Thermal and hydraulic performance of a modified two-stage evaporative cooler

The thermal and hydraulic performance of a modified two-stage evaporative cooler is evaluated. Variables considered are the mode of operation, packing thickness, mass flow rate of the water flowing to the precooler, and the mass flux of water flowing over the packing media. The effectiveness of the system increased with the increase of the mass flow rate of water flowing to the precooler, decreasing the mass flux of water flowing to the packing, and with the increase of the packing thickness. The effectiveness of the system with structured packing was higher than that with sheathy leaf base or natural fiber packing.The air-side pressure drop per unit length in the direction of air flow was nearly constant when the structured packing was used. For the sheathy leaf and natural fiber packings, the air pressure drop increased at a uniform rate as the mass flux of water flowing over the packing increased. The air pressure drop was lowest for the setup with the structured packing.     

Transient model of induction machine using rotating magnetic field approach

Most simulation models of electric machines use the coupled circuit approach, where the machine is considered as an electric circuit element with time-varying inductances (abc model) or with constant inductances (dq0 model). On the other hand, the rotating magnetic field approach, which considers the electric machine as two groups of windings producing rotating magnetic fields and can give insight into internal phenomena of the machines, has not yet received much attention in electric machines modeling, especially for machine transient analysis. Based on the rotating magnetic field approach, this paper presents a transient model of the induction machine including main flux saturation effect. Based on the direct computation of the magnetizing fluxes of all machine windings, the model represents instantaneous main flux saturation by simply introducing a main flux saturation factor. No iteration process is involved to incorporate the saturation effects. The model combines the advantages of the dq0 and abc models advantages, such as rapid computation time and nonsymmetrical conditions simulation, respectively. The simulation results and the experimental tests show advantages and verification of the model.     

Linear and Nonlinear Control Techniques for a Three-Phase Three-Level NPC Boost Rectifier

This paper deals with three control techniques for a three-phase three-level neutral-point-clamped (NPC) boost rectifier to study their relative performance. Linear, nonlinear, and nonlinear model reference adaptive control (MRAC) methods are developed to control power factor (PF) and regulate output and neutral point voltages. These controllers are designed in Simulink and implemented in real time using the DS1104 DSP of dSPACE for validation on a 1.2-kW prototype of an NPC boost rectifier operating at 1.92 kHz. The performance of boost converter with three control methods has been investigated respectively in steady state in terms of line-current harmonic distortion, efficiency, and PF and during transients such as load steps, utility disturbances, reactive power control, and dc-bus voltage tracking behavior. The linear PI controllers are characterized by reduced complexity but poor performance, whereas the nonlinear control technique has improved the converter performance significantly, while nonlinear MRAC exhibits much better performance in a wide operating range     

A new control approach to three-phase active filter for harmonicsand reactive power compensation

This paper deals with a new control scheme for a parallel 3-phase active filter to eliminate harmonics and to compensate the reactive power of the nonlinear loads. A 3-phase voltage source inverter bridge with a DC bus capacitor is used as an active filter (AF). A hysteresis based carrierless PWM current control is employed to derive the switching signals to the AF. Source reference currents are derived using load currents, DC bus voltage and source voltage. The command currents of the AF are derived using source reference and load currents. A 3-phase diode rectifier with capacitive loading is employed as the nonlinear load. The AF is found effective to meet IEEE-519 standard recommendations on the harmonics level     

Equilibrium Gaseous Adsorption at Different Temperatures

The effect of temperature on equilibrium adsorption for pure gases has been investigated. A simple generalized equation already proposed by Khan and colleagues was modified to include temperature dependence for the equilibrium adsorption of pure gas. Eq. (5) was compared with similar published equations that included temperature effects. A statistical test was conducted by comparing the overall average percentage error and the correlation coefficient for the proposed equation with different reported expressions for the experimental gaseous adsorption published data. It was found that the proposed equation was the most accurate and simple to use.     

A new design tool to protect industrial long-cable PWM ASD systems against high-frequency overvoltage problems

This work presents a new tool for use in the design of long-cable adjustable-speed drive (ASD) systems. It allows coordination between the inverter, the motor and the cable type and length in order to protect the motor against overvoltages due to the reflected wave. Detailed mathematical formulas describing the transient voltage and current along the cable are first developed in the frequency domain. Then, by applying the boundary conditions at the cable ends and assuming a lossless cable, delay equations are developed in the time domain. Using these equations, an appropriate simulation scheme is built with the SimPowerSystems (SPS) and Simulink modules of the MATLAB software environment to compute the high-frequency overvoltages (and their associated currents) at the ends of the feeding cable. The proposed tool has been applied to a 5 kVA industrial ASD prototype using a four-wire shielded long cable. The paper includes simulation results and experimental validations.     

Survival of salmonellae in bio-yoghurt

Many yoghurt-like products are sold with pH values of 4.5–4.6, and a potential pathogen, Salmonella infantis , was shown to survive for 10 days in typical commercial samples of 'bio-yoghurt'. Another servovar, Salmonella hadar , could not be detected after 48 h in similar products. The need to avoid contamination during the manufacture of fermented milks with pH values above those traditionally associated with yoghurt (~4.0) is stressed.     

Knowledge-based parametric tools for concrete masonry walls: Conceptual design and preliminary structural analysis

The paper explores the potential uses of parametric modeling to embed construction and structural design knowledge in the form of generative rules and feedback rule-checking functions. The goal of the research reported here is to relate knowledge regarding constructive and structural principles during conceptual design in order to improve early decision-making. For that purpose we have developed a series of functions to guide conceptual design exploration by providing timely evaluation of design alternatives. The research focuses on load-bearing concrete masonry walls, and on basic requirements for doubly curved walls as a design case study. The research extends the Building Object Behavior methodology developed by Lee and others to elucidate, translate and implement design expertise into parametric rules and behaviors. The paper introduces the methodology in the context of a prototype modeling tool for early-stage design of concrete masonry walls and discusses the implications of a parametric modeling approach for conceptual design and collaboration.