Heat Transfer Of CO2 Research Articles

Coupled thermal model for geothermal exploitation via recycling of supercritical CO2 in a fracture–wells system

Geothermal exploitation via recycling of supercritical CO2 (CO2–EGS) in a fracture–wells system is a new method, which can reduce the injection pressure, and improve heat transfer efficiency. In this study, a coupled thermal model for this system is proposed, considering the coupling effects of the temperature and pressure on the CO2 flow and heat transfer. The simulation results show that the thermal breakthrough time is approximately 40 d when the well separation distance is 230 m. After thermal breakthrough, the fracture outlet temperature decreases from 260 °C to 100 °C, and the heat mining rate decreases from 4.6 MW to 3.0 MW over the following 2 months. However, as the deep formation continues to replenish energy into the wellbores and fracture, the decreasing trend in the temperature gradually slows down, and it reaches a quasi-equilibrium state after one year. Owing to the low heat conduction rate of the formation, thermal utilization depth is only approximately 15 m from the wellbore, and 50 m from the fracture after 20 y. Comprehensive analysis shows that the heat mining rate reaches a maximum at an injection rate of 36 kg/s in this case. This model can provide an accurate evaluation and design optimization method for CO2–EGS in a fracture–wells system.

Effects of abnormal gravity on heat transfer of supercritical CO2 in heated helically coiled tube

Based on the RNG k-ε turbulence model, numerical analyses are carried out for the simulation of heating process of supercritical CO2 in the helically coiled tube in the aerospace fields. This study focuses on the influence of gravity on the supercritical CO2 heat transfer in two flow orientations: vertical and horizontal flow directions. The coupling effect of buoyancy and centrifugal force is analyzed to study the mechanism of heat transfer in helically coiled tube. Results reveal that the heat transfer coefficient is enhanced by bigger gravity force in the liquid-like region. The heat transfer coefficient exhibits a noticeable peak near the pseudo-critical temperature, but it remains almost the same in the gas-like region. Regardless of flow orientations, it has a weakening effect on the heat transfer with the low-gravity before the pseudo-critical point, whereas, it has a strengthen effect with the high-gravity. Finally, a new heat transfer correlation is proposed for the supercritical CO2 heating process covering a wide range from 0g to 6g.

Study on wellbore fluid flow and heat transfer of a multilateral-well CO2 enhanced geothermal system

Research efforts towards CO2 enhanced geothermal systems (CO2-EGS) are increasing due to potentially high heat extraction efficiencies. It is well known that CO2 properties are highly sensitive to the temperature and pressure and have noticeable effects on CO2-EGS heat extraction. Consequently, an understanding of wellbore CO2 fluid flow and heat transfer mechanisms is necessary when considering variable CO2 properties. In this paper, a coupled wellbore and reservoir fluid flow and heat transfer model is used to estimate multilateral-well CO2-EGS efficiency. The model is calibrated and validated by field data from the HGP-A well in Hawaii. Schematically, concentric tubulars are used allowing single well injection and production. Multiple cases are analyzed using this model. These include effects of CO2 pressure work on wellbore CO2 fluid flow and heat transfer, assessment of differences in heat extraction using varying wellbore sizes and central tubing insulation lengths, and evaluations of efficiencies under different injection-production well configurations. Results show that CO2 pressure work can induce a dramatic temperature reduction in the central tubing of the multilateral-well CO2-EGS. The majority of pressure loss occurs in the formation and central tubing. The optimized design suggests that a three-layer central tubing with a central diameter of 0.19 m will maximize heat insulation and heat extraction. The effect of annulus diameter on heat extraction is negligible. Also, a lower injection well configuration results in a higher outlet temperature, thermal power output, and lower pressure losses compared to an upper injection well configuration. These results provide significant suggestions for wellbore designs that can potentially optimize multilateral-well CO2-EGS efficiency.

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.

The heat transfer of supercritical CO2 in helically coiled tube: Trade-off between curvature and buoyancy effect

Supercritical CO2 Rankine cycle has great development potential as a power cycle for converting low-grade thermal energy into electricity. Better understanding of supercritical CO2 heat transfer in helically coiled tubes (HCTs) is required for design and operation of supercritical CO2 Rankine cycle power systems. In this work, the SST k∼ω model is employed. A new dimensionless buoyancy parameter Ψ is proposed which denotes the ratio of gravitational buoyancy force to overall curvature effect. Furthermore, a flow regimes map is proposed based on the inclination angle of the dividing streamline between the two vortexes and buoyancy parameter Ψ. The mixed convection region in HCT is decomposed into a gravitational buoyancy force dominated heat transfer region (B Region α>45°) and a curvature effect dominated heat transfer region (C Region α<45°). Subsequently, the effects of HCT geometry on heat transfer mechanisms are respectively investigated in B and C Region, which help us better understanding the relationship of the buoyancy criterion and flow characteristics. The results indicate that the effects of coiled pitch and coiled diameter on heat transfer can be neglected in B Region. In C Region, the heat transfer is suppressed as coiled pitch increases and it will appear oscillation when torsion effect is strong enough. In addition, the heat transfer is enhanced with curvature increases but except for near the pseudo-critical region.

Supercritical “boiling” number, a new parameter to distinguish two regimes of carbon dioxide heat transfer in tubes

The objective of this paper is to develop a criterion to predict the onset of heat transfer deterioration (HTD) for supercritical CO2 heat transfer. A new mechanism is proposed by assuming supercritical pseudo-boiling. Before bulk fluid reaches pseudo-critical temperature (Tpc), the tube cross-section contains a vapor layer and a liquid-like fluid. The saturation temperature interface is defined at T = Tpc, inside and outside which are a vapor layer (T > Tpc) and a subcooled liquid (T < Tpc). The subcritical boiling number is extended to supercritical “boiling” number, defined as SBO = qw/(Gipc), where qw, G and ipc are heat flux, mass flux, and CO2 enthalpy at Tpc, respectively. SBO, by coupling the density ratio between “liquid” and “vapor”, represents the competition between vapor expansion induced momentum force and inertia force. The experiment of supercritical CO2 heat transfer is performed in a 10.0 mm inner diameter tube, covering ranges of P = 7.5–21.1 MPa, G = 488–1600 kg/m2s and qw = 74–413 kW/m2. Surprisingly, the onset of HTD is found to occur at a critical SBO which is 5.126×10−4 and the critical heat flux is expressed as qCHF=5.126×10−4Gipc. It is shown that our new criterion is also suitable for other experiments reported in literature, providing a general guidance to design and operate SCO2 heaters to avoid HTD.

Experimental Testing of a Bi-Functional Material Cu-Cao Composite within the Ca/Cu H2 Production Process

A novel CaO/CuO based composite material (53 wt% CuO/ 22 wt% CaO/ 25 wt% Ca12Al14O33), developed for combined CO2 capture and heat transfer for the Ca/Cu hydrogen production process, showed a maximum CO2 capture rate of 1.53*10-2 kmol CO2/h.kg material, at a linear gas velocity of 0.55 m/s when operating in a fixed bed reactor in presence of a commercial reforming catalyst during methane Sorption Enhanced Reforming (SER). The system was able to convert up to 2.4 kg CH4/h.kg catalyst, producing a gas stream with over 93 % vol. hydrogen (dry basis) at 10 bars for a steam to carbon molar ratio of 3.2.

Condensation heat transfer and multi-phase pressure drop of CO2 near the critical point in a printed circuit heat exchanger

In this study, condensation heat transfer and multi-phase pressure drop of CO2 near the critical point are investigated occurring in a Printed Circuit Heat Exchanger (PCHE) for a supercritical CO2 power cycle application in mind. Homogeneous Equilibrium Model (HEM) approach is used to evaluate and develop appropriate heat transfer and pressure drop correlations. The experiment was performed with the CO2 test facility called SCO2PE (Supercritical CO2 Pressurizing Experiment) in KAIST. The heat transfer and pressure drop test was conducted with the PCHE in the facility. Existing correlations were compared to the test data and a new set of correlations was necessary to be developed. A new set of correlations is newly suggested in this paper which captures physical characteristics reasonably well.

Influence of channel scale on the convective heat transfer of CO2 at supercritical pressure in vertical tubes

Channel scales vary from several centimeters to several micrometers in various industrial applications that conduct convective heat transfer at supercritical pressures. The heat transfer performance reveals relatively different features even under similar Reynolds number and thermphysical property conditions. The authors investigated the influence of the channel scale on the supercritical convective heat transfer based on the experimental results conducted on vertical tubes with inner diameters of 0.27 mm and 2.0 mm. Numerical simulations using several low Reynolds number k-ε turbulence models were also discussed to evaluate the performance of turbulence models when modelling supercritical heat transfer in tubes of various scales. The results exhibited significant heat transfer deterioration due to flow acceleration effect in the 0.27 mm tube at a heat flux to mass flux ratio of about 0.2, whereas the 2.0 mm tube at a similar inlet Reynolds number and heat flux to mass flux ratio exhibited great heat transfer enhancement due to the buoyancy effect. The heat transfer deterioration in the 0.27 mm tube can be explained by its relation to the redistributed mean velocity profiles and the relatively small energy-containing scale while relatively large dissipation scale as compared to those in the 2.0 mm tube.

Flow condensation heat transfer of CO2 in a horizontal tube at low temperatures

This study presents experimental data of CO2 flow condensation heat transfer for mass fluxes ranging from 100 to 500 kg/m2-s inside a 4.73 mm inside diameter, smooth horizontal copper tube, at saturation temperatures between −10 and 0 °C under a wide range of vapour quality conditions. Experimental data were obtained from an open-loop test rig which discharged high-pressure CO2 liquid from bottles to the atmosphere. Experimental results showed that when the test mass flux was greater than or equal to 300 kg/m2-s, for vapour qualities greater than 0.4, the rate of heat transfer increased with increasing mass flux and vapour quality, and increased with decreasing saturation temperature. The flow regimes under these working conditions were predicted as annular from the values of the Soliman Froude number (Frso > 14). Three recently proposed CO2 flow condensation models from the open literature were evaluated against the data from the current experiment. Overall, the model of Li and Norris (2016) gave the most accurate predictions, but under-predicted the low Nusselt number heat transfer rates. This model was modified using the current data, and then re-evaluated against experimental data from the current and other experiments, and was found to have a mean absolute percentage deviation of 7%.

Experimental Study of Supercritical CO2 Heat Transfer in a Thermo-Electric Energy Storage Based on Rankine and Heat-Pump Cycles

Multi-megawatt thermo-electric energy storage based on thermodynamic cycles is a promising alternative to PSH (Pumped-Storage Hydroelectricity) and CAES (Compressed Air Energy Storage) systems. The size and cost of the heat storage are the main drawbacks of this technology but using crystalline superficial bedrock as a heat reservoir could be a readily available and cheap solution. SELECO2 research project considers a thermal doublet consisting in a “hot storage” in a bedrock and a cold storage in an ice pool. The complete system includes a heat pump transcritical CO2 cycle as the charging process and a transcritical CO2 cycle of 1 – 10 MWe as the discharging process. Various technical studies are undertaken to assess the performance of such system. Steady-state thermodynamic models have been realized to optimize system efficiency. In addition, unsteady models of geothermal heat exchanger network were developed for the ground heat storage. An experimental device has been designed and built to test the heat-exchange performance and dynamics. The conditions are intended to reproduce real process dynamics at a laboratory scale. The heat exchanger is at 1/10e scale with a 1.6 m height and 40 mm inner diameter. Temperature (40-130°C) and pressure conditions (~8-12MPa) follow the operating conditions of the real process coupled with a granitic bedrock. First results show that energetic and exergetic performances are better if a specific strategy of short charge and discharge cycles is employed rather than longer charge and discharge phases. Moreover experimental results will be used to improve the above-mentioned numerical simulations and to validate CFD simulations.

Experimental study of convective heat transfer of carbon dioxide at supercritical pressures in a horizontal rock fracture and its application to enhanced geothermal systems

Abstract Enhanced geothermal systems create fractured reservoirs to extract economic quantities of heat from low-permeability and/or low-porosity geothermal resources. Convective heat transfer characteristics of fluids at supercritical pressures in rock fractures are important for optimizing the heat transfer model, which is a key tool for simulating heat extraction and improving the heat recovery factor for such projects. This paper presents the results of experimental investigations of laminar convective heat transfer of CO 2 at supercritical pressures in a horizontal fracture with an aperture of 0.2 mm. The laboratory apparatus operated at temperatures up to 280 °C, fluid pressures up to 14 MPa, and confining pressures up to 28 MPa. The effects of mass flow rate and initial rock temperature on the rock wall and fluid temperatures were examined. A method was proposed for processing the experimental data and local heat transfer performance in the fracture was obtained. Considering the effects of variations in thermophysical properties, a correlation of heat transfer performance in the rock fracture was proposed to improve field simulation models for enhanced geothermal systems.

Characteristics of heat transfer for CO2 flow boiling at low temperature in mini-channel

This paper presents an experimental and theoretical investigation on two-phase flow boiling heat transfer with carbon dioxide (CO2 or R744) as the refrigerant in horizontal small diameter tube at low temperature. The experimental data were obtained in the following condition, the heat flux is 7.5–30kW/m2; the mass flow rate is 300–600kg/m2s; the saturation temperature is 233–273K; the test section is made of stainless steel tubes whose inner diameters are 0.6 and 1.5mm. The experimental results show that heat transfer coefficient increases with the increase of mass flow rate, but decreases with the increase of tubes’ inner diameter and saturation temperature, the boiling heat transfer of CO2 has a greater effect on nucleate boiling and highly depends on heat flux. The comparative study indicates that: during the existed flow boiling heat transfer model, the model which has the best prediction accuracy presented with 79.8% of predicted points within ±30% and 21.8% mean deviation before the dryout phenomenon happened, while it was only 18.4% and 59.9% after the dryout phenomenon happened.

Comparative analysis of a concentric straight and a U-bend gas cooler configurations in CO2 refrigeration system

This paper compares the heat transfer performance of two gas cooler configurations using CO2 as refrigerant. Straight gas cooler vs U-bend gas cooler was analyzed using 3D numerical simulation employing ANSYS FLUENT commercial program. The importance of the structure of the mesh was discussed and the unstructured sweep method was selected. Numerical results were satisfactorily compared with those of eight different empirical correlations for the Nusselt number applicable to similar heat exchangers. Behaviors of the local CO2 thermophysical properties, turbulence parameters, and heat transfer parameters in both straight and U-bend gas cooler configurations were analyzed. The results of this analysis were used to predict the transient behavior of the heat transfer rate and the thermal effectiveness of the two configurations. An increase of 16% was observed in the thermal effectiveness of the gas cooler based on U-bend configuration compared to the straight gas cooler configuration. Finally, the transient behavior of the overall efficiency of the transcritical refrigeration facility in which the two gas cooler configurations are supposed to be installed was assessed. The installation based on U-bend gas cooler geometry has presented higher performance of about 20% of efficiency when compared with the straight geometry.

A variable turbulent Prandtl number model for simulating supercritical pressure CO2 heat transfer

In order to predict heat transfer deterioration (HTD) of supercritical pressure fluid correctly and to investigate the HTD mechanism, the effect of turbulent Prandtl number (Prt) on numerical simulation was theoretically analyzed. Based on such analysis, a new turbulent Prt model (TWL model) was proposed. Numerical simulations of the supercritical pressure CO2 heat transfer in vertical heated tubes were conducted with the proposed Prt model as well as two other previous Prt models and two constant Prt numbers. The performance of the new Prt model was validated by comparing with 14 reported heat-transfer experimental data, especially for the HTD cases. The comparison showed that a better prediction of wall temperature can be achieved with the proposed Prt model in most of the validations, especially for the HTD cases. When HTD occurs, turbulent mixing was restrained in the buffer layer since an “M” shape of velocity profile is formed under the buoyancy effect. The maximum predicted Prt value with TWL model also appears in the buffer layer, while the maximum predicted Prt value with other models appear in the viscous sub-layer. Such Prt profile restrains the turbulent mixing contribution to heat transfer further. Although the turbulent mixing contribution is still several times higher than molecular conduction contribution in the buffer layer when HTD occurs, it’s not strong enough to diffuse energy from near wall region to the bulk region.

Condensation of carbon dioxide in microchannels

Heat transfer coefficients and pressure drops during condensation of carbon dioxide (CO2) are measured in small quality increments in rectangular microchannels of 0.10⩽Dh⩽0.16mm and aspect ratios from 1 to 4. Channels are fabricated on a copper substrate by electroforming copper onto a mask patterned by X-ray lithography, and sealed by diffusion bonding. The test section is cooled by chilled water circulating at a high flow rate to ensure that the thermal resistance on the condensation side dominates. A conjugate heat transfer analysis, in conjunction with local pressure drop profiles allows driving temperature differences, heat transfer rates, and condensation heat transfer coefficients to be determined accurately. Condensation heat transfer coefficients and pressure drops are measured for G=400, 600 and 800kgm−2s−1, for 0<x<1, and saturation temperatures of 15, 20 and 25°C (Pr=0.69, 0.78 and 0.87). The data are used to evaluate the applicability of correlations developed for larger hydraulic diameters and different fluids for predicting condensation heat transfer and pressure drop of CO2.

A numerical investigation on how to efficiently utilise carbon dioxide in convection-based energy systems

Owing to the environmentally benign nature and the special property variation at supercritical pressure, greenhouse gas carbon dioxide is regarded as the best retrofit to meet the future demand on long-term environmental-friendly working fluids. However, how to efficiently utilise carbon dioxide to reduce energy consumption and mitigate global warming is still an unsolved problem. In this paper, the natural convection heat transfer of carbon dioxide under supercritical pressure condition in a small cavity is studied for the first time. The potential of carbon dioxide as working fluid is quantitatively estimated in terms of enhancing heat transfer and reducing the cost of heat exchangers. It is found that the operating conditions including the temperature, temperature difference and pressure all have significant effects on heat transfer rate due to the special property variations of the CO2 fluid. Furthermore, a heat transfer correlation is proposed for the first time to quantitatively describe the natural convection heat transfer of CO2 at supercritical pressure in a small cavity.

A numerical investigation on how to efficiently utilise carbon dioxide in convection-based energy systems

Owing to the environmentally benign nature and the special property variation at supercritical pressure, greenhouse gas carbon dioxide is regarded as the best retrofit to meet the future demand on long-term environmental-friendly working fluids. However, how to efficiently utilise carbon dioxide to reduce energy consumption and mitigate global warming is still an unsolved problem. In this paper, the natural convection heat transfer of carbon dioxide under supercritical pressure condition in a small cavity is studied for the first time. The potential of carbon dioxide as working fluid is quantitatively estimated in terms of enhancing heat transfer and reducing the cost of heat exchangers. It is found that the operating conditions including the temperature, temperature difference and pressure all have significant effects on heat transfer rate due to the special property variations of the CO2 fluid. Furthermore, a heat transfer correlation is proposed for the first time to quantitatively describe the natural convection heat transfer of CO2 at supercritical pressure in a small cavity.

Nanofiltration pretreatment as CO2 deaerator of desalination feed: CO2 release reduction in MSF distillers

In this work, the influence of nanofiltration on carbon dioxide (CO2) release in MSF distillers was investigated for different feed water composition for once-through (OT) and brine recycle (BR) MSF distillers. The results were calculated for the CO2 release from two 20-stage reference MSF once-through and recycle distillers, by coupling mass transfer with chemical reaction. The CO2 release rates decreased exponentially from the first stage to the last stage. The CO2 release rates increased with increasing top brine temperature (TBT). Nanofiltration can be used as CO2 deaeration system in thermal desalination, the amount of CO2 release, in the first stage, decreased by 100% NF from 425 to 140kg/h in brine recycle, and from 650 to 210kg/h in once-through distiller, respectively. The CO2 heat transfer resistances decreased by 100% NF from 5.5×10–6 to 2.0×10–6m2K/W in brine recycle, and from 8.1×10–6 to 3.0×10–6m2K/W in once-through distiller, respectively.

Mixed convective heat transfer of CO2 at supercritical pressures flowing upward through a vertical helically coiled tube

The mixed convective heat transfer of CO2 at supercritical pressures inside a vertical helically coiled tube was experimentally investigated under constant heat flux conditions. Experiments were conducted at three supercritical pressures for various heat and mass fluxes. The buoyancy force was found to have two opposing effects on the heat transfer. When Bo∗≤4×10−8, the forced convection is dominant, and the density is only reduced at the near-wall region, which leads to flow acceleration, relaminarization of the turbulence, and deterioration of the heat transfer. When 4×10−8<Bo∗≤8×10−7, as the buoyancy number increases, natural convection starts to have a positive effect on the heat transfer, and part of the heat transfer ability is recovered. When Bo∗>8×10−7, the natural convection is fully developed, and the relaminarization of the turbulence at the near-wall region is suppressed, which enhances the heat transfer. The coupling effects of the buoyancy force, centrifugal force, and variations in the physical properties were found to determine the temperature and heat transfer coefficient distributions along the circumference edges. Based on the current experimental data, two correlations of the Nusselt number were presented.