- Email: [email protected]
Measurement of ambient air temperature for evaluation of human heat convective losses
Measurement 42 (2009) 62–70
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Measurement of ambient air temperature for evaluation of human heat convective losses Francßoise Thellier, Francßoise Monchoux *, Sandra Spagnol, Michel Bonnis-Sassi Université de Toulouse, PHASE – Université Paul Sabatier-118, route de Narbonne, 31062 Toulouse Cedex 9, France
a r t i c l e
i n f o
Article history: Received 3 February 2006 Received in revised form 4 March 2008 Accepted 3 April 2008 Available online 11 April 2008
Keywords: Temperature Convection Thermocouples Human being
a b s t r a c t To analyze a human being’s local thermal balance through simulation and/or experiments, a good description of the thermal environment is indispensable. Many local physical data are needed to calculate the heat exchanged by the body during transient conditions in a non-homogeneous environment. To evaluate the conditions near the body, measurements have to be made very close to the surface in extremely non-homogeneous thermal conditions, so sometimes the probes have to be fixed on the subject himself. For air temperature measurements, ISO recommends protecting the probe by a low-emissivity protective screen and increasing air velocity that is not possible as it will change heat exchanges on skin surface. If air velocity is low when the air is colder than the surrounding surfaces, the air temperature measured close to the body is overestimated. We performed laboratory experiments to understand the influence of the protective screen. It appeared that the screen and the aerodynamic flow in and around the screen influenced the temperature of the probe. The paper quantifies the correction needed to improve measurements, and the discussion leads to recommendations about how to measure air temperature close to a human being. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Large number of parameters characterizes thermal environmental conditions: air temperature and velocity, humidity, and temperature of the surrounding surfaces. These physical data determine heat exchanges between the human being and his environment. There is a considerable body of literature relating the measurement of these data and their use in the field of thermal comfort [1]. The temperature of the air directly influences the skin and respiratory convective heat exchanges, which represent half the heat exchange in moderate climates. On the other hand, air temperature is often the only parameter used to define and control a thermal environment. Most of the time, the temperature of the air in a room is not uniform, depending on the heating and ventilation system or the temperature of the walls. For example, if the * Corresponding author. Tel.: +33 5 61 55 69 94; fax: +33 5 61 55 81 54. E-mail address: [email protected] (F. Monchoux). 0263-2241/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2008.04.001
heating is through convection, stratification is significant and the variation in temperature between the vicinity of the floor and that of the ceiling can be several degrees. The temperature of the walls (the measurement of which is not the object of this paper) gives rise to convective movements if it is different from that of the air, and may disturb measurements [2]. The ventilation system can also induce distributions of temperature with large variations from one point to another. Finally, the human body itself can create a ‘‘plume” effect thus helping to create a heterogeneous environment around itself [3]. In cars, special problems are encountered; the environment is highly heterogeneous and the space is enclosed. During summer, some surfaces can be very hot (40–60 °C) while the air blown by the air conditioner is quite cold (5–15 °C) and the skin is at about 33 °C [4]. To take account of this lack of homogeneity, several methods have been imagined, according to the final purpose. In particular, many suggestions exist for the calculation of the global indices, i.e., indices based on a calculation
63
F. Thellier et al. / Measurement 42 (2009) 62–70
Nomenclature U DT A, a B, b C, c Er h K, k n R, r
heat flux density, W m2 temperature difference, °C convective coefficient, – convective coefficient, – convection, W measurement error, °C heat transfer coefficient, W m2 K1 conduction, W probe number, – radiation, W
T v
temperature, °C air velocity, m s1
Subscripts 1 reference air air c convective in inside the screen s surface (skin, clothing, plate) scr screen
of human heat balance, for example, PMV [5] where three measurements are made at different heights, and only the average is used. In very heterogeneous situations, a single temperature is no longer sufficient to describe the thermal environment if local discomfort is studied. More often than not, measurements are made far from the body or before the exposure and, because of the air movement mentioned above, the air temperature close to the body might be different. To analyze the effects of a given thermal environment on a human being’s thermal comfort, we have been using, for many years, a multi-node human thermoregulation model [6]. This model needs a precise description of the surroundings. Local heat balances of each body segment are calculated from the local heat fluxes exchanged at skin surfaces. Thus, all the physical thermal parameters are needed: temperatures of the surrounding surfaces, air temperatures, humidity and velocities. These data have to be obtained as close as possible to the human’s surface (skin or clothing). They can come from environmental models or be measured during experiments.
Tair (˚C)
humid cold
40 dry cold
dry warm
20
HR %
dry cold
humid warm
90
dry cold
humid warm
10 40
Mean skin Temperature 38 (˚C) 36 34 32 1.0 Global 0.8 Wetness 0.6 0.4 0.2 0.0 1 2
3
4 Vmin
5
6 VMAX
7
t (h) 8
Fig. 1. Ambient conditions, calculated mean skin temperature and wetness for two air velocities.
2. Human thermal state: an example The difficulty lies in the determination of the inseparable data hc and Tair that are a convenient way to calculate Uc. The analysis of the useful accuracy on hc depends on that accepted for Tair.
An example is given in Fig. 1, where the importance of air temperature and velocity is shown. The model calculated the skin temperature and wetness of a subject for different ambient conditions. The two configurations correspond to different blowing positions of the ventilation system in a car. All local velocities were recorded (local data are given in Table 1) and the mean value over the whole body was calculated [7]. The skin temperature was clearly modified especially during transitions from warm to cold climate. Convective heat transfers (Uc) are generated by the difference in temperature between the ambient air (Tair) and body surface (Ts), skin or clothing according to whether the skin is naked or not. The heat flow by convection is given by the following expression: Uc ¼ hc ðT s T air Þ
2.1. Determination of the convective coefficient hc Various studies contribute to the determination of the local convection coefficient hc [8,9]. In the case of forced convection, many studies show the same type of correlation: hc = B vb. B and b are constants that depend on the body segment. b is between 0.5 and 0.8. v is the air velocity near the surface, which complicates the problem because of the necessity to determine the air velocity. Local measurement is difficult, but actually, it is essential only in extreme cases.
ð1Þ
Table 1 Local and mean value of air velocity for two configurations (m s1)
Vmin Vmax
Head
Trunk
Right arm
Left arm
Hands
Legs
Feet
Mean
0.06 0.57
0.10 0.1
0.14 0.55
0.10 0.24
0.77 1.24
0.22 0.56
0.20 0.20
0.178 0.381
F. Thellier et al. / Measurement 42 (2009) 62–70
2.2. Measuring air temperature Tair The second essential parameter in convective exchanges is the air temperature, which is needed to calculate Uc or to evaluate hc. The International Standards [11] recommend that the probe for air temperature measurement should not be influenced by radiation from the surfaces. Normally, probes are composed of a thermocouple protected against radiation by a low-emissivity protection and have to be ventilated. We will reconsider the effectiveness of this protection further. Calculation of local convective heat transfer depends strongly on the value of the air temperature near the body and then on the accuracy of the measurement; the mean value of the far environment is not sufficient. The great difficulty lies in the fact that measurements must be made near the human while allowing him a certain degree of mobility. During experiments performed in cars, local data are needed to evaluate the thermal comfort of the car driver. Strong transients are studied to understand how the heat fluxes vary while the driver enters the car. Therefore, the driver is equipped with many special probes to measure air temperature. These probes which are described further (Fig. 3), are fixed directly on the skin or clothing surface with adhesive. It appears that this method leads to perturbations at the probe and uncertainty on the measured value. It seems that this measuring method is very sensitive to the temperature of the surrounding surfaces and to the airflow around the probe. Laboratory experiments have been performed to evaluate the measuring errors and to analyze the influence of the protective screen [12]. The study presented here shows that the measurements depend on the experimental configuration. Different methods will be discussed and the differences quantified and analyzed. 3. Material and method The experimental device was designed to reproduce the real measuring conditions used on human being. 3.1. Experimental system The experimental system was composed of a regulated flat heat exchanger (Fig. 2). This plate was made of pol-
ished duralumin and represented the body surface. The regulation was provided by temperature-controlled water flowing through the plate. The flow rate was sufficient to enable the water temperature to be maintained constant or to be changed rapidly if necessary. Six thermocouples (T1–T6) were embedded in the plate, under its surface, to check that it was isothermal. The differences observed over the plate were close to the measurement accuracy of ±0.3 °C, even during strong transients. One thermocouple (no. 11 in Fig. 4, thin thermocouple 0.1 mm diameter, wires covered by glass fibbers) was glued on the surface of the hot plate to measure its temperature. The system was placed in a large regulated room. The air temperature Tair was considered as uniform and only disturbed near the small plate. 3.2. Probes for temperature measurement The probes were those used during experiments with a human body in a car. They were composed of a thermocouple, type-K (Chromel/Alumel) insulated with Teflon and fixed in a protective radiative screen made of aluminum. The screen was cylindrical (1.5 cm high, 1.5 cm in diameter). The ‘‘foot” was also made of aluminum and was used to rigidify the probe and fix it on to the surface (Fig. 3). Measurements were made with different types of probes which are presented in Fig. 4. Two thicknesses of the wire were used: thin (0.1 mm, number n) and thick (0.2 mm, number n0 ). The junction diameter was about 5–6 times the wire thickness.
Regulated Room
Regulated Plate 30 cm
In cases of natural convection, hc depends on the difference in temperature, hc = A (Ts Tair)a. A and a are constants that depend on the segment; a is close to 0.25. Lartigue obtained an experimental value of 4.7 W K1 m2 close to a humid flat dummy that was maintained at 34 °C for various air temperatures [10]. It should be noted that the convection coefficient also has an influence on evaporative heat losses. Indeed, under usual conditions in dwellings, natural convection prevails. In vehicles, the estimated mean velocity is not always sufficient because the variations from one place to another can generate differences in the local exchange coefficients.
Water circulation
32 cm
Thermocryostat
Fig. 2. Experimental system.
THERMOCOUPLE
Regulated plate
64
Insulator Wire Junction
ALUMINUM SCREEN
"FOOT"
1.8 cm 1.5 cm Fig. 3. Measuring probe.
65
F. Thellier et al. / Measurement 42 (2009) 62–70
Vertical Cylinder Probe n˚8
Vertical Cylinder
Plate
7, 7' 8 9, 9'
Horizontal Cylinder
11 10, 10'
Probe n˚9 Horizontal Cylinder
12
13
Plastic tube
Probe n˚10
No screen 1.8cm 100cm
1 cm
Fig. 4. Position of thermocouples (diameter n:/ = 0.1 mm; n0 :/ = 0.2 mm).
– Two thermocouples (nos. 7 and 70 ) were far from the plate (1 m, out of plate influence) to measure the reference air temperature named T1. – Five probes were fixed on the plate (nos. 8, 9, 90 , 10, 100 ), at the same distance (1.8 cm) from its centre to be sure that they were in the same thermal conditions. Some of the protective cylindrical screens were vertical (no. 8), others were horizontal (nos. 9 and 90 ), and two of the probes did not have a screen (nos. 10 and 100 ). – Two other probes (nos. 12 and 13) were fixed on a plastic tube and maintained at the same distance from the plate. Probe no. 13 has a horizontal screen and no. 12 is not protected.
3.3. Controlling sensor position We wanted to determine the position of the probes in the thermal environment created by the plate. In fact, natural convection creates a thermal boundary layer, in which temperature gradient is strong; its thickness is around 1 cm. The probes have to be outside this layer. A holographic picture has been made to visualize this [13]. This method consist in superposing two beams of coherent light (red laser), one going through the analyzed zone, the other being the reference one. It created interference fringes that are directly linked to the air indices, so to temperature. This measuring system has the advantages of not disturbing the phenomenon under observation. The optical index of air is related to its temperature. Therefore, it is quite easy to go from the interference fringes to the temperature (the iso-colour fringes show zones of equal optical index and thus the isotherms). Fig. 5 shows an example of an interferometer picture for steady state conditions at DT = 15 °C. The ambient temperature (T1) was uniform at about 1 cm
Fig. 5. Interferogram of the experimental dummy (the black and white fringes indicate isotherm lines).
from the plate. It is obvious that the probes no. 8, 9 and 10 were outside the natural convective boundary layer. They were in isothermal ambient air. 3.4. Method We imposed a temperature difference between the plate and the environment. Temperatures given by the different probes (Tn) were recorded every 2 s. The thermocouples 7 and 70 were quite far from the plate (1 m) and gave the same temperature. The plate is small compared to the room and radiative effects are negligible. We considered this temperature as the reference one, based on El Khatib work [14] which is comparable to our procedure. Then, the reference for the air temperature was taken as T70 = T1 and the temperature of the plate was Tplate = T11. We considered DT, the imposed temperature difference, and Er(n), the induced measurement error as DT ¼ T plate T 1 ¼ T 11 T 70
ð2Þ
ErðnÞ ¼ T n T 1 ¼ T n T 70
ð3Þ
Because of the inertia of the entire room, its temperature could not change rapidly. So to make variations of DT, the room temperature was maintained stable around 20 °C and only the plate temperature was modified. Many experimental conditions were used (positive and negative DT) but only a few of them are presented here. The conclusions drawn are based on the analyses of all the experiments. 4. Results Fig. 6 gives an example of the experimental results. The monitored conditions are shown in Fig. 6a. This test was
66
F. Thellier et al. / Measurement 42 (2009) 62–70
composed of six phases: during phases II and IV, the plate temperature rose and during the other phases it stayed stable at different DT (0 °C, 15 °C and 30 °C). During the last phase (VI), monitored conditions were stable but the screens were closed with adhesive. The air-conditioning system had to be stopped because it disturbed the general airflow in the room. Thus, the room temperature (T1), rose slowly by about 1 °C during the exposure (Fig. 6b). This point will be discussed further. The temperatures recorded by some of the probes are shown in Fig. 6b. When DT = 0 °C, all the thermocouples give nearly the same temperature as the reference one T1. Then when DT increases, two features become noticeable which are very dependent on DT and on the type of probe:
The recorded data were difficult to analyze because two phenomena were superimposed: the fluctuations were strong and masked the temperature differences. Therefore, it was decided to smooth the curves in a simple manner. Several tests were carried out to smooth the curves sufficiently without losing too much information. Each value was replaced by the mean of the 15 surrounding ones. The result of the smoothed curves is plotted in Fig. 6c. For each probe, the average temperature was calculated on the last 10 min of the phases where DT remained constant. Table 2 gives a summary of the errors, Er(n), for the various DT. 5. Discussion As mentioned above, errors increase with DT and are more significant when the probes are fixed on to the plate. Of course, the temperature measured is that of the junction and is influenced by the plate’s temperature, through different modes of heat transfer. If we look at the very simplified thermal heat balances of the thermocouple junction and of the protective screen in the steady state condition (Fig. 7), it is obvious that the screen, when present, strongly influences the junction temperature. Let us consider the heat transfers (C, K, R) at the screen. Aluminum has low emissivity (polished; e = 0.04) but high thermal conductivity (k = 200 W m1 K1). When the screen is fixed on the warm plate, conductive transfer (K) is induced through the base to the screen, and in all cases, the cylindrical part of the screen is also warmed up by radiation (R). Convective exchanges (C) depend on the air
– large difference Er(n) between measured and reference temperatures; – oscillations having different amplitudes and frequencies. The frequency of the oscillations was not related to the acquisition period; one acquisition point did not correspond to one fluctuation. For example, during a 12 s period, there were six acquisition points and the recorded temperature increased continuously by almost 2 °C. We can also confirm that it was not an electronic or electrical problem as the frequency of the oscillations was different. We performed a Fourier analysis, in which no characteristic frequency appeared. It seems that the thickest thermocouples showed the fewest oscillations.
ΔT≈15˚C
ΔT = 0˚C
I
II
III
ΔT≈30˚C
IV
closed screens
V
VI
T(˚C)
36 32
T8
T9 T12
28 24
T∞
20 36 T(˚C)
T9' T8
32 28
T12
T9
24
T∞
20 0
10
20
30
40
50
60
Fig. 6. Probes temperature variations with increasing plate temperature.
70
t (min)
80
67
F. Thellier et al. / Measurement 42 (2009) 62–70 Table 2 Recapitulation of the Er(n) = Tn T1 for each probe (°C) Phase
DT
Tplate
T1
Er(8)
Er(9)
Er(90 )
Er(10)
Er(100 )
Er(12)
Er(13)
III V VI
13.3 28.3 28.0
34.9 50.0 49.9
21.6 21.7 21.9
2.4 3.2 12.7
4.8 9.3 13.6
6.0 10.9 14.9
1.6 2.8 –
1.6 2.8 –
1.5 3.1 4.7
1.3 2.3 –
K
R
Tin
r
T∞
C
C
C, c : Convection K, k : Conduction R, r : Radiation
k
C R K
Tsup Support: Plate or tube
Fig. 7. Schematic representation of heat exchanges in the probe.
temperature, T1, on the outside of the screen. Nevertheless, inside the cylinders, they depend on Tin, which may be quite different from T1 in some configurations. Let us now consider heat transfers at the thermocouple junction (c, k, r). The junction temperature will be close to T1 if conduction (k) and radiation (r) are low and if convection (c) is high, with the air temperature close to T1. In an ideal system, the probe is sufficiently ventilated, and the temperature of the screen Tscr, and that of the air in the screen Tin should be equal to the reference temperature (Tin = Tscr = T1). In fact, in our case, the measured temperatures are not equal to the reference value because radiation (r) and conduction (k) are high when the screen is close or fixed to the plate. Moreover, there is no ventilation inside the screen and it seems that the air temperature Tin is different from the reference. By comparing the various probe configurations, it is possible to have an idea of each phenomenon and try to separate them. 5.1. Thickness of the wires We have to estimate the influence of the wire thickness on conduction in the wires (k). For the thermocouples without a screen, no. 10 (on plate) and no. 13 (on tube), we observe, for DT 28 °C, that T10 T13 = 0.3 °C, which is about the measuring accuracy. This difference is a little higher for the thicker wires, T100 T13 = 0.5 °C. In this case, conduction in the wires between the plate and the junction is weak compared to other exchanges. For the probes with the screen fixed on the plate, nos. 9 (thin) and 90 (thick), a difference of up to 2 °C appears between the two probes. In this case, the conduction in the wires themselves appears greater over a smaller length. As expected, the thickness of the wires appears as an important parameter. 5.2. Radiation influence For probe no. 13, we assume that the tube is in thermal equilibrium with the environment (Ttube T1) and, conse-
quently, conduction (k) is nearly zero. During phase VI (DT = 28 °C), the error for this probe (Er(13) = 2.3 °C) is due only to the radiation (r) between the junction and the plate. If we compare probes no. 13 and 12, both on the tube, the warmer is no. 12, for which the screen is heated by radiation and then radiates on to the junction. Radiative exchanges (r) seem to be larger with the screen in place even if its emissivity is low. The comparison between the thermocouples with horizontal screens, no. 12 (on tube) and no. 9 (on plate), reveals differences that are more significant. For no. 12, the screen fixed on the tube is heated only by radiation, unlike no. 9 that obviously has a very hot screen because of conduction. 5.3. Convection: air flow in the screens Obviously, other effects have to be taken into account, as no. 8 and no. 9 have very different behavior. The experiments show that the most penalizing configuration is the one with the horizontal screen (probe no. 9) in natural convective airflow. In this configuration the error can reach 10 °C for DT = 30 °C. Such DT values can exist in a car. 5.3.1. Orientation of the cylindrical screen The presence of a screen fixed on the plate increases the measuring errors. Whatever the orientation of the screen, we can consider that radiative and conductive contributions are almost the same, but convection seems to be very different. If we compare no. 8 (vertical screen) and no. 9 (horizontal screen), their temperature differences can only be explained by a different airflow inside the cylinders. It is an aerodynamic problem depending on the screen orientation. Fig. 8 gives a schematic representation of the airflow near the probes. For probe no. 8 (vertical screen) the ‘‘foot” of the screen is in the direction of the vertical ascending airflow of the plate’s convective boundary layer. The air temperature around the junction is close to the reference one (Tin T1) and only radiation and conduction lead to an error, of up to Er(8) = 3 °C (Fig. 8a). In the case of probe no. 9, the horizontal cylindrical screen and the ‘‘foot” are obstacles to the convective airflow along the plate (Fig. 8b). It is difficult to know what occurs. The airflow becomes three-dimensional. Inside the cylinder, the air seems to stagnate then to escape by ‘‘puffs”. When the general airflow is stable (phase V), the air seems to remain a ‘‘long” time in the cylinder and warms up. The air temperature in the screen Tin tends towards Tscr. In this case the error can reach Er(9) = 10 °C. When the general flow is turbulent (phase IV), the puffs are more frequent, the oscillations and the errors are of the same order as for probe no. 8.
68
F. Thellier et al. / Measurement 42 (2009) 62–70
Plate boundary layer
Cylinder
Cylinder
Plate
Plate
"foot"
"foot"
Tin Tscr Air flow
Fig. 8. Airflow around probes: (a) vertical cylinder and (b) horizontal cylinder.
5.3.2. Closing the screens The oscillations of the measured temperatures seem to be due to perturbation of the airflow around the junctions. To check this point, we sealed the extremities of the screens with an adhesive in order to be sure of the immobility of the air near the junctions (Fig. 6, phase VI). When the screens had been sealed, it took a few minutes to reach a new equilibrium, and then the fluctuations disappeared completely. The thermocouples on the plate (nos. 8 and 9) gave nearly the same temperatures (measuring accuracy), which were much higher than for open screens. The junctions were in a small, closed enclosure where the air was confined and probably at the screen temperature, which was quite warm because of conduction. It seems that for DT = 28 °C (Tplate = 48 °C) the screens on the plate were at least at 35 °C. This effect is highlighted if we compare this result to the one for probe no. 12, for which the screen is heated only by radiation and the measured temperature is 26.6 °C. It can thus be deduced that the screens in contact with the plate are heated mainly by conduction but also partly by radiation. In a very simplified way, if the overestimation of the measured temperature by probe no. 12 is due to radiative warming, we can then conclude that Er(9) is due to conduction in the screen for around 70% and to radiation for 30%. 5.3.3. Forced convection In order to determine the effect of the general airflow, tests were carried out with a ventilator placed perpendicular to the plate at a distance of 1 m. Many tests were done with air velocities near the plate between 0.3 and 1.2 m s1, which all corresponded to forced convection. Fig. 9 presents the results: phase I – no ventilator, phase II – ventilator blowing. Plate temperature was not changed (DT = 10 °C), but here the variation of T1 is because of the air-conditioning regulation system which is on. For probe no. 8, the air blown horizontally cooled the vertical screen, but no modification was observed. The effect was more significant for probe no. 9 whose screen was horizontal and parallel to the forced airflow. When the ventilator was blowing, the error diminished and thus the measured temperature approached T1. For each thermocouple, a slight attenuation of the amplitude of the fluctuations was also noticed but the frequencies seemed higher. When the ventilator blowing power was changed, no significant change was observed.
Fig. 9. Forced ventilation.
In forced convection conditions, the radiative transfers were still present but they were no longer predominant, and the screen temperature was close to T1 as convection was enhanced. The influence of the protective radiative screen on the junction was less significant than in natural convection. 5.4. Influence of complex experimental conditions It appears that the air conditioner of the experimental room played a very important role in the airflow around the dummy (Fig. 10). When it is on, the air temperature in the room varied by ±0.5 °C around the set point temperature because of the regulation system, the period being about 12 min. Because of lack of space in the experimental room, the dummy was quite close to the air-conditioning system, which was fixed on the ceiling and blew cold air horizontally. The fan ran continuously. The air velocity was 2.8 m s1 at the outlet, 0.5 m s1 at 1 m, and lower than 0.1 m s1 around the dummy (0.1 m s1: lower measuring limit). Fig. 9 presents the result of the smoothed temperature curves, showing that there was an amplification of the room temperature variations. When the refrigerating unit started, the outlet temperature quickly fell from 20 °C to 15 °C. This cold air conferred a significant downward component that modified the natural convective airflow near the dummy. At the same time, the fluctuation amplitude increased but the error decreased; it seems that the airflow was mixed convection. When it stopped, fluctuations were attenuated and
F. Thellier et al. / Measurement 42 (2009) 62–70
28
T (˚C)
27
7. Conclusion
T9 - Horiz. Cylinder
26 25 24 23 22
T8 - Vertic. Cylinder
21
T∞
20 10
15
25
20
30
35
40
t (min)
Fig. 10. Influence of the air-conditioning system.
the error increased, and the general airflow became more stable, returning to pure natural convection. This perturbation lasted a few minutes after the refrigeration stopped. These observations confirm the strong influence of the aerodynamic flow on the convective coefficient [15]. They also underline the difficulties in drawing a conclusion on the corrections that need to be made to the measured values of the temperature. 6. Correction of the temperature measurements Let us return to the initial problem, i.e., the measurement of local air temperatures needed to calculate convective heat exchanges for the human model. The most significant location is at head level as this temperature is also used to calculate thermal judgments [8]. During experiments in a real car in a wind tunnel, the probe at head level was directly fixed on to the skin surface of the forehead, which was quite warm. An empirical correction law has been obtained from the experimental results. The correction is based on probe no. 8, which appeared to be the best one (thin wire, vertical screen). Plotting Er(8) versus plate – measured temperature difference leads to a correlation for temperature correction. To obtain the ‘‘real temperature” Treal, a correction C can be made to the measured value Tmes (Fig. 11): T real ¼ T mes C
3 2
69
ð4Þ
Thermal comfort will be difficult to estimate if the physical parameters are known only with poor accuracy. The study presented here focuses on analyzing the method used for measuring air temperature near the human being. A screen, initially recommended as protection against radiation, becomes a source of measurement error here. The protected thermocouple in contact with the hot plate, or skin, is heated by conduction but also by radiation. The experimental set-up considered here allows us to point out two phenomena that are superimposed: fluctuations, and differences between the reference and measured temperatures. We conclude that, under all the thermal conditions studied and for all types of probes, there are strongly coupled thermal and aerodynamic problems. It has to be underlined that, in this case, the error is smaller for a thermocouple not protected by a screen and not in contact with the surface, the lowest error being made by probe type no. 13. Many parameters influence the measurements, related to the type of probe: diameter of the wires, temperature of the support, presence and properties of the radiative screen, and especially its emissivity and its orientation relative to the principal airflow. The experiments carried out and analyzed allowed us to quantify the error made on the air temperatures and thus to evaluate a correction law to determine the ‘‘real temperature”. This law could be improved by the analysis of other experiments. Errors cannot be avoided, but we hope to quantify each heat transfer, to obtain an analytical correction law. The major problem is the airflow. There can be different cases: – forced convection – high air velocities, then oscillations and errors are low; – natural convection – airflow is very stable, oscillations are present but small, and errors depend on the orientation of the screen, the best position being vertical to favor natural convection in the screen; – mixed convection – airflow is perturbed, then oscillations are very large and error impossible to determine clearly, especially when the screen is horizontal. Finally, in our configurations, it seems that protecting the thermocouples from radiation by a screen as recommended by ISO [11] is a source of error. This error is significant when the screen is in contact with a support that has a temperature very different from T1. A forced flow will minimize the error but, although this technique is recommended, it cannot be used close the human body, as it changes the heat exchanges on the surface.
Er(8)=Tmes-T∞ (˚C)
References
1 2
C = 0.295 (Tplate-Tmes)- 0.007 (Tplate-Tmes) - 0.021
0
Tplate- Tmes (˚C)
-1 0
2
4
6
8
10
12
14
16
18
20 22
Fig. 11. Experimental correction law (Tsurf: temperature of the support).
[1] K.C. Parsons, Human Thermal Environments. The Effects of Hot, Moderate and Cold Environment on Human Health, Comfort and Performance, Taylor & Francis, London, 1993. [2] D. Quintela, A. Gaspar, A. Raimundo, Development of local heating systems for thermal comfort and energy savings in buildings, in: International Conference UIE 2000, Electricity for a Sustainable Urban Development, Lisbon, Portugal, November 2000.
70
F. Thellier et al. / Measurement 42 (2009) 62–70
[3] C.P. Borges, D. Quintela, G.N. Brites, A. Gaspar, J.J. Costa, Analysis of thermal plumes generated by a seated person, a thermal manikin and a dummy, in: ROOMVENT 2002, Copenhagen, Denmark, September 2002,
and Environmental Engineering (Transactions of Architectural Institute of Japan) 561 (2002) 31–39. [10] B. Lartigue, F. Monchoux, J.L. Breton, Convection sous vestimentaire. Analyse dimensionnelle et mesure des flux échangés, Revue Générale de Thermique (International Journal of Thermal Science) 36 (1997) 469–479. [11] ISO 7726, Thermal environments, Instruments and methods for measuring physical quantities, International Standard Organization, 1985 (NF-EN 27726; ISSN 0335-3931). [12] S. Piniec, Thermique de l’homme dans une voiture, Modélisation et expérimentation: analyse de la métrologie, Ph.D. Thesis, Université Paul Sabatier, Toulouse, France, no. 3453, 1999, 176 p. (in French). [13] E. Piquer, B. Lartigue, M.C. Charrier-Mojtabi, Holographic interferometry: principles and applications in Soret-driven convection, in: Eurotherm 71, Visualization Imaging and Data Analysis in Convective Heat Transfer, Reims, France, 28–30 October 2002, pp. 59–73. [14] H. El Kathib, R. Javelas, Modèle correctif de la mesure de la température d’un gaz par couple thermoélectrique: application à une zone à fort gradient thermique, Revue Générale de Thermique (International Journal of Sciences) 35 (1996) 571–579. [15] R.J. Moffat, Experimental Methods in Heat Transfer: Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Elsevier Sciences Publishing Co., Amsterdam, 1988. pp. 13–31.
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Measurement of ambient air temperature for evaluation of human heat convective losses Francßoise Thellier, Francßoise Monchoux *, Sandra Spagnol, Michel Bonnis-Sassi Université de Toulouse, PHASE – Université Paul Sabatier-118, route de Narbonne, 31062 Toulouse Cedex 9, France
a r t i c l e
i n f o
Article history: Received 3 February 2006 Received in revised form 4 March 2008 Accepted 3 April 2008 Available online 11 April 2008
Keywords: Temperature Convection Thermocouples Human being
a b s t r a c t To analyze a human being’s local thermal balance through simulation and/or experiments, a good description of the thermal environment is indispensable. Many local physical data are needed to calculate the heat exchanged by the body during transient conditions in a non-homogeneous environment. To evaluate the conditions near the body, measurements have to be made very close to the surface in extremely non-homogeneous thermal conditions, so sometimes the probes have to be fixed on the subject himself. For air temperature measurements, ISO recommends protecting the probe by a low-emissivity protective screen and increasing air velocity that is not possible as it will change heat exchanges on skin surface. If air velocity is low when the air is colder than the surrounding surfaces, the air temperature measured close to the body is overestimated. We performed laboratory experiments to understand the influence of the protective screen. It appeared that the screen and the aerodynamic flow in and around the screen influenced the temperature of the probe. The paper quantifies the correction needed to improve measurements, and the discussion leads to recommendations about how to measure air temperature close to a human being. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Large number of parameters characterizes thermal environmental conditions: air temperature and velocity, humidity, and temperature of the surrounding surfaces. These physical data determine heat exchanges between the human being and his environment. There is a considerable body of literature relating the measurement of these data and their use in the field of thermal comfort [1]. The temperature of the air directly influences the skin and respiratory convective heat exchanges, which represent half the heat exchange in moderate climates. On the other hand, air temperature is often the only parameter used to define and control a thermal environment. Most of the time, the temperature of the air in a room is not uniform, depending on the heating and ventilation system or the temperature of the walls. For example, if the * Corresponding author. Tel.: +33 5 61 55 69 94; fax: +33 5 61 55 81 54. E-mail address: [email protected] (F. Monchoux). 0263-2241/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2008.04.001
heating is through convection, stratification is significant and the variation in temperature between the vicinity of the floor and that of the ceiling can be several degrees. The temperature of the walls (the measurement of which is not the object of this paper) gives rise to convective movements if it is different from that of the air, and may disturb measurements [2]. The ventilation system can also induce distributions of temperature with large variations from one point to another. Finally, the human body itself can create a ‘‘plume” effect thus helping to create a heterogeneous environment around itself [3]. In cars, special problems are encountered; the environment is highly heterogeneous and the space is enclosed. During summer, some surfaces can be very hot (40–60 °C) while the air blown by the air conditioner is quite cold (5–15 °C) and the skin is at about 33 °C [4]. To take account of this lack of homogeneity, several methods have been imagined, according to the final purpose. In particular, many suggestions exist for the calculation of the global indices, i.e., indices based on a calculation
63
F. Thellier et al. / Measurement 42 (2009) 62–70
Nomenclature U DT A, a B, b C, c Er h K, k n R, r
heat flux density, W m2 temperature difference, °C convective coefficient, – convective coefficient, – convection, W measurement error, °C heat transfer coefficient, W m2 K1 conduction, W probe number, – radiation, W
T v
temperature, °C air velocity, m s1
Subscripts 1 reference air air c convective in inside the screen s surface (skin, clothing, plate) scr screen
of human heat balance, for example, PMV [5] where three measurements are made at different heights, and only the average is used. In very heterogeneous situations, a single temperature is no longer sufficient to describe the thermal environment if local discomfort is studied. More often than not, measurements are made far from the body or before the exposure and, because of the air movement mentioned above, the air temperature close to the body might be different. To analyze the effects of a given thermal environment on a human being’s thermal comfort, we have been using, for many years, a multi-node human thermoregulation model [6]. This model needs a precise description of the surroundings. Local heat balances of each body segment are calculated from the local heat fluxes exchanged at skin surfaces. Thus, all the physical thermal parameters are needed: temperatures of the surrounding surfaces, air temperatures, humidity and velocities. These data have to be obtained as close as possible to the human’s surface (skin or clothing). They can come from environmental models or be measured during experiments.
Tair (˚C)
humid cold
40 dry cold
dry warm
20
HR %
dry cold
humid warm
90
dry cold
humid warm
10 40
Mean skin Temperature 38 (˚C) 36 34 32 1.0 Global 0.8 Wetness 0.6 0.4 0.2 0.0 1 2
3
4 Vmin
5
6 VMAX
7
t (h) 8
Fig. 1. Ambient conditions, calculated mean skin temperature and wetness for two air velocities.
2. Human thermal state: an example The difficulty lies in the determination of the inseparable data hc and Tair that are a convenient way to calculate Uc. The analysis of the useful accuracy on hc depends on that accepted for Tair.
An example is given in Fig. 1, where the importance of air temperature and velocity is shown. The model calculated the skin temperature and wetness of a subject for different ambient conditions. The two configurations correspond to different blowing positions of the ventilation system in a car. All local velocities were recorded (local data are given in Table 1) and the mean value over the whole body was calculated [7]. The skin temperature was clearly modified especially during transitions from warm to cold climate. Convective heat transfers (Uc) are generated by the difference in temperature between the ambient air (Tair) and body surface (Ts), skin or clothing according to whether the skin is naked or not. The heat flow by convection is given by the following expression: Uc ¼ hc ðT s T air Þ
2.1. Determination of the convective coefficient hc Various studies contribute to the determination of the local convection coefficient hc [8,9]. In the case of forced convection, many studies show the same type of correlation: hc = B vb. B and b are constants that depend on the body segment. b is between 0.5 and 0.8. v is the air velocity near the surface, which complicates the problem because of the necessity to determine the air velocity. Local measurement is difficult, but actually, it is essential only in extreme cases.
ð1Þ
Table 1 Local and mean value of air velocity for two configurations (m s1)
Vmin Vmax
Head
Trunk
Right arm
Left arm
Hands
Legs
Feet
Mean
0.06 0.57
0.10 0.1
0.14 0.55
0.10 0.24
0.77 1.24
0.22 0.56
0.20 0.20
0.178 0.381
F. Thellier et al. / Measurement 42 (2009) 62–70
2.2. Measuring air temperature Tair The second essential parameter in convective exchanges is the air temperature, which is needed to calculate Uc or to evaluate hc. The International Standards [11] recommend that the probe for air temperature measurement should not be influenced by radiation from the surfaces. Normally, probes are composed of a thermocouple protected against radiation by a low-emissivity protection and have to be ventilated. We will reconsider the effectiveness of this protection further. Calculation of local convective heat transfer depends strongly on the value of the air temperature near the body and then on the accuracy of the measurement; the mean value of the far environment is not sufficient. The great difficulty lies in the fact that measurements must be made near the human while allowing him a certain degree of mobility. During experiments performed in cars, local data are needed to evaluate the thermal comfort of the car driver. Strong transients are studied to understand how the heat fluxes vary while the driver enters the car. Therefore, the driver is equipped with many special probes to measure air temperature. These probes which are described further (Fig. 3), are fixed directly on the skin or clothing surface with adhesive. It appears that this method leads to perturbations at the probe and uncertainty on the measured value. It seems that this measuring method is very sensitive to the temperature of the surrounding surfaces and to the airflow around the probe. Laboratory experiments have been performed to evaluate the measuring errors and to analyze the influence of the protective screen [12]. The study presented here shows that the measurements depend on the experimental configuration. Different methods will be discussed and the differences quantified and analyzed. 3. Material and method The experimental device was designed to reproduce the real measuring conditions used on human being. 3.1. Experimental system The experimental system was composed of a regulated flat heat exchanger (Fig. 2). This plate was made of pol-
ished duralumin and represented the body surface. The regulation was provided by temperature-controlled water flowing through the plate. The flow rate was sufficient to enable the water temperature to be maintained constant or to be changed rapidly if necessary. Six thermocouples (T1–T6) were embedded in the plate, under its surface, to check that it was isothermal. The differences observed over the plate were close to the measurement accuracy of ±0.3 °C, even during strong transients. One thermocouple (no. 11 in Fig. 4, thin thermocouple 0.1 mm diameter, wires covered by glass fibbers) was glued on the surface of the hot plate to measure its temperature. The system was placed in a large regulated room. The air temperature Tair was considered as uniform and only disturbed near the small plate. 3.2. Probes for temperature measurement The probes were those used during experiments with a human body in a car. They were composed of a thermocouple, type-K (Chromel/Alumel) insulated with Teflon and fixed in a protective radiative screen made of aluminum. The screen was cylindrical (1.5 cm high, 1.5 cm in diameter). The ‘‘foot” was also made of aluminum and was used to rigidify the probe and fix it on to the surface (Fig. 3). Measurements were made with different types of probes which are presented in Fig. 4. Two thicknesses of the wire were used: thin (0.1 mm, number n) and thick (0.2 mm, number n0 ). The junction diameter was about 5–6 times the wire thickness.
Regulated Room
Regulated Plate 30 cm
In cases of natural convection, hc depends on the difference in temperature, hc = A (Ts Tair)a. A and a are constants that depend on the segment; a is close to 0.25. Lartigue obtained an experimental value of 4.7 W K1 m2 close to a humid flat dummy that was maintained at 34 °C for various air temperatures [10]. It should be noted that the convection coefficient also has an influence on evaporative heat losses. Indeed, under usual conditions in dwellings, natural convection prevails. In vehicles, the estimated mean velocity is not always sufficient because the variations from one place to another can generate differences in the local exchange coefficients.
Water circulation
32 cm
Thermocryostat
Fig. 2. Experimental system.
THERMOCOUPLE
Regulated plate
64
Insulator Wire Junction
ALUMINUM SCREEN
"FOOT"
1.8 cm 1.5 cm Fig. 3. Measuring probe.
65
F. Thellier et al. / Measurement 42 (2009) 62–70
Vertical Cylinder Probe n˚8
Vertical Cylinder
Plate
7, 7' 8 9, 9'
Horizontal Cylinder
11 10, 10'
Probe n˚9 Horizontal Cylinder
12
13
Plastic tube
Probe n˚10
No screen 1.8cm 100cm
1 cm
Fig. 4. Position of thermocouples (diameter n:/ = 0.1 mm; n0 :/ = 0.2 mm).
– Two thermocouples (nos. 7 and 70 ) were far from the plate (1 m, out of plate influence) to measure the reference air temperature named T1. – Five probes were fixed on the plate (nos. 8, 9, 90 , 10, 100 ), at the same distance (1.8 cm) from its centre to be sure that they were in the same thermal conditions. Some of the protective cylindrical screens were vertical (no. 8), others were horizontal (nos. 9 and 90 ), and two of the probes did not have a screen (nos. 10 and 100 ). – Two other probes (nos. 12 and 13) were fixed on a plastic tube and maintained at the same distance from the plate. Probe no. 13 has a horizontal screen and no. 12 is not protected.
3.3. Controlling sensor position We wanted to determine the position of the probes in the thermal environment created by the plate. In fact, natural convection creates a thermal boundary layer, in which temperature gradient is strong; its thickness is around 1 cm. The probes have to be outside this layer. A holographic picture has been made to visualize this [13]. This method consist in superposing two beams of coherent light (red laser), one going through the analyzed zone, the other being the reference one. It created interference fringes that are directly linked to the air indices, so to temperature. This measuring system has the advantages of not disturbing the phenomenon under observation. The optical index of air is related to its temperature. Therefore, it is quite easy to go from the interference fringes to the temperature (the iso-colour fringes show zones of equal optical index and thus the isotherms). Fig. 5 shows an example of an interferometer picture for steady state conditions at DT = 15 °C. The ambient temperature (T1) was uniform at about 1 cm
Fig. 5. Interferogram of the experimental dummy (the black and white fringes indicate isotherm lines).
from the plate. It is obvious that the probes no. 8, 9 and 10 were outside the natural convective boundary layer. They were in isothermal ambient air. 3.4. Method We imposed a temperature difference between the plate and the environment. Temperatures given by the different probes (Tn) were recorded every 2 s. The thermocouples 7 and 70 were quite far from the plate (1 m) and gave the same temperature. The plate is small compared to the room and radiative effects are negligible. We considered this temperature as the reference one, based on El Khatib work [14] which is comparable to our procedure. Then, the reference for the air temperature was taken as T70 = T1 and the temperature of the plate was Tplate = T11. We considered DT, the imposed temperature difference, and Er(n), the induced measurement error as DT ¼ T plate T 1 ¼ T 11 T 70
ð2Þ
ErðnÞ ¼ T n T 1 ¼ T n T 70
ð3Þ
Because of the inertia of the entire room, its temperature could not change rapidly. So to make variations of DT, the room temperature was maintained stable around 20 °C and only the plate temperature was modified. Many experimental conditions were used (positive and negative DT) but only a few of them are presented here. The conclusions drawn are based on the analyses of all the experiments. 4. Results Fig. 6 gives an example of the experimental results. The monitored conditions are shown in Fig. 6a. This test was
66
F. Thellier et al. / Measurement 42 (2009) 62–70
composed of six phases: during phases II and IV, the plate temperature rose and during the other phases it stayed stable at different DT (0 °C, 15 °C and 30 °C). During the last phase (VI), monitored conditions were stable but the screens were closed with adhesive. The air-conditioning system had to be stopped because it disturbed the general airflow in the room. Thus, the room temperature (T1), rose slowly by about 1 °C during the exposure (Fig. 6b). This point will be discussed further. The temperatures recorded by some of the probes are shown in Fig. 6b. When DT = 0 °C, all the thermocouples give nearly the same temperature as the reference one T1. Then when DT increases, two features become noticeable which are very dependent on DT and on the type of probe:
The recorded data were difficult to analyze because two phenomena were superimposed: the fluctuations were strong and masked the temperature differences. Therefore, it was decided to smooth the curves in a simple manner. Several tests were carried out to smooth the curves sufficiently without losing too much information. Each value was replaced by the mean of the 15 surrounding ones. The result of the smoothed curves is plotted in Fig. 6c. For each probe, the average temperature was calculated on the last 10 min of the phases where DT remained constant. Table 2 gives a summary of the errors, Er(n), for the various DT. 5. Discussion As mentioned above, errors increase with DT and are more significant when the probes are fixed on to the plate. Of course, the temperature measured is that of the junction and is influenced by the plate’s temperature, through different modes of heat transfer. If we look at the very simplified thermal heat balances of the thermocouple junction and of the protective screen in the steady state condition (Fig. 7), it is obvious that the screen, when present, strongly influences the junction temperature. Let us consider the heat transfers (C, K, R) at the screen. Aluminum has low emissivity (polished; e = 0.04) but high thermal conductivity (k = 200 W m1 K1). When the screen is fixed on the warm plate, conductive transfer (K) is induced through the base to the screen, and in all cases, the cylindrical part of the screen is also warmed up by radiation (R). Convective exchanges (C) depend on the air
– large difference Er(n) between measured and reference temperatures; – oscillations having different amplitudes and frequencies. The frequency of the oscillations was not related to the acquisition period; one acquisition point did not correspond to one fluctuation. For example, during a 12 s period, there were six acquisition points and the recorded temperature increased continuously by almost 2 °C. We can also confirm that it was not an electronic or electrical problem as the frequency of the oscillations was different. We performed a Fourier analysis, in which no characteristic frequency appeared. It seems that the thickest thermocouples showed the fewest oscillations.
ΔT≈15˚C
ΔT = 0˚C
I
II
III
ΔT≈30˚C
IV
closed screens
V
VI
T(˚C)
36 32
T8
T9 T12
28 24
T∞
20 36 T(˚C)
T9' T8
32 28
T12
T9
24
T∞
20 0
10
20
30
40
50
60
Fig. 6. Probes temperature variations with increasing plate temperature.
70
t (min)
80
67
F. Thellier et al. / Measurement 42 (2009) 62–70 Table 2 Recapitulation of the Er(n) = Tn T1 for each probe (°C) Phase
DT
Tplate
T1
Er(8)
Er(9)
Er(90 )
Er(10)
Er(100 )
Er(12)
Er(13)
III V VI
13.3 28.3 28.0
34.9 50.0 49.9
21.6 21.7 21.9
2.4 3.2 12.7
4.8 9.3 13.6
6.0 10.9 14.9
1.6 2.8 –
1.6 2.8 –
1.5 3.1 4.7
1.3 2.3 –
K
R
Tin
r
T∞
C
C
C, c : Convection K, k : Conduction R, r : Radiation
k
C R K
Tsup Support: Plate or tube
Fig. 7. Schematic representation of heat exchanges in the probe.
temperature, T1, on the outside of the screen. Nevertheless, inside the cylinders, they depend on Tin, which may be quite different from T1 in some configurations. Let us now consider heat transfers at the thermocouple junction (c, k, r). The junction temperature will be close to T1 if conduction (k) and radiation (r) are low and if convection (c) is high, with the air temperature close to T1. In an ideal system, the probe is sufficiently ventilated, and the temperature of the screen Tscr, and that of the air in the screen Tin should be equal to the reference temperature (Tin = Tscr = T1). In fact, in our case, the measured temperatures are not equal to the reference value because radiation (r) and conduction (k) are high when the screen is close or fixed to the plate. Moreover, there is no ventilation inside the screen and it seems that the air temperature Tin is different from the reference. By comparing the various probe configurations, it is possible to have an idea of each phenomenon and try to separate them. 5.1. Thickness of the wires We have to estimate the influence of the wire thickness on conduction in the wires (k). For the thermocouples without a screen, no. 10 (on plate) and no. 13 (on tube), we observe, for DT 28 °C, that T10 T13 = 0.3 °C, which is about the measuring accuracy. This difference is a little higher for the thicker wires, T100 T13 = 0.5 °C. In this case, conduction in the wires between the plate and the junction is weak compared to other exchanges. For the probes with the screen fixed on the plate, nos. 9 (thin) and 90 (thick), a difference of up to 2 °C appears between the two probes. In this case, the conduction in the wires themselves appears greater over a smaller length. As expected, the thickness of the wires appears as an important parameter. 5.2. Radiation influence For probe no. 13, we assume that the tube is in thermal equilibrium with the environment (Ttube T1) and, conse-
quently, conduction (k) is nearly zero. During phase VI (DT = 28 °C), the error for this probe (Er(13) = 2.3 °C) is due only to the radiation (r) between the junction and the plate. If we compare probes no. 13 and 12, both on the tube, the warmer is no. 12, for which the screen is heated by radiation and then radiates on to the junction. Radiative exchanges (r) seem to be larger with the screen in place even if its emissivity is low. The comparison between the thermocouples with horizontal screens, no. 12 (on tube) and no. 9 (on plate), reveals differences that are more significant. For no. 12, the screen fixed on the tube is heated only by radiation, unlike no. 9 that obviously has a very hot screen because of conduction. 5.3. Convection: air flow in the screens Obviously, other effects have to be taken into account, as no. 8 and no. 9 have very different behavior. The experiments show that the most penalizing configuration is the one with the horizontal screen (probe no. 9) in natural convective airflow. In this configuration the error can reach 10 °C for DT = 30 °C. Such DT values can exist in a car. 5.3.1. Orientation of the cylindrical screen The presence of a screen fixed on the plate increases the measuring errors. Whatever the orientation of the screen, we can consider that radiative and conductive contributions are almost the same, but convection seems to be very different. If we compare no. 8 (vertical screen) and no. 9 (horizontal screen), their temperature differences can only be explained by a different airflow inside the cylinders. It is an aerodynamic problem depending on the screen orientation. Fig. 8 gives a schematic representation of the airflow near the probes. For probe no. 8 (vertical screen) the ‘‘foot” of the screen is in the direction of the vertical ascending airflow of the plate’s convective boundary layer. The air temperature around the junction is close to the reference one (Tin T1) and only radiation and conduction lead to an error, of up to Er(8) = 3 °C (Fig. 8a). In the case of probe no. 9, the horizontal cylindrical screen and the ‘‘foot” are obstacles to the convective airflow along the plate (Fig. 8b). It is difficult to know what occurs. The airflow becomes three-dimensional. Inside the cylinder, the air seems to stagnate then to escape by ‘‘puffs”. When the general airflow is stable (phase V), the air seems to remain a ‘‘long” time in the cylinder and warms up. The air temperature in the screen Tin tends towards Tscr. In this case the error can reach Er(9) = 10 °C. When the general flow is turbulent (phase IV), the puffs are more frequent, the oscillations and the errors are of the same order as for probe no. 8.
68
F. Thellier et al. / Measurement 42 (2009) 62–70
Plate boundary layer
Cylinder
Cylinder
Plate
Plate
"foot"
"foot"
Tin Tscr Air flow
Fig. 8. Airflow around probes: (a) vertical cylinder and (b) horizontal cylinder.
5.3.2. Closing the screens The oscillations of the measured temperatures seem to be due to perturbation of the airflow around the junctions. To check this point, we sealed the extremities of the screens with an adhesive in order to be sure of the immobility of the air near the junctions (Fig. 6, phase VI). When the screens had been sealed, it took a few minutes to reach a new equilibrium, and then the fluctuations disappeared completely. The thermocouples on the plate (nos. 8 and 9) gave nearly the same temperatures (measuring accuracy), which were much higher than for open screens. The junctions were in a small, closed enclosure where the air was confined and probably at the screen temperature, which was quite warm because of conduction. It seems that for DT = 28 °C (Tplate = 48 °C) the screens on the plate were at least at 35 °C. This effect is highlighted if we compare this result to the one for probe no. 12, for which the screen is heated only by radiation and the measured temperature is 26.6 °C. It can thus be deduced that the screens in contact with the plate are heated mainly by conduction but also partly by radiation. In a very simplified way, if the overestimation of the measured temperature by probe no. 12 is due to radiative warming, we can then conclude that Er(9) is due to conduction in the screen for around 70% and to radiation for 30%. 5.3.3. Forced convection In order to determine the effect of the general airflow, tests were carried out with a ventilator placed perpendicular to the plate at a distance of 1 m. Many tests were done with air velocities near the plate between 0.3 and 1.2 m s1, which all corresponded to forced convection. Fig. 9 presents the results: phase I – no ventilator, phase II – ventilator blowing. Plate temperature was not changed (DT = 10 °C), but here the variation of T1 is because of the air-conditioning regulation system which is on. For probe no. 8, the air blown horizontally cooled the vertical screen, but no modification was observed. The effect was more significant for probe no. 9 whose screen was horizontal and parallel to the forced airflow. When the ventilator was blowing, the error diminished and thus the measured temperature approached T1. For each thermocouple, a slight attenuation of the amplitude of the fluctuations was also noticed but the frequencies seemed higher. When the ventilator blowing power was changed, no significant change was observed.
Fig. 9. Forced ventilation.
In forced convection conditions, the radiative transfers were still present but they were no longer predominant, and the screen temperature was close to T1 as convection was enhanced. The influence of the protective radiative screen on the junction was less significant than in natural convection. 5.4. Influence of complex experimental conditions It appears that the air conditioner of the experimental room played a very important role in the airflow around the dummy (Fig. 10). When it is on, the air temperature in the room varied by ±0.5 °C around the set point temperature because of the regulation system, the period being about 12 min. Because of lack of space in the experimental room, the dummy was quite close to the air-conditioning system, which was fixed on the ceiling and blew cold air horizontally. The fan ran continuously. The air velocity was 2.8 m s1 at the outlet, 0.5 m s1 at 1 m, and lower than 0.1 m s1 around the dummy (0.1 m s1: lower measuring limit). Fig. 9 presents the result of the smoothed temperature curves, showing that there was an amplification of the room temperature variations. When the refrigerating unit started, the outlet temperature quickly fell from 20 °C to 15 °C. This cold air conferred a significant downward component that modified the natural convective airflow near the dummy. At the same time, the fluctuation amplitude increased but the error decreased; it seems that the airflow was mixed convection. When it stopped, fluctuations were attenuated and
F. Thellier et al. / Measurement 42 (2009) 62–70
28
T (˚C)
27
7. Conclusion
T9 - Horiz. Cylinder
26 25 24 23 22
T8 - Vertic. Cylinder
21
T∞
20 10
15
25
20
30
35
40
t (min)
Fig. 10. Influence of the air-conditioning system.
the error increased, and the general airflow became more stable, returning to pure natural convection. This perturbation lasted a few minutes after the refrigeration stopped. These observations confirm the strong influence of the aerodynamic flow on the convective coefficient [15]. They also underline the difficulties in drawing a conclusion on the corrections that need to be made to the measured values of the temperature. 6. Correction of the temperature measurements Let us return to the initial problem, i.e., the measurement of local air temperatures needed to calculate convective heat exchanges for the human model. The most significant location is at head level as this temperature is also used to calculate thermal judgments [8]. During experiments in a real car in a wind tunnel, the probe at head level was directly fixed on to the skin surface of the forehead, which was quite warm. An empirical correction law has been obtained from the experimental results. The correction is based on probe no. 8, which appeared to be the best one (thin wire, vertical screen). Plotting Er(8) versus plate – measured temperature difference leads to a correlation for temperature correction. To obtain the ‘‘real temperature” Treal, a correction C can be made to the measured value Tmes (Fig. 11): T real ¼ T mes C
3 2
69
ð4Þ
Thermal comfort will be difficult to estimate if the physical parameters are known only with poor accuracy. The study presented here focuses on analyzing the method used for measuring air temperature near the human being. A screen, initially recommended as protection against radiation, becomes a source of measurement error here. The protected thermocouple in contact with the hot plate, or skin, is heated by conduction but also by radiation. The experimental set-up considered here allows us to point out two phenomena that are superimposed: fluctuations, and differences between the reference and measured temperatures. We conclude that, under all the thermal conditions studied and for all types of probes, there are strongly coupled thermal and aerodynamic problems. It has to be underlined that, in this case, the error is smaller for a thermocouple not protected by a screen and not in contact with the surface, the lowest error being made by probe type no. 13. Many parameters influence the measurements, related to the type of probe: diameter of the wires, temperature of the support, presence and properties of the radiative screen, and especially its emissivity and its orientation relative to the principal airflow. The experiments carried out and analyzed allowed us to quantify the error made on the air temperatures and thus to evaluate a correction law to determine the ‘‘real temperature”. This law could be improved by the analysis of other experiments. Errors cannot be avoided, but we hope to quantify each heat transfer, to obtain an analytical correction law. The major problem is the airflow. There can be different cases: – forced convection – high air velocities, then oscillations and errors are low; – natural convection – airflow is very stable, oscillations are present but small, and errors depend on the orientation of the screen, the best position being vertical to favor natural convection in the screen; – mixed convection – airflow is perturbed, then oscillations are very large and error impossible to determine clearly, especially when the screen is horizontal. Finally, in our configurations, it seems that protecting the thermocouples from radiation by a screen as recommended by ISO [11] is a source of error. This error is significant when the screen is in contact with a support that has a temperature very different from T1. A forced flow will minimize the error but, although this technique is recommended, it cannot be used close the human body, as it changes the heat exchanges on the surface.
Er(8)=Tmes-T∞ (˚C)
References
1 2
C = 0.295 (Tplate-Tmes)- 0.007 (Tplate-Tmes) - 0.021
0
Tplate- Tmes (˚C)
-1 0
2
4
6
8
10
12
14
16
18
20 22
Fig. 11. Experimental correction law (Tsurf: temperature of the support).
[1] K.C. Parsons, Human Thermal Environments. The Effects of Hot, Moderate and Cold Environment on Human Health, Comfort and Performance, Taylor & Francis, London, 1993. [2] D. Quintela, A. Gaspar, A. Raimundo, Development of local heating systems for thermal comfort and energy savings in buildings, in: International Conference UIE 2000, Electricity for a Sustainable Urban Development, Lisbon, Portugal, November 2000.
70
F. Thellier et al. / Measurement 42 (2009) 62–70
[3] C.P. Borges, D. Quintela, G.N. Brites, A. Gaspar, J.J. Costa, Analysis of thermal plumes generated by a seated person, a thermal manikin and a dummy, in: ROOMVENT 2002, Copenhagen, Denmark, September 2002,
and Environmental Engineering (Transactions of Architectural Institute of Japan) 561 (2002) 31–39. [10] B. Lartigue, F. Monchoux, J.L. Breton, Convection sous vestimentaire. Analyse dimensionnelle et mesure des flux échangés, Revue Générale de Thermique (International Journal of Thermal Science) 36 (1997) 469–479. [11] ISO 7726, Thermal environments, Instruments and methods for measuring physical quantities, International Standard Organization, 1985 (NF-EN 27726; ISSN 0335-3931). [12] S. Piniec, Thermique de l’homme dans une voiture, Modélisation et expérimentation: analyse de la métrologie, Ph.D. Thesis, Université Paul Sabatier, Toulouse, France, no. 3453, 1999, 176 p. (in French). [13] E. Piquer, B. Lartigue, M.C. Charrier-Mojtabi, Holographic interferometry: principles and applications in Soret-driven convection, in: Eurotherm 71, Visualization Imaging and Data Analysis in Convective Heat Transfer, Reims, France, 28–30 October 2002, pp. 59–73. [14] H. El Kathib, R. Javelas, Modèle correctif de la mesure de la température d’un gaz par couple thermoélectrique: application à une zone à fort gradient thermique, Revue Générale de Thermique (International Journal of Sciences) 35 (1996) 571–579. [15] R.J. Moffat, Experimental Methods in Heat Transfer: Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Elsevier Sciences Publishing Co., Amsterdam, 1988. pp. 13–31.