Experimental investigation of thermal comfort with stratum ventilation using a pulsating air supply

Building and Environment 165 (2019) 106416

Contents lists available at ScienceDirect

Building and Environment journal homepage: www.elsevier.com/locate/buildenv

Experimental investigation of thermal comfort with stratum ventilation using a pulsating air supply

T

Xue Tiana,b, Sheng Zhangc, Zhang Lind, Yongcai Lia,b, Yong Chenga,b,∗, Chunhui Liaoa,b,∗∗ a

National Centre for International Research of Low-carbon and Green Buildings, Ministry of Science & Technology, Chongqing University, Chongqing, 400044, China School of Civil Engineering, Chongqing University, Chongqing, 400044, China c Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China d Division of Building Science and Technology, City University of Hong Kong, Hong Kong, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Thermal comfort Air movement Pulsating air supply Stratum ventilation Dynamic airflows

This study proposes an innovative ventilation strategy combining stratum ventilation with a pulsating air supply to improve thermal comfort. The experiments were conducted in a classroom with two rows of seated occupants. Four cases of pulsating air supply conditions were designed, including different combinations of room air temperatures (27 and 28 °C), supply airflow rates (7.9 and 9.0 air changes per hour), and pulsed cycle durations (2 and 5 min). One case with a constant air supply was also considered for comparison. Air velocity and temperature distributions in the occupied zone were measured, and subjective responses to the thermal environments were collected. Twenty-five subjects were recruited for the subjective investigations, and these subjects performed sedentary work and wore summer clothing with a typical thermal insulation of 0.49 clo. Results showed that with the pulsating air supply, more than 87% of subjects reported thermally comfortable, and the thermal environments satisfied the requirements in ISO 7730 up to Category A. Compared to the conventional constant air supply, the pulsating air supply provided comparable thermal sensation in the two rows, decreased the percentage dissatisfied due to draft from 34% to 8%, and decreased the percentage reporting thermal discomfort from 16% to 4%. These improvements reflect the benefits of the dynamic airflows in the occupied zone created by the pulsating air supply. The time-averaged predicted mean vote (PMV) and predicted percentage dissatisfied (PPD) were demonstrated to be applicable for the dynamic thermal environment created by the proposed stratum ventilation with a pulsating air supply.

1. Introduction Ventilation strategies determine the quality of indoor environments, which has a direct relationship to the health, productivity, and quality of life of occupants [1]. However, it is still challenging to achieve both low energy consumption and satisfactory thermal comfort conditions by applying appropriate ventilation strategies. To reduce energy consumption but deliver an acceptable indoor environment with elevated room temperatures, stratum ventilation has been proposed [2]. With this ventilation strategy, the energy consumption was decreased by approximately 44% and 25% compared with mixing ventilation and displacement ventilation, respectively [3]. Because stratum ventilation directly sends cool air to the head-chest level of occupants, its potential to create draft is of concern, particularly

near the supply diffusers. As a result of this draft, the percentage of dissatisfied occupants (PD) can be greater than 30% at the most-affected locations [4]. Furthermore, as the jet velocity decays, the thermal conditions at different distances from the supply diffusers vary [5]. Therefore, the thermal uniformity is also of concern. Since stratum ventilation was first introduced, there have been several studies aimed at improving its thermal comfort. To avoid draft, supply airflow rates should be below 5.5 ACH (air changes per hour) in a single office, and the supply air temperature should be higher than 20 °C [6,7]. The thermally neutral temperature for stratum ventilation is approximately 27 °C, which is 2.5 °C and 2.0 °C higher than those for mixing ventilation and displacement ventilation, respectively [8]. In this case, an appropriate indoor air temperature is essential for satisfactory thermal comfort, and much effort has been expended to

∗ Corresponding author.National Centre for International Research of Low-carbon and Green Buildings, Ministry of Science & Technology, Chongqing University, Chongqing, 400044, China. ∗∗ Corresponding author. National Centre for International Research of Low-carbon and Green Buildings, Ministry of Science & Technology, Chongqing University, Chongqing, 400044, China. E-mail addresses: [email protected] (Y. Cheng), [email protected] (C. Liao).

https://doi.org/10.1016/j.buildenv.2019.106416 Received 29 June 2019; Received in revised form 30 August 2019; Accepted 11 September 2019 Available online 12 September 2019 0360-1323/ © 2019 Elsevier Ltd. All rights reserved.

Building and Environment 165 (2019) 106416

X. Tian, et al.

Abbreviation PD ACH PMV IAJS BMI PPD IPMV IPPD TAPMV TAPPD

ATS APD ANOVA R1 R2 S.D. (t) S.D. (l)

percentage dissatisfied air changes per hour predicted mean vote intermittent air jet strategy body mass index predicted percentage dissatisfied instantaneous predicted mean vote instantaneous predicted percentage dissatisfied time-averaged predicted mean vote time-averaged predicted percentage dissatisfied

Max Min IQR

actual thermal sensation vote actual percentage dissatisfied analysis of variance first row second row standard deviation of the measuring period standard deviation of the mean value at the six sampling lines maximum value during the measuring period minimum value during the measuring period interquartile range

satisfactory thermal comfort [26,27]. Under IAJS, fresh air is supplied via air inlets above the heads of occupants with a pulsating pattern. IAJS can provide satisfactory ventilation efficiency and thermal comfort in a classroom [26–29]. Nevertheless, the IAJS system is inflexible for rearranging furniture or the seating arrangements in rooms [27]. This study proposes the innovative use of a pulsating air supply with stratum ventilation with the aim of improving thermal comfort. Different indoor thermal environments were created in the experiments, including steady environments with a conventional constant air supply, and dynamic environments with a pulsating air supply. As a result, the occupant perceptions could be investigated under different thermal environments. The proposed method can contribute to the application of stratum ventilation to achieve improved and energy-efficient thermal comfort.

maintain the indoor air temperature at an optimal level [9–12]. Researchers have also tried to satisfy the different thermal preferences of various occupants using stratum ventilation by creating differentiated indoor thermal environments in different zones in a shared room [13,14]. Nevertheless, these previous studies are essentially based on the assumption that the thermal environment remains steady. In contrast, another way to improve thermal comfort is to create transient thermal environments [15–17]. Many types of dynamic airflows (such as simulated natural, sinusoidal, and pulsating) have been shown to be more efficient for body cooling than a constant airflow, and dynamic airflows were preferred by occupants [18–22]. In addition, oscillating air movement produced by fans could also improve the perceived air quality [23]. Generally speaking, the ability of dynamic airflows to provide better thermal comfort than constant airflows has been experimentally verified in neutral–warm environments. Pulsating airflows are a type of dynamic airflow that are used for the air supply in air-conditioning systems. With this strategy, fresh air is supplied periodically following a square wave. Wu and Ahmed [24] found that a pulsating air supply can improve the indoor air quality compared to a constant air supply by enhancing the mixing of fresh supply air and indoor air. The ability of pulsating airflows to improve ventilation efficiency has also been theoretically validated [25]. However, because the indoor air velocity is low, mixing ventilation combined with a pulsating air supply cannot ensure satisfactory thermal comfort for elevated room air temperatures. However, a strategy referred to as the intermittent air jet strategy (IAJS) can achieve

2. Methodology 2.1. Experimental setup Experiments were conducted in a room with dimensions of 8.4 m (length) × 5.4 m (width) × 2.6 m (height) at the Chengdu Research Institute of the City University of Hong Kong, China. The room was configured as a classroom served by stratum ventilation. The room had two windows located on the right exterior wall, while the other three walls were interior walls. Twelve seats were available for occupants, with six seats in each row. For the objective measurements, the

Fig. 1. (a) Layout of the experimental room. (b) Locations of objective measurements (S1–S6: air inlets; L1–L12: sampling lines). 2

Building and Environment 165 (2019) 106416

X. Tian, et al.

thermal comfort. Supply airflow rates within this range have been shown to provide acceptable thermal comfort [7,8]. The supply air temperature and the exhaust air temperature were the mean values of the measurements at the six air inlets and at six exhausts, respectively.

occupant was represented by a rectangular thermal simulator with dimensions of 0.4 m (length) × 0.25 m (width) × 1.2 m (height) [11]. A 100 W light bulb was placed inside each thermal simulator to simulate human body heat [5,11]. Six ceiling-mounted lamps of 29 W each were also present. The temperature fluctuations of the exterior wall and windows during the experiments were small (within 1 °C). Thus, the cooling load can be considered constant. Double deflection grilles were used as the air inlets and exhausts [11,30]. Full fresh air was supplied horizontally to the breathing zone from six grilles on the front wall at a height of 1.35 m above the floor (i.e., S1–S6 in Fig. 1(b)), and then exhausted through six exits on the same wall at a height of 0.49 m above the floor, as shown in Fig. 1. The air inlets and exhausts had the same dimensions of 0.17 m × 0.17 m each. The arrangements resembled those in the previous studies on stratum ventilation [31]. The supply air temperature and supply airflow rate were controlled by a control system through varying the opening of the chilled water valve and the frequency of the supply fan, respectively. For constant and pulsating air supplies, the frequencies of the supply fan were constant and pulsating, respectively. The duration of the whole cycle, duty period during which the air velocity was high, and idle period during which the air velocity was low were set via the control system.

2.3. Measurement instruments All the instruments were calibrated prior to the measurements. In the air supply inlets and exhaust, the air velocity and temperature were measured with SWEMA omnidirectional hot-wire anemometers; in the occupied zone, they were measured with SWEMA and KIMO VT 100 omnidirectional hot-wire anemometers. A portable computer-based data acquisition system was used to record the readings from the air velocity and temperature sensors. Five SWEMA and two KIMO VT 100 anemometers were used. The temperatures of the room and surfaces were measured with WZY-1 thermocouples. The details of the instruments used in this study are summarized in Table 2. 2.4. Subjects and subjective questionnaire Twenty-five healthy subjects were recruited to participate in subjective evaluations of thermal comfort; their anthropometric data are listed in Table 3. The subjects were office workers or university students, and they were all accustomed to spending time in air-conditioned environments. The subjects were instructed not to smoke, drink, or stay up late on days with testing sessions. Before the test sessions, the subjects were informed of the experimental procedure and requirements. No subject reported being sick or feeling unwell during the test sessions. A typical summer clothing level of 0.49 clo (i.e., t-shirt, underwear, short socks, thin trousers, shoes, and a standard office chair) was used for all test sessions. The subjects performed secondary activities during the tests with an activity level of approximately 1.1 met according to ASHRAE 55 [33]. The within-subject design method (each subject participated in all studied cases) was used to reduce error variations. The layout of the questionnaire was designed according to the guidelines in the relevant standards [33,34]. The scales used in the questionnaire are shown in Fig. 3. The thermal sensation was evaluated using the ASHRAE seven-point scale. Thermal acceptability and air movement acceptability were rated using two-point scale votes, i.e., acceptable and unacceptable. Thermal preference and air movement preference were both assessed using three-point scales. However, thermal comfort can differ from the thermal sensation [35–37]. In this study, thermal comfort was assessed on a continuous scale of the comfortable range (i.e., +0.1 to +1) and uncomfortable range (i.e., −0.1 to −1) [29,38,39]. In addition, thermal sensation and thermal comfort of the overall body and local body parts (i.e., head, neck, back, chest, arms, hands, and legs) were also evaluated [29,40].

2.2. Studied cases The detailed conditions of the cases studied are summarized in Table 1. Cases A–D were based on a pulsating air supply. Case E with a constant air supply was designed for comparison with Cases A and C, as these three cases had the same room air temperature and supply airflow rate. Fig. 2 shows the air velocities measured at supply inlet S3 for Cases A and E, and the results demonstrate that the system could achieve both constant and pulsating air supplies successfully. Two cycle lengths were considered for the pulsating air supply: 2 and 5 min [24,29]. Each cycle was divided into the duty period, during which the air velocity was high, and the idle period, during which the air velocity was low. The lengths of the duty and idle periods were the same during a cycle. It took approximately 45 s for the jet to fully distort the thermal plume during the duty period and for the thermal plume to be fully restored during the idle period [32]. However, the length of the duty and idle periods was longer than 45 s. Thus, as designed, the cycles can generate periods with or without convective cooling to create transient velocity conditions. Previous studies showed that the thermal neutral temperature of stratum ventilation with a constant air supply was approximately 27 °C [7,8]. Room air temperatures of 27 °C for the thermal neutral conditions (i.e., except for Case B) and 28 °C for neutral–warm conditions (i.e., Case B) were thus selected. The room air temperature was measured by the monitoring sensor positioned at the geometric center of the room. The room air humidity was monitored at 45%–50%. The supply airflow rate was the sum of the measurements at the six air supply inlets S1–S6. Two supply airflow rates were considered (7.9 and 9.0 ACH) to investigate the effect of the supply airflow rate on

2.5. Experimental procedure The experiments were performed in August 2018. For the objective measurements, the specified conditions for each case were pre-set at

Table 1 Test conditions. Case

A B C D E

Air supply method

Pulsating

Constant

Room air temperature (°C) Nominal

27 28 27 27 27

Actual

26.7 28.0 26.5 27.0 26.6

± ± ± ± ±

0.3 0.3 0.3 0.4 0.1

Supply air temperature (°C)

21.5 23.5 21.6 21.6 21.7

± ± ± ± ±

0.1 0.2 0.3 0.2 0.2

Exhaust air temperature (°C)

26.3 27.9 26.6 27.1 26.6

± ± ± ± ±

0.1 0.1 0.0 0.0 0.2

Air changes per hour (ACH)

9.0 9.0 9.0 7.9 9.0

3

air supply velocity (m/s)

Cycle length (min)

Whole cycle

Duty period

Idle period

Nominal

Actual

Nominal

Actual

Nominal

Actual

1.70 1.70 1.70 1.50 1.70

1.66 1.64 1.78 1.48 1.72

2.21 2.21 2.21 1.70

2.07 1.99 2.21 1.75

1.36 1.36 1.36 1.36

1.25 1.30 1.35 1.21

± ± ± ± ±

0.53 0.38 0.49 0.30 0.13

± ± ± ±

0.29 0.13 0.27 0.15

± ± ± ±

0.11 0.12 0.10 0.07

5 5 2 2

Building and Environment 165 (2019) 106416

X. Tian, et al.

Fig. 2. Measured air velocities with time at air inlet S3 under (a) Case A (pulsating air supply) and (b) Case E (constant air supply). Table 2 Instruments used for study measurements. Type of instruments

SWEMA KIMO VT 100 WZY-1

Air velocity (m/s)

Air temperature (°C)

measurement range

measurement accuracy

measurement range

measurement accuracy

0.07–0.5 0.5–3.00 < 3.00

± 0.02 ± 0.03 ± 0.1

10–40

± 0.2

−20–+80

± 0.3

−20–+80

± 0.3

least 2 h before beginning the tests, which was significantly longer than the pull-down periods (i.e., approximately 1 h at 7 ACH) of stratum ventilation [4]. Measurements were taken within the occupied zone using sensor rigs. The air velocity and temperature sensors were positioned vertically on the sensor rigs at the desired measurement heights (i.e., 0.1, 0.6 and 1.1 m above the floor for seated subjects [33]). The sensor rigs were moved around the room to map out a location grid of 12 sampling lines (i.e., L1–L12 in Fig. 1(b)). The sampling lines were located 10 cm in front of the thermal simulator. Air velocities and temperatures were also measured at the air supply inlets and exhausts. The duration of the air velocity and temperature measurements was three cycles for the pulsating air supply (i.e., 15 min for Cases A and B, 6 min for Cases C and D), and 10 min for the constant air supply. The sampling frequency was 8 Hz. A 20-min interval was applied between moving the anemometers and performing the next measurements to minimize interference. The temperatures of the surfaces (i.e., walls, windows, floor and ceiling) and room air temperature were measured during the entire experimental periods at intervals of 1 min. After the objective measurements were completed, the subjective investigations were carried out (see Fig. 4(a)). Before each test session, one anemometer was placed at Air Inlet S3, and six anemometers were placed on Sampling Lines L3 and L9 at three heights, to determine if the indoor environment was steady. The subjects were required to arrive 20 min before the formal experiment and adapt themselves to the environment. They chose their seats randomly. The formal experiment in each session lasted for 90 min, during which the subjects were not allowed to stand up or move around, which is sufficient duration to produce reliable results [37]. Ten minutes after the beginning of the

Fig. 3. Scales used in the subjective questionnaire.

formal experiment, the subjects answered the first questionnaire (see Fig. 4(b)). Thus, the total time for adaption to the tested thermal condition was 30 min. A shorter acclimatization time of 10–20 min allow the subjects to reach steady conditions [29,41]. The interval time for answering the questionnaire was shorter at the beginning of the experiment to ascertain if the subjects had adapted to the environment. Five questionnaires were answered during each test session (see Fig. 4(b)). In total, 125 questionnaires were analyzed for each case. When the 12 seats in the room were not filled by the subjects, thermal simulators were used to represent occupancy of the empty seats.

2.6. Prediction of thermal comfort The predicted mean vote–predicted percentage dissatisfied (PMV–PPD) model has been applied to evaluate steady thermal environments. The detailed equations for these parameters can be found in ISO 7730 [34]. For transient conditions, the instantaneous PMV (IPMV) and instantaneous PPD (IPPD) were calculated using the instantaneous values. ISO 7730 stipulates that time-weighted average

Table 3 Anthropometric data for the subjects. Sex

Number

Age (y)

Height (m)

Weight (kg)

BMIb

Female Male Total

15 10 25

26.7 ± 6.6a 26.6 ± 6.4 27.1 ± 5.7

1.60 ± 0.05 1.76 ± 0.05 1.66 ± 0.09

50.40 ± 4.88 68.90 ± 10.83 58.00 ± 11.90

19.70 ± 1.14 22.34 ± 3.46 20.86 ± 2.69

a

Standard deviation; b. Body Mass Index = weight (kg)/[height (m)]2. 4

Building and Environment 165 (2019) 106416

X. Tian, et al.

values of IPMV (TAPMV) and IPPD (TAPPD) can be used to predict thermal comfort under transient conditions in which only one or more parameters have minor fluctuations. This study meets this condition as only the air velocity and air temperature in the occupied zone have minor fluctuations, which were close to those reported by Kabanshi et al. [27]. The TAPPD and TAPMV results under transient conditions were also in good agreement with the actual thermal sensation votes [27,42]. The equations for calculation of TAPMV and TAPPD are as follows: T

TAPMV =

∑0 IPMV T

(1)

T

TAPPD =

∑0 IPPD T

(2)

where T is the measurement duration. The time step was 0.125 s, corresponding to the sampling frequency of the anemometers. Cheng et al. [30] validated that the PMV model was appropriate for the evaluation of thermal comfort under stratum ventilation with a constant air supply. Their results showed that the PMV prediction at a height of 1.1 m (i.e., head level) agreed reasonably well with the actual thermal sensation vote (ATS). The actual percentage dissatisfied (APD) is the percentage of respondents reporting thermal unacceptability in subjective investigations. Therefore, in this study, TAPMV and TAPPD were calculated at a height of 1.1 m above the floor. 2.7. Statistical analysis

Fig. 4. (a) Image of a subjective test session; (b) Procedure for questionnaires Q1–Q5 during the subjective test session.

All statistical analyses were carried out using IBM SPSS software version 2017. The normality of the data was tested with the Shapiro–Wilk test. Paired t-tests and repeated analysis of variance (ANOVA) measurements were applied for normally distributed data,

Fig. 5. Air velocity and temperature distributions. 5

Building and Environment 165 (2019) 106416

X. Tian, et al.

Fig. 6(a)–6(d) shows that the local thermal sensation votes for body segments in Cases A–D fluctuated within the ranges of −0.34 to 0.25, −0.07 to 0.41, −0.30 to 0.23, and −0.10 to 0.34, respectively. Overall, they fell between −0.5 and 0.5. Among different body segments, the local thermal sensation votes were significantly different (Wilcoxon test, p < 0.05). The lowest local thermal sensation vote values were reported at the head and arms, while the highest values were found at the back or legs. Additionally, the local thermal sensation votes for lower body segments (e.g., the legs) did not fluctuate much compared with those of the upper body segments (e.g., the arms and back). As indicated in Table 5, the percentages of votes reporting unacceptable thermal conditions were between 2.4% and 9.6%, which are all less than 15% [34]. Table 5 also indicates that the PD due to draft is less than 20% under the pulsating air supply. The PD due to draft was highest for Case A. Cases A and B shared the same supply airflow rate and the cycle length. However, by elevating the room air temperature from 26.7 to 28 °C, the PD due to draft decreased by 4.8%. The PD due to draft for Case A was higher than that for Case C by 6.4%. Cases A and C had the same room air temperature and supply airflow rate, but different cycle lengths. Fig. 7 shows the air movement preference of the subjects, indicating that 65–78% of the respondents preferred to maintain the air movement. Fig. 8 shows the overall and local thermal comfort votes. For Cases A–D, the mean thermal comfort votes were 0.42, 0.30, 0.36, and 0.45, respectively. The mean local thermal comfort votes under the four cases fluctuated in the ranges of 0.37–0.42, 0.29 to 0.31, 0.31 to 0.39, and 0.42 to 0.46, respectively. All of these values were above 0.1. The mean local thermal comfort votes for different body segments were similar to one another, with differences of 0.02–0.08. No statistical difference was found among them (Wilcoxon test, p < 0.001). Fig. 9 shows that 87–95% of the votes were within the comfortable range (i.e., 0.1 to 1).

while differences among non-normally distributed data were assessed by the Wilcoxon rank test. The results were considered statistically significant if p < 0.05. 3. Results 3.1. Air velocity and temperature distributions Owing to the symmetry of the arrangement and the indoor environment, the measurements on Sampling Lines L3 in the first row and L9 in the second row are presented to illustrate the typical air velocity and temperature distributions, as shown in Fig. 5. For both Cases A and E, because the supply air was sent directly to the head level, the air velocity at 1.1 m above the floor were generally higher than that at 0.1 and 0.6 m. The air temperature at 1.1 m was lower by 0.3–1.3 °C than that at 0.1 and 0.6 m, indicating the typical air distributions under stratum ventilation. For Case E, along L3 at 1.1 m, the air velocity and temperature were 0.53 m/s and 25.2 °C, respectively, corresponding to 0.15 m/s and 26.2 °C along L9 at 1.1 m. The different values in the two rows may lead to different thermal comfort levels. For Case A at a height of 1.1 m, in the first row, the high and low pulses corresponded to mean air velocities of 0.8 m/s and < 0.4 m/s, respectively; in the second row, the corresponding values were 0.4 m/s and < 0.2 m/s, respectively. The mean air temperatures in the first and second row were 25.3 °C and 26.0 °C, respectively. Because the air velocities and temperatures oscillated due to the pulsating air supply, the standard deviations for Case A were higher than those for Case E (see Fig. 5). Note: R1: the first row; R2: the second row; S.D. (t): standard deviation of the measuring period; S.D. (l): standard deviation of the mean value at the six sampling lines; Max: maximum value during the measuring period; Min: minimum value during the measuring period. Table 4 summarizes the air velocities and temperatures at a height of 1.1 m for the five cases. The mean air velocities and temperatures were mainly dependent on the supply airflow rate and supply air temperature. The values of S.D. (t) were higher under the pulsating air supply (i.e., Cases A–D) than that under the constant air supply (i.e., Case E). The differences between the Max and Min values were also higher for Cases A–D than Case E. Thus, compared to the constant air supply, the pulsating air supply leads to more significant fluctuations in the air velocity and temperature.

3.3. Comparison of thermal comfort between pulsating and constant air supplies Cases A and C (i.e., pulsating air supply) had the same mean supply airflow rate as Case E (i.e., constant air supply). Fig. 10 shows the overall thermal sensation votes for these three cases. In the first row, the mean overall thermal sensation vote was −0.73 for Case E, while it was approximately −0.2 for Cases A and C. In the second row, the mean overall thermal sensation vote for Case E (0.15) was slightly higher than that for Cases A (−0.12) and C (−0.03). The overall thermal sensation votes for Cases A, C, and E were −0.18, −0.14, and −0.27, respectively. For Case E, the vote in the second row was significantly higher than that in the first row (Wilcoxon test, p < 0.01). However, for Cases A and C, the two rows showed an insignificant difference (Wilcoxon test, p > 0.05). Fig. 11 compares the PD due to draft for Cases A, C, and E. The PD due to draft for Case E was as high as 34.4%, whereas that for Cases A and C was 14.4% and 8%, respectively. A comparison of the overall thermal comfort votes is shown in Fig. 12(a) and (b). The mean overall

3.2. Thermal comfort with the pulsating air supply Fig. 6 shows the overall and local thermal sensation votes of the subjects. For Cases A–D, the mean overall thermal sensation votes of the subjects were −0.18, 0.17, −0.14 and 0.04, respectively. These are all between −0.2 and 0.2, indicating thermal neutrality. The boxplots also showed that most of the overall thermal sensation votes fell into the range of −1 (slightly cool) to +1 (slightly warm) in these four cases. The overall thermal sensation vote for Case B was significantly higher than that for Case A (Wilcoxon test, p < 0.001) by 0.35 scale, which is a result of the higher room air temperature. Table 4 Air velocities and temperatures at a height of 1.1 m

Air velocity (m/s)

Air temperature (°C)

Case

A

B

C

Row

R1

R2

R1

R2

R1

R2

R1

R2

R1

R2

Mean value at 1.1 m S.D. (t) S.D. (l) Mean value at 1.1 m S.D. (t) S.D. (1) Difference between Max and Min

0.53 0.26 0.07 25.1 0.3 0.2 0.9–1.6

0.24 0.11 0.06 26.1 0.2 0.4 0.9–1.2

0.51 0.27 0.06 26.9 0.2 0.3 0.9–1.1

0.22 0.14 0.05 27.8 0.1 0.3 0.5–0.8

0.50 0.25 0.04 25.3 0.2 0.2 0.6–1.3

0.30 0.19 0.07 26.2 0.1 0.4 0.5–1.2

0.42 0.21 0.04 25.4 0.3 0.3 0.7–0.9

0.23 0.12 0.03 26.6 0.2 0.3 0.6–0.9

0.53 0.17 0.04 25.0 0.1 0.3 0.4–0.5

0.17 0.07 0.08 26.2 0.1 0.3 0.2–0.6

6

D

E

Building and Environment 165 (2019) 106416

X. Tian, et al.

Fig. 6. (a)–(d). Thermal sensation votes for body segments under the pulsating air supply (IQR: interquartile range).

pulsating air supply, whereas this number decreased to 18% under the constant air supply.

Table 5 Percentage of votes reporting thermal unacceptability and draft. Case

Thermal unacceptability (%)

PD due to draft (%)

A B C D

8.0 9.6 7.6 2.4

14.4 9.6 8.0 6.4

3.4. Thermal comfort prediction Fig. 13 shows that under the pulsating air supply, the IPMV followed the profile of the supply air velocity, with high pulses decreasing the IPMV and low pulses increasing the IPMV. The IPMV fluctuated within the range of −0.5 to 0.5 for the second row in Case A. For the second row in Case E, the IPMV was above the datum at all times. The IPPDs for Cases A and E were both below 12% at all times, as the IPMV was close to thermal neutrality. Fig. 14 shows comparisons of ATS and TAPMV, as well as APD and TAPPD for all cases. The maximum discrepancy between the ATS and TAPMV was 0.31; the average discrepancy was 0.19. The maximum discrepancy between the APD and TAPPD was only 6%; the average discrepancy was 2%. The TAPMV and TAPPD results agreed well with the ATS and APD, respectively. Therefore, TAPMV and TAPPD can be used as indicators for evaluating thermal comfort in dynamic thermal environments using stratum ventilation with a pulsating air supply. 4. Discussion 4.1. Thermal comfort with a pulsating air supply

Fig. 7. Air movement preference under the pulsating air supply.

Thermal comfort with a pulsating air supply was evaluated comprehensively through votes on the thermal sensation, thermal acceptability, PD due to draft, and thermal comfort levels. In Fig. 6, the mean overall thermal sensation votes were between −0.2 and 0.2 with a room air temperature of 26.5–28 °C. Generally, the strategy proposed here was still suitable for neutral-warm environments, and the thermally neutral temperature was close to that under a constant air supply. Previous studies concluded that subjects were more sensitive to airflows with a fluctuation frequency of approximately 0.5–1 Hz [43,44]. In other words, they may expect a cooler thermal sensation with a

thermal comfort votes for Cases A, C, and E were 0.42, 0.36, and 0.21, respectively. The mean overall thermal comfort votes were thus significantly higher under a pulsating air supply than the constant air supply (Wilcoxon test, p < 0.05). The percentage voting ‘uncomfortable’ for Case E was 16%, which was higher than those for Case A (4%) and Case C (10%), as shown in Fig. 12(b). It can also be observed that the percentage voting in the range of 0.5–1 (close to very comfortable) occupied more than 40% of the total votes under the 7

Building and Environment 165 (2019) 106416

X. Tian, et al.

Fig. 8. Thermal comfort votes for body segments under the pulsating air supply.

Fig. 9. Distribution of the overall thermal comfort votes. Fig. 10. Overall thermal sensation votes under pulsating and constant air supplies.

dynamic airflow having that fluctuation frequency than with a constant air supply, for the same mean airflow rate. In this study, no significant improvement of cooling was observed with use of a pulsating air supply. Other fluctuation frequencies may be used in further studies and the thermally neutral temperature may increase. At present the main concerns are thermal uniformity and draft.

Based on the categories of thermal environments defined in ISO 7730 [34] listed in Table 6, Cases A–C satisfied the requirements of Category B, and Case D satisfied the requirements of Category A. As the thermal sensation under Case D was closest to thermal neutrality (i.e., 0.04), the thermal unacceptability under Case D was the lowest (i.e., 8

Building and Environment 165 (2019) 106416

X. Tian, et al.

Fig. 11. PD due to draft under pulsating and constant air supplies.

Fig. 14. Comparisons of the ATS/APD and TAPMV/TAPPD

Table 6 Categories of thermal environments based on ISO 7730 (2005). Category

PPD

PMV

PD due to draft (%)

A B C

< 6% < 10% < 15%

−0.2 < PMV < +0.2 −0.5 < PMV < +0.5 −0.7 < PMV < +0.7

< 10 < 20 < 30

The subjects were more sensitive to cold stimulus, and thus a higher PD due to draft was reported for Case A [46]. Therefore, the cycle length of the pulsating air supply needs to be designed carefully. For the air movement preference (see Fig. 7), respondents preferred that the air movement with the pulsating air supply should be maintained, which was in accordance with the low PD due to draft. It is worth mentioning that both thermal sensation and thermal comfort were investigated for local body segments. For thermal sensation, subjects felt cooler at the head and arms than at the back and legs. This was still valid with the constant air supply under stratum ventilation and was contrary to what has been reported under displacement ventilation [36]. Thus, the local thermal sensation may still depend on the primary ventilation method used. Moreover, the differences in the local thermal comfort votes among body segments (i.e., 0.02–0.08) were much smaller than that of local thermal sensation votes (i.e., 0.44–0.59). In other words, subjects have different preferences for thermal sensation at different body segments [47].

Fig. 12. (a)–(b). Thermal comfort votes under pulsating and constant air supplies.

2.4%), as also observed by Kabanshi et al. [29]. Most notably, the thermal environments created by the pulsating air supply satisfied the requirements in the standard up to Category A. The thermal comfort levels evaluated under the pulsating air supply in this study were close to those reported in a previous study using IAJS when the room air temperature was higher than 25.5 °C [29], which were both satisfactory. Moreover, this study found that elevating the room air temperature can decrease the PD due to draft, because the subjects preferred more air movement under warm conditions [44,45]. In addition, the duration of pulsed air supply influenced the PD due to draft. The duty period in Case A (150 s) was longer than that in Case C (60s). At the end of the duty period, the air temperature decreased to a lower level in Case A.

4.2. Improvement of thermal comfort with the pulsating air supply Regarding the uniformity between the two rows of seats, under the constant air supply, subjects in the second row felt significantly warmer than those in the first row; under the pulsating air supply, the subjects

Fig. 13. IPMV and IPPD results for (a) A-R2 (Case A, second row) and (b) E-R2 (Case E, second row). 9

Building and Environment 165 (2019) 106416

X. Tian, et al.

second row at a height of 1.1 m under the pulsating air supply was greater than 0.4 m/s during the duty period, whereas it was always lower under the constant air supply. An air velocity of 0.4 m/s can destroy the thermal plume, allowing the fresh supplied air to penetrate into the breathing zone and reduce personal exposure to contaminants [52,53]. Thus, the inhaled air quality can be improved. Furthermore, previous studies showed that a varying supply airflow rate produced secondary vortices, which were shed into stagnant areas in the room [25,54]. In other words, the pulsating air supply can enhance air mixing, and thus improve ventilation efficiency. At present these experiments are carried out under mixing ventilation basically. For stratum ventilation, reducing stagnant areas may also mean increasing the air velocity far from the air inlets, thus improving thermal uniformity. In addition, in this study, the results showed that a lower supply airflow rate with the pulsating air supply (i.e., Case D) could provide better thermal comfort than a higher supply airflow rate with a constant air supply (i.e., Case E). Combined with the possible better air quality, there is a potential for achieving energy-savings, while simultaneously improving the indoor environments by using pulsating air supplies.

in both rows felt similar. A previous study by Cheng and Lin [5] also reported that with a constant air supply, the mean overall thermal sensation vote in the second row was significantly higher than that in the first row (Wilcoxon test, p < 0.05). Under the constant air supply, because the jet throw was short, the air movement was insufficient in the second row. As Table 4 indicates, the air velocity in the second row at 1.1 m for Case E was consistently below 0.24 m/s, and the air velocity in the second row was lower than that in the first row by approximately 0.4 m/s. In contrast, the pulsating air supply enhanced the jet throw and provided more air movement to the second row. As a result, the air velocity in the second row could be up to 0.49 m/s during the duty period (see Case C in Table 4). This increased air movement could improve the cooling in the second row. In the first row, the constant air supply produced a cool sensation which fell outside the acceptable range defined by ISO 7730 [34] (i.e., −0.7 to 0.7), while the pulsating air supply produced a neutral thermal sensation. In other words, subjects in the first row felt cooler under the constant air supply than the pulsating air supply, reaching unacceptable levels. The following explanation can be provided: during the idle period under the pulsating air supply, the air velocity could drop to approximately 0.25 m/s, while the temperature increased by approximately 1 °C (see Table 4); this lower air velocity and increased air temperature could alleviate the cool sensation from air movement [29,48]. Even under the pulsating air supply, the mean air velocities between the two rows differed by approximately 0.3 m/s (see Table 4) due to their different distances from the air inlets. Nevertheless, the subjects in the two rows felt similar thermal sensations. The thermal sensation votes under IAJS with air velocities of 0.4 m/s and 0.8 m/s also exhibited little difference [29]. Overall, the pulsating air supply can significantly improve thermal uniformity compared with the constant air supply. The PD due to draft for Case E was above the maximum value of 30% for Category C defined in ISO 7730 [34], and was significantly higher than the results of Cases A and C (see Fig. 11). During the duty period, the air velocity in the first row could reach approximately 0.8 m/s. Nonetheless, subjects had few complaints on draft. Similarly, in the studies carried out by Kabanshi et al. [28,29], the occupants were satisfied with an air velocity of 0.8 m/s during the duty period without reporting draft discomfort. The following explanations can be suggested: for the pulsating air supply, as indicated in Table 4, dynamic conditions between the low and high pulses were created, with fluctuations in the air velocity of 0.2–0.5 m/s and air temperature of 0.5–1.6 °C. These conditions confer a sensory pleasure known as ‘thermal alliesthesia’ [49,50], which leads to a reduction in draft discomfort. In warm environments, after attaining comfort, occupants may find the constant air velocity unpleasant; in other words, the conditions would be deemed drafty. However, under a pulsating air supply, the air velocity and temperature fluctuate continuously, and the human body receives a constant thermal stimulus, which is described as pleasant feeling [46,50,51]. This thermal pleasure is similar to that felt under natural wind [21] and intermittent airflow [28] conditions. Therefore, the pulsating air supply provided better thermal comfort than the constant air supply, as seen in Figs. 11 and 12. In the study carried out by Kabanshi et al. [29], it was also reported that the IAJS could improve thermal comfort levels by approximately 0.3 scale compared with mixing ventilation at a room air temperature of 28.5 °C. This was due to the enhanced cooling provided by IAJS because the air velocity under mixing ventilation was less than 0.15 m/s. However, this study has demonstrated that a pulsating air supply can improve thermal comfort by increasing the thermal uniformity and decreasing draft.

5. Conclusions In a room served by stratum ventilation, both objective measurements and subjective investigations were conducted on the indoor thermal conditions generated by a pulsating air supply and constant air supply. The conclusions can be summarized as follows: The pulsating air supply created different velocity conditions between the low (i.e., < 0.4 m/s) and high pulses (i.e., > 0.4 m/s) and a temperature fluctuation of approximately 1 °C in the occupied zone. The air velocity in the second row was higher during the duty period than with the constant air supply. These characteristics were responsible for increasing the thermal uniformity and decreasing draft compared with the constant air supply. Stratum ventilation combined with a pulsating air supply can provide acceptable air movement and good thermal comfort at elevated room air temperatures, which can satisfy the requirements for Categories A and B of thermal environments stipulated in ISO 7730. The difference in the mean overall thermal sensation votes between the two rows was reduced from 0.6 to 0.2 scale on the ASHRAE 7-point scale, indicating that the thermal environment was more uniform. The PD due to draft under the constant air supply was greater than 30%, but it decreased to 8% with the pulsating air supply. Thermal comfort was improved by approximately 0.2 scale on the scale of −1 (very uncomfortable) to 1 (very comfortable). The TAPMV and TAPPD results were in good agreement with the ATS and APD, suggesting that these indicators can be used for the thermal comfort evaluation of stratum ventilation with a pulsating air supply. Acknowledgments The work presented in this paper is financially supported by National Natural Science Foundation of China (Grant No. 51608066), and a Research Grant from the Chengdu Research Institute of the City University of Hong Kong, Shuangliu, Chengdu, Sichuan, China (Project No. R-LAAD004). The authors express their gratitude to the subjects who participated in the experiments. References [1] B. Yang, A.K. Melikov, A. Kabanshi, C. Zhang, F.S. Bauman, G. Cao, H. Awbi, H. Wigo, J. Niu, K.W.D. Cheong, K.W. Tham, M. Sandberg, P.V. Nielsen, R. Kosonen, R. Yao, S. Kato, S.C. Sekhar, S. Schiavon, T. Karimipanah, X. Li, Z. Lin, A review of advanced air distribution methods - theory, pratice, limitations and solutions, Energy Build. 202 (2019) 109359 https://doi.org/10.1016/j.enbuild. 2019.109359.

4.3. Other possible improvements Besides thermal comfort, the pulsating air supply may also improve air quality. Based on the objective measurements, the air velocity in the 10

Building and Environment 165 (2019) 106416

X. Tian, et al. [2] Z. Lin, T.T. Chow, C.F. Tsang, K.F. Fong, L.S. Chan, Stratum ventilation – a potential solution to elevated indoor temperatures, Build. Environ. 44 (11) (2009) 2256–2269 https://doi.org/10.1016/j.buildenv.2009.03.007. [3] Z. Lin, C.K. Lee, S. Fong, T.T. Chow, T. Yao, A.L.S. Chan, Comparison of annual energy performances with different ventilation methods for cooling, Energy Build. 43 (1) (2011) 130–136 https://doi.org/10.1016/j.enbuild.2010.08.033. [4] X. Wang, Z. Lin, An experimental investigation into the pull-down performances with different air distributions, Appl. Therm. Eng. 91 (2015) 151–162 https://doi. org/10.1016/j.applthermaleng.2015.08.012. [5] Y. Cheng, Z. Lin, Experimental investigation into the interaction between the human body and room airflow and its effect on thermal comfort under stratum ventilation, Indoor Air 26 (2) (2016) 274–285 https://doi.org/10.1111/ina.12208. [6] L. Tian, Z. Lin, Q. Wang, Experimental investigation of thermal and ventilation performances of stratum ventilation, Build. Environ. 46 (6) (2011) 1309–1320 https://doi.org/10.1016/j.buildenv.2011.01.002. [7] Y. Cheng, Z. Lin, A.M.L. Fong, Effects of temperature and supply airflow rate on thermal comfort in a stratum-ventilated room, Build. Environ. 92 (2015) 269–277 https://doi.org/10.1016/j.buildenv.2015.04.036. [8] M.L. Fong, Z. Lin, K.F. Fong, T.T. Chow, T. Yao, Evaluation of thermal comfort conditions in a classroom with three ventilation methods, Indoor Air 21 (3) (2011) 231–239 http://doi.org/10.1111/j.1600-0668.2010.00693.x. [9] S. Zhang, Y. Cheng, C. Huan, Z. Lin, Heat removal efficiency based multi-node model for both stratum ventilation and displacement ventilation, Build. Environ. 143 (2018) 24–35 https://doi.org/10.1016/j.buildenv.2018.06.054. [10] S. Zhang, Y. Cheng, Z. Fang, C. Huan, Z. Lin, Optimization of room air temperature in stratum-ventilated rooms for both thermal comfort and energy saving, Appl. Energy 204 (2017) 420–431 https://doi.org/10.1016/j.apenergy.2017.07.064. [11] S. Zhang, Y. Cheng, Z. Fang, Z. Lin, Dynamic control of room air temperature for stratum ventilation based on heat removal efficiency: method and experimental validations, Build. Environ. 140 (2018) 107–118 https://doi.org/10.1016/j. buildenv.2018.05.029. [12] S. Zhang, Y. Cheng, C. Huan, Z. Lin, Modeling non-uniform thermal environment of stratum ventilation with supply and exit air conditions, Build. Environ. 144 (2018) 542–554 https://doi.org/10.1016/j.buildenv.2018.08.063. [13] X. Shao, K. Wang, X. Li, Z. Lin, Potential of stratum ventilation to satisfy differentiated comfort requirements in multi-occupied zones, Build. Environ. 143 (2018) 329–338 https://doi.org/10.1016/j.buildenv.2018.07.029. [14] S. Zhang, Y. Cheng, M.O. Oladokun, Z. Lin, Subzone control method of stratum ventilation for thermal comfort improvement, Build. Environ. 149 (2019) 39–47 https://doi.org/10.1016/j.buildenv.2018.11.041. [15] H. Zhang, C. Huizenga, E. Arens, D. Wang, Thermal sensation and comfort in transient non-uniform thermal environments, Eur. J. Appl. Physiol. 92 (6) (2004) 728–733 http://doi.org/10.1007/s00421-004-1137-y. [16] R. Zhao, Investigation of transient thermal environments, Build. Environ. 42 (12) (2007) 3926–3932 https://doi.org/10.1016/j.buildenv.2006.06.030. [17] A.K. Mishra, M.G.L.C. Loomans, J.L.M. Hensen, Thermal comfort of heterogeneous and dynamic indoor conditions — an overview, Build. Environ. 109 (2016) 82–100 https://doi.org/10.1016/j.buildenv.2016.09.016. [18] A. Uğursal, C.H. Culp, The effect of temperature, metabolic rate and dynamic localized airflow on thermal comfort, Appl. Energy 111 (2013) 64–73 https://doi. org/10.1016/j.apenergy.2013.04.014. [19] S.L. Davey, M.J. Barwood, M.J. Tipton, Thermal perceptions and skin temperatures during continuous and intermittent ventilation of the torso throughout and after exercise in the heat, Eur. J. Appl. Physiol. 113 (11) (2013) 2723–2735 http://doi. org/10.1007/s00421-013-2697-5. [20] R. Zhao, J. Li, The effective use of simulated natural air movement in warm environments, Indoor Air 14 (Suppl 7) (2004) 46–50 http://doi.org/10.1111/j.16000668.2004.00272.x. [21] X. Zhou, Q. Ouyang, G. Lin, Y. Zhu, Impact of dynamic airflow on human thermal response, Indoor Air 16 (5) (2010) 348–355 http://doi.org/10.1111/j.1600-0668. 2006.00430.x. [22] Z. Yang, J. Fei, D. Song, Y. Zhao, J. Yu, X. Yu, Effects of simulated natural air movement on thermoregulatory response during head-down bed rest, J. Therm. Biol. 38 (7) (2013) 363–368 https://doi.org/10.1016/j.jtherbio.2013.04.006. [23] W. Pasut, E. Arens, H. Zhang, Y. Zhai, Enabling energy-efficient approaches to thermal comfort using room air motion, Build. Environ. 79 (2014) 13–19 https:// doi.org/10.1016/j.buildenv.2014.04.024. [24] C. Wu, N.A. Ahmed, A novel mode of air supply for aircraft cabin ventilation, Build. Environ. 56 (2012) 47–56 https://doi.org/10.1016/j.buildenv.2012.02.025. [25] B.E.G. Fallenius, A. Sattari, J.H.M. Fransson, M. Sandberg, Experimental study on the effect of pulsating inflow to an enclosure for improved mixing, Int. J. Heat Fluid Flow 44 (2013) 108–119 https://doi.org/10.1016/j.ijheatfluidflow.2013.05.004. [26] A. Kabanshi, H. Wigö, R. Ljung, P. Sörqvist, Human perception of room temperature and intermittent air jet cooling in a classroom, Indoor Built Environ. 26 (4) (2016), https://doi.org/10.1177/1420326X16628931. [27] A. Kabanshi, H. Wigö, M. Sandberg, Experimental evaluation of an intermittent air supply system – Part 1: thermal comfort and ventilation efficiency measurements, Build. Environ. 95 (2016) 240–250 https://doi.org/10.1016/j.buildenv.2015.09. 025. [28] A. Kabanshi, B. Yang, P. Sörqvist, M. Sandberg, Occupants' perception of air movements and air quality in a simulated classroom with an intermittent air supply system, Indoor Built Environ. 28 (1) (2019) 63–76 https://doi.org/10.1177/ 1420326X17732613.

[29] A. Kabanshi, H. Wigö, R. Ljung, P. Sörqvist, Experimental evaluation of an intermittent air supply system – Part 2: occupant perception of thermal climate, Build. Environ. 108 (2016) 99–109 https://doi.org/10.1016/j.buildenv.2016.08.025. [30] Y. Cheng, M.L. Fong, T. Yao, Z. Lin, K.F. Fong, Uniformity of stratum-ventilated thermal environment and thermal sensation, Indoor Air 24 (5) (2015) 521–532 https://doi.org/10.1111/ina.12097. [31] S. Zhang, Z. Lin, Z.T. Ai, F.H. Wang, Y. Cheng, C. Huan, Effects of operation parameters on performances of stratum ventilation for heating mode, Build. Environ. 148 (2019) 55–66 https://doi.org/10.1016/j.buildenv.2018.11.001. [32] H. Wigö, Technique and Human Perception of Intermittent Air Velocity Variation, KTH Royal Institute of Technology, 2005. [33] ANSI/ASHRAE, ANSI/ASHRAE 55-2017: Thermal Environmental Conditions for Human Occupancy, (2017) (Atlanta, GA, US). [34] ISO, ISO 7730, Ergonomics of the Thermal Environment e Analytical Determination and Interpretation of Thermal Comfort Using Calculation of the PMV and PPD Indices and Local Thermal Comfort Criteria. 2005: Geneva, Switzerland, (2005). [35] N.H. Wong, S.S. Khoo, Thermal comfort in classrooms in the tropics, Energy Build. 35 (4) (2003) 337–351 https://doi.org/10.1016/S0378-7788(02)00109-3. [36] K.W.D. Cheong, W.J. Yu, S.C. Sekhar, K.W. Tham, R. Kosonen, Local thermal sensation and comfort study in a field environment chamber served by displacement ventilation system in the tropics, Build. Environ. 42 (2) (2007) 525–533 https:// doi.org/10.1016/j.buildenv.2005.09.008. [37] S. Schiavon, D. Rim, W. Pasut, W.W. Nazaroff, Sensation of draft at uncovered ankles for women exposed to displacement ventilation and underfloor air distribution systems, Build. Environ. 96 (2016) 228–236 https://doi.org/10.1016/j. buildenv.2015.11.009. [38] Y. Zhang, R. Zhao, Relationship between thermal sensation and comfort in nonuniform and dynamic environments, Build. Environ. 44 (7) (2009) 1386–1391 https://doi.org/10.1016/j.buildenv.2008.04.006. [39] H. Zhang, E. Arens, C. Huizenga, T. Han, Thermal sensation and comfort models for non-uniform and transient environments, part III: whole-body sensation and comfort, Build. Environ. 45 (2) (2010) 399–410 https://doi.org/10.1016/j.buildenv. 2009.06.020. [40] L. Schellen, M.G.L.C. Loomans, M.H. de Wit, B.W. Olesen, W.D.v.M. Lichtenbelt, The influence of local effects on thermal sensation under non-uniform environmental conditions — gender differences in thermophysiology, thermal comfort and productivity during convective and radiant cooling, Physiol. Behav. 107 (2) (2012) 252–261 https://doi.org/10.1016/j.physbeh.2012.07.008. [41] C. Sun, H. Giles, G. Xu, L. Lan, Z. Lian, Investigation on thermal comfort and energy conservation of local ventilation, HVAC R Res. 19 (5) (2013) 584–592 http://doi. org/10.1080/10789669.2013.803383. [42] H. Wang, C. Huang, Z. Liu, G. Tang, Y. Liu, Z. Wang, Dynamic evaluation of thermal comfort environment of air-conditioned buildings, Build. Environ. 41 (11) (2006) 1522–1529 https://doi.org/10.1016/j.buildenv.2005.06.002. [43] L. Huang, Q. Ouyang, Y. Zhu, Perceptible airflow fluctuation frequency and human thermal response, Build. Environ. 54 (2012) 14–19 https://doi.org/10.1016/j. buildenv.2012.02.004. [44] Y. Zhu, M. Luo, Q. Ouyang, L. Huang, B. Cao, Dynamic characteristics and comfort assessment of airflows in indoor environments: a review, Build. Environ. 91 (2015) 5–14 https://doi.org/10.1016/j.buildenv.2015.03.032. [45] Y. Zhai, H. Zhang, Y. Zhang, W. Pasut, E. Arens, Q. Meng, Comfort under personally controlled air movement in warm and humid environments, Build. Environ. 65 (2013) 109–117 https://doi.org/10.1016/j.buildenv.2013.03.022. [46] H. Liu, J. Liao, D. Yang, X. Du, P. Hu, Y. Yang, B. Li, The response of human thermal perception and skin temperature to step-change transient thermal environments, Build. Environ. 73 (2014) 232–238 https://doi.org/10.1016/j.buildenv.2013.12. 007. [47] H. Zhang, E. Arens, C. Huizenga, T. Han, Thermal sensation and comfort models for non-uniform and transient environments: Part I: local sensation of individual body parts, Build. Environ. 45 (2) (2010) 380–388 https://doi.org/10.1016/j.buildenv. 2009.06.018. [48] H. Wigö, Effects of intermittent air velocity on thermal and draught perception – a field study in a school environment, Int. J. Vent. 12 (3) (2013) 249–256 https://doi. org/10.1080/14733315.2013.11684020. [49] M. Luo, R. de Dear, W. Ji, C. Bin, B. Lin, Q. Ouyang, Y. Zhu, The dynamics of thermal comfort expectations: the problem, challenge and implication, Build. Environ. 95 (2016) 322–329 https://doi.org/10.1016/j.buildenv.2015.07.015. [50] M. Cabanac, Physiological role of pleasure, Science 173 (4002) (1971) 1103 https://doi.org/10.1126/science.173.4002.1103. [51] Y.G. Lv, J. Liu, Effect of transient temperature on thermoreceptor response and thermal sensation, Build. Environ. 42 (2) (2007) 656–664 https://doi.org/10.1016/ j.buildenv.2005.10.030. [52] D. Licina, A. Melikov, C. Sekhar, K.W. Tham, Human convective boundary layer and its interaction with room ventilation flow, Indoor Air 25 (1) (2015) 21–35 https:// doi.org/10.1111/ina.12120. [53] D. Licina, A. Melikov, J. Pantelic, C. Sekhar, K.W. Tham, Human convection flow in spaces with and without ventilation: personal exposure to floor-released particles and cough-released droplets, Indoor Air 25 (6) (2015) 672–682 http://doi.org/10. 1111/ina.12177. [54] M. Sandberg, P.-Å. Elvsén, Rapid time varying ventilation flow rates as a mean of increasing the ventilation efficiency, ROOMVENT, in 9th International Conference on Air Distribution in Rooms. 2004: September 5-8, Coimbra, Portugal.

11