|Abstract:||A transient model was developed to predict the behavior of the vapor compression
cycle of a mobile air-conditioning system. Mobile air-conditioning systems operate in a
transient mode due to variations in compressor speed, variations in condenser air flow rate, and
the controls strategy such as clutch-cycling. We developed a model to simulate start-up
transients, clutch cycling transients, city-driving transients, and shut-down transients including
the following charge redistribution.
Our transient model treats the components in a vapor compression refrigeration system
including the compressor, condenser, orifice tube, evaporator, and accumulator. The heat
exchangers are divided into a series of constant-volume cells. The conservation of mass,
conservation of energy, and conservation of momentum equations are applied to each cell. The
number of cells and/or the volume of the cells can be changed between simulations in order to
change the resolution of the model. The accumulator model is a modification of the heat
exchanger model which constrains the outlet to always be vapor. The orifice tube model and
the compressor model are semi-empirical.
The model is validated with steady-state and transient data obtained from a test facility
specifically designed to simulate mobile air-conditioners. The steady-state model predicts most
of the system parameters to within ±15%. The transient model predicted the behavior of the
city driving cycle, compressor shut-down, compressor start-up, and clutch-cycling simulations
An important part of the system model is calculating the refrigerant properties correctly.
We developed refrigerant property routines to the calculate equilibrium thermodynamic
properties in the liquid, vapor, two-phase liquid-vapor, and supercritical region using the
Modified-Benedict-Webb-Rubin equation of state. Our property routines accurately solve for a
given output property for applicable combinations of input properties. They also accurately
predict whether the refrigerant is in the single-phase or two-phase region. The property
routines agree extremely well with the experimental data found in the literature.