Pulsate flows

Flow which fluctuates with time, such as pulsating flow in arteries, is more difficult to experimentally quantify than steady-state motion because phase encoding of spatial coordinate(s) and/or velocity requires the acquisition of a series of transients. Then a different velocity is detected in each transient. Hence the phase-twist caused by the motion in the presence of magnetic field gradients varies from transient to transient. However if the motion is periodic, e.g., v(r,t)=VQsin (n t +( )q] with a spatially varying amplitude Vq=Vq(/-), a pulsation frequency co =co (r) and an arbitrary phase ( )q, the phase modulation of the acquired data set is described as follows ... 

The effect of pulsating flow on pitot-tube accuracy is treated by Ower et al., op. cit., pp. 310-312. For sinusoidal velocity fluctuations, the ratio of indicated velocity to actual mean velocity is given by the factor /l + AV2, where X is the velocity excursion as a fraction of the mean velocity. Thus, the indicated velocity would be about 6 percent high for velocity fluctuations of 50 percent, and pulsations greater than 20 percent should be damped to avoid errors greater than 1 percent. Tne error increases as the frequency of flow oscillations approaches the natural frequency of the pitot tube and the density of the measuring fluid approaches the density of the process fluid [see Horlock and Daneshyar, y. Mech. Eng. Sci, 15, 144-152 (1973)]. 

Reciprocating compressors deliver a pulsating flow of gas that can damage downstream equipment or machinery. This is sometimes a disadvantage, but pulsation dampers can be used to alleviate the problem. 

The hot-wire anemometer is very accurate even for very low rates of flow. It is one of the most convenient instruments for the measurement of the flow of gases at low velocities accurate readings are obtained for velocities down to about 0.03 m/s. If the ammeter has a high natural frequency, pulsating flows can be measured. Platinum wire is commonly used. 

Correlations similar to these are available or can be obtained for flow in beds of porous and/or adsorbing solids, in coiled tubes, in flexible channels, for pulsating flow, for non-Newtonians, and so on. These are given in Chapter 64 of Levenspiel (1996). 

Other Flow Disturbances Other examples of deviations from fully developed, single-phase newtonian flow include nonnewtonian flow, pulsating flow, cavitation, multiphase flow, boundary layer flows, and nonisothermal flows. See Sec. 6. 

In a previous paper (16) the hardware design of the SEC/Viscometer system used in this work has been described. The effects of operational variables, e.g. pump pulsations, flow rate and flow irregularities on the performance of the viscometer and the enhancement of the signal-to-noise ratio by using mechanical dampers also has been described. Our current work has focused on the development, implementation and application of automated data... 

The most sensitive detector is the differential UV photometer, which is appropriate for a polymer with a significant UV absorbance with a nonabsorbing eluent. This detector is not appreciably affected by flow pulsations, flow rate changes, and temperature fluctuations. 

Figure 10. Feedback flow control system for high-speed pump with pulsating flow...

Nevertheless the radial-turbo-compressors demonstrate significant advantages with respect to compactness, space requirements, quieter operation, and lower level of vibrations, as well as giving a smooth non-pulsating flow. Their application for high-pressures in the range of above 5000 N/m3 intake volume, is well established up to pressures of 400 bar and more. 

Diaphragm pumps [Fig. 7.12(i)] also produce pulsating flow. They are applied for small flow rates, less than 100 gpm or so, often for metering service. Their utility in such applications overbalances the drawback of their intrinsic low efficiencies, of the order of 20%. 

Turnock, P. H., "The Separation of Nitrogen and Methane by Pulsating Flow Through a Fixed, Molecular Sieve Bed" Ph.D. Thesis, Univ. of Michigan, Dept, of Chemical and Metallurgical Engineering Ann Arbor, MI, 1968. 

Beitler, S. R., Lindahl, E. J., and McNichols, H. B., Developments in the Measuring of Pulsating Flows with Inferential-Head Meters, Trans. ASME 65 337, 1943. 

Figure 3.6 Cam-driven, dual-head reciprocating-piston pump capable of delivering constant flow with relatively low pulsation. Flow rate is controlled by the cam rotation frequency. (Reprinted from Ref. 3 with permission.)...

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