With the ever-increasing availability of high performance computing facilities, numerical simulation through Computational Fluid Dynamics (CFD) is increasingly used to predict the room air distribution. CFD is becoming an important design and analytical tool for investigating ventilation inside the system and thus to increase thermal comfort and improve indoor air quality. The room air supply diffuser flow rates can be reduced for less loading with the help of a variable air volume unit. The reduction in supply flow rate reduces the energy consumption for the unoccupied and reduced load conditions. The present research is to study the comfort consequences for reduced diffuser flow rates and loading and to identify the hot and cold spots inside a room. A small office room with ceiling based room air distribution method is considered for CFD analysis. The CFD results are validated with experimental measured data for the designed diffuser flow rate. A parametric study on different turbulence models, namely, low Reynolds number modification of standard k-epsilon model, re-normalization group k-epsilon model, transition k-kl-w model and Reynolds stress model is carried out, and simulation results in terms of velocity and temperature profiles are compared against the measured data. Other important parameters such as diffuser jet inlet angle and radiation effect are also considered on the benchmark case to validate the results and to recommend the best fit parameters for room air simulations. Analysis has been carried out for a range of flow rates and heat loads. The jet momentum, draft and temperature distribution inside the room are studied for the impact of reduced flow rates and loading. The thermal comfort is quantified in terms of vertical temperature distribution and percentage dissatisfied index. From the research it is found that, for the studied room setup and air distribution method, the diffuser flow rate can be reduced up to 30 percent of the design flow rate, without experiencing a considerable effect on the room air temperature distribution. Also, based on thermal comfort and room air temperature distribution, several recommendations for occupant spacing in a room are suggested for reduced diffuser flow rates.

Omniphobic surfaces, which repel polar and non-polar liquids alike, have proven of value in a myriad of applications ranging from piping networks, textiles, food and electronics packaging, and underwater drag reduction. A limitation of currently employed omniphobic surfaces is their reliance on perfluorinated coatings/chemicals, increasing cost and environmental impact and preventing applications in harsh environments. Thus, there is a keen interest in rendering conventional materials, such as hydrocarbon-based plastics, omniphobic by micro/ nanotexturing rather than via chemical makeup, with notable success having been achieved for silica surfaces with doubly reentrant pillars (DRPs). We discovered a critical limitation of DRPs – they catastrophically lose superomniphobicity in the presence of localized physical damages/defects or on immersion in wetting liquids. In response, we pioneered bio-inspired gas-entrapping microtextured surfaces (GEMS) architecture composed of doubly reentrant cavities (DRCs). DRCs are capable of robustly entrapping air when brought into contact with liquid droplets or on immersion, which prevents catastrophic wetting transitions even in the presence of localized structural damage/defects. This dissertation presents our multifaceted research on DRCs via custom-built pressure cells, confocal laser scanning microscopy, environmental scanning electron microscopy, contact angle goniometry, high-speed imaging, and upright optical microscopy. Specific accomplishments detailed in this thesis include: (i) the microfabrication protocols for silica GEMS developed at KAUST; (ii) the characterization of GEMS’ omniphobicity via apparent contact angles and immersion; (iii) the demonstration of ~ 1000,000,000% delays in wetting transitions in DRCs compared to those in simple cavities (SCs) under hexadecane; (iv) a proposal for immersion of surfaces as a criterion for assessing their omniphobicity in addition to apparent contact angles; (v) effects of surface chemistry, hydrostatic pressure, and cavity dimensions on Cassie-to-Wenzel transitions in DRCs and SCs; (vi) the demonstration of “breathing” (liquid-vapor) interfaces in GEMS under fluctuating hydrostatic pressures; and (vii) the demonstration of directional wetting transitions in DRCs (or cavities in general) arranged in one- and two-dimensional lattices. The last chapter in the thesis presents future research directions such as breathing surfaces capable of preempting vapor condensation and gas replenishment.

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