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The application of ground heat exchanger technology in backfill mines can actualize subterranean heat storage, which is one of the most effective solutions for addressing solar energy faults such as intermittence and fluctuation. This paper provides a 3D unsteady heat transfer numerical model for full-size horizontal backfill heat exchangers (BFHEs) with five configurations in a mining layer of a metal mine by using a COMSOL environment. In order to ensure the fairness of the comparative analysis, the pipes of BFHEs studied have the same heat exchange surface area. By comparing and evaluating the heat storage/release characteristics of BFHEs in continuous operation for three years, it was discovered that the helical pipe with serpentine layout may effectively enhance the performance of BFHEs. Compared with the traditional SS BFHEs, the heat storage capacity of the S-FH type is significantly increased by 21.7%, followed by the SA-FH type, which is increased by 11.1%, while the performances of U-DH and SH type are considerably lowered. Also, the impact of the critical structural factors (pitch length and pitch diameter) was further studied using the normalized parameters C1 and C2 based on the inner diameter of the pipe. It is discovered that BFHEs should be distributed in a pipe with a lower C1, and increasing C2 encourages BFHEs to increase the storaged/released heat of BFHEs. By comparatively analysing the effect of thermal conductivity, it is found that the positive effects of thermal conductivity on the performance of SH, U-DH, SA-FH, and S-FH type BFHEs are found to decrease successively. This work proposes a strategy for improving the heat storage and release potential of BFHEs in terms of optimal pipe arrangement. [ABSTRACT FROM AUTHOR]

• The model of external fin backfill coupled heat exchanger was established. • The thermal performance of FBCHE were simulated. • The materials and structures of different fins are compared. • The heat transfer factors affecting FBCHE were analyzed. There are abundant geothermal resources in mine goaf. The effective exploitation of geothermal energy is of great significance to realize the long-term sustainable development of mines. In this paper, a three-dimensional unsteady heat transfer model of a typical serpentine backfill coupled heat exchanger is established using COMSOL Multiphysics simulation software, and the accuracy of the model is verified. A serpentine external finned backfill coupled heat exchanger is proposed. Comparative analysis of the heat transfer performance of serpentine external finned backfill coupled heat exchanger and smooth backfill coupled heat exchanger is conducted. The effects of different fin materials, fin structures and system design parameters on the thermal performance of serpentine external finned backfill coupled heat exchanger are studied. The results show that the temperature difference between the inlet and outlet, total heat storage and total heat release increases with the increase of fin thickness, fin height, and surrounding rock temperature, decreases with the increase of inlet velocities and inlet temperature. The heat exchanger energy efficiency coefficient increases with increasing fin thickness and height, decreases with increasing surrounding rock temperature, inlet temperature and inlet velocities. The temperature difference between the inlet and outlet, total heat storage and total heat release of copper fins are higher than those of other materials. The energy efficiency coefficient of using round fin is higher than that of using square fin, square H-fin and round H-fin. When copper round fins are used, the heat storage of the system is 1457.03 kJ, the heat release is 2831.95 kJ, and the energy efficiency coefficient is 51.45%. [ABSTRACT FROM AUTHOR]

To reduce the risk of high-temperature geothermal environment in deep mine exploitation, an innovative method for cooling stopes by backfill slurry with ice particles has been focused on. In this paper, aiming at the cooling effect of backfill slurry with ice particles, an experimental device including stope region and ice-filled filling slurry region was established for temperature measurement experimental simulation study. The results showed the ice-filled slurry had a significant cooling effect on the stope region. Orthogonal design experiment and range analysis methods were applied for studying the influencing regularities of four factors, including boundary heat flux, ice-water ratio, sand-cement ratio, and slurry concentration. The effective cooling heat coefficient which is defined by radiation heat flux and boundary heat flux of surrounding rock was applied as an evaluation index for scheme optimization. The influencing rank of the four factors is boundary heat flux >sand-cement ratio >ice-water ratio >slurry concentration. By comprehensive analysis, the optimization of mixture ratio was obtained: the boundary heat flux of the simulated surrounding rocks is 111 W/m2, the ratio of ice to water is 8 : 5, the ratio of sand to cement is 4 : 1, and the slurry concentration is 64%. [ABSTRACT FROM AUTHOR]

The combined mine backfill–geothermal (CMBG) system can be used to effectively extract geothermal energy by installing a heat exchange tube (HET) in the underground backfilled stopes of mines, which can be used as the heat supply for buildings in mines and the surrounding areas. The efficient performance of this system strongly depends on the thermal exchange process between the HET and its surrounding cemented paste backfill body (CPB). In this study, a validated simulation model is established to investigate the heat exchange performance of CPB, in which the nonuniformly distributed thermal properties in CPB are fully considered. The results indicate that the increase in the porosity has a negative effect on the heat exchange performance of CPB. With the increase in the porosity, the decreased rate of the conductive heat transfer in CPB could be up to approximately 18%. In conditions with seepage flow, the heat transfer capacity of CPB could be effectively improved. Generally, a higher hydraulic conductivity corresponds to a higher heat transfer performance of CPB. When the seepage velocity rose from 2 × 10−6 to 6 × 10−6 m/s, the thermal conductivity of CPB achieved a 114% increase from 1.843 to 3.957 W/(m·K). Furthermore, it was found that the thermal energy accumulates along the seepage flow direction, enhancing the thermal influencing radius of the HET in this direction. Thus, the arrangement of HETs should fully take into account the seepage flow effect. This proposed simulation model could provide a reference for parameter determination and optimization of CMBG systems. [ABSTRACT FROM AUTHOR]

One of the most effective methods for geothermal energy extraction in deep mine stopes is the installation of heat exchange tubes within cemented backfill bodies. However, the complex underground environment can cause fracture in the backfill, which may negatively affect the geothermal extraction performance, especially in the presence of groundwater flow. This study establishes a three-dimensional seepage and heat transfer coupling model of cemented backfill heat exchangers with horizontal penetrating rough fractures via a finite element software platform. The model employs the Monte Carlo method combined with linear filtering to generate a rough fracture. The findings demonstrate that for fracture apertures ranging from 0 to 0.3 mm, the predominant mechanism of heat transfer is thermal conduction, with a negligible contribution from groundwater flow. However, as apertures expand from 0.3 mm to 2 mm, groundwater flow significantly enhances heat transfer, stabilizing beyond 2 mm. Increased fracture roughness at a 0.2 mm aperture does not enhance the heat recovery performance of the heat exchange tubes, but at a 4 mm aperture, a strong positive correlation between roughness and heat transfer is observed. Thus, narrow fractures can be treated as smooth, whereas roughness must be considered for wider fractures. The interaction between fracture flow and Darcy seepage increases with increasing groundwater hydraulic head, resulting in a notable improvement in the heat extraction performance of the heat exchange tube. When the relative position transitions from 0.75 °C to 0 °C, the outlet water temperature of the heat exchanger tube increases by approximately 9 °C. [ABSTRACT FROM AUTHOR]

Enhancing the cold load/storage (CLS) functional cemented paste backfill (CPB) technological cooling effect is crucial given the deep mine escalating heat damage. Single optimization schemes such interval boards insulation, higher emissivity, active cooling, and integrated optimization schemes of the aforementioned measures are suggested in order to improve the cooling impact of CLS functional CPB. Firstly, active cooling was implemented to isolate the heat source and protect the cooling capacity. Experiments indicate that this approach can extend the period of time that the stope temperature is maintained below the temperature threshold. Secondly, thermal insulation material is applied to the interval boards of the stope and the CLS functional CPB to decelerate the cooling transfer rate. When the stope temperature reaches the recommended upper limit, remove the insulation material. The results show that 1 mm insulation material is the most economical and environmentally friendly option. However, when the temperature difference inside the backfill is slight, the effect of the radiation coating is not significant. The best-optimized cooling method obtained in this paper is to take active cooling around the stope and CLS functional CPB, and then paste 1 mm insulation material on the interval boards. • Multi-scheme comparison and optimization. • An experimental platform was established to verify the numerical simulation results. • CFD numerical simulation is used to obtain the temperature distribution. • Multi-angle study of cold source and heat transfer process. [ABSTRACT FROM AUTHOR]

• A three-dimensional unsteady state mathematical model of the multi-field coupling of backfill body, tube and fracture was established. • A test bench was designed and built to verify the three-dimensional coupling model. • The effects of a single fracture position, aperture and fractured depth on the heat extraction performance of a full-size BFHEs unit in the fully dry or fully saturated conditions were investigated. • The combined effects of multiple fractures on the performance of a BFHES unit were discussed. Thermal hazards in deep mines can be mined synergistically with the deposit as associated geothermal energy resources. Backfill heat exchangers (BFHEs) as a combination of mine backfilling and ground heat exchanger technology are one of the effective methods to realize ore deposit-geothermal energy synergy mining. However, in the complex environment of deep mines, BFHEs will inevitably suffer from fracture damage during long-term heat storage/release cycle operation, which will affect their working performance. To address this problem, this paper established a scaled-down experimental setup for fractured BFHEs units, and used the experimental data to verify the accuracy of the established three-dimensional transient heat transfer mathematical model of the BFHEs unit. Numerical simulation methods were used to investigate the effects of the position, aperture and fractured depth of a single fracture on the heat extraction performance of the BFHEs unit under fully dry or fully saturated working conditions. Significant differences were found in the effect of different fracture positions on the heat extraction capacity of the BFHEs unit. The extraction of geothermal heat by the BFHEs unit was favored when it was in the middle of two tubes, while the opposite was true for other positions. The effect of fracture on the heat extraction capacity of the BFHEs unit increased linearly with fracture aperture. The effect of the fractured depth was relatively greater in the 1/2–3/4 H b range. The presence of groundwater significantly reduced the effect of the fracture. The effect of the fracture on the heat extraction capacity of the BFHEs unit was reduced by more than 80 % by changing the operating conditions from fully dry to fully saturated condition. The overall effect of a fracture on the heat extraction capacity of the BFHEs unit was not significant, with a 5 mm wide fracture having an effect of no more than 2.5 %, even under the fully dry condition. However, there was a combined effect of multiple fractures, and for a deposit size of 900 × 7 × 80 m, ten 2 mm apertures fractures would results in a maximum reduction of geothermal extraction from BFHEs by 8527 GJ in 10 years, which was equivalent to the heat load of a 13,753 m2 building for one winter in the cold climate of China. [ABSTRACT FROM AUTHOR]

• Raj et al.'s gravity scaling parameters can't be applicable to the controlled wall heat flux condition. • Microgravity boiling mechanisms on chip S 2 × 2 transform from the SDB to BDB regime. • m BDB for controlled wall temperature condition is larger than that for controlled wall heat flux condition. • m BDB for controlled wall heat flux boiling is affected by both T * and L h / L c. • Gravity scaling parameters for controlled wall heat flux boiling were developed. With the ability of dissipating large amount of latent heat in nearly isothermal process, pool boiling heat transfer is an efficient solution for cooling problem of different sized electronic devices in different gravities. For the practical heat dissipation of electronic device usually occurs at controlled wall heat flux condition, it is essential to reveal pool boiling heat transfer characteristics at different sizes and gravities under controlled wall heat flux condition. In this paper, boiling heat transfer mechanisms on two sized smooth silicon chips (chip S 1 × 1 and chip S 2 × 2) in ground gravity and microgravity were studied. It was found that the gravity scaling parameters developed under controlled wall temperature condition cannot be applicable to the controlled wall heat flux condition. Under controlled wall heat flux condition, the microgravity boiling heat transfer performance on chip S 1 × 1 is in surface tension dominated boiling (SDB) regime in the whole heat flux range. While for chip S 2 × 2, it is in SDB regime when heat flux is smaller than 6.44 W·cm−2, but enters into buoyancy dominated boiling (BDB) regime when heat flux is larger than 6.44 W·cm−2. The critical point of the two boiling mechanisms occurs when the threshold gravitational acceleration equals to the experimental microgravity. In BDB regime, the power law coefficient reflecting the gravity effect on nucleate boiling heat transfer performance (m BDB) for controlled wall temperature condition is larger than that for controlled wall heat flux condition, indicating a greater impact of gravity level under controlled wall temperature condition. Moreover, m BDB for controlled wall temperature condition is affected only by non-dimensional wall temperature T *. While for controlled wall heat flux condition, m BDB increases with increasing T * but decreases with increase in heater size, and heater size has a greater influence on m BDB. Based on the available data, the gravity scaling parameters for controlled wall heat flux boiling were developed, which can accurately predict the microgravity boiling heat transfer performance on two sized chips. [ABSTRACT FROM AUTHOR]

• PCM in tube-in-tube heat exchanger can efficiently gather geothermal energy. • 3D heat transfer model of backfill body is built by coupling different regions. • Temperature distribution of backfill body in heat storage process is simulated. • Influence of thermal environment on the heat storage performance is analyzed. Deep mines contain abundant geothermal energy. As a solid heat storage material, the backfill body near the surrounding rock and stope can continuously absorb heat. The phase change material (PCM) embedded in the tube-in-tube heat exchanger is more conducive to the accumulation of geothermal energy. In this paper, the backfill body with tube-in-tube heat exchanger is taken as the research object and its heat storage process is simulated by FLUENT. The influence of surrounding rock temperature, initial temperature of backfill body, airflow temperature and velocity in stope on the heat storage performance of backfill body is analyzed. The results show that adding PCM can increase the heat storage capacity and the average increment is 155.2 kJ within 10 h compared with ordinary backfill body. The total heat storage capacity always increases over time with an increment about 90% occurring within 5 h. This paper provides a theoretical basis for the study of the heat storage performance of the backfill body under different working conditions in mines, and also lays a foundation for the efficient accumulation of geothermal energy and the exploitation of deep geothermal energy. [ABSTRACT FROM AUTHOR]