ASHRAE LV-11-C023-2011 In-tube Boiling Heat Transfer of CO2 Lubricant Mixture at Low Temperatures Preliminary Results.pdf

1、Pradeep Bansal is a professor in the Department of Mechanical Engineering, The University of Auckland, New Zealand. ORNL In-tube Boiling Heat Transfer of CO2Lubricant Mixture at Low Temperatures: Preliminary Results Pradeep Bansal, PhD Fellow ASHRAE ABSTRACT The use of CO2in cascade refrigeration sy

2、stems is increasing to achieve low temperatures, particularly in food and refrigeration industries, where understanding of the flow boiling heat transfer mechanisms is essential for heat exchanger design. This paper presents preliminary experimental results of the flow boiling heat transfer of CO2 l

3、ubricant mixture at low saturation temperatures varying between -30oC to -40oC, oil concentration varying between 0 4%, along with the effects of refrigerant vapour quality. The addition of low oil quantities to the flow (80%). INTRODUCTION Recently there has been a resurgence of CO2as a useful refr

4、igerant in cascade refrigeration systems1-2in food and refrigeration industry down to -45oC due to the phase out of commonly used refrigerants containing chlorofluorocarbons (CFC) and hydro chlorofluorocarbons (HCFC). CO2 is a natural, environmentally friendly, non-GWP alternative refrigerant. The f

5、irst CO2system was built in the 1860s and its use in refrigeration systems continued until the 1930s. The introduction of CFC based refrigerants around this time led to the phase out of CO2due to its inherent disadvantages of high pressure containment, and capacity and efficiency loss at high temper

6、ature. The recent popularity of CO2in air-conditioning and industrial cascade refrigeration systems, has led to a reinvigorated development of technology3in areas such as CO2 lubricants, CO2 compressors and novel heat exchangers that require better understanding of the heat transfer and flow charact

7、eristics of CO2- lubricant mixtures. LV-11-C023186 ASHRAE Transactions2011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmi

8、ssion in either print or digital form is not permitted without ASHRAES prior written permission.There is a variety of commercially available CO2lubricants that are commonly used in industrial cascade refrigeration systems. These lubricants range in solubility with CO2from completely immiscible to mi

9、scible. Non-soluble lubricants such as Alklybenzenes (ABs) and Polyalphaolefins (PAOs) are still applicable due to their flow properties at low temperatures. Polyalkylene glycol (PAG) is partially miscible4with CO2. However, PAG is not miscible with CO2at high concentrations, while Polyol Ester (POE

10、) is found to be completely miscible with CO2 by Li and Rajewski5. With the addition of miscible lubricant, the refrigerant-lubricant mixture usually experiences increased density, surface tension, viscosity and heat conductivity. In CO2-oil mixtures, however, the rapid increase in surface tension a

11、nd viscosity at oil concentrations of more than 3% lead to a reduced heat transfer coefficient when compared to a pure CO2fluid as noted by Zhao and Bansal3. The mixture usually has a higher relative viscosity that results in preferential evaporation of pure refrigerant compared to the lubricant3in

12、the evaporators of vapour compression refrigeration systems. Flow boiling heat transfer of pure COhas been studied by several researchers, including Park and Hrnjak6and Zhao and Bansal7-9. Zhao and Bansal8-9noted that the boiling heat transfer coefficient at low temperatures (-300C) increases with v

13、apour quality until dryout when the maximum heat transfer coefficient occurs. Lower surface tension of CO2facilitates the bubble formation, thus resulting in higher nucleate boiling at low vapour quality. The nucleate boiling of COis more active than the conventional refrigerants, and dominates over

14、 most vapour quality during the flow boiling process. However, the heat transfer coefficient for CO2-lubricant mixture is found to reduce with the increase in the percentage of lubricant10. A review study by Zhao and Bansal3found that at small oil concentrations (75%). At low oil concentrations (2-3

15、%) this relation is well defined. At low vapour qualities (80%). This can be attributed to the increase in local oil concentration reducing the effect of saturation temperature through the creation of oil rich sub layer near the heating surface, in addition to difficulties in effective mixing within

16、 the CO2. Figure 5(d) also illustrates the negligible effect of vapour quality on higher oil concentrations, where heat transfer remains almost independent of quality until dry out occurs. Figure 5: Variation of htpwith CO2-lubricant vapour quality at G=259kg/m2.s, q=16kW/m2,Tsat=-30 and -40C, and o

17、il concentration of (a) 1%, (b) 2%, (c) 3% and (d) 4%. 0.511.522.533.544.555.50.2 0.4 0.6 0.8 1Heat Transfer Coefficient(kW/m2.K)Vapour Quality (x)-30 Saturation Temperature-400.511.522.533.544.555.50.2 0.4 0.6 0.8 1Heat Transfer Coefficient (kW/m2.K)Vapour Quality (x)-30 Saturation Temperature-400.

18、511.522.533.544.555.50.2 0.4 0.6 0.8 1Heat Transfer Coefficient (kW/m2.K)Vapour Quality (x)-30 Saturation Temperature-400.511.522.533.544.550.2 0.4 0.6 0.8 1Heat Transfer Coefficient (kW/m2.K)Vapour Quality (x)-30 Saturation Temperature-40(b) (a) (d) (c) 192 ASHRAE TransactionsCONCLUSIONS AND RECOMM

19、ENDATIONS This paper presented experimental results on the flow boiling heat transfer of CO2-lubricant mixture at low saturation temperatures between -30C to -40C and oil concentrations between 0 and 4%. The study revealed that small concentrations (1-3%) of lubricant increased the local heat transf

20、er coefficients of the refrigerant mixture at low vapour qualities (3%) proved detrimental to heat transfer coefficients at high vapour quality (70%). Lowering saturation temperature for CO2-lubricant mixtures with (2-4%) concentration decreases the heat transfer coefficient significantly at high va

21、pour qualities (70%). The work needs to be extended to collect additional experimental data with other lubricants (including immiscible in CO2) at varying saturation temperatures, heat fluxes, mass fluxes, tube geometries and surface types. This will then enable the development of an empirical corre

22、lation for the prediction of flow boiling heat transfer at low saturation temperatures involving CO2-lubricant mixtures. Such a correlation will help designers to develop more efficient novel heat exchangers. ACKNOWLEDGEMENTS The author is thankful to Messrs Chris Wade and Aaron de Pont for working

23、on the project and collecting data, and Mr Ossama Iqbal for his editorial help. NOMENCLATURE diInner diameter, m doOuter diameter, m G Mass flux, kg m-2s-1h Heat transfer coefficient, W m-2K-1SUBSCRIPTS i ifgEnthalpy, J kg-1Latent heat of evaporation, Jkg-1a Ambient e Outlet of the test section k Th

24、ermal conductivity, W m-1K-1fg Liquid-vapour L Length, m i Inlet of the test section mMass flow rate, kgs-1r Refrigerant null Rate of heat flux, W m-2sat Saturation QRate of heat supplied, W tp Two-phase T Temperature, oC w Wall x Vapour quality w,o Outer wall REFERENCES 1. Bansal, P. K., and Jain,

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26、2-lubricant mixtures, Int. J. Heat & Mass Transfer, 52(3-4), pp. 850-879. 4. Kawaguchi, Y., Takesue, M., Kaneko, M., and Tazaki, T. (2000) Performance study of refrigerating oils with CO2, Proc. SAE Automotive Alternate Refrigerants System Symposium, Scottsdale. 2011 ASHRAE 1935. Li, H. and Rajewski

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