In pool boiling and spray cooling the Leidenfrost point marks the transition from nucleate boiling, in which the evaporating liquid is in contact with the surface, and film boiling, in which a layer of vapor separates the fluid from the surface. For a single evaporating drop, the Leidenfrost point occurs when the capillary and gravitational forces are surpassed by the upward pressure of the escaping vapor. This thesis develops an analytical model to predict the Leidenfrost point for a microstructured surface. The microstructure consists of a regular array of square posts geometrically defined by aspect ratio and spacing ratio. The vapor pressure is modeled using the momentum equation for flow in a porous medium. Varying the geometric parameters indicated that aspect ratio and spacing ratio must be optimized to achieve the maximum Leidenfrost temperature. For a water drop evaporating from a silicon surface, the maximum Leidenfrost temperature is predicted to occur with an aspect ratio of 1.3 and a spacing ratio of 1.5. [mu]L water drops were evaporated from a smooth surface made of silicon and porous surfaces made of aluminum oxide. The microstructure of the surfaces was different from that modeled, but increased wettability and higher Leidenfrost temperatures were observed as porosity increased. Recommendations for further research in this area are made.

This open access book presents the main results of the Collaborative Research Center SFB-TRR 75, which spanned the period from 2010 to 2022. Scientists from a variety of disciplines, ranging from thermodynamics, fluid mechanics, and electrical engineering to chemistry, mathematics, computer science, and visualization, worked together toward the overarching goal of SFB-TRR 75, to gain a deep physical understanding of fundamental droplet processes, especially those that occur under extreme ambient conditions. These are, for example, near critical thermodynamic conditions, processes at very low temperatures, under the influence of strong electric fields, or in situations with extreme gradients of boundary conditions. The fundamental understanding is a prerequisite for the prediction and optimisation of engineering systems with droplets and sprays, as well as for the prediction of droplet-related phenomena in nature. The book includes results from experimental investigations as well as new analytical and numerical descriptions on different spatial and temporal scales. The contents of the book have been organised according to methodological fundamentals, phenomena associated with free single drops, drop clusters and sprays, and drop and spray phenomena involving wall interactions.

Fluid flows that transfer heat and mass often involve drops and bubbles, particularly if there are changes of phase in the fluid in the formation or condensation of steam, for example. Such flows pose problems for the chemical and mechanical engineer significantly different from those posed by single-phase flows. This book reviews the current state of the field and will serve as a reference for researchers, engineers, teachers, and students concerned with transport phenomena. It begins with a review of the basics of fluid flow and a discussion of the shapes and sizes of fluid particles and the factors that determine these. The discussion then turns to flows at low Reynolds numbers, including effects due to phase changes or to large radial inertia. Flows at intermediate and high Reynolds numbers are treated from a numerical perspective, with reference to experimental results. The next chapter considers the effects of solid walls on fluid particles, treating both the statics and dynamics of the particle-wall interaction and the effects of phase changes at a solid wall. This is followed by a discussion of the formation and breakup of drops and bubbles, both with and without phase changes. The last two chapters discuss compound drops and bubbles, primarily in three-phase systems, and special topics, such as transport in an electric field.

Superheated steam drying (SSD) has long been recognized for several major advantages it offers over other convective dryers, including high energy efficiency by utilization of energy in the exhaust steam, higher product quality due to the absence of oxygen, and avoidance of fire and explosion hazards. Offering a global critical overview of the current state of art, Superheated Steam Drying: Technology for Improved Sustainability and Quality assesses future needs and opportunities for industry adoption and further innovation in SSD. It covers SSD technologies for various industrial sectors and mathematical modeling approaches to help with design and scale-up. The effects of SSD on drying kinetics as well as product quality are also discussed with examples. This book serves as a useful reference for technicians, graduate students, and researchers in the field of drying technology. It can also be used in courses on industrial drying, processing and drying of food, advanced drying technology, and superheated steam drying.