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International Heat Transfer Conference 15

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

Mri Measurement and Numerical Modeling of Moisture Transport in Microwave Vacuum Drying of Porous Media

Takaharu Tsuruta
Kyushu Institute of Technology, Sensui 1-1, Tobata, Kitakyushu, Fukuoka 804-8550, Japan

Hirofumi Tanigawa
Kyushu Institute of Technology, Japan

DOI: 10.1615/IHTC15.mtr.008746
pages 5113-5123


KEY WORDS: Mass transfer and drying, Porous media, Microwave vacuum drying, Moisture distribution, MRI technique, Modified Darcy model, Two-phase flow, Food drying

Abstract

Moisture transport inside porous media is an important phenomenon in many technological fields such as materials and foods processing. We are developing a novel drying method of foods with use of microwave to supply only a latent heat for evaporation under low-pressure conditions. Temperature is kept at around a room temperature by controlling the microwave irradiation, which enables a high quality and a rapid drying compared to the traditional drying methods. In this study we have done experimental measurements and numerical simulations focusing on the moisture distribution inside the materials. In the experiment, the microwave is irradiated in pulse or intermittently so as to keep the temperature of material at around room temperature in a vacuum chamber system. MRI measuring technique is used to observe distributions of moisture contents in the materials. The images of the moisture distribution indicate clearly that the central part of materials is dehydrated faster than the outer part. To clarify the mechanism of the inverted moisture profile, we have carried out the numerical analysis based on the extended Darcy model for the two-phase flow in the porous media. The results indicate that the vapor pressure drives the moisture from the inner to the outer because of the internal heating by the microwave irradiation. The numerical moisture-distributions show good agreements with the experimental results obtained by the MRI measurements. It is found that the internal heating and surface evaporation induce the water permeation to the surface from the inside. This is the reason for faster drying.

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