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•CFD modeling of gas and heavy fuel oil wall-fired boilers validated by field data.•Gas operation data from DCS used to obtain correct membrane wall emissivity.•Wall emissivity ...overwhelms fouling effect on heat transfer.•Characteristic of the wall in terms of emissivity is independent of the fuel.•Soot modeling is essential for correct estimation of heat fluxes for HFO operation.
In the present study we develop a complete Computational Fluid Dynamics (CFD) modeling procedure suitable for accurate simulations of industrial boilers fed alternatively with gaseous and liquid fuels. The model is developed and validated by means of data from on-site testing.
Two different boilers are considered: a 6-burner steam generator of a refinery for the model definition, and a 32-burner thermal power plant boiler for validation, fed by flare-gas/natural-gas or Heavy-Fuel-Oil (HFO). Selected reliable experimental data coming from performance testing are used for both the set-up of the CFD simulations as well as to compute the Flue Exit Gas Temperatures (FEGTs) employed as validation criteria. The flare-gas testing on the 6-burner boiler allows us to accurately determine that a membrane wall emissivity around 0.60 is appropriate as a boundary condition for radiation. By a sensitivity analysis, the relevance of wall emissivity with respect to fouling thermal resistance on the overall heat transfer inside the furnace is established. For boilers alternatively operating with gaseous fuel and HFO a value of 2.8 kW/(m2·K) has been found appropriate. HFO testing on the 6-burner boiler provides data to develop the additional soot modeling, crucial to properly catch oil flame emissivity, based on the model of Khan and Greeves. This allows the matching of a mean soot mass fraction of 1%–3% in the flame zone, a range found by in-flame measurements in the literature and confirmed in the present study by overall heat exchange data in the whole furnace coming from HFO boiler testing. Finally, the 32-burner boiler is considered for validation: using the same procedure and parameters, excellent agreement is found between experimental and CFD results, both on a 100% load natural gas and on a 107% load HFO performance testing.
The study demonstrates how field data can be used to validate CFD simulations and confirms that the use of physically meaningful parameters and models exempts from repeated tuning: no change is needed for emissivity and fouling thermal resistance between gaseous and liquid fuel operation, and even a simple soot modeling can be used, just ensuring physically consistent particulate concentration in the oil flame.
The accurate quantification of surface heat and water vapor fluxes is significantly essential for understanding water balance dynamics. In this study, 15-m spatial resolution turbulent fluxes (H and ...LE) in the Zhangye oasis situated the middle reaches of the Heihe River Basin (HRB) were estimated by the remote sensing-based two-source energy balance model (TSEB). The TSEB model uses temperature including land surface temperature (LST) and air temperature (Ta) as the main input variable to compute turbulent fluxes but their spatial resolution is rather limited. To overcome this shortcoming, the 15-m spatial resolution LST and Ta were obtained by using the back-propagation neural network (BPNN). The results indicated that the BPNN was able to obtain finer spatial resolution and LST and Ta; the root mean square error (RMSE) values of LST and Ta are 1.99 K and 0.50 K, respectively. The remotely sensed H and LE predicted by TSEB model utilizing the LST and Ta modeled by BPNN. The results showed that H and LE agreed well with the flux observations from multi-set eddy covariance (EC) systems installed at a number of sites and covering all representative land cover types; particularly for the latent heat flux, its estimates produced mean absolute percent errors (MAPE) of 8.76% for maize, 20.17% for vegetable, 29.06% for residential area, and 16.12% for orchard. This study obtained surface heat and water vapor fluxes at finer spatial resolution than the other flux estimates from the remote sensing models that have been used in the Zhangye oasis. The results produced by combining the TSEB model and BPNN can provide more information for drafting reliable sustainable water resource management schemes and improving the irrigation water use efficiency in arid and semi-arid regions.
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•A finer spatial resolution surface heat and water vapor fluxes have been obtained.•H and LE from TSEB is in good agreement with measurements.•The downscaled LST based on BPNN have a consistency with observation.•BPNN is able to obtain high accuracy and finer spatial resolution Ta.•It is an effective method to obtain regional H and LE combining TSEB and BPNN.
Here we demonstrate with a study of the Lucky Strike hydrothermal field that image mosaicing over large seafloor areas is feasible with new image processing techniques, and that repeated surveys ...allow temporal studies of active processes. Lucky Strike mosaics, generated from >56,000 images acquired in 1996, 2006, 2008 and 2009, reveal the distribution and types of diffuse outflow throughout the field, and their association with high‐temperature vents. In detail, the zones of outflow are largely controlled by faults, and we suggest that the spatial clustering of active zones likely reflects the geometry of the underlying plumbing system. Imagery also provides constraints on temporal variability at two time‐scales. First, based upon changes in individual outflow features identified in mosaics acquired in different years, we document a general decline of diffuse outflow throughout the vent field over time‐scales up to 13 years. Second, the image mosaics reveal broad patches of seafloor that we interpret as fossil outflow zones, owing to their association with extinct chimneys and hydrothermal deposits. These areas encompass the entire region of present‐day hydrothermal activity, suggesting that the plumbing system has persisted over long periods of time, loosely constrained to hundreds to thousands of years. The coupling of mosaic interpretation and available field measurements allow us to independently estimate the heat flux of the Lucky Strike system at ∼200 to 1000 MW, with 75% to >90% of this flux taken up by diffuse hydrothermal outflow. Based on these heat flux estimates, we propose that the temporal decline of the system at short and long time scales may be explained by the progressive cooling of the AMC, without replenishment. The results at Lucky Strike demonstrate that repeated image surveys can be routinely performed to characterize and study the temporal variability of a broad range of vent sites hosting active processes (e.g., cold seeps, hydrothermal fields, gas outflows, etc.), allowing a better understanding of fluid flow dynamics from the sub‐seafloor, and a quantification of fluxes.
Key Points
A temporal decline of hydrothermalism demonstrated by repeated image mosaics
Outflow structure shows clustering that we relate to plumbing geometry
Heat flux estimate at Lucky Strke (200‐1000 MW) and partition to diffuse flow >75%
The maintenance of baroclinicity along mid‐ and high‐latitude storm tracks is a matter of ongoing debate. Using an isentropic framework, a novel diagnostic based on the tendency equation for the ...slope of isentropic surfaces – a measure of the potential for baroclinic development – is presented. The tendency comprises contributions from dynamic processes, latent heat release, radiation and subgrid‐scale turbulence, which incorporates the effect of sensible heat fluxes. A climatology of these tendencies over the North Atlantic is compiled for the winters of 2009 and 2010.
It is found that adiabatic tilting flattens the isentropic surfaces, reflecting the action of growing baroclinic cyclones. This tendency is balanced climatologically by the generation of slope by diabatic processes. In the lower troposphere, the most intense diabatic increase of slope is found along the oceanic frontal zone associated with the Gulf Stream and at higher latitudes in the Labrador Sea, the Nordic Seas and the Barents Sea. Latent heat release and sensible heat fluxes both contribute substantially in these regions. A quantitative analysis of cold‐air outbreaks emphasizes their important role in restoring the slope in the lower troposphere over the Gulf Stream region and off the sea‐ice edge at high latitudes. In the upper troposphere, latent heat release due to cloud microphysical processes is the dominant mechanism maintaining the slope.
Sloping isentropic surfaces are a prerequisite for baroclinic instability. In general, baroclinic instability tends to flatten isentropic surfaces (blue and black arrows), but isentropic upglide (gray arrow) can lead to cloud formation and associated latent heat release, which also acts to deform isentropic surfaces. In addition, warming by upward sensible heat fluxes (red arrow) over warmer waters causes a steepening. In this study, we introduce a diagnostic framework that allows us to quantify the contributions of diabatic and adiabatic processes that change the slope of isentropic surfaces. This framework is applied to North Atlantic winter conditions.
The alarming rate of abrupt change in weather parameters, especially rainfall and temperature, is a cause of major concern not only for the scientific community but also for the lives of common ...people living in a country like India. A broad picture of climate change is not sufficient to carry out an impact assessment on a regional scale. Recent news on the occurrence of floods on a regional scale over India throws an urgent need to look at the patterns of changes in rainfall on a more regional scale. Kerala, the southwestern state of the Indian peninsula, is known to be the gateway of monsoon for the summer monsoon season to the mainland. This study is evaluating the causes of the recent variability in the rainfall pattern over Kerala during the southwest monsoon season. It is found that the Kerala rainfall is decreasing at the rate of 3 mm/year for the past 100 years, with an abrupt decrease of 6.7 mm/year after 1960. The causes of such a decrease are a point of discussion in this study. It is found that the ocean parameters such as sea surface temperature, ocean heat content and ocean heat fluxes are increasing at an alarming rate in the recent decade over the North West Equatorial Indian Ocean, including the Arabian sea. The pattern of the Findlater jet and the ocean response clearly identifies the North West Equatorial Indian Ocean as a hot spot which creates maximum variability in the southwest monsoon dynamics.
•Instead of electron beam guns, the electrically heating method was used to simulate one-side heating conditions.•Effects of system parameters on temperature curves and heat transfer curves were ...discussed in detail.•A set of previous heat transfer correlations were assessed with the experimental results.•A new correlation and a modified correlation were proposed to predict the heat transfer under twisted tape conditions.
In order to investigate the water cooling technology for the divertor in the International Thermonuclear Experimental Reactor (ITER), heat transfer tests of subcooled water flow boiling in a circular channel with a twisted tape (TT) insert under high and non-uniform fluxes have been performed at Xi’an Jiaotong University. The vertically upward channel was off-center in a nickel rectangular block and the twist ratio of the twisted tape is 2. Experimental parameters covered the pressure range of 3–5 MPa, mass flux range of 3000–8000 kg m−2 s−1, the inlet fluid temperature of 40–220 °C and the average heat flux on the inner wall surface of 2–5 MW m−2. The influences of parameters on heat transfer coefficients have been discussed in detail. A wide set of empirical heat transfer correlations from forced convection to fully developed boiling (FDB) have been assessed with the experimental results, and most of the correlations could not fit the experimental data well. A new correlation for the FDB region and a modified Liu and Winterton correlation for the entire subcooled boiling region were proposed to predict the heat transfer coefficients in swirl flow under high and non-uniform heat fluxes. The average deviations (ADs) of the two correlation were 0.46% and 3.65%, respectively, and the root mean square deviations (RSMDs) of them were 4.85% and 13%, respectively.
This study documents the seasonal variation of precipitation over the whole Indochina Peninsula (ICP) and associated changes in atmospheric winds and surface heat fluxes and investigates the impact ...of the ICP precipitation‐related heating and circulation on spring sea surface warming in the South China Sea (SCS). Climatologically, most of the ICP experiences the rainy season from May to October and the period from November to April is the dry season. The summer wet‐winter dry pattern dominates the seasonal rainfall variation in most regions and the maximum rainfall along the west coast occurs during June through August in association with monsoon westerly winds. A pronounced semi‐annual rainfall variation is observed along central Vietnam with a primary peak in October and a secondary peak in May, which is related to northeasterly and southwesterly winds, respectively. Surface heat fluxes change drastically over the ICP in March–April and play an important role in increasing the thermal contrast with the neighbouring waters and heating the atmospheric column, which causes upward motion and precipitation over the ICP. The induced compensatory downward motion over the northern SCS leads to an increase in downward shortwave radiation and a decrease in surface wind speed and turbulent heat fluxes, which contributes to the SCS warming before the summer monsoon onset.
Downward shortwave radiation heats the atmospheric column through sensible heat flux, increasing the thermal contrast with the neighbouring waters, causing upward motion and precipitation over the ICP. The upward motion over the ICP in March–April triggers a regional zonal vertical circulation, inducing compensatory downward motion over the SCS. In turn, the downward motion over the SCS leads to an increase in downward shortwave radiation and a decrease in surface wind speed and turbulent heat fluxes, contributing to the SCS warming.
An intense cold air outbreak affected the northern Adriatic Sea during winter 2012, determining an exceptional persistence of northeasterly Bora wind over the basin, which lasted for about 3weeks. ...The cold air coming from the Balkans produced icing in the Venice lagoon and very intense snowfall in the Apennines Mountains and even near the coasts.
In order to understand the importance and role of air–sea interactions for the evolution of the atmospheric fields, simulations with the Weather Research and Forecasting (WRF) model encompassing the whole period have been performed using sea surface temperature (SST) fields with an increasing level of complexity. Starting from a large-scale static sea temperature, the SST in the initial and boundary conditions has been progressively made more realistic. First, a more refined field, retrieved from a satellite radiometer was used; then, the same field was updated every 6h. Next, the effect of including a simplified 1D ocean model reproducing the Oceanic Mixed Layer (OML) evolution has been tested. Finally, the potential improvements coming from a coupled description of atmosphere–ocean and atmosphere–ocean–waves interactions have been explored within the Coupled Ocean–Atmosphere–Wave Sediment Transport (COAWST) modeling system.
Results highlight that the energy exchange between air and sea does not significantly impact the atmospheric fields, in particular 10m wind and 2m temperature, also because of the geography of the basin and the predominance of synoptic-scale flow in intense events of Bora, in the northern Adriatic. However, when sensible and latent heat fluxes, which are dependent on atmospheric and oceanic variables, are analyzed, the more realistic representation of SST drastically improves the model performances.
•Compared to atmospheric-only runs, ocean–atmosphere coupled models do not change the simulated atmospheric fields•Compared to atmospheric-only simulations, ocean–atmosphere coupled models improve the simulated turbulent heat fluxes;•The additional use of a wave model produces a limited effect (in northern Adriatic Sea) on the results with a ocean–atmosphere coupled model.
Surface eroding thermocouples are developed to measure temperatures and heat fluxes in plasma-facing components of fusion machines, especially in the divertor region. Sensor construction leads to a ...fast response (8ms) and robust design against heat loads around 10-20MW<inline-formula> <tex-math notation="LaTeX">\cdot \text{m}^{-2} </tex-math></inline-formula>. Electrical design of surface thermocouples is developed considering compensation needs and currents expected in the plasma edge. Improved sensor performance is obtained by enhancing the thermal contact surface between the divertor mono-block and the thermocouple carrier body through the integration of conductive collet and bush, both made of copper. The thermal response is analyzed through a finite element non-linear transient model simulating heating due to a plasma discharge. The usual configuration with voltage (emf) measured between the two thermocouple ribbons is compared with a simplified single-ribbon design in which the emf is measured between the thermocouple sole inner ribbon and the divertor support. Measurement errors are discussed for both single-ribbon and double-ribbon designs. The installation of surface thermocouples is studied at different poloidal positions of the DTT divertor vertical targets. A sensor layout with 4mm poloidal resolution is proposed to map power density peaks of plasma-wall interactions. Surface thermocouple measurements can provide useful local information for studying divertor physics, scaling for development of future machines, monitoring of local conditions to assist controlled terminations of plasma discharges. Installed with other diagnostics, they can provide crosschecks for calibration and data validation. The design, development, and integration procedure herein described can be applied to other plasma-facing component embedded sensors measuring plasma parameters like Langmuir probes.
We continue the investigation, started in Jakšić et al. in (J. Stat. Phys. 166:926–1015,
2017
), of a network of harmonic oscillators driven out of thermal equilibrium by heat reservoirs. We study ...the statistics of the fluctuations of the heat fluxes flowing between the network and the reservoirs in the nonequilibrium steady state and in the large time limit. We prove a large deviation principle for these fluctuations and derive the fluctuation relation satisfied by the associated rate function.
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