Convective gas cooling heat transfer and pressure drop characteristics of supercritical CO2/oil mixture in a minichannel tube

The effects of PAG oil concentration on the convective gas cooling heat transfer and the pressure drop characteristics of supercritical CO₂/oil mixture in minichannel tube were investigated. The test results showed that the average gas cooling heat transfer coefficient was decreased by 20.4% and the average pressure drop was increased by 4.8 times when the oil concentration was increased from 0 to 4 wt.%. The effects of the oil concentration on the convective gas cooling heat transfers and the pressure drops of the supercritical CO₂/oil mixture in minichannel tubes were experimentally confirmed to be significant.     

Heat transfer and flow characteristics of flow boiling of CO2‐oil mixtures in horizontal smooth and micro‐fin tubes

Experiments were carried out on the flow pattern, heat transfer, and pressure drop of flow boiling of pure CO₂ and CO₂‐oil mixtures in horizontal smooth and micro‐fin tubes. The smooth tube is a stainless steel tube with an inner diameter of 3.76 mm. The micro‐fin tube is a copper tube with a mean inner diameter of 3.75 mm. The experiments were carried out at mass velocities from 100 to 500 kg/(m²·s), saturation temperature of 10 °C, and the circulation ratio of lubricating oil (PAG) was from 0 to 1.0 mass%. Flow pattern observations mainly showed slug and wavy flow for the smooth tube, but annular flow for the micro‐fin tube. Compared with the flow patterns in the case of pure CO₂, an increase in frequency of slug occurrence in the slug flow region, and a decrease in the quantity of liquid at the top of the tube in the annular flow region were observed in the case of CO₂‐oil mixtures. With pure CO₂, the flow boiling heat transfer was dominated by nucleate boiling in the low vapor quality region, and the heat transfer coefficients for the micro‐fin tube were higher than those of the smooth tube. With CO₂‐oil mixtures, the flow boiling heat transfer was dominated by convective evaporation, especially in the high vapor quality region. In addition, the heat transfer coefficient decreased significantly when the oil circulation ratio was larger than 0.1 mass%. For the pressure drop characteristics, in the case of pure CO₂, the homogeneous flow model agreed with the experimental results within ±30% for the smooth tube. The pressure drops of the micro‐fin tube were 0–70% higher than those predicted with the homogeneous flow model, and the pressure drops increased for the high oil circulation ratio and high vapor quality conditions. The increases in the pressure drops were considered to be due to the increase in the thickness of the oil film and the decrease in the effective flow cross‐sectional area. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20287     

Steam condensing – liquid CO2 boiling heat transfer in a steam condenser for a new heat recovery system

In a new waste heat recovery system, waste heat is recovered from steam condensers through cooling by liquid CO₂ instead of seawater, taking advantage of effective boiling heat transfer performance; the heat is subsequently used for local heat supply. The steam condensing – liquid CO₂ boiling heat transfer performance in a steam condenser with a shell and a helical coil non-fin tube was studied both numerically and experimentally. A heat transfer numerical model was constructed from two models developed for steam condensation and for liquid CO₂ boiling. Experiments were performed to verify the model at a steam pressure range of 3.2–5 kPa and a CO₂ saturation pressure range of 5–6 MPa. Overall heat transfer coefficients obtained from the numerical model agree with the experimental data within ±5%. The numerical estimations show that the boiling local heat transfer coefficient reaches a maximum value of 26 kW/m² K. This value is almost one order higher than that of a conventional water-cooled condenser.     

Development of a new forced convection heat transfer correlation for CO2 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/m² s and 762 kg/m² s, inlet temperatures from 10 °C to 90 °C and the average heat flux was 30 kW/m². Beyond reproducing the regular experimental cases, numerical modeling also implemented higher heat fluxes of 60 kW/m² and 90 kW/m² 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 CO₂ 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%.     

小型CO_2热泵系统用气体冷却器仿真研究

CO_2气体冷却器的结构和换热效果对CO_2跨临界循环影响较大.为设计出高效的气体冷却器,有必要对其性能进行模拟和优化.采用有限单元法建立了小型CO_2热泵热水器中气体冷却器稳态分布参数模型,分别对其CO_2侧和水侧的流动与换热进行了数值仿真,运用该模型分别针对CO_2侧进口压力对气体冷却器设计管长和CO_2换热性能的影响进行了分析.结果表明,CO_2侧进口压力在8~12 MPa时,从8 MPa开始每递增1 MPa,换热系数峰值比压力增加1 MPa前的依次递减约57.14%、33.33%、25.00%、9.83%,设计管长比压力增加1 MPa前的依次递减约55.60%、18.75%、11.33%、9.09%.综合考虑管道耗材与CO_2换热能力,针对小型CO_2热泵系统,气体冷却器CO_2侧进口压力取8.5~10 MPa较合理.研究可为气体冷却器设计提供理论指导.     

An experimental investigation on characteristics of supercritical CO2‐based solar Rankine system

In this paper, the characteristics of the supercritical CO₂‐based Rankine cycle powered by solar energy are experimentally investigated. In order to study the controlling factors of system performances, in the experimental setups an electrical heating section as well as an evacuated solar collect is utilized. Also, corresponding heat transfer measurements of the supercritical CO₂ fluid in the heating section are conducted. Results show that the collecting efficiency will increase with the CO₂ mass flow rate. The increase in solar radiation and the decrease in condensation temperature in the cycle both can lead to the increase in CO₂ mass flow. It is also found that the CO₂ fluid flow in the heating section is not fully developed and the Local Nusselt number decreases along the flow direction of the testing pipe. The influences of pressure as well as other controlling factors on heat transfer are also analyzed in detail in this paper. Copyright © 2010 John Wiley & Sons, Ltd.     

Measurement and correlation of frictional two-phase pressure drop of R410A/POE oil mixture flow boiling in a 7 mm straight micro-fin tube

Two-phase frictional pressure drop characteristics of R410A/POE oil mixture flow boiling inside a straight micro-fin tube with the outside diameter of 7.0 mm were investigated experimentally. Experimental parameters include the evaporation temperature of 5 °C, the mass flux from 200 to 400 kg/(m² s), the heat flux from 7.56 to 15.12 kW/m², the inlet vapor quality from 0.2 to 0.7, and nominal oil concentration from 0% to 5%. The test results show that frictional pressure drop of R410A/POE oil mixture increases with the mass flux, the presence of oil enhances two-phase frictional pressure drop, and the effect of oil on frictional pressure drop is more evident at higher vapor qualities where the local oil concentrations are higher. New correlations to predict the local frictional pressure drop of R410A/POE oil mixture flow boiling inside the straight micro-fin tube are developed based on local properties of refrigerant–oil mixture, and the measured local frictional pressure drop is well correlated with the empirical correlations proposed by the authors.     

A first-principles simulation model for the thermo-hydraulic performance of fan supplied tube-fin heat exchangers

This paper outlines a novel first-principles mathematical model to simulate the thermo-hydraulic behavior of compact fan-supplied tube-fin heat exchangers for light commercial refrigeration applications, i.e., with heat duties ranging from 0.5 to 2.0 kW. The model is based on the mass, momentum and energy conservation equations applied to both the refrigerant and air streams. The model predictions were compared with experimental data taken at several operating and geometric conditions. It was found that the model predictions for the air-side heat transfer and pressure drop were very close to the experimental data with maximum deviations of ±10% and ±15%, respectively. The model was employed to assess the thermal-hydraulic performance of a gas cooler running with supercritical CO₂ as working fluid.     

Application of artificial neural network method for performance prediction of a gas cooler in a CO2 heat pump

The objective of this work is to train an artificial neural network (ANN) to predict the performance of gas cooler in carbon dioxide transcritical air-conditioning system. The designed ANN was trained by performance test data under varying conditions. The deviations between the ANN predicted and measured data are basically less than ±5%. The well-trained ANN is then used to predict the effects of the five input parameters individually. The predicted results show that for the heat transfer and CO₂ pressure drop the most effective factor is the inlet air velocity, then come the inlet CO₂ pressure and temperature. The inlet mass flow rate can enhance heat transfer with a much larger CO₂ pressure drop penalty. The most unfavorable factor is the increase in the inlet air temperature, leading to the deterioration of heat transfer and severely increase in CO₂ pressure drop.     

Cooling of microprocessors using flow boiling of CO2 in a micro-evaporator: Preliminary analysis and performance comparison

The flow pattern based flow boiling heat transfer and two-phase pressure drop models for CO₂, recently developed by Cheng et al. [L. Cheng, G. Ribatski, J. Moreno Quibén, J.R. Thome, New prediction methods for CO₂ evaporation inside tubes: Part I – A two-phase flow pattern map and a flow pattern based phenomenological model for two-phase flow frictional pressure drops, Int. J. Heat Mass transfer 51 (2008) 111–124; L. Cheng, G. Ribatski, J.R. Thome, New prediction methods for CO₂ evaporation inside tubes: Part II – An updated general flow boiling heat transfer model based on flow patterns, Int. J. Heat Mass transfer 51 (2008) 125–135], have been used to predict the thermal performance of CO₂ in a silicon multi-microchannel evaporator (67 parallel channels with a width of 0.223 mm, a height of 0.68 mm and a length of 20 mm) for cooling of a microprocessor. First, some simulation results of CO₂ flow boiling heat transfer and two-phase pressure drops in microscale channels are presented. The effects of channel diameter, mass flux, saturation temperature and heat flux on flow boiling heat transfer coefficients and two-phase pressure drops are next addressed. Then, simulations of the base temperatures of the silicon multi-microchannel evaporator using R236fa and CO₂ were performed for the following conditions: base heat fluxes from 20 to 100 W/cm², a mass flux of 987.6 kg/m²s and a saturation temperature of 25 °C. These show that the base temperatures using CO₂ are much lower than those using R236fa. Compared to R236fa, CO₂ has much higher heat transfer coefficients and lower pressure drops in the multi-microchannel evaporator. However, the operation pressure of CO₂ is much higher than that of R236fa. Based on the analysis and comparison, CO₂ appears to be a promising coolant for microprocessors at low operating temperatures but also presents a great technological challenge like other new cooling technologies.     

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 6.1 mm ID under almost uniform temperature cooling at reduced pressures from 0.55 to 0.95, heat fluxes from 3 to 20 kW m^?2, and superheats from 0 to 40 K. 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.     

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%.     

Natural convective flow and heat transfer of supercritical CO2 in a rectangular circulation loop

Fluid dynamics and heat transfer of supercritical CO₂ natural convection are important for nuclear engineering and new energy system design etc. In this paper, in order to study the flow and heat transfer behavior of supercritical CO₂ natural circulation system, a computational simulation on a closed natural circulation loop (NCL) model has been carried out. The fluid temperature in the loop varies between 298.15 K and 323.15 K, which is across the CO₂ critical temperature, and the density is found to be in the range of 250–800 kg/m³. The results show a small temperature difference of 25 °C between heating and cooling sources can induce a mass flow with the Reynolds number up to 6 × 10⁴ using supercritical CO₂ fluid. A periodic reversal flow pattern is found and presented in this paper. Enhanced heat transfer phenomenon is also found for the supercritical CO₂ natural convective flow. The mechanisms to this enhancement and the heating effect on the flow are also discussed in detail in the present study.     

WSGG models for radiative heat transfer calculations in hydrogen and hydrogen-mixture flames at various pressures

Radiative heat transfer strongly influences pollutant emission prediction in combustion systems. In this work, the weighted sum of gray gas (WSGG) models have been developed for calculating radiative heat transfer in hydrogen and hydrogen-mixture flames. The total pressure effect on cut-off width of the Lorentz line profile is analyzed and properly considered in the line by line (LBL) calculations. Based on the LBL benchmark results, two sets of WSGG model correlations have been proposed for H₂O and its mixture with CO₂ at a molar ratio (Mr) of 3, representing the typical combustion products of the hydrogen and a hydrogen-rich mixture (e.g., 50% hydrogen and 50% methane). The WSGG models are applicable and accurate with a total pressure ranging from 1 to 60 atm. Partial pressure is explicitly applied as an independent variable in the model coefficients to account for its nonlinear effect on gas emissivity, which is particularly important for a participating gas medium with a large amount of H₂O at a total pressure below 5 atm. Detailed studies are carried out to solve radiative heat transfer in non-isothermal and non-homogeneous gas media at different conditions. Results show improvement over the existing WSGG models at the atmospheric pressure and have good agreement with LBL solutions under various conditions.     

Experimental and numerical study of convection heat transfer of CO2 at super-critical pressures during cooling in small vertical tube

Convection heat transfer of CO₂ 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, CO₂ mass flow rate, CO₂ 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 CO₂ 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 CO₂ 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.     

Near-Critical Natural Circulation Flows Inside an Experimental Loop: Stability Map and Heat Transfer

The near-critical CO₂-based natural circulation loop (NCL, or thermosyphon) has been proposed in many energy conversion systems, such as the solar heater, waste heat recovery, next-generation nuclear cooling, and so on. There is an increasing need to obtain detailed information about such systems, as it is less verified from a basic system operation viewpoint. This paper presents an experimental investigation of a near-critical CO₂ thermosyphon. The closed thermosyphon is specially designed for high-pressure (in the critical region, from 6.0 MPa to 15.0 MPa), natural circulation flows. The basic transient flow behaviors and parameter behaviors are found to be dependent on initial pressure. The system stability evolution from subcritical oscillating flow to supercritical stable operations is presented. From the experimental data analysis, the stability map for the current supercritical natural circulation loop system is given. It is found that the stability pressure lines will divide the operation into stable, transition, and unstable regions. It is found that the effectiveness of the cooler will greatly affect the system stability, while the heat transfer efficiency is mainly controlled by the heater conditions. Parameter evolutions of the fluid temperature, mass flow rate, and loop pressure are presented in this paper. The heat transfer dependency on operation pressure and evolution mechanisms are also discussed in detail in this paper.     

Fluid-to-fluid modeling of supercritical water loops for stability analysis

The use of supercritical water as coolant/moderator may induce oscillations in the supercritical light water reactor similar to the density wave oscillations observed in boiling water reactors (BWRs). In order to experimentally investigate the stability of supercritical reactors, a fluid-to-fluid downscaled facility is proposed. It is found that with an appropriate mixture of refrigerants R-125 and R-32, the dimensionless enthalpy and density of the supercritical water can be accurately matched for all relevant operational conditions of the reactor. Moreover, the inertia distribution, the friction factor distribution and the heat transfer mechanism are taken into account in the modeling. As a result of the proposed downscaling, the operational pressure, temperature and power are considerably smaller than those of a water-based system, which in turn helps reducing the construction and operational costs of a test facility. Finally, it is found that the often used modeling fluid supercritical CO₂ cannot accurately represent supercritical water at reactor conditions.     

Development and validation of helical‐coil‐in‐fluted‐tube gas cooler for CO2 heat pump water heaters

A segmented approach [1] for the CO₂ helical‐coil‐in‐fluted‐tube gas cooler is developed. The CO₂ helical‐coil‐in‐fluted‐tube gas cooler consists of helically coiled tube and fluted tube. It is fabricated by twisting a straight copper tube to form helically coiled tube and embedded in the groove of the fluted tube. The available heat transfer and pressure drop correlations for the supercritical CO₂‐side and water‐side are provided to simulate the gas cooler. The simulation is compared with a detailed set of experimental data, for given the inlet conditions. The predicted data matches well with the experimental data with absolute average deviations of 1.15, 4.6 and 4.7% for the CO₂ pressure drop, gas cooler exit temperature and hot water temperature, respectively. Based on the good matches between measured data and predicted data, the detailed thermodynamic processes of gas cooler parameters are predicted and analyzed. Furthermore, different arrangements of the gas cooler within the original package dimensions are simulated and better performance of the gas cooler is obtained under the structural parameters of the 3‐row fluted tube with the inner diameter 12 mm. Copyright © 2010 John Wiley & Sons, Ltd.     

New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside horizontal tubes

A new flow boiling heat transfer model and a new flow pattern map based on the flow boiling heat transfer mechanisms for horizontal tubes have been developed specifically for CO₂. Firstly, a nucleate boiling heat transfer correlation incorporating the effects of reduced pressure and heat flux at low vapor qualities has been proposed for CO₂. Secondly, a nucleate boiling heat transfer suppression factor correlation incorporating liquid film thickness and tube diameters has been proposed based on the flow boiling heat transfer mechanisms so as to capture the trends in the flow boiling heat transfer data. In addition, a dryout inception correlation has been developed. Accordingly, the heat transfer correlation in the dryout region has been modified. In the new flow pattern map, an intermittent flow to annular flow transition criterion and an annular flow to dryout region transition criterion have been proposed based on the changes in the flow boiling heat transfer trends. The flow boiling heat transfer model predicts 75.5% of all the CO₂ database within ±30%. The flow boiling heat transfer model and the flow pattern map are applicable to a wide range of conditions: tube diameters (equivalent diameters for non-circular channels) from 0.8 to 10 mm, mass velocities from 170 to 570 kg/m² s, heat fluxes from 5 to 32 kW/m² and saturation temperatures from −28 to 25 °C (reduced pressures from 0.21 to 0.87).     

Exergy assessment of an optimized capillary tube‐based transcritical CO2 heat pump system

A capillary tube‐based CO₂ heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. A comparative study for various operating conditions, based on system COP and exergetic efficiency, of a capillary tube and a controllable expansion valve‐based transcritical carbon dioxide heat pump systems for simultaneous heating and cooling at 73 and 4°C, respectively, is presented here. Two optimized capillary tubes having diameter of 1.5 and 1.6 mm are compared with an equivalent controllable throttle valve. Heat transfer and fluid flow effects are included in the gas cooler and evaporator model and capillary tube employs the homogeneous flow model to simulate two‐phase flow. Subcritical and supercritical thermodynamic and transport properties of CO₂ are calculated employing a precision in‐house property code. Optimization of effective distribution of total heat exchanger area ratio between gas cooler and evaporator is investigated. The exergetic efficiency is better in case of the capillary tube than that of a controllable throttle valve‐based system. Capillary tube‐based system is shown to be quite flexible regarding changes in ambient temperature, almost behaving to offer an optimal pressure control just like the controllable expansion valve yielding both, maximum system COP and maximum exergetic efficiency. Relatively at a smaller diameter, the capillary tube exhibits better exergetic efficiency. Capillary tube length is the critical parameter that influences system optimum conditions. The exergy flow diagram exhibits that compressor, gas cooler and capillary tube contribute a larger share, in that order, to system irreversibility. It is fairly established in this study that a capillary tube can be a good engineering option for small capacity systems in lieu of an expansion valve, which has been thought of as the only possible solution to attain the pressure optimization, an important feature of all transcritical CO₂ systems. Copyright © 2009 John Wiley & Sons, Ltd.