The impact of temperature on mean local air age and thermal comfort in a stratum ventilated office

The influence of the supply air temperature on the mean local air age and thermal comfort of a typical individual office under stratum ventilation is investigated by a numerical method, which is validated by an experiment carried out by the authors. The results show that for an office, when the supply air temperature is increased from 19 °C to 21 °C, the corresponding mean occupied zone temperature rises from 24.5 °C to 26.5 °C. The inhaled air quality for the occupant is improved when supply air temperature rises from 19 °C to 21 °C. Also, the thermal comfort indices (predicted mean vote or PMV, predicted percentage of dissatisfied or PPD and predicted dissatisfied or PD) fulfill the requirements of ISO 7730 and CR 175 1998. For summer cooling operation, stratum ventilation may offer a feasible solution to elevated indoor temperatures, which are recommended by several governments in East Asia.     

Experimental study of airflow characteristics of stratum ventilation in a multi‐occupant room with comparison to mixing ventilation and displacement ventilation

The motivation of this study is stimulated by a lack of knowledge about the difference of airflow characteristics between a novel air distribution method [i.e., stratum ventilation (SV)] and conventional air distribution methods [i.e., mixing ventilation (MV) and displacement ventilation (DV)]. Detailed air velocity and temperature measurements were conducted in the occupied zone of a classroom with dimensions of 8.8 m (L) × 6.1 m (W) × 2.4 m (H). Turbulence intensity and power spectrum of velocity fluctuation were calculated using the measured data. Thermal comfort and cooling efficiency were also compared. The results show that in the occupied zone, the airflow characteristics among MV, DV, and SV are different. The turbulent airflow fluctuation is enhanced in this classroom with multiple thermal manikins due to thermal buoyancy and airflow mixing effect. Thermal comfort evaluations indicate that in comparison with MV and DV, a higher supply air temperature should be adopted for SV to achieve general thermal comfort with low draft risk. Comparison of the mean air temperatures in the occupied zone reveals that SV is of highest cooling efficiency, followed by DV and then MV.     

Experimental investigation of thermal and ventilation performances of stratum ventilation

Stratum ventilation has been proposed to cope for elevated indoor temperatures. Air speed, temperature and CO₂ concentration of a stratum ventilated office are investigated experimentally. The data obtained under well defined conditions and therefore can be used for validating numerical models. Thermal comfort conditions and ventilation efficiency are studied based on the experimental results of four experimental cases. Thermal comfort indices, i.e. PMV, PPD and PD are calculated from measured data. The values of these indices are found to satisfy the requirements of ISO 7730, CR 1752-1998 and ASHRAE 55-2010. In terms of thermal comfort, the two cases with supply air temperature of 21 °C are found to perform better compared with the two cases with supply air temperature of 19 °C. For all the cases, the ventilation effectiveness is close to 1.5. This ventilation method could therefore be expected to provide indoor air quality in an efficient way.     

Comfort and airflow evaluation in spaces equipped with mixing ventilation and cold radiant floor

In this work the comfort and airflow were evaluated for spaces equipped with mixing ventilation and cold radiant floor. In this study the coupling of an integral multi-nodal human thermal comfort model with a computational fluid dynamics model is developed. The coupling incorporates the predicted mean vote (PMV) index, for the heat exchange between the body and the environment, with the ventilation effectiveness to obtain the air distribution index (ADI) for the occupied spaces with non-uniform environments. The integral multi-nodal human thermal comfort model predicts the external skin and clothing surfaces temperatures and the thermal comfort level, while the computational fluid dynamics model evaluates the airflow around the occupants. The air distribution index, that was developed in the last years for uniform environments, has been extended and implemented for non-uniform thermal environments. The airflow inside a virtual chamber equipped with two occupants seated in a classroom desk, is promoted by a mixing ventilation system with supply air of 28 °C and by a cold radiant floor with a surface temperature of 19 °C. The mechanical mixing ventilation system uses a supply and an exhaust diffusers located above the head level on adjacent walls.     

Analysis on the impact of mean radiant temperature for the thermal comfort of underfloor air distribution systems

Despite the potentially significant advantages of underfloor air distribution (UFAD) systems, the shortcomings in fundamental understanding have impeded the use of UFAD systems. A study has been carried out on the thermal stratification which is crucial to system design, energy efficient operation and comfort performance of UFAD systems with an aim of examining impact of mean radiant temperature (MRT) on thermal comfort. Clear elucidation of the benefit of UFAD systems has been shown by comparing it to the traditional overhead air distribution systems. Keeping the same level of comfortable environment in the occupied zone, UFAD systems require much higher temperature of supply air, which represents significant energy savings. The benefit of UFAD systems is more pronounced at the condition of high ceiling height building. Considerable discrepancies in thermal comfort are found on the assumption that air temperature rather than MRT is used for the evaluation of PMV. However, more rigorous analysis including the full radiation simulation does not show any significant difference in PMV distribution. The result of the full radiation simulations requires much longer simulation time but gives similar air temperature distribution and only slightly higher averaged temperature than present approaches.     

Parametric studies and evaluations of indoor thermal environment in wet season using a field survey and PMV-PPD method

Fanger's PMV-PPD is the most famous thermal sensation indices but it is too complex to be applied in practice. To obtain simple and applicable correlations, taking Qujing of Yunnan province, China, as example, a wet season (six-month) field measurement was conducted in a naturally ventilated residential room. Based on collected data, PMV indices were calculated by using Newton's iterative method. It is shown that the PMV values approximately vary from −1.0 to +1.0 and the indoor thermal environment is basically comfortable. Relationships of the parameters (indoor and outdoor air temperatures, mean radiant temperature, PMV and PPD) and indoor air temperature gradients (vertical and horizontal) were also studied by means of the linear regression and the quadratic polynomial fit techniques. Numerous correlations with high relativities have been developed. Moreover, the vertical and horizontal air temperature gradients range from 0.1 K/m to 0.85 K/m and from −0.208 K/m to 0.063 K/m in wet season. It is convenient to use these results to evaluate and assess the indoor thermal environment under similar climatic conditions. The results of this work enrich and develop the basic theory of the indoor thermal environment design and control.     

Predicted percentage dissatisfied with ankle draft

Draft is unwanted local convective cooling. The draft risk model of Fanger et al. (Energy and Buildings 12 , 21‐39, 1988) estimates the percentage of people dissatisfied with air movement due to overcooling at the neck. There is no model for predicting draft at ankles, which is more relevant to stratified air distribution systems such as underfloor air distribution (UFAD) and displacement ventilation (DV). We developed a model for predicted percentage dissatisfied with ankle draft (PPD_AD) based on laboratory experiments with 110 college students. We assessed the effect on ankle draft of various combinations of air speed (nominal range: 0.1‐0.6 m/s), temperature (nominal range: 16.5‐22.5°C), turbulence intensity (at ankles), sex, and clothing insulation (<0.7 clo; lower legs uncovered and covered). The results show that whole‐body thermal sensation and air speed at ankles are the dominant parameters affecting draft. The seated subjects accepted a vertical temperature difference of up to 8°C between ankles (0.1 m) and head (1.1 m) at neutral whole‐body thermal sensation, 5°C more than the maximum difference recommended in existing standards. The developed ankle draft model can be implemented in thermal comfort and air diffuser testing standards.     

空调工况下空气柱送风仿真模拟与舒适度评价

本文对空调工况下空气柱(上送下回)送风的气流组织,温度场,速度场,空气龄进行了仿真模拟。结果表明。制冷工况下,送风温度为18℃、20℃,风速为3 m/s、空气柱高度为2~2.5 m时,单柱舒适区大小为21 m×21 m×1.7 m,区域ADPI值均大于80%,平均PMV值为-0.34,平均PDD值为8.67%。制热工况下(上送下回),送风温度为26℃、28℃、30℃,风速为2~4 m/s、空气柱高度为2~2.5 m时,区域ADPI值均大于90%,平均PMV值为-0.47,平均PDD为10.2%。单柱区域内制冷工况热舒适性优于制热工况舒适性。     

Comparison of gaseous contaminant diffusion under stratum ventilation and under displacement ventilation

The gaseous contaminant diffusion under stratum ventilation is investigated by numerical method which is validated by experiments carried out. The concentration of gaseous contaminants along the supply air jet is found to be lower than the other parts of the room. Compared with displacement ventilation, the formaldehyde concentration in breathing zone is lower when a contaminant source locates close to the occupant. The concentration is at the same level when the contaminant source locates up-steam to the occupant. The concentration in the occupied zone (<1.9 m from the floor) is also lower when the contaminant source locates on the floor. At supply air temperature optimized for displacement ventilation, the toluene concentration in breathing zone for stratum ventilation is higher than that for displacement ventilation when the area source locates on the four surrounding walls of the room.     

Experimental investigation into the interaction between the human body and room airflow and its effect on thermal comfort under stratum ventilation

Room occupants' comfort and health are affected by the airflow. Nevertheless, they themselves also play an important role in indoor air distribution. This study investigated the interaction between the human body and room airflow under stratum ventilation. Simplified thermal manikin was employed to effectively resemble the human body as a flow obstacle and/or free convective heat source. Unheated and heated manikins were designed to fully evaluate the impact of the manikin at various airflow rates. Additionally, subjective human tests were conducted to evaluate thermal comfort for the occupants in two rows. The findings show that the manikin formed a local blockage effect, but the supply airflow could flow over it. With the body heat from the manikin, the air jet penetrated farther compared with that for the unheated manikin. The temperature downstream of the manikin was also higher because of the convective effect. Elevating the supply airflow rate from 7 to 15 air changes per hour varied the downstream airflow pattern dramatically, from an uprising flow induced by body heat to a jet‐dominated flow. Subjective assessments indicated that stratum ventilation provided thermal comfort for the occupants in both rows. Therefore, stratum ventilation could be applied in rooms with occupants in multiple rows.     

地板辐射与置换通风空调系统运行参数

建立了基于EnergyPlus的地板辐射供冷加置换通风空调系统模型,模拟得到的室内温度和辐射地板所承担冷量与实验结果的误差小于±7%。在此模型基础上,改变送风参数和供水参数,得到置换通风供冷量、辐射地板供冷量、地板表面温度、室内空气平均温度、AUST温度等参数的变化规律。结合热舒适性模型,得到满足室内热舒适性(-0.5≤PMV≤0.5)条件下,置换通风的送风参数和辐射地板的供水参数范围,为复合系统设计和应用提供依据。     

置换通风热力分层高度的数值研究

根据置换通风系统的工作原理,介绍了置换通风热力分层高度的定义及数值计算方法,并对送风温度、送风速度等因素对热力分层高度的影响作了定性分析.在此基础上拟合出量纲一热力分层高度的经验公式.     

Evaluation of thermal comfort using combined CFD and experimentation study in a test room equipped with a cooling ceiling

This paper reports a full-scale experimental campaign and a computational fluid dynamics (CFD) study of a radiant cooling ceiling installed in a test room, under controlled conditions. This research aims to use the results obtained from the two studies to analyze the indoor thermal comfort using the predicted mean vote (PMV). During the whole experimental tests the indoor humidity was kept at a level where the condensation risk was minimized and no condensation was detected on the chilled surface of the ceiling. Detailed experimental measurements on the air temperature distribution, surface temperature and globe temperature were realized for different cases where the cooling ceiling temperature varied from 16.9 to 18.9 °C. The boundary conditions necessary for the CFD study were obtained from the experimental data measurements. The results of the simulations were first validated with the data from the experiments and then the air velocity fields were investigated. It was found that in the ankle/feet zone the air velocity could pass 0.2 m/s but for the rest of the zones it took values less than 0.1 m/s. The obtained experimental results for different chilled ceiling temperatures showed that with a cooling ceiling the vertical temperature gradient is less than 1 °C/m, which corresponds to the standard recommendations. A comparison between globe temperature and the indoor air temperature showed a maximum difference of 0.8 °C being noticed. This paper also presents the radiosity method that was used to calculate the mean radiant temperature for different positions along different axes. The method was based on the calculation of the view factors and on the surface temperatures obtained from the experiments. PMV plots showed that the thermal comfort is achieved and is uniformly distributed within the test room.     

Field study on occupant comfort and the office thermal environment in rooms with displacement ventilation

A field survey of occupants' response to the indoor environment in 10 office buildings with displacement ventilation was performed. The response of 227 occupants was analyzed. About 24% of the occupants in the survey complained that they were daily bothered by draught, mainly at the lower leg. Vertical air temperature difference measured between head and feet levels was less than 3 degrees C at all workplaces visited. Combined local discomfort because of draught and vertical temperature difference does not seem to be a serious problem in rooms with displacement ventilation. Almost one half (49%) of the occupants reported that they were daily bothered by an uncomfortable room temperature. Forty-eight per cent of the occupants were not satisfied with the air quality. PRACTICAL IMPLICATIONS: The PMV and the Draught Rating indices as well as the specifications for local discomfort because of the separate impact of draught and vertical temperature difference, as defined in the present standards, are relevant for the design of a thermal environment in rooms with displacement ventilation and for its assessment in practice. Increasing the supply air temperature in order to counteract draught discomfort is a measure that should be considered carefully; even if the desired stratification of pollution in the occupied zone is preserved, an increase of the inhaled air temperature may have a negative effect on perceived air quality.     

Thermal comfort in environments with different vertical air temperature gradients

The use of displacement ventilation for cooling environments is limited by the vertical temperature gradient. Current standards recommend a temperature difference of up to 3 K/m between the head and the feet. This paper reviews the scientific literature on the effect of vertical temperature gradients on thermal comfort and compares this to the results of our own experiments. Early experiments have demonstrated a high sensitivity of dissatisfied test subjects to changes in the temperature gradient between head and foot level. Recent studies have indicated that temperature gradients of 4‐5 K/m are likely to be acceptable, and the mean room temperature may have a greater sensitivity on the percentage of dissatisfied (PD). In new experiments, test subjects have evaluated the thermal comfort of different vertical air temperature gradients in a modular test chamber, the Aachen comfort cube (ACCu), where they have assessed vertical temperature gradients of ΔT/Δy = 1, 4.5, 6, 8, and 12 K/m at a constant mean room temperature of 23°C. The results of the different temperature gradients are in contrast to ANSI/ASHRAE Standard 55 (Thermal Environmental Conditions for Human Occupancy, Atlanta GA, American Society of Heating, Refrigerating and Air Conditioning Engineers, 2013) as the PD increases almost constantly with higher vertical air temperature gradients. The PD for the overall sensation increases by approximately 7% between gradients of 1 and 8 K/m. The evaluation of our own tests has revealed that vertical temperature gradients of up to 8 K/m or higher are likely to be acceptable for test subjects.     

A new predictive thermal sensation index of human response

The purpose of this paper is to investigate the problem of determining a human thermal sensation index that can be used in feedback control of HVAC systems. We present a new approach based on fuzzy logic to estimate the thermal comfort level depending on the state of the following six variables: the air temperature, the mean radiant temperature, the relative humidity, the air velocity, the activity level of occupants and their clothing insulation. The new fuzzy thermal sensation index is calculated implicitly as the consequence of linguistic rules that describe human's comfort level as the result of the interaction of the environmental variables with the occupant's personal parameters. The fuzzy comfort model is deduced on the basis of learning Fanger's `Predicted Mean Vote' (PMV) equation. Unlike Fanger's PMV, the new fuzzy PMV calculation does not require an iterative solution and can be easily adjusted depending on the specific thermal sensation of users. These characteristics make it an attractive index for feedback control of HVAC systems. The simulation results show that the new fuzzy PMV is as accurate as Fanger's PMV.     

Analysis of thermal comfort in a residential room with insect proof screen: A case study by numerical simulation methods

The present paper aims to investigate the characteristics of airflow inside the room with insect proof screens by employing computation fluid dynamics (CFD) technique. Insect proof screens attached to the window openings were simulated by porous media approach. The simulated pressure drop across the insect proof screen was compared with the experimental result and is having a good agreement with a maximum error of 8.77% for the air velocity of 0.15 m/s. The insect proof screen significantly reduces the airflow rate and increases the indoor air temperature by 3°C. The effect of different window sizes and porosity of the insect proof screen is included for the study of thermal comfort index, predicted mean vote (PMV). From the PMV contours, center portion of the room and portions nearer to the windows are identified as comfort zones.     

Elevated airflow can maintain sleep quality and thermal comfort of the elderly in a hot environment

The effect of elevated airflow on sleep quality was investigated with 18 elderly. Three airflow conditions were set: ceiling fan/30°C/max.0.8 m/s and mean 0.7 m/s, task fan/30°C/max.0.8 m/s and mean 0.6 m/s, and thermally neutral /27°C/0.2 m/s. Sleep quality was evaluated objectively by analysis of electroencephalogram signals that were continuously monitored during the sleeping period. Urinary cortisol concentrations were analyzed to measure the activity of sympathetic nervous system. No significant difference in sleep quality, thermal comfort, or cortisol concentration was found between the ceiling fan and the neutral condition. The duration of total sleep time decreased by 35 minutes, the duration of REM sleep decreased by 15 minutes, and the cortisol concentration in the morning increased by 50 ng/mL in the task fan than the other two conditions. Compared with ceiling fan, less heat load was removed in the task fan condition, possibly due to the lower air speed. This study shows that even small heat load led to reduced sleep quality and overactive sympathetic nervous system of the elderly. By supplying an airflow of 0.8 m/s evenly over the human body, the elderly could maintain sleep quality and thermal comfort at an air temperature that was 3 K higher than the neutral temperature.     

Thermal effect of temperature gradient in a field environment chamber served by displacement ventilation system in the tropics

The effect of vertical air temperature gradient on overall and local thermal comfort at different overall thermal sensations and room air temperatures (at 0.6 m height) was investigated in a room served by displacement ventilation system. Sixty tropically acclimatized subjects performed sedentary office work for a period of 3 h during each session of the experiment. Nominal vertical air temperature gradients between 0.1 and 1.1 m heights were 1, 3 and 5 K/m while nominal room air temperatures at 0.6 m height were 20, 23 and 26 °C. Air velocity in the space near the subjects was kept at below 0.2 m/s. Relative humidity at 0.6 m height was maintained at 50%. It was found that temperature gradient had different influences on thermal comfort at different overall thermal sensations. At overall thermal sensation close to neutral, only when room air temperature was substantially low, such as 20 °C, percentage dissatisfied of overall body increased with the increase of temperature gradient. At overall cold and slightly warm sensations, percentage dissatisfied of overall body was non-significantly affected by temperature gradient. Overall thermal sensation had significant impact on overall thermal comfort. Local thermal comfort of body segment was affected by both overall and local thermal sensations.     

Field study of human thermal comfort and thermal adaptability during the summer and winter in Beijing

This study was conducted during the summer and winter in Beijing. Classrooms and offices in a university were used to conduct the survey. The respondents’ thermal sensation and thermal adaptability in both seasons were analyzed. During the study, indoor environmental parameters including air temperature, mean radiant temperature, relative humidity, and air velocity were measured. The respondents’ thermal sensation was determined by questionnaire.A relationship between indoor temperature and thermal sensation was found. In the summer study, the “scissors difference” between TSV and PMV was observed in the air-conditioned environments if the temperature was out of the neutral zone. People had higher tolerance in the hot environment than PMV predicted. During winter, the outdoor temperature had a prominent influence on thermal adaptability. The low outdoor temperature made people adapt to the cold environment. When the indoor temperature was heated to a high temperature by space heating facilities, respondents felt uncomfortable since their adaptability to the cold environment was nullified.Furthermore, the differences in thermal responses between respondents from North and South China showed that the different climates of people's native regions also affected their thermal comfort and adaptability.