MEASUREMENT OF THE AIRFLOW AND TEMPERATURE FIELDS AROUND LIVE SUBJECTS AND THE EVALUATION OF HUMAN HEAT LOSS

Transcription

1 MEASUREMENT OF THE AIRFLOW AND TEMPERATURE FIELDS AROUND LIVE SUBJECTS AND THE EVALUATION OF HUMAN HEAT LOSS GH Zhou 1, DL Loveday 1, AH Taki 2 and KC Parsons 3 1 Department of Civil and Building Engineering, Loughborough University, UK 2 School of Architecture, De Montfort University, UK 3 Department of Human Sciences, Loughborough University, UK ABSTRACT This paper reports on the successful development of a special garment for measuring convective and radiative heat loss from the clothed human body, thereby enabling the direct determination of local convection coefficients. The garment has been designed to be in the form of a typical office-clothing ensemble and was tested on male subjects who conducted seated office work in a environmental room. Sensors mounted upon the garment permitted measurement of radiation heat transfer from the clothed body to the surroundings; these results were found to agree well with traditional plane radiant temperature data taken with an indoor climate analyser. Airflow and temperature fields around subject were measured from which were derived correlations relating convection coefficients to air speeds in mixed airflow conditions. The correlations obtained are compared with those of earlier studies, in particular those used in the ISO773 Standard, and the implications for thermal comfort prediction are discussed. INDEX TERMS Heat flux measurement, Convection coefficient, Radiation coefficient, Human body, Garment INTRODUCTION Accurate knowledge of the heat loss from the human body is essential for a number of applications, including thermal comfort design and evaluation, and thermal/airflow modelling within indoor spaces. Since 1936, a number of studies have been made of human heat transfer (Winslow et al. 1936; Nielsen and Pedersen 1952; Nishi and Gagge 197). Many of these have considered exercise-related conditions (e.g. cycling, walking and running), and have imposed specific air flow conditions, such as uni-directional flow (Mitchell et al. 1969). For those that have been conducted for sedentary activities in conventional room environments, air flows were of a uni-directional nature and these cannot be easily related to specific room ventilation systems. Such limitations of test conditions were necessarily imposed as a result of the equipment available at the time, in order to obtain the measurements sought. To date, much work in this field has relied almost exclusively upon the use of thermal mannequins to provide a controlled set of conditions for measurement (Ichihara et al. 1996; de Dear et al. 1997). However, the use of living subjects offers a realistic situation, together with natural movement of the body; development of a technique to obtain data in these situations would therefore be an important step forward. * Contact author D.L.Loveday@lboro.ac.uk 683

2 METHOD The Proposed Technique The technique that has been adopted is the direct physical measurement of both the convective and the radiative component of human heat loss separately, using recently-developed sensitive transducers capable of attachment to the surface of interest. For the measurement of human heat loss, a purpose-designed heat transfer garment has been produced which incorporates heat flux sensors within the fabric for determination of the local convection components. Table 1 states the positions of 2 dry heat flux sensors and 7 radiative heat flux sensors. For detailed descriptions of the development of the garment, refer to Zhou et al. (22). Sensors for determining the net local radiative components are mounted just proud of the surface. In addition to the precise and separate direct measurement of human convection and radiation, the technique eliminates the need to measure metabolic rate, evaporative heat loss rate and human heat storage. This results in a new technique which overcomes the uncertainties inherent in earlier methods for determining human heat loss, namely, the deductive methods (partial calorimetry, Winslow et al. 1936), the uncertainties of the direct calorimetry method (Mitchell et al. 1969), and the approximation of the human body as a collection of cylinders (Rapp 1973). The new technology permits the use of human subjects in the study, as opposed to mannequins, leading to more realistic results. The air velocity and air temperature sensors were placed at 12 points around subject. Figure 1 shows sensors' relative position to subject (viewing from both left side and front). The real dots in the figure represent all velocity and temperature measurement points. The blank circles in the figure's right side repeat 7 points from the figure's left side. Mean values and standard deviation of air velocities and mean air temperatures over every 3 minutes were recorded by a computer and were used for calculating convection coefficients and building its relation to air velocity. Table 1. Positions for dry and radiative heat flux sensors and weight factors for each sensor for calculating total dry and radiative heat transfer. Dry Heat Flux Sensor Weight Factor Radiative Heat Flux Weight Factor (% of Position (%)* Sensors Position total body surface)** Forehead 3.4 chest 27 Rear neck 3.4 shoulder 18 chest 9.78 back 27 back 9.4 right thigh-side 21.5 left arm upper outer 1.77 left thigh-side 21.5 left arm upper inner 4.96 left leg front 2.5 right arm upper 2.48 left leg rear 2.5 right arm lower outer 3.53 * de Dear et al right arm lower inner 3.53 ** Fanger 197 right hand outer 5.16 right thigh upper 2.8 left thigh upper 2.72 left thigh inner 5.43 left thigh outer 2.72 right leg rear 1.51 right leg front 1.51 left leg rear 1.51 left leg front 1.51 left leg side 3.2 left ankle 5.71 Figure 1. Air Velocity and Temperature Measurement Points around Subject 684

3 Test Facilities The experiments were conducted in the Environmental Test Room facility in the Department of Civil and Building Engineering at Loughborough University. The facility consists of a test room of dimensions 5.4 m long, 3 m wide and 2.8 m high. Fresh air supplied to the room can be tempered and humidified prior to entry into the space. All environmental parameters within the room are controllable, and include: supply air flow rate, air temperature, relative humidity and room surface temperatures. Variation of room air temperature can be maintained within ±.2 C of the selected temperature set point. Experimental Procedure An investigation of the garment s performance using two male subjects was conducted in the test room. Subjects were individually exposed to 16 different office environment conditions over 16 separate sessions, each session lasting 1.5 hours. Typical office conditions were set up in the form of three methods of ventilation systems - displacement ventilation, combined displacement ventilation and chilled ceiling, and mixed ventilation. Air temperature values ranged from 15 C to 3 C, and air velocities ranged from.5 m/s to.75 m/s. Dry heat fluxes, radiant heat fluxes and clothing surface temperatures were measured every minute from a total of 28 sensors that comprised the heat transfer garment. The mean values of air temperatures and air velocities accumulated over each three minute period were recorded from sensors located at 12 points equally distributed around the subjects(figure 1). Mean air velocities and air temperatures were recorded and used to build correlations between the heat transfer coefficients and air velocity. Other environmental factors including mean radiant temperature and relative humidity were also registered every minute. EXPERIMENTAL RESULTS Figure 2 shows the experimentally measured values of total dry heat loss at 2 body parts for the two test subjects for 8 experimental conditions (4 conditions in a displacement system and 4 conditions in a combined displacement and chilled ceiling). All measured mean velocities were less than.1 m/s, and the mode of heat transfer involved was considered as natural convection only. The weighted average total dry heat loss values were calculated using the weight factors given in Table 1 and are shown in Figure 3. Dry Heat Flux (W/m²) Subject A Subject B Temperature Difference between Clothing Surface and Free Stream Air( C) Figure 2. Total dry heat loss at 2 body parts of two test subjects for 8 experimental conditions (4 conditions in displacement and 4 conditions in combined displacement and chilled ceiling). Dry Heat Flux (W/m²) h c = 7.19 t R 2 =.94 h c = 7.1 t R 2 = Subject A Subject B Difference between Mean Radiant and Clothing Surface Temperature( C) Figure 3. Weighted average dry heat flux of two subjects for eight experimental conditions Local convection coefficients were determined for the clothed surface of the test subjects. In order to do this, it was necessary to measure radiant heat flux and then to subtract it from the 685

4 measured total dry heat flux. Radiant heat fluxes were measured using 7 radiant heat flux sensors mounted on the clothing surface. At the same time, the mean radiant temperatures were also measured at seven locations as described in Table 1, using a plane radiant sensor connected to an Indoor Climate Analyser; radiant heat fluxes then being obtained from these measurements (for comparison purposes). The radiant heat flux values as measured directly by radiant heat flux sensors attached to the clothing surface were then compared with the values obtained from the traditional plane radiant sensor technique (Figure 4). This was carried out in order to evaluate the performance of the new directly attached sensor approach. There is good agreement between the two methods, confirming that the attachment of purpose-designed radiant heat flux sensors to the clothing surface is a suitable method for measuring radiant heat exchange. Weighted average radiative heat loss is calculated using the weighted factors as shown in Table 1 (from Fanger 197). These results are shown in Figure 5. q r -Calculated from Plane Radiant Temperature Sensor (W/m 2 ) y = 1.1x R 2 = q r -Measured with Radiant Heat Flux Sensors (W/m 2 ) Figure 4. Comparison of radiant heat flux q r as measured by: i) radiant heat flux sensors attached to clothing ii) plane radiant temperatures using an indoor climate analyser. Radiative Heat Flux (W/m²) Subject A q r = 4.31 t R 2 =.96 Subject B Difference between Mean Radiant and Clothing Surface Temperature( C) Figure 5. Weighted average radiative heat flux as a function of temperature difference between body surface and mean radiant temperature After subtracting radiant heat loss from the dry heat loss, the general correlation between air/clothing surface temperature difference and convective heat transfer rate at the clothed human surface is deduced. It was found that hc = 3.2 W/m 2 K for the case of the natural convection heat transfer mode, and this agrees with previous studies (Nishi and Gagge 197) where a value for hc of 3 W/m 2 K was found. These convective heat loss values were measured in environments that employed the displacement ventilation system and displacement ventilation in the presence of chilled ceiling. The weighted average coefficients of total dry heat transfer were also calculated for a mixed flow system. After subtracting the radiative heat transfer coefficient (hr = 4.3 W/m 2 K ) from the total dry heat loss coefficient, the corresponding measured values of convective heat transfer coefficients obtained are shown in Figure 6 as a function of mean air velocities. The latter are the average of four points equally distributed around the subjects and at 1.1 m above the floor. These values are from experiments with a mixed ventilation system operating in the room, with a mean air velocity up to.75m/s. Thus, a forced convection heat transfer mode was involved, and the following correlation was deduced: h c = 13.7v.57 (W/m 2 K) (1) 686

5 Convective Heat Transfer Coefficient (W/m 2 C) h c = 13.7v.57 Present results h c = 12.1v.5 Winslaw et al (1936), Used by Fanger (197) Mean Air Velocity(m/s) Figure 6. Convective heat transfer coefficient as a function of mean air velocity for two subjects for 8 experimental conditions in the mixed flow environment. DISCUSSIONS For the case of a natural convection heat transfer mode, where the mean air velocity is usually less than.1 m/s, the value of convective heat transfer coefficient was found to be 3.2 W/m 2 K. This agrees with previous study (Nishi and Gagge 197; de Dear et al. 1997).When the temperature difference between the clothing surface and the free stream air in the room was less than 8 C there is good agreement between the value for natural convective heat flux obtained in the present study and the value predicted using the expression in the PMV model: (Fanger 197: h c = 2.38 t 1.25 W/m 2 C ). However, at larger temperature differences, the expression used in the PMV model gives values greater than those observed in the present study. For the case of both natural and forced convection heat transfer mode, radiative heat transfer coefficient found from this study was 4.31 W/m 2 K which agrees well with what was found by de Dear et al. (1997) with a naked thermal manikin. The measured convective heat transfer coefficient, which are from experiments with a mixed ventilation system operating in the room and with a mean air velocity from up to.75 m/s, are compared with the values predicted by the PMV model (ISO 773) as shown in Figure 6, and are seen to be in good agreement. It should be noted that the results reported related to tests on two human subjects and serve to illustrate the performance and capability of the newly-developed heat transfer garment at this stage. We have been conducting more subjective experiments and any significant differences that may emerge between measured findings and PMV-based predictions will be reported in due course. The final results, including subjective thermal sensations, will be available for use in refining PMV predictions if appropriate and for modelling purposes 687

6 CONCLUSIONS Using a newly-developed heat transfer measuring garment, convective and radiative heat transfer coefficients of actual human subjects were measured under both natural and forced convection modes corresponding to common types of office ventilation systems. The convective heat transfer coefficient, hc, was found to be. 3.2 (W/m 2 K) for the case of natural convection, and 13.7v.57 (W/m 2 K) for the case of forced convection. The radiative heat transfer coefficient, hr, was found to be 4.31W/m2K. The results were in good agreement with that predicted by the PMV model and by a number of previous studies. The preliminary correlations between air velocity and convective heat transfer coefficient indicates that the garment offers a new and convenient method for the measurement of heat transfer from subjects in many indoor climate conditions and is possibly the first garment of its kind in the world that has been designed for this purpose. It is a useful tool that engineers can use to conduct thermal environmental research in other environments with human subjects in the future. ACKNOWLEDGEMENTS The authors express their gratitude to the UK Engineering and Physical Sciences Research Council (EPSRC) for funding this research project. REFERENCES de Dear, R., Arens, E., Zhang, H. and Oguro, M. 1997: Convective and radiative heat transfer coefficients for individual human body segments. International Journal of Bio- Meteorology. 4: Fanger, P.O. 197: Thermal Comfort. Copenhagen: Danish Technical Press. Ichihara M., Saitou M., Nishimura, M and Tanabe, S. 1996: Measurements of convective and radiative heat transfer coefficients of standing human body by using thermal manikin. Proc. Of Indoor Air 96 Vol ISO 773 Moderate thermal environments-determination of the PMV and PPD indices and specification of the conditions for thermal comfort. International Standards Organisation, 1994 Geneva Nielsen, M. and Pedersen, L. 1952: Studies of the heat loss by radiation and convection form the clothed human body. Acta Phys. Scadinav., Vol.79, pp , Nishi, Y. and Gagge, A.P. 197: Direct evaluation of convective heat transfer coefficient by naphthalene sublimation. Journal of Applied Physiology, Vol. 29, No. 6, pp: Mitchell, D., Wyndham, C.H., Vermeulen, A.J., Hodgson, T, Atkins, A.R. and Hofmeyr. 1969: Radiant and convective heat transfer of nude men in dry air. J. Appli. Physiol. 26(1) Rapp, G.M. 1973: Convective heat transfer and convective coefficient of nude man, Cylinders and Spheres at low air velocities. ASHRAE Trans., No.2264 pp75-87, Winslow, C.E.A, Herrington, L.P. and Gagge, A P 1936: The determination of radiation and convection exchanges by partial calorimetry. American Journal of Physiology, 116(3): Zhou, G.H., Loveday D.L., Taki, A.H and Parsons K.C. 22: Design and testing of a garment for measuring sensible heat transfer between the human body and its environment, submitted to ASHRAE Summer Meeting