A three dimensional steady state simulation has been carried out in order to study the riser tubes of a plastic absorber configured by two manifolds and a set of riser tubes without fins.

The physic domain of the studied case is shown in figure 3(a). It consists of a square duct with thin walls (e = 0.4mm), length of 1 m ( ) and different side length (I =

4.6.8.10mm).

The temperature of the water and the air surrounding are assumed constant (Ta = Tin = 25°C), and a mass flow of fn = 501/hrn2 is imposed. In order to simulate solar irradiance and convection losses to the environment, at the top wall (wall facing the cover) a constant heat ^^^Яф2) is imposed and an equivalent convection coefficient ( ) is

Figure 4: Numerical results for riser tube: Heat removal factor (FR) and pressure drop (Ap) along channel vs. side length (I = 4,6.8 and 10 mm).

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2® 29 30 31 32 33 3+ 35 35 36 37 38 38 39 40 41 TiYl

Figure 5: Numerical results riser. Riser tube temperature profiles for the four set of dimen­sions at different sections in у-direction at x-midplane. ,Y* = y/L.

assumed. At the bottom wall, the coefficient imposed is Ub = 0.5W/rn2K and at the outlet a pressure outflow boundary condition is imposed [12].

The computational domain where the governing equations are solved is shown in figure 3(b). The number of control volumes used in each direction is represented by the n parame­ter, where n = 40. Near the solid walls the grid has been intensified by means of a tanh-like function[17], it is indicated by solid triangles in the figure.

As from the CFD simulation local values of all the variables (temperature and velocity field) are know, is possible to evaluate a posteriori any integral quantity of interest. As an example, the values of the heat removal factor Fr and of the pressure drop Ap through the riser in terms of the side of the duct have been computed. They are given in figure 4.

Further information of the numerical results are shown in figure 5. It consist of a compar­ison of the temperature profiles in the central section of the riser tube at different values of y (0.25, 0.50, 0.75 and 1.0 m).