دراسة الأداء الحراري لمواد متغيرة الطور لمبادل حراري.

Rp_ Investigation of Thermal performance of Phase Change Material Heat Exchanger pdf

The present study includes an experimental and numerical investigation of the thermal performance of a phase change material (PCM) inside a concentric two pipes heat exchanger. The RT-42 Rubitherm PCM paraffin wax was used to fill the annular space between the hot copper inner pipe of 25 mm diameter and the external insulated acrylic shell of 75 mm diameter. The experimental setup involves twelve thermocouples to record the transient temperatures of PCM at desired locations. Also, the progress of solid-liquid front during the melting process is tracked photographically by a digital camera. A 2D numerical simulation applied the enthalpy-porosity method was used to solve the problem of PCM melting. The effect of the different temperatures (60, 70 and 80 °C) of hot water as heat transfer fluid (HTF) which flows inside the inner tube on the melting process is examined. Also, the impact of the two orientations (horizontal and vertical) of the heat exchanger on the behavior of melting PCM is researched.
The experimental and numerical results revealed that the melt layer of PCM is formed symmetrically around the inner pipe for both orientations of heat transfer due to the conduction domination at the first stage of melting process. Later, the melt layer of PCM grows and the influence of natural convection is developed causing a higher melting rate at the upper portion of the cavity. It has appeared that the melting time is clearly reduced by increasing the inlet temperature of water in both cases of the heat exchanger, horizontal and vertical. Also, the melting rate of PCM in the vertical direction is faster than the melting rate in the horizontal direction. The experimental results revealed that the melting time in horizontal PCM-annular cavity heat exchanger reduced about 27.5% and 46.3% when the temperature increased from 60 to 70 oC and from 60 to 80 oC, respectively. For the same temperature increase, the melting time is accelerated by about 32.6% and 50.2% in the vertical heat exchanger. Also, the amounts of energy stored by PCM in the vertical heat exchanger are higher about 4, 14.2 and 16.3% than that obtained in the horizontal orientation at 140 min for 60, 70 and 80 ˚C respectively. In addition, the transient Nusselt number has the same behavior for the two orientations and changes positively with the amount of transferred thermal energy from HTF which in turns varies proportionally with an inlet temperature of HTF.
The numerical and experimental results showed a good agreement. The maximum differences between the numerical and experimental results for melting time, Nusselt number and energy storage is found to be 12.4, 22.5 and 22.1% for horizontal heat exchanger at 70 ˚C of inlet water temperature. While those differences are10.7, 14.9 and 15.4% for vertical heat exchanger.