Vertical Temperature Difference Research Articles

Effect of thermal manikin shape on thermal comfort parameters prediction uncertainties: a numerical study

The main goal of the study is to evaluate the effect of the thermal manikin shape on the uncertainty of thermal comfort parameters CFD prediction. Numerical simulation of the airflow and heat transfer was carried out in a model room with displacement ventilation where a standing heated thermal manikin is located (test by P.V. Nielsen et al.). Numerical modelling was carried out using three manikin shapes: the detailed one (close to the human body shape), the simplified (block) one, and the single-parallelepiped shape. The 3D calculations were carried out using the ANSYS Fluent CFD package. Turbulence modelling was performed using the Reynolds-Averaged Navier-Stokes (RANS) approach with the standard k-ε turbulence model. The uncertainties in the CFD-predicted draught rating (DR), percentage dissatisfied caused by vertical air temperature difference (PD), the predicted mean vote (PMV) and the predicted percentage dissatisfied (PPD), are discussed. The main result of the paper is the quantitative evaluation of the changes in the local velocity, temperature and thermal comfort parameters due to the thermal manikin shape simplification. It was found that the shape simplifications are suitable if the goal of the study is to predict the integral parameters.

Prediction of air quality using vertical atmospheric condition and developing hybrid models

Most metropolitan areas experience increasing health and environmental pressure from air pollutants. Air pollutant concentrations are mostly influenced by meteorological factors, pollutant sources, and topography. The vertical atmospheric conditions can affect the surface air pollution concentration. In this paper, to evaluate the relationship between the vertical meteorological characteristics and air pollution concentrations (CO, O3, PM2.5, PM10 and SO2) four different models including multi-linear regression (MLR), multi-linear perceptron (MLP), generalized regression neural network (GRNN), and adaptive Neuro-Fuzzy inference system (ANFIS) were developed. The vertical temperature difference, relative humidity, U and V wind components in different altitudes were extracted from the operational archive of the European Center for Medium Range Weather Forecast (ECMWF 2019) from March 2011 to December 2017. The variance inflation factor (VIF) and tolerance tests were utilized to remove the independent variables, which made multi-collinearity. Results showed that among the studied air pollutant variables, variation of PM10 concentration was more influenced by vertical meteorological characteristics obtained using the GRNN model with R2 = 0.960, root Mean square error (RMSE) = 0.019 and mean absolute error (MAE) = 0.005 in tearing phase and R2 = 0.940, RMSE = 0.054 and MAE = 0.019 in testing phase. The results of ANFIS model for PM2.5 concentration were acquired highest among the other air pollutant variables in tearing phase (R2 = 0.819, RMSE = 0.036 and MAE = 0.025) and in testing phase (R2 = 0.960, RMSE = 0.001 and MAE = 0.001). The results illustrated that GRNN and ANFIS models outperform other models for air pollutants prediction using vertical atmospheric conditions in the Sanandaj, Iran.

Influence of the Uneven Temperature Distribution on the Electrode Warpage of PP IGBTs

The pressure and junction temperature distribution within press-pack insulated gated bipolar transistors had been proven uneven due to the thermomechanical coupling effect caused by electrode warpage. However, the root reason for the electrode warpage remains unclear, and therefore leads to confusion in optimization. In this article, the influence of vertical and lateral temperature difference on the electrode warpage is first separated and analyzed through the proper combination of boundary conditions in finite element simulation. The lateral temperature difference is found to be the main reason for the electrode warpage, which is different from the perception state of the art and provides the possibility to suppress the electrode warpage directly. Furthermore, the role of the lateral temperature difference is analyzed by a simplified model and it is found that the lateral temperature difference could be decreased by only pure thermal design rather than thermomechanical design. The electrode size and pedestal layout are discussed to alleviate the lateral temperature difference of the electrode and the simulation results also show great improvement in pressure and junction temperature distribution. Finally, the junction temperature distribution is measured by the sequential <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CE</sub> ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> ) method to verify the root reason and also the effectiveness of the improvement.

Local thermal comfort chart for non-uniform building environments: Comparison with vehicle environments and its application

There have been many laboratory studies of human subjects exposed to non-uniform thermal environments such as radiant temperature asymmetry from different directions, vertical air temperature differences, and draught. However, there are no final conclusions on the unified quantitative index for general non-uniform environments, which will be stressed in this study. The equivalent temperature (teq) that integrates air temperature, radiant temperature and air velocity at different body parts and well relates with thermal perceptions is adopted as an integral evaluation index. Published data from laboratory measurements of office environments with subjects exposed to different types of radiant asymmetry and measurements with a thermal manikin were analysed. A new comfort chart of teq was developed to evaluate non-uniform building environments and it was compared with the existing comfort chart (in ISO 14505) applied in vehicle. The results indicated that overall sensible heat loss of the human body remained constant, but local thermal discomfort and sensation changed with the non-uniform environments. The new comfort chart identified the source of discomfort and predicted the observed subjective evaluations. Significant discrepancy of the comfort chart between buildings and vehicles was found, indicating the necessity of developing a comfort chart that applied in the building environment. The study provides new insights into the causes of local thermal discomfort and can provide information on the sensitivity of different body parts to heat loss that can be used to optimise the thermal design of indoor environments.

Macroscopic hydro-thermal processes in a large channel-type reservoir

Study regionThis study was conducted in Xiangjiaba Reservoir, located in the Jinsha River, China's upper reach of the Yangtze River. Study focusLarge channel-type reservoirs are characterized by significant runoff, deep water, and narrow-and-long channel shape, creating unique hydrodynamic and thermodynamic conditions for their eco-environmental systems. However, the hydro-thermal processes of this type reservoir are not fully known. A large channel-type reservoir was selected to investigate the macroscopic processes through a field survey and employing a three-dimensional (3D) hydro-thermodynamic model. New hydrological insights for the regionThe reservoir experienced a seasonal varying thermal stratification, with a vertical temperature difference of ∼9 °C. Meanwhile, a conjoint flow pattern was detected, consisting of an upper-layer warm water flow and a lower-layer circulation, which was very different from the lakes' wind-induced pattern. When the reservoir was stratified, the inflow warm water on the thermocline could produce a shear effect, promoting vertical mixing and further contributing to hypolimnion shrinking. The reservoir experienced a water age increase during the non-stratification period, with the near-dam age being ∼20 days. The trap effect of the lower-layer circulation retained the cold water in the hypolimnion during the stratification period, and the water age reached 115 days. The presented varying flow pattern and transport timescale have essential effects on the eco-environmental systems, providing practical insights into managing water quality and aquatic environment in reservoirs.

Real-Time Intelligent Prediction Method of Cable’s Fundamental Frequency for Intelligent Maintenance of Cable-Stayed Bridges

Cable’s fundamental frequency (CFF) is an important characteristic of the working state of long-span cable-stayed bridges. The change in the bridge’s temperature field will influence CFF by altering the cable’s tension and the cables’ sags. An accurate regression model between the temperature-induced variation of CFF and the real-time changing temperature field should be established. Then, the reference value of the temperature-induced variation of CFF can be obtained after inputting the real-time temperature data. In this study, an intelligent real-time prediction model for CFF is proposed based on the full-bridge temperature field, including the average temperature of the main beam, the vertical temperature difference of the main beam, and the temperature of the cable tower. Besides, a machine learning method named the long short-term memory (LSTM) network is exploited to ensure the nonlinear fitting performance of the model, and a paradigm for optimal hyperparameter selection and training strategy selection is provided. To verify the superiority of the LSTM-based model, the output accuracy of the linear regression, BP network, and LSTM network was tested and compared using the monitoring data collected from cable sensors in the main span and side span, which provides an important basis for the intelligent maintenance and sustainable operation of the bridge cables.

Experimental study on heating performance of micro-holes radiant plate under different installation types in severe cold regions

In order to alleviate energy shortage and maintain comfortable indoor environment, radiant air conditioning system is widely used. This paper designs and produces a miniaturized micro-holes radiant plate, which has the characteristics of orifice plate air supply and radiant heat transfer. The micro-holes radiant plate can be installation on the ceiling or sidewall without adding dynamics system and piping system. In order to explore the heating performance of the micro-holes radiant plate under different installation types, an experimental bench of air source heat pump micro-holes radiant plate heating system was built in Harbin, China, for laboratory heating. The experimental results showed that the indoor average temperature under various installation types could basically meet the set requirements, and the deviation was within 1 °C. The indoor horizontal temperature distribution was relatively uniform, and the temperature difference of each measuring point was less than 2 °C. The indoor vertical temperature difference was the smallest under the installation type with sidewall 50 mm from the ground at only 2.68 °C in the sitting position and 3.14 °C in the standing position, which meet China and ASHRAE standards. The ratio of the radiant heat transfer quantity under the sidewall heating was about 60 %, while the ratio under the ceiling heating was only about 30 %. The maximum ratio of the radiant heat transfer quantity was close to 70 % under the installation type with sidewall 50 mm from the ground. The experimental results provide theoretical support for the design and operation of air conditioning systems in severe cold regions.

Evaluation of thermal comfort in air-conditioned rooms based on structure/control-related parameters and data-mining method

Evaluating the thermal environment and thermal comfort in an air-conditioned room is an essential for estimating the performance of air-conditioning systems. However, multiple component structures and control-related parameters often lead to a long test cycle and large number of tests, significantly affecting the testing efficiency and speed. To address these problems, in this study, a data-mining method was proposed to predict and evaluate the thermal environment of an air-conditioned room. Owing to the limited amount of experimental data, the sample data were expanded by the simulation data of a collaborative platform between the air-conditioning system and air-conditioned room. Data-mining models, including the support vector regression (SVR), backpropagation (BP), and multiple linear regression (MLR) models, were developed and achieved good accuracy in evaluating the thermal environment by considering air-conditioning systems with various structures and control parameters. In the multiple-input single-output evaluation method, the prediction accuracy of the SVR model was higher than those of the BP and MLR models with respect to the vertical air temperature difference, temperature uniformity, temperature drop rate, and draft rate, while the result was the opposite in terms of the predicted mean vote indices. In the multiple-input multiple-output evaluation method, there was a decline in prediction accuracy and an increase in efficiency prediction compared with multiple-input single-output evaluation.

A Combinatorial Optimization Strategy for Performance Improvement of Stratum Ventilation Considering Outdoor Weather Changes and Metabolic Rate Differences: Energy Consumption and Sensitivity Analysis

Since occupants spend most of their time indoors, an energy-saving and comfortable indoor environment are particularly important. The differences in the metabolic rate of occupants make them have different requirements for their thermal environment. To save energy under the comprehensive needs of occupants for thermal environment, the combinatorial optimization strategy based on NSGA-II and improved the TOPSIS method is proposed in this study. Firstly, the physical model of the CFD simulation is verified by experiments. Secondly, the specific operation cases corresponding to combinations of different levels of factors are determined via the RSM method, and the ventilation performance prediction model considering the metabolic rate differences and outdoor weather changes is established. Thirdly, supply air velocities and temperatures are optimized by using Pareto-based NSGA-II; the Pareto optimal solution set under different outdoor temperatures is obtained. Finally, based on the Pareto optimal solutions at different outdoor temperatures, the optimal strategy under dynamic outdoor air temperature is obtained by improved TOPSIS by the CRITIC method. The optimization of ventilation parameters significantly improved the ventilation performance, and the results show that the predicted mean vote, energy consumption, vertical air temperature difference between head and ankle levels and the local mean age of air for different metabolic rates decrease by 64.1%, 4.74%, 24.83% and 7.39% on average, respectively. Moreover, the relative energy saving rate increases as the metabolic rate increases, and the strategy facilitates adaptation to outdoor weather changes and meets the individual needs of occupants for the indoor environment. This has important implications for achieving the global goal of energy efficiency and emission reduction.

Study on the effects of personal heated table mats on the comfort level of large office space heated by air-conditioner

In winter, a large vertical temperature difference usually appears in large office spaces with single-mode air conditioner heating, which results in inefficient energy consumption and challenges in meeting the thermal comfort requirements of workers. Aiming at improving the productivity and motivation of employees, it is necessary to improve the thermal environment at the office. In the present study, personal heated table mats in combination with a conventional HVAC system were considered as the research object, and the efficiency of heated table mats in enhancing the ambient temperature and human comfort was analyzed through experiments and questionnaires. The obtained results show that using personal heated table mats significantly improves the thermal comfort of workers. More specifically, the air temperature and black-globe temperature at the seated breathing position were increased by a maximum of 3.5 °C and 4.2 °C, respectively. Moreover, human thermal sensation (TSV) was enhanced from slightly cool (−1.42) to warm (1.64), and thermal comfort (TCV) was increased from uncomfortable (−1.52) to slightly comfortable (0.71), correspondingly. Comparing different input powers, it is found that the highest level of acceptability (0.91) was achieved at an input power of 310w. The performed analyses demonstrate that the personal heated table mats can effectively compensate for the deficiencies of the thermal environment in large office spaces equipped with an air conditioner, thereby improving the thermal comfort of workers.

Effects of vertical air temperature differences on thermal comfort, mental performance, and physiological responses: A review

Effects of vertical air temperature differences on thermal comfort, mental performance, and physiological responses: A review

Control of Natural Ventilation Opening to Achieve Energy Saving Performance and Thermal Comfort in Office Room

In recent years, the number of office buildings that introduce natural ventilation (NV) is being increased to save energy. However, operating NV systems with NV opening device fully open could lower thermal comfort, depending on meteorological condition. Therefore, automatically-controlled NV systems can generally work only for a relatively short period of time. In order to keep both energy saving performance and thermal comfort even when outdoor temperature is low in the mid-seasons, it is necessary to understand the relation between NV conditions and the indoor environment. In this paper, the indoor environment is evaluated changing NV conditions by using CFD for a generalized office room model. Four types of NV inlet system were assumed, i.e., high peri-counter (Type 1), low peri-counter (Type 2), slot-type downward inlet (Type 3), and slot-type horizontal inlet (Type 4). Twenty conditions were analyzed for each type, varying the outdoor air temperature and NV flowrate as parameters. As indices of thermal comfort, average vertical temperature difference within perimeter zone and ADPI in the occupied zone were calculated. For each of NV systems, simplified equations to calculate Nmax from indoor/outdoor temperature difference are finally to be proposed, which may be considered for NV opening control in operating NV systems.

Performance evaluation of ductless personalized ventilation combined with impinging jet ventilation

Despite the high ventilation efficiency of the impinging jet ventilation, two local discomfort risks (draught and excessive thermal stratification) should be highlighted owing to the cool air being supplied directly to the occupied zone. Ductless personalized ventilation, as one type of different personalized ventilation air terminal devices, shows the potential to improve thermal comfort and reduce the draught and excessive thermal stratification risks. In this study, twenty-four cases were studied using the validated numerical simulation method when integrating the impinging jet ventilation and personalized ventilation. According to simulation results, local cooling provided by ductless personalized ventilation reduces the air temperature around the face by a maximum of 3.7 °C. The vertical temperature difference is reduced to less than 1 °C with personalized flow rates greater than 6 l/s. The use of ductless personalized ventilation has no negative effect on the stratification of impinging jet ventilation in the studied conditions. Because of the ductless personalized device, the impinging jet ventilation can be used in a warm environment, reducing the risk of draughts. Finally, it was concluded that the system that combined ductless personalized ventilation and impinging jet ventilation could improve thermal comfort while also having the potential to save energy when used in warm conditions.

Field observations of stratification in stormwater wet ponds

Stratification is one of the fundamental physical processes that may have a significant impact on water quality in stormwater wet ponds. However, the role of thermal and chemical stratifications in governing water quality processes is not fully understood. This is in part due to the lack of detailed field measurements of sufficient governing parameters over time periods that span a wide range of environmental conditions. To fill this gap, a comprehensive 2-year field program was undertaken in two stormwater wet ponds in Calgary, Alberta, Canada, during the ice-free season from May to November in 2018 and 2019. At different locations in each pond, thermal and chemical stratifications were observed, thermocline depth and strength were determined, and continuous water velocity profiles were measured. In addition, the effect of local weather conditions on stratification, thermocline, and hydrodynamics was investigated. The results showed that the ponds had vertical water temperature differences >1 °C for 99% of the time, May to August. In addition, salt-laden inflows from road deicing salts led to strong chemical stratification up to five times stronger in the sediment forebays than in the main cells in spring. Wind-induced surface currents were insignificant, scaling at 0.3% of the wind speed with negligible impact on vertical mixing in the ponds. Our results demonstrate that the ponds' strong and prolonged stratification decreased pollutant retention capacity and caused the water at depth to become anoxic, degrading the quality of the water discharged downstream. Hence, additional consideration of stratification is required when designing new stormwater ponds.

Analysis of Measured Temperature Field of Unpaved Steel Box Girder

This work used the measured data during the whole test period to study the law of temperature change in the steel rail beams, and the distribution characteristics of the sunshine temperature field, in straddle-type monorail tourist transportation systems, employing a field-test and a numerical simulation. The curve form of the temperature gradient was determined by a comparative analysis of the existing domestic and foreign norms. Finally, the generalized extreme value distribution model was used to predict the extreme value of the representative value of the temperature difference, and the value of the temperature base, for different return periods, and the complete temperature gradient model were determined. Results: During the whole test period, the maximum vertical positive temperature difference of the steel box girder was 15.21 °C, and the negative temperature difference was −5.07 °C. In addition, the effect of the ambient temperature, considering solar radiation, was found to be an important factor affecting the distribution of the vertical temperature difference. The analysis determined that the positive and negative temperature difference curves in the unpaved steel box girder were multi-segment polylines and linear straight lines, respectively. The extreme value predicts that the representative temperature differences between T1 and T2 of the 450 mm beam during the 50-year return period were 17.2 °C and 4.58 °C, respectively, and T1 and T2 under the 100-year return period were 17.38 °C and 4.62 °C, respectively.

An experimental comparison of radiant wall and ceiling cooling system integrated with ground source heat pump and direct expansion fan coil system in a highly glazed office room

Hydronic radiant systems, which provide a thermal comfort environment, come to the fore in different building applications due to their energy saving potential and being integrated with renewable energy systems. The aim of this study is to present a full-scale experiment to compare the thermal comfort performances of radiant ceiling, wall and wall + ceiling systems coupled ground source heat pump and direct expansion fan coil system implemented in Yildiz Technical University Science and Technology Application and Research Center, Istanbul, Turkey. Air velocity, horizontal and vertical air temperature differences, mean radiant temperature and humidity values, which are four physical parameters that can affect the thermal satisfaction of office workers for 9 different experimental conditions, are measured for fan coil and all radiant systems. The results indicate that vertical temperature differences between ankle and head level vary from 0.9 °C to 1.4 °C for both fan coil system and all radiant system configurations in different location of the test room. Acceptable overall comfort in indoor environments Class A only showed stable performance with an average of 0.12 PMV and 5.3 % PPD when ceiling + wall radiant panels were actively cooled. The result shows that the difference between room air temperature and room operating temperature varies between 0.45 °C and 0.6 °C when cooling loads are met by radiant systems. This may not make it possible to use indoor air temperature instead of indoor operating temperature in the control of radiant systems. Moreover, the thermal environment of the radiant system are more uniform than fan coil system due to low air temperature stratification, air velocity and small differences between mean radiant temperature and indoor air temperature.

Analysis of Vertical Temperature Difference of the Low-Speed Maglev Steel Girder Based on Measured Data

Analysis of Vertical Temperature Difference of the Low-Speed Maglev Steel Girder Based on Measured Data

Experimental investigation of thermal comfort performance of a radiant wall and ceiling panel system

The number of radiant heating-cooling systems in building applications is increasing it is because of low energy consumption, low operating temperature and high values they provide in terms of working compatibility with renewable energy sources. In this study, ground source heat pump integrated radiant panels installed on the wall and suspended ceiling of the office room in Yıldız Technical University Science and Technology Application and Research Centre were experimentally examined in terms of thermal comfort according to the relevant standards. Vertical air temperature differences and mean radiant temperatures were investigated. The mean air temperature differences at 0.1 m and 1.7 m were found to be 3,9 oC 2.9 oC 3.5 oC, 3.1 oC and 3.4 oC on average for the five different stands, respectively. PMV and PPD values were found to be 0.78 and 18.9% for February 12 (Case 1), 0.36 and 8.4% for February 13 (Case 2), respectively. In the experiment carried out under the conditions of Case 2, while the comfort conditions were provided in almost all of the day, the desired comfort conditions could not be achieved in Case 1 after 11 am.

Statistical Analysis of the Vertical Temperature Gradient of the Web Plate of Steel Box Girder Bridges in Cold Regions

To understand the actual vertical temperature gradient mode of steel box girder webs in cold regions, nearly 1.5 million data measured for over a year by a comprehensive temperature collection system of a steel box girder bridge in a cold region were used to verify the typicality of the temperature screening samples, and a cluster analysis was performed to verify the statistical symmetry of temperature at the symmetry points of webs on both sides of the bridge. One years' worth of daily temperature difference data were used to investigate the vertical temperature difference distribution on the web and to determine the probability distribution curve form. After selecting the extreme temperature difference samples, the standard vertical temperature difference of the web was calculated, and the actual vertical temperature gradient model of the steel box girder web was established. Results show that the probability distribution model of temperature at the symmetrical points of webs on both sides of the steel box girder was symmetric. The vertical temperature differences of the web demonstrated significant seasonal changes. The probability distribution of the positive and negative temperature differences can be described based on the sum of two Weibull distribution functions. The temperature gradients at all levels in the proposed vertical temperature gradient model were generally greater than the values indicated in the relevant specifications.

Stratum-air-distributed radiant-convective room air conditioner for heating

The radiant-convective room air conditioner has been demonstrated to improve the coefficient of performance and the heating capacity effectively. However, the applicability of the radiant-convective room air conditioner in modulating the indoor thermal environment is unclear. This study proposes a stratum-air-distributed radiant-convective room air conditioner and evaluates its effectiveness for providing thermal comfort under heating mode with experimentally validated CFD simulations. Results show that the heat output of the radiant panel of the room air conditioner accounts for 20.7 %- 34.5 % of the total heat output. The supply airflow rate should be properly determined to balance the highly coupled heat output and air distribution. Compared with the conventional room air conditioner, the proposed room air conditioner under proper operation more effectively warms the occupied zone because of the enlarged heat output and improved air distribution, thereby increasing the predicted mean vote from −0.15 to −0.02, reducing the vertical air temperature difference from 6.7℃ to 0.8℃, and enhancing the thermal utilization effectiveness from −0.09 to 0.25. In addition, the draft rating and radiant temperature asymmetry of the proposed room air conditioner are thermally acceptable. Therefore, the proposed room air conditioner can modulate the indoor thermal environment energy-efficiently from the aspects of both overall and local thermal comfort.