Simulated Heat Flux Research Articles

Comparison of experimental and simulated critical heat flux tests with various cladding alloys: Sensitivity of iron-chromium-aluminum (FeCrAl) to heat transfer coefficients and material properties

In this work we analyze the differences between previous transient critical heat flux (CHF) experiments using iron-chromium-aluminum (FeCrAl), Inconel 600, and stainless steel 316 (SS316) alloy test sections and best-estimate modeling results from widely-used nuclear engineering systems and subchannel analysis tools. FeCrAl is an Accident Tolerant Fuel (ATF) candidate cladding material. The thermal hydraulic performance and safety characteristics of FeCrAl are being evaluated to determine viability as a cladding material in Light Water Reactors (LWRs). In this study, the results of the CHF experiments conducted at atmospheric pressure and fixed inlet coolant temperature and mass flux are compared to models built in the fifth version of the Reactor Excursion and Leak Analysis Program (RELAP5-3D) and CTF, the modernized version of COBRA-TF developed by the Consortium for Advanced Simulation of LWRs (CASL). Results from RELAP5-3D and CTF showed differences from the experiments and from each other in predicting CHF. In the Inconel 600 case, both computational tools overpredicted CHF, which led to an underprediction in the tube outer surface temperature. In the SS316 and FeCrAl cases, CHF was underpredicted by the codes, leading to an overprediction of the tube outer surface temperature.To understand the discrepancies in CHF and post-CHF predictions, studies were performed using RELAP5-3D and RAVEN to determine the sensitivity of CHF and peak test section temperature, an analog to peak cladding temperature (PCT), to heat transfer coefficients, a CHF multiplier, and uncertainties in the thermal conductivity and volumetric heat capacity. We found that CHF depends most strongly on the CHF multiplier and thermophysical properties. A combination of these factors that produced the best match to the experiment based on CHF, PCT, and the total energy deposited into the tube was determined. The best match parameters were able to provide best-estimate predictions of the CHF and integral heat flux, but were still conservative when predicting the PCT. The best match set of parameters developed in this paper are intended only as a demonstration of an approach that could be applied in the future with a larger set of experiments to produce more accurate models of CHF and post-CHF behavior.

Using 4-km WRF CONUS simulations to assess impacts of the surface coupling strength on regional climate simulation

Uncertainties in representing land–atmosphere interactions can substantially influence regional climate simulations. Among these uncertainties, the surface exchange coefficient Ch is a critical parameter, controlling the total energy transported from the land surface to the atmosphere. Although it directly impacts the coupling strength between the surface and atmosphere, it has not been properly evaluated for regional climate models. This study assesses the representation of surface coupling strength in a stand-alone Noah-MP land surface model and in coupled 4-km Weather Research and Forecasting (WRF) model simulations. The data collected at eight FLUXNET sites of the Canadian Carbon Program and seven AMRIFLUX sites are used to evaluate the offline Noah-MP simulations. Nine of these FLUXNET sites are used for the evaluation of the coupled WRF simulations. These sites are categorized into three land use types: grassland, cropland, and forest. The surface exchange coefficients derived using three formulations in Noah-MP simulations are compared to those calculated from observations. Then, the default $$ C_{zil} $$ = 0 and new canopy-height dependent $$ C_{zil} $$ are used in coupled WRF simulations over the spring and summer in 2006 to compare their effects on surface heat flux, temperature, and precipitation. When the new canopy-height dependent $$ C_{zil} $$ scheme is used, the simulated Ch exchange coefficient agrees better with observation and improves the daily maximum air temperature and heat flux simulation over grassland and cropland in the US Great Plains. Over grassland, the modeled Ch shows a different diurnal cycle than that for observed Ch, which makes WRF lag behind the observed diurnal cycle of sensible heat flux and temperature. The difference in precipitation between the two schemes is not as clear as the temperature difference because the impact of changing Ch is not local.

Identifying Sensitive Model Parameter Combinations for Uncertainties in Land Surface Process Simulations over the Tibetan Plateau

Model parameters are among the primary sources of uncertainties in land surface models (LSMs). Over the Tibetan Plateau (TP), simulations of land surface processes, which have not been well captured by current LSMs, can significantly affect the accurate representations of the weather and climate impacts of the TP in numerical weather prediction and climate models. Therefore, to provide guidelines for improving the performance of LSMs over the TP, it is essential to quantify the uncertainties in the simulated land surface processes associated with model parameters and detect the most sensitive parameters. In this study, five observational sites were selected to well represent the land surfaces of the entire TP. The impacts of 28 uncertain parameters from the common land model (CoLM) on the simulated surface heat fluxes (including sensible and latent heat fluxes) and soil temperature were quantified using the approach of conditional nonlinear optimal perturbation related to parameters (CNOP-P). The results showed that parametric uncertainties could induce considerable simulation uncertainties in surface heat fluxes and soil temperature. Thus, errors in parameters should be reduced. To inform future parameter estimation efforts, a three-step sensitivity analysis framework based on the CNOP-P was applied to identify the most sensitive parameter combinations with four member parameters for sensible and latent heat fluxes as well as soil temperature. Additionally, the most sensitive parameter combinations were screened out and showed variations with the target state variables and sites. However, the combinations also bore some similarities. Generally, three or four members from the most sensitive combinations were soil texture related. Furthermore, it was only at the wetter sites that parameters related to vegetation were contained in the most sensitive parameter combinations. In the future, studies on parameter estimations through multiobjective or single-objective optimization can be conducted to improve the performance of LSMs over the TP.

Comparative analysis of the meteorological elements simulated by different land surface process schemes in the WRF model in the Yellow River source region

The Weather Research and Forecasting (WRF) model was employed to simulate the water and energy process over the near surface in the Yellow River source region from July to September 2015. The Rapid Update Cycle (RUC), Noah, Noah-Multi-Physics (Noah-MP), and Community Land Model (CLM) version 4 land surface schemes were selected and evaluated in our experiments. The simulated results were also compared with the Global Land Data Assimilation System (GLDAS) outputs, the ground-based automatic weather stations (AWSs), and the Zoige plateau wetlands ecosystem research flux station data. There was substantial agreement between the results of the simulations and observations in terms of 2-m temperature and relative humidity. However, the simulated values of the 2-m temperature were mostly lower than observations, while the values of relative humidity were higher than the observations. The correlation coefficients (R) between the simulations and observations of air temperature in four experiments were all higher than 0.94, and simulated results of the WRF-RUC experiment had the minimum bias compared with other experiments. The root mean square error (RMSE) and bias of 2-m relative humidity between the WRF-CLM4 simulations and observations were lower than other experiments. Moreover, the simulation of the 10-cm soil temperature was lower than the observations. WRF-CLM4 simulations agreed better with the observations, and WRF-Noah-MP simulations had the largest bias in all of the experiments. The soil moisture increased rapidly when precipitation was occurring and decreased slowly after the precipitation. The four experiments overestimate precipitation leading to higher soil moisture compared with the real situations. The peak value of net radiation was appropriately 850 W/m2 during the summertime in the Zoige plateau area. The simulated net radiation agreed well with the observation except in the WRF-RUC scheme. During the summertime, vegetation grew rapidly, and the latent heat flux transport increased and played a dominant role in surface energy transport in the Zoige area. The RMSE between the simulation and observation of latent heat flux had the minimum value of 98.0 W/m2 in the WRF-CLM4 scheme, and the bias of sensible heat flux had the minimum value of 15.4 W/m2 in the WRF-Noah scheme. This study used the WRF model to explore the applicability of the different land surface schemes in the Zoige plateau area. The research related to this study can be used to understand the water-energy cycle process over the Qinghai-Tibet Plateau wetland and can also provide references for the application of regional meteorological models over this area.

Validation and integration of a latent heat storage model into building envelopes of a high-order building model for Modelica library AixLib

Efficient integration of phase change materials (PCMs) into buildings’ envelopes requires the mathematical modeling of PCMs and whole building systems, which is essential for the optimal design and material selection. Therefore, building performance simulation (BPS) is a cost-effective and time-efficient solution. However, currently, only a few BPS tools embed comprehensively verified and validated PCM models. This study presents a procedure of verification and validation of PCM models for building’s envelopes developed in Modelica employing an approach similar to ASHRAE Standard 140, consisting of empirical validation, analytical verification, and comparative testing.In order to characterize specific enthalpy changes and specific heat capacities of PCMs, three temperature-dependent continuous functions were proposed and validated with experimental adiabatic scanning calorimetry (ASC) data for a paraffin based PCM - RT27. Among the models, the arctangent function model behaved accurately across empirical data. Based on the model, the PCM heat transfer model for a semi-infinite PCM slab was verified analytically using the one-phase Stefan problem, where temperature evolutions at various depths of the slab were compared. For a wall with more realistic geometries and boundary conditions, a dynamic wall simulator for a multi-component wall integrating a microencapsulated PCM layer was selected, where simulated heat fluxes on inner surfaces of the wall were compared to the results of a validated numerical model. Lastly, the accuracy of the PCM model was tested on the whole-building level, thereby the variation of the average inner surface temperature of a building’s wall was compared to measured data.

Physics-Based Simulation of Heat Load on Structures for Improving Construction Standards for Bushfire Prone Areas

Australian building standard AS 3959 provides mandatory requirements for the construction of buildings in bushfire prone areas in order to improve the resilience of the building to radiant heat, flame contact, burning embers, and a combination of these three bushfire attack forms. The construction requirements are standardised based on the bushfire attack level (BAL). BAL is based on empirical models which account for radiation heat load on structure. The prediction of the heat load on structure is a challenging task due to many influencing factors: weather conditions, moisture content, vegetation types and fuel loads. Moreover, the fire characteristics change dramatically with wind velocity leading to buoyancy or wind dominated fires that have different dominant heat transfer processes driving the propagation of the fire. The AS 3959 standard is developed with respect to a quasi-steady state model for bushfire propagation assuming a long straight line fire. The fundamental assumptions of the standard are not always valid in a bushfire propagation. In this study, physics based large-eddy simulations were conducted to estimate the heat load on a model structure. The simulation results are compared to the AS 3959 model; there is agreement between the model and the simulation, however, due to computational restrictions the simulations were conducted in a much narrower domain. Further simulations were conducted where wind velocity, fuel load, and relative humidity are varied independently and the simulated radiant heat flux upon the structure was found to be significantly greater than predicted by the AS 3959 model. The effect of the mode of fire propagation, either buoyancy-driven or wind dominated fires, is also investigated. For buoyancy dominated fires the radiation heat load on the structure is enhanced compared to the wind dominated fires. Finally, the potential of using physics based simulation to evaluate individual designs is discussed.

Evaluation of Land Surface Subprocesses and Their Impacts on Model Performance With Global Flux Data

AbstractStudy on the uncertainties in land surface models (LSMs) helps us understand the differences and errors in climate models. Meanwhile, uncertainty in model structure, derived from the many possible parameterization schemes for the same physical subprocess, is a primary source of land model uncertainties. To attribute structural errors and model parameterization scheme uncertainties, it is critical to identify the key subprocesses involved and investigate the interactions of these subprocesses on LSM behavior, which will ultimately help us identify the “optimal” parameterization schemes for various plant functional types, soil types, and different locations. Here, we conduct physical ensemble simulations for multiple sites from FLUXNET and then apply a variance‐based sensitivity analysis method to quantitatively assess the impacts of uncertainties in the parameterization schemes of subprocesses in the Noah with multiparameterization (Noah‐MP) LSM on model performance. The results show that three subprocesses—surface exchange coefficient, runoff and groundwater, and surface resistance to evaporation—have the most significant impacts on the performance of the simulated sensible heat flux, latent heat flux, and net absorbed radiation in the Noah‐MP LSM. The interaction between two subprocesses could contribute up to 50% of the variation in model performance for some sites, which highlights the need for taking into consideration the interactions of subprocesses to improve LSMs. Finally, a statistical optimal combination of the parameterization schemes is recommended for global land modeling, although it is noticed that the optimal schemes vary with regions and can be different even for neighboring sites.

Effect of CeO2 on heat transfer and crystallization behavior of rare earth alloy steel mold fluxes

To improve the heat transfer capability and the crystallization property of the traditional mold flux, CaF2 was replaced with B2O3. Then, the influences of CeO2 on the heat transfer and the crystallization of the CaF2-bearing mold flux and the new mold flux with 10wt% B2O3 were studied using a slag film heat flux simulator and X-ray diffraction (XRD). The results revealed that the addition of CeO2 reduced the heat transfer by increasing the solid slag thickness and the crystallization of two mold fluxes. However, CeO2 had less effect on the B2O3-containing mold flux compared with the CaF2-bearing mold flux. According to the analyses, the CeO2 contents in the CaF2-bearing mold flux and the B2O3-containing mold flux should not exceed 8wt% and 12wt%, respectively. Therefore, these experimental results are beneficial to improve and develop the mold flux for casting rare earth alloy steels.

LiDAR‐based characterization of mid‐altitude wind conditions for airborne wind energy systems

SummaryEngineers and researchers working on the development of airborne wind energy systems (AWES) still rely on oversimplified wind speed approximations and coarsely sampled reanalysis data because of a lack of high‐resolution wind data at altitudes above 200 m. Ten‐minute average wind speed LiDAR measurements up to an altitude of 1100 m and data from nearby weather stations were investigated with regard to wind energy generation and impact on LiDAR measurements. Data were gathered by a long‐range pulsed Doppler LiDAR device installed on flat terrain. Because of the low overall carrier‐to‐noise ratio, a custom‐filtering technique was applied. Our analyses show that diurnal variation and atmospheric stability significantly affect wind conditions aloft which cause a wide range of wind speeds and a multimodal probability distribution that cannot be represented by a simple Weibull distribution fit. A better representation of the actual wind conditions can be achieved by fitting Weibull distributions separately to stable and unstable conditions. Splitting and clustering the data by simulated surface heat flux reveals substate stratification responsible for the multimodality. We classify different wind conditions based on these substates, which result in different wind energy potential. We assess optimal traction power and optimal operating altitudes statistically as well as for specific days based on a simplified AWES model. Using measured wind speed standard deviation, we estimate average turbulence intensity and show its variation with altitude and time. Selected short‐term data sets illustrate temporal changes in wind conditions and atmospheric stratification with a high temporal and vertical resolution.

Evaluating Operational Hydrodynamic Models for Real‐time Simulation of Evaporation From Large Lakes

AbstractMethods for simulating evaporative water loss from Earth's large lakes have lagged behind advances in hydrodynamic modeling. Here we explore use of oceanographic models to simulate lake evaporation from a long‐term water balance perspective. More specifically, we compare long‐term monthly simulations of latent heat flux from two configurations of a current operational hydrodynamic forecasting system (based on the Finite Volume Community Ocean Model, or FVCOM) for the Laurentian Great Lakes. We then compare these simulations to comparable simulations from a legacy conventional lake thermodynamics model, and from a recently developed statistical water balance model. We find that one of the FVCOM configurations that is currently used in operations for short‐term hydrodynamic forecast guidance is also suitable for real‐time simulation of evaporation from very large lakes. The operational versions of FVCOM should therefore be considered a readily available tool for supporting regional water supply management and, pending further research, extended water supply forecasting.

Evaluation of Turbulence Stability Schemes of Land Models for Stable Conditions

AbstractDuring stable conditions, simulated sensible heat fluxes, Qh, in land models exhibit a wide range of behavior, with some models decoupling the atmosphere and land surface, while others continue turbulent exchange. This behavior is dictated by the representation of stability within the model. We explicitly test the representation of Qh during stable conditions using observed surface temperature, air temperature, and wind speed at the Shallow Cold Pool (SCP) Experiment site in Colorado, USA. This site is characterized by frequent stable conditions and weak‐wind conditions, which are known to be difficult to represent in turbulence schemes. We test the log‐linear, Louis, Holtslag and de Bruin, Beljaar and Holtslag, and Cheng and Brutsaert stability schemes against the observations at the SCP site. We also evaluate the minimum conductance assumption commonly implemented in land models. Simulated Qh biases are generally small, due largely to compensating errors. Parameterizations could represent Qh in strong‐wind conditions and during the transition from a weak‐ to strong‐wind regime. The behavior of simulated Qh during strong winds was controlled by the aerodynamic surface roughness, z0. All models, except those that maintain a minimum conductance, were unable to represent turbulence in the weak‐wind regime. We discuss the interacting roles of countergradient fluxes, intermittent turbulence, and decoupled turbulence during this regime. Finally, we list the characteristics that a turbulence parameterization should include and recommend that land models should include a minimum conductance to simulate Qh in the weak‐wind regime.

Impact of endoscopic craniofacial resection on simulated nasal airflow and heat transport.

Endoscopic craniofacial resections (CFR) are performed for extensive anterior skull base lesions. This surgery involves removal of multiple intranasal structures, potentially leading to empty nose syndrome (ENS). However, many patients remain asymptomatic postoperatively. Our objective was to analyze the impact of CFR on nasal physiology and airflow using computational fluid dynamics (CFD). This is the first CFD analysis of post-CFR patients. Three-dimensional sinonasal models were constructed from 3 postoperative images using MimicsTM . Hybrid computational meshes were created. Steady inspiratory airflow and heat transport were simulated at patient-specific flow rates using shear stress transport k-omega turbulent flow modeling in FluentTM . Simulated average heat flux (HF) and surface area where HF exceeded 50 W/m2 (SAHF50) were compared with laminar simulations in 9 radiographically normal adults. Three adults underwent CFR without developing ENS. Average HF (W/m2 ) were 132.70, 134.84, and 142.60 in the CFR group, ranging from 156.24 to 234.95 in the nonoperative cohort. SAHF50 (m2 ) values were 0.0087, 0.0120, and 0.0110 in the CFR group, ranging from 0.0082 to 0.0114 in the radiographically normal cohort. SAHF50 was distributed throughout the CFR cavities, with increased HF at the roof and walls compared with the nonoperative cohort. Average HF was low in the CFR group compared with the nonoperative group. However, absence of ENS in most CFR patients may be due to large stimulated mucosal surface area, commensurate with the nonoperative cohort. Diffuse distribution of stimulated area may result from turbulent mixing after CFR. To better understand heat transport post-CFR, a larger cohort is necessary.

Proxies for heat fluxes to the Arctic Ocean through Fram Strait

Oceanic fluxes through Fram Strait may significantly contribute to climate variations in the Arctic. However, their observations are difficult. Here, a 26-year numerical model simulation is used to derive oceanic proxies for interannual variability in heat fluxes through Fram Strait. It is found that variability in the cross-slope gradient of sea surface height (SSH) across the West Spitsbergen Current (WSC) can explain about 90% of the variance of winter and annual mean volume transports of Atlantic water at 79°N. Given the strong covariance between the simulated heat flux in the slope current along Svalbard and the corresponding volume transport, variability of the SSH gradient across the WSC is also found to account for about 80% of the variance of heat flux associated with the northward flow through Fram Strait. Moreover, variations in the SSH gradient across the Arctic Slope Current (ASC) northeast of Svalbard at 31°E explain about 85% of the variance of heat flux there and about 80% of the variance of the net heat flux upstream through Fram Strait. Finally, about 85% and 75% of the variance of the net heat flux through Fram Strait is associated with anomalies of the eastward volume transport and depth-averaged core velocity in the ASC, respectively. These relations indicate that monitoring of the flow in the ASC, even with a single current meter mooring, or of the SSH gradient across this current derived from either in situ or remote measurements may provide useful proxies for the heat import to the Arctic Ocean.

A New Frozen Soil Parameterization Including Frost and Thaw Fronts in the Community Land Model

AbstractFreeze–thaw processes in soils, including changes in frost and thaw fronts (FTFs), are very sensitive to warming. However, the latest climate models do not predict changes in FTFs directly. In this study, a new frozen soil parameterization including changes in FTFs was incorporated into the Community Land Model version 4.5 for climate modeling, which we denote CLM4.5_FTF. A set of numerical experiments including single points, regions in China, and a global scale were conducted using the model to validate its performance. The simulated FTF depths compare well with observed data from both the D66 station (permafrost) and Hulugou station (seasonally frozen ground). The simulated active layer thickness, defined as the maximum thaw front depth in permafrost, is in general agreement but slightly greater than observations from the Circumpolar Active Layer Monitoring program. The simulated distributions of different types of frozen soil in China and permafrost in the northern hemisphere are in agreement with the frozen soil map of China and the International Permafrost Association map, respectively. The results confirm that the model performs well for FTF simulations. The model was also used for year‐long simulations of soil temperature and freeze–thaw processes to check its applicability in continuous simulation. The results show that CLM4.5_FTF performed better than the original model, and the improvement was better for lower levels than for the upper level. Finally, we give simulated latent heat flux, sensible heat flux, and 10‐cm soil temperature deviations determined via the couple model with and without the new scheme.

A Column Canopy‐Air Turbulent Diffusion Method for Different Canopy Structures

AbstractAn accurate simulation of the sensible heat flux (H) over vegetation from thermal remote sensing requires an a priori estimate of roughness length and the excess resistance parameter kB−1. Despite being the subject of considerable interest in hydrometeorology, there still does not exist a uniform method for estimating roughness length from remote sensing techniques. This study demonstrates a turbulent diffusion method to simulate canopy‐air sensible heat. The performance of the roughness length scheme as described in Chen et al. (2013, https://doi.org/10.1175/JAMC‐D‐12‐056.1) was examined by comparing simulated H to measured values at 28 flux tower stations, which include seven different land covers (needle forest, broadleaf forest, shrub, savanna, grassland, cropland, and sparsely vegetated land). The model predictions of H for grass, crop, and sparsely vegetated land compare favorably with observed values, when actual canopy height is given. H is significantly underestimated at forest sites due to a high value of kB−1. Among the different physical representations for the canopy, canopy‐soil mixture, and soil component, it is found that such a high kB−1 value is caused by the high kB−1 value for the canopy part. The reasons for this high kB−1 were investigated from canopy‐air physical process of turbulent diffusion. This study introduces the vertical foliage density information into a column canopy‐air turbulent diffusion model to include the different momentum and heat transfer efficiencies in the vertical canopy layers to enhance the thermal turbulent transfer intensity above the tall canopy. The new model has been verified to provide accurate simulation over different canopy structures.

Using field data–based large eddy simulation to understand role of atmospheric stability on energy production of wind turbines

A novel and a robust high-fidelity numerical methodology has been developed to realistically estimate the net energy production of full-scale horizontal axis wind turbines in a convective atmospheric boundary layer, for both isolated and multiple wind turbine arrays by accounting for the wake effects between them. Large eddy simulation has been used to understand the role of atmospheric stability in net energy production (annual energy production) of full-scale horizontal axis wind turbines placed in the convective atmospheric boundary layer. The simulations are performed during the convective conditions corresponding to the National Renewable Energy Laboratory field campaign of July 2015. A mathematical framework was developed to incorporate the field-based measurements as boundary conditions for the large eddy simulation by averaging the surface flux over multiple diurnal cycles. The objective of the study is to quantify the role of surface flux in the calculation of energy production for an isolated, two and three wind turbine configuration. The study compares the mean value, +1 standard deviation, and −1 standard deviation from the measured surface flux to demonstrate the role of surface heat flux. The uniqueness of the study is that power deficits from large eddy simulation were used to determine wake losses and obtain a net energy production that accounts for the wake losses. The frequency of stability events, from field measurements, is input into the calculation of an ensemble energy production prediction with wake losses for different wind turbine arrays. The increased surface heat flux increases the atmospheric turbulence into the wind turbines. Higher turbulence results in faster wake recovery by a factor of two. The faster wake recovery rates result in lowering the power deficits from 46% to 28% for the two-turbine array. The difference in net energy production between the +1 and −1 standard deviation (with respect to surface heat flux) simulations was 10% for the two-turbine array and 8% for the three-turbine array. An ensemble net energy production by accounting for the wake losses indicated the overestimation of annual energy production from current practices could be corrected by accounting for variation of surface flux from the mean value.

A numerical study on the thermal transient model with moving laser heat source of AISI 304 stainless steel plate

Abstract Laser beam is commonly used as a moving heat source in various metal processing such as cutting, heat treatment, welding, and recently in additive manufacturing or metal 3D-printing. Understanding of thermal behavior resulting from a moving laser beam are essential to control the product quality in those processes. This research aims to study the 3D thermal transient model of moving laser heat source with changeable direction of motion in order to investigate the effect of process parameters, e.g., power intensity, scanning speed and hatch spacing on the temperature distribution of the AISI 304 stainless steel plate. The numerical study is carried out using the commercial software ANSYS 18.1 and considering the simulation of the low intensity laser heat flux with Gaussian intensity distribution. The results of temperature profile at the probe point from numerical simulation are in agreement with the experiment results.

The Impact of Biomass Heat Storage on the Canopy Energy Balance and Atmospheric Stability in the Community Land Model

AbstractAtmospheric models used for weather prediction and future climate projections rely on land models to calculate surface boundary conditions. Observations of near‐surface states and fluxes made at flux measurement sites provide valuable data with which to assess the quality of simulated lower boundary conditions. A previous assessment of the Community Land Model version 4.5 using data from the Niwot Ridge Subalpine Forest AmeriFlux tower showed that simulated latent heat fluxes could be improved by adjusting a parameter describing the maximum leaf wetted area, but biases in midday sensible heat flux and nighttime momentum flux were generally not reduced by model parameter perturbations. These biases are related to the model's lack of heat storage in vegetation biomass. A biomass heat capacity is parameterized in Community Land Model version 5 with measurable quantities such as canopy height, diameter at breast height, and tree number density. After implementing a parameterization describing the heat transfer between the forest biomass and the canopy air space, the biases in the mean midday sensible heat and mean nighttime momentum fluxes at Niwot Ridge are reduced from 47 to 13 W/m2 and from 0.12 to −0.03 m/s, respectively. The bias in the mean nighttime canopy air temperature was reduced from −5.9 to 0.4 °C. Additional simulations at other flux tower sites demonstrate a consistent reduction in midday sensible heat flux, a lower ratio of the sum of sensible and latent heat flux to net radiation, and an increase in nighttime canopy temperatures.

Shale gas well blowout fire and explosion modelling

The development and application of a computational model for predicting the thermal radiation and explosion over-pressure in the event of an accidental well blowout during shale gas production is described. The transient discharge rate, the fluid phase composition, temperature and pressure at the ruptured wellhead, serving as the source term for fire and explosion modelling are determined based on the numerical solution of the conservation equations using the Method of Characteristics. Two scenarios covering immediate and delayed ignition of the escaping gas respectively leading to a jet fire or an explosion are considered. The efficacy of the well blowout model is demonstrated by simulating the accidental well-head rupture of a real 4000 m deep shale gas production well at a maximum reservoir pressure of 600 bar. The simulated time variant explosion overpressures and thermal radiation heat fluxes are in turn employed to specify the minimum safe distances to people and equipment during the rapid well depressurisation.

3D simulations of pool boiling above smooth horizontal heated surfaces by a phase-change lattice Boltzmann method

Pool boiling heat transfer from smooth horizontal hydrophilic and hydrophobic heaters having a finite thickness under constant bottom wall temperature conditions is simulated numerically with a three-dimensional liquid-vapor phase-change lattice Boltzmann method. For a small heater, single bubble dynamics and temporal variations of heater’s top wall temperature in contact with the bubble are investigated. It is found that top wall temperature drops during initial bubble growth period at low wall superheats on a hydrophilic heater due to the microlayer evaporation process. Because of a residual bubble is left on the top of the heater after bubble departure, no wall temperature drop is found on a hydrophilic surface at high wall superheats nor on a hydrophobic surface at any wall superheats. 3D pool boiling curves in dimensionless forms for smooth horizontal heaters of infinite extent from nucleation to critical heat flux through transition boiling to stable film boiling are presented. It is found that 2D and 3D simulated heat fluxes are identical in nucleate boiling and stable film boiling regimes, although transition boiling heat fluxes differ greatly. It is also found that the 3D simulated CHF is in better agreement with existing theories and correlation equations than 2D simulated CHF. Detailed 3D multiple bubbles dynamics above hydrophobic and hydrophilic surfaces and temporal variations of dry spots distribution on these surfaces at CHF are presented. Effects of wettability and thickness of the heater on boiling curves for smooth heaters are investigated. It is founded that only wettability play predominate roles in nucleate boiling regime from smooth superheated surfaces while all factors influence the transition boiling regime (including maximum and minimum heat fluxes) greatly.