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R134a and PAG Oil Maldistribution and Its Impact On Microchannel Heat Exchanger Performance.

INTRODUCTION

The microchannel heat exchangers have been widely used in the modern air-conditioning, heat pump and refrigeration systems for a variety of automotive, residential, and industrial applications, because of its higher overall heat transfer performance, lower refrigerant inventory, more compactness, and lower cost and weight. However, one of the major problems for the microchannel heat exchangers is the refrigerant maldistribution among the parallel tubes. Due to the significant density difference, it is very difficult to distribute the vapor and liquid evenly among the parallel tubes in the evaporator. Some tubes starved with liquid refrigerant will have large superheated region where the utilization of the heat transfer area is very poor. Thus, the system capacity and COP are usually deteriorated.

The knowledge to achieve good distribution in vertical headers is still limited, although it has been extensively studied. Fei and Hrnjak (2004), Vist and Pettersen (2004), Webb and Chung (2005), Bowers et al. (2006) and Hwang et al. (2007) studied the two-phase flow in the horizontal headers and refrigerant distribution into the vertical parallel tubes, which usually appear in the indoor microchannel heat exchangers. Watanabe et al. (1995), Cho and Cho (2004), Lee (2009), Byun and Kim (2011) and Zou and Hrnjak (2013a and 2013b) investigated the distribution of different working fluid (e.g. R134a, R410A, etc.) in the inlet and/or intermediate vertical header, which are commonly used in the outdoor heat exchangers in heat pump mode. These studies showed that the flow regimes in the header, which was affected by several conditions (e.g. header geometry and orientation, fluid properties and inlet conditions), had a strong influence on liquid distribution into the branch tubes in two-phase flow.

It is noteworthy that most of previous tests on distribution were done for pure refrigerant. To the authors' best knowledge, the effect of oil on distribution was barely studied. However, there were some studies that focused on refrigerant and oil flow in the horizontal tube. Worsoe-Schmidt (1960), Manwell and Bergles (1990) and Poiate and Gasche (2006) examined R12 and mineral oil. The foaming appeared at the interface between liquid and vapor. Wongwise et al. (2002) tested the flow of R134a and 5% PAG oil (180 cSt at 40 [degrees]C (104 [degrees]F)). Depending on the mass flow rate and quality, the foam was either at the interface between liquid and vapor or occupied the whole tube and formed the froth flow. Castro et al. (2004) investigated the flow of R134a and ester oil. It was speculated that the formation of the first bubbles was due to the reduction of refrigerant solubility in the oil. As flow proceeded, more bubbles were formed and the flow became foaming. Bowers and Hmjak (2010) illustrated the flow of R134a and 1.7% PAG oil (46 cSt at 40 [degrees]C (104 [degrees]F)). It was found that the addition of oil created numerous droplets in the vapor and lots of bubbles or froth in the liquid layer. The flow regime transited from Stratified/Wavy to Stratified/Annular due to oil. Kim and Hrnjak (2012) presented the effect of POE oil on CO2 flow patterns. It was shown that the foaming at the interface increased the wetting on the round tube. The size of foaming was affected by mass flux and quality. This paper will investigate the effect of PAG oil on R134a two-phase flow in the vertical header and its effect on refrigerant distribution and heat exchanger performance.

EXPERIMENTAL METHOD

The test loop was constructed to study R134a and PAG oil (46 cSt at 40 [degrees]C (104 [degrees]F)) distribution in the microchannel heat exchanger, as shown Figure 1(a). The subcooled liquid refrigerant was pumped into the inlet header. It was assumed that the single phase subcooled liquid was distributed evenly into the microchannel tubes in the bottom pass, where the refrigerant was heated to the desired quality while the heaters were insulated. The two-phase fluid entered into the test header. In the bottom part of the header, the two-phase fluid turned 90[degrees] to flow upward and reached the upper part of the header. Due to maldistribution, different amounts of liquid exited through the microchannel tubes in the top pass. In each exit tube, the refrigerant was heated again by six heaters and insulated to provide equal superheat at the exit. The single phase superheated vapor was then brought to the condenser. Through the receiver and the subcooler, the subcooled liquid was provided to the pump. Since the oil amount was very small and the exact amount of oil in each microchannel tube was unknown, the thermal physical and transport properties were calculated assuming there was no oil. The liquid mass flow rate in each exit tube was calculated based on the information of outlet superheat, total mass flow rate and the power input. The results are generalized with two metrics: 1) coefficient of variation a, which is dimensionless standard deviation (Uniform distribution is described by [sigma] = 0.)

[mathematical expression not reproducible] (1)

and 2) liquid fraction

[mathematical expression not reproducible] (2)

The measured inner diameter of the transparent header was 15.44 mm (0.051 ft). The tube pitch was 13 mm (0.043 ft). The hydraulic diameter of the microchannel was calculated based on the manufacturer data to be 0.5 mm (0.0016 ft). The saturation temperature was at about 10 [degrees]C (50 [degrees]F) in the header. The inlet quality was from 0.2 to 0.8. The inlet mass flow rate was at 4.19 g [s.sup.-1] (0.0092 lb [s.sup.-1]). The oil circulation rate (OCR) is defined in Equations 3. It was varied from 0% (pure R134a), 0.5%, 2.5% and 4.7%.

OCR = [[m.sub.oil]/[[m.sub.oil] + [m.sub.ref,l] + [m.sub.ref,v]]] (3)

MODEL DESCRIPTION

Tuo et al. (2012) developed an experimentally-validated model of a microchannel evaporator with considering quality and header pressure drop induced maldistribution in the horizontal header. Li and Hrnjak (2013) extended this model to consider oil in the simulation. This model considered two types of refrigerant maldistribution: 1) uneven quality; 2) header-induced pressure drop. In this study, this model was extended to evaluate the impact of the refrigerant maldistribution in the vertical header on the microchannel heat exchanger (MCHX) performance by incorporating the quality distribution based on the experimental results. The discretization schemes have been shown in Figure 1(b).

In each pass of the MCHX, the refrigerant mass flow distribution is based on the fact that added pressure drop along any flow path must be identical to each other. The overall pressure drop for each unique flow path of any pass can be expressed as:

[mathematical expression not reproducible] (4)

The pressure drop in a horizontal microchannel tube includes: frictional, momentum, and sudden contraction as well as sudden expansion pressure drops. The pressure loss in each element of vertical headers includes three components, frictional pressure drop, gravity pressure drop, and local minor loss due to the half-way protrusion of microchannel tubes into the header. The selected heat transfer and pressure drop correlations can be found in Li and Hrnjak (2013).

RESULTS AND DISCUSSION

Distribution Results

Figure 2 shows the distribution profiles and Figure 3 presents the corresponding coefficient of variation. The darkness of the bar color represents the different branches (tubes), with the pale being the lowest exit branch and the dark being the highest exit branch. The distribution usually deteriorates as quality increases regardless of OCR. It is found that with a small amount oil (OCR=0.5%), the distribution is worse than the pure R134a. Increasing OCR to 2.5% and 4.7%, the distribution is better and better. This trend is observed for both low and high inlet qualities. However, at low qualities, the improvement is more significant: at OCR=2.5% and 4.7%, the distribution is even better than the pure R134a case.

The quality maldistribution, though also affected by the pressure drop in the header, is mainly a result of the flow morphology in the header. As shown in Figure 4 and 5, churn and separated flow are identified from the visualization. The flow patterns are also listed in Figure 2, where "C" denotes churn flow while "S" denotes separated flow. Separated flow is the flow pattern that is very similar as annular flow. It is seen that the distribution is better in churn flow because the mixing of vapor and liquid is more uniform. However, in separated flow, in only a small part of the header, the liquid is easily available in front of the entrance to the tube. The flow regime in the header is affected by the inlet conditions. As inlet quality increases, the flow regime transited from churn to separated flow and thus the distribution is worse, as shown in Figure 2.

When oil is present, although generally the flow regime is still churn flow or separated flow, the details of the flow pattern in the header is different. For instance, the flow regime of pure R134a is churn flow at low quality in Figure 4. When OCR=0.5%, the effect oil on the flow regime in the header is not significant, whereas increasing OCR to 2.5% and 4.7%, there are so many small bubbles or foams in the header that even light is hard to go through. Thus, the two-phase flow in the header is more homogeneous and the distribution into the outlet tubes is more uniform. At high qualities, e.g. in Figure 4, the flow regime of pure R134a is separated flow. At OCR=0.5%, the flow regime in the header is similar to that of pure R134a. However, when OCR=2.5% and 4.7%, the liquid film on the wall is thicker; the local churn flow is larger and there are lots of small bubbles or foams in this region.

Microchannel Heat Exchanger Performance Simulation

In this section, the quality distribution results from the experiment are incorporated into a microchannel heat exchanger model (Li and Hrnjak, 2013) to evaluate the effect of refrigerant maldistribution on heat exchanger performance. The quality at each tube inlet is given as input from the corresponding experimental results. On the air side, typical fin geometry of microchannel evaporators is chosen: 12 mm (0.039 ft) fin height, 1.41 mm (0.0046 ft) fin pitch, 10 mm (0.030 ft) louver length. The microchannel tubes are 0.6 m (1.97 ft) long. The evaporator face velocity is assumed uniformly to be 2.1 m [s.sup.-1] (6.89 ft [s.sup.-1]). The air inlet temperature is assumed to be around 32 [degrees]C (89.6 [degrees]F). Figure 2 shows the schematic drawing of the microchannel heat exchanger. This section focused only on the top pass of the heat exchanger because only these experimental results of quality into the tubes are available.

Figure 6 demonstrates the change of predicted capacity of the heat exchanger as a function of OCR. The inlet mass flow rate is fixed at 4.19 g [s.sup.-1] (0.014 lb [s.sup.-1]) for each simulation in this case but inlet quality varies. It can be seen that the capacity decreases with an increasing OCR for all three different inlet quality conditions. This is due to the fact that none-evaporative lubricant takes the place of refrigerant.

In the direct expansion system, the superheat is usually controlled by thermal or electronic expansion valve. Thus, the maldistribution effect is quantified by comparing the evaporator performance of maldistribution case with a uniform distribution case at the same bulk exit superheat. The capacity degradation is defined as following:

[alpha] = 1 - [[Q.sub.mal]/[Q.sub.uni]] (5)

where [Q.sub.mal] and [Q.sub.uni] denote the cooling capacity at maldistribution case and at the uniform distribution case respectively.

Figure 7 shows the effect of OCR on capacity degradation. As OCR increases, although the capacity monotonically decreases as shown in Figure 6, the capacity degradation reaches its maximum at 0.5% OCR and then decreases which is in agreement with the behavior of [sigma] (shown in Figure 3).

CONCLUSION

This paper investigated the effect of PAG oil on two-phase R134a flow in the vertical header and R134a distribution in the microchannel heat exchanger. The distribution was worse with a small amount of oil (OCR=0.5%). With more oil, the distribution was better (OCR=2.5% and 4.7%) and better than the pure R134a in some cases. Visualization showed that when there was enough liquid, a lot of foams were formed in the header with a large amount of oil (OCR=2.5% and 4.7%). The mixing of the two phases was more uniform so that the distribution was better. Through simulation in the heat exchanger model, it is found that as OCR increases, the capacity monotonically decreases though the maldistribution effect is most significant at OCR=0.5%, i.e. the capacity deviates most from the uniform distribution case.

ACKNOWLEDGMENTS

The authors thankfully acknowledge the support provided by the Air Conditioning and Refrigeration Center at the University of Illinois at Urbana-Champaign.

NOMENCLATURE
A         =  Cross section area in the header ([m.sup.2])
G         =  Mass flux (kg m-2-[s.sup.-1])
D         =  Internal diameter of the header (m)
LF        =  Liquid fraction (-)
m         =  Mass flow rate (g s1)
n         =  number of the outlet tubes (-)
OCR       =  Oil circulation rate
Q         =  Power of heaters (kW)/ Capacity (W)
x         =  Quality (-)
[DELTA]P  =  Pressure drop (kPa)
[alpha]   =  Capacity degradation
Subscripts
ave       =  Average pressure
i         =  Index number
ihd       =  Inlet header
l         =  Liquid
M         =  Main pipe (header)
ohd       =  Outlet header
oil       =  Oil
ref       =  Refrigerant
tub       =  Tube
v         =  vapor


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Yang Zou

Student Member ASHRAE

Huize Li

Student Member ASHRAE

Predrag S. Hrnjak, PhD

Fellow ASHRAE

Yang Zou is a PhD student in the Department of Mechanical Engineering, University of Illinois at Urbana-Champaign, Urbana, IL. Huize Li a PhD student in the Department of Mechanical Engineering, University of Illinois at Urbana-Champaign, Urbana, IL. Predrag S. Hrnjak is a professor at the University of Illinois at Urbana-Champaign, Co-Director of the Air Conditioning and Refrigeration Center at the University of Illinois at Urbana-Champaign, and President of Creative Thermal Solutions, Inc.
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Author:Zou, Yang; Li, Huize; Hrnjak, Predrag S.
Publication:ASHRAE Conference Papers
Date:Dec 22, 2014
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