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Abstract Though understanding the gas production behavior from methane hydrate in sediments by depressurization are critical for the utilization of gas hydrate resource and has been widely studied, very few researches are reported using natural marine sediments as porous matrices. Significant differences have been found between the natural sediments and widely used coarse sands; difficulties could be commonly encountered in the formation and production process arising from the fine grain sizes and thereby low permeability. In this work, gas production behavior in hydrate-bearing natural marine sediments from the South China Sea was investigated by depressurization with different gas production pressures (2 MPa, 3 MPa and 4 MPa)and pressure gradients (1 MPa, 2 MPa and 4 MPa). Results show that the gas production process consists of two main stages: the rapid free gas liberation stage when the pressure and temperature drop quickly; the hydrate decomposition stage when the gas production slows down and temperature remains relatively stable and then rises to the backpressure driven by heat transfer from the ambient. A high pressure gradient and low production pressure were found to efficiently facilitate the gas production process. The minimum temperature during production significantly varies, under the combined effects of fast gas release and hydrate decomposition, which could play a crucial role in the production behavior. Furthermore, the difference in the thermal conductivity of the natural marine sediments with common coarse grains seriously affects the heat transfer and gas production behavior, with heat transfer from the ambient to the production well with a radial temperature gradient. The inhomogeneity of hydrate saturation vertically resulting from the low permeability of natural sediments also results in a non-uniform temperature distribution during production, which could locally trigger the secondary hydrate formation and freezing thereby hindering the gas production. Therefore, significant differences could be present in the natural sediments, making this work helpful in understanding the production behavior from natural hydrate deposits. [ABSTRACT FROM AUTHOR]

Abstract Gas hydrates are regareded as a potential alternative energy sources for sustainable development. Thermal properties of gas hydrate-bearing sediments directly govern the heat transfer process during hydrate decomposition which couples with phase transitions and multiphase flows. Latest database on the thermal properties of natural sediments containing gas hydrates still remains limited. Here we report on point heat source measurements of the effective thermal conductivity of hydrate-bearing sediments through a thermistor-based method and evaluation of existing models. Effective thermal conductivity of water saturated natural sediments and partially saturated hydrate-sediment are obtained, showing a significant effect of compaction on the effective thermal conductivity of the sediments. The evolution of thermal conductivity during hydrate formation, decomposition and reformation indicates that the variation of the components in the pores could play a crucial role in the effective thermal conductivity. A higher saturation of hydrates in the pores and a smaller porosity could contribute to a better thermal conductivity. The evolution of the effective thermal conductivity with hydrate formation and decomposition also demonstrates a temperature dependence. Moreover, the reformation process after hydrates are fully decomposed significantly impacts the variation of the effective thermal conductivity, which could be attributed to the solution migration during decomposition and the local inhomogeneity of hydrate reformation. The slightly increase in the thermal conductivity below zero degree Celsius implies the small fraction of ice generation during the cooling process. The results could provide some insights into the effect of hydrate formation and decomposition on the effective thermal conductivity of natural sediments, which would be helpful in the heat transfer evaluation and gas production simulation in the field test. [ABSTRACT FROM AUTHOR]

Hydrate reformation and ice generation may occur under fast depressurizing rate and low production pressure processes caused by lacking sufficient heat transfer in the sediment. This work conducted MRI visualization and analysis for methane hydrate (MH) dissociation by controlling the depressurizing rate gradually to designed production pressures. Obvious hydrate reformation and ice generation can be avoided through this method. MH dissociation behavior was analyzed under different back pressures. Furthermore, ice generation during MH dissociation under rapid depressurization and low back pressure condition was studied. Large amount of ice generated spatially in the vessel. Ambient heat transfer drove ice melt and hydrate dissociation from the surrounding wall in. The saturation of the generated ice in the vessel was also estimated. [ABSTRACT FROM AUTHOR]

Hydrate dissociation incorporates heat transfer and fluid flow with hydrate dissociation kinetics. We previously developed and validated a two-dimensional axisymmetric model to investigate the effect of heat transfer on gas production from methane hydrate dissociation by depressurization, thermal stimulation, and a combination of the two methods. We herein extend our focus to the influence of reservoir permeability on methane hydrate dissociation by depressurization. Initially, pressure reduction propagated slowly from the outlet valve into a low permeability sediment. However, propagation of this pressure reduction was rapidly promoted deep into the sediment core boundary in a high-permeability case. Dissociation behavior was affected by reservoir permeability, with high permeability exhibiting spatial hydrate dissociation, followed by an inward moving dissociation front from the surrounding wall of the sediment core. The hydrate exhibited a sharp dissociation front with pressure reduction regime from the outlet valve to the right end of the sediment in a low-permeability case. The sediment permeability in the presence of hydrate ranged between 0.01 mD and 0.5 mD for the transition from a sharp front dissociation to a spatial dissociation. Furthermore, in the case of a high permeability, a spatial temperature decrement followed by temperature bounce back resulted from heat transfer from the surrounding wall. In the case of a low permeability, ambient heat transfer transmitted from the water bath to the dissociation region with the moving dissociation front. Moreover, gas production from the low-permeability sediment relied significantly on the surrounding ambient heat transfer. Finally, a higher reservoir permeability resulted in an earlier peak gas generation time and a higher peak gas generation rate. [ABSTRACT FROM AUTHOR]

Thermal properties of gas hydrate-bearing sediments directly govern the heat transfer process during hydrate decomposition which couples with phase transitions and multiphase flows. The effective thermal conductivity of a multiphase system represents the composite capacity to conduct heat. Here we report on point heat source measurements of the effective thermal conductivity of methane hydrate-bearing sediments through a thermistor-based method combining with X-ray CT observations. Methane hydrates were formed at different saturations, with various initial water contents, and in porous matrices simulated by grains with differing thermal conductivities. It is indicated that the effective thermal conductivity of sediments negatively correlated with the hydrate saturation, while an increase of initial water contents and thermal conductivity of grains has a positive impact on the elevation of the effective thermal conductivity. Moreover, the effective thermal conductivity was found to slightly increase with the proceeding of hydrate decomposition. Typical effective medium models were evaluated with the measurements of this study, and a hybrid fitting model combining three forms of self-consistent models was proposed, with the optimal weighting parameters determined via the genetic algorithm. The effective prediction of the measurements in this work and results in literatures corroborates the feasibility of the model. This study could help in understanding the evolutions of sediment thermal properties during gas production and their effects on large-scale hydrate decomposition when expanded to field scale tests. [ABSTRACT FROM AUTHOR]

Depressurization is considered to be the most promising method for exploitation of natural gas hydrate. To analyze the characteristics of hydrate dissociation during depressurization, methane hydrate (MH) dissociation was performed at different depressurizing ranges and rates, and the hydrate dissociation process was directly observed using magnetic resonance imaging (MRI). The experimental results indicate that with increased depressurizing rate from 0.01 MPa/min to 0.1 MPa/min, the average dissociation rate increases for a given depressurizing range. Meanwhile, with an increase in depressurizing range from 0.3 MPa to 1.1 MPa, the average dissociation rate increases for a given depressurizing rate. Moreover, the hydrate dissociation process can be divided into two main stages: hydrate saturation remains constant with little fluctuation for several minutes after back-pressure decreases, and then the hydrate dissociates continuously until dissociation completes. In addition, excessively high depressurizing range and rate result in hydrate reformation and ice generation, which slow the rate of hydrate dissociation. Furthermore, it was also determined that MH reformation and ice generation always occur at the higher depressurizing range and rate due to insufficient heat transfer. [ABSTRACT FROM AUTHOR]

Natural gas hydrate dissociation requires the continuous absorption of heat energy from the periphery, which influences both the gas generation rate and cumulative gas production. To predict the potential impact of heat transfer on the hydrate dissociation process, we previously developed and verified a two-dimensional axisymmetric model. In this study, we build on our previous work to investigate the influence of heat transfer on methane gas production by thermal stimulation. The results show that during hydrate decomposition, increasing the specific heat capacity of porous media containing hydrate inhibits the gas generation rate. However, the effects of the initial water content on the gas generation rate and cumulative gas production are weak. The influence of water and methane convection heat on hydrate dissociation is also weak. Increasing thermal conductivity can initially negatively influence hydrate dissociation, but later promote the process. [ABSTRACT FROM AUTHOR]

• Initial gas production rate is enhanced by combining fracturing with depressurization. • Increasing fracturing length accelerates the average gas production rate. • Timely and sufficient boundary heat transfer is necessary for the continual hydrate dissociation. • Hydrate dissociation pattern is visually presented and systemically discussed. Most of the existing field tests of gas recovery from hydrate-bearing sediments suffer from the difficulty in pressure propagation, leading to a low productivity and energy efficiency. Fracturing has shown enormous potential in the shale gas production; thus in this study, we introduced this technique into the laboratory-scale gas hydrate production. The performance of gas production was investigated through numerically analyzing the gas hydrate dissociation behavior under different fracturing patterns. The results indicate that fracturing can significantly facilitate the pressure propagation in the hydrate sediments with a low intrinsic permeability during depressurization, thereby contributing to a better gas production performance. Fracture depth plays a critical role in promoting the gas production efficiency; the maximal enhancement ratio of average production rate by fracturing could attain 13.1%. Moreover, the contribution of reservoir's sensible heat in hydrate dissociation is limited in the core with a low permeability; timely and sufficient heat supply is thus important for the successive gas production. The findings of this study illustrate the effects of fracturing on enhancing the gas production efficiency of depressurization-induced gas hydrate exploitation; this will provide important guidance for its potential application in the field test of marine and permafrost hydrate reservoir where low permeability is commonly encountered and an enhancing technique is much required. [ABSTRACT FROM AUTHOR]

The skeleton shape of the Kelvin cell plays a significant role in influencing forced convective heat transfer behaviors, and it can be tailored to enhance overall heat transfer performance (OHTP). To this end, this study conducted a comprehensive experimental and numerical comparison of the Kelvin cells (KC), Kelvin cells with elliptical skeletons (ES), and Kelvin cells with reversed elliptical skeletons (RES) to elucidate the hydraulic and thermal characteristics. The findings indicated that the pressure drop and the overall Nuseelt number of the ES is 53.0 % and 8.2 % lower than those of the KC. Consequently, the area goodness factor, which serves as an indicator of the OHTP, is 105.5 % higher in the ES compared to the KC. Conversely, the RES does not exhibit a significant advantage in the aforementioned parameters. The reduced energy loss attributable to the presence of elliptical skeletons enhances the hydraulic performance of the ES. Furthermore, the smaller recirculation area on the leeward side of the elliptical skeleton promotes improved heat transfer near the heat substrate. The ES has advantage in the fin efficiency compared with KC. The 1.25 cell height is a significant reference value for the development of heat transfer device with Kelvin cell. • Comparison between anisotropic Kelvin cells investigated. • Effect of skeletons shape on forced convective heat transfer was figured out. • The reference height for the design of heat transfer devices was provided. [ABSTRACT FROM AUTHOR]

· The sheet-network and solid-network TPMS structures considering porosity and other parameters are constructed. · The solid-network TPMS structures with larger through-hole and skeleton concentration ratio have higher overall performance. · Under the same pressure drop, the heat transfer performance of TPMS with D-type sheet structure is better. Triply periodic minimal surface (TPMS) structures are widely used in the field of flow and heat transfer. In this paper, four TPMS structures, Schwartz Diamond-sheet (D-sheet), Schwartz Diamond-solid (D-solid), Schoen IWP-sheet (IWP-sheet), and Schoen IWP-solid (IWP-solid), are designed based on two structure generation strategies. The above structures all are manufactured through additive manufacturing, and the material is AlSi10Mg. The samples were characterized by X-ray scanning. The flow and heat transfer performance of different TPMS structures was studied by computational fluid dynamics (CFD) and experimental methods. The results show that the pressure drop of sheet-network is larger than that of solid-network for the same TPMS type. With the same structure generation strategy, the pressure drop of Schwartz Diamond (D-type) is greater than that of Schoen IWP (IWP-type). The heat transfer performance of the TPMS structure shows the same results as the pressure drop. The performance enhancement criterion (PEC) of the solid-network TPMS is better than that of the sheet-network structure, and the D-type TPMS structure is better than the IWP-type structure. The average PEC of D-solid and IWP-solid is 31 % and 44 % larger than that of D-sheet and IWP-sheet, respectively. The average PEC of D-sheet and D-solid is 11.8 % and 1.7 % larger than that of IWP-sheet and IWP-solid, respectively. The mechanism of the higher PEC of the solid-network structure is the low pressure drop caused by the high through-hole ratio and relatively concentrated skeleton. Under the same pressure drop, the heat transfer performance of TPMS with D-type sheet structure is better. The research content of the article is mainly applied to fin-enhancement, which can replace fins in traditional heat exchangers. In actual engineering applications, the TPMS structures type is reasonably selected according to the requirements. [ABSTRACT FROM AUTHOR]

• Different types of regenerative-cooled channels were studied. • Insertion of ribs inhibits vortex generation from buoyancy. • Enhanced uniformity of heat transfer can significantly reduce thermal stress. • The triangular ribbed channels reduce the average thermal stress by up to 30%. Regenerative cooling plays a critical role in the thermal management for scramjet combustors, and the design of cooling channels is of utmost importance. To comprehensively analyze the performance of cooling channels, a validated thermal-mechanical coupling model was utilized in this study. The thermal and mechanical performance of both smooth and ribbed channels in regenerative cooling was investigated using Finite Volume Method and Finite Element Method. The findings indicate that the ribbed wall has a notable effect on enhancing the overall thermal performance of the channel. Moreover, the presence of ribs weakens the influence of buoyancy in the flow field, which can be one of the contributing factors in inhibiting heat transfer deterioration. Additionally, the design of ribs significantly reduces the average thermal stress experienced by the channel. However, it should be noted that the non-uniform heat transfer in the axial direction leads to a significant increase in local thermal stress. The triangular ribbed channel proves to be an effective approach in reducing thermal stress. Compared to rectangular ribbed ribs, the utilization of triangular ribs can result in a reduction of thermal stress by up to 30%. This observation highlights the advantage of triangular ribs in enhancing system life and safety by minimizing thermal stress. [ABSTRACT FROM AUTHOR]

The triply periodic minimal surface (TPMS) structure is widely used in forced convection heat transfer. Additionally, developing of parametric modeling and additive manufacturing technology makes TPMS manufacturing possible. In this work, the forced convective heat transfer performance of three standard TPMS, the Gyroid (G-type), the Diamond (D-type), and the Primitive (P-type) structures and their hybrids, the Gyroid/Diamond (GD-type), the Gyroid/Diamond (GP-type), and the Diamond/Primitive (DP-type) was studied. The pressure drop, heat transfer coefficient, and overall heat transfer performance are comprehensively investigated. The results show that for the air Reynolds number (Re) range of 412.48 to 1257.96 the average pressure drop of G-type is 43.2% and 38.86% lower than that of P-type and D-type, respectively. The important influencing factor related to pressure drop is structural characteristics. The D-type has the highest heat transfer coefficient (1398 W/m2·K) with an average value exceeding 30.83% of the G-type and 28.39% of the P-type, respectively. Hybrid GD-type and DP-type both show good heat transfer performance. The heat transfer coefficient of the GD-type is 8.41% higher than that of the G-type, and the DP-type was 4.41% higher than of the P-type. • Three hybrid structures based on the TPMS were designed by using equal ratio flexible hybridization method. • Flow and heat transfer experiments on the TPMS hybrid structures were carried out under turbulent conditions • It is confirmed that D type element hybrid can enhance the heat transfer performance of the TPMS structures. [ABSTRACT FROM AUTHOR]

Highlights • Natural gas is recovered from hydrate deposits using different depressurization modes. • The phenomenon of concomitant ice generation is observed using magnetic resonance imaging (MRI) visualization. • The spatial-decomposition characteristic dominated by pressure drop is confirmed. • The radial-decomposition characteristic dominated by ambient heat transfer is determined. • The problem of ice generation is effectively eliminated by controlling the gas production pressure. Abstract Natural gas hydrates have garnered worldwide attention as an important potential non-conventional fossil fuel resource. When extracting natural gas from gas hydrate deposits via depressurization, problematic ice generation and hydrate reformation can occur under conditions of fast depressurizing and low production pressures, due to insufficient heat transfer in the surrounding sediments. In this work we conduct in situ magnetic resonance imaging (MRI) visualization and analysis of hydrate decomposition behavior for different depressurization modes; we visually determine the volumetric and spatial characteristics of the hydrate decomposition during depressurization induced gas production operation. Our results indicate that fast depressurization rate can result in a fast hydrate decomposition rate, therefore, a rapid gas production rate. In addition, the radial extension behavior of the decomposition front confirms that ambient heat transfer is a critical factor driving hydrate decomposition into free gas and liquid water. Obvious hydrate reformation and ice generation phenomenon, seen in some of the sudden depressurization experiments, can be effectively avoided using piecewise and continuous depressurization methods. The findings of this study clearly demonstrate how production pressures affect the gas production behavior from hydrate deposits and provide further insight for establishing optimal production techniques for utilizing hydrate resources in the field. [ABSTRACT FROM AUTHOR]

The hydrate dissociation process involves heat transfer in the decomposing zone, multi-phase fluid flow during gas production, and the intrinsic kinetics of hydrate dissociation. The potential impact of laboratory-scale permeability on hydrate exploitation from hydrate-bearing sediments was predicted from a previously developed and verified two-dimensional axisymmetric model. We herein continue the previous work to investigate the influence of core-scale hydrate sediments’ permeability on gas production by the thermal stimulation method. The results show that the gas production in relatively low permeability reservoirs proceeded at a faster rate, requiring less time to complete the dissociation process, although an optimal permeability was associated with the fastest gas production. In addition, with the temperature continuously increased, the dissociation front displaced from the boundary wall to the core axis along the radial direction. In a lower permeability system, however, the hydrate dissociation process at the zone opposite the outlet valve was delayed. Due to the varying processes associated with hydrate dissociation, the overall thermal conductivity declined faster at an earlier stage in sediments of high permeability as compared with sediments of lower permeability. Furthermore, the effects of boundary heat transfer were more significant for low permeability systems. [ABSTRACT FROM AUTHOR]

As a potential alternative strategic energy source, and for their potential impact on global climate, natural gas hydrates have garnered worldwide attention. This study explored the measurement of effective thermal conductivity of methane hydrate-bearing sediments and evaluated existing models for the prediction of effective thermal conductivity. A thermistor-based method combined with Micro-CT observations was employed in the determination of the effective thermal conductivity of porous matrix materials with various hydrate and water saturation levels and physical characteristics. The effects of sample component characteristics, including the volume content of hydrate and water, phase conversion, and properties of porous materials, on the effective thermal conductivity of hydrate-bearing sediments were systematically evaluated. The effective thermal conductivity positively correlated with hydrate saturation, water content, and the thermal conductivity of porous media. In addition, the effective thermal conductivity slightly increased with hydrate dissociation, indicating an increasing heat transfer capacity during gas production. Existing prediction models were evaluated using our measured results, and a hybrid model combining the parallel and series models was proposed with expanded applicability to the scope of our research. The feasibility of the proposed model was also verified in a comparison with previous research. The results of this study are important for future investigation of the actual thermal properties of hydrate-bearing sediment and the understanding of the heat transfer mechanism during gas production. Furthermore, the results can provide guidance in the selection of an optimal technique for gas production from hydrate deposits at the field scale. [ABSTRACT FROM AUTHOR]

Natural gas hydrates (NGHs) are a promising energy source with huge reserves. The dissociation characteristics of NGHs need to be clarified further for developing safe and efficient technology for its recovery. In this study, Classes 1 and 2 NGH deposits were simulated by forming methane hydrate (MH) in porous media, and MH dissociation induced by depressurization was investigated using magnetic resonance imaging (MRI). MRI showed the liquid water distribution, which was used to analyze MH formation and dissociation. The vessel pressure was also measured during the experiments, which was compared with the MRI mean intensity of liquid water. MH dissociation processes were measured and analyzed under different backpressures, from 2.2 to 2.8 MPa. It was observed that liquid water hindered methane gas output during gas production; hydrate dissociation caused the movement of some liquid water, which usually led to fluctuations in MRI signal intensity. The experimental results also indicated that the MH dissociation pattern was affected by heat transfer; although a larger depressurization range led to faster dissociation, the average dissociation rate was controlled by heat transfer. [ABSTRACT FROM AUTHOR]

Heat transfer affects the pressure and temperature distributions of hydrate sediments, thereby controlling hydrate dissociation. Therefore, its study is essential for planning hydrate exploitation. Previously, a two-dimensional axisymmetric model, to investigate the influence of heat transfer on hydrate exploitation from hydrate-bearing sediments, was developed and verified. Here, we extended our investigation to the influence of heat transfer on methane gas production using a combined method coupling depressurization and thermal stimulation. Our simulations showed that during decomposition by the combined method, a high specific heat capacity of the hydrate-bearing porous media or a high initial water content could inhibit gas generation. However, the initial water content had only a weak influence on the cumulative gas production and generation rate. The influence of water and methane heat convection was also weak. An increase of the thermal conductivity initially inhibited hydrate dissociation but later promoted it. The implementation of the combined method increased gas generation compared with using only thermal stimulation. However, the benefits gradually diminished with an increasing heat injection temperature. [ABSTRACT FROM AUTHOR]

The Kelvin cells are normally applied to represent the open-cell metal foams for simulation due to the similar pore structure. However, the experimental comparison between Kelvin cells and open-cell metal foams is rarely reported. This study compares the forced convective heat transfer properties of four Ni70Cr30 metal foams and one GH4169 Kevin cells manufactured from additive manufacturing. The results indicate that the pressure drop and overall heat transfer coefficient (HTC) increase with the increase of PPI, while decreasing with the increase of porosity. Moreover, the comparisons between present data and data from the literature prove the reasonability of the experiments. The pressure drop and overall HTC of Kelvin cells are 64.4% and 12.3% higher than open-cell metal foams, while showing 26.2% lower overall heat transfer performance at 6 m/s. The pore structure, thicker skeleton, higher porosity, and surface roughness of Kelvin cells contribute to the higher pressure drop than open-cell metal foams. The comparison among experimental correlations and present data indicates that the correlations for predicting the behavior of open-cell metal foams could not be used directly to predict Kelvin cells. [ABSTRACT FROM AUTHOR]

[Display omitted] • Various water saturations simulate Classes I and II hydrate deposits in nature. • Ambient temperature controls periphery dissociation in excess-gas hydrate deposits. • Sensible heat dominates spatially uniform dissociation in excess-water deposits. • Secondary hydrate formation are prevented by adjusted gas production pressure. • Optimized depressurization is crucial to improve water production and CH 4 recovery. Depressurization is considered as the most promising technique for hydrate exploitation, as it achieves the highest energy profit ratio and is the most technologically feasible. However, the exploitation of excess-water hydrate accumulations generates high water production, leading to increased cost, poor energy efficiency, and problems with sand during operation. Thus, water management is crucial to gas recovery by the depressurization of different classes of hydrate accumulations, yet relevant studies remain limited. In this study, synthetic hydrate samples were prepared to simulate two types of natural methane hydrate sediments: Class 1 accumulations (excess-gas hydrate) and Class 2 accumulations (excess-water hydrate). Hydrate dissociation was conducted using a variety of depressurization approaches, and MRI imaging was employed to characterize water performance and methane recovery. Methane hydrate preferentially dissociated along the peripheries of the excess-gas samples due to more efficient heat dissipation. Methane hydrate dissociated more uniformly in the excess-water samples because the high specific heat capacity of water enabled the supply of extra heat. Furthermore, pressure histories, mean intensity change in MRI images, and water variations were monitored to analyze the characteristics of hydrate dissociation, changes in porosity, intrinsic permeability and reservoir heat, water and gas production rates, and possible secondary hydrate formation. The results of this study suggest that an optimized depressurization approach, such as stepwise depressurization, could improve methane recovery from Class 1 and Class 2 methane hydrate accumulations. [ABSTRACT FROM AUTHOR]

The optimization of pore structure for metal foam is considered a feasible approach for improving the overall heat transfer performance. Thus, we numerically investigated Kelvin cells with different throat areas and structures (elliptical Kelvin cell (EKC)) to characterize the influence on pressure drop and heat transfer coefficient using FLUENT 18.0. The standard k – ε model exhibited a better agreement with experimental data and required less time to achieve convergence. The results revealed that the throat area could not feasibly optimize the overall heat transfer performance. Moreover, the area goodness factor j / f that considered the influences of both heat transfer coefficient and pressure drop on the overall heat transfer performance of EKC with the higher than conventional Kelvin cell. Based on comparative analysis between pressure, velocity, turbulence kinetic energy, and temperature distribution, increasing the space and decreasing the angle between the skeleton and flow direction caused a significant pressure drop in the EKC samples. Owing to the existence of a lower temperature area at the leeward of skeletons and a small difference of impingement cooling on windward skeletons, the reduction of HTC was acceptable. Therefore, the EKC exhibited immense potential for enhancing the design of heat transfer devices. • Kelvin cells with varying long and short throat diagonal and axes ratios. • Pressure drop and heat transfer coefficient were investigated comprehensively. • Pressure, velocity, and turbulence kinetic energy distributions were compared. • Increasing the elliptical axes ratio can optimize the overall heat transfer. [ABSTRACT FROM AUTHOR]

• The thermal conductivity of hydrate-bearing fine sediments is in-situ measured. • A compaction of the sediments results in an increase of the thermal conductivity. • Icing and reformation of hydrate could enhance the thermal conduction. An insufficient heat supply has been recognized to majorly hinder the hydrate decomposition which is endothermic and thereby requires large amount of heat to sustain. Consequently, the thermal properties of the sediments play a vital role in the heat transfer process internally. Yet their evolution during hydrate formation and decomposition is complicated which couples phase transition, multiphase flow and component migration; relating knowledge still remain limited. Here, we report on a point-heat-source based measurement of local thermal conductivity of natural fine-grained sediments containing hydrates. The results show a significant effect of compaction on the effective thermal conductivity arising from the compressibility of the silty sediments. The evolution of components and their morphological behavior were found to majorly contribute to the variations of the thermal conductivity during hydrate formation, decomposition and reformation. A local icing and reformation of hydrate could also result in an enhancement of thermal conductivity benefiting from the better connected pore structure. Classic effective medium models were evaluated using our measured data; a hybrid fitting model by combining the self-consistent models was proposed, and the feasibility of the fitting model was verified using the data from the natural reservoir. The results of this study could provide insights into the effect of evolving hydrate phase on the resulting effective thermal conductivity of natural sediments; the findings would be helpful in a potential enhancement of the heat transfer process during gas production from hydrate reservoir to achieve an improved gas productivity. [ABSTRACT FROM AUTHOR]

• Kelvin cell structure was numerically analyzed to evaluate heat transfer performance. • Optimized Kelvin cell skeletons improved heat transfer performance at high velocity. • Elliptical skeletons reduced pressure drop and turbulence at the throat. • Elliptical skeleton achieved uniform velocity distributions. The optimization of pore structure for metal foam is a feasible method for reducing the pressure drop and improving the overall heat transfer performance, especially at high velocity. In this regard, the optimized Kelvin cell with elliptical skeletons with a cross-section ratio of the long axis to the short axis (b/a) of 1.0, 1.4, and 2.0 are considered to evaluate the effects of b/a on the pressure drop and the heat transfer coefficient (HTC). The results indicate that both the pressure drop and HTC decrease with an increase in b/a. However, the pressure drop reduces more significantly. For instance, as b/a increases from 1.0 to 2.0, the pressure drop and the volumetric HTC at 90 m/s decrease by 98.5% and 6.3%, respectively. Therefore, the value of the volumetric area goodness factor (which considers both the effect of heat transfer coefficient and pressure drop on the overall heat transfer performance) j v / f of the sample with b/a = 2.0 is 86.7% higher than the sample with b/a = 1.0 at 90 m/s. As the velocity increases, the effect of b/a on the overall heat transfer performance increases. Compared with the cylindrical skeleton velocity distribution and pressure drop, the elliptical skeletons lead to a more uniform velocity distribution and reduce pressure drop significantly; nevertheless, the temperature distributions barely change. Moreover, the elliptical skeleton increases the specific surface area. Therefore, the j v / f of the elliptical skeleton significantly improves. This study provides a new direction for the design of novel metal foams with excellent overall heat transfer performance for heat transfer devices. [ABSTRACT FROM AUTHOR]

• Forced convective heat transfer characteristics of metal foam were numerically investigated. • 3D models of the metal foam were obtained using X-ray imaging. • Metal foam with higher porosity and PPI had better heat transfer performance. • The effect of closed pores on heat transfer performance was investigated. Previously, extensive efforts have been made in investigating heat transfer in open-cell metal foams in order to better understand and apply them in engineering applications. As a step in this direction, this study prepared five three-dimensional models of copper foam with various porosities (0.82, 0.87, and 0.90) and pores per inch (PPI) (10, 20, and 40) using X-ray computed tomography and several software applications to simulate forced convection heat transfer in a metal foam at a high velocity. The effects of porosity, PPI, and closed pores on the pressure drop and heat transfer properties were investigated. The results indicated that the pressure drop and heat transfer coefficient were more sensitive to porosity than PPI. However, the specific surface area of the metal foam significantly affected the volumetric heat transfer performance. Additionally, the area goodness factors (j/f) were used to evaluate the comprehensive heat transfer performance (CHTP) of each sample. The value of j/f decreased rapidly in the velocity range of 0–20 m/s but slowly in the range of 20–70 m/s, and sample 40–0.87 exhibited a better CHTP. Moreover, the closed pores in the metal foam had a significant effect on the pressure drop, heat transfer properties, and CHTP due to the obstructions posed by the closed pores. Our work helps provide a better understanding for the application of metal foam in heat transfer enhancement at high velocities. [ABSTRACT FROM AUTHOR]