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Heat transfer near geothermal heat exchangers in the presence of groundwater flow has been extensively studied over the past years. However, previous studies often overlooked the presence of an unsaturated layer in the shallow subsurface soil, or disregarded phase change and fluid flow within this layer. In this study, a comprehensive hydro-thermal model is developed to evaluate heat transfer near a partially submerged geothermal borehole in groundwater flow while incorporating phase change and liquid and vapor flow in the unsaturated layer. The effect of unsaturated soil hydraulic analysis on heat transfer and energy efficiency of the geothermal piles is explored under varying soil permeabilities, air-entry suctions, solid thermal conductivities, and soil types. Consideration of water content variation and unsaturated fluid flow in the model enhances heat injection rate by up to 11.23 % in highly permeable soils. However, in scenarios with higher solid thermal conductivity, or lower air-entry suction values, the heat injection rate reduces by as much as 16.13 % due to unsaturated soil hydraulic analysis. Furthermore, it is found that consideration of unsaturated soil hydraulic analysis is of greater importance in Bonny silt than sandy soil, lowering the heat injection rate by 8.53 % and 2.53 %, respectively. [ABSTRACT FROM AUTHOR]

In the present paper, the effect of the external partial magnetic field is studied on the flow and heat transfer of various nanofluids in a microchannel via the incompressible preconditioned lattice Boltzmann method. The simulations are performed for various parameters such as Hartmann number (Ha) ranging from 0 to 40 and surface non-dimensional slip coefficient (B) of 0–0.03. The nanofluid volume fraction is fixed at 1%, and the results are compared with pure water. The effect of using different nanoparticles (Al2O3, CuO, Ag, and Fe) has been investigated on the Nusselt number. The acceptable results are obtained by comparing numerical and experimental data. The results generally show that using Ag and CuO nanoparticles, respectively, leads to the best and worst heat transfer rate. Eventually, the highest and lowest Nusselt numbers are from these nanoparticles, where Ag and Al2O3 give the worst and best stress rates. It is found that for a specific requirement of microchannels in heat transfer application, there would be a certain nanofluid by specific nanoparticles. This novel study opens discussion by proposing a new alternative way to improve the heat transfer in microchannel which is applicable to the systems where microchannel is used as heat sinks. [ABSTRACT FROM AUTHOR]

Heat exchange performance of geothermal piles with single U-shaped circulation tube is quantified as a function of design, operational and site-specific variables. A finite difference model is developed to simulate heat transport by circulation fluid and heat conduction in pile and surrounding soil. Finite difference analyses (FDAs) are performed to quantify the effects of several input parameters on heat transfer performance of a geothermal pile. Based on FDA results, closed-form equations are proposed that can be used in calculation of power output from a single geothermal pile with U-shaped circulation tube embedded in it. Parameter sensitivity study and advanced first order second moment (AFOSM) reliability analysis are performed to determine the hierarchy of different input variables in order of their relative impacts on heat transfer performance. Thermal conductivity of soil, initial temperature difference between circulation fluid and ground, and radius of the circulation tube are identified as the three most important parameters that control heat transfer through geothermal piles. [ABSTRACT FROM AUTHOR]

Generation and dissipation of temperature-induced pore water pressure can alter the mechanical behavior and characteristics of soil media. Experimental observations indicate that heating/cooling cycles alter the soil fabric which may result in irrecoverable deformations. Previous studies suggest that the contribution of thermo-osmotic (non-Darcian) flow in clays may have a significant role in the thermal pressurization diffusion. Hence, this study aims to investigate the effects of thermal, hydraulic, and mechanical constitutive models such as thermo-plastic deformation along with the thermo-osmotic flow and thermal infiltration on the THM response of clayey soils during heating and cooling phases. It is found that considering temperature-dependent parameters in conjunction with the thermo-poroelastoplastic model improves the results to better capture the thermal pressurization, especially during the cooling phase. Moreover, thermo-elastic deformation majorly affects the thermal pressurization in the heating phase, and thermo-plastic deformation (thermal consolidation) has a major role in stress characteristics close to the heater. The magnitude and sign of thermo-osmotic conductivity strongly depend on the temperature and microstructural properties of clays which cannot be ignored. Comparison of the numerical results with and without considering thermo-osmotic flow and thermal infiltration, confirms the importance of the thermo-osmosis phenomenon during thermal loading in clayey soils. [ABSTRACT FROM AUTHOR]

Highlights: [•] An annular cylinder heat transfer model is developed. [•] Assumption of constant heat flux misinterprets pile and ground temperature. [•] Summer operation of an energy pile may increase its thermal efficiency in winter. [•] Parameters affecting thermal efficiency of a heat exchanger pile are identified. [•] Soil thermal conductivity reversely affects energy efficiency and ground temperature. [Copyright &y& Elsevier]

Temperature gradient and heat transfer in saturated porous media may affect pore fluid pressure and/or pore fluid flow depending on thermal, mechanical, and hydraulic properties of the media and the saturating fluid. Therefore, several Thermo-hydro-mechanical (THM) models have been developed to theoretically analyze the thermal behavior of soil media. In this study a coupled thermo-poro-mechanical model is adopted to investigate the heat and mass transfer in deformable porous media in a transient and quasi-steady state conditions. The Effects and importance of different properties of porous media are carefully taken into account to simulate two different cases with different scenarios. The results confirm that in order to accurately predict the thermal pressurization and changes in total stress, temperature dependency of properties of the soil and saturating fluid must be considered carefully. It is found that even a slight perturbation in porosity variation and temperature dependency of the thermal expansion coefficient of the fluid can greatly influence the thermal pressurization of pore fluid in very low permeable soils (e.g. clays), while variations of pore fluid density governs thermally-induced pore fluid flow in high permeable soils (e.g. sands and silty sands). Moreover, the feasibility of heat convection and heat-induced pore fluid flow is discussed in a parametric study with different temperature and permeability values. The results demonstrate that the Boussinesq approximation is a key assumption when dealing with heat-induced pore fluid flow in quasi-steady state condition. [ABSTRACT FROM AUTHOR]

Traditional analyses of ground heat exchangers consider conduction as the sole mode of heat transport within ground. However, research provides evidences of thermally induced pore fluid flow in soil. This paper demonstrates that temperature induced fluid flow should be considered to accurately analyze the performance of geothermal piles. Heat transfer, momentum, and flow continuity equations are coupled to quantify performance of heat exchanger piles. A hybrid explicit-implicit solution scheme is utilized for simultaneous and efficient solution of the system of equations. Results from pile-soil heat exchange analysis of a model geothermal pile installed in saturated sand indicate that temperature increments in saturated ground induces pore fluid flow due to alteration in fluid density with temperature. Direction of pore fluid flow is different at inlet and outlet sides of a pile with U-shaped circulation tube. For the conditions explored within the scope of this paper, power output of a model heat exchanger pile increases by around 12% when contribution of convective heat flow is included in addition to that from conductive heat transfer in saturated sand. Therefore, convective heat transfer due to buoyant pore fluid flow may have considerable contribution towards thermal performance of ground heat exchangers in saturated sand. [ABSTRACT FROM AUTHOR]

• Numerical analysis using a 3D model of a saturated porous medium. • Paramount importance of natural convection while modeling groundwater flows. • Parametric study on the effect of different variables on the thermal behavior in soil. • Proposing a new expression to estimate the mechanism of heat transfer in soil media. Accurate design of energy geostructures such as energy piles requires an exact prediction of soil thermal responses close to the embedded heat sources in the ground. In saturated soil, both heat conduction and heat convection facilitate heat transfer in the soil medium. Conventional analyses of heat transfer in saturated porous medium classified heat convection into two categories: (i) natural convection that could happen in the hydrostatic condition and (ii) forced convection that occurs due to the groundwater flow in the soil medium. This paper demonstrates that, in the presence of groundwater flow, the natural convection in the form of buoyancy-driven flow plays a prominent role in temperature distribution through saturated soil as well as the forced convection. A finite element analysis is performed to study the effects of groundwater velocity and soil permeability on the behavior of a fluid-saturated porous medium (soil). The results confirm that neglecting the natural convection can lead to inaccurate results. Furthermore, a novel expression is defined to estimate the importance of natural convection in heat transfer mechanisms in the soil medium. [ABSTRACT FROM AUTHOR]