Abstract:Although thermal anemometry has been employed for the simultaneous measurement of velocity and gas concentration, the underlying theory and techniques to do so are not well established. To rectify this situation, the present work uses theory and experiments to investigate the design of thermal-anemometry-based probes capable of making such measurements, specifically focusing on those for use in helium/air mixtures. It is demonstrated that an interference probe, which consists of two hotwires placed close enoug… Show more
“…A detailed description of the entire flow facility, including the coaxial jet apparatus and equipment employed to calibrate measurement probes, is given in Hewes (2021). As the calibration equipment is also described in Hewes & Mydlarski (2021 a , b ), only the details of the coaxial jet apparatus are summarized herein. Figure 2.Schematic of the coaxial jet apparatus and 3-wire thermal-anemometry-based probe.…”
Section: Experimental Apparatusmentioning
confidence: 99%
“…In this configuration, the behaviour of the downstream wire is strongly influenced by the thermal field of the upstream wire, and responds differently to changes in the flow's velocity or concentration. As explained in Hewes & Mydlarski (2021 a ), this makes it possible to simultaneously measure the aforementioned quantities in isothermal flows. The cold-wire thermometer is placed approximately 1 mm away from the interference probe, and consists of a 0.63 m diameter platinum wire with a length-to-diameter ratio () of approximately 800.…”
Section: Experimental Apparatusmentioning
confidence: 99%
“…Furthermore, values of the mean concentrations in the comparable flows of air were subtracted from these data to improve the accuracy of the measurements. The resulting filtered concentration () was finally used to calculate the axial velocity by applying King's law to the normalized upstream wire voltage of the interference probe as follows: Derivations and/or justifications of (3.1), (3.2) and (3.3), as well as post-processing methods, are provided in Hewes (2021), Hewes & Mydlarski (2021 a ) and Hewes & Mydlarski (2021 b ). Uncertainties and errors associated with the flow facility, instrumentation and data acquisition are discussed in Appendix A.…”
Section: Experimental Apparatusmentioning
confidence: 99%
“…2020). Additional information on the calibration procedures may be found in Hewes & Mydlarski (2021 a , b ). Data acquisition and post-processing methods for the 3-wire probe are explained in the following section.…”
Although natural and industrial flows often transport and mix multiple scalars, relatively few studies of turbulent multi-scalar mixing have been undertaken. In the present work, a novel three-wire thermal-anemometry-based probe – capable of simultaneously measuring velocity, helium concentration and temperature – is used to investigate the evolution of multiple scalars (
$\phi _1$
,
$\phi _2$
,
$\phi _3$
) and velocity in turbulent coaxial jets. The jets consist of (i) a centre jet containing a mixture of helium and air (
$\phi _1$
), (ii) an annular jet containing pure (unheated) air (
$\phi _2$
) and (iii) a coflow of (pure) heated air (
$\phi _3$
). Axial measurements are made at three different momentum flux ratios (
$M=0.77, 2.1, 4.2$
). Increasing
$M$
was observed to result in complex, competing effects. Larger momentum flux ratios cause the potential core of the centre jet to decrease in size, greater scalar fluctuations and more rapid correlation of
$\phi _1$
and
$\phi _2$
. However, at the same time, certain statistics, including those describing the velocity field, evolve more slowly. Moreover, the flow near the beginning of the fully merged region appears to be less mixed at higher values of
$M$
. The present work finally demonstrates that differences can be observed in the evolution and mixing of coaxial jets between those in which
$M<1$
and those in which
$M>1$
, thereby presenting an opportunity by which the mixing process in coaxial jets may be controlled.
“…A detailed description of the entire flow facility, including the coaxial jet apparatus and equipment employed to calibrate measurement probes, is given in Hewes (2021). As the calibration equipment is also described in Hewes & Mydlarski (2021 a , b ), only the details of the coaxial jet apparatus are summarized herein. Figure 2.Schematic of the coaxial jet apparatus and 3-wire thermal-anemometry-based probe.…”
Section: Experimental Apparatusmentioning
confidence: 99%
“…In this configuration, the behaviour of the downstream wire is strongly influenced by the thermal field of the upstream wire, and responds differently to changes in the flow's velocity or concentration. As explained in Hewes & Mydlarski (2021 a ), this makes it possible to simultaneously measure the aforementioned quantities in isothermal flows. The cold-wire thermometer is placed approximately 1 mm away from the interference probe, and consists of a 0.63 m diameter platinum wire with a length-to-diameter ratio () of approximately 800.…”
Section: Experimental Apparatusmentioning
confidence: 99%
“…Furthermore, values of the mean concentrations in the comparable flows of air were subtracted from these data to improve the accuracy of the measurements. The resulting filtered concentration () was finally used to calculate the axial velocity by applying King's law to the normalized upstream wire voltage of the interference probe as follows: Derivations and/or justifications of (3.1), (3.2) and (3.3), as well as post-processing methods, are provided in Hewes (2021), Hewes & Mydlarski (2021 a ) and Hewes & Mydlarski (2021 b ). Uncertainties and errors associated with the flow facility, instrumentation and data acquisition are discussed in Appendix A.…”
Section: Experimental Apparatusmentioning
confidence: 99%
“…2020). Additional information on the calibration procedures may be found in Hewes & Mydlarski (2021 a , b ). Data acquisition and post-processing methods for the 3-wire probe are explained in the following section.…”
Although natural and industrial flows often transport and mix multiple scalars, relatively few studies of turbulent multi-scalar mixing have been undertaken. In the present work, a novel three-wire thermal-anemometry-based probe – capable of simultaneously measuring velocity, helium concentration and temperature – is used to investigate the evolution of multiple scalars (
$\phi _1$
,
$\phi _2$
,
$\phi _3$
) and velocity in turbulent coaxial jets. The jets consist of (i) a centre jet containing a mixture of helium and air (
$\phi _1$
), (ii) an annular jet containing pure (unheated) air (
$\phi _2$
) and (iii) a coflow of (pure) heated air (
$\phi _3$
). Axial measurements are made at three different momentum flux ratios (
$M=0.77, 2.1, 4.2$
). Increasing
$M$
was observed to result in complex, competing effects. Larger momentum flux ratios cause the potential core of the centre jet to decrease in size, greater scalar fluctuations and more rapid correlation of
$\phi _1$
and
$\phi _2$
. However, at the same time, certain statistics, including those describing the velocity field, evolve more slowly. Moreover, the flow near the beginning of the fully merged region appears to be less mixed at higher values of
$M$
. The present work finally demonstrates that differences can be observed in the evolution and mixing of coaxial jets between those in which
$M<1$
and those in which
$M>1$
, thereby presenting an opportunity by which the mixing process in coaxial jets may be controlled.
“…Accordingly, the probe responds differently to changes in velocity and helium concentration, as shown in figure 4, such that simultaneous measurements of both quantities are possible. As discussed in Hewes [42] and Hewes and Mydlarski [32], the helium concentration (C) can be calculated using both output voltages of the CTA operating the two wires of the interference probe (E up , E down ): The velocity (U ) can then be obtained by applying King's Law to the upstream wire of the probe:…”
Section: Interference Probes In Isothermal Flowsmentioning
Many natural and engineering flows transport more than one scalar. Moreover, to study the scalar mixing therein, knowledge of the velocity field is also essential. For this reason, the present work describes the development of a three-wire thermal-anemometry-based probe to simultaneously measure velocity, helium concentration, and temperature in turbulent flows. It is first demonstrated, both theoretically and experimentally, that the temperature measured by a cold-wire thermometer is effectively insensitive to helium concentration. Then, building on recent work by Hewes and Mydlarski (2021 Meas. Sci. Technol.
32 105305), which pertains to the design of interference probes (i.e. thermal-anemometry-based probes used to measure velocity and gas concentration), a novel temperature compensation technique is proposed to extend their use to non-isothermal flows. The performance of the compensation technique is validated in turbulent coaxial jets by combining the cold-wire thermometer and interference probe to form a three-wire probe. Given that the three-wire probe can be employed to obtain simultaneous measurements of velocity and multiple scalars, it can therefore be used investigate phenomena such as multi-scalar mixing, including differential diffusion.
A detailed study of the gas-dynamic behaviour of both liquid and gas flows is urgently required for a variety of technical and process design applications. This article provides an overview of the application and an improvement to thermal anemometry methods and tools. The principle and advantages of a hot-wire anemometer operating according to the constant-temperature method are described. An original electronic circuit for a constant-temperature hot-wire anemometer with a filament protection unit is proposed for measuring the instantaneous velocity values of both stationary and pulsating gas flows in pipelines. The filament protection unit increases the measuring system’s reliability. The designs of the hot-wire anemometer and filament sensor are described. Based on development tests, the correct functioning of the measuring system was confirmed, and the main technical specifications (the time constant and calibration curve) were determined. A measuring system for determining instantaneous gas flow velocity values with a time constant from 0.5 to 3.0 ms and a relative uncertainty of 5.1% is proposed. Based on pilot studies of stationary and pulsating gas flows in different gas-dynamic systems (a straight pipeline, a curved channel, a system with a poppet valve or a damper, and the external influence on the flow), the applications of the hot-wire anemometer and sensor are identified.
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