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• An analytical model for thermoelastic crack problems in a strip is developed. • Convective heat transfer on lateral surfaces of the strip is considered. • Analytical solutions for thermoelastic field at crack tips are obtained. • Convective heat transfer impacts the SIFs and energy release rate significantly. • It is helpful in improving the understanding of thermoelastic fracture mechanics. Most existing thermoelastic crack models commonly overlook the convective heat exchange occurring on the lateral surfaces of the structures being studied. Consequently, there is often a concern regarding the accurate estimation of temperature fields and the resulting thermal stresses. To address this concern, this paper presents a two-dimensional (2D) thermoelastic model for a cracked strip that incorporates convective heat transfer between the lateral surfaces and the ambient environment. The crack problem is reduced to a set of singular integral equations with Cauchy kernels which are solved numerically. Analytical solutions for temperature distribution, stress intensity factors (SIFs) and energy release rate at crack tips are obtained. The influence of the ratios of convective heat transfer coefficient to thermal conductivity and thickness to height of the strip on the key fracture parameters is investigated through the numerical results. It has been discovered that classical thermoelastic crack models often exhibit a significant underestimation of the SIFs and energy release rates for cracks located near the hot boundary of the strip. The analytical model proposed in this paper has significant potential to advance our understanding of thermoelastic fracture mechanics. [ABSTRACT FROM AUTHOR]

This paper presents numerical simulation of graphe nanoplatelet (GNP) – sodium dodecylbenzene sulfonate (SDBS) – water nanofluid with volumetric mass fractions of ϕ equal to 0–0.1%. The new microchannel is made of 2 layers and has sinusoidal cavities along with the heat sink. The study was conducted for Re = 50, 300, 700 and 1000 (laminar flow). Based on the results, higher Re leads to more convective heat transfer coefficient. Therefore, the value of temperature gradients decreases and on the other hand the effect of cooling fluid temperature in warm areas near solid wall becomes important. Using a higher value of Re results in the penetration of heat in the fluid layer decreases which in general will reduce the temperature of the outlet section in larger Re values. Also, larger Re value results in larger development length which is also a cause for lack of thermal development and dominance of the temperature of the coolant flow in the heat sink resulting in decreased outlet section temperature for higher Reynolds numbers. For Re = 50 and 300 because of lower fluid velocity, the effect of surface geometry factors such as sinusoidal paths or hollow parts (cavities) has a smaller influence on the pressure loss or friction factor of the flow. [ABSTRACT FROM AUTHOR]

Integrating nanotechnology into phase change materials (PCMs) has emerged as a novel approach to improving PCM thermal properties and performance in various thermal energy storage applications. Nanofluids, nanoparticle suspensions in a base fluid, have been identified as a promising method of increasing the thermal conductivity of PCMs and thus reducing thermal energy charging and discharging duration. This review paper investigates recent advances in incorporating nanotechnology into PCMs, focusing on their applications in the solar energy sector. Thermal conductivity, specific heat, viscosity, density, and convective heat transfer coefficient of nanofluids are discussed, as are the thermal properties of nano-enhanced PCMs (NEPCMs), including thermal conductivity, latent heat, specific heat, viscosity, supercooling, and phase-change temperature. The paper also gives an in-depth look at the uses of nanofluids and NEPCMs in solar collectors, photovoltaic/thermal (PV/T) systems, coolants, and desalination, emphasizing their potential to improve system efficiency. The review also considers the potential environmental and human health concerns connected with using nanofluids and NEPCMs, emphasizing the importance of rigorous evaluation and risk assessment in developing and applying these materials. Overall, the review provides valuable insights into the potential benefits and challenges of incorporating nanotechnology into PCMs and emphasizes the importance of ongoing research and development in this field to further advance the use of thermal energy storage technologies in various applications, including solar energy systems. [Display omitted] • Nanofluids and nanomaterials are considered as innovative heat transfer medium. • Phase change materials (PCMs) for harvesting thermal energy are scrutinized. • Recent developments and properties of nanofluids and nano-enhanced PCM. • Potential applications of nanofluids and nano-enhanced PCM are fully presented. • Environmental impacts and human health risks are comprehensively discussed. [ABSTRACT FROM AUTHOR]

To deal with inverse nonlinear heat transfer problems in a two-dimensional heat conduction problem, this paper presents a hybrid approach combining fuzzy logic and the extended Kalman filter (EKF). The proposed algorithm has been applied to the real-time reconstruction of the temperature field, time-varying convective heat transfer coefficient, and time-varying boundary heat flux using temperature data from a set of sensors. The effects of the covariance of the initial estimation error, the time step of data acquisition by the sensors, and the number of installed sensors on the precision and stability of the estimation results has been investigated by numerical experiments. To compare and validate the results, all these parameters were investigated using the EKF method. The results show that the adaptive fuzzy extended Kalman filter (FEKF) method for estimating temperature, convection coefficient, and heat flux at the boundaries is precise and stable. The comparison shows that the performance of the FEKF method in parameter estimation is better than the EKF method, and the highest increase in precision and stability of the results is related to the convective heat transfer coefficient. [ABSTRACT FROM AUTHOR]

As a phase change material, ice slurry has been successfully applied in HVAC&R systems due to its high energy storage density. The heat exchangers are the key equipment realizing energy conversion of ice slurry. In HVAC&R systems, U-tubes are frequently used in shell and tube heat exchanger. However, there are few studies on thermo-fluidic characteristics of ice slurry in U-tubes. Therefore, the thermo-fluidic characteristics of ice slurry in U-tubes are numerically studied in this paper using Euler-Euler approach. The results indicated that as enlarging the inlet ice content and velocity, the ice concentration distribution becomes more uniform along in vertical profile. Due to the secondary flow in bend section, the ice concentration decreases in the inner wall and increases in the outer wall as the inlet velocity increases; At the high Reynolds number, the inlet ice content and ice particle diameter have a more significant fluence on pressure drop (Δ p). The local heat transfer coefficient (h lo) in bend section exhibits a tendency of first increasing and then decreasing, with the maximum value of h lo occurs at θ = 90° of the bend section. Finally, a correlation is suggested between the average resistance coefficient (f ave) and local Nusselt number (Nu lo). • The melting process of ice slurry in U-tubes is investigated. • The factors affecting thermo-fluidic behavior of ice slurry in U-tubes are discussed. • This peak value of h lo is located within the bending section at an angle of θ = 90°. • The correlation of f ave and Nu lo in U-tubes is obtained for ice slurry. [ABSTRACT FROM AUTHOR]

This paper aims to numerically investigate the effects of turbulence-inducing elements with various geometries in a double pipe Heat Exchanger (HEX). The water is considered as a working fluid and the flow is turbulent. Also, CFD simulation has been carried out by finite volume method within commercial software. In the first section, the various geometries, including smooth tube, corrugated tube, tube with spherical elements, and a tube with axial fins, are evaluated. Subsequently, the heat transfer characteristics of two various hybrid nanoparticles comprise Ag-MoS 2 and Fe 3 O 4 -SiO 2 , are compared. Ultimately, the optimized geometry has been selected for adding hybrid nanoparticles of Ag-MoS 2 to enhancing heat transfer performance. The obtain results indicate that the double tube heat exchanger with the spherical elements has the best thermal performance. In addition, with changing the Reynolds number from 4000 to 13,000 in the optimized model with Ag-MoS 2 1%, the convective heat transfer coefficient enhances 62.21%. [ABSTRACT FROM AUTHOR]

Highlights • Particle image velocimetry (PIV) was used to capture flow characteristics near heat transfer fins. • A large eddy simulation (LES) provided deeper insight into the flow structures occurring around the fins. • Near wall turbulence and flow patterns responsible for heat transfer enhancement were discussed. • Two unsolved problems on micro-fins were addressed. Abstract Internally enhanced surfaces such as micro-fins are an important class of heat transfer enhancement in commercial applications. Many research papers discuss the design and optima of these surfaces. However, most previous studies have demonstrated only the macro relationship between the geometries of the micro-fins and heat transfer. The need for a deeper understanding of these fins arose from some currently unsolved problems that limit future development of enhanced surfaces. First, why are increases of heat transfer larger than area increases in micro-finned tubes in most cases? Second, why do internally micro-finned tubes typically have lower heat-transfer-enhanced ratios in laminar and transition flows? This work presents a novel method to analyze the detailed relationship between flow characteristics and heat transfer for one type of micro-fin. The goal of the paper was not to find a new Reynolds number-based correlation, but to find flow patterns responsible for heat transfer enhancement and understand the mechanisms that cause this. First, this paper introduces comprehensive experimental measurements including particle image velocimetry (PIV), measurement of the heat transfer coefficient and accuracy of pressure-drop measurements, all used to validate numerical approaches. Validated large eddy simulations (LES) are then used to predict flow characteristics and coherent structures (Q criterion). The numerical simulation includes both heat conduction in the metal structure and heat convection on the solid–fluid interface. Finally, the paper documents how the flow structures link with the enhancement of heat transfer in the micro-finned duct. [ABSTRACT FROM AUTHOR]

Abstract This paper presents analyses of complex flows and heat transfer induced by twin oblique turbulent slot-jets of different directions (divergent, convergent or parallel) impinging a heated wall. A comparison of the heat transfer characteristics between perpendicular and three cases of twin oblique jets (parallel, convergent and divergent). The twin slot jets are located on a confining adiabatic wall at a distance of 8 slot jet width. Convective heat is investigated numerically examining the effect of Reynolds number (Re) and jet inclination angle (α). This problem is relevant to a wide range of practical applications including nuclear engineering devices, manufacturing, material processing, electronic cooling, drying paper or textile, tempering of glass, etc. All computations are performed using two dimensional large eddy simulations (LES) approach with Smagorinsky sub-grid scale (SGS) models. For all directions and inclinations of the jets, the location of the stagnation points is changed and hence, the location and magnitude of the maximum Nusselt number on the heated wall vary. When Reynolds number increases, Nusselt number is improved for all types of inclination. The averaged Nusselt number shows that the perpendicular impingement gives better heat transfer than that of the oblique jets. The poor heat transfer is obtained for the parallel oblique jets. For the same angle, divergent jets give smallest heat transfer than the convergent jets. Graphical abstract Twin-jets flow configuration. (a) Perpendicular (b) Divergent (c) Parallel (d) Convergent. Image 1 Highlights • Four types of flow patterns of impinging twin jet are studied by Large Dissipation Simulation. • Nusselt number is enhanced for all types of inclination for increasing Reynolds number. • Averaged Nusselt number of the perpendicular impingement gives better heat transfer than that of the oblique jets. • The poor heat transfer is obtained for the parallel oblique jets. • For the same angle, divergent jets give smallest heat transfer than the convergent jets. [ABSTRACT FROM AUTHOR]

At present the ion-wind has been applied to portable electronic equipment heat dissipation because of its excellent heat dissipation performance. In this paper, sawtooth-mesh and sawtooth-plate ion-wind generators are studied. In addition, the best structure of single-stage ion-wind is found through the local maximum velocity, average velocity, and average mechanical efficiency.For single-stage sawtooth-mesh ion wind generator,when the number of sawteeth is 6 and the electrode gap is 20 mm,the average velocity that can achieve the best ion wind 1.7 m/s, and the average mechanical efficiency is 0.84%.For single-stage sawtooth-plate ion wind generation devices.When the number of sawteeth is 4 and the electrode gap is 15 mm.The average wind velocity of ion wind can reach 1.42 m/s, and the average mechanical efficiency is 1.14%.On this basis, the multi-stage sawtooth-plate ion-wind generator is studied. The more number of stages, the higher the average velocity of ion-wind is, but when the number of the stage is more than 2, the average mechanical efficiency decrease then the practicality is limited.When the number of multistage electrode stages is 2, the average heat transfer coefficient can reach 86.1 W/m-2·K-1, which is 34% higher than the single-stage structure. [ABSTRACT FROM AUTHOR]

Flip-chip packaging technology is widely used in the field of electronic packaging. The thermal contact resistances (TCRs) of its double-layer thermal interface materials (TIMs) impact its thermal characteristics. In this paper, a flip-chip package thermal contact model is first constructed considering the internal rough contact interface of double-layer TIMs. Secondly, the effects of friction coefficient, pressure, elastic modulus of double-layer TIMs, thermal conductivity of double-layer TIMs, interstitial medium, and convective heat transfer coefficient on the flip chip package's contact mechanics and thermal properties are analyzed. Finally, the influence of secondary thermal effects on contact mechanics and thermal properties is investigated. The results show that the friction coefficient, pressure, and elastic modulus of double-layer TIMs have a significant influence on the TCRs by changing the actual contact area, but the impact on the heat dissipation of the chip is limited, and the maximum temperature reduction does not exceed 10 K. However, the interstitial medium, convective heat transfer coefficient, and thermal conductivity of double-layer TIMs greatly influence the heat dissipation of the chip and maximum temperature by directly affecting heat transfer performance. Furthermore, secondary thermal effects have varying degrees of influence on contact stiffness, TCRs, and temperature distribution. [Display omitted] • A thermal contact model of flip-chip package considering the TCRs was established. • The effects on contact mechanics and thermal properties of the model were analyzed. • Secondary thermal effects on contact mechanics and thermal properties was analyzed. [ABSTRACT FROM AUTHOR]

Finite amplitude convection in a inclined porous layer heated from below is studied by using local thermal non-equilibrium (LTNE) as mathematical model which takes into account the heat transferred between the solid phase and the fluid phase. Consequently, in addition to Darcy-Rayleigh number Ra and the inclination angle ϕ , two further non dimensional numbers are introduced: the inter-phase heat transfer parameter H and the porosity modified conductivity ratio γ . In a recent paper (Barletta and Rees, 2015), the linear stability analysis of the basic monocellular flow indicated that the inclination angle promotes the appearance of longitudinal rolls as the preferred mode of convection. The current paper focuses on the nonlinear evolution of longitudinal rolls in a supercritical regime of convection. A weakly nonlinear analysis, using a derived amplitude equation, is adopted to determine the nonlinear effects of the parameters Ra , ϕ , H and γ . The results indicate that in inclined layers (i) the nonlinearity decelerates the mean flow; (ii) the heat transfer, determined by the evaluation of the Nusselt number ( Nu ) at the layer boundary, corresponds to the one obtained for horizontal layers by scaling Ra with cos ϕ , i.e. Nu = Nu ( Ra cos ϕ , H , γ ) ; (iii) in accordance with existing laboratory experiments, the slope of Nu is less than 2, where 2 is the value predicted by the local thermal equilibrium model, and the slope represents the derivative of Nu with respect to the distance of the critical parameter from the threshold value for the onset of instability; (iv) increasing values of both H and γ produce an enhancement of the heat transfer across the layer. Finally, the comparison between the LTNE theoretical predictions and existing experiments conducted with various combinations of solid matrix and fluids suggests a possible alternative way to determine the heat transfer coefficient H . [ABSTRACT FROM AUTHOR]

• An adaptive heuristic fluid-solid interface condition is proposed for the first time. This new condition, called "X-R", has been developed to switch automatically between the Dirichlet-Robin (D-R) and Neumann-Robin (N-R) conditions on the basis of the Biot number. • A temporal interpolation of the fluid conditions for dealing with "non-coupling instants" is defined on the basis of a physics-based approach by naturally considering the flux-temperature linearity. This approach replaces the generally adopted time-dependent linearity, which cannot be justified. • The first test case demonstrates that the adaptive interface condition always produces smooth solutions, which is not the case when fixed conditions like D-R or N-R are adopted. • The second test case shows that the physics-base approach based on the flux-temperature linearity is by far the most satisfactory. However, this interpolation method remains valid only if each ramp defined initially remains in a linear domain. • Indeed, a full transient cycle, entirely based on the existence of particular time points can lead to non-linear problems, difficult to deal with. • As a direct consequence of the previous point and shown in the third test case, the initial problem can be not well-posed. In this case, adding a small number of temporal points can keep the coupling algorithm in a quasi-linear domain. This paper is devoted to the study of numerical methods used in the analysis of a thermal transient flight cycle. Two priorities are stressed. The first one concerns a stability issue, namely the fluid-solid interface conditions. The main properties of the Dirichlet-Robin and Neumann-Robin conditions are first recalled, before proposing for the first time an adaptive heuristic condition combining them. The second line of research is dedicated to an accuracy issue, the temporal interpolation of the fluid conditions for dealing with non-coupling instants. We propose here to adopt a physics-based approach by naturally considering the flux-temperature linearity. Then, using a simple numerical test under severe thermal boundary conditions, three test cases are proposed. The first one demonstrates that the adaptive interface condition always produces smooth solutions. The other two cases show that the physical interpolation approach is much more accurate than the one based on temporal linearity, using a costly but accurate reference solution built for the validation. Moreover, important improvements were made by adequately defining the integral contribution of the boundary condition using the Gauss points in the finite element solid code. It was also revealed that a relevant approximation of the heat transfer requires a sufficiently fine solid mesh at the interface. Finally, potential improvements are proposed at the end of this paper and more particularly two other interface conditions. One of them has the remarkable property of not using the convective heat transfer coefficient. [ABSTRACT FROM AUTHOR]

The aim of and inspiration behind this paper was to explain the reasons, also observed by other researchers, of the discrepancy in the results of experimental free convection, which for small Rayleigh and Nusselt numbers in the initial phase of research can sometimes reach several hundred percent. These discrepancies decrease with increasing heating power and plate surface temperature, in proportion to the increase in Ra and Nu , reaching typical values for this type of research. To explain this phenomenon, a comprehensive theoretical and experimental analysis of the influence of the physical properties of a fluid (air and water) as well as primary (t w and t ∞) and secondary (t av and Δ t) temperatures on the Rayleigh number was carried out. The impact was found to be unequal. The plate temperature t w is of greater importance, which is much higher than the much lower and almost constant temperature t ∞ of the undisturbed area, especially since it causes convective movement, generating differences in fluid density and thus driving the phenomenon. Similarly, the direct contribution of the temperature difference Δ t to Ra suggests that it has a greater influence on convective heat transfer than the average temperature of the medium t av. By analysing the effect of each of these temperatures separately, it was possible to show that their mutual, compatible or opposite interaction (t w / t ∞) causes a different scattering of results, or may even lead to unusual Rayleigh numbers (Ra temperature dualism). This study led not only to a better understanding of the phenomenon, but even to a prediction of its unusual behaviour, unheard of in typical experimental studies of free convection. For example, if we consider the theoretical convective heat transfer from a plate l = 0.15 m in air in the context of the interaction of t av and Δ t , it turns out that for the same Δ t = 40 K, the Rayleigh number may assume, depending on t av = (t w + t ∞)/2, different values. So, for t w = 50 °C, t ∞ = 10 °C and t av = 30 °C, Ra = 1.213. 107, whereas for t w = 90 °C, t ∞ = 50 °C and t av = 70 °C it is ≈ 1.7 times smaller (Ra = 0.687. 107). This hypothetical phenomenon, unheard of in typical experimental studies, which could occur, and maybe even does occur in smelting, thermal energy, etc., forces us to think about the values of Nusselt numbers, heat transfer coefficients and heat fluxes for these two cases. This lies beyond the scope of the present paper, but it is a topic for possible future research. • A theoretical analysis of the influence of t w , t ∞ , Δ t and t av on values of Ra has been carried out. • A coefficient of the convective properties of the medium has been introduced. • It has been demonstrated that the influence of t ∞ and t av on convective heat transfer is as important as t w and Δ t. • Theoretically, the possibility of Ra temperature dualism has been demonstrated. [ABSTRACT FROM AUTHOR]

An efficient battery thermal management system is crucial for ensuring the working temperature environment of batteries and extending their lifespan. In this paper, a novel battery thermal management system combining vapor chambers and thermoelectric coolers is proposed to improve the battery's thermal behavior. Also, a complete fluid-thermal-electric multiphysics numerical model for the proposed system is built to predict its thermal performance under different cooling parameters. Results show that the use of thermoelectric coolers and vapor chambers can greatly lower the maximum temperature and temperature difference of batteries. Although the maximum temperature decreases with the increase of air convection heat transfer coefficient and coolant flow rate, the temperature difference increases at the same time. Under the given optimal air and water cooling parameters, it is noticed that as the input current of thermoelectric coolers increases, the maximum temperature and temperature difference show a pattern of decreasing first and then increasing. The optimal air convective heat transfer coefficient, coolant flow rate, and input current of 50 W/(m2·K), 0.04 m/s, and 1.5 A, respectively, are suggested, which corresponds to the maximum temperature of 39.83°C and temperature difference of 5.97°C. The present research introduces a fresh perspective on efficient battery thermal management, offering detailed insights into the utilization of thermoelectric cooling for this purpose. • A novel BTMS integrating with thermoelectric coolers and vapor chambers is proposed. • The use of TECs can greatly reduce the maximum temperature and temperature difference. • The battery temperature first increases and then decreases as the increase of input current. • Optimal input current, air and water parameters of 1.5 A, 50 W/(m2·K) and 0.04 m/s are suggested. [ABSTRACT FROM AUTHOR]

• The retention angle models are modified considering the flow vicious effect. • Convection mechanism is adopted at high heat flux. • The new model has satisfactory predictions for liquid film profile and heat transfer coefficient. • Flow and heat transfer characteristics at high heat flux and low temperature are analyzed. In this paper, a new analytical model is developed for the condensation of refrigerants outside low-finned tubes. It is based on the flow and heat transfer characteristics at low saturation temperatures and high heat flux, such as those in the LNG intermediate fluid vaporizer (IFV). Due to the increase of viscous effect at lower saturation temperature and higher heat flux, the previous condensate retention angle models are modified by adding the viscous term. At high heat flux and condensate rate, the condensate profile at the fin root is simplified to be arc-shaped and obtained directly by force analysis. Furthermore, considering the heat transfer enhancement caused by the circumferential flow at high heat flux, the convective mechanism is included for the heat transfer through the thick liquid film between the fins. The distributions of liquid film and fin surface temperature are also predicted. The present analytical model is in good agreement with different experimental and numerical results, while most deviations are within 10% for predicting the heat transfer coefficients at high heat flux. Because of the increased flow viscosity and surface tension, the optimal fin spacing and fin height increase as the heat flux increases and the saturation temperature decreases. [ABSTRACT FROM AUTHOR]

• Inversion of physical properties of strong extinction materials. • Error filtering using wavelet analysis. • Analysis of the influence of wavelet parameters. • Experimental verification and comparison of traditional methods. The accurate acquisition of the physical properties of strongly extinction materials limits their application development. In order to solve the problem of large errors in the high temperature measurement of these materials, this paper constructs an inversion model for the radiative and conductive coupled heat transfer through wavelet decomposition coefficients, and compares it with traditional methods. The radiative and conductive coupled heat transfer model is solved by finite volume method and Monte Carlo method. Daubechies nine wavelet is used for signal wavelet analysis. It has been discovered that the wavelet decomposition parameters of the temperature field are mainly concentrated in the 4–8 wavelet frequency band of wavelet, it can separate from the random error that is mainly concentrated in the 1–3 frequency band of wavelet. Most of physical properties like the heating power, convective heat transfer coefficient, material thermal conductivity, extinction coefficient has a great impact on the waveform and amplitude of wavelet detail coefficient and approximation coefficient, and the change trend of approximation coefficient and detail coefficient amplitude is consistent with the trend of temperature field. However, the effect of scattering albedo on wavelet parameters is tiny. Through inversion, it is found that the inversion model based on wavelet decomposition parameters of 5–7 frequency band has higher accuracy when random error exists. [ABSTRACT FROM AUTHOR]

Fluid transport through microvascular networks—a hallmark for homeostasis in living systems—has transcended to engineered materials, primarily made possible because of modern manufacturing advancements. Vascular-enabled multifunctionality, including thermal regulation and self-healing, holds great potential for extending the lifetime of structural materials and expanding the operational envelope. Prior studies on vascular-based active cooling use a "combined" heat transfer coefficient (HTC): a single parameter lumps convection and radiation effects. Although the resulting mathematical models are linear—an attractive feature for computational modeling, the combined coefficient approach may not be accurate or even applicable if the operating temperature is unknown, which is the case with many thermal regulation applications (e.g., space probes). In this paper, we illustrate the remarked limitations of the lumped approach and advocate the need to use a decoupled HTC by splitting convective and radiative heat transfer modes. We show the broad applicability of the proposed method by applying it to three material systems: glass and carbon fiber-reinforced polymer composites and an additive manufactured metal. We show, using numerical simulations, the differences in the predictions from the decoupled approach with that of the combined HTC; these differences are prominent at higher heat fluxes. Also, the decoupling has enabled us to establish a scaling law that allows transferring of solutions fields across material systems, strengthening further the validity and utility of our approach. This work's significance is two-fold. First , the research is fundamental, providing accurate measurement protocols for critical model parameters. Second , this work facilitates the development of mathematical models for vascular-based thermal regulation that are predictive even for hostile environments (which are often difficult to realize in laboratories), such as outer space. • Theoretically consistent heat transfer coefficient (HTC) measurements and analyses. • A heat transfer framework for improved prediction of microvascular active-cooling. • Multi-material experimental validation of a reduced-order model. • Self-similar scaling strategy to transfer solution fields across material systems. [ABSTRACT FROM AUTHOR]

Nowadays, a bio-inspired heat exchanger incorporating a triply periodic minimal surface (TPMS) structure has demonstrated great potential in the fields of new energy research and aerospace, which necessitates achieving a balance between low volume and high heat transfer efficiency while maintaining low pressure drop. In this paper, the adjustable heat efficiency of the heat exchanger with TPMS in gradient thicknesses is specially designed. A steady-state conjugate heat transfer (CHT) model is coupled with computational fluid dynamics (CFD) analysis. In addition, the temperature profile and velocity streamline are also checked to analyze the fluid flow behavior of the radiator. The results show that the convective heat transfer coefficient of the Gyroid with gradient level set values is 26.02–60.10% higher than that of the uniform Gyroid model, and the pressure drop is decreased by 9.66–18.05%. Both high heat transfer efficiency and low pressure drop can be achieved when the thickness is 0.2–0.3 mm and Re is 100–125. The heat exchangers with a TPMS thickness gradient in the ratio of 2:4:6 demonstrate a remarkable enhancement in overall heat transfer efficiency, achieving a 30.22% improvement compared to those with a TPMS thickness gradient in 6:4:2. [ABSTRACT FROM AUTHOR]

Vehicle braking systems use sliding friction between discs and pads to convert large amounts of kinetic energy into heat. Thus, sufficient cooling of disc brakes is needed. Convective heat transfer is an efficient way to cool disc brakes and prevent very high temperatures. To enhance convective heat transfer, ventilated disc brakes are commonly used. Therefore, the heat transfer characteristics of the internal passages of ventilated discs are of special interest. This paper describes a new experimental method to study the convective heat transfer characteristics of the interior passages of ventilated disc brakes, in which a wheel rolled a ventilated disc with tapered radial vanes on a circular track to simulate the real working conditions of a ventilated disc brake. The naphthalene sublimation method was used to obtain convective heat transfer characteristics by the heat/mass transfer analogy principle. Different data reduction methods were used to present the experimental results. The correlations among the Nusselt number, Reynolds number and other geometric parameters of the tapered radial vanes are reported in this paper. The results indicate that the heat transfer coefficient has a close relationship with Coriolis acceleration. Thus, this paper reports a correlation between Coriolis acceleration and the convective heat transfer coefficient, which can be used to extend the results obtained by the tested discs to realistic cases. • A new model to study the heat transfer ability of the interior passage of ventilated disc brake is provided. • Correlations of the interior passage of a ventilated disc brake with tapered radial vanes is obtained. • Relationship between Coriolis acceleration and heat transfer capacity is addressed. • Relationship between Coriolis acceleration and heat transfer capacity can extend the experimental results to real case. [ABSTRACT FROM AUTHOR]

Heat transfer due to fluid flow past a sphere is encountered commonly in engineering applications. In this case, the local convective heat transfer coefficient on the surface of the sphere is known to change with azimuthal and polar angles due to various flow phenomena. While surface-averaged convective heat transfer coefficient is commonly used for engineering analysis, however, not much work exists for modeling the temperature field inside the sphere while accounting for the spatially varying convective heat transfer, especially in the context of a sphere with orthotropic thermal conduction properties. This paper presents an analytical approach for a steady state solution of this problem by deriving a set of algebraic equations for coefficients of a series solution of the temperature distribution. The problem is solved using two different approaches, which are shown to lead to equivalent results. Temperature distribution based on the analytical approach is found to be in excellent agreement with finite-element simulation results. The effect of various parameters, such as thermal conduction orthotropic ratio, heat generation rate, power density, flow rate, etc. on temperature distribution in the sphere is presented. Results discussed in this paper contribute towards the fundamental understanding of an important heat transfer problem, and in the design of thermal management techniques for engineering applications involving convective cooling of spherical systems. [ABSTRACT FROM AUTHOR]

Aviation kerosene has been widely used in aviation and aerospace fields as both propellant and coolant for aircraft fuel systems and thermal management systems, where the supercritical heat transfer calculations are required. As a typical aviation fuel in China, RP-3 has gained much research interest on its heat transfer characteristics under supercritical pressure, resulting in many empirical correlations. However, the prediction accuracy of the existing correlations is not satisfactory and the pursuit of accuracy improvement remains needed. This paper proposes a new correlation based on the database of RP-3 heat transfer in vertical tubes under supercritical pressure, which contains 1722 experimental data points compiled from six published papers. It has a mean absolute deviation (MAD) of 11.0%, predicting 83.7% of the entire database within ±20%, while the best existing model available in the literature only has an MAD of 24.1%, predicting 58.4% of the entire database within ±20%. Therefore, the new model improves the prediction accuracy of RP-3 heat transfer under supercritical pressure remarkably. [ABSTRACT FROM AUTHOR]

This paper mainly studied the heat transfer character of super critical carbon dioxide, also named R744. Super critical carbon dioxide can be used in gas cooler for air conditioning system. Compared with traditional Freon coolant, R744 has higher density and lower viscosity, which is benefit for heat transfer process. So it is important to study heat transfer character for super critical carbon dioxide. The experiment research is taken for the temperature ranging from 29 to 55 °C, while pressure is 8, 9 and 10 MPa. The Reynold number is about 2 × 10 5 . Thermal resistance method is used to measure heat transfer coefficient. The results found that convection heat transfer enhances near the pseudo critical point, in the region far away from pseudo-critical point, heat transfer character is just like ordinary single-phase fluid. The heat transfer coefficient violently changeable region is also the region where thermal property change rapidly. It is also found that heat transfer coefficient gets its max at the region near the pseudo-critical point. In addition, the results obtained in this paper are compared with other researchers’ results. [ABSTRACT FROM AUTHOR]

Fluid flow past a cylinder is a classical problem of fluid mechanics and convective heat transfer. In this problem, the local convective heat transfer coefficient on the cylinder surface varies around the cylinder due to boundary layer growth, separation and transition to turbulence. While there is considerable literature on computing the temperature distribution in the fluid, not much work exists on computing the temperature distribution within the solid body, particularly if the solid has anisotropic thermal properties. This paper presents an analytical technique for computing the temperature distribution within a heat-generating cylinder with anisotropic thermal conductivity subjected to spatially varying convective heat transfer coefficient due to fluid flow. As expected, temperature distribution on the cylinder surface exhibits minima and maxima at locations where the convective heat transfer coefficient has maxima and minima respectively. The effect of various parameters, including Reynolds number of the flow and extent of anisotropy are examined. Results presented in this paper contribute towards the fundamental understanding of a classical heat transfer problem. Further, since Li-ion cells that are commonly used for energy conversion and storage exhibit strong thermal conduction anisotropy, these results may be useful for design of convection-based thermal management of Li-ion cells. [ABSTRACT FROM AUTHOR]

In order to improve the heat transfer performance of microchannels, considering from balance flow resistance and increasing heat transfer area, a non-closed droplet pin fin array microchannel has been designed. The purpose of this paper is to study the inner influence of non-closed droplet structure on convection heat transfer by means of simulation. Through the simulation tool ANSYS Fluent, the working conditions of heat flux density from 5 W/cm2 to 85 W/cm2 are simulated respectively, with the inlet flow as 1.5 kg/h and the inlet fluid temperature as 40 °C unchanged. Along with the increase of heat flux density (5 W/cm2 ⩽ q ⩽ 85 W/cm2), the pressure drop, the bottom temperature as well as the heat transfer coefficient continue to rise, but the rising rates are various. Such variation could be explained by the different heat exchange processes: single phase heat transfer, bubble nucleation, growth and migration and then dry phenomenon, which can be observed from the vapor phase distribution results. The distribution of vapor phase under multiple heat flux explains the variation of pressure and temperature in different stages. It could be concluded that the non-closed droplet pin fin has a strong fractal effect on the fluid. After multiple shunting and confluence, the flow velocity of the fluid on both sides is significantly greater than that between the fins, whose average velocity is 0.135 m/s faster than that between the pins. Besides, the heat transfer effect of the corresponding walls on both sides is also better than that between the pin fins. Because the pin fin has the characteristics of droplet structure, the boundary layer on both sides of the pin fin will be separated and the heat transfer performance will be enhanced. The temperature of the pin fin head is lower, and the temperature of the pin fin tail is higher, with an average difference as about 9.32 °C. The comparison research between non-closed droplet and traditional droplet pin fin has been conducted, as well. It could be concluded that when 5 W/cm2< q < 60 W/cm2, the heat transfer coefficient of the opening droplet microchannel is higher, with pressure drop lower. The main reason is that the improvement of the non-closed structure reduces the kinetic energy loss caused by the collision, and promotes the emergence and growth of the vaporization core. • With the increase of heat flux, the pressure drop increases slowly, rapidly and slowly. • With the increase of heat flux, the initial point of phase transition gradually moves forward from the rear section. • The non-closed structure has a strong fractal effect on the fluid. • The boundary layer on both sides of pin fin will be damaged and the heat transfer performance will be enhanced. [ABSTRACT FROM AUTHOR]

• Condensation characteristics of air-vapor mixture is investigated experimentally. • The structural parameters of liquid and gas film are obtained. • The thickness of gas film is an order of magnitude higher than that of liquid film. • Increasing flow rate enhances the mixed vapor condensation heat transfer. • A new experimental correlation equation is established with an error within ±30%. The condensation heat transfer of air–water mixed vapor is an important heat transfer phenomenon in production life. An experimental system for condensation heat transfer of the air–water vapor mixed vapor is built in this paper. The condensation characteristics of the mixed vapor and the double boundary layer structure characteristics were investigated. The study mainly discussed the influence of flow velocity on the condensation performance of mixed vapor and summarized the experimental correlation equations considering flow velocity. Results indicate that the condensation performance decreases significantly as the air concentration increases. The mixed vapor flow velocity was increased from 1 to 3 m/s, and the condensation heat transfer coefficient was increased by 34.9–121.0%. The enhanced effect of flow velocity was also observed at high air concentration and low subcooling. The ratio of convective and condensation heat transfer coefficients increases with the rise in air concentration, reaching a maximum of 0.12. The proportion of convective heat transfer can be reduced by increasing the mixed vapor flow velocity. The gas film thickness is substantially higher than the liquid film thickness in mixed vapor condensation, and the gas/liquid film thermal resistance ratio can be more than 30 times. And increasing the flow velocity can significantly reduce the gas film/liquid film thermal resistance ratio. A new experimental correlation equation is fitted based on the experimental results, and this equation can be used as a reference for the design of related heat exchangers. [ABSTRACT FROM AUTHOR]

• Static mixers for enhancing convective heat transfer were additively manufactured. • Additive manufacturing enabled novel flow geometry, chevron-shaped structures. • Static mixers were monolithically integrated into liquid water channel. • Static mixers enhanced convective heat transfer coefficient about 100%. • A finite volume model suggests a guidance on the static mixer design. This paper investigates heat transfer enhancement by means of additively manufactured static mixers during liquid water cooling of a horizontal, heated flat plate. The static mixers disrupt the thermal boundary layer and induce mixing, resulting in an increased heat transfer rate of about 2X larger than flows without mixers. Simulations of the flows provided insights into the flows near the mixers, and guided selection of specific mixer geometries. The mixers were fabricated directly into the flow channels using additive manufacturing and then assembled onto the heated plate. Two types of mixing structures were analyzed: twisted tape structures that are similar to conventional static mixers; and novel chevron-shaped offset wing structures. Heat transfer performance was measured for liquid water (510 ≤ Re ≤ 1366) cooling the heated section with convective heat flux ranging between 0.1 and 0.8 W/cm2. This work demonstrates the potential of additive manufacturing to enable novel flow geometries that can enhance convection heat transfer whilst minimizing pressure drop penalties and volumes. [ABSTRACT FROM AUTHOR]

Highlights • Thermal convection was suppressed by the dielectrophoretic force. • The electroconvection has a strengthening effect on free convection heat transfer. • Strengthening effect of electroconvection increases with increasing electric field. • The strengthening effect at a low heat flux is better than that at a high heat flux. Abstract In this paper, the electrohydrodynamic (EHD) enhancement of free convection heat transfer is studied experimentally. A smooth copper heating surface is submerged in R113 liquid, the heat fluxes ranging from 4461 W/m2 to 17805 W/m2, and a high voltage ranging from 1 kV to 5 kV are applied on the heating surface. High speed photography is used to record the images of electroconvection. The results show the relationship between the intensity of the electric field, the height of the electroconvection layer and the enhancement of the heat transfer coefficient. Electric field can enhance free convection heat transfer, with the increase of the electric field the fluid disturbance on the heating surface increases obviously, and with the increase of the heat fluxes the enhancement of electric field gradually decreases. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a numerical evaluation of the eddy viscosity turbulence models (the zero-equation, one-equation, k - ε group and k - ω group models) in terms of CFD modelling of convection-radiation coupled heat transfer in indoor environment. The results show that all the models except the zero-equation model have acceptable accuracies to predict the bulk air temperature. However, the zero-equation and one-equation models seriously overpredict the convective heat transfer coefficient at the heater surfaces while the k - ε group models severely underpredict it, ultimately resulting in the incorrect predictions of the thermal buoyancy flows and contaminant dispersion. On the contrary, the k - ω group models perform best to calculate the convection-radiation heat split, which is critical to the successful predictions of the near-wall flow field and distribution patterns of the contaminants. This study also demonstrates the importance of simultaneously considering the convective and radiative heat transfer mechanisms when evaluating the turbulence models for indoor environment simulations. [ABSTRACT FROM AUTHOR]

Abstract This paper deals with the modeling of weakly rarefied and dilute gas flows in micro channels by the continuum approach, valid for Knudsen numbers smaller than about 0.1. It particularly focuses on the modeling of the associated heat transfer. The models proposed in the literature for the forced convection of gas flows in long micro channels between two infinite plates are more specifically discussed. The complete model for such flows is reminded after a compilation and a brief description of their possible applications in industries. The compatibility of the pressure work and viscous dissipation in the energy equation with the power of the viscous stress at the walls is discussed in detail. A dimensional analysis is proposed in the context of long micro channels. Analytical solutions for the velocity and temperature fields and for the Nusselt number are provided in the case of compressible micro-flows in isothermally heated flat plate channels, with pressure work and viscous dissipation included. The choice of an appropriate Nusselt number, including the power of the viscous stress at the wall, is particularly discussed. It is shown analytically and numerically, by solving the complete model for an isothermal wall micro-channel, that the Nusselt number tends to zero when the hydraulic diameter decreases, that is when the Reynolds number decreases and the Knudsen number increases. This could theoretically explain the very small values of the Nusselt number obtained in the experiments by Demsis et al. (2009, 2010). Highlights • Isothermally heated gas slipping flows in long plane micro-channels are considered. • Analytical solution of flow and heat transfer established from an asymptotic analysis. • Viscous stress power at the wall must be included in the total wall heat flux. • Wall Nusselt number tends to zero or to very small values for this flow type. • The very small values of the Nusselt number obtained in experiments are explained. [ABSTRACT FROM AUTHOR]

In classical theory of porous media, a local thermal equilibrium between fluid phase and solid phase is assumed. According to this theory, the fluid and solid temperatures reach an equilibrium temperature value rapidly. However, in porous media, the rate of heat transfer between the fluid and solid may not be fast enough to achieve a local thermal equilibrium (LTE) due to their thermal diffusion properties. Therefore, this paper examines the importance of local thermal non-equilibrium (LTNE) in porous media with conductive heat transfer. The LTNE is constituted by energy equations and defines distinctive temperature profiles for both the solid and fluid phases. The Laplace transform and Stehfest algorithm methods have been employed in formulating an exact solution. Using the weighted average method, a temperature for the porous media is defined. Subsequently, the model is applied to a cylindrical hole subjected to a uniform temperature in an infinite porous medium (rock formation). The exact solution and transient temperature profile are the advantages of this model compared to existing LTNE models. [ABSTRACT FROM AUTHOR]

Helical tube has been widely used in liquefied natural gas (LNG) liquefying applications. However, studies on the heat transfer performance of hydrocarbon mixture refrigerant in a helical tube have rarely been conducted. In this paper, condensation heat transfer characteristics of hydrocarbon mixture refrigerant in a helical tube were investigated numerically. The tests were conducted at saturation pressure of 2–4 MPa, mass flux of 100–400 kg/(m 2 ·s) and vapor quality 0–1. The numerical model was well verified by experimental data. And the effects of different structural parameters (tube diameter, helix angle and curvature diameter) and operating parameters (mass flux, heat flux and saturation pressure) on the heat transfer of hydrocarbons refrigeration were considered. The results showed that the tube diameter and mass flux had significant effect on condensation heat transfer coefficient. Meanwhile, stratified flow, half-annular flow and annular flow were observed during the condensation of methane/propane mixtures refrigerant by simulations, and a new flow pattern transition line for hydrocarbon refrigerants condensation process was proposed. Finally, the simulation results were compared with several well-known condensation heat transfer correlations. The research results will provide some constructive instructions for more effective design of LNG heat exchanger. [ABSTRACT FROM AUTHOR]

Jet impingement cooling is an effective means of inducing high convective heat transfer coefficients for applications such as the cooling of gas turbine surfaces, drying of textiles, and the tempering of metal and glass. Slot jets can be used for applications which require cooling over a line or strip, and these can be realized at the micro-scale if size is constrained. In this paper the application of these jets is of specific interest for high-density photonic integrated circuits (PICs), which can generate device-level heat fluxes as high as 1 kW / cm 2 . In this context, an understanding of the thermal and hydrodynamic behaviour of low Reynolds number, submerged, confined and normally impinging slot jets is currently unavailable in the literature and would be beneficial for the design of micro-scale cooling systems. This investigation utilized the isoflux heated foil technique and Particle-Image Velocimetry (PIV), to measure the single-phase convective heat transfer and velocity fields associated with confined liquid slot jets. The aspect ratios of the jets were varied from 1 to 8, with a constant hydraulic diameter, in order to determine the influence of aspect ratio on the measured parameters. The study was carried out over a jet exit Reynolds number range of 100–500, and a fixed confinement ratio of H / D h = 1 . The results showed that, enhancements of up to 68% in area-averaged heat transfer could be achieved through increasing slot jet aspect ratio, with a corresponding 12% reduction in head loss coefficient. The appearance of off-center peaks in the velocity and Nusselt number distributions were also observed. These peaks were concluded to be as a result of the stagnation zone fluid dynamics and local flow entrainment. [ABSTRACT FROM AUTHOR]

This paper is aimed to investigate the effectiveness of innovative nanostructured surfaces in terms of heat transfer and evaporation rate enhancement during the pool boiling process of argon atoms by employing molecular dynamics simulation. LAMMPS software and Lennard-Jones potential are used for the simulation and determination of the force field. The fundamental thought of creating these novel geometries (tree-root type nanostructures) is to use the branches as a barrier against liquid cluster separation, which leads to fluid temperature enhancement. First, the framework of simulation is validated, and then the results of creating two types of copper tree-root nanostructures are presented and compared with three cases which include: adding platinum, aluminum, and copper nanoparticles to the argon fluid on the plain copper substrate. The results revealed that creating tree-root type nanostructures postponed the separation of the liquid cluster in comparison with adding nanoparticles. In addition, the tree-root type nanostructures can enhance the evaporation rate and the argon temperature up to 60.05% and 17.13% more than adding nanoparticles, respectively. On the other hand, these nanostructures improve the maximum heat flux and corresponding convective heat transfer coefficient by 15.46% and 26.86% compared to the cases of adding nanoparticles, respectively. [ABSTRACT FROM AUTHOR]

• The effect of continuous pressure waves on heat transfer of pipe bundle was studied. • Continuous pressure waves were stimulated by vibrating membrane in CFD models. • A larger affected space was observed in the presence of continuous pressure waves. • The heat transfer is enhanced by 7.01% to 156.22% at the frequency of 20–160 Hz. • Continuous pressure waves have the great potential in enhancing the heat transfer. This paper presents a numerical study of the heat transfer enhancement outside the pipe bundle under the effect of continuous pressure waves (PW) stimulated by a vibrating membrane as a PW generator. Model 1 and 2 made of one and two rows of pipe bundles were investigated, respectively. An oscillating wall boundary condition was applied to the vibrating membrane. Based on the RNG k-ε turbulent model and full cavitation model, the computational fluid dynamics (CFD) model was developed. The prediction results of the model were in good agreement with the experimental data of other researchers. The validity of PW was investigated from the viewpoint of the affected scale by comparing the flow and heat transfer with ultrasound and PW. The effect of the frequency and amplitude of the membrane on the heat transfer enhancement was analyzed. The result shows that the continuous PW had significantly larger affected scale than ultrasonic waves, which can cover the whole pipe bundle space and increase the velocity amplitude of the fluid around the bundle. While the affected scale of ultrasonic waves was limited to the center place of the pipe bundle because of the acoustic stream. In addition, the effect of continuous PW on the convective heat transfer increased with increasing the frequency and amplitude of the membrane. The heat transfer was enhanced by 7.01% to 156.22% at a frequency of 20–160 Hz and an amplitude of 2 mm. Considering the benefits and the energy consumed of PW, the optimal frequencies of models 1 and 2 were 110 and 120 Hz, and the heat transfer coefficient were increased by 119.19% and 150.36% compared to the steady-state flow. The results confirm continuous PW has the great potential in enhancing the heat transfer of large-space pipe bundles. [ABSTRACT FROM AUTHOR]

Accurate determination of time-varying convective heat transfer coefficient (CHTC) is the main content of many engineering applications. In view of some problems existing in the determination of time-varying CHTC, this paper develops an estimation scheme using the boundary condition transfer technique. In the new scheme, the estimation of CHTC is transformed into the estimation of known fluid temperature. Then, according to the estimation results of fluid temperature, the discrete values of characteristic variable are weighted comprehensively to obtain the estimated results of time-varying CHTC. Numerical simulation tests are mainly adopted so as to validate the above scheme. The developed scheme is also compared with the traditional method, which can better reveal the performance of the former. Effects of some important parameters such as the number of unknown variables, fluid temperature and measurement noise on the estimation results are illustrated specifically. Results demonstrate that the proposal can effectively determine the time-varying CHTC and improve the computational efficiency. Moreover, the proposed method also reduces the effects of fluid temperature on the forecast results, which is helpful for the accurate determination of time-varying CHTC. • An efficient method for estimating time-varying convective heat transfer coefficient is developed. • The boundary condition transfer technique is introduced. • It improves computational efficiency, especially for the simultaneous estimation of multiple variables. • It reduces the influence of fluid temperature on the estimation results. [ABSTRACT FROM AUTHOR]

• New theory for heat transfer via microbubble mediated solvent phase change. • Prediction for heat transfer coefficient/microbubble phase fraction correlation. • Consistent with analysis of freezing onset for boiled water placed in a freezer. • Inferred HTCs are inversely correlated related to oxygen solubility at onset temp. • Supports assertion that microbubble phase fraction is controlled by initial temp. Microbubbles dispersions in aqueous solutions can be long lived. For instance, 20micron size microbubbles take on the order of a day to rise one meter. Consequently, any currents in a reasonably sized vessel would be expected to entrain such a microbubble dispersion as the buoyant force is exceeded by the inertial force of liquid currents. This paper argues for the advantages of a microbubble dispersion mediated condenser with two benefits. The obvious advantage over fine bubble direct contact heating or cooling is that the microbubble phase, which can be engineered with a throughput of approximately a hectare per second of interfacial area flux per cubic meter of solution volume, should not be limited by heat transfer to and from the liquid and microbubble phase. Rather the limitation will be on the wetted area for heat transfer of the vessel to its heat exchange configuration. The second potential advantage follows from the theory proposed in this paper. Arranging the condenser in the microbubble mediated airlift configuration will introduce additional heat transfer from microbubbles vaporizing hotter water near the central plume and convecting that additional latent heat to the cold wall, which condenses the water vapor and releases the latent heat. This additional convection of latent heat is proposed as an additional source term for heat transport equation, and the magnitude of the effect is shown to be proportional to the phase fraction of microbubbles. This theory is shown to be consistent with analysis of observations of freezing times measured by Mpemba and Osborne [Phys. Educ. 4:172-5, 1969], that infer heat transfer coefficients from fitting Newton's law of cooling. The inferred heat transfer coefficient ratio from the presumed highest microbubble phase fraction to the lowest is ~7.4:1. Whether or not that enhancement level persists to a microbubble condenser in an airlift vessel, the promise of additional heat transfer should be explored. [ABSTRACT FROM AUTHOR]

Theoretical modeling of thermal conduction in a multilayer cylinder has been studied in multiple past papers due to its significance in engineering applications such as nuclear engineering, energy storage and sensing. Most past papers have assumed a constant convective heat transfer coefficient on the outer surface of the cylinder. However, a circumferentially varying heat transfer coefficient may be appropriate in several applications due to the distributed nature of fluid flow around the cylinder. This paper presents a theoretical model for steady-state thermal conduction in a multilayer cylinder with circumferentially-varying convective heat transfer coefficient on the cylinder surface. The theoretical model is presented for both solid and annular cylinders. A series solution is derived for the temperature distribution in each layer by using the convective boundary condition(s) to derive a set of linear algebraic equations for the coefficients of the inner-most layer. Results are shown to be in good agreement with numerical simulations and with closed-form solutions for special cases. The impact of various problem parameters, such as internal heat generation rate and thermal contact resistances on the temperature distribution is analyzed. It is expected that the results presented in this work will improve the theoretical understanding of multilayer heat transfer and be of practical use in multiple engineering applications. [ABSTRACT FROM AUTHOR]

In this paper, a special flow layout with U-shaped tubes applied in the laboratory was designed for testing the heat transfer performances (HTPs) of molten salt in the shell side of a shell-and-tube heat exchanger (STHE). Based on this design, the transitional convective HTPs (6142 < Re < 9125) of molten salt with higher temperature (209.41–241.49 °C) in the STHE with segmental baffles (STHE-SBs) were experimentally studied, and the corresponding heat transfer correlations were fitted. Then the validation of the traditional correlation applied in the molten salt STHE-SBs was conducted. Finally, the comparison of heat transfer enhancement in the molten salt STHE-SBs was discussed. The results show that a good agreement with 2% deviations exists between the fitted correlations and the test data. Meanwhile, the traditional Kern correlation, with a 7.1% maximum deviation compared with the test data, is still appropriate to analyze the HTPs of molten salt STHE-SBs. The effects of segmental baffles on the molten salt heat transfer enhancement in lower flow rate region are better than those in higher flow rate region, and the maximum increment of Nusselt number is 26%. [ABSTRACT FROM AUTHOR]

This paper demonstrates the feasibility of achieving enhanced microscale heat transfer effects in macro geometry systems using conventional fabrication methods. An annular microchannel, of mean channel gap 300 μm and length 30 mm, is formed by securing a cylindrical insert of mean diameter 19.4 mm within a cylindrical pipe of internal diameter 20 mm. The Fish Scale (FS) profile is introduced on the insert surface to improve the convective heat transfer coefficient, for a constant heat transfer area. The effect of the FS enhancement profile on the heat transfer and flow characteristics are experimentally studied using water, with Reynolds number ranging from 350 to 4600. Results show that the FS profile enhances heat transfer by promoting early laminar-to-turbulent flow transition. The highest convective heat transfer coefficient achieved is 47.9 kW/m 2 ·K, using the FS profile with a scale height of 0.21 mm and scale pitch of 2.1 mm, at Reynolds number of about 4400. New working correlations for the average Nusselt number and friction factor are proposed for the enhanced FS microchannels. These correlations may be used in the future to design macroscale heat exchangers employing economical conventional fabrication techniques and yet exhibiting superior microchannel heat transfer capabilities. [ABSTRACT FROM AUTHOR]

The present work is focused on studying the transition between the laminar and turbulent flow regime in coiled tubes. Wall curvature is a popular heat transfer enhancement technique since it gives origin to a secondary flow in the flowing fluid due to the non-uniformity of the centrifugal force over the cross section. This phenomenon, both in the laminar and the turbulent flow regime, promotes local maxima in the velocity distribution that locally increase the temperature gradient at the wall by enhancing the heat transfer and, at the same time, leading to a significant variation in the convective heat-transfer coefficient along the circumferential angular coordinate. However, this geometry delays the transition from laminar to turbulent regime, transition that, in the majority of the papers available in the scientific literature, has been investigated on the basis of pressure drop data behaviour. In the present work the estimation of the local convective heat transfer coefficient distribution, based on the solution of the inverse heat conduction problem in the tube’s wall, is proposed as a complementary and detailed tool to investigate the transitional flow regime. Moreover, the present research, thanks to the application of the proposed approach to an experimental case, gives additional information on the phenomenon of transition in coiled tubes. [ABSTRACT FROM AUTHOR]

• A transient TLC technique is used to measure the Nusselt number on the rotating cylindrical surface with flow confinement. • The effects of the rotational Reynolds number, jet Reynolds number, and jet angle on heat transfer characteristics are studied. • The mechanism of convective heat transfer for different parameters is analyzed. This paper presents an experimental study on the convective heat transfer characteristics of a circumferentially arranged oblique jet array impinging on the rotating cylindrical surface in the confined space. This cooling structure can be used for the cooling of rotating equipment, such as the cooling of rotating shafts of steam turbines, the quenching of metal materials, and the cooling of cement rotary kilns. The heat transfer coefficient on the rotating cylindrical surface is obtained with the transient thermochromic liquid crystal (TLC) method. Local and circumferential average heat transfer characteristics, overall heat transfer uniformity, and heat transfer capacity are analyzed comprehensively. The effects of rotational Reynolds number (R e r), jet Reynolds number (R e j), and jet angle (θ) on the heat transfer behaviors are investigated where the rotational Reynolds number is varied from 105 , 115 to 420 , 460 and the jet Reynolds number is varied from 20 , 000 to 35 , 000 at three different jet angles (θ = 20 ∘ , 30 ∘ , 45 ∘). The experimental results show that with the increase of the rotational Reynolds number, the overall circumferential average Nusselt number presents a trend of first decreasing and then increasing, mainly depending on the relative velocity of the circumferential velocity of the main flow and the tangential velocity of the rotating cylindrical surface. The average Nusselt number of the cylindrical surface decreases first and then increases gradually with the increase of the rotational Reynolds number and jet Reynolds number. As the jet angle increases from 20 ∘ to 45 ∘ , the average Nusselt number increases gradually. [ABSTRACT FROM AUTHOR]

• Analyzes a two-dimensional two-layer diffusion problem with convective boundary conditions normal to the layered direction. • Generalizes past work that only considered isothermal or adiabatic boundaries. • Derives sufficient linear equations to determine unknown coefficients of a series solution. • Results are in good agreement with past work for special cases. • Improves the theoretical understanding of multilayer diffusion. Theoretical analysis of transient thermal conduction in a two-dimensional multilayer structure has been limited to problems with isothermal or adiabatic boundary conditions along the walls normal to the layered direction. Due to mathematical difficulties pointed out in past work, an analytical solution has not been possible so far for the general case where each layer has a distinct convective boundary condition. This work presents an analytical technique to solve this general problem using Laplace transforms followed by derivation of a sufficient number of linear algebraic equations based on given boundary conditions to determine the coefficients of an eigenfunction-based series solution. This technique makes it possible to solve the problem when each layer may have a different convective heat transfer coefficient along the walls normal to the layered direction. Results from this general analysis are shown to correctly reduce to past work for the special cases of very small or very large Biot number. Good agreement with specific results presented in a past paper is also demonstrated. The technique is used to investigate the impact of key dimensionless parameters on the temperature field. This work significantly generalizes past work that was limited only to adiabatic or isothermal boundary conditions. Results presented here may help improve the theoretical understanding of multilayer diffusion problems, and make theoretical models much more representative of realistic conditions. [ABSTRACT FROM AUTHOR]

In this paper, a new method of estimation of a convective heat transfer coefficient on a hot spot is presented. First of all, a complete numerical model is developed in order to simulate the convection on a wall heated at a hot spot. Then, an analytical solution of the configuration is developed assuming simplified assumptions. Finally, an inverse method is performed to estimate the local convective heat transfer on a hot spot using noisy simulated data. The results of the estimation procedure are accurate and validate this new approach of convection measurement. • A 3D completed model of forced convection of a heated plate at a hot spot is developed. • An explicit analytical calculation using the Fourier and Hankel integral transforms. • Identification of the local convective coefficient is performed using simulated data. [ABSTRACT FROM AUTHOR]

A large amount of energy consumption has been caused by friction and inefficient heat transfer in the industry. Current research suggests a promising future for graphene nanofluids. In this paper, graphene oxide-deionized water (GO-DI) nanofluids with both friction reduction and high efficiency heat transfer properties was prepared by a two-step method in this paper. The graphene oxide was characterized using FTIR spectroscopy, Raman spectroscopy, XPS and SEM. The dispersion stability of nanofluids was analyzed by dynamic light scattering. An experimental system for measuring the convective heat transfer coefficient of nanofluids was established. The convection heat transfer and friction properties of GO-DI nanofluid were tested. The experimental results show that the addition of GO nanoparticles to the base fluid increases the convective heat transfer coefficient. The proportion of Nu nf /Nu f of nanofluids increases from 1.02 to 1.18 as volume fraction rises. For GO-DI nanofluids, the friction coefficient decreased 71% compared to deionized water, when 0.1 vol% of GO and 3 N normal load were applied. The abrasion loss in 0.05 vol% GO-DI nanofluids was 18.5% less than that in deionized water. The association equation for calculating the convective heat transfer coefficient was proposed and the friction reduction mechanism of GO-DI nanofluids was elucidated. Unlabelled Image • Stable dispersant-free graphene oxide deionized water nanofluids were prepared. • Experimental and calculated values of convective heat transfer were well fitted. • The antifriction performances of nanofluids with GNP and GO addition were compared. • Heat transfer and antifriction performances of nanofluids comprehensively improved. [ABSTRACT FROM AUTHOR]

Predicting heat transfer is a primary task in the design of open-cell foams. When a Local Thermal Non Equilibrium (LTNE) model is employed, convection heat transfer between the solid and fluid phases is considered, and a volumetric heat transfer coefficient needs to be defined. Some recent studies pointed out that the effects of developing convection heat transfer between the fluid and the solid in a foam are to be taken into account. The developing thermal flow of air through an open-cell foam, with a uniform heat flux solid/fluid boundary condition, is investigated numerically in this paper. The geometry is modeled with reference to Kelvin's tetrakaidecahedron foam model. A correlation among the porosity, the cell diameter and the ratio of heat transfer surface to volume is derived. Three regions are identified along the flow direction: an impingement region, a thermally developing region and a thermally developed region. Dimensional and dimensionless convection heat transfer coefficients have been predicted numerically as a function of the axial coordinate of the foam, for different values of the Reynolds number and the porosity. A correlation is presented among the predicted values of the volumetric Nusselt number, the porosity, and the Reynolds number in the thermally developed region, which is in good agreement with experimental data and numerical predictions by other authors. Finally, the analysis of the convection heat transfer through a single foam cell, at a local pore-scale, is presented. [ABSTRACT FROM AUTHOR]

A lot of research has been done in order to investigate heat transfer coefficients of a human body in various postures, wind speeds and wind directions. However, there has not been any reference to measurement reproducibility and measurement confidence intervals. The purpose of this study is to determine heat transfer coefficients of a thermal manikin experimentally, while focusing on the repeated determination of the coefficients and statistical data evaluation. The manikin imitates human metabolic heat production; it measures combined dry heat flux from its surface and also its surface temperature. The major part of the radiative heat flux was eliminated by a low-emissivity coating applied to the surface of the nude manikin. The tests were performed across 34 zones that correspond to parts of a human body. Both standing and seated postures were investigated. The tests were conducted at constant air temperature (24 °C) and constant wind speed (0.05 m s −1 ). Based on three repetitions of each case, the mean values of heat transfer coefficients, with their uncertainty intervals, were calculated. Next, the results of this paper were compared to the results of similar experimental work of de Dear et al. (1997) and Quintela et al. (2004). A mismatch of the values is up to 1 W m −2 K −1 , while an extreme was found on the manikin’s seat with a difference of over 1 W m −2 K −1 . The outcomes of this study provide essential information in form of separated values of the convective and radiative heat transfer coefficients that enable us to create detailed computational models of a thermal comfort. [ABSTRACT FROM AUTHOR]

The present paper presents the application of an inverse analysis approach to experimental infrared temperature data with the aim of estimating the local convective heat transfer coefficient for forced convection flow in coiled pipe having corrugated wall. The estimation procedure here adopted is based on the solution of the Inverse Heat Conduction Problem within the wall domain, by adopting the temperature distribution on the external coil wall as input data of the inverse problem: the unwanted noise is filtered out from the infrared temperature maps in order to make feasible the direct calculation of its Laplacian, embedded in the formulation of the Inverse Heat Conduction Problem, in which the convective heat transfer coefficient is regarded to be unknown. The results highlighted the local effects of both wall curvature and of wall corrugation on the convection heat transfer augmentation mechanism. [ABSTRACT FROM AUTHOR]

There is a rising necessity concerning low energy consumption and thermal comfort, thus, new systems combining displacement ventilation and radiant heating are becoming common as a research topic. The current paper covers free, mixed, and forced convection in an experimental chamber consisting of a radiant wall heating system and a baseboard level diffuser. Air temperature in the test room is adjusted using an air blower via a novel designed baseboard level slot diffuser along with a hydronic wall heating circuit. Simultaneously; an upward direction air jet is blown through the heated or the opposite wall under two scenarios. The ranges of air inlet velocity and temperature are selected to be between 0.25 and 12.5 m/s and 14–26 °C, correspondingly. The heat transfer characteristics pertaining to the heated wall are studied with respect to the influences of wall and air temperatures, air inlet velocities, and the location of the diffuser. To derive convective heat transfer coefficient correlations valid for mixed and forced types for a radiant heated wall, acquired data have been processed. The convective, radiative, and total heat transfer coefficient intervals of 3–14.1, 5.2–5.5, and 9.3–21.7 W/m2K have been obtained for the radiant heated wall coupled with airflows, correspondingly. [ABSTRACT FROM AUTHOR]

This paper reports an experimental investigation on the convective heat transfer and flow performance of silica-zinc oxide (SiO 2 –ZnO)/water hybrid nanofluid (HNF) flowing inside a tube with constant heat flux at different Reynolds number (Re) from 7743 to 23,228. A well-stabilized SiO 2 –ZnO/water HNF with 0.025–0.10weight fractions(φ) was produced via two-step method. Thermal conductivity and viscosity of HNF were measured. The convective heat transfer and flow characteristics of the HNF were determined. Thermal conductivity and viscosity of HNF showed notable enhancements as compared to water. The convective heat transfer coefficient (CHTC) of HNF enhanced with increasing φ and Re. A maximum of 29.44% enhancement in CHTC was obtained with φ of 0.10 at Re = 20,131 when compared to water. Further, the friction factor and pressure drop of HNF increased with increasing φ. A maximum of 17.45% increase in friction factor and a maximum increase of 24% in pressure drop were recorded with φ of 0.10 at Re = 23,228 as compared to water. New CHTC and friction factor correlations of high accuracies (R2>90%) have been proposed. A Figure of Merit (FOM) was drawn to describe the thermal performance of HNF. Based on the analysis, the optimum thermal performance was achieved with φ of 0.10 at Re = 20,131. [ABSTRACT FROM AUTHOR]