Constant Wall Research Articles

Take-off threshold velocity of saltating particles under heat radiation

A wind tunnel and a heating device were designed in order to study the influence of solar radiation on the wind-induced motion of solid particles. Velocity and temperature measurements in the boundary layer exhibited clear modifications of flow characteristics with a surprising balance between the viscosity increase and the reduction in the wall velocity gradient. This induces a constant critical wall shear stress under heat flux. Measurements of threshold velocity undertaken to elucidate the possible effects of solar radiation through the increase in turbulence intensity showed a reduction in critical free-stream velocity as the free-stream turbulence increased.

NUMERICAL ANALYSIS OF NON-ISOTHERMAL HELICAL FLOW OF HERSCHEL-BULKLEY FLUIDS

In this article a numerical solution of non-isothermal helical flow for the thermodependent Herschel-Bulkley fluids has been presented. The consistency term (μ0) in comparison to other properties is more temperature dependent; therefore an exponential function in the form of μ0 = exp(−bT) is considered. The governing equations are solved for two different physical situations; constant temperature walls and for a case of constant heat flux using the implicit finite difference method. The results from numerical solution are compared with results of first law of thermodynamics and a good agreement is noticed.

Influence of elongational properties on the contraction flow of polyisobutylene in a mixed solvent

Over the last decade several international programmes have been developed around different standard fluids, one of which is the so-called S1 fluid. This is a solution of polyisobutylene in a mixed solvent and the aim of the programme has been to study the rheology of polymer solutions from the dilute solution to the melt. The focus of this paper will be on the flow visualisation of contraction flows of S1 through orifice dies and on the estimation of some of its extensional properties. The contraction ratios range from 24.4:1 and 124.3:1. The measured entry pressure drops will be correlated with contraction ratio and apparent wall shear rate. Experimental evidence will show that, when regarded as a function of wall shear rate, the entry pressure drops are independent of the contraction ratios. The flow fields for different contraction ratios, at any constant apparent wall shear rate, however, differ substantially. The evolution of the flow fields is monitored and it is shown that an initial increase in vortex size is followed by a slower decrease, this happening at a constant Weissenberg number. At the same Weissenberg number, small scale instabilities start occurring near the office. As the shear rate is increased further, these instabilities grow in size until, eventually, the flow structure is destroyed. An extensional viscosity is evaluated using a modified form of the Binding analysis for contraction flows and we show that the results are not only in qualitative agreement with those from other groups, but also that the analysis is able to predict exactly the onset of the aforementioned flow instabilities.

Dynamic Response of Pressure Sensing Systems in Slip-Flow with Temperature Gradients

Introduction T HE accurate sensing of surface pressures on hypersonic  ight vehicles presents formidable measurement challenges. The hostility of the sensing environment precludes intrusion into the  ow, and measurements must be obtained via sizable lengths of small-diameter pneumatic tubing that connect the surface ports to the remotely located pressure transducers.An idealized pneumatic layout is depicted in Fig. 1. Further complicating the sensing problem is surface boundary layer heating,which causes sizable temperature gradientswithin the pneumatic tubing.Away from the stagnation region, local surface pressure levels,which must be sensed, are extremely low, often less than 0.002 atmospheres (0.2 KPa). The combination of very low pressure levels, small diameter pneumatic lines, and large temperature gradients makes molecular effects no longer negligible. The problem of predicting tube  ow dynamics has been studied extensively. For nonrareŽ ed conditions, Lamb,1 Iberall,2 Schuder and Binder, and Hougen et al., have developed closed-form frequencydomain solutions for simple tubing geometries and constant wall temperatures.Berg and Tijdeman and Tijdeman extended the analyses of Refs. 3 and 4 to develop a recursion formula for complexgeometriesthat consistof cascadesof tubesandvolumes.Parrot and Zorumski7 investigated the dynamic transmission of sound in a simple geometry tube subjected to very large temperature gradients. References 1–7 considered only continuum  ow conditions; rareŽ ed  ow effects were not investigated. Tube  ow for rareŽ ed conditionsat high temperatureswith large temperaturegradientshas been investigated by Maxwell,8 von Knudsen,9 and Tompkins and Wheeler. These rareŽ ed  ow investigations,however, considered only steady- ow conditions. The dynamic in uence of rareŽ ed  ow phenomenaon pneumatic pressure sensing systems has not been generally well understood; consequently, research was initiated at the NASA Dryden Flight Research Center with a primary objective to develop a dynamic response model for pneumatic pressure sensing systems that is applicable to both continuumand rareŽ ed  ows.Whitmore et al.11 presented a detailedanalyticaldevelopmentand empiricalvalidationof one such model. This Note summarizes the information presented in that text.

An upper bound solution for the coating thickness of cylinders in a fluidized bed

This paper presents a solution for the final coating thickness on a solid cylinder under constant object or wall temperature, which will serve as an upper bound to the coating thickness on a solid cylinder.

Effect of mold heat transfer on the curing reaction of epoxy molding compound during transfer molding

The cyclic, transient heat transfer within a transfer molding tool during the EMC (Epoxy Molding Compound) curing stage was analyzed using iterative dual reciprocity boundary element method (DRBEM) and finite difference method (FDM) in a decoupled sequence. It is found that both heat transfer at the mold exterior surface and cyclic, transient variation in mold temperatures are significant and show distinguished influence on the extent of EMC curing. Compared with the presently analyzed results, the simulation based on an assumption of constant mold wall temperature overestimates the conversion profile, whereas the hybrid FESFM (Finite-Element/Shape-Factor Method) underpredicts it around the middle of the cavity surface where the exterior tool transfer effect is less distinguished. However, the FESFM also overestimates the conversion at locations near the mold parting surface where the influence from ambient heat transfer is significant.

Nonaxisymmetric unsteady motion over a rotating disk in the presence of free convection and magnetic field

The nonaxisymmetric unsteady motion produced by a buoyancy-induced cross-flow of an electrically conducting fluid over an infinite rotating disk in a vertical plane and in the presence of an applied magnetic field normal to the disk has been studied. Both constant wall and constant heat flux conditions have been considered. It has been found that if the angular velocity of the disk and the applied magnetic field squared vary inversely as a linear function of time (i.e. as ( 1−λt∗) −1, the governing Navier-Stokes equation and the energy equation admit a locally self-similar solution. The resulting set of ordinary differential equations has been solved using a shooting method with a generalized Newton's correction procedure for guessed boundary conditions. It is observed that in a certain region near the disk the buoyancy induced cross-flow dominates the primary von Karman flow. The shear stresses induced by the cross-flow are found to be more than these of the primary flow and they increase with magnetic parameter or the parameter λ characterizing the unsteadiness. The velocity profiles in the x- and y- directions for the primary flow at any two values of the unsteady parameter λ cross each other towards the edge of the boundary layer. The heat transfer increases with the Prandtl number but reduces with the magnetic parameter.

Unsteady three-dimensional boundary layer flow due to a stretching surface

In this numerical study, the unsteady laminar incompressible boundary-layer flow over a continuously stretching surface has been investigated when the velocity of the stretching surface varies arbitrarily with time. Both the nodal and the saddle point regions of flow have been considered for the analysis. Also, constant wall temperature/concentration and constant heat/mass flux at the stretching surface have been taken into account. The quasilinearisation method with an implicit finite-difference scheme is used in the nodal point region (0≦c≦1) wherec denotes the stretching ratio. This method fails in the saddle point region (−1≦c≦0) due to the occurrence of reverse flow in they-component of velocity. In order to overcome this difficulty, the method of parametric differentiation with an implicit finite-difference scheme is used, where the values atc=0 are taken as starting values. Results have been obtained for the stretching velocities which are accelerating and decelerating with time. Results show that the skin friction, the heat transfer and the mass transfer parameters respond significantly to the time dependent stretching velocities. Suction (A>0) is found to be an important parameter in obtaining convergent solution in the case of the saddle point region of flow. The Prandtl number and the Schmidt number strongly affect the heat and mass transfer of the diffusing species, respectively.

STABILITY ANALYSIS FOR THE PREDICTION OF THE MINIMUM TEMPERATURE OF FILM BOILING

Abstract The stability of film boiling on a vertical, constant temperature wall is analyzed by a steady-state small perturbation method. It was found that large amplifications of the perturbations in the downstream direction are predicted when the vapor Reynolds number falls below a critical value, indicating a great tendency to collapse the vapor film. Using a relatively simple model for the film thickness, the minimum wall temperature required to sustain the vapor film is calculated. The results show a tendency which is in agreement with experimental observation. It is suggested, however, that using a more accurate model for the film thickness can lead to a good prediction of the minimum wall temperature and explain the large effect of liquid subcooling, system pressure, surface tension and other fluid properties.

Mode Characteristics in Chirowaveguides with Constant Impedance Walls

Modal propagation of electromagnetic waves in waveguides filled with chiral media, known as chirowaveguides, is studied. The waveguide walls are assumed to behave as constant impedance surfaces. This can model corrugated surfaces, dielectric coated metallic surfaces, as well as perfectly or imperfectly conducting walls. Conditions leading to modes with pure circular polarization are derived. Under these conditions, a chirowaveguide can act as a low crosspolar feed for reflector antennas.

Mode Characteristics in Chirowaveguides with Constant Impedance Walls

Mode Characteristics in Chirowaveguides with Constant Impedance Walls

Grain-size effects in ferroelectric switching

Abstract We present a model of ferroelectric switching which describes domain growth in crystals composed of a large number of relatively small grains. If the grain boundaries stop or significantly affect domain wall motion then the observed switching transients will deviate from those predicted by the infinite grain model of Ishibashi and Takagi. The model assumes a constant nucleation rate throughout the switching process and constant domain wall velocity. At a characteristic time dependent only on the size of the grains and the domain wall velocity the switching current transient changes from its early-time infinite-grain behavior to an exponential decay. The results are compared with Monte Carlo simulations of switching in the two-dimensional Ising model.

Dielectric anomalies in KH2PO4

Abstract New measurements of the low-frequency dielectric response ec of multidomain KDP in applied field as low as 35 mV/cm show no sign of the threshold field reported in earlier studies. The dependencies of the initial dielectric constant and domain wall width on temperature are correlated and discussed in terms of thermally activated domain motion. The ec data indicate the existence of two regimes of domain dynamics.

Influence of Aperture Height and Width on Interzonal Natural Convection in a Full-Scale Air-Filled Enclosure

The topic of this paper is the influence of aperture height and width on interzonal high Rayleigh number, natural convection heat transfer. Experiments were conducted in an 8 ft. air-filled cube divided into two zones by a vertical partition which was centered between a constant flux hot wall and an isothermal cold wall. The partition was configured to form doorway-like apertures. The aperture height relative to test cell height range from 1/8 to 1 and the aperture width relative to test cell width ranged from 0.009 to 1. The zone-to-zone temperature difference and the overall Nusselt number were determined experimentally, and correlated with the overall Rayleigh number, aperture, and enclosure geometry, using a series resistance model for the enclosure. A turbulent boundary layer resistance was used to represent the hot and cold boundary layer flow, while an orifice resistance was used to represent the aperture flow. For flux Rayleigh numbers between 5*1011 and 5*1012, the enclosure Nusselt numbers ranged between 15 and 165, with a strong dependence on aperture height.

Coefficient of heat transfer during flameless combustion in channels with constant temperature walls

The use of unidimensional equations to define the heat transfer during combustion under conditions of high wall losses, when the temperature of the walls remains unchanged from the initial value (i.e. when combustion occurs in a massive matrix of high conductivity metal) is difficult, due to the indefiniteness of the linear terms. Different methods for determining the average transverse temperature distribution [1-3] lead to different values for the coefficient of heat transfer and this question has not yet been resolved. A theoretical study of this problem, applicable to flameless combustion in flat and cylindrical channels, is now presented.

A feasibility study of stripline-fed slots arranged as a planar array with circular grid and circular boundary

An experimental study of the self-impedance of stripline-fed slots has been undertaken, with slot length and slot offset relative to the strip as parameters. The slot is cut in one wall of a parallel plate waveguide, and locally a cylindrical cavity is created around the slot by two rho =constant walls and two phi =constant walls. Such cavities can be used as modules to build a circular-grid planar array. Anticipating a corporate feed, it is demonstrated that sufficient dynamic range in the self-impedance to overcome the effects of mutual coupling is achievable, thereby opening up the prospect of an efficient circular-grid array for seeker antenna applications that require good sum and difference patterns. >

Mass transport in a converging-diverging nozzle

The local concentration has been determined for a viscous liquid flowing through a converging-diverging tube with constant wall- and initial concentration. The liquid flow was considered as creeping flow and its velocity distribution was determined by solving the biharmonic differential equation of the stream function. The mass transport was presented in form of an infinite series of Legendre functions, which rendered with the Galerkin condition a determinant of finite order for the determination of the eigenvalues. The local concentration was evaluated numerically for differently diverging tubes, of which the case of mass transport for viscous (linearized) flow through an orifice of circular cross-section presented a special case.

Atomization efficiency of a graphite furnace in atomic absorption spectrometry at high heating rates

The efficiency of an HGA-400 for production and containment of atomic vapour was determined for several elements using relatively high heating rates. Two different atomization efficiencies β1 and β2 were experimentally determined by two variations of a method. Assuming that atomization in the HGA-400 can be described by consecutive, first-order reactions [Formula: see text], the method consisted of making the absorbance signal profile (at least the decay part of it) appear at constant tube wall (at the centre) temperature and determining first-order rate constants for atom formation and atom loss, k1 and k2, respectively. Two atomization efficiencies were calculated: β1 and β2, which gave the fraction of analyte which was present as atomic vapour at the peak of the absorbance signal profile. Experimental atomization efficiencies reported for Al, Cu, Co, Fe, Ag, and Ga average: β1, 0.42 and β2 0.30, with a range from 0.30 to 0.50 for β1, and from 0.18 to 0.43 for β2. Significance of the results and limitations of the two variations of the method in terms of validity of their underlying assumptions have been brought out in a critical assessment of the atomization efficiencies measured by the two variations of the method. In both the variations of the method, the heating rates used were much higher than those used by some other authors, with the result that in agreement with the prediction of the theory, the atomization efficiencies measured in this work were considerably higher than those reported by those authors.

Unsteady, three-dimensional, boundary-layer flow due to a stretching surface

In this numerical study, the unsteady laminar incompressible boundary-layer flow over a continuously stretching surface has been investigated when the velocity of the stretching surface varies arbitrarily with time. Both the nodal and the saddle point regions of flow have been considered for the analysis. Also, constant wall temperature/concentration and constant heat/mass flux at the stretching surface have been taken into account. The quasilinearisation method with an implicit finite-difference scheme is used in the nodal point region (0≦c≦1) wherec denotes the stretching ratio. This method fails in the saddle point region (−1≦c≦0) due to the occurrence of reverse flow in they-component of velocity. In order to overcome this difficulty, the method of parametric differentiation with an implicit finite-difference scheme is used, where the values atc=0 are taken as starting values. Results have been obtained for the stretching velocities which are accelerating and decelerating with time. Results show that the skin friction, the heat transfer and the mass transfer parameters respond significantly to the time dependent stretching velocities. Suction (A>0) is found to be an important parameter in obtaining convergent solution in the case of the saddle point region of flow. The Prandtl number and the Schmidt number strongly affect the heat and mass transfer of the diffusing species, respectively.

Laminar Combined Convection in a Horizontal Annulus Subject to Constant Heat Flux Inner Wall and Adiabatic Outer Wall

Steady-state, fully developed velocity and temperature fields in mixed convection through a horizontal annulus (ratio of outside to inside radii of 1.25), with a prescribed constant heat flux on the inner cylinder and an adiabatic outside cylinder are analyzed using finite difference approximations. The effects of the buoyancy-driven lateral flow on the temperature of the inner surface are studied in detail. The results show that, as the buoyancy potential (Rayleigh number) increases, the lateral flow structure changes from one cell (on each side) to two cells. The consequence of these flow regimes is that as Rayleigh number increases the temperature of the upper portion of the inner cylinder first increases significantly above its value for pure forced convection and then decreases significantly as the number of cells increases. The average temperature of the inner cylinder decreases monotonically as the Rayleigh number increases.