Date of Award
Doctor of Philosophy (PhD)
Zhiqiang (John) Zhai
For low-rise buildings, it is well known that envelope systems are the main contributors to heating and cooling energy consumption. Over the last decades, there have been significant research efforts to improve the performance of building envelope systems by integrating passive cooling strategies to reduce cooling loads and maintain acceptable indoor thermal comfort. The ventilated wall cavity system is one of these passive-cooling strategies that have received considerable attention recently due to the significant benefits of reducing building thermal loads. In particular, evaporative cooling inside ventilated wall cavities is an attractive passive cooling technique especially in hot and dry climates.
Very few research studies have investigated evaporative cooling using spray systems within the ventilated wall cavities. Some reported research studies have suggested that the spray droplet size has a significant impact on the evaporative cooling performance. In this dissertation, a detailed analysis of ventilated wall cavities with spray evaporative cooling systems is carried out. First, a simulation environment has been developed using numerical models of droplet evaporation and droplet dynamics for spray systems. Then, the predictions from spray and ventilated cavity models were validated against experimental measurements and CFD analyses. The validation analyses have shown good agreement for all predicted variables. Moreover, thermal comfort indicators have been modeled using energy balance methods to estimate Predicted Mean Value (PMV) for conditioned buildings and Thermal Sensation Vote (TSV) for naturally ventilated buildings. Mold growth has been accounted for in the developed simulation environment using experimental correlations that consider temperature, humidity, and time in addition to favorable and unfavorable conditions for mold growth. The mold growth model was used as an indicator of mold risk inside the ventilated wall cavity. The developed simulation environment has been used to predict the thermal performance of the ventilated wall cavity system by integrating four modules based on the developed and validated models including indoor thermal comfort, cooling energy savings, mold growth potential, and water use.
A simple ON/OFF control algorithm has been developed to control the spray system and three dampers which regulate the amount of water usage and air flow rate. Control sequences have been developed in order to minimize water consumption, prevent mold growth inside the cavity, and eventually to maintain acceptable indoor thermal comfort. Moreover, multi-objective optimization analyses have been performed using the developed simulation environment to identify an optimum control strategy and optimum droplet size depending on both indoor and outdoor conditions. Specifically, four objective functions are considered: cooling energy, indoor comfort, water use, and mold growth index. Near optimum control strategies were provided for conditioned buildings and naturally ventilated buildings. Moreover, a new concept of evaporative cooling spray system has been introduced to generate different droplet sizes according to the changes of the surrounding environment to maintain acceptable indoor thermal comfort.
A series of sensitivity analyses have been performed to evaluate the impact of various design variables on the performance of the ventilated cavity wall and on the selection of the optimum control strategy. Overall, the ventilated cavity wall system has been found to be an energy efficient passive cooling system for buildings located in hot and dry climates especially when operated by the near optimal control strategy developed in this dissertation.
Alaidroos, Alaa Ali, "Thermal Analysis of Ventilated Wall Cavities with Spray Evaporative Cooling Systems" (2016). Civil Engineering Graduate Theses & Dissertations. 432.