Heat Transfer Of CO2 Research Articles

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))

Study on heat transfer and pressure drop characteristics of internal heat exchangers in CO2 system under cooling condition

In order to study the heat transfer and pressure drop on four types of internal heat exchangers (IHXs) of a CO2 system, the experiment and numerical analysis were performed under a cooling condition. The configuration of the IHXs was a coaxial type and a micro-channel type. Two loops on the gas cooler part and the evaporator part were made, for experiment. And the section-by-section method and Hardy-Cross method were used for the numerical analysis. The capacity and pressure drop of the IHX are larger at the micro-channel type than at the coaxial type. When increasing the mass flow rate and the IHX length the capacity and pressure drop increase. The pressure drop of the evaporator loop is much larger than that of the gas cooler loop. The performance of the IHX was affected with operating condition of the gas-cooler and evaporator. The deviations between the experimental result and the numerical result are about ±20% for the micro-channel type and ±10% for the coaxial type. Thus, the new CO2 heat transfer correlation should be developed to precisely predict a CO2 heat transfer.

CFD modeling of heat transfer of CO 2 at supercritical pressures flowing vertically in porous tubes

Computational fluid dynamics (CFD) tool has been used for investigation of convective heat transfer of CO 2 in two porous tubes. Effects of some important parameters such as pressure, inlet temperature, mass flow rate, wall heat flux and porosity on temperature distribution and local heat transfer coefficients have been studied numerically. Near the supercritical conditions, these parameters are very effective on temperature gradient and local heat transfer coefficients. For example at p = 9.5 MPa, under the same conditions, the heat transfer coefficient in a tube with particle diameters of 0.1–0.12 mm is about 20–30% higher than when the particle diameter of 0.2–0.28 mm were used. The heat transfer coefficient increases with decreasing pressure and increasing mass flow rate. Also the porosity of the bed has the important role on the heat transfer. The CFD predictions have been compared to the experimental data and showed pretty good agreement.

Experimental and numerical study of convection heat transfer of CO 2 at super-critical pressures during cooling in small vertical tube

Convection heat transfer of CO 2 at super-critical pressures during cooling in a vertical small tube with inner diameter of 2.00 mm was investigated experimentally and numerically. The local heat transfer coefficients were determined through a combination of experimental measurements and numerical simulations. This study investigated the effects of pressure, cooling water mass flow rate, CO 2 mass flow rate, CO 2 inlet temperature, flow direction, properties variation and buoyancy on convection heat transfer in small tube. The results show that the local heat transfer coefficients vary significantly along the tube when the CO 2 bulk temperatures are in the near-critical region. The increase of specific heat and turbulence kinetic energy due to the density variation leads to the increase of the local heat transfer coefficients for upward flow. The buoyancy effect induced by density variation leads to a different variation trend of the local heat transfer coefficients along the tube for upward and downward flows. The numerical simulations were conducted using several k–ε turbulence models including the RNG k–ε model with a two-layer near wall treatment and three low-Reynolds number eddy viscosity turbulence models. The simulations using the low-Reynolds number k–ε model due to Yang–Shih has been found to be able to reproduce the general features exhibited in the experiments, although with a relatively large overestimation of measured wall temperatures. A better understanding of the mechanism of properties variation and buoyancy effects on convection heat transfer of CO 2 at super-critical pressures in a vertical small tube during cooling has been developed based on the information generated by the simulation on the detailed flow and turbulence fields.

Flow Boiling Heat Transfer, Pressure Drop, and Flow Pattern for CO2 in a 3.5mm Horizontal Smooth Tube

Abstract C O 2 flow boiling heat transfer coefficients and pressure drop in a 3.5mm horizontal smooth tube are presented. Also, flow patterns were visualized and studied at adiabatic conditions in a 3mm glass tube located immediately after a heat transfer section. Heat was applied by a secondary fluid through two brass half cylinders to the test section tubes. This research was performed at evaporation temperatures of −15°C and −30°C, mass fluxes of 200kg∕m2s and 400kg∕m2s, and heat flux from 5kW∕m2 to 15kW∕m2 for vapor qualities ranging from 0.1 to 0.8. The CO2 heat transfer coefficients indicated the nucleate boiling dominant heat transfer characteristics such as the strong dependence on heat fluxes at a mass flux of 200kg∕m2s. However, enhanced convective boiling contribution was observed at 400kg∕m2s. Surface conditions for two different tubes were investigated with a profilometer, atomic force microscope, and scanning electron microscope images, and their possible effects on heat transfer are discussed. Pressure drop, measured at adiabatic conditions, increased with the increase of mass flux and quality, and with the decrease of evaporation temperature. The measured heat transfer coefficients and pressure drop were compared with general correlations. Some of these correlations showed relatively good agreements with measured values. Visualized flow patterns were compared with two flow pattern maps and the comparison showed that the flow pattern maps need improvement in the transition regions from intermittent to annular flow.

Evaporative Heat Transfer of CO2 in a Smooth and a Micro-Grooved Miniature Channel Tube

Grooved surfaces are widely used to enhance heat transfer. In this study, micro-grooves are applied to miniature channels to improve the evaporative heat transfer of CO2. To evaluate the effect of micro-grooves in miniature channels, heat transfer characteristics are investigated for the smooth multichannel tube with 0.8 mm channel diameter and the grooved multi-channel tube with the same diameter and 8 micro-grooves. The tests were carried out at the evaporation temperatures of 0, 5 and 10°C; mass flux between 400 and 800 kg/m2s; and heat flux of 12–18 kW/m2. At lower qualities (less than about 0.3), heat transfer was considerably enhanced by the grooves, however, the micro-grooves showed negative effect in heat transfer at higher qualities. A significant heat transfer enhancement was obtained at high evaporating temperature. Also, heat transfer was enhanced at lower heat and mass flux. The evaporative heat transfer coefficients measured were compared with several correlations. A new correlation was developed to predict the dry-out quality in miniature channel tubes more accurately.

Flow pattern and boiling heat transfer of CO 2 in horizontal small-bore tubes

Increasing attention has been focused on carbon dioxide (CO 2) heat pump system where the temperature level is rather low, while the operating pressure is rather high. In this system, the density difference between vapor and liquid becomes rather small, which significantly affects flow patterns. Low surface tension and latent heat also have significant influence on two-phase flow patterns and heat transfer. This paper describes experimental and numerical investigation on flow patterns and heat transfer characteristics of boiling flow CO 2 at high pressure in horizontal small-bore tubes ranging from 1.0 mm to 3.0 mm I.D. Even though the density difference is rather small at high pressure, phase stratification takes place, which leads to the intermittent dryout at the upper wall. So far developed discrete bubble model by the authors for vertical flows is modified so as to include horizontal flow mechanisms. The predicted flow patterns with this new model agree on the whole with the experimental observation.

Carbon dioxide degassing and thermal energy release in the Monte Amiata volcanic-geothermal area (Italy)

Abstract The quaternary volcanic complex of Mount Amiata is located in southern Tuscany (Italy) and represents the most recent manifestation of the Tuscan Magmatic Province. The region is characterised by a large thermal anomaly and by the presence of numerous CO2-rich gas emissions and geothermal features, mainly located at the periphery of the volcanic complex. Two geothermal systems are located, at increasing depths, in the carbonate and metamorphic formations beneath the volcanic complex. The shallow volcanic aquifer is separated from the deep geothermal systems by a low permeability unit (Ligurian Unit). A measured CO2 discharge through soils of 1.8 × 109 mol a−1 shows that large amounts of CO2 move from the deep reservoir to the surface. A large range in δ13CTDIC (−21.07 to +3.65) characterises the waters circulating in the aquifers of the region and the mass and isotopic balance of TDIC allows distinguishing a discharge of 0.3 × 109 mol a−1 of deeply sourced CO2 in spring waters. The total natural CO2 discharge (2.1 × 109 mol a−1) is slightly less than minimum CO2 output estimated by an indirect method (2.8 × 109 mol a−1), but present-day release of 5.8 × 109 mol a−1 CO2 from deep geothermal wells may have reduced natural CO2 discharge. The heat transported by groundwater, computed considering the increase in temperature from the infiltration area to the discharge from springs, is of the same order of magnitude, or higher, than the regional conductive heat flow (>200 mW m−2) and reaches extremely high values (up to 2700 mW m−2) in the north-eastern part of the study area. Heat transfer occurs mainly by conductive heating in the volcanic aquifer and by uprising gas and vapor along fault zones and in those areas where low permeability cover is lacking. The comparison of CO2 flux, heat flow and geological setting shows that near surface geology and hydrogeological setting play a central role in determining CO2 degassing and heat transfer patterns.

F212 CO_2/DME混合冷媒の水平蒸発管内熱伝達に関する実験的研究(次世代冷凍空調技術に関わる伝熱過程I)

F212 CO_2/DME混合冷媒の水平蒸発管内熱伝達に関する実験的研究(次世代冷凍空調技術に関わる伝熱過程I)

Experimental Investigation on Flow Boiling Heat Transfer of CO2 at Low Temperatures

There is a growing use of CO2 refrigeration to achieve low temperatures, particularly in the food industry; however, very limited information is available in the open literature on its boiling heat transfer characteristics below –30°C. This paper investigates experimentally the flow boiling heat transfer of CO2 at low temperatures down to –40°C. The experimental data were collected from a novel experimental rig, specifically designed to achieve low temperatures, using a 4.5 m long horizontal stainless steel tube of 4.57 mm inner diameter. The effects of heat and mass fluxes and saturation temperature on the flow boiling heat transfer coefficient are also analyzed. Furthermore, this paper highlights the limitations of existing empirical correlations by comparing their predictions with the experimental boiling heat transfer coefficients. 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.

Semi-Empirical Correlation of Gas Cooling Heat Transfer of Supercritical Carbon Dioxide in Microchannels

This paper provides a comprehensive review of existing correlations for supercritical heat transfer of CO2 in microchannels, as well as a comparison of these correlations with experimentally measured data. Based on the experimental data, a new semi-empirical correlation is developed to predict the gas cooling heat transfer coefficient of supercritical CO2 in microchannels, within an error of 15% for most (91%) of the presented experimental data that were obtained in an 11-port microchannel tube with an internal diameter of 0.79 mm and with a pressure range of 8 to 10 MPa and mass flux range of 300 to 1200 kg/m2s.

Critical review of flow boiling heat transfer of CO 2–lubricant mixtures

This paper presents a comprehensive review on the flow boiling heat transfer of CO 2–lubricant mixtures. Some of the immiscible lubricants in CO 2 include alkyl naphthalene/alkylbenzne (AN/AB) and polyalphaolefin (PAO), while polyalkylene glycol (PAG) is partially miscible, and polyol ester (POE) is completely miscible. The effect of oil concentration, vapour quality, heat and mass fluxes and saturation temperature is addressed. One database has been created by collecting the experimental data from the open literature on the flow boiling heat transfer of CO 2–lubricant mixtures, along with empirical correlations. A simple simulation model has been developed in EES software package to compare the empirical correlations with the CO 2–lubricant mixtures experimental database. Most empirical correlations fail to predict the flow boiling heat transfer coefficient in good agreement with the experimental data. Hence, further research is needed to develop appropriate correlations for the flow boiling heat transfer of CO 2–lubricant mixtures.

Experimental and numerical study of convection heat transfer of CO 2 at supercritical pressures in vertical porous tubes

Convection heat transfer of CO 2 at supercritical pressures in vertical sintered porous tubes with particle diameters of 0.1–0.12 mm and 0.2–0.28 mm was investigated experimentally and numerically. The study investigated the influence of the inlet fluid temperature, mass flow rate, pressure, particle diameter, heat flux, flow direction and buoyancy on convection heat transfer in porous tubes. The results show that the inlet temperature, pressure, mass flow rate, particle diameter and heat flux all strongly influence the convection heat transfer at supercritical pressures. When the inlet temperature is much larger than the pseudocritical temperature, T pc, the local heat transfer coefficients in porous tubes are much less than when the inlet temperature is less than T pc. For T 0 < T pc and wall temperatures not much larger than T pc, the local heat transfer coefficients have a maximum for both upward flow and downward flow along the porous tube when the fluid bulk temperatures are near T pc. Buoyancy caused the different variations in the local heat transfer coefficients along the porous tube for upward and downward flows. The results also show that the heat transfer coefficients increase as the particle diameter decrease. The numerical simulations were performed using the local thermal equilibrium model with consideration of the effects of variable porosity, thermal dispersion and area-of-contact stagnant effective thermal conductivity. The experimental and numerical results for the friction factor of CO 2 at supercritical pressures flowing in sintered porous tubes at constant temperature (without heating) corresponded very well with the known correlation. However, the predicted results for the friction factor of CO 2 at supercritical pressures flowing in the heated sintered porous tube differ from those measured in the experiments both for upward and downward flows. The calculated wall temperatures corresponded well with the measured wall temperatures.

Convection heat transfer of CO 2 at supercritical pressures in a vertical mini tube at relatively low reynolds numbers

Convection heat transfer of CO 2 at supercritical pressures in a 0.27 mm diameter vertical mini tube was investigated experimentally and numerically for upward and downward flows at relatively low inlet Reynolds numbers (2900 and 1900). The effects of inlet temperature, pressure, mass flow rate, heat flux, flow direction, buoyancy and flow acceleration on the convection heat transfer were investigated. For inlet Reynolds numbers less than 2.9 × 10 3, the local wall temperature varies non-linearly for both flow directions at high heat fluxes (113 kW/m 2). For the mini tube used in the current study, the buoyancy effect is normally low even when the heating is relatively strong, while the flow acceleration due to heating can strongly influence the turbulence and reduce the heat transfer for high heat fluxes. For relatively low Reynolds numbers ( Re in ⩽ 2.9 × 10 3) and the low heat flux (30.0 kW/m 2) the predicted values using the LB low Reynolds number correspond well with the measured data. However, for the high heat flux (113 kW/m 2), the predicted values do not correspond well with the measured data due to the influence of the flow acceleration on the turbulence.

Experimental and numerical investigation of convection heat transfer of CO 2 at supercritical pressures in a vertical mini-tube

Convection heat transfer of CO 2 at supercritical pressures in a 0.27 mm diameter vertical mini-tube was investigated experimentally and numerically for inlet Reynolds numbers exceeding 4.0 × 10 3. The tests investigated the effects of heat flux, flow direction, buoyancy and flow acceleration on the convection heat transfer. The experimental results indicate that the flow direction, buoyancy and flow acceleration have little influence on the local wall temperature, with no deterioration of the convection heat transfer observed in either flow direction for the studied conditions. The heat transfer coefficient initially increases with increasing heat flux and then decreases with further increases in the heat flux for both upward and downward flows. These phenomena are due to the variation of the thermophysical properties, especially c p . The numerical results correspond well with the experimental data using several turbulence models, especially the Realizable k– ε turbulence model.

경사진 헬리컬 코일 열교환기의 열전달 특성에 관한 연구

The heat transfer coefficient and pressure drop during gas cooling process of CO₂ (R-744) in inclined helical coil copper tubes were investigated experimentally. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and a inclined helical coil type gas cooler (test section). The test section consists of a smooth copper tube of 2.45 mm inner diameter. The refrigerant mass fluxes were varied from 200 to 600 [㎏/㎡s] and the inlet pressures of gas cooler were 7.5 to 10.0 [㎫]. The heat transfer coefficients of CO₂ in the inclined helical coil tubes increases with the increase of mass flux and gas cooling pressure of CO₂. The pressure drop of CO₂ in the gas cooler shows a relatively good agreement with those predicted by Ito’s correlation developed for single-phase in a helical coil tube. The local heat transfer coefficient of CO₂ agrees well with the correlation by Pitla et al. However, at the region near pseudo-critical temperature, the experiments indicate higher values than the Pitla et al. correlation. Therefore, various experiments in the inclined helical coil tubes have to be conducted and it is necessary to develop the reliable and accurate prediction determining the heat transfer and pressure drop of CO₂ in the inclined helical coil tubes.

Experimental and numerical investigation of convection heat transfer of CO 2 at supercritical pressures in a vertical tube at low Reynolds numbers

Convection heat transfer of supercritical pressure CO 2 in a vertical tube at low Reynolds numbers (less than 2500) was investigated experimentally and numerically. The tests investigated the effects of inlet temperature, pressure, mass flow rate, heat flux, buoyancy and flow direction on the heat transfer. For the lower heat flux of 4.49 kW/m 2, the numerical results corresponded well to the experimental data (within 8%). However, for heat fluxes higher than 13.7 kW/m 2, the numerical results for the convection heat transfer coefficient using laminar flow were much less than the experimental data over most of the tube (e.g., difference larger than 74% for x / d = 15 ) due to the strong influence of buoyancy and the decrease of the dynamic viscosity with temperature along the tube which results in an early transition from laminar to turbulent flow. The numerical results for the convection heat transfer coefficient using turbulent flow corresponded much better with the experimental data. The convection heat transfer coefficient increases with increasing heat flux and then decreases with further increases in the heat flux for both upward and downward flows. The flow direction significantly influences the heat transfer. For high heat flux (e.g., 61.0–94.0 kW/m 2) upward flow, the local wall temperature varies in a complex, nonlinear fashion, while for downward flow the local wall temperature increases monotonically and the heat transfer is strongly enhanced by buoyancy.

On CO2 fluid flow and heat transfer behavior in the subsurface, following leakage from a geologic storage reservoir

Geologic storage of CO2 is expected to produce plumes of large areal extent, and some leakage may occur along fractures, fault zones, or improperly plugged pre-existing wellbores. A review of physical and chemical processes accompanying leakage suggests a potential for self-enhancement. The numerical simulations presented here confirm this expectation, but reveal self-limiting features as well. It seems unlikely that CO2 leakage could trigger a high-energy run-away discharge, a so-called “pneumatic eruption,” but present understanding is insufficient to rule out this possibility. The most promising avenue for increasing understanding of CO2 leakage behavior is the study of natural analogues.

Flow boiling heat transfer characteristics of carbon dioxide in a horizontal tube

The flow boiling heat transfer coefficient of CO 2 (R-744) in a horizontal tube was investigated experimentally. The test results showed the boiling heat transfer of CO 2 has great effect on more nucleate boiling than convective boiling. The boiling heat transfer coefficients of CO 2 are highly dependent on the vapor quality, heat flux and saturation temperature. The boiling heat transfer coefficient of CO 2 is about 87.2% and 93% higher than that of R-22 and R-134a. The boiling pressure drop of CO 2 increases with increasing mass flux, and decreases with increasing saturation temperature. The pressure drop of CO 2 is much lower than that of R-22. The boiling pressure drop of CO 2 is about 10–15% of R-22. The lower pressure drop of CO 2 is attributed to its unique thermal properties. In comparison with test results and existing correlations, the best fit of the present experimental data for heat transfer is obtained with Jung et al. correlation, and pressure drop is obtained with Choi et al. correlation. But most of the existing correlations failed to predict the boiling heat transfer coefficient and pressure drop of CO 2.