Variations In Mass Transfer Rate Research Articles

Development of a modified thermal phase change model based on the interfacial area concentration determined by an interface reconstruction algorithm

In order to enhance the physical basis of the mass transfer model in numerical simulation of gas-liquid direct contact condensation and improve the fidelity, a set of methods and models are developed. Based on the concept of the piecewise-linear interface calculation (PLIC), an interface reconstruction algorithm is proposed to calculate the interfacial area concentration in three-dimensional non-orthogonal structured grids. The actual interface in the cell is replaced by a plane, whose position is determined iteratively to satisfy the local volume fraction and orientation of the actual interface. The area of the polygonal cut by the plane is taken as the interfacial area in the cell. The proposed algorithm is validated by calculating the surface area of a series of basic geometries. Obvious superiority of the proposed algorithm over the traditional models (gradient model and algebraic model) is seen. Then, the reconstructed area is used to determine the interfacial area concentration in the thermal phase change model. The process is implemented by ANSYS Fluent user-defined functions. The Stefan problem and steam bubble condensation experiment are used to test the performance of the developed methods and models. The simulation results agree well with the theoretical and experimental results with the quantitative relative errors no more than 4.3 % and 15 %, respectively. The oxygen jets condensation experiments are also used as test cases. The primary dominant frequencies predicted by the simulations are 163.01 Hz and 131.65 Hz, which agree well with the 139.72 Hz and 125.75 Hz in the experiments. The relative errors are only 17 % and 4.7 %. The unstable flow pattern of oxygen jet in liquid oxygen crossflow is explained based on the dynamic characteristics of turbulence, heat and mass transfer. The periodic variation of mass transfer rate is considered as the cause of condensation oscillation. The method for calculating the interfacial area proposed in this paper can serve as an optional and practical improvement to the numerical simulation of direct contact condensation.

A spectroscopic analysis of the eclipsing nova-like EC 21178−5417 – discovery of spiral density structures

ABSTRACT We present phase-resolved optical spectroscopy of the eclipsing nova-like cataclysmic variable EC 21178−5417 obtained between 2002 and 2013. The average spectrum of EC 21178−5417 shows broad double-peaked emission lines from He ii 4686 Å (strongest feature) and the Balmer series. The high-excitation feature, C iii/N iii at 4640–4650 Å, is also present and appears broad in emission. A number of other lines, mostly He i, are clearly present in absorption and/or emission. The average spectrum of EC 21178−5417 taken at different months and years shows variability in spectral features, especially in the Balmer lines beyond Hγ, from pure line emission, mixed line absorption, and emission to pure absorption lines. Doppler maps of the He ii 4686 Å emission reveal the presence of a highly inclined asymmetric accretion disc and a two spiral arm-like structure, whereas that of the Balmer lines (Hα and Hβ) reveal a more circular accretion disc. There is no evidence of a bright-spot in the Doppler maps of EC 21178−5417 and no emission from the secondary star is seen in the tomograms of the He ii 4686 Å and Balmer lines. Generally, the emission in EC 21178−5417 is dominated by emission from the accretion disc. We conclude that EC 21178−5417 is a member of the RW Tri or UX UMa subtype of nova-like variables based on these results and because it shows different spectral characteristics at different dates. This spectral behaviour suggests that EC 21178−5417 undergoes distinct variations in mass transfer rate on the observed time-scales of months and years.

Delayed outburst of H 1743–322 in 2003 and relation with its other outbursts

The Galactic transient black hole candidate H 1743–322 exhibited a long duration outburst in 2003 after more than two and a half decades of inactivity. The 2003 event was extensively studied in multi-wavelength bands by many groups. The striking feature is that the total energy released is extremely high as compared to that in tens of outbursts which followed. In this paper, we look at this event and study both the spectral and temporal properties of the source using two component advective flow (TCAF) paradigm. We extract accretion flow parameters for each observation from spectral properties of the decay phase and determine the mass of the black hole. We computed the energy released during all the known outbursts since 2003 and showed that on an average, the energy release in an outburst is proportional to the duration of the quiescent state just prior to it, with the exception of the 2004 outburst. A constant rate of supply of matter from the companion cannot explain the energy release in 2004 outburst. However, if the energy release of 2003 is incomplete and the leftover is released in 2004, then the companion’s rate of matter supply can be constant since 1977 till date. We believe that erratic behaviour of viscosity at the accumulation radius Xp of matter as well as location the Xp itself, rather than the random variation of mass transfer rate from the companion, could be responsible for non-uniformity in outburst pattern. We discuss several factors on which the waiting time and duration of the next outburst could depend.

ULTRAVIOLET SPECTROSCOPY OF PQ Gem AND V405 Aur FROM THEHSTANDIUESATELLITES

Ultraviolet spectra of two intermediate polars (IPs), PQ Gem and V405 Aur, observed with Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph and Faint Object Spectrograph and International Ultraviolet Explorer (IUE) satellites were analyzed during the period between 1994–2000. We estimated the reddening of the two systems from the 2200 A feature. Six spectra of the two systems revealing modulations of line fluxes at different times are presented. PQ Gem and V405 Aur are featured by spectral lines in different ionization states. This paper focuses on the third ionized carbon emission line at 1550 A and the first ionized helium emission line at 1640 A produced in the optically thin outer region of the accretion curtain for the two systems by calculating spectral line fluxes. From HST and IUE data, we deduced ultraviolet luminosities and ultraviolet accretion rates for the two binary stars. The average temperature of the accretion streams for PQ Gem and V405 Aur are ~4500 K and 4100 K, respectively. The results reveal that there are modulations in fluxes of spectral lines, ultraviolet luminosities, and ultraviolet accretion rates with time for both systems. These modulations are referred to the changes of both density and temperature as a result of the variations of mass transfer rate from the secondary star to the primary star. The current results are consistent with an accretion curtain model for IPs.

An experimental investigation of surface modification of C-40 steel using W–Cu powder metallurgy sintered compact tools in EDM

Surface modification is essential to enhance the surface properties of engineering components. This may be accomplished either in the form of altering the surface chemistry or by providing a protective layer over the work surface. In this paper, the surface modification phenomenon by depositing a protective layer over the work surface by electrical discharge machining (EDM) is presented. The potential of EDM, which is otherwise a useful non-conventional machining process, has been explored for surface alteration by depositing material over work surface using tungsten–copper (W–Cu) sintered powder metallurgy tools. The photographic presentation of the EDMed surface at different parameter settings is given. The variations of mass transfer rate (MTR), deposited layer thickness (LT), and average surface roughness (R a) with various parameter combinations are presented in graphical form and their effects are discussed. A wide spectrum of MTR ranging from nearly 1 to 191 mg/min and average surface roughness values ranging from 3 to 15 μm have been achieved. A wide range of deposited layers with thickness varying from 3 to 785 μm has been achieved by various combinations of process parameters. The microstructure of the deposited layers with microhardness at different zones is presented. It has been observed that the microhardness is gradually increasing from the base material to the deposited layer and its maximum value is found to be 15.7 GPa at the hardest zone. SEM, EDX, and XRD analyses has been also performed for further characterization of the deposited layer. A quantitative analysis of the layer has been carried out by EDX and it is found that the inner part of the layer is richer in tungsten than the superficial surface. This contributed towards the higher hardness of the layer at the core.

Circumbinary Disks and Cataclysmic Variable Evolution

The influence of a circumbinary (CB) disk on the evolution of cataclysmic variable (CV) binary systems is investigated. We show that CB mass surface densities sufficient to influence the evolution rate are plausibly provided by the outflows observed in CVs, if the net effect of these winds is to deliver 10-4 to 10-3 of the mass transfer rate to the CB disk. The torque exerted by the CB disk provides a positive feedback between mass transfer rate and CB disk mass that can lead to mass transfer rates of ~10-8 to 10-7 M☉ yr-1. This mechanism may be responsible for causing the range of variation of mass transfer rates in CVs. In particular, it may explain rates inferred for the nova-like variables and the supersoft X-ray binary systems observed near the upper edge of the period gap (P ~ 3-4 hr), as well as the spread in mass transfer rates above and below the period gap. Consequences and the possible observability of such disks are discussed.

Limiting Current Density on a Line Electrode in Impinging Slot-Jet Flows

The mass-transfer limited current density on a line electrode in impinging slot-jet flows is investigated experimentally. Results are compared with numerical simulations. The feasibility of using computational fluid dynamics as a practical tool for predicting mass-transfer rates in this geometry is evaluated. Flow rates varying by up to a factor of 200 are considered, with the low end flow rates in the laminar flow regime (nozzle-width-based Reynolds numbers, Re, starting at 220) and the high end flows in the high Reynolds number turbulent flow regime, The mass-transfer measurements were conducted using 100 μm platinum electrodes. The use of a small, isolated line electrode makes it possible to resolve partially the sharp variations in mass-transfer rate in the stagnation region. © 2001 The Electrochemical Society. All rights reserved.

An accretion model for the eclipsing cataclysmic variable PG 0859 + 415

The emission lines found in the majority of cataclysmic variables are generally used as tracers of accretion flows which dominate the light at optical wavelengths. It has been suggested from previous observations that the eclipsing nova-like variable PG 0859 + 415 shows individualistic orbital characteristics that are inconsistent with canonical models of stellar accretion. We present spectrophotometry of this star which suggests that the standard picture is not in conflict with observations. We provide evidence that the shallow optical eclipses are of an extended bright spot rather than of the accretion disc, and that the low-excitation lines are dominated by a transient absorption component, perhaps a result of the bright spot or the accretion stream eclipsing the disc. We argue that this object may be another nova-like variable displaying long-term or cyclic variations in mass transfer rate from the secondary star.

Analysis of wall shear stress probes in large amplitude unsteady flows

The measurement of the mass transfer rate to flush mounted wall electrodes is a commonly used method to determine the wall shear stress. An analysis for the performance of these probes in the presence of large amplitude unsteady flows is presented. This is accomplished by using numerical solutions of the direct (the variation of the wall shear stress with time is given) and the inverse problem (the variation of mass transfer rate with time is given). For cases in which flow reversal occurs additional information about the flow direction is needed.

Numerical calculations of mass transfer flow in semi-detached binary systems

The details of the mass transfer flow near the inner Lagrangian point in a semidetached binary system are numerically calculated. A polytropic equation of state with n = 3/2 is used. The dependence of the mass transfer rate on the degree to which the star overfills its Roche lobe is calculated, and good agreement with previous analytic estimates is found. The variation of mass transfer rate which occurs if the binary system has a small eccentricity is calculated and is used to cast doubt on the model for superhumps in dwarf novae proposed by Papaloizou and Pringle (1979).

A non-eclipsing binary model of the symbiotic star AG Dra

The light variation of AG Dra is considered as a result of a variation of free-free emission from a gaseous envelope around the hot component. The emission measure of the envelope at a light maximum is estimated to be about Ne2V = 6.5 × 1059 cm−3. The amplitude of the light variation in the U band should be less than 1.4 mag. which is consistent with the observed results. The emission measure of the envelope might vary according to a variation of mass transfer rate in an elliptical binary system.

The Turbulent Boundary Layer With Mass Transfer and Pressure Gradient

An analysis of the incompressible, turbulent boundary layer, including the combined effects of mass transfer and pressure gradient is presented in this paper. An integral method employing the integral mechanical energy equation forms the basis of the analysis. Stevenson’s velocity profiles are used to obtain the functional dependence of the integral properties and also obtain a skin-friction law. A definition of an equilibrium flow with mass transfer and pressure gradient is given in order to evaluate the dissipation integral (CD) which appears in the integral mechanical energy equation. This definition requires a pressure gradient parameter similar to Clauser’s βT with a modification to include the effect of mass transfer to be held constant. An expression for CD in the case of equilibrium turbulent flows is then obtained which depends directly on this new pressure gradient parameter (βT*). In order to treat the general case of nonequilibrium flows, this expression for CD is uncoupled from βT*, through the use of a single empirical curve fit of the existing no mass transfer equilibrium flow data relating βT to the Clauser shape parameter. In addition to unhooking CD from the pressure gradient parameter, several specified variations in mass transfer rate are assumed in order to obtain an expression for CD which is not a function of the mass transfer rate derivative. The numerical results are found to be weakly dependent on which of these variations is used. Comparisons of the numerical results with a wide variety of experimental data, including cases where the blowing rate and pressure are varying simultaneously, show good agreement. In addition, several problems with discontinuities in blowing or suction are solved and seem to be in good agreement with the data.