Effect of ceramic supports on microwave processing of porous food samples Tanmay Basak 1 , K. Aparna, A. Meenakshi, A.R. Balakrishnan * Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India Received 6 January 2006; received in revised form 4 May 2006 Abstract A theoretical analysis has been carried out to study efficient microwave heating of porous dielectrics. The heating effects are analyzed for two types of porous material: beef-air (b/a) and beef-oil (b/o) with and without ceramic supports (Al 2 O 3 and SiC). Three test cases for porosities (/) 0.3, 0.45 and 0.6 are considered. The maxima in average power corresponding to resonances occur at various sample thick- nesses for all porous materials with and without supports and two dominant resonance modes R 1 and R 2 are considered where the aver- age power at R 1 is larger than that at R 2 . It is interesting to observe that average power absorption is enhanced for samples (b/a and b/o) in presence of Al 2 O 3 support whereas the average power is smaller with SiC support. From the analysis on spatial distribution of electric field, power and temperature, it is seen that runaway heating is observed at the face which is not attached with support for b/a samples, and the intensity of thermal runaway increases with porosity whereas lower thermal runaway is observed for b/o samples at all porosity values. An efficient heating strategy has been investigated for various distributions of microwave incidences. It is observed that one side incidence may correspond to the largest heating rates whereas distributed sources may correspond to smaller thermal runaway for both beef-air (b/a) and beef-oil (b/o) samples. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Microwave; Heating; Ceramic supports; Porous dielectric 1. Introduction Electromagnetic radiations in the frequency range 300 MHz–300 GHz are known as microwaves and the typ- ical wave lengths of microwaves are within few mm to 30 cm. Microwaves propagate through material and the accompanying transport process results in dissipation of electric energy into heat, which led to the term ‘volumetric heat generation’. During microwave heating, the material dielectric loss which is a function of frequency of micro- waves, is responsible to convert electric energy to heat. Dielectric response of various materials plays an important role to carry out efficient material processing and a signif- icant amount of earlier research was devoted to understand the physics on microwave assisted transport and heating characteristics. A detailed analysis on modeling of microwave heating has been carried out by Ayappa et al. [1,2] and their inves- tigations were based on heating of 1D slabs and 2D cylin- ders. They carried out detailed theoretical analyses on coupled microwave and heat transport for pure and multi- layered slabs typically used in the food industry. The local- ized or non-uniform heating in samples occur due to volumetric heating effects, and considerable studies were devoted to analyses of maxima in power or ‘resonances’ due to microwave propagation [3,4]. The heating effects during resonances are considerably greater for specific sample dimensions and the suitable relationships on occur- rence of resonance with sample size were established [3,4]. Microwave heating and transport models were further applied for thawing and heating of multiphase systems in 0017-9310/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2006.05.012 * Corresponding author. Fax: +91 44 2257 0509. E-mail addresses: tanmay@iitm.ac.in (T. Basak), arbala@iitm.ac.in (A.R. Balakrishnan). 1 Fax: +91 44 2257 0509. www.elsevier.com/locate/ijhmt International Journal of Heat and Mass Transfer 49 (2006) 4325–4339