The thermal comfort in subway carriages directly affects passengers’ travel experience and health, while optimizing it enhances energy efficiency and reduces operational costs. The overarching goal of this research is to develop a novel, metro-specific thermal comfort index that overcomes the predictive limitations of existing indices and provides a practical tool for evaluating and optimizing thermal environments in underground railway systems. To achieve this goal, this study investigated passengers’ thermal comfort across Nanjing’s six busiest metro lines through on-site thermal-humidity measurements and subjective questionnaires, yielding 1067 valid datasets. The database covers parameter ranges of air temperature (20.27°C–31.63°C), mean radiant temperature (7.92°C–35.47°C), relative humidity (45.4%–90.1%), and air velocity (0.06–3.74 m/s), with both environmental data and subjective votes exhibiting approximately normally distributed. Analysis revealed significant limitations in existing thermal comfort indices, evidenced by a 17.8% mean absolute deviation (MAD) for the best-performing existing model. To address this deficiency, dimensional analysis and least squares regression established mathematical relationships between environmental parameters and passengers’ thermal comfort responses, deriving a novel metro-specific thermal comfort index. The proposed index comprises three dimensionless parameters: relative humidity, mean radiant temperature to air temperature ratio, and air velocity water vapor partial pressure product to metabolic rate ratio, accounting for key comfort determinants. Demonstrating high predictive accuracy, it achieves a 13.2% MAD across the entire database with 88.2% predictions maintaining errors within ±30%, while showing a systematic 2.4% overprediction tendency. This empirically validated correlation provides valuable references for optimizing metro environmental design and operational strategies.

The introduction of lubricant significantly changes the heat transfer and flow characteristics of CO2 two-phase flow system. Due to the significant differences in physical parameters (viscosity, surface tension) between the lubricant and CO2, its mixing not only changes the dynamic behavior of the gas-liquid interface, but also affects the oil-phase distribution morphology in different flow patterns, which in turn has an important impact on the heat transfer efficiency and flow stability in the evaporator. To obtain the influence mechanism of lubricant on the evolution of CO2 flow pattern, a visualization system is designed to visualize the flow boiling pattern of CO2/ lubricant mixture in a 5 mm horizontal smooth tube under diabatic conditions by using a secondary fluid (water) heating method. The effects of important parameters on the flow pattern characteristics and flow pattern distribution are analyzed, and the experimental results are compared with the conventional flow pattern maps. The results show that there is a large error in describing the flow pattern evolution of the refrigerant/lubricant system using the flow pattern map of a single-component refrigerant. Based on the dynamic characteristics of the flow pattern evolution and the changes of thermophysical properties of the mixtures, the effects of gravity, surface tension, viscous force, inertial force, and local oil concentration on the flow pattern distribution are comprehensively considered, and four dimensionless numbers, including the liquid-phase Froude number Frl, the gas-phase Reynolds number Rev, the local oil concentration omega local, and the liquid-phase Weber number Wel, are introduced, so as to establish a conversion criterion from the intermittent flow to the annular flow for the CO2/ lubricant system.

The recycling and utilization of phosphorus resources in sludge is becoming increasingly important. In this study, we compared the conversion of phosphorus and toxic metal passivation effects of different Ca additives under oxygen-rich combustion conditions and elucidated their specific mechanisms of action. The experimental results indicated that four Ca-based additives improved the recovery rate of total phosphorus, and promoted the generation of stable apatite phosphorus (AP). The effect of CaCl2 and CaO was greater than that of Ca(OH)2 and CaSO4. CaCl2 promoted the formation of Ca3(PO4)2 and Ca2P2O7, and CaSO4 improved the conversion of AlPO4 to Ca(H2PO4)2 with increasing temperature. The conversion capacity of CaO on non-apatite inorganic phosphorus to AP was greater than that of Ca(OH)2, and more CaH2P2O7, Ca(PO3)2, and Ca-Al-P minerals were found. Toxic metal percentages decreased after sludge incineration with CaCl2. Compared with CaO and Ca(OH)2, the toxic metal adsorption effect of CaSO4 was more significant. The influence of Ca additives on the conversion of Zn into stable components was as follows: CaCl2 > Ca(OH)2 > CaO > CaSO4. Ca additives reduced the toxic metal contamination level and ecological risk index values, and the order of toxic metal contamination levels was Ni > Zn > Cr > Cu > Mn. The experiment confirmed the conversion of phosphorus and the toxic metal passivation effect of Ca additives during oxy-fuel combustion of sludge, which is beneficial for its resource utilization.

While jet impingement is recognized for significantly enhancing local convective heat transfer, there remains a considerable challenge in further improving its heat transfer performance and uniformity to meet the growing demand for efficient heat transfer. To address this challenge, this study proposes a method that integrates a piezoelectric fan into a continuous jet, taking advantage of the piezoelectric fan's vibration to deliver pulsating excitation to jet impingement while preserving its continuity. An experimental study is performed to identify the roles of piezo-fan integration on improving convective heat transfer from a continuous circular jet onto a flat target surface. The tests are performed under Rej=2,500∼7,500 (circular jet), s/d = 2∼28 (non-dimensional fan-to-jet distance) and L/d = 2∼8 (non-dimensional jet-to-target distance). In addition, few numerical simulations regarding the flow fields are also made to illustrate the mutual interaction mechanism between piezo-fan and continuous jet. The findings reveal that, in the longitudinal configuration, the average values of heat transfer enhancement factor (χ) and heat transfer uniformity factor (ξ) are approximately 13% and 0.5% higher, respectively, compared to the transverse configuration. In the longitudinal arrangement, the significance of the piezoelectric fan amplifies as the circular nozzle gradually distances itself from the target surface. This fan assumes a pivotal role in augmenting both the uniformity and efficiency of heat transfer associated with the circular jet. Conversely, when the piezoelectric fan moves beyond a specific s/d value (s/d = 10), the interaction between intermittent flow and the wall-jet experiences complete cessation. Despite the existence of a fountain effect resulting from the interaction between the piezoelectric fan and the circular jet in the longitudinal configuration, its discernible impact on overall heat transfer attenuation is minimal. The insights gained from this study will be valuable in improving active strategies for jet impingement and promoting the development of piezoelectric functional devices.

In this paper, the thermal performance of the corrugated surfaces cooled with a piezoelectric fan was studied. A total of 25 corrugated surfaces with different wave shapes have been tested under different Re and gap heights ( G). The results showed that only the corrugated surface with the A/ W PF of 0.167 and T/ W PF of 0.533 produces the highest Nu area values for all conditions, which is approximately 50% higher than that of a flat surface. It is confirmed that the corrugated shape is not only conducive to promoting the development of the vortex structure but also beneficial to increase the effective heat transfer area so that the overall heat transfer is optimal. An empirical correlation for estimating Nu area on the hot corrugated surfaces was developed which has a mean absolute deviation of 11.0% and a mean relative deviation of 0.2%, predicting 75.9% of the entire database within a ±20% band. In addition, a corresponding simulation on the flow field revealed that the gap distance between adjacent wave crests is the key to balance the increasing effective heat transfer area and promoting the development of the vortex structure.

Since the critical temperature of CO2 is low and the working pressure is high, cycles using CO2 as a refrigerant are generally transcritical. Considering the large throttling loss and low system performance of the existing tran-scritical CO2 cycles, this paper proposes a general parameter matching method for transcritical CO2 cycles with vortex tube by using the self-defined functional blocks that can easily and quickly realize the parameter matching of components in refrigeration cycles with diverse architectures. Based on this new method, simulation and optimization of existing transcritical CO2 systems with or without vortex tube are implemented. A two-stage compression transcritical CO2 system with serial vortex tubes is proposed and proven to have a high cooling performance and less exergy loss in comparison with existing transcritical CO2 systems. Furthermore, the number of vortex tube(s) used and the coupling mode of vortex tube(s) with the transcritical CO2 system are also dis-cussed. The results show that for the transcritical system with evaporators greater than two, the performance of the system with parallel vortex tubes is better than that of serial vortex tubes. When there are two evaporators and vortex tubes with a cold mass rate greater than 0.3, the performance of the system with serial vortex tubes is better than that of parallel vortex tubes. Finally, exergy analysis of the improved and existing transcritical CO2 cycles is conducted which shows that the exergy loss caused by the throttle valve is considerably reduced by using the vortex tube.

This paper proposes a new air cycle system by replacing the turbine assembly with vortex tubes and a parameter-matching method independent of the architecture of the air cycle system. Only, in terms of refrigerating and dehumidification, the performance of the newly proposed vortex system is worse than that of a conventional air cycle system under the same bleed air parameters. However, for aircraft practical applications, the overall performance of air cycle systems requires additional considerations such as the heating capacity, weight of the system, and extra aviation fuel used to transport that weight. Therefore, this paper conducts an integrated performance comparison between the new system and a conventional system based on the overall fuel mass penalty criterion. The results show that whether the new system has a smaller total takeoff mass depends on the bleed air parameters, flight duration, and flight speed. For specific bleed air parameters, there is a specific [Formula: see text] curve where the new system and the turbine system have the same total takeoff mass. The performance of the new system is better than that of the turbine system only when the values of flight Mach number and flight duration are at the lower left of the [Formula: see text] curve.

采用现场实测和问卷调研的方式对地铁车厢的热环境参数及乘员的主观热感觉进行了调研,分析了车厢热环境参数对地铁乘员热舒适的影响.引入二节点模型对地铁空调车厢乘员的核心温度和皮肤温度进行建模,采用有限差分法在时间单元内对耦合的非线性常微分方程组进行离散并利用MATLAB软件编程迭代求解.将实测的 1 070 组实验数据与计算结果与进行对比.结果发现,二节点模型对地铁空调车厢人体温度的预测精度较高,可用于分析在车厢环境下的人体温度影响因素.人体特征与空调车厢热环境参数共同决定了人体温度的变化,且各因素的影响能力由大至小分别为环境温度、平均辐射温度、人体代谢率、服装热阻、相对湿度和风速.计算方法和主要结论可为地铁空调车厢乘员热舒适研究提供理论参考.

Vortex tube is widely used as a cooling system in manufacturing, gas separation devices and biological device, etc., for it has the advantages of simple structure, low-cost, lightweight, and no moveable parts. This paper is aimed to study the influence of the parameters related to the temperature separation characteristics of several types of vortex tubes through the flow field structure obtained by an experimentally proven numerical method. The results show that the compressed air passes through the vortex chamber tangentially to generate a high-speed swirl flow, which creates a large temperature gradient in both axial and radial directions of the vortex tubes, especially near the vortex chamber. Five dimensionless parameters concerning the structure and operating conditions are characterized as the prime influential factors in the cooling performance of the vortex tubes, and a promising dimensionless number combination is obtained as mu = 0.3, lambda = 3, SD = 0.038, Pe = 7.91, and LD = 12.7. Further sensitivity analysis of the five dimensionless numbers indicates that the cooling performance of the vortex tubes is mainly affected by the values of mu, lambda and SD. Consequently, the dimensionless number combination of mu, lambda and SD is adopted to develop correlations of the cold end exit temperature and isentropic temperature efficiency of the vortex tubes to balance the simplicity and predictive power of the formula. The evaluation of the prediction performance shows that the two correlations have a high precision for the cooling performance prediction with 77.8 % and 76.0 % of all the data predicted within +/- 10 % error band, respectively.

The thermal comfort of passengers in the carriage cannot be ignored.Thus, this research aims to establish a prediction model for the thermal comfort of the internal environment of a subway car and find the optimal input combination in establishing the prediction model of the predicted mean vote (PMV) index.Data-driven modeling utilizes data from experiments and questionnaires conducted in Nanjing Metro.Support vector machine (SVM), decision tree (DT), random forest (RF), and logistic regression (LR) were used to build four models.This research aims to select the most appropriate input variables for the predictive model.All possible combinations of 11 input variables were used to determine the most accurate model, with variable selection for each model comprising 102 350 iterations.In the PMV prediction, the RF model was the best when using the correlation coefficients square (R 2 / as the evaluation indicator (R 2 : 0.7680, mean squared error (MSE): 0.2868).The variables include clothing temperature (CT), convective heat transfer coefficient between the surface of the human body and the environment (CHTC), black bulb temperature (BBT), and thermal resistance of clothes (TROC).The RF model with MSE as the evaluation index also had the highest accuracy (R 2 : 0.7676, MSE: 0.2836).The variables include clothing surface area coefficient (CSAC), CT, BBT, and air velocity (AV).The results show that the RF model can efficiently predict the PMV of the subway car environment.

This research developed an intelligent ensemble machine learning prediction model for the thermal comfort of passengers inside the compartment of the subway. Data sources used for data-driven modelling were obtained from on-site measurements and passengers’ questionnaires in the compartments of the Nanjing subway. The four models were established using methodologies of Logistic Regression (LR), Random Forest (RF), Support Vector Machine (SVM) and Decision Tree (DT) in machine learning, respectively. The performance of the RF method was compared with DT, LR and SVM in terms of conventional statistical metrics, namely, Mean Squared Error (MSE), Root Mean Square Error (RMSE) and Correlation Coefficients squares (R 2 ). Thermal Sensation Vote with the seven-level indicator (TSV-7) and Thermal Sensation Vote with the three-level indicator (TSV-3) were employed to obtain passengers’ thermal comfort and evaluate the models’ predictions. In this study, the R 2 value of the RF model is 0.6527 and 0.6607 for TSV-7 and TSV-3, which shows higher accuracy than DT, LR and SVM models in predicting the two kinds of Thermal Sensation Vote (TSV). The results show that the predictive performance of the proposed RF model is outstanding, and it can predict the TSV value of passengers inside the compartment of the subway more efficiently.

Refrigeration using thermoelectric effect is a hot spot and frontier issue at present for its advantages of simple structure, no refrigerant, and no moving parts. This paper aims at characterizing the natural convection characteristics of air in the molding chamber of a 3-D printer that is cooled by thermoelectric cooling modules (TECMs). The distributions of the temperature and velocity field in the molding chamber are obtained, and the influences of the number of the TECMs and their arrangement modes on the magnitude and the spatial difference of temperature and velocity are analyzed. It is found that the Case 2 arrangement mode that all the TECMs are located on the top of the chamber is characterized as the optimal mode with the lowest final average air temperature and the smallest temperature non-uniformity coefficient of the molding chamber. Based on the optimal arrangement mode, a computational formula that correlates the final average air temperature of molding chamber with the volume of the molding chamber and the number of the TECMs is proposed. The new correlation can reproduce all of the 104 data points within error band of +/- 1.0 degrees C, among which 76.9% of the data can be predicted within error band of +/- 0.5 degrees C. The conclusions of this paper can provide a reference for the application of 3-D printing technology and the design of temperature control scheme of low-temperature molding chamber.

A numerical investigation is performed to explore the thermal-fluid characteristics of corrugated surfaces with different wave shapes cooled by a vibrating piezoelectric fan. Totally-nine different corrugated surfaces and one baseline flat surface are taken into consideration. Meanwhile, two non-dimensional parameters (A* and T*, representing the amplitude and period of the waves, respectively) are defined to describe the geometric characteristics of these corrugated surfaces quantitatively. Then the effects of A* and T* on flow heat transfer characteristic are investigated in detail. Time-averaged Nusselt number and instantaneous velocity vector are calculated. It is confirmed that the time-averaged Nusselt number distributions on corrugated surfaces is primarily determined by its geometrical shape, and the minimum and maximum values usually appear at the trough and peak, respectively. Compared to that of the flat surface, the corrugated surfaces could trigger the velocity boundary layer separation more easily, and the size of the corrugation directly limits the scale of the counter-rotating vortices. Of all these corrugated surfaces we studied, only the one with the smallest A* of 0.0625 and the largest T* of 2.0 is demonstrated advantageously. It is revealed that the wave shape influences the flow and heat transfer performance substantially.

Nitrogen has been widely used as an expendable coolant in aviation and aerospace fields to cool the high-temperature components in a thermal management system. This paper presents a concept of a thermal management system for hypersonic vehicles that integrates various cooling technologies using supercritical nitrogen as heat sink. Since the thermophysical properties of supercritical nitrogen vary acutely with temperature, existing heat transfer empirical models fail to accurately evaluate the heat transfer coefficient of supercritical nitrogen during the parameter design of the thermal management system. Therefore, a new correlation specialized for supercritical nitrogen is proposed relying on a heat transfer experimental database including 784 data points compiled from seven published papers and the computer optimized dimensionless groups, which commendably solve the considerable deviation caused by the thermophysical property distortion and buoyancy effect. The new correlation has a mean absolute deviation (MAD) of 3.9% and improves the prediction accuracy of supercritical nitrogen heat transfer calculation remarkably by predicting 94.9% of the entire database within ±10% error band, based on which the heat transfer design method of the integrated thermal management system is discussed.

A transient numerical simulation was carried out to investigate the convection heat transfer enhancement of heated asymmetrical concave surfaces using multi piezoelectric fans. And the numerical methodology has been validated through the comparison with the results of validation tests. The effects of the relative curvature (RK), dimensionless fan-to-fan pitch (P/W-PF), vibration phase difference between adjacent piezoelectric fans (phi) and the dimensionless offset distance of multi-fans (Delta y/App) on flow and heat transfer performance were reported. RK was varied from 6 to 2. P/W-PF was varied from 0.25 to 1.0. The value of phi was 0 degrees and 180 degrees. Delta y/App was varied from 0 to 1.0. The heat transfer of the heated surface was characterized by evaluating a time-averaged convection heat transfer coefficient over a complete vibration cycle of the multi-piezoelectric fans after the flow and thermal fields reached a quasi-steady state. In addition, transient and time-averaged flow fields were captured to study the underlying mechanism of the heat transfer enhancement. It was found that higher heat transfer coefficients were observed in stagnation zone with small P/W-PF at smaller RK value of asymmetrical concave surfaces whereas the P/W-PF is observed to affect the data only marginally at larger RK values. The influence of phi on the heat transfer coefficients was primarily reflected in the clearance region of adjacent fans, where operating in-phase could produce stronger flow in clearances than out-of-phase. Finally, it was also observed that proper adjustment of the fan position can reduce its equivalent relative curvature, resulting in improvement in the heat performance of the asymmetrical concave surface with large RK value.

The present work aims at numerically investigating on the flow and heat transfer performances induced by vibrating piezoelectric fan for cooling heated concave surfaces. Two main relevant parameters, relative curvature (K) of concave surface and moving-orientation of the piezoelectric fan, are taken into consideration. The relative curvature (K) is varied from 0.01 to 0.25. The moving-orientations of the piezoelectric fan include transverse oscillation and axial oscillation. The influence mechanism of these parameters are analyzed in terms of the time-averaged heat transfer coefficient, instantaneous streamlines and isosurface of λ2. The heat transfer results indicate that the axial oscillation is more favorable for improving the heat transfer performance than the transverse oscillation, especially in the zone near the fan vibration envelope. Meanwhile, the flow field reveals that the relative curvature of the concave surfaces influences the flow feature under longitudinal configuration substantially.

This paper aims at performing an investigation numerically on the turbulent flow and thermal performances for an asymmetrical concave surface integrated with a slim vibrating piezoelectric fan. The dynamic mesh technique using a user defined function to describe the displacement function of vibrating cantilever beam is employed to model the deformation of the slim piezoelectric fan in time. Meanwhile, the SST k-ω turbulence model is chosen to capture the turbulence behavior of the flow and heat transfer. Two important factors, the relative curvature of the both sides of semicircular surfaces ( RK) and the dimensionless distance of fan offset along y-axis (Δ y/ APP) are taken into considerations during the simulation process. A considerable increase of local time-average heat transfer coefficients is observed in the vicinity of vibration envelope. The results show that the relative curvature ( RK) has a strong influence on the flow and heat transfer at both ends of the asymmetrical concave surface when its value is larger than 2. And by adjusting the dimensionless offset distance of the piezoelectric fan (Δ y/ APP), the area-averaged convective heat transfer coefficient can be increased by 20% on a small zone surrounding the fan with WPF × App (S1). The conclusions of this paper implement a theoretical attempt for expanding the application scenarios of piezoelectric fan.

秸秆焚烧所释放的污染物已成为我国大气污染的主要来源之一,若将秸秆制成保温型建筑材料,不仅可以减少环境污染,还可有效地降低建筑能耗.制作了系列新型秸秆再生保温砖,采用全生命周期模型综合考虑建筑施工、运行及拆除阶段能耗,并应用傅里叶定律对南京某宾馆采用不同墙体的建筑耗能特性进行分析.结果表明,秸秆含量的增加能显著改善秸秆砖的热工性能,但会导致其力学性能变差.含量为4％的样砖兼具良好的热工和力学性能,采用该秸秆含量保温砖替代常用的粘土红砖和混凝土砖,在全生命周期内可分别节约21.8％和68.2％的电能消耗,节能效果显著,具有很强的应用前景.

变风量空调系统的空气处理机组(AHU)出现故障时会使系统舒适性降低,能耗和运维成本增加.本文提出了一种基于改进型主元分析(PCA)和BP神经网络算法,用于AHU的模型建立及故障诊断.结果表明使用改进滤波的PCA检测模型主元数为3个,累计贡献率92.7％.当系统传感器出现5％的故障偏差,模型在送风温度、 新风温度、 冷冻水流量三种故障中的检测率均大于90％.使用BP神经网络算法对系统故障进行诊断,新风温度传感器故障和冷冻水流量传感器故障诊断率均达到了100％.新风温度传感器由于精度较高检测率为92％.但是三种故障类型检测率都超过了90％.通过分析表明改进型主元分析(PCA)和BP神经网络算法可以有效检测与诊断AHU系统故障.

Piezoelectric fan is widely used in the heat removal of high-temperature devices for its advantages of simple structure, low energy consumption, low noise, and good wind orientation. A theoretical and numerical study of characterizing the flow and heat transfer performances induced by single and multi-vibrating piezoelectric fans for cooling heated semi-cylindrical concave surface is conducted in this work. Two kinds of numerical models based on a dynamic meshing scheme and a user defined function describing the time-varying displacement of vibrating fans are employed to capture the instantaneous vortex structures and temperature contours on the semi-cylindrical concave surface. The influence mechanisms on heat removal performance of some vital factors, such as the dimensionless fan-to-fan pitch (P/W), vibrating phase difference and the dimensionless fan tip-to-concave surface distance (G/A) are analysed. The results prove that the evolution of the transient vortical structures around a single vibrating piezoelectric fan for cooling concave surface is almost the same as the case for cooling plane surface. The interaction of the streaming flow induced by multi fans is mainly concentrated in the clearance area between adjacent fans rather than the vibration envelopes. Moreover, the in-phase vibrating condition is more sensitive to the change of P/W and G/A than that of the out-of-phase. The simulations furnish a good reference for the design and optimization of multi piezoelectric fans.