Heat Transfer Characteristics Of CO2 Research Articles

Experimental investigation of the heat transfer characteristics of CO2 at supercritical pressures flowing in heated vertical pipes

Supercritical CO2 shows great potential as a working fluid in power plant cycles due to its moderate critical pressure of 7.38 MPa and critical temperature of 30.98 °C, closely matching typical heat sink temperatures. Understanding the heat transfer characteristics of sCO2 is crucial for designing cycle components. This publication presents a systematic analysis of sCO2 heat transfer in two heated vertical pipes with 4 and 8 mm inner diameters, with both upward and downward flow at pressures of approximately 7.75, 8.00, and 9.50 MPa, and flow inlet temperatures between 5 and 40 °C, based on 196 experiments. The study investigates a range of mass fluxes from 400 to 2000 kg/m2s and heat fluxes from 10 to 195 kW/m2, resulting in a heat to mass flux ratio of 6 to 275 J/kg. The findings reveal enhanced, normal, and deteriorated heat transfer within the experimental dataset. Buoyancy effects are identified as the main cause of deteriorated heat transfer in the investigated parameter range, with flow acceleration showing no significant influence on heat transfer. A new proposed dimensional criterion categorises 36 out of 119 experiments with upward flow in the deteriorated heat transfer regime, accompanied by temperature peaks rising to 47 K. Thermal inflow lengths range from 0 to 480 inner pipe diameters, with some experiments not achieving a thermally fully developed flow over the entire pipe length. Based on a comparison with approximately 8950 experimental data points, two Nusselt correlations are found, capable of reproducing the experimental results with a mean absolute deviation of around 30 %. This publication provides valuable data for validating numerical models and developing correlations to predict the heat transfer of supercritical fluids.

A numerical investigation of the thermal characteristics of supercritical carbon dioxide (sCO2) flowing through mini serpentine channels

In this study, we numerically investigate the heat transfer characteristics of CO2 during phase transitions in its supercritical state in serpentine channels. Four different inclination angles are applied to three different radii of curvature flow channels with the same diameter of 2 mm. The supercritical state is maintained by assuming that the mass flux is approximately 191 kg/m2s, the inlet temperature is 298.15 K, and the operating pressure is 7.65 MPa. To ensure a consistent total heat input, different heat flux values are applied to the three different cases, eliminating the potential impact of the increased surface area on the heat transfer performance and allowing a clearer investigation of the geometric effects within the same temperature range. In a channel with a constant radius of curvature, the centrifugal force has a dominant influence on the heat flow compared to gravitational buoyancy, showing a flow phenomenon independent of the angle. In contrast, in a flow channel with a varying radius of curvature, the gravitational buoyancy fluctuates depending on the local area. As a result, the ratio of the centrifugal force to the gravitational buoyancy decreases, causing more variations with changes in the inclination angle. As the centrifugal force contributes significantly to turbulent mixing, the average heat transfer coefficient is highest in the flow channel with the shortest wavelength. As the wavelength increases, the ratio of centrifugal force to buoyancy begins to affect density stratification, especially in gas-like regions, leading to buoyancy. The local heat transfer coefficient at points around the circumference of the cross-section affected by buoyancy and in the film is analyzed, which is closely related to the centrifugal buoyancy and gravitational buoyancy, and their ratio. To express this influence in a correlation equation, a new Nu correlation with an error of less than 25 % is proposed using De, which combines the centripetal force and Reynolds number, ratio of centrifugal buoyancy and gravitational buoyancy, Ф, and Pr as.Nu=0.849De0.541∅-0.154Pr-0.160

Study on the heat transfer characteristics of CO2 and liquid desiccant CaCl2 aqueous solution in the gas cooler and evaporator

The heat exchangers are essential components in the CO2 heat pump-driven liquid desiccant dehumidification system since the heat transfer performance of heat exchangers directly affects the energy efficiency of the system. However, the heat transfer characteristics of CO2 and liquid desiccant in the heat exchanger have yet to be fully revealed, resulting in a lack of appropriate strategies to improve the heat transfer performance. To address this issue, numerical gas cooler and evaporator models are developed and verified. The effects of parameters such as the weight concentration of the CaCl2 aqueous solution, the CO2 inlet pressure, mass flow rates, and the length of the heat exchanger on the heat transfer performance are investigated respectively under various operating conditions. Results show that the heat exchange load of the gas cooler can be significantly impacted by various inlet parameters of both fluids. In particular, the increase in the weight concentration from 36 to 54 wt.% deteriorates the heat exchange load by over 15 %. Nevertheless, the heat exchange load of the evaporator is hardly influenced by the weight concentration. This is attributed to a great gap in the two fluids’ heat capacity rates, causing the evaporator to be in the maximum heat transfer state. The sign of the maximum heat transfer state of the heat exchanger is whether the thermal effectiveness is close to 1, which is related to the heat transfer area. Therefore, modifying the heat exchanger length could avoid the thermal effectiveness approaching 1. Results indicate that the appropriate length of the evaporator is 3–4 m under the designed operating conditions.

Molecular dynamics simulation of interfacial heat transfer characteristics of CO2, R32 and CO2/R32 binary zeotropic mixture on a smooth substrate

Mixture fluids have the advantage of a flexible adjustment of the physicochemical properties by actively tuning the composition and concentrations, showing promising potential in thermal and power-related applications. However, mixtures are mainly zeotropic with heat transfer deterioration during the boiling process, resulting in a challenge to gain a comprehensive understanding of their microscopic phase change. In this study, molecular dynamics simulations were employed to investigate the nanoscale behavior of CO2, R32 and CO2/R32 mixtures. The equilibrium molecular dynamics (MD) method and the non-equilibrium MD method was utilized to study the vapor-liquid interface characteristics and the boiling heat transfer characteristics of zeotropic mixture, respectively. The results revealed that the thickness of the vapor-liquid interface was linearly related to the temperature, and the surface tension displayed significant fluctuations in both solid-liquid and vapor-liquid interfaces, which indicated a potential barrier for molecular transport. Compared to the pure components, the inception of nucleation boiling and vapor film formation of the mixture fluid was significantly delayed. The heating and evaporation rate, heat flux of mixture were lower, while the thermal resistance was significantly higher. Moreover, the time required for bubble nucleation and the formation of vapor film decreased notably with the enhanced wetting characteristics. The findings in this study provide a molecular-scale insight into zeotropic mixture boiling heat transfer and the possible wettability regulation to improve the heat transfer performance of zeotropic mixtures and promote their wider applications.

Solar tower CSP plants with transcritical cycles based on CO2 mixtures: A sensitivity on storage and power block layouts

In this work three CO2-based binary mixtures, CO2 + C6F6, CO2 + C2H3N and CO2 + C4F8, are compared as innovative working fluids for closed power cycles in CSP plants. Adopted in transcritical cycles, they lead to cycle efficiencies higher than sCO2 cycles at minimum temperatures above 50 °C, a typical condition for arid regions with high solar radiation. The analysis considers four plant configurations: the first with direct storage, solar salts as HTF and cycle maximum temperatures of 550 °C, while the three other plants adopt sodium as HTF and an indirect storage system, designed for cycle maximum temperatures of 550 °C, 625 °C and 700 °C. Detailed models are used to characterize the solar fields optical performance, the receiver thermal efficiency and the HTF pump consumption, both at design and off-design conditions, for large scale plants located in Las Vegas. Different power block layouts are considered, spanning from the more efficient ones to cycles with a high heat recovery capacity. In addition, the impact of the mixtures on the design of heat exchangers is evidenced, with convincing results with respect to the heat transfer characteristics of CO2. Considering the resulting yearly performances and LCOE of each configuration, the adoption of indirect storage systems is considered a viable solution for high temperature solar plants. The three innovative mixtures allow for a reduction in LCOE with respect to sCO2 cycles (up to 10 $/MWh, depending on the configuration), capacity factors above 70% for the specific location, optimal solar multiples around 2.8 and 12 equivalent hours of TES.

Heat Transfer Characteristics of CO2 in a Horizontal Tube under Subcritical and Supercritical Pressures

Supercritical CO2 heat transfer in vertical tubes has been widely studied, but limited work exists for that in horizontal tubes. In this work, experiments were conducted for CO2 in a horizontal tube with inner diameter of 8 mm under wide range of pressures (5.51–20.41 MPa), covering both subcritical and supercritical conditions. The ranges of heat flux and mass flux were 97.5–404.0 kW/m2 and 510.2–1314.2 kg/m2s, respectively. The heat transfer for subcritical pressure far below critical pressure showed typical flow boiling characteristics, with a wall temperature peak along the tube corresponding to departure from nucleate boiling. For near-critical pressure, it showed characteristics similar to supercritical heat transfer, with continuous increase of wall temperature along the tube. The effects of different parameters including pressure, mass flux, and heat flux on the heat transfer behavior were analyzed and discussed. Moreover, using the pseudo-vapor quality concept of the supercritical three-regime-model introduced in a previous work, the heat transfer coefficient was found to behave very similarly between subcritical and supercritical conditions. The experimental results showed significant deviation from single-phase convection within the two-phase/two-phase-like region compared with the single-phase regions, and which validates the similarity between subcritical boiling and supercritical pseudo-boiling.

Experimental investigation on natural circulation heat transfer of supercritical CO2 in a closed loop

The natural circulation system based on supercritical CO2 has attracted attention due to its simple structure and safety compared with the forced circulation system. This work presented an experimental investigation of the heat transfer characteristics of CO2 based on natural circulation. Two heat transfer modes can also be observed in supercritical CO2 natural circulation, normal heat transfer and heat transfer deterioration. The development of heat transfer deterioration criterion and heat transfer correlation is significant for the application of supercritical CO2 natural circulation, but there are few related studies. Direct adoption of the supercritical CO2 forced convective criteria and heat transfer correlations to natural circulation introduces large deviations. By analyzing the existing research results of forced convection in supercritical CO2 and introducing new dimensionless parameters SBO and K, a deterioration criterion and a heat transfer correlation suitable for natural circulation of supercritical CO2 are developed based on experimental data. The new criterion can well distinguish the two heat transfer modes, and the new correlation improves markedly. The results can benefit engineering applications in advanced power cycles.

Experimental investigation of near-critical CO2 heat transfer performance in a closed natural circulation loop

The natural circulation system based on supercritical CO2 has attracted attention due to its simple structure and safety compared with the forced circulation system. This work presented an experimental investigation of the heat transfer characteristics of CO2 based on natural circulation. A closed loop natural circulation experimental system was built, which was specially designed for near-critical pressure experiments. The effects of filling ratio, coolant temperature, heat transfer rate and system pressure were carefully discussed. Experimental results illustrated the loop had excellent heat transfer capability at subcritical pressure, but the heating section showed heat transfer deterioration at supercritical pressures. The heat transfer behaviors were analyzed. From subcritical pressure to supercritical pressure in the loop, the heat transfer mechanism transforms from two-phase transition to supercritical single-phase convection. Gas-like layer and insulating layer will form near the wall in the heating section, resulting in a large thermal resistance at the wall for heat to enter the core flow.

In-tube condensation heat transfer characteristics of CO2 with N2 at near critical pressure

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 kg·m−2s−1. 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.

Study of convection heat transfer of CO2 at supercritical pressures during cooling in fluted tube-in-tube heat exchangers

The heat transfer characteristics of CO2 at supercritical pressures during cooling in two fluted tube-in-tube heat exchangers and a smooth tube-in-tube heat exchanger are investigated. Both the overall heat transfer and the local heat transfer coefficients are analyzed at various CO2 mass flow rates, inlet pressures, and flute pitches. The results demonstrate that the smaller hydraulic diameter of a fluted tube has a higher heat transfer coefficient at any pressure. The fluid is more disturbed in the fluted tube with a smaller flute pitch where the temperature gradient of CO2 near the wall is larger, resulting in greater Nusselt number. The overall heat transfer coefficients of the fluted tubes are 2–3 times that of the smooth tube. A new correlation is developed by considering the fluid properties and the fluted tube structure effects for the convective heat transfer calculation of supercritical CO2 in the fluted tube during cooling.

Simulation Research of Heat Transfer Characteristics of Carbon Dioxide in Microchannel Evaporator

Abstract In this paper, the simulation model of two-dimensional (2D) distribution parameter of a CO2 microchannel evaporator was developed using the finite element method. The simulation model of the CO2 microchannel evaporator was written using matlab both considering the dry and wet conditions on air side, and different heat transfer characteristics of CO2 in two-phase region and overheated region. The experimental and simulation results in terms of CO2 temperature, wall temperature, inlet and outlet air temperatures, and convective heat transfer coefficient were compared. The simulation results have the same tendency with the experimental correlation results. The convective heat transfer efficient increases with the growth of CO2 inlet dryness, mass flow rate and air speed, while decreases along with the increase of evaporation pressure in two-phase region. The dry-out point appears earlier with larger CO2 inlet dryness, and higher air temperature, humidity and speed; however, it appears later with the increasing evaporation pressure and mass flow rate. The convective heat transfer coefficient at the dry-out point decreases dramatically due to the deteriorated heat transfer at this position, which indicates the necessity to prevent or retard the appearance of dry-out point.

Numerical study on the heat transfer characteristics between supercritical carbon dioxide and granite fracture wall

A two-dimensional numerical model was developed to investigate the flow and heat transfer characteristics of supercritical carbon dioxide (scCO2). The numerical results of flow and heat transfer characteristics of scCO2 through straight granite fracture were found to be in good agreement with the experimental results. Moreover, the heat transfer characteristics between scCO2 and rough granite fractures were also investigated by using the proposed model. The confining temperature outside the specimen was set at 200 °C and the pressure of scCO2 was 8 MPa. Four different case studies with the flow rates of 0.75, 0.51, 0.35, and 0.13 kg h−1 were, respectively, designed. The overall and the local heat transfer coefficients (OHTC, LHTC) were used to characterize the heat transfer properties of these cases. Furthermore, the effects of flow rate, fracture roughness, and phase state of CO2 on the heat transfer characteristics of CO2 were investigated. The results indicated that increased roughness or flow rate could improve the heat transfer performance of scCO2. Moreover, on the same area of the fracture, the temperature of the fracture wall was found to be always higher compared to that of scCO2; much similar to the water behavior and the OHTC of scCO2 increased with the increase in the injection flow rate. Surface morphology of the fracture significantly influenced the LHTC distribution of scCO2, and LHTC roughly exhibited a negative correlation with the local waviness of fracture. This was pronounced at the sunken positions of the fracture surface, which exhibited significantly larger LHTCs compared to the prominent positions. The heat transfer characteristics of CO2 were found to be closely related to its phase state. CO2 in the denser phase showed better heat transfer performance.

Heat transfer characteristics and correlation for CO2/propane mixtures flow evaporation in a smooth mini tube

The flow evaporation heat transfer characteristics of CO2 and propane mixtures in a horizontal smooth 2 mm inner diameter tube were experimentally investigated. The working conditions included CO2 and propane composition ratios of 100:0, 75:25, 50:50, 25:75 and 0:100, initial evaporation temperature of 0–10 °C, mass flux and heat flux up to 400 kg m−2 s−1 and 15 kW m−2, respectively. The results show that the mixture heat transfer coefficient is between that of pure CO2 and pure propane for the same conditions, and increases with increasing CO2 proportion and initial evaporation temperature. The dryout vapor quality increases with increasing propane proportion or decreasing initial evaporation temperature. A correlation to predict the heat transfer coefficient for pure CO2, pure propane and various mixtures flow evaporation in small diameter tubes with the predictions agreeing well with the experimental data of this and other studies was developed.

Modeling of a two-phase CO2-filled vertical borehole for geothermal heat pump applications

A numerical model has been developed to study the complex thermal behavior of a CO2-filled vertical geothermal borehole consisting of a long copper U-tube embedded in a solid material (grout). The model accounts for combinations of single-phase and two-phase fluid flow which may occur along the U-tube length. An explicit solution for fully coupled steady-state conservation equations of mass, momentum and energy is applied. The borehole is divided into small volume elements due to relatively significant changes in the thermophysical properties of the fluid. Borehole wall temperature variations as well as thermal interaction between tube sections are taken into account and the model predicts the fluid temperature, pressure and two-phase quality profiles.A case in which the borehole is assumed to be an evaporator of a direct-expansion ground coupled heat pump is studied. The model is used to assess the thermal performance of the borehole as well as pressure, temperature, and quality variations of CO2 along the U-tube. According to the results, the pressure variation of the fluid in the two-phase region has unavoidable consequences on the temperature variation. Results indicate that the proposed borehole can extract relatively large amounts of energy due to the superior two-phase heat transfer characteristics of CO2. Furthermore, the heat exchange rate between the ground and the upward flow leg in the two-phase region is greater than that between the ground and the downward flow, which is the opposite of conventional boreholes with a performance drop from the downward flow to the upward flow.

Comparison of condensation heat transfer and pressure drop of CO2 in rectangular microchannels

The in-tube condensation heat transfer characteristics of CO2 in three different microchannels were investigated. The channels were rectangular, and the numbers of ports were 7, 19, and 23. The hydraulic diameters were 1.5, 0.78, and 0.68mm for the 7, 23, and 19 ports, respectively. The test temperature ranged from −5 to 5°C, and the range of the mass flux was from 400 to 800kg/m2s. The condensation heat transfer coefficient increased with the decrease in hydraulic diameter, which is dominant at condensation temperatures of 0 and −5°C. Increasing and decreasing the heat transfer coefficient at critical vapor quality was observed for the microchannel of 23 ports. The highest pressure drop was found in the microchannel of 23 ports too. The model by Thome et al. showed the smallest mean deviation of heat transfer coefficient, and the Mishima and Hibiki model had a mean deviation of ±30.1% for the frictional pressure drop.

CO2 radiation heat loss effects on NOx emissions and combustion instabilities in lean premixed flames

The effects of CO2 dilution on NOx emissions and combustion instability are studied in an effort to use biogas as a fuel in dual lean premixed gas turbines. More specifically, the influence of the radiation heat transfer characteristics of CO2, which is a major component of biogas, on the combustion characteristics of a dual lean premixed flame of biogas is investigated. OH* chemiluminescence images are used to observe the flame structure for various CO2 dilution rates. The results show little difference in the flame structure for CO2 dilution rates of up to 40%, while higher dilution rates lower the flame intensity. Also, dual lean premixed flames show very different flame structures and temperatures depending on the pilot fuel mass fraction. The results show a decrease in the flame temperature when the dilution rate is increased, resulting in a reduction in the thermal NOx emissions. The present paper also shows that the radiation heat release, which is due to the high heat release rate of CO2, promotes a further drop in the flame temperature, in addition to the thermal effects of CO2. These findings are numerically modeled, empirically verified, and added to the NOx prediction model. CO2 dilution also changes the combustion oscillation frequency. To estimate this, the flame temperature is calculated as a function of the CO2 dilution rate and the pilot fuel mass ratio. The accuracy of the flame temperature calculation can be improved with a more accurate estimation of the radiation heat loss. In calculating the radiation heat loss for an unstable flame, considering the heat release fluctuation (i.e. the temperature fluctuation) improves accuracy, whereas taking the average temperature results in an underestimation. With an improved estimation of the temperature, the combustion oscillation frequency is more accurately predicted.

Condensation heat transfer characteristics of CO2 in a horizontal smooth tube

The condensation heat transfer characteristics of CO2 flowing in a horizontal smooth tube were investigated. The tube diameter was 5.15 mm. The condensation temperature ranged from −10 to 5 °C, and the mass flux was from 600 to 1000 kg m−2 s−1. When the temperature changed from 0 to −10 °C, the increase rate of the heat transfer coefficient was from 9.4 to 14.6%, and the pressure drop increased from 6.2 to 52.9%. When the mass flux increased from 600 to 1000 kg m−2 s−1, the heat transfer coefficient increased from 6 to 35%, and the pressure drop increased from 60 to 165%, which were dependent on the condensation temperature. The increases come from the change of vapor velocity and thermophysical properties with condensation temperature. Considering large variation of mass flux from 200 to 1200 kg m−2 s−1 by including the existing studies, the effect of mass flux on condensation heat transfer coefficient was minor.

CO2 condensation heat transfer coefficient and pressure drop in a mini-channel space condenser

We present the flow condensation heat transfer characteristics of CO2 in a mini-channel condenser. The condenser consists of seven tubes in parallel, which are thermally connected to two aluminium base-plates by using thermal glue. At saturation temperatures ranging from −5°C to 15°C, with mass velocities of 180, 360 and 540kg/(m2s), respectively, and average vapour qualities from 0.2 to 0.8, we obtained the CO2 condensation heat transfer coefficients, ranging from 1700 to 4500W/(m2K). We compared the measured heat transfer coefficients with those predicted by the established correlations, and found that Thome’s was applicable to the condenser under investigation, with deviation less than 30%. We also discussed the measured pressure drop over the condenser, which increased with the mass velocity and the vapour quality, but decreased with the saturation temperature.

Performance of a tube-in-tube CO2 gas cooler

In this study, a tube-in-tube heat exchanger model applicable to supercritical CO2 and water was developed. The developed model is first validated with some existing measurements. Normally, the variation of the heat transfer rate for a constant-property working fluid shows a monotonic decrease from the inlet of minimum heat capacity flow rate (Cmin). By contrast, the CO2 may present a local minimum and a local maximum along the length of the heat exchanger, provided CO2 passes through the pseudo-critical temperature, and this phenomenon becomes more and more pronounced when the pressure is close to the critical pressure. In contrast, it is possible for a local maximum heat transfer rate to occur near the inlet of Cmin even when the CO2 does not pass through the pseudo-critical point. This happens when Cmin is on the water side and the property variation of CO2 is taken into account. The calculation also shows that the effect of the inlet pressure on the variation of the CO2 temperature is not as apparent as the effect of the inlet pressure on the heat transfer rate, even when there is a significant change in the overall heat transfer coefficient, implying that the heat transfer characteristics of CO2 near the pseudo-critical region is similar to normal refrigerants, which show an invariant temperature at the condensation point. Hence, it would be beneficial to extend the influence of the pseudo-critical region when taking the heat transfer augmentation into consideration.

Experimental study of the condensation heat transfer characteristics of CO2 in a horizontal microfin tube with a diameter of 4.95 mm

The condensation heat transfer characteristics for CO2 flowing in a horizontal microfin tube were investigated by experiment with respect to condensation temperature and mass flux. The test section consists of a 2,400 mm long horizontal copper tube of 4.6 mm inner diameter. The experiments were conducted at refrigerant mass flux of 400–800 kg/m2s, and saturation temperature of 20–30 °C. The main experimental results showed that annular flow was highly dominated the majority of condensation flow in the horizontal microfin tube. The condensation heat transfer coefficient increases with decreasing saturation temperature and increasing mass flux. The experimental data were compared against previous heat transfer correlations. Most correlations failed to predict the experimental data. However, the correlation by Cavallini et al. showed relatively good agreement with experimental data in the microfin tube. Therefore, a new condensation heat transfer correlation is proposed with mean and average deviations of 3.14 and −7.6 %, respectively.