ARIANT DEMO PROBLEM
Overview
ARIANT is a complex code that can be used in a variety of applications. It is useful as a start to examine a very simple case where the results are relatively simple to examine and appreciate. Such an example is given below.
Problem Description
The simulation is of a simple 4.0 m long pipe filled with water at 250 deg C pressurised to 7 Mpa (70 atmospheres). The pipe has a rupture disc at one end which is opened starting at t=0 over a period of 0.003 seconds, and the pipe allowed to depressurise.
Observed phenomena are that the water at the open end of the pipe immediately flashes into steam and begins to rapidly exit the pipe into the external environment. Depressurisation moves along the pipe towards the closed end, resulting in vapour generation and continued flow out the opening. Within0.004 seconds the pressure everywhere inside the pipe has fallen to ~3 Mpa and further to atmospheric by 0.4 seconds. Mass flow rises from zero to nearly 160 kg/s within 0.02 seconds, then falls smoothly to near zero by 0.4 seconds. Void fraction near the open end of the pipe rises quickly to ~0.6 by 0.04 seconds, then more slowly to 1.0 (all vapour) at 0.4 seconds, while near the closed end of the pipe, void fraction rises more slowly but approaches 1.0 at the same time as the open end.
The event is essentially complete by 0.5 seconds.ARIANT Idealisation
The file 'blowdown.inp' contains the ARIANT idealisation for this simulation. The portion beginning with 'CONTROL PARAMETERS' contains data that identify simulation start/end times, output frequency and soon. 'COMPONENTS' lays out exactly that. In this particular case, it is just one pipe with a boundary condition on the r.h.s. The 'CONNECTIONS' section shows how the piping network is con-nected - in this case it is trivial. 'BOUNDARY CONDITIONS' identifies the condition at the r.h.s. of the pipe, namely atmospheric pressure with a void fraction of 1.0. Following this is the 'SYSTEM MODELS' section. The DISCHARGE Model entered here is the physical representation of the rupture disc. The 'SYSTEM CONTROL' section contains the OUTPUT models, which write selected data to user-specified disk files, and as well has the INPUT TABLE and FUNCTION TABLE models which control the opening time and break area for the rupture disc. There are no entries in the 'WALL HEAT TRANSFER' section - if wall models were used in this simulation they would be entered here. Finally, the 'TH INITIAL CONDITIONS' section lays out what physical conditions apply to the water in the pipe at the start of the simulation.
ARIANT Output
ARIANT creates two main output files that the user should look at. The first is the synopsis file blowdown.lg. This contains a primitive description of the progress of the simulation solution, noting the step number, timestep size, location and parameter on which the timestep algorithm is controlling. Simulation exceptions such as REDO steps are noted here as well. The second file, blowdown.lis, is the detailed print file. This file contains output which reflects the current situation in the piping network, wall heat transfer models, and the system and system control models at every time and/or step for which the user has requested output. At the end of the detailed print file there is a simulation summary which notes the progress of the simulation during the current run and overall (in the case where the user has stopped and restarted one or more times) including the total simulation time achieved, number of steps, breakdown of time spent in particular areas of the code, number of warning messages, etc.
This particular simulation also produced three additional files (from user-defined OUTPUT models) which were then processed via Gnuplot to create graphs of simulation data versus time. These graphs are represented in Plots 1, 2, 3, 4.
Plot 1 is a graph of pressure in the pipe versus time over the time range 0.0 - 0.01 seconds, both at the rupture disc and at the closed end of the pipe. It can be seen that the pressure near the rupture disc drops off extremely quickly as the liquid there flashes into steam virtually instantaneously, while the pressure at the closed end drops off more slowly.
Plot 2 is the same two variables over the entire simulation time frame, 0.0 to 0.5 seconds. The pressure profile is more exaggerated due to the x-axis scale. Note the pressure recovery starting at about 0.004 seconds. This is caused by a pressure wave being reflected off the closed end of the pipe and moving back towards the open end. Pressure then bleeds down to atmospheric by ~0.4 seconds.
Plot 3 shows mass flow at the rupture disc. This goes from 0.0 to ~160.0 Kg/s at the instant the rupture disc is opened, then more or less smoothly to near zero by 0.4 seconds.
Plot 4 shows void fraction at both pipe ends. The void near the rupture disc goes from 0.0 to ~0.6 within about 0.04 seconds, then more slowly to 1.0 by ~0.4 seconds. Void near the closed end rises more slowly, reaching ~0.2 by about 0.08 seconds, then more quickly to ~0.8 by about 0.15 seconds, then approaching 1.0 asymptotically by the end of the simulation.