Forced Convection Heat Transfer in Power-Law Fluids Around a Semicircular Cylinder at Incidence

Two-dimensional steady flow and convective heat transfer of power-law fluids past a semicircular cylinder are investigated in the reported work. The heated semicircular cylinder is placed in an unconfined domain at different angles facing the incoming free-stream flow of power-law fluids having a generalized Prandtl number (Pr) = 100. Particular emphasis is given to studying the effect of angle of incidence (0 ≤ α ≤ 180°) on fluid dynamics and thermal transport around the semicircular object for varying Reynolds number (10 ≤ Re ≤ 40) and power-law index (0.4 ≤ n ≤ 1.8). A finite volume-based method is adopted for the numerical computation. The flow and heat transfer phenomena are visualized through the streamline and isotherm profiles at various operating conditions. Also, the pressure coefficient, drag coefficient, and Nusselt number on the surface of the object are presented and discussed.     

A Roadmap for Implementing Minichannels in Refrigeration and Air-Conditioning Systems—Current Status and Future Directions

The evolution of compact heat exchangers was driven by the need for reducing their size and weight and enhancing individual components and overall system performance. These needs are also becoming relevant in stationary refrigeration and air conditioning applications. The reduction in volume and footprint of the systems is foremost in the minds of urban architects, while refrigerant charge reduction and performance enhancement remain important safety and operating considerations. Minichannels provide a viable solution to address these concerns and are expected to be used widely in large as well as small commercial, industrial, and residential systems. This paper provides a roadmap for their implementation, with a critical review of the current status of our understanding in this field, recommendations for obtaining fundamental performance data and correlations, and an emphasis on innovative designs of minichannel heat exchangers.     

Similarities and Differences Between Flow Boiling in Microchannels and Pool Boiling

Recent literature indicates that under certain conditions the heat transfer coefficient during flow boiling in microchannels is quite similar to that under pool boiling conditions. This is rather unexpected, as microchannels are believed to provide significant heat transfer enhancement under single-phase as well as flow boiling conditions. This article explores the underlying heat transfer mechanisms and illustrates the similarities and differences between the two processes. Formation of elongated bubbles and their passage over the microchannel walls have similarities to the bubble ebullition cycle in pool boiling. During the passage of elongated bubbles, the longer duration between two successive liquid slugs leads to wall dryout and a critical heat flux that may be lower than that under pool boiling conditions. A clear understanding of these phenomena will help in overcoming these limiting factors and in developing strategies for enhancing heat transfer during flow boiling in microchannels.     

Evolution of Microchannel Flow Passages--Thermohydraulic Performance and Fabrication Technology

This paper provides a roadmap of development in the thermal and fabrication aspects of microchannels as applied in microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, the advantages of using microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Single-phase performance for liquids is still expected to be describable by conventional equations; however, the gas flow may be influenced by rarefaction effects. Two-phase flow is another topic that is still under active research. The evolution of research in microchannel flow passages has paralleled the advancements made in fabrication technology. The earliest microchannels were built in silicon wafers by anisotropic wet chemical etching and sawing. While these methods have been exploited successfully, they impose a number of significant restrictions on channel geometry. A variety of advanced micromachining techniques have been developed since this early work. The current state of fabrication technology is reviewed, taxonomically organized, and found to offer many new possibilities for building microchannels. In particular anisotropic dry etching and other high aspect ratio techniques have removed many of the process-induced constraints on microchannel design. Other technologies such as surface micromachining, microstamping, hybridization, and system-on-chip integration will enable increasingly complex, highly functional heat transfer devices for the foreseeable future. It is also found that the formation of flow passages with hydraulic diameters below the microchannel regime will be readily possible with current fabrication techniques.     

Spatial Decision Support System for Watershed Management

A prototype spatial decision support system (SDSS) is presented for watershed management. The SDSS integrates landuse/landcover derived from the remote sensing data, real-time hydrological data, geographic information system, and a model-based subsystem for computing soil loss, land capability classification and engineering measures. A graphical user interface has been developed to allow effective use by decision makers. The model-based subsystem employs a process-based soil erosion model to compute soil loss in spatial environment. Computed pixel-based soil loss information is an input to the land capability classification and watershed management modules. The developed SDSS can help the end users in avoiding the laborious procedures of soil erosion calculations and analysing various thematic layers to get suitable watershed management practices. The SDSS for watershed management is applied to the Tones watershed in India to compute soil loss, to prioritise watersheds, and to suggest various watershed management practices.     

Thermal management issues in a PEMFC stack – A brief review of current status

Understanding the thermal effects is critical in optimizing the performance and durability of proton exchange membrane fuel cells (PEMFCs). A PEMFC produces a similar amount of waste heat to its electric power output and tolerates only a small deviation in temperature from its design point. The balance between the heat production and its removal determines the operating temperature of a PEMFC. These stringent thermal requirements present a significant heat transfer challenge. In this work, the fundamental heat transfer mechanisms at PEMFC component level (including polymer electrolyte, catalyst layers, gas diffusion media and bipolar plates) are briefly reviewed. The current status of PEMFC cooling technology is also reviewed and research needs are identified.     

Artificial neural networks for recognition of 3-d partial dischargepatterns

Partial discharge (PD) measurements have been carried out over the years to assess insulation systems in power apparatus for their integrity and design deficiencies. Digital PD recording, processing and its presentation as 3-d patterns are recent trends in both industry and testing laboratories. Interpretation of these patterns can lead to evaluation of the cause of PD. A need arises to look for methods in the domain of pattern recognition for automating this process. In this context, this paper presents results to demonstrate the possibility of using pattern recognition capabilities offered by a multilayer neural network to recognize 3-d PD patterns     

Fourier transform infrared-probed O(3P) microreactor: demonstration with ethylene reactions in argon matrix

To demonstrate the development of an oxygen atom microreactor in the form of liquid-helium-cooled solid argon matrix deposited on an infrared (IR) window, the oxidation of ethylene by mobile O atoms has been investigated. O atom diffusion through the argon matrix is confirmed and used to examine ethylene-oxygen atom reactions. In a bench-scale matrix isolation system probed with a Fourier transform infrared (FT-IR) spectrometer, matrices of solid Ar at 8-10 K doped with NO2 and ethylene have been prepared on a ZnSe window within an evacuated cryostat. The matrices have been photolyzed using 350-450 nm photons, and the reaction products resulting from the reaction of O(3P), one of the photolysis products of NO2, with ethylene have been identified using FT-IR and a Gaussian 98W simulation program. These products include oxirane, acetaldehyde, ethyl nitrite radical, and ketene. The temperature effect in the range of 10-30 K on the products formed has also been investigated. The reaction mechanisms are discussed and the viability of the solid Ar matrix being a low temperature microreactor to examine reaction mechanisms of mobile oxygen atoms is elaborated.     

Water-related matrix isolation phenomena during NO2 photolysis in argon matrix

Photolysis (350-450 nm) of NO(2) molecules trapped in argon matrices at 10 K has been studied using Fourier transform infrared (FTIR) spectroscopy to examine the mobility of the photolysis products, O((3)P) and NO, and their subsequent reactions. The formation of N(2)O(5) and N(2)O(3) from reactions of these mobile species with immobilized NO(2) and N(2)O(4) is confirmed. Water molecules from the background gases in the vacuum have been found to be isolated in the argon matrix during deposition of diluted NO(2) in Ar. The entrapped water molecules along with some of their NO(2) adducts have been characterized. Exposure of the matrix to photons to photolyze NO() resulted in not only internal matrix reactions, but also an enhanced deposition of ice over the surface of the argon matrix. This is caused by photodesorption of water molecules from the walls of the matrix isolation chamber and their subsequent condensation on the matrix surface. This ice overlayer has been found to give a very significant dangling OH band and a substantial librational band in the FT-IR spectra, indicating substantial surface area and internal porosity, respectively. The potential of using photodesorbed water to establish high surface area ice interfaces with dangling OH groups for heterogeneous photoreaction studies is discussed.     

Optimal Design of a Stable Trapezoidal Channel Section Using Hybrid Optimization Techniques

A cost effective channel section for a specified flow rate, roughness coefficients, longitudinal slope, and various cost parameters can be determined using an optimization technique. However, the derived optimal channel section may not be feasible for construction because of in situ conditions. The local soil conditions may not support the optimal side slope of the channel and if constructed, the slope may fail. It is therefore necessary to also incorporate the criteria for side slope stability in designing an optimal open channel section. In this paper, a new methodology has been developed to design a stable and optimal channel section using hybrid optimization techniques. A genetic algorithm based optimization model is developed initially to determine the factor of safety of a channel slope for given soil parameters. This optimization model is then externally linked with a separate sequential quadratic programming based optimization model to evaluate the parameters of the stable and optimal channel section. Solution for various example problems incorporating different soil parameters are illustrated to demonstrate the applicability of the developed methodology.