Ultrasonic measurement of gas flow using electrostatic transducers

Abstract

Ultrasonic gas flowmeters typically use narrowband piezoelectric transducer arrangements for interrogating the flow of gas in a pipe. In this work, the suitability of broadband electrostatic transducers operating at frequencies of up to 1 MHz for ultrasonic measurement of gas flow has been investigated. The transit time method of ultrasonic gas flow measurement was adopted and experiments were carried out using a laboratory test rig capable of producing a range of gas flowrates up to 17.5 m/s. The test rig also allowed easy interchange of different prototype flowmetering sections. Times of flight of ultrasonic waves interrogating the gas flow were measured using separate send/receive electrostatic transducer arrangements. Two flowmeter configurations were considered. The first interrogated the flow at 45°in contra-propagating upstream and downstream directions. The second consisted of an upstream interrogation at 45°to the gas flow and an interrogation made normal to the flow direction. k factors correlating the fluid velocity along the ultrasonic path with the mean fluid velocity in the pipe were calculated using experimental ultrasonic data and anemometer measurements. All transducer configurations were numerically modelled using the computational fluid dynamics software package FLOTRAN Ó (ANSYS Inc.). Theoretical gas flow velocities for both transducer arrangements were subsequently compared with experimental values and found to be in excellent agreement. A flow-dependent frequency shift of the received ultrasonic signals was also observed simultaneously with the transit time measurement. Ó

Experimental and simulation characterisation of flexural vibration modes in unimorph ultrasound transducers

“…The field of air-coupled ultrasonics has received an increased interest over the years, as it has expanded into new areas of application, including wireless communication [1], contactless material characterisation [2], gas flow metering [3], [4] and robotics [5]. Each application has its own set of requirements, which has pushed the development of transducer technology.…”

Adaptive Pulse Repetition Frequency Technique for an Ultrasonic Transit-Time Gas Flowmeter for Hot Pulsating Gases

“…Thus, for the application discussed here, the measurement configuration as shown in Fig. 1 basically has two advantages in comparison to a configuration with the transducers showing protrusion into the main flow [12], namely 1) transducer membranes are located in regions where the temperature is lower, and 2) temperature gradients along the transducer membrane surfaces are lower due to the fact that the membranes are not located in the region (close to the center of the pipe) where the temperature distribution normally shows a steep gradient.…”

“…A high-temperature-resistant broadband CUT is used both for transmitting and receiving the ultrasonic pulses [11]. Nowadays, CUTs are accepted as state-of-the-art transducers for gas flow measurement applications, e.g., [12]. By modifying the construction of the commonly used CUTs, which, for instance, utilize a metallized polymer membrane as the moving electrode, and by selecting appropriate transducer materials, we have achieved a wideband CUT for operation at elevated gas temperatures of up to several hundred degrees Celsius.…”

Ultrasonic-based gas flowmeter for harsh environmental conditions

“…(O'Sullivan, Wright, 2002) demonstrated that capacitive ultrasonic transducers (CUTs) are in general a promising transducer technology for UFMs. The advantages are the excellent bandwidth characteristics (up to 100% fractional bandwidth in air at 101.325 kPa), and the reduced acoustic impedance mismatch to gases at lower pressures compared to piezoelectric transducers.…”

“…The advantages are the excellent bandwidth characteristics (up to 100% fractional bandwidth in air at 101.325 kPa), and the reduced acoustic impedance mismatch to gases at lower pressures compared to piezoelectric transducers. The CUTs used in (O'Sullivan, Wright, 2002) had a metallized polymer membrane as moving electrode, which makes them unsuitable for a wider temperature range. The advent of a high-temperature version of these CUTs, as reported in (Schr€ o oder et al, 2004), eliminated this temperature limitation by transferring the insulation layer from the movable electrode to the backplate of the CUT.…”