Airflow Perception and Draught Rating for Varying Thermal Conditions.
The focus point of this study is the draught discomfort and the airflow preferences of subjects for different airflow, temperature, and metabolic rate conditions. Standard 55-2004 (2004) defines draught as the unwanted cooling of the body due to air movement and specifies an allowable airflow of 40 ft/min for neutral temperatures to avoid draught discomfort. However, previous literature (Zhang, Arens et al. 2007) on ASHRAE field studies shows that people want more airflow even when their thermal sensation is cool. This is counterintuitive considering the fact that a cooling stimulus is felt unpleasant when general thermal sensation is on the cool side (Zhang 2003). These two opposing facts imply that thermal conditions alone are not adequate to explain people's perception of airflow which affects people not only physically but also psychologically. In this study, tests were designed accordingly and analyses were conducted to determine the subjects' perception of airflow in multiple thermal conditions.
Fanger et al. (1988) investigated the effect of turbulence intensity on draught sensation and found that highly turbulent airflow increases the sensation of coolness to the point of being uncomfortable. They formulated the percent dissatisfied due to draught equation (Equation 1) which became the basis for the draught discomfort sections of the ANSI/ASHRAE Standard 55 (2004) and ISO 7730 (2005).
DR = ((34 - [t.sub.a])[([bar.v] - 0.05).sup.062]) + (0.37 *[bar.v] * Tu * 3.14) (1)
DR = predicted percentage of people dissatisfied due to draft
ta = local air temperature, C
v = local mean air speed, m/s
Tu = turbulence intensity, %
Griefahn et al. (2002) conducted draught discomfort test under various metabolic and temperature conditions and found that the sensation of draught is increased by increased air velocity and turbulence intensity and by decreased air temperature and metabolic rate. Koskela et al. (Koskela, Heikkinen et al. 2001) developed correction factors for the effect of turbulence intensity on the thermal comfort calculations. Toftum (2002) and Griefahn et al. (2001) focused on the relationship between the metabolic rate and the draught discomfort and found that DR decreases with the increased metabolic rate. Toftum and Nielsen (1996) explained this relationship such that when the deep body temperature is raised due to the increased physical activity, the influence of the impulses from thermoreceptors on the thermoregulation is decreased to maintain the heat balance. The decreased sensitivity to draught in high metabolic conditions ensures utilization of the airflow to lose heat. Mayer (1987) investigated the effect of combinations of turbulence intensities and mean air velocities on draught sensation and developed a curve that estimates the potential for draft for any given combination. Mayer's curve calculates acceptable turbulence intensities of 60%, 40%, and 20% for mean air velocities of 21.5 ft/min, 31.5 ft/min, and 59.0 ft/min respectively.
TEST DESIGN AND METHODOLOGY
A human subject test was designed to test the effect of various thermal conditions on the airflow perception and the draught rating (DR). Metabolic rate varied throughout the test due to 5-minute exercise periods which repeated five times for each subject. The two airflow treatments were the airflow location on the body (head-only or head-hands-feet simultaneously) and the airflow frequency (30 second or 60 second period). The test consisted of three sessions of which the first was the control session and the other two were the treatment sessions in which four combinations of treatments were applied to each subject. The between-subjects variables of the test design were the gender and the room temperature. Approximately, equal number of subjects were randomly assigned to each temperature and gender combination. A total of 19 female and 21 male undergraduate and graduate students participated in the study. This research study was approved by the Institutional Review Board of the Texas A&M University and an informed consent was obtained from each participants before the tests. A more detailed description of the test design and methodology can be found in Ugursal and Culp (2013a, 2013b).
A controlled test room was designed which accommodated a subject test station and two researcher workstations. A test cubicle was designed to provide airflow at various locations within the micro-environment around the human body. A bicycle ergometer which is located under the workstation was used to increase subjects' metabolic rate during the tests. In order to isolate the effect of increased air velocity on subject responses without additional cooling, the airflow at the room temperature was re-directed through the pressurized cubicle walls to the subject without additional cooled air. The test room design allowed various environmental and subjective measurements and exercising using the bicycle ergometer while subjects read magazines or played simple card games on the workstation. A detailed description of the test setup can be found in Ugursal and Culp (2013a).
Environmental measurements were taken and user surveys were conducted during the course of the tests. The environmental measurements include air temperature at three locations next to the subject, mean radiant temperature at the head and head levels, airflow speed behind subjects' neck, dew point temperature in the center of the room, and room and cubicle surface temperatures. In addition, skin temperature measurement at 15 locations on the body, heart rate and electrodermal activity were measured.
Two types of user survey was conducted with each subject. The Background Survey was filled out by the participants before the tests started which is a modified version of ASHRAE field study surveys. An Online Survey was used to collect subjects' responses to thermal conditions. Each person filled this survey a total of 53 times during the tests. Five questions aimed at recording instantaneous thermal comfort, thermal sensation, airflow preference, temperature preference, and thermal environment acceptability responses of the subjects. The main measure in this study is the draught rating (DR) which is the percentage of dissatisfied persons due to unwanted cooling effect of airflow. Following Toftum's (2002) methodology, preference for less air movement was taken as an indicator of the draught dissatisfaction. The draught rating in any given Online Survey instance during the test was calculated based on the percentage of "Less Airflow" responses to the total number of airflow preference responses at that instance. There is a total of 53 Online Survey instances during the test labelled [1.01, 1.02,..., 1.15, 2.01, 2.02,..., 2.19, 3.01, 3.02,..., 3.19].
A total of 2077 subject responses to the Online Survey was pooled and analyzed for airflow preferences in relation to air velocity, skin temperature, airflow location on the body, airflow frequency, metabolic rate, and thermal sensation and comfort. We calculated the draught rating (DR) as a continuous variable for our tests and plotted it against the mean airflow preference and airflow satisfaction (Figure 1). The shaded areas in this graph indicate the exercise periods. The data showed that the DR was consistently below 10% throughout the test and the DR dropped during the exercise periods with increased metabolic rate. The airflow satisfaction percentage was calculated by taking the percentage of "No Airflow Change" responses to the total responses at each instance. The airflow satisfaction consistently dropped during the exercise periods indicating an overall preference for more airflow due to increased metabolic rate.
The Effect of Metabolic Rate and Airflow Location and Frequency
In order the test the effect of metabolic rate on airflow preference, the test data was grouped for high, medium and low metabolic periods. The high metabolic periods were determined as the mid-point and the end of exercise session, the point at which the subject reached the target heart rate to generate 4 Met. metabolic heat. The medium metabolic group was determined based on the average responses of the five survey instances following the high metabolic period during which the heart rate dropped from high to sedentary heart rate levels. The rest of the responses were grouped under low metabolic group. The overall airflow preference data shows that preference for more airflow is highest during the exercise periods which are labelled as 1.07, 1.08, 2.02, 2.03, 2.11, 2.12, 3.02, 3,03, 3.11, and 3.12 (Figure 2). The preference for more airflow drops by up to 50% after each exercise session which indicates an active thermoregulation system that is consistent with instantaneous thermal conditions and thermal stress on the body. The mean airflow preference was 0.25, 0.43, and 0.54 for low, medium, and high metabolic periods of which a score of 1 indicates more airflow and a score of 0 indicates no change. The difference between the three groups statistically significant at 0.05 level.
The mean DR prediction of ANSI/ASHRAE Standard 55-2004 were compared to the actual responses from the test subjects. In this comparison, data corresponding to 28.3[degrees]C (83[degrees]F) room temperature was excluded since the DR formula in Standard 55-2004 is limited to 26.7[degrees]C (80[degrees]F). Air movement dissatisfaction in our study was within 5% while DR in Standard 55-2004 predicted a 16% dissatisfaction for the same conditions. Part of the reason for the discrepancy is the lack of the metabolic rate component in the DR formula. Our test includes high metabolic rate periods and airflow dissatisfaction decreases with the increased metabolic rate (Toftum and Nielsen 1996). The test results of our study were also compared to two other studies from literature which took into account the effect of metabolic rate on DR (Figure 3). Our test results are significantly lower than the other two studies for equivalent metabolic rate conditions. The lower DR in our study is due to the varying airflow rate (between 156 ft/min and 10 ft/min measured behind the neck) instead of a constant stimulus on the body which creates the unwanted cooling effect or higher DR.
The two treatment effects of this study are the airflow location and the airflow frequency. The airflow location (head-only or head-hands-feet simultaneously) did not yield statistically significant differences of airflow preferences. Similarly, 30-second or 60-second airflow period did also not yield significant differences of airflow preference. However, in both cases, the existence of high airflow, regardless of the location or the frequency, yielded statistically significant differences compared to the minimal background airflow condition (p<0.05). This result can be attributed to the high thermal stress during the test (up to 4 Met. under 83[degrees]F) and the difference between having no airflow and having airflow being more noticeable than the actual differences of airflow location or frequency.
The Effect of Air Velocity and Skin Temperature
One of the mechanisms of heat loss from the human body is the convective heat loss on the skin surface. Increased air velocities on the skin surface increases convection heat coefficient which in turn increases heat loss while decreasing the skin temperature. Draught rating (DR) is the unwanted cooling on the body that is a result of excessive heat loss due to higher than desired air velocities.
A non-parametric correlation analysis showed that the air-velocity as measured behind the neck has no statistically significant effect on the DR (p=0.76) while mean skin temperature has a significant effect on the DR scores (p<0.00). In our test, a singular airflow measurement is not adequate to determine the DR tendencies of the subjects. In addition, the measured mean air velocities (0.44 ft/min) and the maximum turbulence intensity (31%) for simultaneous head-hands-feet airflow condition falls within the no-draught zone of Mayer's (1987), which eliminates draught risk for part of the test conditions. However, the average of the 15 skin temperature measurement sites is a good indicator of overall thermal state of the body and is therefore a better indicator of potential DR responses of the subjects.
The Effect of Temperature, Thermal Sensation and Thermal Comfort
The draught rating calculation of ASHRAE Standard-55 considers the effect of ambient temperature such that DR decreases with the increased ambient temperature. When exposed to higher temperatures, the increased heat loss from the body to the environment due to increased air velocities is more desirable than the lower ambient temperature conditions. However, Toftum measured draught sensation for temperatures of 57.2[degrees]F, 62.6[degrees]F, and 68.0[degrees]F and didn't find differences for draught sensation. In our study, the first half of the test is consistent with the Standard-55's calculation such that draught rating was higher under 75[degrees]F conditions than the 83[degrees]F conditions (Figure 4). However, during the second half of the test, draught rating was higher for 83[degrees]F than the 75[degrees]F group. Our findings also confirmed Toftum's findings that ambient temperature did not yield to significant differences in DR. In addition, the airflow satisfaction throughout the test was significantly higher for 83[degrees]F group than the 75[degrees]F group although both groups were exposed to the same airflow conditions. The results suggests that increased ambient temperature, in our case, increased sensitivity to airflow creating an heightened awareness for the existence of airflow.
In a final analysis, the effect of thermal comfort and thermal sensation on airflow perception were investigated. Thermal comfort and thermal sensation responses of the subjects were binned to match the ASHRAE 7-point scale and non-parametric correlation analysis were conducted to determine their effect. In theory, the desire for airflow increases with the increased thermal sensation (or feeling of warmth) while the desire for more airflow should decrease with the decreasing thermal comfort due to cold. The test results suggested that thermal comfort and sensation bins yield significant differences in airflow perception (p<0.00) and preference for more airflow increases with the increased thermal sensation (Figure 5). Some variation is evident between consecutive thermal sensation bins, however, the general trend is significant with Spearman's rho of 0.47.
The maximum air velocity behind the subjects' neck was 156 ft/min. while the average air velocities were 44 ft/min. with a maximum turbulence intensity of 31% for simultaneous head-hands-feet cooling and 55 ft/min. with a maximum turbulence intensity of 38% for head-only cooling conditions. Based on ASHRAE Standard 55-2004, the predicted draught rating (DR) for the average test conditions is 16% due to higher than usual airflow speeds. On average, only 4% of our test subjects expressed a desire for less airflow which was used as the test DR as suggested by Toftum (2002). The subjects' responses were grouped under three metabolic rate conditions, i.e., low, medium, and high metabolic rate. The average DR for those conditions were 5%, 4%, and 3% respectively. The mean airflow preference were 0.25, 0.43, and 0.54 respectively where a score of 1 corresponds to preferences for more airflow and 0 corresponds to no change. The subject's on average demanded more airflow under all conditions. The decreased DR with increased metabolic rate is documented in previous studies (Griefahn, Kunemund et al. 2001, Toftum 2002) and our test results are also consistent with their findings. One major differences is that DR ratings from our study is significantly lower than those of the previous ones for comparable metabolic conditions. This is due to the varying airflow speed in our study instead of constant airflow stimulus on the body which is likely to create more unwanted cooling and a higher DR response.
Subjects' DR was tested against the two treatment effects, i.e., airflow location and the frequency. In both cases, levels of treatment did not yield significant differences of DR response. The frequency of the airflow or the location of the airflow in the micro-environment around the body led to no differences in airflow perception. However, the existences of the treatment for both cases yielded statistically significant differences compared to the no treatment conditions.
The effects of air velocity as measured behind the neck and the room temperature were also tested against the DR responses. Neither the airflow speed, nor the room temperature did not yield to statistically significant differences of DR. It was concluded that a single airflow measurement behind the neck, although one of the most sensitive regions of the body towards airflow, is not adequate to determine the effect of airflow on draught rating. The neutral (75[degrees]F) and warm (83[degrees]F) temperature conditions made no difference on DR which is consistent with some of the previous studies in the literature.
In a final analysis, the effect overall thermal state of the body on airflow perception was analyzed. The thermal sensation and thermal comfort responses of the Online Survey were tested against the airflow perception. Both thermal sensation and thermal comfort has statistically significant effect on airflow perception such that preference for more airflow increased with increased thermal sensation (on 7-point ASHRAE scale) and decreased thermal comfort due to cool sensation. The mean skin temperature was also found to have significant correlation to DR. Therefore, it was concluded that the overall thermal state of the body is a better indicator of airflow perception or DR rather than the instantaneous individual thermal variables.
Airflow perception and the resulting draught rating (DR) of the subjects were investigated for various metabolic rate, room temperature, airflow conditions. A human subject test methodology and the test setup were developed to measure the singular and interaction effect of those thermal factors. Test results showed that the personal factors such as metabolic rate, and the overall thermal state of the body and the person is a more accurate indicator of the airflow perception than the environmental thermal factors such as airflow and the ambient temperature. A more detailed study focusing on determination of the personal thermal states and its relation to airflow perception is noted as future research.
Authors would like to thank to Boston Society of Architects (BSA) and to ASHRAE for research grants and to the Center for Housing and Urban Development (CHUD) of the Texas A&M University for its logistics support.
ASHRAE (2004). Standard 55. Thermal Environmental Conditions for Human Occupancy. Atlanta, ASHRAE.
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Ugursal, A. and C. H. Culp (2013b). "The effect of temperature, metabolic rate and dynamic localized airflow on thermal comfort." Applied Energy 111: 64-73.
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Ahmet Ugursal, PhD
Louis G. Tassinary, PhD
Charles H. Culp, PhD
Ahmet Ugursal is an Engineering Research Associate III at the Texas A&M University, College Station, TX. Louis G. Tassinary is the Executive Associate Dean of the College of Architecture at the Texas A&M University, College Station, TX. Charles H. Culp is a Professor of Architecture and the Associate Director of the Energy Systems Laboratory at the Texas A&M University, College Station, TX.
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|Author:||Ugursal, Ahmet; Tassinary, Louis G.; Culp, Charles H.|
|Publication:||ASHRAE Conference Papers|
|Date:||Jun 22, 2014|
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