Abstract
On the assumption that the heat properties of sandy soil are known, the validity of estimation for the apparent thermal conductivity of gravel was examined by means of an unstationary comparison method. The feature of this method can be stated to be rather simple outdoor practice for the experiment, but in the analytical case, the method must have the Newton's cooling law for the surface boundary condition. Then, the heat transfer coefficient must be known to evaluate the thermal conductivity of the sample exactly. However, the heat transfer coefficient has some troublesome properties depending on the surface characteristics of the object used in the experiment and atmospheric stabilization. But this heat transfer coefficient has a special characteristic, in case of introducing the fluctuation of its value, having an influence on only thin surface layer's soil temperature, meanwhile the thermal conductivity has that, in case of introducing the fluctuation of its value, having an influence on the soil temperature to greater depth. The feature of this method, in other words, arrives at a conclusion of singularity between the heat transfer coefficient and thermal conductivity. In a soil temperature analysis, both values can be obtained by trial and error. Lastly, the availability of the unstationary comparison method was verified by means of the claim of the heat transfer theory.
In this unstationary comparison method, Cary's β that is the phenomenological enhancement factor of the water vapor transfer in the pores, is adopted as an average value of 2.0 for wet sandy soil.
The above two samples were put in the two cylindrical columns A (dia. 50 cm, length, 60 cm) and B (dia. 30 cm, length, 60 cm) bound with insulating material. Columns A was then covered by uplayer gravel (grain size: mm) and down-layer wet sand, and column B was completed by three layers such as top layer dry sand, middle layer gravel and bottom layer wet sand. At the center portion of the cylindrical columns, 7-8 thermocouples (Cu-Co) were inserted for soil temperature measurements.
All the columns were placed on a concrete surface of a building top. The sun was used as the heat source for the columns. Then, the global radiation, albedo of the soil, air temperature, humidity and wind velocity were measured.
In addition, another two experiments were carried out. One was carried out to check on the insulating effect of the columns A and B. The second one was preformed in the glasshouse for the availability of the unstationary comparison method may be said to be good for directly evaluating the thermal conductivity of gravel.
The ratio of the convection value to the total heat transfer in the gravel layer may be evaluated to be about 40% by means of obtaining the difference of the effective thermal conductivity, called the stagnant conductivity estimated by S. Yagi & D. Kunii's theory from the above apparent thermal conduct ivity.
