Theoretical analysis on microwave heating of oil–water emulsions supported on ceramic, metallic or composite plates Sujoy Kumar Samanta a , Tanmay Basak a,b , Bhaskar Sengupta b, * a Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600036, India b School of Planning, Architecture and Civil Engineering, Queen’s University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG Northern Ireland, UK article info Article history: Received 25 July 2007 Available online 2 June 2008 Keywords: Microwave Oil–water emulsion Composite supports Thermal runaway abstract A detailed theoretical analysis has been carried out to study efficient heating due to microwaves for one- dimensional (1D) oil–water emulsion samples placed on various ceramic, metallic (reflective) and cera- mic–metallic composite supports. Two typical emulsion systems are considered such as oil-in-water (o/ w) and water-in-oil (w/o). A preliminary study has been carried out via average power vs emulsion thick- ness diagram to estimate microwave power absorption within emulsion samples for various cases. The maxima in average power, also termed as ‘resonances’, are observed for specific emulsion thicknesses and the two consecutive resonances of significant magnitudes are termed as R 1 and R 2 modes. For both o/w and w/o emulsions, it is observed that microwave power absorption is enhanced in presence of metallic and composite supports during both R 1 and R 2 modes. The efficient heating strategies character- ized by ‘large heating rates’ with ‘minimal thermal runaway’ i.e. uniform temperature distributions within the sample have been assessed for each type of emulsion. Based on the detailed spatial distribu- tions of power and temperature for various cases, SiC-metallic composite support may be recommended as an optimal heating strategy for o/w samples with higher oil fractions (/ P 0:45) whereas metallic and Alumina-metallic composite supports may be favored for samples with smaller oil fractions (/ ¼ 0:3) during R 1 mode. For w/o samples, SiC-metallic composite support may be suitable heating strategy for all ranges of water fractions during R 1 mode. During R 2 mode, metallic and Alumina-metallic composite supports are favored for both o/w and w/o emulsion samples. Current study recommends the efficient way to use microwaves in a single mode waveguide and the heating strategy can be suitably extended for heating of any other emulsions for which dielectric properties are easily measurable or available in the literature. Crown copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. 1. Introduction Electromagnetic radiations in the frequency range 300 MHz to 300 GHz are known as microwaves. During microwave heating, the material dielectric loss which is a function of frequency of microwaves causes electrical energy to convert into heat within material and this involves the mechanism termed as ‘volumetric heating’ which is responsible to carry out uniform and faster pro- cessing of materials. Microwave energy has largely been used for thermal processing in a wide range of food and chemical process industries for pasteurizing, heating, drying, petroleum refining, sintering of ceramics, and many others [1–15]. Oil–water emulsion heating is a very common industrial practice in petroleum refining as about 80% of exploited crude oils exist in the form of emulsion [13]. Various oil-in-water (o/w) and water-in-oil (w/o) emulsions also occur in other industrial operations, such as oil and gas pro- duction, food processing industries, which include dressings, sauces, butter, mayonnaise, and many more. Efficient heating of emulsions is required for a faster processing based on industrial demand and microwave heating has largely been used due to its ‘volumetric heating’ effects. Enhanced power absorption and large volumetric heating ef- fects occur for samples with specific dimensions in presence of ‘resonances’ of microwave power. A number of theoretical and experimental investigations were carried out by earlier researchers on various aspects of microwave heating, especially on resonances. Ayappa et al. [16–18] carried out theoretical and numerical studies on microwave heating of 1D slabs and 2D cylinders. Further, Ayap- pa et al. [19] and Ayappa [20] analyzed resonances occurring only for fixed sample dimensions of 1D slabs and 2D cylinders. They also established suitable relationships on occurrence of resonance with sample size. Microwave heating and transport models were further applied for thawing and heating of multiphase systems 0017-9310/$ - see front matter Crown copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2008.04.003 * Corresponding author. E-mail addresses: sujoysamanta@smail.iitm.ac.in (S.K. Samanta), tanmay@iitm. ac.in (T. Basak), B.Sengupta@qub.ac.uk (B. Sengupta). International Journal of Heat and Mass Transfer 51 (2008) 6136–6156 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt