Transient Analysis of an Improved Finned Tube Heat Exchanger for Thermal Energy Storage System

Abstract

The melting and solidification process of sodium nitrate, which is used as energy storage material, is studied in a vertical arranged energy storage device with two different bimetal finned tube designs (with and without transversal fins) for enhancing the heat transfer. The finned tube design consists of a plain steel tube while the material for the longitudinal (axial) fins is aluminum. The investigation analyses the influence of the transversal fins on the charging and discharging process. 3-dimensional transient numerical simulations are performed using the ANSYS Fluent 14.5 software. The results show that every obstruction given by transversal fins reduces the melting and solidification velocity in direction to the outer shell. In the present study also a comparison of the simulation results between 2D and 3D simulation of the melting and solidification behavior of the sodium nitrate is presented.

Transient Numerical Simulation of the Melting and Solidification Behavior of NaNO3 Using a Wire Matrix for Enhancing the Heat Transfer

“…As we can see from Figure 11 the solidification process starts at the bottom of the computational domain and along the outer tube surface. This result is in good agreement with numerical results presented in [9,10,48,49] and experimental work in [46,47]. In all references a single vertical arranged shell-and-tube heat exchanger is investigated.…”

“…This indicates that the melting front moves from the top to the bottom. This behavior was also observed experimentally by [46,47] as well as numerically by [9,10,48].…”

“…In all numerical simulations, the volume change of the PCM during phase change due to density changes is neglected, the outer shell of the computational region is considered as adiabatic, and the mushy zone constant used for the calculations is C = 10 5 . The numerical simulation presented in [9,10,49] In Figure 11, the development of the liquid fraction of the sodium nitrate during the solidification process at different time steps is depicted. As we can see from Figure 11 the solidification process starts at the bottom of the computational domain and along the outer tube surface.…”

“…To get accurate results, very small time steps as well as a fine structured computational grid is necessary. Therefore long computation time can be expected even for small 3D models [10].…”

“…A widely used method to overcome this heat transport limitation in LHTES is to increase the specific surface of the heat exchanger tubes. Various techniques, like fins [4][5][6][7][8][9][10][11], dispersed high-conductivity particles inside the PCM [12,13], metal and graphite-compound matrices [14][15][16], micro-encapsulation [17][18][19] as well as a combination of metal foam made of copper and sandwich structure fins [20] have been suggested and were analyzed.…”