Study on heat transport of an osmotic heat pipe: Part 2. Flow in the membrane module

An osmotic heat pipe is a top‐heat‐mode heat pipe in which a heat transport medium flowing down is pumped up by means of osmosis in the membrane module. The osmosis is dependent on the concentration difference between the solution at the inside surface and the solvent at the outside surface of the membrane. In addition, convection in the solution inside the membrane affects the concentration of the solution in contact with the inside surface. Thus, the concentration and solution flow rate greatly affect the osmotic pumping rate and the heat transport rate. Therefore, in the present study, the flow in the membrane module was investigated in detail. Using the ratio of the concentration at the inside surface to the mixed mean concentration at the corresponding site, relations for these concentrations and the solution flow rate along the solution channel in the membrane module are derived semitheoretically. These relations can be used to correlate experimental data to within a ±20% error. In addition, a method for increasing the osmotic pumping rate and the heat transport rate of the osmotic heat pipe is proposed. © 2000 Scripta Technica, Heat Trans Asian Res, 29(4): 317–332, 2000     

A new concept for an osmotic energy converter

A new concept for an osmosis power generation system is presented. While the power production of a conventional system is based on continuous, increasing volumetric flow of solution directed to a turbine, the new concept is based on the pressurizing of fluids by osmosis. Two different new concepts were studied. In the first case, the osmotic module consists of the osmotic membranes, fresh water and solution. In the second case, gas is included in the solution part of the module. Consequently, the new system without gas was found to result in more than 2.5 times higher power values than the conventional concept. Copyright © 2001 John Wiley & Sons, Ltd.     

Analysis of temperature-sensitive magnetic fluids in a porous square cavity depending on different porosity and Darcy number

Magnetic fluids are thermo-sensitive and whose flow and energy transport processes can be controlled by temperature and external magnetic field. In the natural convection of porous cavity, magnetic force is not only the driving force, the effective gravity is also a force related to the natural convection, and the effective gravity is closely depending on the porosity and permeability of porous medium. As is known, the porous medium is in solid state with high heat capacity but low heat transfer coefficient, while the magnetic fluid is a kind of fluid with high heat transfer coefficient and easy to be controlled. Combining the complementary characteristic of magnetic fluids and porous medium, we present a study for temperature-sensitive magnetic fluids in a porous square cavity. In the study, a Lattice Boltzmann method is developed to simulate the laminar convection of temperature-sensitive magnetic fluids in a porous square cavity. We present numerical results for the streamlines, isotherms, magnetization for different values of porosity and Darcy number. In addition, Nusselt numbers on heated and cooled wall and the average Nusselt numbers are also investigated.     

Investigation on forced convective heat transfer of molten salts in circular tubes

In this paper, experiments are carried out to obtain convective heat transfer coefficients of turbulent flow and transition flow of molten Hitec salts in a circular tube. The present experimental data together with experimental data of four kinds of molten salts from the existing literature are correlated for transitional and turbulent convective flow respectively. In addition, the Prandtl number dependence of convective heat transfer with different working fluids is examined. It is shown that the present experimental data are in good agreement with existing correlations.     

Second law analysis of reverse osmosis desalination plants: An alternative design using pressure retarded osmosis

A second law analysis of a reverse osmosis desalination plant is carried out using reliable seawater exergy formulation instead of a common model in literature that represents seawater as an ideal mixture of liquid water and solid sodium chloride. The analysis is performed using reverse osmosis desalination plant data and compared with results previously published using the ideal mixture model. It is demonstrated that the previous model has serious shortcomings, particularly with regard to calculation of the seawater flow exergy, the minimum work of separation, and the second law efficiency. The most up-to-date thermodynamic properties of seawater, as needed to conduct an exergy analysis, are given as correlations in this paper. From this new analysis, it is found that the studied reverse osmosis desalination plant has very low second law efficiency (<2%) even when using the available energy recovery systems. Therefore, an energy recovery system is proposed using the (PRO) pressure retarded osmotic method. The proposed alternative design has a second law efficiency of 20%, and the input power is reduced by 38% relative to original reverse osmosis system.     

New correlations for heat transfer coefficients during direct cooling of products

An analytical method for determining the heat transfer coefficients of food products being cooled in water and in air flows is presented. Food products are idealized as geometrical solid objects of regular shapes. New correlations between heat transfer coefficients and cooling coefficients are developed in simple forms for practical use in the refrigeration industry. These correlations are then used to determine the heat transfer coefficient for a cylindrical carrot cooled in air flow as an illustrative example. In addition, evaluating the heat transfer coefficients for several products using the available experimental cooling coefficient values from the literature, two new correlations between the heat transfer coefficient and the cooling coefficient are also obtained for water and air cooling applications. The results show that the correlations presented in this article can determine the heat transfer coefficients of food products forced-convection cooling in a simple and accurate manner.     

Heat transfer to sub-cooled binary solution in horizontal tube

Predicted heat transfer coefficients (HTC) are widely used for calculations of various heat and mass transfer processes. Various empirical and semi-empirical models of non-dimensional groups are used for calculating the heat transfer coefficients. These groups depend on the fluid and flow properties. The common accuracy of the predicted heat transfer coefficient is usually about ±25%; however, the accuracy is failing by the lack of mechanistic model for convective heat transfer and by the inaccurate predictions of the fluid properties. This work is aimed to improve the heat transfer coefficient prediction by reducing the deviations which associated with the fluid properties. When the fluid is a mixture of miscible fluids the predictions of the fluid properties are very rough and therefore the prediction of the heat transfer coefficient is more complicated.In the present study the heat transfer coefficient of sub-cooled organic mixture chlorodifluoromethane (R22)–dimethylacetamide (DMAC) was measured experimentally. In order to compare the experimental value with the predicted one, thermophysical properties of the solution, such as density, viscosity, heat capacity and thermal conductivity have to be known. Since the thermal conductivity of the solution (R22–DMAC) was the only unknown property, various correlations and mixing rules were tested, and the most appropriate was chosen. Based on this method for evaluating the solution’s properties the predicted HTC obtained with an error range of 15%.     

考虑地下水补给的高岭土电渗法固结试验研究

为研究不同补水边界条件对电渗加固软土地基效果的影响,在自制的试验装置上对高岭土进行了不同补水边界条件的电渗固结试验,通过监测电渗过程中电流、电势、排水量、抗剪强度、含水率和沉降等指标,发现不补水条件下的电渗加固软土效果优于补水条件下的电渗加固效果;在不补水条件下软土通过电渗加固还会出现较大裂缝,表明不同的补水边界对电渗加固效果影响较大,在电渗室内试验时必须考虑补水边界条件的影响。     

Oil fouling in double‐pipe heat exchanger under liquid‐liquid dispersion and the influence of copper oxide nanofluid

Dispersions of oil in water are encountered in a variety of industrial processes leading to a reduction in the performance of the heat exchangers when thermally treating such two phase fluids. This reduction is mainly due to changes in the thermal and hydrodynamical behavior of the two phase fluid. In the present work, an experimental investigation was performed to study the effects of light oil fouling on the heat transfer coefficient in a double‐pipe heat exchanger under turbulent flow conditions. The effects of different operating conditions on the fouling rate were investigated including: hot fluid Reynolds number (the dispersion), cold fluid Reynolds number, and time. The oil fouling rate was analyzed by determining the growth of fouling resistance with time and through pressure drop measurements. The influence of copper oxide (CuO) nanofluid on the fouling rate in the dispersion was also determined. It was found that the presence of dispersed oil causes a reduction in heat transfer coefficient by percentages depending on the Reynolds number of both cold and hot fluids and the concentration of oil. In addition, the time history of fouling resistance exhibited different trends with the flow rates of both fluids and its trend was influenced appreciably by the presence of CuO nanofluid.     

Estimation of the maximum power temperature coefficients of PV modules at different time scales

The maximum power temperature coefficients of a-Si modules are negative in indoor measurements, whereas the performance increases with module temperature in outdoor measurements. In order to resolve the discrepancy, we investigate the temperature coefficients of a-Si modules by two different methods corresponding to two different time scales. The results are that the temperature coefficient for a shorter time scale of several hours is negative, although the temperature coefficient for a longer time scale of seasons is positive. These results suggest that the discrepancy in temperature coefficient is partially caused by the effects of thermal annealing and light soaking. Therefore, the history of module temperature and received solar irradiance would be required for the correction of the performance of a-Si modules. In addition, we reveal that the effect of solar spectrum is also a contributory factor in the discrepancy.     

Steam absorption into aqueous lithium bromide solution films falling inside vertical pipes

The realization of a compact and efficient air‐cooled absorber is the key technology for the small‐capacity absorption refrigerator for domestic use. A vertical pipe with absorbent flowing inside and air flowing outside is the best choice for the air‐cooled absorber due to the easy addition of fins to enhance heat transfer on the air side. In this paper, first, the modeling of the absorption process in the vertical pipe for a constant heat transfer coefficient on the outer surface is described. Then, experimental results are presented for pipes of inner diameters 8 to 26 mm, pressure 5 and 6 mm Hg, and outer surface heat transfer coefficient 2000 and 3300W/(m²·K). It was found that for pipes of diameter 13 mm or more, the absorption process is well estimated by the proposed model. The absorption with addition of surfactant is estimated by the pseudo‐turbulent method. The necessary heat transfer area for the air‐cooled condition is about three times that for the water‐cooled condition. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 740–752, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10128     

An estimation for velocity and temperature profiles of nanofluids in fully developed turbulent flow conditions

Theoretically convective heat transfer coefficient depends on velocity and temperature profiles. In this work friction factor and convection coefficient are used in order to compare both profiles for nanofluids and base fluids. For this purpose Al₂O₃/water (due to its present vast experimental study) and carbon nanotube/water (manufactured and examined with our group) are selected. The results show that velocity profile of a nanofluid is similar to the velocity profile of its base fluid. It is proposed that the change of temperature profile for nanofluids compared to the base fluids is the only variable responsible for unpredictable convective heat transfer coefficient of nanofluids using available correlations.     

Mass transfer into liquid falling film in straight and helically coiled tubes

Helically moving liquid films are principally used in many heat and mass transfer processes, which are usually practiced to minimize longitudinal dispersion, maximize secondary currents and provide compact spacing.In the present work, practical experiments are carried out to determine mass transfer coefficients for thin films of liquid falling in case of straight and coiled tubes. The system used in the present investigation is the absorption of CO₂ into liquid films of distilled water, or ethyl alcohol (96.25%), or ethylene glycol of 12% or 5.2%. The obtained experimental results indicated higher mass transfer coefficient in the helically liquid film.Empirical correlations are concluded by using dimensional analysis method. The correlations relate Sherwood number to the following dimensionless groups for laminar and turbulent regimes.

     

Pool pressure-retarded osmosis

In this work, consideration is given for pressure-retarded osmosis (PRO) system in a pool configuration. The motivation behind of such a configuration lies in the possibility to eliminate mechanical moving parts or energy recovery systems—as for example pressure exchangers (PEXs), which are needed in traditional PRO systems. In a pool configuration mobile parts as pumps or PEXs may no longer be needed because advantage is taken not only in the osmotic energy released upon the mixing of the two solutions with different concentrations but also in the buoyant potential from the gradient of density. The preliminary mathematical development for a pool configuration constitutes the core of this work. The main theoretical difficult which aroused when attempting to analyze a pool-PRO system was the determination of the vertical motion of the plume formed immediately after the detachment and the mixing which must be at least equal than that given by the water permeability coefficient of the membrane if it is desired that the system works in a steady stay. Because hydrodynamic entrainment induced by the vertical motion of the plume must be promoted in order to replenish with fresh draw solution the membrane, then it must be arranged in clusters which from computational fluid dynamics (CFD) simulations performed should be in the range of 50 to 100 μm.     

Modeling of low-density lipoprotein (LDL) transport in the artery—effects of hypertension

A robust four-layer model is presented to describe the LDL transport in the arterial wall coupled with the transport in the lumen. The endothelium, intima, internal elastic lamina (IEL) and media are all treated as macroscopically homogeneous porous media and the volume-averaged porous media equations are employed to model various layers, with Staverman filtration and osmotic reflection coefficients introduced to account for selective permeability of each porous layer to certain solutes. The physiological parameters within the various layers are obtained from literature. The set of governing equations for fluid flow and mass transport is descretized using a finite element scheme based on the Galerkin method of weighted residuals. Filtration velocity and LDL concentration profiles are developed at different locations for various clinical conditions. The results are consistent with previous numerical and experimental studies. Effects of hypertension and boundary conditions are discussed based upon the present model. Furthermore, the effects of pulsatile flows on LDL transport in the arterial wall are studied in some detail. Compared to previous transport models, the newly developed model is found to be a more robust tool for investigation of LDL accumulation within different arterial wall layers for various clinical conditions. This will be helpful in understanding the role of transmural transport processes in the initiation and development of atherosclerosis.     

Heat transfer and entropy analysis for a transparent gas flowing in a tube

Heat transfer by forced convection and radiation in tubes is very important for high‐temperature heat exchangers, which find wide applications in power plants. In addition, the entropy analysis gives insight into the qualitative measure of the heat transfer processes. Consequently, in the present study, forced convection and radiation heat transfer in flow through a tube is considered. The wall and fluid sides temperature rise are predicted for different tube lengths. The entropy analysis is carried out and the influence of tube length and heat transfer coefficient on the volumetric entropy generation are examined. It is found that the wall temperature and the volumetric entropy generation increases as the tube length increases. The point of maximum volumetric entropy generation moves close to the tube inlet as the tube length increases. In addition, the maximum volumetric entropy generation becomes independent of tube length for high heat transfer coefficients. Copyright © 1999 John Wiley & Sons, Ltd.     

System identification modeling and unstable behavior of the dynamics of flows within the tip region of an axial compressor blade passage

In recent years, the correlation coefficient of pressure data from the same blade passage in an axial compressor unit has been used to characterize the state of flow in the blade passage. In addition, the correlation coefficient has been successfully used as an indicator for active control action using air injection. In this work, the correlation coefficient approach is extended to incorporate system identification algorithms in order to extract a mathematical model of the dynamics of the flows within a blade passage. The dynamics analyzed in this research focus on the flow streams and pressure along the rotor blades as well as on the unsteady tip leakage flow from the rotor tip gaps. The system identification results are used to construct a root locus plot for different flow coefficients, starting far away from stall to near stall conditions. As the compressor moves closer to stall, the poles of the identified models move towards the imaginary axis of the complex plane, indicating an impending instability. System frequency data is captured using the proposed correlation based system identification approach. Additionally, an oscillatory tip leakage flow is observed at a flow coefficient away from stall and how this oscillation changes as the compressor approaches stall is an interesting result of this research. Comparative research is analyzed to determine why the oscillatory flow behavior occurs at a specific sensor location within the tip region of the rotor blade.     

海水盐差能发电技术的试验研究

介绍了利用海水盐差能发电的三种主要方法：渗透压法、蒸汽压法和反电渗析电池法,建立了一套反电渗析电池试验装置,并对这种电池的电动势、电池内阻和海淡水隔室进行了理论研究和分析,设计了5种浓差电池试验槽,并取得了初步的试验结果和研究结论,为今后我国长江口和珠江口的海水盐差能发电应用提供了理论依据。     

AGTB叶型前缘气膜冷却效果的数值模拟研究

以AGTB叶型为对象,对文献中获取换热系数的两种计算方法进行了对比研究,结果表明维持主流和射流温度一致的等温法不能很好地反映气膜冷却流动中的传热现象,而主流和射流有温差的非等温法使得传热现象和流动现象都得到了很好的反映,且更接近实际物理问题.而后以实际发动机中的AGTB叶型为研究对象,将开槽气膜冷却孔结构应用于该叶片,...     

内燃机润滑油流动特性研究

为提高内燃机中回转运动副,特别是曲轴等运动件的润滑效果,建立一套用于研究粘性流体在管路中流动特性的试验系统。试验研究了三种润滑油的流动摩擦系数、管件出口流量系数、能量损失系数。研究表明,在雷诺数为10~140范围内的流动特性参数试验系统是有效的和适用的,可以为预测和深入研究相应管路中流体的流动特性提供参考。