Good Heat Transfer Research Articles

Experiment on liquid film flow and heat transfer of laminar liquid jet impacting on cylindrical surface

Laminar liquid jet impact has good heat transfer performance with low impact stress on targets. In order to explore the liquid film flow and heat transfer under laminar liquid jet impact on a cylindrical surface, first the flow of a liquid film was analyzed visually; then, the local convective heat transfer characteristics on the cylindrical surface at different impact heights and outlet Reynolds numbers (Re) were obtained by a combination of direct measurement and numerical simulation, followed by a comparative analysis with continuous droplets impacting on the cylindrical surface. The results show that according to flow behavior of the liquid film along the circumferential direction, circumference can be classified into stagnation, thin liquid film, hydraulic jump, stable flow path, and dripping regions. Local convective heat transfer coefficient first drops and subsequently increases marginally along the circumferential direction, while decreasing monotonically along the axial direction. The effect of impact height and outlet Re on local convective heat transfer coefficient is manifested mainly in stagnation, thin liquid film, and hydraulic jump regions. For outlet Re = 984, as impact height rises to a certain degree, there are apparent enhancements of the liquid film perturbation and convective heat transfer performance. Finally, the local Nusselt number correlations in different circumferential regions were proposed.

MORPHOLOGICAL EVOLUTION OF Al2O3 AND CNT NANOFLUID DROPLETS DURING SOLIDIFICATION

Nanofluid is an emerging heat transfer fluid with good heat transfer and thermal conductivity properties. It is important to investigate the phase change properties and morphological evolution during the freezing of nanofluid droplets to understand their practical applications. The effect of dynamic wettability on the deformation of a single droplet of aluminum trioxide (Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O) and graphene (CNT-H<sub>2</sub>O) nanofluids at different mass concentrations and substrate temperatures was investigated by visualizing the droplet freezing. The formation of solid-like and freezing front motions inside the droplet during the freezing process of these droplets was investigated. The solidification process was strongly influenced by the temperature gradient perpendicular to the cold surface and the change in the solid–liquid interface wettability during the phase change, resulting in volume redistribution at the top of the droplet. The freezing shape of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O nanodroplets resembled a "moon crater," and the influence of wettability decreased with increasing concentration, leading to a relative increase in the aperture of the top platform. The fully frozen state of the nanofluid droplet had an increasingly pointed tip, with a strong relationship between the substrate temperature and solidification time when the CNT-H<sub>2</sub>O concentration was 5 times higher and showed no change in the freezing droplet deformation rate under the experimental conditions. The contact angle of the two nanofluid droplets did not fluctuate significantly with increasing concentration, while that of the 1% nanofluid droplets remained at an average value of 85° during freezing. Under different freezing conditions, the freezing shape of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O droplets tended to increase in diameter as the subcooling temperature decreased, with the final deformation rate of 1% Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O being twice that at 5% concentration, while the contact angle of the same mass concentration of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O decreased by 1° as the subcooling temperature dropped. The CNT-H<sub>2</sub>O droplet became sharper at the tip as the subcooling temperature increased, and its contact angle did not change with temperature.

Topological structures for microchannel heat sink applications – a review

The microchannel heat sink (MCHS) has the advantages of small heat transfer resistance, high heat transfer efficiency and small size, which exhibits good heat transfer performance in the field of active heat dissipation of electronic devices integrated with high heat flux density. In this paper, the application of MCHS in thermal management is reviewed in recent years, and the research progress of microchannel topology on enhancing heat transfer performance is summarized. Firstly, the research progress on the cross-sectional shape of the microchannel shows that the heat transfer area and fluid flow dead zone of the microchannel is the keys to affecting the heat transfer performance; Secondly, the microchannel distribution and the bionic microchannel structure have a great role in enhancing heat transfer performance, especially in microchannel temperature uniformity; Thirdly, the disturbing effect caused by interrupted structures in microchannels such as ribs and concave cavities has become a hot topic of research because it can weaken the thermal boundary layer and increase heat dissipation. Finally, the commonly used MCHS materials and cooling media are summarized and introduced. Based on the above reviews of MCHS research and applications, the future trends of MCHS topologies are presented.

Effects of pyrolysis on heat transfer enhancement for hydrocarbon fuel flow in unilateral heated channels with dimples

• Pyrolysis aggravates the non-uniformity of secondary flow intensity by two times. • Pyrolysis causes nonlinear correspondence between heat flux and wall temperature. • Dimples weaken the stratification of secondary flow intensity for pyrolysis fuel. • Dimple cannot disturb aggravated non-uniformity of properties for pyrolysis fuel. • Dimple cannot effectively enhance heat transfer for cracking fuel at high heat flux. Heat transfer enhancement of hydrocarbon fuel is essential to improve the regenerative cooling performance for scramjet engines, especially under hypersonic flights ( Ma >6). Aiming to investigate the effects of fuel pyrolysis on heat transfer enhancement design in the channel, this study exploits the local secondary flow intensities for non-pyrolysis and pyrolysis fuel flow. Flow and heat transfer characteristics of pyrolysis n-Decane in smooth and dimpled channel are compared at 0∼4 MW/m 2 . Parametric analysis of dimple geometry is conducted. Among five dfferent dimensions dimples, h p =0.3 mm, d p =1.5 mm, p =3 mm obtains the best heat transfer performance but it still cannot effectively reduce wall temperature compared with smooth channel. Fuel pyrolysis intensively aggravates the non-uniformity of fluid properties and secondary flow intensities. Besides, the deep-degree pyrolysis in near wall region causes nonlinear correspondence between fluid temperature and heat flux. Above two factors brings enormous difficulties for heat transfer enhancement design in the cooling channel. Dimples weaken the secondary flow intensities stratification and significantly improve the adaptability for the heat transfer of pyrolysis fuel to the changing heat conditions. But dimples cannot sufficiently disturb the strengthened properties stratification induced by severe pyrolysis non-uniformity. Therefore, under high heat flux, dimples cannot perform good heat transfer enhancement for pyrolysis hydrocarbon fuel.

Investigation of the Effects of Using Plastic Instead of Aluminum in Tractor Engines, Intercooler Tanks on Engine Performance

In recent years, researchers have been working on more environmentally friendly engine systems and the efficient use of depleting fuel resources. One of these research topics is intercoolers used in turbocharged engines. Intercooler tanks are generally made of aluminum due to their good heat transfer coefficient. In this study, the suitability of the use of plastic tanks was investigated by examining the engine performance changes as a result of using plastic instead of aluminum, which is the traditional material, in the intercooler tanks of an 81 kW Perkins tractor engine. For this purpose, experiments were carried out at 1400 and 2200 rpm for intercoolers with both materials. According to the results obtained from the experiments, a 0.62% torque increase was obtained at 1400 rpm in the engine with a plastic tank material intercooler compared to the engine with an aluminum material intercooler. According to the data obtained from the experiments carried out at 2200 rpm, a power increase of 0.74% was determined. Similarly, it was determined that the effects of parameters such as radiator upper and lower hose temperatures, turbo inlet and outlet air temperatures, and intake manifold inlet temperature on engine performance were negligible. According to these findings, it has been determined that if the tanks of the intercoolers are plastic, there will be a negligible performance loss compared to the traditional material aluminum. Plastic is lighter, cheaper, and easier to manufacture than aluminum. Considering the production and operating conditions, it was concluded that such materials should be researched and developed by manufacturers.

Heat storage and increasing the rate of heat transfer in polymer electrolyte membrane fuel cell by adding nano-encapsulated phase change material to water in the cooling process

Polymer electrolyte membrane fuel cell (PEMFC) is a promising technology in the field of renewable energy production, especially in electric transportation systems. High heat produced and the temperature sensitivity of the expensive membrane have always reduced the lifespan and efficiency of PEMFCs and prevented their commercialization. Choosing an optimal cooling method is always one of the best solutions studied by researchers. Phase change cooling with nePCM particles is one of the newest microchannel cooling methods, which in addition to good heat transfer and uniform cooling, capability waste heat recovery (WHR) by heat storage. Nano-encapsulated of phase change material (nePCM) is a new type of PCMs that can absorb high heat without noticeable temperature change due to high latent heat during the phase change process while improving heat transfer. These particles consist of a phase change material in the core and a polymer shell, which, significantly improves its heat transfer properties when added to the base fluid. Based on computational fluid dynamics (CFD) simulation, the performance of water-nePCM suspension with an n-nonadecane core and polyurethane shell as a new cooling nanofluid in the microchannel of a PEMFC was evaluated for the first time. The governing equations are solved numerically using the finite volume method (FVM), PIMPLE algorithm, and C++ code development, by OpenFoam software. After examining the effect of different heat transfer parameters, the results show that using 4 % of nePCM particles increases the Nusselt number and heat storing (ηNEPCM %), by 20 % and 18 % respectively. Also, lower Reynolds numbers cause more heat storage in nePCM particles. As a result, this has led to more uniform cooling of the PEMFC plates and the thermal stability of the system.

Design and optimization of micro-perforated ultralight sandwich structure with N-type hybrid core for broadband sound absorption

We propose a novel ultralight multifunctional micro-perforated sandwich structure with N-type hybrid core, which exhibits excellent sound absorption as well as good load-bearing capacity and heat transfer performance. Sound absorption coefficient of the sandwich structure is calculated by establishing an analytical model. To validate the analytical model predictions, both experimental measurements and numerical simulations are carried out, with good agreement achieved. Physical mechanisms leading to the excellent acoustic performance of the structure are revealed, and effects of geometric parameters are quantified. For maximized sound absorption at minimal structural weight, an optimization strategy is developed with the Simulated Annealing algorithm. Compared with non-optimal sandwich structures, the optimal structure enables reduced weight and better absorption within broader frequency range. The proposed ultralight sandwich structure with integrated multifunctional attributes has significance in engineering applications demanding excellent mechanical, thermal, acoustic performances as well as low mass and volume.

The Effect of Tool Rotation on Structure and Mechanical Properties in Friction Stir Welding Of Aluminum AA5052

Abstract: Welding of aluminum alloy materials is often used in the industrial world, including those engaged in shipping, aircraft, and others. The welding method used to bind aluminum materials is Friction Stir Welding (FSW). Friction Stir Welding (FSW) is a simple welding method, namely by utilizing the heat energy generated from the friction of the shoulder tool with the material being welded. This study aimed to determine the effect of tool rotation on the physical and mechanical properties of Friction Stir Welding of Aluminum AA5052. The material used in this study was aluminum with the series AA5052 as the primary material. The friction stir welding process was carried out using CnC milling with parameters of the tool rotation speed of 1000, 1200, and 1500 rpm and a feed rate of 25 mm/minute. Physical analysis was carried out using macro photo observations with a USB Digital Microscope and microstructural observations with optical metallography. Mechanical observations were carried out using tensile testing and hardness testing. The microstructure in the weld nugget area shows good results with fine and dense grains that occur due to the dynamic recrystallization process. The highest tensile strength is 102.49 MPa in the 1000 rpm parameter. This is due to the material mixing process and good heat transfer.

Comparison study of thermoclinic heat storage tanks using different liquid metals for concentrated solar power

Liquid metals are good potential heat transfer materials for thermoclinic heat storage (THS) systems and will play an important role in the next generation solar thermal power system with higher operating temperature. This paper presents a comparative research work on the charging, discharging and mechanical performances of THS tanks based on four different liquid metal materials, which are lead (Pb), lead-bismuth alloy (PbBi), sodium (Na) and sodium-potassium alloy (NaK). The analysis results indicate that for both the charging and discharging processes, the lead-based THS tank can have the shortest operating duration, largest charging and discharging quantities (9.78 × 109 J and 9.21 × 109 J) as well as the highest discharging efficiency (94.17%), revealing its best operating performance. Relatively acceptable operating performances of lead-bismuth-based and sodium-based THS tanks are also demonstrated. Furthermore, in contrast with other three liquid metals, the lead-based THS tank also has the best mechanical performance. It has the smallest peak maximum mechanical stress of the steel wall (58.6 MPa). In summary, the lead-based THS tank has both the best heat storage and mechanical performances.

Ultrathin flexible heat pipes with Microsorum fortunei structural-like wick for cooling flexible electronic devices

Ultrathin flexible heat pipes (FHP) with Microsorum fortunei structural-like wicks were fabricated to meet the cooling demands of flexible electronics. The biomimetic wick was fabricated by high-temperature sintering and composed of multi-layer copper mesh and porous micropillars, which corresponded to the veins and spores of Microsorum fortunei leaves, respectively. The aluminum-plastic film was adopted as the casing and endowed the FHPs with insulating capability. FHPs with the thickness of 0.8 mm and 0.67 mm were fabricated by regulating the height of the porous micropillars to form vapor channels with different thicknesses, which reached the maximum thermal conductivity of 1943.5 W/m·K and 1449.5 W/m·K under heating power of 17.1 W and 8.1 W, respectively. The optimum filling ratio was found to be 100% due to its higher thermal conductivity under lower heating powers. FHPs with Microsorum fortunei structural-like wicks achieved good bending heat transfer performances while incurring thermal conductivity fluctuation to within 5.3% under 0–180° bending angles. The infrared tests highlighted that the FHPs maintained good heat transfer performance under natural convection conditions. The advantages of simple manufacture coupled with good bending heat transfer performance and insulating made the FHPs with Microsorum fortunei structural-like wick highly suitable for thermal management of flexible electronics.

Numerical Study of Heat Transfer by Mixed Convection in a Cavity Filled With Nanofluid: Influence of Reynolds and Grashof Numbers

In this work, a numerical study of stationary laminar mixed convection in a ventilated square cavity has been presented. The cavity is filled with different nanofluids and contains two gates (ports) to enter and exit the flow. The straight vertical wall is maintained at a warm temperature, while the other walls are considered adiabatic. The equations governing flow and heat transfer have been solved by the finite volume method using a second-order centered Upwind scheme. Numerical simulations are carried out in the case of pure water fluid, and mixtures of this basic fluid and nanoparticles (Ag and Cu), for Ri varying from (0.04 to 4) and a volume fraction of the nanoparticles between (0% and 10%). The study presented in this work is devoted to a dynamic study in which the Grashof number is fixed at 104, and the Reynolds number is varied. The numerical results obtained show that the heat transfer increases with the increase in the volume fraction also that the enhancement of the product of entropy generation and heat transfer increases considerably with the increase in the Reynolds number. The most effective nanoparticles for increasing the heat exchange rate are Ag. The latter is characterized by a large local Nusselt number, that is to say, a very good heat transfer compared to that of metallic Cu nanoparticles.

Rotating Hybrid Nanofluid Flow with Chemical Reaction and Thermal Radiation between Parallel Plates.

This research investigates the two different hybrid nanofluid flows between two parallel plates placed at two different heights, y0 and yh, respectively. Water-based hybrid nanofluids are obtained by using Al2O3, TiO2 and Cu as nanoparticles, respectively. The upper-level plate is fixed, while the lower-level plate is stretchable. The fluid rotates along the y-axis. The governing equations of momentum, energy and concentration are transformed into partial differential equations by using similarity transformations. These transformed equations are grasped numerically at MATLAB by using the boundary value problem technique. The influence of different parameters are presented through graphs. The numerical outcomes for rotation, Nusselt, Prandtl, and Schmidt numbers are obtained in the form of tables. The heat transfer rate increases by augmentation in the thermophoresis parameter, while it decays by increasing the Reynolds number. Oxide nanoparticles hybrid nanofluid proved more efficient as compared to mixed nanoparticles hybrid nanofluid. This research suggests using oxide nanoparticles for good heat transfer.

Anisotropic reed-stem-derived hierarchical porous biochars supported paraffin wax for efficient solar-thermal energy conversion and storage

It is valuable to develop and utilize cheap and widely available biowaste for phase change energy storage with high value applications. In this work, by removing lignin and hydrolyzing cellulose with cellulase enzymes, the macropore channels of reed stems were made to be connected via rich micropores on the carbon wall during high-temperature carbonization process. By loading paraffin wax by vacuum impregnation, shape-stable composite phase change materials (CPCMs) with a three-dimensional multi-level pore structure were obtained. The CPCMs have high loading rate (93.45 wt%), high energy storage capacity (141.47 J/g) and good heat transfer performance. The anisotropic reed-stem-derived hierarchical porous biochars supported CPCMs has good solar absorption ability, and as high as 92.28 % solar-thermal conversion efficiency. The good thermal stability as well as cyclic thermal stability are beneficial for practical applications. In addition, this work can achieve the purpose of energy saving and carbon emission reduction via increasing the value utilization of biowaste.

Process Intensification in a Double-Pipe Reactor with Additively Manufactured Internal Inserts

The polycondensation reaction to produce polydextrose can be intensified by using micro-process engineering. Fluid Guiding Elements are additively manufactured internal inserts that have already shown their potential to intensify heat transfer in double-pipe heat exchangers. This study investigated the intensification of the polydextrose yield when these internal inserts were used. Different reactor lengths and internal inserts geometries, as well as different operating conditions, were analyzed. The experiments showed that the reactant concentration had no effect on the product yield. Furthermore, it was shown that the process could be intensified at higher temperatures, with relatively low residence times and lower pressures. It was confirmed that the good heat transfer characteristics of the internal inserts allow them to continuously evaporate water during the reaction and to further reach the required reaction temperature, thus shifting the equilibrium towards the desired product. These findings are of special significance for the optimization of the polycondensation reaction of polydextrose.

Performance tests of dual-phase CO2 cooling for particle detectors

Evaporative CO2 cooling is becoming a popular cooling solution for large-scale, high-energy particle detectors, such as the new ATLAS Inner Tracker (ITk) for the high-luminosity upgrade of the LHC. CO2 offers a high latent heat transfer at reasonable flow parameters and is an environment friendly alternative to many other coolants currently used. This cooling technique is used to investigate the thermal performance of prototypes from the ITk strip detector produced at DESY. The strip end-cap local support structure, called petal core, is designed to allow a good heat transfer between silicon strip modules glued on its surface and the embedded titanium cooling pipe. Studies on the thermal properties using infrared thermography have been performed to analyse the heat dissipation path which allows also to detect eventual imperfections in the assembly as part of the quality control strategy. A similar analysis was executed on a petal loaded with electrical modules to study the heat generation due to active components and its dissipation for each module under different CO2 conditions.

Evaluation of thermal management scheme of engine cowl based on rotating heat pipe

During the flight in icing conditions, ice may accrete on the aircraft engine cowl, and once it falls off, it will seriously affect the engine performance and even the flight safety of the aircraft. The rotating heat pipe performances good heat transfer characteristics in high-speed rotating conditions, and can be coupled with the high-speed rotating main shaft of the engine to implement the reuse of the engine waste heat for anti-icing of engine cowl. A certain type of engine cowl was selected in the paper, and the anti-icing area and heat load of the three-dimensional cowl were determined using CFD method. On this basis, literature researches were carried out, and feasibility of rotating heat pipe applicated on engine cowl was verified by the qualitative analysis of the rotating heat pipe’s thermal characteristics. Furtherly, based on the anti-icing requirements of the cowl, a rotating heat pipe was designed. Through the thermal experiment, the results show that the rotating heat pipe performs good thermal start-up and stability within 200W, and can meet the anti-icing demands of the engine cowl. From the qualitative and quantitative analysis, it can be convinced that the application of rotating heat pipe on thermal management of the engine cowl is feasible.

Structural optimization design of CFRP with ultrahigh in-plane thermal conductivity and mechanical strength

Extreme environments in aerospace require structural materials to have good heat transfer properties. Aiming at ultrahigh in-plane thermal conductivity (TC) and mechanical strength, carbon fiber reinforced plastics (CFRP) laminates were prepared by combining PAN-based (CF) and pitch-based carbon fibers (H-CF) prepregs through hot-pressing method. The 0° tensile strength and modulus of [H-CF6/CF11]T can reach 1400 MPa and 297 GPa, respectively. High interlaminar shear strength (78.58 MPa) and ultrahigh in-plane TC (223 Wm−1K−1) are also obtained. Mechanical properties can be enhanced by heterogeneous structure design due to the decrease of number of weak epoxy/cyanate interfaces. We find that the heterogeneous structure can lead to a higher TC. Finite element simulation results confirm that heterogeneous structure is beneficial to the surface heat transfer under localized surface heating. This circumstance is favorable for the protection of devices from overheating when the CFRP is used as shell of aerospace vehicles.

Annealing treatment on waterborne polyurethane film-forming and structural features induced heat conduction performance in conductive adhesive

Thermal annealing treatment (60, 80, 100, 120 and 150 °C) on waterborne polyurethane (WPU) film-forming and further the structural features induced heat conduction behavior was investigated. It was found that the WPU films annealed at 100 °C or 120 °C showed a higher thermal conductivity than others, which was attributed to more ordered structural sequences as shown by x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Combined with Fourier transform infra-red spectroscopy (FTIR), atomic force microscopy (AFM) and dynamic mechanical analysis (DMA), it is concluded that appropriate annealing can facilitate the creeping of polymer chains and further occurring rearrangement for forming ordered structural sequences. As concluded, annealing is an easy way of regulating morphological structures and further boosting microphase separation in engineering applications. Also for practical application in heat management domains, it is a good sign to see the WPU materials, such as conductive adhesive, with more ordered structural sequences (or high crystallinity) display a good heat transfer performance.

Radially layered configuration for improved performance of packed bed reactors

A particle-resolved computational fluid dynamics model is built and validated to investigate the coupled transfer processes and reactions in radially layered packed beds (RLPBs), with dry reforming of methane as a model reaction. RLPBs are 53.3–69.8% lower in pressure drop and 5.4–21.6 K higher in cold spot temperature than randomly packed beds (RPBs). The methane conversions for RLPBs are slightly lower than these for RPBs, but using smaller catalyst pellets in a RLPB can balance methane conversion and pressure drop. Besides, the diameter of separating wall in RLPBs can be adjusted to meet different requirements. The existence of separating wall in RLPBs makes catalyst pellets orderly distributed and then makes flow channels less tortuous, which leads to low flow resistances and good axial convective heat transfer. This work shows that RLPBs are advantageous in enhancing transfer processes and the results should serve to guide the optimal design of RLPBs.

Boosting the photothermal conversion efficiency of MXene film by porous wood for Light-driven soft actuators

Ti3C2Tx (a typical MXene) has been widely used in light-driven actuators due to its outstanding photothermal conversion capability. However, the response speed of these actuators is always slow because the effective irradiated area is limited to their surface. Herein, we propose a wood-based composite material which is made by coating Ti3C2Tx on delignified wood (DW). The high porosity of DW leads to high loading of Ti3C2Tx and provides large irradiated areas, thus enhancing photothermal conversion efficiency. The delignification on wood can expose cellulose with highly hydrophilic surface for rapid diffusion of Ti3C2Tx suspension, and the hydroxy in cellulose can act as binding sites to form stable combination with Ti3C2Tx. Taking advantage of the good compressibility of DW, a simple densification is conducted on TDW (Ti3C2Tx/DW) to greatly shorten the distance between adjacent oxygen-enriched Ti3C2Tx nanosheets, enhancing the conjugation among nanosheets, thus endowing TDW with good flexibility and high heat transfer efficiency. Moreover, we manufacture a light-driven bilayer actuator comprised of TDW as the passive layer and low-density polyethylene (LDPE) as the active layer. Our light-driven actuator exhibits a tremendous angle variation of 160° at a light intensity of 120 mW/cm2. A series of devices based on the TDW/LDPE actuator are demonstrated, including simulated gestures, a four-finger soft gripper, and a bionic flower. Moreover, we propose a light-controlled smart switch which can be used on non-contact (COVID-19) or dangerous (blasting) occasions. Additionally, we present a finite element simulation to predict the bending deformation, which guides the accurate control of the devices.