Vertical temperature profile in ground heat exchanger during in-situ test

Ground source heat pump systems often use closed-loop heat exchangers, which depend on the soil thermal conductivity and borehole thermal resistance as parameters. An in-situ thermal response test provides a method to estimate these two parameters. In analyzing the in-situ test, one commonly uses the mean of the measured inlet and outlet temperatures of the circulating fluid as a representative fluid temperature along the entire ground loop. This assumption is convenient but not rigorous. In this paper an analytical model of the actual vertical temperature profile in the ground loop has been developed for the late-time period of the in-situ test. With this model one can estimate the soil thermal conductivity and borehole thermal resistance without the mean temperature approximation. In addition, the model identifies dimensionless groups that guide a sensitivity study of the errors introduced by the mean temperature approximation. The error in the total thermal resistance is less than 5% if the volume flow rate is maintained above a threshold, which increases linearly with the total length of the borehole. The analysis also indicates a p-linear averaging method gives smaller errors than the mean temperature approximation.     

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。     

Theoretical Analysis on the Air-Condition Cabin Technique for Satellite Temperature Adjustment on the Ground

The aim of this paper is to study the heat transfer mechanism of the air-condition cabin technique for satellite temperature adjustment on the ground before the launch of those satellites with pressurized structure. By the study, the temperature of the satellite can be set appropriately, and this is very important to the initial satellite temperature after the satellite operates in orbit. In the paper, the simplified physical model is provided and the mathematical equations are obtained, thereafter, the experimental data are compared with the theoretical analysis. The quantitative conclusions from the theoretical analysis can be used to direct the design or the experiment of the temperature adjustment. Finally, we can conclude that, the temperature variation rule of the satellite is exponential, and the temperature adjustment process can be predicted by the exponential formula, which is obtained from the theoretical analysis.     

Development of a numerical model for the simulation of vertical U-tube ground heat exchangers

Vertical U-tube ground heat exchangers are a key component in geothermal energy utilization systems like ground source heat pumps (GSHPs). This paper presents a three-dimensional unstructured finite volume model for them. The model uses Delaunay triangulation method to mesh the cross-section domain of the borefield (borehole field), and consequently may intactly retain the geometric structure in the borehole. To further improve the computational accuracy, the soil is divided into many layers in the vertical direction in order to account for the effect of changing fluid temperature with depth on the thermal process in the borefield. The inlet temperature of the ground heat exchanger (GHE) is used as a boundary condition, and the inside and outside surfaces of the U-tube pipes are treated as the conjugated interfaces in the domain. Thus, the conjugate thermal processes between the fluid in the pipes and the soil around it and between the two pipe legs may be accounted fully. A comparison of the model predictions and experimental data shows that the model has good prediction accuracy.     

Transient numerical and physical modelling of temperature profile evolution in stabilised rammed earth walls

Three established stabilised rammed earth (SRE) mix types (433, 613, 703) were identified for analysis, in the form of 300-mm thick test walls, by being subjected to different static air temperature and relative humidity differentials. The predictive numerical model outputs from WUFI Pro v4.1 hygrothermal simulations displayed good accuracy when validated against experimental data from physical modelling conducted using test walls in a climatic simulation chamber. The wall temperature profile evolution and resultant steady state gradients were very similar regardless of mix type indicating that the majority of the wall remained relatively dry. Unless liquid water is present, the thermal resistance and heat capacity of these materials does not change sufficiently to make significant differences to temperature profile evolution regardless of soil mix type. Little scope exists to intelligently modify the ability of SRE walls to absorb and store heat energy simply by manipulation of particle size distribution (PSD) and the resultant bulk density/void ratio relationships, under these conditions. Only the outer layers of the walls appear to interact with moisture in the air, and the predicted transient responses indicate that significant potential exists to intelligently modify the ability of SRE walls to absorb, store and release moisture vapour from the surrounding air simply by manipulation of PSD and the resultant bulk density/void ratio relationships.     

A numerical model for seasonal storage of solar heat in the ground by vertical pipes

We present a three-dimensional numerical model for seasonal heat storage in the ground using vertical heat exchanger pipes. The model also accounts for convective heat flows in the ground. The storage is employed in a district solar heating system with a heat pump. The effects of storage volume, storage medium, collector area, and collector type on system performances are studied for the Helsinki (60°N) climate. Economic optimization of the storage and collector installation is also briefly discussed. For a 500-house community, a collector area of 35 m² per house and a rock storage volume of 550 m³ per house would provide a solar fraction of 70%.     

间歇过程地温恢复特性及其规律模拟计算分析

通过试验和模拟分析，研究了地源热泵(GSHP)地能利用中，间歇控制过程对系统输出性能的影响、对井壁附近温度恢复的影响以及井的结构参数对系统输出性能的影响。试验和模拟计算结果表明，间歇控制技术能使井壁附近温度得到有效恢复，对发展利用地能供热供冷系统具有非常重要的实用价值。     

Variance of the ground temperature distribution

In order to account for the day-to-day variation in the solair temperature data, the coefficients in its Fourier series representation have been assumed to be time dependent. The variation in the numerical values of the coefficient being random, they have been modelled as a stochastic process. Explicit expressions for the mean and for the autocorrelation function of the ground temperature distribution have been obtained assuming the stochastic processes to be: (i) white noise (delta correlated) and (ii) Markov (exponentially correlated). Whilst the expression for the mean ground temperature distribution is found to be identical with that obtained by a periodic theory, the variance is found to decrease rapidly with depth.     

Continuously distributed sensors for steady-state temperature profile measurements: main principles and numerical algorithm

Principally new method for measurements of steady-state temperature field is suggested. This method is based on the use of so-called continuously distributed sensors (CDS). The basic CDS function is the measurement of temperature profile along the sensor length.The CDS can be manufactured my means of microfilm technologies in the form of a rod, flexible thread, or thin film. Only three electrical cables are needed for connection of CDS and measuring equipment. Nevertheless, it is possible to obtain simultaneously the information about temperature profile at several tens space points along the CDS. So, the CDS is equivalent to several tens of conventional discrete sensors, for example, thermocouples.The measuring process with the use of CDS can be described by the following scheme. A temperature field acts on the CDS causing space non-homogeneity of the electrical parameters of thermo-sensitive films. The space distribution of electrical parameters along CDS can be measured by means of electrical signals of different frequencies. Indeed, the distance to which the electrical signal penetrates the sensor depends on the frequency. So, the measuring impedance of the sensor contains information about space distribution of electrical parameters, which are directly related with the temperature profile.Restoration of the space distribution of electrical parameters from frequency characteristics of the measuring impedance is non-trivial task. From the mathematical point of view it is inverse problem. Some algorithms for solving of this inverse problem are developed and are presented in the paper. Numerical simulations demonstrate possibilities to measure temperature fields by means of CDS.     

Experimental and numerical study of the influence of initial temperature on explosion limits and explosion process of syngas-air mixtures

To study the effect of initial temperature of 30, 60, 90, and 120 °C on the explosion limits and the explosion process of the syngas-air mixtures, the explosion limits were tested by the explosive limit instrument, and the flame propagation process in the spherical pressure vessel was recorded by the high-speed camera. The ANSYS Fluent 3D software was used to simulate the explosion behavior of syngas-air mixtures. The results showed that with the increase of the initial temperature, the lower explosion limit of syngas decreased and the upper explosion limit increased, and the effect of initial temperature on the upper explosion limit of syngas was greater than that on the lower explosion limit. The flame development process in the simulation was consistent with that in the experiment, propagating outward spherically until it filled the entire container. Both experimental and numerical results presented the same trend of accelerating the flame propagation speed with the increase of initial temperature. In addition, the simulation also obtained multi-dimensional transient explosion parameters that were difficult to obtain in the experiment. The explosion process of syngas was analyzed by the explosion parameters such as temperature and pressure field in the explosion area. An increase in temperature decreased the maximum explosion pressure and shortened the time to reach the maximum explosion pressure.     

On the ground temperature below buildings

A transient, numerical model for the prediction of the ground temperature at various depths below buildings is presented in this paper. The proposed model was developed by calculating the heat flow to the ground from a building, which depends on the complicated three-dimensional thermal process in the ground. The main difficulties in obtaining manageable solutions of the heat flow problem were: The three-dimensionality of the thermal process, the strong temporal variability of the outdoor temperature as well as the large number of parameters involved in describing the building foundation geometry as well as the thermal insulation. The techniques of superposition and numerical analysis were used to cope with these difficulties. The model was validated against experimental data and it was found that it could accurately predict the ground temperature under a building.     

汽轮机转子模型振动特性的实验与计算分析

搭建了最高转速为8 000 r·min-1的转子振动测试实验台,使其可以实现单双跨转子动平衡和临界转速的振动测试。用幅频图法测量转子模型的临界转速,实验测得转子的转速—振幅图,由于副临界转速的存在,所以不能确定振幅波峰是转子的临界转速还是副临界转速。为了区分副临界转速和临界转速,利用ANSYS软件对转子进行模态模拟计算,判定了转子的临界转速和副临界转速。用影响系数法将不平衡转子进行实验配平,通过动平衡前后振动波形图、幅频特性图和轴心轨迹图的比较,证明了动平衡实验取得了良好的效果。     

Extending thermal response test assessments with inverse numerical modeling of temperature profiles measured in ground heat exchangers

Thermal response tests conducted to assess the subsurface thermal conductivity for the design of geothermal heat pumps are most commonly limited to a single test per borefield, although the subsurface properties can spatially vary. The test radius of influence is additionally restricted to 1–2 m, even though the thermal conductivity assessment is used to design the complete borefield of a system covering at least tens of squared meters. This work objective was therefore to develop a method to extend the subsurface thermal conductivity assessment obtained from a thermal response test to another ground heat exchanger located on the same site by analyzing temperature profiles in equilibrium with the subsurface. The measured temperature profiles are reproduced with inverse numerical simulations of conductive heat transfer to assess the site basal heat flow, at the location of the thermal response test, and evaluate the subsurface thermal conductivity, beyond the thermal response test. Paleoclimatic temperature changes and topography at surface were considered in the model that was validated by comparing the thermal conductivity estimate obtained from the optimization process to that of a conventional thermal response test.     

An analysis of the numerical path dependence of the J-integral

Path dependent behaviour of the J-integral computed from the results of a finite element analysis may have two sources. The first is the history dependence of the strain energy, which causes J to lose its crack tip strain field characterizing property. The second is rooted in the principle of the finite element displacement method which ensures equilibrium only for each element as a whole, not for points within an element. It is of the utmost importance for a J-based safety assessment that these two sources be clearly distinguished as the latter may be reduced by mesh refinement whereas the former may serve as a criterion for assessing whether J can still be interpreted in its crack tip characterizing sense.The present paper proposes a numerical procedure for this distinction. Two examples confirm the procedure's validity and illustrate the practical necessity of careful evaluation of computed J-values.     

Experimental and numerical analysis of the temperature transition of a suspended freezing water droplet

The objective of this study was to develop a simple experimental and numerical method to study the temperature transition of freezing droplets. One experimental approach and several numerical methods were explored. For the experimental method, a droplet was suspended in a cold air stream from the junction of a thermocouple. The droplet’s temperature transition was able to be accurately measured and the freezing of the droplet observed. The numerical models developed were able to predict the temperature transition and the freezing time of the droplet. Of the numerical methods, a simple heat balance model was determined to be an accurate means of predicting the freezing time of the droplet.     

An average fluid temperature to estimate borehole thermal resistance of ground heat exchanger

Analysis of borehole thermal resistance is important for the standard sizing of ground heat exchanger (GHE). In this paper, a p-linear dimensionless average fluid temperature is proposed to estimate borehole thermal resistance. A p-linear dimensionless fluid temperature and p-linear dimensionless average fluid temperature are introduced, and the p-linear dimensionless fluid temperature is compared with theoretical dimensionless fluid temperature calculated by quasi-three-dimensional model for both single and double U-tubes. Results show that the p-linear dimensionless temperatures with parameters p → 0 and p = ?1/2 are respectively in good agreement with the theoretical dimensionless fluid temperatures of single and double U-tubes. Therefore, the dimensionless logarithmic mean temperature for p → 0 and the dimensionless geometric mean temperature for p = ?1/2 should respectively be adopted to reasonably estimate the thermal resistance of single and double U-tube boreholes.     

Improvement of large scale solar installation model for ground current analysis

Application of a simplified PV model to large-scale PV installations neglects the current distortion, potential rise and losses in the system as consequence of the capacitive coupling inside the dc electric circuit. These capacitive couplings represent a leakage impedance loop for the capacitive currents imposed by the high frequency switching performance of power converters. This paper proposes a suitable method to reproduce these harmonic currents injected not only into the grid, but also into the dc circuit of the PV installation. The capacitive coupling proposed of PV modules with ground is modeled as a parallel resistance and capacitor arrangement which leads to an accurate approximation to the real operation response of the PV installation. Results obtained are compared with those of simplified models of PV installations used in literature. An experimental validation of the proposed model was performed with field measurements obtained from an existing 1 MW PV installation. Simulation results are presented together with solutions based on the proposed model to minimize the capacitive ground current in this PV installation for meeting typical power quality regulations concerning to the harmonic distortion and safety conditions and to optimize the efficiency of the installation.     

温度波动对质子交换膜燃料电池的影响

为了研究温度波动对质子交换膜燃料电池性能的影响,文章提出了一种新的温度计算模型——温度波动模型。将温度波动模型通过自定义函数导入计算流体动力学软件(Fluent)上进行仿真计算,并建立燃料电池试验测试系统,对工作温度为60℃,进气温度分别为43,50,55℃的电池性能进行测试。通过对Fluent模型、温度波动模型和试验值的比较发现:随着进气温度的升高,温度波动趋于平缓,燃料电池的性能逐渐增强;温度波动模型能够较准确地预测燃料电池的性能,尤其在进气温度为43℃、电流密度为1.088 A/cm2时,其误差比Fluent模型减少30%。     

An analysis on the prediction of temperature distribution in an annular radiative recuperator

Axial temperature distribution for the combustion gas, air and walls of an annular radiation recuperator have been predicted with the use of the model, which A. Schack has developed. The model is based on a kind of the zone method to treat radiation exchange between the combustion gas and its boundaries, which can be used for a wide range of design and operating conditions. Parametric calculations have been done to determine the effects of these on recuperator design. Copyright © 1999 John Wiley & Sons, Ltd.