Heat Exchange Efficiency Research Articles

Optimization study of the circuitry for 5 mm small-diameter finned-tube heat exchanger in air-source heat pumps

Finned-tube heat exchangers (FTHXs) are currently extensively utilized in residential air-source heat pumps (ASHPs), but their escalating costs pose an urgent issue needing resolution. This study investigated the feasibility of replacing the 7 mm diameter FTHX in the outdoor unit of an ASHP with a 5 mm small-diameter FTHX, aiming to reduce material consumption without compromising performance. Based on the Intelligent System for Heat Exchanger Design (ISHED), a 6–3-1 circuitry (6 circuitries merging into 3 then 1) optimization scheme tailored for the 5 mm diameter FTHX was proposed under non-uniform airflow distribution. The performance differences between this circuitry and the baseline were then analyzed through numerical simulations. Finally, comparative tests were performed between the original FTHX and the modified 5 mm diameter FTHX. Overall, even with a 5.10 % reduction in total heat exchange area, its heat exchange capacity and efficiency still meet the performance standards for the initial ASHP. Notably, the proposed 5 mm diameter FTHX reduces the material costs and refrigerant charge by 19.76 % and 12.28 %, respectively. This phenomenon illustrates the considerable economic benefits of reducing the volume of FTHX. Additionally, it holds significant practical value in promoting the use of natural refrigerants including hydrocarbons according to the new international legislative framework.

Fractional Analysis of Magnetohydrodynamics Maxwell Flow Over an Inclined Plate with the Effect of Thermal Radiation

This current study provides a comprehensive examination of a novel method for studying the dynamics of a fractionalized Maxwell flow near an inclined plate, considering non-uniform mass transfer through a permeable media. Through the use of partial differential equations, incorporating heat and mass movement effects, the study employs a combination of generalized Fick’s and Fourier’s law with the Caputo operator. Transforming the fractionalized model into dimensionless form using appropriate dimensionless values, semi-analytical solutions for the non-dimensional transmitted fractional model are obtained via the Laplace transformation method. Through graphical analysis, the precise contributions of key parameters such as heat generation, radiation, and chemical reactions are elucidated, including their impacts on the calculated heat generation parameter (Qo), radiation parameter (Nr), and others. The study’s significance lies in its implications for the design of efficient heat exchangers, fluid flow systems, and cooling components in complex engineering systems, including nuclear reactors and power generation plants. Furthermore, the fractional derivative approach offers a more accurate representation of the viscoelastic behavior of materials like polymers, crucial for optimizing fabrication processes such as extrusion and molding. The insights gained from this study extend to the realm of miniaturized fluidic devices, including bio-analysis tools, lab-on-a-chip technology, and microfluidic drug delivery systems, where improved performance and control need a grasp of Maxwell fluid dynamics. The physical outcome of this research lays the groundwork for future investigations that will maximize heat transfer efficiency in real-world systems and give insightful information on the behavior of complicated fluids. We compute and display the skin friction, mass and heat transfer rate in tabular form.

Experimental study on the characteristics of R1150/R600 mixed refrigerant Joule-Thomson low-temperature freezer

In this paper, a low-temperature freezer based on Joule-Thomson system with a variable frequency fan was developed by using the natural binary mixture of R1150/R600. The experiment results demonstrated the feasibility of using this mixture to obtain the low freezing temperature below −70 °C. It could be found that when the concentration of R1150 was 27.5 %, it had the lowest daily energy consumption of 5.94 kWh/24 h. Furthermore, the influences of different fan speeds and active fan speed control on the cool-down characteristics of the system were also studied. The results indicated that lower fan speed in the early stage could effectively improve the cool-down performance. It was 51 min faster than that of the fixed operating fan speed under the optimal fan speed control at the target freezing temperature. Finally, the noise test of the freezer was carried out, and the influences of on–off temperature and fan speed on the system noise were also analyzed. The noise level of the system is between 46.71 dB and 51.62 dB, which is acceptable for the users. Moreover, the influence of fan speed on system noise was greater than that of different on–off temperatures. Therefore, improving the heat exchange efficiency of the condenser to reduce the fan speed is the possible direction of noise reduction for low-temperature freezers in the future.

Local entropy generation optimization and thermodynamic irreversibility analysis of oval-shaped coaxial ground heat exchangers: A detailed numerical investigation

Optimizing the heat transfer and thermodynamic efficiency of Ground Heat Exchangers (GHE) is crucial for maximizing the energy efficiency of Ground-coupled Heat Pump (GCHP) systems utilized in building heating and cooling applications. This study pioneers an effective approach of both the first and second laws of thermodynamics of the newly oval-shaped coaxial GHEs (oval-CGHEs), assessing heat transfer, thermohydraulic performance, and local entropy generation rate to explore the optimum design and operational conditions for heat exchange maximization and entropy generation minimization. Utilizing a three-dimensional numerical model validated by experimental results, this study examines the trade-offs between heat transfer improvements and entropy generation rate intensification of the oval-CGHEs in comparison to the typical circular-shaped CGHE (circle-CGHE) further than exploring the effect of key operational and design parameters and performing sensitivity analysis. The results reveal that oval-CGHEs exhibit higher irreversibility as a penalty for their superior heat transfer capabilities compared to traditional circular-shaped CGHEs (circle-CGHE), which can be mitigated by optimizing the position of the inner tube. Moreover, the study underscores the criticality of maintaining a balance between enhanced heat transfer and the associated rise in irreversibility, particularly concerning the factors of mass flow rates and installation depths.

Vibration-Free 40-80K Turbo-Brayton Cryocooler and its Very High Efficiency Recuperator for Earth Observation Instruments

Several studies have demonstrated that Reverse Turbo-Brayton cryocoolers could be the next space cryogenic revolution, thanks to their capability to provide vibration-free remote cooling in the temperature range of 4 to 150K and for medium to high cooling powers. With this motivation, Absolut System developed a very low vibration 40-80K Reversed Turbo-Brayton cooler, work performed under the ESA Technical Program Technical Research Program - 4000113495/15/NL/KML. The cooler is based on two turbo-compressor stages, a high efficiency recuperator and a cryogenic turbo-expander, for operation between 300 and 40K. Following the fabrication of the cooler, we performed tests on both individual components, compressors, expander and recuperator as well as on the complete cooler. The turbomachines use aerodynamic bearings, i.e. contactless bearings, generate very little vibrations and exhibit no wear over time. The characterizations performed during the project concern exported micro-vibration behaviors of the compressors and the expander, operating respectively up to 250,000RPM and 150,000RPM, and thermodynamic performance of the different elements. The cooler was tested down to its minimum temperature and required the development of specific operating procedures for the conditioning of the circuit. We report here in a first part the main results and observations stemming from the test campaigns carried out during the project. This work showed the necessity of putting additional effort into the recuperator, one of the most critical components of the cooler. Absolut System is thus also working in parallel on developing a very high efficiency and compact heat exchanger, which we report in a second part, for the specific needs of Turbo-Brayton Coolers in response to the ESA Proposal ESA-TDE-TECMTT-SOW-023649. The technology selected for this is based on the common tube and shell but using very thin tubes of small diameters (<1mm). The challenge of this exchanger is not only the manufacturing, but also the very stringent requirements. Obtaining an efficiency higher than 97.5% in a very small mass while maintaining very low pressure drops, makes for a very challenging project. We have performed the design phase, tested, and validated different aspects of the assembly. The design for the first breadboard model has been validated numerically and is in the fabrication process. Testing the exchanger will then take place to characterize the performances of the recuperator and allow for a comparison between experimental results and analytical and CFD models. This effort participates in placing the Turbo-Brayton cryocoolers at the forefront of space coolers and pushing Europe even further in State of Art advancements for space to bring us closer to fulfill future Space Explorations and Earth Observation missions.

Numerical analysis on heat transfer performance of spiral elastic copper tube heat exchanger with helical defectors

Based on the flow-induced vibration mechanism, three kinds of spiral elastic copper tube heat exchangers were proposed to improve the comprehensive heat transfer performance, involving the spiral elastic copper tube heat exchanger with no defector, the spiral elastic copper tube heat exchanger with forward helical deflector and the spiral elastic copper tube heat exchanger with reverse helical deflector. A both-way fluid–structure coupling method was utilized to investigate the effects of helical defectors and their arrangements on the vibration-enhanced heat transfer performance of the spiral elastic copper tubes at different inlet velocities by numerical simulation. The results indicate that: when the inlet velocity ranges from 0.1 m/s to 0.7 m/s, the heat exchanger with reverse helical deflector possesses better heat transfer performance than the other two heat exchangers. For the heat exchangers with no defector and forward helical deflector, the flow-induced vibration always promotes the comprehensive heat transfer performance. Compared with the heat exchanger with no defector, the comprehensive heat transfer performance of the heat exchanger with reverse helical deflector is higher 4.02 % averagely, while that of the heat exchanger with forward helical deflector is lower 29.34 % averagely. Furthermore, a structural optimization is presented to improve the existing disadvantages of the heat exchanger with reverse helical deflector, after structural optimization, the average amplitude of the heat exchanger with forward helical deflector is decreased by 3.26 % averagely and the heat transfer performance is raised to a maximum of 26.24 %, which are of great significance to extend the service life and improve the energy efficiency of heat exchanger.

Thermal performance of a chaotic heat exchanger: an experimental study

Improving the efficiency of heat exchangers requires innovative approaches that reconcile performance and sustainability. This study examines the potential of a curved geometry employing chaotic advection as a passive technique for improving internal heat transfer. The Chaotic Heat Exchanger (CHE) studied was constructed using 18 bends at 90 degree angles, and by simply turning each bend at a 90-degree angle to the previous one. The aim is to investigate the chaotic heat exchanger by focusing on heat transfer efficiency. A comparative analysis is carried out between the chaotic heat exchanger and a conventional Helical Heat Exchanger (HHE), using an experimental approach with water as the internal hot fluid. The Reynolds number examined ranges from 250 to 3500. It is found that the CHE outperforms the HHE both in terms of heating capacity and of inner Nusselt number, particularly in the intermediate Reynolds number range.

Numerical study of heat transfer characteristics of intermittent flow cold storage surface heat exchanger

The intermittent flow cold storage surface heat exchanger is a key component of the new pulse tube expansion cryogenic refrigerator, which is responsible for the periodic heat transfer of cold and hot helium, and its heat exchange efficiency directly affects the thermodynamic efficiency of the whole machine. In this study, a three-dimensional model of the heat exchanger unit is established, and numerical simulation is employed to investigate its heat exchange characteristics. The analysis focuses on understanding the internal temperature distribution patterns within the heat exchanger and explores the influence of wall materials on its heat transfer characteristics. Results show that the existence of the temperature lag loop minimizes the effective heat transfer temperature difference, consequently decreasing the heat exchanger’s efficiency. Moreover, the choice of wall material significantly affects the performance of the heat exchanger, with optimal thermal conductivity and volumetric specific heat enhancing its efficiency. This research provides valuable insights for future design and improvement of similar heat exchangers.

Two-phase cooling system for electric vehicles’ battery

The objective of this research project is to design an innovative cooling system for effectively managing the thermal conditions of batteries in electric vehicles. Electric vehicles commonly utilize Lithium-Ion cells as their power source. Despite significant advancements in Lithium-Ion technology from an electrochemical standpoint, the thermal management of these batteries remains a formidable challenge. This is primarily due to the demanding operational conditions that Lithium-Ion cells face during battery discharge, motion, and charging. The power supply unit in electric vehicles often demands high power outputs within short durations, leading to the generation of substantial heat by the batteries. This elevated working temperature poses a risk of decreased battery performance or even malfunction. Therefore, an efficient battery thermal management system is essential to optimize the performance of the batteries. In our research project, we propose and investigate a cooling system that is directly integrated into the power supply unit. Our study introduces an innovative thermal management system that combines two-phase direct liquid cooling with pulsating heat pipes. This system provides a compelling solution by combining high thermal efficiency, passive operation, and cost-effectiveness. Within this configuration, batteries are immersed in a low-boiling dielectric fluid contained in a Plexiglas container, facilitating efficient heat exchange. Simultaneously, the pulsating heat pipe operates to manage heat spikes by promoting vapor recondensation, thereby maintaining safe operational temperatures. The proposed battery thermal management system has demonstrated remarkable efficiency, ensuring that battery temperatures remain within the recommended range even under high load conditions. A notable advantage of this cooling system is its complete passivity, eliminating the need for energy-consuming coolant circulation.

Матричное моделирование и диагностика состояния регенеративных подогревателей высокого давления

Increasing the efficiency of heat exchangers is a promising area of energy saving, which is one of the priority areas of the development of science, engineering, and technology in the Russian Federation. Improvement of heat and mass transfer in regenerative heaters and condensers of turbine units of thermal power plants can significantly increase the operating efficiency of power equipment. Diagnostics of the condition and improvement of operating modes of the high-pressure regenerative heater which are an integral part of regenerative heating systems for feedwater of turbine units, are urgent scientific and practical tasks. To develop a model of a high-pressure regenerative heater, the methodology of matrix formalization of the description of heat and mass transfer processes has been used. To solve problems of diagnosing the state of heating surfaces of heat exchange equipment, mathematical programming methods have been applied. Within the framework of the matrix formalization methodology, an approach to modeling and solving diagnostic problems of multi-stage heat exchangers considering the phase transition in coolants has been developed. The sensitivity of the resulting model to input signals has been studied. Solutions of the problem of diagnosing the state of a high-pressure regenerative heater have been obtained and analyzed. Analysis of the obtained calculated results has shown an adequate description using the model of the real heat exchange process for the analyzed heater and the possibility to use the proposed model for monitoring and operational diagnostics of power equipment. The proposed approach allows us to formulate and solve inverse problems of diagnosing the state of equipment of heat exchange installations for various purposes.

SCALE FORMATION IN THE OIL AND GAS INDUSTRY AND THE TRANSITION TOWARDS «GREEN» INHIBITORS

Hydrocarbon production in oil field conditions invariably involves the joint production of significant amounts of water. Mineral salts of various compositions can precipitate from produced water under certain conditions. In the oil and gas industry, the deposition of mineral salts on the surface and underground production equipment can cause various problems, such as deterioration of reservoir properties, reduced production and premature failure of well equipment. Scale deposition can also lead to pump malfunctions, reduced efficiency of heat exchangers, reduced quality of commercial products and increased corrosion processes. Salt deposits lead to expensive restoration work, downtime in well operations and, ultimately, to an increase in the cost of produced hydrocarbons.As a result of the geochemical processes between the injected water and the formation water and the formation rock, as well as the complex composition of the formation fluids, controlling the formation of inorganic scales becomes a challenge. In the oil and gas industry, carbonate (calcium), sulfide (iron) and sulfate (calcium, barium, strontium) deposits are most often encountered. The formation of scale deposits depends on several factors, which include, but are not limited to, temperature, pressure, solution saturation and hydrodynamic flow characteristics.There are many approaches to managing scale deposits, but the most common currently is the use of chemical reagents – scale inhibitors. Traditional scale inhibitors contain phosphorus- and nitrogen-based chemicals as their active base, which can be harmful to the environment and human health. In this regard, in the world practice of oilfield chemistry, the direction of «green» chemistry is actively developing – reagents that can effectively prevent the formation of mineral salts with minimal negative impact on the environment and humans.This review discusses various scale types, methods for their removal, scale inhibitors of various natures and the mechanism of their work. Attention is also paid to «green» inhibitors and their development.

Impact of groundwater seepage on thermal performance of capillary heat exchangers in subway tunnel lining

The metro tunnels are currently facing challenges associated with thermal pollution, which significantly affects their operational safety and energy consumption. The subway source heat pump system (SSHPS) with front-end tunnel lining capillary heat exchangers (CHEs) is an effective technology to address this problem. The heat transfer characteristics of SSHPS have been previously investigated; however, there is a lack of research on the thermal performance of tunnel lining CHEs under groundwater seepage. This study established a numerical simulation model of tunnel lining CHEs based on a demonstration project of SSHPS. Subsequently, the influence of groundwater seepage on the heat transfer characteristics of tunnel lining CHEs was comprehensively analyzed. The findings suggest that an increase in groundwater seepage velocity leads to the initial expansion and subsequent contraction of the thermal influence range caused by CHEs. Simultaneously, the performance of CHEs is enhanced with increasing flow velocity. When the seepage velocity increases from 0 to 1 × 10−4 m/s, the heat exchange capacity per meter and efficiency of the CHEs during the cooling season increase by 450 % and 446 %, respectively. During the heating season, they experience a growth of 320 % and 310 %, respectively. In addition, the heat exchange performance of CHEs is also influenced by the relative flow direction between the fluid within CHEs and the groundwater. The counter flow arrangement exhibits a 7.7 % higher efficiency of heat exchange during the cooling season compared to the parallel flow arrangement, and a 3.9 % higher efficiency of heat exchange during the heating season. Therefore, it is highly recommended to evaluate groundwater seepage conditions during the design phase of SSHPS in order to enhance the performance of tunnel lining CHEs. This study serves as a valuable theoretical reference for the design of tunnel lining CHEs.

Numerical investigation of flow boiling characteristics of R134A in a smooth horizontal tube

Three-dimensional transient simulations using ANSYS Fluentʼs mixture multiphase model was conducted to investigate the impact of heat flux, mass flux, and saturation temperature variations on pressure drop behaviour and heat transfer characteristics during flow boiling of R134a refrigerant in smooth horizontal tubes of different diameters. The simulations employ an implicit numerical method to solve the governing equations, with relevant source terms for mass and energy transfer, incorporating the continuum surface force (CSF) and shear stress transport (SST) κ-ω models to account for surface tension and flow turbulence respectively. The results are presented in both dimensional (heat transfer coefficient and pressure drop) and non-dimensional (Nusselt number and two-phase frictional factor) forms. The Nusselt number shows a direct relationship with the Reynolds number, while the two-phase friction factor exhibits different behaviour. At higher vapor quality, mass flux significantly affects heat transfer performance and pressure drop. Heat flux plays a crucial role in heat transfer, but its impact on pressure drop is negligible. Higher saturation temperatures enhance heat transfer while reducing pressure gradients. The obtained results demonstrate good agreement with existing correlations, with mean absolute deviations (MAD) as low as 1.2 % for the heat transfer coefficient and 23.7 % for the pressure drop behaviour. This highlights the model's accuracy in predicting flow boiling behaviour of R134a in smooth horizontal channels, providing valuable insights for thermal analysis. The findings contribute to the understanding of flow boiling processes and aid in designing and optimizing efficient heat exchangers for various engineering applications.

FEA and CFD analysis and optimization of tube in tube heat exchanger for various cases

This research describes the use of advanced simulation techniques to investigate use of different materials for a tube-in-tube heat exchanger. Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) simulations are employed to simulate thermal aspects and optimize the heat exchanger design. The objective is to identify the best-Suitable material combination considering both design and thermal considerations. The findings of this research contribute to the development of more efficient heat exchanger designs.

Investigating the coupling effect of ventilation and GHEs on temperature distribution and heat transfer characteristics

Tunnel lining geothermal heat exchangers (GHEs) offer numerous advantages, including small area, cost-effectiveness and high heat exchange efficiency. It is a good equipment to utilize geothermal energy in tunnels. However, few research has been conducted on the combined impact of tunnel lining GHEs and ventilation on the temperature field of the surrounding rock. In this study, laboratory test model of combined impact of heat transfer between ventilation and GHEs was established, the effects of different flow rates and wind speeds on temperature field and heat exchange characteristics of the rock were investigated, and the changes in heat transfer under different operational conditions were analyzed. The findings reveal that tunnel ventilation initially enhances the heat transfer efficiency of GHEs, but subsequently decreases the overall heat transfer. Moreover, higher wind speeds exacerbate this negative impact. The radius of the heating ring decreases with increased heat transfer, and the temperature of the surrounding rock rises with heightened heat exchange at same locations within the heating ring. There are reasonable ranges of wind speed and GHEs flow rate, which are 1.5 m/s–2 m/s for wind speed and 0.4 m/s–0.6 m/s for flow rate, respectively. These results are important for the evaluation of thermal effect of tunnel lining GHEs and ventilation design.

Irreversibility analysis of Darcy-Forchheimer flow of a Williamson hybrid nanofluids near a stagnation-point across a vertical plate with buoyancy force

PurposeA novel type of heat transfer fluid known as hybrid nanofluids is used to improve the efficiency of heat exchangers. It is observed from literature evidence that hybrid nanofluids outperform single nanofluids in terms of thermal performance. This study aims to address the stagnation point flow induced by Williamson hybrid nanofluids across a vertical plate. This fluid is drenched under the influence of mixed convection in a Darcy–Forchheimer porous medium with heat source/sink and entropy generation.Design/methodology/approachBy applying the proper similarity transformation, the partial differential equations that represent the leading model of the flow problem are reduced to ordinary differential equations. For the boundary value problem of the fourth-order code (bvp4c), a built-in MATLAB finite difference code is used to tackle the flow problem and carry out the dual numerical solutions.FindingsThe shear stress decreases, but the rate of heat transfer increases because of their greater influence on the permeability parameter and Weissenberg number for both solutions. The ability of hybrid nanofluids to strengthen heat transfer with the incorporation of a porous medium is demonstrated in this study.Practical implicationsThe findings may be highly beneficial in raising the energy efficiency of thermal systems.Originality/valueThe originality of the research lies in the investigation of the Darcy–Forchheimer stagnation point flow of a Williamson hybrid nanofluid across a vertical plate, considering buoyancy forces, which introduces another layer of complexity to the flow problem. This aspect has not been extensively studied before. The results are verified and offer a very favorable balance with the acknowledged papers.

Heating and defrosting performance assessment of dual-source heat pump operational modes under various ambient conditions

Multifunctional dual-source heat pump systems, featuring two sets of heat exchangers (air-to-refrigerant and water-to-refrigerant) in series or parallel, aim to boost efficiency. However, these systems often require extensive piping, leading to power losses, higher energy consumption, and increased installation space requirements. Furthermore, few comparative studies have assessed the effectiveness of dual-source versus single-source heat pump systems under various ambient conditions. This study introduces a novel air-water dual-source evaporator design that enables efficient heat exchange using air, water, or both. With this design, the heat pump system can be tuned according to three distinct modes: pure water-source mode (WSHP), pure air-source mode (ASHP), and dual air-water-source mode (AWSHP). The heat pump's efficiency across these modes was evaluated under various weather conditions, adhering to the CNS15466 and GB/T21362 standards. The findings reveal a strong correlation between the operational modes and ambient temperatures, indicating that maximum efficiency is achieved with the WSHP mode below 7 °C, AWSHP mode between 10 °C and 35 °C, and ASHP mode above 35 °C. Moreover, three defrosting methods were evaluated: hot gas bypass, hot water bypass, and a combination of both. The hot water bypass, in conjunction with the proposed dual-source evaporator design, efficiently performed full defrosting, boosting system efficiency. The ability of the heat pump to switch between operating modes enhances the overall energy efficiency during space heating and defrosting, reduces installation space requirements and compressor oil loss, and facilitates hot water production. The proposed system is ideal for various industrial applications requiring efficient operation under diverse ambient conditions.

Characteristics of MWCNT, SWCNT, Cu and water based on magnetized flow of nanofluid with Soret and Dufour effects induced by moving wedge: Consequence of Falkner–Skan power law

Soret and Dufour effects are utilized in designing efficient heat exchangers, optimizing separation processes, and understanding biological transport phenomena. A wedge-shaped structures aid in aerodynamics, optics, seismic resistance, and biological studies. Due to these application the consequences Soret and Dufour effects on flow of nanofluid due to wedge have been investigated. The characteristics of water ( H 2 O ) as a base fluid and single-walled carbon nanotubes ( SWCNTs ) , multi-walled carbon nanotubes ( MWCNTs ) , and copper ( Cu ) nanoparticles are taking into account. The influence of thermal radiation and heat source/sink are part of this investigation. Similarity variables are used to sort out the complexity of fundamental governing equations. Maple via Runge–Kutta–Fehlberg method (RKF45) is used to solve resulting system of ordinary differential equations (ODEs). The graphical and tabulated results of velocity, temperature, and concentration profiles are plotted against different parameters involved in this study. The findings indicate that modifying the moving wedge factor reduces the temperature distribution. By increasing value of magnetic constraint the velocity distribution increased, the temperature and concentration distribution both are increased for varying the value of Dufour, and Soret and source/sink parameters.

Enhancing the efficiency of a double tube heat exchanger via a novel vibrating woven wire turbulator combined with a helical coiled wire; an experimental study

Vibrating turbulators have emerged as innovative methods for enhancing heat transfer in heat exchangers over the past two years. Previous studies have mainly employed string or strip configurations for turbulator construction. In this research, a novel vibrating turbulator, known as the vibrating woven wire turbulator, is introduced to enhance the thermal efficiency of heat exchangers. This study proposes the first-ever application of a turbulator with this unique geometry. This turbulator consists of intertwined metallic wires, providing a lightweight design with a substantial contact surface area for the fluid. Experiments are conducted for Reynolds numbers ranging from 1000 to 8500, covering both laminar and turbulent flow regimes. The findings demonstrate that this turbulator significantly enhances heat transfer by up to 236 %, while only resulting in a marginal increase in pressure drop. This remarkable improvement in heat transfer, while keeping the pressure drop within acceptable limits, gives rise to an impressive thermal enhancement factor (TEF) of 2.45. To achieve an even higher TEF, this study investigates the synergistic combination of the vibrating woven wire turbulator with a helical coiled wire turbulator. Experiments are conducted on coils with diameters ranging from 0.5 to 1.5 mm. The simultaneous utilization of both turbulators leads to a significant improvement in heat transfer coefficient and pressure drop, with an increase of 545 % and 320 %, respectively, resulting in a high TEF of up to 3.13.

Theoretical analysis and numerical study of natural convection inside combined solar chimney

AbstractSolar chimneys may induce natural ventilation through solar radiation. However, sufficient theoretical studies are needed as a basis to fully exploit passive design in practical green building design. In this work, we investigate the heat transfer properties of turbulent natural convective flows in a combined solar chimney with a thermal flux at the absorption wall by means of theoretical analysis and numerical simulations. Two different flow patterns have been found, one with a clear thermal boundary layer flow pattern and the other without, based on high Rayleigh numbers. For flow development in these two flow regimes, the transient scaling analysis is performed separately and the control mechanism for each phase is presented. Some new scale relationships are established to characterize the ventilation performance of solar chimneys, including thermal boundary layer thickness δT, velocity vT, mass flow m, and so on. For the distinct thermal boundary layers, δT,s ~ HΓ2/5/Bo1/5κ2/5, vT,s ~ Bo2/5κ4/5Γ1/5/H, m ~ ρBo1/5κ2/5Γ3/5. For nonobvious thermal boundary layers, vT,f ~ Bo1/3κ/H2/3W1/3, m ~ ρBo1/3κ/A2/3. The important scale relationships are validated using corresponding numerical simulation data, such as the mass flow rate scale M ~ (Γ/κ)3/5Bo1/5 in the distinct thermal boundary layer flow state, and so on. The air changes per hour and heat exchange efficiencies are calculated for a solar chimney with a fixed height‐to‐width ratio to provide a basis for the design of a solar chimney.