Heat Transfer Characteristics Of CO Research Articles

Experimental study on supercritical heat transfer characteristics of CO2/R41 mixture in microchannel

• Supercritical convective heat transfer characteristics of CO 2 /R41 blends in square porous flat tube microchannel were studied. • The peak values of heat transfer coefficient were analyzed. • Experimental data were compared with the heat transfer correlations. Past researches have shown that CO 2 /HFC mixtures and microchannel heat exchanger could benefit solving the problems of high operating pressure and relatively low cycle efficiency. In this paper, the supercritical heat transfer characteristics of R41, CO 2 /R41 (20.5/79.5) and CO 2 /R41 (51.4/48.6) in square porous flat tube microchannel with the hydraulic diameter of 0.715 mm were experimentally studied. The experimental conditions were as follows: the operating pressure of 6.5–7.5 MPa, the mass flow rate of 150–300 kg/m 2 s, the heat flux of 3–9 kW/m 2 , the fluid temperature of 20–90 °C. The results show that, similar to pure CO 2 , heat transfer coefficient (HTC) of mixtures also appears a peak value near the pseudocritical region. At higher operating pressure, the peak value is less and the change of HTC is gentle. Meanwhile, the inflection point temperature is high. When the bulk temperature is less than the inflection point temperature, HTC is higher at low operating pressure. When the bulk temperature is greater than the inflection point temperature, HTC is higher at high operating pressure. Peak value of HTC decreases and the inflection point temperature of mixtures becomes higher when the CO 2 mass fraction is low. With the increase of R41 mass fraction, the change of HTC is gentle. The width of the peak distance in the pseudocritical region becomes wider. The model of Petrov and Popov is recommended as the prediction model for CO 2 /R41 mixtures. This research can serve as a database for the design of CO 2 /R41 mixtures microchannel heat exchangers.

Heat-transfer characteristics of CO[formula omitted] boiling flow in the regenerative cooling channel of an Mg/CO[formula omitted] powder rocket engine for Mars missions

To solve the problem of thermal protection posed by the long working time of Mg/CO2 powder rocket engines, an experimental system is designed to study the heat-transfer characteristics of liquid CO2 in the regenerative cooling channel. How the heat flux, coolant temperature, mass flux, and back pressure influence the heat-transfer coefficient, the thermodynamic quality, and the gas-phase formation point of nearly saturated CO2 is studied. The heat-transfer coefficient is found to increase with increasing mass flux or decreasing inlet temperature. Higher heat flux increases the rate of bubble formation and promotes nucleate-boiling heat transfer, but it also intensifies the film boiling, thereby degrading the heat-transfer performance. Lower back pressure does not affect the heat-transfer performance in the nucleate-boiling region, but it worsens it in the film-boiling region. It is also found that the classical boiling-heat-transfer models do not predict the experimental heat-transfer coefficient well. Combined with experimental data, how reduced pressure affects heat transfer is considered, and a new empirical correlation formula is proposed for the heat-transfer coefficient of CO2 near saturation. The optimized model can successfully predict 90.23% of the experimental data, and the prediction accuracy is improved greatly. The present research provides a theoretical basis for designing a regenerative cooling scheme for an Mg/CO2 powder rocket engine.

Heat Transfer Characteristics of CO2 Desorption from N‐Methyldiethanolamine Solution in a Microchannel Reactor

AbstractThe heat transfer performance and energy consumption of CO2 desorption from rich N‐methyldiethanolamine (MDEA) solution were determined experimentally in a straight microchannel reactor. Nucleate boiling was found to be the dominant heat transfer mechanism in this experiment. The heat transfer coefficients were strongly dependent on the heat flux. The solution flow rate was the most influential factor on the heat flux, followed by desorption temperature, MDEA concentration, and CO2 loading. In addition, an empirical correlation was proposed to predict the experimental heat transfer coefficients.

Numerical Analysis of Flow and Heat Transfer Characteristics of CO2at Vapour and Supercritical Phases in Micro-Channels

Supercritical carbon dioxide (CO 2 ) has special thermal properties with better heat transfer and flow characteristics. Due to this reason, supercritical CO 2 is being used recently in air-condition and refrigeration systems to replace non environmental friendly refrigerants. Even though many researches have been done, there are not many literatures for heat transfer and flow characteristics of supercritical CO 2 . Therefore, the main purpose of this study is to develop flow and heat transfer CFD models on two different phases; vapour and supercritical of CO 2 to investigate the heat transfer characteristics and pressure drop in micro-channels. CO 2 is considered to be in different phases with different flow pressures but at same temperature. For the simulation, the CO 2 flow was assumed to be turbulent, nonisothermal and Newtonian. The numerical results for both phases are compared. From the numerical analysis, for both vapour and supercritical phases, the heat energy from CO 2 gas transferred to water to attain thermal equilibrium. The temperature of CO 2 at vapour phase decreased 1.78% compared to supercritical phase, which decreased for 0.56% from the inlet temperature. There was a drastic increase of 72% for average Nu when the phase changed from vapour to supercritical. The average Nu decreased rapidly about 41% after total pressure of 9.0 MPa. Pressure drop (Δ P ) increased together with Reynolds number ( Re ) for vapour and supercritical phases. When the phase changed from vapour to supercritical, Δ P was increased about 26%. The results obtained from this study can provide information for further investigations on supercritical CO 2 .

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.

Flow and convection heat transfer characteristics of CO 2 mixed with lubricating oil at super-critical pressures in small tube during cooling

This paper describes the heat transfer and the pressure drop characteristics of CO 2 mixed with small amounts of compatible lubricating oil at super-critical pressures inside horizontal tubes with inner diameters of 1.98 mm and 4.14 mm during cooling. The heat transfer coefficients and pressure drops were measured. The results show that for the oil-free cases, the correlation proposed by Dang and Hihara accurately predicted the heat transfer coefficients and Petukhov’s correlation was found to predict the frictional pressure drops reasonably. Entrainment of the lubricating oil reduced the heat transfer coefficients and increased the pressure drops. Analysis showed that the heat transfer coefficients of the CO 2/oil mixture are strongly related to the density and viscosity ratios of the oil to the CO 2. An empirical correlation was developed based on the measured data, which predicts most of the experimental data within a deviation of 20%.

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.

이산화탄소/프로판 혼합냉매의 수평평활관 및 마이크로 핀관에서의 증발열전달에 관한 실험적 연구

Abstract Evaporation heat transfer characteristics of CO 2 /propane mixtures in horizontal smooth and micro-fin tubes have been investigated by experiment. The experiments were carried out for several test conditions of mass fluxes, heat fluxes, compositions of CO 2 /propane refrigerant mixtures and tube geometries. Direct heating method was used for supplying heat to the refrigerant where the test tube was uniformly heated by electric current which was applied to the tube wall. Heat transfer coefficient data during evaporation process of CO 2 /propane mixtures were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 mm, respectively. The tests were conducted at mass fluxes of 318 to 997 kg/m 2 s, heat fluxes of 6 to 20 kW/m 2 and for several mixture compositions (100/0, 75/25, 50/50, 25/75, 100/0 by wt% of CO 2 /propane). The differences of heat transfer characteristics between smooth and micro-fin tubes for various compositions of CO 2 /propane refrigerant mixtures and the effect of mass flux, and heat flux on enhancement factor (EF) and penalty factor (PF) were presented.

Flow boiling heat transfer characteristics of CO 2 at low temperatures

This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO 2 at low temperatures (down to −30 °C). Subsequently, the boiling heat transfer of CO 2 at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57 mm. Due to the large surface tension, the boiling heat transfer coefficient of CO 2 is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0 °C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO 2 in good agreement with the experimental data, suggesting the need for further research in this area.

Forced convection heat transfer of supercritical CO 2 in a horizontal circular tube

The problem of low Reynolds number forced convection of supercritical CO 2 in a horizontal circular tube is studied numerically. The interest has been simulated by an experimental study on solar collector using supercritical CO 2 as working fluid, in which high collector efficiency above 70% has been found for the supercritical CO 2-based collector. In the present study, to reproduce the general features exhibited in the experiments, a circular tube having a diameter of 6.0 mm has been investigated at a pressure of 8.0 MPa and an inlet temperature of 32.0 °C. In particular, this work has been focused upon the convective heat transfer characteristics of CO 2 flowing in a horizontal circular tube with small mass flow rates ranging from 0.003 to 0.03 kg/min and constant heat flux from 100.0 to 800.0 W/m 2. The results show that a threefold increase in Nusselt number has been achieved at the typical Reynolds number of 210 in the experimental tests, compared with that of using water as working fluid in the collector. The heat transfer enhancement is also found to increase with Reynolds number Re and heat flux q for ranges of 210 ≤ Re ≤ 1800 and 100 W/m 2 ≤ q ≤ 800 W/m 2. Furthermore, the mechanisms that are responsible for the heat transfer enhancement are identified. The first is due to thinner thermal boundary layer than that of the water case and the second is that the decrease in viscosity and the increase in heat capacity contribute to the heat transfer enhancement. In addition, the flow does not reach a fully developed velocity and temperature field, which may also contribute to the enhancement phenomena.

Convective boiling heat transfer characteristics of CO 2 in microchannels

Convective boiling heat transfer coefficients and dryout phenomena of CO 2 are investigated in rectangular microchannels whose hydraulic diameters range from 1.08 to 1.54 mm. The tests are conducted by varying the mass flux of CO 2 from 200 to 400 kg/m 2 s, heat flux from 10 to 20 kW/m 2, while maintaining saturation temperature at 0, 5 and 10 °C. Test results show that the average heat transfer coefficient of CO 2 is 53% higher than that of R134a. The effects of heat flux on the heat transfer coefficient are much significant than those of mass flux. As the mass flux increases, dryout becomes more pronounced. As the hydraulic diameter decreases from 1.54 to 1.27 mm and from 1.27 to 1.08 mm at a heat flux of 15 kW/m 2 and a mass flux of 300 kg/m 2 s, the heat transfer coefficients increase by 5% and 31%, respectively. Based on the comparison of the data from the existing models with the present data, the Cooper model and the Gorenflo model yield relatively good predictions of the measured data with mean deviations between predicted and measured data of 21.7% and 21.2%, respectively.

The Effect of Oil Contamination on Evaporator Heat Transfer Characteristics of CO2 Refrigeration Cycle

The Effect of Oil Contamination on Evaporator Heat Transfer Characteristics of CO<SUB>2</SUB> Refrigeration Cycle

408 超臨界圧下における二酸化炭素の冷却伝熱特性

In order to clarify the cooling heat transfer characteristics of CO_2 at a supercritical pressure condition, experimental approach was carried out. The experimental conditions were 9.5MPa for the system pressure and 20 to 70 ℃ for the fluid temperatures. Heat transfer coefficients were measured under the temperature range, and then it was found that the maximum value existed at the fluid temperature of about 45℃. Furthermore, CO_2/Oil mixture was also employed as the experimental fluid. the experiments revealed the heat transfer coefficient decreased by existence of lubricant oil. In order to understand the effect of the lubricant oil on the heat transfer coefficient, flow visualization was achieved through the view section made of acrylic resin tube. It was also clarified that the distribution of the oil changed with varying the fluid temperature, in other words, the flow property of CO_2.

CO 2-heat pump water heater: characteristics, system design and experimental results

CO 2 is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Tap water heating is one promising application for a trans-critical CO 2 process. The temperature glide at heat rejection contributes to a very good temperature adaptation when heating tap water, which inherits a large temperature glide. This, together with efficient compression and good heat transfer characteristics of CO 2, makes it possible to design very efficient systems. A heating-COP of 4.3 is achieved for the prototype when heating tap water from 9°C to 60°C, at an evaporation temperature of 0°C. The results lead to a seasonal performance factor of about 4 for an Oslo climate, using ambient air as heat source. Thus, the primary energy consumption can be reduced with more than 75% compared with electrical or gas fired systems. Another significant advantage of this system, compared with conventional heat pump water heaters, is that hot water with temperatures up to 90°C can be produced without operational difficulties.