Page 211 - Defrosting for Air Source Heat Pump

The influence of refrigerant distribution on defrosting 205

were experimentally investigated, with results indicating that the system frosting COP
improved. Furthermore, experimental studies demonstrated that improving the FEC
of an RCD start could optimize the defrosting performance of an ASHP unit with a
vertically installed three-circuit outdoor coil, no matter whether the downward-
flowing melted frost was locally drained.
In fact, refrigerant distribution not only affects its FEC in heating mode, but also
results in an uneven defrosting phenomenon. The tube internal resistance and gravity
both affect the refrigerant distributed into each circuit. In practical applications, it is
impossible to make the refrigerant totally evenly distributed into each circuit for a ver-
tically installed multicircuit outdoor coil in an ASHP unit. This may be the reason why
studies on the URD effects due to tube internal resistance and gravity on RCD perfor-
mance for an ASHP unit were scarce in the open literature. In the numerical study
in Section 4.3, it is demonstrated that adjusting the refrigerant distributed into each
circuit, at the ratio of 95.6%, 101.1%, and 103.3% for Circuits 1 to 3 in a vertically
installed three-circuit outdoor coil, could alleviate the uneven defrosting phenomenon
and thus improve defrosting performance. To fundamentally study the effects of URD
on defrosting performance, as introduced in the previous section, another experimen-
tal study on system defrosting performance when the refrigerant was evenly and
unevenly distributed into each circuit was carried out, with melted frost both locally
drained, as shown in Figs. 7.10A and B. Experimental results demonstrated the neg-
ative effects of URD, and indicated that system defrosting efficiency was increased
about 6.9% when the refrigerant was evenly distributed.
However, in practical applications, there was always no water-collecting tray
installed between circuits. Thus, the effects of URD and MFDF are always coupled,
which is important and interactive. Also, the coupled effects of uneven heat supplying
and viscous fluid disturbance on the heat and mass transfer performance for a multi-
circuit heat exchanger, with its two sides’ medium continuous changing phases, is a
fundamental problem. Therefore, as a sequential study, an experimental investigation
on system defrosting performance with refrigerant evenly or unevenly distributed into
each circuit in an ASHP unit was carried out in this section, as shown in Figs. 7.10C
and D. After comparative and quantitative analysis conducted using the experimental
data, the coupled effects of MFDF and URD were finally given.

7.3.1 Experimental cases and RDEV adjustment
In order to study the coupled effects of MFDF due to gravity and URD due to the tube
internal resistance and gravity on system defrosting performance, a series of experi-
mental works was carried out using the experimental ASHP unit. To obtain meaning-
ful experimental results, at first it was necessary to ensure that the frost accumulated
on the surface of the three circuits was even. Thus, a series of trial-and-error manual
adjustments on the opening degrees of the stop valves work was conducted to adjust
the FEC higher than 90%. Then, a set of fixed valve opening degrees was obtained,
and the FECs of two cases could be kept the same. For easy reference in this work, this
set of valves’ opening degrees was named State 1. Moreover, the FEC could be