Reduction In Temperature Gradient Research Articles

Infrared on-site heating for material extrusion additive manufacturing of carbon fiber filled polyphenylene sulfide and its nano-graphene-reinforced alternatives

This study investigates the impact of infrared heating and annealing on the mechanical properties of carbon fiber-reinforced polyphenylene sulfide (CF-PPS) and graphene nanoplatelet (GNP)-reinforced CF-PPS composites in Material extrusion (MEX) additive manufacturing. GNP/CF-PPS composites are prepared by mechanically mixing CF-PPS plastic particles with GNP particles and subjecting them to infrared heating and annealing treatments during the three-dimensional (3D) printing process. Three-point bending tests assess the mechanical properties of the samples. The results reveal that the 0.03 wt.% GNP/CF-PPS sample, subjected to infrared heating followed by annealing, exhibits a 30% increase in bending strength compared to pure CF-PPS, demonstrating that the combined treatment significantly enhances mechanical performance. Microscopy observations using a Keyence VH7000 digital optical microscope and a scanning electron microscope (SEM) confirm that GNP addition and infrared irradiation lead to reduced surface roughness and fewer fracture cross-sectional pores. The interlayer bonding improves notably, indicating enhanced internal density and structural stability. Furthermore, differential scanning calorimetry analysis shows a 20% increase in crystallinity for heat-treated samples, contributing to better interlaminar bonding and reduced temperature gradients during printing. Finite element simulations using MATLAB and ABAQUS software corroborate these findings, demonstrating that infrared heating proves more effective than varying GNP content alone in reducing deformation, residual stress, and temperature differences during the MEX process. These results provide valuable insights into improving the mechanical properties and stability of MEX-manufactured large-dimensional composite materials through optimized thermal management.

Numerical study on heat transfer and flow characteristics of symmetric Tesla-type microchannel heat sinks

This study numerically investigates the thermal performance of multi-stage Tesla valve microchannels (TVM) and a symmetric variant (SYMTVM) using single-phase deionized water, with mass flow rates ranging from 0.5459 to 1.6377 g/s. Both designs, particularly under reverse flow, exhibit lower peak temperatures and reduced temperature gradients compared to forward flow. The SYMTVM, characterized by its increased vortices and bifurcations, periodically disrupts and redevelops the thermal boundary layer, enhancing heat transfer efficiency. The TVM exhibits a significantly higher pressure drop than the SYMTVM across all flow conditions, with the reverse flow in TVM reaching up to 2.48 times that of forward flow and exceeding SYMTVM, especially at a peak flow rate of 1.6377 g/s. The performance evaluation criteria (PEC) and thermal resistance criteria (PECTR) demonstrate the superior performance of SYMTVM, with a reduced connection angle between the trunk and helix regions enhancing thermal efficiency. Furthermore, the SYMTVM-Reverse structure with a 30° interconnection angle demonstrates superior performance compared to the conventional rectangular microchannel (RM), achieving a Nusselt number (Nu) that is five times higher than that of the RM and an overall performance up to 2.59 times greater. These results indicate the SYMTVM-Reverse’s high heat transfer efficiency and its capacity to effectively balance thermo-hydraulic performance, even with increased power dissipation.

Optimizing nitrogen removal in PD/A reactors: Effects of influent composition and temperature on system stability and microbial dynamics

This study investigates the performance and microbial community dynamics in two partial denitrification/anammox (PD/A) reactors with different influent wastewater compositions (differ in the presence/absence of NO2−) subjected to a controlled temperature gradient reduction from mesophilic (30 °C) to room temperature (20.92 °C) over 76 days. Two lab-scale PD/A reactors (R1 and R2), both operated with a total inorganic nitrogen (TIN) concentrations of 70 mg N/L. R1 maintained a NH4+/NO2−/NO3− ratio of 3:3:1 and a COD/NO3− ratio of 2.0, while R2 had an NH4+/NO3− ratio of 3:4, and COD/NO3− ratios of 2.0 and 2.5. Our findings reveal distinct responses to the temperature transitions: the optimization of the NH4+/NO2−/NO3− ratio at 3:3:1 facilitated more stable nitrogen removal as temperatures decreased. This stability can be attributed to the enhanced synchronization between anammox bacteria and denitrifiers, promoting a balanced bioconversion process that is less susceptible to temperature-induced disruptions. Notably, the specific anammox activity (SAA) in both reactors declined linearly with the decrease in temperature, but the relative abundance of anammox bacteria (Ca. Brocadia) in R1 increased from 2.1 % to 9.7 %. Furthermore, the percentage of anammox-related key genes was higher in R1 than in R2, suggesting a microbial mechanism underlying the stable performance of R1. These results underscore the significant impact of influent nitrogen composition on PD/A performance amid temperature gradients and highlight the critical role of optimizing influent ratios for maintaining efficient nitrogen removal. This study offers valuable insights into enhancing the stability of PD/A systems under varying thermal conditions.

Enhanced mechanical properties of the additively manufactured IN738LC superalloy through thermal history management during the laser powder bed fusion process

IN738LC superalloy, as one of the hard-to-weld Ni-based superalloys, is relatively difficult to be produced by laser powder bed fusion (LPBF) due to its high cracking susceptibility. In this study, local heat accumulation based on substrate preheating and chessboard scanning strategy modification was proposed to reduce the thermal stress and promote the dynamic precipitation of IN738LC superalloy during LPBF process, which contributes to the crack inhibition and mechanical property enhancement. The as-built IN738LC superalloy exhibited superior room temperature tensile yield strength (YS) of 1011 ± 7 MPa, ultimate tensile strength (UTS) of 1400 ± 12 MPa, and good ductility of 17.4 ± 2.9 %. The superior mechanical performance was achieved mainly due to the successful elimination of micro-cracks, the formation of hierarchical microstructure composed of columnar γ grains, cellular dislocation network, especially in-situ formation of carbide precipitates. The formation of micro-cracks was fully eliminated due to the reduced temperature gradient and residual stress. In-situ formation of carbide precipitates was enabled by well-controlling the thermal history during the LPBF process. The use of a chessboard scanning strategy and substrate preheating was found to be beneficial for achieving a larger melting pool size, coarser dendrite/cellular arm spacing, and a larger volume of carbide precipitates due to enhanced heat accumulation during the LPBF process, which was verified by finite element analysis. This work provides insights into the effects of thermal profiles inherent to LPBF processes on the performance of LPBF-processed IN738LC superalloy.

Influence of heating elements layout on temperature uniformity in a large size heat treatment furnace

Computational Fluid Dynamics simulations were used to optimize heating elements placement on the walls of a large size electrical heat treatment furnace in order to reduce temperature gradients within the furnace and uneven temperatures on different faces of the blocks which could result in variabilities in final mechanical properties. Initial evaluations of different layouts highlighted the effectiveness of equipping two side walls. Subsequently, an optimal percentage of side wall coverage was identified through simulations and multi-objective evolutionary optimization. Genetic and pareto search algorithms yielded 10 % and 14.2 % as optimum values, respectively. Implementing an optimal coverage demonstrated significant reductions in surface temperature non-uniformity, surface-to-center temperature differential, and temperature differences between loaded blocks. These improvements suggest a potential for enhanced uniformity in mechanical properties across a batch of products. This approach presents a promising strategy for obtaining consistent and efficient heat treatment cycles in industrial heat treatment furnaces resulting in energy saving and improved product quality.

Influences of heat treatment on mechanical properties of SCF/PEEK composites in FDM‐3D printing process with UV laser assistance

AbstractFused deposition modeling (FDM) process with laser assistance can effectively enhance the interfacial strength of short carbon fiber/poly‐ether‐ether‐ketone (SCF/PEEK) composites. However, excessive crystallization of the semi‐crystalline matrix generally results in irregular shrinkage, which seriously deteriorates dimensional accuracy of composite specimens. To overcome this issue, we establish a novel heat treatment technique by the combinations of laser assistance and post‐treatment to improve the interlayer properties and dimensional accuracy simultaneously. It is found that lateral cracks at the interlayer interfaces can be completely eliminated at laser pre‐heating temperature , due to the reduction of temperature gradient and residual stresses. The further increased evidently promotes molecular diffusion and crystallization behaviors, thus increasing the interlaminar shear strength of FDM‐printed specimens. The post‐treatment temperature visibly promotes mutual diffusions of the molecules and formations of the perfect crystals, strengthening the interlayer adhesion of specimens. The proposed combination technique can increase the interlayer strengths of composite specimens by approximately 22.5%, while their warpage deformations are suppressed simultaneously.Highlights Pre‐heated temperature was a threshold for eliminating lateral cracks at interlayer interfaces. Post‐treatment significantly promoted molecular diffusion and crystallization behaviors. A novel technique was established to improve interlayer strength while suppressing warpage. The combination technique could increase the interlayer shear strength by approximately 22.5%.

Achieving Equiaxed Transition and Excellent Mechanical Properties in a Novel Near-β Titanium Alloy by Regulating the Volume Energy Density of Selective Laser Melting.

This research investigated the relationship between volume energy density and the microstructure, density, and mechanical properties of the Ti-5Al-5Mo-3V-1Cr-1Fe alloy fabricated via the SLM process. The results indicate that an increase in volume energy density can promote a transition from a columnar to an equiaxed grain structure and suppress the anisotropy of mechanical properties. Specifically, at a volume energy density of 83.33 J/mm3, the average aspect ratio of β grains reached 0.77, accompanied by the formation of numerous nano-precipitated phases. Furthermore, the relative density of the alloy initially increased and then decreased as the volume energy density increased. At a volume energy density of 83.33 J/mm3, the relative density reached 99.6%. It is noteworthy that an increase in volume energy density increases the β grain size. Consequently, with a volume energy density of 83.33 J/mm3, the alloy exhibited an average grain size of 63.92 μm, demonstrating optimal performance with a yield strength of 1003.06 MPa and an elongation of 18.16%. This is mainly attributable to the fact that an increase in volume energy density enhances thermal convection within the molten pool, leading to alterations in molten pool morphology and a reduction in temperature gradients within the alloy. The reduction in temperature gradients promotes equiaxed grain transformation and grain refinement by increasing constitutive supercooling at the leading edge of the solid-liquid interface. The evolution of molten pool morphology mainly inhibits columnar grain growth and refines grain by changing the grain growth direction. This study provided a straightforward method for inhibiting anisotropy and enhancing mechanical properties.

Additive manufacturing inert gas flow path strategies for multi-laser powder bed fusion systems and their impact on lattice structure mechanical responses

Multi-laser Additive Manufacturing systems hold great potential to increase productivity. However, adding multiple energy sources to a powder bed fusion system requires careful selection of a laser scan and inert gas flow strategy to optimize component performance. In this work, we explore four different laser scan and argon flow strategies on the quasi-static compressive mechanical response of Body Centered Cubic lattices. Three strategies employ a swim lane method where laser pathing tends to progress parallel to argon flow. Method one only uses a single laser while method two uses four, both with the laser path working against the argon flow. The third method uses four lasers, each operating in their own lane like the second method, but the laser pathing progresses with the argon flow. The fourth method has all four lasers operating in quadrants and the laser pathing trends against the argon flow.The single-laser strategy generally had the lowest mechanical responses compared to the other three strategies. A quadrant strategy generally had the highest quasi-static mechanical responses and was at least 25% greater in stiffness, yield force, ultimate force, and energy absorption when compared to the single laser strategy. However, the four-laser swim strategy where the laser pathing tends against the argon flow was found to be statistically similar to the quadrant strategy. It is hypothesized that spatter introduced onto the powder layer from the melt pool and particle entrainment may be worse for laser pathing which trends with the argon flow direction. Additionally, the additional energy added to the build volume helps to mitigate inter-layer cool time which reduces temperature gradients. This shows that multi-laser AM systems have an impact on part performance and potentially shows lattices built with multi-laser AM systems may have certain advantages over single-laser AM systems.

Indirect effects of latent heat of condensation on the simulation of the 2021 catastrophic Henan rainfall event in central China

AbstractCloud‐related latent heat is critical to the development and maintenance of atmospheric circulation. However, it is unclear how sensitive the key large‐scale weather systems of an extreme rainfall event could be to the variation in cloud‐related latent heat. In a companion study, by investigating the indirect effects of different microphysics schemes, we hypothesized that the latent heat of condensation (CT) would play a role in the ability to simulate the extreme rainfall event that took place in Henan Province, central China, on July 19–21, 2021. To examine the proposed hypothesis, the impacts of varying the CT and the collection of cloud water by rainwater (ACW) in the Thompson, WDM6, and Morrison microphysics schemes on the simulation of the southerly flow and its indirect effect on this extreme rainfall case were studied using the Advanced Research Weather Research and Forecasting (ARW‐WRF) model. It was found that directly changing the CT and ACW had significant effects on the southerly flow and precipitation. An increase in CT generally led to a reduced temperature gradient, a smaller pressure gradient, a stronger southerly flow, and a precipitation surge of 32%–50%. Similar outcomes resulted from increasing ACW in the WDM6 scheme, which effectively diminished the cloud mixing ratio, generated higher CT, and consequently intensified the southerly flow and precipitation by 31%. This study confirmed that the accurate simulation of microphysical processes, along with latent heating, is desirable for more realistically reproducing the key weather systems in the simulation of extreme rainfall.

Dependence of Uranium Oxide Polymorphism on Plasma Synthesis Conditions.

We synthesized uranium oxide nanoparticles using a plasma flow reactor (PFR) and studied the effects of three different experimental parameters on the resulting morphologies and speciation of the particles: (1) collection duration, (2) collection substrate temperature, and (3) radial collection position due to radial temperature gradients in the PFR. We also induced three distinct temperature histories along the axis of the plasma flow reactor by varying the gas flow rates downstream of the plasma torch. Transmission electron microscopy (TEM) analyses of collected particles showed two phases of uranium oxides (fcc-UO2 and α-UO3). The chemical compositions of the resulting uranium oxide particles were not altered by the three parameters investigated in this work but varied based on the temperature history induced. Preheating of the collection substrate led to deposition of fewer particles, which is attributed to a reduction in thermophoretic force caused by the reduced temperature gradient for preheated substrates. The relative amounts of UO2 to UO3 and particle size varied depending on the cooling history employed during synthesis.

Sustainability of flameless combustion mode with hydrogen addition to liquid kerosene fuel at high-intensity conditions

The application of hydrogen for fuel enrichment and blending has recently gained importance. The sustainability and performance enhancement through hydrogen addition in flameless combustion mode for liquid fuels needs further investigation to enhance our understanding. Therefore, the present study reports the effect of hydrogen addition on the flameless combustion of a kerosene-fired 30 kW swirl combustor with hydrogen addition. A divergence-shaped conical combustor with tangential air injection and asymmetric fuel injection scheme is adopted. The liquid fuel was injected using a solid-cone swirl nozzle with 34 μm Sauter Mean Diameter. The hydrogen addition was varied from 0 to 20% on a thermal input basis, and the thermal input of liquid fuel was maintained at a 30 kW level. Computational studies helped understand the flow physics behavior within the combustor volume. The hydrogen addition was observed to improve the reactant dilution ratio (Rdil). The maximum values of the Rdil increased from 4.47 to 5.44 with hydrogen addition. Hydrogen addition further helped extend the flameless combustion regime to φ = 0.3 without any air preheating. A uniformly distributed reaction with a reduced-temperature gradient is observed with extended combustion stability limits. A marginal increase in NOx emissions was observed with hydrogen addition, albeit within permissible limits, accompanied by ∼8–20 dB reduction in acoustic emissions.

Large thermal heat storages in rock caverns – numerical simulation of heat losses

AbstractIn the future large energy storage facilities will play a key role in district heating systems that transport heat energy through tube systems with water as transport media. Energy storages enable storage of renewable energy and industrial waste heat through flexible buffer heat storage and allow a reduction in installed capacities of heat supply stations. In this article, the application of water‐filled rock caverns for the use of large thermal energy storages is analysed. A key issue is the energy loss over the month/year. Therefore, this study focuses on the quantification of energy losses from water‐filled rock caverns by means of numerical analysis. Three different rock cavern geometries are analysed, varying rock conductivity parameter and varying temperature profiles of the water storage. By simulations with the software COMSOL it could be shown that (i) the energy losses of underground caverns are only 25 % compared to the energy losses from the currently used insulated steel tanks located above ground, (ii) the energy losses can be further decreased by the application of a thermal insulation layer and (iii) the energy losses decrease over the lifetime due to the reducing temperature gradients in the surrounding rock. Since cavern reservoirs can be operated for more than 100 years, these findings are of great relevance and shall be further investigated with respect to economical assessment.

Temperature gradient reduction in a tubular direct ammonia solid oxide fuel cell by fluidizing the cathode particles

Fluidizing cathode particles promotes axial heat transfer and reduces the axial temperature gradient and thermal stress, thus enhancing the stability.

Thermocapillary convection modes and phase change heat transfer characteristics of three-dimensional metal foam composited phase change materials with different aspect ratios under microgravity

This work aims to investigate the flow, heat transfer and phase change characteristics of porous media composite phase change materials with different aspect ratios in the thermocapillary mode under microgravity. A three-dimensional hydrodynamic model of thermocapillary-enhanced phase change heat transfer in the composite is developed. The model considers the shear stress boundary conditions on the porous free surface, employing the enthalpy-porosity method and the Darcy–Forchheimer model to describe the phase change process and fluid flow within the porous matrix, respectively, and the local thermal non-equilibrium model to describe the transient heat energy transfer between the paraffin and copper foam. The effects of different Marangoni numbers (Ma), aspect ratios (AR), Darcy numbers (Da) and porosities (ε) on the phase change heat transfer are compared. The results show that the increase of Ma from 6 × 104 to 2 × 105 accelerates the thermocapillary effect, and positively affect the thermal transport inside the cavity. The total melting time of the composite is reduced by 22.6 %, the heat storage efficiency is improved by 31.1 %, and secondary vortices appear in the convection zone. The large AR is favorable to the development of thermocapillary convection, but it also causes the reduced temperature gradient and heat transfer area of the heat source as well as the increased heat transfer distance. Compared to AR = 1, the total melting time increased by 34.4 % and 46.9 % for AR of 1.95 and 2.25 at a larger Ma, while the heat storage efficiency decreased by 26.5 % and 33.7 %, respectively. For a fixed ε, the decrease in Da hinders the development of convection, thus weakening the heat transfer and storage performance of the composite, and also causes heat conduction to dominate the phase change process in advance.

Investigation on the Attainment of High-Density 316L Stainless Steel with Selective Laser Sintering.

Due to the low density of the green part produced by selective laser sintering (SLS), previous reports mainly improve the sample's density through the infiltration of low-melting metals or using isostatic pressing technology. In this study, the feasibility of preparing high-density 316L stainless steel using 316L and epoxy resin E-12 as raw materials for SLS combined with debinding and sintering was investigated. The results indicated that in an argon atmosphere, high carbon and oxygen contents, along with the uneven distribution of oxygen, led to the formation of impurity phases such as metal oxides, including Cr2O3 and FeO, preventing the effective densification of the sintered samples. Hydrogen-sintered samples can achieve a high relative density exceeding 98% without losing their original design shape. This can be attributed to hydrogen's strong reducibility (effectively reducing the carbon and oxygen contents in the samples, improving their distribution uniformity, and eliminating impurity phases) and hydrogen's higher thermal conductivity (about 10 times that of argon, reducing temperature gradients in the sintered samples and promoting better sintering). The microstructure of the hydrogen-sintered samples consisted of equiaxed austenite and ferrite phases. The samples exhibited the highest values of tensile strength, yield strength, and elongation at 1440 °C, reaching 513.5 MPa, 187.4 MPa, and 76.1%, respectively.

(Keynote) Thermal Performance Optimisation at Cell Level, to Achieve Better, Longer Lasting Battery Packs

Temperature is a major issue in the operation of lithium-ion batteries, with both high and low temperatures causing problems. Cold batteries are inefficient, while hot batteries are at risk of thermal runaway and fire, making it crucial to maintain a safe and optimal temperature range. Temperature gradients can negatively impact battery performance, reducing accessible energy and increasing the degradation rate. This is a critical issue for electric vehicles (EVs), which can experience reduced driving range and deteriorating performance over time.Despite these challenges, current lithium-ion cell designs prioritize maximizing specific energy, which can negatively impact thermal performance. Thermal management systems are used to extract heat from poorly designed cells, but they are a significant parasitic load, demanding on average 8 kW in typical operation. Further, they can introduce uneven distribution of current throughout the battery pack, leading to diminished performance and accelerated degradation. The limited performance of these systems contributes towards range anxiety, which is a major barrier to the widespread adoption of electric vehicles in the UK, Europe, and the USA.To address these challenges, the authors propose the use of a new metric, the cell cooling coefficient (CCC). The CCC is a constant for a given cell and thermal management method (e.g. surface cooling or tab cooling), and is measured in W.K-1 (the reciprocal of thermal resistance). The CCC define a cell's ability to reject heat energy. The CCC is defined as the temperature gradient across a cell in idealized operating conditions, normalized against the heat generation rate in the same conditions. This allows for the comparison of geometrically dissimilar cells and can be used to optimize the thermal performance of lithium-ion cells.Experimental results presented in the paper demonstrate that increasing the thickness of the tabs in a typical lithium-ion pouch cell by 34% can yield a tab cooling performance enhancement of 20%, with a specific energy reduction of just 0.7%. Numerical methods predict that for a large format automotive pouch cell, increasing the tab thickness from 0.2 mm (negative) and 0.45 mm (positive) to 2 mm will lead to a 90% increase in the CCC for tab cooling (see in the attached figure). This yields a 51% reduction in temperature gradient across the cell in any instance of operation. As with all geometric optimisation, our models highlight a law of diminishing returns and we discuss the optimal tab thickness for various operations and applications.The authors also consider the use of the CCC metric for edge cooling in pouch cells and cylindrical cells. They find that tabs used to connect jelly rolls to the external can of cylindrical cells create a considerable thermal bottleneck. Redesign of this tab is well documented through Tesla’s proposed tab-less 4680.The use of thermal models at cell-level alongside existing battery models is critical for the development of future lithium-ion cells. The authors demonstrate that optimisation of thermal performance is entirely achievable within the numerical domain, reducing the requirement for expensive and timely prototyping phases during any product development process.By designing cells to have better thermal performance, it is possible to reduce the temperature gradient required between the hottest and coolest points in the thermal management system. This, in turn, reduces the power required by the thermal management system, allowing for more power to be directed towards extending the driving range of electric vehicles. Reduced temperature gradients at the battery pack level will also extend battery pack lifetime, reducing the lifetime cost to end-users and lessening the strain on raw material supply chains and battery recycling technology.The use of the CCC metric offers a promising approach to optimizing the thermal performance of lithium-ion cells. By addressing thermal management challenges, it is possible to reduce the parasitic load of thermal management systems, improve the lifetime and cost-effectiveness of battery packs, and lessen range anxiety for EV users. Figure 1

Multi-regional microstructure control using ultrasonic-assisted directed energy deposition for Al-Cu alloy

The uneven microstructure and element distribution between the inner-layer zone (INZ) and the inter-layer zone (ITZ) resulting from the thermal cycling effect during the directed energy deposition process with the electric arc energy source (DED-Arc) can lead to inferior mechanical properties. In order to address this problem, the ultrasonic treatment of the 2319 Al-Cu alloy during the DED-Arc process is investigated to control the multi-regional microstructure quantitatively. Therefore, the effect of ultrasonic vibration during the DED-Arc process on grain morphology, distribution of precipitates and mechanical properties in different regions are explored in detail. The formation mechanism of differential microstructure is elucidated by using an ultrasonic-heat-flow coupling simulation model for the DED-Arc process. The results indicate that ultrasonic vibration refines the average grain size, leading to 16% and 40% reductions at the molten pool center (MPC) and boundary (MPB), respectively. However, the grains in the semi-melting zone (SMZ) experience coarsening due to the reduced temperature gradient after ultrasonic treatment. The enhanced melt flow rate is considered responsible for both the homogenization of Cu and the fragmentation of the θ-Al2Cu phase. The higher subgrain boundary density in the ITZ leads to an average microhardness of approximately 60 HV, which is lower than that found in the INZ (72.3 HV). Meanwhile, ultrasonic treatment increases the average strength and elongation by 38.1% and 17.5%, respectively, mainly through precipitation strengthening, which results in a strength increment of 91.6 MPa. The slip directions tend to be consistent after ultrasonic vibration, which is conducive to crystal sliding and displays excellent ductility.

Nanosecond laser grooving of water-immersed silicon carbide assisted by high intensity focused ultrasound (HIFU)

Silicon carbide (SiC) is a very important semiconductor and ceramic material with many applications. However, its micromachining is difficult and it is still challenging to achieve a good combination of high quality, productivity and low cost. The corresponding author previously proposed the “ultrasound-assisted water-confined laser micromachining” (UWLM) process, a new machining technology utilizing in-situ ultrasound to improve laser machining of a water-covered workpiece. The author's previous studies show that UWLM based upon a high intensity focused ultrasound (HIFU) transducer is able to produce a relatively high machining rate and quality for metals. Thus, it is natural to guess the HIFU-based UWLM process might also possibly help provide a good solution for SiC micromachining. However, SiC have properties very different from metals. An experimental study is critically needed to confirm whether or not HIFU-based UWLM can still produce a high machining rate and quality for SiC, and overcome additional challenge(s) arising (if any). However, such a study is seldom reported to the authors' knowledge. This paper reports a study on the micro grooving of SiC via the HIFU-based UWLM process. With the conditions in this study, it has been found that UWLM can produce a much better apparent groove quality (i.e., much less recast material and/or debris re-deposition) than the laser machining process in the ambient air, and a much higher machining depth per pulse than the laser machining process in water with no ultrasound. The likely fundamental mechanism is the in-situ cleaning effect due to ultrasound-induced cavitation flow and pressure waves in water. The laser machining process in water with or without ultrasound can produce cracks at relatively low pulse repetition rates (PRRs). Time-resolved pressure measurements in water have been performed to help analyze stress sources causing the cracks. The crack problem appears to be effectively solved by using a relatively high PRR of 5 kHz. One possible major reason is expected to be the inter-pulse thermal accumulation effect at the high PRR, improving workpiece fracture toughness and/or reducing temperature gradients.

Natural Convection in the Melting of Phase Change Materials in a Cylindrical Thermal Energy Storage System: Effects of Flow Arrangements of Heat Transfer Fluid and Associated Thermal Boundary Conditions

Abstract Latent thermal energy storage systems (LTESS) have received widespread attention due to their high energy density to store a significant amount of thermal energy in the form of latent heat into phase change materials (PCM) at a nearly constant melting temperature. The thermal efficiency of LTESS is usually limited by poor heat conduction in PCM but enhanced by the natural convection of molten PCM. The natural convection increases the uniformity of temperature by mixing in a PCM enclosure and therefore increases the heat transfer rates and accelerates the melting. While there is negligible natural convection, periodic reciprocation of heat transfer fluid (HTF) through the PCM enclosure has been demonstrated to increase the heat transfer rates to PCM by increasing the melt interface area and reducing temperature gradients across PCM compared to fixed-directional flow arrangements. The current study examines the effects of HTF flow direction on the strength and duration of natural convection in PCM in a vertical cylindrical shell-and-tube container. Gallium is used as the PCM because of its low melting temperature and high thermal conductivity, and water is used as the HTF. A 2-D axisymmetric numerical model developed in ANSYS Fluent was used for the study of the cylindrical LTESS. The aspect ratio of the cylindrical container is nearly 1, which allows the generation of natural convection currents of strong magnitude in molten PCM in the vertical orientation. The irregular melting front in the PCM is caused by both natural convection in molten PCM and thermal boundary conditions for different HTF flow arrangements. The temperature and melting front profiles of PCM with the reciprocating flow arrangement are compared to unidirectional flows in upward and downward directions. The influence of HTF operating parameters such as temperature, velocity, and reciprocation period on PCM melting is studied. Scale analysis is also applied to characterize the different melting regimes of PCM under different flow arrangements.

Heat transfer enhancement of a microchannel cooler with V-shaped partitions

The ongoing evolution of electronic systems that operate under extreme conditions has led to persistent concern about potential failures caused by escalating temperature levels which provokes the decline the operating quality. In response to this challenge, cooling solutions based on microchannels have emerged as promising prospects for improving thermal management in such scenarios. This paper explores the importance of these innovative cooling techniques and their potential for mitigating the risks of overheating in electronic systems. Furthermore, the introduction of microfluidic techniques and microchannels, specifically constricted microchannels, offers promising approaches to improve cooling efficiency. These cooling systems enable efficient heat dissipation and thermal regulation, mitigating the risk of overheating and enhancing system performance. Constricted microchannels facilitate compact and efficient heat transfer by leveraging increased surface area-to-volume ratios and improved convective cooling. Nowadays, microchannel-based heat sinks, heat exchangers, and cooling systems have been developed, showcasing improved heat dissipation, reduced temperature gradients, and enhanced energy efficiency. This research focuses on a parametrical study that examines the fluid nature, Reynolds number analysis, and system design. Numerical results demonstrate successful thermal management of high-temperature electronic systems using constricted microchannel cooling. These results mitigate temperature-related failures and support the development of robust systems for harsh operating conditions.