Surrounding Temperature Field Research Articles

The influence of geometry on the heat exchange potential of embedded planar retaining walls with seasonally-balanced thermal load

A number of operational cases exist where embedded retaining walls, used in the construction of underground spaces such as basements and shallow tunnels, have also been utilised as ground-heat exchangers in shallow geothermal energy systems. These are complex structures in terms of their geometry, the surrounding temperature field and boundary conditions, and there are currently no methods to assess their heat exchange capacity in a simple and expedient manner. This contribution uses the finite element method to validate the use of the method in predicting heat flow for this application and then, to assess the influence of wall and excavation geometry in the heat exchange process. The influence of the soil and wall thermal conductivity is shown to be quasi-linear with the latter showing the greatest influence on peak heat exchange. The work identifies a geometric parameter - the ratio of excavation depth to total wall panel depth, H/L which in combination with the wall thickness (D), provide a consistent and simple means by which the heat exchange potential can be estimated, for a given set of wall and soil thermal properties and boundary conditions.

UNet–Based Temperature Simulation of Hot Dry Rock in the Gonghe Basin

Hot dry rock (HDR) geothermal energy, as a clean and renewable energy, has potential value in meeting the rapid demand of the social economy. Predicting the temperature distribution of a subsurface target zone is a fundamental issue for the exploration and evaluation of hot dry rock. Numerical finite–element simulation is currently the mainstream method used to study the variation in underground temperature fields. However, it has difficulty in dealing with multiple geological elements of deep and complex hot dry rock models. A Unity networking for hot dry rock temperature (HDRT–UNet) is proposed in this study that incorporates the matrix rock temperature field equation for relating the three parameters of density, specific heat capacity and thermal conductivity. According to the numerical geological structures and rock parameters of cap rocks, faults and magma intrusions, a new dataset simulated by the finite element method was created for training the HDRT–UNet. The temperature simulation results in the Gonghe basin show that the predicted temperatures within faults and granites were higher than their surrounding rocks, while a lower thermal conductivity of the cap rocks caused the temperature of overlying strata to be smaller than their surrounding temperature field. The simulation results also prove that our proposed HDRT–UNet can provide a certain evolutionary knowledge for the prediction and development of geothermal reserves.

A simplified correction method for thermocouple disturbance errors in solids

When a thermocouple is embedded in a material of lower thermal conductivity, under certain heating or cooling conditions, the presence of the thermocouple can distort the surrounding temperature field. As a result, the measured temperatures may be very different to the ‘undisturbed’ temperatures that would exist without the thermocouple. This study presents the results of a sensitivity analysis of key factors influencing this thermal disturbance. A series of heat transfer models and accompanying experiments are used to demonstrate the effects of thermocouple geometry, contact conditions, thermal properties, and heating regime on the temperature measurement error. These tailored finite element models were validated against experiments on vermiculite insulation board, which confirmed the accuracy of the models in simulating the thermal disturbance for inert heating conditions. Also, a simplified version of the finite element model was used to calculate the thermal disturbance error for a number of conditions, and subsequently to predict a range of corrected temperatures for the experimental measurements. This correction method was found to greatly improve the accuracy of the results for inert heating conditions. Since the method does not account for the effects of moisture in heat transfer, a creep of uncorrected errors could be observed.

Coupled CFD–FEM Simulation Methodology for Fire-Exposed Bridges

Evaluating the fire performance of bridges is necessary for damage mitigation as more fire-related accidents occurred on bridge structures. Current solutions adopting temperature curves can result in significant inaccuracy by neglecting the inhomogeneous characteristic of the fire-induced thermal environment. This paper proposed a numerical methodology for analyzing the coupled thermomechanical response of various bridges exposed to fires. The computational fluid dynamics (CFD) approach was implemented to reproduce the fire condition more realistically by modeling the combustion process and fire-driven flow. Then, an interface was adopted to extract the thermal boundary from the fire model. At last, the thermomechanical finite-element method (FEM) was coupled with the CFD model for determining the fire-induced response of the global bridge, thermally and structurally. By incorporating the multiscale FEM, this methodology can be extended to various large-scale bridges subjected to localized fires. The proposed approach was validated through a real fire experimental study on a steel column. To demonstrate the application of this strategy, a complex case study was carried out. A long-span cable-stayed bridge was investigated considering its girder segment was exposed to an under-deck tanker fire. Numerical results showed that the proposed method was able to capture the surrounding temperature field with strong thermal gradients and can predict not only the localized thermomechanical response of exposed segments but also the global structural performance evolution for large-scale complex bridges. The under-deck fire can introduce a significant impact on the entire cable-stayed bridge. Thereby, the multiscale FEM modeling strategy is required for the long-span bridges exposed to localized fires.

Effective medium theory for thermal scattering off rotating structures.

Controlling heat transfer with artificial functional materials has been a promising route towards the efficient and smart utilization of thermal energy in modern society. At the macroscopic scale, thermal metamaterials have demonstrated versatile functionalities in manipulating thermal conduction. One major method is the effective medium theory, which provides a reliable approximation for the material parameters of the composite. Although most of thermal metamaterials use static components, recent devices with integrated moving parts are attracting great interest thanks to their high efficiency and flexibility. However, the effective medium theory for thermal scattering off such devices has not been well established, due to the fundamental difference between thermal convection and conduction. Here, we provide a thorough study on heat transfer through mechanically rotating structures. It is shown that the effective thermal conductivity of a rotating structure can be rigorously described in a complex plane. The analytical expressions of the effective thermal conductivity for structures with rotating multiple layers are formulated, which explicitly capture their influences on the surrounding temperature field. We validate the theory and numerically demonstrate the rotated and unrotated temperature distributions generated around a single structure. Our theory is expected to become a design recipe for novel thermal metamaterials and meta-devices.

Fracture diagnostic using distributed temperature measurements during a pause in flow-back period

The significant temperature difference between the fractured and non-fractured regions during the stimulation fluid flow-back period can be very useful for fracture diagnosis. The recent developments in downhole temperature monitoring systems open new possibilities to detect these temperature variations to perform production logging analyses. In this work, we derive a novel analytical solution to model the temperature signal associated with the shut-in during flow-back and production periods. The temperature behavior can infer the efficiency of each fracture. To obtain the analytical solution from an existing wellbore fluid energy balance equation, we use the Method of Characteristics with the input of a relevant thermal boundary condition. The temperature modeling results acquired from this analytical solution are validated against those from a finite element model for multiple cases.Compared to the warm-back effect in the non-fractured region after shut-in, a less significant heating effect is observed in the fractured region because of the warmer fluid away from the perforation moving into the fracture (after-flow). Detailed parametric analyses are conducted on after-flow velocity and its variation, flowing, geothermal, and inflow temperature of each fracture, surrounding temperature field, and casing radius to investigate their impacts on the wellbore fluid temperature modeling results.The inversion procedures characterize each fracture considering the exponential distribution of temperature based on the analytical solutions in fractured and non-fractured regions. Inflow fluid temperature, surrounding temperature field, and after-flow velocity of each fracture can be estimated from the measured temperature data, which present decent accuracies analyzing synthetic temperature signal. The estimations are independent on each cluster, which can be combined to analyze the entire production section. The outputs of this work can contribute to production logging, warm-back, and wellbore storage analyses to achieve successful fracture diagnostic.

Four-position heading effect calibration algorithm for rotation inertial navigation system based on fiber optic gyro

Fiber optic gyros (FOGs) are sensitive to the environment fields where they are mounted, and their drifts are easily affected when surrounding temperature field or magnetic field changes. In FOG strapdown inertial navigation system (INS), gyro drifts caused by environmental fields are stable mostly, thus they could be calibrated and compensated beforehand and would not cause obvious alignment and navigation errors. However, in rotation INS (RINS), although navigation errors caused by the constant components of FOG drifts could be well attenuated, the gyro sensing axes are changing relative to the environmental fields in the RINS, which would lead to periodically changing gyro drift components when inertial measurement unit is pointing to different headings, thus producing serious alignment and navigation errors in FOG RINS. To solve this problem, a four-position heading effect calibration algorithm was proposed, and its effectiveness and validity were verified through a dual-axis FOG RINS by turntable experiments. The experimental results show that the azimuth alignment accuracy of the FOG RINS improves from 0.2 deg to about 0.04 deg, increasing five times approximately, which illustrates that the proposed heading effect calibration algorithm could further improve the navigation performance of FOG RINS significantly.

Numerical Simulation Study on Development of Geothermal Resource in the Borehole Wall Surrounding Temperature Field

Geothermal resource is a very clean source of energy, and its exploration and development has a history of almost one hundred years. In this paper, the mathematical model of multi-physical field on the platform of Comsol Multiphysics software was set up about the flow-solid-Thermal numerical model. During the analysis, rock porosity and permeability of stratum were taken into consideration to study change state of the temperature of export wells and injection wells. Besides, porosity-temperature curve fitting and permeability-temperature curve fitting were made respectively to study the relationship between the stratum temperature and permeability and porosity.

Virtual autoclave implementation for improved composite part quality and productivity

State-of-the-art of advanced composite manufacturing uses autoclave devices to ensure the overall curing of the polymer resins for large complex carbon–fibre–polymer composites (CFRP) helicopter structures. A homogeneous temperature distribution on the part surfaces is the key to obtain high and constant quality parts. Local heat transfer coefficients are, therefore, important monitoring factors of autoclave technology processes. Many researchers focussed on the exothermic curing reaction inside the composites, but the surrounding temperature field (inclusive air and mould temperature field) was often neglected. Thus, a thermal uniformity at the surface of the part was often not foreseeable. Due to the 3D arrangements of the parts in the autoclave, together with the design of the tools themselves, strong turbulences and velocity differences are created inside the industrial autoclave, affecting the heat transfer mechanisms at the surface of CFRP parts. Therefore, the temperature field is non-uniform in the spatial and temporal domains. This non-uniform distribution of part and mould temperatures has an important impact on the local degree of cure and on residual stresses in the composite parts. To better understand the process and to optimise the curing cycles, virtual autoclaves have been successfully implemented by MET together with Eurocopter. The validation phase showed very good agreement between measured and simulated temperature fields. On this basis, different internal arrangements of parts for helicopter airframes have been investigated leading to the development of practical solutions to reduce the cycle time, and improve the quality of the parts. Examples of application will be presented together with industrial benefits. This simulation methodology provides Eurocopter with a strong tool for the improvement of composites part quality and productivity. The article content was presented at the Eucomas Conference on February 7, 2012 in Hamburg, Germany.

Computerized tomography technique for reconstruction of obstructed temperature field in infrared thermography

Infrared thermography is a rapid, non-invasive and full-field technique for non-destructive testing and evaluation (NDT&E). With all the achievements on IR instrumentation and image processing techniques attained, it has been extended far beyond simple hot-spot detection and becomes one of the most promising NDT&E techniques in the last decades. It has achieved increasing acceptance in different sectors include medical imaging, manufacturing component fault detection and buildings diagnostic. However, one limitation of IR thermography is that the testing results are greatly affected by object surface emissivity. Surface with various emissivities may lead to difficult discrimination between area of defect and area with different emissivity. Therefore, many studies have been carried out on eliminating emissivity, for example, the time derivative approach, lock-in processing and differential contrast measurements. In these methods, sequence of themo-data/images are recorded and being processed in order to eliminate differences of emissivity. Another problem of IR thermography is that any obstruction may limit stimulations and imaging which leads to the observation of unclear defect image. To solve this problem, this paper proposes an algorithm based on the principle of computerized tomography which permits the reconstruction of unavailable/partially available temperature distribution of the affected area using the measured surrounding temperature field. In the process, a set of imaginary rays are projected from many different directions across the area. For each ray, integration of the temperature derivatives along the ray is equals to the temperature difference between the boundary points intercepted by the ray. Therefore, a set of linear equations can be established by considering the multiple rays. Each equation expresses the unknown temperature derivatives in the affected area in terms of the measured boundary temperature data. Solution of the set of simultaneous equations yields unknown thermal distribution in the area which needs to be reconstructed. Based on the proposed computerized tomography reconstruction (CTR) technique, deviated temperature data due to missing temperature data hidden by obstacle can be reconstructed. With further development, this technique may have the potential to be applied in the reconstruction of any smooth physical fields like phase information in optical techniques.

Modelling the reduction of an iron ore-coal composite pellet with conduction and convection in an axisymmetric temperature field

A mathematical model of the coal-based direct reduction process of iron ore in a pellet composed of coal and iron ore mixture is investigated using a finite-control volume method. Heat transfer by conduction in the solid, convection by gaseous media inside, and radiation from the surroundings of the pellet are included in the model. The pellet is assumed to be spherical initially and the temperature around the pellet is taken to be symmetric about an axis passing through the centre. The parameters of the process, such as thermal conductivity, specific heats, and heats of the reaction, are all temperature dependent. The shrinkage/swelling of the pellet is also considered. We find that the effect of convection on the temperature and on the overall average reduction is small. However, the effect on the local concentration of the reaction components is significant. We predict that a uniform surrounding temperature field around the pellet yields a better average reduction.

Circular cylinder in a periodic temperature field

In a former paper1) the author has considered heat waves in homogeneous and isotropic cylinders surrounded by a periodic temperature field. The present article gives a substantial generalization of previous deductions. It is much more applicable, in technical and physical regards, taking into account various coefficients of radiation at the boundaries of the body and a more general surrounding temperature field.