CAVICAL1 was instrumented with nine thermocouples type T as shown in Fig. 3; seven located at the copper cone, in the wall in contact with water (T1 — T7), to measure the temperature distribution there, and two more to measure the inlet and outlet temperatures of the cooling fluid. A DAS system with other instruments was used to control DEFRAC and to run different set of experiments. In each experiment, mass flow rate, direct incident solar radiation and temperatures were measured and recorded. The experiments were done at the middle of the day and typical runs have duration of 10 minutes.

2. Numerical model

A numerical simulation was performed to evaluate the thermal behavior of the cavity calorimeter. One feature that the simulation allows is the evaluation of the convective energy losses due to natural convection heat transfer from the inner walls to the inner air in the cavity and from the cavity aperture to the atmosphere air. The phenomena that occur inside the air cavity is a free convection driven by the buoyancy force caused by density differences developed due to temperature gradient in the air fluid from the calorimeter wall and the fluid [3].

A three-dimensional numerical model was developed using the computer code FLUENT. A geometric mesh was set on the program according to the geometry of the calorimeter as shown in Fig. 3, together with the thermal properties of the materials (solids and fluids) involved into the experiments.

Mass, momentum and energy equations were solved simultaneously in the fluids domain (water and air) and in the solids domain of the system. The most important assumptions in the mathematical formulation of the cavity calorimeter were the following: the flow is in steady state, the fluid is radiatively non-participating, the Boussinesq approximation is valid, air physical properties are temperature dependent and solid properties are not temperature dependent. The conservation equations in primitive variables are solved using the finite element method.

Boundary conditions for the cooling fluid are: Constant temperature, pressure and mass flow rate at the calorimeter inlet. Physical properties (density, viscosity, thermal conductivity) are not temperature dependent. Boundary conditions for the solid are: External walls are assumed adiabatic. Internal walls (in direct contact with the fluid) are considered as a heater walls and modeled as a boundary conditions with a different heat flux profiles. The calorimeter was modeled with tetrahedrons finite elements (1 millions for air in the cavity, 0.9 millions solid calorimeter walls and 0.5 millions water cooling fluid). Steady state solution of the Navier-Stokes equations with conjugated heat transfer calculation was performed.

To simulate the interaction between the air inside of the cavity with the air outside, an atmospheric air domain, bigger than the air domain inside the calorimeter was considered, as it is shown in Figure 4. It was assumed that the inlet velocity boundary condition for the atmospheric air volume was 1 m/s going form left to right, as if it was win velocity.

In order to get confidence with the simulation, some sensitivity analysis where performed, such as the influence of the cooling flow rate, the calorimeter material (steel, cooper, etc.), the velocity and direction of wind, heat flux profile of incident irradiance, and after that, a comparison with experimental data was carried out.

As an example of typical numerical results, figure 5 shows the steady state temperature field in a central longitudinal plane of the calorimeter. It is possible to see how the heat is absorbed and transferred. The hot zone corresponds to a region on the copper wall and on the air close to it. The coldest zone corresponds to the water field and the solid field which are far from the hot spots. The stratification of the air inside the cavity and in the aperture is very clear; the air tends to go up due to buoyancy forces.

It is possible to observe the energy losses due to the effect of the natural convection mode in the calorimeter aperture and the movement of the air due to the density dependence with the temperature. The air with higher temperature rises to the top of the cavity and there is cooled down by the wall in contact with the water, so the maximum temperature in the calorimeter wall occurs at different position in the region where the highest radiative flux is absorbed.

Figure 6 shows the temperature profile in the calorimeter heated wall and in the fluid inside the cavity. As the figure shows, the maximum wall temperature occur at 8 cm from the aperture and the maximum air temperature near 12 cm. In the upper cone cavity thermal equilibrium occurs, the wall and fluid temperature are the same.