ABSTRACT
Vapour compression chillers represent a high percentage of total power consumption
in developed countries. Therefore it is essential the availability of tools to optimize
their design and facilitate the maintenance. This is achieved by means of mathematical
models which are useful to simulate and predict both their transient and steady state
behavior, and particularly, that of the corresponding heat exchangers.
Moving boundary model (MB) is particularly interesting due to both its degree of
accuracy and execution speed. With this model the refrigerant flow regions are computed using
lumped control volumes with variable boundaries. In the two-phase zone the saturated vapor
and liquid volumes are related by the mean void fraction (MVF) which is often taken as invariant
over time, and only considered as a variable in the problems with sever transients, when some
refrigerant zones can appear or disappear. On the other hand, from the many existing MVF
expressions, those correlated with the slip ratio are often used.
In this work the MB model for the heat exchangers of the cooling plant is presented
along with the stationary model for the mass flow devices. The models are programmed with Matlab.
In the system model the different expressions of the MVF can be inserted, and the results are
analyzed and compared along with different slip ratio expressions. It can be checked that the
results depend on the analyzed transient characteristics, and Zivi’s correlation can be taken
as an intermediate solution.
It can be also verified that the model robustness is improved when the MVF is considered
as time dependent. Also, for the studied transients, any of the exiting wall temperature models
can be used.
The vapor compression system has been solved by modeling each of the individual components
and integrating them together. The obtained results are very similar to those obtained for the
partial models.