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Aims and Scope
Editorial Board

1. A numerical investigation of flow profile and performance of a low cost Crossflow turbine

Chiyembekezo S. Kaunda¹, Cuthbert Z. Kimambo², Torbjorn K. Nielsen¹

1 Waterpower Laboratory, Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway.

2 Department of Mechanical and Industrial Engineering, University of Dar es Salaam, P.O. Box 35091, Dar es Salaam, Tanzania.

Abstract: Low efficiency is the main drawback of a Crossflow turbine, despite the turbine being an important low cost technology for micro hydropower generation. Poor flow profile has been mentioned by other Crossflow turbine performance investigators as one of the reasons for the underperformance. This paper has investigated, using numerical method, the flow profile in the turbine at best efficiency point and at operating conditions that are away from best efficiency point. Numerical method has also been used to calculate and predict the efficiency of the turbine. The flow physics in a Crossflow turbine runner is a two-phase with a movable free surface. Such flow physics is difficult to analyse even numerically. A procedure for numerical analysis was followed and ANSYS CFX® was used to solve the governing equations and to process the simulation results. Actual pictures of the flow were taken so as to compare the actual flow with the numerically determined flow profile. Turbine efficiency results from the previous performance evaluation experiment conducted on the model Crossflow turbine were compared with the numerically obtained efficiency results. It has been observed that the numerically obtained flow profile compare favorably with the actual flow pictures. The numerical analysis over-predicts the efficiency, especially for runner speeds that were more than the best efficiency point speed. Pockets of negative pressures and flow circulation have been observed in the flow profile. At constant head and valve opening, the velocity profile was found to vary more with runner speed than the pressure profile. The numerically obtained flow pattern showed positions where the flow gives maximum efficiency. Therefore, the study has shown that numerical method is a superior design tool for Crossflow turbines.

Volume 5, Issue 3, 2014, pp.275-296.

2. Development of a thermal resistance model to evaluate wellbore heat exchange efficiency

Albert A. Koenig¹, Martin F. Helmke²

1 ARB Geowell, 100 Four Falls Corporate Center Suite 215, West Conshohocken, PA 19428, USA.

2 West Chester University, Department of Geology and Astronomy, 207 Merion Science Center, West Chester, PA, 19383, USA.

Abstract: A new model is proposed to simulate conduction of heat between a pipe loop in a geoexchange system and the ground. The approach employs the thermal resistor technique coupled with a conduction shape factor modified by an occultation factor. The model is compared to available data and demonstrates suitable agreement with previous studies. The model facilitates a parametric study of borehole resistance as a function of geometry and thermal conductivity of the components. By spacing the legs of the loop against the borehole and increasing the pipe size, the study shows that one can maximize the wellbore heat transfer using a moderate (1.2 W/mK) thermal conductivity grout. This study further demonstrates that improved well construction techniques could increase the efficiency of most closed-loop geothermal systems by 10 percent.

3. Using downscaled NCEP/NCAR reanalysis data for wind resource mapping

Arnau Bayon-Barrachina¹, Jeroen van Beeck², Fernando Martinez-Alzamora¹, Petra Amparo Lopez-Jimenez¹

1 Universitat Politècnica de València, Camí de Vera, s/n, 46022 - València, Spain.

2 Von Karman Institute for Fluid Dynamics, Ch. de Waterloo, 74, 1640, Rhode-Saint-Genese, Belgium.

Abstract: Two problems arise when estimating numerically wind energy resources, namely: The low resolution of global-scale atmospheric models and the scarcity of meteorological observations to be used as boundary conditions for smaller-scale models. Downscaling techniques were developed to overcome these issues. These methods consist of using global-scale model output as boundary conditions of smaller-scale models. In this research a downscaling tool to refine NCEP/NCAR Reanalysis data using WAsP is presented. The downscaling technique proposed consists of extrapolating the wind climate at a given point using meteorological observations from another point by means of the WAsP model. Then, using time-marching NCEP/NCAR Reanalysis wind velocity values, on-line refined profiles of wind velocity and direction can be obtained. In order to assess the accuracy of the described tool, data of two episodes of 48 hours are downscaled and compared to meteorological observations at two different climates. A sensitivity analysis is performed in order to assess the effect that atmospheric stability and terrain roughness, among others, exert on the results. Results are not as accurate as expected, probably due to atmospheric instability or other factors neglected by the model. However, the main trends are followed when validating the model output using field measurements.

4. Experimental study on the heat transfer enhancement by Dean Vortices in spiral tubes

Hui Zhu¹, Hanqing Wang², Guangxiao Kou²

1 School of Energy Science and Engineering, Central South University,NO.932, South Lushan Road, Changsha, Hunan Province, 410083, China.

2 School of Civil Engineering, Hunan University of Technology, West Taishan Road, Zhuzhou, Hunan Province, 412007, China.

Abstract: Spiral tubes have been reported to be able to improve the heat transfer performance of the fluid flowing through, which is traditionally believed to be caused by the helix structure. In this literature, however, the mechanism of the heat transfer enhancement of spiral tubes was studied experimentally in terms of the control factors of Dean vortices such as Reynolds number, fluid viscosity and curvature ratio. Meanwhile, the torsion of the spiral tube was also taken into consideration. Experiments were carried out under constant wall temperature condition with one straight copper tube and 7 kinds of spiral copper tubes. The results showed that the Nusselt number of the flow in spiral tubes increases with Reynolds number, viscosity and curvature ratio, but changes anomalously with the torsion. And it also clearly revealed that the pressure drop for spiral tubes increases with Reynolds number, curvature ratio and viscosity, while decreases with the torsion. In addition, the heat transfer performance of spiral tubes is apparently better than that of the straight tube. Based on the experiment results, the effect of Dean number on Nusselt number and pressure drop of the spiral tubes was determined, and the correlation equations of heat transfer enhancement by Dean vortices were obtained through multiple regression.

5. Assessment of thermal comfort in the mosque in Sarawak, Malaysia

S.H. Ibrahim, A. Baharun, Nawi M.N.M., Junaidi E.

Department of Civil Engineering, Faculty of Engineering, University Malaysia Sarawak (UNIMAS), 94300 Kota Samarahan, Sarawak, Malaysia.

Abstract: In hot-humid climate, the mosque should be provided with an acceptable level of thermal comfort in order to seek serenity and focus for worship. The purpose of this study is to investigate the thermal comfort conditions in the Masjid Al-Muttaqin located in Kota Samarahan, Sarawak. The data were analysed using Corrected Effective Temperature (CET) index. The analysis shows that although the air velocity in the mosque is acceptable, due to the influence of high air temperature, thermal comfort is not achieved. A retrofit design by adding new materials and installing insulations on the existing roof are proposed and the results show significant improvement of thermal comfort inside the mosque.

6. Optimization of operating parameters to maximize the current density without flooding at the cathode membrane interface of a PEM fuel cell using Taguchi method and genetic algorithm

S.S.L. Rao, A. Shaija, S. Jayaraj

Department of Mechanical Engineering, National Institute of Technology, Calicut-673601, India.

Abstract: A mathematical model was developed to investigate water accumulation at the cathode membrane interface by varying different operating parameters like fuel cell operating temperature and pressure, cathode and anode humidification temperatures and cathode stoichiometry. Taguchi optimization methodology is then combined with this model to determine the optimal combination of the operating parameters to maximize current density without flooding. Results of analysis of variance (ANOVA) show that fuel cell operating temperature and cathode humidification temperature are the two most significant parameters in the ratio of 56.07% and 27.89% respectively and also that higher fuel cell temperature and lower cathode humidification temperature are favourable to get the maximum current draw without flooding at the cathode membrane interface. The global optimum value of the operating parameters to maximize the current density without flooding was obtained by formulating as an optimization problem using genetic algorithm (GA). These results were compared with the results obtained using Taguchi method and it was found to be similar and slightly better.

7. Numerical computation of solar heat storage in phase change material/concrete wall

Mustapha Faraji

Physics Department, LPMMAT Laboratory, Faculty of Sciences Ain Chock, Hassan II University, PO 5366- Maarif, Casablanca- Morocco.

Abstract: A one-dimensional mathematical model was developed in order to analyze and optimize the latent heat storage wall. Two layers of phase change material (PCM) are sandwiched within a concrete wall. The governing equations for energy transport were developed by using the enthalpy method and discretized with volume control scheme. A series of numerical investigations were conducted. The effect of the melting temperature on the possibility of increasing the energy performance of the proposed heating system was analyzed. Results are obtained for thermal gain and temperature fluctuation. The charging/discharging process was also presented and analyzed.

8. Effect of the number and area of channels on the performance of rectangular, triangular and elliptical channel geometry PEM fuel cells

I. Khazaee

Department of Mechanical Engineering, Torbat-e-jam branch, Islamic Azad University, Torbat-e-jam, Iran.

Abstract: The performance of a PEM fuel cell is an important parameter that can be expressed with the polarization curve of it. In this study a complete three-dimensional and single phase model is used to investigate the effect of increasing the number of serpentine channels in the bipolar plates and also increasing the area (depth) of channels of a PEM fuel cell with rectangular, triangular and elliptical cross-section geometry. The proposed model is a full cell model, which includes all the parts of the PEM fuel cell, flow channels, gas diffusion electrodes, catalyst layers and the membrane. The results show that there are good agreement with the numerical results and experimental results of the previous work of authors. Also the results show that by increasing the number of channels from one to four and eight, the performance improved about 18% and by decreasing the area of channels from 2 sq. mm to 1 sq. mm the performance improved about 13%

9. Improvement the performance of a proton exchange membrane fuel cell by changing the channel geometry

I. Khazaee

Faculty of Mechanical and Energy Engineering, Shahis Beheshti University, A.C., Tehran, Iran.

Abstract: In this study the effect of placing different blocks on the performance of a proton exchange membrane (PEM) fuel cell are investigated numerically for different Aspect Ratios. A complete two-dimensional and single phase model is used to that the proposed model is a full cell model, which includes all the parts of the PEM fuel cell, flow channels, gas diffusion electrodes, catalyst layers and the membrane. Coupled transport and electrochemical kinetics equations are solved in a single domain; therefore no interfacial boundary condition is required at the internal boundaries between cell components. The results show that the predicted polarization curves by using this model are in good agreement with the experimental results. Also the results show that the transverse installation of a rectangular and triangle block in the fuel flow channel can effectively enhance the local cell performance of a PEMFC. The results show that by increasing the aspect ratio of the blocks, the performance of the cell enhances due to enhance the electrochemical reaction at the catalyst layer of the cell.

10. Fabrication and characteristics of 8YSZ/Ni functionally graded materials by applying spark plasma sintering procedure

M. S. EL-Wazery

Department of Production Engineering and Mechanical Design, Faculty of Engineering, Menoufiya University, Shebin El-Kom, Egypt.

Abstract: Functionally graded materials (FGM) in the form of layered structure consisting of yttria stabilized zirconia (YSZ) and nickel were fabricated by spark plasma sintering procedure. The relative density, linear shrinkage and Vickers hardness of each layer of graded materials were measured. The microstructure and the composition of these components were studied. The results obtained show that functionally graded materials produced by spark plasma sintering exhibited a low porosity level and consequently fully dense specimens. Also, the results demonstrate that the composition and microstructure of YSZ/Ni FGM have the expected gradient distribution. There are no macroscopic distinct interfaces in YSZ/Ni FGM due to the gradient change in components. This good continuity of microstructure can eliminate the disadvantage of traditional macroscopic interface in YSZ/Ni joint, and reflect the design idea of FGM. Vicker’s hardness of YSZ/Ni is lower than that of pure zirconia (YSZ) and increases by increasing the relative density of the layer of YSZ/Ni.

11. Comparative analyses of closed-landfill Methane (CH₄) and Carbon dioxide (CO₂) concentrations

Nwachukwu Arthur Nwachukwu

Williamson Research Centre for Molecular Environmental Sciences, School of Earth, Atmospheric and Environmental Science, University of Manchester, M13 9PL, UK.

Abstract: The time series data obtained from in-borehole measurement of CH₄ and CO₂ from a landfill site in Manchester, UK are given. Analysis reveals that they were variable for the period under investigation. There is a significant positive correlation between ground CH₄/CO₂ concentrations and their monitoring time. During this period, CH₄ concentration has increased from 0.5% to 62.7%. Similarly, CO₂ concentration has increased from 0.6% to 35.5%. Both gases have positive correlation coefficients of 0.5671 and 0.6653 respectively with time horizon of June – September 2011. Also, the two gases exhibit positive correlation coefficient of 0.9205 with each other; indicating that emission of CH₄ creates potential for emission of CO₂ and vice versa.