Experimental Study of Vapor Plume Motion and Flame Propagation Characteristics During Ignition of Oil on a Hot Flat Surface

In civil aircraft, there are many hot surfaces in operating condition, such as engine casing and brake discs. Aviation fuels (e.g. RP-3) are widely used in engine and auxiliary power unit compartments. Consequently, there are potential risks of accidental fuel leakage from the fuel pipeline to the hot surface in airplanes, which poses a huge threat to fire safety. In this study, the evaporation and ignition characteristics of RP-3 spray were experimentally investigated at various hot surface temperatures ranging from 893 K to 1053 K. The evaporation process and the vapor movement velocity have been investigated. It was observed that the vapor plume movement velocity accelerated with the combustible mixture vapor forming and developing, exhibiting greater velocity distributed in the lower part of the vapor plume. Three interesting phenomena have been discovered and referred to as “sustained combustion,” “flash burn,” and two occurrences of “flash burn.” Further calculations were conducted on the average and peak flame propagation velocities of “sustained combustion.” The results show that the average flame propagation velocities fluctuated between 40 cm/s and 130 cm/s at the different hot surface temperatures. The maximum peak velocity reached 280 cm/s at the 1053 K surface temperature. The key ignition parameters were studied such as ignition delay time, ignition core height, and combustion duration. The ignition delay time is found to decrease and the maximum ignition core height can reach 60 cm as the surface temperature increases. The negative correlation between the ignition delay time and its corresponding ignition core height has been revealed. Furthermore, a comparative analysis of the fuel vapor temperature of the non-ignition situation and two different “sustained combustion” ignition situations was conducted.

Fire accidents caused by fuel leakage to hot surfaces are of significant concern in the aerospace domain due to their potential for severe consequences. This paper investigates the evaporation and ignition characteristics of fuel leakage to a hot surface through a series of experiments with different volumes of RP-3 droplets. The evolution patterns of evaporation lifetime, ignition position, and ignition delay time are examined. The results show that within the surface temperature range of 500–700°C, the fuel droplets of all volumes almost immediately reach the film boiling state upon contact with the hot surface. The droplets move erratically across the surface, changing shape from irregular to elliptical and finally to spherical. The evaporation lifetime decreases with increasing hot surface temperature. The occurrence of ignition is stochastic, and the ignition probability rises with the hot surface temperature, while the minimum ignition temperature decreases with the increase of fuel volume. The movement of the fuel droplets in the film boiling state leads to the irregularity of the ignition position. Additionally, the ignition delay time decreases with an increase in the temperature of the hot surface. The relationship between ignition delay time and surface temperature in the film boiling state is discussed in the context of the vapor plume model and boiling heat transfer analysis.

Experimental study on the ignition and burning characteristics of liquid fuels on hot surfaces

The ignition process of a single fuel droplet (n-centane, initial diameter approximately 2 mm) on a hot surface has been investigated. The mass evaporated in the very short duration soon after the initiation of contact with the hot surface has been measured. The following conclusions could be obtained. (1) The ignition delay time curve concerning the wall temperature has a maximum and a minimum point as in the case of the evaporation life time. But these critical values in each of them do not coincide with each other, because the evaporation rate in the ignition delay time (τi), directly controlling τi, differs so much from that in time of total evaporation. (2) The ignition delay time is controlled by the chemical reaction rate when the hot surface temperature (tw) is lower than the Leidenfrost point, and by the physical process when tw is higher than that.

To further reveal the pre-ignition characteristics of hydrogen internal combustion engine, the effect of hot surface characteristic parameters on the ignition characteristics of hydrogen-air mixture was investigated in this research. Based on the prototype of the constant volume combustion bomb with an overhead glow plug, the duration from the heating of the hot surface to the combustion of hydrogen-air mixture, the so-called heating duration, was firstly researched under different fuel-air equivalence ratio, initial temperature, initial pressure, hot surface temperature, and hot surface area, and the influence of each factor on the heating duration was analyzed. The results show that the order of the effect of each factor on the hot surface ignition is as follows: hot surface temperature > initial pressure of hydrogen-air mixture > equivalent ratio > initial temperature of hydrogen-air mixture > hot surface area. The influence of the hot surface characteristic parameters on the heating duration was further analyzed in detail. On this basis, the relationship among the critical ignition temperature, the heating duration and the hot surface area was researched and established. The results show that the heating duration is the only major factor affecting the critical ignition temperature. Finally, the research results were applied to analysis the pre-ignition in hydrogen internal combustion engine.

A hot surface is one of the ignition sources which may lead to fires in the presence of aviation fluid leakage. Bleeding ducts and exhaust pipes that are at elevated temperatures are potential sources of ignition. A database of Minimum Hot Surface Ignition Temperatures (MHSIT) resulting from experiments conducted three decades ago at the Air Force Research Laboratory (AFRL), Dayton, OH has served as a valuable source of estimating safe operating temperatures. However, MHSIT for some of the aviation fluids such as Jet-A and MIL-PRF-23699 (lubrication oil) are not readily available. Further, the ranges of the hot surface and flammable liquids’ temperatures and the range of the air stream velocities need to be extended for use in higher pressure ratio and higher performance aircraft engines developed since the generation and interpretation of the original data. The air velocities (V_A) in the modern engines have increased by a factor of two and documenting their effects on the MHSIT for a range of test fluid temperatures and air temperatures (T_F, T_A) is important. The objectives of this study are to develop a generic test apparatus to study MHSIT and to model an air-fuel mixture space to find the range of temperatures and velocities that lead to ignition. Among various leakage scenarios, the test apparatus simulates spray (atomized particles injected through a nozzle) and stream (dripping from a 3 mm tube) injection. A semiempirical ignition model was developed using an ignition temperature and delay time expression based on an energy balance between the heat lost to the cross-stream flow, the heat added from the hot surface and the heat released by the nascent chemical reactions to estimate the MHSIT. MHSIT is measured including the effects of V_A, T_F, T_Aand the effects of obstacles. Ignition probability is evaluated as a function of the hot surface temperature. The probabilistic nature of the hot surface ignition process was established. New flammable fluids (Jet-A & MIL-PRF-23699) have been tested and MHSIT database was expanded. A large number of ignition experiments were completed to evaluate ignition probability at various flow conditions of aviation fluids: (1) Jet-A, (2) Hydraulic oil (MIL-PRF-5606) and (3) Lubrication oil (MIL-PRF-23699). Uncertainty of the experimental measurements for these tests have been documented. Air velocities were extended up to 7 m/s. Effects of flammable liquid and air temperature on MHSIT were studied. The empirical constants for the semi-empirical model were determined using these experimental data.The ignition probability is strongly correlated with hot surface temperature and progressively weakly correlated with air velocity, fluid parcel size, air temperature, and test fluid temperature. Parameters investigated in this study are useful design choices considering MHSIT for a given flow condition.

Measuring hot-surface minimum ignition temperatures of dust clouds – History, present, future

The effect of hot surface temperature on diesel fuel deposit formation

Origin and development of the Godbert-Greenwald furnace for measuring minimum ignition temperatures of dust clouds

<div class="htmlview paragraph">The purpose of this study is to reveal the flame propagation characteristics. Planar OH* image and local radical emission were measured simultaneously. Planar OH* images were used to analyze the flame propagation characteristics by high-speed camera. These images were then used to evaluate the speed of distribution and the direction of flame propagation. By comparing local point radical emission and planar OH*, the flame propagation characteristics was measured and evaluate that. And the time history of the radical intensity and planar OH* distribution were compared. The relation ship between flame propagation speed and initial heat generation was discussed. The variation of flame propagation speed and the difference of propagation speed in both port sides were confirmed.</div>

Evaporation and ignition of a fuel droplet on a hot surface (Part 4, model of evaporation and ignition)

Study on ignition process and flame expansion and propagation characteristics in jet-cooled pilot flameholders using image processing techniques

Hot surfaces in industrial processes and automotive systems present a remarkable fire hazard. Lubricating oil is a widely used oil in these scenarios. Quantifying the ignition characteristics and flame behavior of lubricating oil on hot surfaces is critical for enhancing fire safety in energy-related applications. This paper utilizes a self-developed experimental platform for the hot surface ignition to systematically conduct combustion tests on lubricating oil with varying volumes at different surface temperatures. Through statistical analysis and image processing, the ignition temperature, flame height, flame propagation velocity, and flame temperature were examined to assess the fire risk of a hot surface ignition. The results demonstrate that the ignition and combustion process of lubricating oil on hot surfaces can be categorized into five stages. The ignition temperature decreases as the oil volume increases. The flame height and flame propagation velocity are positively correlated with the hot surface temperature. The maximum flame height increases with the increase in the oil volumes. When the flame height reaches the maximum value, the flame area is the largest, and the average flame temperature is 1540.30 °C, showing a greater fire risk. When the oil content is 0.2 mL, the flame propagation velocity is the fastest, reaching 3.81 m/s. Meanwhile, the flame is very close to the oil pipe, which may cause a secondary fire. Therefore, hot surface ignition of lubricating oil poses a direct threat to vehicle safety.

Study of ignition characteristics of microemulsion of coconut oil under off diesel engine conditions

In experiments of hot surface ignition and subsequent flame propagation, a puffing flame instability is observed in mixtures that are stagnant and premixed prior to ignition. By varying the size of the hot surface, power input, and combustion vessel volume, it was determined that the instability is a function of the interaction of the flame, with the fluid flow induced by the combustion products rather than the initial plume established by the hot surface. Pressure ranges from 25 to 100 kPa and mixtures of n-hexane/air with equivalence ratios between $\phi = 0. 58$ and 3.0 at room temperature were investigated. Equivalence ratios between $\phi = 2. 15$ and 2.5 exhibited multiple flame and equivalence ratios above $\phi = 2. 5$ resulted in puffing flames at atmospheric pressure. The phenomenon is accurately reproduced in numerical simulations and a detailed flow field analysis revealed competition between the inflow velocity at the base of the flame and the flame propagation speed. The increasing inflow velocity, which exceeds the flame propagation speed, is ultimately responsible for creating a puff. The puff is then accelerated upward, allowing for the creation of the subsequent instabilities. The frequency of the puff is proportional to the gravitational acceleration and inversely proportional to the flame speed. A scaling relationship describes the dependence of the frequency on gravitational acceleration, hot surface diameter, and flame speed. This relation shows good agreement for rich n-hexane/air and lean hydrogen/air flames, as well as lean hexane/hydrogen/air mixtures.