Effect of PAG-type lubricating oil on heat transfer characteristics of supercritical carbon dioxide cooled inside a small internally grooved tube (original) (raw)
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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.
2012
It has been reported that lubricating oil has a significant influence on the cooling heat transfer of supercritical carbon dioxide. In this study, numerical analysis of the flow pattern of a carbon dioxide (CO 2)-oil mixture in a gas cooler was conducted to analyze the change in oil film thickness and heat transfer coefficient against bulk temperature. Simulations were conducted using FLUENT software, and the volume-of-fluid (VOF) model was applied to capture the interface of the lubricating oil and CO 2. The compatibility of lubricants was found to have a dramatic influence on cooling heat transfer. The flow regime changes due to CO 2 dissolving into the oil causing changes in viscosity, surface tension, and density of the oil film. The comparison of simulated heat transfer coefficients with experimental results showed that the numerical simulation is an effective approach to analyze the characteristics of the flow and heat transfer of supercritical CO 2-oil mixture, and the numerical approach proposed could be further extended to the actual gas cooler design.
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 flow boiling heat transfer of carbon dioxide
International Journal of Refrigeration, 2013
The flow boiling heat transfer of carbon dioxide with PAG-type lubricating oil entrained from 0% to 5% in horizontal smooth tubes was examined. Experiments were conducted using test tubes with inner diameter of 2-6 mm at mass fluxes of 360-1440 kg m-2 s-1 and heat fluxes of 4.5-36 kW m-2. The saturation temperature was 15°C. At low oil concentrations of 0.5%-1%, the heat transfer coefficient decreased to less than half that under oil-free conditions. The heat transfer coefficient did not decrease further with increasing oil concentration up to 5%. The heat flux positively influenced the heat transfer coefficient in low vapor quality regions, not the high vapor quality regions. The presence of oil caused the mass flux to significantly influence the heat transfer coefficient at a low heat flux till dryout, while no significant influence of the mass flux at a high heat flux was observed. The dryout quality decreased at a large mass flux. The measured pressure drops increased monotonously because of the lubricating oil.
International Journal of Heat and Mass Transfer, 2019
For the different flow orientations and physical models, the influence of buoyancy effect on flow characteristics exist essential difference, the heat transfer behaviors change accordingly. To suppress heat transfer deterioration and improve heat transfer performance, it is quite necessary to discuss the heat transfer behaviors of supercritical CO 2 in various heating tubes. The heat transfer and flow characteristics of supercritical CO 2 heated in the vertically straight tube, horizontal tube, and vertical helical-coiled tube, with inner diameter of 4 mm, are comparatively studied by experiments and numerical simulations. The tests are conducted at operating pressures from 7.5 MPa to 9 MPa, the mass flow rate is in the range of 80-600 kg/(m 2 •s). The heat flux covers a range from 10 kW/m 2 to 70 kW/m 2. The coupling relationship of buoyancy effect and flow characteristics in different physical models are revealed, and the various buoyancy criteria are validated with experimental data. The experimental results confirm that the employment of buoyancy parameter needs to take full account of the interaction between buoyancy force and flow orientations. And experimental data indicate that the heat transfer deterioration in the vertical tube is more serious than it in the horizontal tube. Generally, the helical-coiled tube has a noteworthy advantage for the average heat transfer performance, especially in the case of strong buoyancy. The new empirical correlations for the horizontal tube and helical-coiled tube are proposed with experimental data.