Maximum Heat Research Articles

Preparation of a Polymeric Phosphoramide Flame-Retardant and Its Effect on the Flame-Retardant Properties of Epoxy Resin.

The flammability of epoxy resins and knowing how to achieve curing are particularly important factors during use. A novel approach for enhancing the fire resistance and reducing the smoke emission of epoxy resin during the curing process is suggested, which involves the utilization of a three-source integrated polymerization intumescent flame-retardant. In this study, the synthesis of poly 4,4-diaminodiphenylsulfone spirocyclic pentaerythritol bisphosphonate (PCS) is achieved through using solution polymerization, utilizing 4,4'-diaminodiphenylsulfone (DDS) and spirocyclic pentaerythritol bisphosphorate disphosphoryl chloride (SPDPC) as initial components. Following that, the EP underwent the inclusion of PCS to examine its resistance to heat, its ability to prevent flames, its effectiveness in reducing smoke and its curing effect. Compared to the unmodified epoxy resin, the addition of PCS can not only cure the epoxy resin, but also decompose before the epoxy resin and has a good carbonization effect. With the addition of 7 wt.% PCS, the LOI value can achieve 31.2% and successfully pass the UL-94 test with a V-0 rating. Moreover, the cone calorimeter experiment demonstrated a noteworthy decline of 59.7% in the maximum heat release rate (pHRR), 63.7% in overall heat release (THR), and 42.3% in total smoke generation (TSP). Based on the examination of TG-FTIR and SEM findings, there is ample evidence to suggest that PCS, functioning as a phosphorus-nitrogen intumescent flame-retardant that combines three origins, has the potential to exhibit a favorable flame-retardant impact in both its gas and condensed phases.

Photothermal performance of vitreous products from high-temperature melting of hazardous waste

High-temperature molten vitreous (HTMV), as the primary means of hazardous waste disposal, was produced in large quantities. Exploring its potential in photothermal applications could extend its high-value. Herein, the HTMV was used to explore the photothermal performance potential. It indicates that the HTMV possesses significant photothermal conversion capability, capable of increasing the tested water temperature by 6.65 °C. The HTMV mass and water volume are critical factors affecting the water temperature outcomes. The HTMV exhibits distinct conversion capabilities when applied at different application scenarios on the test object. Application on the bottom primarily focuses on photothermal conversion with high heat production efficiency, while side application additionally demonstrates excellent thermal insulation properties. Based on heat balance and backpropagation neural network predictions, optimal scheme of HTMV on both inside and outside bottom application scenarios of the heated object can achieve a maximum heat input of 660.6 kJ m−2 and 570.1 kJ m−2, respectively, and reduce carbon emissions by 0.062 kg CO2 m−2 and 0.054 kg CO2 m−2. This study not only provides a foundation for the further application of HTMV but also opens a new direction for the development of photothermal conversion materials, which is of great significance in advancing sustainable energy technology.

Fluid flow and heat transfer characteristics of a microstructured microchannel heat sink under flow boiling condition

A microchannel heat sink (MCHS) is known as a heat dissipation device from the miniaturized electronic components and devices due to its capability to dissipate high heat flux from smaller surface area. However, the higher pressure drop and the uneven heat removal along the channel length are the noticeable disadvantages of MCHS. Though there are attempts to address these shortcomings, no existing study has been found to study the effect of positions of microstructures (micro-pin fin and blind holes) and combined effect of micro-pin fins and blind holes under flow boiling condition. Therefore, in the present study, an attempt has been made to logically explain the effect of various arrangements and positions of pin fins and blind holes in the selected twenty-four uniformly distributed stations on the thermohydraulic performance of MCHS through numerically predicted results. The objectives of the study have been attained using the following three analyses, flow boiling, (i) with saturated fluid inlet conditions (inlet Re = 500), (ii) with different inlet subcooling (20, 40, and 60 °C), and (iii) with different heat flux conditions (100, 150, 200 and 250 W/cm2). From the study, it is observed that the pin fin enhances heat transfer in single-phase flow and delays the phase change by disturbing the flow. The blind holes act as nucleation sites and help phase change heat transfer. It is also observed that the ratio of single-phase and two-phase length and arrangement of microstructures in these two regions predominately dominate the heat transfer and fluid flow characteristics of the microchannel heat sink. The optimum performance (maximum heat transfer coefficient and minimum pressure drop) is achieved when twelve pin fins are placed in the upstream half and twelve blind holes are located at the downstream half of the channel. However, design improvement is possible through more computational and experimental analysis.

Research on the Mechanical, Thermal and Induction Healing Properties of Asphalt Wearing Course with Steel Fibers.

Induction healing technology can effectively repair microcracks in asphalt mixtures and is a promising maintenance technology for asphalt pavements. However, it requires the addition of steel wool fibers to asphalt mixtures and cannot be directly used to repair existing pavements. In order to improve the practicality of the induction healing technology, this article designs a wearing course asphalt mixture with induction healing function that is going to be paved above the existing road surface. The AC-10 asphalt wearing course for induction heating was prepared by adding steel fiber (SF). Analysis of the overall temperature of the surface revealed the unevenness of the temperature distribution, and the healing properties were investigated through protective heating that controlled the maximum temperature of the upper surface. The results show that the addition of SF can improve the high-temperature stability, low-temperature and intermediate-temperature crack resistance, and moisture stability of asphalt wearing courses; however, it has adverse effects on volumetric performance and skid resistance. The heating temperature increases with the increase in SF content, but higher maximum temperature heating rate causes worse heating uniformity and lower healing effect. The maximum heating rate of the sample with 10% SF reaches 3.92 °C/s, while its heating rate at minimum temperature is similar to that of the sample with 6% SF, which is only 0.7 °C/s, indicating the worst heating uniformity. The best healing effect occurs when the maximum temperature of the upper surface reaches 160 °C. The recommended optimal SF content is 6% of the asphalt volume. The asphalt mixture with 6% SF has an appropriate volume performance, moisture stability, and skid resistance; additionally, it has the best high-temperature stability, as well as low-temperature and intermediate-temperature crack resistance. Meanwhile, it also has uniform temperature distribution and efficient healing efficiency.

Influence of temperature on the process of preparing homogenised magnesium-zinc alloy spherical powders using the relative vacuum method

The production of magnesium-zinc (Mg-Zn) alloy spherical powders is complex and expensive. As they have extensive applications in a wide variety of fields including military manufacturing, aerospace, and biomedicine, there is an urgent need to explore new production processes for Mg-Zn alloy spherical powders. In this study, 5 wt% homogenised Mg-Zn alloy spherical powder was prepared using the relative vacuum method. The effects of the insulation time, maximum temperature, and heating rate on the morphology, size distribution, and particle size of the Mg-Zn alloy spherical powder were investigated. The results indicated that the insulation time had the greatest influence on the particle size of the spherical powder, whereas the maximum temperature and heating rate had significant impacts on the alloy composition. When the insulation time was 10 min the maximum temperature was 1150 °Cand the heating rate was 15 min, the prepared Mg-Zn alloy spherical powder exhibited a full morphology without satellite particles, uniform composition, narrow size distribution, and a volume median diameter (VMD) of 17.62 μm. The research findings provide a new production process for Mg-Zn alloy spherical powders and establish a foundation for studying the specific effects of temperature on the production of Mg-Zn alloy spherical powders using the relative vacuum method.

Seasonal COP of a residential magnetocaloric heat pump based on MnFePSi

The performance of a magnetocaloric heat pump (MCHP) consisting of active magnetocaloric regenerators (AMR) of 12 layers of MnFePSi magnetocaloric materials (MCM) with a linear distribution of Curie temperatures was investigated using a 1D numerical model. The model predicted the heating power and coefficient of performance (COP) of the AMR for a fixed temperature span of 27K, between 281K and 308K, and variable flow rate and AMR cycle frequency. A maximum applied magnetic field strength of 1.4T was used. A well-insulated house with a maximum heating power demand of 3kW (under quasi steady state conditions) was considered. Ambient temperature in The Netherlands was taken as a reference for the estimation of the seasonal heating power demand. Without optimizing the design of the AMR, the model predicts a maximum single-AMR heating power equal to 43.5W when the AMR operates at 3Hz and 3Lmin-1, and a maximum COP equal to 5.8 when it operates at 1.5Hz and 1Lmin-1 Considering the maximum heating power of a single AMR, approximately 69 AMRs are needed to provide the design heating power demand of the house. It was found that it is possible to achieve an AMR seasonal COP of 5.6 by continuously adjusting the flow rate and frequency of operation of the MCHP along with the ON/OFF switching of some groups of AMRs in order to adjust the heating power of the MCHP to the heating power demand of the house.

Synthesis of silica-encapsulated myristic acid phase-change-assisted nanocapsules for thermal management applications

Myristic acid-based silica (MA/SiO2) nano-encapsulated phase change materials (NePCMs) were synthesized by the sol–gel process. Five different samples of the MA/SiO2 nanocapsules were prepared by varying the mass of the myristic acid. The nanocapsules were characterized by Fourier transform infrared spectrophotometer (FTIR), X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), energy dispersive X-ray (EDX), and dynamic light scattering (DLS). These characterization techniques confirmed the successful encapsulation of the myristic acid inside the silica shell. The maximum latent heat was found to be 114.46 J/g for the sample which was prepared with 25 g myristic acid and 20 ml tetraethyl orthosilicate precursor material. The highest encapsulation ratio of approximately 62% was obtained in the same sample and the mean size of the nanocapsules was 597 nm. The thermal stability of these nanocapsules was assessed using the thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC) results exhibited that the thermophysical properties remained consistent after 50 heating/cooling thermal cycles indicating excellent durability of the NePCMs.

Theoretical analysis of the vapor-liquid slug-train dynamic formation in an ultra-maximum hydraulic diameter oscillating heat pipe in horizontal orientation

The formation of liquid slug is critical to the pulsating flow of vapor-liquid slug-train (VLST) in an ultra-maximum hydraulic diameter oscillating heat pipe (UHDOHP) in horizontal orientation. The high-speed vapor causes the fluctuations in the vapor-liquid interface and forms local low pressure at the wave peak, which promotes the wave peak growth to create a liquid slug. Therefore, how to obtain a sufficient vapor velocity is important. The results show that the required heat to obtain sufficient vapor velocity cannot exceed the maximum load-bearing heat that maintains the effective evaporation of liquid and the unobstructed flow of vapor. And the effects of liquid reflux limit (RL), sonic limit (SL) and critical heat flux (CHF) appear sequentially as the diameter increases. In the effective range of RL, the optimal filling ratio decreases with diameter. And reducing the effective length can weaken the RL limitation and increase the effective range of SL. Moreover, increasing the diameter and filling ratio can weaken the limiting effect of CHF. This study reveals the start-up of UHDOHP in a horizontal orientation and provides a guideline for promoting the VLST formation.

Inflatable aerodynamic decelerators for CubeSat reentry and recovery: Surface properties

In this investigation, Direct Simulation Monte Carlo Method is used to compute the surface properties of inflatable aerodynamic decelerators (IAD) applied for the reentry, recovery, and reuse of cubeSats. Surface properties calculation is of fundamental importance for flexible thermal protection system materials selection that withstand the harsh reentry environment. Furthermore, simulations are extremely important to detect possible hot spots at the IAD before design and flight. In this work, three IAD configurations are exposed to a rarefied hypersonic flow with freestream conditions that correspond to those found at reentry conditions at 105 km altitude. All geometries are assumed to be axisymmetric and have a maximum radius of 0.3 m from the center line to the shoulder edge. The main differences between the geometries are the angle between the IAD and cubeSat body and the IAD curvature for geometry 3. According to the results, all IAD geometries are effective in protecting the cubeSat during the reentry phase. It is shown that the IAD geometry has a significant influence on the heat transfer, pressure, and skin friction coefficient distribution over the flexible heat shield. The maximum heat transfer coefficient computed for geometry 1, 2, and 3 is 0.90, 0.97, and 0.84, respectively.

Sensitivity Analysis and Optimization of Heat Transfer Performance of Ultra-Thin Vapor Chamber With Composite Wick

Abstract An ultra-thin vapor chamber (VC) with the composite wick formed by four spiral woven meshes (SWMs) and a copper mesh was proposed to solve the heat dissipation problem in miniaturized electronic equipment because of its sufficient heat transfer capability under limited thickness. However, the influence factors on the thermal performance of the VC with composite wick are more than that of the VC with a single type of wick. In this study, in order to investigate the thermal performance of the VC with composite wick, a theoretical model was developed to calculate the maximum heat transfer capacity. Besides, a three-dimensional numerical model for the heat transfer characteristics was established, and the simulation results have a good match with the experimental results. The orthogonal test method was adopted to determine that both the width of the vapor channel (wv) and the thickness of the vapor channel (tv) have a significant effect on the maximum heat transfer capacity and thermal resistance, while the porosity of the mesh (εmesh) has a prominent effect on the maximum heat transfer capacity, but has little effect on the thermal resistance. Further optimization of the sensitive factors for VC heat transfer performance was achieved to enhance the maximum heat transfer capacity.

Multi-Temporal Analysis of the Impact of Summer Forest Dynamics on Urban Heat Island Effect in Yan’an City

In this study, MODIS land products and China land cover datasets were used to extract normalized difference vegetation index, land surface temperature, and vegetation cover type in Yan’an City during the summers of 2017–2022. On this basis, analysis of spatial change and correlation were carried out as a way to study the mitigation effect on urban heat islands in Yan’an City with forest. The study showed that: (1) The coverage of normalized difference vegetation index over 0.4 in summer in Yan’an City increased from 59.38% to 69.12%, and the vegetation showed good growth conditions. It has a spatial distribution pattern of more in the south and less in the north. (2) The proportion of the urban heat island in Yan’an City increased from 15.51% to 16.86%. Urban heat island intensity fluctuated year by year, with the maximum urban heat island intensity of 6.26 °C appearing in 2019. It has a spatial distribution pattern of less in the south and less in the north. The transition rate of temperature field grade from low to high is 73.32%, and the transition rate to low is only 0.31%. (3) There is a negative correlation between land surface temperature and normalized difference vegetation index in Yan’an City. Vegetation has a mitigating effect on the UHI and the best cooling effect among the vegetation is shown by forest. The cooling effect of forest in Yan’an City is attenuated by an increase in distance, and the effective range is greater than 1000 m. In this study, the regulation effect of forest on the urban heat island was obtained by digging deeper into the intrinsic connection between spatial change in vegetation cover and land surface temperature change in Yan’an City. It provides an important reference for the formulation of meteorological protection policy as well as the promotion of sustainable development of the urban ecological environment and is of guiding significance for future urban planning and ecological construction.

Experimental investigations of CI engine performance using ternary blends of n-butanol/biodiesel/diesel and n-octanol/biodiesel/diesel

This study explored the ternary blends of biodiesel-diesel-n-butanol and biodiesel-diesel-n-octanol on common rail direct injec-tion (CRDI) diesel engines. The compositions of fuels, which varied from 0% to 100%, were altered by up to 5%. On the basis of their properties, these blends were chosen, with various concentrations of alcohol at 5% and 10%, 5% diesel, and the remainder being biodiesel. Two ternary fuel blends of waste cooking oil biodiesel (90–85%), diesel (5%), and butanol (5–10%), namely BD90D5B5 and BD85D5B10, and subsequently, another two ternary similar blends of waste cooking oil biodiesel (90–85%), diesel (5%), and octanol (5–10%), namely BD90D5O5 and BD85D5O10, were used to conduct the experiments. The experiments were done with varying injection pressure from 17° to 29° crank angle (CA) before top dead centre (bTDC). The optimum con-dition for the blends is achieved at 26°CA bTDC for 80% loading. So, the engine trials were conducted on 26°CA bTDC to attain the results. The BD90D5O10 blend achieved the lowest brake specific fuel consumption (BSFC) reading of 0.308 kg/kWh while operating at full load. The maximum brake thermal efficiency (BTE) was 31.46% for BD90D5B5. The maximum heat release rate (HRR) achieved with BD85D5O5 fuel blend was 58.54 J/°CA. The quantity of carbon monoxide that BD85D5B10 created was the lowest (25.86 g/kWh). BD85D5B10 had a minimal unburned hydrocarbon emission of 0.157 g/kWh while operating at full load. Oxides of nitrogen (NOx) were emitted in the maximum quantity by BD85D5O10, which was equal to 6.01 g/kWh. This study establishes the viability of blends of biodiesel and alcohol as an alternative for petro-diesel in the future to meet the growing global energy demand.

Pool boiling heat transfer characteristics of low-GWP refrigerants in a horizontal tube bundle configuration

Heat transfer enhancement techniques have been adapted on the shell side to improve the overall performance of the flooded evaporators, such as finned tubes. In recent years, it has been demonstrated that the metal foam structure can offer enhanced heat transfer performance under pool boiling conditions. However, there are not much research in the open literature that examine the feasibility of metal foam embedded tubes in horizontal tube bundle configurations. Therefore, this paper proposed a novel metal foam embedded tube (i.e., foam embedded outside the tube) to improve the heat transfer behavior of flooded evaporators. The experiments were performed on a horizontal tube bundle with a staggered arrangement. Moreover, the performance of low global warming potential (GWP) refrigerants (R-1234yf and R-1234ze(E)) is compared against R-134a for both plain tubes and metal foam tubes with porosities of 81%, 75%, and 62%. The results showed that a metal foam tube with a porosity of 62% showed a maximum heat transfer coefficient (HTC) enhancement of 291% compared to the plain tube. As compared with R-134a, the HTCs of R-1234yf and R-1234ze(E) are nearly 10% higher and 5% lower, respectively.

Green hydrogen energy source for a residential fuel cell micro-combined heat and power

In this paper, fuel cell micro combined heat and power with green hydrogen production is investigated for a residential house. Solar and wind energy were investigated for green hydrogen production. The performance of the micro-CHP system was evaluated using yearly energy coverage rate as an index, defined by the energy production and need ratio. A dynamic mathematical model of PEM fuel cell combined with the condensing boiler, supplied by local production of green hydrogen was developed. It was validated based on the yearly measurements in the FC micro-CHP unit installed in a French residential house. The key factors in the micro-CHP operating strategy were the fuel cell heat generation, stop phases, and regeneration which control water temperature, fuel cell life cycle, and annual performance. For conventional residential house in Normandy, the fuel cell operates for a short period during a summer day and the stored hot water was sufficient for heat demand. For winter typical day, the fuel cell operates continuously with maximum heat production. Electricity demand is satisfied and the condensing boiler is used to fulfill heat demand. Operating under heat led strategy, the yearly heat coverage rate is of 100% and the electricity coverage rate is usually upper 100% with 20 to 60% of stored electricity. Impact of the heating degree day and solar irradiance were analyzed using climatic zone factor as an index. Micro-CHP minimum hydrogen self-sufficiency of 57% is obtained in the coldest region where the climatic zone factor is 71°Cm2/W. Micro-CHP 100% hydrogen autonomy is achieved in regions where the climatic zone factor is lower than 39°Cm2/W.

An Experimental Study of Heat Transfer in Pool Boiling to Investigate the Effect of Surface Roughness on Critical Heat Flux

Utilizing pool boiling as a cooling method holds significant importance within power plant industries due to its ability to effectively manage temperature differentials amidst high heat flux conditions. This study delves into the impact of surface modifications on the pool boiling process by conducting experiments on four distinct boiling surfaces under various conditions. An experimental setup tailored for this investigation is meticulously designed and implemented. The primary objective is to discern the optimal surface configuration capable of efficiently absorbing maximum heat flux while minimizing temperature differentials. In addition, this study scrutinizes bubble dynamics, pivotal in nucleation processes. Notably, surfaces polished unidirectionally (ROD), exhibiting lower roughness, demonstrate superior performance in critical heat flux (CHF) compared to surfaces with circular roughness (RCD). Moreover, the integration of bubble liquid separation methodology along with the introduction of a bubble micro-layer yields a microchannel surface. Remarkably, this modification results in a noteworthy enhancement of 131% in CHF and a substantial 211% increase in the heat transfer coefficient (HTC) without resorting to particle incorporation onto the surface. This indicates promising avenues for enhancing cooling efficiency through surface engineering without additional additives.

Induction heating of magnetic nanoparticles: role of thermo-physical properties of the coating moieties

Superparamagnetic nanoparticles (MNPs) are utilized for magnetic fluid hyperthermia (MFH)-based cancer therapeutics, where the MNPs are surface functionalized to impart desired stability and biological properties. In this study, the MFH properties of palmitic (PA) and stearic (SA) acid-coated superparamagnetic Fe3O4 MNPs, with similar sizes (∼ 10 ± 0.8 nm, and ∼ 11 ± 0.7 nm, respectively), and saturation magnetizations (∼ 40.8 emu/g, and ∼ 41 emu/g, respectively), are systematically probed to understand the role of the thermo-physical properties of the coating moieties on the MFH efficiency. PA and SA-coated MNPs are prepared using a one-step microwave-assisted co-precipitation technique, and the presence of the surface coatings is confirmed using Fourier transform infrared spectroscopy and thermogravimetric analyses. Magneto-calorimetric studies show a high induction heating efficiency, surpassing the threshold of MFH for a biologically relevant field-frequency range. The maximum heating efficiencies are found to be ∼ 164.5 ± 5.4 W/gFe and ∼ 223.9 ± 6.4 W/gFe for the PA and SA-coated MNPs, respectively. This indicates ∼ 36.1 % higher specific absorption rate (SAR) for the SA-coated MNPs under similar experimental conditions, even though the magneto-structural properties are identical for the PA and SA-coated MNPs. Further, finite element modelling is utilized to investigate the thermal transport from the magnetic core to the surrounding medium, where the thermo-physical properties of the coating moieties are considered. Finite element simulations indicate ∼ 32.6 % higher SAR for the SA-coated MNPs, which is consistent with the experimental findings. The higher SAR for the SA-coated MNPs is attributed to the ∼ 1.7 times higher thermal diffusivity of SA. The proposed model is experimentally validated using a third system consisting of lauric acid-coated Fe3O4 MNPs with comparable magneto-structural properties. Further, significant temperature rise in a tissue-equivalent agar medium and good bio-compatibility, as indicated by the in vitro cyto-toxicity studies, make the prepared MNPs potential candidates for MFH applications. The obtained results provide deeper insight into the role of the thermo-physical properties of the coating moieties on the induction heating efficiency of superparamagnetic MNPs.

Study on the effect law of jet angle on the cooling of hot-rolled L-beam

The uniformity of cooling affects the organizational properties and product quality of the L-beam. Problems such as bending deformation of the L-beam and excessive residual stresses inside the finished product are mainly caused by uneven cooling. The jet angle is an important cooling parameter in the cooling process of L-beam. In this study, the flow field and cooling law during jet cooling of hot-rolled L-beams were investigated by means of numerical simulation using the jet angle as a variable. The jet angle is −45°∼30° on the lower surface of the short edge of the L-beam due to space limitation, and the jet angle is −45°∼45° on the rest of the parts, and the other process parameters are kept unchanged. The velocity distribution, pressure, water flow, and temperature distribution on the cooling surface and the average heat transfer coefficient were analyzed. The results show that the jet angle has a significant effect on the cooling of each part of the L-beam. With the increase of the jet angle, the asymmetry of water velocity, pressure distribution, water flow distribution, and temperature distribution on the surface of the cooling surface increased significantly. The distribution of heat flux corresponds well to the distribution of temperature. As the jet angle increases the surface average heat transfer coefficient gradually decreases, and the maximum average heat transfer coefficient appears in the vertical jet. Gravity will also have a certain effect on the cooling of the L-beam.

Molecular design and properties of P‐N‐B co‐intrinsic flame retardant thermosetting epoxy resin

AbstractCurrently, epoxy resin (EP) is widely used in various fields, but its flammability greatly limits the wider application of EP. In recent years, additive flame retardants have been widely used to enhance the flame retardancy of EPs, but poor resistance to weather and aging is a prevalent issue encountered with additive flame retardants. To improve the weather fastness, researchers have successfully created and synthesized EP molecules that possess a combined effect of P, N, and B. The results demonstrate a significant enhancement in the mechanical properties of materials containing 20% BVN/10% VPD/EP compared with those of pure EP. 20% BVN/20% VPD/EP exhibited a vertical combustion rating of V‐1 with a limiting oxygen index measuring at 29.8%. The flame retardant epoxy thermosets exhibited a notable decrease in both the maximum heat release rate and total heat release. The findings indicate that the inherent flame‐retardant properties of the P‐N‐B coacting EP are not only characterized by exceptional curing and flame retardancy but also by superior resistance to weathering and aging compared with the additive flame retardant.

Numerical investigation on mixed convection for laminar molten salt in horizontal square tubes

Molten salts are the key working media in nuclear and new energy fields. However, the mixed convection of molten salt is rarely investigated. The mixed convection of laminar flow in horizontal square tubes with constant heat flux is analyzed using Fluent software for the first time. The effects of heat flux, inlet temperature and Reynolds number on the flow and heat transfer performances are evaluated. It is found that as the buoyancy force increases, the friction factor, flow entropy generation rate, Nusselt number, and exergy efficiency increase, while the heat transfer entropy generation rate and exergy loss decrease. Compared to the forced convection, the maximum local friction factor, flow entropy generation rate, mean Nusselt number and mean exergy efficiency for mixed convection increase by 25.8%, 50.5%, 109.0% and 7.6%, respectively. Moreover, the maximum mean heat transfer entropy generation rate and exergy loss decrease by 56.1% and 57.3%, respectively. In addition, approximately 42% of the friction factor predicted by Fluent and 30% of the heat transfer predicted by Fluent exceed 20.0% uncertainties of conventional correlations. Therefore, an accurate friction factor correlation with 12.9% uncertainty and an accurate heat transfer correlation with 9.1% uncertainty are proposed.

Numerical and experimental investigation on boiling heat transfer and flow attributes for the exterior of smooth tubes and T-fin tubes

Experiments and numerical analyses of saturated pool boiling have been carried out on the outer surfaces of horizontal smooth tubes and T-fin tubes. Using a computational model that closely matches the experimental results, the study examined the dynamics of the bubbles as well as the boiling heat transfer coefficients in both the circumferential and axial orientations of the T-fin tubes. For the first time, comprehensive information on the process of high-performance external tube boiling has been made available. The findings demonstrate that an increase in fin density on the surface of the T-fin tubes leads to an abundance of bubble nuclei, with the escaping bubbles exhibiting both increased size and accelerated velocity. Due to the distinctive configuration of the T-fin tube, the local heat flow is lowest at the root fin and highest at the fin tip, indicating that heat transfer is inhibited in the narrow cavity between the two lateral fins. The optimal fin density, which corresponds to the maximum overall heat transfer coefficient has been identified. This states that the enhanced heat transfer mechanism within the T-fin tube is a symbiotic effect of both the increase in heat transfer area and the subsequent degradation of heat transfer fostered by the bubbling of the tube.