Heat Transfer Of CO2 Research Articles

Numerical study on the effects of supercritical CO2-based nanofluid on heat transfer deterioration

The drastic variation of thermoproperties of supercritical CO2 may induce heat transfer deterioration (HTD). To solve the HTD problem, the effects of nanoparticles on supercritical CO2 heat transfer performance was numerically studied. Results show that with the increase of nanoparticle concentration, the local temperature peak is further suppressed. The wall temperature has a maximum decrease of 168.9 K, and the optimal heat transfer enhancement of the supercritical CO2 nanofluid has a maximum increase of 51.4%. Detailed flow field analysis indicates that the decrease of density gradient in the buffer layer significantly suppresses the coupling effect of buoyancy and flow acceleration, which leads to better heat transfer performance. Also, the convective heat transfer benefits from the reduced specific heat and improved thermal conductivity of nanofluid. Eventually, it is the high-density-nanoparticle rather than the high thermal-conductivity-nanoparticle that plays a more effective role in the mitigation of the HTD phenomenon.

Thermodynamic assessment of combined supercritical CO2 cycle power systems with organic Rankine cycle or Kalina cycle

The supercritical carbon dioxide (S-CO2) Brayton cycle is considered to be one of the most promising cycles for the fourth-generation nuclear power system. In this paper, the S-CO2 Brayton combined cycles composed of organic Rankine cycle (ORC) and Kalina cycle as the bottom cycle are proposed to improve the cycle performance. A new system performance parameter η/Qv,max is introduced for the first time to evaluate the economy and compactness balance of the combined cycle. Parametric study based on different working fluids for bottom cycle is conducted, comparison study between the S-CO2 Brayton combined cycles and traditional S-CO2 Brayton reheating cycle are made. The comparison results show that R32 and Ammonia based S-CO2/ORC combined cycle obtain the best performance, with thermal efficiency enhanced by 2.015% and 2.036%, respectively. The S-CO2/Kalina combined cycle obtains an improvement by 1.79% with ammonia-water concentration at 0.85, which is slightly lower than S-CO2/ORC combined cycle. Superheating the organic working fluids can improve the exergy efficiency of the evaporator and increase ηorc/Qv,max, but the efficiency of the cycle is not necessarily improved. The temperature slip contributes in reducing the temperature difference in the evaporator, and this helps the S-CO2/Kalina combined cycle to better match the temperature changing characteristics of the heat source compared with ORC, and the slope of the CO2 heat transfer line near the critical point of CO2 is the key factor that limits the improvement of S-CO2/Kalina combined cycle efficiency.

CFD and exergy analysis of subcritical/supercritical CO2 based naturally circulated solar thermal collector

Solar water heating system is inefficient during winter due to the chances of water freezing and higher viscosity at low temperatures. Several investigations are being done to increase the efficiency of the solar water heater using various secondary fluids for different climatic conditions. This paper emphasises on the study of heat transfer and fluid flow behavior of CO2 based naturally circulated indirect solar water heating system. Subcritical (liquid and vapour) and supercritical CO2 are considered as loop fluid, and the results are compared with water based system. Three-dimensional computational fluid dynamics simulations are carried out for two different weather conditions i.e., winter (278 K) and summer (305 K). Results are obtained for 33° collector inclination angle from horizontal at various operating pressures 50–70 bar for subcritical and 80–100 bar for supercritical CO2. The CO2 based system yields very high Reynolds number (subcritical liquid: ∼160 times; subcritical vapour: ∼204 times; supercritical vapour: ∼260 times) and very high Nusselt number (subcritical liquid: ∼14 times; subcritical vapour: ∼19.5 times; supercritical vapour: ∼48 times) compared to water based system. Supercritical CO2 based system exhibits 12% higher energy efficiency compared to water. Whereas, subcritical vapour based system exhibits 140% higher exergy efficiency relative to water based system.

Comparative Numerical Study on Global Heat Transfer Process in Micro-Channel Gas Coolers with Different Structures

A numerical simulation of global heat transfer process in three types of micro-channel heat exchangers was investigated in this paper: spiral double-pipe micro-channel heat exchanger (DPHE), cross-plate micro-channel heat exchanger (CPHE), and shell and tube micro-channel heat exchanger (STHE). The inner tubes of all three heat exchangers have a diameter of 1 mm and are charged with CO2 as refrigerant. A detailed analysis of the heat exchanger's global heat transfer process was carried out, which is entirely different for different structures. The heat transfer characteristics of supercritical CO2 were analyzed by considering the operating pressure, the refrigerant mass flux, the cooling water mass flux, and the heat exchanger refrigerant inlet temperature. The relationship between the CO2 heat transfer coefficient (hCO2) and CO2 bulk temperature (Tb,CO2) was analyzed in detail. The pseudo-critical temperature (Tpc) mainly determines where the peak CO2 heat transfer coefficient occurs. When Tb,CO2 < Tpc , the rise in Tb,CO2 is accompanied by an increase in the heat transfer coefficient, which reaches a maximum when Tb,CO2 is a little bit higher than the pseudo-critical temperature, the heat transfer coefficient curve begins to decline as Tb,CO2 continues to rise. Higher peak heat transfer coefficients can be achieved at higher pressures. Increased refrigerant mass flux always results in larger heat transfer coefficients. The influence of the refrigerant inlet temperature of the heat exchanger in the Tb,CO2 < Tpc region on the heat transfer coefficient is more significant than expected. In this study, different flow patterns on heat transfer due to different structures were compared. The best heat transfer was achieved using a spiral double-pipe micro-channel heat exchanger (DPHE). It consistently reaches the highest heat transfer and the lowest outlet temperature under the same operating conditions‎.

Coupling Effect between Heat Flux Distribution and Buoyancy of Supercritical Co2 Heat Transfer with Nonuniform Heat Flux in Parabolic-Trough Collector

Coupling Effect between Heat Flux Distribution and Buoyancy of Supercritical Co2 Heat Transfer with Nonuniform Heat Flux in Parabolic-Trough Collector

Improvement of heat transfer performance and unmatched characteristics of a water-cooled carbon dioxide gas cooler

The problem of mismatch in temperature glides in a water-cooled carbon dioxide (CO2) gas cooler has attracted much attention. This paper quantifies the degree of mismatch in temperature glides using an unmatched coefficient and then investigates and improves the unmatched characteristics and heat transfer performance of direct heating (DH) gas coolers with a validated simulation model. The results show that for a DH gas cooler, two zones can be specified with the mass flow ratio of water to CO2. Zone I, with a low flow ratio, produces high-temperature hot water and shows good temperature-glide matching, but the CO2 exit temperature and heat transfer rate are unsatisfactory. Zone II, with a high flow ratio, achieves acceptable CO2 exit temperature and heat transfer rate, but its hot water temperature is low, and its matching characteristic is poor. A segmentation heating (SH) gas cooler is thus developed to compensate for the weaknesses of the two zones while preserving their advantages by selectively adjusting the water flow of a certain part of the gas cooler. The results indicate that an SH gas cooler can increase the heat transfer rate by 4.4% at most and reduce the CO2 exit temperature by 2.3 °C at most. The hot water temperature can be increased by 6.9 °C at most, and the unmatched coefficient can be decreased by 8.2% at most.

Flow boiling heat transfer and two-phase flow phenomena of CO2 in macro- and micro-channel evaporators: Fundamentals, applications and engineering design

• Comprehensive review of fundamentals and applications of CO 2 flow boiling. • Analysis of flow boiling heat transfer data and physical mechanisms. • General CO 2 flow pattern maps, mechanistic model for heat transfer and pressure drop. • CO 2 flow boiling and two phase flow in enhanced tubes and mechanisms. • Analysis of the oil effect on CO 2 flow boiling and two phase pressure drop. • Engineering applications of CO 2 flow boiling and two phase flow. • Discussion of CO 2 evaporator simulation and engineering design. • Future research needs in CO 2 flow boiling and potential practical applications. The paper presents a comprehensive review of fundamentals and engineering applications of CO 2 flow boiling heat transfer, flow patterns and two-phase pressure drops in macro- and micro-channel evaporators. First, distinctions of macro- and micro-channels are discussed. Second, the review addresses the extensive experimental studies on CO 2 flow boiling heat transfer and two-phase flow in macro- and micro-channels. The effects of the physical properties on flow boiling heat transfer, flow patterns and pressure drops are analysed by simulations using various physical property packages. Furthermore, analysis of the existing experimental studies of flow boiling heat transfer is presented and the physical mechanisms are discussed. Next, generalized CO 2 flow pattern map and flow pattern based mechanistic flow boiling heat transfer and two phase pressure drop models specially developed for CO 2 are discussed. New experimental database of flow boiling heat transfer and diabatic two phase frictional pressure drop have been set up to evaluate the models. Comparative results of the flow pattern map, heat transfer and pressure drop models to the experimental database are analysed. In addition, studies of CO 2 flow boiling in enhanced channels are summarized. The oil effects on the flow boiling heat transfer and two phase frictional pressure drop are analyzed and simulated as well. According to the extensive analysis, the physical mechanisms and prediction models of the CO 2 flow boiling heat transfer and two phase flow phenomena in evaportaors have been well understood. In the aspect of engineering application, comparisons of simulation results and the experimental data in the real thermal systems are discuseed. Furthermore, research need of CO 2 evaporators is discussed. Some practical design methods of CO 2 evaporators are recommended according to the analysis. Finally, the potential application of CO 2 for high heat flux cooling, thermal and power systems are also discussed. Future research needs in flow boiling heat transfer and two phase flow of CO 2 in evaporators and engineering applications are discussed and recommended according to this comprehensive review.

Condensation heat transfer of CO[formula omitted] on Cu based hierarchical and nanostructured surfaces

Phase-change processes such as condensation are efficient means of heat transfer. However, condensation is also an energy-intensive process and extensive research is conducted to increase the heat transfer efficiency. Increasing the effective heat transfer area in terms of surface structures on macro or microscale is one such technique of heat transfer enhancement. In this work, we have studied micro- and nanostructured surfaces for their potentials in increasing heat transfer during condensation of CO2. Three Cu-based surfaces on which CuO nanoneedles have been grown, have been investigated. We hypothesize three competing mechanisms govern the overall heat transfer on structured surfaces: (1) increased heat transfer area, (2) lower thermal conductivity of oxides, and (3) condensate flooding of the structures. Our study has shown that in some cases, the effect of these mechanisms can be neutralized. More importantly, the results show that superior heat transfer can be achieved by optimizing the surface structure. The best of the structured surfaces resulted in a heat transfer coefficient 66% higher than that of the unstructured surface.

Lattice Boltzmann simulation of near/supercritical CO2 flow featuring a crossover formulation of the equation of state

In this work, we have incorporated a crossover equation of state into the pseudopotential multiphase Lattice Boltzmann Model (LBM) to improve the prediction of thermodynamic properties of fluids and their flow in near-critical and supercritical regions. Modeling carbon dioxide (CO2) properties in these regions is of increasing interest for industrial processes such as CO2 storage and heat transfer where CO2 is used as a working fluid. Despite the importance of accurately modeling near-critical and supercritical fluids, popular classical cubic equations of state (EoS) are not accurate. The proposed crossover EoS is a proven hybrid equation which uses the original classical EoS far from the critical point where it is valid, and near the critical point, it asymptotically switches to using non-analytic scaling laws. It also transforms into the ideal gas EoS as the density approaches zero. In order to demonstrate the validity and versatility of the crossover approach in the prediction accuracy of LBM in near-critical flows, this formulation was incorporated into the Peng-Robinson (P-R) EoS. First, 2D static droplets of CO2 and water at vapor-liquid equilibrium were modeled using both P-R EoS and its crossover formulation, and the numerically predicted results were then compared against the experimental data. The results demonstrate that the model accuracy in representing the thermodynamic behavior of the fluid in near-critical region is improved after adopting the crossover formulation with respect to the classical analytical EoS. The crossover P-R EoS was further applied to study several two-phase CO2 flows in mini-channels at near-critical temperatures. The flow patterns showed a good qualitative agreement with the experimental data in the near-critical region. The study is further extended to multi-component multiphase systems, specifically to water droplet penetration into porous media filled with CO2.

Research on pressure drop characteristics of CO2 flow boiling based on flow pattern in horizontal Minichannel

The experimental study of the frictional pressure drop characteristics of flow boiling heat transfer of CO2 in a horizontal minichannel with an inner diameter of Φ1.5 mm was carried out. Experimental conditions: heat flux (7.5 ~ 30 kW•m−2), mass flow rate (50 ~ 600 kg•m−2•s−1), saturation temperature (−40 ~ 0 °C), vapor quality (0 ~ 1). The comparison between the experimental results and theoretical flow pattern diagrams shows that the change of heat flux has little effect on the frictional pressure drop at high vapor quality area, but it has a decisive effect on the dryout and mist flow areas. The mass flow rate is the most important factor affecting the frictional pressure drop which has a decisive effect on the flow pattern experienced during the heat transfer process. The saturation temperature has an important influence on the flow pattern transition characteristics and the friction pressure drop decreases with the increase of saturation temperature. The influence of vapor quality on the frictional pressure drop is mainly caused by the flow pattern change in the minichannel. The flow visualization study shows that the theoretical flow pattern diagram can better reflect the flow pattern of CO2 in the phase-change heat transfer process and the transition trend of flow pattern under different working conditions.

Mean velocity and temperature scaling for near-wall turbulence with heat transfer at supercritical pressure

Compared with conventional gaseous and liquid fluids, fluids operating at supercritical pressure undergo drastic variations in thermophysical properties within a small temperature range across the pseudo-critical point. Therefore, the effect of these variations on flow and heat transfer must be studied. This paper presents direct numerical simulations (DNSs) of the turbulent heat transfer of CO2 at supercritical pressure in a fully developed channel flow between two isothermal walls. The thermophysical property tables generated from the REFPROF 9.1 database were used in this DNS. The velocity and temperature scaling and the analogy between momentum and scalar transport are comprehensively explored by using stress balance and semi-local methods. The results show that at small temperature differences, the velocity transformation developed by Trettel and Larsson [“Mean velocity scaling for compressible wall turbulence with heat transfer,” Phys. Fluids 28, 026102 (2016)] with a semi-local coordinate provides a good description of the near-wall turbulence of supercritical fluids. Upon including how large specific-heat variations affect temperature transformation, the logarithmic region of the cooled wall becomes consistent, as does the heated wall in a certain temperature range. In addition, in near-wall turbulence with small temperature differences at supercritical pressure, momentum transport is highly analogous to scalar transport.

Numerical study of heat and mass transfer by reverse water-gas shift reaction in catalyst-coated microchannel reactor

Reverse water-shift reaction (RWGS) is well known as an endothermic reaction to convert carbon dioxide and hydrogen to carbon monoxide and water vapor at an elevated temperature above 600 °C and also considered as a syngas production process before Fischer-Tropsch (FT) synthesis in generating the synthesis of liquid fuels. Unlike the conventional reactor types, micro-structured reactor has various advantages such as enhanced heat and mass transfer, flow uniformity, higher surface-to-volume ratio, safe processing and easier scale-up. In particular, this paper aims to invesitgate the conversion performance of RWGS reaction in catalyst-coated microchannel reactor whose inside surface is coated with Ni-catalyst by utilizing CFD simulations including chemical kinetics. The analytical model is developed to describe heat and mass transfer with chemical reaction in catalyst-coated microchannel. The effect of channel size, gas velocity, operating temperature, inlet gas composition (molar ratio of H2/CO2) and catalytic area ratio is evaluated by displaying CO2 conversion, temperature distribution, heat transfer, and reaction rate.

Parametric study of CO2 separation using carbon molecular sieve, zeolite and silica gel

Performances of Pressure swing adsorption (PSA) dependent on the type of adsorbent and efficiency of the regeneration cycle. In this work, a model describing CO2 mixture flow, mass and heat transfer with multi-component adsorption model are developed in Aspen Adsorption environment to compare the performance of PSA cycles with three different adsorbents (CMS 3K, zeolite 13x and silica gel). Adsorption capacity of CO2 for three adsorbents was found to be in the order of CMS>zeolite>silica gel. Hence, CMS was chosen as the adsorbents to evaluate PSA performances on different feed flowrate and pressure based on the purity, recovery and productivity of the CO2 separation. The result shows that, within a certain range, higher adsorption pressure will increase all purity, recovery and productivity of CO2 while higher feeding rate leads to increase recovery but lower purity and productivity.

Arrangement and three-dimensional analysis of cooling wall in 1000 MW S–CO2 coal-fired boiler

The Supercritical carbon dioxide (S–CO2) Brayton cycle is considered as a promising alternative to traditional steam Rankine cycle due to its high efficiency and compactness. However, in coal-fired power system, the S–CO2 temperature at cooling wall entrance is newly recognized quite high, leading to an over-temperature crisis. A coupled model of combustion and S–CO2 heat transfer was established to predict cooling wall temperature of a 1000 MW S–CO2 Brayton coal-fired boiler. Based on the module arrangement in S–CO2 boiler, the “cold S–CO2-hot fire matching and cascaded temperature control” principle in 1D model was proposed to reduce the cooling wall temperature. Three methods were examined including the low-temperature fluid matching high heat flux burner region, the counterflow, and more cold fluid in circulation drained to match high heat flux zone. The results show that the optimal arrangement can significantly reduce the temperature in overheated region 12–44 °C and eliminate the local hot spot. In addition, the 3D model was developed to obtain the maximum wall temperature and the uneven temperature in the circumferential direction. The employment of spiral cooling wall could alleviate circumferential unevenness and reduce wall temperature. The present work can provide important guidance to design of S–CO2 Brayton coal-fired power system.

Numerical Analysis on Heat Transfer Characteristics of Supercritical CO2 in Heated Vertical Up-flow Tube.

It is great significance to understand the mechanism of heat transfer deterioration of supercritical CO2 for heat exchanger design and safe operation in the supercritical CO2 Brayton cycle. Three-dimensional steady-state numerical simulation was performed to investigate the behavior of supercritical CO2 heat transfer in heated vertical up-flow tube with inner diameter di = 10 mm and heated length Lh = 2000 mm. Based on the characteristics of inverted-annular film boiling at subcritical pressure, the heat transfer model of supercritical CO2 flowing in the heated vertical tube was established in this paper. The mechanisms of heat transfer deterioration (HTD) and heat transfer recovery (HTR) for supercritical CO2 were discussed. Numerical results demonstrate that HTD is affected by multiple factors, such as the thickness and property of vapor-like film near the wall, the turbulence intensity near the interface between liquid-like and vapor-like, and in the liquid-like core region as well as the distribution of radial velocity vector. Among the above factors, the change of turbulent kinetic energy caused by the buoyancy effect seems to be a more important contributor to HTD and HTR. Furthermore, the influences of heat flux and mass flux on the distribution of wall temperature were analyzed, respectively. The reasons for the difference in wall temperature at different heat fluxes and mass fluxes were explained by capturing detailed thermal physical properties and turbulence fields. The present investigation can provide valuable information for the design optimization and safe operation of a supercritical CO2 heat exchanger.

Experimental study of a straight channel printed circuit heat exchanger on supercritical CO2 near the critical point with water cooling

The study presents an experimental examination of heat transfer of straight printed circuit heat exchanger (PCHE) for a precooler of supercritical carbon dioxide (sCO2) Brayton cycle. To perform heat transfer experiment, experimental loop for thermal hydraulic of CO2 in supercritical (ETHICS) was constructed at POSTECH. The straight PCHE was independently manufactured by photochemical etching and diffusion bonding process. An experiment for CO2 cooling with water was conducted via the ETHICS. We focused on heat transfer and flow characteristics of CO2 in the printed circuit heat exchanger. The experiments were conducted at three operating conditions for CO2 cooling, namely the trans-critical case (cooling from supercritical state to subcooled liquid), near the critical case (cooling from gas-like supercritical state to liquid-like supercritical state), and far critical case (cooling just in gas-like supercritical state). Nusselt numbers for different pressure and different operating conditions were compared by following typical analysis used in previous which is using averaged enthalpy based on inlet and outlet data but we conclude that the method of average value using inlet and outlet data is not appropriate for data reduction due to significant changes in the properties of CO2 near the critical point. Instead of that, we propose to use discretization method in data reduction of experimental data of CO2 near the critical point. It is difficult to predict heat transfer performance near the critical point of CO2 in PCHE. Hence, the data from this experiments and discretization method for data reduction are useful in designing a precooler for the sCO2 Brayton cycle.

Effect of cold fluid on supercritical heat transfer characteristics in conjugate cooling

The supercritical CO2 conjugate cooling in double-pipe heat exchanger has been numerically investigated. The results of AKN turbulence model agree well with the experimental data. Compared to the wall condition assumption, the conjugate cooling condition is more suitable for supercritical CO2 heat transfer simulation in double-pipe heat exchanger. For supercritical CO2 conjugate cooling, the mixed convection is the most important factor. The influence of the cooling water temperature on the supercritical CO2 conjugate cooling is more evident than the cooling water Re.

Flow boiling of carbon dioxide: Heat transfer for smooth and enhanced geometries and effect of oil. state of the art review

This paper presents a state-of-the-art review on flow boiling of carbon dioxide, including experimental studies and correlations for smooth and enhanced tubes, with pure CO2 and CO2/lubricant mixtures. Specifically, 5223 CO2 heat transfer coefficient data in smooth tubes are collected, and the effect of the operating conditions is discussed. Additional 883 data points in microfin tubes and 1184 experimental heat transfer coefficients in smooth tubes with CO2/oil mixture are also collected, and the influence of the microfin structure and of the oil presence on the heat transfer mechanism is analyzed.The statistical analysis has highlighted that the CO2-based correlation of Fang et al. is very accurate (MAE = 5.1%) for the smooth tube database, whereas the heat transfer coefficients in microfin tubes are satisfactorily predicted (MAE = 30.5%) with the model of Mehendale. Among the available correlations for CO2/oil mixture in smooth tubes, the method of Gao et al. provides the highest accuracy (MAE = 63.2%).

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.