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In-tube condensation heat transfer characteristics of CO2 with N2 at near critical pressure
International Journal of Heat and Mass Transfer 144 (2019) 118628
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
In-tube condensation heat transfer characteristics of CO2 with N2 at near critical pressure Wonkeun Baik, Rin Yun ⇑ Hanbat National University, 125 Dongseodaero, Yuseonggu, Daejeon 34158, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 May 2019 Received in revised form 19 July 2019 Accepted 22 August 2019 Available online 30 August 2019 Keywords: CO2 transportation CO2 impurity Condensation heat transfer coefficient Pressure drop Near-critical condition
a b s t r a c t The condensation heat-transfer coefficient and pressure drop for pure CO2 and CO2 + N2 mixtures were investigated under the near-critical condition, which simulates the CO2 transporting conditions under Carbon Capture, Transportation, and Storage (CCS). The experimental apparatus consists of a test section, heat exchangers, mass flow meters, temperature sensors, a magnetic gear pump, and a differential pressure transducer. The test section made with a copper tube was assembled with Polyvinyl Chloride (PVC) pipe to form a double tube. The condensation temperature and mole fraction of N2 of the CO2 mixtures ranged from 20 °C to 30 °C, and 1 to 5%, respectively. The mass flux was changed from 500, 600 and 700 kgm2s1. The average heat transfer coefficient of the CO2 + N2 mixtures decreased by 2.23 to 15.9% based on the average heat transfer coefficient of pure CO2, and the pressure drop decreased by 53.4 to 77.3% at the condensation temperature of 25 °C with increase of the mole fraction of N2. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Mankind has achieved tremendous progress by using fossil fuels. However, it has been confirmed that the concentration of greenhouse gases, which are generated by the combustion of the fossil fuels, have been increasing in the atmosphere. Greenhouse gases are the main cause of global warming. The Intergovernmental Panel on Climate Change (IPCC) estimated that the average global temperature will rise by 1.1 to 6.4 °C by the 22nd century because of the quantity of greenhouse-gas emission [1]. Also, NOAA’s Climate Program Office reported that the global mean sea level will rise from 0.2 to 2 m by 2100 because of the global warming, which causes collapse of glaciers [2]. Among the greenhouse gases that have a major effect on global warming, carbon dioxide (CO2) has the lowest global warming potential (GWP). However, CO2 accounts for a large portion of greenhouse gases and must be reduced. To solve the problem of global warming, many countries have discussed ways to decrease carbon-dioxide emission. Carbon Capture, Transportation, and Storage (CCS) is one of the promising ways to significantly reduce CO2 emissions. The International Energy Agency (IEA) reported that CCS will account for 15% of the maximum capture rate in CO2 emissions by 2025 in the 2 °C scenario (2DS) [3]. Also, the Intergovernmental Panel on Climate
⇑ Corresponding author. E-mail address: [email protected] (R. Yun). https://doi.org/10.1016/j.ijheatmasstransfer.2019.118628 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
Change (IPCC) highlighted that almost 450 ppm CO2-eq concentration will be reached by 2100 in the absence of CCS [4]. The CCS process consists of three main parts: capturing, transporting, and storing CO2. In the technology of CCS, CO2 is transported several tens or hundreds of kilometers through pipelines under a supercritical condition. Heat is transferred unavoidably from and to the surrounding seawater or ground during this transportation, and the physical properties of supercritical CO2 change dramatically depending on the transporting temperature and pressure. Also, supercritical CO2 can be changed into two-phase CO2 because of the temperature and the pressure drop by the heat exchange with seawater, whose temperature is lower than that of the transported CO2. Because of the type of power plant, and the methods of combustion and capturing, CO2 contains impurities, such as nitrogen (N2), methane (CH4), argon (Ar), and hydrogen sulfide (H2S). Carbon dioxide is transported under high pressure, and CO2 can undergo drastic changes in thermo-properties, especially in density, by the abrupt change from the supercritical condition to the two-phase. This drastic change in the thermo-physical properties of CO2 near critical pressure with the presence of impurities can cause differences in compressor operational and pipe-line transporting conditions. Without proper control of the operational parameters, CO2 transportation can fail because of the crack propagation of the pipelines. Therefore, it is essential to investigate the condensation heat-transfer characteristics of CO2 in the presence of impurities near critical temperature in order to design a pipeline and to control the parameters related to the transporting of CO2 in the pipelines.
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W. Baik, R. Yun / International Journal of Heat and Mass Transfer 144 (2019) 118628
Nomenclature Cp h G i k L _ m q_ Q_ r T U x
specific heat capacity at the constant pressure, Jkg1K1 heat transfer coefficient, Wm2K1 mass flux, kg m2s1 index for measurement parameters, specific enthalpy, Jkg1 thermal conductivity, Wm1K1 length of tube, m mass flow rate, kgs1 heat flux, Wm2 heat transfer rate, W radius of tube, m temperature, °C uncertainty vapor quality
Kang et al. [5] experimentally studied the condensation heat transfer characteristics of CO2 in the subcritical CO2 refrigeration cycle, and compared the results with the predictions by the existing models. They studied the effects of mass flux and saturated temperature of CO2 on the heat-transfer coefficient and pressure drop in the horizontal tube. Heo and Yun [6] proposed a prediction model of the condensation heat-transfer coefficient of CO2. The model provided better predictions than did the existing models under high condensation temperature, high mass flux and smalldiameter tubes. The mean absolute deviations of the Heo and Yun model with the experimental data of 589 points were 44.8%, and those of Thome et al., Bandhauer et al., Cavallini et al., and Kim and Mudawar were 61.9% 81.8%, 78.3%, and 89.6%, respectively. The existing studies on CO2 flow condensation inside horizontal tubes have been reviewed by Li et al. [7]. The heattransfer coefficients for R-404A and R-410A at near-critical pressures were investigated by Jiang et al. [8]. They mentioned that the average condensation heat-transfer coefficient in the 6.2mm-diameter tubes was higher than that for the 9.4-mm tubes. Also, the heat-transfer coefficients for R-410A were higher than those for R-404A because of the superior thermo-physical properties of R-410A, such as specific heat, thermal conductivity, and latent heat. Ould Didi et al. [9] measured the pressure drop of five refrigerants (R-134a, R-123, R-402A, R-404A, R-502) over mass fluxes from 100 to 500 kgm2s1 and compared the experimental results with two-phase frictional pressure-drop models, which were developed based on the two-phase flow patterns. They reported that the method of Müller-Steinhagen and Heck [10] gave the best predictions for the annular flow, and that the best available method for intermittent flow and stratified-wavy flow was the Grönnerud model. Huang et al. [11] widely reviewed the condensation heat-transfer characteristics in the presence of noncondensable gas. They highlighted that the non-condensable gas accumulated in the liquid film during condensation degraded the condensation heat-transfer coefficient. Minkowycz and Sparrow [12] theoretically analyzed the condensation heat transfer on isothermal vertical plates with a steam-air mixture. They highlighted that the non-condensable gas can have a crucial influence on the heat-transfer rate even with small bulk concentrations of air. They also described that the heat transfer was reduced by more than 50% when the bulk mass fraction of non-condensable gas was 0.5%. Wu and Vierow [13] experimentally studied the fluid flow phenomena and condensation heat transfer of a steam-air mixture in a horizontal condenser tube that had a 27.5-mm inside diameter. They focused on the effects of local liquid-film characteristics, air mass fraction, turbulent mixing, and air concentration on the
X Y
m q s
measured value calculated value kinematic viscosity, ms1 density, kg m3 shear stress, Nm2
Subscript f g i l o s test
fluid, saturated liquid saturate vapor inlet, interface saturated liquid outlet tube surface test section
condensation heat-transfer coefficient. They reported that the turbulent mixing helps the steam pass through the air boundary layer and condense on the heat-transfer surface. Lee and Yun [14] experimentally investigated the in-tube heat-transfer characteristics of CO2 with impurities, such as CO2 + N2, CO2 + CH4, and CO2 + Ar, under the supercritical conditions for the land transportation of the CCS process. They mentioned that the maximum heattransfer coefficient of CO2 mixtures decreased with the increase of the mole fraction of impurities. In addition, they found that the maximum heat-transfer coefficient of CO2 mixtures increased when the operational pressure decreased. Oh and Revankar [15] experimentally and theoretically investigated the film condensation in a vertical tube with a steam-air mixture. They found that the condensation heat transfer decreased with increase of the saturated pressure and of the non-condensable gas fraction, but increased with the inlet steam flow rate. Xu et al. [16] investigated forced-convection condensation of a steam-air mixture inside a horizontal tube with a 28-mm outside diameter experimentally and theoretically. They reported that the local heat transfer coefficient increased with the increase of the inlet velocity of the mixture of gases; however, when the gas mass fraction of the inlet non-condensable gas increased, the local heat-transfer coefficient decreased. Until now, studies in the available literature on the condensation heat-transfer characteristics of CO2 with impurities have been very limited. In addition, the condensation heat-transfer characteristics of CO2 have been studied under low temperature conditions for use as a refrigerant, as shown in Table 1. In this study, in-tube condensation heat-transfer characteristics for pure CO2 and CO2 with impurities were experimentally investigated under the near-critical state for the CCS process. We investigated the effects of the amount of impurities, the condensation temperature, and the mass flux on the condensation heat-transfer coefficients and the pressure drop. The results can be used for the original design of facilities for CO2 pipeline transportation in the CCS process, and for monitoring CO2 transportation in real time.
2. Experimental apparatus and procedures 2.1. Experimental setup Condensation heat-transfer coefficients and pressure drop were measured by using the test facility shown in Fig. 1. The test setup consisted of a test section, heat exchangers that were connected to two chillers, mass-flow meters, sensors for temperature and
W. Baik, R. Yun / International Journal of Heat and Mass Transfer 144 (2019) 118628 Table 1 Existing experimental studies. Reference
Tube geometry
Test conditions
Measurements
Kang et al. [5]
Smooth tube (ID : 5.15 mm),
Heat transfer coefficient, pressure drop
Kim et al. [17]
Smooth (3.48 mm) and microfin tube (3.51 mm), Smooth tube (ID : 6.1 mm),
G : 600, 800, 1000 kgm2s1 Tsat : 10, 5, 0, 5 °C G : 200, 400, 800 kgm2s1 Tsat : 25, 15 °C G : 200, 300, 400 kgm2s1 Tsat : 25, 15 °C G : 100, 200, 300, 400, 500 kgm2s1 Tsat : 10, 5, 0 °C G : 200, 400, 600, 800 kgm2s1 Tsat : 25, 15 °C G : 50, 100, 200, kgm2s1 Tsat : 15, 10, 5, 0 °C
Jang and Hrnjak [18] Li et al. [19]
Park and Hrnjak [20] Iqbal and Bansal [21]
Smooth tube (ID : 4.73 mm)
Microchannel (ID : 0.89 mm) Smooth tube (ID : 6.52 mm)
Heat transfer coefficient Heat transfer coefficient, pressure drop Heat transfer coefficient
Heat transfer coefficient, pressure drop Heat transfer coefficient
pressure, a differential pressure transducer, and a magnetic gear pump that circulated CO2 mixtures. Heat exchangers were used to liquify the CO2 mixtures before they returned to the magnetic gear pump by using the brine solution (40%, EG/water). Fig. 2 shows the details of the test section, which is a double tube. The inner and outside diameters of the test section were 10.7 mm and 12.7 mm, respectively. The total length of the test section was 4 m, and it was composed of six sub-sections for the local measurement of heat-trasnfer coefficients. The test section was a copper tube and was assembled in the form of a double tube with a Polyvinyl Chloride (PVC) pipe to simulate the pipelinetransportation conditions for CO2 under seawater. The outside wall temperatures of the test section was measured by using T-type thermocouples that were firmly soldered to the surface. The heat-transfer rate at each subsection was calculated by inserting
3
the thermocouples through the PVC pipe. The thermocouples were firmly located at the inlet and outlet of each section. Table 2 shows the accuracies of the measuring instruments that were used for the experiment. The pressure transducers had an accuracy of ±0.13% in the full scale of 14.7 to 3000 psig, and a differential pressure transducer had an accuracy of ±0.05% within the full range of 0– 10 kPa. Prior to the experiment, all thermocouples were calibrated by using a constant-temperature water bath, and the differences among the thermocouples were less than ±0.1 °C. A Coriolis-type mass-flow meter that had a full range of 0–18 kgmin1 showed an accuracy of ±0.2%. Two other Coriolis-type mass-flow meters for measuring the mass-flow rate of brine and water to calculate the heat-transfer rate at the test tube and preheater had accuracies of ±0.1% and ±0.2%, respectively. Both mass-flow meters had a full range of 300 kgmin1. 2.2. Experimental procedure We used CO2 and a CO2 + N2 mixture as test fluids. The test procedures for measuring the condensation heat-transfer characteristics of the pure CO2 and the CO2 + N2 mixture were as follows. First, the temperature and flow rate of the brine on the annular side of the test section was adjusted. The CO2 + N2 mixture was injected into the test setup by using a gas booster, which operates through an air compressor. Then, the injected CO2 + N2 mixture was condensed and circulated in the test section. When the CO2 + N2 mixture was sufficiently charged into the system, the inlet temperature and pressure conditions of the test section were adjusted by using a constant-temperature water bath, shown in Fig. 1. Table 3 shows the present test conditions. The condensation pressure and temperature of the CO2 + N2 mixture changed from 57.29 to 81.51 bar, which corresponds to 0.77–0.99 of the reduced pressure, and 20–30 °C, respectively. The mass flux was controlled from 500, 600, and 700 kgm2s1. The mole fraction of N2 in the CO2 + N2 mixture was set at 1, 3, and 5%. The uncertainty of the components for the CO2 + N2 mixture was ±2%. The brine temperature of the test section was fixed at 4 °C to match the actual seawater temperature in winter. The vapor quality of the CO2 + N2 mixture was covered from 0.0 to 1.0.
Fig. 1. Experimental test facility.
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Fig. 2. Details of test section.
Table 2 Specifications of the measuring instruments. Instruments
Range
Accuracy
Thermocouple Pressure transducer Mass flowmeter for brine-side Mass flowmeter for CO2-side Mass flowmeter for preheater Differential pressure transducer
200 °C to 200 °C 14.7 to 3000 psig 0–300 kgmin1 0–18 kgmin1 0–300 kgmin1 0–10 kPa
±0.1 °C ±0.13% full scale ±0.1% ±0.2% ±0.2% ±0.05% full scale
2.3. Data reduction
(Engineering Equation Solver) according to the method presented by the NIST (National Institute of Standards and Technology) [23]. Eq. (4) explains the uncertainty of the calculated value of Y from the measured valued of X. The uncertainty of the measured value, UY, depended on the error of the measuring instruments and the sensitivity of the calculated value to the measured values. The uncertainty of the inlet vapor quality was between ±1.93% and ±5.17%. Also, the uncertainty of the condensation heat transfer coefficient ranged between ±6.19% and ±16.7%. Prior to this experiment, the verification of the test facility was conducted by calculating the heat transfer rate for CO2. We have confirmed that the heat balance between the brine-side and the CO2-side of the test section was within ±5% under single-phase conditions.
The condensation heat-transfer coefficient of the CO2 + N2 mixture was calculated by using Eq. (1). The temperature of tube inner surface, Ts,i, was calculated by using the measurement of the outside surface temperature and by the one-dimensional heat equation.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u uX rY 2 2 UY ¼ t U Xi rX i i
h ¼ q_ test =ðT s;i T f Þ
3. Results and discussion
ð1Þ
The inlet and outlet vapor quality of the CO2 + N2 mixture across the test section was calculated by Eqs. (2) and (3), respectively. The inlet enthalpy of the mixture at the test section can be obtained by measuring the pressure and the temperature of the subcooled CO2 + N2 mixture at the preheater inlet and the heat-transfer rate in the preheater. The outlet vapor quality, xtest,out, was obtained by Eq. (3) at each subsection. The heat removal amount at each subsection was calculated by using the mass flow rate and temperature difference of the brine solution.
xtest;in ¼ itest;in il =ifg xtest;out
ð2Þ
_ CO2 ifg ¼ xtest;in q_ test = m
ð3Þ
The transporting and thermodynamic properties of CO2 + N2 mixture were obtained by REFPROP [22], based on the measured condensation temperature and pressure. The uncertainties of the heat transfer coefficients were calculated by using the EES
ð4Þ
3.1. Prediction of flow regimes Flow regimes of the two-phase flow for the CO2 + N2 mixture were estimated by using the existing models. Fig. 3 shows the prediction of flow regimes for the pure CO2 and the CO2 + N2 mixture by using the Taitel and Dukler flow-regime map [24]. Jang and Hrnjak [18] investigated the condensation flow regime of CO2 in a 6.10-mm smooth tube under low-temperature conditions, as shown in Table 1. They mentioned that the flow-regime map by Taitel and Dukler correctly predicted 19 of the 25 observations. Although our test was conducted at the near-critical temperature, it could be suitable for predicting the flow regimes for the condensation characteristics of CO2 and impurities in a horizontal smooth tube. As shown in Fig. 3, the annular and intermittent flow was predicted during the condensation of CO2 and the CO2 + N2 mixture. Most of the data points were plotted in the annular regions, and
Table 3 Test conditions. Fluid
Brine-side temperature
Mole fraction of N2 in the CO2 mixtures
Condensation temperature
Vapor quality
Mass flux
Pure CO2 CO2 + N2
4 °C 4 °C
– 1, 3, 5%
20, 25, 30 20, 25, 26.5–30
0.0–1.0 0.0–1.0
500, 600, 700 kgm2s1 500, 600, 700 kgm2s1
W. Baik, R. Yun / International Journal of Heat and Mass Transfer 144 (2019) 118628
Fig. 3. Experimental data of CO2 mixture on the flow regime map by Taitel and Dukler [24].
only a few data points were plotted in the intermittent regions. These results suggested that impurities do not have a great effect on the two-phase flow regime compared with that of pure CO2. In the present study, the condensation heat transfer and pressure drop characteristics of CO2 and the CO2 + N2 mixture were explained by the phenomena that are dominant for the annular flow patterns for the in-tube flow condensation. However, we needed to verify the condensation flow characteristics of CO2 with impurities in the horizontal tubes by doing some additional research. 3.2. Condensation heat-transfer coefficient 3.2.1. The effect of the mole fraction of N2 Fig. 4 shows the comparison of the present condensation heat transfer coefficients with the existing experimental data [5,18,19]. Although there were significant differences in the tube diameter and in the condensation temperature among the tests, the heat transfer coefficients were located within the range of 2–4 kWm2K1 except the Jang and Hrnjak’s results [18]. When the vapor quality increased, the condensation heat transfer
Fig. 4. Comparison of the present heat transfer coefficients with the existing data [5,18,19].
5
coefficient increased for all experimental results. Fig. 5 shows the effects of the mole fraction of N2 in the CO2 + N2 mixture on the heat transfer coefficient. The experiments were carried out at a condensation temperature of 20 to 30 °C, mass flux of 500 kgm2s1, and brine-side temperature of 4 °C. As shown in Fig. 5(a), when the mole fraction of N2 was set at 1%, 3%, and 5%, the average of the heat transfer coefficient was lower by 2.23%, 13%, and 15.9%, respectively, based on the average heat transfer coefficient of pure CO2; The average heat transfer coefficient of pure CO2 was 3 kWm2K1. As the vapor quality increased from 0.3 to 0.9, the heat transfer coefficient for the CO2 mixture increased by 8.2%. Increasing the mole fraction of N2 in the CO2 + N2 mixture from 1 to 5%, the condensation heat-transfer coefficient decreased. When the CO2 + N2 mixture was in a steady state, the condensation took place at the interface of a liquid film on the tube wall. Because of the condensation at the interface between the liquid film and the vapor, there is less vapor near the interface than there is far from the interface. At the same time, the non-condensable gas can be regarded as being uniformly distributed in the vapor core. The vapor amount near the interface and the non-condensable gas distribution caused a decrease of the CO2 vapor partial pressure at the vapor-liquid interface of the CO2 + N2 mixture, which lowered the condensation temperature at the liquid film. With the decrease of the condensation temperature at the interface, the amount of condensate decreased, which lowered the diffusion driving force of the CO2 vapor to the interface. These phenomena at the interface when there is non-condensable gas lower the condensation heat transfer coefficient. Also, these effects of noncondensable gas on the condensation heat transfer coefficients become significant as the amount of the non-condensable gas increases. The heat transfer coefficient increased with the increase of vapor quality, and these trends was found to result from the increase in the interfacial shear stress between the liquid and vapor phases with the increase of vapor quality. Soliman et al. [25] suggested the model that the Nusselt number for the condensation heat transfer coefficients was proportional to the square root of the interfacial shear stress. Fig. 5(b) presents the variation of heat transfer coefficient at a condensation temperature of 25 °C under the same conditions. When the mole fraction of N2 was changed to 1 3, and 5% in the CO2 mixture, the average of the heat transfer coefficient decreased by 6.67%, 14.5%, and 15.6%, respectively. Also, the experimental data showed that the heat-transfer coefficient increased with increasing vapor quality of the CO2 mixture. The increasing rate of the average heat transfer coefficient of the CO2 mixture with vapor quality ranged from 6% to 8%. Shah [26] reported that the existing correlations fail to predict the condensation heat transfer coefficients at high reduced pressures and that they are closely related to the reduced pressure and viscosity ratio of each phase. Because the critical temperature was 28.8 °C and 27.1 °C for the mole fraction of 3% and 5% of the CO2 mixture, the condensation temperature was set to 26.5 °C for the CO2 + N2 mixture. This temperature corresponds to the same reduced pressure of the pure CO2 at the condensation temperature of 30 °C. When the mole fraction of N2 increased from 1 to 5%, the mean heat transfer coefficient decreased by 5.8%, as shown in Fig. 5 (c). 3.2.2. The effect of condensation temperature Fig. 6 illustrates the effects of the condensation temperature of the CO2 + N2 mixture on the heat transfer coefficient. The experiments were carried out at a mole fraction of N2 of 1 to 5%, a mass flux of 500 kgm2s1, and brine-side temperature of 4 °C. Fig. 6(a) shows that the heat-transfer coefficient decreased with the increasing condensation temperature. When the condensation temperature increased from 20 to 30 °C, the average heat transfer coefficient decreased by 8.8%. Increasing vapor quality for the CO2
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-2
-1
Heat transfer coefficient (kW m K )
6
5
Pure CO2
was related to the thermo-physical properties of the CO2 mixture. As shown in Table 4, when the condensation temperature increased, the density ratio of liquid to vapor decreased. Increasing the density ratio of liquid to vapor increased the velocity difference between the vapor phase and liquid phase. The increased velocity differences also increased the interfacial shear stress. This phenomenon can be confirmed by Ueda et al.’s experimental results [27]. For internal flow condensation of steam in a tube, the Nusselt number significantly increased with the increase of the nondimensional interfacial shear stress under the same Reynolds number. The non-dimensional interfacial shear is defined as Eq. (5). It is well known that the interface instability, such as the KelvinHelmholtz instability, significantly increased with the increase of the velocity difference between the liquid and vapor phases. This increased instability can highly increase the condensation heattransfer coefficients as related to the interfacial shear.
Mass flux : 500 kg m-2s-1
CO2+N2 (1%) Condensation Temp : 20oC CO2+N2 (3%) CO2+N2 (5%)
4
3
2
1
0 0.0
0.2
0.4
0.6
0.8
1.0
si ¼
Vapor quality (-)
(a) Heat transfer coefficient (kW m -2K-1)
6
5
3
2
1
0.2
0.4
0.6
0.8
1.0
Vapor quality (-)
(b) Heat transfer coefficient (kW m-2K-1)
6
5
Pure CO2 Mass flux : 500 kg m-2s-1 CO2+N2 (1%) Condensation Temp : 26.5oC - 30oC CO2+N2 (3%) CO2+N2 (5%)
4
3
2
1
0 0.0
0.2
0.4
0.6
0.8
ð5Þ
Another factor that affects the condensation heat transfer coefficient is the thermal conductivity of the liquid film. As the condensation temperature increased, the thermal conductivity of the liquid film decreased, which then lowered the condensation heat transfer coefficients and increased the condensation temperature. Fig. 6(b), (c), and (d) show the similar result for the heat-transfer coefficient of pure CO2 under the same temperature conditions. When the condensation temperature changed from 20 to 30 °C, the heat transfer coefficient decreased by 13.3%, as shown in Fig. 6(b). Also, as shown in Fig. 6(c) and (d), the heat-transfer coefficient for the CO2 mixtures decreased with increasing condensation temperature, and the percentage were 13.4% and 9.5%, respectively.
Pure CO2 Mass flux : 500 kg m -2s-1 CO2+N2 (1%) Condensation Temp : 25 oC CO2+N2 (3%) CO2+N2 (5%)
4
0 0.0
s ql ðg v l Þ2=3
1.0
Vapor quality (-)
(c)
3.2.3. The effect of mass flux Fig. 7 shows the effects of mass flux of the CO2 + N2 mixture on the condensation heat transfer coefficient. The experiments were conducted at condensation temperatures of 20 °C to 30 °C, N2 mole fraction of 3%, and brine-side temperature of 4 °C. Fig. 7(a) illustrates the comparison of the heat transfer coefficient of pure CO2 and the CO2 + N2 mixture at the condensation temperature of 20 °C. When the mass flux changed from 600 to 700 kgm2s1, the average heat transfer coefficient of pure CO2 increased by 19.4% and 26.4%, respectively, over the average heat transfer coefficient when the mass flux was 500 kgm2s1. When the mass flux changed from 500 to 600 and to 700 kgm2s1, the average heattransfer coefficient of pure CO2 was 3, 3.7, and 4 kWm2K1, respectively For the CO2 + N2 mixture, the heat transfer coefficient calculated higher at 16.4% and 21.6% based on the average heattransfer coefficient that the mass flux condition was 500 kgm2s1, respectively. Fig. 7(b) and (c) show the trends of the condensation heat transfer coefficients as being similar to that of Fig. 7(a). As shown in Fig. 7, the heat transfer coefficient increased with respect to the mass flux at each condensation temperature. Dukler derived Eq. (6) of the non-dimensional interfacial shear stress from the empirical two-phase pressure drop correlation. A and B are constants that depend on the condensation temperature and the vapor quality. Therefore, the non-dimensional interfacial shear stress can depend on the mass flux under the same condensation temperature and the vapor quality.
si ¼ AðT sat ÞBðxÞG1:4
ð6Þ
Fig. 5. Variation of heat transfer coefficient with N2 mole fraction (a) 20 °C (b) 25 °C (c) 26.5–30 °C.
3.3. Pressure drop mixtures increased the heat-transfer coefficient, and the range of increase was 7.3 to 8.5%. The heat transfer coefficient increased with decreasing condensation temperature, whose main effect
Changes of the average pressure drop with the N2 mole fraction of CO2 mixtures are shown in Fig. 8. The experiments were
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6
Condensation Temp : (20oC) Condensation Temp : (25oC) Condensation Temp : (30oC)
5
Heat transfer coefficient (kW m-2K-1)
Heat transfer coefficient (kW m-2K-1)
6
Mass flux : 500 kg m-2s-1 4
Fluid : Pure CO2
3
2
1
0 0.0
0.2
0.4
0.6
0.8
Condensation Temp : (20oC) Condensation Temp : (25oC) Condensation Temp : (30oC)
5
Mass flux : 500 kg m-2s-1 4
Fluid : CO2 + N2 (1%)
3
2
1
0 0.0
1.0
0.2
Vapor quality (-)
0.4
(a) 6
Condensation Temp : (20oC) Condensation Temp : (25oC) Condensation Temp : (28oC)
5
-2 -1
Mass flux : 500 kg m s 4
Fluid : CO2 + N2 (3%)
3
2
1
0 0.0
0.2
0.4
0.8
1.0
(b) Heat transfer coefficient (kW m-2K-1)
Heat transfer coefficient (kW m-2K-1)
6
0.6
Vapor quality (-)
0.6
0.8
1.0
Condensation Temp : (20oC) Condensation Temp : (25oC) Condensation Temp : (26.5oC)
5
Mass flux : 500 kg m-1s-2 4
Fluid : CO2 + N2 (5%)
3
2
1
0 0.0
0.2
0.4
0.6
0.8
1.0
Vapor quality (-)
Vapor quality (-)
(c)
(d)
Fig. 6. Variation of heat transfer coefficient with condensation temperature (a) 0% (pure CO2) (b) 1% (c) 3% (d) 5%.
Table 4 Variation of thermophysical properties of CO2 mixtures with condensation temperature. Fluid
Condensation temperature ( °C)
Thermal conductivity of liquid (Wm1k1)
Density ratio of liquid to vapor (/)
Surface tension (mNm1)
Viscosity of liquid (106 Pas)
Pure CO2
20 25 30
0.085 0.081 0.095
3.982 2.927 1.719
1.203 0.552 0.054
66.148 57.048 43.768
CO2 + N2 (1%)
20 25 30
0.083 0.079 0.094
3.824 2.762 1.441
1.069 0.435 –
63.844 54.426 38.754
CO2 + N2 (3%)
20 25 28
0.08 0.076 0.077
3.497 2.404 1.067
0.802 0.207 –
59.098 48.728 39.069
CO2 + N2 (5%)
20 25 26.5
0.076 0.072 0.071
3.153 1.991 0.996
0.568 0.034 –
54.151 42.228 36.691
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Pure CO2
Condensation Temp : (20oC)
5
5
4
4
3
3
2
2
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
0 0.0
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
Massflux : 700 kg m-2s-1
Massflux : 700 kg m-2s-1 0.2
0.4
0.6
6
CO2 + N2 (3%)
Condensation Temp : (20oC)
0.8
Heat transfer coefficient (kW m-2K-1)
Heat transfer coefficient (kW m-2K-1)
6
0 1.0 0.0
0.2
Vapor quality (-)
0.4
0.6
0.8
CO2 + N2 (3%) o
Condensation Temp : (25 C)
Condensation Temp : (25oC)
5
5
4
4
3
3
2
2
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
0.2
Vapor quality (-)
0.4
0.6
0 1.0 0.0
0.8
Vapor quality (-)
Massflux : 700 kg m-2s-1 0.2
0.4
0.6
0.8
1.0
Vapor quality (-)
(a)
(b)
6
Heat transfer coefficient (kW m-2K-1)
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
Massflux : 700 kg m-2s-1
0 0.0
1.0
6
Pure CO2
6 CO2 + N2 (3%)
Pure CO2
Condensation Temp : (28oC)
Condensation Temp : (30oC) 5
5
4
4
3
3
2
2
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
0 0.0
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
Massflux : 700 kg m-2s-1 0.2
0.4
0.6
0.8
Massflux : 700 kg m-2s-1
0 1.0 0.0
0.2
Vapor quality (-)
0.4
0.6
0.8
1.0
Vapor quality (-)
(c) Fig. 7. Variation of heat transfer coefficient with mass flux (a) 20 °C (b) 25 °C (c) 28–30 °C.
3
Pressure drop (kPa)
conducted at condensation temperatures of 20 to 30 °C, mass flux of 500 kgm2s1, and brine-side temperature of 4 °C. The pressure drop decreased with the increasing mole fraction of N2 at each condensation temperature. As the N2 mole fraction of the CO2 mixture increased from 1 to 3 and to 5%, the pressure drop measured as 23.7%, 43.9%, and 54.5% lower than the condensation temperature of 20 °C based on those of pure CO2, respectively. When the condensation temperature was 25 °C, the pressure drop was decreased by 53.4%, 66.9%, and 77.3% under the same conditions of a condensation temperature of 20 °C, respectively. When the N2 mole fraction of CO2 mixture increased from 1 to 3 and to 5%, based on the pure CO2, the pressure drop decreased by 78.6%, 77.7%, and 79.2%, respectively, at the condensation temperature of 26.5 to 30 °C. The pressure drop decreased with the increasing mole fraction of N2 with all mixtures. Fig. 9 and Table 4 show the variation of the density ratio between liquid to vapor with the change of the mole fraction of N2. It can be clearly shown that the density ratio decreased with the increase of the mole fraction of N2. Accordingly, the vapor velocity in the two-phase flow significantly decreased with the increase of the impurities, resulting in a lower pressure drop for the lower mole fraction of the impurities. Fig. 10 shows the effect of condensation temperature on the mean pressure drop. The mean pressure drop decreased by 43.8% on average when the temperature was increased from 20 to 30 °C for pure CO2. When the condensation temperature changed from 20 to 30 °C, the mean pressure drop decreased by 69.9% at a mole fraction of N2 of 1%. For a mole fraction of N2 of 3%, the mean pressure drop decreased by 64.5%. When the condensation
Condensation Temp : 20oC Condensation Temp : 25oC Condensation Temp : 26.5 - 30oC Massflux : 500 kg m-2s-1
2
1
0 Pure CO2
CO2+N2 (1%)
CO2+N2 (3%)
CO2+N2 (5%)
Fig. 8. Variation of pressure drop with N2 mole fraction.
temperature increased from 20 to 26.5 °C, the mean pressure drop decreased 71.2%, and as the condensation temperature increased from 25 to 26.5 °C, the decreasing rate was 48.7% for the mole fraction of 5% of N2. The pressure drop at 20 °C was higher than at 25 °C, 26.5 to 30 °C in all mixtures. The effects of the condensation
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W. Baik, R. Yun / International Journal of Heat and Mass Transfer 144 (2019) 118628
5
Pressure drop (kPa)
4
3
Condensation Temp : 28 - 30 oC Condensation Temp : 25 oC Condensation Temp : 20 oC
: Pure CO2 : CO2 + N2 (3%)
2
1
0 500
600
700 -2 -1
Massflux (kg m s ) Fig. 9. Variation of density ratio of liquid to vapor with condensation temperature.
temperature on the pressure drop can be explained by the same reasoning used to explain the condensation heat transfer coefficients. With the increase of the condensation temperature, the non-dimensional interfacial shear stress decreased, which decreased the pressure drop. Besides, it should be noted that the viscosity of the liquid film decreased with the increase of the condensation temperature too. The effect of mass flux on the mean pressure drop of pure CO2 and of the CO2 + N2 mixture is shown in Fig. 11. The N2 mole fractions of the CO2 + N2 mixture were 0% and 3%; 0% means pure CO2. As expected, when the mass flux increased by an interval of 100 kgm2s1, the pressure drop increased in all CO2 + N2 mixtures. When the mass flux varied 500 to 700 kgm2s1, the mean pressure drop increased by 35.6% on average for pure CO2 at all condensation temperatures. Also, the increasing rate was 38.8% for the CO2 + N2 mixture. We compared the average heat transfer coefficients between the experimental data and the predictions by using the Lee and Rose model [28] for each mole fraction as shown in Fig. 12. When the mole fraction of N2 in the mixture and the condensation temperature increased, the experimental data and the predictions from
Pressure drop (kPa)
3
Massflux : 500 kg m-2s-1
Pure CO2 CO2+N2 (1%) CO2+N2 (3%) CO2+N2 (5%)
Fig. 11. Variation of pressure drop with mass flux.
Fig. 12. Comparison of experimental data with the predictions by the Lee and Rose model [28].
the Lee and Rose model show the decreasing trends of the condensation heat transfer coefficients. The maximum mean deviation between them was 25.4%, and the minimum was 2.8%.
2
4. Conclusions
1
0 20
25
30 o
Condensation Temperature ( C) Fig. 10. Variation of pressure drop with condensation temperature.
In-tube condensation heat-transfer characteristics of pure CO2 and a CO2 + N2 mixture were experimentally investigated under seawater transportation conditions for the CCS process. The heat transfer coefficient and pressure drop of the CO2 + N2 mixture during pipeline transportation were significantly dependent on the quantity of the impurities, condensation pressure, and mass flux. When the mole fraction of N2 increased, the condensation heattransfer coefficient of the CO2 + N2 mixture decreased because of the thermophysical properties of the mixture and the effects of a non-condensable gas on the film condensation. As the condensation temperature of the CO2 + N2 mixture increased, the heat transfer coefficient decreased from the low density ratio between liquid and vapor, the decreased interfacial shear stress and less interfacial
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movements. The pressure drop of the CO2 + N2 mixture increased with decreasing N2 mole fraction; however, as the condensation temperature decreased, the pressure drop increased. The effects of the mole fraction of N2 and the condensation temperature on the pressure drop can be well explained by the velocity difference between phases. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education of the Republic of Korea (NRF-2016R1D1A1B02010075). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijheatmasstransfer.2019.118628. References [1] IEA, World Energy Outlook, OECD/IEA, London, 2011. [2] R. Balnk, J. Lubchenco, R. Rietrick, Global Sea Level Rise Scenarios for the United States National Climate Assessment, NOAA, Silver Spring, 2012, pp. 1–2. [3] IEA, 20 Years of carbon capture and storage: Accelerating future deployment, OECD/IEA, Paris, 2016. [4] IPCC, Climate Change 2014: Synthesis Report, in: Core Writing Team, R.K. Pachauri, L.A. Meyer (Eds.), Contribution of Working Groups, I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC, Geneva, Switzeland, 2014, pp. 151. [5] P.J. Kang, R. Yun, Condensation heat transfer characteristics of CO2 in a horizonal smooth tube, Int. J. Refreg. 36 (2013) 1090–1097. [6] J. Heo, R. Yun, Prediction of CO2 condensation heat transfer coefficient in a tube, Int. J. Refrig. 89 (2015) 254–263. [7] P. Li, J.J.J. Chen, S. Norris, Review of flow condensation of CO2 as a refrigerant, Int. J. Refrig. 72 (2016) 53–73. [8] Y. Jiang, B. Mitra, S. Garimella, U. Andresen, Measurement of condensation heat transfer coefficients at near-critical pressures in refrigerant blends, J. Heat Transfer 129 (8) (2007) 958–965.
[9] M.B. Ould Didi, N. Kattan, J.R. Thome, Prediction of two-phase pressure gradients of refrigerants in horizontal tubes, Int. J. Refrig. 25 (2002) 935–947. [10] H. Müller-steinhagen, K. Heck, A simple friction pressure drop correlation for two-phase flow in pipes, Chem. Eng. Process. Process Intensif. 20 (6) (1986) 297–308. [11] J. Huang, J. Zhang, L. Wang, Review of vapor condensation heat and mass transfer in the presence of non-condensable gas, Appl. Therm. Eng. 89 (2015) 469–484. [12] W.J. Minkowycz, E.M. Sparrow, Condensation heat transfer in the presence of noncondensables, interfacial resistance, superheating, variable properties, and diffusion, Int. J. Heat Mass Transf. 9 (10) (1966) 1125–1144. [13] T. Wu, K. Vierow, Local heat transfer measurements of steam/air mixtures in horizontal condenser tubes, Int. J. Heat Mass Transf. 49 (2006) 2491–2501. [14] W. Lee, R. Yun, In-tube convective heat transfer characteristics of CO2 mixtures in a pipeline, Int. J. Heat Mass Transf. 125 (2018) 350–356. [15] S. Oh, S.T. Revankar, Experimental and theoretical investigation of film condensation with noncondensable gas, Int. J. Heat Mass Transf. 49 (2006) 2523–2534. [16] H. Xu, Z. Sun, H. Gu, H. Li, Forced convection condensation in the presence of noncondensable gas in a horizontal tube; experimental and theoretical study, Prog. Nucl. Energy 88 (2016) 340–351. [17] Y.J. Kim, J. Jang, P.S. Hrnjak, M.S. Kim, Condensation heat transfer of carbon dioxide inside horizontal smooth and microfin tubes at low temperatures, J. Heat Transf. 131 (2) (2009), 021501-1-021501-10. [18] J. Jang, H.S. Hrnjak, University of Illinois at Urbana-Champaign, 2004, ACRC CR56. [19] P. Li, J.J.J. Chen, S. Norris, Flow condensation heat transfer of CO2 in a horizontal tube at low temperatures, Appl. Therm. Eng. 130 (2018) 561–570. [20] C.Y. Park, P. Hrnjak, CO2 flow condensation heat transfer and pressure drop in multi-port microchannels at low temperatures, Int. J. Refrig. 32 (2009) 1129– 1139. [21] O. Iqbal, P. Bansal, In-tube condensation heat transfer of CO2 at low temperatures in a horizontal smooth tube, Int. J. Refrig. 35 (2012) 270–277. [22] E.W. Lemmon, M.L. Huber, M.O. Mclinden, Reference fluid thermodynamic and transport properties (REFPROP) version 9.1 user’s guide, National Institute of Standards and Technology, 2013. [23] N.N. Taylor, C.E. Kuyatt, Guidelines for evaluating and expressing the uncertainty of NIST measurement results, NIST Technical Note 1297 (1994). [24] Y. Taitel, A.E. Dukler, A model for predicting flow regime transitions in horizontal and near horizontal gas-liquid flow, Am. Inst. Chem. Eng. J. 22 (1976) 47–55. [25] M. Soliman, J.R. Schuster, P.J. Berenson, A general heat transfer correlation for annular flow condensation, J. Heat Transfer 90 (1968) 267–276. [26] M.M. Shah, An improved and extended general correlation for heat transfer during condensation in plain tubes, HVAC&R Res. 15 (2009) 889–913. [27] T. Ueda, T. Kubo, M. Inoue, Heat transfer for steam condensing inside a vertical tube, Proc. 5th Int Heat Transfer Conference, vol. 3, 1976, pp. 304–308. [28] W.C. Lee, J.W. Rose, Forced convection film condensation on a horizontal tube with and without non-condensing gases, Int. J. Heat Mass Transf. 27 (4) (1984) 519–528.
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
In-tube condensation heat transfer characteristics of CO2 with N2 at near critical pressure Wonkeun Baik, Rin Yun ⇑ Hanbat National University, 125 Dongseodaero, Yuseonggu, Daejeon 34158, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 May 2019 Received in revised form 19 July 2019 Accepted 22 August 2019 Available online 30 August 2019 Keywords: CO2 transportation CO2 impurity Condensation heat transfer coefficient Pressure drop Near-critical condition
a b s t r a c t The condensation heat-transfer coefficient and pressure drop for pure CO2 and CO2 + N2 mixtures were investigated under the near-critical condition, which simulates the CO2 transporting conditions under Carbon Capture, Transportation, and Storage (CCS). The experimental apparatus consists of a test section, heat exchangers, mass flow meters, temperature sensors, a magnetic gear pump, and a differential pressure transducer. The test section made with a copper tube was assembled with Polyvinyl Chloride (PVC) pipe to form a double tube. The condensation temperature and mole fraction of N2 of the CO2 mixtures ranged from 20 °C to 30 °C, and 1 to 5%, respectively. The mass flux was changed from 500, 600 and 700 kgm2s1. The average heat transfer coefficient of the CO2 + N2 mixtures decreased by 2.23 to 15.9% based on the average heat transfer coefficient of pure CO2, and the pressure drop decreased by 53.4 to 77.3% at the condensation temperature of 25 °C with increase of the mole fraction of N2. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Mankind has achieved tremendous progress by using fossil fuels. However, it has been confirmed that the concentration of greenhouse gases, which are generated by the combustion of the fossil fuels, have been increasing in the atmosphere. Greenhouse gases are the main cause of global warming. The Intergovernmental Panel on Climate Change (IPCC) estimated that the average global temperature will rise by 1.1 to 6.4 °C by the 22nd century because of the quantity of greenhouse-gas emission [1]. Also, NOAA’s Climate Program Office reported that the global mean sea level will rise from 0.2 to 2 m by 2100 because of the global warming, which causes collapse of glaciers [2]. Among the greenhouse gases that have a major effect on global warming, carbon dioxide (CO2) has the lowest global warming potential (GWP). However, CO2 accounts for a large portion of greenhouse gases and must be reduced. To solve the problem of global warming, many countries have discussed ways to decrease carbon-dioxide emission. Carbon Capture, Transportation, and Storage (CCS) is one of the promising ways to significantly reduce CO2 emissions. The International Energy Agency (IEA) reported that CCS will account for 15% of the maximum capture rate in CO2 emissions by 2025 in the 2 °C scenario (2DS) [3]. Also, the Intergovernmental Panel on Climate
⇑ Corresponding author. E-mail address: [email protected] (R. Yun). https://doi.org/10.1016/j.ijheatmasstransfer.2019.118628 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
Change (IPCC) highlighted that almost 450 ppm CO2-eq concentration will be reached by 2100 in the absence of CCS [4]. The CCS process consists of three main parts: capturing, transporting, and storing CO2. In the technology of CCS, CO2 is transported several tens or hundreds of kilometers through pipelines under a supercritical condition. Heat is transferred unavoidably from and to the surrounding seawater or ground during this transportation, and the physical properties of supercritical CO2 change dramatically depending on the transporting temperature and pressure. Also, supercritical CO2 can be changed into two-phase CO2 because of the temperature and the pressure drop by the heat exchange with seawater, whose temperature is lower than that of the transported CO2. Because of the type of power plant, and the methods of combustion and capturing, CO2 contains impurities, such as nitrogen (N2), methane (CH4), argon (Ar), and hydrogen sulfide (H2S). Carbon dioxide is transported under high pressure, and CO2 can undergo drastic changes in thermo-properties, especially in density, by the abrupt change from the supercritical condition to the two-phase. This drastic change in the thermo-physical properties of CO2 near critical pressure with the presence of impurities can cause differences in compressor operational and pipe-line transporting conditions. Without proper control of the operational parameters, CO2 transportation can fail because of the crack propagation of the pipelines. Therefore, it is essential to investigate the condensation heat-transfer characteristics of CO2 in the presence of impurities near critical temperature in order to design a pipeline and to control the parameters related to the transporting of CO2 in the pipelines.
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Nomenclature Cp h G i k L _ m q_ Q_ r T U x
specific heat capacity at the constant pressure, Jkg1K1 heat transfer coefficient, Wm2K1 mass flux, kg m2s1 index for measurement parameters, specific enthalpy, Jkg1 thermal conductivity, Wm1K1 length of tube, m mass flow rate, kgs1 heat flux, Wm2 heat transfer rate, W radius of tube, m temperature, °C uncertainty vapor quality
Kang et al. [5] experimentally studied the condensation heat transfer characteristics of CO2 in the subcritical CO2 refrigeration cycle, and compared the results with the predictions by the existing models. They studied the effects of mass flux and saturated temperature of CO2 on the heat-transfer coefficient and pressure drop in the horizontal tube. Heo and Yun [6] proposed a prediction model of the condensation heat-transfer coefficient of CO2. The model provided better predictions than did the existing models under high condensation temperature, high mass flux and smalldiameter tubes. The mean absolute deviations of the Heo and Yun model with the experimental data of 589 points were 44.8%, and those of Thome et al., Bandhauer et al., Cavallini et al., and Kim and Mudawar were 61.9% 81.8%, 78.3%, and 89.6%, respectively. The existing studies on CO2 flow condensation inside horizontal tubes have been reviewed by Li et al. [7]. The heattransfer coefficients for R-404A and R-410A at near-critical pressures were investigated by Jiang et al. [8]. They mentioned that the average condensation heat-transfer coefficient in the 6.2mm-diameter tubes was higher than that for the 9.4-mm tubes. Also, the heat-transfer coefficients for R-410A were higher than those for R-404A because of the superior thermo-physical properties of R-410A, such as specific heat, thermal conductivity, and latent heat. Ould Didi et al. [9] measured the pressure drop of five refrigerants (R-134a, R-123, R-402A, R-404A, R-502) over mass fluxes from 100 to 500 kgm2s1 and compared the experimental results with two-phase frictional pressure-drop models, which were developed based on the two-phase flow patterns. They reported that the method of Müller-Steinhagen and Heck [10] gave the best predictions for the annular flow, and that the best available method for intermittent flow and stratified-wavy flow was the Grönnerud model. Huang et al. [11] widely reviewed the condensation heat-transfer characteristics in the presence of noncondensable gas. They highlighted that the non-condensable gas accumulated in the liquid film during condensation degraded the condensation heat-transfer coefficient. Minkowycz and Sparrow [12] theoretically analyzed the condensation heat transfer on isothermal vertical plates with a steam-air mixture. They highlighted that the non-condensable gas can have a crucial influence on the heat-transfer rate even with small bulk concentrations of air. They also described that the heat transfer was reduced by more than 50% when the bulk mass fraction of non-condensable gas was 0.5%. Wu and Vierow [13] experimentally studied the fluid flow phenomena and condensation heat transfer of a steam-air mixture in a horizontal condenser tube that had a 27.5-mm inside diameter. They focused on the effects of local liquid-film characteristics, air mass fraction, turbulent mixing, and air concentration on the
X Y
m q s
measured value calculated value kinematic viscosity, ms1 density, kg m3 shear stress, Nm2
Subscript f g i l o s test
fluid, saturated liquid saturate vapor inlet, interface saturated liquid outlet tube surface test section
condensation heat-transfer coefficient. They reported that the turbulent mixing helps the steam pass through the air boundary layer and condense on the heat-transfer surface. Lee and Yun [14] experimentally investigated the in-tube heat-transfer characteristics of CO2 with impurities, such as CO2 + N2, CO2 + CH4, and CO2 + Ar, under the supercritical conditions for the land transportation of the CCS process. They mentioned that the maximum heattransfer coefficient of CO2 mixtures decreased with the increase of the mole fraction of impurities. In addition, they found that the maximum heat-transfer coefficient of CO2 mixtures increased when the operational pressure decreased. Oh and Revankar [15] experimentally and theoretically investigated the film condensation in a vertical tube with a steam-air mixture. They found that the condensation heat transfer decreased with increase of the saturated pressure and of the non-condensable gas fraction, but increased with the inlet steam flow rate. Xu et al. [16] investigated forced-convection condensation of a steam-air mixture inside a horizontal tube with a 28-mm outside diameter experimentally and theoretically. They reported that the local heat transfer coefficient increased with the increase of the inlet velocity of the mixture of gases; however, when the gas mass fraction of the inlet non-condensable gas increased, the local heat-transfer coefficient decreased. Until now, studies in the available literature on the condensation heat-transfer characteristics of CO2 with impurities have been very limited. In addition, the condensation heat-transfer characteristics of CO2 have been studied under low temperature conditions for use as a refrigerant, as shown in Table 1. In this study, in-tube condensation heat-transfer characteristics for pure CO2 and CO2 with impurities were experimentally investigated under the near-critical state for the CCS process. We investigated the effects of the amount of impurities, the condensation temperature, and the mass flux on the condensation heat-transfer coefficients and the pressure drop. The results can be used for the original design of facilities for CO2 pipeline transportation in the CCS process, and for monitoring CO2 transportation in real time.
2. Experimental apparatus and procedures 2.1. Experimental setup Condensation heat-transfer coefficients and pressure drop were measured by using the test facility shown in Fig. 1. The test setup consisted of a test section, heat exchangers that were connected to two chillers, mass-flow meters, sensors for temperature and
W. Baik, R. Yun / International Journal of Heat and Mass Transfer 144 (2019) 118628 Table 1 Existing experimental studies. Reference
Tube geometry
Test conditions
Measurements
Kang et al. [5]
Smooth tube (ID : 5.15 mm),
Heat transfer coefficient, pressure drop
Kim et al. [17]
Smooth (3.48 mm) and microfin tube (3.51 mm), Smooth tube (ID : 6.1 mm),
G : 600, 800, 1000 kgm2s1 Tsat : 10, 5, 0, 5 °C G : 200, 400, 800 kgm2s1 Tsat : 25, 15 °C G : 200, 300, 400 kgm2s1 Tsat : 25, 15 °C G : 100, 200, 300, 400, 500 kgm2s1 Tsat : 10, 5, 0 °C G : 200, 400, 600, 800 kgm2s1 Tsat : 25, 15 °C G : 50, 100, 200, kgm2s1 Tsat : 15, 10, 5, 0 °C
Jang and Hrnjak [18] Li et al. [19]
Park and Hrnjak [20] Iqbal and Bansal [21]
Smooth tube (ID : 4.73 mm)
Microchannel (ID : 0.89 mm) Smooth tube (ID : 6.52 mm)
Heat transfer coefficient Heat transfer coefficient, pressure drop Heat transfer coefficient
Heat transfer coefficient, pressure drop Heat transfer coefficient
pressure, a differential pressure transducer, and a magnetic gear pump that circulated CO2 mixtures. Heat exchangers were used to liquify the CO2 mixtures before they returned to the magnetic gear pump by using the brine solution (40%, EG/water). Fig. 2 shows the details of the test section, which is a double tube. The inner and outside diameters of the test section were 10.7 mm and 12.7 mm, respectively. The total length of the test section was 4 m, and it was composed of six sub-sections for the local measurement of heat-trasnfer coefficients. The test section was a copper tube and was assembled in the form of a double tube with a Polyvinyl Chloride (PVC) pipe to simulate the pipelinetransportation conditions for CO2 under seawater. The outside wall temperatures of the test section was measured by using T-type thermocouples that were firmly soldered to the surface. The heat-transfer rate at each subsection was calculated by inserting
3
the thermocouples through the PVC pipe. The thermocouples were firmly located at the inlet and outlet of each section. Table 2 shows the accuracies of the measuring instruments that were used for the experiment. The pressure transducers had an accuracy of ±0.13% in the full scale of 14.7 to 3000 psig, and a differential pressure transducer had an accuracy of ±0.05% within the full range of 0– 10 kPa. Prior to the experiment, all thermocouples were calibrated by using a constant-temperature water bath, and the differences among the thermocouples were less than ±0.1 °C. A Coriolis-type mass-flow meter that had a full range of 0–18 kgmin1 showed an accuracy of ±0.2%. Two other Coriolis-type mass-flow meters for measuring the mass-flow rate of brine and water to calculate the heat-transfer rate at the test tube and preheater had accuracies of ±0.1% and ±0.2%, respectively. Both mass-flow meters had a full range of 300 kgmin1. 2.2. Experimental procedure We used CO2 and a CO2 + N2 mixture as test fluids. The test procedures for measuring the condensation heat-transfer characteristics of the pure CO2 and the CO2 + N2 mixture were as follows. First, the temperature and flow rate of the brine on the annular side of the test section was adjusted. The CO2 + N2 mixture was injected into the test setup by using a gas booster, which operates through an air compressor. Then, the injected CO2 + N2 mixture was condensed and circulated in the test section. When the CO2 + N2 mixture was sufficiently charged into the system, the inlet temperature and pressure conditions of the test section were adjusted by using a constant-temperature water bath, shown in Fig. 1. Table 3 shows the present test conditions. The condensation pressure and temperature of the CO2 + N2 mixture changed from 57.29 to 81.51 bar, which corresponds to 0.77–0.99 of the reduced pressure, and 20–30 °C, respectively. The mass flux was controlled from 500, 600, and 700 kgm2s1. The mole fraction of N2 in the CO2 + N2 mixture was set at 1, 3, and 5%. The uncertainty of the components for the CO2 + N2 mixture was ±2%. The brine temperature of the test section was fixed at 4 °C to match the actual seawater temperature in winter. The vapor quality of the CO2 + N2 mixture was covered from 0.0 to 1.0.
Fig. 1. Experimental test facility.
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W. Baik, R. Yun / International Journal of Heat and Mass Transfer 144 (2019) 118628
Fig. 2. Details of test section.
Table 2 Specifications of the measuring instruments. Instruments
Range
Accuracy
Thermocouple Pressure transducer Mass flowmeter for brine-side Mass flowmeter for CO2-side Mass flowmeter for preheater Differential pressure transducer
200 °C to 200 °C 14.7 to 3000 psig 0–300 kgmin1 0–18 kgmin1 0–300 kgmin1 0–10 kPa
±0.1 °C ±0.13% full scale ±0.1% ±0.2% ±0.2% ±0.05% full scale
2.3. Data reduction
(Engineering Equation Solver) according to the method presented by the NIST (National Institute of Standards and Technology) [23]. Eq. (4) explains the uncertainty of the calculated value of Y from the measured valued of X. The uncertainty of the measured value, UY, depended on the error of the measuring instruments and the sensitivity of the calculated value to the measured values. The uncertainty of the inlet vapor quality was between ±1.93% and ±5.17%. Also, the uncertainty of the condensation heat transfer coefficient ranged between ±6.19% and ±16.7%. Prior to this experiment, the verification of the test facility was conducted by calculating the heat transfer rate for CO2. We have confirmed that the heat balance between the brine-side and the CO2-side of the test section was within ±5% under single-phase conditions.
The condensation heat-transfer coefficient of the CO2 + N2 mixture was calculated by using Eq. (1). The temperature of tube inner surface, Ts,i, was calculated by using the measurement of the outside surface temperature and by the one-dimensional heat equation.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u uX rY 2 2 UY ¼ t U Xi rX i i
h ¼ q_ test =ðT s;i T f Þ
3. Results and discussion
ð1Þ
The inlet and outlet vapor quality of the CO2 + N2 mixture across the test section was calculated by Eqs. (2) and (3), respectively. The inlet enthalpy of the mixture at the test section can be obtained by measuring the pressure and the temperature of the subcooled CO2 + N2 mixture at the preheater inlet and the heat-transfer rate in the preheater. The outlet vapor quality, xtest,out, was obtained by Eq. (3) at each subsection. The heat removal amount at each subsection was calculated by using the mass flow rate and temperature difference of the brine solution.
xtest;in ¼ itest;in il =ifg xtest;out
ð2Þ
_ CO2 ifg ¼ xtest;in q_ test = m
ð3Þ
The transporting and thermodynamic properties of CO2 + N2 mixture were obtained by REFPROP [22], based on the measured condensation temperature and pressure. The uncertainties of the heat transfer coefficients were calculated by using the EES
ð4Þ
3.1. Prediction of flow regimes Flow regimes of the two-phase flow for the CO2 + N2 mixture were estimated by using the existing models. Fig. 3 shows the prediction of flow regimes for the pure CO2 and the CO2 + N2 mixture by using the Taitel and Dukler flow-regime map [24]. Jang and Hrnjak [18] investigated the condensation flow regime of CO2 in a 6.10-mm smooth tube under low-temperature conditions, as shown in Table 1. They mentioned that the flow-regime map by Taitel and Dukler correctly predicted 19 of the 25 observations. Although our test was conducted at the near-critical temperature, it could be suitable for predicting the flow regimes for the condensation characteristics of CO2 and impurities in a horizontal smooth tube. As shown in Fig. 3, the annular and intermittent flow was predicted during the condensation of CO2 and the CO2 + N2 mixture. Most of the data points were plotted in the annular regions, and
Table 3 Test conditions. Fluid
Brine-side temperature
Mole fraction of N2 in the CO2 mixtures
Condensation temperature
Vapor quality
Mass flux
Pure CO2 CO2 + N2
4 °C 4 °C
– 1, 3, 5%
20, 25, 30 20, 25, 26.5–30
0.0–1.0 0.0–1.0
500, 600, 700 kgm2s1 500, 600, 700 kgm2s1
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Fig. 3. Experimental data of CO2 mixture on the flow regime map by Taitel and Dukler [24].
only a few data points were plotted in the intermittent regions. These results suggested that impurities do not have a great effect on the two-phase flow regime compared with that of pure CO2. In the present study, the condensation heat transfer and pressure drop characteristics of CO2 and the CO2 + N2 mixture were explained by the phenomena that are dominant for the annular flow patterns for the in-tube flow condensation. However, we needed to verify the condensation flow characteristics of CO2 with impurities in the horizontal tubes by doing some additional research. 3.2. Condensation heat-transfer coefficient 3.2.1. The effect of the mole fraction of N2 Fig. 4 shows the comparison of the present condensation heat transfer coefficients with the existing experimental data [5,18,19]. Although there were significant differences in the tube diameter and in the condensation temperature among the tests, the heat transfer coefficients were located within the range of 2–4 kWm2K1 except the Jang and Hrnjak’s results [18]. When the vapor quality increased, the condensation heat transfer
Fig. 4. Comparison of the present heat transfer coefficients with the existing data [5,18,19].
5
coefficient increased for all experimental results. Fig. 5 shows the effects of the mole fraction of N2 in the CO2 + N2 mixture on the heat transfer coefficient. The experiments were carried out at a condensation temperature of 20 to 30 °C, mass flux of 500 kgm2s1, and brine-side temperature of 4 °C. As shown in Fig. 5(a), when the mole fraction of N2 was set at 1%, 3%, and 5%, the average of the heat transfer coefficient was lower by 2.23%, 13%, and 15.9%, respectively, based on the average heat transfer coefficient of pure CO2; The average heat transfer coefficient of pure CO2 was 3 kWm2K1. As the vapor quality increased from 0.3 to 0.9, the heat transfer coefficient for the CO2 mixture increased by 8.2%. Increasing the mole fraction of N2 in the CO2 + N2 mixture from 1 to 5%, the condensation heat-transfer coefficient decreased. When the CO2 + N2 mixture was in a steady state, the condensation took place at the interface of a liquid film on the tube wall. Because of the condensation at the interface between the liquid film and the vapor, there is less vapor near the interface than there is far from the interface. At the same time, the non-condensable gas can be regarded as being uniformly distributed in the vapor core. The vapor amount near the interface and the non-condensable gas distribution caused a decrease of the CO2 vapor partial pressure at the vapor-liquid interface of the CO2 + N2 mixture, which lowered the condensation temperature at the liquid film. With the decrease of the condensation temperature at the interface, the amount of condensate decreased, which lowered the diffusion driving force of the CO2 vapor to the interface. These phenomena at the interface when there is non-condensable gas lower the condensation heat transfer coefficient. Also, these effects of noncondensable gas on the condensation heat transfer coefficients become significant as the amount of the non-condensable gas increases. The heat transfer coefficient increased with the increase of vapor quality, and these trends was found to result from the increase in the interfacial shear stress between the liquid and vapor phases with the increase of vapor quality. Soliman et al. [25] suggested the model that the Nusselt number for the condensation heat transfer coefficients was proportional to the square root of the interfacial shear stress. Fig. 5(b) presents the variation of heat transfer coefficient at a condensation temperature of 25 °C under the same conditions. When the mole fraction of N2 was changed to 1 3, and 5% in the CO2 mixture, the average of the heat transfer coefficient decreased by 6.67%, 14.5%, and 15.6%, respectively. Also, the experimental data showed that the heat-transfer coefficient increased with increasing vapor quality of the CO2 mixture. The increasing rate of the average heat transfer coefficient of the CO2 mixture with vapor quality ranged from 6% to 8%. Shah [26] reported that the existing correlations fail to predict the condensation heat transfer coefficients at high reduced pressures and that they are closely related to the reduced pressure and viscosity ratio of each phase. Because the critical temperature was 28.8 °C and 27.1 °C for the mole fraction of 3% and 5% of the CO2 mixture, the condensation temperature was set to 26.5 °C for the CO2 + N2 mixture. This temperature corresponds to the same reduced pressure of the pure CO2 at the condensation temperature of 30 °C. When the mole fraction of N2 increased from 1 to 5%, the mean heat transfer coefficient decreased by 5.8%, as shown in Fig. 5 (c). 3.2.2. The effect of condensation temperature Fig. 6 illustrates the effects of the condensation temperature of the CO2 + N2 mixture on the heat transfer coefficient. The experiments were carried out at a mole fraction of N2 of 1 to 5%, a mass flux of 500 kgm2s1, and brine-side temperature of 4 °C. Fig. 6(a) shows that the heat-transfer coefficient decreased with the increasing condensation temperature. When the condensation temperature increased from 20 to 30 °C, the average heat transfer coefficient decreased by 8.8%. Increasing vapor quality for the CO2
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-2
-1
Heat transfer coefficient (kW m K )
6
5
Pure CO2
was related to the thermo-physical properties of the CO2 mixture. As shown in Table 4, when the condensation temperature increased, the density ratio of liquid to vapor decreased. Increasing the density ratio of liquid to vapor increased the velocity difference between the vapor phase and liquid phase. The increased velocity differences also increased the interfacial shear stress. This phenomenon can be confirmed by Ueda et al.’s experimental results [27]. For internal flow condensation of steam in a tube, the Nusselt number significantly increased with the increase of the nondimensional interfacial shear stress under the same Reynolds number. The non-dimensional interfacial shear is defined as Eq. (5). It is well known that the interface instability, such as the KelvinHelmholtz instability, significantly increased with the increase of the velocity difference between the liquid and vapor phases. This increased instability can highly increase the condensation heattransfer coefficients as related to the interfacial shear.
Mass flux : 500 kg m-2s-1
CO2+N2 (1%) Condensation Temp : 20oC CO2+N2 (3%) CO2+N2 (5%)
4
3
2
1
0 0.0
0.2
0.4
0.6
0.8
1.0
si ¼
Vapor quality (-)
(a) Heat transfer coefficient (kW m -2K-1)
6
5
3
2
1
0.2
0.4
0.6
0.8
1.0
Vapor quality (-)
(b) Heat transfer coefficient (kW m-2K-1)
6
5
Pure CO2 Mass flux : 500 kg m-2s-1 CO2+N2 (1%) Condensation Temp : 26.5oC - 30oC CO2+N2 (3%) CO2+N2 (5%)
4
3
2
1
0 0.0
0.2
0.4
0.6
0.8
ð5Þ
Another factor that affects the condensation heat transfer coefficient is the thermal conductivity of the liquid film. As the condensation temperature increased, the thermal conductivity of the liquid film decreased, which then lowered the condensation heat transfer coefficients and increased the condensation temperature. Fig. 6(b), (c), and (d) show the similar result for the heat-transfer coefficient of pure CO2 under the same temperature conditions. When the condensation temperature changed from 20 to 30 °C, the heat transfer coefficient decreased by 13.3%, as shown in Fig. 6(b). Also, as shown in Fig. 6(c) and (d), the heat-transfer coefficient for the CO2 mixtures decreased with increasing condensation temperature, and the percentage were 13.4% and 9.5%, respectively.
Pure CO2 Mass flux : 500 kg m -2s-1 CO2+N2 (1%) Condensation Temp : 25 oC CO2+N2 (3%) CO2+N2 (5%)
4
0 0.0
s ql ðg v l Þ2=3
1.0
Vapor quality (-)
(c)
3.2.3. The effect of mass flux Fig. 7 shows the effects of mass flux of the CO2 + N2 mixture on the condensation heat transfer coefficient. The experiments were conducted at condensation temperatures of 20 °C to 30 °C, N2 mole fraction of 3%, and brine-side temperature of 4 °C. Fig. 7(a) illustrates the comparison of the heat transfer coefficient of pure CO2 and the CO2 + N2 mixture at the condensation temperature of 20 °C. When the mass flux changed from 600 to 700 kgm2s1, the average heat transfer coefficient of pure CO2 increased by 19.4% and 26.4%, respectively, over the average heat transfer coefficient when the mass flux was 500 kgm2s1. When the mass flux changed from 500 to 600 and to 700 kgm2s1, the average heattransfer coefficient of pure CO2 was 3, 3.7, and 4 kWm2K1, respectively For the CO2 + N2 mixture, the heat transfer coefficient calculated higher at 16.4% and 21.6% based on the average heattransfer coefficient that the mass flux condition was 500 kgm2s1, respectively. Fig. 7(b) and (c) show the trends of the condensation heat transfer coefficients as being similar to that of Fig. 7(a). As shown in Fig. 7, the heat transfer coefficient increased with respect to the mass flux at each condensation temperature. Dukler derived Eq. (6) of the non-dimensional interfacial shear stress from the empirical two-phase pressure drop correlation. A and B are constants that depend on the condensation temperature and the vapor quality. Therefore, the non-dimensional interfacial shear stress can depend on the mass flux under the same condensation temperature and the vapor quality.
si ¼ AðT sat ÞBðxÞG1:4
ð6Þ
Fig. 5. Variation of heat transfer coefficient with N2 mole fraction (a) 20 °C (b) 25 °C (c) 26.5–30 °C.
3.3. Pressure drop mixtures increased the heat-transfer coefficient, and the range of increase was 7.3 to 8.5%. The heat transfer coefficient increased with decreasing condensation temperature, whose main effect
Changes of the average pressure drop with the N2 mole fraction of CO2 mixtures are shown in Fig. 8. The experiments were
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6
Condensation Temp : (20oC) Condensation Temp : (25oC) Condensation Temp : (30oC)
5
Heat transfer coefficient (kW m-2K-1)
Heat transfer coefficient (kW m-2K-1)
6
Mass flux : 500 kg m-2s-1 4
Fluid : Pure CO2
3
2
1
0 0.0
0.2
0.4
0.6
0.8
Condensation Temp : (20oC) Condensation Temp : (25oC) Condensation Temp : (30oC)
5
Mass flux : 500 kg m-2s-1 4
Fluid : CO2 + N2 (1%)
3
2
1
0 0.0
1.0
0.2
Vapor quality (-)
0.4
(a) 6
Condensation Temp : (20oC) Condensation Temp : (25oC) Condensation Temp : (28oC)
5
-2 -1
Mass flux : 500 kg m s 4
Fluid : CO2 + N2 (3%)
3
2
1
0 0.0
0.2
0.4
0.8
1.0
(b) Heat transfer coefficient (kW m-2K-1)
Heat transfer coefficient (kW m-2K-1)
6
0.6
Vapor quality (-)
0.6
0.8
1.0
Condensation Temp : (20oC) Condensation Temp : (25oC) Condensation Temp : (26.5oC)
5
Mass flux : 500 kg m-1s-2 4
Fluid : CO2 + N2 (5%)
3
2
1
0 0.0
0.2
0.4
0.6
0.8
1.0
Vapor quality (-)
Vapor quality (-)
(c)
(d)
Fig. 6. Variation of heat transfer coefficient with condensation temperature (a) 0% (pure CO2) (b) 1% (c) 3% (d) 5%.
Table 4 Variation of thermophysical properties of CO2 mixtures with condensation temperature. Fluid
Condensation temperature ( °C)
Thermal conductivity of liquid (Wm1k1)
Density ratio of liquid to vapor (/)
Surface tension (mNm1)
Viscosity of liquid (106 Pas)
Pure CO2
20 25 30
0.085 0.081 0.095
3.982 2.927 1.719
1.203 0.552 0.054
66.148 57.048 43.768
CO2 + N2 (1%)
20 25 30
0.083 0.079 0.094
3.824 2.762 1.441
1.069 0.435 –
63.844 54.426 38.754
CO2 + N2 (3%)
20 25 28
0.08 0.076 0.077
3.497 2.404 1.067
0.802 0.207 –
59.098 48.728 39.069
CO2 + N2 (5%)
20 25 26.5
0.076 0.072 0.071
3.153 1.991 0.996
0.568 0.034 –
54.151 42.228 36.691
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Pure CO2
Condensation Temp : (20oC)
5
5
4
4
3
3
2
2
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
0 0.0
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
Massflux : 700 kg m-2s-1
Massflux : 700 kg m-2s-1 0.2
0.4
0.6
6
CO2 + N2 (3%)
Condensation Temp : (20oC)
0.8
Heat transfer coefficient (kW m-2K-1)
Heat transfer coefficient (kW m-2K-1)
6
0 1.0 0.0
0.2
Vapor quality (-)
0.4
0.6
0.8
CO2 + N2 (3%) o
Condensation Temp : (25 C)
Condensation Temp : (25oC)
5
5
4
4
3
3
2
2
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
0.2
Vapor quality (-)
0.4
0.6
0 1.0 0.0
0.8
Vapor quality (-)
Massflux : 700 kg m-2s-1 0.2
0.4
0.6
0.8
1.0
Vapor quality (-)
(a)
(b)
6
Heat transfer coefficient (kW m-2K-1)
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
Massflux : 700 kg m-2s-1
0 0.0
1.0
6
Pure CO2
6 CO2 + N2 (3%)
Pure CO2
Condensation Temp : (28oC)
Condensation Temp : (30oC) 5
5
4
4
3
3
2
2
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
0 0.0
Massflux : 500 kg m-2s-1 Massflux : 600 kg m-2s-1
1
Massflux : 700 kg m-2s-1 0.2
0.4
0.6
0.8
Massflux : 700 kg m-2s-1
0 1.0 0.0
0.2
Vapor quality (-)
0.4
0.6
0.8
1.0
Vapor quality (-)
(c) Fig. 7. Variation of heat transfer coefficient with mass flux (a) 20 °C (b) 25 °C (c) 28–30 °C.
3
Pressure drop (kPa)
conducted at condensation temperatures of 20 to 30 °C, mass flux of 500 kgm2s1, and brine-side temperature of 4 °C. The pressure drop decreased with the increasing mole fraction of N2 at each condensation temperature. As the N2 mole fraction of the CO2 mixture increased from 1 to 3 and to 5%, the pressure drop measured as 23.7%, 43.9%, and 54.5% lower than the condensation temperature of 20 °C based on those of pure CO2, respectively. When the condensation temperature was 25 °C, the pressure drop was decreased by 53.4%, 66.9%, and 77.3% under the same conditions of a condensation temperature of 20 °C, respectively. When the N2 mole fraction of CO2 mixture increased from 1 to 3 and to 5%, based on the pure CO2, the pressure drop decreased by 78.6%, 77.7%, and 79.2%, respectively, at the condensation temperature of 26.5 to 30 °C. The pressure drop decreased with the increasing mole fraction of N2 with all mixtures. Fig. 9 and Table 4 show the variation of the density ratio between liquid to vapor with the change of the mole fraction of N2. It can be clearly shown that the density ratio decreased with the increase of the mole fraction of N2. Accordingly, the vapor velocity in the two-phase flow significantly decreased with the increase of the impurities, resulting in a lower pressure drop for the lower mole fraction of the impurities. Fig. 10 shows the effect of condensation temperature on the mean pressure drop. The mean pressure drop decreased by 43.8% on average when the temperature was increased from 20 to 30 °C for pure CO2. When the condensation temperature changed from 20 to 30 °C, the mean pressure drop decreased by 69.9% at a mole fraction of N2 of 1%. For a mole fraction of N2 of 3%, the mean pressure drop decreased by 64.5%. When the condensation
Condensation Temp : 20oC Condensation Temp : 25oC Condensation Temp : 26.5 - 30oC Massflux : 500 kg m-2s-1
2
1
0 Pure CO2
CO2+N2 (1%)
CO2+N2 (3%)
CO2+N2 (5%)
Fig. 8. Variation of pressure drop with N2 mole fraction.
temperature increased from 20 to 26.5 °C, the mean pressure drop decreased 71.2%, and as the condensation temperature increased from 25 to 26.5 °C, the decreasing rate was 48.7% for the mole fraction of 5% of N2. The pressure drop at 20 °C was higher than at 25 °C, 26.5 to 30 °C in all mixtures. The effects of the condensation
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5
Pressure drop (kPa)
4
3
Condensation Temp : 28 - 30 oC Condensation Temp : 25 oC Condensation Temp : 20 oC
: Pure CO2 : CO2 + N2 (3%)
2
1
0 500
600
700 -2 -1
Massflux (kg m s ) Fig. 9. Variation of density ratio of liquid to vapor with condensation temperature.
temperature on the pressure drop can be explained by the same reasoning used to explain the condensation heat transfer coefficients. With the increase of the condensation temperature, the non-dimensional interfacial shear stress decreased, which decreased the pressure drop. Besides, it should be noted that the viscosity of the liquid film decreased with the increase of the condensation temperature too. The effect of mass flux on the mean pressure drop of pure CO2 and of the CO2 + N2 mixture is shown in Fig. 11. The N2 mole fractions of the CO2 + N2 mixture were 0% and 3%; 0% means pure CO2. As expected, when the mass flux increased by an interval of 100 kgm2s1, the pressure drop increased in all CO2 + N2 mixtures. When the mass flux varied 500 to 700 kgm2s1, the mean pressure drop increased by 35.6% on average for pure CO2 at all condensation temperatures. Also, the increasing rate was 38.8% for the CO2 + N2 mixture. We compared the average heat transfer coefficients between the experimental data and the predictions by using the Lee and Rose model [28] for each mole fraction as shown in Fig. 12. When the mole fraction of N2 in the mixture and the condensation temperature increased, the experimental data and the predictions from
Pressure drop (kPa)
3
Massflux : 500 kg m-2s-1
Pure CO2 CO2+N2 (1%) CO2+N2 (3%) CO2+N2 (5%)
Fig. 11. Variation of pressure drop with mass flux.
Fig. 12. Comparison of experimental data with the predictions by the Lee and Rose model [28].
the Lee and Rose model show the decreasing trends of the condensation heat transfer coefficients. The maximum mean deviation between them was 25.4%, and the minimum was 2.8%.
2
4. Conclusions
1
0 20
25
30 o
Condensation Temperature ( C) Fig. 10. Variation of pressure drop with condensation temperature.
In-tube condensation heat-transfer characteristics of pure CO2 and a CO2 + N2 mixture were experimentally investigated under seawater transportation conditions for the CCS process. The heat transfer coefficient and pressure drop of the CO2 + N2 mixture during pipeline transportation were significantly dependent on the quantity of the impurities, condensation pressure, and mass flux. When the mole fraction of N2 increased, the condensation heattransfer coefficient of the CO2 + N2 mixture decreased because of the thermophysical properties of the mixture and the effects of a non-condensable gas on the film condensation. As the condensation temperature of the CO2 + N2 mixture increased, the heat transfer coefficient decreased from the low density ratio between liquid and vapor, the decreased interfacial shear stress and less interfacial
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movements. The pressure drop of the CO2 + N2 mixture increased with decreasing N2 mole fraction; however, as the condensation temperature decreased, the pressure drop increased. The effects of the mole fraction of N2 and the condensation temperature on the pressure drop can be well explained by the velocity difference between phases. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education of the Republic of Korea (NRF-2016R1D1A1B02010075). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijheatmasstransfer.2019.118628. References [1] IEA, World Energy Outlook, OECD/IEA, London, 2011. [2] R. Balnk, J. Lubchenco, R. Rietrick, Global Sea Level Rise Scenarios for the United States National Climate Assessment, NOAA, Silver Spring, 2012, pp. 1–2. [3] IEA, 20 Years of carbon capture and storage: Accelerating future deployment, OECD/IEA, Paris, 2016. [4] IPCC, Climate Change 2014: Synthesis Report, in: Core Writing Team, R.K. Pachauri, L.A. Meyer (Eds.), Contribution of Working Groups, I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC, Geneva, Switzeland, 2014, pp. 151. [5] P.J. Kang, R. Yun, Condensation heat transfer characteristics of CO2 in a horizonal smooth tube, Int. J. Refreg. 36 (2013) 1090–1097. [6] J. Heo, R. Yun, Prediction of CO2 condensation heat transfer coefficient in a tube, Int. J. Refrig. 89 (2015) 254–263. [7] P. Li, J.J.J. Chen, S. Norris, Review of flow condensation of CO2 as a refrigerant, Int. J. Refrig. 72 (2016) 53–73. [8] Y. Jiang, B. Mitra, S. Garimella, U. Andresen, Measurement of condensation heat transfer coefficients at near-critical pressures in refrigerant blends, J. Heat Transfer 129 (8) (2007) 958–965.
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