The need for better accuracy and reliability of high-frequency dynamic pressure measurements is driven from a variety of industrial sectors. In the automotive industry, increasingly stringent regulations on greenhouse gas emissions require the development of internal combustion engines with ever better fuel efficiency. For optimizing the engine performance, i.e. reducing fuel consumption and emissions, accurate measurements of the time-varying pressure due to engine misfire, or engine knocking, with the amplitudes up to few MPa and frequencies up to 10 kHz are required. Improved dynamic pressure measurements are also needed in many safety critical applications, such as crash testing of cars, to reduce currently very wide safety margins and thus ensure user safety in a cost effective way. In the development of airbag systems, it is necessary to accurately measure the time evolution of pressure inside the bag having a dynamic content of interest up to few kHz. Accurate measurements of high-frequency time-varying pressure are also vital in the steam and gas turbines found in power plants. In recent decades, the development of the aerospace industry has brought the urgent need also for the accurate measurement of rapidly changing pressures in many components, such as aero-engine, aircraft exhaust and aircraft surface. In the engineering of turbine engines and rocket propulsion systems dynamic pressure measurements with the frequencies up to 30 kHz are used for feedback sensing, thrust measurement and overpressure indication.
Generally, the project will answer three leading questions. How to manipulate the shock waves for the purpose of extending the amplitude pressure range of the shock tubes? How to decrease the uncertainty contribution that covers the incompleteness of the current shock tube measurement model? How to provide calibrations of pressure measurement system (PMS) in few 10 kHz range with the amplitude and phase uncertainty of 1% and 5°, respectively?
The objective of the project is to develop advanced system for high-frequency pressure dynamic calibration with a calibration and measurement capability to act as a primary dynamic pressure calibration standard. The proposed concept is based on diaphragmless shock tube with a fast opening valve (FOV), which has been developed in the Laboratory for Measurements in Process Engineering at the Faculty of Mechanical Engineering, University of Ljubljana, and is shown in figure 1. The developed shock tube will be upgraded to enable the generation of well-defined pressure step changes up to 10 MPa. The developed system will enable to determine the amplitude and phase frequency characteristics of the PMS with the use of a generated pressure step as an input signal in the frequency range up to few 10 kHz with a measurement uncertainty of 1% and 5°, respectively. The configuration will also be adapted to allow dynamic calibrations of pressure meters with the use of liquid, which will be the first proof-of-principle of liquid dynamic calibrator, in which the shock waves are used to generate time-varying pressure. Our proposed concept of liquid shock tube system is based on a double-acting actuator, in which the incident plane shock wave generated within the gas-filled part of the shock tube will be smoothly transformed into a spherical shock wave that converges, accelerates and thereby amplifies its strength. Such extreme conditions created by the shock amplification at the focal point of the spherical shock wave will excite the piston separating the gas-filled part of the shock tube and a liquid-filled cylinder with the integrated pressure meter under calibration, see figure 2. The proposed configuration of the calibrator will therefore combine the advantages of both aperiodic pressure generators, shock tube as a high-frequency pressure generator and drop weight system as a high-amplitude liquid pressure generator.
Figure 1. Schematic view of the developed diaphragmless shock tube with FOV.
Figure 2. A schematic of the proposed principle of a liquid shock-tube-based generator.
To achieve the final objective of the project, first, a complete numerical model for analysing the effects of the operating conditions on the supersonic velocity distribution of the shock waves will be developed in a commercially available CFD simulation tool OpenFOAM, which is an open-source CFD library written in C++. The goal of numerical analyses is to determine general functional dependence of the shock wave velocity along the tube on different thermodynamic and transport properties of the used gas and geometrical parameters of the shock tube, which will decrease the uncertainty contribution that covers the incompleteness of the current shock tube measurement model. Furthermore, the findings from numerical analyses will help us to define optimal operating conditions and geometrical parameters to increase pressure and frequency calibration range of the shock tube and will lay a solid foundation for upgrading the mechanical implementation of the diaphragmless shock tube. In adaptation of the shock tube for dynamic high-frequency calibrations of pressure meters with the use of liquid, the shock tube will be coupled with a developed double-acting actuator filled with liquid. As the dynamic characteristics of such dynamic pressure generator are defined by the properties of the piston, cylinder chamber and the liquid in the cylinder chamber, the findings of mathematical analyses of such pressure calibrator will give an important insight into the interaction between the generated shock waves, piston movement and pressure step generated in the liquid filled cylinder. Furthermore, they will give crucial directives to achieve the main objective of designing and developing the first experimental proof-of-principle liquid shock tube based calibrator for calibration of PMS in few 10 kHz range. To achieve the target uncertainty of the calibration of PMSs in the frequency range of few 10 kHz, a suitable digital signal processing method for determining the amplitude and phase frequency characteristics of the device under test using a pressure step envisaged by the shock tube measurement model as the reference pressure step input will be developed.
