Heat transfer in a combination microwave–jet impingement oven S. Geedipalli, A.K. Datta *, V. Rakesh Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA 1. Introduction Microwave ovens are popular for heating foods because they are quicker and more convenient than conventional ovens. However, problems associated with microwave heating are numerous such as non-uniform heating, edge over-heating, soggy texture, lack of browning and concerns about inade- quate microbial destruction. To overcome these disadvan- tages of microwave heating, researchers have proposed various solutions like designing better ovens, changing food composition, using active packages like shields and suscep- tors, etc. Some ovens include mode stirrers to change the electromagnetic field continuously while most ovens are provided with a turntable to rotate food and thereby heat it more uniformly. In spite of the above measures, major issues remain about the quality of food heated in a microwave oven. Microwave combination ovens are ovens in which the food is heated by a combination of microwaves and other heat sources such as infrared radiation, convection and jet impingement. These ovens are intended to overcome some of the problems associated with microwave ovens while maintaining the advantages of speed and convenience. 1.1. Jet impingement ovens Jet impingement ovens have been used for many years in meat processing and have been the standard for pizza cooking. The advantages of jet impingement heating over more traditional convection heating include faster cooking, higher efficiency and better water retention. Research on combination baking (Ovadia and Walker, 1998) showed that jet impingement required half the cooking time when compared to a convection food and bioproducts processing 86 (2008) 53–63 article info Article history: Received 16 July 2006 Accepted 21 June 2007 Keywords: Heating uniformity Electromagnetics Heat transfer coefficient abstract Combination heating of food using microwave and jet impingement has been simulated by coupling Maxwell’s equations of electromagnetics with energy equation and using experi- mentally measured heat transfer coefficient values for jet impingement in a novel domestic oven. Transient food temperatures from the model and experiment for each separate heating mode and their combination revealed the characteristic nature of each of the heating modes. Contour plots of temperature show that with combination heating, surface can be heated faster (for crispness) and edge over-heating can be partially avoided. Mea- sures of non-uniformity in temperatures in the heated food are developed using coefficient of variation and middle 80-percentile range as the parameters. Using these measures, it is shown that combination heating leads to more uniform heating, without compromising the speed or convenience. A 22–30% increase in uniformity has been observed for combination microwave–jet impingement heating over microwave-only heating. Jet impingement is a good complement to microwave heating as it has different spatial and time variation of heating rates. During the initial period, jet impingement dominates over microwave heating near the surface, with microwave heating being more significant in the interior. At later times, the roles switch with microwaves becoming more dominant on the surface while jet impingement takes a more significant role in heating the interior of the food. These findings should help the product, process and equipment designer achieve the balance between speed and uniformity of heating in a more precise manner. # 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. * Corresponding author at: 208 Riley-Robb Hall, Cornell University, Ithaca, NY 14853, USA. Tel.: +1 607 255 2482; fax: +1 607 255 4080. E-mail address: akd1@cornell.edu (A.K. Datta). available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/fbp 0960-3085/$ – see front matter # 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fbp.2007.10.016