1、2009 ASHRAE 599This paper is based on findings resulting from ASHRAE Research Project RP-1394.ABSTRACTExperimental study of condensation of carbon dioxide in brazed plate heat exchangers is the main objective of this research project. However, it is essential to characterize the single-phase flow th
2、rough these minichannel heat exchangers in order to analyze and formulate the two-phase flow. In this manuscript, the open literature on the subject is reviewed first, the facility for testing the entire system is then described, and the initial results on the single-phase flow are presented at the
3、end. Three brazed plate heat exchangers with different interior configurations, each consisting of three channels, are consid-ered and tested in this study. For the two-phase analysis, carbon dioxide is the working fluid, flowing through the middle channel, while dynalene is the cooling fluid, flowi
4、ng through the side channels. For the single-phase analysis, data was taken using hot and cold water flow through the middle and side channels, respectively. Data was also taken using hot water in the middle and chilled dynalene in the surrounding channels. The modified Wilson plot technique was app
5、lied to obtain single-phase heat transfer coefficients, and Fanning friction factor was estimated for the pressure drop. The resulted correlations were within reasonable range of stan-dard deviation and uncertainty, and compared well with other relevant studies.INTRODUCTIONGlobal warming concerns ar
6、e gaining momentum in the twenty-first century and as such, environmentally friendly refrigerants are quickly becoming a necessity rather than just an interesting topic to speculate about for the future. R-744 or carbon dioxide (CO2) is a top contender, as instigated in the Montreal protocol, to pha
7、se out the use of ozone depleting chlorofluorocarbon (CFC) and the greenhouse gas contribut-ing hydrochlorofluorocarbon (HCFC) refrigerants, especially in low temperature applications. As mentioned by Bodinus (1999), the idea of using carbon dioxide as a refrigerant started in the mid 19th Century b
8、y Alexander Twining; however, it was not readily implemented until Franz Windhausen made a CO2compressor in 1886. CO2refrigeration systems gained popularity until the late 1920s and early 1930s in the great depression. Due to the rise in demand for smaller systems in the non-commercial refrigeration
9、 market, coupled with the extremely high pressures required to use CO2as a refrigerant, an innovation was needed. Companies such as General Motors and DuPont funded research to develop new refrigerants that could operate under much lower pressures, Pearson (2005). Thus, synthetic CFC refrigerants we
10、re invented, which allowed refrigeration units to be sized much smaller and cheaper due to lower working pressure requirements. When these new refrigerants were developed, research and use of R-744 was greatly reduced.With the introduction of new CFCs/HCFCs refrigerants, such as R-12 and R-22, the r
11、efrigeration markets needs were met, but at the same time these innovations created environmen-tal problems. The chlorine molecules in CFCs/HCFCs proved to be harmful when leaked and released into the atmosphere. The ozone molecules in the stratosphere, which protect humans from harmful ultra-violet
12、 rays, are absorbed and destroyed by the chlorine molecules through a chemical reaction. In the early 1990s, a few hydrofluorocarbon (HFC) refrigerants, such as R-134a, were developed in which chlorine molecules were totally Dynalene/Water Correlations to Be Used for Condensation of CO2in Brazed Pla
13、te Heat ExchangersNiel Hayes Amir Jokar, PhDStudent Member ASHRAE Member ASHRAENiel Hayes is a graduate research assistant and Amir Jokar is an assistant professor in the School of Engineering and Computer Science, Wash-ington State University Vancouver, Vancouver, WA.LO-09-57 (RP-1394) 2009, Americ
14、an Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior wr
15、itten permission.600 ASHRAE Transactionsreplaced by hydrogen molecules. In spite of the improvement regarding the ozone depletion dilemma, HFCs are still not perfect refrigerants due to their excess carbon, which contrib-utes to the potential for global warming. In general, the existing refrigerants
16、 in todays market have two measured potential harmful effects on the environment: Global Warming Potential (GWP) and Ozone Depletion Potential (ODP). Global Warming Potential measures how harmful a greenhouse gas can be compared to carbon dioxide, which is defined as the GWP base reference having a
17、value of 1. Ozone Depletion Potential char-acterizes how harmful a chemical compound can be in depleting the ozone layer on a scale of 0 to 1. These two measured poten-tials are typically presented for common refrigerants in Table 1.Comparing GWPs and ODPs of R-744 to R-134a, R-22, and R-12 in Table
18、 1, one can find that more research and appli-cation must be established to further the progress of CO2as an eco-friendly refrigerant.Not only is carbon dioxide promising as a refrigerant with respect to the environments protection, but it also works as a great refrigerant due to its abundance, safe
19、ty, as well as its thermophysical properties. Halder and Sarkar (2001) found that CO2had several advantages over conventional refriger-ants, which included lower pumping power requirements (attributed to a lower required volumetric flow rate), higher efficiency in heat exchangers, and higher latent
20、heat. However, one of the leading factors in the decline of CO2as a practical refrigerant in the early 20th century was due to the lack of tech-nology in using this refrigerant under its high pressure demands in smaller applications. With the substantial recent development in heat exchanger and comp
21、ressor technologies however, CO2can now be seriously considered a potential working refrigerant in industrial as well as non-commercial applications.Since the 1930s, plate heat exchangers have served well for single-phase heat transfer applications, e.g., beverage and food processing, pharmaceutical
22、 industries, paper and rubber industries, and dairy pasteurization, as mentioned by Shah and Wanniarachchi (1992). The introduction of brazed plate heat exchangers in the 1970s, which brazed the plates together rather than using gaskets, bolts, and carrying bars, unlocked the possibilities of using
23、refrigerants that require higher oper-ating pressures and larger temperature ranges. Plate heat exchangers are innovative in several different aspects of heat transfer; total heat transfer area per total volume is quite large compared to other types of heat exchangers, and high turbu-lence can be ac
24、hieved even at low flow rates which results in high heat transfer coefficients. One of the drawbacks to the brazed plate heat exchangers however, is the difficulty of free-ing the plates of fouling, unlike the earlier plate heat exchang-ers that could be disassembled, cleaned, and reassembled.The fo
25、cus of this research is to understand the condensa-tion behavior of CO2in three brazed plate heat exchangers with differing interior plate geometries. To date, no research has been reported in literature on condensing CO2in brazed plate heat exchangers. Nevertheless, there has been research on other
26、 refrigerants being condensed in brazed plate heat exchangers as well as CO2being used in refrigeration systems.LITERATURE REVIEWSingle-Phase Flow in Plate Heat ExchangersTo be able to formulate two-phase flow in the BPHEs, comprehensive single-phase flow experimentation is first required to establi
27、sh single-phase formulation. Table 2 and Table 3 show typical well-established experimental correla-tions for single-phase heat transfer coefficients and friction factors, respectively, in PHEs, as presented by Ayub (2003).Condensation in PHEsWhile heat transfer coefficients and pressure drops have
28、been studied extensively in the single-phase realm in PHEs, as documented by Ayub (2003), two-phase vaporization and condensation have not received as much attention, let alone in BPHEs.A comprehensive condensation table of plate channels has been presented by Wrfel and Ostrowski (2004). Various ent
29、ries of that table, as well as more recent findings, are focused on in the present section of literature review.Panchal (1985) observed condensation heat transfer of ammonia in Alfa-Laval PHEs. Two exchangers with chevron angles of 60 and 30 were used. Heat transfer was observed using the different
30、parameters of film Reynolds numbers (200-2000) in the high angle plate and low angle plates. Experimen-tal data was compared against theoretical calculations. The conclusion that was made stated that for both low and high angle plates the heat transfer coefficients increased with the film Reynolds n
31、umber or remained constant. This behavior was attributed to high interfacial shear stress for laminar-film condensation and the occurrence of the shear-stress-controlled condensation at low Reynolds numbers. Although no analytical relationship was made from Panchals (1985) experiments, it was compar
32、ed to Tovazhnyanskis (1984) Nusselt numbers for high and low angle plates:Nu = 0.34924 Re0.75Pr 0.4(high-angle plates) (1)Nu = 0.11159 Re0.75Pr0.4(low-angle plates) (2)Table 1. Adverse Affects of Refrigerants on the Environment from the United Nations Environment Program (UNEP-2006)Refrigerants ODPG
33、WP (100 Year Time Horizon)R-12 (CFC) 1.00 10,890R-22 (HCFC) 0.050 1810R-134a (HFC) 0 1430R-744 or CO2(nature friendly) 0 1ASHRAE Transactions 601Water-steam condensation was studied by Wang and Zhao (1993) in a plate heat exchanger with a chevron angle of 45 degrees, consisting of three channels. Pa
34、rameters measured were flow, steam content, temperature differences, and pres-sures to obtain heat transfer coefficients. Condensation heat transfer was directly proportional to pressure drop. The heat flux in the plate heat exchanger was consistently larger than shell and tube condensers, this was
35、due in part by the complex configurations of the channels, small cross sections, and shearing of steam flow with high velocities.Arman and Rabas (1995) reported their results using Table 2. Single-Phase Heat Transfer Correlations Using an Enlargement Factor of Unity ( = 1)General Single-Phase Heat T
36、ransfer Correlation: Nu = C1ReC2PrC3(/w)C4 Re C1C2C3C4Muley and Manglik (1999)30 1000 0.09 0.70 1/3 0.1445 1000 0.08 0.76 1/3 0.1460 1000 0.08 0.78 1/3 0.14Focke et al. (1985)3020150 1.89 0.46 0.50 0.00150600 0.57 0.70 0.50 0.0060016,000 1.11 0.60 0.50 0.004545300 1.67 0.44 0.50 0.003002000 0.41 0.7
37、0 0.50 0.00200020,000 0.84 0.60 0.50 0.00601201000 0.77 0.54 0.50 0.00100042,000 0.44 0.64 0.50 0.00Thonon (1995)30 50 Re 15,000 0.29 0.70 1/3 0.0045 50 Re 15,000 0.30 0.65 1/3 0.0060 50 Re 15,000 0.23 0.63 1/3 0.00Table 3. Single-Phase Pressure Drop Correlations using an Enlargement Factor of Unity
38、 ( = 1)General Friction Factor Correlation: f = C5ReC6+ C7 Re C5C6C7Muley and Manglik (1999)30 10100 19.40 0.59 045 15300 18.29 0.65 060 40400 3.24 0.63 0Focke et al. (1985)3090400 188.75 1.00 1.2640016,000 6.70 0.21 0451501800 91.75 1.00 0.30180030,000 1.46 0.18 0602603000 57.50 1.00 0.09300050,000
39、 0.90 0.26 0Thonon (1995)30 160 45.57 0.67 0160 3.7 0.17 045 200 18.19 0.68 0200 0.69 0.17 060 550 26.34 0.83 0550 0.57 0.22 0602 ASHRAE Transactionsammonia in their PHEs with chevron angles of 30 and 60 to test single component condensation correlation equations. A propane/butane mixture was used i
40、n a plate heat exchanger (angle not specified) to study binary-component condensation as well. The single component condensation results revealed that the high chevron angle achieved high pressure drops at Reynolds numbers (5000-7500), where the low angle plate achieved the same pressure drop at alm
41、ost three times the Reynolds numbers. Also, from the tables in the paper, it appeared that the high angle chevron plate required a lower mass flow rate than the low angle chevron plate to achieve the same heat transfer coefficients.Yan et al. (1999) found that condensation trends for R-134a in PHEs
42、showed greater heat transfer coefficients as well as pressure drops as vapor quality increased. Their experimen-tal parameters included mass flux, average imposed heat flux, saturated pressure, and vapor quality. Using an inclination angle of 60 in the 500 mm (19.68 in.) by 120 mm (4.72 in.) plates,
43、 it was noted that the chevron configuration in PHEs create high turbulence at low Reynolds numbers which increased heat transfer at slower flow rates. The following heat transfer coefficient and friction factor were correlated:Nu = 4.118 Reeq0.4Prl1/3(3)ft pRe0.4Bo0.5( pm /pc)0.8= 94.75 Reeq0.0467(
44、4)Jokar et al. (2006) explored single and two-phase heat transfer and pressure drop correlations of R-134a that were found in three heat exchanger depths (34, 40, and 54 plates) of 112 mm (4.41 in.) by 311 mm (12.24 in.) plates all having a corrugation inclination angles of 60. These heat exchangers
45、 were used as both evaporators as well as condensers in an auto-motive dual refrigeration system. The two-phase flow corre-lations proved to be much more complex than that of the single-phase. The following correlations were proposed for the two-phase condensation heat transfer and pressure drop:(5)
46、Cf, tp= 2.139 107(GDhyd /m, sat)1.6for 960 (GDhyd /m, sat) 4169 (6)Important two-phase behaviors were observed in the experimentation of condensation in the BPHEs; e.g., when the temperature differences were not large, film condensation proved to be the dominant characteristic when determining heat
47、transfer correlations. Also, as the mass flow rates increased within the minichannels of the BPHEs, convection proved to be more important in driving heat to transfer.Longo and Gasparella (2007) studied the condensation of R-134a in BPHEs. Three parameters were observed to conclude how each affected
48、 the heat transfer coefficients and pressure drop in the BPHE; mass flux, saturated R-134a vapor, and super heated R-134a vapor. The BPHEs used consisted of 10 plates, 72 mm (2.83 in.) by 310 mm (12.20 in.) that had the inclination chevron angle of 65. It appeared that gravity controlled condensatio
49、n occurred at mass fluxes less than 20 kg/m2s (4.10 lbm/ft2s), but when the flux exceeded this, forced convection condensation was achieved, producing a 30% increased heat transfer coefficient when the mass flux was 40 kg/m2s (8.19 lbm/ft2s). The behavior of the saturated vapor versus the super heat was similar, with the super heat yielding only an 810% increase in heat transfer over the satu-rate. Saturation temperature however played no great signifi-cance in heat transfer in the BPHE. For saturated vapor condensation Lo
