Author: Hyunjun OhPublisher:
ISBN:
Category :
Languages : en
Pages : 261
Book Description
Soil critical temperature is the soil temperature where moisture outflow under a thermal gradient is balanced with moisture inflow caused by an increase in hydraulic gradient due to change in matric potential. Uncompensated moisture outflow occurs when soil temperature exceeds the critical temperature. The critical temperature and moisture migration above the critical temperature play important roles for backfills used in applications in thermal geotechnics due to significant impacts on soil thermal properties. The critical temperature is also an important design parameter for cable ampacity with respect to IEC Standard 60287. In this dissertation, critical temperatures of seven sandy soils were comprehensively evaluated using two laboratory approaches (vertical and radial) and computational analyses. Then, cable ampacities and coupled heat and moisture transfer at a field site were simulated using CYMCAP and SVOffice[superscript TM], respectively. In the laboratory testing, seven sandy soils were compacted with temperature and moisture probes for target void ratio and gravimetric moisture content ([i.e.], 1%, 3%, and 5%). Heat (90 [degrees]C) was generated from a heating element and, temperature and moisture data were recorded for the 48-h test duration. While the vertical approach measured soil temperature and moisture content from top to bottom of the vertical column, the radial method obtained measurements from the center to the outer perimeter of the radial cell. To observe whether gravity-induced heat or moisture flow occurred, in the radial testing, temperature and moisture data were measured both above and below the heater. Soil temperatures at the first measurements ranged approximately from 75 [degrees]C to 42 [degrees]C, depending on the thermal and physical properties (e.g., moisture content, void ratio), and the temperatures decreased with an increase in the distance from heating element. Temperature discontinuities were observed in the radial testing at 1% and 3% initial moisture contents while gradual temperature gradients were shown in radial testing at 5% initial moisture content and all vertical testing due to potential limitations in laboratory apparatuses ([e.g.], heat losses). In radial testing, gravity-induced heat transfer was minimal while moisture contents for the lower section (especially at 3% initial moisture content) were slightly higher than those for upper section, due to gravity-induced moisture flow, except at the last measurements where evaporation may significantly affect the moisture content near the upper edge. In contrast to the gradual temperature gradients, moisture content profiles showed significant variation near the boundary between the dry and wet zones. The critical temperature was thus determined based on both temperature and moisture profiles. As the initial moisture content increased from 1% from 5%, the critical temperature averagely increased from 31.14 [degrees]C, 46.35 [degrees]C, and 53 [degrees]C, respectively, with a decrease in dry zone size. When void ratio increased, critical temperature also decreased with an increase in size of the dry zone. In comparisons of the data obtained from vertical and radial testing, final void ratios, time to steady-state temperatures, and critical temperatures obtained from radial testing were slightly higher on average than those obtained from vertical testing. To validate testing results and further examine coupled heat and moisture transfer under various conditions, computational analyses were performed in MATLAB using non-equilibrium model equations. The modeling results indicated that temperature at 0 cm ([i.e.], temperature on the heating element surface) was much lower than the heater temperature of 90 [degrees]C potentially due to heat losses in the heating element itself, as well as the effect of heat dissipation through moist soil. In contrast to the gradual temperature gradients in laboratory testing, temperature profiles obtained from modeling clearly included a temperature discontinuity near the boundary between dry and wet zones. While only slight coupled heat and moisture transfer occurred in the radial testing for 5% initial moisture content cases (no dry zone formed), dry zone with temperature discontinuity was observed in the modeling result as soil size was increased from 10 cm to 30 cm. An increase in soil bulk density (from 1.4 g/cm^3 to 1.8 g/cm^3) caused decreases in dry zone sizes particularly in 5% initial moisture content case (from about 1.2 cm to about 0.5 cm) involving an overall decrease in temperature gradient. When thermal conductivity of soil mineral decreased from 14 W/m*K to 4 W/m*K, temperature gradient increased, particularly near the heating element, resulting in an increase the dry zone size from about 2.2 cm to about 2.8 cm. Ampacities and temperatures at perimeter boundaries of three cables directly buried at 1 m depth with 0.5 m spacings were simulated using CYMCAP. Derating factors broadly ranged from 0.58 to 1.00 based on the laboratory testing results ([e.g.], a wide range of the critical temperatures based on the initial moisture content). Average temperatures of the middle cable and the side cables were 68.38 [degrees]C and 57.31 [degrees]C, respectively, at 1% initial moisture content, and the temperatures decreased from 68.38 [degrees]C to 54.28 [degrees]C and 57.31 [degrees]C to 36.44 [degrees]C, respectively, with an increase in the initial moisture content. In addition to the CYMCAP simulations, field-scale numerical modeling was conducted using SVFlux and SVHeat in SVOffice[superscript TM] software package to investigate how coupled heat and moisture transfer occurs when the three cables used in CYMCAP simulations generated heat. The modeling results demonstrated that dry zone was formed around the three cables, and thus these results implied ampacities of the cables may significantly fluctuate due to the dry zone formation ([i.e.], a significant change in soil thermal resistivity). That is, in this case, engineered backfill that has a high critical temperature is recommended for maintaining moist condition under the thermal gradient.