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Description
INTRODUCTION
Airflows in ventilated spaces are typically turbulent, with velocities varying in magnitude, direction, and fluctuation frequency. Mean air velocities from below 0.05 m/s up to 0.6 m/s, turbulence intensities from less than 10% up to 70%, and frequency of velocity fluctuations as high as 2 Hz that contribute up to 90% of the measured standard deviation of fluctuating velocity (RMS) have been identified in the occupied zone of rooms (Finkelstein et al. 1996).
Accurate measurement of a low air velocity is difficult. Several methods, such as thermal anemometry, laser Doppler anemometry, flow visualization, sonic anemometry, etc., may be used to measure air velocity in rooms. At present, a thermal anemometer with an omnidirectional velocity sensor is most often used in practice due to its low price and easy and convenient operation. These anemometers, in fact, measure speed but not velocity.
Draft is one of the most frequent complaints indoors. Draft is defined as an unwanted local cooling of the body caused by air movement. Draft discomfort increases when the air temperature decreases and the mean speed and turbulence intensity increase (Fanger et al. 1988). The same airflow from the back causes more draft discomfort than that from the front (Mayer and Schwab 1988; Mayer 1992; Toftum et al. 1997). Research also identifies that the frequency of the speed fluctuations is important for the sensation of air movement (Fanger and Pedersen 1977). Room airflow is felt most uncomfortable when the equivalent frequency of the speed fluctuations is around 0.5 Hz (Zhou and Melikov 2002; Zhou et al. 2002).
Present standards acknowledge the importance of draft discomfort and define requirements for either summer and winter allowable maximum mean speed in the occupied zone or maximum percent of dissatisfied occupants due to draft (EN 1995; ASHRAE 2004, 2005). The percentage of occupants in a space that may experience draft discomfort is assessed by a draft rating index, %, calculated by the following equation:
DR = (34-[t.sub.a])([[V.sub.mean]-0.05).sup.0.62](0.37 . [V.sub.mean] . Tu + 3.14) (1)
or
DR = (34-[t.sub.a])([[V.sub.mean]-0.05).sup.0.62](37 . [V.sub.RMS] + 3.14) (2)
In these equations, [t.sub.a] ([degrees]C) is the air temperature, [V.sup.mean] (m/s) is the mean speed, [V.sub.RMS] (m/s) is standard deviation of speed fluctuations, and Tu (%) is the turbulence intensity of the flow. The mean speed is defined by the instantaneous speed averaged over an interval of time, while the turbulence intensity is the standard deviation of the speed divided by the mean speed, Tu = [V.sub.RMS]/[V.sub.mean]. The above equation is valid when [V.sub.mean] is higher than 0.05 m/s; for smaller than 0.05 m/s, [V.sub.mean] = 0.05 m/s should be used; for DR< 100 %, DR = 100% should be used. This equation is based on results from human subject experiments (Fanger et al. 1988). The importance of airflow direction and frequency of speed fluctuations on draft sensation is not addressed in the standards.
Apart for evaluation of occupants' thermal comfort, air velocity measurements in rooms are needed with regard to air quality research, validation of CFD predictions of room air distribution, balancing and commissioning of air-conditioning systems, etc.
The accuracy of measuring mean speed, standard deviation of speed, and turbulence intensity by a low-velocity thermal anemometer (LVTA) can be affected by several error sources defined later in this paper. Comprehensive analyses for the impact of the error sources on the measurement accuracy of the mean speed and the standard deviation of speed have been performed (Loomans and Schijndel 2002; Popiolek et al. 2007; Joergensen et al. 2004). Measurements without considering the impact of these factors on the accuracy of speed measurement and draft assessment may lead to erroneous results (Melikov and Sawachi 1992; Popiolek et al. 1998).
Requirements for the characteristics of instruments measuring low air velocity are given in the present indoor climate standards (EN ISO Standard 7726, Ergonomics of the Thermal Environment--Instruments for Measuring Physical Quantities [EN 1998]; ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy [ASHRAE 2004]; ANSI/ASHRAE Standard 113-2005, Method of Testing for Room Air Diffusion [ASHRAE 2005]). The standards define range of velocity, required and desirable accuracy of its measurement, and measuring time, as well as requirements regarding dynamic response of the anemometer and directional sensitivity of the velocity transducers. However, the standards do not specify requirements for design of the velocity transducer or for the sampling rate. Several other requirements that will ensure low uncertainty of velocity measurements are not specified in the standards either. ASHRAE (2004) requires DR < 20%, while ISO Standard 7730, Moderate Thermal Environments--Determination of the PMV and PPD Indices and Specification of the Conditions for Thermal Comfort (EN 1995a), requires DR < 15%. The accuracy of determination is not known and is not defined in the standards. CEN Report 1752, "Ventilation for buildings: Design criteria for the indoor environment" (CEN 1998), defines three categories of indoor thermal environment, A, B and C, with draft... |
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