RICERCA ITALIANA     

THERMAL AND FLUID DYNAMICS ANALYSIS OF JETS AND FLAMES WITH FLUID/STRUCTURE INTERACTION AND ACOUSTIC PHENOMENA.

Università degli Studi di Roma "Tor Vergata"

Abstract

The present research is aimed to investigate some thermal and fluid dynamics phenomena of gas jets (without chemical reaction) and flames which can be split into three subjects:
- Thermal and fluid dynamics of the interaction jet/transversal flow.
- Thermal and fluid dynamics of the fluid/structure interaction.
- Thermal and fluid dynamics of flames/jets with associated acoustic phenomena.

Each subject can be split in topics which will be treated by the research unit according to the following scheme.

Thermal and fluid dynamics of the interaction jet/transversal flow.

TOR VERGATA
This topic investigates the fluid dynamics, with anemometric measurements and flow visualizations, of a jet of air which enters into a transversal flow of air. The goal is to study the effect of the thermal and fluid dynamics conditions of the jet onto its evolution inside the transversal flow.

Thermal and fluid dynamics of the fluid/structure interaction.

TOR VERGATA
This topic is related to the numerical investigation, with commercial codes, of the fluid flow and heat transfer inside thermal protections used in the nozzles of space vehicles and in the protective surfaces of spacecrafts during the re-entry into the Earth atmosphere.

POTENZA-RUOCCO
This topic is aimed to study numerically and experimentally the thermal and fluid dynamics interaction between a jet of a given shape and a material of biotechnogical interest, where the joint phenomena of heat and mass transfer are investigated simultaneously.

Thermal and fluid dynamics of flames/jets with associated acoustic phenomena.

TOR VERGATA
This topic is aimed to investigate diffusive or premixed flames, rich in methane, and is focused on two aspects. The first one is the flow structure of a flame originating from a duct where methane is entering into stagnant air or a mixture of methane and air entering into stagnant air. The second aspect is an acoustic experiment at variable Reynolds numbers.

POTENZA-NINO
This topic is aimed to investigate the fluid dynamics characteristics, i.e. velocity and turbulence, of a coaxial burner interacting with a synthetic jet driven by an acoustic signal with several frequencies and amplitudes.

PISA
This topic is relative to a technique to enhance the convective heat transfer coefficient driven by the destabilization of the fluid dynamics with acoustic waves. <<<

Principal Investigator

Fabio Gori Università degli Studi di ROMA "Tor Vergata"

Research Objectives

The goal of the present research is to study some thermal and fluid dynamics phenomena of gas jets (without chemical reaction) and flames which can be split in three subjects:
- Thermal and fluid dynamics of the interaction jet/transversal flow.
- Thermal and fluid dynamics of the fluid/structure interaction.
- Thermal and fluid dynamics of flames/jets with associated acoustic phenomena.

The goals of each subject can be split into the goal of the topic which is treated by the research unit according to the following scheme.

Thermal and fluid dynamics of the interaction jet/transversal flow.

TOR VERGATA
The goal of this topic is to study the effect of the thermal and fluid dynamics conditions, including the presence of the undisturbed region, of an air jet onto its thermal and fluid dynamic evolution inside a transversal flow of air.

Thermal and fluid dynamics of the fluid/structure interaction.

TOR VERGATA
The goal of this topic is the numerical investigation, with commercial codes, of the flow inside nozzles with thermal protections, on protective surfaces of spacecrafts during the re-entry into the Earth atmosphere, of jets impinging thermal protections and the heat transfer in the surface itself.

POTENZA-RUOCCO
The objective of this topic is to study heat/mass transfer features of gaseous jet impingement on solid/porous substrate, typical of the process industries such as for biotechnology and food applications, especially in characterizing the local effects, that is, the multi-dimensional distribution of temperature and concentration on modified surfaces of various shapes (protrusions), even when multi-physics effects are present, as during the exposition to electromagnetic fields.

Thermal and fluid dynamics of flames/jets with associated acoustic phenomena.

TOR VERGATA
The goal of this topic is to investigate diffusive or premixed flames, rich in methane, to determine the position of the anchor point of the flame, the length of the flame, the flow structure with the variation of the Reynolds number, and to study the relation between acoustics and fluid dynamics.

POTENZA-NINO
The present topic is aimed to study experimentally gaseous flows, both isothermal and reactive, combined with synthetic jets used as flame stabilizers, to understand the mechanisms regulating the interaction between two coaxial jets. The first jet is obtained with a premixed flow of air and fuel, close to the flammable limit, and the second one is obtained with the activation of a synthetic jet.

PISA
The goal of this topic is the enhancement of the heat transfer coefficient obtained with the destabilization of the fluid dynamics with acoustic waves, outside the audible range, also as a function of frequency and intensity. <<<

Timescale

24 months

National and international background

The scientific background of the present research is presented on several thermal and fluid dynamics phenomena of gas jets (without chemical reaction) and flames which can be split in three subjects:
- Thermal and fluid dynamics of the interaction jet/transversal flow.
- Thermal and fluid dynamics of the fluid/structure interaction.
- Thermal and fluid dynamics of flames/jets with associated acoustic phenomena.

The scientific background of each subject can be split into the topic which is treated by the research unit according to the following scheme.

Thermal and fluid dynamics of the interaction jet/transversal flow.

TOR VERGATA
In the cooling of surfaces with jets, i.e. transpiration or film cooling problem, the fluid jet coming out from a surface is used to protect the surface itself from the heating due to the hot fluid flowing on it.
The thermal and fluid dynamics interaction between the transversal flow and the incoming jet has been investigated for several years. An important state of art of the literature is reported in [1] where the main aspects of the problem are presented and discussed. The fluid dynamics of a single jet is reviewed in [2] where the results of the literature [3-12] are critically analysed. The rows of holes have been investigated in [13-22] while the applications to the cooling of turbine blades are examined in [22-33].

Thermal and fluid dynamics of the fluid/structure interaction.

TOR VERGATA
In the degradation of thermal protections the materials involved produce gas and char. There is a distinct region which separates the virgin material from the char. This region propagates with time inside the thermal protection while the gas diffuses throughout the char.
Boully [34] used three equations of conservation: energy, mass and the kinetics of chemical reactions. Torre [35-36] made the characterization of an ablative composite to be used as thermal protection. The work [37] employed these techniques in order to obtain the main parameters to be used in the simulation of space vehicles in the re-entry into the Earth atmosphere. Composite ceramic materials have been proposed in [38]. The plasma wind tunnel [39] has been used to qualify and test thermal protections employed in the last re-entry phase of some spacecrafts of important space missions like Huygens. Some NASA researchers [40] developed a series of mathematical equations to analyse the penetration into thermal protections employed in the Space Shuttle Orbiter and other spacecrafts with tiles of ceramics with low density, carbon-carbon, flexible ceramic insulations and composites. EADS Space Transportation Company [41] is engaged in space explorations, especially Mars, with the responsibility of Mars Premier Netlander Thermal Protection Systems. For the front-shield of the spacecraft the thermal protections are formed of composite materials based on phenyl resins. Torre [42] presented an ablative material made with a polymer (methyl phenyl) with microspheres, phenolic, quartz and inorganic. The Institute of Space and Astronautical Science is conducting two missions (MUSES-C e DASH) where the re-entry of small capsules are forecast in 2007 directly from the interplanetary orbit at the velocity of 12 km/s. The thermal shield of the capsules works as ablative material and structural component. The external wall of the thermal protection, carbon-phenyl, is resistant mainly to the stresses, and is developed with a new industrial system to avoid counter effects due to the high aerodynamic heating. The capsules are designed to be ventilated during the re-entry [43]. Borisov [44] considered the heat transfer by radiation close to the nose of the space capsules.

RUOCCO POTENZA
Jet impingement is claimed to have very favourable heat and mass transfer characteristics, and can be used to introduce innovation in many processes of biotechnology and food industry, as it can be applied to thermal control and material conditioning and process. This technique consists in directing a jet flow from a nozzle of a given configuration to a target surface. For planar configurations (a jet impinging on an indefinite plate), or when an extruded blunt object is subject, a large heat and mass transfer coefficient can be obtained in the vicinity of the stagnation region upon impact, deflection and acceleration of the flow along the surface. The fluid flows, even in the lateral regions to stagnation, due to the impact, may be important on extrusions or for multiple jets configurations. Either circular or planar nozzle geometry are usually employed, the choice being dictated by the fact that the slot jet provides a larger impingement zone, while the circular nozzle insures a more localised high transfer rate.
In many industrial applications the heat transfer is often coupled to mass transfer, that is, the transport of species even when physical transformation (phase change) or/and chemical reaction has to be taken into account. The first and last available review on jet impingement can be found in [45,46]. The heat transfer to plates has been taken into consideration in the open literature as in [47-49], but industrial processes that present simultaneously the aforementioned physical and chemical transformations still need to be described in depth as in [50], relative to the evaporation from a moist saturated substrate. To this end, an adequate design and verification of convection and mass transfer processing can be carried out with computational thermal and fluid dynamics (TCFD) analysis (whereas commercial packages sometime need to be integrated to account for complex simultaneous energy and mass transfer), specially by means of conjugated experimental activities.

Thermal and fluid dynamics of flames/jets with associated acoustic phenomena.

TOR VERGATA
Several lines of research are present in this field, as analysis of efficiency in engines [51], combustion of coal [52], flame stability and flow configuration [53-56], acoustic phenomena [57-58]. Two reference points are [59-60]. An experimental analysis of the performances and the efficiency of an i.c. engine using hydrogen is carried on in [51]. An experimental analysis of the chemical and granular composition of the solid residuals in the coal combustion (ashes) is carried on in [52]. An experimental analysis of the flow structure of a detached flame, partially premixed of methane and air, in the point of stability is done in [53-54]. A DNS numerical simulation of a turbulent diffusive flame of hydrogen and air, where is evidenced the formation of islands of diffusion of the hydrogen in a certain region of the flame is done in [55]. A numerical-empirical investigation on detached flames, partially premixed of methane and air, in gravity and microgravity conditions is done in [56]. An experimental analysis of the sound generation in a turbulent flame is done in [57] while an analysis of the coupling between combustion and noise is done in [58].

NINO POTENZA
Modern design of low emission combustors is characterized by swirling air in the combustor’s dome coupled with distributed fuel injection to maximize mixing. Various flow dynamics processes control the mixing between the fresh fuel/air mixture and hot combustion products and fresh air in premixed combustors. They include large-scale vortices that evolve in a separating shear layer downstream of a sudden expansion or bluff body flame holders, and swirling vortices that undergo vortex breakdown in swirl-stabilized combustors. Interaction between these vortices which are related to flow instabilities, acoustic resonant modes in the combustion chamber and heat release process was shown to cause undesired instabilities in combustors [61].
The stability of industrial burners like jet or swirl burners depend on the coupling between fluid mechanical and combustion mechanisms. The structure of the flow field and the formation of the jet break-up as well as the onset for vortex breakdown in case of a swirled exit is predetermined by the fluid dynamic. This is due to the large temperature and density differences, incorporated in the flow field combustion, that can modify the flow field to a wide extent [62]. The sensitivity of vortex breakdown to pressure gradients can cause coupling between pressure perturbations in the combustion chamber and the heat release from the flame which is anchored at the recirculating region produced by the breakdown, forming a feedback loop that may lead to combustion instability and change in pollutants formation [63]. Beside the flow properties stability of the flame with respect to pulsations can be influenced by the air/fuel mixing profile [64]. In particular for lean flame, characterized by low NOx production, the quenching effect increase with an unsustainable flame instability.
Gagnepain et al [65] report that the evaluation of turbulent premixed flame regimes by measuring the conditional (in the reactants) turbulence parameters is meaningful as the non-conditional ones incorporate part of the flame response to the turbulence structure in the reactants. The experiments showed a large difference between cold flow turbulence and the conditional velocity measured in the reactants with flame. All this information indicates that turbulent mixing is reduced due to the presence of the flame. A similar burner was investigated by Lee et al [66]. The burner consists of two concentrically premixed burners that were found to provide a stable turbulent premixed flame for a wide range of turbulence conditions. Both exits have independent gas supply and can be independently controlled. The paper reports that the stabilization is achieved via the exit velocity of the inner premixed burner which increases the turbulence intensity.
Durox et al [67] studied a conical burner with acoustical perturbation. Two different burner exits were examined, one thin edged burner and one wide edged burner. While the first represents a tube, the latter represents an exit in a wall. It was found, that beyond a given (strong) acoustic amplitude, the wide edged burner generates cells oscillating at half of the excitation frequency. It is stated that theses cells are characteristics of a secondary parametric instability. The instability could be maintained because of a better anchorage of the flame on it than on the thin-edged burner. The conical shape of the burners interior upstream the exit is explained to have a rather strong influence on the exit velocities in interplay with the acoustics.
Lieuwen and Zinn [68] reported on theoretical investigations on the role of equivalence ratio oscillations in driving combustion instabilities in Low-NOx gas turbines showing that equivalence oscillations play a key role in driving instabilities. The author claims for measurements of equivalence ratio oscillations during instable burner combustion to be missing and needed. Instability diagnostics on gas-turbine burners under high-pressure full load operation have been performed by Essman et al. [69] in a power station. They conducted phase matched OH-PLIF diagnostics with high frame acquisition rate. The laser pulse frequency was chosen close to the humming frequency and digital filtering of the image sequences in the Fourier space with backward transformation into image sequences. The sequence then showed, which part of the burner flame reacts resonantly and how it moves in the light sheet plane. Therefore it was concluded, that the axial pulsation detected from the PLIF images was in fact a rotating helical flow-field structure. The 2-D representation of such a 3-D structure is a Von Karman vortex flow and vortices like that where periodically passing through the OH-PLIF images.
The same was actually detected by Ji and Gore [70] when finding coherent vortex structures from time to time in PIV images taken downstream a swirl burner. These vortices were described as instantaneously appearing and vanishing at different locations.
Unsteady simulations as well as phase matched laser diagnostic imaging have demonstrated that the formation of a stable axisymmetric flow field is limited to defined parametric areas and that a swirl burner is not in general a simple solution to stabilize lean premixed flames with equivalence ratios close to the flammability limits [71, 72]. As representing a periodic system by itself, swirl flames tend to periodic instabilities and might couple into resonant processes leading to malfunctioning of the system and, in worst cases, to the resonance catastrophe (quenching flame).

BARTOLI PISA
The influence of acoustic waves into flames has been investigated from several years, as reported in the scientific literature [73-76]. It is then possible to make the hypothesis that the acoustic waves can have a relevant importance also in the thermal and fluid dynamics of jets without chemical reaction.
On the other hand, the enhancement of the convective heat transfer coefficient with acoustic waves outside the audible range, obtained by destabilizing the boundary layer or in forced convection with air jets, does not seems to have been investigated, as reported in the scientific literature. <<<