Thermal Enhancement Research Articles

Unlocking energy and economic benefits of integrated green envelopes in office building retrofits

Green envelopes show significant potential in energy conservation and thermal comfort enhancement in office buildings. This study presents a comprehensive method integrating field tests, building energy simulations, and optimization in office buildings in Chongqing, China. A 20-year life-cycle cost analysis was conducted to evaluate the benefits of green roofs (using sedum lineare container-type planting modules) and green façades (featuring vertical kudzu greenery systems). The results indicate that green roofs could reduce peak temperatures by up to 34.3 °C, leading to daily energy savings of 12.5 % during summer. Green façades decreased the wall exterior surface temperatures by up to 3.4 °C and 5.1 °C on the test cold and hot days, respectively. The maximum annual building electricity savings per green-modified area was 22.8 kWh/m2 for the poorly-insulated building and 5.4 kWh/m2 for the well-insulated one. This study highlights the cost-effectiveness of green envelopes in improving building efficiency. The optimal retrofit measure for the poorly-insulated building was the combination of green roofs and daylighting dimming control systems, yielding a 20-year life-cycle net savings per floor area of 81.5 CNY/m2. For the well-insulated building, the daylighting dimming control system alone provided optimal net savings per floor area of 26.4 CNY/m2.

Unlocking energy efficiency: Experimental investigation of bamboo fibre reinforced briquettes as sustainable solution with enhanced thermal resistance

Energy consumption data reveals that buildings account for 40 % of overall energy usage. Without urgent intervention, this figure is poised to escalate alongside population growth, exacerbating the dependence on fossil fuels for energy production. Addressing this pressing issue demands effective measures to curtail energy consumption in buildings, a realm where research on bamboo's thermal performance still needs to be improved. This study fills a critical gap by investigating the integration of bamboo fibre in the briquettes, known for their high heat transfer coefficient (U-value). Leveraging the innovative coheating test method, the research pioneers an unexplored avenue to evaluate thermal performance. This novel approach distinguishes the study as a unique and original endeavour in the field. The findings demonstrate significant thermal enhancements by substituting bamboo fibre: 2 %, 4 %, and 6 % bamboo additions yield U-values of 4.698 W/m2K, 3.94 W/m2K, and 2.77 W/m2K, respectively. Notably, the 6 % bamboo-reinforced briquette showcases a remarkable 49.9 % improvement in thermal resistance compared to conventional counterparts. This study marks a pioneering effort towards achieving energy efficiency through sustainable materials in designing low/zero carbon buildings. By showcasing the potential of bamboo as a thermal insulator, the research illuminates a promising path for future construction practices. Embracing such innovations is paramount in mitigating the environmental impact and securing a sustainable future.

Transient cold-front-water through y-shaped aluminium ducts: nature of turbulence, non-equilibrium thermodynamics, and velocity at the converged and diverged outlets

Abstract The interaction between water motion efficiency, outlet control mechanisms, and energy dynamics management hinges significantly on turbulence characteristics. However, understanding the influence of input velocities and duct features on outlets remains elusive. This study employs the realizable k − ɛ viscous model and Reynolds-averaged Navier–Stokes equations (RANS equations) to explore transient water dynamics encountering a cold front through ducts leading to convergence or divergence. Using Ansys Fluent 2023R2 and the waterlight workflow, meticulous meshing of the ducts is executed to capture flow intricacies accurately. Grid independence, suitable boundary conditions, and solver settings are carefully considered to ensure reliable results for investigating four key research questions. Duct bending introduces non-uniformities in velocity distribution, impacting exit velocity and altering flow characteristics and turbulence. In Case III, centrifugal forces from a 90° bend result in higher outlet velocities at the convergent exit and secondary flow patterns like swirls and vortexes. Additionally, entrance velocities influence Reynolds numbers, affecting mixing, heat transfer coefficients, and flow regimes, thereby optimizing thermal conductivity. This comprehensive investigation sheds light on optimizing water dynamics and energy management across various duct configurations, offering valuable insights into efficient flow control and thermal performance enhancement.

Mixed convective viscous dissipative flow of Casson hybrid nanofluid with variable thermal conductivity at the stagnation zone of a rotating sphere

AbstractMixed convection flows across the revolving bodies have eminent applications in science and technology, such as fibre coating, polymer deposition, centrifugal blood pumps, rotatory machinery, and so forth. In the current work, magnetohydrodynamic (MHD) flow and heat transfer characteristics of Casson hybrid nanofluid (Ag/MgO as nanoparticles) over the rotating sphere at the stagnation zone are being studied. Moreover, an analysis of heat transmission is conducted by considering the influence of thermal radiation, temperature‐dependent thermal conductivity, magnetic field, and viscous dissipation. The relevant partial differential equations are reformed into ordinary differential equations by appropriate transformations, which are solved using the successive linearization method (SLM). The thermal field, velocity components in x and z directions, heat transfer rate, and skin friction coefficient are computed for various physical quantities like rotation parameter mixed convection parameter, radiation parameter, Eckert number, Casson parameter, magnetic parameter, variable thermal conductivity parameter, and so forth. The current findings align well with the literature in a limiting sense. Thermal enhancement in hybrid nanofluid is observed for the viscous dissipation parameter (Ec), thermal conductivity parameter (), and radiation parameter (Rd). The degree of heat transfer rises from 12.8% to 20% when the Casson parameter's value () increases. Also, a decrease of approximately 33% is depicted between the peak values of the velocity magnitude with an increase in rotation parameter.

Magneto-Hydrodynamic Stagnation Point Flow of Casson Williamson Hybrid Nanofluid Incorporating Viscous Dissipation and Suction/Injection Effect Past an Exponentially Stretching Cylinder

The ongoing inquiry aims to analyze stagnation point flow characteristics of magneto-hydrodynamic (MHD) Cason-Williamson hybrid nanofluids over an exponentially stretched cylinder, incorporating phenomena like viscous dissipation and suction/injection effects, as no prior investigation has been conducted on it, which represents the distinctiveness of the flow model. To facilitate analysis, the original partial differential equation (PDE) formulation of the flow model is transformed into non-dimensional ordinary differential equations (ODEs) employing dimensionless quantities, a process facilitated by the MATLAB bvp4c approach. Various non-dimensional variables are examined for their impacts on velocity profiles, temperature distribution, shearing stress, and Nusselt number. Results conveyed through graphs and detailed tables show thermal profile enhancement with escalating Weissenberg, Eckert, and Biot numbers for Casson Williamson hybrid nanofluid. Increasing copper nanoparticle volume in this fluid raises friction drag compared to the Casson hybrid nanofluid, with a 9% enhancement in shear stress. Conversely, the heat transport rate is reduced by about 1.5% for Casson Williamson hybrid nanofluid compared to Casson hybrid nanofluid. These findings significantly advance fluid dynamics and nanofluid exploration, offering opportunities for improved heat and mass transmission in various industries.

Publisher's Note: “Phase change-related thermal property characterization and enhancement in carbon-based organic phase change composites” [Appl. Phys. Rev. 11, 021322 (2024)

Publisher's Note: “Phase change-related thermal property characterization and enhancement in carbon-based organic phase change composites” [Appl. Phys. Rev. <b>11</b>, 021322 (2024)

Twisted tape inserts in parabolic trough solar collectors: Assessment of Energy, Exergy, and Environmental impacts

The parabolic trough solar collector is a solar technology extensively employed to focus and harness solar energy for diverse heating purposes. One effective technique to enhance its thermal performance involves the insertion of swirl generators within its absorber tube. In this research, experimental investigation on performance enhancement of parabolic trough solar collector using single twisted tape and dual twisted tape insert across Reynolds number values ranging from 1,000 to 9,000 is performed. Collector efficiency, Nusselt number, friction factor, performance improvement factor, and exergy efficiency are examined for single twisted tape inserts with pitch to width ratios (y/w) of 1, 1.5, 2, 2.5, and 3, and dual twisted tape inserts with y/y0 ratios of 2, 3, and 4. Maximum enhancement in collector efficiency is reported at the lowest flow rate for both inserts. The single twisted tape insert with pitch to width ratio of 1 and the dual twisted tape insert with a pitch ratio of 2 exhibit the most notable enhancements in the evaluated performance parameters. Specifically, at lower flow rates (Re < 4400), the dual twisted tape insert (y/y0 = 2) outperforms the single twisted tape insert (y/w = 1). This dual twisted tape insert configuration provides a thermal efficiency enhancement of 14 %, while the Nusselt number and friction factor enhancement are found to be 372 % and 1600 % compared to bare tube. Additionally, the exergy efficiency exhibits a peak increase of 20.8 % at the lowest flow rate using the dual twisted tape insert (y/y0 = 2). This improvement translates to an annual CO2 reduction between 1.53 tons/year offering a highest enviro-economic benefit of 22.19 US dollars per year.

Effect of Upwind and Downwind Directions of Right-Trapezoidal Winglet-Vortex-Generators in a Solar-Air-Heater

Two directions of right-trapezoidal winglet vortex generators, namely, upwind and downwind directions, were numerically investigated to understand how they affect the thermo-hydraulic performance of the heat exchanger in a solar air heater. The results showed that the Nusselt number, friction factor, and thermal enhancement factor were higher when the vortex generator was installed in an upwind direction. This was because of the magnitude and the position of the airflows’ velocity. That is, faster airflows were induced in the region adjacent to the absorber plate when the vortex generator was installed in the upwind direction. As a result, high convection rates occurred and then increased the overall convective heat transfer rates of the solar air heater. Therefore, Nusselt number and thermal enhancement factor were higher. Nevertheless, the faster airflows also caused increased form drags due to vortex generator blockage, resulting in higher friction factor.

Correction: Black glass balls as thermal enhancers in conical solar distillation: performance optimization and comparative insights

Correction: Black glass balls as thermal enhancers in conical solar distillation: performance optimization and comparative insights

Experimental and Numerical Study of a Trapezoidal Rib and Fan Groove Microchannel Heat Sink.

A novel microchannel heat sink (TFMCHS) with trapezoidal ribs and fan grooves was proposed, and the microchannel was manufactured using selective laser melting technology. Firstly, the temperature and pressure drop at different power levels were measured through experiments and then combined with numerical simulation to explore the complex flow characteristics within TFMCHSs and evaluate the comprehensive performance of microchannel heat sinks based on the thermal enhancement coefficient. The results show that, compared with rectangular microchannel heat sinks (RMCHSs), the average and maximum temperatures of TFMCHSs are significantly reduced, and the temperature distribution is more uniform. This is mainly caused by the periodic interruption and redevelopment of the velocity boundary layer and thermal boundary layer caused by ribs and grooves. And as the heating power increases, the TFMCHS has better heat dissipation performance. When P=33 W and the inlet flow rate is 32.5 mL/min, the thermal enhancement factor reaches 1.26.

Phase change-related thermal property characterization and enhancement in carbon-based organic phase change composites

By utilizing the significant amount of energy absorbed and released during their phase transitions, phase change materials (PCMs) can capture and store thermal energy to fill gaps between supply and demand. Due to their many favorable properties, organic PCMs have gained attention in a wide range of applications. Nevertheless, their inherent low thermal conductivity has limited the direct use of organic PCMs in thermal energy storage (TES). Extensive research has been conducted on enhancing organic PCM thermal conductivity by incorporating high thermal conductivity materials. Owing to their high thermal conductivity and low density, carbon-based materials have been extensively used for thermal conductivity enhancement in phase change composites (PCCs). Carbon-based organic PCCs, which incorporate highly thermally conductive carbon allotropes and their direct chemical derivatives with organic PCMs, are a group of diverse PCCs with highly promising potential for TES applications. Adequate latent heat and shape stability performances are crucial to the success of the applicational performances of these PCCs. Much empirical research has pushed efforts to enhance these phase change properties, yet a logical understanding of these enhancement efforts based on the thermodynamics and intermolecular interactions of carbon-based organic PCCs has been elusive. In particular, the effect of characterization methods on the evaluation of phase change properties has been largely understudied. This review strives to provide novel physical and chemical insights into latent heat and shape stabilization evaluation processes and enhancement efforts in carbon-based organic PCCs through a detailed review and analysis of recent literature. The review provides an unprecedented comprehension of newly developed PCCs that challenge the traditional understanding that the latent heat of PCCs cannot exceed that of its base PCM. Efforts on phase change property enhancement driven by these new insights have the potential for carbon-based organic PCCs to succeed in a variety of TES applications, including solar-thermal harvesting, thermal management of batteries and electronics, thermoregulating textiles, and infrared stealth and infrared responsive materials.

Cattaneo–Christov heat and mass flux model and thermal enhancement in three‐dimensional MHD Jeffrey hybrid nanofluid flow over a bi‐directional stretching sheet with convective boundary conditions

AbstractInspired by the progressive relaxation characteristics of the Jeffrey model and its applied advantages in the rheological modeling of various dynamic fluids, the current study is focused to investigate the heat and mass transfer of magnetohydrodynamic (MHD) Jeffrey hybrid nanofluid flow over bi‐directional stretching sheet with convective boundary conditions. Additionally, the Cattaneo–Christov model of heat and mass flux is employed to take into consideration the time relaxation effects. The energy and concentration equation are taken into account to explore the effects of thermophoresis and Brownian motion. Homotopy analysis method (HAM) is employed for the solution of the current problem. Solution methodology is verified by comparing present results with those already published in open literature. The physical aspects of obtained graphical and numerical results are explained in detail to justify acquired trends. From the investigation, it is inferred that the magnetic and viscoelastic factors have a reducing influence on the flow profile along primary and secondary directions, while the stretching parameter has an increasing behavior on the flow profile in the secondary direction. Furthermore, the Brownian motion, magnetic parameter, and thermophoretic parameter have an escalating behavior on thermal distribution; however, the Brownian motion has a declining consequence on the concentration profile. The larger Biot number heightens the thermal and concentration distributions.

Experimental investigation of effect of different configurations of semi-circular or half-round ribs on heat transfer enhancement in a rectangular duct

This research study investigated the impact of various configurations of semi-circular or half-round ribs on the thermal performance and frictional factor characteristics of a rectangular duct. During this research study, the ribs were provided on the inner surface of the test section’s bottom wall. Six distinct configurations of semi-circular ribs varying from continuous to hybrid arrangement were used. The range of Reynolds number was varied from 13,500 to 34,800; while the ratio of rib pitch to rib height (P/e ratio) was kept constant at 8. The ratio of the height of rib to hydraulic diameter of duct (also known as blockage ratio) was kept at 0.114. The results of experimental investigation showed that the Nusselt number ratio obtained was ranging from 1.36 to 2.74, while the friction factor ratio ranges from 1.70 to 4.51. The overall enhancement ratio for all configurations obtained was in the range of 1.06–1.80 times to that of plain duct. Among all rib configurations, the hybrid rib configuration showed superior ratio of Nusselt number and intermediate increase in ratio of friction factor as compared to all the other rib configurations. Hybrid rib configuration showed overall enhancement ratio values ranging from 1.45 to 1.80 times as compared to that of plain duct (duct without ribs) within the specified range of Reynolds number. The results of study showed that the hybrid configuration of semi-circular ribs exhibited superior overall thermal enhancement as compared to that of remaining tested rib configurations.

Impact of eliminating ungated access regions on DC and thermal performance of GaN-based MIS-HEMT

The impacts of eliminating access regions (AR) on DC and thermal performance of GaN-based MIS-HEMT have been studied using a device of 4 µm gate length. Nominal absence of ARs was achieved by a metal-organic chemical vapor deposition-regrown heavily n-doped GaN contact region extended to the two-dimensional electron gas (2DEG) channel (with sheet resistance as low as 14 Ω/ ◻ and ohmic contact resistance of 0.20 Ω⋅ mm) and 22-nm-thick AlN/Al2O3/HfO2 insulator layers acting as both gate dielectrics and sidewall spacers. As a result, a low knee voltage (2.5 V @ VGS = +2 V) comparable to deeply-scaled devices was attained, revealing the dominant role of ARs in knee voltages. High linearity at lower supply voltages (gate voltage swing of 7.3 V @ VDS = 5 V) and a faster GVS saturation trend with V DS increasing were observed, benefiting from the improved utility of applied lateral voltage. Moreover, a much lower thermal resistance compared to that of the conventional MIS-HEMT structure (146 vs. 202 K W−1) was extracted by a static-pulsed I-V measurement method. Simplified TCAD simulations were conducted to explain the underlying mechanisms, demonstrating that the enhanced surface heat flow covered by the gate metal as well as the more uniform electric field along the 2DEG channel accounts for the better capability of heat management in the device free of ARs. Our results indicate the size of DC and thermal performance enhancements obtained by eliminating ARs in a GaN-based MIS-HEMT structure.

Thermal enhancement of a heat exchanger of engine via U-cut and V-cut zigzag strip turbulator combined with bubble injection method

This experimental study proposes a new approach to enhance heat transfer performance inside heat exchangers of engine by tandem utilization of bubble injection and a punched zigzag strip turbulator. The side edges of the zigzag turbulator underwent two distinct geometry modifications, involving V and U shape cuts. These cuts were made at varying zigzag angles, ranging from 60° to 150° and four diffirent surface area. The potential improvement in thermal efficiency of the heat exchanger was evaluated at different inlet Reynolds number ranging from 5390 to 10,860. Based on survey results, it has been demonstrated that at a Reynolds number of 5390, the Nusselt number and friction factor increase by 48% and 154% for U-Cut with a bending angle of 150°, and by 61% and 170% for V-Cut, respectively. Hence, V-cut boasts a TEF of 1.15, representing a 6.5% improvement over U-cut. Additionally, to assess further enhancement, air bubbles with flow rate ranging 2–6 LPM were introduced to the central tube while maintaining the superior geometry. The data suggests that incorporating air bubbles can enhance heat transfer up to 142%, achieving a TEF of 1.17 at an air flow rate of 6 LPM.

Numerical Investigation of Innovative Photovoltaic–Thermal (PVT) Collector Designs for Electrical and Thermal Enhancement

Photovoltaic–thermal (PVT) technology is gaining popularity due to the diminishing availability of traditional fossil fuels and escalating environmental concerns. Enhancing the heat dissipation of PVT to improve its electrical and thermal performance remains a significant task. This study simulates the thermodynamic and heat transfer characteristics in multiple novel PVT structures by examining the impact of various factors such as collector materials, radiation intensity, mass flow rate, and inlet temperature. This work also identifies the optimal mass flow rate for locations with different solar radiation. The numerical results indicate that the electrical efficiency of a designed cylindrical structure has increased by 1.73% while the thermal efficiency has increased by 8.29%. Aluminum is identified as the most cost-effective material for the collector. The optimal mass flow rates in selected locations of Xining, Taiyuan, and Turpan are 0.36 kg/s, 0.35 kg/s, and 0.30 kg/s, respectively. The numerical results provide valuable insight into optimizing the design and operating conditions of PVT systems.

Experimental and numerical investigations of tube inserted with novel perforated rectangular V-shape vortex generators

The utilization of punched vortex generators has been established as an effective means to enhance heat transfer in heat exchangers. However, there are few studies on the use of perforated rectangular winglet pairs in heat exchange tubes and the study of perforation parameters based on this approach in the existing literature. In this paper, three parameters, including three height ratios, four perforation indices, and three pitch ratios, are considered experimentally and numerically. In the Reynolds number range of 7454–13,664, the effects of perforated rectangular vortex generator pairs on fluid flow and heat transfer are obtained by scaling parameters such as Nusselt number ratio, friction factor ratio, thermal enhancement factor, field synergy number, entropy generation, and exergy efficiency. The numerical results indicate that the perforation will lead to a reduction in turbulent kinetic energy in the local multi-longitudinal vortices on the rear side of the vortex generator, resulting in a decrease in local heat transfer as well as friction loss. Meanwhile, the pitch of perforated rectangular vortex generator pairs plays a crucial role in heat transfer and friction loss, enabling an increase in the Nusselt number of smooth tubes by 68–257%. Furthermore, the results are proved using the principle of entropy generation and exergy efficiency analysis. As Reynolds number increases, the thermal enhancement factor gradually decreases. Notably, the maximum thermal enhancement factor of 1.43 is achieved when the Reynolds number is equal to 7454, the height ratio is 0.08, the perforation index is 0.18, and the pitch ratio is 1.04.

Full-scale experimental study on combustion characteristics and smoke temperature of double-source fires in different tunnels

Through a series of full-scale fire tests conducted in three different tunnels, the common characteristics of combustion and smoke temperature of double pool fires in actual operation tunnel environments are investigated. The evolution process of the coupling effects of shielding entrainment and thermal feedback enhancement is revealed. It is found that the ventilation state and the fire source spacing can alter the dominant relationship between the shielding entrainment effect and the thermal feedback enhancement effect, resulting in different evolution patterns of combustion characteristics and smoke flow patterns of double pool fires in tunnels. The heat release rate increases by 4 times when the double pool fire flames merge under the longitudinal ventilation compared with the double pool fire flames without fusion under the natural ventilation. The flame tilt angles of double pool fires were determined through the image processing method, and the prediction model of the flame inclination angle of downstream fire sources was established, considering fire source spacing. At the same time, the characteristic of temperature measured by Fiber Bragg Grating in double-source fire scenarios was investigated, developing a model to predict the longitudinal temperature rise distribution. It is found that the maximum temperature rise prediction model based on steady-state fire has poor applicability in the weak-plume fire scenario of full-scale tunnels. The analysis of the smoke flow patterns shows that the two-stage ventilation is more suitable for smoke control compared to full longitudinal ventilation in tunnel double-source fire accidents, especially for ensuring the safety of individuals trapped in the area between the two fire sources.

Thermal enhancement of PEM fuel cell cooling with novel configurations of Tesla valve and hybrid nanofluids: A numerical study

The operating temperature and thermal distribution play a significant role in the appropriate performance of the proton exchange membrane fuel cell (PEMFC). A suitable cell cooling method can guarantee uniform temperature distribution and preservation of membrane water content under harsh conditions in a PEMFC. The compact structure of PEM fuel cells and the size of the cooling system impose a great challenge to achieve a proper design of cooling plate. In this numerical study, four designs of the multi-stage Tesla valve with straight and reverse configuration are proposed to be employed as a cooling channel in which three different hybrid nanofluids such as TiO2–Cu, Al2O3–CuO, and Ag–MgO dispersed in water and Ethylene glycol flow inside the channels. A validated numerical model is developed using a commercial computational fluid dynamics (CFD) code to investigate the effect of inlet temperatures and mass flow rates of cooling fluid and volume fraction of nanoparticles on heat transfer coefficient, pressure drop, index of uniform temperature (IUT), and performance evaluation criteria (PEC). The results demonstrated that the reverse configuration of the Tesla valve (CASE E) experienced outstanding uniform thermal distribution while the heat transfer coefficient increased by 15% in this case compared to the straight-line cooling channel. The Ag–MgO hybrid nanoparticles showed better thermal performance than the two other nanoparticles with a 4% reduction in temperature difference in the cooling plate.

Insight into the bidirectional dynamics of hyperbolic tangent hybrid nanomaterial subject to Nield’s conditions using numerical approach

A hyperbolic tangent fluid model characterized by shear rate dependent viscosity together with the effects of hybrid nanoparticles, Brownian motion, thermo-diffusion, and Nield’s conditions, is utilized for thermal enhancement purpose. Five percent copper nanoparticles and twenty percent alumina nanoparticles are submerged into ethylene glycol with the parametric influence of tangent hyperbolic fluid to obtain the tangent hyperbolic hybrid nano suspension. Numerical solutions of the nondimensionalized conservation laws are found by the Keller-box method. The influence of various parametric factors on velocity, temperature, concentration, Nusselt number, and Sherwood number has been discussed. A constant Nusselt number is noted with the influence of Nield’s constraints. A 90% increase in thermal conductivity is achieved by dispersing 5% copper and 20% alumina nanoparticles into the ethylene glycol with the influence of hyperbolic tangent fluid. The error and convergence analysis of the numerical solution have been discussed, along with the validation of the obtained numerical results.