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-temperature sensible heat storage water heater using cast iron as a storage material. An experimental set-up consisting of a cast iron cylinder and a stainless-steel tube running through its centre was fabricated and tested. The experimental data were compared with a theoretical model. It was observed that 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 contacting surfaces was modelled as the composite of two parallel resistances: one due to the points where two surfaces contact each other and the other due to a gap between the surfaces, which is often air filled. An approximate contact resistance prediction was obtained by assuming the resistance due to the air gap modulated by a correction factor which accounts for the contacting surface area. Based on the results from the experimental set-up and theoretical modelling, a prototype storage water heater using cast-iron blocks as the storage material was designed, fabricated and tested. Copyright (C) 2001 John Wiley & Sons, Ltd.