Longitudinal Heat Research Articles

Influence of fin configuration on the thermal performance of nanoparticle-enhanced latent heat storage system

Storing the thermal energy using the materials' latent heat capacity is a prominent and well-established technique; these materials are termed phase-change materials (PCMs). For low-temperature applications mostly organic PCMs are available. These PCMs possess very less thermal conductivity, which results in a low rate of heat transfer. To overcome this problem usage of fins and the addition of nanoparticles (NPs) to PCM are two potential methods. In the present study, three-dimensional numerical analysis of melting and solidification is performed on the radial, spiral, and longitudinal finned PCM-based shell and tube heat exchangers with the addition of NPs to PCM. Numerical analysis is carried out with various volume fractions of NPs in the PCM. Based on the study it is observed that the PCM-based heat exchanger with radial fins has the lowest melting time, average temperature, and energy storage ratio. For longitudinal fins melting time is maximum and it also has the maximum average temperature and energy storage ratio. It is observed that the addition of NPs showed a significant effect on the phase change process. For all the selected geometries the average exergy efficiency is not more than 4.4% during solidification.

Heat recovery ventilation design limitations due to LHC for different ventilation strategies in ZEB

Today's buildings are becoming more insulated and airtight to reduce transmission heat losses. Energy use for ventilation can represent up to half of these buildings' total energy use. Heat recovery in ventilation and demand-controlled ventilation (DCV) are energy-efficient measures to reduce ventilation energy use, especially when combined. However, this study revealed that the often-overlooked longitudinal heat conduction (LHC) in aluminium rotary heat exchangers might yield less efficient heat exchangers, particularly for intended high-efficiency heat recovery at low ventilation rates in DCV. This study presents a theoretical method to assess the effect of LHC on the amount of energy used to heat ventilation air for several ventilation strategies. The method is demonstrated in a case study for a virtual office building in a cold climate (Oslo, Norway). When neglecting the LHC effect, the energy used to heat the supplied air using DCV with a rotary heat exchanger is about three times smaller than when considering LHC. Unlike earlier studies, we find that DCV may consume more ventilation heating energy than constant air volume (CAV) ventilation when the selected wheel is deep and oversized due to LHC. This study highlights the need to design rotary heat exchangers carefully in order to account for the LHC effect, particularly when targeting zero emission buildings (ZEB).

Phonon thermal Hall effect in a metallic spin ice

It has become common knowledge that phonons can generate thermal Hall effect in a wide variety of materials, although the underlying mechanism is still controversial. We study longitudinal κxx and transverse κxy thermal conductivity in Pr2Ir2O7, which is a metallic analog of spin ice. Despite the presence of mobile charge carriers, we find that both κxx and κxy are dominated by phonons. A T/H scaling of κxx unambiguously reveals that longitudinal heat current is substantially impeded by resonant scattering of phonons on paramagnetic spins. Upon cooling, the resonant scattering is strongly affected by a development of spin ice correlation and κxx deviates from the scaling in an anisotropic way with respect to field directions. Strikingly, a set of the κxx and κxy data clearly shows that κxy correlates with κxx in its response to magnetic field including a success of the T/H scaling and its failure at low temperature. This remarkable correlation provides solid evidence that an indispensable role is played by spin-phonon scattering not only for hindering the longitudinal heat conduction, but also for generating the transverse response.

Development of Intelligent CAD Technology for New Longitudinal Shell-Side Heat Exchange Equipment

In order to correct the errors of “periodic full-section calculation model” and “unit flow channel model,” the author proposes a numerical simulation method of a longitudinal flow shell-side heat exchanger based on CAD. The numerical simulation of the fluid flow and heat transfer characteristics of the three-blade orifice plate heat exchanger is carried out by using FLUENT software, and the “periodic full-section calculation model” and “unit flow channel model” are established, as well as the calculation results of comparative analysis. Experimental results show the following: With the increase of the inner diameter D of the heat exchanger shell, the calculation results of the two models are gradually reached. When D > 800 mm , the error of the calculation results of the two models has been reduced to about 10%; at this time, the “unit flow channel model” has practicability and applicability. When D ≤ 800 mm , the correction algorithm of the “unit flow channel model” is proposed, and the correction correlation formula of the pressure gradient and convective heat transfer coefficient is given. The revised results of the simulation can not only meet the engineering needs but also save computer resources and improve the calculation efficiency, which provides a theoretical basis for the development, improvement, and further industrial application of the “unit flow channel model” of the longitudinal shell-side heat exchanger.

A Two-Temperature Model Based on Fin-Approximation for Transient Longitudinal Heat Transfer in Unidirectional Composite

The reliability of a two-temperature model is assessed in the case of longitudinal heat transfer in unidirectional composites. One interest is that it makes it possible to apply separate boundary conditions or source terms on the fibre and the matrix (emissivity for example), without necessitating an explicit description of the fibre and matrix domains. For the sake of simplicity, the model under study is based on a fin-approximation in both fibre and matrix, which implies a high interfacial thermal resistance. The range of validity of this assumption is investigated by comparing the model to an axisymmetric one-temperature model, using non-dimension variables and Dirichlet boundary conditions. It turns out that this range of validity is strongly dependent on the parameters.

Thermal conductivity of single crystals zirconia stabilized by scandium, yttrium, gadolinium, and ytterbium oxides

The phase composition and heat conductivity of (ZrO2)0.9(R2O3)0.1 solid solution single crystals have been studied, where R = (Gd, Yb, Sc, Y), (ZrO2)0.9(Sc2O3)0.09(Gd2O3)0.01 and (ZrO2)0.9(Sc2O3)0.09(Yb2O3)0.01. Single crystals have been grown by directional melt crystallization in a cold skull. The phase composition of the crystals has been studied using X-ray diffraction and Raman spectroscopy. The heat conductivity of the crystals has been studied using the absolute steady-state technique of longitudinal heat flow in the 50—300 K range. We show that at a total stabilizing oxide concentration of 10 mol.% the phase composition of the crystals depends on the ionic radius of the stabilizing cation. The (ZrO2)0.9(Sc2O3)0.1 crystals have the lowest heat conductivity in the 50—300 K range while the (ZrO2)0.9(Gd2O3)0.1 solid solutions have the lowest heat conductivity at 300 K.Analysis of the experimental data suggests that the heat conductivity of the crystals depends mainly on the phase composition and ionic radius of the stabilizing cation. Phonon scattering caused by the difference in the weight of the co-doping oxide cation has a smaller effect on the heat conductivity.

The Propagation of Thermoelastic Waves in Different Anisotropic Media Using Matricant Method

Thermoelasticity is a generalization of classical theories of elasticity and thermal conductivity and describes a wide range of phenomenon. The theory can precisely predict the propagation of thermoelastics waves in case of an isotropic medium. However, the propagation of thermoelastic waves in the anisotropic medium is not fully understood. In this case, the theory of elasticity employs an approximate theory of temperature stress which does not take into consideration the interactions of temperature and deformations. In this paper, an analytical study has been carried out by using method of matricant to investigate the propagation of longitudinal elastic and heat waves in the anisotropic medium of a monoclinic, trigonal, hexagonal, and cubical crystal systems. In this article, a solution to the problem of the propagation of thermal waves and the propagation of a thermal wave along z -axis has been obtained. The attenuation coefficient and phase velocity of thermal waves for various materials are determined. Specifically, the problem of propagation of heat waves in one dimension has been solved.

Thermo-hydraulic performance investigation of heat pipe used annular heat exchanger with densely longitudinal fins

To enhance the heat transfer capacity of the condenser end in the heat pipe heat exchanger, heat transfer and flow characteristics investigations of three densely longitudinal fin-equipped annular heat exchangers have been carried out by using a combined method of experimental testing and numerical simulation. One of the heat exchangers as a baseline is featured by the long straight fins. The other featured folded fins and arc-shaped fins, in which 40 rows of short fins were arranged just like the blade-lattice. The attained results showed that the short fold fins and arc-shaped fins had Nusselt numbers that were 27% and 35% higher than the long straight fins, and had friction factors that were 1088% and 2209% higher than the long straight fins. It indicates that the dense fin arrangement has a relatively limited influence on the wall temperature and overall heat transfer capacity. However, the friction loss is significantly sensitive to the fin arrangement because of the severe flow drag and flow separation induced by the blade-lattice like fins. The thermo-hydraulic performance evaluation suggests that the three densely longitudinal fins are suitable for different application scenarios with different heat transfer and pressure loss requirements. This work serves as a reference for the heat exchanger design in the heat pipe-based thermal power conversion system.

Atmospheric Gravitational Tides of Earth-like Planets Orbiting Low-mass Stars

Temperate terrestrial planets orbiting low-mass stars are subject to strong tidal forces. The effects of gravitational tides on the solid planet and that of atmospheric thermal tides have been studied, but the direct impact of gravitational tides on the atmosphere itself has so far been ignored. We first develop a simplified analytic theory of tides acting on the atmosphere of a planet. We then implement gravitational tides into a general circulation model of a static-ocean planet in a short-period orbit around a low-mass star—the results agree with our analytic theory. Because atmospheric tides and solid-body tides share a scaling with the semimajor axis, we show that there is a maximum amplitude of the atmospheric tide that a terrestrial planet can experience while still having a solid surface; Proxima Centauri b is the poster child for a planet that could be geophysically Earth-like but with atmospheric tides more than 500× stronger than Earth’s. In this most extreme scenario, we show that atmospheric tides significantly impact the planet’s meteorology—but not its climate. Two possible modest climate impacts are enhanced longitudinal heat transport and cooling of the lowest atmospheric layers. The strong radiative forcing of such planets dominates over gravitational tides, unlike moons of cold giant planets, such as Titan. We speculate that atmospheric tides could be climatologically important on planets where the altitude of maximal tidal forcing coincides with the altitude of cloud formation and that the effect could be detectable for non-Earth-like planets subject to even greater tides.

Development and optimization of highly efficient heat recoveries for low carbon residential buildings

To achieve energy efficiency and lower carbon emissions, building envelopes have become tighter and more insulated. These buildings must have mechanical ventilation with high efficient heat recovery to provide adequate indoor air quality using low energy in cold climates, and requirements for heat recovery efficiency are expected to increase. Today’s aluminum heat wheels, one of the most commonly used heat recovery systems, may not be capable of providing such a high efficiency due to the presence of longitudinal heat conduction. This study focuses on developing highly efficient heat wheels by reducing the longitudinal heat conduction and enhancing heat transfer in different channel shapes and matrix materials for heat wheels. Previously often neglected, but in large heat wheels, the longitudinal heat conduction can result in a significant efficiency reduction effect. The highly efficient heat wheels are sought via parametric analysis, experimental verification, and optimization in this study. Reducing matrix wall thickness for high conductive materials and using low conductive materials can significantly improve temperature efficiency. The calculated and experimental results show that the developed plastic and stainless steel heat wheels can exhibit high temperature efficiency of over 90% with acceptable pressure drops. However, stainless steel's high cost and stiffness may limit its use in heat wheels. Wheels made of plastic with circular channels that are symmetrical could be a promising solution to meet the high temperature efficiency needs of the future. Additionally, the optimization shows that the temperature efficiency of plastic heat wheels is more sensitive to design parameters than aluminum or stainless-steel wheels.

Thermal conductivity of single crystals zirconia stabilized by scandium, yttrium, gadolinium, and ytterbium oxides

The phase composition and heat conductivity of (ZrO2)0.9(R2O3)0.1 solid solution single crystals have been studied, where R = (Gd, Yb, Sc, Y), (ZrO2)0.9(Sc2O3)0.09(Gd2O3)0.01 and (ZrO2)0.9(Sc2O3)0.09(Yb2O3)0.01. Single crystals have been grown by directional melt crystallization in a cold skull. The phase composition of the crystals has been studied using X-ray diffraction and Raman spectroscopy. The heat conductivity of the crystals has been studied using the absolute steady-state technique of longitudinal heat flow in the 50–300 K range. We show that at a total stabilizing oxide concentration of 10 mol.% the phase composition of the crystals depends on the ionic radius of the stabilizing cation. The (ZrO2)0.9(Sc2O3)0.1 crystals have the lowest heat conductivity in the 50–300 K range while the (ZrO2)0.9(Gd2O3)0.1 solid solutions have the lowest heat conductivity at 300 K. Analysis of the experimental data suggests that the heat conductivity of the crystals depends mainly on the phase composition and ionic radius of the stabilizing cation. Phonon scattering caused by the difference in the weight of the co-doping oxide cation has a smaller effect on the heat conductivity.

Experimental and numerical study of a 3D-printed aluminium cryogenic heat exchanger for compact Brayton refrigerators

Some cryogenic applications, such as for superconducting generators of wind power plants, require compact refrigerators. Heat exchangers are the most bulky components within the cold box of a cryogenic refrigerator. Thus, a reduction of their size would be advantageous for the development of a compact system. Volume and performance of a heat exchanger depend strongly on the hydraulic diameter of the channels and of the efficiency of the heat transfer surface. To improve the performance, a concept of aluminium 3D printed heat exchanger with optimised fin geometry has been developed. A fin design with hydraulic diameters of 1.48 mm and 1.07 mm for the high and low pressure sides, respectively, has been realized.Two prototype aluminium heat exchangers with improved fins have been produced by 3D printing and studied within this project. The accuracy of the geometry reproduction within the printing process has been investigated. The heat exchangers have successfully withstood pressure tests. The pressure drop has been measured at atmospheric and at higher pressure using helium and air as working fluids. Furthermore, the heat transfer with a temperature difference of 40 K along the heat exchanger has been investigated using helium. The Colburn and Darcy friction factors of designed and printed fin geometry have been calculated using Ansys CFX. A full heat exchanger simulation model with consideration of longitudinal heat conduction through inner and outer walls has been developed in Python. The results of the numerical simulations have been successfully validated against measurements. Based on the obtained data, suggestions for an improved heat exchanger geometry for cryogenic applications have been worked out.

The impact of ambient heat ingression on performance of cryogenic three-fluid cross-flow compact heat exchanger

The upshot of the improper insulation of the thermal system is the substantial instability and degradation in the thermal effectivity. The heat interaction between the ambient and heat exchanger shroud is initiated due to insufficient insulation, poor insulating material and/or the deterioration of the insulation over time. The dynamic investigation of the compact three-fluid heat exchanger (3-FHE) with cross-flow configuration (C1,2,3and4) under ambient heat ingression (both non-uniform and uniform) is conducted. The ‘Implicit Finite Difference’ scheme is implemented to perceive the numerical solution which is followed by experimentation to check the authenticity of the solution. Additionally, the ‘Dankwert's boundary condition’ which signifies the dispersive heat conduction in the fluids at the inlet section of the core and along the flow length; all along with the conduction of heat longitudinally in the exterior and parting walls is involved which has come up as the most practical model. It is learnt that the expeditious attainment of steady-state is observed under ambient heat ingression. It is engrossing to mention that the axial dispersion (AD) and longitudinal heat conduction (LHC) escorts in lower deterioration in cooling effectivity by 20.22% and 9.14% respectively under ambient heat ingression. The C1 configuration offers maximum drop of 41.8% in thermal effectivity under the varying magnitude of uniform heat ingression factor from 1 to 10. The adjacent fluids which are adhered to the non-insulating exterior wall reflect considerably early ‘temperature cross’ with the central hot fluid.

Study on Behavior of the Heat Exchanger with Conically-Corrugated Tubes and HDD Baffles

Baffles with holes in different diameters (or HDD baffles) and conically-corrugated tubes are respectively longitudinal flow baffle and high-efficiency heat exchange tubes proposed by the author. In this paper, vibrations of tube bundles with HDD baffles and fluid flow as well as heat transfer inside conically-corrugated tubes were numerically simulated, and the heat exchanger with conically-corrugated tubes and HDD baffles was tested for the heat transfer efficiency. It is found that compared with the traditional segmental baffles, tube bundle vibrations in heat exchangers, if using the HDD baffles, can be significantly reduced. Regarding heat transfer efficiency, conically-corrugated tubes are much better than smooth tubes and even better than other high-efficiency heat transfer tubes. Compared with the traditional heat exchangers, heat exchangers constructed with conically-corrugated tubes and the HDD baffles can provide better heat transfer efficiency and less tube bundle vibration.

Фазовые переходы в эпитаксиальных слоях карбида кремния, выращенных на кремниевой подложке методом согласованного замещения атомов

Temperature dependences of the longitudinal resistance and heat capacity of silicon carbide epitaxial films grown on monocrystalline silicon substrates by the method of coordinated substitution of atoms are investigated. Peculiarities in the behavior of these dependences have been found at temperatures equal to 56°C, 76°C, 122°C and 130°C. The observed peculiarities of the behavior of heat capacity and longitudinal resistance, considering appearance of a giant value of diamagnetism previously discovered in the samples at these temperatures, are interpreted as phase transitions of charge carriers into a coherent (superconducting, if we consider diamagnetism) state.

Influence of contact pressure on the thermal contact conductance of layered metallic sheets

For the optimisation of the annealing process of aluminium coils, simulation of the process is often performed. To simulate the process with higher accuracy, reliable input parameters are required, and thermal conductivity (thermal contact conductance) is one of them. In the present study, a method to measure the thermal conductivity and thermal contact conductance of metallic sheets were developed based on the steady-state comparative longitudinal heat flow. The apparatus was built with a compression test machine, and thus it allows to control the pressure to the sample and carry out the measurements at different contact pressure. An equipped heater allows to heat the sample to 573 K. To evaluate the thermal conductance at the interface, a thermal resistance network model was applied. The measurements were carried out with an aluminium alloy (AA3003 sheets). In addition to the thermal contact conductance measurements, the surface roughness of the sheets was also investigated. The semi-empirical equation for the relationship between thermal contact conductance and contact pressure was obtained based on the measurement results.

Implementation of novel triangular fins at a helical coil heat exchanger

Various techniques have been utilized to improve the performance of heat exchangers. In this study, multiple passive enhancement techniques have been applied. Triangular fins have been fabricated on the outside surface of the longitudinal helical heat exchanger. The performance of the new heat exchanger has been investigated experimentally and mathematically. In counterflow, hot water flowed inside the helical tube at 60 ᵒC while cold water entered the shell at 30 ᵒC. A triangle copper fin with 1 cm height, 2 cm base length, and 0.3 cm thickness with 4 cm pitch distance was welded at the external surface of the helical Copper coil tube. Along the helically coiled tube, 123 fins have been welded on the outside surface with 25 overall coil turns heat exchangers.The results showed a good enhancement ratio compared to the unfinned helical heat exchanger. The thermal performance factor showed an improvement of more than 10% compared to plain tube coils with an increase in the effectiveness of 11%. The improvement in the Nusselt number was 16.5%. These results confirmed that the fins addition on the helical coil heat exchanger modified and improved the helical heat exchanger and supported the industrial design with better performance.

Experimental study and comparative performance analysis on thermal-hydraulic characteristic of a novel longitudinal flow oil cooler

The longitudinal flow heat exchanger possesses lots of merits but it exhibits poor heat transfer coefficient at low Re condition, which makes it difficult to be applied in the high viscosity fluid area. In this paper, a heat transfer augment solution for lubricating oil flowing on shell side of a novel longitudinal flow twisted trifoliate tube oil cooler was proposed. Experiments were conducted to comparatively investigate the thermal–hydraulic performance of a twisted trifoliate tube oil cooler and a conventional segmental baffle oil cooler. The tube side study demonstrates that the Nu and f of the twisted trifoliate tube are 17.5–75.8% and 16.9–62.9% larger than that of the smooth circular tube. The shell side study shows that at the same oil mass flow rate, the shell side heat transfer coefficient of the twisted trifoliate tube oil cooler is 66.3–148.9% higher than that of the conventional segmental baffle oil cooler with pressure drop increased by 31.6–65.2%. Furthermore, the shell side performance evaluation criterion h/ΔP of the twisted trifoliate tube oil cooler is 1.46 times larger than that of the conventional segmental baffle oil cooler on average. The possible mechanisms responsible for the heat transfer enhancement are analyzed. In addition, the predictive correlations of tube side and shell side of the twisted trifoliate tube bundles for Nu and f are deduced based on the experimental data. Results from current research could provide useful correlations and beneficial guidance for design and application of the longitudinal flow heat exchanger in low Re ranging of 40–250.

Enhanced Longitudinal Heat Transfer in Oscillatory Channel Flow—A Theoretical Perspective

Abstract Enhanced longitudinal heat transfer in viscous, laminar, single-phase, oscillatory channel flow is investigated in this paper. Kurzweg (ASME J. Heat Transfer-Trans. ASME. 107, 1985) analyzed this case theoretically and derived a correlation for a nondimensionalized effective thermal conductivity in terms of Prandtl and Womersley numbers. The present investigation contributes analysis of limiting cases and physical interpretation to the results of Kurzweg. A simplified model with isothermal walls is proposed, applicable if working fluid and channel wall material exhibit sufficiently large differences in thermal inertia. Examined over a wide range of Womersley numbers, this model reveals six distinct regimes characterized by the Prandtl number of the fluid. The respective thickness of hydrodynamic and thermal boundary layers relative to the channel width is relevant in this context. Maximum effective thermal conductivity is attained when the thermal boundary layer expands over the full channel width. The influence of Womersley number is discussed and explained in terms of the interplay of hydrodynamic and thermal flow characteristics. These patterns reveal either quasi-steady parabolic or oscillating bulk characteristics. The importance of the thermal boundary layer thickness motivates the introduction of a new nondimensional group, making it easier to classify the various regimes of enhanced longitudinal heat transfer.

Dominant roles of eccentricity, fin design, and nanoparticles in performance enhancement of latent thermal energy storage unit

In this paper, numerical and experimental investigations of thermal performance enhancement of horizontal longitudinal shell and tube latent heat thermal energy storage unit (LHTESU) via eccentricity (e), fin design optimization, and nanoparticles in the phase change material (PCM) of stearic acid are presented. An enthalpy-porosity-based two-dimensional, transient numerical methodology is used after validating against the experimental results and the literature. Five different eccentric positions, e = 0.14, 0.28, 0.42, 0.56, and 0.63 are investigated by modifying the design of Y-finned tube. The eccentricity and fin design modifications improved natural convection effects as compared to concentric Y-finned tube. Thermal performances of LHTESU are analyzed on the basis of melting time, heat storage capacity, heat storage rate, and performance enhancement ratio. Based on the performance analysis, an optimum eccentric configuration of LHTESU is proposed. For optimized eccentric unit (e=0.42), a reduction of 34.14% in melting time along with 30.7% improvement in thermal energy storage rate is achieved as compared to concentric (e=0) LHTESU. The effect of temperature of the tube on PCM's melting time and heat transfer is also analyzed and two important correlations are proposed. The thermal performance of the optimized eccentric unit is further enhanced by adding nanoparticles of Al2O3 and CuO ranging from 0.5% to 10% by volume in the pure PCM. The nano-enhanced PCM with 1% of Al2O3 further improves melting and energy storage rates of optimum LHTESU by 10%.