1、Sarath Mulugurthi is MS student and Ardiyansyah Yatim is PhD student in the School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK, USA. Lorenzo Cremaschi is an Associate Professor in the School of Mechanical and Aerospace Engineering, Oklahoma State University, St
2、illwater, OK, USA. Lubricant Retention and its Effects on Heat Transfer and Pressure Drop of a Microchannel EvaporatorSarath K. Mulugurthi Ardiyansyah S. Yatim Lorenzo Cremaschi, PhD Student Member ASHRAE Associate Member Member ASHRAEABSTRACT In vapor compression air conditioning systems a small po
3、rtion of the compressor oil circulates with the refrigerant through the components. The oil that is carried with the refrigerant flow typically ranges from 0.5 to 1 percent of the flow rate. This oil accumulates in the heat exchangers, increases the pressure losses and creates an additional thermal
4、barrier to the heat exchange processes. In the present paper, oil retention in a microchannel type evaporator was experimentally investigated and its effects on heat transfer rate and two-phase flow pressure drop are presented. The microchannel heat exchanger was a single pass, aluminum louvered-fin
5、 type evaporator with vertical multi-port microchannel tubes. Refrigerant R410A and Polyolester oil mixture was studied at saturation temperature ranging from 33 to 48 F (0.5 to 9 C) and for mass fluxes that are commonly observed in 3 ton nominal capacity AC units for residential applications. The r
6、esults showed that at oil mass fractions (OMFs) of 0.5 to 1 weight percent, the oil volume retained in the microchannel evaporator ranged from 1 up to 11% of the microchannel evaporator internal volume. The oil retention was depended on the OMF and, at same OMF and saturation temperature, the oil re
7、tention increased if the refrigerant mass flux decreased. This was due to the reduced driving force that carried the oil throughout the microchannel evaporator. At low OMFs the heat transfer rate diminished by up to 3.5% and the pressure drop was up to 16% higher than that of oil free conditions. Th
8、e experimental results of this paper are helpful to decrease the energy use and increase the estimated useful life of air conditioning systems. INTRODUCTION Lubricants are used in compressors of air conditioning systems but some of the oil tends to escape from the compressors and circulates through
9、the other components. When oil is retained in condensers and evaporators, it increases the pressure losses and is likely to generate an additional thermal barrier to the heat exchange processes. These phenomena penalize the thermal performance of the heat exchangers and ultimately decrease the COP o
10、f air conditioning systems. In addition, a lack of proper lubrication inside the compressors can compromise the reliability and might lead to failure of the compressors. Abundant studies exist in the literature on the oil return characteristics of various refrigerant and oil mixtures but only few st
11、udies focused on quantifying the oil retained in the various components of an air conditioning system (Jin and Hrnjak, 2014). Alonso et al. (2010), measured the oil hold up during the condensation and evaporation of refrigerant. The saturation temperature was 77 F (25 C), the oil mass fraction (OMF)
12、 varied from 3 weight percent (wt.%) to 5 wt.%, the mass flux varied from 20 to 61 lbm/ft2-s (100 to 300 kg/m2-s), and the quality was from 20 % to 95 %. Their results showed that oil hold up was the lowest in the mid-range of quality and it increased when the refrigerant approached saturated vapor.
13、 They also pointed out that oil hold up was greater in enhanced tubes compared to the smooth tubes. Lee et al. (2002) measured oil retention characteristics in a CO2 air-conditioning system. They measured the oil retained in an evaporator and in a gas cooler by using an oil injection and extraction
14、method. Their results suggested that the amount of oil retained in the evaporator was larger than that of the gas cooler. Cremaschi et al. (2005) measured the oil retained in fin-and-tube condenser and evaporator by using a similar oil injection and extraction experimental technique and for various
15、refrigerant and oil mixtures, including refrigerant R410A and Polyolester (POE) lubricant. They observed that at OMF of 5 wt.%, the mass of oil held up in the evaporator and suction line was about 25% of the total mass of oil initially charged into the compressor. The cooling capacity and COP of the
16、 air conditioning system were penalized when oil was retained in the evaporator. Both Lee et al. (2002) and Cremaschi et al. (2005), reported that the oil retention in the evaporator and suction line decreased if the mass flux of the refrigerant increased. Authors previous work (Yatim et al. (2014)
17、reported that if the OMF increased then the oil retained in a microchannel condenser also increased and the thermal performance of the condenser were degraded. In the open literature domain there is a lack of studies that quantify the oil retention in microchannel evaporators and that isolate the ef
18、fects of oil retention on the thermal performance of microchannel type heat exchangers. This paper focuses on addressing these gaps for refrigerant R410A and POE oil mixture and for one type of microchannel evaporator adopted in commercially available air conditioning systems for residential applica
19、tions. EXPERIMENTAL SETUP, METHODOLOGY AND TEST CONDITIONS The oil retention experiments were conducted by using a laboratory test apparatus referred as to pump-boiler loop system, shown in Figure 1. Liquid refrigerant from a gear pump (component 4) was circulated to a Coriolis mass flow meter and t
20、hen to preheater tube heat exchangers. In the preheaters, the refrigerant was heated up to near saturation conditions before entering the microchannel evaporator. The refrigerant inlet conditions were controlled at near saturated liquid conditions with the aim to promote uniform distribution of refr
21、igerant and of the refrigerant and oil mixture. Infrared thermal images of the microchannel evaporator were taken to confirm that the distribution was uniform. An example is shown in Figure 2, in which the refrigerant flow provided uniform surface temperature measurements along the horizontal sectio
22、ns. The liquid refrigerant entered at the bottom header, evaporated in the vertical microchannel tubes, and exited as superheated vapor refrigerant at the top header. Two sight glasses (indicated by the symbols S2 and S3 in Figure 1) were installed at the outlet of the test section. The refrigerant
23、vapor and oil mixture circulated toward the oil separators of the oil extraction device. From the outlet of the oil separators, the vapor refrigerant circulated to the condenser and was brought to subcooled liquid conditions before it went back to the gear pump. A large-scale psychrometric chamber (
24、Cremaschi this image indicates refrigerant flowing vertically from bottom to top of the heat exchanger and fairly uniform refrigerant flow distribution Where g1839g1853g4666uni0040g2899g2897g2890g2880g2934uni0009g2933g2930uni0025uni0040g2919g2924g2922g2915g2930g4667g3404g3505 g1865g4662g3042g3036g30
25、39,g3036g3041g3037g3032g3030g3047g3032g3031uni0040g3036g3041g3039g3032g3047uni0009g1856g1872g3047g3117,g3284g3289g3047g3116,g3284g3289g3404g1839g1853uni0009uni0009uni0009uni0009g4670g1864g1854g3040g4666g1863g1859g4667g4671 (3) and g1839g1854g4666uni0040g2899g2897g2890g2880g4666g2934g2879g2934g4667un
26、i0009g2933g2930uni0025uni0040g2925g2931g2930g2922g2915g2930g4667g3404g3505 g1865g4662g3042g3036g3039,g3036g3041g3037g3032g3030g3047g3032g3031uni0040g3042g3048g3047g3039g3032g3047uni0009g1856g1872g3047g3117,g3290g3296g3295g3047g3290,g3290g3296g3295g3404g1839g1854uni0009uni0009uni0009uni0009g4670g1864
27、g1854g3040g4666g1863g1859g4667g4671uni0009uni0009 (4) Here the g1865g4662g3042g3036g3039,g3036g3041g3037g3032g3030g3047g3032g3031uni0009was the measured mass flow rate of oil metered at the inlet and at the outlet of the test sectionand, by accounting for the oil solubility, it excluded the amount o
28、f refrigerant dissolved in the oil. The time t0,in and t0,out were the times at which the oil was first released to the inlet and outlet of the test section. The time t1,in and t1,out were the times at which the oil was detected at the sight glass S2 for oil injections at the inlet and outlet of the
29、 test section. The oil retention volume in the test section was calculated as follows: g1841g1844g1874g1867g1864g1873g1865g1857uni0040g3016g3014g3007g2880g3051uni0009g3050g3047.uni0025mg2871 = g1841g1844g1865g1853g1871g1871uni0040g3016g3014g3007g2880g3051uni0009g3050g3047.uni0025uni0009g1864g1854g30
30、40(g1863g1859)g2925g2919g2922uni0040g2874g2876g2890uni0009uni0009g1864g1854g3040/g1858g1872g2871(g1863g1859/g1865g2871) (5) Here the oil was the density of the oil at reference temperature of 68 F (20 C). The oil retention inside the microchannel heat exchanger was normalized with respect to the tot
31、al internal volume of the heat exchanger, that is, internal volume of the tubes plus the internal volume of the headers: g1841g1844g1848g3015,uni0040g3016g3014g3007g2880g3051uni0009g3050g3047.uni0025uni0009 = g1841g1844g1874g1867g1864g1873g1865g1857uni0040g3016g3014g3007g2880g3051uni0009g3050g3047.u
32、ni0025ftg2871(mg2871)uni0009g1848g3047g3042g3047g3028g3039,g3036g3041g3047g3032g3045g3041g3028g3039uni0009g3049g3042g3039g3048g3040g3032uni0009ftg2871(mg2871)uni0009 (6) The ORVN was a dimensionless number that varied from 0 for the case of no oil retained in the heat exchanger up to 1, if the heat
33、exchanger was completely filled with oil. It the present experiments the ORVN varied from 0.01 to 0.13, that is, the oil retained in the heat exchanger ranged from 1 up to 13 percent of the total internal volume of the heat exchanger. The oil effect on pressure drop was estimated by measuring the pr
34、essure drop in the test section at various mass flow rates and OMFs. The refrigerant side pressure drop at the given OMF, pwith oil, was compared to the corresponding pressure drop across the test section at the same total mass flow rate but with no oil present, pwithout oil. The pressure drop facto
35、r (PDF) was used to represent the effect of the oil and it was defined as follows: oilwithoutoilwithppPDF= (7) The PDF is a cumulative factor that accounts for both acceleration and friction pressure drop components e.g. alteration of two-phase flow regime, increase in the liquid phase viscosity and
36、 the reduction of the free-flow area available to the refrigerant flow due to the oil retention. Similarly, the microchannel heat exchanger heat transfer capacity factor, HTF, was calculated based on the heat transfer rate measured from the air-side during tests with oil and the corresponding heat t
37、ransfer rate measured during the baseline tests without oil, as follows: oilwithoutairoilwithairQQHTF,the solid circle points (series G) are for low mass flux while the void circle points (series J) are for high mass flux. The results indicate that if the oil mass fraction Figure 3 ORVN vs. OMF for
38、microchannel evaporator at two mass fluxes 0.000.050.100.150 1 2 3 4 5 6ORV N-OMF wt %Lowmass fluxHigh mass fluxTsatF -Mass FluxG- 33 - LowH- 38 - LowI - 48 - LowJ - 33 - HighK- 38 - HighL - 48 - High(OMF) increased, the heat transfer factor of microchannel evaporator decreased from 1 to 0.87. This
39、represents a reduction of the refrigerant-side heat transfer rate due to oil by about 13%. However, at OMF of 1 wt.%, the reduction of the refrigerant side heat transfer rate was within the experimental uncertainty of 4.5%. From Figure 4 it is clear that the effect of oil on heat transfer rate was t
40、o decrease the refrigerant-side heat transfer rate but the impact of oil was not significant if OMF ranged from 0.5 wt. % to 1 wt. %. The impact of oil on the heat transfer rate was measurable for OMFs of 3 wt. % and of 5 wt. % and the heat transfer rate decreased by about 8 and 13%. At OMF of 3 wt.
41、%, a reduction of mass flux from high mass flux of 8.4 lbm/ft2-s (20.4 kg/m2-s, series K) to low mass flux of 4.2 lbm/ft2-s (20.4 kg/m2-s, series H) decreased the HTF from 0.96 (series K in Figure 4) to 0.90 (series H) for the same saturation temperature of 38F (3.3C). Hu et al. (2011) conducted hea
42、t transfer experiments to study the heat transfer coefficient of R410A and POE oil mixture during flow boiling in a 0.28 inches (7 mm) diameter smooth tube. Their analysis showed that decreasing the mass flux of R410A/POE mixture at particular quality and oil concentration resulted in diminished loc
43、al heat transfer coefficient. From this point of view, their finding was in agreement with the experimental results of Figure 4, in which the HTFs for low mass flux series (letters G, H, and I) tended to be slightly lower than the HTFs for the high mass flux series (J, K and L). Furthermore, it was
44、recalled that for the same OMF and saturation temperature, oil retention in the low mass flux series (G, H, and I) was higher when compared to that of high mass flux series (J, K and L). Due to higher oil retention for the lowmass flux test series G, H, and I, the oil was likely to create a higher t
45、hermal resistance to heat transfer process in microchannel tubes than for the high mass flux test series J, K, and L. Pressure Drop Factor in the Microchannel Evaporator The PDF isolates and quantifies the effect of oil on the refrigerant-side pressure drop and the results are summarized in Figure 5
46、, in which OMF is on the x-axis and the PDF is on the y-axis for two mass fluxes. When OMF was 0 wt.% (i.e., no oil was present inside the evaporator), the PDF resulted 1 by its own definition and it decreased if OMF increased as shown in Figure 5. The results indicated that if the oil mass fraction
47、 (OMF) increased, the pressure drop factor of microchannel evaporator increased from 1 up to 1.47. Figure 4 HTF vs. OMF for a microchannel evaporator at three sat. temp. and two mass fluxes Figure 5 PDF vs. OMF for a microchannel evaporator at three sat. temp. and two mass fluxes This represents an
48、augmentation of the refrigerant-side pressure drop due to oil by about 47%. For typical air-conditioning applications, when OMF is typically less than 1 wt. %, the increase in pressure drop was about 16%. The 0.80.91.01.11.20 1 2 3 4 5 6HTF -OMF wt %TsatF -Mass FluxG- 33 - LowH- 38 - LowI - 48 - Low
49、J - 33 - HighK- 38 - HighL - 48 - High0.50.81.01.31.50 1 2 3 4 5 6PDF -OMF wt %High mass fluxLow mass fluxTsatF -Mass FluxG- 33 - LowH- 38 - LowI - 48 - LowJ - 33 - HighK- 38 - HighL - 48 - HighPDFs were slightly above 1 for OMF less than 0.5 wt. % and for both high and low mass flux. For OMF the impact of oil was still significant even when OMF ranged from 0.5 wt. % and 1 wt. %. For OMF higher than 1 wt. %, the impact of mass flux on PDF was measurable. For example, at OMF of 3 wt.%, an increase of mass flux from low mass flux o
