An improved approach for measurement of coupled heat and water transfer in soil cells

Authors: HEITMAN, JOSHUA - IOWA STATE UNIVERSITY; HORTON, ROBERT - IOWA STATE UNIVERSITY; REN, TUSHENG - CHINA AGRICULTURAL UNIV; Ochsner, Tyson

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/19/2007
Publication Date: 5/16/2007
Citation: Heitman, J.L., Horton, R., Ren, T., Ochsner, T.E. 2007. An improved approach for measurement of coupled heat and water transfer in soil cells. Soil Science Society of America Journal. 71:872-880.

Interpretive Summary: In soil, heat and water flow are closely linked. The linkages are not well understood because inadequate experimental methods have hampered research. The objectives of this research were to develop and demonstrate improved methods for measuring coupled heat and water flow in soil. A new design for closed soil cells with embedded sensors was developed and successfully tested. The cells achieved the necessary high degree of thermal insulation. The sensors accurately measured the primary experimental variables: soil water content, temperature, and thermal properties. The nondestructive measurements enabled repeated experiments on the same soil, a capability which has not previously been available in this type of research. Scientists and engineers who study heat and water movement in soil and other geological media will benefit from these improved methods.

Technical Abstract: Laboratory experiments on coupled heat and water transfer in soil have been limited in their measurement approaches. Inadequate temperature control creates undesired two-dimensional distributions of both temperature and moisture. Destructive sampling to determine soil water content prevents measurement of transient water content distributions and provides no direct information on soil thermal properties. The objectives of this work are 1) To develop an instrumented closed soil cell, providing one-dimensional conditions and allowing in situ measurement of temperature, water content, and thermal conductivity under transient boundary conditions, and 2) To demonstrate this cell in a series of experiments using four soil type-initial water content combinations and ten transient boundary conditions. Experiments were conducted using soil-insulated cells instrumented with thermo-time domain reflectometry (TTDR) sensors. Temperature distributions measured in the experiments show non-linearity, which is consistent with non-uniform thermal properties provided by thermal moisture distribution but differs from previous studies lacking one-dimensional temperature control. TTDR measurements of water content based on dielectric permittivity, volumetric heat capacity, and change in volumetric heat capacity agree well with post-experiment sampling, providing r2 values of 0.87, 0.93, and 0.95, respectively. Measurements of water content and thermal conductivity are also consistent with the shapes of the observed temperature distributions. Techniques implemented in these experiments allowed observation of transient temperature, water content, and thermal conductivity distributions on the same soil sample for ten sequentially imposed boundary conditions, including periods of rapid redistribution. This work demonstrates that through improved measurement techniques the study of heat and water transfer processes can be expanded in ways previously unavailable.