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A visualized experimental test rig of flow boiling in high temperature thermal storage module was presented. The effects of high-temperature sensible heat thermal storage boundary condition on the oscillation characteristics at the onset of dynamic instability were investigated in this work. The flow boiling tests were carried out in a 1.6-meter-long horizontal tube with an inner diameter of 16.0 mm, which was embedded in a sensible thermal storage module using solid graphite. A series of thermal storage temperatures (200–340 °C), inlet subcooling degrees (5–45 °C) and mass flow rates (0–593.5 kg/m2s) were applied. Based on the dimensionless stability map, an early onset of dynamic instability was observed in comparison with the traditional constant heat flux condition. Meanwhile, both the pressure drop oscillations (PDO) and density wave oscillations (DWO) were observed. Based on the development of flow pattern and heat transfer distribution characteristics, the regulation mechanisms of the high-temperature thermal storage boundary on the development of dynamic instability were analyzed. The results show that the characteristics of dynamic instability are highly related to the development of flow pattern. The thermal storage boundary has affected the development of flow pattern by determining the wall superheat. The comprehensive effects of superheat, subcooling degree and mass flux on the development of flow pattern were also interpreted.

Clean energy is now developing rapidly, especially in the United States, China, the Britain and the European Union. To ensure the stability of power production and consumption, and to give higher priority to clean energy, it is essential for large power plants to implement peak shaving operation, which means that even the 1000 MW steam turbines in large plants will undertake peak shaving tasks for a long period of time. However, with the peak load regulation, the steam turbines operating in low capacity may be much more likely to cause faults. In this paper, aiming at peak load shaving, a fault diagnosis method of steam turbine vibration has been presented. The major models, namely hierarchy-KNN model on the basis of improved principal component analysis (Improved PCA-HKNN) has been discussed in detail. Additionally, a new fault diagnosis method has been proposed. By applying the PCA improved by information entropy, the vibration and thermal original data are decomposed and classified into a finite number of characteristic parameters and factor matrices. For the peak shaving power plants, the peak load shaving state involving their methods of operation and results of vibration would be elaborated further. Combined with the data and the operation state, the HKNN model is established to carry out the fault diagnosis. Finally, the efficiency and reliability of the improved PCA-HKNN model is discussed. It’s indicated that compared with the traditional method, especially handling the large data, this model enhances the convergence speed and the anti-interference ability of the neural network, reduces the training time and diagnosis time by more than 50%, improving the reliability of the diagnosis from 76% to 97%.

The temperature-dependent rheological behaviors of five selected binary eutectic mixture sugar alcohols, with great potential for latent heat storage in the range of 353.15 K to 523.15 K, were investigated. It was found that the rheological behaviors of the mixture sugar alcohols depend on those of the pure compounds as well as their molar ratios. The two mixtures of xylitol (75 mol%)+erythritol and erythritol (84 mol%)+d-mannitol behave like pseudoplastic fluids with typical non-Newtonian shear-thinning behaviors, as indicated by the power law index of 0.99 (

The quality of output temperature is one of the key parameters for the reliability of water heating system. In this work a simulation work was done to regulate the output temperature in a novel water heating system using solid graphite as thermal energy storage medium. The effects of water tank on the quality of output temperature was considered. Two different strategies were thus considered: (i) single tank system, and (ii) double-tank system. And two different working conditions were analyzed: (i) standard design condition, and (ii) demand change conditions. The results showed that the application of water tanks can reduce the standard deviation of output temperature by 68.49% and 68.81% under the standard design condition, respectively. And the improvement is highly related to the capacity of hot water tank. For the demand change condition, the regulation strategy was carried out based on the criterion of heat power equilibrium. The results showed that the single tank system will deteriorate the reliability of water heating for demand change condition, though it can reduce the oscillation of the output temperature. For the double-tank system, a two-step adjustment method was applied. The results showed that the average output temperature can be quite close to the target value when demand ratio changes to 0.8 and 0.9. And the standard deviation of temperature has also reduced by 24.19% and 63.06%, respectively.

With the development of new energy, the primary frequency control (PFC) is becoming more and more important and complicated. To improve the reliability of the PFC, an evaluation method of primary frequency control ability (PFCA) was proposed. First, based on the coupling model of the coordinated control system (CCS) and digital electro-hydraulic control system (DEH), principle and control mode of the PFC were introduced in detail. The simulation results showed that the PFC of the CCS and DEH was the most effective control mode. Then, the analysis of the CCS model and variable condition revealed the internal relationship among main steam pressure, valve opening and power. In term of this, the radial basis function (RBF) neural network was established to estimate the PFCA. Because the simulation curves fit well with the actual curves, the accuracy of the coupling model was verified. On this basis, simulation data was produced by coupling model to verify the proposed evaluation method. The low predication error of main steam pressure, power and the PFCA indicated that the method was effective. In addition, the actual data obtained from historical operation data were used to estimate the PFCA accurately, which was the strongest evidence for this method.

The durability of a superhydrophilic surface upon consecutive quenching in dilute aqueous saline solutions was studied, with emphasis on the effects of the solution concentration (up to 0.01 mol/L) and the type of salts. It was shown that the quenching process slows down, as a result of the deteriorated pool boiling heat transfer, during consecutive quenching runs in the saline solutions. Progressive changes in the surface morphology and wettability were observed on the superhydrophilic surface, due to the accumulative deposition of the dissolved salts during quenching. The deteriorated quenching performance of the superhydrophilic was found to become more significant upon increasing both the solution concentration and the number of consecutive quenching runs. Comparison was also made among three different dissolved salts at a constant concentration of 0.0025 mol/L, in order to reveal the effects of both cation and anion. It was observed that the KCl and NaCl solutions decelerate significantly the quenching process, whereas the K2SO4 solution only has a slight impact. Through the morphological and elementary analysis on the quenched surfaces, the rational surface deposition mechanisms are hypothesized. It was concluded that the chloride salts are prone to deposition due to their stronger corrosive ability than the sulfate salts.

The effects of sample length on the transient measurement results of the water vapor diffusion coefficient of autoclave aerated concrete (AAC) were studied based on the transient technique that solves an inverse problem of one-dimensional mass transfer in a semi-infinite domain. Tests were performed on square-rod-shaped AAC samples having various porosities. The axial sample length was varied from 120 mm to 250 mm while maintaining a constant cross-sectional area of 50 mm × 50 mm. Humidity sensors having two sizes of 6 mm and 12 mm were also adopted, and the transient measurement results were compared with reference values obtained using standardized steady-state method. Results showed that smaller humidity sensors are always preferred and that the appropriate sample length varies with the type of materials. For the specific types of AAC samples studied, the appropriate sample length was found to lie between 150 mm and 200 mm. The use of a shorter (or longer) sample was shown to cause overestimation (or underestimation) of the transient results. It was suggested that the appropriate sample length should be kept short as long as the semi-infinite domain assumption is valid, which could guide the sample length selection for other porous building materials.

Current demonstration projects show that the power capacity of redox flow batteries can span a large range from kW- to MW-scale. The large-scale, especially MW-scale, flow battery system can usually benefit from cell's large active area, due to that a large cell can reduce the required number of cells and thus assembling difficulties. However, the lack of practical pathways for scaling-up lab-scale toward large-scale flow field designs has been one of the barriers to the commercialization of flow batteries. The present study investigates the interdigitated flow field design for a large-scale (900 cm2 active area) vanadium redox flow battery cell, based on a three-dimensional, multi-physical model. Four pathways for scaling up the flow field are investigated, including (i) geometric similarity, (ii) channel length extension, (iii) same pressure drop, and (v) split-interdigitated flow field. The relation between the width and length of the channel and the concentration overpotential is formulated. The results show that the split-interdigitated flow field outperforms the other scaling-up methods in terms of the overall energy efficiency, while at the cost of the increased pressure drop. To alleviate the high pressure drop, the design can be improved by widening the main channels or adding one extra flow inlet.

Forced convection heat transfer in a laminar Falkner-Skan boundary layer flow with constant wall temperature is considered over a wide range of pressure gradient parameter β and entire range of Prandtl number Pr. A new asymptotic behavior for the Nusselt number Nu x for large β and arbitrary Pr is discovered by examining the dependence of the ratio of the heat transfer for arbitrary Pr to its small Pr limit on Pr / β . Correlations for Nu x are developed for −0.19883774 ≤ β β → ∞, is analyzed. The peculiar behavior of Nu x = 0 as β → ∞ in the Falkner-Skan thermal problem is caused by the loss of self-similarity for the solution of boundary layer temperature in the point-sink flow.

In this work, an experimental investigation on the regulation of gas-liquid stratified flow boiling dynamic instabilities was carried out in a 1530 mm long horizontal arranged copper tube with an inner diameter of 12 mm. Two different thermal boundary conditions, namely (i) heat load distribution, (ii) wall thermal capacity were investigated. Three different heat load distributions (uniform heating, power decrease and power increase) and three kinds of tube with different thickness of graphite casing tubes (bare, 6 mm and 12 mm) were analyzed, respectively. The results showed that the oscillation intensity of gas-liquid stratified flow boiling dynamic instabilities was highly related to occurrence of the onset of nucleating boiling (ONB). In comparison with the based cases, the power increase heat load condition especially that with lower inlet subcooling degree and the case with thicker graphite casing tube had delayed the occurrence of ONB. And finally a suppression of dynamic instabilities was obtained. On the contrary, an early occurrence of ONB and an enhancement of gas-liquid stratified flow boiling instability were obtained for the other two cases.

The vanadium redox flow battery (VRB) is considered to be one of the most promising technologies for large-scale energy storage, with the electrolyte flow rate capable of significantly affecting the mass transfer, temperature rise, and pump power losses of the VRB system. Although the flow-rate optimization under constant current has been addressed in the literature, few studies have investigated the control strategy for the electrolyte flow rate under varying (dis-)charge power that is common in practical applications. Moreover, fewer studies have considered the concentration discrepancy of the active species in the tank and stack in the flow-rate optimization. In this paper, the electrolyte flow-rate optimization is investigated by incorporating the influences of the flow rate on the mass transfer, temperature rise, and required pump power. A transient model of the VRB system is developed to derive the total power losses (by which the overall system energy efficiency is determined; include losses resulting from overpotentials, ohmic drops, and required pump power) as a function of the applied current, concentration of the active species in the stack, and flow rate of the electrolyte. Based on this model, a dynamic flow-rate control strategy is proposed for determining the optimal flow rate under varying (dis-)charge power and state-of-charge conditions. The simulation results show that the proposed control strategy can deliver a high VRB system efficiency of 87.7%, and manage the electrolyte temperature to the safe range during mild summer days.

Pool boiling heat transfer during quenching in subcooled water at atmospheric pressure was investigated on stainless steel rodlets with a superhydrophilic surface. It was shown that both the surface superhydrophilicity and degree of subcooling can accelerate significantly the quenching process. Under saturated condition, the quenching time is reduced by 50% for the superhydrophilic surface in comparison to the untreated one, due to the remarkable heat transfer enhancement with the critical heat flux being increased by 128%. The increase of the degree of subcooling can further reduce the quenching time. Frequent and intensive liquid-vapor interface fluctuations were observed visually during the initial phase of quenching for both saturated and subcooled conditions, indicating the absence of stable film boiling regime even though film-boiling-like boiling curves were obtained for the subcooled cases. Through visualized observations, the quench front was found to propagate upwards from the bottom of both the untreated and superhydrophilic rodlets. The propagation velocity could be accelerated noticeably with increasing the degree of subcooling. In addition, good durability and stability of the superhydrophilicity were exhibited by consecutive quenching tests up to 100 times.

A gas–liquid stratified flow boiling was performed in a 1530 mm long copper tube with an inner diameter of 12 mm under forced convection condition. Four different heat fluxes (q = 15.43 kW/m2, 18.52 kW/m2, 21.60 kW/m2 and 24.69 kW/m2) and inlet subcooled temperatures (ΔT​sub = 10 °C, 15 °C, 20 °C and 25 °C) were applied to investigate the characteristics of flow boiling instabilities, respectively. The results show that the instability features were closely related to the heat flux and inlet subcooling degree. With the increase of heat flux or decrease of inlet subcooling degree, two types of instability oscillation, DWO (density wave oscillation) and PDO (pressure drop oscillation) were identified. Moreover, an additional enhancement of heat transfer coefficient was found when the transition of the oscillation type from DWO to PDO happened. In the meantime, the frequency and amplitude of the PDO type oscillation were also increased with the rise of heat flux or the drop of subcooling. The intensification of fluctuation in PDO, however, has deteriorated the flow boiling heat transfer.

The thermal contact resistance (TCR) between the interfaces of phase change materials (PCMs) and their encapsulation materials, usually thin-walled metals, have a great impact on the heat storage/retrieval rates of PCM-based latent heat storage (LHS) systems. However, the TCR data beteween metal/PCM interfaces are rarely reported in the literature. In this work, an improved self-designed steady-state apparatus was adopted to measure the TCR between the interfaces of erythritol, a cost-effective sugar alcohol with great potential for medium-temperature LHS, and multiple metals including SS304, nickel silver (a copper-nickel-zinc ternary alloy), and 1060 Al. The metal/erythritol composites were prepared by naturally solidifying molten erythritol on the surface of thin metal plates. The variation of the TCR between metal/erythritol interfaces was measured as a function of the surface roughness (of the metal plates) and the contact pressure applied (between the interfaces). With the decrease of surface roughness, the TCR also decreases due to the better contact between the interfaces as observed by a microscope. Comparing to SS304 and nickel silver, the 1060 Al/erythritol composites exhibit a significant reduction of TCR by ~40% when rising the contact pressure from 0 MPa to 7 MPa, for all surface roughness levels tested. The in situ measured sample thickness variations confirmed that upon pressurization, the softer metal, i.e., 1060 Al, experiences more micro-deformation at the interface. In addition, correlations of the TCR data between metal/erthyritol interfaces were proposed with the surface roughness and contact pressure. The results and correlations attained in this work could serve as a reference for the thermal design of LHS units using erythritol.

Currently, solar energy technologies are in the stage of intensive development. With booming solar industry, there is a challenge to seek for appropriate solar energy solutions for different district typologies. This paper presents a comparative study on solar energy utilization patterns for different types of districts located in Kunming, China. Four district typologies are investigated: residential district (RD), official district (OD), commercial district (CD) and industrial district (ID). For each district, the objective is to identify such solar energy utilization patterns that result in an optimal design and operation of solar energy system. The optimum system is defined and obtained through minimizing life cycle CO 2 emissions and costs as well as maximizing exergy efficiency. To that end, a multi-objective optimization approach based on Genetic Algorithm is proposed. The results of the case study suggest that solar energy is to represent 3.7%, 5.9%, 7.9% and 21.4% of annual energy consumption for RD, OD, CD and ID, respectively. For each district, the portfolio of solar energy technologies is different. Solar power systems factually contribute to the energy supply of ID only. The final work aims at investigating the effects of different solar energy parameters on the solar utilization patterns for these districts.

In this Technical Note, a visualized study at the pore-scale was carried out to explore the pore size effects on melting heat transfer of a paraffin wax saturated in a copper foam. Three types of copper foams having a pore size of 15 ppi, 30 ppi and 50 ppi were used. The pore-scale visualization of the melting process was enabled using an infrared video camera equipped with a macrolens. Thermocouples were also embedded in the melting system at different locations to monitor the local temperature variations. It was observed that at the wall superheat of 20 °C, the 30 ppi and 50 ppi copper foams lead to almost identical overall melting rates, which are both much faster than that for the 15 ppi one. However, when the wall superheat was increased to 30 °C, the 30 ppi copper foam appears to be the best filler that causes much faster melting than that of the 50 ppi one. In addition, transient evolutions of the pore-scale temperature field were observed through the infrared thermal imaging snapshots at representative time instants during melting. The local thermal nonequilibrium between the paraffin wax and its surrounding copper ligaments was clearly demonstrated to decrease with reducing the pore size. In sum, the use of copper foams with relatively small pores was deemed to be desirable, which may lead to both faster melting and less significant local thermal nonequilibrium effect.

Quenching experiments were performed with hot stainless steel spheres in a pool of saturated water-based nanofluids with carbon nanotubes (CNTs) of various sizes. A test matrix was developed by choosing multi-walled CNTs having four combinations of nominal lengths and outer diameters, while the concentration was fixed at 0.5% by mass. It was shown that the quenching process is progressively accelerated upon consecutive runs due to the accumulative deposition of CNTs. The relative quenching acceleration and boiling heat transfer enhancement were found to be strongly related to the size discrepancy among the CNTs, as a result of the various surface morphologies and properties of the deposition layers. The CNTs with a length of 5 μm and a diameter of 60 nm were exhibited to lead to the most remarkable enhancement on the quenching and boiling behaviors, with the critical heat flux and Leidenfrost point being increased by about 60% and 100% respectively, due to the easy formation of micro-porous layers by this type of longer and thicker CNTs. It was identified that the significantly increased surface roughness with porous structure, instead of the nearly unvaried surface wettability, is responsible for the enhanced boiling heat transfer during quenching. The findings of this work suggested an active approach to optimization of quenching performance of CNT-based nanofluids.

In this Short Communication, the water vapor diffusion coefficient of autoclaved aerated concrete (AAC) samples with various apparent densities was measured using a transient method that is based upon solving an inverse problem of one-dimensional water vapor diffusion. The transient measurements were performed over a relative humidity (RH) range from 1% to 90% at temperatures between 15 °C and 45 °C. It was revealed that the dependence of water vapor diffusion coefficient on the presence of pores cannot be simply correlated to either apparent porosity or open porosity. The pore structure, as quantified by pore size distribution, was found to be responsible for the relative comparison among the various AAC samples. It was also exhibited that the water vapor diffusion coefficient is enhanced at higher temperatures, whereas its RH-dependence is significantly suppressed at lower temperatures.

The knowledge on pressure drop over the packed layers of sintered ore particles (SOPs) is deemed essential to have a deep understanding on the flow and heat transfer in emerging vertically arranged sinter coolers for waste heat recovery. In this work, the wall effects on pressure drop over a packed bed of SOPs were studied experimentally. Experiments for SOPs with bed-to-particle diameter ratio ranging from 7 to 35 and glass beads with the ratio between 6 and 22 were performed. The results revealed that the voidage effect tends to be pronounced for relatively low ratio values, while the friction effect becomes more significant for relatively high ratio values when it is lower than 40. Comparing to the confining walls, the particle characteristics were shown to have a dominant effect on the flow regime in the voids of the SOP-filled beds, whereas an opposite comparison was observed for the smooth glass beads as a reference. The correlations for the wall corrected friction factor proposed in the literature (Exp. Heat Transfer 12 (1999) 309), with varying coefficients, fit well to predict the pressure drop through packed beds for both glass beads and SOPs with considering the wall effects. Improved correlations were also proposed for spherical particles, SOPs and their blends with fairly good predictive accuracy.

The gas–liquid stratified flow boiling heat transfer of water in a natural circulation thermosyphon was investigated experimentally. The flow boiling test was carried out in a visual thermosyphon loop system with a 0.8-m-long, horizontally arranged, evaporator tube with an inner diameter of 15.0 mm and under a heat load of 1.0 kW. The development of the gas–liquid stratified flow in the evaporator and its effects on thermal performance of the thermosyphon loop were analyzed. The results show that flow pattern in the evaporator was gradually developed from the slug/stratified-wavy flow to the stratified-wavy/stratified-mist flow along the flow direction. Simultaneously, a periodic transition of the flow pattern was observed. A countercurrent flow at the exit of evaporator has emerged to prevent the development of the stratified-wavy flow to the mist flow which subsequently resulted in a drop in superheat. Moreover, two scales of dynamic instabilities were observed. The large scale was found to be the typical pressure drop type oscillation. The small scale, however, was found to be highly related to the flow pattern transition. Due to the countercurrent flow and backflow, a continuous density oscillation was observed. Finally, a modified correlation was developed for the local heat transfer coefficient that could account for the effect of the subcool and flow pattern transition.

A superhydrophilic surface was prepared on stainless steel spheres by spray coating of silica nanoparticles to enhance the surface roughness at the nanoscale. The quenching performance of such surface was tested in both saturated and subcooled water. It was shown that quenching is accelerated on the superhydrophilic surface by a factor of 60%, due to the marked boiling heat transfer enhancement with the critical heat flux being increased by 78%. Under saturated condition, the initial temperature of 800 °C is not high enough to lead to stable film boiling on the superhydrophilic surface. Instead, quenching begins with an extended transitional film boiling sub-regime that is dominated by point-contact mode of liquid-solid contacts. The quenching time becomes shorter with increasing the subcooling degree, in agreement with the enhancement in quasi-film-boiling heat transfer and CHF. Bubble jetting phenomenon is observed at the liquid-vapor interface for the subcooled cases, which becomes more violent at a deeper subcooling. The quenching performance maintains upon consecutive tests for up to 100 times, because the superhydrophilicity is sustained on the surface. The spray coating method adopted in this work is not only fast and cost-effective, but is also scalable to large and curved metallic surfaces for a variety of applications in thermal engineering including quenching.

In this Technical Note, the use of a liquid metal, i.e., a low melting point Pb–Sn–In–Bi alloy, as the phase change material (PCM) in thermal energy storage-based heat sinks is tested in comparison to an organic PCM (1-octadecanol) having a similar melting point of ∼60 °C. The thermophysical properties of the two types of PCM are characterized, revealing that the liquid metal is much more conductive while both have nearly identical volumetric latent heat of fusion (∼215 MJ/m3). By using at the same volume of 80 mL, i.e., the same energy storage capacity, the liquid metal is shown to outperform significantly over the organic PCM under the various heating powers up to 105.3 W/cm2. During the heating period, the use of the liquid metal leads to a remarkable extension of the effective protection time to nearly twice longer as well as a reduction of the highest overheating temperature by up to 50 °C. The cool-down period can also be shortened significantly by taking advantage of the much higher thermal conductivity of the liquid metal. These findings suggest that liquid metals could serve as a promising PCM candidate for particular applications where the volume limit is very rigorous and the penalty in weight increment is acceptable.

Constrained melting heat transfer of a phase change material (PCM) in a circumferentially finned spherical capsule was studied with application to latent heat thermal energy storage (TES). Attention was paid primarily to revealing the influence of fin height on melting heat transfer and TES performance of the PCM system. Visualized experiments were performed to observe the liquid–solid interface evolutions during melting, and to validate the numerical simulations that were conducted based on the enthalpy method. By means of measuring the instantaneous volume expansion upon melting, an indirect experimental method was proposed and implemented to acquire quantitatively the variations of melt fraction and heat transfer rate. Good consistency was observed between the experimental and numerical results. A combination of the two was able to offer an in-depth understanding on the fin effects by providing detailed knowledge on the interface evolutions and natural convective flow and heat transfer as well. It was shown that the TES performance is enhanced with increasing the fin height, and that the melting duration time is shortened up to nearly 30% at the highest fin height studied. The enhancement was attributed to the combined positive effects due to the presence of the fin, which are enhanced heat conduction by the extended heat transfer area and local natural convection induced in the vicinity of the fin.

Chitosan, as a low-price and eco-friendly raw material, was used to synthesize a graphene-based carbon aerogel (CA). Afterwards, 1-hexadecanol was infiltrated into CA to prepare a form-stable phase change material (PCM) for thermal energy storage. The morphologies and thermal properties of samples were analyzed. Even at 70 °C, the form-stable PCM still retains form stability without any leakage. TGA results reveal a remarkable absorption capacity of CA, which can hold up to 50 times its weight in 1-hexadecanol. A thermal conductivity enhancement of 107.9% along x-y direction of CA was achieved at the loading of nearly 2 wt%, while that in z direction is less remarkable. DSC results show excellent thermal stability of the form-stable PCM with a satisfactory latent heat capacity of around 220 kJ/kg.

Wicking surfaces have been widely proven to enhance critical heat flux (CHF) by improving liquid replenishment when approaching the dry-out area during vapor bubble evolution within the nucleate boiling regime. However, different than the vapor column growth mechanism for CHF of regular boiling by heating up a surface, during rapid-cooling-induced quenching boiling processes, CHF occurs upon collapse of vapor film within the transition boiling regime, resulting in a difference in the effect of surface properties on CHF. In this work, we show experimentally that the surface wickability is unable to fully elucidate the enhancement of quenching CHF on wicking surfaces with superhydrophilicity, and that the surface roughness could facilitate collapse of vapor film, thus leading to an extra boost of quenching CHF over the contribution of surface wickability. We propose an improved CHF correlation based on the introduction of a roughness-weighted wicking number, which accounts for the extra contribution of surface roughness in breaking the vapor film barrier and promoting the occurrence of wicking phenomena.

An asymptotic local solution to the axisymmetric conduction problem with a discontinuous temperature boundary condition in the radial direction is presented. The Laplace's equation in cylindrical coordinates (r, z) is expanded near the discontinuity at r = 1, z = 0. The leading order term arises from the step-change in the boundary condition. The next term reflects the effect of curvature at the position of the temperature discontinuity and is a particular solution to the Poisson's equation. Subsequent higher-order terms involve general solutions to the Laplace's equation in Cartesian coordinates and depend on conditions on the far end boundaries. Three domain sizes are thus considered: (i) infinite solid cylinder (0 ≤ r

Improving the thermal insulating performance of porous building material is of great practical significance for energy-efficient buildings. In this work, silica aerogels (SA) having ultralow thermal conductivity were incorporated into autoclaved aerated concrete (AAC) via physical solution impregnation method to further improving its thermal insulating performance. As expected, the addition of SA lowers the thermal conductivity of the composites and the improvement in thermal insulating performance depends on porosity of the pristine AAC. At the SA loading of ~7 wt.%, the highest relative improvement was found to be nearly 30%. Two types of fractal models, i.e., parallel and serial, were established to predict the thermal conductivity of the SA-AAC composites. It was observed that the serial model could match well with the experiment data of the AAC-300 and AAC-400-based composites. A hybrid model of the two types was proposed so as to predict the data for the composites more satisfactorily. This modeling effort helped us better understand the role of the impregnated SA on the heat conduction mechanisms in the composites. As an extra bonus, the compressive strength and the capillary water absorption coefficient of the composites were found to increase and decrease slightly, respectively, with increasing the SA content. The lower water absorptivity indicates a better potential durability. The results shed light on the great potential of such composites in real-world applications if the cost issue can be addressed by future advances in materials fabrication technologies.

Graphene oxide (GO) nanosheets were employed as the additive to make composites of erythritol, a promising medium-temperature PCM candidate. GO nanosheets modified with hydroxyl groups were applied to improve the dispersion stability of the composites. A systematic characterization on the latent heat storage performance was performed for both pure and composite erythritol, in order to identify the benefits and challenges of the composites. It was found that the thermal conductivity is increased by nearly twice and the degree of supercooling was lowered from ~64 °C to ~48 °C at the loading of 1.0 wt% GO nanosheets (the maximum loading tested). The addition of GO nanosheets also leads to an increase of the retrievable latent heat during crystallization, from ~187 kJ/kg to ~225 kJ/kg at the same loading, by increasing the crystallinity. However, the introduction of GO nanosheets can also lead to a rise in the dynamic viscosity of erythritol. As a result, the crystallization rate is slowed down and accordingly, the duration of crystallization becomes 62% longer when the loading reaches 1.0 wt%. In addition, favorable dispersion stability of the erythritol composites is observed, and their melting point (~117 °C) remains almost unchanged during 50 melting-crystallization cycles. Functionalized GO nanosheets have been shown to be an efficient additive for improving the performance of erythritol, but a trade-off analysis on the loading would be required to achieve the best overall performance.

Thin-walled materials, such as alloys, polymers, and glasses, with a thickness between 0.1 and 1 mm have been widely utilized in the encapsulation structures of electronic devices (e.g., the shells of laptops, cellphones, and cameras). The thermal contact resistance (TCR) and bulk thermal conductivity of these thin-walled materials are the key parameters for thermal design of electronic devices. In order to measure the TCR of thin-walled materials having a sub-millimeter thickness, an improved steady-state method was proposed in this work, which is based on the ASTM D5470 standard with the improvement being relied on in situ detection on the sample thickness variations. Such an improved apparatus was designed and constructed, which is equipped with a high-precision laser displacement sensor for measuring the sample thickness. Numerical simulations were performed to verify the heat dissipation from the insulated rods and the experimental uncertainties were analyzed. The measured bulk thermal conductivity was compared with those obtained by traditional steady-state, laser flash and transient plane source methods. The overall TCR of thin-walled stainless steel (SS304) was determined to be of the order of 10-4 m2·K/W, which was in good agreement with that of bulk SS304 samples having similar surface roughness (TCR = 3.13 × 10−4 m2·K/W).

Aerated concrete (AC) was incorporated with micro-encapsulated phase change materials (PCMs) to form a novel PCM-composite AC with improved thermal storage capacity. RT25 paraffin was selected as the PCM and the composite materials were prepared by adding micro-encapsulated paraffin as an ingredient at various loadings. The effective thermal conductivity of the composite samples was measured at both 17 ºC and 35 ºC, while the paraffin was in solid and liquid phases, respectively. The volumetric heat capacity of the composites were also measured. Results showed that both the thermal conductivity and volumetric heat capacity increase upon adding the micro-encapsulated paraffin. However, they were found to decrease when further increase the paraffin loading. The maximum thermal conductivity and volumetric heat capacity were enhanced by approximately 35% and 30% when the paraffin loadings were 1% wt. and 3% wt., respectively. Since the increase of thermal conductivity leads to the deterioration of the thermal insulation performance, the composite samples with 3% wt. micro-encapsulated paraffin with lower thermal conductivity but the highest volumetric heat capacity was exhibited to be more appropriate.

In this work, the water vapor permeability of autoclave aerated concrete (AAC) with various porosities, as a typical porous building material, were predicted based on the fractal theory, with emphasis on the effects of relative humidity (RH). The tortuosity fractal dimensions were first determined based on the porosities and pore sizes of the various AAC samples. The water vapor permeabilities were then predicted based on the determined tortuosity fractal dimensions, porosities, pore sizes and the isothermal adsorption curves. The model was validated by comparing the predicted results with experimental data. Results indicated that in this model, the porosity and pore size of the AAC samples have considerable effects on the predicted water vapor permeability. For all types of AAC, the predicted water vapor permeabilities were found to agree well with experimental data in the medium RH range of 30%–60% with the maximum deviation being less than 15%. In the low RH range below 30%, the water vapor permeabilities were underestimated, mainly due to the disconnected water vapor phase in the realistic scenario. In conclusion, this fractal model can be utilized to predict the water vapor permeability of AAC with various porosities and pore size distributions in the medium RH range of 30%–60%.

The servo system in steam turbine digital electric-hydraulic control system (DEH) is affected by nonlinear factors when it is working. To accurately simulate dynamic characteristics of the DEH, a new nonlinear servo system is proposed, which has limit, dead zone and correction coefficient caused by unknown factors. The model parameters are divided into linear parameters and nonlinear parameters to be identified, respectively. Neural networks are used to identify linear parameters. The nonlinear parameters should be identified according to flow characteristic curve. To verify the validity of the proposed model and parameter identification method, the actual data of primary frequency control from a 1000 MW Ultra Supercritical Unit is adopted. Meanwhile, the linear model with no nonlinear factors is used for comparison. Where the fitting degree of valve opening is 98.445% and power is 96.986%, the output of nonlinear model coincides with actual output well. Where the relative error of stable result is 5% of valve opening and 1.58% of power, the error of linear model is larger. The simulation results of the proposed method show that the nonlinear factors of high-power units cannot be ignored and the nonlinear model of servo system is more accurate.

The pressure drop over a packed bed with sintered ore particles was studied experimentally. The sintered ore particles were characterized to determine their equivalent particle diameter, bed voidage, and sphericity. The pressure drop experiments were performed on unsorted and sieved particles with various size distributions for a superficial velocity up to 2.4 m/s, covering flow regimes from laminar to turbulent. It was shown that the Ergun equation underestimates the pressure drop for such highly irregular-shaped particles by about 40%. The measured modified friction factor was well correlated to a scaled Ergun equation, which was verified to be valid for the modified particle Reynolds number up to 12 000 toward design and optimization of vertically arranged sinter coolers for waste heat recovery.

Autoclaved aerated concrete (AAC) has widely been utilized as a lightweight, porous insulation material for energy-efficient buildings. The knowledge on the thermal conductivity of AAC is required for thermal design of building envelopes. The effective thermal conductivity of AAC is strongly dependent on the moisture content. Such dependence, however, is not well documented in available literature. In this work, AAC bricks with three different bulk densities of 415, 520, and 630 kg/m3, were obtained as the raw materials, and the samples were prepared by humidification to a set of moisture content levels up to 100% by mass. The effective thermal conductivity of the moisturized samples was measured by means of the transient plane source technique. Meanwhile, fractal models for predicting the effective thermal conductivity were proposed based on construction of the porous structure of AAC by self-similar Sierpinski carpet. A two-phase fractal model was first proposed for dry AAC samples, and then an extension to a three-phase model was developed by considering the presence of water phase in the pores for unsaturated, moist samples. It was shown that the thermal conductivity increases with increasing the moisture content, by a factor up to 3.8 over the studied range of moisture content, following a two-section piecewise linear variation. A high-to-low slope change was found to be around a moisture content of 15% for all the AAC samples. A correlation was proposed for the measured thermal conductivity as a function of both moisture content and porosity. Appropriate parameters for the two-phase model were determined by comparing the predicted results to the measured data at dry state. The three-phase fractal model was exhibited to be able to predict the hygric dependence of thermal conductivity. The discrepancy among the predictions by the three-phase model with different geometric parameters was discussed in relation to the constructed pore structures. The predicted results by the two configurations of the three-phase model, i.e., with and without considering the presence of connected water bridges in the pores, were also presented. A reasonable elimination of the presence of connected bridges was shown to lead to better predictions in the low moisture content regime.

Waste heat recovery from sintered ore particles (SOPs) is deemed to be a promising approach to improving the thermal efficiency in iron and steel industry. In this work, industrial SOPs sampled from a steel plant were characterized to obtain a comprehensive set of fundamental structural and thermophysical properties. The SOPs investigated were identified as self-fluxed sinters having a basicity of 1.175 after composition analysis, and two peaks were observed on the pore size distribution. The specific heat capacity of the SOPs at elevated temperatures up to 200 °C was measured, with a power-law fit curve being established for extrapolation to higher temperatures up to 900 °C. The thermal conductivity of the SOP samples with various porosities was determined experimentally, while a Sierpinski-carpet-based fractal model was also utilized to predict the effective thermal conductivity as a function of porosity. The fractal model was validated by the experimental results, which can be applied over a wide range of porosity.

This work is concerned with the influence of cooling rates on the thermal conduction behavior of nano-enhanced PCMs for thermal energy storage. 2-dimensional graphite nanosheets of different sizes have been applied to prepare the eicosane-based composite PCMs. Comprehensive measurements on the thermal storage properties of the prepared composite PCMs were also performed considering the factors including particle size as well as loading level. Results strongly revealed the tunable thermal conduction behavior of the solid-state composite PCMs versus cooling rate. As the cooling rate was ascended from 2.03 to 30.94 °C/min, more than 25% drop of the solid-state thermal conductivity was observed for the samples containing graphite nanosheets. Besides a percolation threshold around 1 wt.%, measured thermal conductivities increased linearly with the particle concentration, achieving the highest value of 1.57 W/m K by GNS-30 at 5 wt.%. Moreover, the graphite nanosheets with a larger size were found to cause higher improvement in thermal conductivity, and also the relatively higher viscosity of PCM suspensions. DSC results indicated the decrease in latent heat capacity of samples is negligible after the incorporation of graphite nanosheets.

A numerical study has been carried out for a laminar steady mixed convection flow in a 2D triangular enclosure with an inner rotating coaxial cylinder, with the enclosure filled with ethylene glycol-silicon carbide (SiC-EG). The thermal conductivity and viscosity of the SiC-EG nanofluids were experimentally determined by using a Decagon Devices KD2 Pro thermal property meter and a rotational Brookfield viscometer, respectively. Various pertinent parameters, such as the dimensionless rotation velocity, solid volume fraction, dimensionless radius of the inner cylinder, and Rayleigh numbers, were analyzed to determine their influences on heat transfer and fluid flow. Results clearly show how the direction of rotation of the cylinder affects the thermal performance in a triangular enclosure. It is found that the average Nusselt number increases with rise in the Rayleigh number or as more nanoparticles are added to the base liquid. It was also observed that at constant Rayleigh number, different rotational conditions have remarkable effects on the flow and heat transfer characteristics.

Transient pool boiling experiments were performed by quenching of stainless steel spheres in dilute aqueous nanofluids in the presence of graphene oxide nanosheets (GONs) at various concentrations (by weight) up to 0.1 wt.%. All the experiments were performed for saturated boiling at atmospheric pressure. Quenching and boiling curves were obtained for the nanofluids in comparison to the baseline case of pure water. It was shown that quenching is accelerated upon increasing the concentration of GONs. The enhanced boiling heat transfer by the nanofluids was interpreted in relation to the modified surface properties, including morphology, wettability, and roughness, on the quenched surfaces. Unlike the findings in available relevant studies that point to surface wettability change, however, the primary cause of critical heat flux (CHF) enhancement was observed to be related to the increased surface roughness serving as paths to facilitate solid−liquid contacts. The increases of both nucleation site density and liquid agitation intensity as a result of the presence of porous structures at relatively high concentrations were also found to be responsible for the enhanced CHF.

An indoor experimental test rig of parabolic trough collector (PTC)-based natural circulation steam generation system consisting in a thermosyphon loop was presented. A series of five heat loads (0.6–1.2 kW) were applied to investigate effect of heat load on thermal performance of the system. Effect of heat load on flow pattern, thermal efficiency and two phase heat transfer coefficient was discussed, respectively. An extended correlation equation was provided for two flow patterns, which is characterized by heat pipe thermal resistance. The critical heat pipe thermal resistance for flow pattern transition was ranged from 34.37 K/kW to 33.35 K/kW. Simultaneously, thermal efficiency shows a continuous increase as heat load kept rising. The effect of backflow was found to be negligible when heat load increased to 1.1 kW. Additionally, the average two-phase heat transfer coefficient in receiver also went up with the rising of heat load for the same flow pattern. Due to the flow pattern transition, which resulted in a dryness fraction drop in receiver, a maximum heat transfer coefficient of 285.86 W/m2 K was obtained at heat load of 1.0 kW under a steam discharging pressure of 0.15 MPa.

This work is concerned with the size effects of Ag nanowires on thermal conductivity and rheological behavior of EG-based suspensions. The influences of inclusion concentration and temperature on the thermophysical properties of specimens containing three types of Ag nanowires were also investigated. It was shown that the measured thermal conductivity of EG-based suspensions increased with the rising temperature and loading. Besides, the relative enhancement in thermal conductivity exhibited a linear relationship with respect to the specific surface area of Ag nanowires. A theoretical approach was developed to predict the effective thermal conductivity of suspensions containing nanowires by introducing liquid layer into account. The Ag nanowires/EG interface thermal resistances were extracted from the experimental results, which ranged from 2.0 × 10 8 to 5 × 10 8 m 2 K/W. Furthermore, a comparative study revealed the excellent performance of Ag nanowires used in present work on improving thermal conductivity compared with the reported studies. Finally, the presence of Ag nanowires with the highest aspect ratio (250) was concluded as the main explanation of a noticeable rise in dynamic viscosity and non-Brownian fluid behavior of EG-based suspensions at the highest loading (10 mg/mL).

The effect of using composite phase change materials (PCMs) filled with high aspect-ratio carbon nanofillers on the transient performance of a thermal energy storage (TES)-based heat sink was investigated experimentally under pulsed heat loads of various powers. The composite PCMs were prepared with carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) at various loadings. Several important thermophysical properties of the composite PCMs were characterized as functions of the loading and temperature. It was shown that the presence of the carbon nanofillers is in general undesirable for the heating periods because the enhanced heat conduction was compensated by the weakened natural convection effect. As compared to cases in the presence of CNTs, the GNP-based composite PCMs performed much better due to their greater thermal conductivity enhancement along with much lower viscosity increase. The use of GNP-based composite PCMs was exhibited to be able to improve transient performance of the heat sink at certain conditions. Expedited recovery rates of the TES-based heat sink during the cooling periods, however, were achieved for almost all the cases using composite PCMs.