Numerical simulation on heat transfer performance of vertical U-tube with different borehole fill materials

Heat exchange performance of vertical U-tube heat exchanger was studied with two different borehole fill materials and CFD software. Borehole surface temperature and water temperature distribution were simulated on the condition of continuous operation for 8 h in winter with inlet water temperature being 10℃. The results show that there is no obvious difference on heat exchanger performance between the two different borehole fill materials.

Advances in ground heat exchangers for space heating and cooling:Review and perspectives

As a renewable energy source,geothermal energy has been widely used to provide space heating and cooling for buildings.The thermal performance of ground heat exchanger(GHE)is significant for the operating efficiency of the ground source heat pump(GSHP)systems.This paper presents a comprehensive review of developments and advances of three kinds of GHE,including vertical borehole GHE(VBGHE),Pile GHE(PGHE),and deep borehole GHE(DBGHE)which are currently popular in larger GSHP systems.Firstly,analytical models proposed to ana-lyze heat transfer process of VBGHE with different geological conditions are summarized,such as homogenous or heterogeneous ground,with or without groundwater advection.Numerical and short-time step models and measures to improve GHE thermal performance are also reviewed.Secondly,a summary of research advances in PGHE is provided,which includes the heat transfer models of PGHE,the effects of geometric structure,oper-ation modes,pile spacing,use of phase change material(PCM),thermal properties of PCM,thermo-mechanical behavior and/or thermal performance of PGHE.The effects of groundwater flow direction and velocity on PGHE are also summarized in brief.Lastly,models of three kinds of DBGHEs,i.e.,deep coaxial GHE(DCGHE),deep U-bend GHE(DUGHE)and super-long gravity heat pipe(SLGHP),are reviewed.The physical bases of the dif-ferent analytical models are elaborated and also their advantages and disadvantages are described.Advances in numerical modelling and improving numerical model calculation speed of DCBHE,DCBHE array,and DUBHE are summarized.The review provides a meaningful reference for the further study of GHEs.

Experimental evaluation of a“U”type earth-to-air heat exchanger planned for narrow installation space in warm climatic conditions

The thermal performance of a“U”type earth-to-air heat exchanger is presented in this experimental study.The device has a serial-connected vertical configuration.The wells where tubes were installed have a depth of fewer than 3 m and are separated every 1.5 m,using an installation area of 3m2.The experimentation was carried out in March in Morelos,Mexico when the environmental temperature reaches 35℃ during the day.The performance of the device was measured and compared to the requirements of an office for cooling purposes within a university campus to reproduce the space restrictions found in urbanized areas.By using a small land surface,it is feasible for urbanized areas.The air temperature inside the“U”type earth-to-air heat exchanger,the surrounding soil temperature,the airspeed,and the power consumed by the fan were measured.The air temperature and the fan’s power consumption data were obtained by modifying the airspeed in four constant values,from 1.3 m/s to 6.6 m/s.Results show that the device evaluated in this work has adequate thermal performance for cooling purposes compared to the requirements of an office.A decrease in air temperature was recorded in a range of 5.1℃ to 9.4℃.Over 70%of the total temperature difference was reached in the first well,where the average soil thermal disturbance at 5 cm was 2.8℃.The device achieved a maximum COP of 12.8 and a maximum effectiveness of 88.4%.With these results,it is concluded that the system is suitable for cooling purposes in areas with space restrictions.This work is novel since the dimensions available for installation in urbanized areas are considered and compared with the thermal requirements of an office.In addition to the fact that there are no published works with vertical heat exchangers connected in series.

Submesoscale-enhanced filaments and frontogenetic mechanism within mesoscale eddies of the South China Sea

Submesoscale activity in the upper ocean has received intense studies through simulations and observations in the last decade,but in the eddy-active South China Sea(SCS)the fine-scale dynamical processes of submesoscale behaviors and their potential impacts have not been well understood.This study focuses on the elongated filaments of an eddy field in the northern SCS and investigates submesoscale-enhanced vertical motions and the underlying mechanism using satellite-derived observations and a high-resolution(∼500 m)simulation.The satellite images show that the elongated highly productive stripes with a typical lateral scale of∼25 km and associated filaments are frequently observed at the periphery of mesoscale eddies.The diagnostic results based on the 500 m-resolution realistic simulation indicate that these submesoscale filaments are characterized by cross-filament vertical secondary circulations with an increased vertical velocity reaching O(100 m/d)due to submesoscale instabilities.The vertical advections of secondary circulations drive a restratified vertical buoyancy flux along filament zones and induce a vertical heat flux up to 110 W/m^(2).This result implies a significant submesoscale-enhanced vertical exchange between the ocean surface and interior in the filaments.Frontogenesis that acts to sharpen the lateral buoyancy gradients is detected to be conducive to driving submesoscale instabilities and enhancing secondary circulations through increasing the filament baroclinicity.The further analysis indicates that the filament frontogenesis detected in this study is not only derived from mesoscale straining of the eddy,but also effectively induced by the subsequent submesoscale straining due to ageostrophic convergence.In this context,these submesoscale filaments and associated frontogenetic processes can provide a potential interpretation for the vertical nutrient supply for phytoplankton growth in the high-productive stripes within the mesoscale eddy,as well as enhanced vertical heat transport.

Insights into multidimensional transport flux from vertical observation and numerical simulation in two cities in North China

This study represents the first quantitative evaluation of pollution transport budget within the boundary layer of typical cities in the Beijing-Tianjin-Hebei (BTH) region from the perspective of horizontal and vertical exchanges and further discusses the impact of the atmospheric boundary layer (ABL)-free troposphere (FT) exchange on concentration of fine particulate matter (PM_(2.5)) within the ABL during heavy pollution.From the perspective of the transport flux balance relationship,differences in pollution transport characteristics between the two cities is mainly reflected in the ABL-FT exchange effect.The FT mainly flowed into the ABL in BJ,while in SJZ,the outflow from the ABL to the FT was more intense.Combined with an analysis of vertical wind profile distribution,BJ was found to be more susceptible to the influence of northwest cold high prevailing in winter,while sinking of strong cold air allowed the FT flowing into the ABL influence the vertical exchange over BJ.In addition,we selected a typical pollution event for targeted analysis to understand mechanistic details of the influence of ABL-FT exchange on the pollution event.These results showed that ABL-FT interaction played an important role in PM_(2.5)concentration within the ABL during heavy pollution.Especially in the early stage of heavy pollution,FT transport contributed as much as 82.74% of PM_(2.5)within the ABL.These findings are significant for improving our understanding of pollution transport characteristics within the boundary layer and the effect of ABL-FT exchange on air quality.

Numerical simulation for thermal response test performance in closed-loop vertical ground heat exchanger

In this study, a series of numerical analyses was performed in order to evaluate the performance of full-scale closed-loop vertical ground heat exchangers constructed in Wonju, South Korea. The circulating HDPE pipe, borehole and surrounding ground formation were modeled using FLUENT, a finite-volume method (FVM) program, for analyzing the heat transfer process of the ground heat exchanger system. Two user-defined functions (UDFs) accounting for the difference in the temperature of the circulating inflow and outflow fluid and the variation of ground temperature with depth were adopted in the FLUENT modeling. The thermal conductivities of grouts (cement vs. bentonite) measured in laboratory were used as input values in the numerical analyses to compare the thermal efficiency of the cement and bentonite grouts used for installing the closed-loop vertical ground heat exchanger. A series of numerical analyses was carried out to simulate in-situ thermal response tests performed in the construction site. From the comparison between the in-situ thermal response test results and numerical simulations, the average thermal conductivity of the ground formation in the construction site is back-calculated as approximately 4 W/mK. This value can be used in evaluating the long-term performance of the closed-loop vertical ground heat ex changer.