Masud Behnia

Improving Photovoltaic Module Efficiency Using Water Cooling

An effective way of improving efficiency and reducing the rate of thermal degradation of a photovoltaic (PV) module is by reducing the operating temperature of its surface. This can be achieved by cooling the module and reducing the heat stored inside the PV cells during operation. In this paper, long-term performance modeling of a proposed solar-water pumping system is carried out. The system, which is used for irrigation purposes, consists of a PV module cooled by water, a submersible water pump, and a water storage tank. Cooling of the PV panel is achieved by introducing water trickling configuration on the upper surface of the panel. An experimental rig is developed to investigate and evaluate PV module performance with the proposed cooling technique. The experimental results indicated that due to the heat loss by convection between water and the PV panels upper surface, an increase of about 15% in system output is achieved at peak radiation conditions. Long-term performance of the system is estimated by integrating test results in a commercial transient simulation package using site radiation and ambient temperature data. The simulation results of the systems annual performance indicated that an increase of 5% in delivered energy from the PV module can be achieved during dry and warm seasons.

Intracoronary shear-related up-regulation of platelet P-selectin and platelet-monocyte aggregation despite the use of aspirin and clopidogrel

Recent in vitro studies have shown that shear stress can cause platelet activation by agonist-independent pathways. However, no studies have assessed the extent of shear-induced platelet activation within human coronary arteries. We sampled blood from the coronary arteries proximal and distal to coronary lesions and from the coronary sinus in humans with stable coronary disease who were taking both aspirin and clopidogrel. A novel, computationally based technique for estimating shear stress from 3-dimensional coronary angiographic images of these arteries was developed, and the effect of stenosis severity and calculated shear stress on in vivo platelet and related leukocyte activation pathways were determined. We provide evidence of intracoronary up-regulation of platelet P-selectin, platelet-monocyte aggregation, and monocyte CD11b without platelet glycoprotein IIb-IIIa activation or soluble P-selectin up-regulation. This correlates with intracoronary stenosis severity and calculated shear stress and occurs despite the concurrent use of aspirin and clopidogrel. Our results show for the first time shear-related platelet and monocyte activation in human coronary arteries and suggest this as a potential therapeutic target that is resistant to conventional antiplatelet agents.

Flow recirculation zone length and shear rate are differentially affected by stenosis severity in human coronary arteries

Flow recirculation zones and shear rate are associated with distinct pathogenic biological pathways relevant to thrombosis and atherogenesis. The interaction between stenosis severity and lesion eccentricity in determining the length of flow recirculation zones and peak shear rate in human coronary arteries in vivo is unclear. Computational fluid dynamic simulations were performed under resting and hyperemic conditions on computer-generated models and three-dimensional (3-D) reconstructions of coronary arteriograms of 25 patients. Boundary conditions for 3-D reconstructions simulations were obtained by direct measurements using a pressure-temperature sensor guidewire. In the computer-generated models, stenosis severity and lesion eccentricity were strongly associated with recirculation zone length and maximum shear rate. In the 3-D reconstructions, eccentricity increased recirculation zone length and shear rate when lesions of the same stenosis severity were compared. However, across the whole population of coronary lesions, eccentricity did not correlate with recirculation zone length or shear rate (P = not signficant for both), whereas stenosis severity correlated strongly with both parameters (r = 0.97, P < 0.001, and r = 0.96, P < 0.001, respectively). Nonlinear regression analyses demonstrated that the relationship between stenosis severity and peak shear was exponential, whereas the relationship between stenosis severity and recirculation zone length was sigmoidal, with an apparent threshold effect, demonstrating a steep increase in recirculation zone length between 40% and 60% diameter stenosis. Increasing stenosis severity and lesion eccentricity can both increase flow recirculation and shear rate in human coronary arteries. Flow recirculation is much more sensitive to mild changes in the severity of intermediate stenoses than is peak shear.

Using Postgraduate Students' Evaluations of Research Experience to Benchmark Departments and Faculties: Issues and Challenges.

BACKGROUND The introduction of the Australian Research Training Scheme has been a strong reason for assuring the quality of the research higher degree (RHD) experience; if students experience poor supervision, an unsupportive climate, and inadequate infrastructure, prior research suggests RHD students will be less likely to complete their degree, with negative consequences for the student, the university, and society at large. AIMS The present study examines the psychometric properties of a survey instrument, the Student Research Experience Questionnaire (SREQ), for measuring the RHD experience of currently enrolled students. The core scales of the SREQ focus on student experiences of Supervision; Infrastructure; Intellectual and Social Climate; and Generic Skills Development. SAMPLE Participants were 2,213 postgraduate research students of a large, research-intensive Australian university. RESULTS Preliminary factor analyses conducted at the student level supported the a priori four factors that the SREQ was designed to measure. However, multi-level analyses indicated that there was almost no differentiation between faculties or departments nested with faculties, suggesting that the SREQ responses are not appropriate for benchmarking faculties or departments. CONCLUSIONS Consistent with earlier research based on comparisons across universities, the SREQ is shown to be almost completely unreliable in terms of benchmarking faculties or departments within a university.

A novel approach to the design of complex heat transfer systems: portable computer design-a case study

The paper describes a novel approach to solving heat transfer problems in geometrically complex systems. In the present approach, instead of simulating the detailed complex geometry, templates of components and devices are assumed rather than working on actual components. By varying the template dimensions a systematic study on the effects of geometrical configurations on cooling airflow and heat transfer is made possible. The application of the approach is illustrated drawing examples from heat transfer analysis of portable computers. The purpose of the study is to develop a methodology whereby the packaging designer is freed from the task of performing detailed numerical analysis. Currently available numerical analysis tools such as computational fluid dynamics (CFD) codes are given a role in an undertaking to create databases from which fast design codes are developed.

Thermal Performance of an Air-Cooled Data Center With Raised-Floor and Non-Raised-Floor Configurations

In this paper, the thermal performances of an air-cooled data center with raised-floor and non-raised-floor configurations are compared with respect to the room and ceiling return strategies. The thermal performance of the data center is evaluated in terms of supply heat index, rack cooling index, total irreversible loss, and the number of racks with at least one server exceeding the maximum recommended and allowable inlet air temperature according to American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) thermal guidelines. The numerical simulations are conducted providing an insight into the flow and temperature distributions, and thus giving a better understanding of the cooling issues. It is found that using a ceiling return strategy for the return of hot exhaust air to the computer room air conditioning units gives a better thermal performance of the data center, for both raised- and non-raised-floor strategy, as compared to the room return. The findings are then extended to a geometrically complex operational data center to improve its cooling effectiveness. The paper also highlights the drawback of using supply heat index alone as a performance metric.

Foam Sclerosants are More Stable at Lower Temperatures

OBJECTIVES Sclerosant foams are aqueous and break down under the influence of gravity, pressure, and temperature. The aim of this study was to investigate the effects of temperature on foam stability. METHODS Sodium tetradecyl sulphate (STS) and polidocanol (POL) liquid and foam (1 + 4, liquid-plus-air fraction) were investigated in a range of concentrations (0.5%, 1.5%, 3.0%) and temperatures. Surface tension was measured by the Du Nuoy ring method. Liquid drainage from foam was measured and documented by serial photography. Both pre- and post-cooling variations were investigated. RESULTS Surface tension decreased at higher temperatures. Surface tension of POL was higher than STS at concentrations tested. POL foam half-time increased significantly at higher concentrations while the half-time of STS foam was not affected by concentration. Heating the sclerosant foam above the ambient temperature reduced its half-time while cooling below the ambient temperature prolonged the half-time. Both pre- and post-cooling of the foams resulted in significant prolongation of half-times when compared to no cooling. Maximum stability of the two sclerosant foams tested was achieved at 10 °C. CONCLUSIONS Foam sclerosants are more stable at cooler temperatures.

Modeling of Air to Air Enthalpy Heat Exchanger

The thermal performance of a Z-shaped enthalpy heat exchanger utilizing 45-gsm Kraft paper as the heat and moisture transfer surface for heating, ventilation, and air conditioning (HVAC) energy recovery is experimentally investigated through temperature and moisture content measurements. A mathematical model is developed and validated against the experimental results using the effectiveness-NTU method. In this model the paper moisture transfer resistance is determined by paper moisture permeability measurements. Results showed that the paper moisture transfer resistance is not constant and varies with moisture gradient across the paper. Furthermore, the model is used to predict the heat exchanger performance for different heat exchanger flow configurations. The results showed that higher effectiveness values are achieved when the heat exchanger flow path width is reduced. Temperature and moisture distribution in the heat exchanger is also studied using a computational fluid dynamics package (FLUENT). To model the moisture transfer through the porous materials a nondimensional sensible–latent effectiveness ratio was developed to obtain the moisture boundary conditions on the heat exchanger surface.

Heat transfer from micro-finned surfaces to flow of fluorinert coolant in reduced-size channels

Experiments were conducted in order to investigate the forced convective heat transfer from a micro-finned surface to a fluorocarbon liquid FX3250. The heat transfer surface is made from copper, has a base area 2/spl times/2 cm/sup 2/, and is equipped with 20 longitudinal fins, each fin being 0.5 mm high and 0.5 mm wide. Particular attention was directed to the effect of the channel height (H) on the heat transfer performance, changing H from 1 to 3 mm. The Reynolds number based on the hydraulic diameter covered a range from 1000 to 12000. Also conducted was an experiment using a flat heat transfer surface. It was found that the sensitivity of heat transfer performance to the change of H depends on the type of heat transfer surface. Comparisons are presented in terms of the heat transfer coefficient based on the actual heat transfer area. In order to interpret the experimental data the numerical simulation of flow and heat transfer was performed. The results render support to the heuristic assumption that the increased flow velocity in the free flow area above the fins in higher-height channels yield high heat transfer coefficients on the fin tips, offsetting the degradation of heat transfer in the grooves.

Sclerosant foam structure and stability is strongly influenced by liquid air fraction

OBJECTIVES To determine the effects of sclerosant foam preparation and composition on foam structure, the time course of liquid drainage, and foam coarsening. METHODS Sodium tetradecyl sulphate (STS) and polidocanol (POL) foams were investigated in a range of concentrations (0.5-3%) and liquid-plus-air fractions (LAF; 1 + 2 to 1 + 8). Foam was injected into a vein simulation model (polyvinyl chloride tubing, inner diameter 3 mm, constant pressure 5-7 mmHg) filled with saline or blood. Liquid drainage, bubble count, and diameter were measured and documented by serial photography. RESULTS Liquid drainage was faster in the vertical position than the horizontal one. In all variations, very small bubbles (diameter <30 μm) were generated initially that coarsened to form micro-foams (<250 μm). By 3 minutes mini-foams (>250 μm) and by 7.5 minutes macro-foams (>500 μm) were formed. Following injection, the upper regions of foam coarsened faster as liquid drained to the bottom of the vessel. Wet preparations produced significantly smaller bubbles. Low concentration POL foam produced significantly higher bubble counts and coarsened slower than STS. CONCLUSIONS Foam structure is strongly influenced by the LAF. Despite the initial formation of micro-bubbles in the syringe, mini- and macro-bubbles are formed in target vessels with time post-injection.