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Temperature Fluctuations at a Wetting Front: I. Characteristic Temperature-Time Curves¹

ABSTRACT

Temperatures at localized sites in various porous media were monitored during infiltration of water. The temperature at any given point was observed to rise gradually and then to drop sharply as the water approached and passed the measurement site. For water and medium at the same temperature, the observed temperature fluctuations are in accord with the following hypothesis.

When a volatile liquid such as water invades a dry porous substance, evaporation at the liquid front continually supplies a gas phase which by displacement and diffusion moves ahead through the pore space, colliding with and being sorbed by the medium. The medium ahead of the liquid front is thus continually warmed by the heat of sorption. Part of this heat is subsequently transferred to the liquid phase as it envelops the medium and is used again in the evaporation-sorption cycle. Continual evaporation at the liquid front tends to cool it and is dominant in establishing a temperature there lower than that of the medium immediately ahead. The intensity of the heating and cooling processes depends upon the nature of the medium and the invading fluid.

Correlation of the position of a tracer ion with the temperature fluctuations and results obtained when a gap in the medium permitted only vapor movement also support this hypothesis.

¹ Technical paper 633, Arizona Agr. Exp. Sta., Tucson. Contribution from the Department of Agricultural Chemistry and Soils. This work was supported by the Signal Corps, Department of Meteorology, USAEPG, Ft. Huachuca, Ariz.

² Associate Soil Physicist and Research Associate, respectively.

Received for publication March 6, 1961. Accepted for publication June 7, 1961.