Surface Thermal Boundary Conditions Research Articles

Applying satellite sea surface temperature as Dirichlet-type surface thermal boundary condition in an ocean model

Following the Dirichlet-type boundary condition that specifies value at boundary of a system in mathematics and physics, this study suggests applying satellite sea surface temperature (SST) as Dirichlet-type surface thermal boundary condition (STBC) in ocean models. Numerical experiments with Dirichlet-type and Combined-type STBC with different configurations of satellite SST, for the period of January to April 2019 in the Northwest Pacific Ocean, were carried out based on the Princeton Ocean Model. The experiments were assessed using satellite and in situ observations of temperature, GOFS3.1 analysis and GLORYS12v1 reanalysis. Results show that applying satellite SST as Dirichlet-type STBC could constrain the modeled SST and near-surface temperature in upper 50 m depth well, which is better than the scheme that uses satellite SST as the relaxation terms of Combined-type STBC. The temperature section along the 137°E resulted from the Dirichlet-type STBC is comparable with GOFS3.1 analysis and better than GLORYS12v1 reanalysis in upper 50 m depth. These results suggest that applying high-accuracy satellite SST as Dirichlet-type STBC in ocean models has a promising prospect in numerical simulation.

Effects of the ground surface thermal boundary condition on the long-term thermal performance of energy diaphragm walls

Energy diaphragm walls have attracted increasing attention due to their high heat transfer performance. Thus, the factors that affect the thermal performance of energy walls have been investigated to provide insights for further practical applications. Numerical simulation is widely used in energy wall analysis due to the complexity of conducting field tests. Therefore, the choice of boundary conditions in the simulations is crucial; it refers to whether the model reflects reality and has a significant impact on the results. In most cases, the assumed ground surface thermal boundary is not the same (e.g., thermal insulation, constant temperature), and the ground temperature is also assumed to be constant with depth, which may result in a large loss of thermal energy or inaccurate assessment of thermal performance for the energy wall. In this respect, this paper investigates the long-term differences in thermal performance under these boundaries and considers the ground temperature that varies with depth and time, which has not been comprehensively considered before in the analysis of energy walls. The results show that accounting for air temperature or ground temperature variations with depth and time can result in a great increase in thermal performance, e.g., a 34% increase in the cooling thermal load compared with the thermal insulation boundary. In the performed parametric analysis, the results highlight the significant influence of the thermal load and wall height on the thermal differences between these boundaries. These findings provide a reference for choosing ground surface thermal boundaries and better thermal design of energy diaphragm walls.

A numerical and experimental study of natural convective heat transfer from two-sided circular and square horizontal plates having a finite thickness

Several numerical and experimental studies of natural convective heat transfer from two-sided heated horizontal plates are available. In the numerical studies it has usually been assumed that the plate is thin, effectively having no thickness, whereas plates of finite thickness have been used in the experimental studies. The main aim of the present study is to determine whether the plate thickness has a significant influence on the heat transfer rates from the plate surfaces. The effect of the vertical side surface thermal boundary condition and the effect of the plate shape on the heat transfer rates have been also studied. Heat transfer results have, therefore, been numerically obtained for circular and square plates having various thicknesses. The upper and lower plate surfaces have been assumed to be isothermal and at the same temperature. Both the cases where the vertical side surface of the plate is isothermal and at the same temperature as the upper and lower plate surfaces and the case where this vertical side surface is adiabatic have been considered. The solution has been obtained using ANSYS FLUENT©. Variations of the mean Nusselt number with Rayleigh number for various dimensionless plate thicknesses for the two vertical side surface conditions have been obtained for both plates. To validate the numerical results, a limited range of experiments has been undertaken for both shapes. Mean heat transfer rates have been obtained using the lumped capacity method. Results have only been obtained for the case where the plates are exposed to air.

Development of a Building-Scale Meteorological Prediction System Including a Realistic Surface Heating

Microscale urban meteorological models have been widely used in interpreting atmospheric flow and thermal discomfort in urban environments, but most previous studies examined the urban flow and thermal environments for an idealized urban morphology with imposing neutral or homogeneous thermal forcing. This study has developed a new building-scale meteorological prediction system that extends the ability to predict microscale meteorological fields in real urban environments. A computational fluid dynamics (CFD) model has been developed based on the non-hydrostatic incompressible Reynolds-averaged Navier-Stokes (RANS) equations with a standard k-ε turbulence model, and the microscale urban surface energy (MUSE) model was coupled with the CFD model to provide realistic surface thermal boundary conditions in real urban environments. It is driven by the large scale wind and temperature fields predicted by the Korean operational weather prediction model. The validation results of the new building-scale meteorological prediction system were presented against wind tunnel data and field measurements, showing its ability to predict in-canyon flows and thermal environments in association with spatiotemporal variations of surface temperatures in real urban environments. The effects of realistic surface heating on pedestrian level wind and thermal environments have been investigated through sensitivity simulations of different surface heating conditions in the highly built-up urban area. The results implied that the inclusion of surface thermal forcing is important in interpreting urban flow and thermal environment of the urban area, highlighting a realistic urban surface heating that should be considered in predicting building-scale meteorology over real urban environments.

Numerical modelling of heat exchanger pile in pyroclastic soil

Energy piles are an innovative technology that couples the role of the structural foundation to the role of the heat exchanger to satisfy the energy needs of buildings and infrastructures. In the last decade insight into the mechanisms governing the thermo-mechanical response of single energy piles have been gained via full-scale in-situ tests, numerical analyses and model-scale experiments. The numerical approaches allow testing a large variety of configurations, geometries, materials, thermo-mechanical loadings and boundary conditions in a rather controlled but relatively easy and cost-effective manner. As a widespread habit it should be remarked that the temperature changes adopted in the piles have been only rarely justified based on realistic heating and cooling demands of live buildings. This numerical study investigates the effects of combination of mechanical and thermal loads on the mechanical behaviour of a single energy pile embedded in a typical pyroclastic soil deposit of Campania region, in Southern Italy. Thermal loads over one year have been determined by means of Design Builder software analysing a building in Napoli; the intermittent operation of the heat pump over daily time scale is properly modelled in the coupled thermo-hydro-mechanical axisymmetric FE analyses. A sensitivity analysis has been carried out to investigate the effects of the surface thermal boundary conditions.

Calibration of a plug-type gauge for measurement of surface heat flux and temperature using data from in-depth thermocouples

This study presents an investigation of the calibration and testing of a stainless steel plug-type gauge that utilizes temperature data from its two in-depth thermocouples (TCs) to predict its surface thermal boundary conditions caused by an unknown heating scenario regardless of its back surface boundary condition. Calibration of the sensor was accomplished using the two-probe calibration integral equation method (CIEM) which was modified to incorporate the effects of known temperature dependent properties. The precise locations of the in-depth TCs are not required to implement this method. The two-probe CIEM requires data from two calibration tests consisting of accurate measurements of the surface boundary conditions along with data from the two in-depth TCs to estimate the surface boundary conditions of any unknown (reconstruction) heating process when data from its in-depth TCs are measured. High temperature data was collected from three calibration tests and four reconstruction tests. Accurate predictions of the surface heat flux and surface temperature of four high temperature reconstruction tests (up to 770 °C) were obtained where the back surface of the gauge was heated, cooled, insulated and unaltered. Amplified experimental noise was added to the reconstruction temperature data to simulate noisy temperature data that may be gathered in a field test. The addition of amplified noise to the reconstruction temperatures had almost no effect on the overall accuracy of the surface heat flux and temperature predictions.

Experimental Evidence of Temperature Ratio Effect on Turbine Blade Tip Heat Transfer

Determination of a scalable Nusselt number (based on “adiabatic heat transfer coefficient”) has been the primary objective of the most existing heat transfer experimental studies. Based on the assumption that the wall thermal boundary conditions do not affect the flow field, the thermal measurements were mostly carried out at near adiabatic condition without matching the engine realistic wall-to-gas temperature ratio (TR). Recent numerical studies raised a question on the validity of this conventional practice in some applications, especially for turbine blade. Due to the relatively low thermal inertia of the over-tip-leakage (OTL) flow within the thin clearance, the fluids' transport properties vary greatly with different wall thermal boundary conditions and the two-way coupling between OTL aerodynamics and heat transfer cannot be neglected. The issue could become more severe when the gas turbine manufacturers are making effort to achieve much tighter clearance. However, there has been no experimental evidence to back up these numerical findings. In this study, transient thermal measurements were conducted in a high-temperature linear cascade rig for a range of tip clearance ratio (G/S) (0.3%, 0.4%, 0.6%, and 1%). Surface temperature history was captured by infrared thermography at a range of wall-to-gas TRs. Heat transfer coefficient (HTC) distributions were obtained based on a conventional data processing technique. The profound influence of tip surface thermal boundary condition on heat transfer and OTL flow was revealed by the first-of-its-kind experimental data obtained in the present experimental study.

Heating and cooling induced stresses and displacements in heat exchanger piles in sand

Heating and cooling of heat exchanger piles causes changes in the axial stresses in the pile and vertical pile displacements. Changes in axial stress (thermal stresses) are more significant in the case of fixed head piles, while vertical displacements are more important in the case of free head piles. This paper presents a series of finite element analyses that was performed in order to investigate the effect of the heating or cooling duration, the surface thermal boundary condition, the mechanical and thermal parameters of the soil and the temperature change in the pile, on the thermal stresses that develop in fixed head piles and the thermal vertical displacements of free head piles in sand. The numerical modelling procedure is first validated through the simulation of a set of published centrifuge experiments of heat exchanger model piles in sand. The results of an extensive parametric numerical investigation are then presented and the effect of the abovementioned potentially influencing factors is presented and discussed. Based on the numerical results, the most important parameters that affect the mechanical response of heat exchanger piles to heating or cooling are identified. Finally, design charts are proposed for the calculation of the maximum thermal stresses that develop in fixed head piles and the pile head thermal vertical displacement of free head piles.

An investigation of the thermomechanical features of Laohugou Glacier No. 12 on Qilian Shan, western China, using a two-dimensional first-order flow-band ice flow model

Abstract. By combining in situ measurements and a two-dimensional thermomechanically coupled ice flow model, we investigate the thermomechanical features of the largest valley glacier (Laohugou Glacier No. 12; LHG12) on Qilian Shan located in the arid region of western China. Our model results suggest that LHG12, previously considered as fully cold, is probably polythermal, with a lower temperate ice layer overlain by an upper layer of cold ice over a large region of the ablation area. Modelled ice surface velocities match well with the in situ observations in the east branch (main branch) but clearly underestimate those near the glacier terminus, possibly because the convergent flow is ignored and the basal sliding beneath the confluence area is underestimated. The modelled ice temperatures are in very good agreement with the in situ measurements from a deep borehole (110 m deep) in the upper ablation area. The model results are sensitive to surface thermal boundary conditions, for example surface air temperature and near-surface ice temperature. In this study, we use a Dirichlet surface thermal condition constrained by 20 m borehole temperatures and annual surface air temperatures. Like many other alpine glaciers, strain heating is important in controlling the englacial thermal structure of LHG12. Our transient simulations indicate that the accumulation zone becomes colder during the last two decades as a response to the elevated equilibrium line altitude and the rising summer air temperatures. We suggest that the extent of accumulation basin (the amount of refreezing latent heat from meltwater) of LHG12 has a considerable impact on the englacial thermal status.

Australian mean land-surface temperature

The mean land-surface temperature represents an important boundary condition for many geothermal studies. This boundary is particularly important to help constrain the models made to analyse resource systems, many of which are shallow in nature and observe relatively small thermal gradients. Consequently, a mean land-surface temperature map of the Australian continent has been produced from 13 years of MODIS satellite imagery, for the period 2003–2015. The map shows good agreement with independent methods of estimating mean land-surface temperature, including borehole surface-temperature extrapolation and long-term, near-surface ground measurements. In comparison to previously used methods of estimating mean land-surface temperature, our new estimates are up to 12°C warmer. The MODIS-based method presented in this study provides spatially continuous estimates of land-surface temperature that can be incorporated as the surface thermal boundary condition in geothermal studies. The method is also able to provide a quantification of the uncertainties expected in the application of these estimates for the purposes of thermal modelling.

Determination of surface heat flux and temperature using in-depth temperature data – Experimental verification

This paper presents an experimental investigation of the accuracy of two previously reported formulations of the modified one-probe calibration integral equation method (CIEM) using high temperature (to 837°C) and heat flux (to 83.1W/cm2) experimental data. The data was collected from an AISI type 304 stainless steel plug-type test sample heated with a laser power source. The one-probe CIEM requires the knowledge of the surface thermal boundary condition of a calibration test along with temperature data from one in-depth probe in order to predict the surface thermal boundary condition of an unknown (reconstruction) heating process given the temperature data from the same probe. Two modified versions of the CIEM that account for temperature dependent properties are investigated. The first modified version utilizes rescaling the time domain to account for temperature dependent thermal diffusivity and rescaling the heat flux to account for temperature dependent thermal conductivity. The Kirchhoff transformation is used in the second version to account for temperature dependent thermal conductivity while employing the same time domain rescaling approach. Seven calibration tests and six reconstruction tests were conducted which allows forty-two test cases consisting of pairs of calibration test data and reconstruction test data to be investigated. The average value of the standard deviations of the 42 net surface heat flux prediction errors obtained from the rescaling model was 0.82W/cm2. The average value of the standard deviations of the 42 surface temperature prediction errors obtained from the Kirchhoff transformation model was 1.03°C. This study demonstrates that a calibrated plug-type sensor can be used in high temperature applications to accurately reconstruct the surface heat flux and temperature of an unknown surface heating process. The stainless steel test sample is a proof of concept for a future prototype of this kind of calibrated dual measurement gauge.

Thermo-electro-elasticity solution of functionally graded carbon nanotube reinforced composite cylindrical shell embedded in piezoelectric layers

Based on theory of elasticity, static analysis of functionally graded carbon nanotube reinforced composite (FG-CNTRC) cylindrical shell imbedded in piezoelectric sensor and actuator layers under thermo-electro-mechanical load is carried out. Thermo-elastic constants of the CNTRC cylindrical shell is supposed to be independent of temperature difference. In this study, uniformly as well as different cases of functionally graded CNT distribution, along the radial coordinate of host layer is considered. Governing ordinary differential equations are derived by expanding all of quantities in term of axial coordinate and using state space technique along the radial direction and then they are solved analytically to compute temperature, stress and displacement fields as well as induced electric voltage in sensor layer. Validity of the present approach is assessed by comparing the obtained numerical results with the available results in literature. Influence of CNT volume fraction, kind of CNT distribution along the radial coordinate, surface thermal, mechanical and electrical boundary conditions on static behavior of hybrid FG-CNTRC cylindrical shell are evaluated.

Fully coupled electromagnetic‐thermal‐mechanical comparative simulation of direct vs hybrid microwave sintering of 3Y‐ZrO2

AbstractDirect and hybrid microwave sintering of 3Y‐ZrO2 are comparatively studied at frequency of 2.45 GHz. Using the continuum theory of sintering, a fully coupled electromagnetic‐thermal‐mechanical (EMTM) finite element simulation is carried out to predict powder samples deformation during their microwave processing. Direct and hybrid heating configurations are computationally tested using advanced heat transfer simulation tools including the surface to surface thermal radiation boundary conditions and a numeric proportional‐integral‐derivative regulation (PID). The developed modeling framework shows a good agreement of the calculation results with the known experimental data on the microwave sintering of 3Y‐ZrO2 in terms of the densification kinetics. It is shown that the direct heating configuration renders highly hot spot effects resulting in nonhomogenous densification causing processed specimen's final shape distortions. Compared with the direct heating, the hybrid heating configuration provides a reduction of the thermal inhomogeneity along with a densification homogenization. As a result of the hybrid heating, the total densification of the specimen is attained without specimen distortions. It is also shown that the reduction of the sample size has a stabilization effect on the temperature and relative density spatial distributions.

Influence of building surface solar irradiance on environmental temperatures in urban neighborhoods

For buildings located in actual urban neighborhoods, modeling of outdoor airflow with Computational Fluid Dynamics (CFD) requires solar radiation at building surfaces to predict local environmental temperatures. This study conducts a parametric analysis to support the development of coupled simulations of outdoor airflow and solar radiation simulations at building surfaces. To account for different assumptions used in the outdoor modeling, this study uses OpenFOAM CFD and couples it with three different simulation engines, including EnergyPlus, Daysim, and Radiance to predict simulated outdoor solar irradiance for implementation in outdoor CFD simulations. The primary aim of selecting these three simulation engines is to evaluate tradeoffs between the model complexity and accuracy for simulated outdoor solar irradiance for outdoor CFD simulations. Examined parameters include: (i) surface representation with different mesh types, (ii) urban plan area density, and (iii) the impact of simulated solar irradiances on the simulated air temperature and velocity values in the CFD simulations. The study results showed that the surface representation has up to a 7.6% and 133% influence on the simulated outdoor global and local solar irradiances, respectively. The surface thermal boundary conditions have up to 1.5°C difference on the air temperature and negligible impacts on the air velocity.

Revisiting Surface Heat-Flux and Temperature Boundary Conditions in Models of Stably Stratified Boundary-Layer Flows

Two formulations of the surface thermal boundary condition commonly employed in numerical modelling of atmospheric stably stratified surface-layer flows are evaluated using analytical considerations and observational data from the Cabauw site in the Netherlands. The first condition is stated in terms of the surface heat flux and the second is stated in terms of the vertical potential temperature difference. The similarity relationships used to relate the flux and the difference are based on conventional log-linear expressions for vertical profiles of wind velocity and potential temperature. The heat-flux formulation results in two physically meaningful values for the friction velocity with no obvious criteria available to choose between solutions. Both solutions can be obtained numerically, which casts doubt on discarding one of the solutions as was previously suggested based on stability arguments. This solution ambiguity problem is identified as the key issue of the heat-flux condition formulation. In addition, the agreement between the temperature difference evaluated from similarity solutions and their measurement-derived counterparts from the Cabauw dataset appears to be very poor. Extra caution should be paid to the iterative procedures used in the model algorithms realizing the heat-flux condition as they could often provide only partial solutions for the friction velocity and associated temperature difference. Using temperature difference as the lower boundary condition bypasses the ambiguity problem and provides physically meaningful values of heat flux for a broader range of stability condition in terms of the flux Richardson number. However, the agreement between solutions and observations of the heat flux is again rather poor. In general, there is a great need for practicable similarity relationships capable of treating the vertical turbulent transport of momentum and heat under conditions of strong stratification in the surface layer.

Milliscale confined impinging slot jets: Laminar heat transfer characteristics for an isothermal flat plate

Heat transfer and overall visualized flow characteristics of confined, laminar milli-scale slot jets are investigated, as they impinge upon an isothermal flat target plate, with a fully-developed profile at the nozzle exit. The effects of Reynolds number Re and normalized nozzle-to-plate distance ratio H/B are investigated for Re=120–200, H/B=2–10, and B=1.0mm, with a nozzle aspect ratio of y/B=50. Instantaneous visualizations of overall slot jet flow structure show unsteady lateral distortions of jet columns at experimental conditions corresponding to the presence of continuous sinusoidal oscillations. Also apparent in flow visualization sequences are smoke signatures associated with instantaneous vortex structures which form as secondary flows develop in fluid which, initially, is just adjacent to the jet column. For each Reynolds number considered, local stagnation region Nusselt numbers Nuo decrease dramatically as H/B increases to become greater than 7.2–13.2, as the Reynolds number is maintained constant at a value from 200 to 120, changes which occur just as continuous sinusoidal oscillation of the jet column begins to develop. The further development of continuous sinusoidal oscillating motion results in an approximate collapse of stagnation region Nusselt numbers measured at different Re and H/B values. When surface thermal boundary condition data are compared, the constant surface temperature data are generally higher than the constant surface heat flux data near the stagnation location, and lower at locations where x/B is greater than 1–2. The constant surface temperature data also show relatively low values with only very small changes with x/B, for x/B values which are greater than about 5.0. As such, the results illustrate the sensitivity of Nusselt numbers for laminar boundary layer and laminar slot jet flows to thermal boundary condition, as well as the restrictions on near-wall temperature gradients which result from a constant surface temperature thermal boundary condition.

Effects of thermally induced inhomogeneous shear and surface thermal boundary conditions on the shear stress in sliding elastohydrodynamic contacts

AbstractNumerous research work has shown that significant thermally induced cross‐film inhomogeneous shear or thermal shear localisation may be developed in sliding elastohydrodynamic lubrication (EHL) contacts with pronounced consequences. This paper uses the theoretical framework established in previous research to further analyse the effects of the shear localisation on the lubricant shear stress and thus the EHL traction. Results obtained suggest that the shear localisation significantly accelerates the thermally induced reduction of the shear stress in sliding EHL contacts. The study also shows dramatic reduction of the shear stress in EHL contacts with one‐insolated surface, which is significantly attributed to the more intensified thermal shear localisation near the insolated surface. The practical significance of the dramatic shear stress reduction is discussed. Copyright © 2009 John Wiley & Sons, Ltd.

Modeling geographic impacts on early Eocene ocean temperature

A general circulation model is used to evaluate the impact of geographic differences between the early Eocene and the present day on ocean thermal structure. The model includes reconstructed Eocene bathymetry and surface thermal boundary conditions that mimic a diffusive atmosphere. The model estimates that bathymetric changes alone caused surface waters to cool by 3–4°C in high latitudes and warm by 1–2°C in low latitudes over the last 55 million years, which would account for as much as half (∼4–5°C) of the increase in the meridional sea surface temperature gradient during this time. Other changes include a cooling (∼2°C) of deep ocean waters, a warming of low‐latitude intermediate waters (∼2°C), and decreases in Southern Ocean zonal sea surface temperature gradients.

Evaluation of Haney-type surface thermal boundary conditions using a coupled atmosphere and ocean model

A coupled atmosphere-ocean model developed at the Institute for Space Studies at NASA Goddard Space Flight Center (Russell et al., 1995) was used to verify the validity of Haney-type surface thermal boundary condition, which linearly connects net downward surface heat fluxQ to air/sea temperature difference ΔT by a relaxation coefficientk. The model was initiated from the National Centers for Environmental Prediction (NCEP) atmospheric observations for 1 December 1977, and from the National Ocean Data Center (NODC) global climatological mean December temperature and salinity fields at 1°x 1° resolution. The time step is 7.5 minutes. We integrated the model for 450 days and obtained a complete model-generated global data set of daily mean downward net surface fluxQ, surface air temperatureTA, and sea surface temperatureTO. Then, we calculated the cross-correlation coefficients (CCC) betweenQ and ΔT. The ensemble mean CCC fields show (a) no correlation betweenQ and ΔT in the equatiorial regions, and (b) evident correlation (CCC≥0.7) betweenQ and ΔT in the middle and high latitudes. Additionally, we did the variance analysis and found that whenk=120 W m−2K−1, the two standard deviations, σQ and σκδT, are quite close in the middle and high latitudes. These results agree quite well with a previous research (Chu et al., 1998) on analyzing the NCEP re-analyzed surface data, except that a smaller value ofk (80 W m−2K−1) was found in the previous study.

A Study of Dynamically Loaded Finite Journal Bearings in Mixed Lubrication Using a Transient Thermohydrodynamic Analysis

A transient analysis of dynamically loaded finite journal bearings in mixed lubrication is made by solving the modified generalized Reynolds equation, 2-D energy equation in the oil, and heat conduction equation in the journal simultaneously including mass conserving cavitation. ISOADI (isothermal shaft and adiabatic bushing inner surface) thermal boundary condition is used. Journal temperature is treated as quasi-steady over one loading cycle. Two kinds of contact model are used. Numerical solutions using the finite difference method are presented. Results shows that the bearing behavior is closely tied to the roughness texture and topography, asperity contact load, bearing geometry, and operating conditions. Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999