Heat Transfer Of Carbon Dioxide Research Articles

Critical Heat Flux Condition and Post-Critical Heat Flux Heat Transfer of Carbon Dioxide at High Reduced Pressures in a Microchannel

Abstract Flow boiling heat transfer around the critical heat flux (CHF) condition at high reduced pressures of carbon dioxide in a 296-μm hydraulic diameter microchannel was experimentally studied. The CHF conditions for developing flow and fully developed flow were measured and compared to established correlations. The post-CHF heat transfer coefficient was obtained for l/d of 3.2, 7.4, and 11.6 for inlet Reynolds numbers, based on the homogeneous two-phase flow model, ranging from 6622 to 32,248. The critical heat flux conditions seemed to peak around a reduced pressure of about 0.5 and gradually decreased with reduced pressure. However, the typical rapid increase in the surface temperature following the CHF condition decreased with increasing pressure, and the post-CHF heat transfer coefficient was appreciably high (up to about 50 kW/m2K) at high reduced pressures. The enhancement in the heat transfer coefficient and CHF condition near the inlet were quantified. The experimental results were compared to established CHF correlations and heat transfer coefficient correlations with some limited success. Thus, the Katto CHF correlation (Katto and Ohno, 1984, “An Improved Version of the Generalized Correlation of Critical Heat Flux for the Forced Convective Boiling in Uniformly Heated Vertical Tubes,” Int. J. Heat Mass Transfer, 27(9), pp. 1641–1648) and the Bishop correlation (Bishop et al., 1964, “Forced-Convection Heat Transfer to Water at Near-Critical Temperatures and Supercritical Pressures,” Westinghouse Electric Corp, Atomic Power Division, Pittsburgh, PA.) for the post-CHF heat transfer coefficient were adjusted to better predict the experimental results. Additionally, an enhancement factor was derived to predict the increase in the heat transfer coefficient in the developing region.

High pressure saturated flow boiling of CO2 at the micro scale

Saturated flow boiling heat transfer of carbon dioxide (CO2) at the microscale was studied with pressures ranging from 2.4 MPa to 6 MPa (i.e., reduced pressure ranging from 0.3 to 0.81, respectively), mass fluxes ranging from 1199 kg/m2s to 2428 kg/m2s, mass qualities up to 0.94, heat fluxes up to 270 W/cm2, and ΔTsat up to 150 K. A maximum heat transfer coefficient (HTC) of about 150 kW/m2K, and a maximum critical heat flux (CHF) of about 200 W/cm2 was recorded. Comparison to established correlations were made and discussed based on a flow map of saturated flow boiling of carbon dioxide by Cheng et al. The map classified the flow into different patterns (i.e., bubbly, annular, dry-out, and mist flows), and provided insight about the observed HTC trends based on the mass quality. The Shah and the Cheng et al. correlations predicted the HTC reasonably well for mass qualities of up to 0.45 with mean average errors of about 23%, but for a mass quality of 0.94, the Kandlikar, the Shah, and the Cheng et al. correlations underpredicted experiments with mean average errors of 80%, 24%, and 98%, respectively. The critical heat flux condition was predicted well with different models, and the Katto et al. correlation provided the lowest mean average error of 23.0%.

Heat transfer correlation for sCO2 cooling in a 2 mm tube

The heat transfer of carbon dioxide near the critical point cooled in a horizontal tube with 2 mm inner diameter was investigated experimentally. The local heat transfer coefficients (htc) were determined by temperature measurements inside the tube wall along the 1.2 m cooled length. The effects of the CO2 mass fluxes in the range of 400–1300 kg/m2s, CO2 inlet pressures between 77 and 85 bar, CO2 bulk fluid temperatures between 10 and 85 °C and cooling water temperatures between 10 and 40 °C on the heat transfer were examined. The experimental results are compared with existing correlations for sCO2 heat transfer in small diameter tubes. A modified Dittus-Boelter equation was developed to predict the heat transfer coefficients with an accuracy of 20%.

Subcooled Flow Boiling of Carbon Dioxide Near the Critical Point Inside a Microchannel

Subcooled flow boiling heat transfer of carbon dioxide (${\mathrm{CO}}_{2}$) at the microscale near the critical pressure is experimentally studied. The onset of nucleate boiling and the local heat-transfer coefficient are obtained under 26 experimental conditions, consisting of 17 sample points each, yielding 442 data points. Excess temperatures of about 1 K at the onset of nucleate boiling and heat-transfer coefficients of up to $200\phantom{\rule{0.25em}{0ex}}\mathrm{kW}/{\mathrm{m}}^{2}\phantom{\rule{0.25em}{0ex}}\mathrm{K}$ are recorded. The heat-transfer coefficient and the onset of nucleate boiling models and correlations developed for low reduced pressures are compared to current experimental results. Modifications to these models are introduced to account for conditions pertinent to reduced pressures around unity. Overall, the correlations do not fully capture current measurements, suggesting that the physics of flow boiling heat transfer near the critical conditions exhibits some unique characteristics. Furthermore, it is observed that the experimentally obtained heat-transfer coefficient is better predicted by a model developed for flow boiling of ${\mathrm{CO}}_{2}$. However, with increasing pressure, deviations between experiments and predictions increase.

Experimental comparison of the heat transfer of carbon dioxide under subcritical and supercritical pressures

The carbon dioxide (CO2) trans-critical cycle is widely used in refrigerating systems, air conditioning, and heat pumps. Heat transfer behaviors at subcritical and supercritical pressures play essential roles in trans-critical thermohydraulic systems. In past decades, boiling heat transfer at subcritical pressure and `single-phase' convective heat transfer at supercritical pressure have been broadly discussed; however, evaluation of the heat transfer difference of CO2 between supercritical and subcritical pressures remains lacking. In this study, experimental investigations are conducted to explore the heat transfer characteristics of the vertical flow of CO2 in a wide pressure range (6.2 MPa–8.0 MPa) within a circular tube (inner diameter = 5 mm). The range of operating conditions includes a mass flux (G) between 0 and 190 kg/m2s and a heat flux (q) of up to 105 kW/m2. Parametric effects (e.g., heat flux, inlet temperature, and mass flux) within these experimental conditions are also analyzed. Measurement results show that the inner-wall temperature at subcritical pressure is higher than that at supercritical pressure when a boiling crisis occurs. Closer to the critical pressure, a boiling crisis occurs more easily at lower heat fluxes, but the corresponding temperature peak declines. Higher temperatures are measured farther from critical pressure, indicating that overheating induced by boiling heat transfer at subcritical pressure is far more significant than that governed by the deterioration of convective heat transfer at supercritical pressure. Additionally, a qualitative analysis of physical mechanisms governing observed heat transfer differences is performed, confirming variations in fluid thermophysical properties and a bubble-like flow pattern.

Numerical study on nonuniform heat transfer of supercritical pressure carbon dioxide during cooling in horizontal circular tube

A three-dimensional numerical investigation has been performed to study the nonuniform heat transfer of carbon dioxide (CO2) flows in cooled horizontal tube under supercritical pressures. It is found that the standard k-ε turbulence model with enhanced wall treatment can predict the heat transfer coefficients fairly well as compared with available experimental data. Detailed information about velocity, temperature, and thermophysical properties are captured and discussed. The results show that the heat flux on the wall-fluid coupled interface is highly nonuniform with maximal heat flux located at the top surface and minimal heat flux at the bottom surface although constant heat flux is fixed at the outer solid wall. Both the thermally induced flow acceleration and the buoyancy force contribute to the heat transfer enhancement. The effect of heat flux is further investigated and the results indicate that the decrease of heat flux leads to a narrower and sharper heat transfer coefficient profiles. Meanwhile, a larger heat flux causes much more strongly nonuniformity in the circumferential directions. It is also observed that the maximal heat transfer coefficient is decreased significantly as the operating pressure deviates away from the critical pressure. This study provides a fundamental basis for further engineering applications.

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

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

Evaporation heat transfer of carbon dioxide at low temperature inside a horizontal smooth tube

In this paper, the evaporation heat transfer coefficient of carbon dioxide at low temperature of −30 to −20 °C in a horizontal smooth tube was investigated experimentally. The test devices consist of mass flowmeter, pre-heater, magnetic gear pump, test section (evaporator), condenser and liquid receiver. Test section is made of cooper tube. Inner and outer diameter of the test section is 8 and 9.52 mm, respectively. The experiment is conducted at mass fluxes from 100 to 300 kg/m2 s, saturation temperature from −30 to −20 °C. The main results are summarized as follows: In case that the mass flux of carbon dioxide is 100 kg/m2 s, the evaporation heat transfer coefficient is almost constant regardless of vapor quality. In case of 200 and 300 kg/m2 s, the evaporation heat transfer coefficient increases steadily with increasing vapor quality. For the same mass flux, the evaporation heat transfer coefficient increases as the evaporation temperature of the refrigerant decreases. In comparison of heat transfer correlations with the experimental result, the evaporation heat transfer correlations do not predict them exactly. Therefore, more accurate heat transfer correlation than the previous one is required.

Numerical Study on the Heat Transfer of Carbon Dioxide in Horizontal Straight Tubes under Supercritical Pressure.

Cooling heat transfer of supercritical CO2 in horizontal straight tubes with wall is numerically investigated by using FLUENT. The results show that almost all models are able to present the trend of heat transfer qualitatively, and the stand k−ε with enhanced wall treatment model shows the best agreement with the experimental data, followed by LB low Re turbulence model. Then further studies are discussed on velocity, temperature and turbulence distributions. The parameters which are defined as the criterion of buoyancy effect on convection heat transfer are introduced to judge the condition of the fluid. The relationships among the inlet temperature, outlet temperature, the mass flow rate, the heat flux and the diameter are discussed and the difference between the cooling and heating of CO2 are compared.

Numerical study of the characteristic influence of the helically coiled tube on the heat transfer of carbon dioxide

The cooling heat transfer of supercritical CO2 is simulated numerically in the horizontal helically coiled tube using RNG k–ε turbulence model. The parameters of the tube are: inner diameter of 4mm, a length of 2000mm, a pitch of 10mm, radius of helically coiled tube of 20mm, curvature of 0.1. It is found that the heat transfer coefficient fluctuates from top to bottom along the flow direction. On this basis, the effect of the pitch p, tube diameter of helically coiled tube d, radius of helically coiled tube R on the heat transfer coefficient (h) are studied. The results show that the h increases as the p, d, R increases and is insensitive when the p, d, R further increases to higher values. The change condition of the pressure is different with the p, d, R increasing. At the same time the contours show that the effect of the buoyancy force on h is weaker than that of the centrifugal force. A quantitative analysis is carried on to study the effect of gravity based on the setting conditions and the results shows that the effect of gravity can be ignorable on the heat transfer coefficient. The change condition of Rig is consistent with that of Gr/Re2.7 in judging the scope of the buoyancy force. The change condition of Ric/Rig indicates that the gravitational buoyancy force is more important than the centrifugal buoyancy force on the heat transfer coefficient under the setting conditions.

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

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

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

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

Flow boiling heat transfer of carbon dioxide with PAG-type lubricating oil in pre-dryout region inside horizontal tube

A flow boiling heat transfer model for horizontal tubes is proposed for CO2 with entrained polyalkylene glycol (PAG) type lubricating oil in the pre-dryout region. A general power law-type model with a power number of 3 is used together with the average thermodynamic properties of the CO2–oil mixture. A convective enhancement factor (F) is recommended according to the relationship between the Lockhart–Martinelli parameter and the ratio αtp/αl, which was obtained based on previous experimental results for CO2 and oil. A new suppression factor (S) is introduced that comprises a suppression term for forced convection and oil concentration term for bubble generation. A comparison of six correlations showed that the proposed correlation can depict the influence of the mass and heat fluxes on both nucleate and convection boiling reasonably well.

Convection heat transfer of supercritical pressure carbon dioxide in a vertical micro tube from transition to turbulent flow regime

This paper presents experimental investigations of the convection heat transfer of carbon dioxide at supercritical pressures in a vertical tube with inner diameter of 99.2μm for various Reynolds numbers, heat fluxes and flow directions. The effects of buoyancy and flow acceleration due to heating and pressure drop are evaluated and analysed. The results show that the effects of flow acceleration are significant and the local wall temperature varies non-linearly for both upward and downward flows at the pressures in the vicinity of critical point and low inlet Reynolds numbers when the heat fluxes are relatively high. The buoyancy effect on the heat transfer is negligible in micron scale tubes at inlet Reynolds (from 2600 to 6700) and various heat fluxes (from 85kW/m2 to 748kW/m2). The flow acceleration due to heating and pressure drop can strongly influence the turbulence and reduce the heat transfer for high heat fluxes and low inlet Reynolds. Comparison of numerical predictions with the experimental data showed that the AKN low Reynolds number turbulence model gave better agreement than the k–ε realizable turbulence model with the enhanced wall treatment.

Effect of lubricating oil on flow boiling heat transfer of carbon dioxide

The flow boiling heat transfer of carbon dioxide with PAG-type lubricating oil entrained from 0% to 5% in horizontal smooth tubes was examined. Experiments were conducted using test tubes with inner diameter of 2–6 mm at mass fluxes of 360–1440 kg m−2 s−1 and heat fluxes of 4.5–36 kW m−2. The saturation temperature was 15 °C.At low oil concentrations of 0.5–1%, the heat transfer coefficient decreased to less than half that under oil-free conditions. The heat transfer coefficient did not decrease further with increasing oil concentration up to 5%. The heat flux positively influenced the heat transfer coefficient in low vapor quality regions, not the high vapor quality regions. The presence of oil caused the mass flux to significantly influence the heat transfer coefficient at a low heat flux till dryout, while no significant influence of the mass flux at a high heat flux was observed. The dryout quality decreased at a large mass flux. The measured pressure drops increased monotonously because of the lubricating oil.

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%.

Flow boiling heat transfer of carbon dioxide inside a small-sized microfin tube

This study investigated the flow boiling heat transfer of carbon dioxide inside a small-sized microfin tube (mean inner diameter: 2.0 mm; helix angle: 6.3°) at a saturation temperature of 15 °C, and heat and mass flux ranges of 4.5–18 kW m −2 and 360–720 kg m −2 s −1, respectively. Although, experimental results indicated that heat flux has a significant effect on the heat transfer coefficient, the coefficient does not always increase with mass flux, as in the case of conventional refrigerants such as HFCs or HCFCs. Under certain conditions, the heat transfer coefficient at a high mass flux was lower than that at a lower mass flux, indicating that convective heat transfer had a suppression effect on nucleate boiling. The heat transfer coefficients in the microfin tubes were 1.9∼2.3 times the values in smooth tubes of the same diameter under the same experimental conditions, and the dryout quality was much higher, ranging from 0.9 to 0.95. The experimental results indicated that using microfin tubes may considerably increase the overall heat transfer performance.

Thermal analysis of slurry ice production system using direct contact heat transfer of carbon dioxide and water mixture

This research studies the heat transfer characteristic during ice formation of a direct contact heat transfer between carbon dioxide and water mixture. This research is divided into two parts. For the first part, the low temperature carbon dioxide, between − 15 and − 60 °C, is injected into water initially at 28 °C and exchanges heat directly. The flow rate of carbon dioxide is varied between 0.003 and 0.017 kg/s while the volume of water is between 1 and 3 L. From the experiment, it is found that the effectiveness of the direct contact heat transfer between the carbon dioxide and the water is closed to 100%. Moreover, the lumped model is found to be used for predicting the temperature of water and the mass of ice formation quite well. However, for this technique, the blockage of ice around the injector always occurs. To avoid this problem, slurry ice is considered instead of pure ice. In the second part of this work, the thermal behavior of the slurry ice by direct contact heat transfer technique between carbon dioxide and water mixture is investigated. Compressor oil and Tween 60 are mixed with water for making slurry ice and it is found that the suitable composition is water/oil/Tween 60 at 100/6/1 by volume. Again, the lumped model can be used for predicting the temperature of water mixture and the mass of slurry ice quite well.

D124 臨界圧近傍における二酸化炭素の水平管内強制対流熱伝達(混相流2)

D124 臨界圧近傍における二酸化炭素の水平管内強制対流熱伝達(混相流2)

A computational study of convection heat transfer to CO 2 at supercritical pressures in a vertical mini tube

Computational simulations of experiments on turbulent convection heat transfer of carbon dioxide at supercritical pressures in a vertical tube of diameter 0.948 mm have been carried out using low-Reynolds number eddy viscosity turbulence models. The simulations were able to reproduce the general features exhibited in the experiments, although, in some cases, the details between the simulations and the experiments were very different. A better understanding of the problem has been developed based on the information generated by the simulations on the detailed flow and turbulence fields. It has been shown that for mini tubes such as the one used in the current study, the buoyancy effect is generally insignificant. However, heat transfer can still be significantly impaired as a result of flow acceleration when the heating is strong, which causes a reduction in turbulence production. Such an effect can be described in terms of the heating acceleration parameter Ω 1 . This parameter correlates reasonably well the data from all the cases considered in the current study.