Convective heat transfer enhancement techniques of heat exchanger tubes: a review

Heat exchangers are widely used in almost all industrial activities. Turbulent promoters used in heat exchanger tubes are an effective way of enhancing the performance. This paper summarises various investigations using twisted tapes, wire coil ribs, baffles, and swirl flow generators. The main objective of this paper is to review various studies, in which different obstacle roughness elements are used to enhance the rate of heat transfer in the heat exchanger tubes. It has been found that a lot of experimental and analytical studies reported in the literature. On the basis of correlations developed by various investigators an attempt has been made to compare the thermal hydraulic performance of obstacles in heat exchanger tubes. In this work, a comparative study is also carried out to select the best obstacle roughness shapes for higher heat transfer rate and low pressure drop losses.     

A Novel In Situ Method for Producing a Dispersion of a Ceramic Phase into Copper That Remains Stable at 0.9T M

We apply an in situ approach, whereby a polymer is incorporated into copper and evolves within the metal into the ceramic phase, to create a dispersion of hard particles in a metal. All constituents for the ceramic phase are contained within the organic polymer. The temperature for this polymer to ceramic conversion lies in the 1073 K to 1273 K (800 °C to 1000 °C) range. The process produces a nanoscale dispersion of the ceramic, which leads to high microhardness that remains unaltered at temperatures up to 1223 K (950 °C) (0.9T _M). Apparently, the introduction of the ceramic phase leads to the retention of copper crystallite size of a few hundred nm, despite exposure to heat treatments at these very high temperatures. We call these materials polymer-derived metal-matrix composites.     

Effect of Microstructural Parameters on the Relative Densities of Metal Foams

A detailed quantitative microstructural analyses of primarily open cell FeCrAlY and 314 stainless steel metal foams with different relative densities and pores per inch (p.p.i.) were undertaken in the present investigation to determine the effect of microstructural parameters on the relative densities of metal foams. Several elements of the microstructure, such as major and minor cell sizes, cell areas and perimeters, ligament dimensions, cell shapes, and area fractions of closed and open cells, were measured. The cross-sections of the foam ligaments showed numerous pores, and their circularity factors and average sizes were determined. The area fractions of the open cells and ligaments decreased, whereas that of the closed cells increased linearly with increasing relative density. The relative densities and p.p.i. were not significantly dependent on cell size, cell perimeter, and ligament dimensions within the limits of experimental scatter. A phenomenological model is proposed to rationalize the present microstructural observations.     

Characterisation of solution annealed VT-14 titanium alloy using eddy current based electrical resistivity measurements

This paper discusses characterisation of microstructures in Ti-4.5Al-3Mo-1V (VT-14) alloy specimens solutionized in the temperature range of 923K–1373K, using room temperature electrical resistivity measurements. Using phase angle of impedance change in eddy current coil as a parameter, calibration curve between resistivity and phase angle has been established using three reference specimens of known resistivity. The changes in the electrical resistivity of the specimens have been correlated with optical microscopy investigations. The electrical resistivity of unstable β phase has been compared with α and α′ phases and possible reasons for higher resistivity of β phase and lower resistivity of α′ have been given based on the rule of mixtures and scattering and mobility of electrons. The electrical resistivity of α′ martensite phase of the chosen VT-14 alloy has been determined as 1.04 αΩ-m. The studies reveal that it may be possible to identify the β transus temperature of α+β titanium alloys from the electrical resistivity data.     

Nitrogen redistribution, microstructure, and elastic constant evaluation using ultrasonics in aged 316LN stainless steels

The presence of even dilute concentrations of nitrogen (0.08 mass pct) is found to have a strong influence on the microstructure of a nuclear grade 316LN austenitic stainless steel, thermally aged at 1123 K. High-resolution transmission electron microscopy (TEM) evidence of pre-precipitation reactions involving formation of Cr-N rich clusters prior to Cr₂N precipitation has been presented. A tendency for cellular precipitation has been observed on continued aging above 500 hours. Beyond 1000 hours, chi precipitates are the most frequently seen phase. The observed microstructural variations correlate well with ultrasonic velocity and attenuation measurements. The present study reveals that nitrogen in solid solution decreases all the elastic constants, namely longitudinal modulus, elastic modulus, shear modulus, and bulk modulus in this steel but has a negligible effect on Poisson’s ratio. The stages associated with the precipitation of intragranular coherent Cr₂N are, however, associated with an increase in all the elastic constants including Poisson’s ratio.     

Bibliography on Nullors and Their Applications in Circuit Analysis, Synthesis and Design

A bibliography on research work related to nullors and their applications in circuit analysis, synthesis and design, covering the period 1961–2000, is given.     

Improved design procedure for double‐ridged waveguides

This article presents a novel methodology for designing double‐ridged waveguides by numerically optimizing their geometric shape such that they sustain the two prescribed lowest order modes. The field solution to the problem is obtained by using the finite element method. The performance of the microgenetic algorithm and the quasi‐Newton methods is studied for carrying out geometry optimization. This generalized formulation is capable of handling inhomogeneous material fillings in the guide, and computational results are presented to demonstrate the versatility of the proposed technique. © 2002 Wiley Periodicals, Inc. Int J RF and Microwave CAE 12, 530–539, 2002. Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mmce.10053     

Modelling of the disulphide-swapped isomer of human insulin-like growth factor-1: implications for receptor binding

Insulin-like growth factor-1 (IGF-1) is a serum protein whichunexpectedly folds to yield two stable tertiary structures withdifferent disulphide connectivities; native IGF-1 [18–61,6–48,47–52]and IGF-1 swap [18–61,6–47, 48–52]. Here we demonstratein detail the biological properties of recombinant human nativeIGF-1 and IGF-1 swap secreted from Saccharomyces cerevisiae.IGF-1 swap had a ~30 fold loss in affinity for the IGF-1 receptoroverexpressed on BHK cells compared with native IGF-1.The parallelincrease in dose required to induce negative cooperativity togetherwith the parallel loss in mitogenicity in NIH 3T3 cells impliesthat disruption of the IGF-1 receptor binding interaction ratherthan restriction of a post-binding conformational change isresponsible for the reduction in biological activity of IGF-1swap. Interestingly, the affinity of IGF-1 swap for the insulinreceptor was ~200 fold lower than that of native IGF-1 indicatingthat the binding surface complementary to the insulin receptor(or the ability to attain it) is disturbed to a greater extentthan that to the IGF-1 receptor. A 1.0 ns high-temperature moleculardynamics study of the local energy landscape of IGF-1 swap resultedin uncoiling of the first A-region -helix and a rearrangementin the relative orientation of the A- and B-regions. The modelof IGF-1 swap is structurally homologous to the NMR structureof insulin swap and CD spectra consistent with the model arepresented. However, in the model of IGF-1 swap the C-regionhas filled the space where the first A-region -helix has uncoiledand this may be hindering interaction of Val44 with the secondinsulin receptor binding pocket.