Doppler flow meter

Doppler Flow Meters. Doppler flow meters sense the shift in apparent frequency of an ultrasonic beam as it is reflected from air bubbles or other acoustically reflective particles that ate moving in a Hquid flow. It is essential for operation that at least some particles ate present, but the concentration can be low and the particles as small as ca 40 p.m. CaUbration tends to be influenced by particle concentration because higher concentrations result in mote reflections taking place neat the wall, in the low velocity portion of the flow profile. One method used to minimize this effect is to have separate transmitting and receiving transducers focused to receive reflections from an intercept zone neat the center of the pipe. 

Laser Doppler Velocimeters. Laser Doppler flow meters have been developed to measure Hquid or gas velocities in both open and closed conduits. Velocity is measured by detecting the frequency shift in the light scattered by natural or added contaminant particles in the flow. Operation is conceptually analogous to the Doppler ultrasonic meters. Laser Doppler meters can be appHed to very low flows and have the advantage of sensing at a distance, without mechanical contact or interaction. The technique has greatest appHcation in open-flow studies such as the deterrnination of engine exhaust velocities and ship wake characteristics. 

Matsuno et al. (1991) reports a method to induce thrombosis in the rat femoral artery by means of a photochemical reaction after injection of a fluorescent dye (rose Bengal, 10 mg/kg i.v.) and transillumination with a filtered xenon lamp (wave length 540 nm). Blood flow is monitored by a pulsed Doppler flow meter. Occlusion is achieved after approximately 5-6 min. Pretreatment with heparin dose-dependently prolongs the time required to interrupt the blood flow. The model also enables the study of thrombolytic mechanisms, which had been evaluated with t-PA. A comparative data for hirudin in various models was carried out by Just et al. (1991). 

The Doppler flow meter sends out sound waves similar to the time-of-flight meter, but the waves are reflected back to a detector by small particles or bubbles moving with the fluid. The sound wave reflected back from the fluid will have a slightly lower frequency than the transmitted sound wave due to the Doppler effect. The difference between transmitted frequency and the reflected frequency is used to determine the flow rate. 

Commercially available Doppler flow meters requires that the liquid contain at least 100 parts per million of 100 micron or larger suspended particles or bubbles [22]. 

Ultrasonic Flow Meters. Ultrasonic flow meters can be divided into three broad groups passive or turbulent noise flow meters, Doppler or frequency-shift flow meters, and transit time flow meters. 

The Doppler effect is used in practice to visualize directional blood flow on ultrasound, to estimate cardiac output and in some types of flow meter. 

Kostiuk et al. [40] measured experimentally the flow field of the vertical co-axial turbulent impinging streams with a two-component Laser Doppler velocity meter. The opposing gas streams were ejected from two burner nozzles, which were designed to produce a uniform axial velocity profile at their exits. The turbulence in the flow was generated by a perforated plate located at the end of the contraction section in each nozzle. The air velocity at the exit of the nozzle was varied from 4.1 to 11.4 m s and... 

Figure 3-24 A Schematic Diagram of In-Line Measurement of Flow Properties with an Ultrasonic Doppler Velocity Meter.

Doppler-shift meters, on the other hand, depend on reflections of the pressure wave from suspended particles or bubbles in the stream, which are assumed to be moving at the velocity of the stream. The pressure wave is projected into the fluid at an angle to the direction of flow. The difference between the frequency of the projected wave and that of the reflected wave is proportional to the fluid velocity. 

Ultrasonic Flowmeters. Ultrasonic methods have been used to measure flow velocity and concentration in slurry pipelines (22) and emulsion pipelines (65). There are three methods of ultrasonic flow meter applications transmission of ultrasonic wave, beam deflection, and frequency shift method (22). The frequency shift method (the ultrasonic Doppler flowmeter) consists of a transducer and an electronic control box. The transducer is either clamped on the outside of the pipe or inserted into the pipe so that it is flush with the inside of the pipe wall. The transducer comprises the sensors to transmit and receive the Doppler signal. These sensors are either in a single transducer or in two separate transducers. The control box processes transmitted and received signals (Figure 25). 

Crabtree (2009) detailed most of the flow meters used in industrial plants. His classification for selecting measuring technology with respect to process application is reproduced in Table 6.3. All flow meters are suitable for clean liquids except for the Ultrasonic-Doppler instrument and only electromagnetic instruments are unsuitable for low conductivity fluids. Most instruments are suitable for high temperature operation or application under certain conditions except for the ultrasonic instruments. Many flow meters are suitable for gases. Few instruments can be used for open channel flow or pipes that are semifllled with the exception of weirs and flumes. 

The temperature distribution in the loop is measured with chromel-alumel thermocouples and two Pt-100 temperature sensors for reference measurements. Absolute pressure is measured at the top of the riser and at the inlet of the core. The liquid level in the steam dome is measured with a differential pressure sensor. The differential pressure over the friction settings of the individual channels is a measure for the flow distribution over the coolant channels and bypass channels. The total flow in the loop is measured at 2 different positions with electromagnetic flow meters. The void fraction at a given height can be measured with gamma transmission techniques. At a fixed height at the top of the riser the radial void distribution is measured with a wire-mesh sensor, which measures the conduction of the two-phase mixture on a two-dimensional grid. Furthermore, laser doppler anemometry is used to study the local liquid velocity in the core or in the riser. 

Both wetted-sensor and clamp-on Doppler meters ate available for Hquid service. A straight mn of piping upstream of the meter and a Reynolds number of greater than 10,000 ate generally recommended to ensure a weU-developed flow profile. Doppler meters ate primarily used where stringent accuracy and repeatabiHty ate not requited. Slurry service is an important appHcation area. 

Transit-time flowmeters measure the time taken for an ultrasonic energy pulse to traverse a pipe section both with and against the flow of the liquid within the pipe (Figure 3.97). The flow rate is the difference in transit times. Transit-time flowmeters are widely used in water treatment and chemical plant applications. This type of ultrasonic meter is considerably more expensive than the Doppler version, but it offers better accuracy. Unlike the Doppler meter, it is usable only on relatively clean fluid applications. Its advantages... 

In 1842, Christian Doppler discovered that the wavelength of sound is a function of the receiver s movement. The transmitter of a Doppler flowmeter projects an ultrasonic beam into the flowing stream and detects the reflected frequency, which is shifted in proportion to stream velocity. The difference between the transmitted and reflected velocities is called the beat frequency, and its value relates to the velocity of the reflecting surfaces (solid particles and gas bubbles) in the process stream. For accurate readings it is important that the ultrasonic radiation be reflected from a representative portion of the flow stream. The main advantage of Doppler meters is their low cost, which does not increase with pipe size, whereas their main limitation is that they are not suitable for the measurement of clean fluids or gases. 

Instrument used (measured response) Dia-Stron erythema meter , Minolta Chromameter , Cortex Dermaspectrometer (all measure a redness index of erythema) laser Doppler Velocimeter (blood flow) Servo-Med evaporimeter (transepidermal or skin surface water loss)... 

Ultrasonic meters are of two types transit time and Doppler shift. In the first type a high-frequency pressure wave is beamed at an angle across the pipe. The velocity of the wave is found from its time of transit. When the wave is transmitted in the direction of the flow, its velocity is increased, and vice versa. From the change in transit time from that in a quiescent fluid the fluid velocity can be determined. Transit-time meters are applicable to clean fluids only. 

S.2.2 Ultrasonic Doppler Flowmeter Probe. The ultrasonic Doppler flowmeter (UDF) instrument measures the velocity of a fluid by measuring the Doppler shift in an ultrasonic signal reflected from particles being carried by the fluid (Figure 4-20). This type of meter is normally used for measuring flows in pipes, but when mounted beneath the vessel base can be used to measure... 

Current measurements are usually taken by acoustic Doppler meters, which emit an acoustic signal from either a boat or bottom-mounted transmitter. The signal is reflected from sediment or other particles transported by the flow and recorded. The reflected signal is analyzed to detect the Doppler shift in frequency, yielding a measure of flow velocity in three dimensions. 

A better way of measuring flows than the ordinary orifice plate method is by inducing vortex shedding across a tube in the flowing liquid and then measuring the velocity of the vortices. This is a nice method, as there are no orifice taps to plug. Then there are Doppler meters, which measure the velocity of a fluid based on how the speed of sound is affected by the flow in a pipe. More commonly, we have rotometers, which measure how far a ball or float is lifted in a vertical tube by the velocity of the liquid. But regardless of their relative merits, perhaps simply for historical reasons, the vast majority of flows in most process plants are measured with the orifice plate flowmeter, shown in Fig. 10.7. 

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