This thesis is concerned with the effects of flow maldistribution in fin-and-tube
A-coil evaporators for residential air-conditioning and compensation potentials
with regards to system performance. The goal is to create a better understanding
of flow maldistribution and the involved physical phenomenons. Moreover,
the study investigates the individual and combined effects of non-uniform inlet
liquid/vapor distribution, different feeder tube bending and non-uniform airflow.
In addition, the possible compensation of these maldistribution sources is
investigated by control of individual channel superheat by distributing individual
channel mass flow rate continuously (perfect control). The compensation
method is compared to the use of a larger evaporator in order to study their
trade-off in augmenting system performance (cooling capacity and COP).
The studies are performed by numerical modeling in the object-oriented programming
language Modelicar and by using the commercial modeling environment
Dymola 7.4 (2010). The evaporator model needs to be capable of
predicting the flow distribution and circuitry effects, and for these reasons
the dynamic distributed one-dimensional mixture two-phase flow model is implemented.
The model is verified in steady state with commercial software
Coil-Designer (Jiang et al., 2006) and compared to steady state experiments
with acceptable results considering the unknown degrees of flow maldistribution
for these experiments. Furthermore, the system dynamics in the model
were validated and showed that a slip flow model need be used.
A test case 8.8 kW residential air-conditioning system with R410A as refrigerant
is chosen as baseline for the numerical investigations, and the simulations
are performed at standard rating conditions from ANSI/AHRI Standard
210/240 (2008). The investigations are performed on a simplified evaporator
tube circuitry (two straight channels), a face split evaporator circuitry and an
interlaced evaporator circuitry. The first case is a generic study and serves
to provide general results independent of specific type of tube circuitry. The
second and third cases are standard tube circuitry designs and these results are thus tube circuitry specific. In addition, a novel method of compensating
flow maldistribution is analyzed, i.e. the discontinuous liquid injection principle.
The method is based upon the recently developed EcoFlowTM valve by
Danfoss A/S, and controls the individual channel superheat by distributing
individual mass flow rate discontinuously (on/off injection).
The results in this thesis show that flow maldistribution decreases system performance
in terms of cooling capacity and COP, but may be compensated significantly
by control of individual channel superheat. The generic study (two
straight channels) shows that the airflow maldistribution has the largest effect,
whereas the liquid/vapor maldistribution has smaller effect and the different
feeder tube bending has a minor effect on system performance. The comparison
between the face split and interlaced circuitry shows that the face split
evaporator performs better at uniform flow conditions, whereas the interlaced
evaporator performs better at flow maldistribution conditions. When compensating,
the face split evaporator always performs best. A similar result is also
obtained as the airflow profile across the A-coil evaporator was predicted by
means of CFD simulation software STAR-CD 3.26 (2005) and applied in the
numerical model. The main reason for the better face split evaporator performance
at uniform conditions or when compensating, is that the superheated
"weak" zones with low UA-value is located in the first tube row, where the heat
transfer driving potential (temperature difference) is highest.
The discontinuous liquid injection principle showed that the cycle time is an
important parameter for the performance of this compensation method. The
cycle time is essentially the time it takes for distributing mass flow to each
evaporator channels. It should be kept as low as possible. Furthermore, it
is better to use a partial secondary flow into the remaining channels while
distributing the main flow to each individual channel. The discontinuous liquid
injection simulations showed spurious fluctuations in pressure, which have not
been observed as high in any experiments carried out at Danfoss with high
enough sampling frequency. It is believed that the absence of the interfacial
dynamics in the mixture model and the use of correlations developed from
steady state experiments may be the reasons for these fluctuations.
|Place of Publication||Kgs. Lyngby, Denmark|
|Publisher||Technical University of Denmark|
|Number of pages||225|
|Publication status||Published - Aug 2011|
|Series||DCAMM Special Report|