Abstract

The remarkable class of substances known as phase change materials (pcms) possesses an intriguing property known as latent heat storage. These materials have the unique ability to undergo a physical transformation between solid and liquid states, or between liquid and gas states, while absorbing or releasing a substantial amount of latent heat. This latent heat phenomenon enables pcms to store and release thermal energy during phase transitions, thereby making them indispensable for a vast array of industrial applications. Their relatively low thermal conductivity is a significant drawback, especially for organic pcms. This limitation can affect the efficiency of heat transfer during phase transitions, leading to slower response times and reduced overall performance in certain applications. This work aims to investigate the effectiveness of different approaches of heat conduction enhancement between the tube that carries the cooling medium (water) and the rt42 phase change material (pcm). In the first one, a finned tube with annular fins is employed, while in the second one, aluminum foam is incorporated. The latter is modelled by applying a novel algorithm, that is based on the voronoi tessellation technic. With this algorithm it is possible to generate stochastic lattice structures with different characteristics such as cell size, strut thickness, porosity and even anisotropy. The rt44hc pcm is a paraffin wax with high thermal energy storage (250 kj/kg), density of 0.7 kg/l in liquid phase, a heat conductivity of 0.2 w/mk and is chemically inert. Where possible, temperature-dependent thermal properties, provided by the producer, are used for the definition of the material model. The geometry of the model consisted of a shell-and-tube heat exchanger, with two different heat conduction enhancement structures. The tube had an outside diameter of 21mm and a thickness of 2mm, the shell had an inside diameter of 50mm, the annular fins had a diameter of 49mm and a thickness of 1mm, the aluminum foam had an outside diameter of 49mm and porosity was around 85%. In order to reduce computational effort, a simplified model was developed using only a quarter of the geometry. Also, since it was not of interest the water flow characteristics but rather the pcm liquid fraction along the simulation duration, only the geometry of the pcm material was considered. The simulation results revealed a huge reduction of the melting time between the two configurations. In particular, the solution that employs the aluminum foam needs less than half the time to start the paraffines phase change with respect to the corresponding configuration with the annular fins. Keywords: cfd, pcm, lhtes