Volumetric Heat Loss Research Articles

Effective Lewis number and burning speed for flames propagating in small-scale spatio-temporal periodic flows

Propagation of premixed flames having thick reaction zones in rapidly-varying, small-scale, zero-mean, spatio-temporal periodic flows is considered. Techniques of large activation energy asymptotics and homogenization theory are used to determine the effective Lewis number Leeff and the effective burning speed ratio ST/SL, which are influenced by the flow through flow-enhanced diffusion. The resultant effective diffusivity matrix is, in general, neither a scalar nor a diagonal matrix and therefore induces anisotropic effects on the propagation of multi-dimensional flames. As the flow Peclet number Pe becomes large, the flow-enhanced fuel diffusion coefficient and the thermal diffusivity behave respectively like (PeLe)σ and Peσ, where Le is the Lewis number and σ≤2 is a constant which depends on the flow and the direction of flame propagation. The maximal value σ=2 is achieved for steady, unidirectional, spatially periodic shear flows, while for steady two-dimensional square vortices, we have σ=1/2. In general, the constant σ is determined by solving a linear partial differential equation. The scaling laws for the diffusion coefficients lead to corresponding scaling laws for the effective Lewis number and the effective burning speed ratio of the form Leeff≃Le1−σ and ST/SL∼(Pe/Le)σ/2. Effects of thermal expansion and volumetric heat loss on the flame are also briefly discussed. In particular, it is shown that the quenching limit is enlarged by a factor 1/Leσ for Le<1 and diminished by the same factor for Le>1, due to the flow-enhanced diffusion. The potential implications of the results to better understand turbulent combustion are discussed. A special emphasis is placed on the dependence of the flame on Le in the presence of high-intensity, small-scale flows. In particular, it is shown that this dependence is intimately linked to the flow through Taylor-dispersion like enhanced diffusion, rather than through the traditional molecular diffusion coupled with curvature effects. The flow-dependent effective Lewis number identified may also provide an explanation to the peculiar experimental observation that turbulence appears to facilitate ignition in Le>1 mixtures and to inhibit it in Le<1 mixtures. Novelty and significance statementAn original study, combining asymptotic analysis and homogenization theory, is applied to describe flame propagation in small-scale, spatio-temporal periodic flow fields. Scaling laws are derived for the effective burning speed and the effective Lewis number for high-intensity small-scale flows, which are useful to better understand the behaviour of turbulent premixed flames in the distributed reaction zone regime. The formula for the flow-dependent effective Lewis number identified herein may explain the peculiar experimental observation that turbulence appears to facilitate ignition in Le>1 mixtures and to inhibit it in Le<1 mixtures. The high-intensity small-scale flows are shown to increase the quenching limit due to volumetric heat losses in Le<1 mixtures and decrease it in Le>1 mixtures

Modeling of 2D self-drifting flame-balls in Hele-Shaw cells

The disintegration of near limit flames propagating through the gap of Hele-Shaw cells has recently become a subject of active research. In this paper, the flamelets resulting from the disintegration of the continuous front are interpreted in terms of the Zeldovich flame-balls stabilized by volumetric heat losses. A complicated free-boundary problem for 2D self-drifting near circular flamelets is reduced to a 1D model. The 1D formulation is then utilized to obtain the locus of the flamelet velocity, radius, heat losses and Lewis numbers at which the self-drifting flamelet exists. Novelty and significance statementThe existence and limits of self-propagating 2D flame-balls have been substantiated theoretically for the first time. They have been investigated analytically, displaying the crucial role of heat losses. The regime arises in praxis in thin enclosures such as Hele-Shaw cells. The design of flame arresters may well be influenced by these findings.

On the radiation extinction of opposed flame spread over curved solid surface in low flow velocity conditions

The controlling mechanisms of the radiation extinction of the flame spreading over cylinders in low flow velocity conditions are investigated with an analytical approach. Attention is focused on the interaction among solid surface curvature, solid surface radiation, and gas-phase volumetric radiation heat loss in the flame spread process. The analysis uses a classical thermal model, which considers the heat balance on the unburned fuel surface near the flame front. A non-dimensional number that evaluates the effects of volumetric radiation heat loss and finite rate chemistry on the flame temperature is also proposed. It is found that solid surface radiation cannot be a dominant mechanism for the radiation extinction of a flame spreading over a thin cylinder. Furthermore, in the case of flame spread over a cylinder, flame spread rate significantly increases with decreasing opposed flow velocity due to curvature effect, thus the consideration of the relative gas flow velocity seen by the spreading flame becomes essential in low flow velocity conditions. However, when the volumetric radiation heat loss in the gas phase is taken into account in the thermal model, flame spread rate for a thin cylinder once increases with decreasing opposed flow velocity and then turns to decrease. As a result, the flame spread rate of a thin cylinder shows a peak value in low flow velocity conditions. This trend is in good agreement with previous microgravity experiments (Fujita et al., 2002 [1]). Moreover, the limit condition of flame spread appears as a bifurcation point where the stable and unstable solutions of flame spread rate coincide in the low flow velocity condition. The analytical results indicate that it is essential to consider the volumetric radiation heat loss in the gas phase to predict the flame spread over a curved surface in low flow velocity conditions.

Correlation of heat loss with quenching distance during transient flame-Wall interaction

Detailed numerical simulations of stoichiometric premixed methane/air flames have been conducted for a side-wall-quenching (SWQ) setup, in order to study the effect of transient motion of the flame on flame-wall interaction (FWI). A W-shaped plane-jet (2D) flame is considered, which is bounded in its lateral direction by walls. The left wall oscillates in its normal direction with pre-defined frequencies f and stroke lengths L (distance between the minimum and maximum wall locations), exciting the entire flow and resulting in an unsteady motion of the flame at the left and right wall. While the instantaneous flame contours near the moving and the stationary wall differ, the instantaneous quenching distance dq and wall heat flux q˙w at the walls yield a quasi-linear correlation in the case of low f and L. However, hysteresis loops with a weakened correlation for q˙w vs. dq have been detected for large f and L, which leads to a considerably increased time-averaged dq and decreased time-mean q˙w compared with results obtained from the steady-state solution. The reason is attributed to the delayed response of the flame to the rapidly varying temperature and flow fields caused by the transient motion of the flame with respect to the wall. In this case, the flame requires a relaxation time to perceive the heat flux to the wall and to adjust its dynamics. The results indicate an essential impact of the unsteady flow nature on FWI, which has to be considered for turbulent combustion modeling. A quasi-linear correlation between the volumetric heat loss rate caused by the wall and the time-mean dq has been proposed for a first-order estimate of the effect of FWI, which could be implemented into the energy balance as sink term for modeling turbulent flame propagation under the influence of a cold wall.

A thermomechanical finite element model and its comparison to inherent strain method for powder-bed fusion process

In this study, a thermomechanical model is developed to predict the melt-pool dimensions and residual stresses for laser powder bed fusion process. Inherent strain method is also used to predict residual stresses by using only the thermal solution that is free the complexities involved with the mechanical solution. A unique approach is developed to define the surface heat losses as volumetric heat losses in order to avoid the definition of traction-free surfaces and their re-definition after layer deposition. The thermal process simulations predict melt-pool dimensions of experimental cross-sections of single tracks within approximately 10% agreement. The thermomechanical process model is used to forecast the effect of process parameters on the melt-pool dimensions and residual stresses. The inherent strain method reproduces the residual stresses within 15% accuracy, approximately six times faster in comparison to the thermomechanical model, and free of any convergence issues related with the displacement field solution.

Lambert W function for effect of gas compression on premixed flames with ignition temperature approximation

The assumption that the gradient of temperature at the burned-side edge of a premixed flame front is zero has been usually imposed in investigating the adiabatic profile of flames for quasi-isobaric flow or in the zero-Mach-number limit. However, the non-monotonic behaviour of temperature is also observed for flames, or deflagrations, in the process of deflagration-to-detonation transition inside tubes or in the context of laser ablation or thermonuclear explosions in the universe. This non-adiabatic profile of a flame front is mainly due to the effect of gas compression, which is different from the conductive or volumetric heat loss. To clarify this point, we derive the non-zero condition of temperature gradient by considering the additional region, or the compression zone, behind a freely propagating flame front. The method of singular perturbations, or matched asymptotic expansions, is employed to calculate the inner and outer solutions of the compression zone of thickness with the squared Mach number assumed to be small, . The inner solutions are expressed by use of Lambert W function, and the pressure and dissipation terms in the heat-conduction equation play an important role to capture the non-adiabaticity of a flame front for non-zero values of Mach number. The strong ignition temperature approximation is imposed to obtain exact solutions in the outer region, composed of the reaction and preheat zones. The adiabatic value of ignition temperature is determined. The non-adiabatic feature is categorised into two cases. One case shows the non-monotonic behaviour of temperature. The other case presents the monotonic profile of temperature, but its maximum value is beyond the adiabatic value. The boundary, which determines whether the compressibility effect increases or decreases the flame temperature, is given with respect to the values of ignition temperature, heat release and Prandtl number.

Flame Balls in a non-uniform reactive mixture: preferential diffusion, heat-loss and stability

The paper examines the existence and stability of axisymmetric flame balls in a non-uniform reactive mixture corresponding to a mixing layer taking into account preferential diffusion and volumetric heat-loss. The mixture's non-uniformity is measured with a non-dimensional parameter ε which is inversely proportional to the square root of the Damköhler's number. The investigation is carried out analytically in the limit of large activation energy and small values of ε, and numerically in the general case. New simple formulas accounting for preferential diffusion are derived which determine in particular the thermal energy and location of the flame ball; these may be argued to represent the minimum ignition energy and optimum spark location for a successful forced ignition of the diffusion flame in the mixing layer. A new free boundary problem (FBP) with two dependent variables is derived which describes non-adiabatic flame balls subject to volumetric heat-loss from the burnt gas. For small ε, the analytical solution to the FBP shows that the main effect of weak non-uniformity can be understood in a simple way if the volume of the distorted flame ball is characterised by an equivalent radius which is plotted versus a heat-loss parameter κ. Specifically, the curve is the same inverse-C shaped curve found in the literature in the uniform case () but shifted to the left by an amount, proportional to , which explicitly accounts for all parameters. The numerical investigation addresses the existence of the axisymmetric flame balls and their stability within two models familiar in studies on flame balls in uniform mixtures, namely the ‘far-field losses model’ where heat-losses from the burnt and unburnt gas are accounted for, and the ‘near-field losses model’ adopted in our analytical investigation, where heat-loss from the unburnt gas is neglected. Typically four regions are determined in the κ-ε plane for fixed Lewis numbers which identify conditions for the existence of either the flame ball, the diffusion flame or of both. This subdivision is argued to provide useful insight regarding the possible modes of burning in the mixing layer. A particularly interesting type of solutions identified for moderate values of ε corresponds to ring-shaped flame balls, termed ‘flame rings’, in regions where the diffusion flame cannot exist. As for the stability of the flame balls, we have found these to be typically unstable, as expected for their spherical counterparts. However, we have also determined these to be stable in special circumstances requiring low Lewis numbers and the presence of heat-losses and depending on the non-uniformity parameter ε. Furthermore, an increase in ε was found to play a stabilising effect, at least for the cases considered.

Forced thermal ignition of a polydisperse fuel spray

ABSTRACTA new mathematical model is presented for determining critical conditions for forced thermal ignition of an arbitrary polydisperse fuel spray/oxidant mixture. An asymptotic analysis is used to ascertain both the necessary initial applied temperature as well as the time needed to accumulate enough fuel vapor through droplet evaporation in order that thermal runaway, indicating ignition, should occur. The influence of the combined effect of heat loss due to evaporation and volumetric heat loss on critical ignition conditions is accounted for. The quenching distance is analyzed and found to match the expression used in the literature. It is demonstrated that, for practical purposes, even when volumetric heat loss alone does not prevent ignition, the presence of the fuel droplets can do so. The impact of liquid fuel-related parameters on ignition conditions is discussed in detail.

Albanian Building Stock Typology and Energy Building Code in Progress Towards National Calculation Methodology of Performance on Heating and Cooling.

The expertise on building stock typology used openly available data from the Albanian statistical office. As the CENSUS was not especially designed for gathering data for the energetic evaluation of the building stock, some data were not available on the required level of detail. Estimations were necessary to extrapolate data to the existing stock. Technically, the study selected and described twenty representative categories of residential buildings typology for Albania. Were identified the level and the structure of final energy consumption at present and in the future by building age category, building type, climate zone, and energy end-use. Using an original template excel data sheet, were conducted the calculations of their thermal energy performance in three climate zones, designed standardized retrofit packages, calculated possible energy savings, and investment required by building type. The engineering principle of the Regulation in force, regarding to the legislative act of Energy Building Code in Albania, is beyond any doubt correct and carefully studied. The act is a rule book or the Regulation (energy building code - here The Code) which contains information that is sufficient to perform calculations of the different insulating layers for new construction after the year 2003. Also, the Regulation’s algorithms are still relevant in terms of calculation to provide Energy for heating demands in Albania. After 12 years, the Code remains the same and could provide either an optimal potential energy savings to the existing buildings, or an optimal cost-effective of building’s insulation without imposing a burden of high financial housing builders to multifamily prospective buyers. Based today Europe’s developments on Energy Performance of Buildings, the study is found relevant to provide a methodology for calculation of the energy performance in buildings (kWh/ m2a) based on volumetric coefficient heat losses (Gvt) for heating only, the existing indicator of the existing Code. The following study deals with the possibility of transposing the methodology used to the Code into an energy Performance based on minimum requirement for a new Regulation and/or EP Calculation Methodology based on efficient use of energy for heating and cooling purposes.

Effect of external heat loss on the propagation and quenching of flames in small heat-recirculating tubes

This study investigates the effect of external heat loss on the slow propagation of strongly-burning flames inside narrow heat-recirculating tubes. The system is studied using a two-dimensional numerical model for reactive flow, including conjugate heat transfer, over a range of tube diameters spanning the micro- and the meso-scale. Increasing external heat loss decreases the propagation speed of the slowly-propagating flames, leading to a transition from upstream to downstream propagation, until a heat loss limit is reached. Flames in micro-scale tubes are subjected to an extinction limit, while flames in larger diameter tubes, in the meso-scale range, are subjected to a blowout limit. While the absolute value of the external convective heat loss coefficient at the extinction/blowout limit increases with diameter, the dimensionless volumetric heat loss coefficient decreases with an increase in diameter until an approximately constant value, for the studied conditions, is reached at the blowout limit in the mesoscale range. Discrepancies in the predicted trends of 1-D and 2-D models indicate that 2-D effects play a significant role at larger diameters, not only at the meso-scale, but also in the upper-range of the micro-scale. These 2-D effects, associated with changes in flame shape that allow an increase in burning surface area, are seen to promote stability of the system. Results have implications on the choice of tube diameter to be used in the design of a stable burner optimized for heat transfer to an external heat load.

Modelling and experimental validation of moving tilted volumetric heat source in gas metal arc welding process

In this work, the effect of electrode’s tilt angle on transient temperature distribution, heat affected zone width and weld bead geometry in gas metal arc welding process are investigated. Experimental, microstructure and analytical modelling studies of heat affected zone and fusion zone have been performed for different electrode tilt angles. Gaussian heat density distribution and ellipsoidal heat source shape were assumed to predict the transient temperature distribution in the welded plate. The analytical model for the transient temperature distribution in the welded plate considers heat transfers from molten droplets of the filler material, moving volumetric heat source, and convective and radiative heat losses from the welded plate. Decent agreement between the predicted and the experimental temperature distribution, heat affected zone and weld bead geometry is obtained. The comparison suggested that ellipsoidal heat source shape is quite appropriate for predicting the transient temperature distribution on the welded plate for gas metal arc welding process. It was found that the heat affected zone width increases with the decrease in tilt angle. Microstructural examination on samples revealed prominent grain growth in the heat affected zone, however, fine grain structure was observed in the fusion zone. The predictions for heat affected zone width and weld bead geometry are also validated with experiments performed in shop floor welding conditions.

Sustainable New Brick and Thermo-Acoustic Insulation Panel from Mineralization of Stranded Driftwood Residues

There is considerable interest recently in by-products for application in green buildings. These materials are widely used as building envelope insulators or blocks. In this study, an experimental study was conducted to test stranded driftwood residues as raw material for possible thermo-acoustic insulation panel and environmentally sustainable brick. The thermal and acoustic characteristics of such a natural by-product were examined. Part of samples were mineralized by means of cement-based additive to reinforce the material and enhance its durability as well as fire resistance. Several mixtures with different sizes of ground wood chips and different quantities of cement were investigated. The thermo-acoustic in-lab characterization was aimed at investigating the thermal conductivity, thermal diffusivity, volumetric specific heat, and acoustic transmission loss. All samples were tested before and after mineralization. Results from this study indicate that it is possible to use stranded driftwood residues as building materials with competitive thermo-acoustic properties. In fact, the thermal conductivity was shown to be always around 0.07 W/mK in the unbound samples, and around double that value for the mineralized samples, which present a much higher volumetric specific heat (1.6 MJ/m3K) and transmission loss capability. The lignin powder showed a sort of intermediate behavior between the unbound and the mineralized samples.

On Laminar Rich Premixed Polydisperse Spray Flame Propagation with Heat Loss

A mathematical analysis of laminar premixed spray flame propagation with heat loss is presented. The analysis makes use of a distributed approximation of the Arrhenius exponential term in the reaction rate expression and leads to an implicit expression for the laminar burning velocity dependent on the spray-related parameters for the fuel, gas-related parameters and the intensity of the heat losses. It is shown that the initial droplet load, the value of the evaporation coefficient, and the initial size distribution are the spray-related parameters which exert an influence on the onset of extinction. The combination of these parameters governs the manner in which the spray heat loss is distributed spatially and it is this feature that is the main factor, when taken together with volumetric heat loss, which determines the spray’s impact on flame propagation and extinction.

N-Heptane/air combustion in perfectly stirred reactors: Dynamics, bifurcations and dominant reactions at critical conditions

The dynamics of n-heptane/air mixtures in perfectly stirred reactors (PSR) is investigated systematically using bifurcation and stability analysis and time integration. A skeletal mechanism of n-heptane constructed by entropy production analysis is employed, which is extensively validated for different conditions with respect to the ignition delay time, laminar flame speed, and the typical hysteretic behavior observed in PSRs. The significantly reduced size of the skeletal mechanism, enables the extension of the bifurcation analysis to multiple parameters. In addition to residence time, the effect of equivalence ratio, volumetric heat loss and the simultaneous variation of residence time and inlet temperature on the reactor state are investigated using one- and two-parameter continuations. Multiple ignition and extinction turning points leading to steady state multiplicity and oscillatory behavior of both the strongly burning and the cool flames are found, which can lead to oscillatory (dynamic) extinction. The two-parameter continuations revealed isolas and codimension-two bifurcations (cusp, Bogdanov–Takens, and double Hopf). Computational Singular Perturbation (CSP) and entropy production analysis were used to probe the complex kinetics at interesting points of the bifurcation diagrams.

Influence of radiation losses on the stability of premixed flames on a porous-plug burner

Analysis of the planar premixed flames on a porous plug was performed numerically for finite activation energy within the diffusive-thermal model. The paper is focused on the influence of radiation heat loses on the flame standoff distance and its linear stability. We show that the presence of volumetric heat losses limits the range of the mass flow range as well as it can promote the flame instabilities of different kinds, both oscillatory and cellular. The oscillatory instability, which for freely propagating flames can be usually observed for the Lewis number larger than one, in the porous-plug case occurs also for flames with unity and lower than unity Lewis number. For flames with Le<1 both cellular and oscillatory instabilities can be observed simultaneously in a certain range of the mass flow rate.

Strained premixed flames: Effect of heat-loss, preferential diffusion and reversibility of the reaction

We provide an analytical description of the effect of preferential diffusion and volumetric heat-loss on strained premixed flames within a reversible chemistry model. The model comprises a single reversible reaction of the form F⇌P whose forward and backward rates follow an Arrhenius law. An asymptotic analysis of the problem is carried out in the limit of infinitely large activation energy of the forward reaction. The study allows for non-unit Lewis numbers for the fuel F and the product P. Two main contributions are made. The first contribution consists in identifying the fundamental differences in the asymptotic description of the non-adiabatic flame between the reversible and irreversible cases, and resolving the difficulties by a suitable generalization of the so-called near-equidiffusion flame approximation to the reversible case, including the derivation of appropriate jump conditions at the reaction sheet. This is used to demonstrate that the one-step reversible chemistry model is reducible to the classical irreversible chemistry model, provided that an effective reduced Lewis number and an effective heat-loss parameter are used. The second contribution is the determination of the domain of the solutions and their multiplicity, for selected values of , in the -ε plane, where ε is a non-dimensional measure of the strain rate. For , the diagrams are found to consist of four regions with one, two, three, and four solutions respectively, including the frozen solution. For , an additional region with three solutions is identified. For larger values of , the two regions with three solutions merge, and the region with four solutions is of insignificant size. Our diagrams are found to be in good agreement with, and complementary to, numerically determined diagrams in flammability limit studies based on one-step and detailed chemistry models. They are also a valuable component in studying non-adiabatic premixed edge-flames.

Analysis of energy consumption patterns in multi-family housing in a moderate cold climate

This paper analyzes the energy consumption during the period 2001–2006 of 192 flats distributed in three-storey buildings, in order to understand how current policies related to energy use could be improved for increased residential energy efficiency in Argentina. The buildings (1, 2 and 3 bedrooms) are located in La Pampa, central Argentina, in a moderate cold climate. The dimensional and energy-consumption variables are studied (area, envelope's area, FAEP=envelope's area/floor area; envelope's thermal resistance R, volumetric heat loss G and auxiliary heating Q aux). The natural gas consumption is analyzed at annual and seasonal levels. Consumption variability among buildings, storeys and flats is calculated. The quantitative analysis is coupled to a qualitative description through direct observation of the buildings. The results show: (a) a high incidence of natural gas consumption in the total annual energy consumption (natural gas+electricity), (b) seasonality of natural gas consumption, with a maximum value in the cold period July–August (variability=80%), (c) little variability among buildings of the annual natural gas consumption (4.17%), (d) the lowest average energy consumption at the first floor, (e) high variability among flats on each storey, (f) winter consumption of a multi-family dwelling lower than a single-family dwelling of similar area and (g) little seasonal variability of the electricity consumption.

A multi-scale approach to the propagation of non-adiabatic premixed flames

Flame propagation involves physico-chemical processes that occur over a range of temporal and spatial scales. By use of a multi-scale analysis it is shown that diffusion processes occurring on relatively small scales can be resolved analytically when the overall activation energy of the chemical reactions is large, thus providing, by asymptotic matching, explicit conditions for the state of the gas and for the flow field across the flame zone. The mathematical formulation on the larger hydrodynamic scale reduces to a free-boundary problem, with the free surface being the flame front. The front propagates into the fresh unburned gas at a rate that depends on both the local strain that it experiences and the local curvature, with coefficients that depend on the diffusion rates of heat and mass, the equivalence ratio of the mixture and the chemical kinetic parameters. The simplified model, properly termed a hydrodynamic model, involves the solution of the Navier Stokes equations with different densities and viscosities for the burned and unburned gas. The present work extends earlier studies by including volumetric heat loss, such as radiative loss, which affects the dynamics and may lead to flame extinction.

Flame propagation in a small-scale parallel flow

We consider the propagation of laminar premixed flames in the presence of a parallel flow whose scale is smaller than the laminar flame thickness. The study addresses fundamental aspects with relevance to flame propagation in narrow channels, to the emerging micro-combustion technology, and to the understanding of the effect of small scales in a (turbulent) flow on the flame structure. In part, the study extends the results of a previous analytical study carried out in the thick flame asymptotic limit which has in particular addressed the validity of Damköhler's second hypothesis in the context of laminar steady parallel flows. Several new contributions are made here. Analytical contributions include the derivation of an explicit formula for the effective speed of a premixed flame U T in the presence of an oscillatory parallel flow whose scale ℓ (measured with the laminar flame thickness δ L ) is small and amplitude A (measured with the laminar flame speed U L ) is (1). The formula shows a quadratic dependence on both the amplitude and the scale of the flow. The validity of the formula is established analytically in two distinguished limits corresponding to (1) frequencies of oscillations (measured with the natural frequency of the flame U L /δ L ), and to higher frequencies of (A/ℓ) (the natural frequency of the flow). The analytical study yields partial support of Damköhler's second hypothesis in that it shows that the flame behaves as a planar flame (to leading order) with an increased propagation speed which depends on both the scale and amplitude of the velocity fluctuation. However our formula for U T contradicts the formula given by Damköhler in his original paper where U T has a square root dependence on the scale and amplitude. Numerical contributions include a significant set of two-dimensional calculations which determine the range of validity of the asymptotic findings. In particular, these account for volumetric heat loss and differential diffusion effects. Good agreement between the numerics and asymptotics is found in all cases, both for steady and oscillatory flows, at least in the expected range of validity of the asymptotics. The effect of the frequency of oscillation is also discussed. Additional related aspects such as the difference in the response of thin and thick flames to the combined effect of heat loss and fluid flow are also addressed. It is found for example that the sensitivity of thick flames to volumetric heat loss is negligibly affected by the parallel flow intensity, in marked contrast to the sensitivity of thin flames. Interestingly, and somewhat surprisingly, thin flames are found to be more resistant to heat loss when a flow is present, even for unit Lewis number; this ceases to be the case, however, when the Lewis number is large enough.

Performance assessment of low-energy buildings in central Argentina

This paper summarizes the results obtained from the energy and thermal performance assessment of residential and non-residential low-energy buildings that were designed to minimize fossil energy use. They are located in the province of La Pampa, central Argentina, in a temperate continental climate that shows extreme hot and cold records during the summer and winter seasons, respectively. The common applied technologies for saving energy were passive solar heating, natural ventilation for cooling and daylighting. The glazing area in the principal functional spaces facing to the North oscillates between 11 and 17% of the building useful areas. All the studied buildings are massive, with the exception of an auditorium that was designed with a lightweight insulated technology. The mean thermal transmittance of the envelope is 0.45 W/(m 2 K). Double glazing and hermetic carpentry were used to reduce thermal losses ( U-value = 2.8 W/(m 2 K)). The volumetric heat loss coefficient ( G-value) oscillates between 0.90 and 1.00 W/(m 3 K). During the design and thermal simulation convective-radiative heat transfer coefficients were estimated through a dimensional equation ( h = 5.7 + 3.8 ws, wind speed). On internal surfaces, convective-radiative heat transfer coefficients of 8 and 6 W/(m 2 °C) (for surfaces with and without solar gain, respectively) were applied. The monitoring process provided information on the energy and thermal behaviour under use and non-use conditions. The measured value of energy consumption was similar to the expected value that was used during the pre-design stage. Building technologies work well during the winter season, allowing 50–80% of energy savings. However, overheating is still an unresolved problem during the summer. Interviews with occupants revealed that they need both, information about functional details, and good-practice guidance to manage thermal issues of the building. In most cases, the annual consumption of energy was lower than those established by the Low Energy Housing German Standards and the Minirgie Switzerland Certificate. Despite their relative cost increase during the last years, the use of insulation technology and the application of passive solar devices involved an extra cost of only 3% in our works. Provided the expected depletion of natural gas production in the coming decade, the importance of applying energy-efficiency guidelines will increase very soon in Argentina in order to match the requirements of a new national energy matrix.