Shuhn-Shyurng Hou

The influence of external heat transfer on flame extinction of dilute sprays

Abstract The extinction of a dilute spray flame burning in a steady, one-dimensional, low-speed, sufficiently off-stoichiometric, two-phase flow, and experiencing the external heat transfer from the spray to a tube wall upstream is further analyzed. The external heat transfer results in globally external heat loss, excess enthalpy burning and external heat gain, respectively, to the spray system with increasing the wall temperature. However, the droplet gasification provides the overall internal heat loss and heat gain for rich and lean sprays, respectively. Therefore, the burning and extinction of the dilute spray flame can be fully described by the interaction between external and internal heat transfers in two spray models which were identified to be the completely and partially prevaporized burnings. The C-shaped and S-shaped extinction curves are clearly classified and mapped with parameters of the wall temperature, the overall external heat transfer and the initial droplet size. Variations of the extinction curves under the influence of transition from overall external heat loss to heat gain, and the jump between the completely and partially prevaporized burnings on flame extinction, are reported and discussed for both lean and rich sprays.

Extinction of stretched spray flames with nonunity Lewis numbers in a stagnation-point flow

The extinction of stretched premixed flames under the influence of dilute fuel sprays is studied using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. Three parameters for flame extinction in the analysis consist of the mass fraction of liquid fuel and the initial droplet size of the spray, indicating the internal heat loss and heat gain for rich and lean sprays, respectively, and the flow stretch coupled with Lewis number (Le) that intensifies and weakens the burning strength of the Le 1 flame, respectively. The study presents sample calculations on lean ethanol-spray flames (Le>1), rich ethanol-spray flames (Le>1), and rich methanol-spray flames (Le 1 is weakened by the flow stretch: however, it is enhanced (further reduced) when the lean (rich) spray has a larger amount of liquid fuel loading or a smaller initial droplet size. It is generally found that the external heat loss associated with the flow stretch dominates the trend for flame extinction. The coupling effects of flow stretch and internal heat gain result in that there exists flame flashback instead of flame extinction for rich methanol-spray flames (Le

LAMINAR DIFFUSION FLAMES IN A MULTIPORT BURNER

ABSTRACT The influence of stream concentrations and velocities on the flame shape in a multiple-port burner is theoretically and experimentally studied in this work. A general solution of normal and inverse diffusion flame configurations in a multiple-port burner is obtained with the inclusion of the effects of axial diffusion and unequal stream velocities. The theoretical results show that not only the flame height but also the flame shape is affected by the Peclet number. In the theoretical analysis, the fuel and oxidizer concentrations, stream temperature, and stream velocity are three important parameters. Flame configurations can be predicted well by including the temperature effect in calculating the Peclet number. For relatively weak (or strong) flame intensity, the prediction of flame configurations agrees well with the experimental results if a lower (or higher) temperature is used. In the experiment, different flame configurations are observed and discussed for various stream velocities and concentrations. The transition of inverse diffusion flame from a single cone-shaped flame to a double cone-shaped flame to an envelop flame occurs when the inner stream velocity is adjusted for fixed middle and outer stream velocities. It is of interest to note that under the same operating conditions the flame has a history-dependent configuration decided by increasing or decreasing the inner stream velocity.

Effects of internal heat transfer and preferential diffusion on stretched spray flames

The extinction of dilute spray flames propagating in a stagnation-point flow under the influence of flow stretch, preferential diffusion, and internal heat transfer is analyzed using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. The internal heat transfer, associated with the liquid fuel loading and the initial droplet size of the spray, provides heat loss and heat gain for rich and lean sprays, respectively. It is found that the flow stretch coupled with Lewis number (Le) reduces and enhances the burning intensity of the lean methanol-spray flame (Le>1) and rich methanol-spray flame (Le 1 flame is dominated by the flow stretch, while the S-shaped extinction curve for the Le<1 flame is mainly influenced by the internal heat loss associated with the droplet gasification process.

Interactions for flames in a coaxial flow with a stagnation point

The possible burning structures existing in two co-flowing combustible mixtures with different compositions, and their implications to the field of turbulent combustion are examined in this study. A coaxial burner with a quartz plate was used to experimentally investigate the flames of methane/air and propane/air mixtures propagating in a coaxial flow impinging onto a stagnation surface. The possible burning structures were observed to be: (1) a single-flame (a lean or rich premixed flame); (2) a double-flame (two lean or rich premixed flames, or a rich premixed flame and a diffusion flame); and (3) a triple-flame (a rich premixed flame, a diffusion flame and a lean premixed flame). An inner (or outer) mixture, far beyond the flammability limit, can still burn if a stronger outer (or inner) flame supports it. The extinction limit of the top part of the inner hat-shaped premixed flame is nearly independent of the burning intensity of the outer flame. It was found that the inner flame has a wider flammable region than the outer flame, and that the latter has a narrower flashback region than the former. Both propane and methane flames may exhibit flame-front instability, although the former displays much more clearly than the latter. Cellular and polyhedral instabilities can exist individually or appear simultaneously in the inner flame. However, only polyhedral (stripped-pattern) instability was observed in the outer flame. Finally, the experiments were analyzed theoretically using a simple geometrical model incorporated with the numerical simulations. The predicted shapes and locations of the flames are in good agreement with the experimental observations qualitatively.

A theoretical study on Bunsen spray flames

Abstract The structure of Bunsen flame tip under the influence of dilute, monodisperse inert (water) or fuel (methanol) sprays is theoretically studied using large activation energy asymptotics. A completely prevaporized mode is identified, in which no liquid droplets exist downstream of the flame. Parameters for open and closed flame tips in the analysis consist of the amount of liquid loading indicating the internal heat loss for the water spray or the internal heat loss and heat gain for the rich and lean methanol-sprays, respectively, and the (negative) stretch coupled with Lewis number ( Le ) which strengthens the burning intensity of the Le >1 flame but weakens that of the Le Le >1) with water sprays (or lean methanol-spray flames with Le >1), closed-tip solutions are obtained. The burning intensity of the flame tip is enhanced with either decreasing liquid-water loading (or increasing liquid-fuel loading) or increasing stretch. Conversely, the negative stretch weakens the burning intensity of a lean methane–air flame ( Le Le Le >1) and thus leads to the transition of flame configurations from conventional Bunsen cone through planar flame to inverted flame cone (a convex flame shape with respect to the upstream reactants). The critical value of liquid-fuel loading, at which there exists a planar flame rather than a Bunsen cone flame, is increased with either increasing upstream flow velocity or decreasing equivalence ratio.

Analysis of finite laminar opposed-jets with and without rigid-body rotation

Abstract The fluid mechanics and mixing process of steady, laminar, finite and opposed-jets are theoretically investigated. A finite-volume method is employed to numerically solve the corresponding transport equations. Calculated results are compared with the analytical similarity solutions and the experimental data for two cases of the non-identical opposed-jets, constituted by two different fluids, without rigid-body rotation and the identical opposed-jets with rigid-body rotation. Agreements show that the numerical solutions obtained from the complete transport equations yield accurate predictions, while the similarity solutions yield fair predictions under some limitations.

Numerical investigation of CuO-water nanofluid turbulent convective heat transfer in square cross-section duct under constant heat flux

Purpose – The purpose of this paper is to numerically investigate the convective heat transfer of water-based CuO nanofluids flowing through a square cross-section duct under constant heat flux in the turbulent flow regime. Design/methodology/approach – The numerical simulation is carried out at various Peclet numbers and particle concentrations (0.1, 0.2, 0.5, and 0.8 vol%). The finite volume formulation is used with the semi-implicit method for pressure-linked equations algorithm to solve the discretized equations derived from the partial nonlinear differential equations of the mathematical model. Findings – The heat transfer coefficients and Nusselt numbers of CuO-water nanofluids increase with increases in the Peclet number as well as particle volume concentration. Also, enhancement of the heat transfer coefficient is much greater than that of the effective thermal conductivity at the same nanoparticle concentration. Research limitations/implications – Simulation of nanofluids turbulent forced convect...

The interaction between internal heat loss and external heat loss on the extinction of stretched spray flames with nonunity Lewis number

Abstract The influences of flow stretch, preferential diffusion, internal heat transfer and external heat loss on the extinction of dilute spray flames propagating in a stagnation-point flow are analyzed using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. The internal heat transfer, associated with the liquid fuel loading and the initial droplet size of the spray, provides heat loss for rich sprays but heat gain for lean sprays. The flow stretch respectively weakens and intensifies the burning intensity of the lean methanol-spray flame (Le>1) and rich methanol-spray flame (Le 1 flame can be extinguished with or without external heat loss. Flame extinction characterized by a C-shaped curve is dominated by the external heat loss or the flow stretch. For the Le

The influence of preferential diffusion and stretch on the burning intensity of a curved flame front with fuel spray

Abstract In our most recent paper on Bunsen spray flames, only a completely prevaporized mode of a normal Bunsen flame was considered; inverted Bunsen flame and droplet size effects had not been examined yet. In the present study, we consider two flame structures: normal and inverted Bunsen flames, and two spray modes: completely and partially prevaporized burning, by the method of large activation energy asymptotics. In this way, a complete parametric study of flame tip intensification or extinction (opening) can be conducted. Four parameters are used in the analysis. The first two are the droplet size and amount of liquid-fuel loading, which indicate internal heat loss for a rich spray but heat gain for a lean spray. The other two are the stretch and Lewis number (Le). Stretch is negative for a normal Bunsen flame but positive for an inverted Bunsen flame. Stretch strengthens (or weakens) the burning intensity of the Le>1 (or Le 1 (or Le 1 or a rich methanol-spray inverted Bunsen flame with Le 1, or rich methanol-spray normal Bunsen flames with Le 1, if liquid loading is large enough and droplet size is sufficiently small, there exists flame transition from normal (or inverted) Bunsen through planar to inverted cone (or normal Bunsen) flame. Finally, the critical value of droplet size, at which there exists a planar flame rather than a normal (or an inverted) Bunsen flame, increases with increasing liquid loading.