Heat Transfer Effects Research Articles

Effects of extended pin fins on the hydrothermal performance of double pipe heat exchanger

Double pipe heat exchangers (DPHEs) are crucial for effective heat transfer between two fluids in industrial processes, enhancing energy recovery and optimizing thermal performance. This study aims to enhance the efficiency of DPHEs by incorporating extended pin fins. DPHEs are vital in industrial applications for their efficient heat transfer, but optimizing their performance remains challenging. Extended pin fins, which are minor protrusions added to the heat exchanger’s surface, increase the surface area for heat transfer. A numerical study on DPHEs with pin fins of varying length ratios (R=1, 2, 3, and 4) is conducted, validating our model by comparing results with prior experimental data. Parameters such as Nusselt number (Nu), friction factor, thermal–hydraulic performance factor (TPF), and exergy efficiency across Reynolds numbers from 4400 to 9400 are calculated. The results indicate that pin fins significantly enhance heat transfer. The Nu increases by 163 %-242 % for different length ratios, while the friction factor decreases, showing reductions of 161 %-290 %. Exergy efficiency improves by 216 % with an R=4 design compared to a standard DTHE, and the TPF increases by 152 % with the R=4 configuration. These findings demonstrate the substantial benefits of using extended pin fins in improving the thermal performance of DPHEs.

Surface structures and wettability: Their roles in enhancing nucleate boiling heat transfer of refrigerants

Nucleate boiling is an effective heat transfer mechanism. Previous studies have demonstrated that modifying surface structure or wettability can independently improve nucleate boiling efficiency; however, the combined effects and optimal modification sequence remain underexplored. This research aims to comprehensively analyze the influence of surface structure and wettability on the nucleate boiling heat transfer performance of refrigerants. Different surface structures and wettability were prepared using surface polishing, laser ablation, mechanical cutting, and a combination that involved initially employing laser ablation, followed by the application of chemical etching, and finishing with the treatment using materials possessing low surface energy. The static contact angles on these surfaces were measured via the capillary rise method, followed by pool boiling experiments and bubble behavior visualization. The results show that increasing surface wettability can enhance bubble departure frequency and capillary wicking, thereby improving critical heat flux and heat transfer coefficient. The key to optimizing surface modification to enhance critical heat flux involves constructing alternating high and low nucleation core regions to reduce interactions between bubbles; for enhancing heat transfer coefficient, the key lies in constructing compact nanoscale structures that expand the heat transfer and nucleation area, followed by improving wettability.

Experimental studies and pore-scale modeling of hydrate dissociation kinetics at different depressurization rates in a cubic hydrate simulator

Natural gas hydrate (NGH) is a promising energy resource that has triggered a race for scientific innovation in energy and environmental research. However, there still faces the challenges of low-efficiency hydrate dissociation and unstable gas production during the hydrate production. Clarifying the kinetic mechanism of hydrate dissociation is the key to improving the efficiency of “phase-transition to gas-supply” in hydrate-bearing sediment. This study successfully conducts hydrate dissociation experiments with different depressurization rates in the Cubic Hydrate Simulator (CHS). A novel dissociation kinetic model integrating the hydrate pore-scale morphology and its evolutions are established, and the sensitivities of the kinetic and thermal parameters are discussed in detail. The mathematical relationship between the hydrate saturation and reaction area is quantitatively analyzed for the hydrate formation and dissociation kinetics. The history matching simulation with the newly developed model achieves an excellent agreement with the experimental results. The full-lifecycle kinetic behaviors of the hydrate dissociation in the depressurization experiments are finely characterized. It is found that the evolutions of the hydrate saturation (SH) and temperature (T) spatial distributions exhibit strong heterogeneous characteristics in the vessel due to the phase transition and heat transfer effects. In addition, this reveals that the relatively high depressurization rate can lead to a more significant dissociation rate in the depressurization stage. This provides a new reliable and adaptable method for describing the hydrate dissociation kinetics in the porous media.

Experimental Study on the Effect of Skin Friction Drag and Convective Heat Transfer for Viscoelastic and Pseudoplastic Fluid Flow

Drag Reducing Agents (DRAs) have a huge impact and major concern in engineering and industrial applications. It makes the fluid flow turbulent to laminar, dampens eddy reduces head loss by up to a certain limit, and saves pumping energy costs. Viscosity is the property that dampens eddy due to the viscous effect, which increases the fluidity up to a certain limit. Pseudoplasticity is the shear thinning effect that decreases viscosity when the flow rate increases. So, for the viscoelastic effect, we can increase the concentration up to a certain limit to reduce head loss. Still, during flow due to the pseudoplastic effect, the viscosity will start decreasing which is a negative effect. So, these combined effects are studied to reduce skin friction drag in the pipeline and save energy costs which will be convenient for the food industry, chemical, and medicine industries. In this investigation, investigation is carried out for 0.3 g/L, 0.2 g/L, and 0.15 g/L of xanthan gum in turbulent flow to observe the pressure drop and heat transfer rate. The study reveals that after increasing viscosity the pressure drop reduced significantly. Conversely, the heat transfer rate was also reduced due to the poor mixing effect. A higher performance and less vibration of the pump was also observed. It was concluded that the frictional pressure drop was reduced up to 85 % and the heat transfer rate was reduced by up to 90% by increasing the concentration of the DRA up to 0.3 g/L at 10 LPM than the pure water or base fluid as a working substance on the double pipe heat exchanger. As the heat transfer rate reduced up to 90% with reducing pressure drop another aim of the study was to establish a concentration and flowrate for which the heat transfer rate is maximum and it was found at a concentration of 0.15 g/L of DRAs (drag reducing agents) at 22 LPM (maximum flowrate at this setup).

Magnetohydrodynamic Stagnation Point Flow and Heat Transfer of Casson Fluids Over a Stretching Sheet

This research investigates the effects of heat transfer on the stagnation-point flow of a non-Newtonian Casson fluid in a two-dimensional magnetohydrodynamic (MHD) boundary layer over a stretched sheet, considering thermal radiation impacts. By employing similarity transformations, the governing partial differential equations are transformed into nonlinear ordinary differential equations. The obtained self-similar equations are numerically solved using the Optimal Homotopy Analysis Method (OHAM). The numerical results are graphically represented, showcasing the influence of various parameters on fluid flow and heat transfer characteristics. The study uncovers important dynamics in transport phenomena. Examining and illustrating the effects of dimensionless parameters on velocity, temperature, and concentration profiles reveal significant insights. Moreover, skin friction and Nusselt number results for Casson fluids are analyzed and presented. The findings indicate that the Casson parameter and Hartman number act in opposition to fluid momentum, while the thermal conductivity parameter enhances fluid temperature. Thus, this research provides valuable insights into MHD boundary layer flows of non-Newtonian Casson fluids with thermal radiation effects, and the OHAM solution method proves effective in predicting flow transport properties.

Impact of graphene oxide lateral sizes on the mechanical and thermal properties of carbon fiber composites

AbstractThe impact of carbon fiber (CF) composites sizing with graphene oxide (GO) to form brick‐and‐mortar structures on their mechanical properties and thermal conductivity has not been closely examined. This study systematically investigates the effect of GO sheet size on the interfacial properties and thermal conductivity of CF composites. Three sizes of GO—small (0.11 μm), medium (0.33 μm), and large (0.97 μm)—were used to modify CF surfaces via layer‐by‐layer self‐assembly, forming the brick‐and‐mortar structures. The mechanical properties were assessed through flexural tests, interlaminar shear strength (ILSS) tests and tensile tests, while thermal conductivity was measured using a thermal constant analyzer. The results revealed that medium‐sized GO (GO‐M) significantly enhanced the interfacial strength and toughness of the composites. This improvement is attributed to the orderly arrangement of GO‐M on the CF surface, which enhances stress transfer between the fiber and the matrix. In contrast, small‐sized GO (GO‐S) tended to scatter, resulting in less effective reinforcement, while large‐sized GO (GO‐L) exhibited stacking and agglomeration, limiting its reinforcement effectiveness. However, GO‐L provided the highest thermal conductivity due to the formation of continuous heat conduction pathways. These findings suggest that GO‐M offers a balanced improvement in mechanical properties, whereas GO‐L is more suitable for applications requiring high thermal conductivity. This work underscores the importance of selecting the appropriate GO sheet size to optimize the interfacial properties and thermal conductivity of CF composites, which is crucial for enhancing their performance in high‐end engineering applications, such as aerospace, automotive, and marine industries.Highlights Modification of CF composites by GO‐M sheets significantly improved toughness and strength. GO‐L sheets formed an effective heat transfer channel with the highest thermal conductivity. The study emphasized the critical role of selecting the appropriate GO sheet size to optimize the mechanical and thermal properties of CF composites.

Double pass solar air heater with aerofoil and bio-inspired fins: A numerical analysis of thermohydraulic performance

In the present study, a numerical flow analysis is performed on a double pass solar air heater (DPSAH) with a bottom plate surface roughened using aerofoil fins to study its thermohydraulic performance. The experimentally validated DPSAH model was studied considering factors like different fin shapes, diurnal variation of solar radiation, absorber plate material, and the flow conditions at different relative height ratios (h/H) between 0.17 and 0.50 and fin length ratio (l/H) of 0.29. Using the RNG k-ε turbulence model, the relative Nusselt number (Nu/Nus), relative friction factor (f/fs), and thermohydraulic performance factor (THPF) were found for Reynolds number between 3000 and 21000 with an increment of 3000. The maximum enhancements of 4.23 and 2.51 times were achieved in the Nu/Nus and THPF values, respectively, for a Re value of 3000 in a 2-row setup with h/H of 0.50. The DPSAH with bio-inspired fins performed better than optimized aerofoil fins for majority of the tested range even with similar increase in effective heat transfer area. The values predicted using correlations developed in this study and numerically obtained values of the Nusselt number and friction factor matched closely with an extreme deviation of 5 % in f values.

A new approach to evaluate the overall heat transfer coefficient of a fluidized bed biomass pyrolyzer

The heat transfer characteristics of non-pretreated rice husk pyrolysis in a fluidized bed with alumina beads (500 μm - 590 μm) as bed material are studied. Assuming instantaneous heating of biomass to the reaction temperature, the effective overall heat transfer coefficient of the system (UA) is initially calculated based on the compositions of the collected products and reaction temperature. UA ranges from 0.82 to 2.95 W/K. This heat transfer coefficient study allows ad hoc product distributions to be predicted from theoretical bases. The effects of the biomass feeding rate (F) and the bed height (H/D) on UA values are also analyzed. Highest UA and specific heat of the pyrolysis reaction of −3.35 MJ/kg are found at F = 5 g/min and H/D = 1.0, where H/D is the largest and F is the lowest in the range studied. Upon feeding of the low thermal conductivity biomass, UA typically decreases. However, if the bed materials are limited, the fed biomass and its produced char act as an auxiliary heating medium to improve the effective heat transfer coefficient. High UA improves the efficiency of the pyrolysis reaction, but it decreases the bio-oil fraction in the product due to secondary decomposition.

Broadband Unidirectional Thermal Emission

AbstractDirectional control of far‐field thermal emission plays a key role in effective heat and energy transfer. However, conventional photonic strategies are challenging to concurrently control the polar and azimuthal angle of thermal emission over broadband. Here both polar and azimuthal angles of thermal emission are constrained to narrow ranges over broadband by introducing in‐plane anisotropy combined with magneto‐optical materials in the epsilon‐near‐zero (ENZ) wavelength range. The physical mechanism of tunable perfect absorption/emission is explored by investigating the evolution of multiple topological phase singularity pairs (TPSPs). The structure consisting of a magnetized gradient‐ENZ emitter and anisotropic spacer that exhibits high (>0.8) unidirectional emissivity (θ: 55°–79°, φ: 163.5°–196.5°) in the p‐polarization for a broad range of wavelength (22–26 µm) is demonstrated. The unveiled physics synergizing ENZ, anisotropy, and magneto‐optical properties that support broadband unidirectional thermal emission will bring new opportunities in applications such as thermal camouflaging, thermal photovoltaics, and infrared light sources.

An experimental investigation into the use of biomimetic methods for thermal regulation and heat retention with PCMs in buildings

To meet the goal of net-zero emissions by 2050, integrating renewable energy into building energy supplies is a key solution, particularly through the application of solar energy. Due to the intermittent nature of solar energy, effective thermal regulation and heat transfer using thermal mass materials play critical roles in energy efficiency, safety, and performance in building applications. For solar electricity production in building integration, the high temperatures in Building Integrated Photovoltaics (BIPV) need to be regulated to avoid the reduction of solar power conversion efficiency and to maximize the utilization of excess thermal energy. This requires effective energy charging and discharging to meet energy demands.A biomimetic method has been experimentally investigated to enhance the thermal regulation and the thermal energy retention capacity of hybrid PVT-PCM/EG systems for building thermal management. By mimicking natural systems, the PVT-PCM-EG system promotes sustainability and energy efficiency, contributing to reduced energy consumption and lower carbon emissions. Based on the testing condition of around 480 Wm-2 and 15 °C, it is recommended that the combined two transient temperature PCM/EG A28 and A36 for the PVT-PCM/EG system can achieve the best performance for both heat retention and PV temperature regulation.

Effective lateral dispersion of momentum, heat and mass in bubbling fluidized beds

The lateral dispersion of bed material in a bubbling fluidized bed is a key parameter in the prediction of the effective in-bed heat transfer and transport of heterogenous reactants, properties important for the successful design and scale-up of thermal and/or chemical processes. Computational fluid dynamics simulations offer means to investigate such beds in silico and derive effective parameters for reduced-order models. In this work, we use the Eulerian-Eulerian two-fluid model with the kinetic theory of granular flow to perform numerical simulations of solids mixing and heat transfer in bubbling fluidized beds. We extract the lateral solids dispersion coefficient using four different methods: by fitting the transient response of the bed to (1) an ideal heat or (2) mass transfer problem, (3) by extracting the time-averaged heat transfer behavior and (4) through a momentum transfer approach in an analogy with single-phase turbulence. The method (2) fitting against a mass transfer problem is found to produce robust results at a reasonable computational cost when assessed against experiments. Furthermore, the gas inlet boundary condition is shown to have a significant effect on the prediction, indicating a need to account for nozzle characteristics when simulating industrial cases.

Numerical Study on Heat Transfer and Thermal–Mechanical Performance of Actively Cooled Channel of All-Movable Rudder under Supercritical Pressure

The utilization of an actively cooled thermal protection system is widely recognized as an effective approach to decrease the temperature of components exposed to severe aerodynamic heating. In this study, two cooling schemes with different flow paths and structural configurations were proposed, and six cooling channel designs were developed by modifying the leading-edge details. A numerical analysis on the heat transfer and thermal–mechanical performance was conducted under actual flight conditions (30 km altitude, Mach 8). The results highlight an optimal design scheme that balances temperature control and minimized coolant flow rates. The channel flow field demonstrated its superiority by effective convective heat transfer and improved fluid mixing facilitated through recirculation zones and turbulence at the bends. Structural assessments showed that the optimal scheme not only provided better cooling but also preserved the structural integrity. Overall, the study offers a practical and effective thermal protection approach for air rudders subjected to severe heat.

Study of microfluidic heat transfer in 2D hydrophobic walls with different slip mechanisms

Numerous studies have shown that the gas at the solid-liquid interface in microflow can increase the slip length, reduce the flow resistance. However, the relationship between nanobubbles and fluid boundary slip is complicated, and the impact of nanobubble morphology must be considered. To investigate the combined effect of the bubble model on slip and heat transfer, we investigated the drag reduction and heat transfer effect of microfluidic flow on a 2D hydrophobic wall by theoretical derivation combined with numerical simulation. The comparison of the results revealed good agreement between the analytical solution and the numerical simulation results. It was shown that the gas slip generated by the rarefied gas significantly affected the flow heat transfer effect. Further analysis of the bubble meniscus shape revealed that minimizing the protruding angle of the meniscus is crucial for enhancing drag reduction. In addition, the simulation results also revealed that increasing the gas coverage fraction effectively reduces flow resistance while maintaining heat transfer performance. At the same time, the higher pressure enhanced the heat transfer effect while maintaining the stability of the drag reduction effect.

Convective heat transfer in porous channel with multi-layer fractures under Local thermal non-equilibrium condition

Based on Brinkman-extended-Darcy model and Local thermal non-equilibrium (LTNE) model, analytical solutions of velocity and temperature fields, pressure drop and Nusselt number of porous channels containing different number of fracture layers are obtained. The characteristics of fluid flow and heat transfer in multi-layer parallel fractured channels are further studied. Results show that for a single-layer or multi-layer fractured channel, the pressure drop changes sensitively at small hollow ratio and increases as the number of fracture layers increases with decreasing increase degree, but the pressure drop tends to be consistent regardless of the number of fracture layers when Darcy number is high. For small ratio of effective thermal conductivity, LTE model is valid regardless of Biot number, and the heat transfer intensity in multi-layer fractured channels is always higher than that in single-layer fractured channel at any hollow ratio, but the heat transfer intensity is still low. For large ratio of effective thermal conductivity, increasing Biot number or fracture layer number makes the Nusselt number increase under very small hollow ratio for both single and multiple fracture cases, and when both the ratio of effective thermal conductivity and Biot number are high, there is an intersection point occurs at hollow ratio near 0.1, which makes same heat transfer effect under different fracture numbers.

Heat and mass transfer effects on an unsteady MHD boundary layer flow of fractionalized fluid

This research investigates the transient magnetohydrodynamic (MHD) flow of a non-integer Maxwell fluid over a vertical plate, taking into account the effect of diffusion-thermo and parameter of heat absorption. The fractional derivative Caputo-Fabrizio is employed to model the problem. Using the Laplace integral transform technique, we solve the dimensionless governing equations to obtain the profiles of velocity, concentration, and temperature, which are then presented in graphical form for easy comparison and interpretation. The influence of several parameters—specifically, the fractional parameter and heat absorption is thoroughly analyzed and illustrated through a series of graphical representations. Furthermore, a comparison between traditional as well as fractionalized velocity fields is provided. The results show that chemical reactions and magnetic fields reduce the velocity profile, while diffusion-thermo and mass Grashof number increase the fluid velocity.

Experimental study on energy storage characteristics of packed bed using different solid materials

The packed bed energy storage system can solve the mismatch between solar energy supply and demand at a low cost. The physical properties of storage materials have a decisive impact on the performance of storage systems. Different scenarios may require different storage materials. Studying the impact of storage materials on storage characteristics is crucial. However, the comparative analysis and research on different TES materials are focused on characterization and numerical simulation but less on packed bed TES experiments. This paper evaluates the thermal durabilities of three materials, including sintered ore particles, alumina balls, and rock particles, through thermal cycling tests. Through packed bed heat storage experiments, the energy storage characteristics and thermocline evolution characteristics of three beds under different operating conditions are compared and analyzed. The results indicate that all materials exhibit excellent thermal durabilities. A larger bed voidage and smaller bed heat capacity are beneficial for thermal diffusion in the bed. When reverse charging, the natural convection of air in the bed enhances the convective heat transfer effect, resulting in a higher charging power. The existence of natural convection promotes the mixing of cold air and hot air in the tank, increases the irreversibility of the process, and leads to a significantly lower efficiency in reverse flow compared to that in forward flow. When changing the airflow direction, the system overall exergy efficiencies of SOP, alumina, and rock bed decrease from 74.1 %, 72.4 %, and 77.2 % to 47.0 %, 39.9 %, and 45.8 %, respectively. Regardless of the airflow direction, the smaller the bed voidage and the larger the bed heat capacity, the better the maintenance characteristics of the thermocline.

Modelling carbon capture from power plants with low energy and water consumption using a novel cryogenic technology

Global warming mitigation requires modern energy generation technology to integrate low-cost carbon capture techniques. Despite volumes of research on the latter, the significantly high unit energy (MWh) of capture and high water consumption, do not enable any current carbon capture techniques to be adopted at a scale. To address this problem, we theoretically explore a novel low-cost carbon capture technology (NLCCT). In this paper, a detailed thermodynamic analysis of the effects of the operating conditions and parameters of NLCCT, such as the initial feed gas concentration, compressor intercooler temperature, number of compressor stages, efficiencies of the compressors, and turbines, ambient temperatures on the energy consumption for CO2 capture (ECC) is carried out with a view to obtain costs much lower than current techniques. The study identifies the inlet feed concentration, turbine and compressor efficiencies, and compressor intercooler temperatures (heat transfer effectiveness) as the significant parameters. For a drop of 20 % efficiency, the ECC was found to increase by 40 %, and for an increase in inlet CO2 feed concentration from 4 % to 10 %, the ECC was found to decrease by 31 %. Considering the effect of intercooler temperatures, the ECC increases by 25 % when the intercooler temperature is raised by 6 K. The effect of ambient conditions was also analyzed and observed that for an increase of 27C in the ambient temperature, the ECC would rise by 15 %.

Redesigning heat exchanger channels: A local entropy generation approach

This paper proposes a redesign technique for heat exchanger channels based on a Local Entropy Generation (LEG) analysis and numerical simulations. The approach first divides the computational domain into zones and then identifies the zones responsible for increasing irreversibilities due to viscous and heat transfer effects. By evaluating different geometric features and flow conditions, the wall shape is modified (zone by zone) with the configurations that offer the lowest local entropy generation rates. The redesigned channels demonstrate that sudden changes in geometry along the streamwise direction reduce the size of regions with high thermal entropy generation and promote the creation of vortices, thereby reducing the magnitude of thermal irreversibilities. Smooth wall shape transitions might not be desirable when the goal is to improve heat transfer. The redesign process enhances the thermal performance of the system without significantly increasing power demand. The proposed methodology is evaluated using three wavy channels with a non-uniform amplitude-to-wavelength ratio (β1,β2,andβ3) since they allow for better management of wall geometry. As demonstrated for case β1, the LEG analysis enables the construction of new channels with a thermal performance 3.9 % superior to those with a uniform amplitude-to-wavelength ratio of α = 0.16 (best-case scenario), but with approximately 20 % less pumping power. While this study employs wavy channels as a case study, the methodology is broadly applicable to various heat exchanger configurations, including trapezoidal, triangular, and zigzag channels, making it a versatile tool for improving thermodynamic performance across different designs.

Comparison of the effects on heat transfer through oil oscillation in a steel piston cooling gallery: Friction welding versus laser welding

Piston cooling gallery plays a critical role in managing the temperature of the piston and its performance. This paper aims to study the effect of oscillatory heat transfer within the cooling galleries, focusing on two distinct types: flanged (friction-welded) and smooth structures (laser-welded). To do so, the Eulerian multiphase flow model and the k-ω SST turbulence model were used to simulate the oil and air flow through the cooling gallery. Key parameters such as the oil filling rate, wall heat transfer coefficient, oil outlet flow rate, heat removal rate by the oil, and overall wall heat transfer rate were assessed for both gallery designs at an engine speed of 1800 r/min. The results indicate significant disparities between the two cooling gallery structures: the smooth structure exhibits an oil filling rate that is 5.41% higher and an oil outlet flow rate that is 33.48% greater compared to the flanged structure. The flange structure impedes internal oil movement, which leads to boundary layer separation, secondary wall contact, and the emergence of a double vortex phenomena. Collectively, these factors influence the oil fill ratio. This, in turn, affects the distribution of oil within the gallery and, subsequently, the efficiency of heat transfer. Comparative analyses of heat transfer within both galleries, under the same studied conditions, revealed that the smooth structure outperforms the flanged one. However, the presence of a flange significantly alters the heat contribution from different walls, thereby highlighting the substantial impact of flanges on the overall efficiency of heat transfer.

Experimental study on concentrating characteristics during static flash of aqueous NaCl solution at different flash speeds

This experimental study explores the concentration characteristics of aqueous NaCl solution in static flash evaporation, focusing on the equilibrium mass and concentration, at different flash speeds (FS). An aqueous NaCl solution, with an initial concentration (mass fraction) of 0.15 to 0.26, was employed as the working fluid. The superheat varied from 1.49K to 54.9K, while the flash speeds were adjusted from 0.006s-1 to 0.53s-1. The study delineates the flash evaporation process into two regions based on the interplay of heat transfer and steam-carrying effects: one where both factors are co-dominant, and another where heat transfer is the primary controlling element. The results indicate that increasing the flash speed effectively raises the equilibrium concentration in the co-controlled region but has no impact in the region dominated solely by heat transfer. Furthermore, initial saturation conditions at various flash speeds to predict whether the flash evaporation process is occurring in the co-controlled or heat transfer-controlled region are proposed. Calculating method of equilibrium mass/concentration at different flash speeds was set up.