Heat transfer characteristics of a two-phase closed thermosyphon using different working fluids

In this paper, an enhanced FORTRAN code was combined with the EES software to predict the vapor flow thermal resistance effects on the heat transfer characteristics of a two- phase closed thermosyphon (TPCT). Different refrigerants such as water, ammonia, R-11, R-22 and R-134a were tested. Also an enhanced time integration scheme was recommended for solving the governing equations in FORTRAN code.     

Efficiency analysis of organic Rankine cycle with internal heat exchanger using neural network

In this study, artificial neural network (ANN) has been used for efficiency analysis of the organic Rankine cycle with internal heat exchanger (IHEORC) using refrigerants R410a, R407c which do not damage to ozone layer. It is well known that the evaporator temperature, condenser temperature, subcooling temperature and superheating temperature affect the thermal efficiency of IHEORC. In this study, thermal efficiency is estimated depending on the above temperatures. The results of ANN are compared with actual results. The coefficient of determination values obtained when the test set were used to the networks were 0.99946 and 0.999943 for the R410a and R407c respectively which is very satisfactory.     

Efficiency comparison of various friction models of a hydraulic cylinder in the framework of multibody system dynamics

Nonlinear Dynamics - Dynamic simulation of mechanical systems can be performed using a multibody system dynamics approach. The approach allows to account systems of other physical nature, such as...     

Slosh dynamics of inviscid fluids in two‐dimensional tanks of various geometry using finite element method

This paper brings into focus some of the interesting effects arising from the motion of the liquid free surface due to sloshing in partially filled containers of several geometrical shapes in two dimensions. The slosh characteristics that include frequencies, free surface profiles and the hydrodynamic pressure over the container walls have been reported in this study. The equations of motion of the fluid, considered inviscid, are expressed in terms of the pressure variable alone. It is assumed that the frequency of the exciting oscillation is not in the immediate neighborhood of the natural slosh frequency, so that the slope of the free surface is small. Simple harmonic oscillation and earthquake excitations are used as the prescribed boundary conditions. A finite difference‐based iterative time‐stepping technique is employed to advance the solution in the time domain. The paper presents numerical solutions for rectangular, vertically mounted annular cylindrical, trapezoidal and horizontal circular cylindrical containers. Numerical results obtained are compared with the available existing solutions to validate the code developed. The parametric study of the slosh dynamic systems shows the importance of the nature of excitation, fluid height and the geometry of the container. Copyright © 2007 John Wiley & Sons, Ltd.     

Modelling the flow of power law fluids in a packed bed using a volume-averaged equation of motion

The development of a theoretical model for the prediction of velocity and pressure drop for the flow of a viscous power law fluid through a bed packed with uniform spherical particles is presented. The model is developed by volume averaging the equation of motion. A porous microstructure model based on a cell model is used. Numerical solution of the resulting equation is effected using a penalty Galerkin finite element method. Experimental pressure drop values for dilute solutions of carboxymethylcellulose flowing in narrow tubes packed with uniformly sized spherical particles are compared to theoretical predictions over a range of operating conditions. Overall agreement between experimental and theoretical values is within 15%. The extra pressure drop due to the presence of the wall is incorporated directly into the model through the application of the no-slip boundary condition at the container wall. The extra pressure drop reaches a maximum of about 10% of the bed pressure drop without wall effect. The wall effect increases as the ratio of tube diameter to particle diameter decreases, as the Reynolds number decreases and as the power law index increases.     

Selection of working fluids for electrohydrodynamic experiments in terrestrial conditions and microgravity

Electrohydrodyamic (EHD) heat transfer enhancement and flow control methods are becoming increasingly popular in engineering science and applications both in terrestrial and low gravity applications. The correct choice of the working fluid is essential for the design and performance of EHD hardware and can pose challenge because some working fluids with favorable EHD properties can be unstable or hazardous. In this paper key properties and criteria for the selection of working fluids for single-phase (liquid) as well as gas–liquid and vapor–liquid two-phase electrohydrodynamic experiments and applications are discussed. Key physical and electrical properties as well as environmental and safety issues are reviewed for the sample fluids PF-5052, FC-72, R141b, cyclohexane and pure water. Microgravity experiments impose additional demands on the selection of the working fluids. Some of these demands are addressed by contrasting bubble dimensions and shapes at detachment, estimated using a simple thermodynamic model, in terrestrial and microgravity conditions with and without electric fields. Data are obtained using a simplified analytical model and verified experimentally.     

A detailed comparison of various FENE dumbbell models

The rheological behaviour of dilute solutions of finitely extensible non-linear elastic (FENE) dumbbells in both steady state and transient shear and simple elongational flow is investigated. Three dumbbell models are compared: the original FENE model with the Warner spring force, which is treated by brownian dynamics simulations, and the FENE-P model based on the Peterlin approximation and the FENE-CR model as suggested by Chilcott and Rallison, which are treated by standard numerical techniques. It is shown that in the linear viscoelastic limit and in steady state flows the behaviour is similar, except for the FENE-CR dumbbell in shear flow, modelling a Boger fluid. In transient flows larger differences appear.     

On the comparison of the solutions obtained by using two different transform methods for the second problem of Stokes for Newtonian fluids

The unsteady flow over a plane wall which is initially at rest and the plate begins suddenly to oscillate in own plane is considered. The solution subject to the boundary and initial conditions is obtained by applying to the governing equation the Laplace transform method or Fourier transform method. A comparison of the solutions obtained by two transform methods for flow considered is given. It is shown that the solution obtained by the Laplace transform method or Fourier transform method is the sum of the steady-state and the transient parts. The transient parts are found in terms of definite integrands whose integrals are oscillatory functions. Therefore, the transient parts are expressed in terms of the tabulated functions.     

Head-flow and npsh performance of an axial piston pump working with organic fluids at different temperatures

A modified version of the standard axial piston pump, normally used with fuels, has been tested with organic fluids R11 and R113. Head-flow characteristics, volumetric and global efficiency, and npsh curves, have been determined at different speeds and fluid temperatures and the results compared with those obtained with kerosene. Pump efficiencies remain satisfactory with high density, very low viscosity and high vapour pressure fluids. In the absence of cavitation, pump performance seems to be a function of kinematic viscosity, while the npsh curves appear to be a complex function of density, viscosity and vapour pressure     

Supercritical and subcritical Hopf bifurcation and limit cycle oscillations of an airfoil with cubic nonlinearity in supersonic\hypersonic flow

In this paper, the Hopf bifurcations and limit cycle oscillations (LCOs) of an airfoil with cubic nonlinearity in supersonic\hypersonic flow are investigated. The harmonic balance method and multivariable Floquet theory are applied to analyze the LCOs of the airfoil. Four distinct cases of the LCOs response are detected in this system: (I) supercritical Hopf bifurcation, (II) a single subcritical Hopf bifurcation, (III) two subcritical Hopf bifurcations, and (IV) no Hopf bifurcation. Furthermore, the parameter variations domains separating the supercritical and subcritical Hopf bifurcations are presented using singularity theory.     

Similitude considerations in laminar flow of modified power law fluids in circular ducts

A numerical solution is presented for the friction factorReynolds number relation for a fluid which is Newtonian at low shear rates, power law at high shear rates with a transition region at intermediate shear rates. The solution makes possible the conservation of similitude when designing duct systems for such fluids since both Reynolds numbers and shear rates are considered.     

A comparison of extensional viscosity measurements from various RME rheometers

The transient uniaxial extensional viscosity η _e of linear low density polyethylene (LLDPE) has been measured using the commercial Rheometric Scientific RME and the Münstedt Tensile Rheometer in an effort to compare the performance of available extensional rheometers. The RME indicated a significant strain hardening of the LLDPE, especially at a strain rate of 1 s⁻¹. In contrast, the Münstedt rheometer showed the LLDPE to be only slightly strain hardening. This artificial strain hardening effect in the RME resulted from the strain rate applied to the sample, determined from the sample deformation, being up to 20% less than the set strain rate. These results initiated a round-robin experiment in which the same LLDPE was tested on several RMEs in various locations around the world. All but one of the RMEs indicated a deviation between set and applied strain rates of at least 10%, especially at strain rates above 0.1 s⁻¹. The strain rate deviation was found to depend strongly on the value of the basis length L ₀ , and may result from the upper pair of belts not properly gripping the sample during extension. Thus visual inspection of the sample deformation is necessary to determine the applied strain rate. The most accurate measurements of η _e with respect to the strain rate deviation were obtained when the correct L ₀ value and belt arrangement were used. A list of recommendations for running an RME test is provided. Future work focusing on the fluid mechanics during the test may identify fully the cause of the strain rate deviation, but from a practical point of view the problem can be corrected for in the determination of η _e . Received: 27 September 2000/Accepted: 5 February 2001     

Forced response of quadratic nonlinear oscillator: comparison of various approaches

The primary resonances of a quadratic nonlinear system under weak and strong external excitations are investigated with the emphasis on the comparison of different analytical approximate approaches. The forced vibration of snap-through mechanism is treated as a quadratic nonlinear oscillator. The Lindstedt-Poincaré method, the multiple-scale method, the averaging method, and the harmonic balance method are used to determine the amplitude-frequency response relationships of the steady-state responses. It is demonstrated that the zeroth-order harmonic components should be accounted in the application of the harmonic balance method. The analytical approximations are compared with the numerical integrations in terms of the frequency response curves and the phase portraits. Supported by the numerical results, the harmonic balance method predicts that the quadratic nonlinearity bends the frequency response curves to the left. If the excitation amplitude is a second-order small quantity of the bookkeeping parameter, the steady-state responses predicted by the second-order approximation of the LindstedtPoincaré method and the multiple-scale method agree qualitatively with the numerical results. It is demonstrated that the quadratic nonlinear system implies softening type nonlinearity for any quadratic nonlinear coefficients.     

A new look at the laminar flow of power law fluids through granular beds

Summary The paper deals with laminar flow of power law fluids through granular beds. A critical review of the assumptions concerning the capillary model of the bed, applied by various authors, led us to the conclusion that the derivation of the correlation eq. [13] given byChristopher andMiddleman was based on a too simplified model of the granular bed. Taking advantage of the approach presented in the classical works ofKozeny andCarman (which seems to be partly overlooked by some authors, including our own previous works) a modified correlation equation for power law fluids [21], a corrected formula for shear rate in the bed [29] and for Deborah number [32], as well as corrected correlation equation for fluids exhibiting memory effects [34] were presented.

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Zusammenfassung Diese Arbeit betrifft laminare Strömungen von Potenzgesetzflüssigkeiten durch Kornschüttungen. Eine kritische Prüfung der Annahmen, die von verschiedenen Autoren für das Kapillar-Modell der Schüttung gemacht worden sind, führt uns zu der Folgerung, daß die Herleitung der Korrelationsgleichung [13] nachChristopher undMiddleman auf einem übervereinfachten Modell der Kornschüttung basiert. Unter Nutzbarmachung der Annahmen, die in den klassischen Arbeiten vonKozeny undCarman dargestellt worden sind (sie wurden sowohl von manchen anderen Autoren als auch in unseren früheren Arbeiten nicht beachtet), werden nun eine modifizierte Korrelationsgleichung für die Potenzgesetzflüssigkeiten [21], eine korrigierte Formel für die Schergeschwindigkeit in der Schüttung [29], eine korrigierte Formel für die Deborah-Zahl [32] und eine korrigierte Korrelationsgleichung für Flüssigkeiten, die Gedächtnis-Effekte zeigen [34], angegeben.

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Notation A constant in eq. [9] - d _p effective particle diameterd _p = 6/a (wherea is the specific surface of the bed), m - f _BK modified friction factor, defined by eq. [1] - k power law parameter, N s ⁿ /m² - K Kozeny constant, defined by eq. [8] - K ₀ constant depending on the shape of the channel cross-section - K ₁ constant, defined by eq. [5] - l bed height, m - l _e channel length, m - n power law parameter - p pressure drop due to friction, N/m² - r _h hydraulic radius, defined by eq. [6], m - s bed permeability, defined by eq. [16], m² - v ₀ mean linear velocity related to an empty crosssection of the column, m/s - v _e mean linear velocity in the channel, m/s - shear rate at the wall of the channel, s^–1 - shear rate at the wall of the channel calculated according to the formula [29], s^–1 - bed porosity - characteristic time of the fluid, s - friction factor, defined by eq. [25] - µ dynamic viscosity of the fluid, N s/m² - parameter, defined by eq. [15], N s ⁿ /m¹⁺ⁿ - De Deborah number, defined by eq. [33] - De ^* Deborah number, defined by eq. [32] - Re _BK modified Reynolds number, defined by eq. [2] - Re _BK modified Reynolds number, defined by eq. [26] - Re _BK ^* modified Reynolds number, defined by eq. [23] - Re _CM modified Reynolds number byChristopher andMiddleman, defined by eq. [14] - Re _CM modified Reynolds number, defined by eq. [17] With 3 figures and 1 table     

Similarity solutions of a class of laminar three-dimensional boundary layer equations of power law fluids

An analysis of the possibility of finding similarity solutions to the three-dimensional, steady, incompressible, boundary layer equations in rectangular coordinates for a power law fluid is investigated. It is found that, in general, the two components of the mainstream flow must differ by at most a multiplicative constant and that these components are powers or exponentials of the x'-coordinate.

By assuming small cross-flows, the cross flow component may be generalized and found to be representable by a polynomial in the through flow variable, x'.     

Modeling of complex fluids using micro-macro approach with transient network dynamics

In this work, the micro-macro approach is used to simulate the flow of dilute polymer solutions by means of a kinetic model coupled with the dynamics of a transient network. The transient network modeling is based on the original formulation, in which the kinetics of microstates describes the complexity of interactions among the macromolecules suspended in a Newtonian solvent (Rincón et al, J Non-Newton Fluid 131:64–77, 2005). The average concentration of microstates, at a given time, defines a variable maximum segment length (variable extensibility) of the molecular FENE model. The non-Newtonian contribution to the extra stress tensor is computed according to the Brownian configuration-fields method. Comparisons with the Oldroyd-B model validates its limiting behavior. Numerical results show the influence of the solvent to total viscosity ratio and shear rate, on the transient and steady rheological phenomena of complex fluids with microstates.     

Reduced-order models for control of fluids using the eigensystem realization algorithm

As sensors and flow control actuators become smaller, cheaper, and more pervasive, the use of feedback control to manipulate the details of fluid flows becomes increasingly attractive. One of the challenges is to develop mathematical models that describe the fluid physics relevant to the task at hand, while neglecting irrelevant details of the flow in order to remain computationally tractable. A number of techniques are presently used to develop such reduced-order models, such as proper orthogonal decomposition (POD), and approximate snapshot-based balanced truncation, also known as balanced POD. Each method has its strengths and weaknesses: for instance, POD models can behave unpredictably and perform poorly, but they can be computed directly from experimental data; approximate balanced truncation often produces vastly superior models to POD, but requires data from adjoint simulations, and thus cannot be applied to experimental data. In this article, we show that using the Eigensystem Realization Algorithm (ERA) (Juang and Pappa, J Guid Control Dyn 8(5):620?C627, 1985) one can theoretically obtain exactly the same reduced-order models as by balanced POD. Moreover, the models can be obtained directly from experimental data, without the use of adjoint information. The algorithm can also substantially improve computational efficiency when forming reduced-order models from simulation data. If adjoint information is available, then balanced POD has some advantages over ERA: for instance, it produces modes that are useful for multiple purposes, and the method has been generalized to unstable systems. We also present a modified ERA procedure that produces modes without adjoint information, but for this procedure, the resulting models are not balanced, and do not perform as well in examples. We present a detailed comparison of the methods, and illustrate them on an example of the flow past an inclined flat plate at a low Reynolds number.     

Efficient computation of mean drag for the subcritical flow past a circular cylinder using general Galerkin G2

General Galerkin (G2) is a new computational method for turbulent flow, where a stabilized Galerkin finite element method is used to compute approximate weak solutions to the Navier–Stokes equations directly, without any filtering of the equations as in a standard approach to turbulence simulation, such as large eddy simulation, and thus no Reynolds stresses are introduced, which need modelling. In this paper, G2 is used to compute the drag coefficient c_D for the flow past a circular cylinder at Reynolds number Re=3900, for which the flow is turbulent. It is found that it is possible to approximate c_D to an accuracy of a few percent, corresponding to the accuracy in experimental results for this problem, using less than 10⁵ mesh points, which makes the simulations possible using a standard PC. The mesh is adaptively refined until a stopping criterion is reached with respect to the error in a chosen output of interest, which in this paper is c_D. Both the stopping criterion and the mesh‐refinement strategy are based on a posteriori error estimates, in the form of a space–time integral of residuals times derivatives of the solution of a dual problem, linearized at the approximate solution, and with data coupling to the output of interest. Copyright © 2008 John Wiley & Sons, Ltd.