Slip velocity/coefficient, measurement

The power dissipation influence on the liquid-phase mass transfer coefficient (/cl) is highly debated in STRs, especially at higher power densities. The slip velocity model and eddy turbulence model have been used to explain mass transfer, but they come to different conclusions with respect to power. The slip velocity model predicts a decrease in mass transfer with increasing power dissipation while the eddy turbulence model predicts an increase. Linek et al. (2004) postulate that the main reason for the confusion stems from the miscalculation of They investigated different measurement methods and models used by others and concluded that the slip velocity models were underestimating and, hence,... 

Due to chain entanglements the self diffusion coefficient of the labeled macromolecules is very small (<10 mVs), and the transport of the photobleached tracers takes place predominantly through convection. If there is no slip at the polymer - prism interface, the fluorescence intensity decreases steadily with time as the concentration pattern is progressively tilted by the shear. If slip occurs, the concentration pattern is translated (and tilted) in front of the interference fringes creating a damped periodic oscillation of the fluorescence intensity with a period T = iA s, if Vs is the average slip velocity within the distance A from the prism surface. Measuring T and i yields directly Vj. 

Measurements have been made on a wide variety of molecules adsorbed on Au, Ag, or Pb surfaces [3,4,131,132]. The phase of the adsorbed layer changes from fluid to crystal as the density is increased. As expected, motion of fluid layers produces viscous dissipation that is, the friction vanishes linearly with the sliding velocity. The only surprise is that the ratio between friction and velocity, called the drag coefficient, is orders of magnitude smaller than would be implied by the conventional no-slip boundary condition. When the layer enters an incommensurate phase, the friction retains the viscous form. Not only does the incommensurate crystal shde without measurable static friction, the drag coefficient is as much as an order of magnitude smaller than for the liquid phase ... 

Thus, we take advantage of the accuracy, robustness and efficiency of the direct problem solution, to tackle the associated inverse heat transfer problem analysis [26, 27] towards the simultaneous estimation of momentum and thermal accommodation coefficients in micro-channel flows with velocity slip and temperature jump. A Bayesian inference approach is adopted in the solution of the identification problem, based on the Monte Carlo Markov Chain method (MCMC) and the Metropolis-Hastings algorithm [28-30]. Only simulated temperature measurements at the external faces of the channel walls, obtained for instance via infrared thermography [30], are used in the inverse analysis in order to demonstrate the capabilities of the proposed approach. A sensitivity analysis allows for the inspection of the identification problem behavior when the external wall Biot number is also included among the parameters to be estimated. 

The effect of slip coefficient on concentration polarisation (CP) was mathematically modeled for flat membrane and tubular membrane systems [12,13,15,16]. Lowering of CP due to slip coefficient as a function of product water recovery ( ) for different normalised diffusion coefficients (a) is shown in Figure 6.8. The data show that CP decreases both with and a. Since a is a measure of particle diffusion from the membrane surface to the bulk solution, slip-flow possibly augments diffusive back-transport of particles from the membrane surface to the bulk solution. Thus, the slip-flow velocity model possibly accounts for higher or actual UF/MF flux, which is under-predicted by the gel polarisation model discussed in Chapter 1. 

When a steady friction regime is reached, the tangential force is measured for different normal load (-50 nN < P < 250 nN). The friction force varies linearly with the normal load but does vanish at zero load. The slope of the curve allows a dynamic coefiRcient of friction to be defined. This friction coefficient is observed to decrease when the velocity increases (Figure 13). This effect is to be linked to the existence of the stick-slip motion 18). 

Variable slip technique is used to measure the friction coefficient between tire and pavement. The test equipment consists of an automotive vehicle. Each test wheel contains a variable brake system. The resulting resistive force caused by friction between the tire and the pavement surface is sampled and recalculated to slip friction numbers. Frictional properties of pavement surface as a function of speed are measured by a d5mamic friction tester. A disk spins with its plane parallel to the test surface. The rubber sliders come into contact with the pavement, and torque is monitored when rotational velocity reduces, due to friction. A graph of friction vs. speed is plotted. 

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