Effect of lubricating oil on flow boiling heat transfer of carbon dioxide (original) (raw)
Related papers
International Journal of Refrigeration, 2014
A flow boiling heat transfer model for horizontal tubes is proposed for CO 2 with entrained polyalkylene glycol (PAG oil) in the pre-dryout region. A general power law-type model with a power number of 3 is used together with the average thermodynamic properties of the CO 2-oil mixture. A convective enhancement factor (F) is recommended according to the relationship between the Lockhart-Martinelli parameter and the ratio α tp /α l , which was obtained based on previous experimental results for CO 2 and oil. A new suppression factor (S) is introduced that comprises a suppression term for forced convection and oil concentration term for bubble generation. A comparison of six correlations showed that the proposed correlation can depict the influence of the mass and heat fluxes on both nucleate and convection boiling reasonably well.
2012
Flow boiling heat transfer characteristics of CO 2 with and without oil were investigated experimentally in horizontal smooth and enhanced tubes with an inner diameter of 11.2 mm. The visualization of flow pattern was also performed to provide the detailed attributes of the nucleate and the convective boiling heat transfer. In order to investigate the effect of the miscible oil on the heat transfer of CO 2 , POE (polyolester) RENSIO C85E oil is added to give an oil circulation rate (OCR) between 0.5 % and 2 %. Results are compared with those of pure CO 2. The experimental conditions include evaporation temperatures of-15 °C, mass fluxes from 40 to 200 kg/m 2 s, heat fluxes from 0.5 to 10 kW/m 2 , and vapor qualities from 0.1 to 0.8. Oil generally deteriorates the heat transfer coefficient of pure CO 2. The reduction in heat transfer coefficient is most apparent at low vapor qualities, 0.1 to 0.4, and at low mass fluxes, 100 and 200 kg/m 2. It is caused by the suppression of nucleate boiling due to increased surface tension. At conditions where the convective boiling contribution is dominant, vapor qualities above 0.5, oil increases heat transfer coefficients. Through visualization, it is shown that the wetted area on the perimeter of inner tube is enhanced due to formation of foaming in the smooth tube. However, such enhancement of heat transfer due to forming is negligible in the enhanced tube, because the enhanced factor due to micro-finned structures is dominant.
Effect of lubricating oil on cooling heat transfer of supercritical carbon dioxide
International Journal of Refrigeration-revue Internationale Du Froid, 2007
In this research, the cooling heat transfer coefficient and pressure drop of supercritical CO 2 with PAG-type lubricating oil entrained were experimentally investigated. The inner diameter of the test tubes ranged from 1 to 6 mm. The experiments were conducted at lubricating oil concentrations from 0 to 5%, pressures from 8 to 10 MPa, mass fluxes from 200 to 1200 kg m À2 s À1 , and heat fluxes from 12 to 24 kW m À2 .
2010
In this research, we studied the effects of lubricating oil on the heat transfer performance of supercritical carbon dioxide by applying three different lubricants: polyalkylene glycol (PAG), polyvinyl ether (PVE), and PAG-PVE copolymer (ECP). The heat transfer coefficient measurement and the flow pattern visualization were conducted using a horizontal smooth tube with inner diameter of 2 mm at pressures ranging from 8 to 10 MPa and mass fluxes from 800 to 1200 kg m -2 s -1 . The compatibility of the lubricants with supercritical CO 2 was found to have a remarkable influence on the heat transfer coefficient. Changes in the flow patterns of CO 2 mixed with the three different oils are observed with increasing temperature as a result of the different properties of the oils. For the lubricant in which CO 2 has the highest solubility, i.e., PVE, the oil film does not form at temperatures lower than the pseudocritical temperature, and the heat transfer coefficient does not greatly change with oil concentration. At higher temperatures, a decrease in the heat transfer coefficient with increasing oil concentration was observed for all three lubricants because of the formation of an oil film. ECP is inferior to PVE but provides better heat transfer characteristics than PVG.
International Journal of Refrigeration, 2012
Effects of lubricating oil on heat transfer performance of supercritical CO 2 were studied by applying three lubricants: PAG, PVE, and ECP. Heat transfer coefficient measurements and flow-pattern visualization were conducted in a horizontal tube of 2 mm I.D. at CO 2 pressures from 8 to 10 MPa and mass fluxes from 800 to 1200 kg m-2 s-1. The solubility of lubricants with CO 2 was found having remarkable influence on both the flow pattern and heat transfer coefficient. For PVE, which has the highest CO 2 solubility, oil droplets can only be observed occasionally and the oil film can hardly be identified at temperatures lower than T pc , and the heat transfer coefficient does not greatly change with oil concentration. At higher temperatures, a decrease in the heat transfer coefficient with increasing oil concentration was observed for all three lubricants due to the formation of oil film. The experiments show that while ECP is inferior to PVE, it provides better heat transfer performance than PVG.
2010
Flow boiling heat transfer characteristics of R744 with and without oil were investigated experimentally in horizontal smooth tubes with an inner diameter of 11.2 mm. In order to investigate the effect of the miscible oil on the heat transfer of R744, POE (polyolester) RENSIO C85E oil is added to give an oil circulation rate (OCR) between 0.5% and 4%. Results are compared with those of pure R744. The experimental conditions include evaporation temperatures of -15 and -30 °C, mass fluxes from 40 to 200 kg/m 2 s, heat fluxes from 2 to 10 kW/m 2 , and vapor qualities from 0.1 to 0.8. The heat transfer coefficients of pure R744 show strong dependence on heat flux due to the dominance of the nucleate boiling. However, heat transfer also depends on mass flux and evaporation temperature due to enhanced convective boiling contribution at higher mass flux, 200 kg/m 2 s, and change in thermophysical properties. The pressure of oil generally deteriorates the heat transfer coefficient of pure R744. The reduction in heat transfer coefficient is most apparent at low qualities, 0.1 to 0.4, and at low mass fluxes, 40 kg/m 2 . It is caused by the suppression of nucleate boiling due to increased surface tension. At conditions where the convective boiling contribution is dominant, qualities above 0.5 and mass fluxes above 100 kg/m 2 s, oil increases heat transfer coefficients.
2006
This paper will develop an equation based approach for the VLE and VLLE properties of CO 2 -lubricant mixtures. Since carbon dioxide is only partially soluble in many of the working lubricants now available, knowing the separation temperature (miscibility) and phase inversion temperature is of importance in the design of heat exchangers, compressors and auxiliary equipment. Also, of special interest to compressor design engineers is the knowledge of the CO 2 -lubricant liquid viscosity; and for microchannel heat exchanger designers, the interfacial surface tension can be vitally important.
Proceedings of the 4th World Congress on Momentum, Heat and Mass Transfer, 2019
In order to design evaporators for the CO2 thermal systems effectively, it is essential to understand the fundamentals and mechanisms of flow boiling heat transfer, flow patterns and two-phase flow characteristics of CO2 inside horizontal tubes including both the macro-and micro-channels. The proper prediction models for CO2 flow boiling heat transfer should be relevant to the physical mechanisms and corresponding flow patterns. This paper presents a comprehensive review of flow boiling heat transfer and two-phase flow of CO2 characteristics and their prediction methods. First, the review addresses the extensive experimental studies on flow boiling heat transfer and two-phase flow in macro-channels and micro-channels. Then, The studies of CO2 two-phase flow patterns are summarized. Furthermore, the effects of oil on the flow boiling heat transfer and two-phase pressure drops are analyzed. The generalized mechanistic models for flow boiling heat transfer and two-phase pressure drops of CO2 and a new flow pattern map specially for CO2, which cover both macro-and micro-channels developed by Cheng et al. are presented and compared to the experimental data in the literature. It has been proved that the models and flow maps favorably agree with the experimental data. In the application aspect, comparison of simulation results and the experimental data in the real thermal systems are presented.