Borehole Ground Research Articles

Design and optimization of ground‐coupled refrigeration heat exchanger in Dubai: Numerical approach

AbstractGround‐coupled heat exchangers (GCHE) have received significant attention over several decades as a result of increasing the world's energy demand and the need for reducing fossil fuels consumption. Prior studies have demonstrated the effectiveness of utilizing GCHE with refrigeration and heating systems. However, optimizing the performance of GCHE coupled with chillers for heat rejection, especially in extreme hot‐humid climates (where cooling towers are not very effective) is lacking in the literature. In this work, a ground borehole fitted with a coaxial‐tubes heat exchanger (BHE) is numerically simulated. Based on a wide range of data collected for the soil of Dubai, its real in situ thermophysical properties are characterized. The soil's upper layer thickness is relatively small and dry that operates in conduction mode, while the lower one is water‐saturated that works in coupled conduction‐advection mode. The study aims at optimizing the parameters advancing heat rejection into ground considering the actual properties of the soil of Dubai. The results indicate that the more feasible high‐density polyethylene pipes can perform as good as the steel ones. Also, a great finding based on the presented novel design is that insulating the inner pipe can increase the temperature duty by 55%. The proposed design of BHE is relatively inexpensive, more feasible and efficient, which is achieved for the first time based on a deep analysis of Dubai climate and soil. This makes the technology ready to be implemented for industrial applications in Dubai and other regions having a similar climate and soil nature.

Research on the long-term operation performance of deep borehole heat exchangers array: Thermal attenuation and maximum heat extraction capacity

The long-term operation performance and thermal attenuation of deep borehole heat exchangers (DBHEs) array are important for their stable and safe operation. This paper proposed a semi-analytical method to quantitatively evaluate the 100-years operation performance of DBHEs array. Numerical simulation was utilized to determine the heat flux per depth of DBHE, while the finite line heat source model was modified to calculate the ground temperature variation. This allowed to obtain the ground borehole wall temperature, serving as a boundary condition for calculating the water temperature distribution in DBHE using an analytical solution. The results revealed that the line space of DBHEs array directly affected the allowed maximum accumulated heat extraction per DBHE per heating season (Qa,max). With line space decreasing from 100 m to 10 m, the Qa,max decreased from 0.3% to 19.1% compared to the single DBHE. Therefore, a line space larger than 30 m was recommended. Considering practical project with limited space, the inclined DBHEs were recommended in depth direction. For DBHEs array with a line space of 10 m at the surface ground and 100 m at a depth of 2500 m, Qa,max was only 3.2% smaller compared to the single DBHE, which showed an improvement over DBHEs array with a line space of 50 m. The proposed semi-analytical method could be applied in system design, to ensure the long-term safety and stability of DBHEs array.

Minimum Quantity Lubrication (MQL) multiphase dynamics of a vibration-assisted drilling process

Metalworking fluid (MWF) consumption in machining processes can be minimised using minimum quantity lubrication (MQL). In the vibration-assisted drilling process, the MWF is transported as aerosol (dispersed droplets in gas) to the cutting zone through microchannels inside the drilling tool. Numerical simulations have been applied to elucidate the multiphase flow dynamics of the MWF inside a helical microchannel of a twist drilling tool. Numerical results indicate a transition from the inserted MQL aerosol flow and a wall film already after the first helical bend of the microchannel. The wall film contains > 90 % of the lubricant and is strongly localised on only 10 % of the microchannel circumference at the tool tip. This MQL-flow behaviour has to be considered in future investigations of the processes at the borehole ground, where even more complex phenomena occur.

Performance analysis of a solar thermal system coupled with a ground source heat pump in Nordic conditions

The main objective of this work is to investigate the energy and exergy performance of a solar assisted ground source heat pump for a school building designed according to Norwegian passive house standard. The system is designed in such a way that the solar collector prioritizes to provide heat to the hot water tank and whenever there is excess heat from the solar collector it can be used to charge the ground borehole. A detailed thermodynamic analysis of the system has been carried out in Engineering Equation Solver (EES). Isobutane (R600a), propane (R290), ammonia (R717) and solstice (R1234ze(E)) have been investigated as working fluid for the heat pump. The result revealed that the solar collector and the different working media have affected both the COP and the exergy of the system. Moreover, the mass flow rate of the brine and the length of the borehole affect the performance of the system.

A non-Darcy gas flow model for coalbed methane in mine gobs

The expected extraction efficiency of coalbed methane (CBM) depends significantly on the laws considered to govern its gas flow. This study applies a non-Darcy gas flow model to describe the CBM migration in mine gobs; by mine gob, we mean that it is a fractured zone along with massive cracks and the primary place where gas flows after mining activities. A permeation experiment involving crushed sandstone is first conducted to prove the CBM undergoes Forchheimer-type non-Darcy flow. Subsequently, the three-dimensional continuous distribution functions of the permeability parameters are determined. The non-Darcy flow model includes the influence of inertial force on the gas flow, which is neglected in the Darcy model. A coupling model is established based on the experimental results and the gas flow characteristics in different regions. Thereafter, the model and distribution functions are applied to a series of numerical simulations of CBM extraction at the Sihe coal mine in China, to ascertain the most appropriate location for a ground borehole. These simulations involve boreholes placed in three different zones: the natural accumulation zone (NAZ), the load affected zone (LAZ), and the compaction stable zone (CSZ). The simulation results show that the total extraction quantity expected from the borehole in the NAZ is 2.4 and 13.5 times that from the boreholes in the LAZ and CSZ, respectively. This confirms that the NAZ is the most suitable zone for a borehole. This research ultimately provides a realistic gas flow model for CBM extraction from mine gobs.

Research on Strong Ground Pressure of Multiple-Seam Caused by Remnant Room Pillars Undermining in Shallow Seams

Numerous room-and-pillar mining goaf are apparent in western China due to increasing small coal mining activities, which causes the collapse of the overlying coal pillars and the occurrence of strong ground pressure on the longwall face and surface subsidence. In this study, Yuanbao Bay Coal Mine, Shuozhou, Shanxi, was selected to study the collapse of the overlying coal pillars on the longwall face and reveal the mechanism of the pillar collapse and the disaster-causing mechanism caused by strong ground pressure. Results show that the dynamic collapse process of coal pillars is relatively complicated. First, the coal pillars on both sides of the goaf are destroyed and destabilized, followed by the adjacent coal pillars, which eventually cause a large-scale collapse of the coal pillars. This results in a large-scale cut-off movement of the overlying strata, and the large impact load that acts on the longwall face causes an unmovable longwall face support. Moreover, the roof weighting is severe when strong ground pressure occurs on the longwall face, causing local support jammed accidents. Furthermore, the data of each measurement point of the strata movement inside the ground borehole significantly increases, and the position of the borescope peeping error holes in the ground drill hole rise steeply. The range of movement of the overlying strata increases instantaneously, and the entire strata begin to move. Research on the mechanism of strong ground pressure can effectively prevent mine safety accidents and avoid huge economic losses.

Treatment Technology of Closed Tunnel Filled with Water

During the construction of No.3 connection gallery between huyong station and lvdaohu station of first phase project of Line 2 Foshan Rail Transit, water and sand kicks caused the large-scale settlement of tunnel segments. After comparison and selection of schemes, the rehabilitation scheme of abandoning the existing right line tunnel, shield machine starting from huyong station and heading to Lvdaohu Station by changing the line, is finally adopted. In order to prevent secondary disasters caused by the collapse of abandoned tunnels, the ground boreholes are used to fill the abandoned tunnels. During the construction, a series of key construction technologies have been formed, such as sectional backfill of abandoned tunnel, alternate backfill of underwater concrete and mortar, and prevention of pipe blockage. The results of core-drilling test and subsequent construction procedures show that the treatment scheme of abandoned tunnel is reliable and feasible, which can provide reference for similar projects.

Influence of the Aggregate-Pouring Sequence on the Efficiency of Plugging Inundated Tunnels through Drilling Ground Boreholes

This paper presents an experimental and field investigation on the efficiency of plugging by pouring aggregate in different sequences through multiple boreholes in a tunnel with flowing water. There have been controversies surrounding the selection of the pouring order for different particle sizes of aggregates and the order in different boreholes. A visualized experimental setup is used to investigate the influence of the pouring orders on the efficiency of plugging through multiple boreholes under the flowing-water condition. A case study of the salvage of a flooded mine using ground directional boreholes was investigated and compared with the experimental results. The water-pressure difference at the aggregate-capping moment, when fine aggregate was poured first and coarse aggregate later, was relatively small, compared to that when fine aggregate was poured upstream and coarse aggregate, downstream. The result implies that the efficiency of plugging with the order of pouring fine aggregate first and coarse aggregate later in different boreholes is better than that with the order of pouring fine aggregate upstream and coarse aggregate downstream. When the poured aggregate is about to be capped, increasing the pouring intensity with the same or a larger particle size is more conducive to capping. The case study shows that pouring fine materials in the early stage reduced the cross-sectional area; in the later stage, the aggregate particle size was gradually increased, which can be helpful in forming an effective water-barrier section in the tunnel. The pouring of aggregate provided a base for cement grouting to form a water-plug section with a length of 106 m, resulting in a sealing efficiency of 100% for the case.

Dynamic simulation and multi-objective optimization of a solar-assisted desiccant cooling system integrated with ground source renewable energy

This study presents a dynamic simulation-optimization of a solar-assisted desiccant cooling system integrated with a ground source heat exchanger (SDCS-GSHE). Solar and ground source energies are used for regenerating the desiccant wheel (DW) and a pre-cooling process, respectively. The system is considered as an alternative for extremely hot and humid regions. Determinant design parameters of the SDCS-GSHE that are associated with the DW, GSHE, and solar loop components directly affect the system behavior and, consequently, the provided thermal comfort, as well as the solar fraction (SF). Therefore, a multi-objective genetic algorithm optimization is invoked to determine all viable optimum design parameters to set up the system. Also, an economic assessment of the system is performed to demonstrate its economic feasibility. With the results, the optimum regeneration temperature, number of ground boreholes, collector area can be determined based on the required level of thermal comfort and the SF. The results reveal that, in the absence of the GSHE, the system cannot provide thermal comfort in extremely humid regions even with high regeneration temperatures (around 120 °C). Instead, using the GSHE dramatically improves the established thermal comfort. For regeneration temperatures below 90 °C, it is determined that the total necessary energy can be supplied entirely by solar energy. For the best case, in which the system successfully provides thermal comfort, the cost payback period for the system is found to be 5.7 years.

A New Flank Face Design Leading to an Improved Process Performance when Drilling High-Temperature Nickel-Base Alloys

In previous investigations, the concept of a modified flank face design with a retraction behind the cutting edge was developed. Based on fundamental investigations regarding the mechanical and thermal loads, combined with simulation-based analyses, a new design of the flank face has been created with the largest possible free volume behind the cutting edge while maintaining the mechanical stability required for the challenging machining of nickel-base alloys. This flank face modification allows a greater amount of cutting fluid to flow along the back of the cutting edge at higher velocities while at the same time creating the potential of a friction reduction between the tool tip and bore hole ground. The described concept was implemented on solid carbide twist drills which were then used to machine Inconel 718. The results show that, while tools equipped with a flank face retraction perform up to four times longer compared to standard tools, they tend to reach the end of tool life rather suddenly due to cutting edge chipping. This typically occurred when the maximum width of flank wear land equalled the width of the remaining cutting edge. Since then, further investigations have been carried out in order to refine the concept of the flank face modification. A new design was implemented with regard to a more effective cooling of the flank face, presumably caused by an improved cutting fluid flow velocity particularly at the cutting edge corners. Simulation-based findings, combined with experimental tests of the new prototypes, could reveal that the improved cutting fluid flow in the area of the cutting edges leads to significantly extended tool life and machining performance when drilling Inconel 718.

Insights into parameter estimation for thermal response tests on borehole heat exchangers

Thermal response tests (TRTs) are performed on borehole heat exchangers in order to evaluate ground and borehole thermal properties, which are needed in the design of ground-source heat-pump systems. When parameter estimation techniques are applied to evaluate the ground thermal conductivity and borehole thermal resistance, the map of the root mean square error (RMSE) sometimes shows a shallow descent to the global minimum. This study obtains a steeper descent by estimating the component of borehole resistance without heat capacity as a separate parameter. A temperature derivative curve is introduced to identify the boundaries between three distinct periods of a TRT, which are borehole-dominated, transition, and steady heat-flux periods. The effective borehole resistance and ground thermal conductivity can be evaluated independently from each other during the borehole-dominated and steady heat-flux periods, respectively. Meaningful estimates of the ground volumetric heat capacity can be obtained from the TRTs in this study in only limited cases. The best chance is when the grout and ground thermal conductivities are nearly equal and the borehole thermal resistance is small.

Development and Application of ZMK5200QJY40 type Mining Rescue Vehicle for Hoisting

ZMK5200QJY40 type Mining Recovery Vehicle for Hoisting was designed for quick borehole rescue. In this paper, the traits between underground rescue and ground borehole rescue were analyzed, technical principle of the vehicle was introduced, the technical parameters of rescue capsule and hoisting control system were listed. Of the 11 key parts in the vehicle, the rescue capsule design and the design of hoisting winch with large rope capacity were introduced in detailed. It was tested in Pingshang Coalmine of Qinxiu Coal Co., Ltd in Jincheng, Shanxi. It was validated that the vehicle is useful for normal borehole or sleeve with the diameter of 580mm to 620mm. With the help of the vehicle, the miners trapped in 900 meters underground can be rescued quickly and safely. ZMK5200QJY40 type Mining Recovery Vehicle for Hoisting was developed successfully; it can be used for vertical rescuing in large borehole. Matching with the new-designed rescue drilling rig, a package of emergency rescue technology and equipment were formed.

A thermomechanical analysis leading to a novel flank face design providing longer tool lives for tools used in the drilling of Inconel 718

Inconel 718 is a versatile material due to its excellent properties. However, machining and in particular drilling of Inconel 718 present great challenges for the tools. In order to increase process reliability and tool life, a novel modification for the applied drilling tools was developed. It consists of a flank face retraction behind the cutting edge which is created by grinding. While regular drilling tools feature a continuous flank face on the tool tip, the retraction provides a minimized frictional surface between the tool tip and the bore hole ground. Furthermore, computational fluid dynamics simulations have shown that the created vacuous volume behind the cutting edge becomes filled with cutting fluid during the drilling process. The described effects result in longer tool lives which could be shown in experimental investigations. This paper presents the conducted experiments in which both the occurring mechanical and thermal loads were examined in order to determine the radius-dependent thermomechanical loads along the cutting edge. The results have shown that an increase in either cutting speed or feed result in significantly higher temperatures at the cutting edge corners while temperatures at the inner diameter of the tool remained almost constant. At the same time, a rise in feed leads to notably higher mechanical loads along the whole cutting edge. In addition, reference tool life tests were conducted using standard tools and under variation of the cutting values. The resulting tool wear as well as the tool-workpiece contact zones were investigated in detail. The gathered findings were then used to develop a flank face modification whose sufficient thermomechanical stability was ensured by FEM simulations. Afterwards, the new tool design was applied to drilling tools and then validated in field tests. The results show that a considerably higher drilling distance as well as lower tool wear and thus a longer tool life could be achieved with the modified drills.

A case study of water inrush incident through fault zone in China and the corresponding treatment measures

In 2010, a water inrush accident through a fault with a high flow rate occurred at Tianzhuang coal mine in Jining, Shandong Province, China, which was subject to the fault excavation. Grouting is an important treatment method before and after water inrush accidents. In this paper, the evolution of this water inrush accident is studied, and the effective treatment method is presented. The obtained results indicate that groundwater from Ordovician limestone aquifer 28.5 m below the floor flowed into the roadway through the fault zone. Results also show that this water inrush through the fault zone can be divided into a flow rate steady increase phase, a rapidly increase phase, a decrease phase, and a steady phase. The first two phases resulted from permeability increase of the fault, which was caused by seepage failure, and water level of the aquifer led to last two phases. Additionally, grouting using the ground borehole successfully controlled this water inrush accident, and it is proven to be a valid method.

A steady and quasi-steady state analysis on the CO2 hybrid ground-coupled heat pumping system

This article contains the steady and quasi-steady state analysis on a CO2 hybrid ground-coupled heat pumping system for warm climates. The hybrid system uses a combination of ambient air and ground boreholes as a heat sink for the cooling mode, while only the ground boreholes are used as a heat source in the heating mode. The steady state analysis suggests that the optimal control strategy of gas cooler pressure for a CO2 hybrid transcritical cycle is based on the optimal cooling COP value and the ratio of heat rejected to ambient air. This optimal control strategy is important for decreasing the annual ground thermal imbalance performance of ground boreholes. In addition, the quasi-steady state model of a CO2 hybrid ground-coupled heat pumping system is constructed for the hourly simulation with different boundary conditions. Simulation results show the details of the system operating characteristics both for heating and cooling modes and the COP values with different operating and design conditions are presented.

Geophysical and Hydrogeological Groundwater Prospectivity Mapping in the Kraaipan Granite-Greenstone Terrain, Southeast Botswana

Application of regional geophysical methods for hydrogeological purposes has increased over the last two decades especially in arid and semi-arid areas. A project to map the Kraaipan granite-greenstone terrain in southeast Botswana has recently been undertaken using regional aeromagnetic and gravity data with the aim to map the rocks at depth to understand the geology while the secondary objective was to subsequently assess the mineralization and groundwater potential in the area. An integrated analysis of the aeromagnetic and gravity data and their derived/processed products is hereby investigated for groundwater for drinking and agricultural purposes. The studies include: subsurface characterisation and delineation of structural framework suitable for groundwater exploration and determination of petrophysical relationships used to link the geophysical properties (e.g., density) to hydrological properties (e.g., porosity). The results of interpretation indicate that the rocks are under ~50 m of Kalahari cover and the study area is composed of three aquifers: the extensive hard rock aquifer (granitic and volcanic), the important (fractured) karst aquifer and the minor sedimentary aquifer. The area is dissected by an ENE-to-EW-trending dyke swarm visible on the regional aeromagnetic data and much clearer on high resolution aeromagnetic data. Minor fault and/or dyke elements of NW-SE and NE-SW trend are observed. Spectral analysis reveals three main average ensample interfaces at depths of 0.7 km, 1.99 km and 4.8 km. The linear Euler solutions maps reveal that the majority depths to top of magnetic bodies range from 40 m to 400 m throughout the survey area. The shallowest depths are the most significant one in this case as they probably relate to depth of bedrock and thickness of regolith or thickest sediments. For 2695 existing boreholes analysed, maximum borehole depth is 482 m (mean 108 m), and almost half (1263) were dry with another 972 having low yield (1 - 5 m3/hr) and 432 yielding 6 - 49 m3/hr and only 28 above 50 m3/hr (maximum ~160 m3/hr) and an average water strike of 64 m. There is very little correlation between interpreted hydrogeological features and the existing borehole locations. The study shows the importance of preliminary geophysical investigations before ground borehole siting and drilling in order to improve borehole success rates and/or reduce costs inherent in groundwater projects.

Evaluation of the impact of the thermal shunt effect on the U-pipe ground borehole heat exchanger performance

A U-pipe ground borehole heat exchanger (BHE) is the most commonly used device for heat extraction from the ground. The amount of extracted heat depends on two interacting heat transfer processes – between the ground and the carrier fluid, and between the pipes. Heat flow between the U-pipe legs is called the thermal shunt effect (TSE). In this paper, a steady-state numerical model has been used to assess the contribution of TSE to the overall U-pipe performance. As the model requires borehole wall temperature as input, a CFD analysis has been performed to determine how this temperature is influenced by elimination of TSE. The analysis has shown that reducing pipe-to-pipe heat flow has no substantial impact on the borehole wall temperature. Fluid temperature profiles have been calculated for all possible pipe arrangements in a heat exchanger with the borehole diameter of 152mm, pipe diameters in the range of 30.4–45.6mm and borehole depths ranging from 30 to 200m. Results indicate that thermal shunting becomes aggravated by decreasing pipe-to-pipe distance or increasing pipe diameters and borehole depths. The maximum loss of 35% in the U-pipe thermal performance has been observed in the range of investigated parameters, which proves the necessity of TSE elimination.

Study on High Prestressed Anchor Beam Supporting Optimization Technology in Deep Roadway

Coal mine has stepped into deep mining period in China, in which the ground pressure is great, the stability of the surrounding rock is poor and roadway is seriously destroyed. 3302 trick roadway of Zhaolou coal mine is the kilometers depth roadway, under the condition of original supporting, the roof settling amount and the surrounding rock deformation were large, which made bad effect on safety production. The surrounding rock failure region was monitored by borehole televiewer and ground penetrating radar, and the loosening failure law was analyzed. The scheme of high prestressed anchor beam was proposed according to the detection results, the crack distribution of surrounding rock and the stress of blots and anchors of original scheme and optimized scheme were analyzed by UDEC. The results of numerical simulation and field monitoring showed that increasing prestress of blot and anchors was better for the control of roof separation in anchorage zone and maintaining the integrity of roof, and high prestressed anchor beam can effectively improve the effect of surrounding rock control in roadways.

Investigating internal structure of permafrost using conventional methods and ground-penetrating radar at Honhor basin, Mongolia

A ground-penetrating radar (GPR) survey was conducted at the end of August 2009 in the suburb region of Ulaanbaatar, Honhor basin, Mongolia, in combination with conventional methods such as borehole drilling and measurement of ground temperatures. The interface of frozen and unfrozen sediment was distinctly resolved in the interpreted GPR images, verified by the borehole drilling records and 6-month measurement of ground temperatures. The location of the permafrost table was assessed to be at the depth of 2–4 m in the study region. A conspicuous ice-saturated soil layer (massive ground ice) was detected in the interpreted GPR images with a thickness of 2–5 m. The GPR investigation results were consistent with the borehole drilling records and ground temperatures observation. The borehole logs and ground temperatures profile in the borehole indicates that permafrost at Honhor basin is characterized by high ground temperature and high ice content, which implies that ongoing climatic warming would have great influence on permafrost dynamics. The research results are of great importance to further assess permafrost dynamics to climatic change in the boundary of discontinuous and sporadic permafrost regions in Mongolia in the future.

Numerical modeling of ground borehole expansion induced by application of pulse discharge technology

Pulse discharge technology (PDT) is an innovative technique that can be used to enhance bearing capacity of piles and resisting capacity of anchors. It enlarges the section area and compacts the surrounding soil by high-powered shock wave pressure induced by an underwater electrical discharge. This study aims to establish a suitable numerical model for the simulation and prediction of ground borehole expansion induced by PDT. In order to examine the relationship between electricity and the characteristics of shockwaves generated by PDT, laboratory pulse discharge tests were performed using PDT equipment used in current practice. Then, based on the underwater explosion (UNDEX) model and a coupled acoustic–structural analysis scheme, the results of laboratory PDT tests were analyzed and numerically benchmarked to determine the equivalent UNDEX model parameters for providing shock loading input in a ground borehole expansion simulation. A series of expansion simulations for undrained clayey and sandy soils were performed, and the predicted borehole expansion behaviors were compared with the test results. Moreover, a parametric study was conducted to examine the effects of soil properties on the expansion behavior. The results of the numerical work in this study appeared to be consistent with field test results published in the literature and showed that the soil characteristics related with packing, state of stresses, and degree of saturation were important when analyzing borehole expansion behavior.