Heat Transfer Of CO2 Research Articles

Experimental study on heat transfer and pressure drop of supercritical CO2 cooled in a large tube

In order to reveal the heat transfer and pressure drop characteristics of supercritical CO2 cooled in the large diameter tube, an experimental system was established. The heat transfer and pressure drop performances of CO2 cooled in horizontal tubes having inner diameters of 4, 6 and 10.7 mm were experimentally investigated. The effects of the CO2 mass flow rate, inlet pressure, mean bulk temperature and tube diameter on the heat transfer coefficient and pressure drop were examined. The results show that the tube diameter significantly affects the heat transfer performance. As a consequence, when the universal heat transfer correlations for the small diameter tube are used to predict the heat transfer coefficient of CO2 in a large diameter tube, the predicted results present obvious deviation compared to the experimental results. Then based on the experimental data, a new heat transfer correlation for the large diameter tube was proposed. The maximum error between the predicted results of the new correlation and the experimental data is ±15%.

A comparative analysis of a CO2 evaporator model using experimental heat transfer correlations and a flow pattern map

This work analyses and compares two approaches to evaluate the boiling heat transfer of CO2 in a real evaporation process: classical empirical correlations and flow pattern maps. We develop a finite-volume mathematical model of an evaporator to contrast both approaches and simulate it with four empirical correlations, not developed for CO2 but tested successfully with it, and a flow pattern map specific for CO2. We contrast the results of the simulation with 73 experimental steady-states performed with a transcritical CO2 refrigeration plant that incorporates a concentric counter current evaporator. We observe that both approaches can predict the evaporation process of CO2, nonetheless, in spite of the more complex mathematical formulation, the considered flow pattern map allows to obtain more precision.

Heat transfer and various convection structures of near-critical CO2 flow in microchannels

Supercritical/near-critical fluid is very dense and much expandable, and with its preferable flow and heat transfer properties, it has been proposed in various kinds of energy conversion systems. The fluid critical transitions and diverges are very important for both hydrodynamic study and heat transfer applications. The current study deals with the near-critical CO2 horizontal flow and heat transfer performance in microchannels. Careful numerical procedures are carried out with Navier–Stokes equations, energy and state equations, which are treated with special care for micro-scale investigations. Due to the thermal–mechanical effects of critical fluid, abnormal thermal convection structure and transient micro-scale vortex mixing evolution mode are found in microchannels, which is identified to be governed by basic Kelvin–Helmholtz instability. New sources of near-critical fluid thermal–mechanical perturbations, instead of gravity waves, are found to be responsible for the microscale instabilities for the transient vortex flow development. It is also found that while in closed systems the Piston Effect (PE) is dominant, the thermal–mechanical expansion characteristics yield new convective structures and evolutions in open system (microchannels). At the same time the microchannel heat transfer is greatly enhanced due to the vortex evolutions. Possible model extensions are also discussed, and the near-critical fluid convection problem is then characterized from a more general viewpoint in this study.

Numerical Analysis on the Evaporative Heat Transfer of CO2 at Different Inlet Reynolds Numbers

With the rapid development of electronic devices for more compact structure and rapidity speed, the traditional single-phase radiator cannot meet the demand of an increasing heat flux from the electronic components. The effects of inlet Reynolds number on the evaporative heat transfer of CO2 in a loop heat pipe evaporator are numerically investigated in this paper. The objective is to seek an achievable approach to enhance the heat-dissipating efficiency and provide a low operating temperature for the electronic components. The temperature of the bottom oscillates continuously during the evaporation process and is influenced by the vapor; the pattern of two-phase flow is quite different from single-phase flow. It is also found that the heat transfer coefficient of CO2 increases with the inlet Reynolds number largely.

Convection Heat Transfer of CO2 at Supercritical Pressures in Microtubes

AbstractConvection heat transfer of supercritical pressure CO2 in microtubes as well as characteristics of fluid flow and heat transfer were investigated. Alterations of physical properties of supercritical CO2 and the impact of buoyancy on the heat transfer were analyzed when the inlet Reynolds (Re) number of CO2, diameter of heat exchanger, and inlet Re of cooling water were changed. The temperatures of hot fluid in numerical simulations from several classical turbulence calculation models were compared with experimental results from the literature. The physical properties of CO2 changed significantly around the critical point, causing a distinct decline of buoyancy and strengthening of heat transfer.

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.

Condensation heat transfer and pressure drop characteristics of CO2 in a microchannel

The condensation heat transfer coefficient and pressure drop of CO2 in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm−2s−1 and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.

Experimental investigation of CO2 thermosyphon flow and heat transfer in the supercritical region

Due to the unique properties of near-critical fluid, trans-critical/supercritical CO2 based natural circulation loop (NCL, or thermosyphon) has been proposed in many energy conversion systems, such as solar heater, waste heat recovery, next generation nuclear cooling, etc. This paper presents an experimental investigation of a near-critical CO2 thermosyphon. The closed thermosyphon is specially designed for high pressure experiments, where natural convection is achieved only by heating and cooling of fluid in the loop. The system is operated in wide range of pressures from around 6.0 to 15.0MPa in the near-critical region. It is found that the NCL flow will change from unstable sub-critical two-phase flow to stable liquid flow, and then become stable supercritical circulation with the increase of system initial pressure. The heat transfer behaviors are analyzed for stable flow conditions mainly for supercritical region. General variations of the fluid temperature, mass flow rate and loop pressure are presented in this paper. In the steady supercritical region, the heat transfer performance of the cooler show very small changes with bulk mean fluid temperature, and at the heater better heat transfer is found for conditions close to the pseudo-critical point. The heat transfer dependency on operation pressure and evolution mechanisms are also discussed in detail in this paper.

Boiling heat transfer of co- and counter-current microchannel heat exchangers with gas heating

In this paper, we present an experimental study on a two-phase boiling heat transfer of liquid methanol in co- and counter-current microchannel heat exchangers (MCHEs) with gas heating. The silicon-based MCHE with dimensions of 20×20×2mm is designed with 18 microchannels. The working fluids on the hot and cold sides of the MCHE are helium and liquid methanol, respectively. During the study, the helium fluid inlet temperature is varied from 40 to 250°C, while the mass flux is fixed at 10kg/m2s; additionally, the mass flux of the liquid methanol is varied from 3 to 9kg/m2s, and the inlet temperature is varied from room temperature to the saturation temperature of methanol, depending on the hot-side heat flux. The results demonstrate that the convective boiling heat transfer increases with the mean vapor quality until approaching the critical heat flux (CHF) condition. The counter-current MCHE exhibits a higher CHF due to a higher inlet subcooling temperature than the co-current MCHE. The single-phase and convective boiling heat transfer coefficients are measured and compared with the existing correlations. Moreover, two empirical correlations for the two-phase boiling heat transfer coefficient for the MCHEs are proposed in this study.

The cooling heat transfer characteristics of the supercritical CO2 in micro-fin tube

This study intended to verify the cooling heat transfer characteristics of supercritical gas for refrigerating and air-conditioning devices that use CO2, a natural refrigerant, as the operating fluid. Experiments were performed with a gas cooler, which was the test part. The gas cooler was a heat exchanger made of a micro-fin tube with an inner diameter of 4.6 mm and an outer diameter of 5.0 mm. The experiment results are summarized as follows. The heat transfer coefficient, according to the mass flux, peaked at the low cooling pressure of 8.0 MPa in the gas cooler, and reached its minimum at the high pressure of 10.0 MPa. Furthermore, when the mass flux of the refrigerant increased, the coefficient increased faster with the lower cooling pressure in the gas cooler. The heat transfer coefficient, according to the shape of the heat transfer tube, showed that the maximum values of the CO2 cooling heat transfer coefficients of the smooth tube and the micro-fin tube were found at 44.7 °C, which were the pseudo-critical temperatures for the entrance pressures. It was found that the cooling heat transfer coefficient of the micro-fin tube increased by 12–39 % more than that of the smooth tube. The experiment results for the CO2 heat transfer coefficients of the smooth tube and the micro-fin tube were compared with the results estimated from previous correlations. It was found that the experiment values generally significantly differed from and the experiment values greater than the estimated values. The differences were especially greater in the vicinity of the critical temperature points. Based on these results, a new correlation was suggested that includes the density ratio and the specific heat ratio.

Numerical Investigation for Heat Transfer of Supercritical CO2 Cooled in a Vertical Circular Tube

Convective heat transfer of supercritical CO2 cooled in a vertical circular tube has been numerically investigated by using the computational fluid mechanics code FLUENT. The objectives of this paper are to predict the heat transfer coefficient and to study the supercritical convective heat transfer of CO2 under uniform heat flux. Simulation starts with four cases, including a physical model, mathematical model, mesh independency, and solution method. Based on the mesh size of 80 × 500 in the radial and axial directions with near-wall resolution of less than 1, the low Reynolds number turbulence model proposed by Lam and Bremhorst (LB) model is adopted to investigate the heat flux effect, buoyancy effect on flow direction, and buoyancy effect on fluid flow. The results show that the LB low-Reynolds-number turbulence model agrees well with the experimental data. Buoyancy effect significantly affects the fluid flow and heat transfer characteristics in turbulent flow, which enhances heat transfer in upward flow, and impairs it in downward flow.

Detecting CO2 leakage in vertical wellbore through temperature logging

One of the risks associated with CO2 geologic sequestration is CO2 migrating upwards from the storage reservoir along vertical leakage paths, which may be abandoned or improperly cemented wellbores, or reactivated faults. CO2 migrating upwards is a process generally controlled by formation pressure gradient and CO2 buoyancy as controlled by the changes of CO2 properties such as density, viscosity, enthalpy and phase state along the paths, and companied by heat transfer between escaping CO2 and the leakage paths and heat conduction in the formation.Considering absorption or release of latent heat associated with phase changes, this paper develops models for CO2 flow and heat transfer during the leakage process. Then a procedure is developed to couple these models. Both steady and un-steady state models are presented in this paper. The steady state and unsteady state models generate similar temperature profiles with different magnitude. In terms of the temperature profile, the CO2 cooling effect caused by forming the super heated gas is dominant. It is also discovered that, during the CO2 migrating upwards process, CO2 cooling effect along the paths is essentially described by the saturation line in the CO2 P–T phase diagram. Case study shows that a simple relationship between the CO2 leakage rate and the maximum temperature difference between the original and the altered temperature profiles can obtained, so that the CO2 leakage rate could be estimated with the temperature profile logged along the wellbore in the field practice.

In-tube condensation heat transfer of CO2 at low temperatures in a horizontal smooth tube

This paper presents the experimental investigation of CO2 condensation in a horizontal smooth tube at saturation temperatures between 0 and −15°C with mass fluxes between 50 and 200kgm−2s−1 and for various vapour qualities. One tube-in-tube counter flow heat exchanger was designed together with an open-loop CO2 cycle to make up an experimental rig that condenses CO2 at these conditions. The inner test tube, with internal diameter of 6.52mm, is cooled using Glycol Water flowing in the outer tube. Experimental trends show that the heat transfer coefficient decreases with increasing saturation temperatures. Decrease in heat transfer coefficients is also observed for lower mass fluxes and lower vapour qualities in two-phase. Based on the experimental data collected, a new empirical correlation was developed to improve CO2 heat transfer coefficient prediction. The new correlation agreed well with experimental and data from the literature, with an average deviation of −4.4%.

Development of a new forced convection heat transfer correlation for CO 2 in both heating and cooling modes at supercritical pressures

Experimental and numerical investigations on forced convection heat transfer of carbon dioxide at supercritical pressures in a prototypic printed circuit heat exchanger under both cooling and heating conditions have been performed in this present study. The experiment test section has nine semi-circular channels with a hydraulic diameter of 1.16 mm and a length of 0.5 m. Primary operational parameters include inlet pressure of 7.5–10 MPa, mass fluxes of 326 kg/m 2 s and 762 kg/m 2 s, inlet temperatures from 10 °C to 90 °C and the average heat flux was 30 kW/m 2. Beyond reproducing the regular experimental cases, numerical modeling also implemented higher heat fluxes of 60 kW/m 2 and 90 kW/m 2 in order to investigate the effect of heat flux. Good agreement was found between the experiments and FLUENT simulations using an SST k– w model with the near-wall region being completely resolved. The distinctive behavior of convection heat transfer at supercritical pressures between heating and cooling modes was systematically analyzed. A more physically reasonable property-averaging technique, Probability Density Function (PDF)-based time-averaged property, was developed to account for the effect of nonlinear dependency of properties on instantaneous local temperature. Furthermore, experimental and computational data were compared to empirical predictions by the Dittus–Boelter and Jackson correlations. The results showed that Dittus–Boelter correlation has better precision for the average value of the predicted heat transfer coefficient but cannot take account of the effect of heat flux. In contrast, the Jackson correlation, with property ratio correction terms to account for the distribution of the properties in the radial direction, could predict the distinction of heat transfer characteristics under heating and cooling conditions. However, it overestimates the average value of heat transfer coefficient in the whole range of the experiment conditions. Finally, a new correlation evaluated by PDF-based time-averaged properties for forced convection heat transfer of CO 2 in both heating and cooling mode at supercritical pressures was developed. Comparison of experimental and computational data with the prediction results by the new developed correlation reveals that it works quite well; i.e., more than 90% data in either heating or cooling mode with various heat fluxes are predicted within an accuracy of ±25%.

An experimental study on heat transfer of CO 2 solid–gas two phase flow with dry ice sublimation

Knowledge on heat transfer characteristics of CO 2 solid–gas two phase flow is important to design of some new refrigeration systems. Continuously obtaining CO 2 solid–gas fluid flow in a closed loop is necessary to measure its heat transfer. In this paper, an experiment set-up is designed, constructed and tested in order to measure the temperatures, pressures of the CO 2 solid–gas two phase flow, and also further its heat transfer characteristics. As the first step of the investigation, the measurement work is conducted in a horizontal circular tube. Based on the obtained results, it is verified the present experiment set-up can continuously obtain CO 2 solid–gas fluid flow in the closed loop and also heat transfer measurement is possible. The results show an average value 310 W/(m 2 K) of heat convective coefficient is experimentally obtained in the present range, which is higher than that of gas flow. The Nusselt number is found to increase along the tube length in the sublimation area. In addition, some knowledge on the heat transfer characteristic of the CO 2 solid–gas flow is learned from the present study.

Subcooled Boiling Heat Transfer of CO2 in A Horizontal Tube at Low Temperatures

This article presents an experimental investigation on the subcooled flow boiling heat transfer characteristics of CO2 in a horizontal tube with inner diameter of 6.16 mm below −30°C. The effects of mass and heat fluxes and saturation temperature on the heat transfer coefficient are discussed. Large deviations are noted between the predictions from previous empirical correlations and the current CO2 experimental data. Hence a new empirical correlation is developed, which agrees within ±30% with the current CO2 experimental data. It is expected that the data presented in this study would be beneficial to industry and designers of compact heat exchangers for CO2 at low temperatures.

Flow boiling heat transfer and pressure drop characteristics of CO 2 in horizontal tube of 4.57-mm inner diameter

Abstract This paper presents the flow boiling heat transfer and pressure drop of CO 2 (R744) in a horizontal tube. The test tube had an inner diameter of 4.57 mm, with a 0.89-mm wall thickness and length of 4.2 m. Experiments were carried out for heat fluxes ranging from 10 to 40 kW/m 2 K, mass fluxes ranging from 200 to 1000 kg/m 2 s, and saturation temperatures ranging from 0 to 40 °C. The boiling heat transfer of CO 2 has a greater effect on nucleate boiling than convective boiling and highly depends on the vapor quality, heat flux, and saturation temperature. The boiling heat transfer coefficient of CO 2 increases with a decrease in the inner tube diameter. In a comparison to previous boiling heat transfer correlations, the correlation of Cheng et al. showed fairly good agreement with the experimental data for CO 2 . The boiling pressure drop of CO 2 increased with an increase in the mass flux and decreased with an increase in the saturation temperature. None of the empirical correlations from the existing literature were able to predict the pressure drop of CO 2 in good agreement with the experimental data. Therefore, it is necessary to develop a reliable and accurate prediction for estimating the boiling pressure drop of CO 2 .

Experimental study on evaporation heat transfer of CO 2/DME mixture refrigerant in a horizontal smooth tube

In-tube evaporation heat transfer coefficient of pure dimethyl ether (DME) and CO 2/DME mixtures have been measured with water heated double tube heat exchanger under the conditions of mass flux of 100, 200, 300 kg m −2 s −1 and inlet saturation temperature of 10 °C. Test section was horizontally installed smooth tube with inner diameter of 4.2 mm. The heat transfer coefficient of mixture decreases with mixing CO 2 into DME in which the degradation was about 20% for 10%CO 2 and 48% for 25%CO 2. Existent correlations predict the heat transfer coefficients of pure DME within ±30% error. On the other hand, heat transfer coefficient of the mixture with higher concentration 25%CO 2 could not be predicted well by existent correlations, though those of 10%CO 2 could be correlated fairly well.

Experimental investigation of convection heat transfer of CO 2 at supercritical pressures in a vertical circular tube

The convection heat transfer characteristics of supercritical CO 2 in a vertical circular tube of 2 mm inner diameter were investigated experimentally for pressures ranging from 78 to 95 bar, inlet temperatures from 25 to 40 °C, and inlet Reynolds numbers from 3800 to 20,000. The effects of the heat flux, thermo-physical properties, buoyancy and thermal acceleration on the convection heat transfer were analyzed. The experimental results show that for high inlet Reynolds numbers (e.g. Re = 9000) and high heat fluxes, a significant local deterioration and recovery of the heat transfer was found for upward flows but not for downward flows. Comparison of the experimental data for inlet Reynolds numbers from 3800 to 20,000 with some well-known empirical correlations showed large differences especially when the heat transfer deteriorates and then recovers when the effect of buoyancy is significant. The experimental data was used to develop modified local turbulent Nusselt number correlations for supercritical CO 2 flowing in vertical small circular tubes.

H121 二酸化炭素-潤滑油混合物の水平平滑管内における沸騰熱伝達(OS-2:沸騰・凝縮伝熱の最近の進展(II)(相変化研究会としての話題提供))

H121 二酸化炭素-潤滑油混合物の水平平滑管内における沸騰熱伝達(OS-2:沸騰・凝縮伝熱の最近の進展(II)(相変化研究会としての話題提供))