Numerical study on geometrical specifications and operational parameters of multi-tube heat storage systems

In this paper, a numerical investigation as well as a comparison among multi tube heat exchangers is carried out for which variations of both geometrical and operational parameters are studied. The internal part of the shell is filled with RT35 as the phase change material which is heated by the heat transfer fluid (HTF) carrying inner tubes. This paper studies the effect of number of the tubes as well as their arrangement as geometrical parameters on liquid fraction, phase front and melting time. The details of the process progression are presented via liquid fraction contours and streamlines. The effective operational parameters considered are inlet temperature and flow rate of the HTF. Results show that conduction is the dominant heat transfer mechanism at initial stages of the melting process which is proceeded by a convection dominated interval. As the number of tubes increase, the contact surface between PCM and HTF tube increases, thus the convective heat transfer accelerates the melting process which reduces melting time. An increase in both operational parameters shortens the melting time for which the inlet temperature more effectively reduces the time. The results also show that among the arrangements, the ones whose tubes are located within the upper half of the shell are candidates for the longest melting time. Generally, for same inner tube numbers, at cases whose tubes occupy the bottom section of the system, a faster melting rate is obtained.