A reduction of fan power is required to develop high efficiency air conditioner. Fan-power reduction is achieved by reducing of the pressure loss of flow channels and/or improving the efficiency of a fan. An indoor unit of the air conditioner in the present study is installed on a ceiling in a room. The indoor unit consists of a centrifugal fan, a heat exchanger, an air inlet and 4 air-outlet nozzles. One of the areas of the highest pressure-loss in the indoor unit is around the air-outlet nozzles since the cross-section of the flow channel in the air-outlet nozzles is smaller than those in other areas. In addition, the path of air flow after passing through the heat exchanger is sharply turned downward by a cabinet wall of the indoor unit. The air flow separates in the air-outlet nozzles when the air flow gets over a drain pan which receives water condensed on the surface of the heat exchanger. As a result, the effective cross-section of the air-outlet nozzles is further reduced due to the flow separation. This is main cause of the pressure-loss in the air-outlet nozzles.
The optimum nozzle shape to suppress flow separation in air-outlet nozzles of the indoor unit of an air conditioner was determined. An edge, shaped on the wall of the drain pan, minimized the flow separation by corresponding to the location between the edge and the attachment point of the flow separation. The location of the edge is defined by two parameters, and the influence of the parameters on reduction of fan power was determined by using Computational Fluid Dynamics (CFD). A CFD model of a whole indoor unit (including fan, heat exchanger, air-outlet nozzles) was applied to accurately predict fan power for different locations of the edge. Furthermore, the flow separation in the air-outlet nozzle was visualized on the basis of the CFD results. To obtain the appropriate combination of parameters to suppress the flow separation, a response surface based on the CFD results and approximate values given by a Kriging-based method, was used. The Kriging-based model is one of the response-surface methods and is characterized by approximating a nonlinear function based on Bayesian probabilistic estimation. The response surface provided the area containing the appropriate parameters for reducing fan power (“parameter area”, hereafter). The parameter area of fan-power reduction on the response surface was found. The CFD results confirm that the flow separation corresponds to the edge location given by this parameter area and that the edge minimizes the flow separation. To experimentally verify the effect of the edge on fan-power reduction, four points in the parameter area for fan-power reduction were selected, and four nozzle shapes with these parameters were prototyped. It was found that fan power was reduced (at most) by 9.9% by the optimized nozzles shapes in comparison with the current shape of air-outlet nozzles.