4.2 The Experiment Case M1-1 CFD Modelling
4.2.1 Boundary Condition
The climate chamber utilised for the case M1-1 has the dimensions of 4.2 m× 4.0 m × 2.4 m (height) with the simulated window with an area of 8 m2, and the orientation of the building and the materials and thickness of the walls were not reported in the study [8]. The important internal features in the chamber during the test include the air supply inlet and a return outlet, the window and the thermal manikin. The experimental conditions such as mass flow rate and temperatures are provided in Appendix C1.
The air supply inlet and return outlet utilised in the experiment were the wall-mounted air diffusers made by Uponor [115]. The diffuser has 210 air valves with the diameter of 4 mm as shown in Fig. 4.3 (a). In the case M1-1, the air inlet was mounted 10 cm above the outlet on the same panel as shown in Fig. 4.3 (b) and located on the opposite wall to the window. Directly modelling all geometric details of the semi- porous surface of the diffuser would increase the complexity, size, and computational cost as well as potentially causing solution convergence and stability problems. There are several common methods can be found in the literature to model such nozzle diffuser [92, 116-119]. The simplest method is known as the simple rectangle slot method which models the diffuser with an equivalent area of the air valves. Although the method is simple, the modelling result is less accurate. The multi-slot method simulates the diffuser with numbers of rectangles which have a total area equivalent to the area covered by valves. Rather than using single rectangle in the centre of the diffuser for modelling, the multi-slot method allows rectangles to be positioned far apart to simulate the surface of nozzle diffuser and maintain appropriate incoming jet width. The momentum method models nozzle diffusers by maintaining the correct mass and momentum of the flow. The mass and momentum are decoupled and specified separately while the area of the modelled diffuser is equivalent to the surface of real nozzle diffuser. Chen, Qingyan, Srebric and Jelena [119] examined different simplified diffuser models and indicated the box method is the best approach that can model the diffuser with high accuracy. However, the method requires appropriate estimation of the box size and the measurement of the air flow parameters on the boundaries of the box, which let the implementation become difficult. The modelling of the air supply inlet for test case 1 was based on the multi-slot method which area of the valves, mass flow rate and velocity at the inlet were identical to the settings in the experiment. The surface of the air diffusers was replaced by rectangles with a total area equivalent to the holes on the diffusers. The air inlet was replaced by two rectangles as shown in Fig. 4.3 (c) to maintain the correct air spreading rate in the horizontal plane in the far field. The mass flow rate at the air inlet in the CFD model was identical to the measurement in the test, which the simulated inlet could deliver same amount of momentum as in the
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experiment. The modelled air inlet was moved 10 cm away from the wall to avoid the risk of causing the supply air being sucked directly into the outlet by a recirculation vortex when air diffusers are at high level. An example of the modelling velocity profile at the inlet with the simplified approach is shown in Fig. 4.3 (d). The incoming air jets from the two rectangular inlets merge into a single jet at a certain distance with the correct spreading rate in the horizontal plane.
For vertical walls and ceiling in the CFD model, the stationary and non-slipped conditions were applied to wall boundary conditions. As the information of the wall surface temperature in the case M1-1 was not provided, the wall thermal condition in the model was set as isothermal where the walls produced zero heat flux. The window was modelled as a stationary wall with a non-slipped boundary condition. The surface temperature of the window was constant and had a value of 18.6 ℃ as measured in the experiment. In the simulation, gypsum was used as the material for vertical walls and ceiling, and glass was used for the window.
Figure 4.3 Air supply inlet and return outlet modelling. (a) Wall-mounted air diffuser, with 210 holes with the diameter of 4 mm and a total area of 0.0032 m2on each diffuser. (b) Air supply inlet and return outlet installed on a panel. (c) Mesh of the inlet and outlet in the CFD model, front view. (d) The modelling velocity contour with the simplified approach, top view. Figures are taken and reproduced [8, 115].
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In the case M1-1, a thermal manikin that produced 70 W and a lamp generated 20 W of energy were utilised. To simulate the heat sources in the CFD model for domain level comfort modelling, the heat dissipation was re-assigned to the floor by summing the heat dissipation rate from every source. Therefore, the impact of the heat sources can be maintained while the manikin and lamp could be removed from the CFD model. For occupant level comfort modelling, the CFD model utilised the proposed physiological model to simulate the effect of the thermal manikin used in the experiment. The physiological model was in a sedentary position, and it would calculate the corresponding amount of heat that is transferred from the manikin to the environment.
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