High Temperature Heat Transfer Research Articles

AN ASSESSMENT OF PHASE CHANGE MATERIAL, PREPARATION TECHNIQUES AND CONTAINMENT MATERIAL WITH DIFFERENT MOLTEN SALTS FOR THERMAL ENERGY STORAGE SYSTEM

Molten salts are extensively used as high-temperature heat transfer media and thermal energy storage (TES) materials in concentrating solar power (CSP) plants. Molten salt corrosion is a critical factor influencing the safe operation of the system. This work comparatively summarizes various factors influencing molten salt corrosion, including temperature, impurities, alloy composition and gas atmosphere. Additionally, the corrosion mechanisms of nitrate, carbonate, chloride, and fluoride-based mixture salts are meticulously reviewed. The demand for high-temperature storage is indeed increasing, driven by the need for more efficient and sustainable energy solutions. High-temperature storage systems, particularly those utilizing phase change materials (PCMs) and molten salts, play a key role in enhancing energy efficiency and reducing carbon footprint. By improving the efficiency of energy storage and reducing the need for fossil fuels, high-temperature storage systems help decrease greenhouse gas emissions, contributing to the fight against global warming. The paper focused on enhancing the corrosion resistance of container materials in terms of improving PCM thermal conductivity and stability.

Effect of Na2CO3 content on thermophysical properties, corrosion behaviors of KNO3-NaNO2 molten salt

Molten salt is popularly used as high temperature heat transfer and thermal energy storage medium. In order to decrease the melting point and increase decomposition temperature of molten salt, phase diagrams for KNO3-NaNO2 binary nitrates were calculated. And the binary salt with the lowest eutectic point was prepared. Furthermore, the effect of Na2CO3 addition on optimal selected KNO3-NaNO2 binary molten salt thermophysical properties and corrosion behavior are experimentally investigated. The results show that the addition of 7 wt% Na2CO3 to KNO3-NaNO2 binary molten salt can lower the melting point by 5.8 %. The addition of 9 wt% Na2CO3 to KNO3-NaNO2 binary molten salt can increase the decomposition temperature by 20.6 %, enhance the specific heat capacity by 4.8 % and increase the thermal conductivity by 7.5 %, but at the same time viscosity increased about 70 % at 300 °C. Meanwhile, the density increases with the increase of Na2CO3 content addition. The average corrosion rate of 316 stainless steel increases with the increasing of Na2CO3 content, which is consistent with the cross-section SEM results.

Experimental study on thermal–hydraulic performance of a nature-inspired mini-channel heat exchanger manufactured by 3D printing

Zigzag-channel printed circuit heat exchangers (PCHEs) are widely used in nuclear energy, solar energy, and aerospace due to their good heat transfer performance. However, the high flow resistance of zigzag channels leads to large pumping power consumption and thus limits the overall performance. Although previous modified channels could reduce the flow resistance of PCHEs, they also bring a decrease in the heat transfer efficiency. Inspired by river islands, this study proposes a novel nature-inspired channel. The nature-inspired channel heat exchanger, along with a traditional zigzag-channel heat exchanger, were manufactured by 3D printing. The high-temperature air-air flow and heat transfer experiments show that the nature-inspired channel heat exchanger increases the average heat transfer rate by 1.5% and significantly reduces the pressure drop by 34.85%. The addition of airfoil fins at the corners effectively alleviates the flow resistance caused by flow separation, reattachment, and collision, leading to a higher Nusselt number and a lower Fanning friction factor. The Nusselt number of nature-inspired channel is 59% higher than that of the zigzag channel with a 15° inclined angle. This work demonstrates that the nature-inspired channel could significantly reduce the flow resistance while maintaining high heat transfer efficiency compared with the traditional zigzag channel.

Corrosion of 316 SS in chloride molten salt for thermal energy storage: Inhibitory effects of Al powder

Abstract MgCl2-KCl-NaCl is regarded as one of the most prospective high-temperature thermal energy storage mediums and heat transfer fluids (HTF) for 3rd generation concentrated solar power (CSP) systems. However, high corrosion to alloys limits its application. In this paper, corrosion tests were conducted on 316 SS, in MgCl2-KCl-NaCl at 800°C with different content (0 wt.%,1 wt.%, and 10 wt.%) of Al powder addition as a corrosion inhibitor. The impact of Al powder was assessed through electrochemical methods, specifically impedance spectroscopy (EIS) and potentiodynamic polarization (PDP). Following corrosion tests, the morphologies and phase compositions of 316 SS were determined by using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD). The addition of Al powder can significantly reduce the corrosion current density of 316 SS in MgCl2-KCl-NaCl at 800°C, which was 183.29 times higher than that with 10 wt.% without Al addition. Al and the degree increased with increasing content of Al. With the addition of 1 wt.% Al, the thickness of the diffusion layer is significantly reduced, which was 54.6 μm (100 h), 275.1 μm (200 h), 370.4 μm (300 h), and 500 μm (400 h), respectively. When the addition of Al reaches up to 10 wt.%, the inwards diffusion of Al caused the formation of Al enriched layer, which was identified as the FeAl phase, on the surface of 316 SS during the high-temperature corrosion processes. The thickness of the Al enriched layer was associated with the diffusion time of Al, and its depth was 40.4 μm (100 h), 45.3 μm (200 h), 103.5 μm (300 h), and 139.5 μm (400 h).

A Planar-Cavity Receiver Configuration for High-Temperature Solar Thermal Processes

Next-generation concentrating solar thermal power (CSP) technologies target a wide spectrum of applications including electricity generation, thermochemical processes, and industrial process heat for broad decarbonization potential. Many of these applications require higher temperatures than those of current commercial nitrate salt systems. Particulate materials are promising candidates for next-generation high-temperature heat transfer and low-cost storage media and can facilitate operation over a wide temperature range. However these materials necessitate novel receiver configurations to accept the high incident flux concentrations that enable high solar-to-thermal receiver efficiency. One option is a novel light-trapping planar cavity receiver configuration in which small cavity-like structures are formed from opaque planar surfaces such that a high incident flux concentration at the cavity aperture is reduced to a wall-absorbed solar flux concentration that is manageable within limited wall-to-particle heat transfer rates. The paper introduces the receiver configuration and provides calculated optical performance for a preliminary 50 MWt receiver design.

Enhanced broadband infrared radiation from rare earth orthochromites for High-Temperature radiative heat transfer

The development of broadband, high-performance infrared radiation materials is crucial for energy conservation and applications in aerospace and industrial sectors. Rare earth orthochromites, such as PrCrO3, exhibit good thermal stability and high infrared emissivity beyond the 6 μm wavelength range. However, their large bandgap limits their infrared emissivity before 6 μm, significantly impacting their potential applications. Herein, we have successfully enhanced the thermal emissivity of PrCrO3 by manipulating oxygen vacancies (OV) and accompanying impurity levels, leading to the synthesis of five rare earth orthochromites. Compared with pristine PrCrO3, co-doping of Ca, Co, and Mn introduced oxygen vacancies (OV) and impurity energy levels, effectively reducing the band gap and improving emissivity in the wavelengths below 6 μm. In particular, the infrared emissivity of (Pr0.3Y0.3Ca0.4)CrO3 in the wavelengths of 0.78–2.5 μm reaches 0.89, which is double than that of pristine PrCrO3. Furthermore, its thermal stability is excellent, as demonstrated by thermal shock experiments at 1500 ℃ for five cycles. More importantly, the developed orthochromites can be applied as a coating on substrates such as alumina ceramic sheets, exhibiting an average infrared emissivity of 0.95 and remarkable radiative heat transfer capability. This research contributes to the advancement of high-performance infrared radiation materials by vacancy engineering.

Elucidating the Transition of 3D Morphological Evolution of Binary Alloys in Molten Salts with Metal Ion Additives.

Molten salts serve as effective high-temperature heat transfer fluids and thermal storage media used in a wide range of energy generation and storage facilities, including concentrated solar power plants, molten salt reactors and high-temperature batteries. However, at the salt-metal interfaces, a complex interplay of charge-transfer reactions involving various metal ions, generated either as fission products or through corrosion of structural materials, takes place. Simultaneously, there is a mass transport of ions or atoms within the molten salt and the parent alloys. The precise physical and chemical mechanisms leading to the diverse morphological changes in these materials remain unclear. To address this knowledge gap, this work employed a combination of synchrotron X-ray nanotomography and electron microscopy to study the morphological and chemical evolution of Ni-20Cr in molten KCl-MgCl2, while considering the influence of metal ions (Ni2+, Ce3+, and Eu3+) and variations in salt composition. Our research suggests that the interplay between interfacial diffusivity and reactivity determines the morphological evolution. The summary of the associated mass transport and reaction processes presented in this work is a step forward toward achieving a fundamental comprehension of the interactions between molten salts and alloys. Overall, the findings offer valuable insights for predicting the diverse chemical and structural alterations experienced by alloys in molten salt environments, thus aiding in the development of protective strategies for future applications involving molten salts.

Molecular dynamics simulation on thermal and transport properties of LiF-KF

Molten fluoride is considered as an attractive heat transfer and storage medium in advanced energy utilization facilities, but it is short of the necessary thermal and transport properties at high temperature. In this paper, the thermal and transport properties of the binary LiF-KF mixtures at 1073–1473 K are obtained by classical molecular dynamics. The deviation of simulated thermal conductivity of binary LiF-KF at 1073 K from experimental value is less than 19 %. Moreover, the deviation of simulated viscosity of pure LiF at different temperatures from experimental value is around 10 %. It is indicated that reverse non-equilibrium molecular dynamics (RNEMD) is acceptable for describing fluoride salts. An exponential correlation is established between molar fraction of LiF and thermal conductivity at different temperatures. Furthermore, the macroscopic thermal and transport properties correlated with local structure are analyzed, especially the dependence of local structure on temperature and composition of LiF is discussed deeply. With temperature increasing, ion clusters are considered to be formed and overall structure become loose, which leads to reductions in density, thermal conductivity and viscosity. An increase in composition of LiF results in higher specific heat capacity, thermal conductivity and viscosity, which is related to the reduction of Li-Li bond length and the enhancement of interactions between F−. The comparison of properties enhancement of different compositions is completed through the analysis of the synergy coefficient of macroscopic properties. This work provides the basic data for the accurate design of high-temperature heat transfer system.

Optimized design of large-diameter directly buried heating pipe

Abstract The elbow of a large-diameter directly buried hot water heating pipeline is the weak part of the whole pipe system. The foam cushion is often used to reduce the stress on the elbow. However, the foam cushion raises the temperature on the outer protective pipe’s surface, which may trigger thermal stress damage to the pipes. In this paper, the insulating pipe at the elbow section is designed with a variable diameter. The numerical simulation of the thermal stress for the outer protective pipe and the working pipe was conducted. The maximum stress for the outer protective pipe is about 3 MPa, and the average stress for the working pipe is 83 MPa, much smaller than the yield stress. Combined with the simulation results, the design ensures the reliability of the elbow and provides a reference for the design of large-diameter and high-temperature heat transfer pipelines.

Predicting thermophysical properties of molten salts in the MgCl2-NaCl-KCl-LiCl system with a shell-model potential

Ternary eutectic salts composed of MgCl2, NaCl, and KCl, referred to as MNK salts, have recently emerged as promising candidates as high-temperature heat transfer fluids and thermal energy storage media. In this study, we performed classical molecular dynamics (MD) simulations to predict the densities, specific heat capacities, viscosities, and ionic self-diffusivities for MNK salts over a wide temperature range. The impact of LiCl additive on their thermophysical properties was also investigated. To capture the electronic polarization of Cl anions by neighboring cations, we developed a novel shell-model potential using the force-matching method and a dataset of ab initio calculated interatomic forces. Our extensive MD simulations predict structure and properties for pure salts and binary/ternary salt mixtures in the MgCl2-NaCl-KCl-LiCl system in overall good agreement with available experimental and theoretical data, which corroborates the accuracy and reliability of our developed potential.

Turbulent flow and heat transfer in a new type of internally torsion-ribbed helical tube

Adding inserts is an effective technique to enhance heat transfer in tubes. In this paper, a new internally torsion-ribbed helical tube is designed to enhance the heat transfer capability and improve the temperature uniformity inside the pipe by adding torsional ribs inside the tube. The flow and heat transfer characteristics of high-temperature heat transfer oil in a straight tube, a common helical tube, and a new internally torsion-ribbed helical tube are numerically compared. The results show that the presence of internally torsional ribs can effectively strengthen the fluid perturbation inside the tube and further weaken the boundary layer, thus improving the heat transfer effect. Moreover, the convergence of strong vortices in the fluid is weakened by 29.3 % to 41.2 %. The comprehensive heat transfer factor (PEC) of the internally torsion-ribbed helical tube is between 1.1 and 1.2, which is 11.1 % to 12.0 % higher than that of the common helical tube, and the temperature uniformity TU = 0.18 % is 72.3 % better than that of the common helical tube. These data show that the new helical tube has excellent performance in strengthening heat transfer and preventing carbon deposition and coking caused by high local temperature of high-temperature heat transfer oil.

An improved laser flash method for thermal conductivity measurement of molten salts

Thermal conductivity measurement of high-temperature heat transfer fluids provides a crucial basis for designing utility-scale thermal systems. Molten salts are promising heat transfer and thermal storage fluids in high-temperature thermal energy storage systems, while the molten salt thermal conductivity obtained in existing studies exhibits large deviations due to the high experimental complexity and unstandardized test procedures. In this work, we improve the conventional laser flash analysis method by proposing a theoretical heat transfer model for multi-layer heat conduction and providing a near-optimal molten salt container design. With water as a test sample, the relative error of thermal conductivity measurement using the improved method is 6.3%. The thermal conductivity of Solar Salt from 250 to 400 ​°C, and of Hitec Salt from 160 to 250 ​°C are measured and compared with the previous work. Both results show that the thermal conductivity increases with the temperature rising. This work will promote the technology standardization for accurately acquiring the thermal conductivity of molten salts or other similar high-temperature heat transfer fluids.

Experimental and Numerical Investigation of Effect of Inclination on sCO2 Heat Transfer in a Circular Pipe

Abstract Supercritical carbon dioxide (sCO2) can be utilized as a working fluid in various thermal systems including large-scale power cycles; portable power production units, centralized coolant systems, and standalone cooling devices. However, the lack of accurate prediction tools such as heat transfer coefficient correlations and insufficient research studies about the mechanisms controlling heat transfer processes are hindering its practical realization for key energy and cooling systems. The overall objective of this study is to extend fundamental knowledge about heat transfer and fluid dynamic processes in conduits pertinent to sCO2 with an emphasis on flow direction and inclination effects. This paper presents the study on effects of gravity, buoyancy on sCO2 flow at temperatures near and away from the pseudo-critical temperature. The experimental setup consists of a high temperature and pressure sCO2 heat transfer loop and flow testing facility. Recently, researched sCO2 heat exchangers can have tubes oriented at different angles such as 45 deg or 90 deg to horizontal. For optimized design of efficient and cost-effective turbomachinery components utilizing sCO2 as the heat transfer fluid, an understanding of convective heat transfer inside a tube/pipe is equally as important as external heat transfer. This paper presents an experimental and numerical study on sCO2 heat transfer at various inclinations with angles ranging from 0 deg (horizontal) to 90 deg (vertical) along with upward and downward flow direction with different inlet temperatures. Thermocouple-based temperature measurement is utilized at multiple locations within the test section axially and circumferentially to study the temperature distributions on the tube surface. Computational fluid dynamics (CFD) simulations have been performed using ANSYS Fluent to complement experimental data. The CFD and experiment have been analyzed against the well-known Gnielinski Nusselt number correlation.

Geothermal Power Generation: Harnessing Electrical Energy Through The Organic Rankine Cycle (ORC) System

The research employs a thermodynamic simulation method using an Engineering Equation Solver (EES), relying on theoretical calculations. This method is integrated into a geothermal power plant, precisely focusing on geothermal source temperatures of approximately 95ºC. The investigation centers on the heat transfer process within a high-temperature heat transfer fluid from geothermal sources, conveying stored heat to the Organic Rankine Cycle (ORC) evaporator. Three specific working fluids, R134a, R11, and R22, examine working fluid selection for ORC at 95ºC. The results highlight the R11 organic fluid as an optimal compromise, excelling in two crucial criteria. Firstly, R11 exhibits the highest net mechanical power, = 34.81 kW compared to alternative fluids. Secondly, it boasts the best energetic efficiency of the cycle, registering = 16.01%, outperforming both R134a ( = 13.17%) and R22 ( = 12.64%). In summary, this study conducts a focused analysis of the energy aspects of an Organic Rankine Cycle (ORC) for electricity production using geothermal sources and organic fluids. Operating at a geothermal source temperature of 95ºC with a water flow rate of 80 lt/s and environmental conditions at 20ºC, the parametric study emphasizes the superiority of the R11 organic fluid. R11 emerges as the optimal choice, demonstrating the highest net mechanical power and superior energetic efficiency compared to alternative fluids, thereby contributing valuable insights to advancing sustainable and efficient energy technologies.

Numerical and experimental characterization of high-temperature heat transfer in a ceramic foam with dual-scale porosity

In this paper, we compare numerical predictions and measurements of the high-temperature apparent thermal conductivity of an insulating alumina foam with dual-scale porosity: the foam contains ▪-sized cells and also ▪-sized pores in the highly scattering foam skeleton. Simulations of the apparent thermal conductivity are performed on tomography-reconstructed foams, with a three-part multi-scale approach to account for the dual-scale porosity. The effective phonon thermal conductivity of the foam is first computed using a finite element homogenization technique. The spectral and temperature-dependent effective radiative properties of the foam are next computed through an improved Monte Carlo ray-tracing approach, with foam skeleton scattering modeled using a physical-optics-based approach. The apparent thermal conductivity of the foam is then simulated by resolving the coupled conductive-radiative heat transfer through a semi-infinite homogeneous slab. The simulated high-temperature apparent thermal conductivity is found to be in good agreement with experimental data up to ▪, obtained by parallel hot wire measurements coupled with an improved quadrupolar model for low-density insulating materials. The large influence of scattering at the cell-skeleton interface is shown through a numerical parametric study. It is also shown that the Rosseland conductivity calculated directly from the simulated spectral radiative properties provides a good estimation of radiative heat transfer for optical thicknesses exceeding 30.

Exploring Cr and molten salt interfacial interactions for molten salt applications.

Molten salts play an important role in various energy-related applications such as high-temperature heat transfer fluids and reaction media. However, the extreme molten salt environment causes the degradation of materials, raising safety and sustainability challenges. A fundamental understanding of material-molten salt interfacial evolution is needed. This work studies the transformation of metallic Cr in molten 50/50 mol% KCl-MgCl2via multi-modal in situ synchrotron X-ray nano-tomography, diffraction and spectroscopy combined with density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Notably, in addition to the dissolution of Cr in the molten salt to form porous structures, a δ-A15 Cr phase was found to gradually form as a result of the metal-salt interaction. This phase change of Cr is associated with a change in the coordination environment of Cr at the interface. DFT and AIMD simulations provide a basis for understanding the enhanced stability of δ-A15 Cr vs. bcc Cr, by revealing their competitive phase thermodynamics at elevated temperatures and probing the interfacial behavior of the molten salt at relevant facets. This study provides critical insights into the morphological and chemical evolution of metal-molten salt interfaces. The combination of multimodal synchrotron analysis and atomic simulation also offers an opportunity to explore a broader range of systems critical to energy applications.

Experimental and numerical investigation on the cold harvest of composite phase change materials for building energy conservation

This paper investigates the cold energy storage of a horizontal shell-tube latent heat thermal energy storage (LHTES) unit for the cold harvest during night time. An experimental apparatus containing a horizontal circular tube with circumferential heating/cooling was designed and developed. Paraffin based phase change material (PCM) with a melting temperature of 27 °C was adopted as the cold harvest media. To improve the heat transfer rate, multi-walled carbon nanotubes (MWCNTs) were mixed with the paraffin to form a composite PCM with high thermal conductivity. A corresponding model of the composite PCM was developed and verified against the experimental data. Numerical simulations were performed for different boundary temperatures and concentrations of MWCNTs. The results show that when the heat transfer temperature difference is 20 °C, 0.05 wt% nanoparticles increase the melting rate of PCMs. When the heat transfer temperature difference was 40 °C, 0.10 wt% nanoparticles significantly affect the PCM melting rate. For low or high temperature (heat transfer temperature difference of 10 °C and 80 °C), the addition of nanoparticles is not conducive to improving the melting rate.

Development of Rotary Solar Receiver and Solar Simulator Facility for Concentrated Solar Power Applications

A prototype rotary solar receiver and a solar simulator facility have been designed, built and commissioned by Odqa Renewable Energy Technologies in conjunction with The Oxford Thermofluids Institute, University of Oxford for the validation of concentrated solar power technologies. The key features of the rotary solar thermal receiver include the rotating absorber surface which incorporates a high temperature heat transfer design. The innovative design also includes a compact solar power inlet aperture for very high incident heat flux to reduce the radiant heat loss from the high temperature surface. Air is used as the heat transfer fluid as this has system integration advantages over other types of heat transfer media, such as CO2, steam, molten salt, or solid particles. A 30kWe Xenon-arc lamp high heat flux incident light has also been developed to enable experimental validation of the solar receiver technologies. The Xenon-arc lamp array consists of three lamps combined with a custom designed, 5-degree-of-freedom focusing mechanism to achieve precise focus and maximise the heat flux. The receiver has successfully produced working fluid temperatures reaching 1200°C, demonstrating the concept viability for high temperature processes.

Improving the Heat Transfer Performance of the Tower Molten Salt Solar Receiver With the Novel Folded Flow Tubes

Abstract The molten salt has been widely used in concentrated solar power generation as an effective high-temperature heat transfer and heat storage working fluid. However, due to the concentrating characteristic of the tower receiver, the solar flux distribution of the molten salt receiver is extremely non-uniform, and thus the circumferential non-uniform heat flux has a prominent effect on the heat transfer performance and reliability of the traditional solar molten salt receiver tube (TRT). In this contribution, in order to solve above problems, we propose some novel folded flow tubes (NFTs), which add a partition in the tube and seal the top with end cap so that the inflow and outflow of the fluid can only proceed from the same cross section. Then, we apply the binary nitrate (solar salt) as a heat transfer fluid, which is a mixture of 60% sodium nitrate and 40% potassium nitrate. First, we analyze some effects such as flow parameters, structure, and heat flux loading direction on the convective heat transfer performance of the NFTs. The results show that the circumferential temperature difference of NFTs is about 17–92 K lower than that of TRT, and the molten salt temperature distribution is more uniform accordingly. Moreover, the heat transfer coefficient is increased about 88.37–122.85%, which can provide a guidance for the structural optimization of practical solar molten salt receivers to improve the heat transfer performance and reliability.

In-situ thermophysical measurement of flowing molten chloride salt using modulated photothermal radiometry

Molten salts are leading candidates for high-temperature heat transfer fluids (HTFs) for thermal energy storage and conversion systems in concentrated solar power (CSP) and nuclear energy power plants. The ability to probe molten salt thermal transport properties in both stationary and flowing status is important for the evaluation of their heat transfer performance under realistic operational conditions, including the temperature range and potential degradation due to corrosion and contamination. However, accurate thermal transport properties are usually challenging to obtain even for stagnant molten salts due to different sources of errors from convection, radiation, and corrosion, let alone at flowing status. To the best of authors’ knowledge, there is no available in-situ technique for measuring flowing molten salt thermal conductivity. Here, we report the first in-situ flowing molten salt thermal conductivity measurement using modulated photothermal radiometry (MPR). We could successfully perform the first in-situ thermal conductivity measurement of flowing molten NaCl-KCl-MgCl2 in the typical operating temperature (520 and 580 °C) with flow velocities ranging from around 0.3 to 1.0 m s−1. The relative change of the molten salt thermal conductivity was measured. Gnielinski’s correlation was also used to estimate the heat transfer coefficient h of the flowing NaCl-KCl-MgCl2 in the given experimental condition. The work showed the potential of the MPR technique serving as an in-situ diagnostics tool to evaluate the heat transfer performance of flowing molten salts and other high-temperature HTFs.