Numerical Simulation Of Underground Heat Exchangers Under The Influence Of Groundwater Advection

In the north of China,the consumption of heating energy in winter is an important part of housing energy consumption.The ground source heat pump(GSHP)is a device to transfer heat from the cold source to the heat source,it is usually get renewable low-grade heat from nature and transfer into high-grade heat in electromagnetic induction,then output for people to use.In addition,GSHP is a system not only energy-saving,environmentally friendly,but also meets the requirements for sustainable development.But,due to the heating load in winter is greater than cooling load in summer,so the absorbing heating is less than the releasing heating in GSHP,it will lead to cold or heat accumulation,resulting in ground heat imbalance after long-time running,make the temperature of the soil reduced,which will reduce the heating load and coefficient of performance(COP),and it will affect the operational efficiency.Therefore,to maintain the balance between heating load and cooling load is important to ensure GSHP system running high efficiently.The domestic and foreign workers focus on how to improve the operational efficiency of the GSHP,such as,storage heat through solar collection-heating-storage system in non-heating season,changing the operating modes of GSHP,the space of GSHP,the depth of GSHP and the parameters of material in soil.In practical engineering,the influence of groundwater will effects the heat transfer of the heat exchanger.In this thesis,three-dimensional heat transfer model between U-tube underground heat exchanger and soil was developed,the Matlab software was used to simulate every condition and analysed the datas in order to make the best condition of GSHP.The concrete research content is as follow: 1)Three-dimensional heat transfer model that is one-dimension heat transfer in vertical direction of U-tube and two-dimension on a horizontal surface of soil,which is coupled between the out surface temperature and the soil temperature.2)The finite difference method(FDM)is used to discrete the U-tube heat transfer model,first order upwind difference is used to solve the soil heat transfer model due to the direction of groundwater.3)Simulated and analysed the soil temperature field and the exchanging heat quantity for different flow velocity(61 10 m / s-? ?62 10 m / s-? ?63 10 m / s-?).4)Simulated and analysed the soil temperature field and the exchanging heat quantity for different parameters of material in soil(soil density,heat capacity,porosity and thermal conductivity)under the influence of groundwater.5)Simulated and analysed the soil temperature field and the exchanging heat quantity for different inlet water temperature(35? ?55??75?)and inlet flowrate(0.3kg / s ?0.5kg / s?0.7kg / s)of U-tube under the influence of groundwater.6)Simulated and analysed the soil temperature field and the exchanging heat quantity for different operation modes(continuous and intermittent system)under the influence of groundwater.7)Simulated and analysed the exchanging heat quantity under intermediate condition in layered soils.8)Simulated and analysed the soil temperature and exchanging heat quantity of multilayered soils.The results show that,1)Under the influence of groundwater,the soil temperature field will offset along the flow direction,the faster groundwater flows,the greater the distance offset;and the heat exchanging quantity with groundwater flow is greater than it without groundwater flow.2)Different soil physical proper have different impacts on heat exchanging: the greater the thermal conductivity is,the smaller the thermal resistance will be,and the exchanging heat quantity is greater,so greater thermal conductivity is beneficial to heat exchanging;the greater the heat capacity is,the smaller the soil temperature field will be,and the exchanging heat quantity is smaller too,so greater heat capacity is bad for heat exchanging;the greater the soil density is,the weaker the heat transfer ability will be,so greater soil density is bad for heat exchanging;the greater the porosity is,the weaker the heat transfer ability will be.3)The greater the inlet water temperature or inlet flowrate is,the greater the exchanging heat quantity will be,so greater the inlet water temperature or inlet flowrate is beneficial to heat exchanging.4.The exchanging heat quantity is greater under intermittent system than it under continuous system.Through the above research,it is worthy to guide the design of U-tube underground heat exchanger.