Teaching thermal wave physics with soils
E. Marín*, L.E. Jean-Baptiste** and M. Hernández
Facultad de Física, Universidad de La Habana, San Lázaro y L, Vedado 10400, Ciudad de La Habana, Cuba.
* Present address:
CICATA-IPN Legaria 694, ]]>
Colonia Irrigación, 11500 México D.F.
emarin@ff.oc.uh.cu.
** Present address:
Ministère de l'Education Nationale d'Haïti (MENJ),
Direction de l'Enseignement Secondaire (DES).
Recibido el 21 de junio de 2004;
Aceptado el 8 de junio de 2005.
Abstract
]]> In this paper, we discuss the features of a possible student experiment related to the conduction of heat in soils excited by a natural periodically time dependent source, namely the daily periodical oscillations in the earth's temperature, which can be denoted as thermal waves. A measuring device was designed and constructed for automatic measurements of the daily time air temperature variations as well as of the daily time temperature variations, at different depths beneath the soil's surface. Measurements were performed using LM-335 solid state temperature sensors incorporated into a computer-controlled probe. The data acquisition software was developed using a programming environment LabVIEW from National Instruments. In order to obtain characteristic parameters governing the physical phenomena involved, the results of our measurements were fitted to a thermal wave like solution of the heat diffusion equation in the presence of periodical heat sources. The phase shift as well as the attenuation of the temperature waves with depth was demonstrated, as well as their dependence on soil thermal properties, in particular its thermal diffusivity.Keywords: Thermal waves; soils; thermal properties; thermal diffusivity.
Resumen
En este trabajo se discuten las peculiaridades de un posible experimento docente relacionado con la conducción del calor en suelos excitados por una fuente térmica variable periódicamente en el tiempo, como son las oscilaciones cíclicas diarias de la temperatura de la corteza terrestre, que pueden considerarse como ondas térmicas. Se diseñó y construyó un dispositivo para la medición automática de las variaciones periódicas en la temperatura del aire así como a diferentes profundidades de la superficie del suelo. Las mediciones fueron realizadas utilizando sensores de estado sólido LM 335 incorporados en una sonda controlada por ordenador. El programa de adquisición de datos fue desarrollado en el ambiente LabVIEW de National Instruments. Para obtener los parámetros característicos que gobiernan los fenómenos físicos involucrados, los resultados de las mediciones fueron ajustados a la solución tipo onda térmica de la ecuación de difusión del calor en presencia de fuentes periódicas. Fueron demostrados el corrimiento de fase y la atenuación de las ondas de temperatura con la profundidad, así como su dependencia de las propiedades térmicas del suelo, en particular su difusividad térmica.
Descriptores: Ondas térmicas; suelos; propiedades térmicas; difusividad térmica.
PACS: 44.10.+I;44.50.+f
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]]>Acknowledgements
The authors gratefully acknowledge the technical assistance given by I. Estevez, E. Vega, and D. Seuret during the development of this work. L.E. Jean Baptiste also expresses his gratitude to the postgraduate program at Havana University. This work was partially funded by the "Alma Mater" project 01/2001.
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