State space model extraction of thermohydraulic systems Part II: a linear graph approach applied to a Brayton cycle–based power conversion unit
Abstract
This second paper in a two part series presents the application of a developed state space model
extraction methodology applied to a Brayton cycle-based PCU (power conversion unit) of a PBMR (pebble
bed modular reactor). The goal is to investigate if the state space extraction methodology can cope with
larger and more complex thermohydraulic systems. In Part I the state space model extraction methodology
for the purpose of control was described in detail and a state space representation was extracted
for a U-tube system to illustrate the concept. In this paper a 25th order nonlinear state space representation
in terms of the different energy domains is extracted. This state space representation is solved
and the responses of a number of important states are compared with results obtained from a PBMR PCU
Flownex model. Flownex is a validated thermo fluid simulation software package. The results show that
the state space model closely resembles the dynamics of the PBMR PCU. This kind of model may be used
for nonlinear MIMO (multi-input, multi-output) type of control strategies. However, there is still a need
for linear state space models since many control system design and analysis techniques require a linear
state space model. This issue is also addressed in this paper by showing how a linear state space model
can be derived from the extracted nonlinear state space model. The linearised state space model is also
validated by comparing the state space model to an existing linear Simulink model of the PBMR PCU
system.
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