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As an alternative energy for fossil fuels, geothermal reserve is abundant. In recent years, scholars have investigated geothermal wells through numerical simulations. To the best of our knowledge, many studies have explored the influence factors of thermal power over time, and few studies have investigated the heat transfer mechanism between wellbore and formation in different circumstances. This paper aims to established a transient numerical heat transfer model of U-shaped closed-loop geothermal system with long horizontal section, which can simulate the temperature distribution and heat transfer mechanism during the whole wellbore formation. In the model, the convection term is discretized by using the second-order upwind scheme, and the finite volume method is used to solve it. The numerical results are proven to be correct by comparing with the field data of a well in China. Subsequently, the influence mechanism of fluid type, insulation casing length, fluid injection temperature, flow rate, and horizontal section length on thermal power is explored. The optimal depth of insulation casing under different injection temperatures, flow rates, and horizontal section lengths is suggested so as to provide crucial guidance for engineering practice. [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]

The thermal conductivity of rock is an essential thermophysical parameter in geothermal engineering. Experiments have shown that when rock changes from an unsaturated to a saturated state, thermal conductivity can be increased by 50% at most. In this paper, the variation of thermal conductivity with saturation of 101 samples of six different rock types from Songliao Basin, Gonghe Basin, and Ordos Basin in China was measured in the laboratory. Correlations between saturation, porosity, internal structural characteristics of rocks and thermal conductivity were investigated. The results show that the effect of saturation on the thermal conductivity of rock increases exponentially with porosity. Based on three classical models and the relationship between porosity, saturation, and thermal conductivity, improved models and a fitted model of thermal conductivity are proposed. By comparing the improved models to the fitted model, the applicable scope of each model is obtained. The errors of each model are analyzed in detail. The average MAE value of the improved models is 0.405 and the average RMSE value is 1.207 over the applicable model scopes. The R 2 of the fitted model is 0.94. Compared with experimental data from literature, MAE values of the improved models and the fitted model are 0.040 and 0.003 respectively. Unsaturated rock with a single fracture heat transfer model is established by simulation. A thermal-Hydrological coupling model is used to simulate seepage heat transfer through fracture. The effect of variable thermal conductivity simulation is the best with the experimental value, and the MAPE value was 8.3%. This study fills a gap in available models of unsaturated thermal conductivity of rock, and provide theoretical support for the accurate calculation of heat transfer efficiencies in related geothermal engineering applications. • Theoretical saturation coefficient equation on thermal conductivity was proposed. • Three unsaturated thermal conductivity models were improved and verified. • Propose an empirical unsaturated model based on laboratory data. • The relationship between unsaturated model and rocks structure was revealed. [ABSTRACT FROM AUTHOR]

The Experimental Advanced Superconducting Tokamak (EAST) had been under major upgrading from 2020 to 2021 in China. In this campaign of major upgrading, the new lower tungsten divertor is successfully installed to achieve the targets of long pulse and high β H-mode plasma operation. High temperature and high pressure Divertor primary heat transfer system (DIV PHTS) is also developed to remove the plasma deposition heat load on the Divertor. This paper presents a description of the design and construction of DIV PHTS for EAST Tokamak. Starting from the requirements specified in the Technical Specifications of DIV PHTS, the process to meet the Divertor cooling and baking requirement is designed and validated by the supported thermal-hydraulic analysis. Then, the piping stress analysis under different types of load combinations is performed to verify the structural design requirements. General layout design is also performed based on the component requirement and the allocated space in the EAST Tokamak hall. Finally, the procurement and test of the system main equipment is also given. [ABSTRACT FROM AUTHOR]

• A new structural design method for earth-to-air heat exchangers in greenhouses is described. • The structural design method incorporates greenhouse heat balance model to better match greenhouse characteristics. • The system is more energy-efficient and stable in intermittent operation mode. • Application of an earth-air heat exchanger is a green technology for greenhouse sustainability. An earth-air heat exchanger (EAHE) is a type of environmental regulation equipment that is suitable for greenhouses. In the future, it could be a green technique for greenhouse energy supplementation. However, EAHEs lack structural design methods for application in greenhouses. Consequently, this paper presents a structural design method for an EAHE based on the architectural characteristics of greenhouses. Firstly, the EAHE design heat exchange amount was calculated according to the greenhouse heat and cold load, then the pipe length, diameter, air velocity, and burial depth were determined by combining the heat transfer efficiency and number of transfer units (NTU), and finally the final parameters were optimized and selected according to the energy loss and construction cost. The design method was validated in a greenhouse in northern China, and the heat transfer performance was analyzed in both intermittent and continuous operation modes. The results showed that the novel structural design method could guide the application of EAHE in a greenhouse and satisfy the energy demand of greenhouses. The measured average greenhouse temperature at night increased by 1.57 °C and 1.11 °C, respectively, under intermittent operation and continuous operation and decreased by 1.19 °C and 1.32 °C, respectively, during the day. The EAHE performance coefficients for heating and cooling conditions under intermittent operation were 21.49 and 18.15, respectively, significantly higher than those for continuous operation. [ABSTRACT FROM AUTHOR]

• A review of the difficulties faced when burning high-alkali coals in boilers. • The effective combustion utilization methods of high-alkali coals are summarized. • The ash deposition solutions of burning high-alkali coals are summarized. • Proposed future research directions for the utilization of high-alkali coals. High-alkali coals are distributed in many countries all over the world, mainly characterized by contains abundant alkali metals and alkaline earth metals (AAEMs) elements, direct combustion in boilers will bring serious ash deposition, fouling, and slagging (ash-related issues) on the metal heat transfer surface of the boilers. In particular, the Zhundong (ZD) coals in Xinjiang, China, as typical high-alkali coal, have a huge reserve that can effectively ensure China's coal demand in the long term. The main purpose of this paper is to clarify the problems and difficulties faced by the combustion of high-alkali coals in boilers, explore and summarize the effective utilization methods. First, the primary difficulties and problems faced by direct combustion of high-alkali coals in boilers are summarized. Second, the research progress of current efficient utilization methods and solutions of high-alkali coals combustion in boilers are summarized and reviewed, it mainly includes the following aspects: (1) pretreatment technology before combustion; (2) control and optimization of combustion parameters such as temperature and atmosphere; (3) co-combustion with inorganic additives, other coals, biomass fuels, and other fuels, respectively. The above methods have been proved can be effectively reduced and alleviated the ash-related issues, but no methods can be eliminated or solved the problems completely at present. Based on the existing research and practice results reviewed in this paper, aiming at the efficient utilization of high-alkali coals combustion in utility boilers, further feasible research directions or key points for the future are proposed. [ABSTRACT FROM AUTHOR]

In response to the energy conservation and emission reduction targets proposed by China's Fourteenth Five-Year Plan, it is necessary for oilfield enterprises to achieve the purpose of reducing energy consumption and carbon emissions of crude oil storage under the premise of ensuring safe production of oil depots. As the preferred oil storage facility for crude oil depots, the internal heat flow transmission process of storage tanks plays a vital role in the overall energy consumption of oil depots. In this paper, a three-dimensional theoretical model of the heating process of large waxy crude oil tank storage is established, and the influence law of the circumferential effect of the coil on the heating process of waxy crude oil is put forward. The external environment influence area, the internal temperature rise heat flow influence area and the transition area are put forward. The multi-nonlinear regression method is innovatively used to establish the main influencing factor model for each influence area so as to realize the quantitative characterization of the influence mechanism between the heat flux at the tank boundary and the dynamic thermal environment and the variable physical properties parameters of crude oil. The results show that the convective heat transfer of oil in the tank bottom area is greatly influenced by the circumferential effect of the coil, and the circumferential effect of the coil is enhanced with the gradual proximity of the coils on both sides, and the low temperature area at the tank bottom is obviously improved. The heat loss of the storage tank is mainly concentrated at the boundary of the tank. According to the change of heat flux in each boundary area, the external environment influence area, the internal heating heat flow influence area and the transition area are divided. Among them, the top of the storage tank forms an external environmental impact area dominated by solar radiation heat transfer, supplemented by natural convection of temperature difference, and the bottom of the storage tank forms an external soil impact area dominated by heat conduction to the external soil. Due to the effect of the thermal insulation layer, the influence of the external dynamic thermal environment on the position of the tank wall is weakened, and only the influence area of the internal temperature rise heat flow dominated by the natural convection of crude oil is formed. At the same time, there is a transition area with different thickness between the internal and external influence areas, which is affected by the external dynamic thermal boundary conditions and the physical properties of crude oil. The internal heat transfer direction is different, which causes the influence weight of each factor to fluctuate, and with the reduction of the external dynamic thermal environment, the transition area becomes the external condition influence area. • The influence of circumferential effect of coils on heat transfer of crude oil is proposed. • The heat transfer characteristics of the tank boundary region under dynamic thermal conditions are studied. • The external influence area, internal influence area and transition area of the tank are divided. • The main influence factors model of influence area is established. • The relationship between the boundary heat flux of tank and internal and external influencing factors is quantitatively characterized. [ABSTRACT FROM AUTHOR]

The content and distribution of pore unfrozen water directly reflect the coal thawing process. It is the key to break through the evolution of coal pore structure under low temperature to study the pore ice melting speed. Taking bituminous coal in Yuan Zhuang, China as the research object, this paper studied pore thawing characteristics of coal samples with different liquid nitrogen freezing time through nuclear magnetic resonance technology. The experimental results clearly demonstrate that appropriate liquid nitrogen freezing time could greatly expand the macropore of bituminous coal, while the expansion effect of micropore and mesopore was less affected by the freezing time. As the freezing time increased, the pores and pore throats of saturated bituminous coal underwent two phases of ice-water transition expansion and liquid nitrogen low-temperature shrinkage. There were differences in the local thermal conductivity of coal samples with different liquid nitrogen freezing time, resulting in different thawing speeds of pore water at the same thawing temperature. When the freezing time increased, the thermal conductivity of coal samples increased due to the pore freezing shrinkage, which can greatly improve the thawing speed of coal samples. Combine the experimental date with heat transfer mechanism to study the thawing speed of the coal frozen by liquid nitrogen. Analyze the unfrozen water distribution and local thermal conductivity in thawing process, which effectively connects the macro-level and micro-level of coal. [Display omitted] • Liquid nitrogen freezing has the best effect on the expansion of macropore in bituminous coal. • The pore structure undergoes two stages of expansion and shrinkage with the increase of freezing time. • The increased freezing time leads to pore freezing shrinkage, which can improve the thawing speed of coal sample. [ABSTRACT FROM AUTHOR]

Heat and mass transfer of pyrolytic hydrocarbon fuels at supercritical pressure has important implications for regenerative cooling technology in advanced engines. Based on the detailed chemical kinetic mechanism of China No. 3 jet fuel (a.k.a. RP-3), the heat and mass transfer of RP-3 in a horizontal square cooling channel under asymmetric heating and buoyancy at supercritical pressure is investigated numerically. The results show that buoyancy induces secondary flow. The secondary flow enhances the heat and mass transfer between the near wall region and the core flow field, which can effectively improve the utilization level of fuel heat absorption capacity and suppress the carbon deposition on the surface of the cooling channel. In the case of bottom heating, the dramatic change in fuel density with fuel pyrolysis induces the strongest secondary flow, resulting in the highest fuel heat absorption capacity and the least carbon deposition. In addition, as the operating pressure increases, the buoyancy effect becomes stronger duo to fuel pyrolysis, which is significantly different from the buoyancy effect in the non-cracking zone. Under the buoyancy effect, the increased operating pressure enhances the secondary flow intensity, which reduces the heated wall temperature and suppresses the carbon deposition on the cooling channel surface. The results of this paper are expected to provide further insight into the design and optimization of regenerative cooling technologies. • Heat transfer and surface coking under asymmetric heating and buoyancy were analyzed. • Secondary flow reduced thermal stratification and improved fuel conversion. • Stronger secondary flow intensity resulted in lower wall temperature and carbon deposition. • The increasing pressure accelerated secondary flow and enhanced heat and mass transfers. [ABSTRACT FROM AUTHOR]

This method is based on the measured data of the efficiency of the heat pipe PV/T system under different solar radiation, and the typical meteorological year data, the system efficiency proposed in the existing literature, and the annual distribution of solar radiation in the region where the system located is comprehensively considered, then the annual average efficiency of heat pipe PV/T systems in different regions is obtained, and the annual heat collection and power generation of heat pipe PV/T systems are calculated and analyzed by this method. [Display omitted] • Efficiency of heat pipe PV/T systems under different radiation are tested. • The duration of solar radiation of different intensities in the typical year is statistically analyzed. • Efficiency differences of different radiations and solar radiation distribution are considered comprehensively. • An annual average efficiency evaluation method of heat pipe PV/T systems is proposed. As a passive heat transfer system, the heat pipe is becoming an important way to solve the problem of heat loss and heat accumulation in PV/T systems. However, the evaluation of the annual average efficiency of heat pipe PV/T systems in the existing studies does not consider the efficiency under different solar radiation and the difference in the annual solar radiation distribution in different regions. The purpose of this study is to propose a method for the annual average efficiency evaluation of heat pipe PV/T systems (AEHP method), which comprehensively considers the efficiency differences under different radiation and the distribution of solar radiation of different intensities in the typical year. The calculation results are more accurate, which will promote the reasonable evaluation and engineering application of heat pipe PV/T systems. Firstly, the dimensionless models of different radiation and their performance (photovoltaic efficiency, photothermal efficiency, and comprehensive efficiency) are established by experimental tests on the heat pipe PV/T system in this paper. Secondly, the frequencies and frequency of solar radiation in different intervals of the typical year in major cities of China are analyzed statistically, and the radiation frequency method is proposed. Thirdly, the average efficiencies of heat pipe PV/T systems in the existing literature are obtained by statistical analysis. Combined with the radiation frequency method, an annual average efficiency evaluation method of heat pipe PV/T systems (AEHP method) is proposed. The annual average efficiency of heat pipe PV/T systems in different regions is obtained. Finally, the annual heat collection and power generation of heat pipe PV/T systems are calculated based on the AEHP method, and provide a scientific and reasonable evaluation method for the promotion and application of heat pipe PV/T systems. [ABSTRACT FROM AUTHOR]

The aero-engine manufacturing industry is booming in China, while sustainability is neglected. For example, the grinding superalloy part uses a high-volume low-velocity flood delivery strategy for grinding cooling, which is harmful to health and the environment, consumes more resources and energy, emits more CO 2 and increases production costs. To achieve sustainability in the aero-engine manufacturing industry, it is worth considering eliminating coolant from the grinding to reduce the environmental impact and production cost. In this study, a novel oscillating heat pipe grinding wheel (OHP grinding wheel) is introduced to enhance the heat transfer in the eco-benign grinding process. The core concept is to design, fabricate particular oscillating heat pipes and position them in the grinding wheel so that the inside oscillating heat pipes transfer out the heat directly through the grinding wheel to avoid heat gathering, lower the grinding temperature, and suppress the thermal damages. Moreover, the use of harmful coolant can be diminished or even avoided, and an eco-benign grinding technology can therefore be achieved. Holistic sustainability analysis is then conducted in this paper. Compared with grinding by Non-OHP grinding wheel, the water-based coolant is avoided, the energy consumption is reduced by 26%–42%, and the carbon emission is reduced by 45%–56%. The OHP grinding wheel shows a great advantage in eco-benign grinding. • A novel oscillating heat pipe grinding wheel for eco-benign grinding is designed. • Carbon emission reduces 45%, production cost drops 41% by the proposed technology. • Energy consumption reduces 26%–42% by the proposed grinding wheel. • The proposed grinding wheel improves the material removal rate by 2.4 times. [ABSTRACT FROM AUTHOR]

Piston action can affect the thermal environment inside the tunnel by changing the air pressure and velocity. This is extremely unfavorable for tunnels in cold regions because it causes frost damage in cold season. Understanding the influence of piston action on the air temperature distribution inside the tunnel can help us determine the length and location of frost damage. In this paper, a heat transfer numerical model considering piston action is developed. And then, a high-speed railway tunnel in the northeast of China is taken as an example to explore the temperature distribution laws via computational fluid dynamic (CFD). Afterwards, the effects of air temperature outside the tunnel and train velocity on temperature distribution are analyzed. The results show that the piston action can accelerate the heat transfer between cold air outside and inside the tunnel, the anti-freezing length near tunnel entrance and exit must be extended based on the calculation result of existed empirical formula. Before the train enters the tunnel, the freezing length near tunnel entrance is 1158 m. After the train passing through the tunnel, the freezing length near tunnel entrance is between 1495 m and 1497 m when the air temperature outside the tunnel decreases from −1 °C to −15 °C. Under different train velocities (80 km/h, 130 km/h, 160 km/h, 200 km/h and 250 km/h), the freezing lengths near tunnel entrance are 1458 m, 1469 m, 1472 m, 1497 m, 1498 m and its near tunnel exit are 330 m, 342 m, 349 m, 356 m, 357 m, respectively. In the section of 50 m away from tunnel entrance, the air temperature near the train changes suddenly, while the air temperature far away from the train changes gently. After the train passing through the tunnel, the air temperature inside the tunnel is consistent to that outside the tunnel within a specific distance from entrance. This study provides a guideline and references for the structure design and the construction of anti-freezing measurements for the tunnels in cold regions. [ABSTRACT FROM AUTHOR]

Clean district heating systems are needed in large cities, especially in Northern China. This paper presents a low carbon district heating system that features a low return water temperature, use of low grade waste heat as the main heat source, long distance heat transmission with a large temperature difference, distributed peak heating load addition by natural gas and heat-power decoupling with heat pump and thermal storage. This Low carbon district heating 2025 system is suitable for large cities, large waste heat sources, high heating densities and the utilization of using existing large heating networks in almost all the cities of Northern China. These clean heating systems in Chinese cities have great potential to reduce energy use, reduce emissions and improve the district heating system economics. [Display omitted] • Low carbon district heating 2025 system is designed for large cities, large waste heat sources, high heating densities. • It describes a district heating system which uses low-grade thermal energy. • It features with a large temperature difference for heat transmission. • It solves heat-power decoupling with heat pump and thermal storage. • It reduces the energy consumption and emissions by 80% compared to coal boilers. [ABSTRACT FROM AUTHOR]