Constant temperature anemometry

Fluid Metering, Fig. 2 Example of a MEMS structure for metering the flow rate in a microchannel, based on the theory of constant-temperature anemometry... 

Hot wire anemometry (HWA) or constant temperature anemometry (CTA), laser Doppler anemometry (LDA) or laser Doppler velocimetry (LDV), and particle image velocimetry (PIV) are currently the most commonly used and commercially available diagnostic techniques to measure fluid flow velocity. The great majority of the HWA systems in use employ the constant temperature anemometry (CTA) implementation. A quick comparison of the key transducer properties of each technique is shown in Table 4-1, with expanded details on spatial resolution, temporal resolution, and calibration provided in the following sections. 

For experimental determination the flow pattern produced by the stirrer was initially visualized using different photographic methods (e.g. [574, 497]), but hydraulic probes were also used to determine the pressure distribution (e.g. [135]) and velocity distribution (e.g. [437]). Also, convection probes (spherical probes) and pressure probes (Prandtl s Pitot tube) were used. Later constant temperature hotwire/hot-film anemometry was used. Currently contactless laser doppler velocim-ctry (LDV)/anemometry (LDA) is exclusively utilized. 

Hubbard, P.G. (1954). Constant-temperature hot-wire anemometry with appheation to measurements in water. PhD Thesis. Iowa State University Iowa lA. 

When the sensor is heated and fluid flows through the sensor, heat will be taken away downstream. We can use this effect to measure the fluid flow. One application of the thermal effect is similar to the hot-wire anemometry used to measure wind velocity. Let us imagine that the sensor is made of tungsten wire, which will produce heat when electric power is supplied. If we try to keep the wire at a constant temperature, a higher current is needed for a faster fluid velocity. When thermal balance is reached, the heat generated by the electric power equals that dissipated owing to fluid flow. Therefore,... 

ABSTRACT. Characteristics and fluid dynamics of gas phase recirculation in a novel Riser Simulator Reactor have been investigated using constant temperature hot wire anemometry. In situ concentration and velocity measurements enabled to evaluate the mixing time and the inner recirculation ratio of the gas phase. In addition, fibre optic techniques allowed to characterize the degree of fluidization of the catalyst particles and the effect of gas phase density changes. By combining the anemometry and the fibre optic techniques, mixing patterns in the Riser Simulator have been evaluated. The importance of the study can be realized in the context of the potential use of the Riser Simulator for gas-solid reaction kinetics. 

Cold-flow velocity field measurements were made, employing hot-wire anemometry to determine the flow characteristics without the influence of combustion. With this diagnostic technique, fluid velocity is measured by sensing the changes in heat transfer from a small, electrically heated sensor exposed to the fluid motion. Under conditions in which fluid temperature, composition, and pressure are constant, the dominant mechanism of heat transfer from the wire is through forced convection, which is directly related to the velocity of the fluid. By measuring the voltage required to maintain the desired hot-wire temperature, the fluid velocity can be accurately determined. 

Temperature compensated anemometry was used for the gas velocity measurements. By sensing the changes in heat transfer from a heated sensor exposed to the fluid motion, thermaJ anemometers measure fluid velocity in terms of voltage. Measurement at conditions should be such that fluid temperature and composition are constant. The system consists of a sensor and a control box which supplies current to keep the temperature constant around the sensing... 

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