1、4753 Comparison Listiani Nurul Huda, DrEng Thermal Sensation of Local Airflows with Different Temperatures and Velocities: between Summer and Winter Hiroshi Homma, TeknDr Member ASHRAE ABSTRACT Local airflow gives diferent sensations at different loca- tions on a human body as the thickness of natur
2、al convection is not equal over the body surface. This study is intended to examine the effect of horizontal local airflows with different temperatures and velocities on thermal sensation, airflow perception, and thermal comfort to develop economical air conditioning especially for hot and humid are
3、as. The subjects votes concerning the back of the neck and the ankles in summers and winters were examined statistically. The results indicated that airflow caused stronger thermal sensation and aivlowperception at the necks than at the ankles in thesummer experiments. The winter results were mostly
4、 different from the summer results. Only the airflow perception in the veloci change experiments was the same in the winter and the summer: The percentage dissatisjed (PO) was smaller for the neck than the ankles. INTRODUCTION Local airflow is a disturbance defined as “draft“ for people in cold area
5、s, but it seems to be acceptable, or rather is felt to be comfortable, in the summer for people in temperate or mild areas. There may be differences in perception of local airflow between people, between seasons, and between geographic locations. The different perceptions of airflow may cause differ
6、ent sensations, which suggest the possibility of attaining the same comfort or physical performance with different combinations of temperatures and velocities for different people and for different seasons. In air-conditioning practise, it can be more practical to direct airflow to certain locations
7、 ofa body surface, where it is sensitive but is not disturbed by the airflow, than to a whole body. Physically, different sensations may be caused by the difference in thickness and velocity of the natural convection layer over a body surface. Natural convection, which arises from a persons own meta
8、bolic heat, is not equal over a body (Homma 1988,200 I). It plays an important role in heat dissi- pation from a body surface in a room. A room airflow should travel across this layer before it reaches and stimulates the body surface. The study by Rapp (1 973) showed that the heat transfer of a heat
9、ed and naked dummy was found to depend only slightly on the room air movement and to depend prima- rily on natural convection of the dummy when room air veloc- ity did not exceed 0.09 to O. 1 m/s (1 8 to 20 fpm). There are various research results on local sensation that were conducted in the summer
10、 and winter. Some of the exper- iments conducted in winter or in cold conditions are those by Houghten et al. (1 938) and Fanger and Christensen (1 986). These suggested that the back of the neck was more intolerant to cold drafts than the ankles. On the other hand, the impor- tance of the temperatu
11、re in the lower part of a room was suggested by Wyon et al. (1969) and Gonzales and Nishi (1976). Experiments have shown that there is inconsistency in thermal comfort conditions between sensations at the back of the neck and at the ankles. Homma (1988) indicated that an upward airstream envel- oped
12、 a human body, and its velocity reached the highest acceptable air velocity for comfort. Toftum et al. (1997) showed that at 20C (68.O“F) and23“C (73.4“F), airflow from below was felt as most uncomfortable, followed by airflows toward the back and front. At 26“C, airflow from above and toward the ba
13、ck caused the most dissatisfaction due to draft, but generally only a few of the subjects felt discomfort at this temperature. Homma (200 1) studied local heat transfer by Listiani Nurul Huda is a lecturer in the Department of Industrial Engineering, University of Sumatra Utara, Medan, Indonesia. Hi
14、roshi Homma, is a professor in the Department of Architecture and Civil Engineering, Toyohashi University of Technology, Toyohashi, Japan. 02005 ASHRAE. 123 local airflows with a thermal manikin. The effect of the airflows was compared at the back ofthe neck and at the ankles of the thermal manikin.
15、 The results indicated that the local airflows affected the ankles more strongly than the necks. In ASHRAE Standard 55- 1992 or IS0 Standard 7730, the allowable mean air velocities in an office space are O. 18 mis and0.15ds (35.64fpmand29.7fpm)forsummerand winter, respectively. The experiments that
16、were conducted in hot areas such as Thailand by Kehndari et al. (2000) and Australia by Cena et al. (1999) reported that the comfort condition was maintained with airflow as fast as 1 mis (198 fpm) in air-condi- tioned spaces. Even in experiments conducted with US and British subjects by Arens et al
17、. (1998) and McIntyre (1978), comfort was reported to be maintained even when air velocity exceeded 0.8 mis (158 fprn). Even in an experiment conducted in winter in Denmark by Fanger et al. (1974), it was reported that comfort was maintained when the subjects were exposed to avelocityofO.8 mis (158
18、fpm) andatemperature of27.7“C (49.9“F). From these experiments it seems that airflow of a faster velocity than that in the present standards may be acceptable by people in hot or temperate areas. Tanabe et al. (1987) investigated the thermal comfort of Japanese subjects during the summer. From this
19、study it was found that the neutral temperature for Japanese was 26.3“C (79.3“F). This is not significantly different from studies with Danish and US subjects. Xia et al. (2000) investigated the effect of turbulent airflow on thermal sensations in a warm isothermal environment. The experiments were
20、performed at air temperatures of 26“C, 27.5“C, 29“C, and 30.5“C (78.8“F, 81.5“F, 84.2“F, 86.9“F), two turbulence levels of 25% and 40%, and two relative humidities of 35% and 65%. During the experiments the air velocity was controlled by the subjects. The airflows were directed to the area from the
21、chest to the knee of sitting subjects, and results showed that most subjects could attain comfort after adjusting the air velocity as they liked. The preferred velocity was in an approximate range of 0.3 to 1.2m/s(59.4 to237.6fpm). Anewmodelofpercentage dissatisfied at preferred velocity (PDV) was p
22、resented to predict the percentage of the feeling of a draft in warm isother- mal conditions. In the perception and sensitivity examination of airflow at the head region by Todde (2000), the subjects were exposed to isothermal local airflows in a velocity range of O to 1 m/s (O to 198 fprn). The vot
23、e scale for the air velocity was divided into five levels. The gradient of the votes was at the level of about 3.5 for an air velocity change of 1 mis (198 fprn). The gradient of the temperature sensitivity vote was at a level of about -3.1 on a scale of -4 to O. The vote for level of pleasantness w
24、as about -2.5 on a scale of -3 to O. In the present study, airflows of velocities in a wider range than what is allowable in the present standards were directed to two locations-the back of the neck and the ankles-on the subjects. These locations for local airflow application were chosen because the
25、 natural convection produced by body heat starts at the ankle level and develops fully at the neck level (Cena and Clark 1981). The differences in perception of -0- tuvc/tcvu -cid vc/tcvd 10 E -1 o -30 -15 O 15 30 45 80 75 time Imid Figure 1 Schedule of temperature or velocity change. temperature an
26、d airflow and the feeling of comfort were exam- ined by the subject votes. It was intended to find a range of comfort at these locations from the experiments using human subjects. If there is a location where local airflow causes comfort, airflow may be concentrated at this location. By this measure
27、, a more economical air-conditioning system may be designed. If a draft is felt more strongly at any location, it is effective to limit the draft at this location. In order to distin- guish the physiological responses of a human body, the following laboratory experiment was conducted. EXPERIMENTAL S
28、ETUP Arrangements in Climate Chamber The experiments were carried out in the climate cham- ber at a technological university in Japan during the winters of 2002 and 2003 and the summers of 2001,2002, and 2003. Four different types of temperature and air velocity changes were investigated. In the two
29、 temperature change experi- ments, the temperature of the local airflow was adjusted in steps of 5C (9F) while the air velocity was maintained at 0.5 m/s (99 fpm). In one experiment, the temperature differ- ence from the laboratory temperature was changed upward, starting from -10C (-18F) and finish
30、ing at +1O“C (+18“F). This combination was labeled TUVC. The reverse tempera- ture sequence was labeled TDVC. In the velocity change experiments, the velocities of the local airflow were changed from zero to 1 ms (1 98 fpm) and vice versa in steps of 0.25 mis (49.5 fprn), while the temperature was m
31、aintained at the ambient temperature. In the increasing-velocity experiment, labeled TCW, the local airflow velocity started at zero and finished at 1 m/s (198 fprn). In the decreasing-velocity experiment, labeled TCVD, the velocity change schedule was reversed, starting at 1 mis (198 fpm) and endin
32、g at zero. The schedules of the experiments are shown in Figure 1. In both the temperature change and the velocity change experi- ments, step-up and step-down procedures were employed to 124 ASHRAE Transactions: Research Table 1. Dta Subjects in the Temperature and Velocity Change Experiments curlin
33、g short middle* 12 curling 12 curling O 36 short 25 short 11 33 middle* 5 middle* 28 Winter * = reaching the shoulder Total: 60 persons I male: 30 persons I female: 30 persons I eliminate the effect of the step changes, which might have influenced the adaptation of the subjects. The laboratory was t
34、hermally well isolated from outdoor temperature changes. The laboratory air temperature was controlled to between 22C and 24C (71.6“F and 75.2“F) in the winter experiments and controlled to between 26C and 28C (783F and 82.4“F) in the summer experiments. In an experiment lasting 95 minutes, the temp
35、erature change was less than 0.5“C (0.9“F). The temperature gradient between 0.1 m (0.33 ft) and 1.1 m (3.6 ft) above the floor was less than 03C (32.5“F). The mean radiant temperature was identical to the air temperature in the areas where the experi- ments were conducted. During the winter experim
36、ents, the average outdoor temperature was 5C (41 OF) and the relative humidity was about 45%. In the summer experiments, it was 28C (82.4“F) and the relative humidity was about 60%. Two local airflow producers were constructed using thermo- modules. The local airflow was blown from a nozzle of a dia
37、meter of 50 mm (2 in.). The details are in Homma (2001). A change in the temperature of the airflow was completed in five minutes by supplying more power than was required to maintain the temperature. The applied airflow had a low turbulent intensity. The standard deviation of the airflow was 0.02 m
38、is (3.96 fpm) when the velocity at the nozzle face was 0.25 ms (49 fpm). It increased to 0.08 ms (1 5.84 fpm) when the velocity at the nozzle face was 1 .O0 ms (1 98 fpm). Experimental Procedure The subjects were students at a Japanese technical univer- sity, who were paid for their participation. I
39、n the experiment there were 144 subjects (72 males and 72 females) who were 18 to 24 years old. Average physical data on the subjects, as well as the thermal resistance of their clothes for each season and data on their body surfaces, are listed in Table 1. Each subject was exposed to all four types
40、 of experiments. In the summer of 2001 and the winter of 2002, the subjects wore their own clothes used in ordinary classes in the summer or winter. In the summers of 2002 and 2003 and the winter of 2003, the ASHRAE Transactions: Research 125 Table 2. Enquiries, their Scales and Semantics of the Que
41、stionnaire Questions thermal sensation TOP Bottom Scale Level Semantic Value Semantic Value -2 - +2 5 hot 2 cold -2 thermal preference airflow perception lairflow Dreference I -2 - +2 I 5 I prefer faster I 2 I prefer slower I -2 I -2 - +2 5 prefer warmer 2 prefer cooler -2 O-+3 4 not at all 3 a lot
42、O effect of airflow thermal comfort I feeling of air I -2-+2 I 5 I fresh I 2 I stamant I -2 I -2 - +2 5 pleasant 2 disturbing -2 -2 - +2 5 comfortable 2 uncomfortable -2 encouragement I Season Summer -2 - +2 5 encouraging 2 tiresome -2 I Winter L subjects were requested to wear a combination of unif
43、orm clothes that were prepared in the laboratory. During each experiment, a sedentary subject sat on a metal folding chair at a desk and was allowed to read. One of the airflow producers was directed to the back of the neck horizontally. The other one was directed to the left side of the ankle, also
44、 horizontally. The distance between the nozzle and the objective location was adjusted to 0.4 m (1.3 1 ft). During the experiment, the subject was exposed to the airflow and asked to fill out a questionnaire consisting of eight semantic differential questions includ- ing thermal sensation, perceptio
45、n of airflow, and thermal comfort at the two body locations. Table 2 shows the scales of the semantic differential questions. The initial adaptation time was more than 20 minutes (Fanger et al. 1988; Toftum and Nielsen 1996; Toftum et al. 1997; Todde 2000). In several previous step-change studies, t
46、he subjects were exposed to five consecutive 15-minute peri- ods, and during that time the subjects were asked for their local or general thermal sensation at the beginning of or several times in each step (Fanger et al. 1988; Toftum and Nielsen 1996; Tofturn et al. 1997). In the present study, the
47、subjects were allowed 20 minutes to acclimate to the thermal conditions in the laboratory. During this period, they received instruction on filling out the ques- tionnaire. During the next 75 minutes, the subjects were exposed to local airflows of one of the above four types. Five consecutive 15-min
48、ute periods consisted of stepped changes in either temperature or velocity. Three times during each 15- minute period, the subjects noted their responses on the ques- tionnaire (see also Figure 1). At the beginning of each step change, the observer recorded the pulse rate of the subject remotely; at
49、 the same time the subjects measured their audi- tory temperature. At the beginning and the end of the experi- ment, the weight of the subject was measured. After the experiment, the responses were converted into numbers for statistical evaluation. RESULTS AND DISCUSSION The means and standard deviations of the responses for the eight inquiries are shown in Table 3. The responses showed that the uppermost and the lowermost semantics were rarely entered for both of the locations and seasons. In this paper, further analysis was focused on the results of thermal sensation,
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