D. Lentini

Radiation Modelling in Non-Luminous Nonpremixed Turbulent Flames

A finite-rate chemistry model for nonpremixed turbulent combustion, based on the stretched laminar flamelet (slf) approach, is extended to account for radiative heat transfer in non-luminous flames, in the optically thin limit. The model is applied to a methane/air flame, and results in greatly improved predictions of mean temperature, and of its probability density function. Mean carbon monoxide concentration predictions are instead shown to be somewhat deteriorated. The model is further tested to reproduce radiative heat flux levels; solutions obtained by determining the average heat flux divergence by full convolution with the pdT, and by the simpler but less fundamentally correct ‘mean-property’ method, are compared, and show a significant gap. A formulation to deal with non-unity Lewis number flames is proposed; the model holds potential for further extension.

Assessment of the stretched laminar flamelet approach for nonpremixed turbulent combustion

Abstract An assessment is sought of the stretched laminar flamelet approach, in particular as far as its capability to account for finite-rate chemistry effects in nonpremixed turbulent combustion is concerned. It is used here with the k - e - 9 turbulence model, in order to obtain a computational model which can easily be implemented on current codes. A particularly convenient form is adopted, which limits the computational overhead over nonreacting computations to a minimum. An appropriate plane is identified to check the combustion regime in the different regions of the flame. Test cases are reported which involve quantities most directly affected by finite-rate chemistry, namely mean and variance of OH, CO and NO concentration in different flames. Numerical predictions worked out with the present model are compared to experimental results and predictions by other authors. Results confirm the potentialities of the proposed approach, which in addition can beextended to include insofar neglected effects....

Stretched laminar flamelet modeling of turbulent chloromethane-air nonpremixed jet flames

Abstract An experimental and numerical investigation of a nonpremixed turbulent flame burning chloromethane (CH3Cl) in air is presented. Finite-rate chemistry plays an important role in halogenated flames due to the inhibitory effect of halogens on hydrocarbon combustion. The objective of the study is to assess the applicability of the stretched laminar flamelet (slf) model that accounts for finite-rate chemistry and differential diffusion effects. The slf approach is a convenient tool to incorporate these effects into computations in view of its largely lenient assumptions, and its ease of inclusion into already existing codes based on the popular k-ϵ-g turbulence modeling. In the experimental set-up, a flame is established at Re = 11700. Velocity measurements are made using laser doppler velocimetry, species measurements by means of gas chromatography, and temperature measurements by thermocouples. A library of computationally obtained laminar flamelet profiles is used in the slf calculations. For sake of comparison, a single flamelet profile is also determined experimentally; notwithstanding the detailed chemical description adopted in the computational model, significant discrepancies are evident, possibly indicative of the weight of neglected effects (sooting, radiation, etc.). The applicability of the slf approach to the turbulent flame is checked by comparing the characteristic time of energetically significant reactions to the characteristic turbulent time scales, and the results show that the flame under study operates in the flamelet regime. Predictions for the turbulent flame indicate that the slf model gives improvement for predictions of the velocity, temperature, and concentrations of reactive species, as well as of the conserved scalar (which is affected by finite-rate chemistry through the effect of flame extinction on the mixture density), with respect to a near-equilibrium model. In particular, the location of temperature and concentration peaks are closely reproduced. An improvement is obtained regarding predictions of the concentrations of the important species CO, HCl, CH3Cl, O2, and N2. At any rate, due to remaining discrepancies, further investigation is called for to include insofar neglected effects.

The influence of real-gas thermodynamics on simulations of freely propagating flames in methane/oxygen/inert mixtures

Abstract The modeling of high-pressure combustion presents a continuing challenge in the context of chemical kinetics and mixture transport properties. This is mainly due to a lack of experimental data at high pressures. Consequently, there is limited confidence in the accuracy of simulations above moderate pressures. In this paper real gas thermodynamics is included in reacting flow simulations, e.g., by adopting the Redlich–Kwong equation of state, modifying the expression of the equilibrium reaction rate constants that are used to determine reverse reaction rates in a detailed reaction mechanism, and considering high-pressure effects on the enthalpy and specific heat. These real gas effects are characterized through simulations of freely propagating flames in methane/oxygen/inert mixtures at pressures up to 150 atm. Our results indicate that the laminar flame speed decreases more rapidly with increasing pressure when the real gas formulation is employed than for an ideal gas, particularly for pressures greater than 40 atm. The differences can be ascribed to high-pressure effects on the mixture specific heat. The incorporation of real gas thermodynamics improves the agreement of predictions with measurements.

Fast numerical solver for transonic flows

Abstract A fast numerical solver of the Euler equations is presented. The method is based on the Lambda formulation and is an improved version of the scheme proposed by Moretti [1, 2], further revised by Moretti—Onofri [3] and Dadone—Moretti [4]. Results for internal flows are presented, obtained with both orthogonal and non-orthogonal grids. The solutions show second order accuracy and high rate of convergence.

Convective and Radiative Wall Heat Transfer in Liquid Rocket Thrust Chambers

The relative weight of convective and radiative wall heat transfer in liquid rocket engine thrust chambers is estimated by means of dedicated computational fluid dynamics tools. In particular, alth...

Opportunities for a Liquid Rocket Feed System Based on Electric Pumps

A feed system for liquid-propellant rocket engines based on electric pumps powered by batteries is proposed. It is proven to stand as a viable alternative to the pressure-gas feed system. The dependence of the feed system mass on the different operating parameters is obtained so as to identify the conditions favoring its adoption, that is, a relatively long burning time and a fairly high chamber pressure. Under such conditions, the proposed system is shown to offer significant mass savings with respect to the pressure-gas system when advanced batteries are used. This advantage is further enhanced by the beneficial effect of chamber pressure on the engine effective exhaust velocity. A test case for a low Earth orbit to geostationary equatorial orbit transfer is also presented to identify the optimum value of the burning time, deriving from the competition between the feed system mass and the effect of gravitational losses.

Identification of chemical and vibrational relaxation regimes in rocket nozzle flow via a quasi-linear formulation

Abstract A quasi-linear formulation is proposed for high-speed finite-rate chemically reacting mixtures of imperfect gases, i.e. thermally perfect gases with specific heat varying with temperature, as an extension of a previously developed formulation for perfect gases. The form is suitable for application of accurate and fast algorithms. In particular, the resulting equations keep the same formalism already derived for reacting mixtures of perfect gases, thus indicating the potential for a straightforward extension of existing computational algorithms. In order to assess the applicability of the approach, the assumption of vibrational equilibrium needs to be verified. Accordingly, vibrational, as well as chemical, relaxation regimes are checked in a high expansion ratio rocket nozzle, indicating that the assumption under consideration is fully warranted. The effect of nozzle size on engine performance is also predicted.

Electric Feed Systems for Liquid-Propellant Rockets

Liquid-propellant rocket feed systems based on electric pumps are compared with the more classical pressure–gas and turbopump systems. The design parameters entering in the definition of the system mass are highlighted, and a careful choice of the figures of merit is performed, in particular for the electric motors and batteries. Indeed, recent developments, taking into account new electric motors based on rare earth permanent magnets (neodymium–iron–boron), and different lithium-based cells, show that the specific mass of the electric-pump system can be reduced to such an extent to make the proposed system competitive not only with the pressure–gas system but also with the turbopump one, at least for some applications such as small launchers and upper stage rockets. Further, electric motor and battery cell technologies currently under development could extend the proposed feed system convenience. Critical points related to electric-pump systems are also discussed.

A COMPUTATIONAL ALGORITHM FOR SECOND-MOMENT CLOSURE TURBULENCE MODELLING

A computational formulation is proposed for second-moment closure turbulence models, especially suited to models intended to ensure physical realizability. It enables to cast the quite complicated model equations in a compact form. It is specifically applied here to a two-dimensional parabolized flow, though it lends itself to extension to more complex flows. An effective computational algorithm is proposed, based on a staggered grid and a block tridiagonal solver. The algorithm is applied to a turbulent mixing layer, and the comparison between the predictions obtained by standard modelling tools and a realizable second-moment closure clearly points out the superiority of the latter.