Application of metal foams in air-cooled condensers for geothermal power plants: An optimization study

Optimized design of metal foam heat exchangers, as replacements for finned-tubes in air-cooled condensers of a geothermal power plant, is presented here. Two different optimization techniques, based on first and second law (of thermodynamics) are reported. While the former aims at the highest heat transfer rate with as low pressure drop as possible, the latter minimizes the generated entropy in the thermodynamic system. Interestingly, the two methods lead to the same optimal design. The new design has been compared to the conventional air-cooled condenser designed and optimized by using the commercially available software ASPEN. It is shown that while the heat transfer rate increases significantly (by an order of magnitude) compared to the finned-tube for the same main flow obstruction height, the pressure drop increase is within an acceptable range. Further comparison between the two systems are carried out, making use of Mahjoob and Vafai's performance factor developed specifically for metal foam heat exchangers.     

On the use of body biasing to improve linearity in low LO-power CMOS active mixers

In a radio-frequency (RF) transceiver, the linearity of the mixer has a profound effect on the overall transceiver performance. In many RF transceivers, active mixers are used due to their higher gain which also improves the overall receiver noise figure. In a typical RF active mixer where the transistors in the LO stage switch abruptly, most of the nonlinear distortions come from the transconductance or RF stage and thus the linearity of the mixer can be enhanced by proper design of the RF stage. In low-power receivers, however, to reduce the power consumption of the local oscillator (LO) circuit, the amplitude of LO signal is low and thus the switching of the transistors in the LO stage of the mixer is gradual. In this paper, we propose a technique to improve the linearity of such low-power mixers by enhancing the linearity of the LO stage. In particular, body biasing is utilized in the LO stage to improve the linearity. To verify the effectiveness of the proposed technique, two proof-of-concept double-balanced down-conversion active mixers have been designed and fabricated in 0.13-µm CMOS. The maximum IIP3 of +2.7 dBm and −4.9 dBm at a conversion gain of 13 dB and 16 dB are achieved for the first and second prototype respectively. For a 2.4 GHz RF input signal and an intermediate-Frequency (IF) of 50 MHz, the first prototype consumes 2.4 mW from a 1.2 V supply while the second one consumes only 780 µW from a 0.7 V supply.     

Towards a Deleuzoguattarian methodology for urban design

The philosophy of Gilles Deleuze and Felix Guattari (Deleuzoguattarian) is now widely thought to be apropos for today’s world. This paper proposes Deleuzoguattarian philosophy as a new methodology for urban design research and practice. First, existing methodologies in the field of urban design are examined and their strengths and limitations outlined in relation to current urban problems. A reading of Deleuzoguattarian philosophy from an urban design perspective is then provided in order to propose a new methodology for research and practice. This reading mainly concentrates on the ontology and epistemology offered by the philosophy, aspects that have been neglected in the literature. The concept of territorialization as a complementary concept to assemblage theory is highlighted in order to illustrate the characteristics of the ontology and epistemology. Finally, normative goals are discussed. It is concluded that Deleuzoguattarian philosophy is not only an ethical response to modernism but is an epistemological need.     

A Novel Self-aligned and Maskless Process for Formation of Highly Uniform Arrays of Nanoholes and Nanopillars

Fabrication of a large area of periodic structures with deep sub-wavelength features is required in many applications such as solar cells, photonic crystals, and artificial kidneys. We present a low-cost and high-throughput process for realization of 2D arrays of deep sub-wavelength features using a self-assembled monolayer of hexagonally close packed (HCP) silica and polystyrene microspheres. This method utilizes the microspheres as super-lenses to fabricate nanohole and pillar arrays over large areas on conventional positive and negative photoresist, and with a high aspect ratio. The period and diameter of the holes and pillars formed with this technique can be controlled precisely and independently. We demonstrate that the method can produce HCP arrays of hole of sub-250 nm size using a conventional photolithography system with a broadband UV source centered at 400 nm. We also present our 3D FDTD modeling, which shows a good agreement with the experimental results.     

Lattice Boltzmann Simulation of Conjugate Heat Transfer from Multiple Heated Obstacles Mounted in a Walled Parallel Plate Channel

In the present study, lattice Boltzmann simulation of conjugate heat transfer from multiple heated obstacles mounted in a walled parallel-plate channel is carried out. The aim is to investigate the effects of the pertinent thermophysical and geometrical parameters on the local Nusselt number around the obstacle's periphery. The result showed that increasing the obstacle to fluid thermal conductivity ratio and decreasing wall-to-obstacle thermal conductivity ratio results in increasing the local Nusselt number around the obstacle's periphery. The results of the present study are compared with those available from the conventional numerical methods, and good agreement is observed.     

TBM performance prediction developing a hybrid ANFIS-PNN predictive model optimized by imperialism competitive algorithm

Neural Computing and Applications - A reliable and accurate prediction of the tunnel boring machine (TBM) performance can assist in minimizing the relevant risks of high capital costs and in...     

A miniature cell for gas solubility measurements in oils and bitumen

A miniature cell has been designed and constructed to measure gas solubility in crude oils and bitumen. The cell was made of stainless steel with a total internal volume of 1.835 cc and only an oil sample of 0.4 cc was required for one set of measurements at different pressures. By using this small cell, the waiting time for reaching equilibrium was less than 10 min. The technique was validated by measuring CO(2) gas solubility in two bitumen samples. The results were compared and found to be in very good agreement with available data. The apparatus was also used to study the effect of ashphaltene on CO(2) solubility in bitumen. It was shown that ashphaltene had a negligible effect on CO(2) solubility in bitumen.