ASHRAE OR-16-C014-2016 Lubricant Retention and its Effects on Heat Transfer and Pressure Drop of a Microchannel Evaporator.pdf
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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
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