Heat Flow Research Articles

Processing, experimental characterization and thermal analysis for friction stir welded joint of 2219Al alloy

ABSTRACT Present study aimed at thermal modelling and numerical analysis of dynamic heat flow behaviour, for friction stir welded (FSW) joint of 2219Al alloy, further experimentally correlated with microstructural evolution and microhardness profile through different weld zones. FSW was performed on cast 2219Al alloy, with varying process parameters of welding and rotational speed of the tool, and transient temperatures measured at different distances from weld line. From microstructural analysis, three different heat affected weld zones having separate grain morphologies were identified, namely Weld Nugget Zone (WNZ), Thermo-Mechanical Affected Zone (TMAZ) and Heat Affected Zone (HAZ). Vickers microhardness was evaluated at varying distances from weld line, through different weld zones. To investigate transient temperature profiles over the welded plate, a 3-D Finite Element (FE) thermal model was developed, incorporating volumetric heat source as heat generation. Transient temperature profiles computed from FEM analysis, were successfully compared and validated with experimental results, with fairly good accuracy within 4.86 %. Individual influences of welding parameters, on transient temperature profile, weld zones, microstructural and mechanical property were investigated, to optimise and achieve desired weld quality. Present study established structure-property correlation, along with thermal analysis, to validate and optimise the potential application of FSW on 2219Al alloy.

Two-phase closed thermosiphon with grooved the evaporator section applied for low-grade energy recovery: Thermo-hydrodynamic performance and potentials

High thermal conductivity and simple structure of two-phase closed thermosyphons (TPCTs) facilitate it extensively applied in low-grade thermal recovery, such like geothermal and solar energy exploitation. In present work, by incorporating threaded grooves into the evaporator section of the TPCTs, heat transfer performance could be significantly enhanced. Thermal transport mechanism of thermosyphons with various threaded groove structures was comprehensively investigated, and corresponding thermo-hydrodynamic performance was modelled through CFD/VOF simulations. Numerical procedures were validated well after comparison with experimental ones felled within 5.0 %. Under various heating power levels and filling ratios, heat transfer performance of thermosyphons with threaded grooves evidently surpassed that of smooth thermosyphons. Depending on delicate numerical results, critical point at 33 % filling ratio and 90 W heating power were confirmed that total thermal resistance of the grooved thermosyphon decreased almost close to 4.6 %. Furthermore, deviating from that critical point, higher heating power and lower filling ratio instead confined heat transfer performance of the threaded TPCT, which was primarily due to the rate of bubble generation and growth being significantly was slower than its detachment rate from the wall. In addition, heat and mass transfer performance of thermosyphons with threaded grooves of various pitches and apex angles were further analyzed under various conditions. Our results indicated that, when the groove apex angle turned to 90°, overall thermal transportation capability of the TPCT achieved optimal, with a decline in thermal resistance of up to 6.5 % compared to thermosyphons with smaller apex angles. Apex angle primarily affected the number of nucleation sites and the bubble detachment rate. Additionally, as the screw pitch decreased, boiling heat transfer enhancement, leading to more robust boiling heat transfer of TPCTs. Under high heating power, thermosyphons with larger screw pitch exhibited superior heat transfer performance. Overall, this innovative TPCTs with threaded grooves could be well applied in low-grade energy exploitation, particularly like as geothermal and solar energy fields.

Organic compounds as temperature calibrants for fast scanning calorimetry

Organic compounds can be used as temperature calibrants in fast scanning calorimetry. Their advantages include ease of surface cleaning of the calorimetric chip and good thermal contact with the chip surface. Among several compounds tested, benzoic acid was identified as a convenient and reliable calibrant for temperatures below approximately 130 °C. However, organic calibrants often exhibit unusual heating rate dependencies of the onset temperatures of melting. This phenomenon can be semi-quantitatively explained by considering different heat flows within the sensor. Notably, the thermal resistance between the heater and thermopile, often overlooked, introduces an additional time constant that can sometimes result in a negative apparent thermal lag. In addition, the onset temperatures are influenced by factors such as sample position, thickness, surface wetting, and spreading. These factors limit the accuracy of transition temperature determinations to approximately ±1 K below 130 °C and ±5 K up to 220 °C.

Paraffin/graphite/boron nitride composite as a novel phase change material for rapid heat absorption in battery thermal management technology

Phase change material (PCM) cooling is widely employed in the thermal management of compact battery packs. Thermal protection triggered by rapid temperature rise shortens the effective discharge time in high-rate discharge, making it essential to prepare PCM that rapidly absorbs heat at high temperatures. However, research on the thermal absorption performance and insulating properties of PCM at high-rate discharge remains limited. Herein, we innovatively propose a thermally conductive and insulating PCM with a wide phase transition temperature to address the overheating issue in batteries at 7 C, 8 C and 9 C discharge rates. The C236 consists of paraffin (PA236), graphite and boron nitride with a ratio of 6: 2: 2, which achieves outstanding thermal performance with a latent heat of up to 138.0 J/g and a high-volume resistivity of 1.6 × 108 Ω·m. It is found that at a discharge rate of 7 C, the C236 with a thickness of 3 mm effectively maintains the temperature below 80 °C. At discharge rates of 8 C and 9 C, the PCM significantly reduces the battery temperature by 26.9 % and 32.5 %, respectively, highlighting the rapid heat absorption capability at high temperatures. Furthermore, after four cycles at a 9 C discharge rate, the temperature differential of the battery remains below 5 °C. The low-cost PCM provides a new experimental reference for battery thermal management in high-rate discharge, with broad application prospects.

Experimental study on the pool boiling heat transfer of R134a outside various enhanced tubes

The paper aims to experimentally study the pool boiling heat transfer properties of R134a outside various enhanced tubes under saturation temperatures ranged from 3 to 40 °C and heat fluxes of 5.5∼76.5 kW·m-2. One smooth tube and three micro-fin tubes were selected as the tested tube. Based on the experimental results, it can be found that the pool boiling heat transfer coefficient increases firstly and decreases later with increasing heat flux, but saturation temperature and water velocity have little effect on the heat transfer coefficient. Under the same experimental conditions, refrigerant thermal resistance ratio is greater than 0.5 during the heat transfer process. The micro-fin facilitates the bubble nucleation with the heat transfer enhancement ratio varying in the range of 2.08∼4.59. Moreover, the experimental values are compared with the calculated values of three existing correlations for the pool boiling heat transfer coefficient. The experimental variables, which mainly includes saturation temperature, heat flux and tube structure, have great influences on the correlation accuracy. Finally, to eliminate the influence of the experimental variable, a new correlation was proposed by introducing ratio of fin height to outer diameter of the tested tube. After the further verification, the proposed correlation can predict the heat transfer coefficient with the mean deviation of less than 10%.

Microscale heat transfer and phase change mechanisms of carbon dioxide at near-critical conditions

Recent studies highlight the potential benefits of two-phase carbon dioxide (CO2) systems operating near their thermodynamic critical point. Significant heat transfer coefficients and resistance to critical heat flux from improved vapor phase properties can enable heat transfer and reliability enhancements. However, the physical mechanisms of phase change that govern heat transfer behavior in the near-critical regime of reduced pressures (0.85<PR<1, where PR=P/Pcrit) are not well understood. In the present study, subcooled microscale (hydraulic diameter, Dh=500μm) flow boiling experiments are conducted to elucidate the heat transfer behavior near the critical point and the underlying mechanisms that influence it. Experimental data reveals a deviation from the trends predicted by the Cheng correlation (Cheng, L et al., 2008, “New prediction methods for CO2 evaporation inside tubes: Part II—An updated general flow boiling heat transfer model based on flow patterns.” Int. J. Heat Mass Transfer), in particular, displaying a decreasing peak heat transfer coefficient as pressure increases. Heat transfer in the near-critical regime can be further partitioned into two regions: near-critical boiling (0.85<PR<0.96) that exhibits heat transfer highly dependent on surface temperature, and supercritical-like behavior (0.96<PR<1) exhibiting a strong heat transfer dependence on mass flux. High-speed side-view flow visualization reveals the dominant nucleation mechanism (heterogeneous or homogeneous) governs the heat transfer behavior and is adequately predicted using flow inlet conditions. The present study provides a method for predicting nucleation behavior, which, in turn, facilitates the prediction of heat transfer near the critical point. The continuous alteration of heat transfer in the near-critical regime as a function of PR is underscored by this mechanistic insight, bridging the gap between typical flow boiling (PR<0.85) and supercritical heat transfer (PR>1).

Thermal effectiveness analysis of heat exchange tube with staggered louver-punched V-baffles

An experiment has been conducted to ascertain the optimal approach for enhancing the convective transmission of heat in a heat exchanger tube by periodically positioning staggered louver-punched V-baffle (SLVB) vortex generators on a perforated tape. The louver-punched V-baffles were staggered and set up with two patterns: V-apex directing upstream (VU) and downstream (VD), and each featured a louver-punched aperture to minimize friction loss. The aim of this work was to rise the thermal effectiveness as well as the Nusselt number ratio (NuR) at optimal effectiveness in order to cut down on overall dimension of heat exchangers. Consequently, the research findings focused on behaviors of heat transmission and frictional loss, incorporating generated entropy, through a variety of Reynolds numbers (Re) spanning 29,270 to 4762. The SLVB elements were designed and placed in VU and VD forms using three relative pitches (PR = 0.5, 1.0, and 1.5) as well as six louver-flapped angles (θ = 0°, 10°, 20°, 30°, 45°, and 90°), all at the same attack angle (α = 52°), a baffle height ratio (BR = 0.3) and a louver size ratio (LR = 0.73). As demonstrated by the experiment, the θ = 20° generated the best heat transfer of all PR values, up to 6.55 times greater than the smooth tube, despite having a lower friction loss than the θ = 0° (solid baffle). At PR = 0.5, θ = 20° and smaller Re, the optimum thermal effectiveness factor (TEF) and NuR values were, respectively, about 2.65 and 6.55 for the VU SLVB and 2.52 and 6.04 for the VD one. It implied that the TEF strategies can be utilized to anticipate the best effectiveness under identical scenarios. Also, correlations for the critical parameters, namely, Nu, f, and TEF, were estimated and documented.

Radiogenic heating sustains long-lived volcanism and magnetic dynamos in super-Earths.

Radiogenic heat production is fundamental to the energy budget of planets. Roughly half of the heat that Earth loses through its surface today comes from the three long-lived, heat-producing elements (potassium, thorium, and uranium). These three elements have long been believed to be highly lithophile and thus concentrate in the mantle of rocky planets. However, our study shows that they all become siderophile under the pressure and temperature conditions relevant to the core formation of large rocky planets dubbed super-Earths. Mantle convection in super-Earths is then primarily driven by heating from the core rather than by a mix of internal heating and cooling from above as in Earth. Partitioning these sources of radiogenic heat into the core remarkably increases the core-mantle boundary (CMB) temperature and the total heat flow across the CMB in super-Earths. Consequently, super-Earths are likely to host long-lived volcanism and strong magnetic dynamos.

Numerical study of fluid flow and entropy optimization subject to second order slip condition across a permeable curved surface

The entropy generation analysis has great significance in the industrial sectors with heat transmission and fluid flows, for evaluating the irreversibility aspect of a system in heat transfer operations. The numerical simulation of entropy generation and the nanofluid flow across a permeable curved surface subject to cross-diffusion and irregular heat source/sink is reported in the current investigation. The thermodynamics 2nd law is used to simulate the entropy optimization. The flow phenomena are mathematically described by partial differential equations (PDEs), which are derived in a curvilinear coordinate system. The system of ODEs (ordinary differential equations) is derived by using the similarity conversion, which is further numerically calculated through the parametric continuation method (PCM) using MATLAB software. The results reveal that the velocity slip and curvature parameters improve the velocity profile whereas the inverse effect is observed against the surface permeability parameter. It can also be noticed that the entropy optimization enhances with the variation in Brinkman number and temperature ratio parameter. The impact of Schmidt number and chemical reaction decline the mass transmission ratio.

Influence of corrugated jet plates on internal heat transfer and flow dynamics of double-wall cooling: experimental-numerical approach

In the present study, combined experimental and numerical investigations were carried out to analyze the fluid flow dynamics and heat transfer of impinging jets in a corrugated double-wall structure for exhaust nozzle cooling operating at high temperatures. The sinusoidal wavy plate, with multi-jet holes positioned at its trough, served as the jet plate, creating a staggered configuration with respect to the effusion holes. The transient thermochromic liquid crystal (TLC) method was employed to determine the local distribution of the Nusslet number on the flat effusion plate in a wind tunnel at small Reynolds numbers (ranging from 450 to 2700 based on the jet hole diameter (d)). In the numerical part of the study, the k-ω SST turbulence model verified with the TLC data was used to solve the RANS equations. The effects of jet-to-plate spacing (Have/d = 1.6–3.6), corrugated plate amplitude (A/d = 0.2–0.8), effusion hole diameter (de/d = 0.6–1.5), and Reynolds numbers were examined in detail. Our results showed that the introduction of the corrugated structure in the jet plate altered the high-velocity region pattern within the jet hole, leading to a 16 %-81 % reduction in jet core length compared to the flat jet plate. At a relatively high wave amplitude (A/d ≥ 0.5), the vortex structure underwent significant changes, inducing a higher level of turbulence kinetic energy in the near-wall region of the target chamber. This led to enhanced heat transfer near the stagnation and effusion hole regions compared to the flat double-wall cooling. The effusion hole diameter exerted a minimal effect on the overall Nusselt number of the corrugated double-wall cooling. At very low Reynolds numbers (Rej 〈900), the flow and heat transfer characteristics in corrugated double-wall cooling resembled those observed in flat double-wall cooling.

Saturated pool film boiling of cryogenic fluids: Review of databases, assessment of existing models and correlations, and development of new universal correlation

The present study is motivated by the absence of a comprehensive pool film boiling database for cryogenic fluids. Amassing such a database is deemed essential for the thorough evaluation of existing models and correlations, simultaneously laying the groundwork for the development of new advanced predictive methodologies. To address this research gap, a Consolidated Cryogenic Pool Film Boiling Database was meticulously compiled, encompassing 1,209 data points for heat transfer coefficient (HTC) from flat surfaces across various orientations, from horizontal (θ = 0°) to vertical (θ = 90°). The exhaustive database enabled a comprehensive evaluation of prior models and correlations, revealing significant errors, particularly for elevated superheat conditions. In response, a finely tuned universal correlation for all cryogenic fluids is proposed, surpassing the predictive capabilities of its predecessors. The inclusion of a radiation term in the correlation contributes to its superior performance, especially in scenarios involving elevated temperatures, resulting in a Mean Absolute Error (MAE) of 12.94 % for the entire database, with 91.07 % of predictions falling within ±30 % of the data, and 98.92 % within ±50 %. Furthermore, outstanding performance is realized for specific orientations, with MAEs of 15.47 %, 7.92 %, 3.78 %, and 11.59 % for orientations θ = 0°, 30°, 60°, and 90°, respectively. This achievement situates the new correlation as a robust tool for thermal design and performance assessment across a diverse spectrum of devices and systems, also marking a significant advancement in understanding of cryogenic pool film boiling heat transfer phenomena.

Simulation-Directed Construction of Bamboo-Forest-Like Heat Conduction Networks to Enhance Silicon Rubber Composites' Heat Conduction Properties.

Highly vertically thermally conductive silicon rubber (SiR) composites are widely used as thermal interface materials (TIMs) for chip cooling. Herein, inspired by water transport and transpiration of Moso bamboo-forests extensively existing in south China, and guided by filler self-assembly simulation, bamboo-forest-like heat conduction networks, with bamboo-stems-like vertically aligned polydopamine-coated carbon fibers (VA-PCFs), and bamboo-leaves-like horizontally layered Al2O3(HL-Al2O3), are rationally designed and constructed. VA-PCF/HL-Al2O3/SiR composites demonstrated enhanced heat conduction properties, and their through-plane thermal conductivity and thermal diffusivity reached 6.47W(mK)-1 and 3.98mm2s-1 at 12vol% PCF and 4vol% Al2O3 loadings, which are 32% and 38% higher than those of VA-PCF (12vol%) /SiR composites, respectively. The heat conduction enhancement mechanisms of VA-PCF/HL-Al2O3 networks on their SiR composites are revealed by multiscale simulation: HL-Al2O3 bridges the separate VA-PCF heat flow channels, and transfers more heat to the matrix, thereby increasing the vertical heat flux in composites. Along with high volume resistivity, low compression modulus, and coefficient of thermal expansion, VA-PCF/HL-Al2O3/SiR composites demonstrate great application potential as TIMs, which is proven using multiphysics simulation. This work not only makes a meaningful attempt at simulation-driven biomimetic material structure design but also provides inspiration for the preparation of TIMs.

Experimental study on the thermal performance of non-white near-infrared solar reflective coatings in a permafrost region

Solar reflective coating (SRC) offers a method to reduce surface temperature without the need for additional energy consumption, making it highly applicable in permafrost regions. However, many SRCs with high solar reflectance are white, which can cause glare and not meet the aesthetic pavement requirements of high-grade highways. Herein, we developed two non-white SRCs (black and gray) designed to achieve low reflectance in the visible spectrum while maintaining high reflectance in the near-infrared region. We then evaluated the thermal performance of a white SRC, two non-white SRCs and asphalt (as a control) by analyzing surface temperature, ground temperature, and the heat flux between the soil and pavement in a permafrost region of the Qinghai-Tibet Plateau. The results showed that three SRCs reduced the annual mean surface temperature and surface temperature fluctuation compared to the asphalt. The white SRC exhibited annual heat release, whereas the non-white SRCs (black and gray) reduced heat absorption by 34.53% and 69.44%, respectively, compared to the asphalt. By directly reducing the heat absorption of asphalt, the non-white SRCs can potentially slow permafrost degradation beneath asphalt pavements. This study provides a theoretical foundation and technical support for applying non-white near-infrared SRC on high-grade highways in permafrost regions.

Theory and Measurement of Heat Transport in Solids: How Rigidity and Spectral Properties Govern Behavior.

Models of heat transport in solids, being based on idealized elastic collisions of gas molecules, are flawed because heat and mass diffuse independently in solids but together in gas. To better understand heat transfer, an analytical, theoretical approach is combined with data from laser flash analysis, which is the most accurate method available. Dimensional analysis of Fourier's heat equation shows that thermal diffusivity (D) depends on length-scale, which has been confirmed experimentally for metallic, semiconducting, and electrically insulating solids. A radiative diffusion model reproduces measured thermal conductivity (K = DρcP = D × density × specific heat) for thick solids from ~0 to >1200 K using idealized spectra represented by 2-4 parameters. Heat diffusion at laboratory temperatures (conduction) proceeds by absorption and re-emission of infrared light, which explains why heat flows into, through, and out of a material. Because heat added to matter performs work, thermal expansivity is proportional to ρcP/Young's modulus (i.e., rigidity or strength), which is confirmed experimentally over wide temperature ranges. Greater uptake of applied heat (e.g., cP generally increasing with T or at certain phase transitions) reduces the amount of heat that can flow through the solid, but because K = DρcP, the rate (D) must decrease to compensate. Laser flash analysis data confirm this proposal. Transport properties thus depend on heat uptake, which is controlled by the interaction of light with the material under the conditions of interest. This new finding supports a radiative diffusion mechanism for heat transport and explains behavior from ~0 K to above melting.

Heat and mass transfer effects on an unsteady MHD boundary layer flow of fractionalized fluid

This research investigates the transient magnetohydrodynamic (MHD) flow of a non-integer Maxwell fluid over a vertical plate, taking into account the effect of diffusion-thermo and parameter of heat absorption. The fractional derivative Caputo-Fabrizio is employed to model the problem. Using the Laplace integral transform technique, we solve the dimensionless governing equations to obtain the profiles of velocity, concentration, and temperature, which are then presented in graphical form for easy comparison and interpretation. The influence of several parameters—specifically, the fractional parameter and heat absorption is thoroughly analyzed and illustrated through a series of graphical representations. Furthermore, a comparison between traditional as well as fractionalized velocity fields is provided. The results show that chemical reactions and magnetic fields reduce the velocity profile, while diffusion-thermo and mass Grashof number increase the fluid velocity.

Numerical study on flow-boiling heat transfer in square channels partially centrally filled with gradient porous media

Boiling heat transfer is widely used in industry and daily life and can be enhanced by porous media structure. Previously HPM have been shown to significantly improve heat transfer performance, but how the porosity gradient affects boiling heat transfer remains unknown. In this paper, the subcooled flow boiling in vertical square channels partially filled with gradient porous media was numerically investigated by using a newly developed solver named GPMPhaseChangeFoam embedded in the open-source platform OpenFOAM. The results indicate that in positive GPM inserted channels, when z ≤ 0.16 m Nuz increases with increasing Hp*, and when z ≥ 0.16 m Nuz increases with increasing Hp* from 0.2 to 0.8 but deceases with Hp* rises from 0.8 to 1.0, and the effect of Hp* on Nuz is more pronounced in the entrance region over the exit region. In negative GPM inserted channels, Nuz increases with increasing Hp* throughout the channel length in positive GPM inserted channels, and the effect of Hp* on Nuz is less pronounced in the entrance region than the exit region. Flow resistance increases with Hp*, but changes little with Δε, and Nufd increases with Hp* in both positive and negative GPM inserted channels. However, Nufd decreases with an increase in ∆ε from 0 to 0.3 in positive GPM inserted channels, from -0.3 to 0 in negative GPM inserted channels, and ∆ε = -0.3 at Hp* = 1.0 is 13.4 % higher than that of ∆ε = 0.3. Furthermore, the negative GPM inserted channel behaves higher flow boiling heat transfer performance than the HPM and positive GPM.

Stacking Ensemble Method to Predict the Pool Boiling Heat Transfer of Nanomaterial-Coated Surface

Abstract The boiling heat transfer coefficient is important information for designing thermal devices for effective thermal management. It is affected by several factors like surface roughness and wettability of the surface. So, it is necessary to create a model for the accurate prediction. This article aims to use the stacking ensemble method to predict the boiling heat transfer coefficient (BHTC). To improve the performance of the prediction of the stacking model, AdaBoost regression and Random Forest regression are chosen as the base learner, and meta estimator linear regression is selected. Datasets are generated from a pool boiling experiment of carbon nanotube and graphene oxide (CNT + GO)-coated surface. Results have depicted that the stacking method outperformed individual models. It is found that the accuracy of the stacking ensemble model is 99.1% efficient with mean absolute error (MAE), mean square error (MSE), and root mean square error (RMSE) values of 0.016, 0.0004, and 0.021, respectively.

Heat-Induced Liquid Hovering in Liquid-Gas Coexistence under Gravity.

We study a liquid-gas coexistence system in a container under gravity with heat flow in the direction opposite to gravity. By molecular dynamics simulation, we find that the liquid buoys up and continues to float steadily. The height at which the liquid floats is determined by a dimensionless parameter related to the ratio of the temperature gradient to gravity. We confirm that supercooled gas remains stable above the liquid. We provide a phenomenological argument for explaining the phenomenon from a simple thermodynamic assumption.

Dynamic ablation of C/C-SiC-AlSi and C/C-SiC-ZrB2-AlSi at changing impacted angle

Thermal protection component and jet vane of aircraft with trajectory transfer ability are attacked by heat flow from variable direction. To understand the dynamic ablation characteristics, C/C-SiC-AlSi and C/C-SiC-ZrB2-AlSi were ablated in plasma at cyclic changing impacted angle (±45o) and compared with the conventional test at steady state. Accompanied with higher and fluctuant surface temperatures, the periodical changed stress from scouring accelerated the consumption of AlSi matrix and strengthened the mechanical denudation of carbon fiber, which led to severer ablations in the dynamic environment.

Comparative Study of Photovoltaic Thermal Performance with Water and Aloe Vera Heat Extracting Fluids

Crystalline solar panels are widely used in households and for public road lighting. Mono-crystalline panels are well-known for their higher efficiency and long service life. However, their efficiency decreases as the module temperature increases under consistent solar radiation conditions. To enhance module power generation and efficiency, effective temperature reduction techniques are necessary. This study investigates the use of water and aloe vera fluid as cooling agents for a mono-crystalline photovoltaic thermal (PVT) system. The system was designed with a circulating mass flow rate of 0.016 kg/s or 1 LPM (liter per minute) and tested under the climate conditions of Phnom Penh city, Cambodia. The specific heat of aloe vera fluid was determined and found to be 4236 j/kg·K. The optical efficiency of PVT systems using water and aloe vera fluid was compared in this paper. The experiment results indicate that cooling with aloe vera fluid led to a 0.21% higher electrical power generation compared to water cooling, due to more effective temperature reduction and thermal heat absorption rate of aloe vera fluid is higher than water 32.41%.