Effect Of Heat Loss Research Articles

Numerical investigation of azimuthal thermoacoustic instability in a gas turbine model combustor

Self-excited spinning mode azimuthal instability in an annular combustor with non-swirling flow is investigated using large eddy simulation (LES). Compressible Navier–Stokes equations are solved with a flamelet combustion model to describe the subgrid chemistry-turbulence interactions. Two flamelet models, with and without heat loss effects, are compared to elucidate the non-adiabatic wall effects on the thermoacoustic instability. The azimuthal modes are captured well by both models with only marginal differences in the computed frequencies and amplitudes. By comparing with the experimental measurements, the frequencies given by the LES are approximately 10% higher and the amplitudes are well predicted. Further analysis of the experimental and LES data shows a similar dominant anti-clockwise spinning mode, under which a good agreement is observed for the phase-averaged heat release rate fluctuations. Dynamic mode decomposition (DMD) is applied to shed more light on this spinning mode. The LES and experimental DMD modes reconstructed for their azimuthal mode frequencies agree very well for the heat release fluctuations. The DMD mode structure for the acoustic pressure from the LES shows a considerable non-zero profile at the combustor outlet, which could be essential for azimuthal modes to establish in this annular combustor. Finally, a low-order modelling study was conducted using an acoustic network combined with the flame transfer function extracted from LES. The results show that the dominant mode is associated with the plenum showing a first longitudinal and azimuthal mixed mode structure. By tuning the plenum length to match the effective volume, the predicted frequency becomes very close to the measured value.

The effect of hydrogen addition on methane/air explosion characteristics in a 20-L spherical device

The addition of hydrogen to methane changes its deflagration characteristics and increases the combustion rate. However, studies on the effect of hydrogen on methane deflagration remain insufficient. Therefore, based on the CFD code GASFLOW-MPI, a four-step combustion-mechanism model was established for methane/hydrogen mixtures. The deflagration characteristics of a premixed combustible gas in a 20-L spherical device was numerically simulated using a methane/hydrogen/air equivalence ratio of 1 and hydrogen addition in the range of 0–50%; subsequently. The results were compared with experimental data. The four-step methane/hydrogen combustion-mechanism could effectively reproduce the methane/hydrogen deflagration process on considering the heat losses. With an increase in hydrogen addition, the laminar burning velocity increases, and the deflagration duration reduces. It decreases the explosion heat loss and increased the maximum deflagration pressure. Under adiabatic simulation, the maximum deflagration pressure decreased with an increase in hydrogen addition, in contrast with the experimental results. This indicates that the heat-loss effect of the methane/hydrogen/air-mixture deflagration process should not be ignored. Moreover, the heat loss during the methane/hydrogen/air-mixture deflagration was mainly caused by thermal radiation. Thus, the influence of the thermal-radiation and convective heat-transfer mechanisms should be considered in the numerical simulations of methane/hydrogen/air-mixture deflagration.

The effect of heat losses and heat transfer coefficient on the efficiency of the solar air heater collector

In this Article developed the heat balance equations for a solar air heater with a concave air pipe absorber, calculating the heat wastes lost from the barrier structures of the device. In addition, a special emphasis is placed on the issues of optimal placement of concave air pipes, optimal distances are selected between the air pipes. Issues of increasing its heat giving capacity are solved by the shading placement of concave air pipes. The result is an increase in the performance of the device by increasing the coefficient of heat transfer of the solar air heater collector with a concave air pipe absorber. To do this, the movement of concave tube absorbers with different geometric arrangements was studied. Also in this article, a heat balance equation was developed to determine the optimal geometric location of air pipes and the reliability of the results obtained was confirmed. According to the simulations carried out, the placement of a concave-tube in the form of a checkerboard on the surface of the absorber is calculated, perpendicular to the air flow, parallel to the air flow and in the form of a flat air pipe.

Heat loss effect on bitumen ageing in tanks

The article is devoted to laboratory research on the influence of convective motion as a result of thermal losses and their compensation in tanks on the physical and chemical ageing of bitumen BND 60/90 and BND 90/130. Laboratory tests of bitumen samples confirmed the hypothesis about the key role of free movement of bitumen in tanks on its degradation under conditions of maintaining operating temperatures and free air access. Thus, bitumen degradation has a functional relationship with thermal losses from the tank surface and the total energy input for bitumen heating.

Steady state performance of molten salt natural circulation loop with different orientations of heater and cooler

Molten salts are proposed as a coolant and sensible heat storage medium for Solar Thermal Power Plant (STPP) as well as fuel and coolant for Molten Salt Reactor (MSR). The objective of the present study is to investigate the steady state behavior of molten salt in natural circulation flow condition along with effect of different orientations of heater and cooler on the steady state mass flow rate. For which a Molten Salt Natural Circulation Loop (MSNCL) has been setup with molten nitrate salt (mixture of Sodium Nitrate and Potassium Nitrate in 60:40 ratio by wt.) as working fluid. Steady state experiments in four different orientations of heater and cooler [Horizontal Heater Horizontal Cooler (HHHC), Horizontal Heater Vertical Cooler (HHVC), Vertical Heater Horizontal Cooler (VHHC) and Vertical Heater Vertical Cooler (VHVC)] have been performed and data generated are compared with the steady state correlations available in open literature which are derived considering either standard friction factor correlations or friction factor correlations deduced from their experiments and theoretical studies. Based on the deviations observed in the experimental values and correlation predictions, an improved formulation has been derived. The steady state formulation has been improved by incorporating Wall Thermal Inertia (WTI) effect and Heat Loss (HL) effect considering the importance of WTI & HL in dealing with high temperature molten salt loop. The improved formulation has been compared with the experimental results for different orientations of heater and cooler. Further, a new friction factor correlation has also been deduced from the experiments for vertical heater orientation. Description of MSNCL along with validation studies performed for different steady state correlation against experimental data generated in MSNCL, are reported in this paper.

Thermal recovery of offshore coalbed methane reservoirs: Flow characteristics of superheated steam in wellbores

Methane in coalbed methane (CBM) reservoirs is mainly in the adsorbed state, and has both free and dissolved states. The proportion of adsorbed methane can be as high as 90% or more. Field practice shows that low methane desorption efficiency is one of the keys affecting CBM development. The adsorption of methane is significantly affected by temperature, and the desorption efficiency increases with the increase of temperature. In this paper, the wellbore flow during the development of CBM reservoirs by injection of superheated steam is taken as the research object, and the superheated steam flow model is established considering the influence of seawater disturbance caused by the phase change of superheated steam. The study found: (a) In the formation section wellbore, the superheated steam has the highest temperature value. (b) The temperature of the superheated steam is affected by both the Joule Thompson effect and the heat loss effect. (c) The longer the wellbore in the seawater section, the lower the temperature of the superheated steam in the wellbore. (d) Superheated steam is only suitable for shallow sea conditions.

Thermal conductivity of single silk fibroin fibers measured from the 3ω method

Silkworm silk, a potential material used as the matrix of conductive fibers, has attracted extensive attention due to its excellent mechanical properties. However, researches on its thermal properties, especially in the axial direction of the single silk fibroin fiber are less. In this study, the axial thermal conductivity of a single silk fibroin fiber after coating a gold film on the surface is measured using the 3ω method. The effects of the radiation heat loss on the intrinsic thermal conductivity are eliminated by varying the length of the test samples. The results show that the thermal conductivity of the single silk fibroin fiber decreases slightly as the temperature increases from 260 to 300 K. The extracted intrinsic axial thermal conductivity is about 0.775 W/m-K at room temperature, an order of magnitude higher than most textile fibers. The method proposed in this study can be extended to accurately measure the thermal conductivity of the other non-conductive micro-fibers.

Large Eddy Simulation of a Turbulent Dilute Ethanol Flame Using the Two-Phase Spray Flamelet Generated Manifold Approach

ABSTRACT A recently developed spray Flamelet Generated Manifolds (SFGM) approach has been extended to include turbulence–chemistry interactions for studies of turbulent spray flames based on Eulerian–Lagrangian multiphase Large Eddy Simulation (LES). The Eulerian–Eulerian two-fluid formulation of the SFGM was extended to include the mixture fraction variance to construct a 4-D manifold library. The effect of heat loss due to droplet evaporation was considered in the SFGM tabulation, and the SFGM approach was evaluated using the Sydney ethanol spray flame experimental database. The two-phase interaction was investigated from complete analyses of the continuous- and dispersed-phase dynamics. Comparisons between the numerical results and the experimental data indicate that the SFGM formulated from counterflow spray flames can produce a good agreement with the gaseous flow field and the dispersed-phase statistics. By comparing the results in terms of the temperature field with other existing models, it can be inferred that the discrepancies between the flamelet-based results and the experimental data may not depend on the choice of the flamelet approach but on the modeling of the turbulence/chemistry interaction model. A Beta-function of the normalized progress variable can be an alternative to improve the temperature predictions. Moreover, using a well-designed definition of the progress variable by finding the optimal weights of several appropriate species is another possible path to improve the predictions.

Thermal recovery of coalbed methane: Modeling of heat and mass transfer in wellbores

Coalbed methane (CBM) reservoirs have the characteristics of low porosity, low permeability, strong heterogeneity and high content of adsorbed gas. A large number of studies have shown that the adsorption performance of coalbed methane reservoirs on methane is significantly affected by temperature. Therefore, heat injection development of coalbed methane reservoirs has received extensive attention. This paper takes the superheated steam (SS) flow in the wellbore after fracturing the coalbed methane reservoir as the research object, and establishes a mathematical model of the SS flow. The model consists of momentum conservation equation and energy conservation equation as the main control equation, combined with auxiliary models to form a complete mathematical model. In this paper, the discrete method is used to obtain the numerical solution of the model, and then the influence of the injection parameters on the SS flow is analyzed. The research conclusions of this paper have reference significance for the development of coalbed methane reservoirs with SS injection. This study found that: (a) When the injection rate is low enough (0.2 kg/s in this paper), the effect of gravity on SS pressure is stronger than that of pressurized gas; (b) When the steam injection speed is within a certain range (0.5 kg/s in this example), the pressure of the SS first decreases and then increases, forming a U shape; (c) The SS temperature increases with increasing of injection rate. During this period, the Joule-Thompson effect replaces the heat loss effect as the main factor controlling the temperature drop of the SS; (d) In order to maintain the development advantages of SS, the SS should still have a certain degree of superheat at the bottom of the well. Therefore, the steam injection speed should not be too high (eg 0.2 kg/s in this article); (e) Therefore, with the increase of SS pressure inside the wellbore, the temperature value decreases due to the influence of Joule Thompson effect, resulting in the continuous decrease of superheat degree.

Effects of radiation, curvature, and preferential diffusion on the extinction of laminar non-premixed flames

The combined effects of radiative heat loss, curvature, and preferential diffusion on laminar non-premixed flames (or flamelets) are investigated in this work by using asymptotic analysis. A general theoretical description of flame temperature and extinction is derived for curved flames with non-unity Lewis numbers and radiative heat loss. Special attention is paid to the effects of curvature and radiative heat loss on the flammability limits. The results show that (1) a curved flamelet always has two extinction limits: one is the kinetic extinction limit, and the other is the curvature-induced extinction limit for the adiabatic case or the radiative extinction limit for the radiative case; (2) the curvature exerts a different influence on the adiabatic and radiative flames. Specifically for the adiabatic flame, it is found that both flame temperature and flame position significantly decrease as the curvature increases and that a new extinction limit at a low stretch rate occurs due to the existence of curvature. Furthermore, a higher curvature coupled with the increase in the Lewis number results in a lower flammability limit and narrower flammable zone. Therefore, the presence of curvature has a negative impact on the adiabatic flame. On the contrary, for the radiative flame, the results show that the increase in curvature has a positive effect on the flammability limit and thereby increases the flammable zone. It is expected that curved flamelets hold smaller (larger) flammable zones than planar flamelets under the adiabatic (radiative) condition. All results show that the change in flame curvature has a stronger effect on the flame structure and extinction than the deviation of the Lewis number from unity.

Combined effects of heat loss and curvature on turbulent flame-wall interaction in a premixed dimethyl ether/air flame

This study investigates the effects of curvature on the local heat release rate and mixture fraction during turbulent flame-wall interaction of a lean dimethyl ether/air flame using a fully resolved simulation with a reduced skeletal chemical reaction mechanism and mixture-averaged transport. The region in which turbulent flame-wall interaction affects the flame is found to be restricted to a wall distance less than twice the laminar flame thickness. In regions without heat losses, heat release rate and curvature, as well as mixture fraction and curvature, are negatively correlated, which is in accordance with experimental findings. Flame-wall interaction alters the correlation between heat release rate and curvature. An inversion in the sign of the correlation from negative to positive is observed as the flame starts to experience heat losses to the wall. The correlation between mixture fraction and curvature, however, is unaffected by flame-wall interactions and remains negative. Similarly to experimental findings, the investigated turbulent side-wall quenching flame shows both head-on quenching and side-wall quenching-like behavior. The different quenching events are associated with different curvature values in the near-wall region. Furthermore, for medium heat loss, the correlations between heat release rate and curvature are sensitive to the quenching scenario.

Experimental study of spontaneous ignition of overloaded electrical wires under transverse wind

Spontaneous ignition of electrical wire caused by the Joule heat generated in core due to overloading or a short circuit is common, and is significantly influenced by wind velocity, but has been very limited addressed yet. This paper, for the first time, presents an experimental investigation in spontaneous ignition of overloaded wire under transverse wind (perpendicular to wire). Polyethylene (PE)-insulated wires with a NiCr core diameter of 0.8 mm and the insulation layer thickness of 0.3 mm and 0.15 mm are employed as samples. It can be found that with the increase of transverse wind velocity, the ignition delay time demonstrates a non-monotonic tendency (firstly decreases then increases, and decreases again, finally increases) until ignition limit, where four regimes are clarified according to different controlling mechanisms. The ignition delay time declines with the increase of electrical current. One of the most noteworthy findings is that the ignition delay time declines as the insulation thickness increases, which shows an opposite trend with the results of piloted ignition for thermally thin materials. In order to distinguish the ignition and non-ignition zones, an ignitability map considering both the gas phase kinetic effects and the solid phase heat loss effect is established based on two non-dimensional parameters [Damköhler number (Da) and Rloss]. A theoretical ignition model combining the effects of pyrolysis time, mixing time and chemical time is established, which well interprets the variation trend of ignition delay time and verifies the significant role of the chemical time in spontaneous ignition. This work provides essential knowledge on spontaneous ignition of overloaded electrical wires under practical transverse wind.

Numerical modeling of filtration gas combustion in a cylindrical radiation burner for contactless heating of materials

• A new configuration of radiant burner is considered. • The study of the effect of heat loss on the stability of the filtration combustion in a porous medium is carried out • Numerical modelling of the combustion inside a burner with axial fuel supply has shown the possibility of stable operation at high temperature levels. • There is a range of pressure drops at which the process of contactless heating of the material to high temperature levels can be carried out. The filtration gas combustion in a porous cylindrical tube with axial injection of combustible mixture is numerically investigated. This configuration can be potentially used as new type of heater for the contactless heating of materials in industrial processes. The material under treatment is placed inside the cylindrical porous tube and is heating by radiative flux from the cylindrical tube wall. The simulation of the filtration gas combustion in porous burner was carried out within the framework of the two-temperature thermal-diffusion model. Conducted numerical modeling yields qualitative description of the effects of heat removal from the burner on the limits of a stable combustion regime. The data about range of the gas flow rates and the intensity of the heat losses on the burner's inner surface corresponding to the stable combustion were obtained. The temperature distributions in the gas and the porous burner were as well gas flow field were obtained in general case, without addresing to the details of the material processing.

Flame-acoustics interaction for flames propagating from the open to the closed end of a channel: effects of heat losses and the Lewis number

In this paper we present a numerical study of the impact of heat losses and the differential diffusion effect on the flame-acoustics interaction, for flames propagating from the open to the closed end of a planar channel. It has been recently demonstrated [1] that for Le=1 symmetric and non-symmetric flames produce different acoustic responses in adiabatic channels. The results of the present study show that the interaction of the flame with the acoustic field is significantly influenced by heat losses and the Lewis number through a change in the shape of the flame: increasing the heat losses tends to make the flames symmetric and low Lewis numbers result in non-symmetric flames.

Effect of flame retardants on side-wall quenching of partially premixed laminar flames

A combined experimental and numerical investigation of partially premixed laminar methane-air flames undergoing side-wall quenching (SWQ) is performed. A well-established SWQ burner is adapted to allow the seeding of the main flow with additional gaseous products issued from a (secondary) wall inlet close to the flame’s quenching point. First, the characteristics of the partially premixed flame that quenches at the wall are assessed using planar laser-induced fluorescence measurements of the OH radical, and a corresponding numerical simulation with fully-resolved transport and chemistry is conducted. A boundary layer of enriched mixture is formed at the wall, leading to a reaction zone parallel to the wall for high injection rates from the wall inlet. Subsequently, in a numerical study, the wall inflow is mixed with dimethylmethylphosphonat (DMMP), a phosphor-based flame retardant. The DMMP addition allows the assessment of the combined effects of heat loss and flame retardants on the flame structure during flame-wall interaction. With an increasing amount of DMMP in the injected mixture, the flame stabilizes further away from the wall and shows a decrease in the local heat-release rate. Thereby, the maximum wall heat flux is significantly reduced. That results in a lower thermal load on the quenching wall. The flame structure analysis shows an accumulation of the intermediate species HOPO at the wall similar to the CO accumulation during the quenching of premixed flames without flame retardant addition. The study shows how the structure of a partially premixed flame is influenced by a wall that releases either additional fuel or a mixture of fuel and flame retardant. The insights gained from the canonical configuration can lead to a better understanding of the combined effects of flame retardants and heat losses in near-wall flames.

Large eddy simulation investigation of pressure and wall heat loss effects on rich ammonia-hydrogen-air combustion in a gas turbine burner

Ammonia is currently investigated as a sustainable energy source. Its mixture with hydrogen may present combustion characteristics that are similar to those of hydrocarbon, which motivates its use in gas turbines burners. Such similarities were discussed at atmospheric pressure in previous works for a fuel blend with a molar fraction of hydrogen XH2Fuel=0.46, which is further studied here. The influences of pressure and wall heat loss on ammonia/hydrogen/air flames are for the first time investigated via large eddy simulation. A first campaign is led at both 1 and 5 atm, to estimate the effect of pressure. It demonstrates that NO emissions are favoured by flame interactions with a hot wall, along which NO is convected. Accordingly, the flame length reduction observed at high pressure, due to higher heat release rates, leads to a more efficient NO consumption. Increasing the pressure shifts the equivalence ratio for optimal NH3 and NOX emissions towards the lean side. It results in lower hydrogen emissions and therefore an increase of the combustion efficiency. Finally, the NH3 and NOX emissions at optimal equivalence ratio are reduced from 450 ppmv for φ = 1.27 at 1 atm, to ∼ 100 ppmv for φ = 1.20 at 5 atm. A second campaign is led at both 1 and 5 atm, by varying the burner wall thermal boundary conditions. Lean combustion with cold walls presents high N2O emissions of 607 ppmv while in rich cases, the higher gas temperatures and the excess of H radicals in the burned gases yield complete N2O consumption. It is shown that heat loss effect on N2O fractions distribution is reduced at high pressure due to weaker flame interaction with the cold walls. Finally, thermal boundary conditions are found to significantly affect NOX, N2O and NH3 emissions, showing that heat losses should be considered when modelling such configuration.

Experimental and analytical natural circulation analysis of the Jordanian 5 MW research reactor using plate type fuel by RELAP5 MOD3.3

The 5-MWth Jordanian research reactor is an open-pool type research reactor using plate-type fuel. It is cooled by the upward natural flow at low power-operation. Natural circulation cooling is important in the safety analysis because it is the last scenario of all accidents. Therefore, studying natural circulation is important in determining the safety limits, such as the maximum allowed temperature and the minimum critical heat flux ratio of fuel. In this paper, the reactor operated at 50KW by the natural circulation to investigate its neutronics and thermal–hydraulic characteristics. Then, the results were benchmarked with RELAP5 MOD 3.3 by implementing models with and without considering the effect of heat loss through the Upper Guide Structure. As a result of this study, different models show minor differences in thermodynamic parameters and an additional safety margin. Eventually, benchmarking the results and the calculations of RELAP5 led to a reasonable agreement in both simulated models.

Method-of-characteristics model for a low-enthalpy, detonation-driven shock tube

A reduced-order model for a detonation-driven shock tube was developed using the method of characteristics (MoC). The scope of this work was limited to calorically perfect slugs of gases. Effects of momentum and heat losses were included. The governing equations for inviscid, one-dimensional flow of a calorically perfect gas were simplified using MoC. These simplified equations represented and resolved various gasdynamic phenomena such as weak compressions, rarefactions, shocks, and contact surfaces. The momentum losses in the governing equations were estimated using established friction factors. Various empirical methods were explored to determine an appropriate heat-transfer model. Based on the expected ideal wave processes in a detonation-driven shock tube, MoC subroutines were assembled into a global algorithm representing detonation tube operation. To validate the results from the reduced-order model, experiments were carried out in a small-scale detonation tube. The experiments used nitrogen as the high-pressure driver gas, stoichiometric oxyhydrogen as the detonation driver gas, and nitrogen or helium as the driven gas. Comparison with experiments showed that the detonation tube model reasonably replicated detonation tube operation for all the experimental cases. Specifically, the decaying incident shock trajectory in the driven section was replicated well, and so was the peak pressure at the driven end wall. The quasisteady plateau pressure in the detonation driver was replicated reasonably, with experimental pressure traces showing earlier decay than MoC pressure traces. The wave system produced by the reflected shock wave–contact surface interaction in the driven section was also predicted accurately by the MoC model.

Critical Role of Layer Thickness in Frontal Polymerization.

Thermal frontal polymerization (FP) is a chemical process during which a cold monomer-initiator mixture is converted into a hot polymer as a polymerization front propagates in the system due to the interplay between heat diffusion and the exothermicity of the reaction. The theoretical description of FP generally focuses on one-dimensional (1D) reaction-diffusion (RD) models where the effect of heat losses is encoded into an effective parameter in the heat equation. We show here the limits of such 1D models to describe FP under nonadiabatic conditions. To do so, the propagation of a polymerization front is analyzed both analytically and numerically in a rectangular two-dimensional (2D) layer. The layer thickness is shown to control the dynamics of the front and to determine its very existence. We find that for given heat losses, a minimum thickness is required for front propagation as recently observed in FP experiments of 2D thin films on wood. Moreover, when the thickness exceeds a critical value, the front is observed to survive independently of the rate of heat losses. This result cannot be predicted with 1D models where front extinction is always possible. A scaling analysis is proposed to highlight the physical interpretation of such a front survival. The influence of dimensionality on thermal instabilities is also analyzed, with a focus on the differences with the 1D predictions.

Effect of heat loss on the syngas production by fuel-rich combustion in a divergent two-layer burner

The effect of heat loss on the syngas production from partial combustion of fuel-rich in a divergent two-layer burner is numerically studied using two-dimensional model with detailed kinetics GRI-Mech 1.2. Both the radiation and wall heat losses to the surrounding are considered in the computations. It is shown that two types heat losses have different effects on the syngas production. The radiation heat loss has significant effect on the syngas temperature and the syngas temperature is dropped as radiation heat loss is increased, but it has neglected effect on the reforming efficiency and methane conversion efficiency. The wall heat loss has a comprehensive effect on the syngas production. The wall heat loss not only reduces the conversion efficiency, but also significantly decreases the syngas temperature. The effect of wall heat loss becomes weak as the equivalence is increased. The reforming efficiency drops from 0.440 to 0.424 for equivalence ratio of 2 and mixture velocity of 0.17 m/s for the predictions between adiabatic wall and non-adiabatic conditions. • Heat loss on syngas production in divergent porous burner is conducted with detailed kinetics. • Radiant heat loss to the surrounding significantly reduces the syngas temperature. • Radiant heat loss has little effect on the methane conversion efficiency and reforming efficiency. • Wall heat loss has a comprehensive effect on the reforming efficiency and syngas temperature. • Effect of wall heat loss on the syngas production becomes strong with the equivalence ratio.