Temperature dependence of thermal conductivity of soil Influence de la température sur la conductivité thermique du sol

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1 Scientific registration n 435 Symposium n 1 Presentation : poster Temperature dependence of thermal conductivity of soil Influence de la température sur la conductivité thermique du sol KASUBUCHI Tatsuaki, HIRAIWA Yoshiki Faculty of Agriculture, Yamagata University, Tsuruoka 997, Japan INTRODUCTION Soil thermal conductivity is depended on temperature. So it is important to understand the mechanism to estimate the thermal regime of soil in different temperatures. But the mechanism is not yet known sufficiently and we could not estimate the temperature dependence of thermal conductivity. In order to make clear the mechanism, it is indispensable to obtain the accurate data on temperature dependence of soil. In this report, we examined the thermal conductivity of two kinds of soils which have different physical properties on different water content and temperaturefrom 5 to 75 degrees centigrade). Thermal conductivity was accurately measured by the improved twin transient heat probe method. Using these data, we investigated the mechanism of the temperature dependence of soil. MATERIALS AND METHODS Soils. Volcanic ash soils and red yellow soil (diluvial soil) were used. Soils were air dried and sieved through 2mm screen. Specific gravity of each soil were 2.81 and 2.50, respectively. Air dried soils were packed into the plastic samplers ( 5.0cm in diameter and 5.1cm in height ) with the same volumetric solid content and the required amount of water was taken into it. Volumetric water contents of each soil were 6, 15, 25, 30, 35, 45, 55 and 7, 15, 25, 30, 35, 45, 55, 65%, respectively. Soil samplers were covered with thin plastic film and heated in the microwave oven for about one minutes and allowed to cool for about one day in the laboratory. The amount of scatter of the soil water distribution in the sampler was checked within about one percent. The samplers were covered with the hard plastic plate and adhered with silicon bond to protect drying. Measurement of thermal conductivity. Thermal conductivity of the sample was measured by the improved twin heat probe method (Kasubuchi, 1977; Kasubuchi and Hasegawa, 1994). The heat probes were made of stainless steel pipe, 0cm outer diameter, 5 cm inner diameter and 5.0 cm long, and the heating wire (enameled constantan wire 1cm in diameter) and a 1

2 thermocouple were placed into the pipe, and the remaining space was filled with the epoxy-resin. Heat probe was inserted into the sampler through the small hole in the center of the plastic plate. Samples were set in the box with constant temperature which was controlled by the heater and refrigerator with the accuracy of degree centigrade. Temperature of the sample was set 5, 15, 25, 35, 45, 55, 65 and 75 degree centigrade. Soil water potential. Soil water potential were measured by the pressure plate method. RESULTS AND DISCUSSIONS The relation between water content and the thermal conductivity for each temperature was shown in figure 1 and 2. These figures show that the thermal conductivity is affected not only by temperature but also by the water content. In order to make clear the temperature dependence, the difference between the thermal conductivity of each temperature and that of 5 degree centigrade for each soil was shown in figure 3 and 4. (W/(m K)) Ther 1.2 mal cond 1.0 uctiv ity Volumetric water content (v/v) 5 C 15 C 25 C 35 C 45 C 55 C 65 C 75 C Fig. 1. Thermal conductivity of Red Yellow Soil At the air dried condition the temperature dependence is very small rather than other wetness. The degree of temperature dependence increases rapidly as water content increases from dry condition to the certain wetness then decreases linearly until saturated condition. This means that the temperature dependence is caused by the latent heat transfer. But the maximum difference at 75 degree centigrade in the red yellow soil is 1.5 times larger than that in the volcanic ash soil. 2

3 Volumetric water cont Fig.3 Temperature Dependence of 3

4 Volumetric water c Fig.4 Temperature Dependence o The latent heat transfer is caused by the H*D*ddwhere H is the heat of vaporization of waterj kg -1 ), D is the diffusion coefficient of water vapor ( m 2 s -1 ), is the vapor density (kg m -3 ) and T is temperature. So the dimension of H*D*ddis the same to thermal conductivity (W m -1 K -1 ). The vapor density at 5 degree centigrade is very low (6.6*10-3 kg m -3 ) and the contribution to the thermal conductivity by the latent heat transfer is thought to be negligible. The relation between H*D*ddand the difference of thermal conductivity of each temperature from that of 5 degree centigrade is effective to analyze the latent heat transfer of the soils. These relation were shown in Fig. 5 and 6. 4

5 1 g H*D*d ρ/dt (W/(m Fig.5. H*D*d ρ/dt and the Difference Conductivity ( Red Yellow 5

6 H*D*d ρ/dt (W/(m Fig.6. H*D*d ρ/dt and Difference of Therma (Volucanic ash soil) These figures show that the relation between H*D*dd and the difference of thermal conductivity for each soil wetness are almost linear. These means that the temperature dependence of thermal conductivity is mainly depend on the latent heat transfer. The relation between the air ratio and the gradients of those lines which are the phenomenological enhancement factor () ( Cary, 1979) is shown in Fig.7. Figure 7 shows that the lines of the gradients are divided into two parts. That is, as the air ratio increases the gradient increases and reach the maximum then the gradient decreases as the air ratio increases. The air ratios at which the gradient reaches maximum are about 23% for the red yellow soil and 30 % for the volcanic ash soil and the corresponding water ratios are 35% for both. The relations between the maximum points of gradients and the soil water potentials are not clear. The gradient of the lines in Fig.7 are linear in the range of the air ratio 0-23 and 0-30, respectively. This means that the temperature dependence of these parts are linearly affected by the air ratio. But the gradients of the lines are not the same and the gradient of the line of the red yellow soil is about two times larger than that of the volcanic ash soil. These show that the 6

7 temperature dependence relies on not only temperature but also other physical properties of the soils. Kasubuchi (1984) estimated the thermal conductivity of the soil solids of these soils and that of the red yellow soil is several times higher than that of the volcanic ash soil. These properties might be related to the temperature dependence of the thermal conductivity. The linearity of the lines are not clear in the region that the gradient decreases as the air ratio increases. This relates to that the relative humidity changes exponentially from 9 (1MPa of soil water potential) to zero ( oven dry condition). CONCLUSION The temperature dependence of the thermal conductivity was investigated and the results are as follows. (1) Temperature dependence is affected by the water content. The degree of temperature dependence increases rapidly as water content increases from dry condition to the certain wetness then decreases linearly until saturated condition. (2) Temperature dependence of soil is based on the latent heat transfer which depends on the temperature, soil water potential, air ratio of the soil and other physical properties. (3) To estimate the temperature dependence of the thermal conductivity of the soil, it is needed to take into consideration not only of the value of H*D*dd but also of the other physical informations such as the thermal conductivity of the soil solid and the thermal structure of the soil. Acknowledgment: We are much obliged to Mr. Hiroyuki Yokoyama and Miss Takako Watanebe for their help of our work. LITERATURE Cary, J.W Soil heat transducers and water vapor flow. Soil. Sci.Soc. Am. J Tatsuaki Kasubuchi Twin transient-state cylindrical-probe method for the determination of the thermal conductivity of soil. Soil Sci Tatsuaki Kasubuchi Heat Conduction Model of Saturated Soil and Estimation of Thermal conductivity of Soil Solid Phase. Soil Sci Tatsuaki Kasubuchi and Shuich Hasegawa Measurement of Spatial Average of the Soil Water Content by the Long Heat Probe Method. Soil Sci. Plant Nutr Key words : temperature dependence, thermal conductivity Mots-clés : dépendance de la température, conductivité thermique 7