Transfer Intensity Research Articles

A novel central bank digital currency framework design for offline and foreign transactions based on blockchain

Purpose Fiat money production necessitates physical commodities, increasing costs and its flow is challenging to monitor, making it vulnerable to criminal exploitation. Cryptocurrencies offer decentralized solutions, but their decentralization has led to illegal activities. Current cross-border transactions face high costs, resource intensity and lack of instant currency transfers. Offline transactions are essential in unreliable networks. Design/methodology/approach Here, the authors proposed the methodology to perform offline transactions based on card, quick response (QR) code and a foreign transaction framework with universal identification (UID) to perform cross-border transactions using blockchain-dependent central bank digital currencies (CBDCs). Implications for the financial system are also analyzed. Findings The proposed CBDC framework reduces illegal transactions, corruption and the cost of producing fiat money; eases overseas transactions; and eventually increases international tourism, trade and business between countries. It also reduces the processing fees. Offline framework found useful for performing retail-level transactions. Research limitations/implications The research methodology may face limitations due to diplomatic relations, political instability, sanctions and the need for robust offline transaction infrastructure. Practical implications The proposed CBDC framework simplifies debt and insurance management, tax collection, international trade, tourism and global stock market participation. However, implementing CBDCs in low-income countries presents challenges like extensive training, infrastructure and user acceptance issues. Social implications The adoption of CBDCs can enhance financial stability by reducing corruption and illegal transactions through improved traceability and monitoring, thereby curbing activities like terrorism. Originality/value Common framework for foreign transactions is based on the UID, and offline transaction framework is based on the sender’s QR code for multiple user applications.

Liquid metal-based flexible heat sink for adaptive thermal management

The deformable heat sinks are critical for flexible electronics, wearable personal thermoregulation and energy management. However, the available soft polymer materials with poor thermal characteristics face challenges in achieving efficient heat transfer. Herein, we report an excellent flexible heat sink enabled by a gallium-based liquid metal (LM) for adaptive thermal management. The flexible heat sink is prepared by embedding LM droplets and copper particles into the elastic matrix forming solid–liquid multiphase composites, achieving a considerable thermal conductivity (3.5 W/mK@300 % stretching deformation), outstanding high-temperature resistance, and excellent conformity. The cooling and hydrodynamic characteristics of the flexible heat sink with different structures, including parallel (plane), S-type (plane), and annular (three-dimensional) channels, are investigated and optimized by the thermal experiment and developing the fluid–structure coupling numerical simulation. The results show that the maximum temperature and heat flux achieved by the S-type heat sink are 36.7 °C and 0.68 W/cm2 respectively, due to the secondary flow enhancing convection thermal transfer intensity. The annular heat sink has outstanding advantages in suppressing temperature and improving uniformity because the thermal transfer path is optimized by stretching, especially for larger deformation. Furthermore, the annular heat sink has been proven feasible for lithium battery cooling and pre-insulation.

Mechanism chaotic movement of Leidenfrost droplets

One of the most effective methods for cooling overheated surfaces is drip irrigation. If the surface temperature exceeds the Leidenfrost temperature, then a vapor film is formed between the droplet and the surface, which leads not only to a decrease in heat transfer intensity but also causes droplet mobility. For a number of applications, the mobility of droplets is an undesirable phenomenon, so the analysis of the factors responsible for their movement is a relevant task. Here we analyze the movement mechanism of the Leidenfrost droplets with variations in the composition and volume of the droplets. The data obtained show that the droplet speed increases with an increase in the droplet volume. However, smaller droplets change direction of motion more often than larger droplets. To substantiate the experimental data, a hypothesis is proposed, according to which the mechanism of movement of Leidenfrost droplets is caused by the reactive force that arises due to the evaporation of liquid. A Leidenfrost droplet changes the direction of its movement due to the deformation of its surface under the influence of gravity and capillary force. To substantiate the experimental data a simple phenomenological model is proposed.

Convective Heat Transfer Analysis of Supercritical CO2 in Artificial Rock Fractures

Geothermal energy has emerged as a promising renewable and sustainable resource. This research article explores the utilization of supercritical CO2 and convective heat transfer in porous rock to enhance geothermal extraction. Artificial cement rock with a smooth and manmade slotted surface fracture was used as a working sample for heat transfer source in the test. Heat transfer characteristics under various flow rates, fluid inlet temperature and pressure, rock initial temperature are analyzed. The results indicate that the geometry characteristics of fracture surface impact the overall heat transfer intensity to a certain extent, whereas lithology has little influence on the heat transfer intensity. In addition, it is found that increasing injection pressure affects the operating temperature and density of the injected fluid, which helps to improve the production effect, to obtain more heat. However, limited by the heat transfer area, low injection flow is required. The maximum error of the heat transfer coefficient model is 7.05% compared with the empirical value and less than 10% compared with the experimental results. By examining the interaction between fluid and porous fractures, we aim to provide insights into the potential of these techniques for maximizing energy efficiency and tapping into deep geothermal resources.

Experimental investigation of the structure effects on the energy recovery and jet impinging process in a fast cooling Joule-Thomson cryocooler

Compared to other Joule-Thomson (J-T) refrigeration systems, open-cycle miniature J-T cryocoolers offer exceptional rapid cooling capabilities, making them ideal for applications such as infrared guidance in missiles. The energy recovery in the heat exchanger enables the refrigerant reach saturation temperature quickly and improve the jet liquefaction rate. The heat transfer intensity of the impinging jet determines the cooling rate of the target. Hence, heat recovery and the impact jet process are the primary factors behind this rapid cooling, with distinct roles that require separate consideration. The structural differences will directly affect the energy recovery and jet impact process. To investigate these affects, an experimental system for rapid cooling J-T cryocoolers was established and three distinct cryocoolers with substantial structural variations were designed. The important structures, including jet height, orifice diameter, enhanced heat transfer treatment of the cold plate, heat exchanger height, and heat exchanger cone angle, were closely studied. In the range of our experiments, it was found that larger heat exchanger cone angle leading better energy recovery performance, while the length of the heat exchanger is limited by the type of refrigerant. Longer heat exchanger actually introduce too much thermal mass for the refrigerant with better energy recovery performance. In the aspect of jet impingement, enhanced heat transfer treatment and larger jet height will improve the jet heat transfer intensity.

Contrasting role of Ca2+ on the Gd2TiO5: Eu3+ light emission under charge transfer and f-f band excitation: Interplay of oxygen vacancies and structural distortion

The possibility of generating new lanthanide ion doped inorganic material that could work efficiently towards solid-state lighting was explored. The material studied in this work is Gd2TiO5 doped with Eu3+ which displays the lowest lattice strain, high solubility, and lacks both charge-compensating defects and structural deformation. In this study, the impacts of exchanging Gd with up to 10 mol% of Ca, an optically inactive ion, on the luminescence characteristics are investigated. Only small variations in the positron lifetimes suggest the Ca2+ doping only enhanced oxygen vacancies in the system.. Time resolved photoluminescence suggested that the emission intensity increases with Ca2+content when excited with 395 nm (f-f band) while it reduced when excited with 270 nm (charge transfer). The host to dopant energy transfer is attributed to the charge transfer from O2− to Eu3+ and the creation of oxygen vacancies due to Ca2+ doping reduced the charge transfer intensity. The intensity with excitation at 395 nm is almost doubled upon 10 % Ca doping and is attributed to enhanced distortions around the dopant favouring the direct excitation of Eu3+. The creation of oxygen vacancies via Ca substitution and the resultant distortions around Eu3+ ion have resulted in opposite variations in the emission intensity under excitation using charge transfer band and direct excitation. The study offers the possibility of exploring more methods of creating vacancies and distortions around Eu3+ to induce the spectral shifts.

Influence of an inclined thick partition on free convection heat transfer performance under surface radiation in a hollow block

In the present work, a computational analysis of the combined energy transfer by free convection, radiation, and heat conduction in 2D hollow block with an inclined partition was carried out. The inclined partition was made of a heat-conducting material and had a finite thickness. The closed space inside the brick was filled with air (Pr = 0.71) and the airflow is laminar. The external surfaces of the vertical borders are considered to be isothermal, and the remaining horizontal surfaces are supposed to be adiabatic. The finite difference technique is employed to work out numerically the differential equations. The in-house computational code was verified in detail using various problems. The major characteristics that determine the considered phenomena are surface emissivity of internal walls, Ra number, and material of solid walls and partition. As a result of the research, the distributions of streamlines and isotherms combined with the average Nusselt numbers were obtained. Depending on the material of the partition, the flow structure in the cavity changes significantly, which reflects the distribution of streamlines, namely, for the materials with low thermal conductivity the convective cell in the upper part of the cavity is elongated, while in the lower part it is shifted to the lower and left borders. The isotherms reflect the more intensive heating in the left part of the area. With an increase in the thermal conductivity coefficient, the streamlines reflecting the nature of the flow are restructured. It was shown that the presence of an inclined partition significantly reduces the intensity of convective heat transfer.

Investigation on uneven flow distribution in triply periodic minimal surface heat exchangers

The development in manufacturing engineering has made it possible to adopt the unique porous structure known as the triply periodic minimal surface to increase energy efficiency in a variety of energy-related sectors. With the unique morphological characteristics, triply periodic minimal surface exhibits remarkable mechanical strength and thermal performance, which makes it a potential candidate for compact, high-performance heat exchanger constructions. In this study, a theoretical model based on triply periodic minimal surface porous media is developed. Based on the hypothesis that the triply periodic minimal surface is isotropic and the heat transfer intensity of fluid and solid is uniform throughout the core of the heat exchanger, the simulation of the full-size heat exchanger is simplified by using a porous media model. The flow distribution characteristics of triply periodic minimal surface heat exchangers, as well as the effect mechanism of key factors, including the channel porosity, header configuration, the type of triply periodic minimal surface, the core length and Reynolds number on the flow distribution performance, are investigated. A dual-header configuration is proposed to improve the flow uniformity of triply periodic minimal surface heat exchangers. The results indicate that the flow distribution performance of triply periodic minimal surface based heat exchangers can be enhanced by the reduction of channel porosity. When the cold-side porosity is 0.7, the flow rate deviates from the mean value by 220% at its maximum for a Primitive heat exchanger. A dual-header configuration eliminates the columnar extreme high velocity space in the core and greatly reduces the “weak zone” of flow distribution, which improves the flow non-uniformity by 45–90% over the single-inlet headers. This paper provides data and theoretical support for the optimal design and practical application of the complex, high-performance triply periodic minimal surface based heat exchangers.

Effect of arrangement patterns of spheres encapsulating phase change material in packed beds on thermal performance

The NaNO3-KNO3 salt has the disadvantage of poor heat transfer capacity and low heat transfer efficiency in single-tank heat storage systems. However, molten salt can be used as phase change materials (PCM) and encapsulated with highly conductive sphere-shaped metals as PCM spheres. Arranging these spheres can appropriately change the situation. In this study, a heat storage system of a multilayer packed bed of PCM spheres consisting of NaNO3-KNO3 salt and type 316 stainless steel sphere shell was designed. This system's heat transfer and heat storage performance were also investigated using CFD simulation. For the same sphere diameter, the heat storage efficiency of hexagonal close packing (HCP) was 1.1 times that of cubic close packing (CCP). With the same porosity, the heat charging efficiency for the case using the 47.5 mm sphere diameter was the highest, at 99.3 %. When the number of layers of spheres was increased from 3 to 7, the heat flux through the sphere wall and the heat storage density of PCM spheres decreased from 40.8 kW and 67.3 × 105 kJ/m3 to 35.7 kW and 59.2 × 105 kJ/m3, respectively. Therefore, the heat charging efficiency of HCP was better than that of CCP. Reducing sphere size could shorten the heat transfer time and improve the heat storage efficiency. These numerical simulation results are consistent with the commonly known fact that the volumetric heat transfer intensity in the porous media is proportional to the reciprocal particle size since the smaller the particle size, the larger the contact area between the packing and the fluid. The conclusion drawn from the present findings provides an idea for designing a PCM heat storage system with high temperature and high efficiency of molten salt. The idea could also be used as a reference for the optimal design of a packed bed reactor.

Thermo‐hydraulic analysis of wavy microchannel heat sinks with porous fins based on field synergy principle

AbstractIn order to enlarge the area and intensity of convective heat transfer among the coolant and heated surface, the vertical fins of microchannel heat sinks (MCHSs) with microencapsulated phase change material slurry (MPCMS) as coolant are arranged into wavy porous channels to realize more heat being dissipated to the outside. The phase transition of microencapsulated particles in laminar flow state is described, and the Brinkman–Forchheimer–Darcy model based on volume average approach and the energy equation for local heat equilibrium are adopted to portray flow and heat transfer in porous medium. The impacts of geometrical parameters on flow and heat transfer behaviour of wavy porous MCHS are numerically analyzed, and performance evaluation factor (PEF) is defined to estimate the thermo‐hydraulic capability of heat exchanger. The numeric outcomes match well with the experiments. Results indicate that MPCMS has a significant heat transfer improvement in the newly designed channel configuration compared with the coolant fluid flowing in the straight microchannel. Based on field synergy principle, the comprehensive capability enhancement mechanism of MPCMS in new MCHS is explored, and its superior thermal performance can be attributed to the improvement of the synergistic degree among flow and temperature fields, and its reasonable structural design can effectively improve the heat rejection capacity in the limited space.

Mathematical modeling of slab heating in a furnace with walking beams due to their curvature

Slab heating before hot rolling process is necessary for obtaining required metal ductility. The most effective for this purpose are furnaces with walking beams that provide heat supply to all sides of the slab. However, the places of slabs lower surfaces, contacting with water-cooled beams, are shielded from the radiation of the furnace lower heating zones and take the heat from the beams. Previously, the authors developed and programmatically implemented a mathematical model of slab heating in a furnace with walking beams, based on the numerical solution by finite difference method of the three-dimensional heat conduction problem with piecewise defined boundary conditions on the slab bottom surface. In this model, for the open zones of the slab bottom surface, boundary conditions were similar to those on the top surface, and for the zones of contact with the beams were set effective boundary conditions assuming duration of this contact. In this paper, the model was modified to take into account the curvature of the beams and to recalculate the configuration of zones with different boundary conditions on the slab bottom surface for each position of the slab along the furnace. By variant calculations at different values of heat transfer intensity from the slab bottom surfaces to the beams it was determined that curvature of a single beam can significantly change the characteristic of the corresponding “cold” spot, but it practically does not affect the general characteristic of the slab heating non-uniformity. If all fixed beams are subjected to curvature, the final drop across the slab is significantly reduced due to an increase in its minimum temperature. It was found that the influence of beam curvature on the temperature field at the end of heating process is higher the more intensive the heat transfer to the beams is.

Effect of impinging jet on rapid cooling of a miniature Joule-Thomson cryocooler: Experimental study of operating conditions

A miniature open-cycle Joule-Thomson (J-T) cryocooler possesses excellent potential for rapid cooling from ambient temperature to about 100 K within seconds. In this rapid cooling process, the interplay of the heat transfer process between the cryogenic fluid and the cold plate plays a crucial role, in addition to the J-T effect and the energy recovery within the heat exchanger of the cryocooler. The phase state of the impinging jet is determined by the jet temperature and has different heat transfer mechanism. To investigate this impinging jet behavior separately, an experimental system for the rapid cooling J-T cryocooler was set up and the jet temperature was measured besides other important parameters. The operating conditions, including the cylinder pressure, cylinder volume, ambient temperature, and the refrigerant, were studied orthogonal. It was observed that argon, compared to nitrogen, can be more rapidly throttled into its two-phase region within a second. However, nitrogen will perform better than argon in terms of cool-down time. This is attributed to the temperature difference between the jet and the cold plate, which significantly influences the boiling mode of the two-phase jet, and in turn affects the cryocooler's cooling rate. Nevertheless, due to the smaller heat transfer intensity of single-phase jet, the cool-down time of nitrogen deteriorates quickly at high ambient temperatures. Experimental results also revealed that the cooling rate of the cryocooler still can be enhanced even if cylinder pressure beyond its saturation curve, caused by the dynamic cooling process and the pressure drop within the finned tube.

Analysis of the Effects of Structural Parameters on the Thermal Performance and System Stability of Ventilation Air Methane-Fueled Reverse-Flow Oxidation Reactors

Ventilation air methane (VAM) from coal mining is a low-grade energy source that can be used in combustion systems to tackle the energy crisis. This work presents a numerical analysis of the thermal and stabilization performance of a VAM-fueled thermal reversal reactor with three fixed beds. The effects of the combustion chamber/regenerator height ratio (β), heat storage materials, and porosity on the oxidation characteristics are evaluated in detail. It is shown that the regenerator temperature tends to vary monotonically with β due to the coupling effect of the gas residence time and heat transfer intensity. The optimal β is determined to be 4/6, above which the system may destabilize. Furthermore, it is found that regardless of the methane volume fraction, the regenerator with mullite inserted has the highest temperature among the heat storage materials investigated. In contrast, the temperature gradually decreases and the system becomes unstable as SiC is adopted, signifying the importance of choosing proper thermal diffusivity. Further analysis reveals that the porosity of the heat storage materials has little effect on the system stability. Decreasing the porosity can effectively reduce the oscillation amplitude of the regenerator temperature, but it also results in greater pressure losses.

Transmembrane helix interactions regulate oligomerization of the receptor tyrosine kinase EphA2

The transmembrane helices of receptor tyrosine kinases (RTKs) have been proposed to switch between two different dimeric conformations, one associated with the inactive RTK and the other with the active RTK. Furthermore, recent work has demonstrated that some full-length RTKs associate into oligomers that are larger than dimers, raising questions about the roles of the TM helices in the assembly and function of these oligomers. Here we probe the roles of the TM helices in the stability of EphA2 RTK oligomers in the plasma membrane. We employ mutagenesis to evaluate the relevance of a published NMR dimeric structure of the isolated EphA2 TM helix in the context of the full-length EphA2 in the plasma membrane. We use two fluorescence methods, Förster Resonance Energy Transfer and Fluorescence Intensity Fluctuations spectrometry, which yield complementary information about the EphA2 oligomerization process. These studies reveal that the TM helix mutations affect the stability, structure, and size of EphA2 oligomers. However, the effects are multifaceted, and point to a more complex role of the TM helix than the one expected from the “TM dimer switch” model.

V-shaped effect of primary and secondary airflow momentum ratio on performance of a 660 MW tangentially fired lignite boiler: Identification and verification

To explore how the primary and secondary air ratios affect the overall performance of lignite-fired power generation boilers, a 660 MW wall-tangentially fired lignite boiler was chosen as research subject. 17 combustion scenarios were designed and then numerically simulated through a validated model, and the in-furnace coal combustion behavior, heat transfer process, and NOx conversion characteristics under various conditions were analyzed in detail. Results show that, except for the monotonous increase in NOx emission, the overall boiler performance follows a V-shaped trend with the continuous increase in primary air ratio (PAR). Initially, as PAR is slightly increased, the in-furnace combustion temperature, heat transfer intensity and coal utilization rate notably decrease, followed by a subsequent increase in these parameters as PAR is significantly increased. The separated over fire air (SOFA) ratio doesn’t alter this V-shaped pattern; however, a higher SOFA ratio does reduce the critical PAR at which the overall boiler performance is most compromised. This is attributed to changes in the momentum ratio between primary and secondary airflow caused by varying PAR. Boiler performance deteriorate significantly when their momentum ratios are nearly equal, and a higher SOFA ratio makes it easier. These observations are confirmed under different thermal load conditions, where the close proximity of primary and secondary air momentum is avoided by reducing SOFA ratio. Based on these findings, a substantial increase in PAR should be avoided in practice, if necessary, lowering the SOFA ratio helps mitigate significant deterioration in boiler performance.

Using Quality Function Deployment to Assess the Efficiency of Mini-Channel Heat Exchangers

This article addresses the design of a compact heat exchanger for the cooling of electronic systems. The Quality Function Deployment (QFD) method is used to identify crucial product features to improve device performance and key customer requirements. The QFD simplifies management processes, allowing modifications to device components, such as design parameters (dimensions and materials) and operating conditions (flow type and preferred temperature range). The study was applied to analyse the fundamental features of a compact heat exchanger, assessing their impact on enhancing heat transfer intensity during fluid flow through mini-channels. The thermal efficiency of the compact heat exchanger was tested experimentally. The results allow to verify the results obtained from the numerical simulations due to Simcenter STAR-CCM+. Consequently, the experimental part was reduced in favour of numerical simulations conducted using this commercial CFD software version 2020.2.1 Build 15.04.01. The numerical simulations performed with the aid of CFD showed increases in the heat transfer coefficient of up to 180% compared to the case treated as a reference. The application of the QFD matrix significantly reduces the time required to develop suitable design and material solutions and determine the operating parameters for the cooling of miniature electronic devices.

Sweeping jet impingement heat transfer on concave surface: Influence of trapped vortex ring revealed by generalized k-ω (GEKO) model

Jet impingement-based heat transfer enhancement is widely employed due to the significant mass, momentum, and energy it accompanied. While the steady jet with high energy flux is commonly favored, it still exhibit few defects, including heat transfer deterioration away from the stagnation point and mutual flow interference in the jet arrays, which are particularly critical in demanding applications. Recently, the sweeping jet has been proposed to resolve the above challenges owing to its self-excited and self-sustaining unsteady oscillation. However, current research predominately focuses on impinging on the flat surface, neglecting impinging on the concave surface, such as internal cooling at the gas turbine blade leading-edge as well as the hot gas de-icing at the aircraft wing leading-edge. Understanding the heat transfer characteristics in these scenarios is crucial as they notably differ from those on flat surfaces. Therefore, this paper investigates the heat transfer characteristics and the related essential flow dynamics of a sweeping jet with a Reynolds number of 2577 impinging on a concave surface, and those of a steady jet under the same conditions are compared. The jet-to-wall distance and the curvature of the impingement surface are 4 and 10 times the jet's hydraulic diameter, respectively. To correctly capture the three-dimensional characteristics of the sweeping jet, this research employs the Reynolds stress model for turbulence modeling and the Generalized k-ω (GEKO) model for better calibration of the jet's spreading and dissipation rates, as well as the characteristics of trapped vortex generated due to the confined concave flow channel. The study initially focuses on the average Nusselt number on the impingement surface, revealing a 15.2% increase in heat transfer intensity of the sweeping jet compared to the steady jet, and the sweeping jet covers a larger area with enhanced uniformity. Subsequent analysis on streamlines in two perpendicular planes and Q criterion iso-surfaces in the spatial domain shows the formation of a trapped vortex ring with a constant location around the jet, restricting the effective cooling range to its inner side and thus resulting in a steep temperature rise on the two lateral sides of the vortex ring. The stability of the trapped vortex is enhanced by the curved impingement wall in the oscillation plane, which on the one hand forces the high-temperature fluid that has exchanged heat with the wall to repeatedly wash the same location, and on the other hand significantly reduces the jet oscillation angle compared to a free sweeping jet. Consequently, the heat transfer intensity of the sweeping jet is lower in the main oscillation plane than in the non-oscillation plane. Analysis of the near-wall flow parameters indicates that the sweeping jet generates higher velocity and turbulence kinetic energy in the wall jet zone far away from the stagnation point compared to the steady jet, which results in a higher local heat transfer intensity. The unsteady transverse oscillation of the sweeping jet disturbs the wall jet zone and disrupts the formation of the wall boundary layer, causing a higher velocity gradient at the wall and hence the lower thicknesses of the velocity boundary layer and the thermal boundary layer than those of the steady jet. In conclusion, this study reveals the flow and heat transfer features of the sweeping jet impingement on confined concave surface, with the influence of the trapped vortex ring mattering significantly. The findings provide a reference for designing a more effective jet arrangement. Future work will focus on investigating the effects of jet Reynolds number and impingement distance variations on the trapped vortex ring, and how to mitigate the adverse effects of the trapped vortex ring on impingement heat transfer.

Quantifying flow regime impacts on hydrodynamic heat transfer in rough-walled rock fractures

Hydrodynamic heat transfer in rock fractures is crucial for many geophysical processes and engineering activities, notably in harnessing geothermal energy. Despite extensive research, characterizing hydrodynamic heat transfer in rock fractures, complicated by their geometric heterogeneity, remains a formidable challenge. To address this challenge, we experimentally investigated the hydrodynamic heat transfer in rock fractures with diverse geometric features at steady-state elevated temperatures. We found that both heterogeneous fracture geometry and elevated temperatures promote the transition of fracture flow from a linear to nonlinear regime. This flow regime transition enhances the heterogeneity of heat transfer intensity within the fracture and exacerbates the increase in heat transfer efficiency with flow rate. At a fortyfold increase in flow rate, the heat transfer coefficient in the most heterogeneous fracture rises by approximately 72 times, compared to about 28 times in the least heterogeneous one. Leveraging experimental data, we developed parametric models linking heat transfer coefficients, outflow temperature, and heat recovery rate to flow rate and geometric features, yielding a theoretical formulation for the maximal heat recovery rate. By introducing the concept of an energy conversion rate, we quantitatively evaluated the effect of nonlinear flow resistance on heat extraction feasibility, revealing that energy conversion rates in highly heterogeneous fractures can be up to 452 times lower than in less heterogeneous ones. This highlights the pivotal role of flow resistance in heat extraction feasibility. Our findings are instrumental for understanding and predicting the hydrodynamic heat transfer within fractured geothermal reservoirs and analogous hydro-thermal systems.

An experimental and numerical investigation into the sensitivity of Rayleigh–Bénard convection to heat loss through the sidewalls

Thermal buoyancy flows and related heat transfer problems play a crucial role in a variety of fields and industrial applications. The present study is structured around the main objective of systematically investigating the response of fluid convection of the Rayleigh–Bénard type to heat loss taking place through the lateral walls of the considered fluid container. We elucidate, both experimentally and numerically, new aspects which deeply affect the dynamics when the classical assumptions of adiabatic or conducting walls cease to be valid. The emerging spatio-temporal behaviors are examined with respect to several parameters or conditions, including the dimensionality of the system (made accessible by dedicated numerical simulations based on the time-dependent and non-linear governing equations), the average temperature of the liquid, the applied temperature difference, the dependence of fluid properties on temperature and the intensity of heat transfer to the ambient. The results reveal the interesting triadic relationship among the heat exchanged through non-thermally controlled walls, the hierarchy of bifurcations displayed by the flow and the related multiplicity of solutions.

Superbiphilic hierarchical aluminum surfaces for exceptional pool boiling performance

The advancement in high-power electronic devices coupled with the need to ensure reliable and efficient heat dissipation of two-phase cooling systems underscores the urgent necessity for breakthroughs in enhancing boiling performance and especially critical heat flux (CHF) to minimize the risk of system failure. In this field, surfaces with tailored wettability have already demonstrated their potential to enhance boiling heat transfer intensity, while surfaces featuring wickable structures like micropillar arrays have shown significant improvements in CHF. In this study, we investigate the use of aluminium micropillar surfaces with tailored wettability to simultaneously enhance nucleate boiling heat transfer performance and specifically increase of the CHF. We fabricated the micropillar surfaces using a combination of nanosecond laser texturing and chemical etching in hydrochloric acid, while the wettability of selected surfaces was further tailored by application of a fluoroalkyl phosphonic acid and an additional laser texturing step. Three micropillar patterns were tested under pool boiling conditions using saturated twice-distilled water at atmospheric pressure. Importantly, our results revealed that the bottom part of the boiling interface (i.e., the superhydrophilic area) ensured increased liquid supply, while the top parts (i.e., the superhydrophobic area) tend to serve as nucleation sites. When combined, these two effects allowed us to simultaneously improve the CHF and the heat transfer coefficient, resulting in enhancements of up to 113% (2343 kW m−2) and 450% (205 kW m−2 K−1), respectively, compared to the benchmark untreated surface. This research provides a practical and reliable approach to enhancing heat transfer by fabricating hierarchical surfaces, offering potential applications in ultrahigh heat flux thermal technologies.