Thermophysical Properties Of CO2 Research Articles

Structural analysis and optimization of flattened tube gas cooler for transcritical CO2 heat pump systems

The performance of gas cooler is acknowledged as a vital factor in CO2 heat pump systems. Consequently, its optimal design can significantly impact the overall efficiency, attributed to the pronounced changes in thermophysical properties of CO2 during pseudo-critical operating conditions. This research examines, for the first time, various configurations of flattened tubes (FTs) utilized as the conduit for CO2 flow within the gas cooler. It is established that changing the cross-sectional configuration represents an effective approach for enhancing the thermal performance of the gas cooler, resulting in a significant increase of up to 60 % in heat transfer coefficient. All FTs exhibit superior overall performance across most CO2 cooling conditions compared to circular tubes. Regarding the performance index, the overall improvement can be reached up to 31.8 % and 18.6 % under near-critical and trans-critical conditions, respectively. Furthermore, an optimization is conducted using BBD-RSM for a specific alternating FT due to its potential to enhance the thermal performance not only on the CO2 side but also on the cold side of the gas cooler. The analysis highlights the significant positive influence of the minimum axis on the thermal and hydraulic specifications of gas cooler, followed by the transition length.

The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation for the CO2 around pseudo-critical region

Trans-critical CO2 Rankine cycle has great potential in converting low-grade heat into electricity, because of its good temperature glide matching between working medium and heat source. However, the drastic variation in thermo-physical properties of CO2 around the pseudo-critical region causes the abnormal flow and heat transfer behaviors. In this paper, the heat transfer deterioration and thermal oscillation in the heat addition of trans-critical CO2 cycle are investigated by experiments. The test section is heated uniformly by a DC power (0∼6 kW) and the minimum Reynolds number in the test section inlet is about 8000 (it is much larger than 2300) for the all cases. A shear stress reconstruction model is corrected by introducing the velocity profile of turbulence in the boundary layer to attempt to provide a quantitative criterion for deterioration and oscillation. The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation is elucidated by connecting the shearing-stress reconstruction model and experimental results. It is found that, the thermal acceleration in the near-wall zone provides the key motivation to drive the heat transfer deterioration. The thermal oscillation is driven by the transition between partial and full re-laminarization flows in the local zone of heat transfer deterioration. When (Δτa/τw)<1&(Δτ/τw)tot>1 or 0.1<(Δτ/τw)tot<1, the turbulence is partially re-laminarized. When(Δτa/τw)>1&(Δτa/τw)>>(Δτbo/τw), the intense thermal acceleration causes the turbulence to be fully re-laminarized in the near-wall zone and it maintains the new laminar boundary . The turbulence is partially re-laminarized in the lower boundary and it is almost full re-laminarization in the upper boundary of the oscillation wall temperatures. The results confirm that the emergence of heat transfer deterioration and oscillation instability in mixed turbulent convective relate with the change of flow states caused by acceleration and buoyancy effects.

On the Sublimation of Dry-Ice: Experimental Investigation and Thermal Modelling of Low-Temperatures on a Sandy Soil

In the last decade, growing awareness about CO2 emissions is supporting the authorities in a more sustainable society. The proposed solutions embrace different topics, such as renewable energy implementation, lower waste production, and carbon capture and storage technologies (CCS). The latter is based upon the best available knowledge about the thermophysical properties of CO2, which are not always satisfactory for its complete characterization. In this work, it is investigated the interaction of the CO2 in solid phase (dry-ice) with sandy soil, a phenomenon that can potentially occur following pipeline ruptures. An experimental setup and a numerical model have been developed to measure and validate the temperature profiles beneath the dry-ice bank at steady-state conditions. The model has been validated with the experimental data by defining a suitable range of the thermal conductivity at the solid phase (0.25–0.30 W m−1 K−1) that led to the best match (deviation of 7.81%). Finally, the overall heat transfer coefficient (85.56–86.35 W m−2 K−1) has been numerically calculated.

A REVIEW OF HEAT TRANSFER OF CO2 AT SUPERCRITICAL PRESSURE IN THE CRITICAL AND PSEUDO-CRITICAL REGION

This paper presents a comprehensive analysis of heat-transfer characteristics and correlations for CO2 at supercritical pressure in the critical and pseudo-critical region. Firstly, the thermophysical properties of CO2 are discussed along with their influence on heat-transfer characteristics. This is followed by a review of existing experimental and numerical studies on heat transfer and pressure drop for different channel geometries (smooth tubes, porous tubes, concentric annular passages, micro-fin tubes, and helical coils), covering hydraulic diameters from 0.27 to 22.8 mm and bulk temperature from 0 to 120° C and pressure from 74 to 150 bar, as well as factors influencing heat transfer. The review of published works shows that the heat-transfer characteristics are influenced by the geometry configuration and operating conditions, including channel shape and dimension, mass flux, heat flux, bulk temperature and pressure, flow direction, buoyancy, and heating or cooling conditions. Detailed comparisons and analysis of available heat-transfer correlations for CO2 at supercritical pressure are discussed and the review shows that there is lack of universal correlations able to accurately describe local heat transfer and pressure drop for different channel geometries and in particular for the pseudo-critical region. The paper identifies research gaps and proposes research and development needs to fill these gaps to ensure that reliable heat transfer and pressure drop correlations are developed to cover a wider range of operating conditions and applications.

Review of stationary and transport CO2 refrigeration and air conditioning technologies

Since its resurgence as a refrigerant in the 1990s, Carbon Dioxide (CO2) has grown in popularity and breadth of application. Its negligible global warming potential (GWP) eliminates the risk of being phased out due to legislation, and its non-toxicity, non-flammability, and low cost allows its use in many vapor compression cycle applications. However, the high critical pressure and low critical temperature require significantly more compressor power under moderate and high-ambient conditions and thus, necessitate the addition of cycle modifications to reach coefficient of performance (COP) values equal to or greater than those of other working fluids. This paper provides a review of research conducted on the use of CO2 vapor compression cycles in refrigeration and air conditioning (AC) cycles for both transportation and stationary refrigeration. A primary intention of writing this review is to offer evidence and justification for many common cycle modifications, then connect these modifications to a broad array of applications. Thus, advanced cycles in mobile and stationary applications are shown and the individual modifications present have their own independent justification to create a complete picture of the topic. The designs of complete systems, as well as specific components within the system, are included, as are the relations between the thermo-physical properties of CO2 and their benefits in these particular applications. Additionally, economic analyses of the feasibility of using CO2 in place of existing fluids are reviewed. Challenges facing the use of CO2 refrigeration cycles in stationary and transportation refrigeration are discussed, as are potential future advancements to overcome them.

Numerical investigation of thermal and hydraulic characteristics of sCO2-water printed circuit heat exchangers with zigzag channels

Since the precooler and the recuperator are the largest components of a supercritical carbon dioxide Brayton cycle, their design can substantially affect the performance and size of the whole system. Although the design of a precooler with zigzag channel geometry as an alternative to straight channels can reduce its size significantly, the applicability of available correlations (e.g. 0.7 < Pr<2.2) for the zigzag channel geometries is limited to the operational range of recuperators only. The current study, therefore, aims to develop correlations and understating of the complex flow and heat transfer characteristics in the zigzag channel printed circuit heat exchangers (PCHEs) operating under precooler conditions (2.2<Pr<13) of supercritical carbon dioxide Brayton cycle(sCO2-BC). Thermal and hydraulic characteristics of the PCHEs are computed numerically for a wide range of Reynolds numbers (5000<Re<7000) and Prandtl number (2.2<Pr<13). Also, a new data reduction method based on segmental averaged values has been proposed to handle adverse variations in the thermophysical properties of CO2 under precooler conditions. To ensure accurate evaluations, steep variations in the thermophysical properties of CO2 are implemented by supplying high-resolution real gas (RGP) property tables. Results suggest that thermal and hydraulic characteristics associated with zigzag channel vary substantially along the length of heat exchanger thus the conventional data reduction methods based on the channel average values cannot be used for true evaluations. Instead, segmental average values are used to develop pressure drop and heat transfer correlations for a broader range of Reynolds number and Prandtl number. The proposed correlations should be useful in the design of compact heat exchanger systems using zigzag channels for a wider range of cooling loads.

That’s Pretty Cool. Using Work to Freeze Water. The Vapor-Compression Refrigerator and How It Works

The fundamental principles and applications of thermodynamics and single-component phase equilibrium are used to explain and quantitatively analyze the operation and performance of the vapor-compression refrigerator (VCR). Carbon dioxide is used as the refrigerant. The four processes (or steps) that comprise the VCR refrigeration cycle are explained. The thermophysical properties of CO2 are used to calculate the heat, work, and changes in internal energy, enthalpy, and entropy for each step. Plots of pressure versus molar volume (Vm), temperature versus Vm, and temperature versus molar entropy are presented. The coefficient of performance, an index of refrigerator efficiency, is calculated.

Experimental study on oxy-fuel combustion behavior of lignite char and carbon transfer mechanism with isotopic tracing method

In this work, the conventional combustion in O2/Ar and O2/CO2 atmospheres at temperatures of 700–1000 °C and isotopic tracing experiments at 700–800 °C were conducted respectively in a micro fluidized bed reactor to clarify the conversion characteristics of lignite char and influence mechanism of elevated CO2 concentrations. The isotopic method employing C18O18O provides an intuitive insight into the CO2 gasification behavior by tracking the evolution of the 18O atom throughout the oxy-fuel combustion. The results reveal that char conversion rate significantly reduces when the atmosphere is switched from O2/Ar to O2/CO2 at low temperature range (e.g. 700–800 °C), primarily caused by thermo-physical properties of CO2. However, at higher temperature range (e.g. 950–1000 °C), the C-O2 reaction is limited by the external O2 diffusion and the C-CO2 reaction contributes to the carbon consumption, resulting in a higher combustion rate, especially at O2-deficient conditions. The presence of the atomic 18O in the product C18O16O originating from C18O18O confirms that CO2 is indeed involved in the whole oxy-fuel combustion process and the overall reaction could be described as C + C18O18O+16O16O → 2C18O16O. Besides, total contributions to carbon conversion by the C-CO2 reaction decrease as O2 concentrations and particle sizes increasing.

Effect of Impurities on Compressor and Cooler in Supercritical CO2 Cycles

With the increasing demand for electric power, the development of new power generation technologies is gaining increased attention. The supercritical carbon dioxide (S-CO2) cycle is one such technology, which has relatively high efficiency, compactness, and potentially could provide complete carbon capture. The S-CO2 cycle technology is adaptable for almost all of the existing heat sources such as solar, geothermal, fossil, nuclear power plants, and waste heat recovery systems. However, it is known that optimal combinations of operating conditions, equipment, working fluid, and cycle layout determine the maximum achievable efficiency of a cycle. Within an S-CO2 cycle, the compression device is of critical importance as it is operating near the critical point of CO2. However, near the critical point, the thermo-physical properties of CO2 are highly sensitive to changes of pressure and temperature. Therefore, the conditions of CO2 at the compressor inlet are critical in the design of such cycles. Also, the impurity species diluted within the S-CO2 will cause deviation from an ideal S-CO2 cycle as these impurities will change the thermodynamic properties of the working fluid. Accordingly, the current work examines the effects of different impurity compositions, considering binary mixtures of CO2 and He, CO, O2, N2, H2, CH4, or H2S on various S-CO2 cycle components. The second part of the study focuses on the calculation of the basic cycles and component efficiencies. The results of this study will provide guidance and define the optimal composition of mixtures for compressors and coolers.

Experimental study on heat transfer of supercritical carbon dioxide in a long silica-based porous-media tube

The heat transfer phenomena of supercritical carbon dioxide were experimentally investigated in a vertical tube containing silica-based porous media. The experiment was conducted at various levels of static pressure, flow rates, and initial wall temperatures as well as with silica sand of porous media in a long test section to study the heat transfer characteristics of supercritical carbon dioxide (CO2). The results indicated that the average heat transfer coefficient and outlet temperature at an initial wall temperature of 150 °C were higher and lower than that of 200 °C. The heat transfer performance was significantly influenced by flow rate of supercritical CO2. The porous media was provided large heat exchange surface between particles and CO2 to increase the heat transfer coefficient, especially when small diameter of particles. When the inlet temperature was higher than the pseudocritical temperature, the heat transfer coefficient sharply dropped when x/L ≥ 0.5, because of the development of a thermal boundary and the decrease of CO2 thermophysical properties of CO2 in a far pseudocritical temperature. When the pseudocritical temperature was higher than the inlet temperature of the fluid, the local heat transfer coefficient was affected by a thermal boundary and thermophysical properties of CO2 in pseudocritical point at a higher initial wall temperature or lower supercritical pressure when x/L ≤ 0.75; only the thermophysical properties of supercritical CO2 in pseudocritical point played a pivotal role when x/L > 0.75 at a lower initial wall temperature or higher supercritical pressure. In the present study, the supercritical pressure of 10.5 MPa constituted an optimal operating condition for supercritical CO2 a long silica-based porous-media tube because of the high heat transfer performance at 150 and 200 °C.

Wellbore–reservoir coupled simulation to study thermal and fluid processes in a CO2-based geothermal system: identifying favorable and unfavorable conditions in comparison with water

Using CO2 as a heat transmission fluid to extract geothermal energy is currently considered as a way to achieve CO2 resource utilization and geological sequestration. As a novel heat transmission fluid, the thermophysical properties of CO2 are quite different from those of water. CO2 has many advantages, such as larger mobility and buoyancy resulted from the lower density and viscosity. This will reduce the consumption of pressure driving the circulation, and save the energy of external equipment. The cycle even can be achieved by siphon phenomenon under a negative circulating pressure difference. However, there are still some disadvantages for CO2 as the heat transmission fluid, such as small heat capacity, leading to a less heat at the same mass flow rate. At the same time, because of the lager expansion and compression coefficient for CO2, changes in temperature and pressure may cause a more complex flow and thermodynamic processes. The lager compressibility makes it possible to get high temperature at the bottom of the injection well, whereas the lager expansion coefficient makes the temperature drop rapidly along the production well. Therefore, how to scientifically control the production pressure to guarantee sufficient high temperatures at the head of production well and, thereby, improve the efficiency of heat extraction are the key issues needed to be further addressed. The geological and geothermal conditions correspond to the central depression of the Songliao Basin located in the Northest of China. This depression has a high geothermal gradient and heat flow. In this article, a classic idealized “five-spot” reservoir model coupled with wellbores is used for simulations and analyses. The objectives of the present work are: (1) to investigate the fluid flow and thermal processes of supercritical CO2 along the wellbore and in the reservoir, (2) to understand the heat-extracting mechanism, (3) to identify advantages and disadvantages of using CO2 as the heat transmission fluid, and (4) to provide a theoretical basis for the selection of heat transmission fluid.

Experimental study of heat transfer characteristics of supercritical CO2 fluid in collectors of solar Rankine cycle system

Heat transfer characteristics of supercritical CO2 (S-CO2) fluid in collectors of a solar Rankine cycle system is experimentally investigated at different flow conditions. In the experiments, temperature, pressure and mass flow rate are measured, and local and average heat transfer coefficients are investigated. Study shows that the thermophysical properties of CO2 near the critical point have significant effects on the S-CO2 local heat transfer behaviors. The heat transfer of S-CO2 is also dependent on the mass flow rate, input heat flux and pressure at inlet of test section.

Modeling CO2 leakage scenarios, including transitions between super- and sub-critical conditions, and phase change between liquid and gaseous CO2

Abstract Storage of CO 2 in saline aquifers would be made at supercritical pressure and temperature conditions, but CO 2 leaking from a geologic storage reservoir and migrating towards the land surface (through faults, fractures, or improperly abandoned wells) would reach sub-critical conditions above 500-750 m depth for typical temperature and pressure conditions in terrestrial crust. At shallower horizons, subcritical CO 2 can form two-phase mixtures of liquid and gaseous CO 2 , with significant latent heat effects during boiling and condensation processes. Additional strong non-isothermal effects can arise from decompression of gas-like subcritical CO 2 , the so-called Joule-Thomson effect. Evaluation of leakage scenarios requires a capability to model nonisothermal flows of brine and CO 2 at conditions that range from supercritical to subcritical, including three-phase flow of aqueous phase, and liquid and gaseous CO 2 . In this paper we describe the development of comprehensive simulation capabilities that can cope with all possible phase conditions in brine- CO 2 systems. Our model formulation includes • an accurate description of phase properties as function of temperature, pressure, and composition, including the mutual dissolution of CO 2 and H 2 O in aqueous and CO 2 -rich phases; • transitions between super- and sub-critical conditions, including phase change between liquid and gaseous CO 2 ; • one-, two-, and three-phase flow of brine- CO 2 mixtures, including heat flow; • non-isothermal effects associated with phase change, mutual dissolution of CO 2 and water, and (de-) compression effects; • effects of dissolved NaCl, and the possibility of precipitating solid halite, with associated porosity and permeability change. Applications to specific leakage scenarios demonstrate that the peculiar thermophysical properties of CO 2 provide a potential for positive as well as negative feedbacks on leakage rates. The interplay of self-enhancing and self-limiting effects on different space and time scales can induce non-monotonic behavior of CO 2 flow rates. Such behavior may facilitate monitoring of CO 2 storage systems, as it may provide signals that may be more easily distinguished from background noise.

Optimizing geologic CO 2 sequestration by injection in deep saline formations below oil reservoirs

The purpose of this research is to present a best-case paradigm for geologic CO 2 storage: CO 2 injection and sequestration in saline formations below oil reservoirs. This includes the saline-only section below the oil–water contact (OWC) in oil reservoirs, a storage target neglected in many current storage capacity assessments. This also includes saline aquifers (high porosity and permeability formations) immediately below oil-bearing formations. While this is a very specific injection target, we contend that most, if not all, oil-bearing basins in the US contain a great volume of such strata, and represent a rather large CO 2 storage capacity option. We hypothesize that these are the best storage targets in those basins. The purpose of this research is to evaluate this hypothesis. We quantitatively compared CO 2 behavior in oil reservoirs and brine formations by examining the thermophysical properties of CO 2, CO 2–brine, and CO 2–oil in various pressure, temperature, and salinity conditions. In addition, we compared the distribution of gravity number ( N), which characterizes a tendency towards buoyancy-driven CO 2 migration, and mobility ratio ( M), which characterizes the impeded CO 2 migration, in oil reservoirs and brine formations. Our research suggests competing advantages and disadvantages of CO 2 injection in oil reservoirs vs. brine formations: (1) CO 2 solubility in oil is significantly greater than in brine (over 30 times); (2) the tendency of buoyancy-driven CO 2 migration is smaller in oil reservoirs because density contrast between oil and CO 2 is smaller than it between brine and oil (the approximate density contrast between CO 2 and crude oil is ∼100 kg/m 3 and between CO 2 and brine is ∼350 kg/m 3); (3) the increased density of oil and brine due to the CO 2 dissolution is not significant (about 7–15 kg/m 3); (4) the viscosity reduction of oil due to CO 2 dissolution is significant (from 5790 to 98 mPa s). We compared these competing properties and processes by performing numerical simulations. Results suggest that deep saline CO 2 injection immediately below oil formations reduces buoyancy-driven CO 2 migration and, at the same time, minimizes the amount of mobile CO 2 compared to conventional deep saline CO 2 injection (i.e., CO 2 injection into brine formations not below oil-bearing strata). Finally, to investigate practical aspects and field applications of this injection paradigm, we characterized oil-bearing formations and their thickness (capacity) as a component of the Southwest Regional Partnership on Carbon Sequestration (SWP) field deployments. The field-testing program includes specific sites in Utah, New Mexico, Wyoming, and western Texas of the United States.

Flow boiling heat transfer characteristics of CO 2 at low temperatures

This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO 2 at low temperatures (down to −30 °C). Subsequently, the boiling heat transfer of CO 2 at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57 mm. Due to the large surface tension, the boiling heat transfer coefficient of CO 2 is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0 °C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO 2 in good agreement with the experimental data, suggesting the need for further research in this area.

Theoretical Evaluation of Carbon Dioxide Refrigeration Cycle

Concerns of ozone depletion and global warming call for investigation of natural refrigerants. In this study the performance potential of the carbon dioxide refrigeration cycle is investigated theoretically. For this purpose, two cycle simulation models were developed. One is an Evans-Perkins cycle model for R-22, and the other is a transcritical cycle model for CO2. By using these models, the CO2 refrigeration cycle and heat exchangers for the CO2 refrigeration cycle were optimized. The water chilling and tap water heating performances of CO2 were compared to those of R-22. The thermophysical properties of CO2 and the proper heat exchanger design appropriate to CO2 offer the opportunity of reducing heat exchanger size and mass for tap water heating applications.