The store analysed is a tank with a length of L=1m and an internal radio of R=0.25m. The tank is made of 8mm thick Plexiglass material and the inlet flow rate is directed by a rigid manifold (see figure 1a, b). The rigid manifold selected is the one designed by Davidson et al. [5] consisting of a diffuser that reduces the inlet stream momentum and, at the same time, has different rings with open orifices that force the fluid to exit at the height closest to its temperature. An 8mm thick baffle plate and a radio of 0.15m is located at 0.04m from the bottom of the tank. This plate is also used as a diffuser to reduce the momentum of the inlet stream during the tank withdraw.

The principles of operation of the manifold are studied considering two geometrical mod­ifications. The first one consists of the reduction of the momentum diffuser tube height (Lin) from 0.133m to 0.04m. While in the second one, all the manifold outlet rings are obstructed except the ones located at the middle of the tank, forcing the fluid to exit the manifold through these rings.

Thus, three different situations, hereafter refererred to as cases A, B and C, are analysed. Case A considers the original manifold geometry provided by [5] . In Case B the tube height is reduced, and in case C, both the tube’s height is reduced and outlet rings are obstructed.

Figure 1: Non scaled geometry and mesh. a) Tank geometry, b) Detail of the manifold. c) Mesh

A test procedure to characterise the level of mixing produced by natural convection when the fluid entering the tank is colder than the surrounding fluid is proposed. This test is considered as a complement of test sequences of the European Standard EN 12977-3 [6] for testing the thermal performance of storage tanks. Hereafter, this test will be referred to as test P The test sequence consists of the following phases:

Phase P1: Conditioning of the store at

Phase P2: One half tank charging with a constant mass flow rate of 0.25 v„ and at a constant temperature of 60°C.

Phase P3: One half tank charging with a constant mass flow rate of 0.5 vn and at a constant temperature of 40°C.

Phase P4: Discharge of the tank at a constant mass flow rate of 0.25 vn and constant

inlet temperature of C until the steady state is reached.

As in EN 12977-3 [6], in order to determine the energy balance in the store, the test sequence starts and ends with the same temperature of the store. During phase P2, the tank is charged at constant temperature of C. The objective of this phase is to study the
formation of the thermocline and the influence of the inlet design in the thermocline height at the end of the phase. In phase P3, the tank is charged at a constant temperature of C. The effects that provoke mixing and, consequently, the degradation of stratification can be studied. As at the beginning of this phase the tank has been half charged, the effects of the plume entrainment phenomena and how it enhances the natural convection inside the tank can be considered.