Anemometry thermal

Measurement of the velocity of a large particle. The investigation of the turbulence characteristics in the liquid phase of a bubbly flow has generated detailed studies on the use of thermal anemometry and optical anemometry in gas-liquid two-phase flows. These techniques have been proved to be accurate and reliable for the measurement of the instantaneous liquid velocity in bubble flow. However, the velocity of the gas bubbles—or, more precisely, the speed of displacement of the gas-liquid interfaces—is still an active research area. Three techniques that have been proposed to achieve such measurement were reviewed by Delhaye (1986), as discussed in the following paragraphs. 

The first test case, useful for future comparison, involved the measurements without any obstructions. That is, a typical boundary layer over a relatively smooth surface (the wind tunnel ground plane covered with plywood) was studied. No unexpected phenomena were found, thereby validating the thermal anemometry technique and acquisition algorithms employed, for flows with a predominant wind direction. For a freestream speed of / , the time-averaged velocity and turbulence intensity distributions are shown in Fig. 3.39,(A). These data allow the determination of the boundary... 

M. Chirtoc. and J.F. Hemy, 3co hot wire method for micro-heat transfer measurements From anemometry to scaiming thermal microscopy (SThM), Europ. Phys. J., Special Topics, 153, 343-348 (2008). 

The concept of hot wire anemometry is similar to that of thermal mass flow meters as well as the Pirani gauge to measure pressure a fine wire is placed in a flow stream and then heated electrically. The heat transfer rate from the fluid to the wire equals the rate heat is generated by the wire. 

In this Chapter we have made an attempt to describe mixing and heat and mass transfer in supercritical fluids. Using Laser Doppler Anemometry, Computational Fluid Dynamics and High-Pressure Calorimetry, some basic guidelines have been derived. When compared to the behavior of ordinary liquids, the behavior of SCCO2 is quite different, especially near the critical point. However, when the thermodynamic behavior of CO2 is taken into account (in terms of constant-pressure heat capacity, viscosity, and thermal conductivity), its behavior is consistent with that of other liquids. 

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,... 

The indirect methods include ultrasonic Doppler, magnetic resonance imaging (MRI), and thermal anemometry, which measure the shear stress indirectly by correlating this parameter to flow rate. The ultrasonic Doppler method and MRI are noninvasive however, the spatial resolution near the wall is not as precise. Another indirect method is to correlate shear stress with convective cooling of a heated element based on the heat transfer principle. The advent of MEMS technology has elevated the significance of microfluidics in biomedical applications. 

Micro- and Nanoscale Anemometay Implication for Biomedical Applications, Figure 3 Two-dimensional physics model for MEMS/nanoscale anemometry. The fluid flow is denoted in the x-direction. At steady state, a parabolic veiocity profile reflects the fully developed flow. The heat gener-ating/sensing element is denoted in a red rectangular block which is heated by the electric cunent to form a thermal boundary layer... 

Stainback PC, Nagabushana KA (1996) Review of Hot-Wire Anemometry Techniques and the Range of their Applicability for Various Hows. Electron J Huids Eng, Transaction of the ASME King LV (1914) Phil. Trans R Soc, London 4. Liu CJ, Huang J, Zhu Z, Jiang F, Tung S, Tai YC, Ho CM (1999) A Micromachined How Shear-stress Sensor Based on Thermal Transfer Principles. J MEMS 8(l) 90-99 Sheplak M, et al (2002) Characterization of a silicon-micro-machined thermal shear-stress sensor. AIAA J 40(6) 1099-1104 Hsiai TC, SK Wong P, Ing M, Salazar A, Hama S, Navab M, Demer L, Ho CM (2003) Monocyte Recruitment to Endothelial Cells in Response to Oscillatory Shear Stress. EASED J 17 1648-1657... 

RTDs are used in a great variety of applications. At one extreme, carefully-designed platinum RTDs are used in the very definition of temperature between 13.8033 K and 961.78 °C [2], because of their potential for extreme stability and accuracy (uncertainties of better than 1 mK). At a more practical level, commercial RTDs are widely used for industrial and research applications from 14 K to around 600 - 700 C, with typical absolute accuracies of around 0.15 K to 2 K uncalibrated, or 0.1 K or better after calibration. Bare platinum RTDs also have a long and illustrious history as velocity sensors for fluid flow, a technique known as hot-wire anemometry. More recently, special RTD configurations have also been developed to measure the thermal properties of fluids and solids [7-9]. 

A direct measurement of the current field in the area of the droplet, for example, with thermal anemometry was not possible. For this purpose, the gas velocity arotmd the droplets in the levitator was simulated with a k-e-model using CFD simulations (Ansys Huent) [23], These simulations were carried out with different reflector geometries (see Fig. 4.8) in order to investigate their influence on the flow characteristics. 

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