Technical Paper

This paper reports on the first phase of an experimental evaluation of five different methods for the mathematical modelling of unsteady gas flow in engine ducting. The five methods under investigation are the homentropic method of characteristics, the non-homentropic method of characteristics, the two-step Lax-Wendroff method with flux corrected transport, the Harten-Lax-Leer upstream difference method and the Blair method of pressure wave propagation through finite spaces.

A single cycle pressure wave generator consisting of a cylinder, connected via a sliding valve to a long duct, has been designed and built. The pressure waves it creates closely mimic those to be found in i.e. engines. The cylinder and the ducts of the device can be filled with any gas and at elevated temperatures. A perfect seal exists between the cylinder and the valve thus enabling mass-flow correlation. The initial cylinder pressure may be set to simulate an induction or an exhaust process. The duct attached to the pressure wave generator can simulate virtually any configuration to be found on an i.c. engine. Pressure and temperature are recorded by transducers positioned at various locations in the apparatus and stored using a high speed data acquisition system.

A series of tests have been conducted to simulate exhaust and intake flow in a constant area duct sufficiently long, to permit wave observation unclouded by superposition effects and to determine the accuracy of the simulation methods and the cylinder to pipe boundary conditions.

A computer simulation of the test apparatus has been written for each of the five theoretical methods using the Queen's University of Belfast (QUB) non-isentropic treatment of the boundary conditions. The accuracy of each prediction method is then correlated with the experimental results. As might be expected, most of the computer codes produce good correlation for unsteady flow in a constant area duct. However, this paper is the first in a series of papers which will report the exposition of the various computer codes to increasingly difficult pipe modelling geometry and thermodynamic discontinuities.

You may obtain a copy of this paper at the SAE International website.