土壤水热耦合模型在三江源冻土活动层水热变化中的应用

为揭示青海三江源区水文活动规律,描述季节性冻土分布区内冻、融变化过程中土壤内部热量交换和水分迁移等物理过程,采用有限体积法离散热传导方程和非饱和土壤水运动方程并对其进行耦合求解,建立了冻土区土壤水热耦合模型。利用2005～2007年间9个测站的土壤水热观测资料,从不同角度分析了冻土活动层内的土壤水热特征,对土壤融化深度、表层土壤温度及表层土壤含水量等变化过程的模拟验证结果表明,该模型的模拟结果符合当地的水热运动规律。并定量分析检验了模型方法的有效性,揭示了三江源区的土壤水热运动规律,为该地区的生态系统服务提供了有力的支持。     

Computer analysis,design and simulation of horizontal ground heat exchangers

This paper presents a new computerized procedure for dealing with the design of horizontal ground heat exchangers (HGHE). The computer program is based on the transient model of coupled nonlinear partial differential equations governing heat and mass flow in soils. The model is two-dimensional and delineates the operation of ground heat storage with the HGHE and such phenomena as freezing/thawing and drying/rewetting of soil moisture. Comprehensive climatological data, such as ambient temperature, solar radiation, wind velocity, rainfall, snowfall, snow characterstics, and water vapour pressure is used to simulate conditions at the ground surface over any required length of time. The package can be applied to any geographical location by changing climatic and soil data input. The designer has the possibility of selecting any of 12 types of soils from sand to clay, 12 commercial heat pumps, nine different configurations of the HGHE, 16 plastic pipes for ground coils, and 13 ground coil fluids. The program, however, does not calculate the length of the HGHE but it evaluates the thermodynamic performance of a ground heat pump system and provides comprehensive data on thermal and hydraulic conditions in ground heat storage. The length of the ground heat exchanger is obtained from a line source theory model or from site dimensions and pipe spacing. Computed results for ground heat exchanger operation correlate fairly well with experimental data. Simulation of temperature and moisture content in the ground for natural conditions (no heat extraction/deposition) showed a fair agreement with field data. The entire computer program is user-friendly, interactive, menu-driven, and written in FORTRAN 77.     

Modeling of horizontal geoexchange systems for building heating and permafrost stabilization

We present a new analytical model based on the finite line source that extends the steady state results for parallel horizontal pipes to the transient case and for any desired horizontal pipe layout. The analytical model is validated, when there is no freezing/thawing, by a 3D finite element numerical model. When the phase change is accounted for in the numerical model, the analytical model still provides good approximation to the ground temperature during the heating season and the heat extracted by the ground heat exchanger. However, summer ground temperature and thaw depth are overestimated by the analytical model. A case study for a typical building in Kuujjuaq (northern Canada) area is analyzed. The ground heat exchanger layout follows a spiral pattern characterized by three parameters: length L, depth D, and spacing S. The influence of each parameter on the amount of heat extracted from the ground and on the ground temperature at a control point is assessed. The results show that increasing depth D favors keeping the ground frozen at this depth and increases the amount of heat that may be extracted. Conversely, increasing S and/or L is beneficial for the amount of heat extracted, but it enhances the risk of thawing around the pipes. The model and case study provides useful ground heat exchanger design guidelines in cold regions for the double purpose of ground freezing and heat extraction.     

Ground heat storage with a double layer heat exchanger

A one-dimensional model of ground heat storage is used to simulate complex heat and moisture processes in the vicinity of double-and single-layer ground heat exchangers. Simulated results indicate that a ground heat pump with the double-layer heat exchanger has certain advantages over the single-layer system in terms of a higher seasonal performance factor, smaller ground area, and reduced ecological risk in the soil subsurface region. Solar energy injection into the ground improves the seasonal performance factor both in the single and in the double layer heat exchanger system. Latent heat released by the freezing of soil moisture is an important component in the total amount of heat extracted from the ground when a phase change takes place.     

Performance study on heat and moisture transfer in soil heat charging

In some cold areas, the system performance of the soil source heat pump system is reduced by the decreasing underground soil temperature, which is caused by the thermal imbalance between the heating demand in winter and the cooling demand in summer. Soil heat charging with solar energy in non-heating seasons is proposed for solving the problem. It has been found from previous studies that the effect of the moisture transfer on the heat transfer within porous media could not be neglected especially under higher temperature difference. Therefore, this paper provides an investigation on the heat and moisture transfer in soil during soil heat charging at high temperature. A numerical model is developed for the study. The simulation results are compared with the testing data from the authors' previous study for the model verification. Based on the verified model, the performance of the heat and moisture transfer in soil during soil heat charging in a longer time and a larger area is investigated in the paper. The results show that the testing data match very well with the simulation results within a relative error of ±9% and the mathematical model is reliable for the performance prediction of heat and moisture transfer in soil heat charging. The soil volumetric water content (VWC) distribution tends to be stable after soil heat charging for 13 days and the heat source has an effective influence on soil VWC distribution within 2.4?m. The effect of the heat source temperature and initial VWC on the soil temperature and VWC distribution and heat power is proved to be obvious. Loam has a better performance in soil heat charging than sand.     

Heat and mass transfer model of a Ground Heat Exchanger: theoretical development

A new approach to the simulation of a horizontal type Ground Heat Exchanger is proposed resulting in a better accuracy and at the same time a reduced computational effort. These results come from the concentration of the computational effort at the locations with the largest temperature and moisture gradients, i.e. the pipe–soil interface. The model takes into account heat and moisture transfer in the soil allowing for more accurate predictions of the soil thermal response to the heat fluxes induced by the GHE operation. This in turn allows for a more accurate prediction of the soil temperature field and the circulating fluid temperature profile. A comparison of the results obtained by using the implicit and explicit methods of solving the set of governing equations is discussed. The implicit method requires partial linearization of the heat and mass transfer equations but results in a considerably shorter simulation time. The explicit formulation allows for the solution of the fully nonlinear set of heat and mass transfer equations at the expense of increased simulation time. The following analysis shows that the difference between the solutions obtained using these two methods is minimal, thus favouring the implicit formulation. Copyright © 1999 John Wiley & Sons, Ltd.     

Three dimensional numerical modeling of simultaneous heat and moisture transfer in a moist object subjected to convective drying

The drying behavior of a moist object subjected to convective drying is analyzed numerically by solving heat and moisture transfer equations. A 3-D numerical model is developed for the prediction of transient temperature and moisture distribution in a rectangular shaped moist object during the convective drying process. The heat transfer coefficients at the surfaces of the moist object are calculated with an in-house computational fluid dynamics (CFD) code. The mass transfer coefficients are then obtained from the analogy between the thermal and concentration boundary layer. Both these transfer coefficients are used for the convective boundary conditions while solving the simultaneous heat and mass transfer governing equations for the moist object. The finite volume method (FVM) with fully implicit scheme is used for discretization of the transient heat and moisture transfer governing equations. The coupling between the CFD and simultaneous heat and moisture transfer model is assumed to be one way. The effect of velocity and temperature of the drying air on the moist object are analyzed. The optimized drying time is predicted for different air inlet velocity, temperature and moisture content. The drying rate can be increased by increasing the air flow velocity. Approximately, 40% of drying time is saved while increasing the air temperature from 313 to 353 K. The importance of the inclusion of variable surface transfer coefficients with the heat and mass transfer model is justified.     

Heat conduction with seasonal freezing and thawing in an active layer near a tower foundation

In this paper, seasonal freezing and thawing of an active layer near a power transmission tower foundation are investigated experimentally with a metal rod buried in an enclosure filled with soil. The measured soil temperatures are used to examine thermal effects of the tower footing and snow cover on seasonal freezing and thawing cycles. It was found that the metal tower has significant thermal effects on the local region around the tower footing. This increases the thaw depth of the active layer and lengthens the freezing/thawing cycle, which adversely affects the foundation stability and safety. These results provide useful new insight of practical utility in the design and maintenance of power transmission line foundations and other infrastructure in northern regions.     

地源热泵间歇运行下土壤热湿迁移的实验研究

建立地埋管换热土壤热湿迁移过程的实验装置,对地源热泵间歇运行时不同进口流体温度及不同土壤体积含水率下土壤温湿度场的变化特性进行实验研究。实验结果表明∶间歇运行时,入口流体温度的升高会使土壤温度最大值升高,但不利于土壤温度的恢复,土壤体积含水率的增加在一定程度上有利于地下换热和土壤温度的恢复。系统开机后存在土壤温度上升的主上升区,此区温度增幅超过65%,关机后第18小时土壤温度基本恢复至初始温度;系统关停后在温湿度梯度的作用下会出现温度和含水率最大值后移的现象,热源对土壤温度和含水率的作用半径约为280和375 mm;开停比为1∶2时温湿度较1∶1能恢复得更低,合理设置停机时间有利于机组长期有效运行。     

土壤热湿传递下的直埋电缆载流量数值计算

针对土壤热湿过程对直埋电缆载流量的影响,修正土壤热湿耦合模型,联合MAXWELL方程组,建立电磁一热湿三场耦合模型.利用三场耦合模型和COMSOL Multiphysics软件按正交法计算不同敷设土壤类型、干密度和粒径下的YJV220.6/13×6直埋电缆载流量.通过对比正交计算结果的最大、最小载流量组的磁通密度、发热...     

Study of heat and moisture transfer in soil with a dry surface layer

Mathematical model for describing simultaneous heat and moisture transfer in the porous soil with a dry surface layer was developed by using the volume-averaging method. Numerical simulation was conducted to investigate water evaporation, transient distributions of temperature and moisture in the porous soil at environmental conditions, which might be useful for agricultural application. In order to validate the mathematical model and numerical method, an experiment was conducted under natural environmental conditions. An additional experiment was conducted in a closed-loop wind tunnel to investigate the temperature effect on soil moisture transport. Theoretical and experimental results indicate that the dry surface layer has an important effect on heat and moisture migration in soil and the influence of temperature on moisture transport in unsaturated soil is significant.     

Dynamic Simulation and Operating Characteristics of Ground-Coupled Heat Pump with Solar Seasonal Heat Storage System

Abstract

A hybrid ground-coupled heat pump (GCHP) is an efficient and sustainable technology for space heating and cooling. A demonstration house equipped with GCHP with a solar seasonal heat storage (SSHS) system had been built in Harbin, a severe cold zone of China. A dynamic simulation model was built for the house and GCHP with the SSHS system using TRNSYS. The model used a newly developed vertical ground heat exchanger (VGHE) module which considered coupled heat and moisture transfer (CHMT) in ground with variable soil properties (VSPs) and phase change of soil moisture (PCSM). In the simulation, a large amount of computing is consumed for VSP and PCSM, while the computing amount for moisture transfer is small. The model with the new VGHE module produced better simulated results, compared with the field data. So, CHMT-VSP-PCSM affects the performance of VGHE and system to some extent, especially CHMT. Hourly variation laws of temperatures and energy parameters were analyzed, and different characteristics were showed up at different operating stages in heating and cooling seasons for both long and short terms. The GCHP with the SSHS system can meet the heating and cooling demands of the house in general. In cooling season, adjusting the ratio of the two groups of VGHE for heat storage and cooling will increase the utilization efficiency of VGHE and make the soil temperature more balanced.     

Modeling of Air to Air Enthalpy Heat Exchanger

The thermal performance of a Z-shaped enthalpy heat exchanger utilizing 45-gsm Kraft paper as the heat and moisture transfer surface for heating, ventilation, and air conditioning (HVAC) energy recovery is experimentally investigated through temperature and moisture content measurements. A mathematical model is developed and validated against the experimental results using the effectiveness-NTU method. In this model the paper moisture transfer resistance is determined by paper moisture permeability measurements. Results showed that the paper moisture transfer resistance is not constant and varies with moisture gradient across the paper. Furthermore, the model is used to predict the heat exchanger performance for different heat exchanger flow configurations. The results showed that higher effectiveness values are achieved when the heat exchanger flow path width is reduced. Temperature and moisture distribution in the heat exchanger is also studied using a computational fluid dynamics package (FLUENT). To model the moisture transfer through the porous materials a nondimensional sensible–latent effectiveness ratio was developed to obtain the moisture boundary conditions on the heat exchanger surface.     

Soil temperature distribution around a U-tube heat exchanger in a multi-function ground source heat pump system

The imbalance of heat extracted from the earth by the underground heat exchangers in winter and ejected into it in summer is expected to affect the long term performance of conventional ground source heat pump (GSHP) in territories with a cold winter and a warm summer such as the middle and downstream areas of the Yangtze River in China. This paper presents a new multi-function ground source heat pump (MFGSHP) system which supplies hot water as well as space cooling/heating to mitigate the soil imbalance of the extracted and ejected heat by a ground source heat pump system. The heat transfer characteristic is studied and the soil temperature around the underground heat exchangers are simulated under a typical climatic condition of the Yangtze River. A three-dimensional model was constructed with the commercial computational fluid dynamics software FLUENT based on the inner heat source theory. Temperature distribution and variation trend of a tube cluster of the underground heat exchanger are simulated for the long term performance. The results show that the soil temperature around the underground tube keeps increasing due to the surplus heat ejected into the earth in summer, which deteriorates the system performance and may lead to the eventual system deterioration. The simulation shows that MFGSHP can effectively alleviate the temperature rise by balancing the heat ejected to/extracted from underground by the conventional ground source heat pump system. The new system also improves the energy efficiency.     

Performance of a spiral ground heat exchanger for heat pump application

A high-efficiency ground heat exchanger has been developed for use with ground-source heat pumps. The exchanger is made of copper tubing, shaped in the form of a spiral, which can be installed in a vertical borehole backfilled with sand. Thermal performance of a full-scale prototype indicated that this heat exchanger can achieve very high heat extraction rates if subfreezing operating temperatures are used. For most soil types cyclic freezing and thawing is not a problem; however, for the sensitive Leda clay in which the prototype tests were conducted, substantial settlement occurred after the first freeze-thaw cycle owing to initial collapse of the soil structure.     

冰球式蓄冷槽内冰球冻结和放热特性的数值模拟分析

在冰蓄冷系统中,冰球式蓄冰槽是一种常用的蓄冰设备;通过相变材料的潜热保存冷量。对冰球式蓄冰槽内冰球的冻结和解冻过程进行了数值模拟,通过对比冻结曲线和解冻曲线,探究了冰球不同的排布方式对冰球冻结速率和解冻速率的影响。结果显示:采用顺排方式的蓄冰槽冰球冻结时间为29 400 s,而采用错排方式的蓄冰槽冰球冻结时间为25 200 s,错排方式能够提高冰球在预冷阶段的冷却速率,从而降低冻结时间;在解冻过程中,错排方式融冰相变过程的时间明显小于顺排方式,而融冰后的显热阶段两种排列方式的差异不大。     

Lattice Boltzmann simulation of heat transfer with phase change in saturated soil during freezing process

A lattice Boltzmann model is presented for simulating heat transfer with phase change in saturated soil. The model includes a quartet structure generation set for creating soil structure, double distribution functions for simulating temperature field evolution of soil particles and water, respectively, and an enthalpy-based method for tracing phase interface. The model is validated by two cases with analytical solutions. Then, we investigate the influence of porosity on freezing process in saturated sandy loam soil. The results demonstrate that porosity is the predominant factor when the location is far from the cold source; otherwise, thermal gradient is more important.     

PREDICTIONS OF DEVELOPING FLOW WITH BUOYANCY-ASSISTED FLOW SEPARATION IN A VERTICAL RECTANGULAR DUCT: PARABOLIC MODEL VERSUS ELLIPTIC MODEL

ABSTRACT

This article reports the unsteady-state heat transfer analysis of unpaved and paved granite soil media in a closed system in order to predict the temperature profiles along a buried pipeline and the frozen penetration depth (FPD). These studies focused on the development of a computational scheme by applying the effective heat capacity model to numerical procedures. The proposed model took into consideration the phase-change effect of in-situ pore water in the frozen fringe. To realize these objectives, several physical values for granite soil of South Korea at various subzero temperatures were determined in laboratory tests. The comparison of results obtained from an unpaved freezing soil medium by the proposed model and the actual performances was valuable in establishing a level of confidence in the application of the theory introduced.     

太阳辐照下湿份分层土壤中热湿传输的数值模拟

考虑温度对土壤湿分迁移的影响，建立描述存在干饱和层时的土壤热湿传递的数学模型，并就自然环境和恒定太阳辐照下两种情况进行数值模拟，获得不同环境条件下土壤中温度和湿分分布以及水分蒸发的动态特性，分析干饱和土壤层对土壤热湿迁移与水分蒸发以及温度对土壤湿分传输的影响。     

埋地换热器理论模型与周围土壤温度数值模拟

提出内热源型埋地换热器理论模型，建立换热器周围土壤热湿传递物理数学模型。内热源模型综合考虑热湿迁移、土壤物性等各方面因素，将埋设于土壤中的换热器处理为等效内热源。采用专业多孔介质计算软件Autough2对模型进行模拟计算。着重模拟计算不同土壤物性、不同换热器运行方式对单根U型垂直埋管换热器周围土壤温度场影响的模拟计算与分析。