Abstract This study presents the results of an experimental campaign on a large-diameter bubble column operating with air or CO2 as the gaseous phase and water as the liquid phase. The experimental setup consists of a 5.3 m bubble column with an inner diameter of 0.24 m. The initial liquid height was 3m, corresponding to an aspect ratio of 12.5. The column was operated in batch mode, and a range of superficial gas velocities that extended from 0.0037 to 0.0233 m/s were investigated. Measurements were performed focusing on global gas holdup. The impact of the gas phase on the gas holdup curve was examined, and particular attention was given to the transition between flow regimes. With the present configuration, the only observed flow regimes are the poly-dispersed homogeneous and the pure-heterogeneous flow regimes. The analysis of flow regime transition points was carried out by adopting two methods derived from the drift flux approach: the swarm velocity method and the Wallis plot method. The CO2 caused the flow to stabilize compared to air, exhibiting a higher gas holdup and delayed transition. Additionally, the operation of the sparger was investigated, revealing maldistribution at low flow rates.

Abstract Bubble columns are gas-liquid reactors widely used in various engineering disciplines, their most notable advantage being a substantial contact area between the gas and liquid phases, facilitating efficient heat and mass transfer processes. However, despite the apparent simplicity of the configuration, flows in bubble columns can be quite complex because instabilities occur that lead to different flow regimes. This work investigates flow stability in bubble columns using computational fluid dynamics (CFD). It addresses the implications of introducing instabilities through a spatially non-uniform inlet condition for gas fraction injection in batch mode, then investigates the destabilizing effect of a counter-current liquid flow and finally delves into controlling flow stability by introducing a co-current liquid flow. 3D transient simulations are performed using the OpenFOAM Foundation software with the Euler-Euler multifluid model. The results indicate that the non-uniformity imposed at the inlet is completely smoothed at L/D=5. Starting from an unstable flow condition (i.e, chaotic gas volume fraction and liquid velocity fields) the flow can be stabilized (i.e., transition to a state where the flow regime becomes homogeneous) with the addition of a co-current liquid flow. Conversely, the addition of a counter-current flow results in a destabilization of the homogeneous flow regime.

We present the results of an experimental study of a large-diameter bubble column operating in batch mode with air or CO2 2 as the gas phase and water as the liquid phase. The bubble column has an inner diameter of 0.24 m and is 5.3 m in height. The superficial gas velocity varied between 0.0037 m/s and 0.0233 m/s. The experimental investigation consists of gas holdup measurements and image analysis. Flow regime transitions were detected by gas holdup measurements, and image analysis was used to investigate the bubble size distributions. The results suggest that the homogeneous flow regime is stabilized when CO2 2 is used as the gas phase. The gas holdup is higher for the CO2-water 2-water system at fixed superficial gas velocity, especially in the heterogeneous flow regime. In the homogeneous flow regime, the mean bubble diameter and the fraction of large bubbles are lower for the CO2-water 2-water system, explaining the higher gas holdup.

This vision article accompanies a Special Issue of Applied Thermal Engineering dedicated to Heat Transfer in Geothermal Energy Extraction. This issue contains original research articles selected for publication in Applied Thermal Engineering, all connected by a focus on processes, technologies and systems for the exploitation of geothermal energy. Geothermal resources, which can be found at depths ranging from a few m to several km below the earth's surface, are amongst the most promising renewable, zero carbon, and clean energy sources. Geothermal energy utilization technologies can be mainly classed into four categories: ground source heat pumps, energy geo-structures, enhanced geothermal systems, and closed-loop geothermal systems. Ground source heat pumps are the most well-developed geothermal technology for shallow depths, and have been widely used for building cooling and heating applications. Current and future heat pump research is focused on the coupling and hybridization of different heat pump configurations, using supplementary energy sources and phase change materials as storage media to overcome the thermal imbalance of the ground. Energy geo-structures, including energy piles, energy walls and energy tunnels, are an emerging and promising geothermal technology, with current research mainly focusing on simulating the effects of influencing factors on heat extraction effectiveness. Enhanced geothermal systems are a technology that can be used to extract deep geothermal energy for power generation and direct use. This technology has attracted significant renewed attention in the last decade. Future research should focus on the heat transfer mechanisms in high temperature rock fractures, and how the layout of inlet and outlet wells and different heat transfer fluids-such as CO2-affect performance. Closed-loop geothermal systems, including coaxial and U-shaped systems, have no direct physical contact between working fluid and the reservoir, and may overcome some shortcomings of enhanced geothermal systems, such as leakage of the working fluid and water-rock interactions; however, further research is urgently needed on understanding the thermal recovery performance of these systems.

This vision article accompanies aSpecial Issueof Applied Thermal Engineering dedicated to the 14th International Conference on Boiler Technology (ICBT 2022) on ‘Transition technologies towards carbon neutrality and reduced environmental impacts’. The aim of the conference was to discuss recent progress, achievements and new solutions in the field of thermal power production, covering scientific research from a wide range of universities and research centres on topics relating to the design and operation of boilers and auxiliary devices for thermal power generation. This article discusses a selection of papers presented at this conference and selected for publication in Applied Thermal Engineering.

This vision article accompanies a Special Issue of Applied Thermal Engineering dedicated to the 18th Sustainable Development of Energy, Water and Environment Systems (SDEWES) conference held in Dubrovnik, Croatia from 24 to 29 September 2023. This article presents a selection of papers presentet at the conferences, published in Applied Thermal Engineering journal, all focused on advancing sustainable thermal energy systems together with clean energy technologies. The research encompasses a diverse range of innovations, with particular attention to heat exchangers, thermal energy storage, thermal system simulations, and their integration with chemical processes and pollutant control. Based on recent advancements in the field, this vision article examines the development and application of cutting-edge technologies while addressing key challenges that require further investigation. The progress of both emerging and established technologies will rely on breakthroughs in fundamental research, cost reduction measures, expansion of operational capabilities, and the implementation of advanced control strategies. Furthermore, optimizing systems will necessitate the integration of multiple renewable energy sources and sophisticated modeling tools. By synthesizing the latest trends and critical research needs, this article offers valuable perspectives for scientists, researchers, and engineers, providing direction for future studies and innovations in sustainable thermal energy systems.

Bubble columns (BCs) are widely used gas-liquid contactors in the chemical industry. Their hydrodynamic and mass transfer performance depends on the prevailing flow regime (FR) and the gas sparger (GS) type (particularly in the homogeneous FR). In this work, numerous gas holdup profiles measured by means of different techniques in various air-water BCs (with inner diameters between 0.1 and 0.33 m) have been carefully analyzed. A comparative analysis of the power-law dependence of gas holdup on the superficial gas velocity UG has been performed. Eight different GS types have been used. The dependence of the constant C and the exponent n (in the power-law fit) on the GS type in both main FRs has been investigated. The effect of the operating pressure P on these parameters has been studied in a BC equipped with a perforated plate GS with an open area (OA) of 2.25%. The previous knowledge about the exponent n was corrected based on our database of gas holdups. It was shown that in some cases the n value could exceed the limit of 1.2 in the homogeneous FR. On the other hand, in the case of gas maldistribution, the n value could be lower than 1. It was also found that the n value increases with P, which implies that the n value is associated with the behavior of the small bubbles, which predominate at elevated P values. It turn out that in the heterogeneous FR the n value at ambient P was as low as 0.17 and then gradually it increased to 0.44 at P = 3.0 MPa. In the case of heterogeneous BCs equipped with the other GS types and operated at ambient P, the n values varied between 0.11 and 0.73. Such a low n value has not been reported hitherto.

The aim of this work was to experimentally analyze the variable geometry gas ejector with a spindle designed for operation with the natural working fluid propane (R290). The performance of the ejector was evaluated based on the mass entrainment ratio and critical temperature, being the most crucial parameters for optimizing the cooling capacity in ejector refrigeration systems, as well as the ejector efficiency using common literature notation. Additionally, local pressure drop measurements allowed for the determination of pressure profiles inside for different spindle positions used for capacity regulation. The experimentally defined ejector efficiency curves demonstrated the ability to control the ejector capacity by means of the spindle, irrespective of the unfavorable operating conditions. By decreasing the effective throat area using spindle, the ejector mass entertainment ratio increased by 35%. Spindle movement allowed for the decrease of the motive nozzle flow rate by up to 65%, while still maintaining the suction of the secondary flow of the ejector. Moreover, the behavior of a slight increase in suction nozzle flow with a decrease in motive nozzle flow has been observed for the initial movement of the spindle followed by a huge drop for further reduction of the effective throat area, confirming the previous conclusions shown in the literature.

The poor entrainment performance of the conventional ejector is a significant problem that makes it hard to use in proton exchange membrane fuel cell (PEMFC) systems. This study aims to evaluate the entrainment performance of a bypass ejector for a 100 kW PEMFC system. The effects of three critical geometric parameters, namely the axial position, width, and angle of the bypass inlet, on the entrainment performance are thoroughly investigated. The results demonstrate that the bypass flow exhibits a significant performance improvement in the critical mode. In contrast, the performance improvement is negligible and even negative in the subcritical mode. After careful evaluation of the entrainment performance across various stack powers, the optimal axial position, width, and angle of the bypass inlet are found to be 1.1, 2 mm, and 10(degrees), respectively. A comparative analysis between the bypass ejector and the conventional ejector underscores a significant advantage for the former, exhibiting a remarkable 22.1 % increase in the hydrogen entrainment ratio at the stack power of 101 kW. Nevertheless, the entrainment performance of the bypass ejector diminishes when operating at low stack powers below 24 kW.

The aim of this study was to analyze and compare the performance of an 8 kW ejector-based R290 heat pump system for hot water heating applications that operates with two different expansion devices: a throttling valve and a two-phase ejector. The vapor compression system was designed to compare the unit operating in direct expansion and ejector-based working modes under similar operating conditions. The test results were assessed for overall system performance measured by COP and heating capacity and for single-component operation. The ejector mode allowed operation at the same range of ambient temperatures as direct expansion mode with the potential to completely replace the expansion valve. The COP of ejector system was up to 2.6, which was 38% higher than direct expansion when working under similar conditions for an ambient temperature of −12.8 °C. Moreover, the COP in direct expansion mode decreased significantly along the decrease of ambient temperature, whereas for the ejector mode this decrease was considerably smaller.

The special issue "AI in Thermal Engineering" covers the most recent studies with a focus on the applications of artificial intelligence (AI) technologies in thermal engineering systems. The overall aim is to report the latest advances of research and development, discuss the pros and cons, and explore the future perspectives on the synergy of AI and thermal engineering. Articles reporting original research contributions and critical reviews on adopting AI to address engineering problems of modeling, prediction, control, optimization, performance assessment, diagnosis of thermal engineering devices, components and systems are welcome. Special focus is given to those problems which have not been adequately addressed by adopting traditional methods due to limited knowledge and information, computation efficiency, capability of generalization and adaptation in applications, etc. The targeted engineering systems include energy storage devices, power plants, heat pumps and cooling or refrigeration plants, combined heat and power plants, buildings and district energy systems, renewable and clean energy systems, and other engineering systems involving thermal engineering processes.The special issue has received over 30 submissions, and a total of 10 technical papers are selected for publication which cover a broad range of applications including building energy systems, thermal power units, vehicles, and heat transfer processes. These papers addressed challenges and research gaps in adopting AI in real applications, such as model development and adaption, model interpretability, data imbalance, missing data imputation, etc.

Approximately half of the global primary energy consumption is wasted in the form of low-grade (i.e., low-temperature) thermal energy, which has been traditionally overlooked and rejected to the environment, leading to low overall energy utiliization efficiencies. In recent years, there has been a growing urgency for deep decarbonization and energy security, as well as a change to global economic conditions, which have sparked an increased interest in the utilization of low-grade thermal energy. Research has increasingly focused on a diverse range of technologies and applications for harnessing this energy resource. The Special Issue ‘Low-grade thermal energy utilization: technologies and applications’ served as a platform for presenting state-of-the-art research on the management and utilization of low-grade thermal energy. This editorial article reviews the articles featured in the Special Issue, and provides commentary on these contributions.

The fluid dynamics in large-diameter bubble columns can be described by an analytical relation between two global flow parameters, the drift flux and the gas holdup. This relation, named bubble column operating curve, builds on five flow regime transitions. In order to determine the variables influencing the flow regime transitions, a statistical approach was derived by coupling: (1) the ordinary least squares method (OLS) to determine the relationship between the variables, (2) the variance inflation factor (VIF) to check for multicollinearity issues, and (3) the least absolute shrinkage and selection operator (LASSO), to select suitable variables. It was found that the geometrical characteristics of the sparger strongly influence the flow regime transitions, and uniform aeration is essential for all the regimes to exist. Increasing the superficial liquid velocity in the counter-current mode destabilises the mono-dispersed and poly-dispersed homogeneous flow regimes. As for the aspect ratio, an increase in the column aspect ratio slightly destabilises the existing flow regimes. The statistical method identifies viscosity as the only significative variable concerning the liquid phase properties.

Abstract This study proposes a CFD model to simulate large-scale bubble columns operating in different flow regimes. Transient 3-D simulations were performed employing a commercial code (ANSYS Fluent), and the numerical results were compared with available experimental data. The superficial gas velocity ranges between 0.0037 m/s and 0.2 m/s, covering both the mono-dispersed and pure-heterogenous flow regimes, where bubbles coalescence and breakup were modelled. The results have been critically analysed, and the discrepancies between the numerical and experimental results have been deeply commented on, setting the stage for future improvements.

This vision article accompanies a Special Issue of Applied Thermal Engineering dedicated to the Sustainable Development of Energy, Water and Environment Systems (SDEWES) conference series held during 2022, including the 5th SEE SDEWES Conference Vlore, 3rd LA SDEWES Conference Sao Paulo, and 17th SDEWES Conference Paphos. The article discusses a selection of papers presented at these conferences and selected for publication in Applied Thermal Engineering, all connected by a focus on clean energy technologies and systems for the advancement of sustainable thermal energy systems. This research area covers a wide range of technologies but is primarily focused on the power generation sector, energy storage and utilization, efficiency improvements, sustainable technical solutions, and the facilitation of the robust integration of renewable energy resources into wider energy systems. Additional work on previously established research topics is also present, but new directions have crystallized, especially research into thermal management for power devices and microelectronics. Based on the recent contributions to the research field presented in the included papers, this vision article discusses the development and application of cutting-edge technological solutions and identifies the challenges that require focused attention. Current shortcomings of developing and established technologies will be developed by the underlying fundamental research, reduction of prohibitively high costs, expansion of operating conditions, and the inclusion of advanced control approaches, sophisticated combinations of renewable energy sources, and modeling tools for system investigations. Scientists, researchers, and engineers can benefit from the summary of the topics at the forefront of the research field and find guidelines and ways forward for future research activities.

Although the global flow characteristics of annular gas-liquid flows have been studied experimentally for more than 50 years, the spatiotemporally-resolved details of these flows have remained relatively unexplored until recently, with data provided via advanced experimental methods based, e.g., on optical techniques. Similarly, the numerical modelling of annular flows is still an immature process. The present work aims to provide a computational fluid dynamics (CFD) model based on the volume of fluid (VOF) method for simulating annular gas-liquid flows, setting the stage for a deeper investigation of these flows at global and local scales. The work focuses on the most common downwards annular flow (DAF) flow pattern: the regular wave regime. 3-D and 2-D axisymmetric transient simulations have been performed using a commercial code (ANSYS Fluent 2021 R1). The code is validated through available experimental data regarding topological flow properties, mainly film thickness and wave statistics. The validation results suggest that 3-D simulations are needed to provide predictions that agree with the experimental data, highlighting strong 3-D features in the flow.