A water heater using very high-temperature storage and variable thermal contact resistance

This paper describes the role of thermal contact resistance in a high-tempe rature sensible heat storage water heater using cast iron as a storage mate rial. An experimental set-up consisting of a cast iron cylinder and a stain less-steel tube running through its centre was fabricated and tested. The e xperimental data were compared with a theoretical model. It was observed th at the variation in thermal contact resistance between the cast-iron blocks and the tube with temperature plays a dominant role in extracting the heat at a reasonably constant temperature. The contact resistance between two c ontacting surfaces was modelled as the composite of two parallel resistance s: one due to the points where two surfaces contact each other and the othe r due to a gap between the surfaces, which is often air filled. An approxim ate contact resistance prediction was obtained by assuming the resistance d ue to the air gap modulated by a correction factor which accounts for the c ontacting surface area. Based on the results from the experimental set-up a nd theoretical modelling, a prototype storage water heater using cast-iron blocks as the storage material was designed, fabricated and tested. Copyrig ht (C) 2001 John Wiley & Sons, Ltd.