The effect of thermal plumes on the performance of ventilated ceilings in commercial kitchens

The efficiency of the exhaust system is especially emphasised with a ventilated ceiling system where the exhaust is located at ceiling level. The removal efficiency of the total system must be guaranteed and the spread of impurities throughout the kitchen should be prevented. At the moment, none of the existing calculation standards are specially tailored for a kitchen ceiling environment. In the normal design practice, empirical knowledge of the existing installations together with heat load based calculation has been used for airflow rate determination. The starting point for this research was to study the effect of the thermal plumes and supply air systems on the efficiency of a ventilated ceiling. A special consideration was to analyze the effect of a capture jet on the contaminant removal efficiency. In that capture air concept, the air jet is projected horizontally across the ceiling, which helps to direct heat and air impurities towards the exhaust. From the practical point of view, the objective of this study was to develop a design process to compute the required air flow rate more accurately. In this study, the measured convection flows of kitchen appliances during idle and cooking modes were compared with the generic plume equation in which the virtual origin is constant. The generic plume equation derived in the region of complete flow similarity is not fully valid in the intermediate zone (0.8-2.0 m from appliances). Still, it is possible to reach a reasonable accuracy for practical applications with the adjusted virtual origin. The cooking process does not have any significant effect on the velocity and temperature distribution of the convection flow. The reason for this is that the mass flow rate of water during boiling is small compared with the induced air flow rate and therefore does not have a significant effect on the convection flow. Thus, the actual convection load and the product specific virtual origin can describe the plume during the cooking process. In the previous studies of thermal plumes, the velocity and temperature distribution factors are much higher than in the present study. In addition, the entrainment factors of the previous studies are much smaller than in this study. The measurements indicate that the heat gain has a significant effect on the spreading angle and the entrainment factor close to the heat source. The plumes with high heat gains are narrower and the convection flow induces more room air than the previous studies have pointed out. The efficiency of the exhaust system can be improved with a small capture jet installed at the ceiling surface. Both the measurement and simulated data give lower contaminant levels when the capture jet was introduced. The plume equation gives a platform to calculate the air flow rate that is theoretically required to remove the convective heat output of the appliance block. In this study, the flush-out factor of the supply air on the theoretical plume equation was derived for the centralized capture jet concept. For practical design work, the target for the containment removal efficiency should be 85 %. To obtain 85 % containment removal efficiency requires to a flush-out factor of 1.2.reviewe