High-efficiency Heat Research Articles

Micro-diamond assisted bidirectional tuning of thermal conductivity in multifunctional graphene nanoplatelets/nanofibrillated cellulose films

Environmentally friendly thermal interface materials (TIMs) with bidirectional high thermal conductivities have aroused considerable interests for addressing the heat dissipation issue in integrated circuits. Although graphene-based TIMs exhibit excellent in-plane thermal conductive performance, their through-plane thermal conductivity is commonly less than 3 Wm−1K−1 owing to the vast interfacial phonon scattering, significantly limiting their practical applications. In this study, a strategy aimed at building TIMs with controllable heat transfer pathways both along the in-plane and through-plane directions is proposed by incorporating micron-diamonds (MDs) in graphene nanoplatelets/nanofibrillated cellulose (GNPs/NFC) composite film via a facile and green self-assembly method. The morphology of the obtained MDs@GNPs/NFC composite film can be precisely tailored from a hierarchical structure to a 3D solid foam-like structure to tailor heat transfer paths. By adjusting the loading and particle size of MDs, a through-plane thermal conductivity of 8.85 Wm−1K−1 was achieved accompanied with a simultaneously high in-plane thermal conductivity of 32.01 Wm−1K−1. The excellent bidirectional thermal conductive performance is integrated with high-efficiency Joule heating capability, outstanding nonflammability, as well as superior electromagnetic shielding performance, showing a promising future in advanced electronic devices.

An Evaluation of the Performance of a Ground-to-Air Heat Exchanger in Different Ventilation Scenarios in a Single-Family Home in a Climate Characterized by Cold Winters and Hot Summers

In the present study, the real-world performance of a ground-to-air heat exchanger (GAHE) was analyzed in the Polish climate which is characterized by warm summers and cold winters. The heat exchanger’s performance was monitored over a period of three years (2017 to 2019), and real-world conditions were compared with a Typical Meteorological Year (TMY). The aim of the study was to assess the exchanger’s energy-efficiency potential in various ventilation scenarios in a single-family home under variable real-world conditions, rather than to simply determine its heating and cooling capacity. The analyzed single-family home was a modern, single-story building with a usable floor area of 115 m2. The building’s thermal insulation and airtightness met stringent energy-efficiency standards. Energy consumption in a building equipped with a natural ventilation system was compared with three other scenarios: ventilation coupled with a GAHE, mechanical ventilation with heat recovery and a high-efficiency heat exchanger (HE), and mechanical ventilation with heat recovery coupled with a GAHE. Sensible heating and cooling loads were calculated based on standard ISO 13790:2008, and latent heating and cooling loads were also included in the energy balance. During the year, the GAHE generated around 257.6 W of heating energy per hour and 124.7 W of cooling energy per hour. Presented results can be used to select the optimal HVAC system scenarios for engineering projects as well as private investors.

Glass fiber reinforced graphite/carbon black@PES composite films for high-temperature electric heaters

Carbon based conductive polymer composites as electrothermal materials offers many merits in high flexibility, light weight, low cost, excellent processability, and rapid heating ability, however, the maximum working temperature is usually lower than 200 °C, hampering seriously the medium–high temperature applications. Herein, a series of rationally designed glass fiber reinforced graphite/carbon black@polyethersulfone (G/CB@PES) composites are developed to significantly promote the maximum working temperature over 300 °C. To be specific, the introduced glass fibers acted as rigid skeleton greatly enhance the mechanical strength of PES polymer matrix, thereby offering robust structural stability in high temperature working scenario. Meanwhile, glass fibers not only endow G/CB@PES composite with excellent flame-retardant ability, but also provide rapid thermal conduction channels to reduce the risk of heat accumulation and overheating. More importantly, glass fiber reinforced G/CB@PES composites as electrothermal film heaters exhibit superior electric heating performance in terms of fast temperature responsiveness, high electric heating efficiency, stable maximum working temperatures, and high electric power efficiency under the relatively low applied voltages of 3–21 V. This work provides new insights on the innovative design and facile fabrication of high working temperature electrothermal film heaters in emerging applications.

Carbothermal treated iron oxide nanoparticles with improving magnetic heating efficiency for hyperthermia

Iron oxide nanoparticles with high magnetic heating efficiency is highly desired for magnetic hyperthermia applications. In this paper, carbothermal treatments with graphene are used for improving the magnetic heating efficiency of iron oxide nanoparticles. With the increasing treatment temperature from 400 °C to 700 °C, the specific absorption rate (SAR) increases 2–7 folds. Oxygen vacancies resulted from carbon reduction process, along with the rapid grain growth (from 5.1 nm to 225.6 nm), improve eddy current loss. SAR calculations suggest that eddy current loss accounts up to 23.9%-78.5% of the total SAR after carbothermal treatments, which is the main factor for SAR improvements. This work may provide a methodological reference of preparing iron oxide nanoparticles for magnetic hyperthermia.

A Numerical Study on the Performance of the H2 Shaft Furnace with Dual-Row Top Gas Recycling

Given the urgent pursuit of carbon neutrality and stringent climate policies, the H2 shaft furnace (H2-SF) is starting to gain widespread attention in the steel industry. In this study, the performance of the H2-SF under operation with a dual-row injection top gas recycling system was investigated by a one-dimensional mathematical model. The potential of microwave heating as a means to supply thermal energy in regions of energy deficit was also assessed briefly. The results showed that for scenarios without microwave heating, increasing the upper-row injection rate can improve the furnace performance, and increasing the distance of the upper-row injection level from the furnace top also has a positive effect. A high microwave heating efficiency is expected in regions above the upper-row injection level. For scenarios with microwave heating, a higher microwave power leads to a better furnace performance. Thus, a higher furnace productivity can be achieved by increasing either the upper-row injection rate or the microwave power. However, the latter seems more promising as it decreases the total energy demand due to a better utilization of thermal energy. Based on the comparison of two representative examples, the decrease in the total energy demand is about 0.2 GJ/t-Fe.

Enhanced capillary performance of grooved nanocarbon foams as wicks for heat pipes

An effective heat pipe needs a great amount of working liquid to be moved quickly from the cold end to the hot end. That is why capillary performance works as a critical factor to evaluate the potential of a wick for high-efficient heat pipes. However, good capillary performance only means that the liquid can be transferred quickly; the amount of liquid transferred during capillary rising also needs to be taken into account. In this work, novel grooved nanocarbon foam (GNCF) wicks were fabricated with good capillary performance and a large capillary rise amount. Grooves with different numbers, depths, and widths were formed on the nanocarbon foams (NCFs) and the effect of these factors on the capillary performance was studied. The results indicate that the groove is beneficial to improve the capillary performance of NCFs due to the enhanced permeability and feeding effect. The capillary performance is achieved with a capillary rise height of 45 mm in 14 s. It is 2.5 times as high as that of the NCF and better than some other published results. Moreover, because the GNCFs have a superhigh porosity of 0.99, they are capable to transfer much more working liquid per time than other wicks with similar capillary performance.

Algorithm for searching optimal set values of absorption chiller system using Bayesian optimization

HVAC systems account for at least 40% of the energy consumption of general office buildings. Therefore, reducing the energy use of HVAC systems is indispensable. HVAC systems used in public office buildings mostly adopt a central air conditioning system. To save energy in the central air conditioning system, high-efficiency heat source machines have been adopted, and the inverter control of pumps has been generally introduced. As a further measure to save energy, an optimal control has been proposed. Model-based control has been mainly studied. However, the target HVAC system must be appropriately equipped with sensors for model-based control to use previous operation data. To solve this problem, we proposed a model-free optimal control method using Bayesian optimization. We verified its effectiveness in one-on-one (one cooling tower and one chiller) HVAC systems. As a result, the energy-saving ratio when compared to the rated specification control is 10.38% for our proposed method and 11.34% for the model-based approach, which shows the equivalent performance. In addition, the training results indicate that the optimal set values can be automatically determined in 4 weeks as the training proceeds under rated specification control with Bayesian optimization.

Additive-manufactured single-piece thin multi-layer tungsten heater for an electrothermal thruster.

In this study, a novel single-piece thin multi-layer tungsten resistive heater was successfully fabricated using additive manufacturing and tested as an electrothermal thruster. The heater has 12 resistive layers, with each layer having a thickness and height of 0.15 and 81mm, respectively, and can provide high heating efficiency. A single-piece or monolithic heater was manufactured via additive manufacturing technique, which drastically improved its reliability and decreased its manufacturing cost. In the heating and thrust measurement tests that used nitrogen gas as a propellant, the heater reached a gas temperature of ∼2000K at a 140-A heater current without experiencing any failure. The tungsten-heater resistance linearly increased with an increase in temperature due to the temperature dependence of tungsten's resistivity. The specific impulse and thrust increased with the heater temperature in accordance with the theoretical prediction. Even including a voltage drop due to a contact resistance, the achieved heater efficiency reached 63% at a 100-A heater current even without a thermal insulation around the thruster. The heater efficiency decreased with an increase in the heater temperature due to heat loss to the surroundings. The heat-loss analysis indicated that both thermal conduction and radiation heat losses were crucial for improving the heater performance at a high-temperature operation of over 2000K.

Research on modeling and parameter sensitivity of flow and heat transfer process in typical rectangular microchannels: From a data-driven perspective

With the miniaturized and high-powered development of power equipment and electronic devices, the microchannel heat sink is becoming an important device widely used in the modern cooling field. The heat transfer performance of the microchannel heat sink is affected by many influence factors, and amongst them there exists a complex coupling effect. So far, the mechanism of the microchannel heat sink has not been clearly elucidated. To figure out the dominant characteristics that affect the heat transfer performance of the microchannel heat sink and the interaction mechanism between relevant characteristics, this work carried out numerical simulations and parameter sensitivity analysis to investigate the key factors of the fluid flow and heat transfer performance in rectangular microchannels, and considered variables included key physical parameters of the fluids and dimensional parameters of the microchannels. To satisfy the enormous amount requirement of sample size in the parameter sensitivity analysis, a machine learning model, Gradient Boosting Tree (GBT), was constructed based on computational fluid dynamics (CFD) simulation results; then, the Sobel sensitivity analysis algorithm was coupled in to analyze the multi-parameter contribution of each input parameter. Results show that the number of channels N and Reynolds number Re have a greater impact on heat transfer performance under low Re conditions, and the cross-sectional area A and aspect ratio α have a much higher effect on the pumping power rather than Nusselt numbers Nu; N and A have a positive second-order interaction on Nu. When moving to the conditions with a higher Re, the number of channels N became the dominant factor that influenced both heat transfer and flow performance of the microchannel heat sink. This work also provides effective guidance for the design and optimization of high-efficiency microchannel heat sinks.

Local softening deformation and phase transformation induced by electric current in electrically-assisted micro-compression of Ti–6Al–4V alloy

Electric current induced local softening behaviors were investigated using electrically-assisted micro-compression (EAMC) tests of Ti–6Al–4V alloy in this study. It was found that the flow curves showed a S-shape characteristic at higher current density and larger strain rate, i.e. the local softening dominated by the uneven distribution of Joule heat temperature. Numerical simulations were adopted to analyze the Joule heat temperature and the strain filed distributions in different deformation zones during EAMC. The time functions of the instantaneous strain in the center of softening deformation regions were calculated to contrast the differences between the EAMC and the uniform deformation. It turned out that the plastic strain at the center of the sample was accelerated compared to the rest of the test section during EAMC. Based on the microstructural observations, a α→β→αʹ phase transformation mode was put forward to illustrate the electric current induced phase transformation, in which the “hotspots” effect of local Joule heat was a main driving for the nucleation of new β phase at a relatively low temperature. The proposed local softening deformation method induced by electric current in the fabrication of micro-parts for titanium alloy is possible due to the high heating efficiency for material and controllable temperature field for billet.

Cascade heat utilisation via integrated organic Rankine cycle and compressor-assisted absorption heat pump system

Absorption heat pumps have attracted increasing attention due to the recent interest in high-efficiency waste heat utilisation in buildings. By ways of the outstanding performance of organic Rankine cycle (ORC) for low-temperature heat recovery applications, this paper proposes a new hybrid system integrating an organic Rankine cycle and compressor-assisted absorption heat pump (OCAHP) to improve the operating range and overall performance of the heat pump via cascading heat utilization. A thermodynamic model of the OCAHP system is developed for the evaluation of its performance at different operating parameters. The results show that the system has an excellent performance even at an ambient temperature as low as −20 °C, and its primary energy efficiency is improved significantly compared to both conventional and compressor-assisted absorption heat pumps. The performance improvement is at least 10% over a wide ambient temperature range. Also, it has a greater potential to maintain high-performance operation at a low heat source temperature (below 100 °C) while still meeting the heating requirements. Furthermore, the system shows a good tolerance to the variation of hot water supply temperature. Under a hot water temperature of 60 °C, the primary energy efficiency of the OCAHP is up to 1.5. The above features have proved that the proposed system has a great potential to be used in practical applications and achieve energy efficiency enhancement in typical winter condition.

THEORICAL STUDY ON PROPULSIVE PERFORMANCE OF OBLIQUE DETONATION ENGINE

Detonation combustion is characterized by the high thermodynamic efficiency and fast heat release. Benefitting from these potential advantages, an oblique detonation wave (ODW) is introduced into the combustion chamber and oblique detonation engine (ODE) plays an important role in hypersonic air-breathing propulsion systems. Previous studies mainly focused on the initiation structures, standing features and wave systems of oblique detonation, but the global analysis of ODE propulsive performance is still absent at the macro-level. In this paper, the flow and combustion processes of an ODE are decomposed into four basic modules, named as inlet model, mixing model, combustion mode and nozzle model, respectively. We solve these four basic flow processes using theoretical methods and propose a systematically theoretical approach that can be used to predict the ODE propulsion performance. On the basis of previous ODW initiation structures and waves systems, four different combustion modes, i.e., over-driven ODW, Chapman-Jouguet ODW, over-driven normal detonation wave and oblique shock-induced constant-volume combustion, are chosen to describe the heat release processes of combustible mixture in the ODE combustor. The effects of different combustion modes on fuel specific impulse of the ODE are also analyzed. In addition, the influence mechanisms of inflow parameters, combustor parameters and intake-exhaust parameters on the thrust performance of ODE are also obtained, and the results show that the major factor of fuel specific impulse of an ODE consists mainly of the inflow Mach number and the expansion ratio of engine nozzle. Finally, combined with precious detonation research results, such as the standing features and initiation structures of oblique detonation in a confined space, the preliminary design direction of oblique detonation engine are proposed, which mainly involve some constrained conditions, such as geometrical constraints, inflow velocity limitations and stability ranges of a detonation wave in ODE combustor.

The effect of using forced airflow on the thermal performance of the skirting board heating system

The heating performance of two new modified hydronic skirting board heating systems with air supply is investigated in the present work. In the modified system indoor airflow is forced through the system. The main aim of the present work is to achieve a system with higher heating efficiency and lower energy consumption than the conventional cases. The heat output rate of the system is evaluated experimentally for different water flow rates and inlet water temperatures based on standard No. En-442. Comparison of the results between the new system and the conventional model with air supply shows that heating performance improved about 40%. Thermal comfort conditions in the room are analyzed numerically according to Fanger’s method. Also to CFD validation, the temperature distribution of the simulation is compared with experimental data and obtained results show that there is a good agreement between them. Based on the obtained results, people dissatisfied (PDD) in the different parts of the room is less than 30%.

Frequency thermal characteristic and parametric study of multi-functional building envelope for coolth recovery and thermal insulation: Modelling and experimental validation

The exhaust air insulation (EAI) wall provides a new solution to utilize the low-grade coolth energy of exhaust air in conditioned rooms for preventing the heat gain through the building envelope. Such a multi-functional building envelope takes advantage of the exfiltration process of the cool indoor exhaust air to block the inward conductive heat flux within building envelope in cooling season, and then cooling load is effectively decreased. Previous studies demonstrate the excellent thermal insulation performance of the EAI wall. However, current studies still lack a highly efficient compactional method for simulating the dynamic thermal performance of this wall. Moreover, thermal characteristics of the wall still have not been investigated in frequency domain, which would offer significant design guidance. To address these critical challenges, a frequency-domain finite-difference (FDFD) model of the EAI wall was developed in this study to analyze its transient heat transfer with high computational efficiency and no stability problem. An experimental platform was established to conduct the experimental analysis of the EAI wall and verify the proposed FDFD model. Moreover, the validated FDFD model was used to numerically evaluate the thermal characteristic of EAI wall in frequency-domain. The influences of some design parameters on the decrement factor and phase delay of the wall were also considered in numerical investigation. The result demonstrates that the frequencies of outdoor thermal disturbances exhibit a remarkable impact on the thermal characteristic of EAI wall. The decrement factor of the EAI wall can decrease to near-zero for all the frequencies of outdoor thermal disturbances, when airflow velocity reaches to 0.005 m/s or thickness of porous medium increases to 50 mm. This means that the EAI wall could effectively moderate the impact of outdoor thermal disturbances towards the indoor thermal environment under most scenarios of outdoor thermal disturbances. The outcomes of this study can provide a high-efficiency transient heat transfer model and some important design guidelines for this wall.

Study of the high heating efficiency and uniformity by multi-port sweep frequency microwave irradiations

As a clean energy, microwaves have been widely used in thermal engineering. However, in the traditional microwave heating process, it is difficult to obtain high heating efficiency and uniformity. In this paper, a novel multi-port sweep frequency heating method is proposed to solve this problem. Multi-physics modelling and experiment are carried out to study the mechanism, efficiency, and uniformity of multi-port sweep frequency heating. The simulation and experimental results agree well, which demonstrates the proposed method can make heating more efficient and uniform. In addition, important factors affecting the heating efficiency and uniformity are also discussed, including the multi-port and frequency sweep effects, the polarization direction of the electromagnetic waves, and the bandwidth of the sweep frequency. This study can help us make better use of microwave energy.

Operation strategy and performance investigation of a high-efficiency multifunctional two-stage vapor compression heat pump air conditioning system for electric vehicles in severe cold regions

Cooling and heating are indispensable for electric vehicles (EVs) in severe cold regions due to the hot summer caused by global warming. A multifunctional two-stage vapor compression (MTSC) heat pump air conditioning system (HPAC) with its operation strategies is proposed, which can operate in not only single-stage vapor compression (SSC) cooling and heating mode, but also two-stage vapor compression (TSC) heating mode under ambient temperatures as low as −35 °C. Then a simulation model is developed, and comparative analyses among the MTSC HPACs with different refrigerants, and between MTSC HPACs and SSC HPACs are conducted. The influences of intermediate pressure and pressure drop on performance are discussed. Besides, exergy analysis and second law performance are presented. It is found that the MTSC HPAC attains a 6% to 30% higher coefficient of performance (COP) than the SSC HPAC when the evaporating temperature changes from −10 °C to −20 °C, and 156% to 85% higher energy efficiency than the positive temperature coefficient (PTC) heaters when the evaporating temperature varies from −25 °C to −45 °C. The COP of R290 is slightly higher than that of R410A, R32 and R454C. Furthermore, the main exergy destruction appears in compressors, accounting for 40% to 45%.

Maghemite (γ-Fe2O3) and γ-Fe2O3-TiO2 Nanoparticles for Magnetic Hyperthermia Applications: Synthesis, Characterization and Heating Efficiency.

In this report, the heating efficiencies of γ-Fe2O3 and hybrid γ-Fe2O3-TiO2 nanoparticles NPs under an alternating magnetic field (AMF) have been investigated to evaluate their feasible use in magnetic hyperthermia. The NPs were synthesized by a modified sol-gel method and characterized by different techniques. X-ray diffraction (XRD), Mössbauer spectroscopy and electron microscopy analyses confirmed the maghemite (γ-Fe2O3) phase, crystallinity, good uniformity and 10 nm core sizes of the as-synthesized composites. SQUID hysteresis loops showed a non-negligible coercive field and remanence suggesting the ferromagnetic behavior of the particles. Heating efficiency measurements showed that both samples display high heating potentials and reached magnetic hyperthermia (42 °C) in relatively short times with shorter time (~3 min) observed for γ-Fe2O3 compared to γ-Fe2O3-TiO2. The specific absorption rate (SAR) values calculated for γ-Fe2O3 (up to 90 W/g) are higher than that for γ-Fe2O3-TiO2 (~40 W/g), confirming better heating efficiency for γ-Fe2O3 NPs. The intrinsic loss power (ILP) values of 1.57 nHm2/kg and 0.64 nHm2/kg obtained for both nanocomposites are in the range reported for commercial ferrofluids (0.2–3.1 nHm2/kg). Finally, the heating mechanism responsible for NP heat dissipation is explained concluding that both Neel and Brownian relaxations are contributing to heat production. Overall, the obtained high heating efficiencies suggest that the fabricated nanocomposites hold a great potential to be utilized in a wide spectrum of applications, particularly in magnetic photothermal hyperthermia treatments.

Numerical investigation on the thermal-hydraulic performance of helical twine printed circuit heat exchanger

The printed circuit heat exchanger (PCHE) is a high-efficiency heat exchanger with high compactness, pressure resistance, and stability. It can also withstand high temperatures and ultra-low temperatures. Therefore, it has attracted a large number of scholarly interests in recent years. In order to improve the thermal and hydraulic performance of traditional printed circuit heat exchangers, a three-dimensional helical twine printed circuit heat exchanger (HTPCHE) is designed and the laminar flow is numerically investigated using water as the working fluid. Taking Colburn factor j, Fanning friction factor f and comprehensive index j/f as parameters, the thermal-hydraulic performance of HTPCHE and zigzag-type PCHE with different inclined angles are compared. It is found that when the inclination angle of HTPCHE is 10–25°, its overall performance is better than that of traditional PCHE. When the inclined angle is 10° and Rein = 828, the overall performance is 37.6% better than that of 15° zigzag channel. In addition, the impact of the pitch of HTPCHE and the thickness of the transition plate on its heat transfer and flow characteristics is explored. It is discovered that when the given fixed inclined angle, the overall performance can be improved by increasing the pitch while the thickness of the transition plate does not have any significant impact.

Exergy Analysis of Phase-Change Heat-Storage Coupled Solar Heat Pump Heating System.

With the rapid development of industrialization, the excessive use of fossil fuels has caused problems such as increased greenhouse gas emissions and energy shortages. The development and use of renewable energy has attracted increased attention. In recent years, solar heat pump heating technology that uses clean solar energy combined with high-efficiency heat pump units is the development direction of clean heating in winter in northern regions. However, the use of solar energy is intermittent and unstable. The low-valley electricity policy is a night-time electricity price policy. Heat pump heating has problems such as frosting and low efficiencies in cold northern regions. To solve these problems, an exergy analysis model of each component of a phase-change heat-storage coupled solar heat pump heating system was established. Exergy analysis was performed on each component of the system to determine the direction of optimization and improvement of the phase-change heat-storage coupled solar heat pump heating system. The results showed that optimizing the heating-end heat exchanger of the system can reduce the exergy loss of the system. When the phase-change heat-storage tank meets the heating demand, its volume should be reduced to lower the exergy loss of the tank heat dissipation. Air-type solar collectors can increase the income exergies of solar collectors.

A Review on Welding and Fatigue Behaviour of Titanium and Its Alloys

Titanium is metal element which comprises about 0.44% of earth’s crust. The abundance and reliable properties of this element made it essential in field of engineering works. Welding is a fabrication process which fuse the materials together. Welding of titanium and its alloys expands the applications of usage of titanium. The idea of this review, is to sum up the welding and fatigue behavior of titanium and its alloys. It has been identified that laser beam welding (LBW) is feasible for titanium and its alloys due to its flexibility, high heat capacity input and efficiency.