Application Cases

Research Direction: Pressure Sensor

Objective:
Accurately measuring the internal flow fields of high-speed ramjet isolators and aero-engine compressor components is one of the effective means to enhance engine performance and reliability. However, the internal flow fields of isolators and high-pressure compressors are characterized by high temperature, high pressure, and high-frequency dynamic pressure fluctuations. Therefore, to achieve accurate measurements in such complex dynamic machinery, dynamic pressure sensors must simultaneously possess small size, high-frequency response (MHz and above), and stable operation in high-temperature and high-pressure flow fields. Currently, due to the limitations of mass inertia, the highest usable frequency response of existing sensors is below 500 kHz, and they often suffer from issues such as large size (6–10 mm) and temperature drift, making them unsuitable for unsteady measurements within complex engine machinery. In the exploration of new technologies, plasma technology has attracted researchers' attention due to its inherent advantages, such as theoretically high-frequency response, no limitations from thermal inertia, and small probe dimensions (down to the millimeter scale) based on plasma principles. Therefore, it is believed to have significant development potential and is expected to overcome the technical bottleneck of high-temperature and high-pressure flow field measurements.

Testing Equipment:
Plasma pressure sensor, signal source, oscilloscope, pressure sensor, ATA-8202 RF power amplifier, electric heating high-temperature chamber.

Figure 1: Schematic and Physical Diagram of the Plasma Pressure Sensor

Experimental Procedure:
During the experiment, the plasma pressure sensor was placed inside a pressure tank. Wires were connected to the discharge chamber through an insulating sleeve outside the gas discharge chamber. The RF AC power supply was turned on, and the output power was adjusted to 5 W. At this point, the voltage waveform across the plasma pressure sensor resembled an approximate sine wave, indicating stable plasma discharge. A voltage probe with an attenuation ratio of 1000:1 was used to record the sustaining voltage across the sensor, which was displayed and recorded by an oscilloscope.

First, a stable RF plasma was generated under atmospheric pressure with an AC frequency of 1.25 MHz. Then, the pressure inside the discharge chamber was gradually increased from 0.4 atm to 4.5 atm in increments of 0.1 atm, and the relationship between the voltage across the plasma pressure sensor and the pressure inside the discharge chamber was recorded. During this process, the output power of the RF power supply was adjusted to maintain stable continuous discharge. The applicable pressure range for the plasma pressure sensor with an electrode gap of 220 μm was 0.4–4.5 atm, corresponding to an output power range of 5–9 W. As the pressure inside the tank increased, the stable operating voltage also increased, indicating a positive correlation between the sustaining voltage of the plasma pressure sensor and pressure. Different output power levels corresponded to different pressure ranges, with an overall sensitivity of 0.22 V/kPa. The experimental results at an output power of 8 W are shown in Figure 2.

Figure 2: Calibration Curve

Experimental Results:
This study investigated the steady-state response of an RF discharge plasma pressure sensor under an excitation carrier frequency of 1.25 MHz, a pressure range of 0.4–4.5 atm, and different output power levels, as well as its high-temperature operating characteristics from 25°C to 400°C. Finally, the plasma pressure sensor was applied to a dynamic pressure measurement experiment at the tip of a single-rotor axial compressor. The conclusions are as follows:

(1) Steady-state calibration experiments showed that the RF plasma pressure sensor with a 220 μm electrode gap operated effectively within a pressure range of 0.4–4.5 atm, corresponding to an output power range of 5–9 W. The output power varied with changes in pressure, and different output power levels corresponded to different pressure ranges. Overall, the sensitivity of the plasma pressure sensor was 0.22 V/kPa.

(2) High-temperature characteristic experiments demonstrated that within the temperature range of 25°C to 400°C, the sustaining voltage of the plasma pressure sensor remained largely unchanged, fluctuating only around 3.56 V. This confirmed that the plasma pressure sensor is not only heat-resistant but also insensitive to temperature within this range, eliminating the need for temperature calibration or compensation. This is one of the advantages of plasma pressure sensors compared to other types of sensors.

(3) Based on the above research, the plasma pressure sensor and a piezoresistive dynamic pressure sensor (Kulite) were simultaneously installed on the casing of an axial compressor to measure the dynamic pressure flow field at the compressor tip. Two operating conditions were selected for analysis: a high-flow condition (flow coefficient of 0.53) and a near-stall condition (flow coefficient of 0.45). Frequency domain analysis revealed that, like the Kulite dynamic pressure sensor, the plasma dynamic pressure sensor was capable of capturing the blade-passing frequency (2410 Hz) and the self-excited unsteady characteristic frequency of tip leakage flow (970 Hz).

(4) However, from the dynamic pressure measurement results, there remains a gap between the current plasma pressure sensor and the Kulite sensor, mainly in signal processing, signal-to-noise ratio, and accuracy. These are the challenges that need to be addressed for plasma pressure sensors in the future. Subsequent research will focus on improving the signal-to-noise ratio and measurement accuracy of the plasma pressure sensor. Building on this, studies will be conducted on its application in actual high-temperature and high-pressure environments. Ultimately, the goal is to achieve measurement and acquisition of dynamic pressure fluctuation signals in the highly demanding high-temperature, high-pressure, and high-speed environments inside aero-engines and ramjet engines.

Aigtek ATA-8000 Series RF Power Amplifier:

Figure: Specifications of the ATA-8000 Series RF Power Amplifier