Non-Steady Flow in Compressor Pipes Simulated by a Modified Inverse Method of Characteristics

Renzo Tosato

D.I.I. University of Padova


A correct prediction of transient pressures in reciprocating compressors is considered. By method of characteristics partial differential equations are transformed into three total differential equations along three characteristic directions (u, u+c and u-c). Riemann variables lambda l and beta b are used to define all physical parameters and the fluid velocity u in a pipe. A modified inverse method reduces the errors of smaaring because the number of linearizations is much less than with mesh methods: only linear interpolation to determine delta lamda Dl, delta beta Db, and delta DCA along characteristic lines but not to determine lambda l, beta b, and CA at initial points. I suggest to divide pipes in three sections:

  • the sections at the right and left ends have to be very short (one cell) and mesh methods are used to apply the boundary conditions;
  • the intermediate section is the longest part (more cells) and uses a modified inverse method which makes linear interpolations to determine Dl, Db and DAA along characteristic lines, but not to determine l, b and AA at initial points of characteristic lines.