Local thermal sensation and comfort study in a field environment chamber served by displacement ventilation system in the tropics

This paper presents a study of local thermal sensation (LTS) and comfort in a field environmental chamber (FEC) served by displacement ventilation (DV) system. The FEC, 11.12 m (L)×7.53 m (W)×2.60 m (H), simulates a typical office layout. A total of 60 tropically acclimatized subjects, 30 male and 30 female, were engaged in sedentary office work for 3 h. Subjects were exposed to three vertical air temperature gradients, nominally 1, 3 and 5 K/m, between 0.1 and 1.1 m heights and three room air temperatures of 20, 23 and 26 °C at 0.6 m height. The objective of this study is to investigate the mutual effect of local and overall thermal sensation (OTS) and comfort in DV environment. The results show that in a space served by DV system, at OTS close to neutral, local thermal discomfort decreased with the increase of room air temperature. The OTS of occupants was mainly affected by LTS at the arm, calf, foot, back and hand. Local thermal discomfort was affected by both LTS and OTS. At overall cold thermal sensation, all body segments prefer slightly warm sensation. At overall slightly warm thermal sensation, all body segments prefer slightly cool sensation.     

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.     

Assessment of thermal environment using a thermal manikin in a field environment chamber served by displacement ventilation system

This paper presents a thermal comfort study using a thermal manikin in a field environment chamber served by the Displacement Ventilation (DV) system. The manikin has a female body with 26 individually heated and controlled body segments. The manikin together with subjects was exposed to 3 levels of vertical air temperature gradients, nominally 1, 3 & 5 K/m, between 0.1 and 1.1 m heights at 3 room air temperatures of 20, 23 and 26 °C at 0.6 m height. Relative humidity at 0.6 m height and air velocity near the manikin and the subjects were maintained at 50% and less than 0.2 m/s, respectively. The aims of this study are to assess thermally non-uniform environment served by DV system using the manikin and correlate the subjective responses with measurements from the manikin. The main findings indicate that room air temperature had greater influence on overall and local thermal sensations and comfort than temperature gradient. Local thermal discomfort decreased with increase of room air temperature at overall thermally neutral state. The local discomfort was affected by overall thermal sensation and was lower at overall thermally neutral state than at overall cold and cool sensations.     

Computer modeling of displacement ventilation systems based on plume rise in stratified environment

A model for displacement ventilation system based on plume rise of single point heat source was developed. The errors for temperature gradient ratio were less than 6% in most cases. Errors for temperature gradient and displacement zone height were relatively higher (up to 28.1%) which might be due to the derivation of the parameters from experimental data. Still, the errors were lower than those from design model/method of some other workers (68.5% for the temperature gradient ratio and 15.7% for the temperature difference between the supply air and at 0.1 m above floor level). With a room height of 2.4 m (common for office in Hong Kong) and design room temperature 25.5 °C defined at 1.1 m above floor level under the normal load to air flow ratio of 12,000 W/m³/s (typical values for sub-tropical region) and minimum supply temperature of 18 °C, there existed a zone capacity range from 1000 to 5000 W that stand alone operation displacement ventilation system was feasible and that the displacement zone height (minimum 2.2 m) was above normal breathing level. The feasible zone capacity range diminished with decrease in design room temperature and/or room height. In this case, the load to air flow ratio had to be reduced, resulting in a higher flow rate when compared to a mixing ventilation system, or an auxiliary cooling facility such as a chilled ceiling had to be used.     

Performance of “ductless” personalized ventilation in conjunction with displacement ventilation: Impact of intake height

The importance of the intake positioning height above the floor level on the performance of “ductless” personalized ventilation (“ductless” PV) in conjunction with displacement ventilation (DV) was examined with regard to the quality of inhaled air and of the thermal comfort provided. A typical office room with two workstations positioned one behind the other was arranged in a full-scale room. Each workstation consisted of a table with an installed “ductless” PV system, PC, desk lamp and seated breathing thermal manikin. The “ductless” PV system sucked the clean and cool displacement air supplied over the floor at four different heights, i.e. 2, 5, 10 and 20 cm and transported it direct to the breathing level. Moreover, two displacement airflow rates were used with a supply temperature adjusted in order to maintain an exhaust air temperature of 26 °C. Two pollution sources, namely air exhaled by one of the manikins and passive pollution on the table in front of the same manikin were simulated by constant dosing of tracer gases. The results show that the positioning of a “ductless” PV intake height up to 0.2 m above the floor will not significantly influence the quality of inhaled air and thermal comfort.     

Numerical study of the lock-up phenomenon of human exhaled droplets under a displacement ventilated room

This paper adopts an Eulerian-Lagrangian approach to investigate the lock-up phenomenon (or trap phenomenon) of human exhaled droplets in a typical office room under displacement ventilation (DV). A particle-source-in-cell (PSI-C) scheme is used to correlate the concentration with the Lagrangian particle trajectories in computational cells. Respiratory droplets with sizes of 0.8 μm, 5 μm and 16 μm are released from a numerical thermal manikin (NTM). The influence factors including indoor temperature gradient, heat source configuration and exhalation modes are studied. It is found that large temperature gradient would result in trap phenomenon of small exhaled droplets (smaller than 5 μm). The intensive heat source near the NTM could help to transport the small droplets to the upper zone and decrease the concentration level in the trapped zone. Both nose-exhaled and mouth-exhaled small droplets would be trapped at the breathing height when temperature gradient is sufficiently high. However, the trap height of the droplets from mouth is a little bit higher. Because of large gravitational force, it is difficult for the thermal plume to carry 16 μm respiratory droplets to the upper zone.     

Effect of warm air supplied facially on occupants’ comfort

Human response to air movement supplied locally towards the face was studied in a room with an air temperature of 20 °C and a relative humidity of 30%. Thirty-two human subjects were exposed to three conditions: calm environment and facially supplied airflow at 21 °C and at 26 °C. The air was supplied with a constant velocity of 0.4 m/s by means of personalized ventilation towards the face of the subjects. The airflow at 21 °C decreased the subjects' thermal sensation and increased draught discomfort, but improved slightly the perceived air quality. Heating of the supplied air by 6 K (temperature increase by 4 K at the target area) above the room air temperature decreased the draught discomfort, improved subjects' thermal comfort and only slightly decreased the perceived air quality. Elevated velocity and temperature of the localized airflow caused an increase of nose dryness intensity and number of eye irritation reports. Results suggest that increasing the temperature of the air locally supplied to the breathing zone by only a few degrees above the room air temperature will improve occupants' thermal comfort and will diminish draught discomfort. This strategy will extend the applicability of personalized ventilation aiming to supply clean air for breathing at the lower end of the temperature range recommended in the standards. Providing individual control is essential in order to avoid discomfort for the most sensitive occupants.     

Influence of fire source locations on the actuation of wet-type sprinklers in an office fire

An experiment is conducted on a full-scale model office and an actual sprinkler system to explore the influence of fire source locations on sprinkler actuation. The office space is a brick structure that measures 5.7 m in interior length, 4.7 m in width and 2.4 m in ceiling height, and equipped with a sprinkler system. The investigated fire source (100 kW LPG burner) locations include the room center, wall centers, room corner, and other locations at different distances from sprinklers. The results show that actuation of the sprinklers is affected by the fire source locations and the heat conduction properties of the glass temperature-sensing bulb. Average actuation time of all the tests is 102 s, around 40 s faster than if the fire source is located in the room center. For fire sources in corners, sprinklers are quickly activated at the experimental time 75 s, showing concentrated hot gas flow.     

Air quality in rooms conditioned by chilled ceiling and mixed displacement ventilation for energy saving

A transient-contaminant-transport model is developed for assessing IAQ in the breathing zone when introducing return air into rooms conditioned by CC/DV system to save energy. The steady state transport model of [1] is extended to transient conditions while accounting for significant wall plumes associated with external loads.Experiments are performed to validate the extended model predictions of IAQ expressed in the level of CO₂ concentration. Experiments are conducted in a chamber with two external walls in Kuwait Climate. Measurements are recorded in time of the air temperature and CO₂ concentration at selected locations in the room and compared with values predicted by the model. Experimental results agreed well with model predictions. The maximum errors in predicted CO₂ concentrations are less than ±25 ppm in presence of external load. 60% fresh air fraction resulted in 37% less energy consumption compared with 100% fresh air CC/DV system.The validated model is applied to a case study in Kuwait to evaluate energy saving over the cooling season for a typical office space while using mixed DV air. Energy savings of up to 20.6% can be realized using mixed supply air while maintaining IAQ compared with energy used for the 100% fresh air.     

Numerical study on temperature stratification in a room with underfloor air distribution system

In this study, numerical prediction using computational fluid dynamics (CFD) was utilized to investigate air temperature stratification in a room with an underfloor air distribution (UFAD) system. The numerical modeling using CFD computation was validated with physical test in a full size experimental room with an UFAD system. The different supply air conditions and heat loads were discussed. The results show that the effect of three parameters, heat load, supply volume flux and supply air velocity, on room air temperature would be expressed by the length scale of the floor supply jet. When the length scale increased from 0.8 to 1.56 m, the ratio of vertical temperature difference between 2.5 and 0.1 m at the occupied zone to the difference between return and supply air temperature decreased from 0.62 to 0.25. When there was only one local heat source in the room, there was a thermal stratified interface at the occupied zone. The interface height was about 1.42 times the length scale. The results may suggest ways to optimize UFAD design and operation.     

Chilled ceiling displacement ventilation design charts correlations to employ in optimized system operation for feasible load ranges

This paper expands Ghaddar et al. [N. Ghaddar, K. Ghali, R. Saadeh, A. Keblawi, Design charts for combined chilled ceiling displacement ventilation system (1438-RP), ASHRAE Transactions, 143 (2) (2008) 574-587] design charts of combined chilled ceiling (CC) displacement ventilation (DV) system to operating sensible load ranges from 40 W/m² to 100 W/m². It develops a global correlation of system load and operational parameters, with comfort measured by vertical temperature gradient and indoor air quality measured by the stratification height. The correlations are used for a known transient load profile in generating optimal settings of the CC/DV system operational parameters and associated energy consumption.An example is illustrated to show how the correlation could be used to size the system and to provide optimized control of the CC/DV system operation at low computational cost. Results of the current model are compared to the published case study of an optimized operation based on transient simulations of the space thermal model to achieve minimum operation cost [M. Mossolly, N. Ghaddar, K. Ghali, L. Jensen, Optimized operation of combined chilled ceiling displacement ventilation system using genetic algorithm, ASHRAE Transactions, 143 (2) (2008) 541-554]. The design correlations resulted in good agreement with published data (within 3% error in energy consumption and average 6% error in predictions of comfort and stratification height) at 1/4 of the computational time. The presented methodology provides an alternative for using the correlation for supervisory online controllers for the CC/DV system based on physically derived correlations.     

The effects of outdoor air supply rate in an office on perceived air quality, sick building syndrome (SBS) symptoms and productivity

Perceived air quality, Sick Building Syndrome (SBS) symptoms and productivity were studied in a normally furnished office space (108 m3) ventilated with an outdoor airflow of 3, 10 or 30 L/s per person, corresponding to an air change rate of 0.6, 2 or 6 h-1. The temperature of 22 degrees C, the relative humidity of 40% and all other environmental parameters remained unchanged. Five groups of six female subjects were each exposed to the three ventilation rates, one group and one ventilation rate at a time. Each exposure lasted 4.6 h and took place in the afternoon. Subjects were unaware of the intervention and remained thermally neutral by adjusting their clothing. They assessed perceived air quality and SBS symptoms at intervals, and performed simulated normal office work. Increasing ventilation decreased the percentage of subjects dissatisfied with the air quality (P < 0.002) and the intensity of odour (P < 0.02), and increased the perceived freshness of air (P < 0.05). It also decreased the sensation of dryness of mouth and throat (P < 0.0006), eased difficulty in thinking clearly (P < 0.001) and made subjects feel generally better (P < 0.0001). The performance of four simulated office tasks improved monotonically with increasing ventilation rates, and the effect reached formal significance in the case of text-typing (P < 0.03). For each two-fold increase in ventilation rate, performance improved on average by 1.7%. This study shows the benefits for health, comfort and productivity of ventilation at rates well above the minimum levels prescribed in existing standards and guidelines. It confirms the results of a previous study in the same office when the indoor air quality was improved by decreasing the pollution load while the ventilation remained unchanged.     

Distribution of respiratory droplets in enclosed environments under different air distribution methods

The dispersion characteristics of respiratory droplets are important in controlling transmission of airborne diseases indoors. This study investigates the spatial concentration distribution and temporal evolution of exhaled and sneezed/coughed droplets within the range of 1.0 − 10.0μm in an office room with three air distribution methods, specifically mixing ventilation (MV), displacement ventilation (DV), and under-floor air distribution (UFAD). The diffusion, gravitational settling and deposition mechanism of particulate matter were accounted by using an Eulerian modeling approach with one-way coupling. The simulation results indicate that exhaled droplets up to 10μm in diameter from normal human respiration are uniformly distributed in MV. However, they become trapped in the breathing zone by thermal stratifications in DV and UFAD, resulting in a higher droplet concentration and an increased exposure risk to other room occupants. Sneezed/coughed droplets are more slowly diluted in DV/UFAD than in MV. Low air speed in the breathing zone in DV/UFAD can lead to prolonged human exposure to droplets in the breathing zone.     

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.     

Room air temperature affects occupants’ physiology,perceptions and mental alertness

Thermal environment that causes thermal discomfort may affect office work performance. However, the mechanisms through which occupants are affected are not well understood. This study explores the plausible mechanism linking room air temperature and mental alertness through perceptual and physiological responses in the tropics. Ninety-six young adults participated as voluntary subjects in a series of experiment conducted in the simulated office settings. Three room air temperatures, i.e. 20.0, 23.0 and 26.0 °C were selected as the experimental conditions. Both thermal comfort and thermal sensation changed significantly with time under all exposures (P < 0.0001). Longer exposure at 20.0 °C led to cooling sensations due to lower skin temperatures (P < 0.0001) and was perceived as the least comfortable. Nevertheless, this moderate cold exposure induced nervous system activation as demonstrated by the increase of α-Amylase level (P < 0.0001) and the Tsai–partington test (P < 0.0001). A mechanism linking thermal environment, occupants' responses and performance is proposed.     

Effects of ventilation rate per person and per floor area on perceived air quality,sick building syndrome symptoms,and decision‐making

Ventilation rates (VRs) in buildings must adequately control indoor levels of pollutants; however, VRs are constrained by the energy costs. Experiments in a simulated office assessed the effects of VR per occupant on perceived air quality (PAQ), Sick Building Syndrome (SBS) symptoms, and decision‐making performance. A parallel set of experiments assessed the effects of VR per unit floor area on the same outcomes. Sixteen blinded healthy young adult subjects participated in each study. Each exposure lasted four hours and each subject experienced two conditions in a within‐subject study design. The order of presentation of test conditions, day of testing, and gender were balanced. Temperature, relative humidity, VRs, and concentrations of pollutants were monitored. Online surveys assessed PAQ and SBS symptoms and a validated computer‐based tool measured decision‐making performance. Neither changing the VR per person nor changing the VR per floor area, had consistent statistically significant effects on PAQ or SBS symptoms. However, reductions in either occupant‐based VR or floor‐area‐based VR had a significant and independent negative impact on most decision‐making measures. These results indicate that the changes in VR employed in the study influence performance of healthy young adults even when PAQ and SBS symptoms are unaffected.     

The effect of turbulence intensity on local skin temperature and subjective responses to draft

The aim of the present study was to investigate whether turbulence intensity affects local skin temperature drop, and whether subjects’ dissatisfaction with draft can be correlated to the local skin temperature drop. Eighteen subjects were exposed to two levels of turbulence intensity (15% and 30%) at mean air velocities of 0.3 and 0.6 m/s respectively for 1 h. Subjective responses to draft were registered every 10 min and skin temperatures at the back of the neck were recorded throughout the sessions. More subjects felt uncomfortable due to draft when exposed to high turbulence than exposed to low turbulence. Skin temperature drop after 1 h exposure under high turbulence intensity is significantly larger than that under low turbulence intensity. Percentage of subjects dissatisfied with draft positively correlated to the local skin temperature drop.     

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.     

Thermal evaluation of a chair with fans as an individually controlled system

It is difficult for a total air-conditioning system to satisfy the thermal comfort of all workers in an office. Therefore, an individually controlled system that can create a comfortable thermal environment for each worker is needed. In the present study, two chairs incorporating two fans each, one under the seat and one behind the backrest, were developed to provide isothermal forced airflow to the chair occupant. The chairs differed in the size of the fans. Experiments were conducted in a climate chamber during the summer. Seven subjects, who were healthy male college students, were allowed to freely control the two built-in fans by adjusting dials on the accompanying desk. The room air temperatures were set at 26 °C, 28 °C, 30 °C and 32 °C. The following findings were obtained. At a room air temperature of 28 °C, the whole-body thermal sensations were almost thermally neutral, regardless of the type of chair. At a room air temperature of 30 °C, the occupants were able to create acceptable thermal environments from the viewpoints of whole-body thermal sensation and comfort by using the chairs with fans. Their local discomfort rates at the back and lower back, which were affected by the isothermal airflows, were greatly improved at this room air temperature. However, at a room air temperature of 32 °C, the chairs tested in the present study were not able to provide acceptable thermal environments. In order to provide a more comfortable environment to the chair occupants, additional local systems to cool the head, arms, and hands are needed.     

Impact of individually controlled facially applied air movement on perceived air quality at high humidity

The effect of facially applied air movement on perceived air quality (PAQ) at high humidity was studied. Thirty subjects (21 males and 9 females) participated in three, 3-h experiments performed in a climate chamber. The experimental conditions covered three combinations of relative humidity and local air velocity under a constant air temperature of 26 °C, namely: 70% relative humidity without air movement, 30% relative humidity without air movement and 70% relative humidity with air movement under isothermal conditions. Personalized ventilation was used to supply room air from the front toward the upper part of the body (upper chest, head). The subjects could control the flow rate (velocity) of the supplied air in the vicinity of their bodies. The results indicate an airflow with elevated velocity applied to the face significantly improves the acceptability of the air quality at the room air temperature of 26 °C and relative humidity of 70%.