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.     

Critical review of flow boiling heat transfer of CO2–lubricant mixtures

This paper presents a comprehensive review on the flow boiling heat transfer of CO₂–lubricant mixtures. Some of the immiscible lubricants in CO₂ 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 CO₂–lubricant mixtures, along with empirical correlations. A simple simulation model has been developed in EES software package to compare the empirical correlations with the CO₂–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 CO₂–lubricant mixtures.     

Numerical Investigation of Conjugate Heat Transfer to Supercritical CO2 in a Vertical Tube-in-Tube Heat Exchanger

Conjugate heat transfer to supercritical CO₂ in a vertical tube-in-tube heat exchanger was numerically investigated. The results demonstrate that most models considered are able to reproduce the heat transfer processes qualitatively, and the Abe, Kondoh, and Nagano model shows optimal agreement with the experimental data. The influences of hot fluid mass flux and temperature of the shell side, supercritical fluid mass flux of the tube side, flow direction, and pipe diameter on conjugate heat transfer were investigated based on velocity and turbulence fields. It is concluded that hot fluid mass flux and temperature of the shell side significantly affect heat transfer of the tube side. Mixed convection is the main heat transfer mechanism for the supercritical CO₂ conjugate heat transfer process when the inner diameter of the tube is greater than 1 mm. In addition, density variation is highly significant for heat transfer of supercritical CO₂ while high viscosity hinders the distortion of the flow field and reduces deterioration in heat transfer.     

New prediction methods for CO2 evaporation inside tubes: Part II—An updated general flow boiling heat transfer model based on flow patterns

Corresponding to the updated flow pattern map presented in Part I of this study, an updated general flow pattern based flow boiling heat transfer model was developed for CO₂ using the Cheng–Ribatski–Wojtan–Thome [L. Cheng, G. Ribatski, L. Wojtan, J.R. Thome, New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside horizontal tubes, Int. J. Heat Mass Transfer 49 (2006) 4082–4094; L. Cheng, G. Ribatski, L. Wojtan, J.R. Thome, Erratum to: “New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside tubes” [Heat Mass Transfer 49 (21–22) (2006) 4082–4094], Int. J. Heat Mass Transfer 50 (2007) 391] flow boiling heat transfer model as the starting basis. The flow boiling heat transfer correlation in the dryout region was updated. In addition, a new mist flow heat transfer correlation for CO₂ was developed based on the CO₂ data and a heat transfer method for bubbly flow was proposed for completeness sake. The updated general flow boiling heat transfer model for CO₂ covers all flow regimes and is applicable to a wider range of conditions for horizontal tubes: tube diameters from 0.6 to 10 mm, mass velocities from 50 to 1500 kg/m² s, heat fluxes from 1.8 to 46 kW/m² and saturation temperatures from ?28 to 25 °C (reduced pressures from 0.21 to 0.87). The updated general flow boiling heat transfer model was compared to a new experimental database which contains 1124 data points (790 more than that in the previous model [Cheng et al., 2006, 2007]) in this study. Good agreement between the predicted and experimental data was found in general with 71.4% of the entire database and 83.2% of the database without the dryout and mist flow data predicted within ±30%. However, the predictions for the dryout and mist flow regions were less satisfactory due to the limited number of data points, the higher inaccuracy in such data, scatter in some data sets ranging up to 40%, significant discrepancies from one experimental study to another and the difficulties associated with predicting the inception and completion of dryout around the perimeter of the horizontal tubes.     

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.     

Predictions of In-Tube Cooling Heat Transfer Coefficients and Pressure Drops of CO2 and Lubricating Oil Mixture at Supercritical Pressures

The in-tube cooling heat transfer and flow characteristics of supercritical pressure CO₂ mixed with small amounts of lubricating oil differ from those for pure CO₂ due to the entrainment of the lubricating oil as well as the sharp property variations of the supercritical CO₂ working fluid. In-tube gas cooling flow and heat transfer models were developed in this study for CO₂ with entrained polyol ester type lubricating oil in a CO₂ gas cooler at supercritical pressures. A “thermodynamic approach,” which treats the CO₂–oil mixture as a homogenous mixture was used with the heat transfer coefficients and frictional pressure drops evaluated based on the thermophysical properties of the CO₂–oil mixture. Thermophysical property variation correction terms as a function of the wall temperature and the oil concentration were included in the models. The frictional pressure drop correlation predicts more than 90% of the experimentally measured data within ±10%, while the heat transfer coefficient correlation predicts more than 90% of the experimentally measured data within ±20%.     

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).     

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.     

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     

Evaluation of Mg‐MOF‐74 for post‐combustion carbon dioxide capture through pressure swing adsorption

This paper presents a computational study of an energy‐efficient technique for post‐combustion CO₂ capture using novel material, namely, Mg‐MOF‐74, using pressure swing adsorption (PSA) processes. A detailed one‐dimensional, transient mathematical model has been formulated to include the heat and mass transfer, the pressure drop and multicomponent mass diffusion. The PSA model has been further extended by incorporating a heat regenerating process to enhance CO₂ sequestration. The heat dissipated during adsorption is stored in packed sand bed and released during desorption step for heating purpose. The model has been implemented on a MATLAB program using second‐order discretization. Validation of the model was performed using a complete experimental data set for CO₂ sequestration using zeolite 13X. Simulation of the PSA experiment on fixed bed has been carried out to evaluate the capacity of Mg‐MOF‐74 for CO₂ capture with varying feed gas temperature of 28 and 100 °C, varying pressurization and purge times and heat regeneration. It was discovered that the PSA process with heat regeneration system might be advantageous because it achieves equivalent amount of CO₂ sequestration in fewer PSA cycles compared with PSA without heat regeneration system. Based on the simulated conditions, CO₂ recovery with Mg‐MOF‐74 gives high percentage purity (above 98%) for the captured CO₂. Copyright © 2015 John Wiley & Sons, Ltd.     

An Experimental Study of Carbon Dioxide Condensation in Mini Channels

In this study, the local characteristics of pressure drop and heat transfer were investigated experimentally for carbon dioxide condensation in a multi-port extruded aluminum test section, which had 10 circular channels each with 1.31 mm inner diameter. The CO2 was cooled with cooling water flow inside the copper blocks that were attached at both sides of the test section. The temperatures at the outer surface of the test section were measured with 24 K-type thermocouples embedded in the upper and lower surfaces along the length. Local heat fluxes were measured with 12 heat flux sensors to estimate the local enthalpies, temperatures and heat transfer coefficients. Bulk mean temperatures of CO2 at the inlet and outlet of the test section were measured with 2 K-type thermocouples. The measurements were performed for the pressure ranged from 6.48 to 7.3 MPa, inlet temperature of CO2 from 21.63 to 31.33℃, heat flux from 1.10 to 8.12 kW/m2, mass velocity from 123.2 to 315.2 kg/m2s, and vapor quality from 0 to 1. The results indicate that pressure drop is very small along the test section, heat transfer coefficient in the two-phase region is higher than that in the single-phase, and mass velocity has important effect on condensation heat transfer characteristics. In addition, experimental data were compared with previous correlations and large discrepancies were observed.     

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.     

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.     

Axial and radial heat transfer studies in a circulating fluidized bed

The axial and radial variation of the heat transfer coefficient in a circulating fluidized bed riser column, and the effect of operating parameters thereon, are investigated. The experimental set-up consists of a riser column of 102 mm×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions. The unit is fabricated with plexiglass columns of 0·6 m in length which are interchangeable with one another. Two axial heat transfer test sections of 102 mm×102 mm in cross-section, 500 mm in height, and made of mild steel, are employed for the axial heat transfer study and one horizontal tube section of 22·5 mm OD made of mild steel is employed for the radial heat transfer study. The primary air velocity is varied between 4·21 and 7·30 m s⁻¹. Local sand of mean size (d_p) 248 μm is used as the bed material. One empirical model with the help of dimensional analysis has been proposed to predict the heat transfer coefficient to a bare horizontal tube in a CFB riser column and the model results are validated with the experimental data; good agreement has been observed. © 1997 John Wiley & Sons, Ltd.     

Enhanced specific heat and thermal conductivity of ternary carbonate nanofluids with carbon nanotubes for solar power applications

Specific heat and thermal conductivity are important thermal properties of high-temperature heat transfer fluids and thermal storage materials for supercritical solar power plants. In the present work, nanofluids composed of ternary carbonate Li₂CO₃-K₂CO₃-Na₂CO₃ (4:4:2, mass ratio) and 1.0 wt.% carbon nanotubes (CNT) were prepared to obtain high-temperature heat transfer and storage media with enhanced specific heat and thermal conductivity. The dispersion of CNTs in the nanofluids was tuned by changing the evaporation temperature (100, 140, 180 and 220 °C) and adding surfactants such as sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), or gum Arabic (GA). The results showed that GA and SDS facilitate good dispersion of CNT in nanofluids at the evaporation temperatures of 140 °C and 180 °C, resulting in the formation of more needle-like nanostructures. The higher increase in the specific heat and thermal conductivity of the nanofluids with SDS at 500 °C was 78.3% and 149.2%, respectively. Additionally, the specific heat of as-prepared ternary carbonate nanofluids exhibits a good thermal stability after 30 cycles of thermal shock experiments.     

Experimental investigation on two-phase flow boiling heat transfer of five refrigerants in horizontal small tubes of 0.5, 1.5 and 3.0 mm inner diameters

An experimental investigation on two-phase flow boiling heat transfer with refrigerants of R-22, R-134a, R-410A, C₃H₈ and CO₂ in horizontal circular small tubes is presented. The experimental data were obtained over a heat flux range of 5–40 kW m^?2, mass flux range of 50–600 kg m^?2 s^?1, saturation temperature range of 0–15 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 0.5, 1.5 and 3.0 mm, and lengths of 330, 1000, 1500, 2000 and 3000 mm. The experimental data were mapped on Wang et al. (1997) [5] and Wojtan et al. (2005) [6] flow pattern maps. The effects of mass flux, heat flux, saturation temperature and inner tube diameter on the heat transfer coefficient are reported. The experimental heat transfer coefficients were compared with some existing correlations. A new boiling heat transfer coefficient correlation that is based on a superposition model for refrigerants in small tubes is presented with 15.28% mean deviation and ?0.48% average deviation.     

Preliminary investigation of a transcritical CO2 heat pump driven by a solar‐powered CO2 Rankine cycle

In this paper, a transcritical carbon dioxide heat pump system driven by solar‐owered CO₂ Rankine cycle is proposed for simultaneous heating and cooling applications. Based on the first and second laws of thermodynamics, a theoretical analysis on the performance characteristic is carried out for this solar‐powered heat pump cycle using CO₂ as working fluid. Further, the effects of the governing parameters on the performance such as coefficient of performance (COP) and the system exergy destruction rate are investigated numerically. With the simulation results, it is found that, the cooling COP for the transcritical CO₂ heat pump syatem is somewhat above 0.3 and the heating COP is above 0.9. It is also concluded that, the performance of the combined transcritical CO₂ heat pump system can be significantly improved based on the optimized governing parameters, such as solar radiation, solar collector efficient area, the heat transfer area and the inlet water temperature of heat exchange components, and the CO₂ flow rate of two sub‐cycles. Where, the cooling capacity, heating capacity, and exergy destruction rate are found to increase with solar radiation, but the COPs of combined system are decreased with it. Furthermore, in terms of improvement in COPs and reduction in system exergy destruction at the same time, it is more effective to employ a large heat transfer area of heat exchange components in the combined heat pump system. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Effect of Volume Fraction of γ‐Al2O3 Nanofluid on Heat Transfer Enhancement in a Concentric Tube Heat Exchanger

In this work, γ-Al₂O₃ nanoparticles with mean diameter of 10 nm are dispersed in deionized water with four nanoparticle volume concentrations of 0.25, 0.5, 0.75, and 1%. The effect of γ-Al₂O₃/water nanofluids on the heat transfer enhancement of heat exchangers is investigated under turbulent regime for four different volumetric flow rates of 150, 200, 250, and 300 L/h. The experimental results showed that the convective heat transfer is increased by increasing particles volume fraction as well as flow rate. The maximum enhancement obtained in Nusselt number and heat transfer coefficient was 20 and 22.8%, respectively, at Reynolds number of 6026 and particle volume fraction of 1%. The experimental Nusselt numbers of nanofluids showed good agreement with the available empirical correlation at particle volume fractions of 0.25 and 0.5%. An empirical correlation is obtained to estimate the Nusselt number of nanofluid under the conditions of this work.     

Carbon dioxide local heat transfer coefficients during flow boiling in a horizontal circular smooth tube

Carbon dioxide is gaining renewed interest as an environmentally safe refrigerant. In order to improve the energy efficiency of R744 systems, an accurate knowledge of heat transfer coefficients is fundamental.In this paper experimental heat transfer coefficients during flow boiling of R744 in a smooth, horizontal, circular, 6.00 mm inner diameter tube are presented. We obtained 217 experimental points in 18 operating conditions commonly encountered in dry-expansion evaporators investigating the effect of the mass flux within the range from 200 to 349 kg/m² s, the saturation temperature within the range from ?7.8 to 5.8 °C, the heat flux within the range from 10.0 to 20.6 kW/m² and the vapor quality within the range from 0.02 to 0.98.An interpretation of the experimental trends based on the local circumferential distribution of heat transfer coefficients, the flow regimes and the thermophysical properties is proposed.Besides the measured data are compared with those predicted by the Cheng et al. [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, International Journal of Heat and Mass Transfer 51 (2008) 125–135] and Yoon et al. [R. Yun, Y. Kim, M.S. Kim, Y. Choi, Boiling heat transfer and dryout phenomenon of CO₂ in a horizontal smooth tube, International Journal of Heat and Mass Transfer 46 (2003) 2353–2361] correlations to determine the best predictive method for the tested operating conditions.