Centerline Temperature Research Articles

Thermal characteristics of externally venting flames and their effect on the exposed façade surface

In a compartment fire, Externally Venting Flames (EVF) may significantly increase the risk of fire spreading to adjacent floors or buildings, especially when combustible insulation materials are installed on the building façade. An increasing number of recent reports suggest that existing fire engineering design methodologies cannot describe with sufficient accuracy the characteristics of EVF under realistic fire load conditions. In this context, a series of fire safety engineering design correlations used to describe the main EVF thermal characteristics, namely EVF centreline temperature and EVF-induced heat flux on the exposed façade surface, are comparatively assessed. Towards this end, measurements obtained in a medium- and a large-scale compartment-façade fire test are employed; aiming to broaden the scope of the validation study, predictions of the investigated correlations are further compared to measurements obtained in 6 large-scale fire tests found in the literature. It is found that the correlation proposed in EN1991-1–2 (Eurocode 1) for the estimation of the EVF centreline temperature is under-predicting the measured values in large-scale fire tests. In addition, it is concluded that estimation of the local flame emissivity should take into account the specific fuel type used in each case.

Uncertainty assessment for loss of flow accident of a 5 MW pool-type research reactor

Best estimate plus uncertainty (BEPU) is a promising approach to the safety analysis of nuclear reactors, and the uncertainty calculation is a very important concern about it. BEPU ensures realistic safety margins and secures higher reactor effectiveness by adopting best-estimate codes and realistic input data with uncertainties, whereas the previous conservative analysis generates excessive conservatism by considering each input parameter separately. A loss of flow accident (LOFA) of a 5MW open-pool type research reactor was selected as a sample problem for a BEPU uncertainty assessment. We selected the failures of all primary cooling system (PCS) pumps, which would cause the abrupt reduction of flow and the reversal of core flow. The significant contributors to the reactor safety were identified and then input sets were sampled. For the uncertainty evaluation, 124 calculations were performed. This is the number of code runs required for a 95%/95% level with the 3rd order Wilk’s formula. The MOSAIQUE software developed by Korean Atomic Energy Research Institute (KAERI) was used for automated sampling of the uncertainty parameters, a global uncertainty calculation, and post processing of the results. The critical heat flux ratio (CHFR) and the fuel centerline temperature (FCT) were calculated at the 95%/95% level and were compared with those from conservative analyses. In addition, the impact of each design variables on the safety parameters was estimated by sensitivity analysis.

Effect of different turbulence models on combustion and emission characteristics of hydrogen/air flames

This paper aims to present modeling results of hydrogen/air combustion in a micro-cylindrical combustor. Modeling studies were carried out with different turbulence models to evaluate performance of these models in micro combustion simulations by using a commercially available computational fluid dynamics code. Turbulence models implemented in this study are Standard k-ε, Renormalization Group k-ε, Realizable k-ε, and Reynolds Stress Transport. A three-dimensional micro combustor model was built to investigate impact of various turbulence models on combustion and emission behavior of studied hydrogen/air flames. Performance evaluation of these models was executed by examining combustor outer wall temperature distribution; combustor centerline temperature, velocity, pressure, species and NOx profiles. Combustion reaction scheme with 9 species and 19 steps was modeled using Eddy Dissipation Concept model. Results obtained from this study were validated with published experimental data. Numerical results showed that two equation turbulence models give consistent simulation results with published experimental data by means of trend and value. Renormalization Group k-ε model was found to give consistent simulation results with experimental data, whereas Reynolds Stress Model was failed to predict detailed features of combustion process.

Manufacture of a UO2-Based Nuclear Fuel with Improved Thermal Conductivity with the Addition of BeO

The low thermal conductivity of oxide nuclear fuels is a performance-limiting parameter. Enhancing this property may provide a contribution toward establishing accident-tolerant fuel forms. In this study, the thermal conductivity of UO2 was increased through the fabrication of ceramic-ceramic composite forms with UO2 containing a continuous BeO matrix. Fuel with a higher thermal conductivity will have reduced thermal gradients and lower centerline temperatures in the fuel pin. Lower operational temperatures will reduce fission gas release and reduce fuel restructuring. Additions of BeO were made to UO2 fuel pellets in 2.5, 5, 7.5, and 10 vol pct concentrations with the goals of establishing reliable lab-scale processing procedures, minimizing porosity, and maximizing thermal conductivity. The microstructure was characterized with electron probe microanalysis, and the thermal properties were assessed by light flash analysis and differential scanning calorimetry. Reliable, high-density samples were prepared using compaction pressure between 200 and 225 MPa and sintering times between 4 and 6 hours. It was found that the thermal conductivity of UO2 improved approximately 10 pct for each 1 vol pct BeO added over the measured temperature range 298.15 K to 523.15 K (25 °C to 250 °C) with the maximum observed improvement being \(\sim \)100 pct, or doubled, at 10 vol pct BeO.

Neutronic-thermal hydraulic coupling analysis of the fuel channel of a new generation of the small modular pressurized water reactor including hexagonal and square fuel assemblies using MCNP and CFX

The main purpose of this paper is neutronic-thermal hydraulics coupling analysis of the fuel channel of an advanced Small Modular Reactor (SMR), which is nominated as a near term option of the generation IV reactors. The CAREM25 is chosen as the reference SMR. By considering hexagonal and square lattices of the fuel assemblies in the SMR reactor core, neutronic-thermal hydraulic coupling of hot and average fuel channels were conducted via utilizing of the MCNPX and CFX codes. Central fuel assemblies are determined as the hottest fuel assembly in the proposed SMR cores with hexagonal and square fuel assemblies with power peaking factor of 1.778 and 1.674, respectively. The hottest fuel rod power peaking factor is calculated as 1.846 and 1.954 for hexagonal and square fuel channels, respectively. The calculated linear power densities of the hottest and the average fuel rod of hexagonal channels are 20.18(kWm)and10.77(kWm), respectively, as well as of square channels are24.75(kWm) and10.36(kWm), respectively. Results show that the maximum axial power along the fuel rods occurred below the mid-plane of the rod. The ratio of the hot to average rod axial power peaking factor as a safety parameter used to calculate the maximum heat flux in the hottest channel, is calculated close to 2 and 2.4 for the core with hexagonal and square fuel assemblies, respectively. The calculated power peaking factors and linear power densities of fuel rods are in suitable range and in good agreement with PWR safety issues.The maximum temperature of fuel centerline in both types of the proposed SMRs is much lower than that of large PWRs (e.g., 2156 K for hot rod in VVER-1000). Between hexagonal and square fuel assemblies, the hexagonal FAs are preferred because of the two reasons; first, the axial fuel centerline temperature in the hottest hexagonal channel is lower than that of hottest square channel, so it is a good feature with respect of safety margins. Second, the outlet temperatures of the channels in the SMR core with hexagonal FAs are higher than the outlet temperature of the channels in the SMR core with square FAs. Thus the thermal efficiency of the reactor core with hexagonal FAs is more than that of the core with square FAs.

Prediction of Peak Temperatures of Nigeria Research Reactor Core Components under Several Reactivity Accident Tests

The Nigeria Research Reactor-1 (NIRR-1) consists of small water cooled square cylindrical core of 23cm in diameter and 23cm high. The small dimension of the core of this reactor facilitated our choice of PARET to perform reactivity accident analysis for NIRR-1 system. Our goal in this work is to predict the peak temperature of some important Nigeria Research Reactor (NIRR-1) core components under several reactivity accident tests. At power levels below 80kW, there were no significant differences between the peak fuel centerline temperatures, the peak fuel surface temperature and the peak clad surface temperature in the hot channel as well as in the average channel. The result from the reactivity accident test shows that power can never rise to an uncontrollable level in the core of NIRR-1 under ramp or step insertion of up to 4mk of reactivity. The calculated temperature of the important core components (e.g. fuel and clad) in the two channels (during this reactivity accident test) were far below their melting point temperatures. Boiling of any kind was not observed during this reactivity accident test. Therefore, NIRR-1 can be operated safely even if there is an inadvertent addition of up to 4mk of positive reactivity

A numerical model and experimental verification for analysing a new vacuum spray flash desalinator utilising low grade energy

This study investigates the performance of a new vacuum spray flash desalinator, a core component of the open water cycle in a discharge thermal energy combined desalination (DTECD) technology using theoretical and experimental techniques. The feedwater contains 3.5wt% of NaCl while the inlet temperature range can vary over a range of 55°C to 75°C based on the low temperature utilised in the DTECD system. In order to design an efficient desalinator, physical aspects of the proposed vacuum spray flash evaporation (VSFE) should be studied. Thus, an experimental study was undertaken to verify the theoretical evaporation rate and centreline temperature data. The proposed desalinator was modelled using a CFD model implemented in the available package ANSYS FLUENT 16.2 and some results are compared with a thermodynamic model embedded in ASPEN/HYSY 8.0. It was observed that the defined thermodynamic models based on vapor-liquid equilibrium in the Aspen and Fluent can predict the evaporation rate with the average errors of 5% and 17%, respectively. Moreover, discrete phase model (DPM) approach can analyse the thermo-fluid field in the desalinator with acceptable accuracy about 9%. Droplets size, velocity, temperature and concentration profiles are predicted and the underlying physics are discussed regarding the VSFE geometry.

Experimental and numerical simulation studies on heat transfer to calorimeters engulfed in diesel pool fires

Characterization of heat transfer to calorimeters engulfed in pool fires is extremely important. To estimate the heat flux to the calorimeters, experiments are performed with horizontal stainless steel 304L pipes engulfed in diesel pool fires. The concept of adiabatic surface temperature is applied to predict the incident heat flux to horizontally oriented calorimeters engulfed in diesel pool fires. Plate thermometers are used to measure the adiabatic surface temperature for diesel pool fires. The estimated subsurface temperatures inside the steel pipes using the adiabatic surface temperature concept and the measured temperatures are in good agreement. Adiabatic surface temperature is also computed from fire simulations. The incident heat fluxes to the steel pipes engulfed in fire predicted from the simulations are found to be in good agreement with the experiments. The fire numerical code is validated against the 1 m pool fire experimental results of centerline temperature distribution and irradiances away from fire. A correlation is provided for the estimation of adiabatic surface temperature for large diesel pool fires. These results would provide an effective way for thermal test simulations.

Large Eddy Simulations of CH4 Fire Plumes

Large eddy simulations of large-scale CH4 fire plumes (1.59-2.61 MW) with two different CFD packages, FireFOAM and FDS, are presented. It is investigated how the vorticity generation mechanism and puffing behavior of large-scale fire plumes differs from previously studied iso-thermal buoyant plumes of the same scale. In addition, the predictive capabilities of the turbulence and combustion models, currently used by the two CFD codes, to accurately capture the fire dynamics and the buoyancy-generated turbulence associated with large-scale fire plumes are evaluated. Results obtained with the two CFD codes, typically used for numerical simulations of fire safety applications, are also compared with respect to the average and rms velocities and temperatures, puffing frequencies, average flame heights and entrainment rates using experimental data and well-known correlations in literature. Furthermore, the importance of the applied reaction time scale model in combination with the Eddy Dissipation Model is examined. In particular, the influence of the considered mixing time scales in the predicted centerline temperatures is illustrated and used to explain the discrepancies between the two codes.

Experimental study on premixed methane-air catalytic combustion in rectangular micro channel

Abstract The catalytic combustion process of pre-mixed methane-air in a rectangular micro channel was studied experimentally. Infrared thermal imager and flue gas analyzer were used to measure the temperature distribution of the outer wall and main components of exhaust gas respectively. Flammability limits of premixed methane-air in catalytic and non-catalytic micro channel were obtained by changing flow rates of methane, and the effects of equivalence ratio, inlet velocity and channel height on combustion characteristics were analyzed. Results showed that the flammability limits improved significantly when platinum was added into the micro channel. The highest centerline temperature of the outer wall was obtained at an equivalent ratio of 0.9 at the same inlet velocity with or without catalyst in the micro channel. Addition of catalyst in the channel not only gave a uniform temperature distribution on the outer wall of the channel but also improved methane conversion. With the increase of inlet velocity, the centerline temperature of the outer wall increased and the highest points of the temperature shifted to downstream of the channel gradually. Combustion intensity in the channel increased with the increase of channel height at the same inlet velocity.

Effects of H2O and CO2 diluted oxidizer on the structure and shape of laminar coflow syngas diffusion flames

The effects of up to 30% H2O and CO2 replacement of N2 in air on the structure and shape of laminar coflow syngas (COH2) diffusion flames were experimentally and numerically studied. Temperatures along the flame centerline were measured using a type-B thermocouple. The OH*-chemiluminescence and flame luminance were captured respectively by an intensified CCD camera and a CCD camera to determine the flame height and radii. The syngas diffusion flames were numerically modeled using detailed thermal and transport properties and the chemical reaction mechanism of Davis et al. (2005). Four pairs of artificial species were introduced in additional numerical calculations to isolate the chemical, thermal, transport, and radiative effects of H2O and CO2. The experimental and numerical results show that H2O and CO2 replacement of N2 in the oxidizer reduce the peak flame temperature, but they influence the flame centerline temperature distributions differently. The thermal and radiative effects of both H2O and CO2 addition decrease the flame temperatures. The chemical and transport effects of CO2 and H2O affect flame temperatures differently. H2O addition promotes the OH concentration though H+O2=O+OH and O+H2O=2OH, while CO2 addition decreases the OH concentration by suppressing those reactions. The higher concentrations of OH under H2O addition signify higher combustion intensity and hence lead to decreased flame height and radius. In contrast, the addition of CO2 suppresses flame temperature and the overall combustion process, resulting in increased flame height and radius.

Numerical Simulation of Transient Development of Flame, Temperature and Velocity under Reduced Gravity in a Methane Air Diffusion Flame

A methane air co flow diffusion flame has been numerically simulated with the help of an in-house developed code at normal gravity, 0.5 G, and 0.0001 G (microgravity) for the study of transient behavior of the flame in terms of flame shape, temperature profile and velocity (streamlines). The study indicates that lower is the gravity level, the higher is the time of early transience. The flame developments during transience are marked by the formation of a secondary flamelet at different heights above the primary flame at all gravity levels. The development of temperature profile at microgravity takes a much longer time to stabilize than the flame development. At normal gravity and 0.5 G gravity level, streamlines, during transience, show intermediate vortices which are finally replaced by recirculation of ambient air from the exit plane. At microgravity, neither any vortex nor any recirculation at any stage is observed. Centerline temperature plots, at all gravity levels during transience, demonstrate a secondary peak at some instants as a consequence of the secondary flamelet formation. The centerline velocity at microgravity decreases gradually during transience, unlike at other two gravity levels where the fall is very sharp and is indicative of negligible buoyancy at microgravity.

Combined Radiation/Natural Convection in a Participating Medium Using Novel Lattice Boltzmann Method

This paper deals with analysis of combined natural convection and volumetric radiation in a square enclosure containing an absorbing, emitting, and scattering medium using the lattice Boltzmann method. Unlike all previous studies, the proposed lattice Boltzmann method is used to compute both convective and radiative terms of the energy equation. The fluid flow, energy, and radiative heat transfer equations are solved by using separate distribution functions in the lattice Boltzmann method. This formulation regarding natural convection with and without radiation has been validated. Streamlines, isotherms, and centerline temperature are analyzed to investigate the influence of parameters such as Rayleigh number, extinction coefficient, and scattering albedo. It is found that the present lattice Boltzmann method not only provides accurate results but also decreases time consumption to a great extent.

Effect of power variations across a fuel bundle and within a fuel element on fuel centerline temperature in PHWR bundles in uncrept and crept pressure tubes

The neutron flux and fission power profiles through a fuel bundle and across a fuel element are important aspects of nuclear fuel analysis in multi-scale/multi-physics modelling of Pressurized Heavy Water Reactors (PHWRs) with advanced fuel bundles. Fuel channels in many existing PHWRs are horizontal. With ageing, pressure tubes creep and fuel bundles in these pressure tubes are eccentrically located, which results in an asymmetric coolant flow distribution between the top and bottom of the fuel bundles. The diametral change of the pressure tube due to creep is not constant along the fuel channel; it reaches a maximum in the vicinity of the maximum neutron flux location. The cross-sectional asymmetric positioning of fuel bundles in a crept pressure tube contributes to an asymmetric power distribution within a ring of fuel elements. Modern reactor physics lattice codes (such as WIMS-AECL1AECL stands for Atomic Energy of Canada Limited.1) are capable of predicting the details of power distribution from basic principles. Thermalhydraulics subchannel codes (such as ASSERT-PV) use models to describe inhomogeneous power distribution within and across fuel elements (e.g., flux tilt model, different powers in different ring elements, or radial power profiles). In this work, physics and thermalhydraulics codes are applied to quantify the effect of eccentricity of a fuel bundle on power variations across it and within a fuel element, and ultimately on the fuel temperature distribution and fuel centerline temperature, which is one of the indicators of fuel performance under normal operating conditions (NOC). Due to fissioning, Xe and Kr gases are produced, which contribute to the internal gas pressure within an element. The increase in gas pressure may jeopardize the fuel element integrity (sheath rupture). On the other hand, fission gas is produced as a function of neutron flux/power and diffuses to the grain boundaries and fuel-pellet interface as a function of fuel temperature. Therefore, it is important to know the fuel temperatures and distributions within the fuel.

Accident tolerant composite nuclear fuels

Investigated accident tolerant nuclear fuels are fuels with enhanced thermal conductivity, which can withstand the loss of coolant for a longer time by allowing faster dissipation of heat, thus lowering the centerline temperature and preventing the melting of the fuel. Traditional nuclear fuels have a very low thermal conductivity and can be significantly enhanced if transformed into a composite with a very high thermal conductivity components. In this study, we analyze the thermal properties of various composites of mixed oxides and thoria fuels to improve thermal conductivity for the next generation safer nuclear reactors.

Coupled neutronics–fuel behavior calculations in steady state using the Serpent 2 Monte Carlo code

This paper describes the recent developments in the multi-physics capabilities of the Serpent Monte Carlo code concerning coupled multi-physics calculations with fuel behavior feedback. We cover the structure and operation of the fuel behavior interface in Serpent 2 as well as the coupled calculation routines implemented for steady state multi-physics calculations with any internally or externally coupled solver. The intended solution flow and code-to-code communication in internally and externally coupled multi-physics simulations is described alongside with the stochastic approximation based solution relaxation methods implemented in Serpent.The two-level multi-physics coupling scheme in Serpent 2 is demonstrated by obtaining a coupled solution for the neutronics–fuel behavior problem using first the internally coupled FINIX fuel behavior module and then the externally coupled ENIGMA fuel performance code in a 3D assembly geometry. Parameters such as maximum pellet centerline temperatures can be evaluated from the coupled solution. The temperature fields obtained from the coupled solution are also used to estimate the effect of the detailed radial representation of the fuel temperature distribution compared to various radially averaged effective fuel temperature representations. The convergence of the coupled solution is investigated alongside the possibility to speed up the convergence by using the Uniform Fission Sites method.

Estimation of centerline temperature of the waste form for the rare earth waste generated from pyrochemical process

Estimation of centerline temperature of nuclear glass waste form for each waste stream is very essential in the period of storage because the centerline temperature being over its glass transition temperature results in the increase of leaching rate of radioactive nuclides due to the devitrification of glass waste form. Here, to verify the effects of waste form diameter and transuranic element content in the rare earth waste on the centerline temperature of the waste form, the surrogate rare earth glass waste generated from pyrochemical process was immobilized with SiO2Al2O3B2O3 glass frit system, and thermal properties of the rare earth glass waste form were determined by thermomechanical analysis and thermal conductivity analysis. The estimation of centerline temperature was carried out using the experimental thermal data and steady-state conduction equation in a long and solid cylinder type waste form. It was revealed that thermal stability of waste form in case of 0.3 m diameter was not affected by the TRU content even in the case of 80% TRU recovery ratio in the electrowinning process, meaning that the waste form of 0.3 m diameter is thermally stable due to the low centerline temperature relative to its glass transition temperature of the rare earth glass waste form.

Thermodynamic performance of pre-pressurization in a cryogenic tank

To supply more actual initial physical field for launch of cryogenic propellant tank, it is necessary to investigate the thermodynamic performance of propellant tank before launch. As cryogenic propellant tank will experience the ground open parking and ground pre-pressurization phase, the present study is particularly to investigate the pressurization performance and temperature distribution in a cryogenic liquid oxygen (LOX) tank for the above parts. One CFD model is established with both the vapor-liquid interface phase change and external forced convection heat exchange considered. The results show that during the ground parking part, the intense nucleate boiling appears in cryogenic tank firstly, then the boiling intensity becomes weak, finally, it tends to be stable. With the high temperature gas oxygen (GOX) injecting into tank, tank pressure fluctuates among the setting pressure limits. The ullage is condensed during the whole pre-pressurization part. Influenced by heat transferring from the ullage and the external heat leakage, the thermal stratified layer becomes thick with time continuing. With the environmental temperature varying from 253.0K to 313.0K, its effect on tank pressure and liquid temperature distribution is not so obvious in this study. The unique large change is that the ullage center line temperature increases with the rise of environmental temperature, while the other parameters have slight changes.

A COMPARISON IN THERMAL-HYDRAULICS ANALYSIS OF PWR1000 USING FIXED AND TEMPERATURE FUNCTION OF THERMAL CONDUCTIVITY

A COMPARISON IN THERMAL-HYDRAULICS ANALYSIS OF PWR-1000 USING FIXEDAND TEMPERATURE FUNCTION OF THERMAL CONDUCTIVITY. A study to analyze theinfluence of the fuel-cladding’s thermal conductivity on the sub-channel of pressurized waterreactor 1000 (PWR-1000) using COBRA-EN computer code was conducted. The purpose ofthis research is to gain complete understanding of sub-channel thermal-hydraulic aspectsrelated to fuel performance, especially the appropriate range of thermal conductivity of UO2fuel (kf) and zircaloy-4 cladding (kc) in order to obtain an accurate sub-channel analysisrelated to its safety behavior. The research was conducted by comparing the calculation withthe combination values of the fixed kf and kc, as well as the calculation using kf and kc astemperature function. The fixed kf using in this calculation were 5.26 W/m.K, 3.85 W/m.K,3.60 W/m.K, 3.18 W/m.K, 2.90 W/m.K, 2.53 W/m.K and 2.34 W/m.K, while the kc were13.0 W/m.K, 15.57 W/m.K, 16.75 W/m.K, 17.94 W/m.K and 18.69 W/m.K. The maximumfuel center linet emperature using kf and kca s temperature function (MATPRO) for hot sub -channel was 1717.65°C and taken as the reference in accepting the calculation result usingfixed thermal conductivity. The analysis was accepted, if the deviation between bothtemperature was in the range of -10% to 10%. This analysis results for hot sub-chan nelwas accepted for the calculation using value of kf in the range of 3.18 - 2.90 W/m.K for allall variation value of kcW. hile the calculation using value of koff 2.53 W/m.K was accepet dfor value of kc in the range of 16.76 - 18.69 W/m.K

An experimental study of vertical centreline temperature and velocity profile of buoyant plume in cubical compartment

In a compartment fire, the plume temperatures and velocities are important parameters which are to be investigated. A proper understanding of behavior and temperature of the flame helps fire engineers and researchers deal with the fire situation. A series of four full scale steady state fire experiments were performed in a fire compartment of dimension 4 m × 4 m in plan and 4 m in height to study the vertical temperature profile of a flame. A pure grade of propane gas was supplied to a square burner of size 0.17 m × 0.17 m to create a steady state fire. The centerline vertical temperature rise of the buoyant plume was measured at seventeen different locations with the help of K-type thermocouples and was compared with several correlations available in the literature. The centerline velocities of buoyant plume were also measured and compared with various plume models. The prediction of Alpert (Fire Technol 8:181–195, [11]) and Cox and Chitty (Combust Flame 39:191–209 [10]) model of plume temperature rise and centerline velocities were found to be closer to that of experimental values.