A continuously growing focus on lowering the energy consumption and improving the indoor climate gives challenges when designing buildings. In densely occupied spaces, large amounts of fresh air must be supplied to keep a good indoor air quality, often with the result that draught or high temperature gradients occur. Diffuse ceiling ventilation is an air distribution system in which the air is supplied from the entire ceiling surface, giving a low supply velocity. Therefore the flow pattern in the room is controlled by the present heat sources. The system typically generates high mixing and the air velocities in the room are expected not to be influenced by the flow rate to the room. Previous studies have shown that diffuse ceiling ventilation has an ability to remove large heat loads without compromising the indoor climate, however recent experiments indicate that problems can occur at large room height and with certain heat load distributions.

The experimental part in this thesis is an investigation of the importance of three parameters: room height, heat load distribution and air supply geometry. Results consist of measured air velocities and temperatures in the test room and are evaluated using a method where the thermal climate in the room determines the cooling capacity of the system. The results of the measurements showed that both room height and heat load distribution had great importance on the performance of the system, but also that air velocities in the room increased when increasing the flow rate opposite previous experimental investigations.

CFD simulations were set up in the program FloVENT to either further investigate the nature of the flow in some experimental situations. A reference model with conditions similar to one of the experiments was created, and validation showed good accordance between experiments and CFD model. The reference model was modified to test some tendencies experienced in the experiments, namely what happens when changing flow rate, room height or supply geometry. The CFD predictions showed very similar results as the experiments. Air velocities in the room was dependent on the flow rate, and room height had a great significance on the thermal indoor climate in the room. It also showed very vague changes when changing supply geometry from small slot diffusers to a fully diffuse ceiling.