Numerical and experimental study of personalized ventilation

Abstract

Conventional air conditioning systems in buildings are designed to create a uniform environment across the entire occupied zone. Individual thermal preferences cannot be accommodated, and the fresh supply air is polluted by indoor contaminants before its inhalation by the occupants. Personalized ventilation (PV) is a novel development in the field of heating, ventilation, and air conditioning (HVAC) that has the potential to eliminate the deficiencies of conventional systems. The primary aim of this work is to measure the ventilation effectiveness of PV, to find out the human response to PV, and to develop a numerical thermal manikin (NTM) for the evaluation of the non-uniform thermal environment that is generated by PV. Experimental and numerical studies are reported in this thesis. The work that is presented consists of five parts: 1) a review of PV and computational fluid dynamics (CFD) studies of the thermal environment around a human body, 2) an experimental study of the ventilation seat type of PV, 3) the development of an NTM and its application to investigate the micro-environment that surrounds a human body and the performance of the ventilation seat, 4) the coupling of CFD and an inner-body thermoregulation model to predict local thermal sensation and comfort in the non-uniform thermal environments that are created by three different PV systems, and 5) a comparison of PV performance in a displacement ventilated room and a mixing ventilated room and a discussion of some significant issues of PV. Experiments using tracer gas and a heated, breathing thermal manikin with an artificial lung are carried out to measure the pollutant exposure reduction effectiveness (PER) of a chair-based PV system (a ventilation seat) that has an air supply nozzle that is located at the microphone position and serves cool, fresh air directly to the nose and mouth. The personalized airflow rate and temperature are maintained at 0.4-2.5 l/s and 15-22 oC, respectively. The PER is found to increase when the air supply flow rate and temperature increase. At an operating ventilation rate of 2.5 1/s, the pollutant level in the air that the user inhales can be reduced by around 76%. Subjective measurements are taken to evaluate human responses to the chair-based PV, and the perceived air quality; irritation at the eyes, nose, and lips; thermal sensation in different body parts; and overall thermal comfort are surveyed. In the personalized air temperature range of 15-22 oC, the perceived air quality is greatly improved without any facial discomfort, such as eye irritation and the sensation of a local draft feelings through correct design. Personalized air with a temperature that is below the air temperature in a room creates a "cool head" and increases the thermal comfort level in comparison to conventional mixing ventilation. People are found to be more sensitive to the flow rate of personalized air than to its temperature. To enrich thermal comfort theory, it is highlighted that the perceived air quality is significantly affected by the velocity of air on the face. Based on a review of existing work on the simulation of the micro-environment around a human body, an NTM with a realistic geometry of the body is developed to visualize the airflow in the breathing zone. This NTM, the skin surface of which is composed of small patches, is obtained by the laser scanning of a thermal manikin. In a CFD simulation using a commercial code (Fluent), the NTM surface is discretized into triangular elements with an average length scale of 4 mm. With the help of the NTM, the interaction between PA, respiration airflow, and the thermal plume around the body is clarified. The PER values from the simulation are compared to the experimental values, and a reasonable agreement is found. Parametric studies are conducted to investigate the effects of room air movement, personalized air temperature, and the turbulence intensity of personalized air on the performance of PV. Equipped with an up-to-date thermoregulation model that was developed at the University of California at Berkeley, the NTM, which mimics a real person in terms of heat and mass loss to the surrounding environment, is integrated into the CFD model for the evaluation of thermal comfort. The NTM is divided into 16 segments. The local air conditions, that is, the air velocity and air temperature, are input into the thermoregulation model manually to calculate the various skin temperatures, which are then fed back to the CFD as the body boundary conditions. A new set of local air conditions is created and the process is repeated until convergence. This coupling simulation is validated against some experimental data from the literature. In further steps, the facial thermal comfort level and ventilation effectiveness are comprehensively assessed using a state of the art local thermal comfort prediction model. The developed NTM may also be applied in other fields, such as the evaluation of airborne infection control technologies and thermal comfort levels in non-uniform and transient environments in vehicle HVAC engineering. The performance of PV in displacement ventilation and mixing ventilation systems is modeled and compared. In general, it is found that the combination of PV systems with displacement ventilation systems provides better indoor air quality and more opportunities for energy saving than the combination of PV with mixing ventilation systems. In the both modes, the inhaled air quality and overall thermal comfort level are determined by the conditions of the personalized air and the room air. With the head cooling function of PV, the limitation on the vertical temperature difference in the occupied zone of a building can be increased by up to 6 oC.