Study on heat transfer of leakage flow in tooth tip clearance for single screw compressor based on fuzzy contourlet finite element method

Heat transfer of leakage flow in tip clearance will reduce the working performance, and therefore it is necessary to study it in depth. To improve the computing effectiveness of heat transfer of leakage flow in the tooth tip clearance of the single-screw compressor, the fuzzy contourlet finite element model is constructed by combining the contourlet finite element method and fuzzy finite element method. First, the related research progresses are summarized. Second, heat transfer model of leakage flow in the tip clearance of single-screw compressor is constructed. Third, the fuzzy contourlet wavelet finite element model is established. Finally, heat transfer simulation of leakage flow in the tip clearance of single-screw compressor is performed, and the variables with fuzziness are chosen, which are transformed to random variables based on information entropy theory. The comparing analysis among simulation analysis and test results is performed, and results show that the contourlet finite element method has highest computing precision and efficiency. In addition, the temperature of leakage flow in the tip clearance (L1) is also obtained, and results show that the fuzzy contourlet wavelet finite element method can effectively obtain the temperature distribution rules with fuzziness.     

波纹板换热器板间流动与传热的有限元计算分析

通过有限元法计算了波纹板式换热器板间温度场与速度场分布,情况，对八种板型进行了计算，计算结论通过图示表明不同的波纹板板型的传热与流动性能。     

不同管间距的垂直U型地埋管换热实验研究

对不同管间距的垂直U型地埋管进行了夏季工况连续实验,比对单U地埋管换热器不同管间距下的单位井深换热量、管群内土壤温度变化和系统运行情况,结果表明,管间距越大,单U换热器和土壤之间换热效果越好,管群内的热干扰越弱;管间距过小,系统内换热器的换热情况将恶化,导致不能长期稳定运行。     

Numerical studies on flow and heat transfer in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger

The flow and heat transfer characteristics of synthesis gas (syngas) in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger under different operating pressures, inlet velocities and pitches are investigated numerically. The three-dimensional governing equations for mass, momentum and heat transfer are solved using a control volume finite difference method. The realizable k-ε model is adopted to simulate the turbulent flow and heat transfer in heat exchangers. There flows syngas in the channels consisting of the membrane helical coils or membrane serpentine tubes, where the operating pressure varies from 0.5 to 3.0 MPa. The numerically obtained heat transfer coefficients for heat exchangers are in good agreement with experimental values. The results show that the syngas tangential flow in the channel consisting of membrane helical coils is significant to the heat transfer enhancement to lead to the higher average heat transfer coefficient of membrane helical-coil heat exchanger compared to membrane serpentine-tube heat exchanger. The syngas tangential velocity in the membrane helical-coil heat exchanger increases along the axial direction, and it is independent of the gas pressure, increasing with the axial velocity and axial pitch rise and decreasing with the radial pitch rise.     

Heat transfer analysis of single screw compressor under oil atomization based on fuzzy random wavelet finite element method

Oil atomization is an effective enhanced heat transfer method for single screw compressor, which can make the compression process of single screw compressor be close to the isothermal process, then the working efficiency can be improved. In order to obtain optimal atomization technology effectively, the heat transfer of single screw compressor is analyzed based on fuzzy random wavelet finite element method. Firstly, the relating researching progresses are summarized. Secondly, the heat transfer governing equation of single screw compressor is constructed based on large eddy simulation technology. Thirdly, the heat transfer wavelet finite element model of single screw compressor is established through using Hermitian wavelet function, and then the fuzzy random Hermitian wavelet finite element mode is established based on λ level set theory. Finally, the heat transfer rules of single screw compressor is obtained based on simulation analysis, the effect of atomization on heat transfer of single screw compressor is obtained, results can offer effective theoretical guidance for designing the novel atomization system for single screw compressor.     

基于数值计算的U型地埋管井群换热器计算模型研究

针对U型地埋管换热器的特点,在分析单井换热器换热量的基础上,考虑到多井换热器井与井之间传热的相互干扰,提出了一种用于U型地埋管井群换热器数值计算的9井模型,这种模型在数值计算时既能代表一般井群换热的规律,又便于计算机处理,为实际工程中分析井群换热规律提供了很好的理论依据。     

地源热泵换热器可靠性设计方法研究

介绍地源热泵换热器传热模型和设计方法的研究现状,分析地源热泵换热器传热影响因素的随机特征,提出地源热泵换热器的可靠度分析方法,讨论采暖热能指标及设计值的变异性。结合工程实例提出基于可靠性理论的地源热泵换热器设计与分析方法。     

Numerical analysis of heat transfer and flow field around cross-flow heat exchanger tube with fouling

Fouling is one of the main problems of heat transfer which can be described as the accumulation on the heat exchanger tubes, i.e.; ash deposits on the heat exchanger unit of the boiler. A decrease in heat transfer rate by this deposition causes loss in system efficiency and leads to increasing in operating and maintenance costs. This problem concerns with the coupling among conduction heat transfer mode between solid of different types, conjugate heat transfer at the interface of solid and fluid, and the conduction/convection heat transfer mode in the fluid which can not be solved analytically. In this paper, fouling effect on heat transfer around a cylinder in cross flow has been studied numerically by using conjugate heat transfer approach. Unlike other numerical techniques in existing literatures, an unstructured control volume finite element method (CVFEM) has been developed in this present work. The study deals with laminar flow where the Reynolds number is limited in the range that the flow field over the cylinder is laminar and steady. We concern the fouling shape as an eccentric annulus with constant thermal properties. The local heat transfer coefficient, temperature distribution and mean heat transfer coefficient along the fouling surface are given for concentric and eccentric cases. From the results, we have found that the heat transfer rate of cross-flow heat exchanger depends on the eccentricity and thermal conductivity ratio between the fouling material and fluid. The effect of eccentric is dominant in the region near the front stagnation point due to high temperature and velocity gradients. The mean Nusselt number varies in asymptotic fashion with the thermal conductivity ratio. Fluid Prandtl number has a prominent effect on the distribution of local Nusselt number and the temperature along the fouling surface.     

The combined effects of longitudinal heat conduction,flow nonuniformity and temperature nonuniformity in crossflow plate-fin heat exchangers

An analysis of a crossflow plate-fin compact heat exchanger, accounting for the combined effects of two-dimensional longitudinal heat conduction through the exchanger wall and nonuniform inlet fluid flow and temperature distribution is carried out using a finite element method. A mathematical equation is developed to generate different types of fluid flow/temperature maldistribution models considering the possible deviations in fluid flow. Using these models, the exchanger effectiveness and its deterioration due to the combined effects of longitudinal heat conduction, flow nonuniformity and temperature nonuniformity are calculated for various design and operating conditions of the exchanger. It was found that the performance variations are quite significant in some typical applications.     

Predicting the fluid temperature at the exit of the vertical ground heat exchangers

The energy analysis of ground source heat pump systems is based on the instantaneous fluid temperature at the ground heat exchanger outlet. This temperature defines the ground source heat pump coefficient of performance (COP) and hence the electricity consumption required in order to fulfill the energy demands of the building. The aim of this work is to present a model able to predict the fluid temperature at the ground heat exchanger outlet, taking into account the heat transfer phenomena in the soil and the temporal variation of the thermal load of the ground heat exchanger. The model developed was verified using experimental data, expanding over a three years period, of a vertical ground heat exchanger. It is proved that the model is able to satisfactorily predict the recorded temperature values throughout the verification period. The differences between measured and estimated outlet water temperatures impose a deviation between the estimated and the actually recorded electricity consumption of less than 4%.     

Economic optimization of shell and tube heat exchanger based on constructal theory

In this paper, the new approach of constructal theory has been employed to design shell and tube heat exchangers. Constructal theory is a new method for optimal design in engineering applications. The purpose of this paper is optimization of shell and tube heat exchangers by reduction of total cost of the exchanger using the constructal theory. The total cost of the heat exchanger is the sum of operational costs and capital costs. The overall heat transfer coefficient of the shell and tube heat exchanger is increased by the use of constructal theory. Therefore, the capital cost required for making the heat transfer surface is reduced. Moreover, the operational energy costs involving pumping in order to overcome frictional pressure loss are minimized in this method. Genetic algorithm is used to optimize the objective function which is a mathematical model for the cost of the shell and tube heat exchanger and is based on constructal theory. The results of this research represent more than 50% reduction in costs of the heat exchanger.     

地源热泵地下岩土热物性的测定方法

提出了可用于现场测量计算地下岩土综合热物性参数的方法。垂直埋设在地下的换热器与周围岩土换热过程可以近似地看作半无限大介质中常热流边界条件下的非稳态导热过程来处理。这种方法克服了其他常用研究模型对钻孔中埋管的具体位置、上升管及下降管之间的距离、换热器材料和回填材料的物性参数的要求,相应地消除了上述各个参数所带来的误差。通过测量地埋换热器的循环水流量、进出水口温度,加热器的加热功率等数据确定地下岩土综合的热物性参数。     

The influence of a vertical ground heat exchanger length on the electricity consumption of the heat pumps

The use of heat pumps combined with vertical ground heat exchangers for heating and cooling of buildings, has significantly gained popularity in recent years. The design method for these systems, as it is proposed by ASHRAE, is taking into account the maximum thermal and cooling loads of the building, the thermophysical properties of the soil at the area of installation and a minimum Coefficient of Performance (COP) of the heat pumps. This approach usually results in larger than needed length of the ground heat exchanger, thus increasing the installation cost.A new analytical simulation tool, capable to determine the required ground heat exchanger length has been developed at the Process Equipment Design Laboratory (PEDL) of the AUTh. It models the function of the system as a whole over long time periods, e.g. 20 years, using as input parameters the thermal and cooling loads of the building, the thermophysical properties of the borehole and the characteristic curves of the heat pumps. The results include the electricity consumption of the heat pumps and the heat absorbed from or rejected to the ground.The aim of this paper is to describe the developed simulation algorithm and present the results of such a simulation in a case study. It is proved that the total required length of the ground heat exchanger is less than that calculated using the common numerical method.     

A new model and solutions for a spiral heat exchanger and its experimental validation

A spiral heat exchanger was applied in a ground source heat pump (GSHP) system that is primarily used for residential indoor heating. Studies that have been performed on the heat transfer of spiral heat exchanger have focused on field measurements and numerical analysis; however, theoretical research on the subject is absent in the literature. In this study, a methodology is proposed to analyze the heat performance of a spiral heat exchanger. A ring source model was established and solved analytically to describe the temperature variation of the ground caused by a spiral heat exchanger. The validity of the model was examined by an experiment on the soil temperature variation with a spiral heater. The virtual ring tube surface temperature response of unit ring circle was calculated by a superposition of the contributions of the ring source itself and adjacent ring sources. Furthermore, a fast algorithm was created to compute the average tube surface temperature resulting from the dimensionless temperature rise at a point far from the ring source that is constant when the non-dimensional distance is less than 0.13. The author confirmed that the calculation time of this proposed algorithm decreased by a factor of 100 compared with the traditional integration method. A system designer will find this algorithm helpful when determining the size of a heat exchanger under a required heating load, particularly for different arrangement of spiral heat exchangers.     

深井地埋管换热器配置参数模拟计算研究

针对深井地埋管换热系统运行原理,根据地埋管换热器热阻-热容优化模型,建立深井地埋管井孔内、外非稳态柱坐标传热模型.基于环渤海湾盆地埋深1000～2000 m热储层水文地质条件,采用双连续介质空间耦合有限元数值计算方法,分析深井地埋管典型配置参数取值对于地埋管换热性能的影响程度.研究结果表明:深井地埋管换热性能随着系统运...     

Second law analysis on the heat transfer of the horizontal concentric tube heat exchanger

In the present study, the theoretical and experimental results of the second law analysis on the heat transfer and flow of a horizontal concentric tube heat exchanger are presented. The experiments setup are designed and constructed for the measured data. Hot water and cold water are used as working fluids. The test runs are done at the hot and cold water mass flow rates ranging between 0.02 and 0.20 kg/s and between 0.02 and 0.20 kg/s, respectively. The inlet hot water and inlet cold water temperatures are between 40 and 50 °C, and between 15 and 20 °C, respectively. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer characteristics, entropy generation, and exergy loss are discussed. The mathematical model based on the conservation equations of energy is developed and solved by the central finite difference method to obtain temperature distribution, entropy generation, and exergy loss. The predicted results obtained from the model are validated by comparing with the present measured data. There is reasonable agreement from the comparison between predicted results and those from the measured data.     

Evaluation of estimation method of ground properties for the ground source heat pump system

Technology directed at geothermal energy, one of our renewable energy sources, to heat and air-condition buildings has become very attractive in recent years following the significant developments in ground-source heat pump (GSHP) systems. In general, although the energy efficiency of GSHP systems is far superior to conventional air-source heat pump (ASHP) systems, GSHP system is still expensive. Therefore, GSHP system employs the foundation pile of buildings as heat exchanger is introduced in order to reduce the initial cost. When designing a GSHP system (especially in case of the energy pile system), it is necessary to accurately predict the heat extraction and injection rates of the heat exchanger. The thermal and hydraulic properties of the ground are very important to accurately predict heat transfer between the ground heat exchanger and the ground. In particular, those are the most important design parameters because energy pile system is installed only a few tens of meters deep. In this paper, an estimation method is suggested in order to determine the thermal and hydraulic properties of the ground for design the heat exchanger of energy pile system base on geotechnical investigation for the design the building's foundations. The use of results from generally applied geotechnical site investigation methods to estimate ground thermal and hydraulic properties was evaluated.     

Thermal analysis on metal-foam filled heat exchangers. Part II: Tube heat exchangers

The forced convection heat transfer characteristics in high porosity open-cell metal-foam filled tube heat exchangers are analysed in this paper. The Brinkman-extended Darcy momentum model and two-equation heat transfer model for porous media are employed for the analysis of the heat transfer performance. The morphological effects of metal foams on overall heat transfer are examined. The optimal foam-area ratio for a metal-foam filled counter-flow tube-in-tube heat exchanger is predicted. The study shows that the thermal performance of a metal-foam heat exchanger can be superior to that of conventional finned tube heat exchangers.     

土壤源热泵地下垂直换热埋管周围非稳态温度场的数值模拟

针对土壤源热泵地下垂直U型换热埋管,建立了周围土壤的非稳态温度场的数学模型,并利用隐式有限差分法进行了数值模拟。通过对制冷和制热工况的模拟,得到土壤温度沿径向的变化规律、埋管出水温度的变化规律及埋管的热作用半径的变化规律。     

Heat transfer analysis of pile geothermal heat exchangers with spiral coils

This paper investigates the transient heat conduction around the buried spiral coils which could be applied in the ground-coupled heat pump systems with the pile foundation as a geothermal heat exchanger. A transient ring-coil heat source model is developed, and the explicit analytical solutions for the temperature response are derived by means of the Green’s function theory and the image method. The influences of the coil pitch and locations are evaluated and discussed according to the solutions. In addition, comparisons between the ring-coil and cylindrical source models give that the improved finite ring-coil source model can accurately describe the heat transfer process of the pile geothermal heat exchanger (PGHE). The analytical solutions may provide a desirable and better tool for the PGHE simulation/design.