Non-uniform Thermal Environment Research Articles

Analysis of the non-uniform thermal environment of local floor and ceiling radiant heating: Numerical simulation and thermal comfort experiment

Local radiant heating is a crucial method for avoiding inefficient heating and reducing energy consumption in buildings. To supplement existing research, which primarily focuses on local vertical-wall radiant heating, this paper investigates the non-uniform thermal environment of local floor and ceiling radiant heating rooms under various radiant positions, areas, and temperatures. The study utilizes numerical analysis by Ansys Fluent and thermal comfort experiments in Xi'an, China, which has a Dwa Köppen climate classification. When the local heating zone is located near windows and doors, the radiant film preferentially exchanges heat with them, resulting in a decrease in air temperature and mean radiant temperature. In such cases, it is recommended to increase the radiant temperature or area. With a radiant area of 5 m2 and a radiant temperature of 25 °C, expanding the radiant area to 15 m2 increases the average air temperature in the local heating zone by 5.0 °C, while raising the radiant temperature to 29 °C increases it by 1.4 °C. Moreover, enlarging the radiant area can effectively improve occupants' skin temperature, thermal sensation, and overall thermal comfort. When the radiant area and temperature are determined, priority should be given to local floor radiant heating over local ceiling radiant heating due to its lower air temperature, larger temperature gradients, and smaller mean radiant temperatures. This study aims to provide references for proposing refined design and regulation parameters of local floor and ceiling radiant heating systems in the future.

Transient simulation and experimental analysis for the response of dual-cycle air-carrying energy radiant diffuse terminal

A novel air-carrying energy radiant diffuse terminal is proposed, and the response of the terminal is investigated by field experiment and numerical simulation. This paper explores the dynamic operation characteristics of the new system to guide the optimal operation of the system. A transient simulation method of three factors (conduction, convection, and radiation) with dual-circulation (air-carrying energy zone and occupied zone) is put forward for the radiant diffuse terminal with a fresh air system. This transient simulation sets up the porous baffle model coupling radiation model concurrently, revealing the dynamic thermal performance for the system's response accurately, quickly, and comprehensively. This method avoids the limits for the complexity of mini porous (Pore size of 1 mm–3mm) geometric models and the transient uncertainty in multiple combination models. The results show that the average error between the convective-conductive model and the convective-conductive-radiative model is about 8% at five different heights. This system has high response performance, which can quickly heat the room while maintaining the comfort requirements of vertical temperature. The radiated heat transfer between the radiant diffuse terminal and the occupied zone is greater than the convective heat transfer (Radiation accounts for 74%). This system has a short response time, high energy efficiency, uniform airflow, and eliminates draught rating. Moreover, the indoor non-uniform transient thermal and humid environments, the thermal comfort, and air quality for the response of the terminal with fresh air system under four different diffuser placements are compared using computational fluid dynamic simulation. The results indicate that introducing heating strategies featuring higher-positioned fresh air inlet and lower-positioned recirculation return air outlet for this system leads to a temperature efficiency increase by 5%, velocity uniformity by 8%, and age of air reduction by 16–50% in human-breathed zone and the response time by 25%, which shows substantial potential for energy savings. The transient simulation and experiments in the paper are beneficial to optimize design parameters and carry out energy-saving applications of operation energy consumption for the response of this terminal with a fresh air system in the future.

The Effects of Exterior Glazing on Human Thermal Comfort in Office Buildings

As a major component of the building envelope, the energy-saving design of exterior windows is key to energy savings in office buildings. The conventional design of exterior windows mainly focused on their impact on heating and cooling energy but ignored the indoor thermal comfort problems caused by the direct solar radiation transmitted by windows and the fluctuation of their internal surface temperatures. This study analyzed the influence of exterior windows on the indoor thermal environment of office buildings by carrying out field experiments. The experiments were carried out in a typical office building in Xi’an during December and January. The impact of exterior windows on human thermal comfort was studied from two perspectives: longwave radiation from the surface of window glass and solar shortwave radiation. It was found that solar radiation was the main cause of temperature fluctuations on the internal surface of windows and created non-uniform thermal environments. The mean radiant temperature fluctuations in the near-window area could reach up to 7.8 °C due to outdoor solar radiation in winter. Solar radiation transmitted by windows directly affects thermal sensations. Since conventional thermal comfort models or indices underestimated the thermal sensations of occupants in the presence of solar radiation, the additional thermal loads caused by solar radiation needed to be taken into account. The allowable operative temperature range for maintaining thermal comfort should be reduced by 0.5 °C when occupants are exposed to solar radiation.

Analysis of asymmetric thermal sensation under the coupling effect of temperature difference in multiple parts of human body

Responding to thermal comfort based on physiological indicators is a broad and reliable research method. How to use skin temperature to predict thermal comfort in a non-uniform environment is of great significance. 24 volunteers were selected for testing with local radiant heating in Kitakyushu, Japan. It was found that the most obvious skin temperature difference value occurred between left and right thighs while heating after 30 min, which would exceed 5 °C. The asymmetric thermal sensation votings (ATSV) and the skin temperature difference were significantly influenced while changing radiation intensity and radiation distance, the ATSV of the subjects in the small space was about 0.3 lower than that of the subjects in the large space. Increasing the radiation intensity, the TSV of the small space decreased by about −0.5, which can be seen that higher radiation intensity may bring lower value of the TSV. A prediction model of the ATSV and skin temperature difference was established and the sensitivity analysis was performed on the different body parts that significantly affected the ATSV, and the dominant factors affecting the ATSV were obtained. This paper complements the non-uniform thermal environment prediction method.

Subzone division optimization with probability analysis-based K-means clustering for coupled control of non-uniform thermal environments and individual thermal preferences

Collective air distribution is widely used for thermally comfortable indoor environments but is limited to unified thermal comfort requirements. Advanced collective air distribution has the potential to generate non-uniform thermal environments for individual thermal preferences. This study proposes a subzone division optimization method for nonuniform thermal environments to improve thermal preference satisfaction. The proposed method divided non-uniform thermal environments into subzones using probability analysis-based K-means clustering, minimizing and maximizing the thermal environment differences within and among the subzones, respectively. The number of subzones was determined to maximize thermal preference satisfaction with the coupled control between the nonuniform environments and individual thermal preferences while limiting the percentage of thermal discomfort. The results based on experiments of advanced collective air distribution, i.e., stratum ventilation, showed that thermal preference satisfaction and the percentage of thermal discomfort increased with an increasing number of subzones. Compared with the conventional method (i.e., uncoupled control without subzone division optimization) and recent method (i.e., coupled control without subzone division optimization), the proposed method improved the thermal preference satisfaction by 34.7 % and 11.7 %, respectively, for four subzones. Moreover, with the proposed method, increasing the system control flexibility increased thermal preference satisfaction. Compared with the variable air volume system, the constant air volume system and the system with variable supply airflow rate and temperature improved thermal preference satisfaction by 5.8 % and 22.5 %, respectively. The proposed method contributes to extending the capability of collective air distribution for individual thermal preferences.

Thermal sensation and physiological responses in non-uniform bathing thermal environment: A comparative study on 25-, 52-, and 71-year-old participants

A non-uniform thermal environment significantly affects human physiological responses and even harms health. Specifically, a non-uniform bathing thermal environment is particularly deadly for older adults. Although each body part's arterial blood flow changes in the bathing thermal environment, little research has been conducted on the physiological responses of participants of different ages. Herein, we conducted a series of bathing experiments on three healthy participants aged 25, 52, and 71. The aim of these experiments to elucidate and analyze the thermal sensations and physiological responses of the participants in a non-uniform bathing thermal environment (i.e., the upper body was exposed to air, whereas the lower body was immersed in hot bathwater). We found that the non-uniform thermal environment greatly influenced a variety of physiological parameters and increased the participants' heart rates and blood flow. The youngest participant (i.e., the 25-year-old) and oldest participant (i.e., the 71-year-old) had the fastest and slowest perception of thermal sensation, respectively. The physiological responses changed significantly with the perception of thermal sensation change for the three participants. Unlike the other two participants, the oldest participant had decreased blood pressure and slowly increased thermal sensation. In addition, the oldest participant in a short bathing time had minor thermal health risks than younger participants due to exhibiting the slowest thermal response. However, older adults may experience greater thermal health risks due to their relatively slow thermal response in long-time and high-temperature bathing environments.

A coupled analysis on human thermal comfort and the indoor non-uniform thermal environment through human exergy and CFD model

The importance of human thermal comfort evaluation lies of the development of healthy buildings as well as low-carbon buildings. In recent years, the applications of the second law of thermodynamics to thermal comfort offered a new perspective and this study, for the first time, an integrated human exergy model and the indoor CFD model for a coupled analysis is put forward, which supplements a critical link between human and building environment. In this paper, a predefined office model is investigated to improve the efficiency and thermal comfort of occupants. Based on the human exergy model, the parameters of metabolism, respiration, evaporation, convection, radiation, skin, and core heat storage terms are optimized. Additionally, the effects of gender, age, and clothing thermal resistance on the exergy model values are also separately explored. Finally, the optimized human exergy model is combined with computational fluid dynamics (CFD) to calculate, under different air supply conditions, the distribution of exergy consumption values in the predefined room model. The results show that the factors of gender, age, and thermal resistance of clothing have some influences on human exergy consumption, while the above factors do not have a significant enough influence on the model in the thermal comfort interval. Moreover, the combination of the human exergy model and CFD can offer some suitable air supply conditions for the development of indoor thermal comfort and work efficiency. This study presented a new tool that bridges human thermal sensation and indoor thermal environment.

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.

Investigation on buckling of Timoshenko nanobeams resting on Winkler-Pasternak foundations in a non-uniform thermal environment via stress-driven nonlocal elasticity and nonlocal heat conduction

This study investigates the size-dependent buckling behavior of Timoshenko nanobeams resting on Winkler-Pasternak foundations in a non-uniform thermal environment. A non-uniform temperature distribution is established through nonlocal heat conduction. Subsequently, the equivalent thermal load due to the obtained temperature distribution and boundary constraints is derived using the governing equations of the axial thermal deformation of the beams based on the stress-driven nonlocal elastic model. To obtain the critical buckling load of the nanobeams, the quadrature element method is used to numerically resolve the eigenvalue problem. In the numerical simulation section, we have presented a series of examples to analyze the effects of length-to-height ratios, nonlocal scale parameters, and Winkler-Pasternak foundation parameters on the buckling loads of the nanobeams under various boundary conditions. Moreover, we examine the effect of comprehensively considering both the elastic and thermal nonlocality on the thermal loads and finally mechanical buckling loads.

Measurement of heat transfer characteristics by a child manikin with embedded cyclic temperature control

Thermal manikin is an effective tool to study the heat transfer between the human body and the surrounding environment. Combining the heat transfer characteristics with a human-body thermal model enables the detailed analysis of the interaction of the human body and the surrounding thermal environment. Most thermal manikins adopt steady surface temperature control. However, this control method can evaluate the heat transfer coefficient only in a uniform thermal environment while daily thermal environments are non-uniform. In the present study, a theory for cyclic control method is proposed to determine the heat transfer characteristics in a non-uniform environment. A new child-shaped thermal manikin with embedded the cyclic control method is developed. The theory to determine heat transfer characteristics is verified in both uniform and non-uniform thermal environments by using the developed thermal manikin.

Topology optimization of regenerative cooling channel in non-uniform thermal environment of hypersonic engine

Regenerative cooling technology is widely used in the active thermal protection system of hypersonic airbreathing engine. However, the overall efficiency of active cooling with limited total coolant quantity needs to be improved due to flow non-uniformity, hydrocarbon fuel pyrolysis and other factors. This paper presents a fluid–structure coupled topology optimization design of the regenerative cooling channel to improve heat transfer efficiency. To further verify the advantages of topology-optimized cooling channels under real operating conditions, detailed three-dimensional numerical simulations were conducted to investigate the supercritical flow and heat transfer processes of hydrocarbon fuel in the topology-optimized channels. Results reveal that compared to the traditional straight cooling channel, the topology-optimized channel has improved the heat transfer efficiency and flow distribution significantly. Many micro-channels perpendicular to the mainstream direction which can be termed as “capillary-like” zone in the optimized channel distribute the inlet fuel to the adiabatic walls, thus significantly improving the overall heat transfer. Remarkable cooling capacity robustness of the topology-optimized cooling channels has been found under different non-uniform heat flux conditions. Comparing to the straight channels, the averaged temperature of the heated wall in the optimized channels can be decreased up to 5.8% and the pressure drop can be decreased up to 20.6%. The uniformity of the fuel temperature distribution is about 2 times better than that of the straight channel. However, although overall heat transfer efficiency is high, complex flow and heat transfer phenomenon occurs in the topology-optimized cooling channel, such as the reverse flow phenomenon induced by adverse pressure gradient near the entrance and the stagnated flow in the “capillary-like” zone near the exit, which impair the local heat transfer and cause the existence of high temperature regions.

More intelligent and efficient thermal environment management: A hybrid model for occupant-centric thermal comfort monitoring in vehicle cabins

Online monitoring of thermal comfort in non-uniform and asymmetry thermal environments is essential for achieving more intelligent and efficient vehicle cabin thermal environment management. This study introduces a new approach that provides real-time feedback to a Heating, Ventilation, and Air Conditioning (HVAC) system controller concerning the occupants' thermal sensation through non-intrusive measurements. Specifically, the cabin thermal environment is divided into multiple thermal zones and a hybrid model is proposed. The hybrid model contains two sub-models, a data-driven model and a physics-based model. The data-driven model is a temperature soft sensor that non-intrusively estimates the thermal environment parameters for each thermal zone and passes the results to the physics-based model. The physics-based model can predict the overall thermal sensation of each occupant and consider the weighted effect of local thermal sensation on the overall thermal sensation. Two vehicles' cabin thermal environment data and twenty young subjects' thermal comfort votes were collected, including both cooling and heating conditions. We conclude that the hybrid model can infer cabin thermal environment parameters and thermal comfort based on limited sensors, specifically providing thermal comfort monitoring for occupants in each seating position, which has not been much investigated in previous research. In this regard, our research will contribute to broadening the consideration of individualized thermal comfort in existing vehicles.

Investigation into effect of non-uniform thermal environment on thermal sensation under stratum ventilation for heating by using interpolation-based multi-level fuzzy comprehensive evaluation

Stratum ventilation for heating is applied to provide satisfactory indoor thermal environments with great potentials in winter. Indoor non-uniform environments are important for both overall and local thermal sensations in the same occupied subzone (i.e., vertical spaces) as well as different subzones of the occupied zone (i.e., horizontal spaces) under the effect of supply air jets. This study carried out field chamber experiments to consider the effect of non-uniform environments on both overall and local thermal sensations under stratum ventilation for heating using the multi-level fuzzy comprehensive evaluation (MFCE) method. This method has a multi-level structure to simultaneously investigate thermal sensation in both vertical spaces and horizontal spaces. The interpolation models of thermal sensation are helpful to enlarge the experimental sample size investigated by MFCE. The results showed that the interpolation-based MFCE method can quantitatively analyze the effect of non-uniform environments on thermal sensation in different subzones of the occupied zone from vertical spaces and horizontal spaces under the effect of supply air parameters. Compared with supply air temperature, supply air velocity has the opposite influence on the variation range of both integrated thermal sensations of feet, head and overall body and local thermal sensation of feet between the occupied subzones 1 and 2. Supply air temperature and velocity of 25–27 °C and no less than 1.1 m/s were recommended to improve the non-uniformity of thermal environments.

Study on physiological response considering blood flow volume in transient and non-uniform bathing thermal environment using thermo-cardiovascular regulation model

In transient and non-uniform thermal environments, such as bathing, the human thermal physiological response changes dramatically and accidents often occur owing to cardiovascular system diseases. Blood flow volume is a crucial index of the cardiovascular system; thus, studying the human thermal physiological response considering blood flow volume in a bathing thermal environment is essential. In this paper, we proposed a novel integrated thermo-cardiovascular regulation model that merged a human thermoregulation model (HTM) and a cardiovascular regulation model (CRM), to predict the human physiological response in a transient and non-uniform bathing environment considering blood flow volume and the thermal environment, metabolic rate, and heart rate changes. First, based on the Stolwijk model, more nodes representing different human body parts have been added to the HTM, and it takes into consideration metabolic rate changes. Second, based on the Windkessel model, the circulatory system of the CRM is divided into four parts. Meanwhile, the changes in heart rate over time is used to adjust the cardiac output of the CRM's circulation in real time. To assess the accuracy of the prediction model, we conducted a bathing experiment with a 50-year-old man. The results revealed that the proposed model could accurately predict the participant's thermal physiological response, including skin temperature, core temperature, blood pressure, and blood flow volume during bathing.

Three-dimensional and high-resolution building energy simulation applied to phase change materials in a passive solar room

Phase change materials (PCMs) reduce energy consumption and improve indoor thermal comfort in buildings. Various numerical models have been developed to evaluate the thermal performance of the PCMs integrated into building enclosures. To remain computationally efficient, these models typically adopt a trade-off between the ability to investigate complex and realistic configurations and the prediction accuracy of the heat transfer associated with the phase change. This study developed a model to simulate a room with three-dimensional heat conduction in PCM-integrated walls and to analyze the surface heat balances with high resolution. The model was validated using experimental data of a passive solar test cell with a non-uniform and dynamic thermal environment. The model was then applied to investigate the PCMs integrated into the test cell during the summer. The PCM with a narrow phase change temperature range and a thickness of approximately 10mm was highly effective in reducing the indoor temperature fluctuations; the peak surface temperature was reduced by up to 6°C in the sun patch, and the operative temperature fluctuations decreased by up to 2.6°C. The PCM integration could be limited to the sun patch trajectory on the wall surfaces to optimize its utilization and limit the installation cost.

Nonlinear vibration responses of lattice sandwich beams with FGM facesheets based on an improved thermo-mechanical equivalent model

An investigation on the nonlinear vibration behaviors of a lattice sandwich beam with pyramidal truss core and functionally graded material (FGM) facesheets under thermo-mechanical loads is performed with consideration of the temperature-dependency of material properties. In present work, an improved equivalent methodology is proposed to determine the transverse shear modulus of pyramidal core subjected to a non-uniform thermal field across thickness direction and makes lattice truss core transformed to a continuous layer. Finally, the lattice sandwich beam is modeled as a three-layered sandwich structure. The facesheets are made of FGMs with mixture of ceramics and metals in a pow-law form. The outer surfaces of facesheets are ceramic-rich, while the inner surfaces are metal-rich. The nonlinear strain–displacement relations are established based on the von Kármán large deflection and classical beam theory. The Lagrange method are implemented to yield equations governing the free and forced vibration behaviors. With the help of Newton-Raphson iteration technique as well as the Newmark-β method, the nonlinear time-independent responses of the sandwich beam under thermal environment are solved. A comprehensive parameter study is directed to address the effects of non-uniform thermal environment, FMG facesheets, external load, and lattice core on the nonlinear vibrations of the beams.

Acoustic Radiation Performance of a Composite Laminated Plate Subjected to Local Temperature

Composite structures are extensively applied because of their high specific stiffness-to-weight ratios and other advantages. In various scenarios, composite structures are elastically coupled and usually exposed to a nonuniform thermal environment. This paper presents numerical simulation studies on the vibroacoustic characteristics of a composite laminated plate with elastic supports subjected to local temperature. The thermal stiffness matrix and stiffness matrix of the elastic boundary are first derived based on the finite element method, and then a coupled vibroacoustic model is developed to calculate the acoustic properties of the structure. A comprehensive study is performed to highlight the effect of the temperature loading position, temperature range, and heated area on the acoustic radiation responses of composite laminated plates with different elastic supports. The results show that the existence of thermal stress has a great influence on the acoustic radiation of the structure at low frequencies, which is affected by the boundary constraints and heated areas.

Occupant-centered evaluation on indoor environments and energy savings of radiant cooling systems with high-intensity solar radiation

Solar radiation has an overwhelming influence on indoor thermal and light environments, as well as building energy consumption. In this study, radiant cooling systems were applied to reduce the negative effects of solar radiation. The tests were carried out with the floor radiant cooling system (FRC), ceiling radiant cooling system (CRC), ·and conventional air conditioning system (AC), under typical warm days. An occupant-centered body model was presented to comprehensively evaluate the non-uniform thermal environment caused by solar heat. Besides, the effects on the cooling performance of cooling systems were evaluated by simulation and experiments. The results demonstrate that the cooling capacity rapidly increased with the intensity of the solar radiation. During beam-exposed period, cooling capacity of FRC system increased from 55.6 W/m2 to 210.0 W/m2 and the proportion of shortwave radiant heat flux qsr reached 84%. Furthermore, solar radiation also deteriorated the non-uniform thermal environment. The FRC system could reduce the mean radiant temperature by the maximum of 8.0 °C. Exposed to the beam solar radiation was the main reason that caused large vertical stratification of temperature and dissatisfaction. The FRC system effectively performed better with the maximum percentage dissatisfied value reduced by 14.76 %. Finally, the energy-saving potential of RC systems was analyzed. After the set-point temperature was corrected, the FRC system reduced the typical day’s energy consumption by 21.7% and annual energy consumption by 12.2%. Appropriate adoption of RC systems is an energy-saving method to improve thermal comfort of indoor environments with high-intensity solar radiation.

Method of spherical triangle-division for solving view factors in built environment and its application for non-uniform thermal radiation environments

Radiative heat transfer is the focus of many studies on heat transfer issues in the built environment, and the view factor (VF) between the surfaces of built objects in space is a crucial parameter for quantitative analysis of radiative heat transfer. In the built environment, not only the types of space are diverse, but also the forms of the objects of interest are often different from each other. Studying the thermal radiation heat transfer in complex space usually requires to simplify the calculation of VF between the surfaces, based on specific studied problems. In this paper, the method of Spherical Triangle-division is constructed by mathematical derivation through spatial analytic geometry and surface integral. The analytical formula is then developed to calculate VF from the surface element to the surface of the polygonal plane, with arbitrary orientation in space. Using the geometric transformation in terms of planar projection and polygonal intersection, the method can also be used to solve the VF in occlusion conditions, and it is proven to be more accurate. Finally, the application of the method is discussed, which aims at providing novel ideas and tools for the quantitative analysis of non-uniform thermal radiation issues in the built environment.

Assessment of thermal comfort of frail people in a sitting posture under non-uniform conditions using a thermal manikin

Thermal comfort has gradually become an important research field in the context of sustainable living. Several thermal comfort studies have been conducted in thermally uniform environments, though far fewer are being performed in non-uniform thermal conditions. Recently, the evaluation of thermal responses in non-uniform thermal environments, has come to the attention of researchers. This study aims to evaluate the thermal responses of frail people using a “Newton” thermal manikin and a test chamber. Personalized cooling and heating floor systems were used to produce non-uniform conditions by supplying global cooling air flow and vertical heat flow to the whole manikin body. In this paper, skin temperature, core temperature, and thermal responses were examined and discussed in both males and females. The analysis showed a significant difference in thermal sensation and dynamic thermal sensation between females and males over time, and under the same experimental conditions. On the other hand, the ANOVA analysis revealed a significant difference in the main effect of gender in terms of thermal comfort. Results show that thermal discomfort in frail people originated largely from air temperature. It was found that air temperature increments of 5 °C–30 °C can modify thermal comfort by 1.5 scale units’ in comparison with the thermal sensation.