Surface Regression Rate Research Articles

On the numerical approach to the prediction of flame spread over non-planar surface of solid combustibles

The flame spread over a non-planar surface of solid fuel has been studied numerically by the coupled model of heat transfer using the proposed approach for the evaluation of burning surface regression. The boundary conditions for the surface regression rate are formulated by the combination of flame spread modes over the vertical and horizontal surfaces resulted from the staircase shape of the burning surface. Numerical results showed a good agreement with the experiment on surface regression profile and mass loss of PMMA solid fuel.

Comparison of focal cryoballoon ablation with 10- and 8-second doses for treatment of Barrett’s esophagus–related neoplasia: results from a prospective European multicenter study (with video)

Focal cryoballoon ablation (FCBA) is currently being investigated for the treatment of Barrett's esophagus (BE)-related neoplasia in a European multicenter study (Euro-Coldplay study). After inclusion of 28 of 107 patients, the initial dose of 10 seconds was lowered to 8 seconds. The current study aimed to compare the efficacy and safety of a single FCBA treatment session with 10 seconds versus 8 seconds. Treatments were performed at 7 European BE referral centers. All 28 patients treated with 10 seconds were compared with 28 consecutive patients treated with 8 seconds. The gastroesophageal junction was ablated circumferentially followed by all visible BE. To assess efficacy and safety, 3 expert adjudicators, blinded to physician and dose, compared pre- and post-treatment images. Primary outcomes were median BE surface regression and stricture rate after single-session FCBA. We included 56 patients (10-second cohort, n= 28; 8-second cohort, n= 28) with a median BE length of C0M2 (Prague classification). Baseline characteristics did not significantly differ between the cohorts. The median BE surface regression after a single FCBA session was comparable for 10 seconds and 8 seconds (80% [95% confidence interval {CI}, 75-90] and 80% [95% CI, 66-90], respectively; P= .65). Strictures requiring dilation were seen in 19% (95% CI, 4-33) and 15% (95% CI, 4-30) of the 10-second and 8-second groups, respectively (P= 1.00). Two patients in the 10-second group developed a severe stricture requiring >3 dilations. In patients with limited BE, single-session FCBA with 8 seconds showed similar BE surface regression as compared with 10 seconds and may theoretically result in fewer and less severe strictures. Therefore, we suggest using 8 seconds as the standard dose for FCBA. (Clinical trial registration number: NL7253.).

Experimental Studies of the Evaporation of Pure Liquid Droplets in a Single-Axis Non-Resonant Levitator

Though a simple daily observation, evaporation of drops is still poorly understood due to the complex nature that involves hydrodynamic effects in the bulk fluids and transport phenomena at the liquid-vapor interface. This paper reports on the evaporation of single component droplets (water, ethanol, acetone, and glycerol) levitated in a single-axis non-resonant levitator. It was observed that the acetone and ethanol evaporated faster than water, although the acetone is the most volatile. The estimated lifetime of acetone is less than 5min, which is much shorter as compared to 56min for ethanol or about 90min for water droplets. On the other hand, glycerol showed no tendency to evaporate. With increasing the evaporation time, the ratio of large and small semi-axis decreases and tends to 1 corresponding to changes in drops shape from oblate ellipsoid to a sphere. Based on the classical D2-law, the surface regression rates have been estimated.

Gas-phase thermography of droplet combustion and its application to characterize nanofuels

An IR thermography-based methodology is proposed to compute the instantaneous burning rate in droplet combustion. Temperature distribution around the droplet surface is measured using an infrared camera with a narrow bandpass filter of 4.25-μm wavelength. The technique holds importance in multicomponent fuels, densely concentrated nanofuels and nanofuels prepared by the addition of surfactant wherein the droplet surface regression rate is not linear due to micro explosions. For such applications, the conventional high-speed videography shows limitations, highlighting the importance of IR thermography. The methodology has first been validated against the results of classical interferometry. Thereafter, it has been applied to characterize the droplet combustion of alumina nanoparticles (AlONPs)-loaded methanol nanofluids. The results of the study showed that burning rates calculated through IR thermography are in good agreement with those determined using the conventional videography approach for the low mass loading of alumina nanoparticles. However, at higher mass loading ratios, the random occurrence of microexplosions towards the end of droplet combustion leads to inherent errors in the computation of the mass burning rate through videography. An increase in the average burning rate of the combusting droplet is observed at higher values of mass loading ratios, an observation that has been explained using IR thermography measurements.

Downward burning of PMMA cylinders: The effect of pressure and oxygen

Better understanding the burning behavior of solid fuels is necessary in order to be able to estimate more accurately the fire risk that they might represent. The burning rate is an essential component when assessing the growth of a fire, since it determines the heat released during the fuel burning. The objective of the present work is to study the effect of ambient pressure and oxygen concentration on the burning of cylindrical samples of polymethyl-methacrylate (PMMA) in opposed flow flame spread conditions. Experiments are conducted using pressures ranging between 100 and 30 kPa and oxygen concentrations between 19% and 23%, with a forced flow velocity of 100 mm/s. The experimental conditions reflect environments potentially encountered in future spacecraft cabins. The data collected show that during downward opposed flame spread the surface regression rate, and consequently the mass burning rate, decreases with decreasing ambient pressure and increases with decreasing oxygen concentration. The data presented is correlated with a simplified model to estimate the surface regression rate based on the mixed heat convection coefficient and the B number. Model predictions are made based on fuel properties as well as the relevant environmental conditions. The predictions of the model agree well capturing the effect of ambient pressure and oxygen concentration. The correlations provide information about the variations in the burning rate in environments with variable pressure and oxygen concentration, providing guidance for potential for fire safety testing and design in different ambient conditions.

Regression Rate Design Tailoring Through Vortex Injection in Hybrid Rocket Motors

In the last few decades, vortex injection has been proposed as a possible solution to main hybrid rocket motor drawbacks. The aim of this work is to understand the effects of the swirl intensity and the postchamber length on a 300 N hybrid rocket motor by using as the oxidizer and high-density polyethylene as the solid fuel with a classical cylindrical port grain. Three different injection plates have been designed in order to obtain three different geometrical swirl numbers; at the same time, three different postchamber lengths have been used. The numerical and experimental investigations have proved that swirl injection allows a good motor efficiency without the need for a long postchamber. From the analysis of fuel consumption data, a linear relation between the grain surface regression rate and the intensity of the swirling flow given by vortex injection has been obtained. This information could be used in future design phases to tailor the fuel regression in order to achieve the best packaging of a hybrid rocket motor for a desired mission profile.

An improved categorization of vapor bubble collapse: Explaining the coupled nature of hydrodynamic and thermal mechanisms

This work presents a numerical and theoretical analysis of the intermediate bubble collapse, where in contrast to the thermally-induced or inertia dominated collapse, both the effects of liquid-vapor interfacial heat transfer and the advection of the surrounding liquid play an important role. A key distinguishing characteristic of this type of collapse is that the interfacial temperature and bubble pressure are continuously changing throughout the collapse process. This behavior is caused by a non-negligible value for the interfacial vapor velocity, which is characterized by a magnitude of the same order as the bubble surface regression rate. This behavior is in contrast to the inertial or thermal collapse, where, in these regimes, the vapor velocity is essentially zero. Incorporating the rate of change of background system pressure, a regime map for bubble collapse is also presented where the parameter space is defined by Bsat and ξ and extends the previous categorization of Florschuetz and Chao (1965) where only Beff was used. Specifically, the present work quantitatively shows that the collapse behavior changes when the process is initiated by a gradual rate of pressurization in the surrounding liquid phase instead of the often used approximation of a step change.

Burn angle in forced and natural convection: a simplified scaling approach

The surface regression of burning solid fuels facing a forced flow has been observed and measured by several authors, because of its connection with mass burning rate and heat transfer between flame and fuel. The pyrolysis region of non-spreading flames is not uniform in space and changes in shape with time. On the contrary, for moving flames experiments showed that the competition between surface regression and spread rate causes the pyrolysis region to assume a steady-state geometry. In particular, the declination of the pyrolysis surface is constant for given burning conditions, although it is highly influenced by their variations. The geometry of the pyrolysis region is characterised by the surface declination angle, called burn angle, which can be used to study the relation between mass burning rates and flame spread rates. Despite the amount of experimental work in literature, a theoretical description of the burn angle and its dependence on burning parameters such as flow velocity is still missing. The scale analysis in this work shows an alternative approach to predict the qualitative behaviour of the burn angle and therefore of the average mass burning rate, qualitatively describing its dependence on burning conditions.

Regression Rate Estimation for Swirling-Flow Hybrid Rocket Engines

In the present study, an analytical model based on convective heat feedback is developed for the estimation of the solid fuel surface regression rate of hybrid rocket engines with head-end swirling-flow oxidizer injection. The convective heat transfer between the axial core flow and the burning fuel surface, coupled with the convective heat feedback between the effective tangential flow and the burning fuel surface, is the means by which the fuel regression rate is presumed to be increased by swirl, above that due to the axial mass flux. The representation of the effective boundary layers used in this study includes the influence of transpiration, effective hydraulic diameters (for flows in the axial and tangential direction), and fuel surface roughness. From the literature, a variety of propellant combinations, engine sizes, and flow swirl numbers are evaluated for engines having circular-port fuel grains, with sample model results provided. The predicted fuel regression rates for the most part compare quite well with the corresponding experimental data.

One-dimensional Raman spectroscopy and shadowgraphy for the analysis of the evaporation behavior of acetone/water drops

This paper reports – for ambient conditions – experimentally determined surface regression rates of acoustically levitated acetone/water drops as a function of the mixture composition at the drop boundary. The fluid levitated as a drop was a mixture of acetone and water with vapor pressures of 24.5kPa and 3.17kPa, respectively, at ambient conditions. As expected the acetone evaporated faster from the acetone/water drop than water. Already small quantities of acetone in the mixture significantly increased the surface regression rate of the drop.Temporally and spatially resolved composition profiles were measured along a line through the drops using one-dimensional Raman spectroscopy. Shadowgraphs of the evaporating drops were acquired, from which the drop shrinkage was derived. Due to the good spatial (120μm) and temporal (1s) resolution of the one-dimensional Raman experiment the evolution of the radial composition profiles through the drop could be followed showing that the diffusion-process inside the drop plays an important role in the binary evaporation process.

Pool Fires Of Biofuels And Their Blends With Petroleum Diesel

The combustion characteristics of pool fires of biofuels, canola methyl ester (CME), soy methyl ester (SME), and their blends with a petroleum fuel (No. 2 diesel) were studied. The fuels were burned in cups of two sizes (0.042 m and 0.057 m in diameter and 0.038 m height) that simulated convection-dominated small pool fires of liquids. Blends of CME and SME with diesel fuel were tested with biofuel concentrations of 25, 50, and 75% by volume. The mass burn rate, the fuel surface regression rate, the radiation emission from the flames, the flame temperature field, and the emission indices of CO and NOx were recorded. The fuel surface regression rate in both containers was comparable. Both the fuel mass burning rate and surface regression rate varied non-monotonically with the volume concentration of biofuel in the blend. The radiation fraction of heat release and the temperature profiles were similar for all the flames. The CO emission index decreased with the biofuel content in the fuel. The NOx emission index did not show a systematic and significant dependence on the fuel blend; however, it was smaller than that measured in the turbulent gas jet and liquid spray flames of the corresponding fuels.

Heat release effects on drag reduction in high speed flows

Heat addition due to exothermic reactions of ablated chromium in hypersonic flow on coated blunt bodies is numerically investigated. The advection upstream splitting method is used as computational scheme. Conjugate heat transfer is considered by simultaneous solution of flow and solid phase governing equations to compute the surface regression rate. The results show that using such coatings on blunt noses in high speed flows will increase the shock standoff distance and decrease the aerodynamic drag but not so much that previously reported. Using these simulations, the chromium proper thickness will be estimated more precisely for specific flight mission.

Interaction features of different propellants under plasma impingement

The development of electrothermal chemical launch technology and the effective ignition of energetic thermoplastic elastomer (ETPE) propellants all call for a thorough understanding of the plasma propellant interaction mechanism. Therefore in this work, a numerical model is developed to study this process. In this model, special attention is paid to deal with the regression of propellant surface, which is caused by plasma ablation, and is assumed to have considerable influence on the interaction process. By this model, interaction features of four propellants, including two ETPE propellants are studied with the aid of experiments, which are carried out to obtain plasma parameters utilized in the model. Surface temperature, ablation rate, and surface regression rate are obtained for different propellants under plasma impingement. Finally, the influence of propellant composition and charging voltage are analyzed.

Reaction Dynamics and Probability Study of Aluminum-Viton-Acetone Droplets

The fluoroelastomer, Viton® A (C5H3.5F6.5), is a binder used in energetic material composites which can be dissolved in acetone and mixed with solid metal fuel particles, such as aluminum (Al). The slurry can be molded into any configuration and the acetone is evaporated off leaving a solid homogeneous reactant mixture. Fundamentally understanding the reaction dynamics of acetone, Viton, and Al is useful for not only consolidating reactants but also for the potential to use nanometric (nm) Al particles in liquid combustion applications. The objective of this study is to examine acetone-Viton droplet burning as a function of Al particle size and concentration. A diagnostic was developed that suspends the multiphase 3 mm droplet on a fiber and enables droplet surface regression rates to be monitored with a high speed video camera. Results show that acetone combined with Viton (5%) and either 2.5% micron Al (10-14 µm) or nm Al (80 nm) burned with two distinct stages. But, acetone combined with Viton and more than 5% nm Al burned with a distinct 3-stage behavior. The three stages are described as evaporation, combustion, and burning of carbon residue. Addition of micron Al to Viton and acetone droplets decrease the droplet regression rate 82% when compared with Viton and acetone droplets with nm Al particles. A probabilistic analysis was performed using NESSUS© software to determine the occurrence of the third stage in the multicomponent droplets. The probability of the third stage occurring in Acetone-Viton is 92.1% but with Acetone-Vitonnm Al is 1.45%. These results suggest that the presence of nm Al particles generates localized hot-spots that consume the carbon present from Viton decomposition thereby eliminating the third stage.

Evaporation of liquid fuel droplet at supercritical conditions

A comprehensive analysis of liquid fuel droplet evaporation at supercritical conditions is performed. The numerical model is based on complete time-dependent conservation equations, with a full account of variable thermophysical properties and vapor-liquid interfacial thermodynamics. And the model employs the Peng-Robinson (PR) equation of state (EOS). As a specific example, problems involving n-heptane droplet in nitrogen gas are investigated. The results indicate that the increase of ambient pressure and temperature results in the increase of surface temperature rise rate and surface regression rate. The transition from subcritical state to supercritical state can occur at the droplet surface when the droplet evaporates in a strongly supercritical environment.

RECENT ADVANCES IN HYBRID PROPULSION

The idea of the hybrid rocket is to store the oxidizer as a liquid and the fuel as a solid, producing a design that minimizes the chance of a chemical explosion. While the hybrid enjoys many safety and environmental advantages over conventional systems, large hybrids have not been commercially viable. The reason is that traditional systems use polymeric fuels that evaporate too slowly, making it difficult to produce the high thrust needed for most applications. Research at Stanford University and Space Propulsion Group (SPG) has led to the development of paraffin-based fuels that burn at regression rates 3-4 times that of polymeric fuels. Under the action of the oxidizer flow, the new fuels form a thin, hydrodynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid-gas interface can substantially increase the rate of fuel mass transfer, leading to a much higher surface regression rate than can be achieved with a conventional fuel. To demonstrate the use of these fuels, a series of scale-up tests using several oxidizers has been carried out on intermediate-scale motors. The data from these tests are in agreement with small-scale, low- pressure, and low-mass-flux laboratory tests and confirm the high regression rate behavior of the fuels at chamber pressures and mass fluxes representative of commercial applications. Recently, SPG has developed a new class of oxidizers based on refrigerated mixtures of N2O and oxygen. The mixtures combine the high vapor pressure of dissolved oxygen with the high density of refrigerated N2O to produce a self-pressurizing oxidizer with high density and good performance. The combination of these technologies leads to a hybrid rocket design with reduced system size and mass.

Regression rate estimation for standard-flow hybrid rocket engines

The present effort is towards predicting with some accuracy hybrid rocket engine fuel regression rates under standard flow conditions. A convective heat feedback modelling approach is applied in tying the mass-flux-dependent heat flux directed into the regressing fuel surface, to the subsequent solid fuel grain regression rate. Factors such as transpiration, hydraulic port diameter, and effective fuel surface roughness are incorporated into the phenomenological surface regression rate model. A number of comparisons between the model's predicted results and corresponding experimental data are made, in illustrating the efficacy of the present approach for a classical head-end-injection engine. Where substantial differences between theory and experiment exist, this might be due to one of several identifiable factors related to non-standard flow, such as the presence of radiant heating, swirl or flow impingement in or at the boundaries of the experimental core flow.

Pyrolysis/Evaporation Study of Succinic Acid/Polyvinyl Acetate for Reducing Nozzle Throat Erosion

A nozzle boundary-layer control system is under consideration for application in high-pressure rockets to mitigate the erosion rates of a graphite nozzle. The current design contains multiple center-perforated solid grains of fuel-rich materials consisting of succinic acid and polyvinyl acetate. This combination of the fuel-rich grains was selected due to high carbon and hydrogen contents along with relatively low evaporation temperature for generating fuel-rich gases. The characterization of the pyrolysis behavior of fuel-rich grains is a requirement for any subsequent quantitative analysis pertaining to the effect of the nozzle boundary-layer control system on graphite rocket nozzle erosion rates. Two separate experiments were conducted: 1) to determine the regression rate of solid fuel-rich grains under controlled heat flux or temperature conditions, and 2) to characterize its chemical decomposition and/or evaporation behavior. An empirical correlation between heat flux and surface regression rate of fuel-rich grains was developed. From Fourier transform infrared spectroscopy measurements, the fuel-rich grains were found to melt and evaporate at temperatures up to 773 K. These results have been used in parallel study nozzle throat erosion processes using computational simulation.

Laser-induced fluorescence imaging of acetone inside evaporating and burning fuel droplets

Laser-induced fluorescence was used to visualize acetone fields inside individual droplets of pure acetone as well as droplets composed of methanol or 1-propanol initially mixed with acetone. Droplets were supported on a horizontal wire and two vaporization conditions were investigated: (1) slow evaporation in room air and (2) droplet combustion, which leads to substantially faster droplet surface regression rates. Acetone was preferentially gasified, causing its concentration in droplets to drop in time with resultant decreases in acetone fluorescence intensities. Slowly vaporizing droplets did not exhibit large spatial variations of fluorescence within droplets, indicating that these droplets were relatively well mixed. Ignition of droplets led to significant variations in fluorescence intensities within droplets, indicating that these droplets were not well mixed. Ignited droplets composed of mixtures of 1-propanol and acetone showed large time-varying changes in shapes for higher acetone concentrations, suggesting that bubble formation was occurring in these droplets.

Experimental Study of the Nozzle Surface Regression Rate to the Heat Transfer

The surface regression is occurred by three kinds of mechanisms, namely ablation, chemical corrosion, and mechanical erosion. The total surface regression can be represented by a function of the heat transfer coefficient only based on the theoretical relations of these mechanisms. For this, the Reynolds Analogy between the skin coefficient and Stanton Number was adopted, also Least-Square Fit method was used between the Mach Number and cross section area ratio. The surface regression rate was strongly dependent on the ablation including chemical corrosion and could be expressed by a linear relation to the heat transfer coefficient.