On the validity of quasi-steady assumption in transient droplet combustion

Published on Jan 1, 2009in Combustion and Flame4.12
· DOI :10.1016/j.combustflame.2008.09.005
Mehdi Jangi12
Estimated H-index: 12
(Tohoku University),
Satoru Sakurai3
Estimated H-index: 3
+ 1 AuthorsHideaki Kobayashi6
Estimated H-index: 6
(Tohoku University)
A study on droplet combustion in unsteady force convection at high pressure under microgravity conditions was performed. The hysteresis loop of the instantaneous burning rate of a single suspended 1-butanol droplet was observed for the first time. Results showed that the classical quasi-steady film model cannot describe droplet combustion in an unsteady flow. Based on precise experimental observation and by utilizing dimensional analysis of the energy conservation equation, a new criterion is herein proposed for the condition in which the quasi-steady assumption is valid and for that in which it is not. The dimensional analysis led to formulation of a new time scale. Based on the time scale which we call the response-time-scale, a new Damkohler number, termed the response-Damkohler-number was formulated. Using the definition of the new time scale and that of the Damkohler number, unsteady behaviors of droplet combustion under conditions of various pressures and varying force convection were examined. Finally, using the response-Damkohler-number and the deviation factor between the actual instantaneous burning rate and the burning rate predicted by the quasi-steady theory, droplet combustion was categorized into four specific regimes. This study is also of fundamental interest in terms of the effects of turbulence on droplet evaporation and combustion in spray flames.
  • References (15)
  • Citations (8)
📖 Papers frequently viewed together
30 Citations
22 Citations
1 Author (K. Madooglu)
78% of Scinapse members use related papers. After signing in, all features are FREE.
#1Srinivasan Dattarajan (UCLA: University of California, Los Angeles)H-Index: 1
#2A. Lutomirski (UCLA: University of California, Los Angeles)H-Index: 1
Last. Ann Karagozian (UCLA: University of California, Los Angeles)H-Index: 24
view all 5 authors...
This experimental study focused on methanol droplet combustion characteristics during exposure to external acoustical perturbations in both normal gravity and microgravity. Emphasis was placed on examination of excitation conditions in which the droplet was situated (1) at or near a velocity antinode (pressure node), where the droplet experienced the greatest effects of velocity perturbations, or (2) at a velocity node (pressure antinode), where the droplet was exposed to minimal velocity fluctu...
20 CitationsSource
#1Madjid Birouk (UM: University of Manitoba)H-Index: 18
#2Iskender Gökalp (CNRS: Centre national de la recherche scientifique)H-Index: 33
This article reviews the available literature results concerning the effects of turbulence on the transport (heat and mass transfer) rates from a droplet. The survey emphasizes recent findings related specifically to physical models and correlations for predicting turbulence effects on the vaporization rate of a droplet. In addition, several research challenges on the vaporization of fuel droplets in turbulent flow environments are outlined.
83 CitationsSource
#1Masaki MitsuyaH-Index: 1
#2Hironao Hanai (Chiba University)H-Index: 4
Last. Hideaki Kobayashi (Tohoku University)H-Index: 34
view all 5 authors...
Abstract A new microscopic model of the interaction between droplet flames and fine vortex tubes which compose a coherent structure of turbulence was developed. Three non-dimensional numbers were introduced to extend the length scale and time scale so as to be suitable for microgravity experiments using droplets of combustion of about 1 mm in diameter. An experimental apparatus for combustion of a single droplet and that of an array of two droplets in varying airflow was developed, and experimen...
7 CitationsSource
#1Randall T. Imaoka (UCI: University of California, Irvine)H-Index: 4
#2William A. Sirignano (UCI: University of California, Irvine)H-Index: 43
Abstract Three-dimensional droplet-array combustion with an unsteady liquid-phase and a quasi-steady gas-phase is studied computationally by a generalized approach using a mass-flux potential function. Symmetric and asymmetric droplet arrays with non-uniform droplet size and non-uniform spacing are considered. Burning rates are computed and correlated with the number of droplets, an average droplet size, and an average spacing for the array through one similarity parameter for arrays as large as...
40 CitationsSource
#1Sergei Sazhin (University of Brighton)H-Index: 39
#2W.A. Abdelghaffar (University of Brighton)H-Index: 5
Last. Morgan Heikal (University of Brighton)H-Index: 31
view all 5 authors...
New approaches to numerical modelling of droplet heating and evaporation by convection and radiation from the surrounding hot gas are suggested. The finite thermal conductivity of droplets and recirculation in them are taken into account. These approaches are based on the incorporation of new analytical solutions of the heat conduction equation inside the droplets (constant or almost constant h) or replacement of the numerical solution of this equation by the numerical solution of the integral e...
70 CitationsSource
#1Jong-Shinn Wu (NCTU: National Chiao Tung University)H-Index: 25
#2K. H. Hsu (NCTU: National Chiao Tung University)H-Index: 1
Last. Horn-Jiunn Sheen (NTU: National Taiwan University)H-Index: 15
view all 4 authors...
A general evaporation model of a single liquid fuel droplet under various ambient turbulence conditions at intermediate droplet Reynolds numbers is proposed combining the current and previous experimental results. Extensive evaporation experiments of single fuel droplets are conducted at room temperature in quasi-laminar and turbulent environments at Reynolds numbers in the range of 72–333. Tested hydrocarbon fuels include by pentane, hexane, heptane, octane and decane. Flow fields, behind a per...
27 CitationsSource
The evaporation process of a suspended droplet was examined for several kinds of drops of paraffin hydrocarbons, distilled water, and ethanol in a hot, turbulent air flow. As the turbulence intensity increases, the evaporation rate constant increases for all materials because the Kolmogorov microscale becomes smaller and it breaks the concentration boundary layer of the droplet. When the boundary layer thickness of vapor concentration around a droplet is thin, the droplet evaporation is accelera...
17 Citations
#1Iskender Gökalp (CNRS: Centre national de la recherche scientifique)H-Index: 33
#2Christian Chauveau (CNRS: Centre national de la recherche scientifique)H-Index: 22
Last. Xavier Chesneau (CNRS: Centre national de la recherche scientifique)H-Index: 1
view all 4 authors...
Abstract An experimental apparatus and strategy have been developed to investigate the influence of turbulence on the global mass transfer rates from fuel droplets. Heptane and decane droplets suspended in grid-induced turbulent flows have been investigated in the regime where the integral length scales of turbulence are, on average, 5 times larger than the initial droplet diameter. The turbulence intensity has been increased up to 44%. A new mass transfer parameter that distinguishes between th...
51 CitationsSource
#1B. Abramzon (UCI: University of California, Irvine)H-Index: 2
#2William A. Sirignano (UCI: University of California, Irvine)H-Index: 43
Abstract The re-examination of the classical droplet vaporization model is made in order to develop the simple but sufficiently accurate calculation algorithm which can be used in spray combustion calculations. The new model includes the effects of variable thermophysical properties, non-unitary Lewis number in the gas film, the effect of the Stefan flow on heat and mass transfer between the droplet and the gas, and the effect of internal circulation and transient liquid heating. To evaluate the...
873 CitationsSource
#1R. J. Haywood (UW: University of Waterloo)H-Index: 1
#2R. Nafziger (UW: University of Waterloo)H-Index: 1
Last. Metin Renksizbulut (UW: University of Waterloo)H-Index: 16
view all 3 authors...
A finite volume numerical technique has been used to model the evaporation of an n-heptane droplet with an initial Reynolds number of 100 in air at 800 K, 1 atm. The effects of variable thermophysical properties, liquid phase motion and heating, and transient variations in droplet size and velocity are included in the analysis. With appropriate corrections for the effects of variable properties and liquid phase heating, quasi-steady correlations are shown to predict accurately the transient hist...
97 CitationsSource
Cited By8
#1Hiroshi Nomura (College of Industrial Technology)H-Index: 10
#2Shinji Nakaya (UTokyo: University of Tokyo)H-Index: 9
Last. Mitsuhiro Tsue (UTokyo: University of Tokyo)H-Index: 17
view all 3 authors...
#1Kai Han (BIT: Beijing Institute of Technology)H-Index: 8
#2Hao Chen (BIT: Beijing Institute of Technology)H-Index: 2
Last. Yuchuan Li (BIT: Beijing Institute of Technology)H-Index: 3
view all 6 authors...
Abstract In this work, the droplet burning characteristics of neat benzyl azides, diesel-benzyl azides blends (10%, 30%, 50% of benzyl azides (wt.)) and diesel fuel were investigated under atmospheric pressure. The ambient temperature around the flat-flame burner is about 1123 K. The initial droplet temperature is about 300 K. The effects of ambient oxygen concentration (14%, 21% and 30%) and initial droplet diameter (∼257 μm and ∼456 μm) on the burning characteristics were also studied. Experim...
3 CitationsSource
#1Ann Karagozian (UCLA: University of California, Los Angeles)H-Index: 24
3 CitationsSource
Fuels derived from biofeedstocks are receiving attention for their potential as additives to conventional petroleum-based transportation fuels. Normal butanol, in particular, can enhance performance compared to ethanol because of its higher energy density. To better understand the combustion dynamics of n-butanol in the context of gasoline, experiments are reported here to examine the isolated droplet combustion characteristics of an 87 octane (ethanol-free) gasoline and a mixture of gasoline (0...
14 CitationsSource
#1Shinji Nakaya (UTokyo: University of Tokyo)H-Index: 9
#2Kotaro Fujishima (UTokyo: University of Tokyo)H-Index: 1
Last. Daisuke Segawa (OPU: Osaka Prefecture University)H-Index: 10
view all 5 authors...
Abstract The burning characteristics of isolated fuel droplets for ethanol, 1-butanol and n -decane were experimentally examined in carbon dioxide-rich or argon-rich environments under microgravity. The ambience consisted of oxygen, nitrogen, carbon dioxide and argon. The concentration of oxygen was fixed to be 21% in volume percentage, and that of carbon dioxide or argon was varied from 0% to 60% in volume percentage. Detailed measurements of the droplet surface areas were conducted using a hig...
11 CitationsSource
#1Shinji Nakaya (OPU: Osaka Prefecture University)H-Index: 6
#2Daisuke Segawa (OPU: Osaka Prefecture University)H-Index: 10
Last. Tomoya Furuta (OPU: Osaka Prefecture University)H-Index: 1
view all 5 authors...
Abstract The burning and sooting behaviors of isolated fuel droplets for ethanol and n -decane are examined in high concentration of the ambient carbon dioxide under microgravity. A quartz fiber with the diameter of 50 μm maintains the droplet in the center of the combustion chamber and the range in the initial droplet diameter is from 0.30 to 0.80 mm. The ambience consists of oxygen, nitrogen and carbon dioxide. The concentration of oxygen is 21% in volume, and that of carbon dioxide is varied ...
6 CitationsSource
#1Mehdi Jangi (UGent: Ghent University)H-Index: 12
#2Benjamin Shaw (UC: University of California)H-Index: 1
Last. Hideaki Kobayashi (Tohoku University)H-Index: 6
view all 3 authors...
Numerical studies on the behaviors of combustion of 1-butanol fuel droplet at presence of upstream velocity oscillation are performed. Fuel droplet has an initial diameter of 1.25 mm and ambiance pressure and temperature are 0.4 MPa and 300 K, respectively. These conditions are those in which the microgravity experiments in literature conducted. In the excellent agreement with the experimental data, numerical results show a significant enhancement of the burning rate of droplet compared to what ...
#1Mehdi Jangi (Tohoku University)H-Index: 12
#2Hideaki Kobayashi (Tohoku University)H-Index: 6
The enhancement and hysteresis behavior of the burning rate of single droplet combustion in the presence of airstream oscillation observed in previously performed microgravity experiments at elevated pressure up to 1.0 MPa were numerically investigated. Excellent agreement with the experimental results was obtained and the mechanisms of these phenomena were examined based on precise numerical data on instantaneous droplet diameter variations corresponding to the unsteady airstream velocity, flow...
7 CitationsSource
#1Mehdi Jangi (Tohoku University)H-Index: 12
#2Hideaki Kobayashi (Tohoku University)H-Index: 6
A numerical study of the heat and mass transfer from an evaporating fuel droplet in oscillatory flow was performed. The flow was assumed to be laminar and axisymmetric, and the droplet was assumed to maintain its spherical shape during its lifetime. Based on these assumptions, the conservation equations in a general curvilinear coordinate were solved numerically. The behaviors of droplet evaporation in the oscillatory flow were investigated by analyzing the effects of flow oscillation on the eva...
9 CitationsSource