Heat Transfer Coefficient Of CO2 Research Articles

Flow and heat transfer investigation of supercritical carbon dioxide in a novel biomimetic honeycomb fractal gas cooler of transcritical CO2 heat pumps

Flow and heat transfer characteristics of gas coolers, one of the most significant components in transcritical CO 2 heat pumps, significantly influence the overall performance of the heat pumps. To ensure high heat transfer efficiency and increased structural stability of gas coolers, a novel biomimetic honeycomb fractal heat exchanger is proposed in this study. The conjugate heat transfer between supercritical CO 2 and water in this novel heat exchanger is then investigated by numerical methods under different operating and geometric parameters. The results indicated that the heat transfer coefficients of CO 2 were quite different in different flow channels and different locations in each flow channel, especially near the supercritical region, which was significantly influenced by the buoyancy effect. Based on the thermophysical parameters, buoyancy, and hydraulic diameter, a new correlation for supercritical CO 2 was developed. The relative errors between the data calculated by the correlation and the numerical results were mostly in the range of ±15% and the root mean square deviation of the ratio of the calculated data to the numerical results for the Nusselt number was 5.4%.

Numerical simulation of supercritical CO2 characteristics in a double-pipe microchannel gas cooler

In this paper, numerical simulation is considered as a method to explore the heat transfer characteristics of sCO2 under different pressures, CO2 mass flow and CO2 inlet temperature in the double-pipe microchannel heat exchanger. 0.89 mm was adopted as the microchannel diameter. No matter it is under different pressures, different CO2 mass fluxes or different inlet temperatures. The heat transfer coefficient of supercritical CO2 rises sharply to the highest point and then falls slowly. The maximum value of the heat transfer coefficient of CO2 always appears at the position where Tb<Tpc with all working conditions. Lower pressure, CO2 mass flux and CO2 inlet temperature lead to higher heat transfer coefficient. No matter what kind of working conditions, CO2 has not been cooled to water temperature.

A THEORETICAL INVESTIGATION ON THE EVAPORATIVE HEAT TRANSFER OF CO2 IN SMOOTH AND MICROFIN TUBE

Nowadays, global environmental events such as thinning of the ozone layer and climate changes are increasing. These types of events are not only affecting all creatures living on the Earth, but also decreasing the quality of life. For this reason, natural refrigerants which are not harmful to environment, have been preferred in cooling systems. In this study, heat transfer coefficient of CO2 was investigated during the evaporation process in smooth tube and designed microfin tube. Features of the tube used in evaporator of these cooling systems directly affect to the heat transfer coefficient. The geometric parameters of the microfin tubes are an outer diameter of 9.52 mm, number of fins 50, apex angle of 38°, helix angle of 20° and fin height of 0.12 mm. Theoretical model was created on MATLAB environment. The heat transfer coefficient was investigated based on vapor quality. When theoretical results obtained for microfin tube were compared with experimental results, 6% approach was seen. A theoretical model was created for smooth and microfin tube by selecting heat flux as 10 kw/m^2 and mass flux as 380 kg/m2s and heat transfer coefficients in different evaporation temperatures were compared based on vapor quality. It has been concluded that heat transfer coefficients of microfin tube at 5°C, 0°C and -8°C were 52%, 44% and 34% higher than that of smooth tube, respectively.

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

The condensation heat-transfer coefficient and pressure drop for pure CO2 and CO2 + N2 mixtures were investigated under the near-critical condition, which simulates the CO2 transporting conditions under Carbon Capture, Transportation, and Storage (CCS). The experimental apparatus consists of a test section, heat exchangers, mass flow meters, temperature sensors, a magnetic gear pump, and a differential pressure transducer. The test section made with a copper tube was assembled with Polyvinyl Chloride (PVC) pipe to form a double tube. The condensation temperature and mole fraction of N2 of the CO2 mixtures ranged from 20 °C to 30 °C, and 1 to 5%, respectively. The mass flux was changed from 500, 600 and 700 kg·m−2s−1. The average heat transfer coefficient of the CO2 + N2 mixtures decreased by 2.23 to 15.9% based on the average heat transfer coefficient of pure CO2, and the pressure drop decreased by 53.4 to 77.3% at the condensation temperature of 25 °C with increase of the mole fraction of N2.

Heat transfer and pressure drop characteristics of CO2 mixtures in a pipeline under the seawater condition

Captured CO2 from CO2 emission sources contains impurities such as N2, CH4, H2O, O2 and Ar, and these are transported with the CO2 in pipelines under supercritical condition. The aim of this study is to experimentally investigate the effects of impurities in CO2 + N2 and CO2 + CH4 on the in-tube heat transfer and pressure drop under seawater environmental conditions during pipeline transportation. The experimental apparatus consisted of a test section, heat exchangers that were connected to two chillers, and a magnetic gear pump. The test section was made by a cooper tube that was inserted into a Polyvinyl Chloride (PVC) pipe to form a double-tube. The operational temperature and pressure of the CO2 mixture ranged 25–55 °C and 80–100 bar, respectively. The mass flux was varied by 300, 500, and 700 kg m−2 s−1. The mole fraction of CO2 in the CO2 mixtures was varied from (1.00–0.95). When the CO2 mole fraction decreased from 1.00 to 0.95, the maximum heat transfer coefficients of CO2 + N2 and CO2 + CH4 decreased by 4157 and 1224 W m−2 K−1, respectively, and the average pressure drop of CO2 + CH4 increased by 29.87%.

Prediction of heat transfer coefficient and compressor power consumption for CO2 with impurities under pipeline transporting condition

In CCS (carbon capture & storage) process, carbon dioxide is captured with impurities of various componets and different concentration. Under pipeline transporting conditions of near-critical states for CO2, the impurities such as nitrogen, methane, and hydrogen sulfide significantly affect the in-tube heat transfer coefficient and compressor power consumption, which are different from those of the pure carbon dioxide. In the present study, in-tube heat transfer coefficient and compressor power consumption of CO2 with impurities under pipeline transporting of its diameter 600 mm were predicted by using the theoretical models; the Gnielinski correlation and the Darcy- Weisbach equation. Thermal and transport properties were estimated by the Chung model and REFPROP. This prediction of the in-tube heat transfer coefficient was conducted under various parameters; CO2 mole fraction from 0.90 to 1.00 in mixture, pressure from 80 to 120 bar, temperature from -20 to 60 °C, and velocity from 1 to 4 m/s. The compressor power consumption was estimated in the pipeline of 100 km. In-tube heat transfer coefficient and compressor power consumption follows the trends of pure CO2, and were decided by the type and quantity of impurity. The maximum heat transfer coefficient of CO2 mixture decreased with increasing pressure in supercritical condition. When CO2 mole fraction was 0.95 in CO2+N2 mixture, the effect of pressure on the compressor power consumption increased after 10 °C.

A research on the dryout characteristics of CO2'S flow boiling heat transfer process in mini-channels

The experimental and theoretical researches have been carried out to get the flow boiling heat transfer characteristics of carbon dioxide (CO2 or R744) as a refrigerant in horizontal mini-channel. Based on infrared thermal imaging tests and experimental studies on heat transfer coefficients, the heat transfer coefficients and dryout characteristics of CO2 are analyzed qualitatively and quantitatively in following conditions: Heat flux: 2~35 kW m-2, Mass flux: 50~1350 kg m-2 s-1, saturation temperature: −10~15 °C, mini-channel inner diameter: 1 mm and 2 mm. Primary conclusions can be drawn from the results of the experiments: The increase of heat flux enhances the nucleate boiling heat transfer of the refrigerant inside mini-channel, which leads to the remarkable increase of heat transfer coefficient. But it speeds up the process of dryout. It also has a certain influence on vapor qualities of dryout at both the starting and the ending stage. The effect of mass flux on heat transfer enhancement depends on the dominant heat transfer mode in the tube. With the increase of mass flow rate, the vapor quality at the start of dryout has a decreasing trend. But the heat transfer coefficient increases at the end of dryout process or even after dryout process; the heat transfer coefficient does not vary monotonically with the saturation temperature: when the saturation temperature is high and even close to CO2's critical temperature, the heat transfer coefficient increases with the increase of saturation temperature; when the saturation temperature is low, the heat transfer coefficient increases with the decrease of saturation temperature. Besides, during the heat transfer process, the dryout vapor quality falls monotonically with the increase of saturation temperature. It is reasonable to conclude that dryout characteristics have significant influence on heat transfer coefficient. Fang correlation that predicts the heat transfer coefficient of CO2 is in good agreement with the experimental data, which has a mean absolute deviation of 15.7%, and predicts 71.98% of the entire database within ±20% and 86.84% of the entire database within ±30%.

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%.

INFLOW CONDENSATION HEAT TRANSFER CHARACTERISTICS OF CO2 IN MICROCHANNEL

In this study, we investigated two-phase flow patterns and effects of oil concentration on the heat transfer coefficient and pressure drop of CO 2 undergoing condensation process in a microchannel, and from the data collected, we developed a prediction model of CO 2 condensation heat transfer coefficient in smooth tube and microchannels. The narrow single rectangular channel was utilized to observe flow patterns of CO 2 under the condensation process. Experimental results show that the transition of vapor quality from intermittent flow to annular flow advances with increase of mass flux and with decrease of condensation temperature. The heat transfer coefficient decreased by 50% as compared to that of the pure CO 2 when the oil concentration was increased from 0.7 to 1.2 wt.% for the mass flux of 600 kg ⋅ m-2 ⋅ s-1. The pressure drop slightly decreased with oil concentration as compared to that of pure CO 2. We developed the prediction model for the heat transfer coefficient of CO 2 undergoing condensation process in microchannel by considering the effects of the liquid film thickness and of the interface shape between liquid and vapor phases on the heat transfer coefficient. The present prediction model estimated the experimental data within 18.9% of mean deviation. For the pressure drop in microchannel tubes, the existing models developed by Mishima and Hibiki, and Garimella showed the marginal predictability.

Performance of a flat-plate solar thermal collector using supercritical carbon dioxide as heat transfer fluid

Energetic and exergetic performance analyses of flat-plate solar collector using supercritical CO2 have been done in this study. To take care of the sharp change in thermophysical properties in near critical region, the discretisation technique has been used. Effects of zonal ambient temperature and solar radiation, fluid mass flow rate and collector geometry on heat transfer rate, collector efficiency, heat removal factor, irreversibility and second law efficiency are presented. The optimum operating pressure correlation has been established to yield maximum heat transfer coefficient of CO2 for a certain operating temperature. Effect of metrological condition on heat transfer rate and collector efficiency is significant and that on heat removal factor is negligible. Improvement of heat transfer rate is more predominant than increase in irreversibility by using CO2. For the studied ranges, the maximum performance improvement of flat-plate solar collector by using CO2 as the heat transfer fluid was evaluated as 18%.

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.

Prediction of gas cooling heat transfer coefficients for CO2–oil mixtures

The transport properties and the gas cooling heat transfer coefficients of CO2–oil mixtures were investigated under gas cooling conditions for a CO2 refrigeration system. The viscosity and thermal conductivity of the CO2–oil mixture were predicted using the TRAPP model under the high pressure condition of around 10 MPa. Depending on the oil concentration, two different models: the homogenous model and separate model, were utilized to estimate the gas cooling heat transfer coefficient. Significant variations of the thermophysical properties of pure CO2 with oil concentration heavily influenced the gas cooling characteristics of the CO2–oil mixture. The homogenous model for low oil concentrations (i.e. 1.0 wt.%) and the separate model for relatively high oil concentrations (i.e. 5.0 wt.%) predictions matched the previous experimental data with mean deviations of ±13.2% and ±33.4%, respectively.

Condensation from superheated vapor flow of R744 and R410A at subcritical pressures in a horizontal smooth tube

This paper presents experimentally determined heat transfer coefficients for condensation from a superheated vapor of CO2 and R410A. The superheated vapor was flowed through a smooth horizontal tube with 6.1mm ID under almost uniform temperature cooling at reduced pressures from 0.55 to 0.95, heat fluxes from 3 to 20kWm−2, and superheats from 0 to 40K. When the tube wall temperature reaches the saturation point, the measured results show that the heat transfer coefficient gradually starts deviating from the values predicted by a correlation valid for single-phase gas cooling. This point identifies the start of condensation from the superheated vapor. The condensation starts earlier at higher heat fluxes because the tube wall temperature reaches the saturation point earlier. The heat transfer coefficient reaches a value predicted by correlations for condensation at a thermodynamic vapor quality of 1. The measured heat transfer coefficient of CO2 is roughly 20–70% higher than that of R410A at the same reduced pressures. This is mainly because the larger latent heat and liquid thermal conductivity of CO2, compared to that of R410A, increase the heat transfer coefficient.

C223 CO_2冷媒の水平平滑管内における沸騰熱伝達整理式の評価(OS-13:ヒートポンプの高性能化)

For the development of high performance and small sized carbon dioxide (CO_2) heat pump water heater, high performance heat transfer tube is very important. In the previous studies, parts of the authors experimentally clarified average boiling heat transfer coefficient of CO_2 in horizontally located smooth tubes takes a maximum against mass velocity but not in grooved tubes. In this study, the experimental data are compared with conventional correlations based on local coefficients to clarify the phenomena, and also predicting performance of the correlations is examined. It is concluded that more experimental data in dryout and post-dryout region should be obtained.

Carbon dioxide flow boiling in a single microchannel – Part II: Heat transfer

Flow boiling heat transfer coefficients of CO2 have been measured in a single microchannel. Experiments were carried out in a horizontal stainless steel tube of 0.529mm inner diameter, for three temperatures (−10, −5 and 0°C), with the mass flux ranging from 200 to 1200kg/m2s and the heat flux varying from 10 to 30kW/m2. The investigation covered qualities from zero to the dryout inception, i.e. pre-dryout conditions. Compared to larger microchannels and positive temperatures, a higher contribution of convective boiling was found, with a larger heat transfer coefficient than for pure nucleate boiling. Mainly two heat transfer regimes were found, depending on the boiling number (Bo). For Bo>1.1×10−4, the heat transfer coefficient was highly dependent on the heat flux and moderately influenced by the quality and the mass flux. For Bo<1.1×10−4, the heat transfer coefficient was hardly affected by the heat flux but strongly influenced by the quality and the mass flux. In addition, dryout results were reported. The effect of the mass flux on the dryout inception quality was found to be highly dependent on the heat flux and the saturation temperature.

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 .

Effect of PAG-type lubricating oil on heat transfer characteristics of supercritical carbon dioxide cooled inside a small internally grooved tube

Heat transfer characteristics of supercritical CO 2 cooled inside a small internally grooved tube were investigated. The mean inner diameter (ID) and helix angle of this tube were 2 mm and 6.3°, respectively. Experiments were conducted in the pressure range of 8–10 MPa and mass flux range of 400–1200 kg m −2 s −1. The effect of PAG-type lubricating oil on the heat transfer characteristics was investigated by changing the oil concentration from 0% to 5%. The drop in the heat transfer coefficient was 30–50% and 50–70% at oil concentrations of 1% and 3%, respectively. The heat transfer coefficient of CO 2 in the grooved tube was higher than that in the smooth tube at all oil concentrations tested. The experimental results indicated that the grooved tube has a large heat transfer area and facilitates the breaking up of the oil film.

브레이징식 동세관내 CO2의 냉각 열전달 특성

Abstract ： The cooling heat transfer coefficient of CO 2 in a brazing type small diameter tube was investigated experimentally. The main components of th e refrigerant loop are a receiver, a CO 2 compressor, a mass flow meter, an evaporator and a brazing type small diameter tube as a test section. The mass flux of CO 2 is 400~1600 [kg/m 2 s], the mass flowrate of coolant were varied from 0.15 to 0.3 [kg/s], and the cooling pressure of gas cooler were from 8 to 10 [MPa]. The cooling heat transfer coefficients of the brazing type small diameter copper tube is about 4~11.7% higher than that of the conventional type small diameter copper tube. In comparison with test result s and existing correlations, correlations failed to predict the cooling heat t ransfer coefficient of CO 2 in a brazing type small diameter copper tube. therefore, it is nec essary to develope reliable and accurate predictions determining the cooling heat transfer coefficient of CO 2 in a brazing type small diameter copper tube.

Experimental studies on the characteristics of evaporative heat transfer and pressure drop of CO 2/propane mixtures in horizontal and vertical smooth and micro-fin tubes

This study presents evaporative heat transfer characteristics of CO 2/propane refrigerant mixtures in horizontal, and vertical smooth and micro-fin tubes. The effect of mass flux, heat flux, inlet temperature and several compositions have been investigated and analyzed. The copper tube with outer diameter of 5 mm and length of 1.44 m was selected as the test sections. Average inner diameters of test tubes are 4.0 mm and 4.13 mm, respectively. The tests were conducted at mass fluxes from 212 to 656 kg/m 2, heat fluxes from 15 to 60 kW/m 2, inlet temperatures from −10 to 30 °C, and for several compositions (75/25, 50/50, 25/75 wt%). Among CO 2/propane refrigerant mixtures, the heat transfer characteristics are much better than those of any other compositions when the composition is 75/25 (wt%). Finally, the heat transfer coefficients of CO 2/propane refrigerant mixtures in the vertical tube are 5–10% higher than those in horizontal tubes.

경사평활관 및 마이크로핀관에서의 이산화탄소의 증발열전달 특성과 압력강하에 관한 실험적 연구

New alternative refrigerants have been developed due to the ozone layer depletion and global warming. For this reason, carbon dioxide is believed to be a promising refrigerant for use in air conditioners and heat pumps. Evaporative heat transfer characteristics and pressure drop of CO₂ with outer diameter of 5 ㎜ in inclined (45°) smooth and micro-fin tubes have been investigated by the experiments with respect to several test conditions such as mass fluxes, heat fluxes, evaporation temperatures in this study. The inclined (45°) smooth and micro-fin tubes with length of 1.44 m were installed to measure the evaporative heat transfer coefficients of CO₂ and heat was supplied to the refrigerant by direct heating method where the test tube was uniformly heated by electricity. The tests were conducted at mass fluxes from 212 to 656 ㎏/㎡s, heat fluxes from 15 to 60 ㎾/㎡ and evaporation temperatures from -10 to 20℃. The heat transfer coefficients of CO₂ are slightly increased with increasing mass flux, and the heat transfer characteristics in the inclined (45°) tubes are enhanced about 5~10% compared with those in horizontal or vertical tubes.