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The fault detection (FD) problem is investigated for event-triggered discrete-time Markov jump systems (MJSs) with hidden-Markov mode observation. A dynamic-event-triggered mechanism, which includes some existing ones as special cases, is proposed to reduce unnecessary data transmissions to save network resources. Mode observation of the MJS by the FD filter (FDF) is governed by a hidden Markov process. By constructing a Markov-mode-dependent Lyapunov function, a sufficient condition in terms of linear matrix inequalities (LMIs) is obtained under which the filtering error system of the FD is stochastically stable with a prescribed H∞ performance index. The parameters of the FDF are explicitly given when these LMIs have feasible solutions. The effectiveness of the FD method is demonstrated by two numerical examples.

The manufacturing industry undergoes a new age with major changes taking place on several fronts. Companies devoted to digital transformation take their future plants inspired by the Internet of Things (IoT). The IoT is a worldwide network of interrelated physical devices,which is an essential component of the internet, including sensors, actuators, smart apps, computers, mechanical machines, and people. The effective allocation of the computing resources and the carrier is critical in the industrial internet of Things (IIoT) for smart production systems. Indeed, the existing assignment method in the smart production system cannot guarantee that resources meet the inherently complex and volatile requirements of the user are timely. Many research results on resource allocations in auction formats which have been implemented to consider the demand and real-time supply for smart development resources, but safety privacy and trust estimation issues related to these outcomes are not actively discussed. The paper proposes a Hierarchical Trustful Resource Assignment (HTRA) and Trust Computing algorithm (TCA) based on Vickrey–Clarke–Groves (VGCs) in the computer carriers necessary resources to communicate wirelessly among IIoT devices and gateways, and the allocation of CPU resources for processing information at the CPC. Finally, experimental findings demonstrate that when the IIoT equipment and gateways are effective, the utilities of each participant are improved.

The pyrolysis mechanism of pure HFO-1234yf and R32 is investigated by ReaxFF molecular dynamics simulation(MD) and density function theory (DFT) calculation. There are 15 initiation reaction pathways observed in the process of pyrolysis. The pyrolysis processes of mixed working fluid (HFO-1234yf/R32) are carried out at different temperatures and different ratios of mixture. The results show that R32 can restrain the decomposition of HFO-1234yf at 2600 K–2800 K and promote one of HFO-1234yf after 3000 K. This is also demonstrated by the kinetic analysis. The radical CHF2 from the decomposition of R32 plays a significant role and can obviously decrease the energy barrier of all the decomposition pathways of HFO-1234yf after 3000 K based on the analysis of DFT. HF is the dominant product in the all pyrolysis processes. In the mixture, R32 slows down the formation of HF at 2600 K–3100 K, but increase the quantity of HF at 3200–3500 K.

Experimentally and numerically studied the heat transfer performance of supercritical CO2 (S-CO2) cooled in small (d ≤ 4 mm) horizontal helically coiled tube (HCT). Five HCTs with fixed coil pitch b = 34 mm, coil diameters D ranging from 36–140 mm and tube diameters d ranging from 2 to 4 mm were used. The results showed that the heat transfer coefficient (HTC) has tripled as the tube diameter decreases from 4 to 2 mm. HTC increases up to 30% with the coil diameter decreasing from 140 to 36 mm. Based on the modified theory derived from the straight tube (ST) with upward flow, the onset value of buoyancy effect in horizontal HCT was obtained, which confirmed that the buoyancy in the present experimental study can be neglected. Totally, 33 test runs and 426 points were performed, and from which a new correlation was proposed to evaluate the HTC of S-CO2 in the horizontal HCT under cooling condition.

A series of ReaxFF-MD simulations are employed to illuminate the oxidation mechanism of HFO-1336mzz(Z) as an environmentally friendly refrigerant in O2/H2O environment. The results showed that H2O molecule has a significant impact on the HFO-1336mzz(Z) oxidation reactions and promotes the rate of HFO-1336mzz(Z) oxidation decomposition. H2O molecule can react with H and O radicals to form OH radical and will provide more H atoms to combine with F radicals to generate HF molecule. The OH radical plays a crucial role in the HFO-1336mzz(Z) oxidation reactions because the reactivity of OH radical. CO2, COF2 and HF are dominant products and the other radicals COF, O, F, CF3 and CO2F have a significant impact on the HFO-1336mzz(Z) oxidation reactions. The chemical equations of HFO-1336mzz(Z) oxidation in different conditions are presented to expound the comprehensive oxidation mechanism.

Reducing energy consumption in buildings through behavioural changes has been regarded as a relatively low-cost and sustainable method. However, studies that focus on occupant building energy consumption in student residences are few. In the context of shared residences, the energy behaviour could be very different. This is because student–student and student–building system interactions are complex. To address this research gap, this study developed an agent-based simulation model regarding students as heterogeneous individuals. Simulation parameters include student basic information, status of staying at dormitories and applicances using behaviours. All data were obtained from a university campus in Shenzhen, China. Energy-saving scenarios under different strategies were simulated, including different energy-saving strategies and interaction behaviour energy-saving advertising. Results show that (1) occupancy is the most important factor for dormitory energy consumption; (2) reducing the time of air conditioner use has the largest impact on energy-saving; (3) reducing computer standby time has a great energy-saving potential; (4) students’ attitude and awareness of energy conservation and the communication and exchange of energy information amongst students play an important role in energy saving. According to these results, the university can take measures to promote energy saving, which include strengthening education, establishing a energy saving championship system for dormitory energy consumption and implementing a reward-and-punishment mechanism.

Construction is a major contributor to pollution in the natural environment. Among all construction pollutants, construction dust is the most significant pollutant that endangers human health. To expand the limited scope of studies on construction dust exposure, this study investigates current dust control practices in the construction industry in Hong Kong through dust monitoring data compilation. The database constructed and compiled by this study is larger than any other previous dust-related datasets in the construction industry in Hong Kong. A total of 837 samples are collected from 33 construction sites and 16 contractors, among which 783 valid samples are analyzed. Descriptive statistics are reported in terms of “construction process”, “trade”, “tools”, “data source type”, and “dust control measure”. Overall geometric mean for personal exposures are 0.314 (geometric standard deviations: 3.929) mg/m3 for respirable dust and 0.003 (geometric standard deviations: 5.105) mg/m3 for quartz concentration. It is found that the top three dust respirable exposures are from cement mixing, concrete breaking, and manual demolition, whereas grinding and rock breaking are recorded the two highest processes in terms of quartz exposure. Analytical results indicate that respiratory protection commonly used on construction sites is often inadequate for exposures encountered. Data variability within task and tool is large, with high exposures reported for a broad spectrum of tools. This study attains a large dataset to represent as many dust production construction activities as possible, determining the most influential factors for predicting dust exposure and evaluating the effectiveness of current dust control practices. This knowledge should, in turn, lead to enhanced practices of dust control in Hong Kong and highlight areas that need improvements.

The Organic Rankine Cycle (ORC) system is a rather promising technology for waste heat recovery. However, waste heat source generally presents a fluctuating behavior, which is a big challenge to the security and efficiency of the ORC. An improved dynamic model of ORC system with zeotropic mixture is firstly developed, which can be better adapted to the phase change process of working fluid. Accordingly, dynamic behavior of the ORC and dynamic distribution of evaporation stages are investigated. Besides, three control strategies are proposed to improve the performance of steadiness with external controllable conditions. Results show that a sudden abnormal change occurs in specific enthalpy of working fluid at the evaporator outlet when different disturbances are imposed to the system. It is found that the proposed control strategies are adapted to different heat source fluctuation. The control strategies with feedforward control system work well with low frequency variation of heat source temperature.

Supercritical CO2 Rankine cycle has great development potential as a power cycle for converting low-grade thermal energy into electricity. Better understanding of supercritical CO2 heat transfer in helically coiled tubes (HCTs) is required for design and operation of supercritical CO2 Rankine cycle power systems. In this work, the SST k∼ ω model is employed. A new dimensionless buoyancy parameter Ψ is proposed which denotes the ratio of gravitational buoyancy force to overall curvature effect. Furthermore, a flow regimes map is proposed based on the inclination angle of the dividing streamline between the two vortexes and buoyancy parameter Ψ . The mixed convection region in HCT is decomposed into a gravitational buoyancy force dominated heat transfer region (B Region α > 45 ° ) and a curvature effect dominated heat transfer region (C Region α 45 ° ). Subsequently, the effects of HCT geometry on heat transfer mechanisms are respectively investigated in B and C Region, which help us better understanding the relationship of the buoyancy criterion and flow characteristics. The results indicate that the effects of coiled pitch and coiled diameter on heat transfer can be neglected in B Region. In C Region, the heat transfer is suppressed as coiled pitch increases and it will appear oscillation when torsion effect is strong enough. In addition, the heat transfer is enhanced with curvature increases but except for near the pseudo-critical region.

Construction waste minimization in design stage can reduce construction waste from sources. Design unit is regarded as a key stakeholder in this stage and plays a critical role in reducing construction waste. However, the limited studies focused on factors affecting their willingness towards construction waste. To fill this research gap, this study explored the critical factors which affect design units' willingness towards construction waste minimization using exploratory sequential mixed method. Literature bibliometric and semi-structured interviews were initially conducted to identify influencing factors of the willingness of design units towards construction waste minimization. Questionnaire survey was used for data collection and Structural equation modeling was then used for analyzing factorial interactions. It is indicated that social and market environment exerts the greatest positive effects on design units' willingness towards construction waste minimization, followed by government supervision and minimization attitude of designers, while effect of project constraints seems minimal. Furthermore, enterprise internal culture indirectly affects design units' willingness towards construction waste minimization by influencing minimization attitude of designers. Moreover, strategies and suggestions were proposed from perspectives of society, government, design units, and clients, respectively. Findings of this empirical study provide theoretical reference for improving relevant standards and regulations regarding construction waste minimization design with establishing incentive mechanisms.

Because of the urgent need of environment protection and energy saving, Chinese government has proposed ‘coal-to-electricity’ policy in north China to use air-source heat pump for space heating. Therefore, a cascade heat pump working at −20 °C and providing 75 °C hot water is studied in this work to match original cast iron radiator in cold climate in north China, which was seldom reported. It is also able to provide hot water for industrial and commercial use and work under single-stage mode in summertime. An experimental prototype of the cascade heat pump is designed and set up. Experimental results show COP of the cascade heat pump system can remain 1.69 at environment temperature as low as −21 °C and the COP is around 2 for most of the tested cases. In the tested temperature range, heating capacity varies from 11.32 to 18.93 kW, and discharge temperature varies from 93.95 to 91.42 °C. Thus, the cascade heat pump reveals good energy saving potential and can work stably demonstrating the feasibility to supply high-temperature water for space heating in coal-to-electricity retrofit. To study the switch between cascade and single-stage modes, computational models of the heat pump are established, validated with experimental data. The results imply cascade mode should be switched to single-stage mode when ambient temperature is higher than 20 °C, taking into account the COP, heating capacity, and discharge temperature. The studied system and results can be also used in other cold weather worldwide to help save energy.

Improvement of building energy performance is crucial in achieving energy conservation goals. Previous studies have indicated that effective collaboration among stakeholders can help to address the building energy performance gap. However, only a few studies have examined the effects of underlying collaboration between designers and contractors. To fill this research gap, this study proposes an evolutionary game model to investigate the underlying logic of the collaboration by analyzing the dynamic behaviors of designers and contractors. First, the main possible drivers and barriers of the collaboration between designers and contractors were identified through a literature review and an expert interview. Second, a Nash equilibrium model was utilized to analyze the decision of designers and contractors under different scenarios. The results show that the best means to promote the collaboration is “reducing collaboration costs”, followed by “increasing the collaboration benefits” and “decreasing the loss caused by a lack of collaboration”. Based on the parameter discussion of the evolutionary game model, five solutions were proposed for the effective collaboration between designers and contractors. This study helps to clarify the dynamic collaboration decision process of designers and contractors, who are considered players with bounded rationality. The proposed evolutionary game model provides a theoretical basis for quantitative research on the collaboration between designers and contractors. This study could help researchers to consider the dynamics of collaboration among stakeholders in building projects, and it could also help practitioners to select proper strategies for improved collaboration.

Effective energy saving and environmental protection are the global topics. Organic Rankine cycle (ORC) that efficiently converts the waste heat into electricity has drawn widespread attention. The sustainability assessment of ORC can explicitly reveal its contribution to mitigating the ecosystem degradation. Meanwhile, the ecological cumulative exergy consumption (ECEC) analysis is generally used to evaluate the sustainability of industrial activities. Therefore, we established the ORC-ECEC model based on the ecological life cycle assessment involving three fundamental aspects, the resource consumption, economic capital and the environmental impact. Particle swarm optimization algorithm was adopted to optimize the system parameters. The working fluid, R134a, is used in ORC to illustrate the features of proposed framework and conduct the comparison with coal-fired power generation system. Further, systems using working fluids of different hydrogen fluoride olefins (HFOs) are selected to investigate the effect of working fluid on ORC sustainability. The results showed that the economic capital exhibited the largest ECEC during the life-time of ORC, followed by environmental impact and resource consumption. The ECEC of system using R134a was about 1.89E+10 sej/kWh, which was much smaller than that of coal-fired power plant, 1.41E+12 sej/kWh. In addition, the ECEC analysis of systems using different HFOs indicated that ORC using R1336mzz as working fluid was significantly superior compared to systems using other working fluids from the point of view of the sustainable development.

Citrus wastes contain large amounts of pectin as the main constituent in addition to cellulose, lignin and hemicellulose. The thermal degradation behaviors and products distribution from the fast pyrolysis of citrus wastes are different from lignocellulosic biomass due to the high content of pectin. In view of this, the detailed decomposition behaviors of pectin are investigated by using density functional theory (DFT). The calculation results indicate that the most energetically favored initial reaction of pectin is the cleavage of α-1,4-glycoside bond (C1 O bond) with an energy barrier of 126.4 kJ/mol. In addition, compared with the reaction energy barriers, it can be seen that the formation of CO, CH4 and furan are more difficult than that of CO2, methanol and furfural, respectively. The comprehensive kinetic analysis indicates that Path-A2-2 should be the most favorable pathway for the formation of acetaldehyde. The rate-determining steps of the formation of furan and furfural are dehydration reactions while decarbonylation reaction is the result of the rate-determining step of the formation of methyl 2-furoate and propanone.

Environmentally-friendly working fluid selection and multi-objective evaluation have been paid a great attention in organic Rankine cycle (ORC). Based on a modified analytic hierarchy process, a multi-factor evaluation method for trans-critical ORC (TORC) is developed first. The multi-factor indicator consists of eight indicators including energetic, exergetic, economic and environmental criteria (4E). The net power output, system efficiency, total exergy loss, exergy efficiency, cost per unit of time, electricity production cost, dynamic payback period, and emissions of CO2 equivalent (ECE) during life-time are considered. R134a, R32, R1270, R290 and R1234yf are selected as potential working fluids. The results show that the performance of R134a is the best followed by R32, R290, R1270 and R1234yf when only thermo-economic performances are considered. R290 shows the maximum comprehensive emission reduction of 8019 tons CO2, eq during life-time followed by R1270 being 7529 tons CO2, eq. The primary source of ECE from R134a, R32, R1270 and R290 is fluid leakage, accounting for over 68–83%. In multi-factor evaluation, R290 and R1270 exhibit higher feasibility level than R134a when the environmental impact is paid great attention. The multi-factor evaluation method is effective and valid to conduct comprehensive feasibility evaluation for decision-making.

The ad-ranking module is the core of the advertising recommender system. Existing ad-ranking modules are mainly based on the deep neural network click-through rate prediction model. Recently an innovative ad-ranking paradigm called DNN-MAB has been introduced to address DNN-only paradigms’ weakness in perceiving highly dynamic user intent over time. We introduce the DNN-MAB paradigm into our ad-ranking system to alleviate the Matthew effect that harms the user experience. Due to data sparsity, however, the actual performance of DNN-MAB is lower than expected. In this paper, we propose an innovative ad-ranking paradigm called DNN-VMAB to solve these problems. Based on vectorization and clustering, it utilizes latent collaborative information in user behavior data to find a set of ads with higher relativity and diversity. As an integration of the essences of classical collaborative filtering, deep click-through rate prediction model, and contextual multi-armed bandit, it can improve platform revenue and user experience. Both offline and online experiments show the advantage of our new algorithm over DNN-MAB and some other existing algorithms.

Numerical simulations are performed to investigate the turbulent heat transfer characteristics of supercritical CO2 in heated vertical helically coiled tube, and primary focus is to analyze the mechanism of buoyancy force and flow acceleration on the heat transfer. The results show similar effect from buoyancy force and centrifugal force, and both forces induce a secondary flow in the cross section that improves the heat transfer efficiency. The buoyancy parameter ϕ2 and flow acceleration parameter q+ are established with reasonably good validation against numerical results. On the basis of the two parameters, the buoyancy factor Fb and the acceleration factor FAc are proposed to quantify buoyancy and flow acceleration effect, respectively. Furthermore, a temperature difference correction factor Ft is introduced to consider variation of thermo-physical properties. A new semi-empirical heat transfer correlation is proposed for supercritical CO2 in function of Fb, FAc and Ft for the vertical helically coiled tube.

Design, manufacturing, storage, transportation, and on-site assembly are fundamental processes in prefabricated housing production (PHP) in Hong Kong. However, these processes are fragmented and entail various risks that adversely affect the schedule performance of PHP, thereby causing frequent delay problems in PHP projects and hindering the government from satisfying the high housing demands. Accordingly, many researchers have examined these schedule risks to resolve these delay problems. However, none of these studies developed an effective tool for managing schedule risks of PHP by envisaging the key characteristics of schedule risks and PHP. Most of previous research regarding to the management of prefabrication construction tends to consider risks from static and isolated perspectives, despite that these risks are coherently interrelated with each other and their influence varies throughout the whole PHP process. To fill the research gaps, a hybrid dynamic model is developed in this research to evaluate and simulate the impact of identified schedule risks on the schedule performance of PHP in view of underlying interrelationships and interactions, employing the hybrid system dynamics (SD) and discrete event simulation (DES) method. The resulting hybrid model is validated through a serial of model structure tests and model behavior tests, with the use of data collected from a PHP project in Hong Kong. This study offers an in-depth understanding of how schedule performance of PHP are dynamically influenced by interrelationships and interactions underlying various schedule risk variables. The developed model not only has the benefits of ease of modifying model structure to reflect real schedule situation of PHP project, performing various risk analyses and communicating with simulation results, but also is of value of providing an experiment platform for identifying and determining managerial and technical solutions proposed to minimize and mitigate the influence of corresponding schedule risks prior to implementation.

The study investigates the comprehensive effects of superheat and internal heat exchanger (IHX) on the thermo-economic performance of organic Rankine cycle (ORC). Exergy efficiency, net power output, and electricity production cost (EPC) are compared based on the working fluid properties and heat sources. The results indicate that under a lower heat source temperature and load, exergy efficiency of IHX-ORC does not always exceed that of simple ORC (S-ORC) when EPC is selected as an objective function, and IHX-ORC exhibits a worse economic performance than S-ORC for all fluids (R161, R1234ze, R152a, cyclopropane, butane, R123, cyclopentane, heptane, and cyclohexane). However, IHX-ORC with dry fluid achieves a better thermo-economic performance than that with wet fluid when the heat source temperature and load increase to a high level. The EPC of IHX-ORC is close to that of S-ORC with the increase in heat source temperature and load, and thus, IHX-ORC exhibits approximately 10–17% higher thermal efficiency and 5–10% higher exergy efficiency than those of S-ORC. With respect to butane and R123, the net power output exhibits approximately 22.5% and 23.5% growth, respectively. In order to evaluate the feasibility of IHX-ORC, a judgement indicator [α > 1.90625 + 0.4258ξ] with respect to six factors is proposed.

The oxidation decomposition mechanism of HFO-1336mzz(Z) were studied by ReaxFF simulations. The initial oxidation dissociation reactions of HFO-1336mzz(Z) can be divided into two types: thermal decomposition of HFO-1336mzz(Z) and the collision of O2 with HFO-1336mzz(Z). Most of the latter reactions are more likely to occur than the former ones. By comparing the energy barriers of initiation pathways, the pathway 2 and pathway 3 which involve the collision of C C bonds with O2 molecules are main reaction pathways and the energy barriers of these reactions are 153.3 and 135.3 kJ mol−1, respectively. HF, COF2 and CO2 are main products and the formation pathways of these products are observed from ReaxFF MD simulations. The main small radicals CF3, F, O, COF and COF3 play an important role in the oxidation decomposition of HFO-1336mzz(Z) because they can promote the decomposition of molecules and radicals and the formation of main products.

Organic Rankine cycle (ORC) is a promising heat recovery technology for low grade energy. The liquid receiver is an important equipment in ORC system for it stores the organic fluids. This paper established a leakage model of zeotropic mixture in liquid receiver. The main impacts of the parameters, including the mass fraction of fluid, vapor mole fraction, and leak rate, on the thermodynamic performance of ORC system were conducted with environmental-friendly mixtures of R152a/R600 and R290/R245fa. The results revealed that the leak of zeotropic mixtures had a great influence on the mass fraction of fluid. It would further impact on the thermodynamic performance of system. The effect of liquid leak in liquid receiver on the net power output of system is less than that of vapor leak. The maximum increasing rate of net power output through vapor and liquid leakage were 0.19% and 0.04% of R152a/R600 at different initial mass fraction, and the maximum decreasing rate were 20.81% and 0.25%, respectively. The sensitive analysis showed that the net power output of system monotonously decreases as leakage rate increases.

The construction industry in Hong Kong has been exposed to serious housing production issues such as insufficient manpower, safety, environmental concerns and inefficient housing supply. Prefabrication housing production has shown as efficient construction model which enables to overcome these issues and it is applied in construction industry in Hong Kong. In this paper, hybrid modeling techniques that combine system dynamics and discrete event simulation are used to analyze the interrelationships of schedule risks within the six-day cycle assembly of prefabrication housing production. Moreover, a hybrid dynamic model is developed to simulate and evaluate the impact of schedule risks on the schedule performance of the six-day cycle assembly of PHP via the Anylogic software package. The resulting model is validated by data which is collected from a PHP project in Hong Kong. Based on the simulation results, the first most influential level contains five schedule risks, namely, inefficient verification of precast components because of ambiguous labels, misplacement on the storage site because of carelessness, owner crane breakdown and maintenance, slow quality inspection procedures, and inefficient design data transition that cause average schedule delays of more than 300 min. The second most influential level includes delay of the delivery of precast element to site, design information gap between designer and manufacturer, installation error of precast elements, and logistics information inconsistency because of human errors, with average schedule delays less than 300 min and greater than 200 min. Design change, low information interoperability between different enterprise resource planning systems, and inefficiency of design approval belong to the third level that imposes the least influence on the schedule delay of the assembly. This research contributes to the current knowledge of the management of prefabrication construction by developing an effective model that offers an in-depth understanding of how schedule performance of PHP is dynamically influenced by interrelationships of various risk variables. Also, it provides an experimental platform for simulating and analyzing schedule risks that significantly affect the schedule performance of the six-day cycle assembly of prefabrication housing production. Such critical schedule risks could be identified and managed prior to the implement of prefabrication housing production projects. Compared with traditional techniques, the hybrid dynamic model simultaneously considers macro and micro levels, thereby enabling project managers to gain a deeper insight into schedule management and acquire a multidimensional understanding of schedule delay.

Occupant behavior is viewed as a main source causing the building energy performance gap between the predicted and the actual consumption. The nature of occupants’ energy behavior research requires a combination of social science and natural science, which indicates that a mixed methods design would be useful. However, researchers often do not know when and how mixed methods approach should be used. To fill this gap, this paper first reviewed the research methods adopted in 230 relevant articles published in the past decade. The results show that 83.48% of articles applied quantitative methods, followed by mixed methods (5.22%) and qualitative methods (0.87%) with rest being pure review or conceptual papers. This shows that researchers in the field of occupant behavior mainly adopt the objectivist philosophical position. Subsequently, an in-depth analysis of research methodologies was conducted in relation to worldviews and philosophical assumptions, and the advantages and disadvantages of qualitative and quantitative methods were discussed. Finally, a mixed methods research design framework was proposed as a point of departure for researchers to gain a comprehensive understanding of mixed methods design. It is expected that this framework could help researchers develop a proper mixed methods research design according to the nature of their research problem.

Experimental investigation of the cooling heat transfer of supercritical CO2 in helically coiled tubes with constant wall heat flux is analyzed based on exergy analysis. The experiment covers an inlet temperature range of 295–330 K, a mass flux range of 159.1–954.9 kg/m2s, a tube diameter range of 2–5 mm and a Reynolds number range of 11,000–90,000. The effects of mass flux, heat flux and diameter of helically coiled tube on the dimensionless exergy destruction are investigated. Experimental results show that the dimensionless exergy destruction caused by the irreversibility of heat transfer is much larger than that by flow friction for supercritical CO2. The optimal Re is more sensitive with mass flux and tube diameter rather than heat flux. For the tube diameters of 3, 4, 5 mm considered, there exists an appropriate range of Re that have the smaller NI and the better exergy utilization with certain diameter. A correlation for the optimal Reynolds number as function of main dimensionless parameters related to wall heat flux, mass flux, fluid properties and geometric dimensions is proposed. These results provide helpful information for the optimal design of helically coiled heat exchanger with supercritical CO2 based on the thermodynamic analysis approach.

The heat transfer deterioration (HTD) of supercritical CO2 encountered in trans-critical CO2 Rankine cycle is an important issue related the safety of the whole unit. For the purpose, numerical simulations are performed to get a further insight into the mechanism of heat transfer characteristics of supercritical CO2 flow in helically coiled tube (HCT) both in horizontal and vertical orientations with parameters in a range of p = 8 MPa, G = 100–800 kg/m2s and q = 15–140 kW/m2. The Shear-Stress Transport (SST) k–ω turbulence model with enhanced wall treatment method is employed to handle the coupled wall-to-fluid heat transfer. Results show that secondary flow induced by the coil curvature produces a transverse transport of the fluid over the cross section of the pipe and therefore enhances the heat transfer. Additionally, in the vertical oriented HCT, the HTD at supercritical pressure observed in a smooth straight tube (ST) is significantly alleviated. At a higher q/G, the HTD still exists irrespective of coil orientations. But, different from ST, the HTD in HCT is caused by both gravitational and centrifugal buoyancy force. Therefore, the onset of HTD in HCT cannot be predicted by the empirical correlation (q = 0.0002G) derived from analytical and experimental results for ST. Based on numerical calculation, an improved buoyancy parameter is developed to predict buoyancy effect on vertical HCT with an internal upward flow.

An exergoenvironmental model of the Organic Rankine Cycle (ORC) system is established in this paper. The manufacture and leakage of working fluid are important in a life cycle assessment (LCA). The environmental impact of the working fluid is allocated to each component of the ORC system according to the exergy destruction of the corresponding component. R134a, R227ea, R152a, and R245fa are selected as the working fluids. In addition, the components in the ORC are manufactured with material steel or copper. The environmental impacts of the working fluids and components are analyzed, and the exergoenvironmental impact factors of the components are discussed. The results showed that the environmental impact of the working fluid cannot be neglected. The environmental impacts of R134a, R227ea, R152a, and R245fa are 13.76%, 26.04%, 2.62%, and 14.77% in the environmental impacts of overall system, respectively. The environmental impact caused by working fluid leakage is 67.52%, 75.62%, 35.71%, and 68.34% in the environmental impacts of the working fluid, respectively. The ORC system with R245fa has a minimal exergoenvironmental impact (steel: 70.06 mPts/h, copper: 83 mPts/h).

HFO-1234yf is considered as one of the most promising working fluids for Organic Rankine Cycle (ORC) owing to its good environmental performance and thermal performance. In this study, an experimental system was designed to determine the thermal stability of HFO-1234yf. The results showed that pyrolysis temperature ranges of HFO-1234yf were 170–190 °C. The pyrolysis products of HFO-1234yf were gas and liquid products. Six gases detected in vapor phase products were HF, CHF3, CH2F2, CF4, C2F6 and C3F8. The concentration of HF in vapor phase had an absolute advantage in the gas products. The thick yellow oily liquid product was observed and their chemical composition and the corresponding forming mechanism were studied. There mainly were 3 monomer units (A, B and C) in the liquid polymers. The optimal generation pathways of the liquid polymers were studied by density functional theory (DFT) method, and the theoretical calculation results were consistent with the experimental results. Finally, the effects of pressure on the thermal stability of HFO-1234yf were discussed.

The inorganic salt hydrate PCM, CaCl2·6H2O, has a promising potential for thermal energy storage. However, this salt hydrate has tremendous issues of supercooling (the supercooling degree was 25.5 °C as found in this research) which restricts its utilization in practical applications. The present study aimed to reduce the supercooling degree of CaCl2·6H2O through the innovative use of hydrophilic graphene oxide (GO) nano-sheets and SrCl2·6H2O as nucleating agents. The percentages of these nucleating agents in PCM were 0.005, 0.01, 0.02, 0.05 and 0.08 wt% for GO and 0.2, 0.5 and 0.8 wt% for SrCl2·6H2O, respectively. Based on the synergistic/additive effects, a novel composite PCM based on CaCl2·6H2O, GO and SrCl2·6H2O was proposed and developed. The test results showed that the composite PCM containing 0.02 wt% GO nano-sheets and 0.8 wt% SrCl2·6H2O effectively lowered the supercooling degree of CaCl2·6H2O from 25.5 °C to as low as 0.2 °C (approximately 99.2% supercooling degree of CaCl2·6H2O was reduced). To the best of authors' knowledge, it was the first time that the combination of GO and SrCl2·6H2O was found effective in reducing supercooling degree of CaCl2·6H2O. Moreover, 99.2% supercooling reduction of CaCl2·6H2O is considered the best achievement so far when compared to other studies related to supercooling reduction of CaCl2·6H2O. Finally, the solidification enthalpy of the composite PCMs was found to be as high as 207.90 J/g. Therefore, the developed composite PCM is an excellent candidate for thermal energy storage applications in buildings.

The thermal decomposition of HFO-1234yf with O2 was studied by ReaxFF molecular dynamics simulation at the wide temperature ranges of 1900–3500 K. The results show that the initial reactions of HFO-1234yf pyrolysis including C C cleavage, C H cleavage and C F cleavage starts at 2100 K and then release free radicals ( CF3, F and H). The oxidation reaction starts at 2500 K and numerous intermediates (H2O, CF3OH and FCOOH) are produced. The main products observed from simulation are HF, COF2 and CO2 and these products agree with the result of related experiments. Ten formation pathways of primary products are observed from ReaxFF. The concentrations of free radicals (FCOO , F, H, HOO and OH) have a great impact on the reaction activity of HFO-1234yf oxidation. The chain transfer process is primarily carried by the conversion from HOO to OH radical. Our work presents the decomposition mechanisms of HFO-1234yf with O2 from the molecule level and provides a reference for the study of the thermal stability of organic working fluids.

Every country is dealing with its own housing problems; however, none compares with Hong Kong where housing has always been a major concern as a result of low supply over the past decades. Against the constraints in delivering sufficient houses, prefabrication as a sustainable solution for housing has been increasingly advocated for its potential merits of better quality, construction safety and cleaner built environment. However, schedule delay caused by various risks affected the prefabrication housing production (PHP) in Hong Kong. This problem can be further worsened when the manufacturing sector of PHP has entirely moved to offshore areas in the Pearl River Delta region. This study applies system dynamics to recognize and investigate the potential effect of various risks on the scheduling of prefabrication housing construction projects through the employment of Vensim software package. The simulation results show that schedule risks, namely low information interoperability between different enterprise resource planning systems (LIIBDERPS), logistics information inconsistency due to human errors probability (LIIHEP), Delay of delivery of precast element to site (DDPES), and Design information gap between designer and manufacturer (DIGDM) significantly contribute to the schedule delay in PHP. However, schedule is more sensitive toward LIIBDERPS than for the other three risks, indicating that LIIBDERPS should be monitored and given priority. The system dynamic model serves as an effective tool for quantitatively evaluating the effect of various risks on the schedule of PHP, offering valuable references for managers though comparing simulation results under different risk scenarios, so that potential risks that might lead to schedule delay could be identified and handled in advance.

The heat transfer characteristics of supercritical CO2 cooled in the helically coiled tube are investigated experimentally and numerically. The inner diameter, coil pitch and coil radius of the helically coiled tube are 4 mm, 34 mm and 36 mm, respectively. The effects of heat flux, pressure and mass flux on the heat transfer are analyzed based on 512 sets of experimental data, a new modified correlation is developed to calculate the heat transfer coefficients of supercritical CO2 in the cooled helically coiled tube. To study the buoyancy effect, the heat transfer coefficients of upward flow are compared to that for horizontal flow, and it is found that the effect of buoyancy is negligible in the liquid-like region, but significant in other region under the experimental conditions. On this basis, the three buoyancy parameters: Bup, Grth/Grq and Ri are applied to predict the effect of buoyancy on the heat transfer. The three parameters overestimate the impact of buoyancy. Numerical analysis of the cooling heat transfer coefficient in helically coiled tube is conducted by using shear-stress transport (SST) model. The flow fields are analyzed and the effect of heat flux is mainly related to the distribution of specific heat (cp) in the radial direction of tube transverse section.

The fully developed turbulent flow of supercritical CO2 in a heated helically coiled pipe is investigated by numerical simulation. The supercritical CO2 in the helically coiled tube is heated under constant heat flux with the inclination angles range from −90° to 90°. This work analyzes the effects of the heat flux, pressure, mass flux, flow direction, buoyancy force and centrifugal force on the convection heat transfer characteristics of supercritical CO2. Results show that the non-uniformity of the circumferential heat transfer coefficients is more evident with the decrease of the incline angle. When the helically coiled tube is oriented vertically, the heat transfer coefficient firstly increases and then decreases, exhibiting a maximum near the pseudo-critical temperature. When the helically coiled tube is positioned at horizontal or inclined angles, the heat transfer coefficient (HTC) oscillates dramatically as a result of the fact that the buoyancy force becomes a dominant factor compared to the centrifugal force.

The cost of heat exchangers account for a large proportion of total investment in organic Rankine cycle (ORC). In this paper, plate heat exchanger (P), shell-and-tube heat exchanger (S) and finned-tube heat exchanger (F) are used as evaporator and condenser of four subcritical ORC configurations: ORC-PP, ORC-SS, ORC-FP and ORC-FS. The thermo-economic models are built and a thermo-economic evaluation and comparison of four ORC configurations is presented in order to recover the low-temperature waste heat. The optimal evaporating pressure, pinch point temperature difference, net power output and dynamic payback period corresponding to the minimum electricity production cost (EPC) are obtained for different ORC configurations under different heat source temperatures. Results show that the EPCs of ORC-PP and ORC-SS are apparently higher than that of ORC-FP and ORC-FS. Among them, ORC-FS is the most cost-effective configuration. The optimal pinch point temperature difference in evaporator has a decreasing trend with the increase of critical temperature of working fluid for ORC-FS and ORC-FP, while the optimal pinch point temperature difference in condenser keeps nearly constant.

Twenty-two chemical reaction pathways of thermal decomposition of 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf, CF 3 CF CH 2 ) are proposed to investigate the formation mechanism of some possible products (CF 3 H, CF 4 , HF, H 2 ) by using density function theory (DFT) simulations with M06-2X/6–311++(d,p) level of theory. The results point out that the ground state CF 3 CF CH 2 will excite into the lowest triplet state CF 3 CF-CH 2 favorably in the first step with an energy barrier of 264.67 kJ mol −1 and pathway 5 is the most preferred route of homolytic cleavage reactions with the lowest energy barrier of 205.70 kJ mol -1 . F radical is hard to generate during thermal decomposition processes because of its higher energy barrier. H radical and CF 3 radical play a dominant role in thermal decomposition of HFO-1234yf. H-abstraction and F-abstraction reactions are proposed in subsequent radical attacking chain reactions. CF 3 H and H 2 are easier to be generated due to their lower energy barriers. Our work presents the mechanism of thermal decomposition of HFO-1234yf from the molecule level and provides a reference for studying the thermal stability of other working fluids.

Zero-shot learning has been a tough problem since no labeled data is available for unseen classes during training, especially for classes with low similarity. In this situation, transferring from seen classes to unseen classes is extremely hard. To tackle this problem, in this paper we propose a self-training based method to efficiently leverage unlabeled data. Traditional self-training methods use fixed heuristics to select instances from unlabeled data, whose performance varies among different datasets. We propose a reinforcement learning framework to learn data selection strategy automatically and provide more reliable selection. Experimental results on both benchmarks and a real-world e-commerce dataset show that our approach significantly outperforms previous methods in zero-shot text classification

This proposes a novel dual-ejector based organic flash combined power and refrigeration cycle, which replaces the two throttle valves with two ejectors for the basic flash cycle, to provide power and cooling simultaneously for users. Detailed mathematical models of the proposed system are built and validated. The preliminary analysis results show that the exergy efficiency reaches 45.59% under geothermal water at 150℃. Then a parametric analysis is conducted to investigate the effects of five key parameters on system performance. The results show that both an optimal flash pressure and an extraction pressure exist to maximize the exergy efficiency. Finally, the proposed cycle, the separated power and refrigeration cycle consisting of a basic organic flash cycle and an ejector refrigeration cycle, and the basic organic flash cycle are optimized and compared by examining seven different organic fluids such as R245fa, R141b, R123, R245ca, R601, R365mfc, and R600. The results show that the R245fa brings the highest exergy efficiency to both the proposed cycle and organic flash cycle among seven involved organic fluids. Compared with the basic organic flash cycle, the proposed cycle with R245fa has a 4.91% percentage point higher exergy efficiency, 10.44% higher net power output, and an extra 172.6 kW of refrigeration output. Meanwhile, the optimal exergy efficiency and net power output of the proposed cycle are 5.82–6.80% percentage point higher and 15.06–23.87% higher than those of the separated power and refrigeration cycle. An exergy analysis suggests that the throttling losses are indeed significantly reduced by replacing the throttle valves with the ejectors for the organic flash cycle.

The mixed turbulent convection heat-transfer characteristics of supercritical CO2 (S-CO2) heated in both vertical and horizontal oriented helically coiled tube (HC-tube) is investigated via experimental and numerical methods. Firstly, the coupled equations of flow direction, buoyancy force and centrifugal force in HC-tube are established. Then, the influence of the three factors on axial and radial heat transfer coefficient (HTC) performance are discussed. In vertical oriented HC-tube, the HTC first increases progressively and then drop swiftly, presenting a peak value at the pseudo-critical point. But, in horizontal oriented HC-tube, due to buoyancy force effects, the HTC oscillates dramatically which indicates the poor heat-transfer stability. Meanwhile, the numerical simulation, corresponding to the experiment condition, is carried out. To capture the flow structures, 6 characteristic lines on each cross-section are selected and the corresponding velocity and the turbulent kinetic energy distributions are comparative analyzed. The results showed that the coupling effects of centrifugal force and buoyancy force mainly affect heat-transfer on the inside of a cross-section. In addition, different from the M-type velocity in straight tube, the salient point of M-type velocity in HC-tube always locate at the outer side of the buffer-layer and the heat-transfer is slightly affected. The local heat-transfer deterioration mainly occurs at inside of a cross section where the velocity is obviously suppressed.

The prediction of building energy consumption plays a crucial role in building energy management and conservation because it contributes to effective building operation, energy efficiency evaluation, fault detection and diagnosis, and demand side management. Although a large number of energy prediction methods have been proposed, each method has its pros and cons and still exhibits the potential to be improved. This study proposes an evolutionary double attention-based long short- term memory model and introduces binary features by using feature combination. The proposed model is adopted to analyse the building energy consumption data of a green building in Shenzhen, China. The prediction performance of the proposed hybrid model measured via root-mean-square-error and mean absolute error are 4.02 and 2.87 respectively, which are evidently better than those of the base models. Results also show that an attention mechanism can improve the efficiency of the long short-term memory algorithm with which the model uses the input time series data. Meanwhile, binary features exert a significant effect on energy consumption. The proposed model is valuable to researchers and practitioners. It helps researchers apply artificial intelligence-based methods to building energy prediction from the perspective of paying selective attention to input data. Practitioners will benefit from developing accurate diagnosis of building energy efficiency and decision support for building retrofitting.

Energy conservation in the building sector is a major target worldwide, and individual’s energy conservation behavior is considered a critical means to meet energy conservation goals. Although individual’s energy conservation behavior has been explored, its formation mechanism in the workplace remains unclear. This study aims to investigate the effect of the determinants of energy conservation behavior in the workplace based on the extended theory of planned behavior model. The novelty of this study is that it extends the theory of planned behavior with the consideration of actual behavior controls. This study is also one of the first studies to identify the effect of user experience on energy saving behavior. Questionnaire survey was used for data collection, and structural equation modeling was applied to develop the structural model. Results show that individual’s comfort is the most critical impact factor, followed by user experience and supervision. Intention has a limited effect on energy saving behavior. Moreover, three strategies were proposed to promote energy conservation behavior in the workplace from the perspectives of design, supervision, and punishment. These findings will not only help researchers in conducting further in-depth research on the mechanism of individual’s energy conservation behavior but also practitioners in proposing other effective strategies to promote energy conservation behavior.

Energy-related stakeholders play a critical role in energy-saving and emission reduction. However, most previous studies focused on only a few stakeholders (e.g., occupant, designer and owner). Therefore, there is a lack of systematic analysis of energy-related stakeholders. Improving building energy efficiency requires the collaboration of all stakeholders in the building lifecycle. This study applies a two-mode social network model to investigate the stakeholders’ power over the impact issues of building energy efficiency. Results show that all issues can be addressed by at least two stakeholders, thereby indicating the potential of collaboration in building energy management. Designers, manufacturers, commissioning agents and researchers are the most influential stakeholders with the highest degree centrality, betweenness centrality and eigenvector centrality. Insufficient knowledge and experience, lack of information integrity and unclear responsibility are the most influential impact issues. With the density of interaction being 1.000, nine core stakeholders (i.e. owner, designer, contractor, subcontractor, supervisor, manufacturer, commissioning agent, energy manager and researcher) and four core issues (i.e. unclear responsibility, poor collaboration and communication, insufficient knowledge and experience, and lack of information integrity) have intensive relationships. The core stakeholders account for 75% of the total stakeholders, whereas the core issues account for 25% of the total issues. Moreover, five strategies that are useful in promoting collaboration are proposed and validated. After the proposed strategies are implemented, the density of the collaboration network increased from 1.242 to 2.652. This study is not only expected to help researchers in understanding the complex interrelationships between stakeholders and issues but also practitioners in promoting effective collaboration strategies.

For the advantages of high efficiency and low impact to the environment, CO2 air source heat pump water heater (ASHPWH) is applied to produce domestic water, which also reveals good potential in cold regions. In order to boost the system performance and practicability under low ambient temperature, optimization for CO2 ASHPWH is conducted using non-dominated sorting genetic algorithm (NSGA-II). A validated artificial neural network (ANN) predicts energy parameters for the optimization. And an economic model provides economic and environmental parameters, which considers the influence of housing price, tank volume, and on/off-peak electricity price, rarely taken into account in published studies. Then the optimizing progress is conducted under −20 °C ambient temperature and 9–65 °C water temperature, in which four optimized variables are selected: gas cooler outlet temperature (Tgc), heat rejection pressure (Pgc), compressor displacement (qvh) and water tank volume (Vwt). The final solution of Tgc = 15 °C, Pgc = 8294.1 kPa, Vwt = 0.3647 m3, qvh = 401.33 mL/s results in two objectives (CO2 emission and total annual cost) of 8599.4 kg and 1626.9 $/year, revealing advantages both in energy and economy. It is noteworthy that the cost of the space occupied by system is the fourth important factor in capital cost. These results lay solid foundation for further studies and system application.

An experimental investigation on the heat transfer characteristics of supercritical CO2 during gas cooling process in a helically coiled tube is conducted. The experimental data are obtained over a mass flux range of 79.6–238.7 kg m−2 s−1, an inlet pressure range of 7.5–9.0 MPa and a mean bulk temperature of 23.0–53.0 °C. The effects of mass flux, bulk temperature and pressure on the heat transfer coefficient for helically coiled tubes are investigated. A comparative analysis of the gravitational buoyancy and the heat transfer coefficient is carried out between helically coiled tubes and straight tubes. A new heat transfer correlation of the supercritical CO2 in the horizontal helically coiled tube is proposed based on the experimental data. The maximum error between the predicted results of the new correlation and the experimental data is 20%.

The trans-critical CO2 Rankine cycle is a promising low-grade waste heat recovery technology. The research about the heat transfer deterioration of supercritical CO2 in tube has been attracted more and more attentions recently, but there is not much attention that it produces the flow instability of supercritical CO2 under a certain range of working conditions as well in the trans-critical CO2 Rankine system. The present experimental work is aimed at the investigation of flow instability of supercritical CO2 in vertically upward flow tube. The experiments are conducted at the conditions of the inlet temperature from 10 to 20 °C, the inlet pressure from 7.5 to 8.5 MPa, the mass flux from 315 to 477 kg/m2·s and the heat flux from 59.95 to 90.05 kW/m2. The influences of the operating parameters on the heat transfer deterioration and the flow instability are discussed. For the upward flow, it is observed that the flow instability occurs with the continued increasing of the heat flux after the heat transfer deterioration. Moreover, the flow instability occurs in the transition from the heat transfer deterioration to the heat transfer recovery, and the flow remains stability after heat transfer recovery with the increasing of the heat flux, pressure, inlet temperature or mass flux. Compared with the upward flow, the downward flow is stable without any oscillation. It follows from analysis that the oscillation in the upward flow tube results from the buoyancy effect with the variation of heat transfer mode. The experimental results also show that the periodical oscillation of local temperature occurs within the region where the density variation is greatest. The growth and collapse of the fluid particle between the near-wall flow and the core flow makes the volumes of the fluid vary greatly, resulting in the occurrence of oscillation phenomenon.

The building sector is responsible for 32% of global energy consumption and 19% of all energy-related greenhouse gas emissions. The urgent requirement for energy conservation and greenhouse gas emission reduction in the building sector has been recognised at the highest level of governments around the world. One potential solution, which has yet to be critically considered, is the application of Building Information Modeling (BIM) to overcome building energy performance gap (BEPG), defined as the discrepancy between the designed and actual energy consumption in buildings. This study performs a systematic and comprehensive literature review to identify the specific causes of the BEPG, and then analyses the application of BIM for addressing the BEPG. A life-cycle BIM engaged framework was developed, including the function of “information exchange”, “design review”, “energy-related quality control”, “life-cycle commissioning”, and “real-time operation and maintenance management”. It is expected that the proposed framework will assist researchers and practitioners better understand application of BIM to systematically improve building energy performance.

A novel combined cooling and power (CCP) system integrating a supercritical carbon dioxide recompression Brayton cycle with an ejector transcritical carbon dioxide refrigeration cycle (E-TCRC) is proposed to realize the effective utilization of nuclear power. In the proposed system, a portion of CO2 exiting the pre-cooler is used to drive the E-TCRC for generating cooling and recovering partial waste heat of sCO2 turbine exhaust. The mathematical model and economic model of the proposed system are established under steady-state conditions. Besides, the exergy efficiency and total product unit cost of the system are selected as the main criteria to evaluate system performance. Parametric analysis is applied to study the effects of four key parameters on the system performance. The CCP system, conventional separated cooling and power (C-SCP) system and ejector separated cooling and power (E-SCP) system are optimized by single-objective and multi-objective optimization. Single-objective optimization reveals that the exergy efficiencies of the CCP system are up to 1.08%pt (percentage point), 0.80%pt and 0.47%pt higher than those of the C-SCP system at the corresponding evaporation temperatures (−20 °C, −10 °C and 0 °C). Besides, the CCP system performs better than the E-SCP system at lower turbine inlet pressures. The multi-objective optimization shows that when the evaporation temperature increases from −20 °C to 0 °C, the total product unit cost of the CCP system decreases from 10.087 $/GJ to 9.668 $/GJ, and exergy efficiency increases from 59.25% to 60.97%.

The organic flash cycle is an efficient low-grade energy conversion technology. However, a large space is left for the organic flash cycle to improve the useful energy output due to the throttling processes. To utilize the thermal energy before the low-pressure throttling process in the organic flash cycle driven by the geothermal energy, an ammonia-water absorption refrigeration cycle is adopted to generate cooling by using this part of heat, thereby forming a novel geothermal combined cooling and power (Geo-CCP) system. Eight different organic flash cycle working fluids are investigated, including n-Nonane, n-Octane, n-Heptane, Cyclopentane, i-Pentane, n-Pentane, R365mfc, and R245fa. Detailed thermodynamic modeling, thermoeconomic analysis, parametric analysis, system optimization, and exergy analysis are carried out for the proposed system. The results show that the total product unit cost and exergy efficiency of the proposed systems are 6.52–11.03% lower and 4.10–5.31%pt (percentage point) higher than those of the geothermal separate cooling and power systems. Among the eight considered organic fluids, the n-Nonane brings the lowest total product unit cost and highest exergy efficiency to the Geo-CCP system (11.20 $·GJ−1 and 37.58%). Exergy analysis indicates that the flasher and condenser-I have the first and second highest exergy destructions.

The mixed turbulent heat transfer characteristics of supercritical CO2 heated in a straight tube of diameter d = 4 mm are experimentally and numerically investigated at a pressure p = 8 MPa, mass flux G = 278 kg/(m2s), heat flux q = 15–35 kW/m2, and inlet Reynolds number Rein ≥ 14000. To identify the key factor affecting the heat transfer deterioration (HTD), the flow structures at different conditions are comparatively analyzed. The results show that the HTD accompanying abrupt increase in the wall temperature is mainly caused by buoyancy force rather than thermal acceleration. The M-type velocity profiles are observed at the initial, stable, and recovery stages of the HTD. The location of the salient point of the M-type velocity plays a major role in the different regions of the HTD. Similar to the salient point of the M-shaped velocity, the location at which the local fluid temperature Tb(r) is equal to the pseudo-critical temperature Tpc can also be an effective criterion for predicting the onset of HTD. Both the salient point of the M-shaped velocity and the location of Tb(r) = Tpc demonstrate that the flow structure at the buffer layer (y+ = 5–30) is the dominant factor that influences the onset of HTD. An analysis of the thermal resistance in cross section also confirms that the buffer layer is the dominant region for the HTD. Based on the analysis of the flow states at the buffer layer, a new correlation model is developed for accurately predicting the heat transfer coefficient of supercritical CO2 under the HTD condition.

The non-azeotropic natural mixture, propane/CO2, in trans-critical Rankine cycle shows some special advantages in operation safe, thermodynamic performance and exergo-economic performance. The improvement of the system performance depends on further in-depth investigation on the heat transfer of propane/CO2 mixture above critical pressure which has not been performed yet. In this paper, the heat transfer behaviors of CO2, propane and their non-azeotropic mixtures in helically coiled tube are investigated and compared by experiments. The mass fractions of the mixtures are measured by Fourier Transform Infrared Spectrometer. The effects of operating parameters under various mass fractions on heat transfer are discussed. The general law of mass fraction on heat transfer is got under the same pressure ratio (P/Pcr) by experimental results. The peak of heat transfer coefficient increases as propane increases when the mass fraction of propane changes from 22.24% to 100%. The heat transfer coefficient of the mixtures has little variations with pressure because the specific heat of the mixtures has a slight decrease as pressure increases. The complicated heat transfer characteristics of the propane/CO2 mixtures are analyzed based on the buoyancy force and the thermal effect in heat surface. In the helically coiled tube, the heat transfer of the mixture is mainly dominated by curvature effect and the effect of the low-density in boundary layer becomes smaller and smaller as propane increases. The experimental data are predicted by a correlation. The results show that for various compositions, 80% of the data are predicted within ±20%, and 95% are within ±30%.

The gap between the designed and measured building energy consumption is one of the biggest obstacles to the realization of energy conservation goal. The factor analysis method has been typically used in previous studies to explore the individual factors affecting building energy performance gap, while few studies examined the complex interrelationships among the influencing factors. In this research, the interpretive structural modelling approach was developed and applied to identify the key factors and explore the interrelationships among the factors affecting building energy performance gap. This paper first identified 47 factors by reviewing 276 relevant articles. Sixteen representative factors were further identified by interviewing 12 interviewees with 7 to 25 years’ practical experience on building energy management. Second, a six-level hierarchical structure was developed based on interpretive structural modelling to illustrate the intricate interrelationships among the representative factors. Third, “Matrice d’Impacts croises-multipication applique a classement” was applied to demonstrate the driving force and dependence power of the representative factors. Four clusters of factors were identified, namely dependent factors, driving factors, autonomous factors and linkage factors. Fifth and finally, 10 strategies were proposed to address these factors and their interrelationships. The findings of this research will help not only researchers in understanding the factors and their hierarchical structures affecting BEPG but also policymakers and practitioners in understanding the priority of allocating limited resources to address these factors and issues.

The Guangdong–Hong Kong–Macao Greater Bay Area (GBA) was recently proposed by the Chinese government for socioeconomic–political–environmental development. As one of the most important parts for the development of the GBA, the transportation sector has become a nonnegligible consumer of energy and source of emission. Therefore, vehicle energy consumption and related emissions in the GBA must be evaluated. However, an effective framework for accurately estimating the real-time transportation performance, vehicle energy consumption and emission in large urban areas is still lacking. This study aims to develop a novel method for examining the regional integration and the spatial connection that affect vehicle emission via crowdsourced traffic data and an emission model. The novelty of this study is that it explores the acquisition of large-scale and real-time data as well as the calculation of corresponding energy consumption and environmental impact. Based on the analysis of transportation in GBA, it is found that the vehicle energy consumption and emission on the expressway are nearly half as that on the other urban roads. Moreover, a significant difference exists in vehicle energy consumption and emission between passenger cars and medium-duty trucks. Regarding transportation performance, vehicle energy consumption and emissions, they are closely related to departure time. It is also found that most of the adjacent cities tend to have high vehicle energy consumption and emission. The contribution of the Hong Kong–Zhuhai–Macao Bridge to transportation from Hong Kong, Macao and Zhuhai is different. Hong Kong benefits most from the bridge in terms of traffic energy efficiency. This study would be valuable to both researchers and practitioners. It helps researchers apply a large dataset in replicating real-world travel, energy consumption and emission pattern at a large scale. Policymakers and practitioners would benefit from developing effective strategies for the sustainable development of the GBA.