|Abstract:||An experimental study of two-phase flow patterns and droplet entrainment in a
horizontal-tube evaporator was conducted. The study is an element of the worldwide change
from the current generation of refrigerants to chemicals with reduced ozone-depletion and
global-warming potential. Measurements were made with serpentine aluminum and glass
evaporators with geometries typical of those used for domestic refrigeration. The refrigerant in
the majority of tests was R134a (tetrafluoroethane), which will replace R12
(dichlorodifluoromethane) for domestic refrigeration and automotive air-conditioning
applications in 1995. Baseline data with R12 and R22 (chlorodifluoromethane, currently used in
residential air conditioners) were also recorded.
The phenomenon of primary interest was the non-equilibrium transport of liquid droplets
within slightly superheated vapor at the evaporator exit. In sufficient quantity, such liquid would
prove detrimental to evaporator performance by increasing the vapor superheat level at a given
heat duty and reducing the effective temperature difference between refrigerant and air. Also of
concern were substantial variations in carry over rate with time, and corresponding fluctuations
in exit temperature. These variations are due to the formation of slugs far upstream in the
evaporator which rapidly transport a surplus of liquid toward the exit.
A flow loop was constructed to circulate oil-free refrigerant through the evaporator under
conditions spanning those of domestic refrigerator operation. Liquid carry over (expressed as a
dimensionless entrained mass fraction, or EMF) was measured as a function of inlet quality, heat
flux, mass flux and exit superheat level. A laser-based phase/Doppler particle analyzer was used
to measure droplet diameters and velocities within an optical probe volume at the evaporator
exit. Videotapes and still photographs of flow patterns within the glass evaporator were made.
Two types of experiments -- time-averaged and time-resolved -- were conducted. The former
revealed the quantitative dependence of EMF on the independent variables, while the latter
documented the relationship between EMF and flow-regime transitions within the evaporator.
Tests with three refrigerants over wide ranges of operating conditions revealed time averaged
EMFs of no more than 0.1 percent Thus, liquid carry over does not significantly affect
the performance of the evaporator as a heat exchanger. Analysis of variance (ANDY A) revealed
exit superheat to have the strongest effect on entrained mass fraction, followed by mass flux,
inlet quality and heat flux. Time-averaged EMFs varied with operating conditions by several
orders of magnitude, decreasing with increasing superheat level (due to lower entrainment rates
near the exit and more rapid droplet vaporization) and mass flux and inlet quality (due to a lower
liquid inventory in the first few evaporator passes). An expression relating EMF to
dimensionless forms of the independent variables was developed via nonlinear multiple
regression.The incidence of slug flow within the evaporator and its effect on evaporator exit
conditions were documented in the time-resolved experiments, using techniques such as autoand
cross-correlation and Fourier transform. Time-resolved EMFs as high as one percent were
observed, as well as sharp reductions in exit superheat of as much as 8°C. These coincided with
the arrival at the exit of slug remnants in the form of entrained droplet clusters. The distribution
of superheat readings with time was found to follow a Rayleigh probability density function.
This finding will facilitate the use of the superheat reading in control strategies, should it become
necessary to actively control refrigerant flow rate.