumption is valid for laminar flow pressure drop correlations. • If a phenomenon does not occur near (0,0), the assumption is obviously invalid. For example, turbulent flow does not occur near (0,0)—it occurs only after a critical Reynolds number is reached. Therefore the assumption is invalid for turbulent flow pressure drop correlations. When the assumption is invalid, the correlation methodology widely used in heat transfer and fluid flow is lacking in rigor. The impact of the lack of rigor is evidenced by examples that demonstrate that, when this methodology is applied to phenomena that do not occur in the vicinity of (0,0), highly nonlinear power laws oftentimes result from data that exhibit highly linear behavior. Because the widely used methodology lacks rigor when applied to phenomena that do not occur near (0,0), power laws based on this methodology are suspect if they purport to describe phenomena that do not occur near (0,0). Data cited in support of such power laws should be recorrelated using rigorous correlation methodology. Rigorous correlation methodology is also used in heat transfer and fluid flow. It is described in the text, and should become the methodology in general use.

Slip-Flow Pressure Drop in Microchannels of General Cross Section

2009 ◽  
Vol 131 (3) ◽  
Author(s):  
M. Bahrami ◽  
A. Tamayol ◽  
P. Taheri
Keyword(s):  
Boundary Condition ◽  
Pressure Drop ◽  
Cross Section ◽  
Slip Flow ◽  
Homogeneous Boundary Condition ◽  
Slip Boundary Condition ◽  
Arbitrary Cross Section ◽  
Geometrical Parameters ◽  
Cross Sectional ◽  
Slip Boundary

In the present study, a compact analytical model is developed to determine the pressure drop of fully-developed, incompressible, and constant properties slip-flow through arbitrary cross section microchannels. An averaged first-order Maxwell slip boundary condition is considered. Introducing a relative velocity, the difference between the bulk flow and the boundary velocities, the axial momentum reduces to Poisson’s equation with homogeneous boundary condition. Square root of area is selected as the characteristic length scale. The model of Bahrami et al. (2006, “Pressure Drop of Laminar, Fully Developed Flow in Microchannels of Arbitrary Cross Section,” ASME J. Fluids Eng., 128, pp. 1036–1044), which was developed for no-slip boundary condition, is extended to cover the slip-flow regime in this study. The proposed model for pressure drop is a function of geometrical parameters of the channel: cross sectional area, perimeter, polar moment of inertia, and the Knudsen number. The model is successfully validated against existing numerical and experimental data collected from different sources in literature for several shapes, including circular, rectangular, trapezoidal, and double-trapezoidal cross sections and a variety of gases such as nitrogen, argon, and helium.

Simulation of Fluid Flow through a Elastic Microchannel Deformed by a Piezoelement in Microgrip Cooling Systems

2019 ◽  
Vol 20 (12) ◽  
pp. 740-750 ◽  
Author(s):  
I. Sh. Nasibullayev ◽  
E. Sh. Nasibullayeva ◽  
O. V. Darintsev
Keyword(s):  
Fluid Flow ◽  
Pressure Drop ◽  
Cooling System ◽  
Fluid Dynamic ◽  
Three Dimensional ◽  
Piezoelectric Element ◽  
Practical Applications ◽  
Calculation Results ◽  
Liquid Cooling System ◽  
Cylindrical Microchannel

The flow of the fluid in an elastic cylindrical microchannel, the central part of which is located inside the piezoelectric ring, is simulated numerically. It arises as due to channel deformation by piezoelement according to the harmonic law, and pressure drop at the inlet and outlet to the microchannel. The aim of the work is to create a three-dimensional computer model of controlling the flow of a fluid by means of a pressure drop and a tube compression piezoelectric element. The model of an element of a computational bench that allows you to find fluid flow using specified analytical formulas, built using an approximation of the calculation results for the full model for individual sets of parameters. Modeling an element of a computing bench will allow real-time calculations with direct integration into the control system of a technical device. The model is based on the obtained analytical dependencies taking into account the restrictions introduced, which can significantly reduce the amount of computation and improve the quality of the result. The solution of the full equations of elasticity for the tube and the equations of hydrodynamics in the microchannel was carried out numerically by the finite element method in the package of numerical simulation FreeFem++. Numerical results are obtained for the flow rate of a fluid as a function of time, the physical properties of the fluid (dynamic viscosity and density) and external influences (the magnitude of the pressure gradient, the amplitude and frequency of compression of the piezoelectric element). The variants of using the obtained results in practical applications are shown. For example, in a liquid cooling system, the obtained relationship between the system parameters allows one to determine the flow regime that prevents the flow of heated liquid through the channel outlet. It is planned to use the results in the development of a computing stand for capillary micro-capture, containing two tubes (at the input and output) with piezoelectric elements, dividing the device into two parts (with dynamically changing and unchanged geometries) which will greatly simplify the full simulation.

Slip-Flow Pressure Drop in Microchannels of General Cross-Section

2008 ◽  
Author(s):  
A. Tamayol ◽  
M. Bahrami ◽  
P. Taheri
Keyword(s):  
Boundary Condition ◽  
Pressure Drop ◽  
Cross Section ◽  
Slip Flow ◽  
Homogeneous Boundary Condition ◽  
Slip Boundary Condition ◽  
Arbitrary Cross Section ◽  
Geometrical Parameters ◽  
Cross Sectional ◽  
Slip Boundary

In the present study, a compact analytical model is developed to determine the pressure drop of fully-developed, incompressible, and constant properties slip-flow through arbitrary cross-section microchannels. An averaged first-order Maxwell slip boundary condition is considered. Introducing a relative velocity, the difference between the bulk flow and the boundary velocities, the axial momentum reduces to the Poisson’s equation with homogeneous boundary condition. Square root of area is selected as the characteristic length scale. Bahrami et al.’s model, which was developed no-slip boundary condition, is extended to cover the slip-flow regime in this study. The proposed model is a function of geometrical parameters of the channel: cross-sectional area, perimeter, polar moment of inertia and the Knudsen number. The model is successfully validated against existing numerical and experimental data from different sources in the literature for several shapes, including: circular, rectangular, trapezoidal, and double-trapezoidal cross-sections and a variety of gases such as: nitrogen, argon, and helium.

Stokes Slip Flow in Channel Bend

2019 ◽  
Vol 142 (1) ◽  
Author(s):  
C. Y. Wang
Keyword(s):  
Exact Solution ◽  
Pressure Drop ◽  
Slip Flow ◽  
Velocity Slip ◽  
Meandering Channel ◽  
Channel Bend ◽  
Slip Factor ◽  
Small Ratio ◽  
Gap Width

Abstract Using intrinsic coordinates, the slip flow in a minute meandering channel is studied by perturbation about the small ratio of curvature to inverse half gap width. The exact solution for an annulus shows this ratio can be as large as 0.5 with less than 1% error. Velocity slip on the walls and the pressure drop depend on the slip factor. Formula for the pressure drop in a channel with a single bend is derived.

A General Model for Predicting Low Reynolds Number Flow Pressure Drop in Non-Uniform Microchannels of Non-Circular Cross Section in Continuum and Slip-Flow Regimes

2013 ◽  
Vol 135 (7) ◽  
Author(s):  
M. Akbari ◽  
A. Tamayol ◽  
M. Bahrami
Keyword(s):  
Pressure Drop ◽  
Cross Section ◽  
General Model ◽  
Circular Cross Section ◽  
Numerical Data ◽  
Creeping Flow ◽  
Geometrical Parameters ◽  
Cross Sectional ◽  
Flow Pressure ◽  
The Cross

A general model that predicts single-phase creeping flow pressure drop in microchannels of a noncircular cross section under slip and no-slip regimes is proposed. The model accounts for gradual variations in the cross section and relates the pressure drop to geometrical parameters of the cross section, i.e., area, perimeter, and polar moment of inertia. The accuracy of the proposed model is assessed by comparing the results against experimental and numerical data collected from various studies in the literature for a wide variety of cross-sectional shapes. The suggested model can be used for the design and optimization of microsystems that contain networks of microchannels with noncircular cross sections resulting from different fabrication techniques.

Compressibility and Rarefaction Effect on Heat Transfer in Rough Microchannels

2007 ◽  
Author(s):  
Giulio Croce ◽  
Paola D’Agaro
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Mach Number ◽  
Pressure Drop ◽  
Fluid Dynamic ◽  
Slip Flow ◽  
Flow Conditions ◽  
Rarefied Flow ◽  
Wide Range ◽  
Poiseuille Number

High pressure drop and high length to hydraulic diameter ratios yield significant compressibility effects in microchannel flows, which compete with rarefaction phenomena at the smaller scale. In such regimes, flow field and temperature field are no longer decoupled. In presence of significant heat transfer, and combined with the effect of viscous dissipation, this yields to a quite complex thermo-fluid dynamic problem. A finite volume compressible solver, including generalized Maxwell slip flow and temperature jump boundary conditions suitable for arbitrary geometries, is adopted. Roughness geometry is modeled as a series of triangular shaped obstructions, and relative roughness from 0% to 2.65% were considered. The chosen geometry allows for direct comparison with pressure drop computations carried out, in a previous paper, under adiabatic conditions. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 300 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to around 0.05, and the competing effects of rarefaction, compressibility and roughness are investigated in detail. Compressibility is found to be the most dominant effect at high Mach number, yielding even inversion of heat flux, while roughness has a strong effect in the case of rarefied flow. Furthermore, the mutual interaction between heat transfer and pressure drop is highlighted, comparing Poiseuille number values for both cooled and heated flows with previous adiabatic computations.

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