An experimental study on local interfacial area concentration using a double-sensor probe

Interfacial area concentration is an important parameter in modeling the interfacial transfer terms in the two-fluid model. In this paper, the interfacial area concentration, void fraction, and bubble Sauter mean diameter for air-water bubbly flow through a vertical transparent pipe with 40 mm internal diameter was investigated experimentally using both digital high-speed camera system and a double-sensor conductivity probe. Based on the experimental data of digital high-speed camera system, the statistical models derived by different researchers for local interfacial area concentration measurement using double-sensor conductivity probe were evaluated. The results show that there are obvious differences among the values of local interfacial area concentration calculated by different statistical models even from the same probe signals. The section-averaged values of the local interfacial area concentration calculated using the statistical model by Kataoka et al. agree best with experimental data of digital high-speed camera system. Therefore, the statistical model developed by Kataoka et al. is recommended for the local measurement of interfacial area concentration using a double-sensor conductivity probe in bubbly two-phase flow. Using the verified double-sensor probe method, we carry out experiment to study the local distribution characteristic of the interfacial area concentration and void fraction in air-water bubbly flow through a vertical pipe.     

Mass transfer in the case of interfacial turbulence induced by the marangoni effect

A discussion concerning the mechanism of mass transfer is made by means of a physical model. In this model one considers that owing to interfacial turbulence a roll-cell structure is organized near the interface. The elements of liquid have a translatory motion with the average local velocity and a circulatory one in the roll-cell. Two limiting cases are discussed: (1) the elements of liquid keep their individuality without any change during the motion along the inside paths of the roll-cell; (2) the elements of liquid are completely refreshed during their inside motion. In the first limiting case the rate of mass transfer is increased as compared to that predicted by the penetration theory, since the time interval which an element of liquid spends at the interface is reduced. The second limiting case is similar to the renewal model. It is shown that these limiting cases have physical existence. By using a simulation procedure one concludes that the thickness of the roll-cell is of the same order of magnitude as its length. It is stressed that the treatment may be extended to wave motion and turbulent motion.     

An analytical study of a hybrid wind-passive solar system

This paper presents the results of an analytical performance and economic evaluation of a combined passive solar and wind powered residential heating and electrical energy system. Simulated in a New England wind and weather environment, the modeled system is based on the coupling of a vertical axis wind turbine with a super-insulated passive solar house. The analytical model is composed of four major sections: (1) residential heating and energy load; (2) wind turbine generator system; (3) energy system performance model; and (4) life-cycle costing economic analysis. Results for the heating, electrical supply and economic performance of the system are presented, varying such key parameters as residence window area, insulation, storage size, wind turbine size, and site average wind speed. These results show that when sited in adequate wind regimes (average wind speed greater than 5.4 m/sec (12 mph)) and coupled with super-insulated passive structures, wind turbines can provide significant fractions of the electrical and heating requirements in the New England environment.     

An experimental study of the sensitivity to freestream turbulence of heat transfer in wakes of cylinders in crossflow

An experimental study of the effect of mainstream turbulence on heat transfer from a cylinder is presented. The cylinder is electrically heated and the effect of turbulence on the pressure distribution and local heat-transfer coefficient round the cylinder is reported; the latter data are compared with other published information.     

An experimental study of a solar-regenerated liquid desiccant ventilation pre-conditioning system

A solar-regenerated liquid desiccant ventilation pre-conditioning system has been installed and experiments were carried out for a period of nine months covering rainy, cold, and hot seasons in a hot and humid climate (Thailand). A heat exchanger was used to cool the dehumidified air instead of typical evaporative cooling to maintain the dryness of the air. The use of solar energy at the regeneration process and cooling water from a cooling tower makes the system more passive. The evaporation rate at the regeneration process was always greater than the moisture removal rate at the dehumidification process indicating that the concentration of the desiccant in the system would not decrease and so the performance would not drop during continuous operation. The system could reduce the temperature of the delivered air by about 1.2 °C while the humidity ratio was reduced by 0.0042 kg_w/kg_da equivalent to 11.1% relative humidity reduction. The experimental results were also compared with models in literature.     

An experimental study of natural convection in a differentially heated cavity through a 2D-PIV system

This paper provides an experimental analysis of the natural convection in a differentially heated H side square enclosure. The cavity is full of air and is heated by a hot strip with a height of H/2. The effect of the position of this source on the dynamic structures generated by the natural convection heat transfer was analyzed at the steady state and under laminar conditions. The experimental apparatus is a 2D-PIV system while the experimental data consist of vector maps, velocity maps and streamlines at different Rayleigh numbers. During the study the presence of two small vortexes was noted on the upper surface of the source. These are dependent both on the Rayleigh numbers and on the position of the strip.     

新型太阳能膜蒸馏聚光系统光学性能模拟研究

利用太阳能加热膜蒸馏淡化苦咸水是一项水处理技术。通过对完整型抛物面聚光系统进行改造,建立了完整型抛物面、非完整型抛物面、同轴非完整型平移抛物面3种聚光系统的三维模型。利用蒙特卡洛光线追踪法模拟了经聚光器聚光在平板接收器表面的辐射强度分布。与其它两种聚光系统比较,同轴非完整型平移聚光系统的辐射强度峰值最小、均值相差不大,且有两条光斑,光学效率达到85.65%。针对同轴非完整型平移聚光系统,在焦距不变的条件下,模拟环数和边缘角对接收器表面辐射强度分布的影响,从而得出辐射强度随环数和边缘角的变化规律。     

An experimental study of air-solid two-phase flow in a 90° bend using LDV system

The measurements of the mean streamwise and radial velocities, the associated turbulence and the relative particle densities were made in an air-solid two-phase flow in a square sectioned (30mm×30mm) 90° vertical to horizontal bend using laser Doppler velocimetry. The radius ratio of the bend was 2.0. Glass beads of 100μm in diameter were employed to form the solid phase. The measurements of air and solid phases were performed separately at the same bulk velocity 19.34m/s, corresponding to a Reynolds number of 3.87×10⁴. The mass ratio of solid to air was 1.6%. The results indicate that the particle trajectories are very close to straight lines. The streamwise velocity profiles for the gas and the solids cross over near the outer wall with the solids having the higher speed. At θ=30° and 45°, particle-wall collisions happen mostly in the region from θ=30° to θ=75°, and cause a sudden change in solid velocity. The particles tend to move towards the outer wall in 90° bend. The particle concentration near the outer wall is much higher than that near the inner wall in the bend, and there are few particles in the inside of the bend. The bend leads to apparent phase separation: at θ=45°, the solids concentrate in the half of the duct near the outer wall. After θ=60° the second peak concentration appears, and goes gradually towards the inner wall.     

An experimental and modelling study of a photovoltaic/proton-exchange membrane electrolyser system

An experimental model of a photovoltaic (PV) module-proton exchange membrane (PEM) electrolyser system has been built. A model has been developed for each device separately based on the experimental results. Output current–voltage (I–V) characteristics of the PV module are modelled in respect to different irradiance and temperature conditions by experimental tests. Similarly, input I–V characteristic and hydrogen formation characteristic of the PEM electrolyser are measured and modelled. After these studies, combined PV module–PEM electrolyser system model is defined. There is a good agreement between model predictions and measurements. At 18–100% irradiance interval, operating points of PEM electrolyser on the PV module are predicted with relative errors of 0.1–0.8%. Furthermore, the study shows that these simple model system devices can easily be defined in MATLAB/Simulink and used to model similar systems of different size.     

A molecular dynamics study of interfacial thermal transport in heterogeneous systems

Non-equilibrium molecular dynamics software was developed to study thermal transport at the nanoscale. Lennard–Jones parameters of pure argon (mass and bond strength) were systematically modified to create heterogeneous thin film systems, including layered systems and nanocomposites, to investigate the influence of interfaces on thermal conductivity. Results were analyzed using combinations of kinetic theory and a thermal resistance network model together with the acoustic mismatch model (AMM). The introduction of a second material into an argon film generally decreased its overall thermal conductivity. Moreover, the presence of a nanoparticle was less influential in reducing thermal conductivity than the addition of a thin layer.     

An optical method of measuring the temperature in a fluidised bed combustor

The paper analyses the dynamic aspects of the temperature field in a fluidised bed of solids particles (e.g., sand) in which a gaseous fuel is being burned. Such a hot bed emits electromagnetic radiation within the visible range and this can be recorded using a digital video camera. This fact has been used to develop a method for measuring the bed’s temperature in the line of sight, through the quartz sides of the reactor. A solid probe is only used for calibration.Video recordings were obtained covering different regions of the bed over three wavelength bands, red, green and blue. In the course of an experiment, the mean temperature of the bed, measured with thermocouples, was raised from ambient to 1300 K, at a rate of ∼ 1 K/s. The data collected were used for calibration, with the brightness of individual pixels converted to a temperature scale. The calibration can then be used to investigate the dynamic temperature distribution within the field of view, in individual elements of the bed. This can also help the study of heat transfer in the bed, its distribution and dissipation.Using this method, it is possible to make direct observations of the intermittent combustion of gaseous fuels in a bubbling fluidised bed. The results provide direct proof that the temperature gradients observed within such beds are associated with exothermic processes within fast moving bubbles. The method could be adapted to studying, e.g., the combustion of solid fuels.     

关于浮升力增强锅炉系统中湍流换热的研究

在锅炉系统中会遇到流体在一个表面加热,而另一个表面近似绝热的环形通道的湍流混合对流。这一问题中由于有浮升力的存在,使湍流换热被强化,从而使能源的利用率提高了。本文应用Fortran程序对锅炉内浮升力增强换热这一问题进行了数值模拟求解。     

Study of a novel sunlight concentrating and optical fibre guiding system

This paper presents the experimental investigation of a novel sunlight concentrating and optical fibre guiding system for daylighting applications. The key part of the system is a sunlight concentrator unit which includes a novel mirror image co-focus compound parabolic concentrator. The structure and working principle of an experimental sunlight concentrating and optical fibre guiding system are introduced. Under a real sky condition, the experimental system was investigated to determine its performance and characteristics. The relationship curves between the internal illuminance of the integrating box and external illuminance are given for different tracking accuracy values and optical fibre geometries. The transmittance of the experimental system is calculated from the measured illuminance values. The results demonstrate the feasibility of the novel sunlight concentrating and optical fibre guiding system for daylighting applications.     

Heat rejection pressure optimization for a carbon dioxide split system: An experimental study

Recent studies indicate carbon dioxide (R744) as a valid alternative to classical substances such as HFCs used in vapour compression plants. However a transcritical refrigeration cycle is needed because the critical temperature of carbon dioxide is usually near the ambient temperature. Consequently the carbon dioxide refrigerator performances are significantly influenced by the heat rejection pressure. In this paper an experimental investigation on working optimization for a “split-system” to cool air in residential applications is presented: by varying the heat rejection pressure an optimum working condition has been found at different ambient temperatures. Furthermore a simplified model to predict the optimum heat rejection pressure is shown and a comparison with experimental results is carried out. Both the model validation and the experimental results suggest that the heat rejection pressure optimization is an convenient method to improve the performance of a carbon dioxide split system. Finally an algorithm based on the aforementioned model has been proposed in order to control an electronic back pressure valve by means of a PLC.     

An experimental and analytical study of non-conventional air conditioning system

Out of three sorbents studied, the silica gel bed, either in adiabatic or diabatic condition, has been found to be best suited for dehumidification of air. The time for its regeneration by a flat plate solar collector is about 2 h as compared to a 1 h dehumidification period for air. Further the distinct changes in its colour during dehumidification and desorption processes provide an easy guide line for changing the cycle. In addition, a comparative analysis has been carried out for five systems in order to select the best one. System III exhibits the overall best performance and is recommended for exploitation. The condition of air attained at the inlet of the room falls near the comfort line on the comfort chart. Hence, in the light of increasing cost of energy, the proposed system would become attractive in the future.     

A theoretical study of parallel flow condensation with interfacial drag in an annulus

The in-tube condensation model of Bae, Maulbetsch and Rohsenow (BMR) is extended to condensation in an annulus with coolant on the tube side. An iterative calculation procedure is developed and used for predicting the variations of local condensing heat transfer coefficient, quality and wall temperature with condenser length for the systems-steam and Freon-12. Comparison of the predictions with Shah's correlation suggests the applicability of BMR's model for the annulus condensation.     

A numerical study of the effect of turbulence on mass transfer from a single fuel droplet evaporating in a hot convective flow

A three-dimensional numerical model is developed to investigate the effect of turbulence on mass transfer from a single droplet exposed to a freestream of air. The freestream temperature, turbulence intensity and Reynolds number are varied to provide a wide range of test conditions, whereas the ambient pressure is kept atmospheric. The turbulence terms in the conservation equations of the gas-phase are modelled by using the shear-stress transport (SST) model. A Cartesian grid based blocked-off technique is used in conjunction with the finite-volume method to solve numerically the governing equations of the gas and liquid-phases. This study showed that the vaporization Damköhler number proposed in the literature to correlate the effect of turbulence on the droplet's vaporization rate is invalid at air temperatures higher than room temperature. Additionally, an attempt is made to correlate the effect of the freestream turbulence on the droplet's mass transfer rate by using Sherwood number over a wide range of freestream temperatures.     

Comparative study of turbulence models performance for refueling of compressed hydrogen tanks

Compressed hydrogen gas is a popular mode of fuel storage for hydrogen powered vehicles. When hydrogen gas is filled at high pressure, the gas temperature increases. The maximum gas temperature should be within acceptable safety standards. Numerical studies can help optimize the filling process. There is a high level of turbulence in the flow as the high velocity inlet jet is penetrating the nearly stagnant gas in the tank. Selection of a suitable turbulence model is important for accurate simulation of flow and heat transfer during filling of hydrogen tanks. In the present work, a comparative study is performed to identify suitable turbulence model for compressed hydrogen tank filling problem. Numerical results obtained with different turbulence models are compared with available experimental data. Considering accuracy, convergence and the computational expenses, it is observed that the realizable k-ε model is the most suitable turbulence model for hydrogen tank filling problem.     

Wind tunnel study on wind and turbulence intensity profiles in wind turbine wake

In recent years, there has been a rapid development of the wind farms in Japan. It becomes very important to investigate the wind turbine arrangement in wind farm, in order that the wake of one wind turbine does not to interfere with the flow in other wind turbines. In such a case, in order to achieve the highest possible efficiency from the wind, and to install as many as possible wind turbines within a limited area, it becomes a necessity to study the mutual interference of the wake developed by wind turbines. However, there is no report related to the effect of the turbulence intensity of the external flow on the wake behind a wind turbine generated in the wind tunnel. In this paper, the measurement results of the averaged wind profile and turbulence intensity profile in the wake in the wind tunnel are shown when the turbulence intensity of the external wind was changed. The wind tunnel experiment is performed with 500mm-diameter two-bladed horizontal axis wind turbine and the wind velocity in wake is measured by an I-type hot wire probe. As a result, it is clarified that high turbulence intensities enable to the entrainment of the main flow and the wake and to recover quickly the velocity in the wake.