Study of Pressure Losses of Unsteady Compressible Flows in Three- Way Junctions

The aim of this paper is to extend the evaluation of the accuracy of published 1-D pressure loss coefficients which are used in 1-D gas dynamics models, in unsteady compressible flows propagating in the exhaust pulses in manifolds. These pressure loss coefficients were derived from the conservation of linear momentum over finite control volumes based on assumptions including steady flow. The objectives of this work were to evaluate the accuracy of the pressure loss coefficients over the type of flows generated by engine-like pressure pulses propagating in a range of three-pipe junctions. The evaluation was performed by reference to results from unsteady, compressible, 3-D Reynolds-averaged computational fluid dynamic (CFD - open source software OpenFOAM) simulations. Two of the junction branches represented the exhaust pipes from two cylinders and the remaining was the outlet pipe. All pipes had a diameter of 25mm with length ratio 1:2 between inlet and outlet. Y junctions of 30° and 60° degrees and a T junction of 60° were tested with a blow-down 1.8bar isothermal pulse at 45Hz as the inlet boundary condition. The results have shown that the temporal pressure loss ‘signal’, as calculated between stations 5 diameters away of each of the junction ends, has also the shape of a pulse. The discrepancies observed with the 1D model occur at the beginning of the pulse and during the deceleration phase for all three geometries tested. For the Y30° discrepancies also occurred in the intermediate regions leading to less satisfactory results.