A Review of Recent Passive Heat Transfer Enhancement Methods

[EN] Improvements in miniaturization and boosting the thermal performance of energy conservation systems call for innovative techniques to enhance heat transfer. Heat transfer enhancement methods have attracted a great deal of attention in the industrial sector due to their ability to provide energy savings, encourage the proper use of energy sources, and increase the economic efficiency of thermal systems. These methods are categorized into active, passive, and compound techniques. This article reviews recent passive heat transfer enhancement techniques, since they are reliable, cost-effective, and they do not require any extra power to promote the energy conversion systems' thermal efficiency when compared to the active methods. In the passive approaches, various components are applied to the heat transfer/working fluid flow path to improve the heat transfer rate. The passive heat transfer enhancement methods studied in this article include inserts (twisted tapes, conical strips, baffles, winglets), extended surfaces (fins), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), and nanofluids (mono and hybrid nanofluids).Ajarostaghi, SSM.; Zaboli, M.; Javadi, H.; Badenes Badenes, B.; Urchueguía Schölzel, JF. (2022). A Review of Recent Passive Heat Transfer Enhancement Methods. Energies. 15(3):1-55. https://doi.org/10.3390/en1503098615515

Numerical Simulation of Convective-Radiative Heat Transfer

This book presents numerical, experimental, and analytical analysis of convective and radiative heat transfer in various engineering and natural systems, including transport phenomena in heat exchangers and furnaces, cooling of electronic heat-generating elements, and thin-film flows in various technical systems. It is well known that such heat transfer mechanisms are dominant in the systems under consideration. Therefore, in-depth study of these regimes is vital for both the growth of industry and the preservation of natural resources. The authors included in this book present insightful and provocative studies on convective and radiative heat transfer using modern analytical techniques. This book will be very useful for academics, engineers, and advanced students

Effect of diameter, twist angle, and blade count on the thermal-hydraulic performance of a decaying twisted swirler

Inserting a decaying swirler into a heat exchanger has been shown to improve heat transfer with minimal effect on the friction factor. The study analyses the effect of diameter, twist angle, and blade count on the thermal-hydraulic performance of a Decaying Twisted Swirler (DTS) in a horizontally heated tube. The diameter, twist angle, and DTS's blade count are examined for 13.5 mm–15.5 mm with a 0.5 mm interval, 0°–360° with a 60° gap, and 2 to 6 blades, respectively. The Nusselt number, friction factor, and thermal-hydraulic performance are examined for Reynolds numbers between 4583 and 35000. The relative Nusselt number and friction factor increase as DTS diameter and twist angle increase, reaching a maximum value at Re = 4583. Despite this, the relative Nusselt number dispersed as the blade count increased. The relative friction factor increases as the blade count increases. Maximum relative Nusselt number and friction factor reached 1.64 and 3.25, respectively with DTS's 15.5 mm diameter, 360° angle, and 4 blades. Nonetheless, the thermal-hydraulic performance is greatest when the DTS has a diameter of 15.5 mm, a twist angle of 180°, and 2 blades with 1.17

Acta Universitatis Sapientiae - Electrical and Mechanical Engineering

Series Electrical and Mechanical Engineering publishes original papers and surveys in various fields of Electrical and Mechanical Engineering

Heat Transfer

Over the past few decades there has been a prolific increase in research and development in area of heat transfer, heat exchangers and their associated technologies. This book is a collection of current research in the above mentioned areas and describes modelling, numerical methods, simulation and information technology with modern ideas and methods to analyse and enhance heat transfer for single and multiphase systems. The topics considered include various basic concepts of heat transfer, the fundamental modes of heat transfer (namely conduction, convection and radiation), thermophysical properties, computational methodologies, control, stabilization and optimization problems, condensation, boiling and freezing, with many real-world problems and important modern applications. The book is divided in four sections : "Inverse, Stabilization and Optimization Problems", "Numerical Methods and Calculations", "Heat Transfer in Mini/Micro Systems", "Energy Transfer and Solid Materials", and each section discusses various issues, methods and applications in accordance with the subjects. The combination of fundamental approach with many important practical applications of current interest will make this book of interest to researchers, scientists, engineers and graduate students in many disciplines, who make use of mathematical modelling, inverse problems, implementation of recently developed numerical methods in this multidisciplinary field as well as to experimental and theoretical researchers in the field of heat and mass transfer

Numerical Heat Transfer and Fluid Flow 2021

This reprint focuses on experiments, modellings, and simulations of heat transfer and fluid flow. Flowing media comprise single- or two-phase fluids that can be both compressible and incompressible. The reprint presents unique experiments and solutions to problems of scientific and industrial relevance in the transportation of natural resources, technical devices, industrial processes, etc. In the presented works, the formulated physical and mathematical models together with their boundary and initial conditions and numerical computation methods for constitutive equations lead to solutions for selected examples in engineering

Solidification Enhancement in a Triple-Tube Latent Heat Energy Storage System Using Twisted Fins

Copyright: © 2021 by the authors. This work evaluates the influence of combining twisted fins in a triple-tube heat exchanger utilised for latent heat thermal energy storage (LHTES) in three-dimensional numerical simulation and comparing the outcome with the cases of the straight fins and no fins. The phase change material (PCM) is in the annulus between the inner and the outer tube, these tubes include a cold fluid that flows in the counter current path, to solidify the PCM and release the heat storage energy. The performance of the unit was assessed based on the liquid fraction and temperature profiles as well as solidification and the energy storage rate. This study aims to find suitable and efficient fins number and the optimum values of the Re and the inlet temperature of the heat transfer fluid. The outcomes stated the benefits of using twisted fins related to those cases of straight fins and the no-fins. The impact of multi-twisted fins was also considered to detect their influences on the solidification process. The outcomes reveal that the operation of four twisted fins decreased the solidification time by 12.7% and 22.9% compared with four straight fins and the no-fins cases, respectively. Four twisted fins improved the discharging rate by 12.4% and 22.8% compared with the cases of four straight fins and no-fins, respectively. Besides, by reducing the fins’ number from six to four and two, the solidification time reduces by 11.9% and 25.6%, respectively. The current work shows the impacts of innovative designs of fins in the LHTES to produce novel inventions for commercialisation, besides saving the power grid.Jiangsu Provincial Basic Research Program (Natural Science Fund); Natural Science Research Project of Jiangsu Province Colleges and Universities; Philosophy and Social Science Project of Jiangsu Province Colleges and Universitie

On the dynamics of heat transfer and combusting flows in porous media

Dynamics of heat transfer and combusting flows has attracted increased attention in porous media in recent years. A growing number of technologies require prediction of unsteady forced convection in porous media when the inlet flow is unsteady. Also, in practical combustion systems fluctuations in the fuel flow rate can occur and result in flame destabilisation, in particular in lean and ultra-lean modes of operation. This is due to heat transfer being dominant in combusting flows in porous media. To address these challenges a joint numerical and experimental approach is adopted. A numerical study of heat convection response of a reticulated porous medium to the harmonic and ramp disturbances in the inlet flow is investigated taking a porescale approach. The developed model consists of ten cylindrical obstacles aligned in a staggered arrangement with set isothermal boundary conditions. A few types of fluids, along with different values of porosity and Reynolds number, are considered. Assuming laminar flow, the system is first modulated by sine waves superimposed on the inlet flow velocity, and the spatio-temporal responses of the flow and temperature fields are calculated. The results are then utilised to assess the linearity of the thermal response represented by the Nusselt number on the obstacles. In general, it is found that for low Reynolds numbers, the dynamics of heat convection can be predicted decently by taking a transfer function approach. However, the dynamical relations between the inlet flow fluctuations as the input and those of Nusselt number as the output, can be non-linear. Second, the thermal system is subject to a ramp disturbance superimposed on the entrance flow temperature/velocity. A response lag ratio (RLR) is defined to further characterise the transient response of the system. The results reveal that an increase in amplitude increases the RLR and interestingly, the Reynolds number has almost negligible effects upon RLR. An experimental investigation is undertaken to examine the response of ultra-lean flames, stabilised in a porous burner, to the fluctuations imposed on the fuel flow rate. The employed porous burner includes layers of silicon carbide porous foam placed inside a quartz tube. The burner is equipped with a series of axially arranged thermocouples and is imaged by a digital camera. The fuel streams are measured and controlled separately by programmable mass flow controllers, which impose sinusoidal fluctuations with variable amplitude and frequency on the steady flow. To replicate realistic fluctuations in the fuel flow rate, the period of oscillations is chosen to be in the order of minutes. The flame embedded in porous media is imaged while the fuel flow is modulated. First, methane and blends of methane and carbon dioxide (mimicking biogas) are mixed with air and then fed to the burner at equivalence ratios below 0.3. Amplitude of the flame oscillations for methane is found to be higher than that for biogas. Further, it is observed that exposure of the burner to the fuel fluctuations for a long time (180s) eventually results in flame destabilisation. In a separate set of experiments, the hydrogen and methane blends are premixed with air at equivalence ratios below 0.275 and fed to the porous burner. It is found that fuel mixtures are noted to be rather insensitive to hydrogen flow fluctuation with a modulation amplitude below 30% of the steady flow. This study reveals the strong effects of unsteady heat transfer in porous media upon the fluctuations in flame position

Heat transfer and pressure drop characteristics in the transitional flow regime with twisted tape inserts

Studies on heat transfer and pressure drop characteristics in a tube with twisted tape inserts have been receiving research attention in the laminar and turbulent flow regimes since 1921. However, several gaps exist in the transitional flow regime. The purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics in a smooth circular copper tube with conventional twisted tape (CTT) inserts, alternating clockwise and counter clockwise twisted tape (CCCTT) inserts and peripheral u-cut twisted tape (PUCTT) inserts without and with ring (PUCTTR) inserts. An experimental set-up was designed and constructed in this study. The set-up was validated by comparing the results of the heat transfer and pressure drop characteristics in a smooth tube (without twisted tape inserts) with literature. The smooth circular copper tube had a wall thickness, an inner diameter, and a length of 1.5 mm, 19 mm, and 5.27 m respectively. The twisted tape inserts considered in this study were fabricated from 1 mm thick and 18 mm wide copper strips. The strips used for the CTT inserts were twisted to form tapes with twist ratios of 3, 4 and 5. A total of five 900 mm long CTT inserts were connected longitudinally to an additional 770 mm insert to form a tape with overall length of 5.27 m. The strips used for CCCTT inserts were twisted to obtain a twist ratio of 5 and 12 tapes were joined longitudinally so that a clockwise direction twisted tape insert was connected to a counter clockwise direction twisted tape insert. The assembling was at connection angles of 0°, 30° and 60°, to form CCCTT inserts with an overall length of 5.27 m. For the PUCTT inserts, the peripheries of the strips were cut to achieve depth ratios of 0.105 and 0.216. The strips were twisted to form tapes with a twist ratio of 5 and ring inserts were soldered on the PUCTT inserts to form PUCTTR inserts with ring space ratios of 1.25, 2.5 and 5. A total of five 900 mm long PUCTT inserts were connected longitudinally to an additional 770 mm insert to form a tape with overall length of 5.27 m. Water was circulated as test fluid and experiments were conducted at constant heat flux boundary condition, at Reynolds numbers of 300 – 11 404. This Reynolds number range covered the transitional flow regime, as well as sufficient parts of the laminar and turbulent flow regimes. This study focused on the identification of the transitional flow regime with the CTT, CCCTT, PUCTT and PUCTTR inserts. With the CTT inserts, it was the influence of twist ratio and heat flux on the transitional flow regime. For the CCCTT inserts, it was the influence of connection angle and heat flux on the transitional flow regime. Modified Grashof number which is a function of heat flux was used to describe free convection effects in the CTT and CCCTT inserts. The PUCTT and PUCTTR inserts it was the influence of depth ratio as well as ring space ratio on the transitional flow regime. When twist ratios and heat fluxes of the CTT inserts were compared, a reduction in twist ratio and heat flux caused the transitional flow regime to occur earlier. When the CCCTT inserts were compared it was found that both the start and end of the transitional flow regime were influenced by the connection angle and heat flux. When different connection angles of the CCCTT inserts were compared it was found that an increase in connection angle enhanced the heat transfer in the transitional flow regime. An increase in heat flux significantly enhanced the heat transfer in the laminar flow regime and delayed transition. When depth ratios of the PUCTT inserts were compared, an increase in depth ratio caused the transitional flow regime to occur earlier. Furthermore, the transitional flow regime occurred earlier with PUCTTR inserts than with PUCTT inserts and transition occurred even earlier as the ring space ratio was reduced. An increase in depth ratio and reduction in ring space ratio significantly enhanced heat transfer in the transitional flow regime. It can be concluded that when the CTT, CCCTT and PUCTT inserts were compared, transition first occurred with the CCCTT inserts and delayed the most with the CTT inserts. Heat transfer and pressure drop correlations were developed to predict the experimental data in the laminar, transitional and turbulent flow regimes. Where applicable the correlations were developed as a function of Reynolds number, twist ratio, modified Grashof number, connection angle, depth ratio and ring space ratio.Thesis (PhD)--University of Pretoria, 2019.• Department of Science and Technology (DST); • National Research Foundation (NRF); • University of Pretoria.Mechanical and Aeronautical EngineeringPhDUnrestricte

Towards COP27: The Water-Food-Energy Nexus in a Changing Climate in the Middle East and North Africa

Due to its low adaptability to climate change, the MENA region has become a "hot spot". Water scarcity, extreme heat, drought, and crop failure will worsen as the region becomes more urbanized and industrialized. Both water and food scarcity are made worse by civil wars, terrorism, and political and social unrest. It is unclear how climate change will affect the MENA water–food–energy nexus. All of these concerns need to be empirically evaluated and quantified for a full climate change assessment in the region. Policymakers in the MENA region need to be aware of this interconnection between population growth, rapid urbanization, food safety, climate change, and the global goal of lowering greenhouse gas emissions (as planned in COP27). Researchers from a wide range of disciplines have come together in this SI to investigate the connections between water, food, energy, and climate in the region. By assessing the impacts of climate change on hydrological processes, natural disasters, water supply, energy production and demand, and environmental impacts in the region, this SI will aid in implementation of sustainable solutions to these challenges across multiple spatial scales