Wall Thermal Boundary Conditions Research Articles

The influence of geometry on the heat exchange potential of embedded planar retaining walls with seasonally-balanced thermal load

A number of operational cases exist where embedded retaining walls, used in the construction of underground spaces such as basements and shallow tunnels, have also been utilised as ground-heat exchangers in shallow geothermal energy systems. These are complex structures in terms of their geometry, the surrounding temperature field and boundary conditions, and there are currently no methods to assess their heat exchange capacity in a simple and expedient manner. This contribution uses the finite element method to validate the use of the method in predicting heat flow for this application and then, to assess the influence of wall and excavation geometry in the heat exchange process. The influence of the soil and wall thermal conductivity is shown to be quasi-linear with the latter showing the greatest influence on peak heat exchange. The work identifies a geometric parameter - the ratio of excavation depth to total wall panel depth, H/L which in combination with the wall thickness (D), provide a consistent and simple means by which the heat exchange potential can be estimated, for a given set of wall and soil thermal properties and boundary conditions.

Effect of thermal boundary condition and turbulent models on the combustion simulation of ethylene-fueled scramjet combustor

Wall thermal boundary conditions and turbulent models can affect flow and combustion simulations but are seldom considered in the turbulent modeling of supersonic combustors. This work investigated the effect of thermal boundary conditions and four turbulent models on turbulent combustion in a cavity-stabilized scramjet combustor. Results showed that the thermal boundary condition had a noticeable influence on the temperature fields. Changing the thermal boundary condition from zero gradient to a fixed lower temperature considerably reduced the maximum temperature but did not affect the temperature distribution. The fixed temperature boundary condition generated a slightly larger reaction heat release near the upper region of the cavity. However, the mass fraction of carbon dioxide was low for a fixed low temperature. The pressure increased near the rear of the cavity but decreased elsewhere at a fixed temperature. Reynolds-averaged models (k-epsilon, k-omega, and realizable k-epsilon) tend to over-predict the temperature and turbulent kinetic energy but under-predict the mass fraction of carbon dioxide. The detached Eddy simulation also under-predicts carbon dioxide but predicts a more accurate temperature.

Influence of Flow Configuration and Thermal Wall Boundary Conditions on Turbulence During Premixed Flame-Wall Interaction within Low Reynolds Number Boundary Layers

The influence of flow configuration on flame-wall interaction (FWI) of premixed flames within turbulent boundary layers has been investigated. Direct numerical simulations (DNS) of two different flow configurations for flames interacting with chemically inert isothermal and adiabatic walls in fully developed turbulent boundary layers have been performed. The first configuration is an oblique wall interaction (OWI) of a V-flame in a turbulent channel flow and the second configuration is a head-on interaction (HOI) of a planar flame in a turbulent boundary layer. These simulations are representative of stoichiometric methane-air mixture under atmospheric conditions and the non-reacting turbulence for these simulations corresponds to the friction velocity based Reynolds number of Re_{tau }=110. It is found that the mean wall shear stress, mean wall friction velocity and the mean velocity statistics are affected during FWI and the behaviour for these quantities varies under the different flow configurations as well as for the different thermal wall boundary conditions. The behaviour of the quenching distance and mean wall heat flux under isothermal wall conditions is found to be significantly different between the two flow configurations. The variation of the non-dimensional temperature in wall units for cases with isothermal walls suggests that the temperature in the log-layer region is significantly altered by the evolving wall heat flux in both flow configurations. Statistics of the mean Reynolds stresses and turbulence dissipation rate show that the flame significantly alters the behaviour of turbulence due to thermal expansion effects and flow configuration plays an important role.

Effect of wall temperature in streamwise supersonic corner flow

The interaction between corner flow and heat transfer is very common in the internal and external flow of hypersonic vehicle surface, but it is not clear how the wall thermal boundary condition affects the supersonic corner flow boundary layer. In the present study, the Reynolds stress model is used to solve the Reynolds-averaged Navier–Stokes equations. The results show that when the symmetrical wall temperature Tw is less than or equal to the recovery temperature Taw, the mean velocity profile in the corner zone is similar to the plate zone, and the temperature is higher in the corner zone. At heating wall condition of Tw>Taw, both velocity and temperature profile of outer corner boundary layer are different from that in the plate zone. The near-wall temperature distribution increases relatively, and the temperature profile is no longer similar to the law of the wall when Tw rises. Along the spanwise wall, the deviation of the velocity–temperature quadratic curve and the generalized Reynolds analogy (GRA) relation is within 10%. In the supersonic corner flow, the velocity–temperature relation under non-adiabatic wall condition satisfies the GRA relation. The effect of wall temperature on corner boundary layer velocity and temperature is significant. It is necessary to consider wall thermal boundary condition in the supersonic corner flow boundary layer analysis.

Wavelet-based adaptive large-eddy simulation of supersonic channel flow with different thermal boundary conditions

This work investigates the effect of different thermal wall boundary conditions on the wavelet-based adaptive large-eddy simulation of supersonic turbulent channel flow. The compressible flow governing equations are expressed in terms of wavelet-based Favre-filtered variables and are supplied with the anisotropic minimum dissipation closure model. Various computations are performed, where the resolved temperature field is constrained by either Dirichlet (isothermal) or Neumann (adiabatic) boundary conditions at the walls. The turbulence diagnostics include mean flow features and turbulent fluctuations statistics. The successful comparison with reference direct numerical simulations demonstrates the validity and the efficiency of the wavelet-based adaptive approach for wall-bounded turbulent compressible flow, regardless of the thermal boundary conditions that are imposed.

Numerical simulation of laminar premixed hydrogen-air flame/shock interaction in semi-closed channel

Highly refined simulations of flame/shock interactions (FSI) are performed and analyzed in the context of hydrogen/air combustion in a two-dimensional shock-tube configuration (height 7 cm). The chemical mechanism used for the hydrogen oxidation contains 9 reactive species, without nitrogen oxides, and 23 kinetic reactions. An initially planar laminar premixed flame (ϕ = 0.8, P=20 kPa) is left to evolve until the ratio between its burning flame velocity and the laminar flame speed reaches 1.45. Two thermal wall boundary conditions are envisaged: isothermal with Twall fixed to 300 K and adiabatic. The species transport is described with a unity Lewis number for all species or by complex transport. Once the flame is established, a shock is installed in the domain which propagates toward the flame. Two values of the Mach number for the incident shock are considered: Ms=1.4 and Ms=1.9. The relative impacts of the wall thermal condition, of the species transport modeling and of the incident shock Mach number on the FSI process are discussed. It is observed that thermal boundary conditions and transport modeling have a weak impact during the first stages of the FSI, i.e. the two successive interactions of the incoming and reflected shock with the flame, but a significant one once the reflected shock has crossed the flame front.

Influence of Gas-to-Wall Temperature Ratio on the Leakage Flow and Cooling Performance of a Turbine Squealer Tip

In high-pressure turbines, there is a large difference in temperature between the mainstream and the turbine blade surface. Most of the turbine blade tip heat transfer studies were conducted under the assumption that the Over-Tip-Leakage (OTL) flow field is independent of the wall thermal condition. Recent numerical and experimental studies have revealed that the two-way coupling effect between aerodynamics and heat transfer should not be neglected. The heat transfer coefficient obtained by the conventional method is not able to match the realistic engine condition accurately. This study investigates the impact of the wall thermal boundary condition on the tip cooling performance of squealer turbine blades. The RANS CFD result was validated against experimental tip heat transfer data obtained from a high-speed test rig with the effect of high-speed relative casing motion. The aerothermal performance for both uncooled and cooled squealer tips was studied at two different gas-to-wall temperature ratios, 1.7 and 1.1; the reference temperature is 204 K. It was found that the location and strength of cavity vortices varied with different wall thermal boundary conditions, leading to different signatures in tip heat transfer and cooling performance. It is recommended that the experimental heat transfer data and film cooling effectiveness obtained at the near-adiabatic wall boundary condition should be corrected before their application to the tip cooling design process. It would be more reliable to match the wall-to-gas temperature ratio during the tip experimental study.

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.

Prandtl number effects on the hydrodynamic stability of compressible boundary layers: flow–thermodynamics interactions

Hydrodynamic stability of compressible boundary layers is strongly influenced by the Mach number ( $M$ ), Prandtl number ( $Pr$ ) and thermal wall boundary condition. These effects manifest on the flow stability via the flow–thermodynamics interactions. Comprehensive understanding of stability flow physics is of fundamental interest and important for developing predictive tools and closure models for integrated transition-to-turbulence computations. The flow–thermodynamics interactions are examined using linear analysis and direct numerical simulations in the following parameter regime: $0.5 \leq M \leq 8$ ; and $0.5 \leq Pr \leq 1.3$ . For the adiabatic wall boundary condition, increasing Prandtl number has a destabilizing effect. In this work, we characterize the behaviour of production, pressure–strain correlation and pressure dilatation as functions of the Mach and Prandtl numbers. First and second instability modes exhibit similar stability trends but the underlying flow physics is shown to be diametrically opposite. The Prandtl number influence on instability is explicated in terms of the base flow profile with respect to the different perturbation mode shapes.

Experimental and computational fluid dynamics modeling of double‐partition divided‐wall column for flow hydrodynamics and wall heat transfer characteristics

AbstractThe aim of this study is to investigate the flow hydrodynamics and wall heat transfer characteristics on the tray of a two‐staggered walled region in a double‐partition divided‐wall column (DPDWC). An experimental setup of DPDWC is designed, and a theoretical temperature gradient (TG) model based on Fourier's equation, which describes the wall heat transfer, is proposed. A coupled computational fluid dynamics‐TG model is developed to analyze the heat transfer characteristics across partitions of DPDWC. The effects of liquid velocity, gas velocity, and different wall thermal boundary conditions on the flow and temperature fields are studied. The results indicate that the lack of a gas cone near partitions led to few vertical backflows of the liquid phase on the tray. Furthermore, the TGs near the liquid phase outlet increased with an increase in the liquid phase velocity. This work provides a scope for design and operation of DPDWC for industrial implementation.

Comparative appraisal of nanofluid flows in a vertical channel with constant wall temperatures: an application to the rocket engine nozzle

ABSTRACT This study aims to investigate the entropy generation of fully developed nanofluid flow in a spongy medium between two fixed absorbent vertical plates with a transpiration cooling mechanism. For heat transfer analysis exponential space-dependent heat source and thermal dependent heat source along with linear radiation are employed. Three distinct scenarios for thermal wall boundary conditions are investigated: i) Constant walls temperature ii) Convective boundary condition on the left wall of the channel and iii) convective boundary conditions on both walls of the channel. Two nanofluids combinations i.e. copper (cylindrical shape) nanoparticles immersed in water and silver (platelet shape) nanoparticles in water are used. It is also noted that studies on the transpiration of nanofluid in rocket engines for cooling purposes are scrutinized when walls are stretchy. But the walls of the rocket engine are fixed. Keeping this in mind, we analyzed the cooling process of the rocket nozzle with a fixed wall channel. The numerical solution of the model equations is attained using the bvp4c method in MATLAB. Findings indicate that the copper/water nanofluid combination performs better than silver/water as for as cooling the nozzle of a rocket engine is concerned. Moreover, convective boundary conditions lower the fluid temperature.

Enstrophy evolution during head-on wall interaction of premixed flames within turbulent boundary layers

The statistical behaviors of mean enstrophy and its evolution during head-on interaction of premixed flames propagating toward a chemically inert flat wall across the turbulent boundary layer have been analyzed using direct numerical simulations for a friction velocity-based Reynolds number of Reτ=110. The enstrophy dynamics have been analyzed for both isothermal and adiabatic thermal wall boundary conditions. The contributions of vortex-stretching and viscous dissipation are found to be leading order source and sink, respectively, to the mean enstrophy transport in both non-reacting and reacting flows irrespective of the wall boundary condition. However, the contributions due to dilatation rate and baroclinic torque play important roles in addition to the leading order contributions of the vortex-stretching and viscous dissipation terms in the enstrophy transport in turbulent premixed flames. The thermal boundary condition has been demonstrated to affect the near-wall behavior of the enstrophy transport contribution due to dilatation rate, which also affects the near-wall distribution of the enstrophy. The magnitudes of the leading order contributors to the enstrophy transport decrease with the progress of head-on interaction for both wall boundary conditions. Moreover, the overall sink contributions to the enstrophy transport dominate over the source contributions, giving rise to a drop in the mean enstrophy with the progress of head-on interaction. The enstrophy distribution changes significantly during flame-wall interaction, which gives rise to a modification of the relative proportion of the coherent structures in the reacting flow turbulent boundary layer compared to the corresponding non-reacting flow features.

Influence of thermal wall boundary condition on scalar statistics during flame-wall interaction of premixed combustion in turbulent boundary layers

The influence of thermal wall boundary conditions on the behaviour of scalar statistics during flame-wall interaction (FWI) of premixed flames within turbulent boundary layers has been investigated. Three-dimensional direct numerical simulations (DNS) of two different flow configurations for flames interacting with chemically inert isothermal as well as adiabatic walls in fully developed turbulent boundary layers have been performed. The first configuration is an oblique wall interaction (OWI) of a V-flame in a turbulent channel flow and the second configuration is representative of head-on interaction (HOI) of a planar flame in a turbulent boundary layer. The turbulence in the non-reacting conditions for these simulations is representative of a friction velocity based Reynolds number of Reτ=110 while the combustion characteristics are representative of stoichiometric methane–air mixture under atmospheric conditions. Differences in the mean behaviours of the progress variable and non-dimensional temperature have been observed for the two configurations in response to different wall boundary conditions. Mean scalar variance of the progress variable and temperature and their respective scalar dissipation rates have been investigated and it is found that the variances of the progress variable and temperature remain coupled in the case of adiabatic walls, while significant differences exist between these quantities in the vicinity of the wall under isothermal wall conditions for both flow configurations. The mean scalar dissipation rates of progress variable and temperature during FWI decrease towards the wall in both flow configurations under different thermal wall boundary conditions, but in the case of isothermal wall conditions a higher scalar dissipation rate for the non-dimensional temperature is observed when compared with the scalar dissipation rate for the progress variable. The behaviours of the mean turbulent scalar fluxes of progress variable and the non-dimensional temperature suggest that the scalar fluxes can show both gradient and countergradient type behaviour depending on the flow configuration and also on the proximity of the flame to the wall. In the case of V-flame OWI, the choice of the wall boundary condition has a significant influence on the behaviour of the scalar fluxes, whereas in the case of turbulent boundary layer HOI, no significant differences in the scalar fluxes are observed for different wall boundary conditions.

Impact of wall heat transfer in Large Eddy Simulation of flame dynamics in a swirled combustion chamber

Large Eddy Simulation (LES) is a fundamental research tool to study gas turbines and aero-engine combustors. In LES, although rarely addressed systematically, it is known that thermal boundary conditions control the heat transfer between the flow and the combustor walls. This work presents a study on the impact of thermal wall boundary conditions for the PRECCINSTA test bench, operated by the German Space Agency (DLR). Two approaches are tested: Heat Resistances Tuning (HRT), where a local resistance is tuned using experimental temperature data, and full Conjugate Heat Transfer (CHT), where the chamber wall-temperature is solved and coupled to the flow computation. Results reveal that the HRT method captures the mean flame correctly but the predicted flame becomes unstable and responds to a thermoacoustic oscillation which is not observed experimentally. On the contrary, using CHT, the flame is correctly predicted and stable as in the experiments. Finally, to understand the differences between the HRT and the CHT simulations, Dynamic Mode Decomposition (DMD) analysis is performed showing that the correct response of the flame branches to the pressure oscillations is recovered only in the CHT simulations for which thermoacoustically stable operations are retrieved.

The Effect of Wall Thermal Boundary Condition on Natural Convective Shutdown Cooling in a Gas Turbine

Abstract As a civil gas turbine cools down, asymmetric natural convective heat transfer causes the bottom sector of the rotor to cool faster than the top; this circumferential thermal gradient can potentially cause the shaft to deflect – a phenomenon called thermal or “rotor” bow. Rotor bow is tremendously difficult to predict due to its dependence on a number of engine design parameters, in addition to the complex nature of natural convective flows. A novel experimental facility has been developed to gain further understanding into shutdown cooling of a gas turbine. The scope of this paper is to quantify the effect of basic design features on natural convective cooling in an engine annulus during shut-down; specifically the influence of the thermal boundary wall conditions and the annular diameter ratio. In addition to this, a low-cost, robust thermocouple probe has been developed and validated, which allows for accurate temperature measurements in a natural convective boundary layer. An extensive experimental campaign has been completed. The key finding is that the local radial wall temperature difference was found to be the most influential parameter on the local heat transfer. Non-isothermal walls did not alter the overall distribution of the inner wall equivalent conductivity. This was true for both cylindrical and conical sections. An appropriate characteristic length for use in the Rayleigh number definition for both the concentric cylinder and conical sections have been validated. The conical inner section (5 deg hade angle) did not affect the overall heat transfer in the range of conditions tested. Therefore, the mean surface heat transfer for non-isothermal inner and outer profiles, within the range −0.4<ΔRa/RaLc<0.4, where the thermal gradient is negative in the clockwise from top-dead-center, can be predicted using isothermal correlations for RaLc<5.0×105 and Dr < = 1.5.

Effect of Mach number and plate thickness on the flow field and heat transfer characteristics of supersonic turbulent flow over a flat plate at different thermal boundary conditions

Conjugate heat transfer analysis of supersonic turbulent flow over a finite thickness flat plate is studied numerically. The flow field is modeled by employing the Favre averaged Navier–Stokes (FANS) equations. Turbulence is modeled with the Menter’s shear-stress-transport (SST) k−ω model. The comparison of NCHT (non-conjugate heat transfer) and CHT (conjugate heat transfer) analysis shows that the interface temperature and fluid temperature variation for the CHT analysis are lower compared to the NCHT analysis. The results indicate that the interface temperature is significantly increased for the low thermal conductivity plate material, due to the reduced rate of heat transfer. The effect of plate thickness on the flow field temperature variation is negligible. However, it is observed that the maximum interface temperature Ti,max increases with the increase in plate thickness. The values of maximum interface temperature difference between the 5 mm and 2 mm thick Steel-1006 plates at Mach numbers 2, 4, and 5 in non-dimensional (non-dimensionalized with free-stream temperature) form are 0.05594, 0.129, and 0.1816 respectively for the isothermal boundary condition. Results indicate that for Mach 2, the heated wall thermal boundary conditions acts like heat source, leading to the higher interface temperature, whereas for Mach 4, and 5 the heated wall thermal boundary conditions acts like heat sink, thus lowering the interface temperature. The results show that for NCHT analysis the leading edge shock strength, and mean pressure along the wall increases with an increase in Mach number. The non-conjugate results (flow and heat transfer characteristics) of the present numerical study is validated with the direct numerical simulation (DNS) results available in the literature.

Steady natural convection Couette flow with wall conduction and thermal boundary condition of third kind

AbstractThis study theoretically investigates the effects of viscous dissipation and boundary plate thickness on an incompressible heat generating/absorbing fluid with non‐uniform internal temperature unlike lumped heat capacitance assumption. The flow, which is laminar is induced by free convection caused by asymmetric heating of the channel boundaries and internally generated energy caused by viscous dissipation. In addition, convection through the boundary plates is also considered in the flow formation. One of the plates channel moves along the flow direction while the other is stationary. Due to the non‐linear and coupling nature of the governing flow equations, homotopy perturbation method (HPM) has been adopted to analytically find the approximate solution to the problem. The effects of thermodynamic and hydrodynamic parameters are depicted in graphs and tables. It is discovered from the investigation that, both the velocity and temperature profiles increase with increase in viscous dissipation. Velocity distribution decreases with increase in Biot number while the temperature distribution near the heated plate increases with increase in Biot number. The increase in boundary plate thickness d causes a boost in the fluid flow across the medium and reduces the temperature of the fluid near the heated plate. It is further discovered that the rate of heat transfer on both plates increase with increase in Biot number while they drastically dropped when the boundary plate thickness d is increased. The shear stresses on the surface of both plates increase with increase in heat generation while reverse cases were observed with increase in heat absorption . It is further observed that the volume flow rate within the channel increases with increase in viscous dissipation .

Comparison of urban airflow between solar-induced thermal wall and uniform wall temperature boundary conditions by coupling CitySim and CFD

Previous studies investigated the urban wind airflow under the isothermal condition, solar-induced thermal wall and uniform wall temperature boundary conditions. The differences in spatially-averaged airflow properties among those three boundary conditions have not been investigated yet. This study investigated different scenarios to identify: (1) whether the solar-induced non-uniform wall temperatures could be replaced by the uniform wall temperatures (average of all wall temperatures from solar-induced scenario) in the analysis of urban wind airflow and (2) the wind conditions that the wall thermal boundary conditions might be neglected. In this study, urban energy model, CitySim, was employed to calculate the solar-induced walls’ temperatures of an idealized 5 × 5 building array, which were set as thermal boundary conditions in computational fluid dynamic (CFD) simulations. The simulation results were compared to those obtained from cases with uniform wall temperatures and isothermal conditions. The results showed that excluding wall thermal boundary conditions underestimated the spatially-averaged flow properties when reference wind speed (Uref) was lower than 2.0 m s−1 under the designed scenarios and noticeable difference of airflow pattern was observed between isothermal and non-isothermal conditions. The solar-induced wall temperature condition might be replaced by uniform wall temperature condition with Uref of 3.0 m s−1 under parallel wind condition, and with Uref of 5.0 m s−1 under oblique wind conditions. Meanwhile, the airflow structures of the street canyons inside the three-dimensional building array differed significantly from the previous results obtained from isolated street canyon model.

Prechamber ignition: An exploratory 2-D DNS study of the effects of initial temperature and main chamber composition

A numerical investigation was conducted to study ignition and the initial phases of methane combustion in a two-dimensional setup consisting of a pre- (PC) and main (MC) chamber. The effect of the wall thermal boundary condition (isothermal Tw=500 K or adiabatic), initial mixture temperature (Tu=300 and 800 K) and equivalence ratio in the main chamber (ϕMC=0.5 and 1.0) was studied. A stoichiometric mixture was used in the PC and the mixtures were initially quiescent in all cases. Flame propagation in the prechamber is affected mainly by the initial temperature, while the thermal state of the wall plays a minor role. The transient jet that is generated at the exit of the orifice connecting the two chambers creates an intense flow field with vortical structures that, depending on the initial temperature, persist for a long time or dissipate quickly affecting combustion in the main chamber. Depending on the local flow and mixing conditions close to the orifice exit, the hot jet can be broken into small kernels at low Tu or forms quickly a flame torch at hot conditions, strongly affecting ignition of the MC mixture and flame propagation in the main chamber. In the lean MC cases, the intense mixing with the stoichiometric PC mixture creates local compositions that are more favorable for ignition by the hot turbulent reactive jet that subsequently exits from the PC at a temperature that is significantly lower than the adiabatic flame temperature of the corresponding mixture. Despite the short residence time, the reactive state of the mixture is affected as it flows through the nozzle. The flame structures in the MC are described in terms of the progress variable and mixture fraction and compared to flamelet-type calculations. The local flame structure differs strongly from that of the 1-D unstrained premixed flame, particularly for the low Tu cases. The flamelet-type calculations show that ignition of the most reactive mixture is enhanced by the radicals in the hot reactive jet, while scalar dissipation rate accelerates the ignition of the whole mixture. The 2-D simulations show that ignition is significantly longer than what is predicted by the flamelet calculations.

Modelling of thermal wall boundary conditions with temperature-dependent material properties for use in RANS

The present work extends a recently proposed P-function based model for describing the near-wall variation of temperature in forced convective turbulent flow to the case with temperature-dependent material properties. The extension essentially modifies the model formulations for describing the local variation of the turbulent mixing length and the turbulent Prandtl number. Direct Numerical Simulations (DNS) and experimental measurements are carried to provide comprehensive validation data for a wide range of Reynolds numbers, considering molecular Prandtl numbers well beyond unity. The observed good agreement of the predictions with the DNS data and experiments proves the present extended model as a well-suited approach for prescribing reliable thermal boundary conditions in Reynolds Averaged Navier-Stokes (RANS) simulations, assuming temperature-dependent material properties.