ELSEVIER Temperature and Pressure Field Visualizations in a Porous Medium Dried in Superheated Steam F. Topin 0. Rahli H. Tadrist Laboratoire de I'lnstitut Universitaire des Sysdmes Themiques Zndustriels, Technopble de ChBteau-Gombert, 5, rue Enrico Femzi, 13453 Marseille Cedex 13, France .This work focuses on heat and mass transfers with phase change in porous media. The experimental analysis was carried out in an aerody- namic return-flow wind tunnel, with very small cylinders of cellular con- crete. For the local analysis, the samples were fitted with thermocouples and pressure sensors. A method of temperature and pressure field visual- ization is developed to highlight the dynamics of transport phenomena. We show two specific phenomena: (1) liquid outflow generated by the overpres- sure and (2) vaporization of the water inside a two-phase zone that progressively pervades the sample. O Elsevier Science Inc., 1997 Keywords: porous media, superheated steam, local analysis, drying curves, temperature and pressure fields INTRODUCTION In the past two decades, a great deal of research has been carried out on porous media. This field has gone through a rapid acceleration owing to widespread concern about issues such as energy conservation and environmental pollution. The areas of application include drying, insula- tion for buildings and equipment, energy storage and recovery, geothermal storage, nuclear waste disposal, chemical reactor design, and the storage of heat-gener- ating substances. Despite their practical and theoretical interests, few experimental studies have been carried out on the transport phenomena with phase-change in porous media. The phase change and transfer phenomena occur dur- ing drying when the temperature reaches at least the saturation temperature. Yet few studies have been canied out on high-temperature drying and superheated steam drying. Reviews dealing with high-temperature drying phe- nomena are given by several authors [I-31. Most of these studies were conducted at the levels of the drier or of the individual board for wood drying application and numeri- cal modeling [4-81. These transfer phenomena are also analysed by several authors for boiling in porous media studies (reviews are given in 3 and 9 for example). These works reveal the complexity of the phenomena and the fundamental role of a two-phase (boiling) zone developing inside the porous medium when the tempera- ture reaches the saturation temperature. It appears that the precise behavior of the material depends on the relative intensity of heat and mass transfer, on their orientations, and on the structure of the porous material. Consequently, there still remains a great deal of work, both experimental and theoretical, to determine the cou- pled mechanism of phase-change, fluid flow, and heat transfers in a wet porous medium. The purpose of the present study is to gain a better understanding of the coupled heat and mass transport phenomena within small wet porous cylinders dried in superheated steam a local approach using temperature and pressure measurements at several locations is devel- oped. The specificity of this experimental work resides in the delicacy of the data acquisition system design. We made optimal use of the metrological techniques devel- oped in this domain. A greater number of sensors (relative to the volume of the studied object) and more frequent acquisition procedures were used than has been generally encountered in previous studies. EXPERIMENTAL SETUP Drying Setup The experiments were carried out in superheated steam in different aerothermal conditions, temperatures (120, 140, 160, 180 "C), and velocities (2, 5, and 8 m/s). The experi- mental difficulties are mainly linked to the positioning of the sensor (which should be accurate and reproducible to allow comparison of measurements) or to the quality of the steam ambience (stability of both temperature and velocity, presence of air) or the both. Consequently, a data acquisition and processing system was developed and adapted to the Superbes drying setup developed by the Electricite' de France laboratory [lo-121 (Fig. la). This aerodynamic return-flow wind tunnel operated in super- heated steam (or in air) at atmospheric pressure, from ambient temperature to 200°C. This installation is entirely Address correspondence to Dr. FrCdCric Topin, Laboratoire de I'Institut Universitaire des Systkmes Thermiques Industriels (I.U.S.T.I.), UMR CNRS 6595 UniversitC de Provence, Technopale de Chlteau-Gombert, 5, rue Enrico Fermi, 13453 Marseille Cedex 13, France. Experimental Thermal and Fluid Science 1997; 15:359-374 O Elsevier Science Inc., 1997 655 Avenue of the Americas, New York, NY 10010