Uneven Heat Transfer Research Articles

Entropy generation analysis and thermal performance of absorber tube in parabolic trough solar collector inserted with eccentric structure insert

To address the issue of uneven heat transfer in parabolic trough solar collectors, this study introduces an innovative insert (composed of vortex generators) layout. The vortex generators are strategically placed in areas with high heat flux density, while their number is reduced in regions with low heat flux density. By adjusting the position of the central rod, the inserts are defined as three different layout strategies (central structure, low eccentric structure, and upper eccentric structure), and their thermal performance is analyzed. The differences in fluid flow and heat transfer induced by different structures are also compared. The results show that the upper eccentric structure can induce unevenly distributed high-intensity mixing vortices, and through the ejection and sweeping movements of these vortices, the high-temperature fluid in areas with high heat flux density is transported to positions with low geothermal flow. Among all the studied configurations, the upper eccentric structure achieves a Nusselt number increase ranging from 1.16 to 2.34 times and a friction number value increase between 1.47 and 5.38 times. In terms of heat transfer performance, the highest value is 1.43 when the number of vortex generators is 6 and Reynold number is 13,750. Additionally, the thermal performance of the inserted tubes is analyzed using the field synergy principle and entropy generation method.

Study on the control of transverse cracks at the corners of large square billets for continuous casting of marine 945 hypo-peritectic micro-alloyed steel

Hypo-peritectic micro-alloyed steel has strong crack sensitivity, and marine 945 steel, as a kind of sub-encapsulated crystalline micro-alloyed steel, has frequent corner cracks during production, which seriously affects the production rhythm and reduces the product quality. Through the study of the morphology and composition of the matrix near the crack, the study and analysis of the high-temperature mechanical properties of the steel itself, and combined with the actual production situation, the causes of the corner cracks were clarified and the process optimisation was carried out accordingly. The results of the study show that the generation of corner cracks is due to the improper deoxidation process, uneven heat transfer inside the crystallizer and inappropriate cooling water parameters in the steelmaking process. By changing the deoxygenation method, crystallizer protective slag performance, crystallizer cooling water volume, water volume in the second cooling zone and other measures, the occurrence of billet corner cracks was effectively reduced, and the frequency of billet corner cracks was reduced from 6.3 to 2/m.

Breakup regimes and heat transfer of an isolated bubble and Taylor bubble flow in the T-type microchannel

Current research on Taylor bubble flow heat transfer mainly focuses on straight channels, with less emphasis on the transport characteristics and heat transfer regimes of Taylor bubble flow in branching channels. Understanding the flow and heat transfer regimes of Taylor bubble flow in branching channels is crucial for enhancing microfluidic and microchemical applications. Numerical simulations can provide detailed flow information that experiments might miss, deepening our understanding of Taylor bubble flow for enhanced heat transfer. The research results indicate the presence of three bubble breakup regimes at the T-junction: Non-breakup (NB), Tunnel breakup (TB), and Obstructed breakup (OB). The phase diagrams were established based on Capillary number and bubble length to predict the transitions between these three breakup regimes. Pressure within the channel increases with bubble deformation but decreases after breakup. Bubble breakup converts potential energy to kinetic energy, enhancing the heat transfer coefficient in the branching microchannels. The stagnation of bubbles at the T-junction temporarily reduces the heat transfer coefficient but increases after the breakup. Up to 115 % of the best performance improvement was achieved for Taylor bubble flow when the channel width ratio was 1.2. When the channel width ratio was greater than or equal to 1, the TB regime influenced bubble breakup within the channel, resulting in asymmetric breakups and uneven heat transfer. When employing tree-like branched microchannels and Taylor flow for enhanced heat transfer, a channel width ratio of 0.8 is recommended.

Simulation study on the effect of liquid ammonia spray on the heat transfer performance of multiple heat exchange tubes

Abstract In this paper, aiming at the problems of low heat transfer efficiency and uneven heat transfer of heat exchange tubes in refrigeration and air conditioning chillers, a liquid ammonia spray cooling method is proposed, and a flow and heat transfer model is established. The distribution of refrigerant droplets in the fluid domain and the heat transfer on the surface of the heat exchange tube are simulated under different spray speeds, spray directions, and double nozzle sprays. The results show that with the increase of spray velocity, the heat transfer performance of heat exchange tubes decreases first and then increases. The effect of spray velocity on the heat transfer performance of each row of heat exchange tubes is not consistent. The farther away it is from the nozzle, the greater the speed corresponding to the peak value of the heat transfer performance improvement of the heat exchange tube will be; the spray direction (gravity) has little influence on the heat transfer performance of the heat exchange tube, which is within 5.1%. Compared with single-nozzle spray, double-nozzle spray greatly improves the heat transfer performance of multi-row heat exchange tubes.

Enhanced comprehension of the cross-scale forming mechanism in CF/PEEK resistance welding: Analysis of temperature gradients and meso–microscopic phase transitions

Resistance welding technology is an effective solution for connecting thermoplastic composites. However, limited research on non-uniform distribution of interfacial temperature and complex evolution mechanism of phase-change molding hinders its application. This reliance on traditional experimental and trial-and-error methods severely impedes its development. Therefore, this study employed a novel cross-scale numerical simulation method, being applied for the first time in the field of resistance welding for elucidating the underlying mechanism of joint formation through multi-component phase transformation. The constructed model accurately represented the transient heat transfer of a realistic wire mesh structure in three dimensions, while incorporating complex gradient effects arising from both spatial and temporal variations in heating element temperature due to air interference. Further, melting, flow, and consolidation behavior of the resin matrix in resistance welding of carbon fiber/polyetheretherketone (CF/PEEK) thermoplastic composite was investigated herein at a mesoscopic scale. To accomplish this objective, a three-phase flow spatial forming and evolution model for resistance welding was developed at the mesoscopic scale by integrating principles from fluid dynamics. Interestingly, introduction of an air phase reinstated the spatial voids formation and flow in resistance welding, providing robust evidence for the initiation mechanism of internal defects. Moreover, this study revealed variations in the melting mode of PEEK resin across different regions within the weld and highlighted distinct distributions of voids resulting from uneven heat transfer. The interfacial phenomenon and flow diffusion mechanism between components in resistance welding tests were further verified and discussed through analyzing scanning electron microscopy and energy dispersive spectroscopy results. Temporal and spatial correlation in void distribution within the welding zone was observed, which was found to be consistent with simulation results. Specifically, with the progress of the welding process, air near the wire mesh gradually dispersed into the laminate, with significantly more voids at weld seam edges than in central regions. This research methodology based on stepwise scale reduction offers a promising avenue for investigating resistance welding and other connection forming applications, in particular, within the context of engineering underlying logical behavior.

Microstructural characterization and mechanical behaviour of laser powder Bed Fusion stainless steel 316L

Laser Powder Bed Fusion (L-PBF) is a highly precise and customizable additive manufacturing (AM) technique that uses a high-energy laser to selectively melt and fuse powdered material into a three-dimensional object. However, depending on the process parameters, the final components may have potential flaws that can affect their quality and mechanical properties, due to porosity, melting and incomplete fusion of powder particles; and because the process involves local heating and sometimes uneven heat transfer, the processed components may warp or crack due to residual stresses or thermal gradients. The manufacturing process itself reflects in the final component structure having a detrimental effect on the strength, durability, fatigue resistance, and corrosion.In this work, static tensile and fatigue tests were performed on traditional and L-PBF manufactured AISI 316L stainless steel specimens. The energetic release has been evaluated with an infrared camera during the static and fatigue tests aiming to identify material thermal response to the loading and to predict the failure in rapid way adopting Thermographic Methods. Differences were observed comparing the fatigue data of the L-PBF processed specimens with the traditional material. However, analysis of internal structure, porosity, and surface characteristics of the AM material in combination with fractographic analysis helped to explain the differences in the fatigue life. The observed energy release, different for both material types, was discussed based on the structural characteristics. The results show that the crack originates from a defect on the surface or just below the surface, with a transgranular propagation.

Research on flow distribution optimization of flue gas cooler based on numerical simulation

Based on Fluent, the flow distribution in flue gas cooler and its inlet and outlet flue in a power plant was numerically simulated. The calculation results show that the flue gas velocity distribution at the import of flue gas cooler was imbalance. The no uniform distribution of flue gas velocity can easily lead to problems such as wear, ash deposition and uneven heat transfer. To address the problem of uneven distribution of flue gas velocity, a diversion structure is designed and installed in the inlet flue of flue gas cooler. The numerical calculation results show that compared with the original flow distribution structure, the diversion structure can make the flow distribution of the flue gas cooler and its inlet and outlet flue more uniform, which can effectively avoid the problems of wear, ash deposition and uneven heat transfer.

Numerical investigations of the effect of the flip method on charging/discharging performance of a vertical shell-and-tube latent heat thermal energy storage unit

To address the issues of uneven heat transfer and low heat storage rate in the vertical shell-and-tube latent heat thermal energy storage (LHTES) unit, in the paper, the flip method is proposed to be applied to melting and solidification processes of units, aiming at prolonging the time for natural convection to work, while alleviating the temperature stratification phenomenon, so as to strengthen thermal performance. The melting and solidification characteristics of vertical units with flipping are revealed by numerical simulation methods. More importantly, the effect of the dimensionless flipping time t* on the charging and discharging processes is explored. Results show that the implementation of flipping significantly boots both charging and discharging rates, with maximum reductions in melting and solidification times of 33.44 % and 13.31 %, respectively, compared with units without flipping. Furthermore, with the increase of t*, the melting and solidification time both show a trend of first decreasing and then increasing, where the optimal melting tm* and solidification ts* are 0.5 and 0.25, respectively. In this mode, the melting-solidification full cycle time is shortened by 14.93 %. The present study provides new ideas for optimal designs and engineering applications of LHTES units.

Study on flow and heat transfer characteristics of 3D molten aluminum droplet printing process

The molten metal droplet printing technology can effectively realize a precise preparation of complex electronic devices at the micro scale by relying on the metallurgical bonding among the tiny molten metal droplets, which can improve the metallographic structure and mechanical properties of the fabricated parts. In this paper, the interaction process between metal droplets and movable substrates during the preparation of electronic devices is studied based on the coupled level set volume of fluid method (CLSVOF) and equivalent heat capacity method. The influence of the micro-bubbles and wall infiltration characteristics on the impingement spreading flow of metal droplets and their heat transfer cooling process is also investigated. The evolution mechanisms of droplet geometry and the distribution characteristics of droplet internal temperature and heat flow density during the impingement spreading process are discussed. The mechanism of high-temperature region formation under superhydrophobic wall conditions is analyzed. Results show that the microbubbles inhibit the spread of molten metal droplet. Under the same conditions, the spreading radius of hollow metal droplet is smaller than that of solid droplet, and their spreading height is higher than that of solid droplet. Different wall infiltration characteristics have similar degrees of influence on the spread of solid and hollow droplets. A larger wall contact angle for the same impingement mode at the same moment corresponds to a smaller droplet spreading radius and the higher the spreading height. With the participation of microbubbles, the heat transfer cooling process inside metal droplet becomes slow and demonstrates a less uniform temperature and heat flow density distribution. A high-temperature region is easily formed inside droplet under superlyophobic wall conditions, which triggers an uneven heat transfer cooling process at the bottom of the droplet. The results benefit to reveal the law of metal microdroplet impingement molding on different working surfaces and provide a theoretical reference for the preparation of electronics-related devices.

The effect of the aftercooler on the regenerator temperature non-uniformity in a high-capacity pulse tube cryocooler

High-power Stirling-type pulse tube cryocoolers (SPTC) are expected to be an ideal candidate for cooling high temperature superconductivity (HTS) for its advantages of compact structure, low maintenance and long service life. However, the regenerator temperature non-uniformity is still a key problem needs to be solved since it significantly deteriorates the cooling performance. As a main heat exchanger, the insufficient and asymmetric heat exchange in the aftercooler may be one of the factors affecting the temperature inhomogeneity inside the regenerator. However, it has been seldom investigated. In this study, the relationship between the performance of the aftercooler and the temperature non-uniformity of the regenerator was investigated based on a high-power SPTC working at ∼ 80 K. With the help of Sage simulation, it is found that the inlet temperature of the regenerator has a large effect on its inside temperature distribution. Even an inapparent temperature inhomogeneity at the inlet of the regenerator will induce a large temperature difference in the middle of the regenerator, due to an obvious DC flow generated in the regenerator. By changing the water inlet and outlet directions of the aftercooler, the temperature non-uniformity of the regenerator will consequently change, which means the uneven heat transfer of aftercooler can be one of the main inducements to the temperature non-uniformity inside the regenerator. Further, methods of using copper wire mesh filling at the hot end of the regenerator, and optimizing heat exchange tubes inside the aftercooler were studied to reduce the temperature non-uniformity.

The deformation behavior of high temperature steel plates for different surface heat transfer strength

Cooling after hot rolling is an important part of hot rolling process, which affects the internal structure and quality of hot rolling products. In order to deeply understand the deformation degree for the different cooling intensity of up and down strip surface, the method of numerical simulation is adopted. The heat transfer coefficient for 5000W/(m2• °C ), 7500W/(m2• °C) and 10000W/(m2-°C), the change of the temperature distribution for the steel plate thickness and the position of the key nodes are studied. The results show that with the increase of the heat transfer coefficient ratio of the up and down surface, the temperature difference between the up and down surface of the strip increases gradually at first and then remains basically unchanged. Appropriately increasing the heat transfer intensity can improve the strip deformation along the width caused by the uneven heat transfer between the up and down surface.

Effect of Closed and Opened the Door to Temperature on PID-Based Baby Incubator with Kangaroo Mode

The uneven distribution of the baby incubator temperature can cause the temperature in the baby incubator to be different at each point. The purpose of this study was to analyze the effect of the door closed and opened to the temperature at each point of sensor placement that has been determined. The study was conducted as an experimental research design. In this experiment, an Incu Analyzer comparison was used as a calibrator unit, a baby skin temperature thermistor sensor, and four LM35 sensors for baby incubator room temperature with one LM35 sensor as a PID control system carried out by trial-and-error method. Based on the results of measurements was made with the design, when the chamber is open, it produces an average error value of T1 4.083%, T2 6.06%, T3 3.78%, T4 4.88%, and T5 1.48%, while when the chamber is closed, it produces an average error value T1 0.75. %, T2 0.88%, T3 1.15%, T4 0.74%, and T5 0.87%. Measurement of skin temperature using a thermometer has an average error value of 1.1%. The results showed that uneven heat transfer, lack of air distribution, different sensor placements at each point, and non-standard chamber sizes were factors that were uneven at each point. Based on the results of the study, it was found that the use of a working system on this device can be implemented to control the temperature of the baby incubator by knowing the temperature distribution at each point

Finite Element Simulation of the Autoclave Curing Process of Aerospace Resin-based Composite Material

Due to the influence of the wall thickness, uneven heat transfer, and heat generation inside the resin during the molding of the composite material, the uneven temperature field distribution of the composite material is caused. This paper uses ABAQUS to perform finite element simulation on the temperature field of the forming process and obtains the distribution of the internal temperature field of the composite material. On the premise of obtaining the temperature field, through thermo-solid coupling simulation experiment, the influence of various factors on the entire curing process is compared. Based on the use of finite element simulation software, several comparative tests were carried out, and the residual stress problems in the curing process were verified by calling the stress and strain values of representative points. This article focuses on the analysis of temperature and energy data in heat transfer and 15 sets of stress-strain data under coupling action. On the premise of giving the reasons for each data, the accuracy and practicality of the data are analyzed.

An Extended Multilayer Thermal Model for Multichip IGBT Modules Considering Thermal Aging

An accurate and real-time knowledge of temperatures in insulated-gate bipolar transistor modules is crucial for reliability analysis and thermal management of power electronic converters. For this purpose, this paper establishes an integrated thermal equivalent circuit model comprising self-heating thermal impedances and cross-heating thermal impedances to provide a temperature profile of the junction and solder joints during various operations and in the case of thermal aging. The thermal resistance and capacitance parameters of the thermal impedances are characterized in terms of different electro-thermal operating conditions and solder joints aging conditions with the help of three-dimensional finite element simulations. Also, the effect of the heatsink, which brings an uneven heat transfer coefficient distribution at the module baseplate, is investigated and modeled into the thermal impedances. The introduced thermal model can work even if the conditions change simultaneously. The accuracy of the model is verified by experiments and finite element simulations, all of which agree with negligible error unlike thermal models given in the datasheet and fixed-parameter thermal models.

Study of the Ni–Cr–Fe‐Based Alloy Casting Process using a Mold Simulator Technique

To meet the balance between supply and demand, a Ni–Cr–Fe‐based alloy is tried to be casted using a lab‐scale continuous casting mold simulator. The results show that the temperature of mold wall increases from 73 °C (346 K) maximum to 117 °C (390 K) maximum, whereas the amplitudes of fluctuation of both temperature and heat flux generally reduces, from MF (original Mold Flux) to NF1 (New Mold Flux 1), and then to NF2 (New Mold Flux 2) mold fluxes during the casting process. Ti and Al transfer from molten alloy to mold flux cause the precipitation of high‐melting point CaTiO3 and NaCaAl3(SiO4)3 in mold fluxes, which reduce the infiltration of slags in the gap. In addition, herein, all ten oscillation marks on the alloy shells maintain a strong one‐to‐one correlation with the heat flux cycles. The irregular oscillation marks together with the depression and entrapped slag on the shell mainly result from the uneven heat transfer and slag infiltration.

Numerical and Experimental study on the performance of a new two-layered high-temperature packed-bed thermal energy storage system with changed-diameter macro-encapsulation capsule

In this paper, a new two-layered high-temperature packed-bed thermal energy storage system (PBTES) with changed-diameter macro-encapsulation capsule has been established to improve the thermal performance of PBTES. The effects of different diameters on thermal energy storage (TES) charging rate and TES density are numerically analyzed primarily. Second, the optimized hierarchical structure of PBTES is further constructed based on the proposed optimal comprehensive index called TES rate density. The optimal diameter pair of capsules exists when one of the diameters is fixed to obtain the optimized TES charging rate and TES rate density. Moreover, the thermal performance of diameter-changed two-layered PBTES is obtained compared to that of single-layered PBTES system. Finally, thermal performance of two-layered PBTES is experimentally examined. The influence of different inlet temperature and mass flow on the thermal performance of the system are investigated. The results are concluded as follows. (1) The phase change material of the lower-layer is melted faster by adopting the two-layered PBTES system. The improvement of heat transfer performance and enhancement of uneven heat transfer temperature can further increase the thermal performance of the system. (2) When the air inlet temperature increases from 425 °C to 465 °C, the TES charging rate can be increased by 60.5% and the heat storage efficiency rises from 84.8% to 91.1%. The TES charging rate is raised by 23.5% when the mass flow rate increases from 180 kg·h−1 to 260 kg·h−1 and the heat storage efficiency is decreased slightly. (3) With the same inlet temperature and mass flow, the TES charging rate of diameter-changed two-layered PBTES is better than that of single-layered PBTES. The maximum TES charging rate of the former system can be increased by 12.4% compared to that of the latter system, and TES rate density of two-layered PBTES can be improved by 13% as well. In summary, the new diameter-changed two-layered PBTES is an optimized system of heat storage. The study provides a design of such the PBTES for a first step implementation of the technology and the improvement of thermal performance optimization.

Numerical investigation on the oscillating flow and uneven heat transfer processes of the cooling oil inside a piston gallery

In previous research of achieving high cooling efficiency of engine pistons, the phenomenon of uneven heat transfer has not been investigated or recognized. These issues lead to a greater piston temperature gradient and its effects on engine durability have not been paid sufficient attention. In this research, a piston gallery was classified into four regions and several zones per region to investigate oscillating oil flows and uneven heat transfer distributions with a Relative Displacement Method (RDM). The RDM allows the cooling gallery to be treated as a rigid body, and the original constant boundary conditions could be translated into varying conditions that change as a function of engine crank angle. The relationships are investigated between the heat transfer performance and some factors such as the movement conditions of the air-oil two-phase flow inside the gallery, the instantaneous oil distributions, the relative oil velocity, the instantaneous acceleration and the velocity of the piston. The results reveal that the instantaneous oil charge ratio decreases and the area-weighted heat transfer coefficient increases as the engine speed increases. Different regions exhibit more apparent uneven heat transfer at higher engine speeds, and the top and bottom regions play a dominant role in the overall heat transfer coefficient. When the gallery was positioned closer to the oil inlet passage, the uneven heat transfer is more intense.

Effect of the laser surface treatment on structure and mechanical properties of AlZn10Si8Mg cast alloy

The laser surface melting/hardening is method which provides increasing mechanical properties of secondary (recycled) Al-Si cast alloys for automotive industry. Improvement of mechanical properties and structure of secondary aluminium alloys can often significantly increase the lifetime of casting and reduce costs for fuel and reduction of environmental loading. Self-hardening Al-alloys (Al-Zn-Si-Mg alloys) introduce an innovative class of light Al-alloys. AlZn10Si8Mg alloy is used for engine and vehicle constructions, hydraulic unit and mould making without the need of heat treatment because this alloy is self-hardened. For this study, a laser beam of Nd: YAG laser, BLS 720, with laser power 50 and 80 W, was used. The final microstructure of Al- alloys depend on the laser process parameters. The laser beam action on the surface of the experimental alloy has changed the microstructure of the material. Melting area is alpha-phase with much fine columnar dendrites morphology without the presence of Si-particles and intermetallic phases. In the transition area, grain refinement of eutectic Si (finer and rounder Si particles) as the modify action of the laser was observed. By increasing the laser power the microhardness of surface layers decreases. In the surface layer (after the 80 W laser beam action), cracks due to uneven heat transfer of the material were observed.

Unconventional Metallographic Methods for the Structural Characterization of Laser Hardened Al-Zn-Si Cast Alloy

The laser surface hardening is method which provides increased mechanical properties of secondary (recycled) Al-Si cast alloys for automotive industry. Improvement of mechanical properties and structure of secondary aluminium alloys can often significantly increase the lifetime of casting and reduce costs for fuel and reduction of environmental loading. For study was used a laser beam Nd: YAG lasers, BLS 720, on the test samples of secondary cast alloy AlZn10Si8Mg. AlZn10Si8Mg cast alloy are used for engine and vehicle constructions, hydraulic unit and mould making without the need of heat treatment because this alloy is self-hardened. The effect of laser beam was evaluated with the laser power 50 W and 80 W on the surface of samples. The final microstructure of Al-alloys depend on the laser process parameters. The changes of microstructure after laser surface hardening was observed by using classical techniques of etching (standard black-white contrast - etching by 0.5 % HF, 10 % H3PO4and colour contrast - etching by Weck-Al) and deep etching with HCl. Due to the action of laser beam on the surface of the secondary alloy AlZn10Si8Mg there have been changes in the microstructure of the material. Melting area is alpha-phase with much fine columnar dendrites morphology without the presence of Si-particles and intermetallic phases. In the transition area were observed grain refinement of eutectic Si (finer and rounder Si particles) as the modify action of the laser. By increasing the laser power the microhardness of surface layers decreases. In the surface layer (80 W) were observed cracks due to uneven heat transfer of the material.

A computational fluid dynamics and effectiveness-NTU based co-simulation approach for flow mal-distribution analysis in microchannel heat exchanger headers

Refrigerant flow mal-distribution is a practical challenge in most microchannel heat exchangers (MCHXs) applications. Geometry design, uneven heat transfer and pressure drop in the different microchannel tubes are three main reasons leading to the flow mal-distribution. To efficiently and accurately account for these three effects, a new MCHX co-simulation approach is proposed in this paper. The proposed approach combines a detailed header simulation based on computational fluid dynamics (CFD) and a robust effectiveness-based finite volume tube-side heat transfer and refrigerant flow modeling tool. The co-simulation concept is demonstrated on a ten-tube MCHX case study. Gravity effect and uneven airflow effect were numerically analyzed using both water and condensing R134a as the working fluids. The approach was validated against experimental data for an automotive R134a condenser. The inlet header was cut open after the experimental data had been collected. The detailed header geometry was reproduced using the proposed CFD header model. Good prediction accuracy was achieved compared to the experimental data. The presented co-simulation approach is capable of predicting detailed refrigerant flow behavior while accurately predicts the overall heat exchanger performance.