EDITORIAL

The editorial of Thermal Engineering of this issue continues the discussion on scientific research needs in vital areas in which thermal engineering has important participation. The main goal is to motivate the readers, within their specialties, to identify possible subjects for their future research. Natural Convection is present in the most diverse applications of Thermal Engineering, such as controlling and reducing temperatures in electronic systems, reducing the thermal efficiency of cooling in machining processes by the Leidenfrost effect and even in biological systems. With the increasing technological evolution and the development of industrial automation, microelectronics, quantum computing, signal processing, mobile telephony, etc., transmission systems operate increasingly with smaller spacing and higher integration rates between components, with greater power density and heat generation. As a result, there is a growing demand for cooling systems with greater safety, reliability, and efficiency. Therefore, natural convection cooling systems are viable alternatives due to their characteristics of: (A) protection and safety of the transmission system, especially in cases of mechanical and/or electrical failures of the forced cooling system; (B) high reliability and safety of operation; (C) low maintenance costs and (D) no noise. However, due to their low thermal efficiency, such cooling systems are still limited to applications with the low power density and/or combined with forced convection cooling systems. In this sense, the natural convection area is increasingly being researched to create and enable even smaller and more robust high power density transmission systems, with greater economic feasibility (lower costs of acquisition, manufacturing, and maintenance) and exclusively refrigerated (or with minimal use of forced cooling components) by natural convection; all without reducing the efficiency or reliability of these systems. One of the main technologies for thermal optimization of cooling systems researched is the inclusion of geometric surface modifications, through fins (extended surfaces) or corrugated surfaces. The use of corrugated surfaces has been gaining more space in the academic community and industry, standing out for: (A) increasing the area of exposure to the heated surface and the transfer of energy to the circulating fluid; (B) induce changes in the flow in the vicinity of the heated surface, such as the formation of vortices, recirculations, and zones of rarefaction and stagnation; and (C) anticipate and facilitate the flow transition process for the turbulent regime. The study of natural convection – in its most diverse applications and areas of theoretical, applied, and experimental investigation – has been widely explored by Thermal Engineering, arousing more and more the academic community's interest and motivating further research in this area. The mission of Thermal Engineering is to document the scientific progress in areas related to thermal engineering (e.g., energy, oil and renewable fuels). We are confident that we will continue to receive articles’ submissions that contribute to the progress of science. Sílvio Aparecido Verdério JúniorProfessor of Mechanical Engineering

Surface Diffusion by Means of Stochastic Wave Functions. The Ballistic Regime

Stochastic wave function formalism is briefly introduced and applied to study the dynamics of open quantum systems; in particular, the diffusion of Xe atoms adsorbed on a Pt(111) surface. By starting from a Lindblad functional and within the microscopic Caldeira–Leggett model for linear dissipation, a stochastic differential equation (Ito^-type differential equation) is straightforwardly obtained. The so-called intermediate scattering function within the ballistic regime is obtained, which is observable in Helium spin echo experiments. An ideal two-dimensional gas has been observed in this regime, leading to this function behaving as a Gaussian function. The influence of surface–adsorbate interaction is also analyzed by using the potential of two interactions describing flat and corrugated surfaces. Very low surface coverages are considered and, therefore, the adsorbate–adsorbate interaction is safely neglected. Good agreement is observed when our numerical results are compared with the corresponding experimental results and previous standard Langevin simulations.

Microswimmers near corrugated, periodic surfaces

We derive an analytical theory for the hydrodynamic interactions between microswimmers and corrugated surfaces and study the impact of a periodic surface on the velocities of active agents.

Spoof Zenneck waves in terahertz spectral range

We theoretically predict existence of Spoof Zenneck waves in the terahertz spectral range that are supported by corrugated surfaces with subwavelength patterning made of materials with high positive dielectric permittivities.

Resonant and Sensing Performance of Volume Waveguide Structures Based on Polymer Nanomaterials

Organic–inorganic photocurable nanocomposite materials are a topic of intensive research nowadays. The wide variety of materials and flexibility of their characteristics provide more freedom to design optical elements for light and neutron optics and holographic sensors. We propose a new strategy of nanocomposite application for fabricating resonant waveguide structures (RWS), whose working principle is based on optical waveguide resonance. Due to their resonant properties, RWS can be used as active tunable filters, refractive index (RI) sensors, near-field enhancers for spectroscopy, non-linear optics, etc. Our original photocurable organic–inorganic nanocomposite was used as a material for RWS. Unlike known waveguide structures with corrugated surfaces, we investigated the waveguide gratings with the volume modulation of the RI fabricated by a holographic method that enables large-size structures with high homogeneity. In order to produce thin photosensitive waveguide layers for their subsequent holographic structuring, a special compression method was developed. The resonant and sensing properties of new resonant structures were experimentally examined. The volume waveguide gratings demonstrate narrow resonant peaks with a bandwidth less than 0.012 nm. The Q-factor exceeds 50,000. The sensor based on waveguide volume grating provides detection of a minimal RI change of 1 × 10−4 RIU. Here we also present the new theoretical model that is used for analysis and design of developed RWS. Based on the proposed model, fairly simple analytical relationships between the parameters characterizing the sensor were obtained.