Hot wire anemometry

H. H. Bruun. Hot-Wire Anemometry. New Y ork Oxford University Press, 1995. 

Hsu, Y. Y, F. F. Simon, and R. W. Graham, 1963, Application of Hot-Wire Anemometry for Two-Phase Flow Measurements Such as Void-Fraction and Slip Velocity, pp. 26-34, ASME Symp. on Multiphase Flow, Philadelphia, PA. (3)... 

In homogeneous turbulence, the one-point joint velocity PDF can be written as /u(V t), and can be readily measured using hot-wire anemometry or laser Doppler velocimetry (LDV). 

The soot formation and its control was studied in an annular diffusion flame using laser diagnostics and hot wire anemometry [17, 18]. Air and fuel were independently acoustically forced. The forcing altered the mean and turbulent flow field and introduced coherent vortices into the flow. This allowed complete control of fuel injection into the incipient vortex shedding process. The experiments showed that soot formation in the flame was controlled by changing the timing of fuel injection relative to air vortex roll-up. When fuel was injected into a fully developed vortex, islands of unmixed fuel inside the air-vortex core led to... 

The quantities pvy vt are the Reynolds stresses and have been extensively studied by hot-wire anemometry techniques (81, G7). The other turbulent fluxes have not yet received much attention. 

The foregoing analysis of free-convection heat transfer on a vertical flat plate is the simplest case that may be treated mathematically, and it has served to introduce the new dimensionless variable, the Grashof number, which is important in all free-convection problems. But as in some forced-convection problems, experimental measurements must be relied upon to obtain relations for heat transfer in other circumstances. These circumstances are usually those in which it is difficult to predict temperature and velocity profiles analytically. Turbulent free convection is an important example, just as is turbulent forced convection, of a problem area in which experimental data are necessary however, the problem is more acute with free-convection flow systems than with forced-convection systems because the velocities are usually so small that they are very difficult to measure. Despite the experimental difficulties, velocity measurements have been performed using hydrogen-bubble techniques [26], hot-wire anemometry [28], and quartz-fiber anemometers. Temperature field measurements have been obtained through the use of the Zehnder-Mach interferometer. The laser anemometer [29] is particularly useful for free-convection measurements because it does not disturb the flow field. 

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. 

Lomas, G. G. Fundamentals of Hot Wire Anemometry. New York Gambridge University Press, 1986. 

Once the basic scheme for hot-wire anemometry was worked out, numerous variants on it were developed [6]. Generally, hot-wire anemometers are delicate, temperamental instruments which require expensive electronic circuitry and considerable care and training to use. 

A. E. Perry, Hot-Wire Anemometry, Clarendon Press, Oxford, 1982. 

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. 

King, L.V., 1914. On the convection of heat from small cylinders in a stream of fluid, with applications to hot-wire anemometry. Philosophical Transactions of the Royal Society of London 214 (14), 373M33. 

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,... 

To be able to design devices based on microfluidics and nanofluidics, it is crucial to quantitatively visualize the flow of fluids in the microfluidic and nanofluidic channels. There have been many flow visualization methods being developed for macroscale fluid flow (e.g., hot-wire anemometry), but most of them are not suitable for micro- and nanoscale measurements because they are too intrusive for micro- and nanoscale fluid flows [3]. Fluorescence measurements are very suitable for quantitatively visualizing flow in micro- and nanoscales, because it is nonintrusive and it allows for measurements with a high spatial resolution. 

Micro- and Nanoscale Anemometry Implication for Biomedical Applications, Fig. 2 The operating principle of hot-wire anemometry. The electrical current is passed to the hot wire via the electrodes. The changes in resistance of the hot wire in response to the fluid flow are calibrated for temperature, flow rate, and shear stress... 

Cardiovascular disease, namely, coronary artery disease, remains the leading cause of death in the developed nations. Over the last few years, MEMS sensors have advanced the understanding of blood flow, namely, fluid shear stress, in arterial circulation. Fluid shear stress is defined as the frictional force acting tangentially on the surface of a blood vessel wall. Furthermore, the measurement of wall shear stress is important to study the durability of prosthetic valves and to monitor thrombosis or blood clots in cardiopulmonary bypass machines, artificial hearts, and left ventricular assist devices. Luminal shear stress measurement predicts the development of atherosclerotic plaque in patients at risk for acute heart attacks. In this context, the application of microscale hot-wire anemometry bridges fluid mechanics of blood flow with vascular biology. 

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