Measurement of Laminar Burning Velocity of Liquified Petrolium Gas

May 22, 2012 - burning velocity for LPG−air mixture can also be observed in the available literature. It is also clear from Table 1 that the experim...
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Measurement of Laminar Burning Velocity of Liquified Petrolium Gas Air Mixtures at Elevated Temperatures Mohammad Akram and Sudarshan Kumar* Department of Aerospace Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India ABSTRACT: The present work reports the measurement of the laminar burning velocity of liquefied petroleum gas (LPG)−air mixtures at high temperatures using the planar flame propagation mode appearing in the preheated mesoscale diverging channel. The experiments were carried out for a range of equivalence ratios, 0.7 ≤ Φ ≤ 1.3. The present data for LPG−air mixtures are reported for a temperature range of 370−650 K in comparison to maximum mixture temperatures of 400 K reported in the literature. Experimental studies complimented with computational studies for conditions similar to present experiments confirm that the effect of heat loss from flame to channel walls on burning velocity is minimal and measured burning velocities are nearly equal to adiabatic burning velocity. The stabilized flame is nearly flat in both transverse and depth directions. The power law form of correlations from present experiments help in understanding the variation of laminar burning velocity with mixture temperatures and equivalence ratios. An increase in mixture temperature significantly enhances the burning velocity. Maximum burning velocity is obtained for slightly rich mixtures, unlike for the highly rich mixtures reported in the literature. A minimum value of the temperature exponent is observed for slightly rich mixtures.

1. INTRODUCTION With increased awareness about the emissions of greenhouse gases, efficient energy usage and reduction of pollutant emissions from combustion systems has challenged combustion researchers in the past decade.1 Alternative fuels, such as liquefied petroleum gas (LPG), are usually considered as clean fuels compared to diesel and gasoline. Therefore, the introduction of these alternative fuels is beneficial for achieving clean combustion with higher combustion efficiency, reduced fuel consumption, and reduced pollutant emissions. LPG finds wide applications ranging from domestic heating and cooking to powering of automotive vehicles. Despite many advantages of LPG over gasoline and other conventional fuels, very few studies related to its combustion characteristics have been reported in the literature.1−9 For instance, laminar burning velocity is an important parameter describing many features related to the reactivity, diffusivity, and exothermicity of a particular fuel−air mixture. Laminar burning velocity of a combustible mixture is of fundamental importance and enables the validation of various chemical kinetic mechanisms of the fuel and prediction of the performance and emissions from various combustion systems such as internal combustion engines, industrial furnaces, and gas turbine engines.3 The accurate knowledge of laminar burning velocity is essential for carrying out the feasibility studies of LPG as an alternate fuel in various combustion systems.3 The determination of burning velocity for various mixtures is also important from a design viewpoint of fire safety equipment, flame flashback, explosion protection, and fuel tank venting systems. In the available literature, many experimental techniques have been reported to measure the burning velocity of fuel−air mixtures subjected to heat loss and stretch effects due to hydrodynamic strain rates.10−24 One of the most popular methods is the closed vessel method.10−16 The closed chamber is filled with the reactive mixture and ignited at the center. The © 2012 American Chemical Society

laminar burning velocity is extracted from the pressure-time records or change in flame radius using a high-speed imaging technique. Corrections for flame stretch in many cases have not been made.10−12 Opposed jet method or a single jet with a stagnation plate17−20 is another commonly used technique for measuring the laminar burning velocity at ambient temperature. Although the effect of stretch has been carefully dealt through linear extrapolation of burning velocity to zero stretch rate conditions, Goey et al.21 have pointed out that linear extrapolation may result in certain inaccuracies due to the nonlinear effect of stretch on the laminar burning velocity. Also, there is some amount of heat loss from stabilized flame to the stagnation plate or jet nozzle which authors have not accounted.17−20 In other methods, stabilizing a flame using a porous disk and recording the variation of flame speed with heat carried away by the porous disk and through cooling water was reported by Botha and Spalding.22 A similar technique called the heat flux method has also been reported in the literature.21,23 In this method, to achieve the adiabatic conditions, the heat loss from the burner to the surrounding walls is compensated by heating the burner with hot water. Authors23 claim that heat flux method gives adiabatic burning velocities of the mixtures with a planar flame stabilized using a number of small flames. Recently, Kim and Kim24 used an annular diverging tube to predict the laminar burning velocity of fuel−air mixtures at ambient conditions. The measured burning velocities of fuel−air mixtures match well with the existing results. However, they have not considered the effect of heat loss and thermal regeneration between flame and walls in such small channels. Received: January 18, 2012 Revised: May 15, 2012 Published: May 22, 2012 3267

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metric methane−air mixture was extracted from these planar flames and matched well with the existing experimental correlations and numerical predictions.27,28 In the present work, this study has been extended to LPG−air mixtures for a range of equivalence ratios at elevated mixture temperatures.

Very few researchers have reported the burning velocity of LPG−air mixtures,1,3,7−9 which has been summarized in Table 1. A large variation in the magnitude of burning velocity of Table 1. Existing Literature of LPG−Air Mixture Burning Velocity ref

Φrange

Trange (K)

2 2 correlation 7, 8 9 9 correlation 6 6, INSFLA model

0.95−1.9 0.8−2.0 0.6−1.4 0.75−1.9 0.745−1.9 0.8−2.0 0.6−1.8

298 298 300−400 290−400 290−400 298 298

Su,o (Φ = 1.0) 0.16 0.18 0.403 0.26 0.3 0.35

2. EXPERIMENTAL SECTION In the present work, a mesoscale quartz channel with inlet dimension of 25 × 2 mm (aspect ratio 12.5) and 10° divergence angle was chosen. A schematic diagram of the experimental setup with detailed channel dimensions is shown in Figure 1. The starting point of the diverging section is considered as a reference point and marked as “o”. X-axis is the axial direction and Y-axis is transverse direction. A premixed fuel−air mixture at ambient conditions (300 K, 1.0 atm pressure) was supplied to the channel through electric mass flow controllers connected to a personal computer through a command module. A sintered metal burner was used to externally preheat the channel walls to stabilize the flame with different preheat temperatures and to minimize the heat loss from flame to the solid walls. This also helped in initial flame ignition and subsequent stabilization in the channel. LPG fuel containing 40% propane and 60% n-butane by volume was used. The flow velocity and gas mixtures were regulated through calibrated mass flow controllers (MFC). The accuracy of these AALBORG MFCs is ±1.5% of the full scale. Temperature measurements were carried out with K-type thermocouples. The movement of the thermocouple was controlled through a precisely controlled traverse (0.25 mm minimum resolution). Measured temperatures were accurate to ±5 K of the actual value. The bottom wall temperature profiles in two directions (axial and transverse) of the inner side were measured in advance with airflow. Figure 2 shows the measured temperature distribution in the transverse direction for a particular constant heating rate with an airflow velocity of 0.3 m/s. Since for Hele-Shaw flow,29 the velocity remains uniform in the transverse direction, it is clear that the measured temperature is uniform in the transverse direction except near the walls and outlet. The measured axial temperature along the

(Su,o, max) Φ (0.35) 1.35 (0.35) 1.40 (0.405) 1.05 (0.43) 1.35 (0.445) 1.5 (0.42) 1.30 (0.425) 1.15

LPG−air mixtures can be observed in the literature. For instance, the burning velocity of the stoichiometric LPG−air mixture varies from 0.16 to 0.403 m/s, as can be observed from Table 1. A major discrepancy in the existence of maximum burning velocity for LPG−air mixture can also be observed in the available literature. It is also clear from Table 1 that the experimental data for burning velocity are available only for a very small range of mixture temperatures varying from 298 to 400 K. However, in many practical devices such as IC engines, industrial furnaces, and gas turbine engines, the local temperature of the mixture is significantly higher than this reported temperature range. Further, in recent years, direct preheating of mixtures is being actively pursued to enhance thermal efficiency and simultaneously reduce pollutant emissions including NOx from combustion systems.25,26 This necessitates the need for accurate data of flame burning velocity at elevated mixture temperatures. In earlier studies on flame propagation in preheated divergent channels, authors have observed the formation of a planar flame propagation mode in high aspect ratio diverging channels for certain conditions of flow rate and mixture equivalence ratios.27,28 The burning velocity of the stoichio-

Figure 1. Schematic of experimental setup with detailed dimensions of diverging channel. 3268

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to 0.05 mm in both directions. The grid was subsequently refined within the reaction zone through grid adaptation scheme and the grid size was as small as 2−5 μm within the reaction zone. This provided sufficient resolution to the propagating flame front within the computational domain and the results obtained are grid-independent.30,31 The present computations were carried out with propane− air mixtures since the laminar burning velocity and adiabatic flame temperature is very close to the LPG−air mixture (less than 5 K). To examine the effect of flow divergence on flame stability, a single-step global reaction mechanism was chosen along with Fick’s law of diffusion for species diffusion. The activation temperature, Ta is taken to be 15 098 K and constant parameters m and n are taken to be 0.1 and 1.65, respectively, as recommended by Westbrook and Dryer.32 The preexponential factor, A, was taken as 8.6 × 1011 (k mol cm−3)−0.5 s−1. Initially, the flow was solved for a nonreacting case with complete species transport. Once a converged solution is obtained for a nonreacting case, a flame is initiated near the exit plane of the diverging channel by introducing a high-temperature patch in the computational domain. A similar procedure has been reported in literature.30,31 The computations were carried out for a 2-mm thick quartz channel with different wall heat transfer conditions such as adiabatic, isothermal and various convective heat transfer conditions. As a special case, for comparison of present experimental conditions with various wall heat transfer conditions, the channel walls were externally heated with the flow of hot gases from bottom side of the channel to simulate the realistic experimental conditions. The only air flow axial wall temperature profile obtained by blowing the hot air over the channel matches closely with the experimentally measured temperature profile as shown in Figure 3 for a typical airflow velocity of 0.5 m/s. 3.1. Preliminary Computational Results. 3.1.1. Adiabatic Burning Velocity. The external preheating of the channel walls compensates for the part of the heat loss from flame to the channel walls. Figure 4 shows the variation of heat flux on the inner and outer side of the channel walls for a mixture velocity of Uinlet = 0.5 m/s with preheated wall conditions. Such

Figure 2. Wall temperature distribution inside the channel for Uairflow = 0.3 m/s.

center line was near linear in the axial direction as shown in Figure 3.

Figure 3. Comparison of experimental and computational temperature profile for an airflow velocity of 0.5 m/s.

3. COMPUTATIONAL DETAILS Numerical simulations were carried out using a general purpose CFD code Fluent 6.3.26, and the results obtained were analyzed along with the experimental results for understanding the effect of energy interaction between flame and solid walls, flame structure, and thermal feedback through solid walls. Two dimensional Navier−Stokes equations were solved along with energy and species conservation equations. The flow Reynolds number varies in the range of 80−720 for the given flow rate conditions. Therefore, laminar flow calculations were carried out to obtain the distributions of velocity, pressure, species mass fraction, reaction rate, and temperature in the computational domain. The solution was considered converged when scaled residuals of mass, momentum, energy, and species had dropped by 5 orders of magnitude, and there was no further appreciable change in the respective residuals. Numerical simulations were carried out for the same operating conditions as that of experiments. The initial grid system scales were equal

Figure 4. Variation of heat flux on the inner and outer wall for stoichiometric propane−air mixture at Uinlet = 0.5 m/s with preheated wall condition. 3269

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the heat loss from flame to the wall in the present experiments and corresponding arrows (horizontal lines) show the respective deviation of present burning velocity from adiabatic value. It is clear from this figure that for present experiments, the deviation of the value of laminar burning velocity is less than 4% from adiabatic burning velocity. Reduction in burning velocity magnitude with heat flux is considered in the uncertainty analysis and subsequently corrected. 3.1.2. Thermal Feedback. In the postcombustion zone, the hot combustion products lose heat to the solid walls. A small part of this energy is transferred in the upstream direction through solid walls, resulting in preheating of fresh reactants. This is termed as thermal feedback or heat recirculation from hot combustion products to the fresh reactants. Thermal feedback essentially increases the enthalpy of combustion, resulting in increased burning velocity of the mixture. In Figure 4, the negative heat flux on the inner wall surface shows the thermal feedback via conduction through solid walls. This magnitude is very small compared to the magnitude of the heat transferred from the flame to the walls. This is due to the fact that the channel walls are already preheated to a sufficiently high temperature through external heating. Further, the thermal conductivity (1.4 W/m K) of the channel wall material is relatively very small, which makes the thermal feedback or heat recirculation relatively less important. The increase in burning velocity with such small heat flux is also considered in the uncertainty analysis. 3.1.3. Flame Structure. To understand the variation of flame shape in depth direction due to a change in wall heat transfer boundary conditions, detailed computational studies were carried out. For a case with fully developed flow and adiabatic wall conditions, the observed flame shape is relatively positively stretched, as shown by the reaction contours (nondimensionalized with maximum reaction rate) of the stabilized flame in Figure 6a. As the wall convective heat transfer coefficient is

studies were carried out for a range of equivalence ratios (Φ = 0.7−1.3) and inlet velocities (0.3−0.7 m/s) with experimentally observed wall temperature profiles. The heat flux on the inner wall near the flame is observed to be in the range of 100−300 kW/m2 for different mixture and temperature conditions. The heat flux on the outside wall is observed to be in the range of 10−30 kW/m2. The heat transfer from stabilized flame to the walls influences the burning velocity of the mixture.22 However, in the present case, external preheating of the channel substantially reduces the amount of heat loss, thus making the conditions very close to adiabatic. Botha and Spalding22 have reported the variations of laminar burning velocity with the heat carried away from the flames. Figure 5 shows the variations of normalized burning

Figure 5. Variation of burning velocity with heat transferred from flame to solid walls for stoichiometric mixture with preheated wall condition (Data taken from Botha and Spalding22).

velocity with heat flux reported for stoichiometric propane−air mixtures. In Figure 5, dashed (vertical) lines show the range of

Figure 6. Nondimensionalized reaction rate contours of stabilized flames in the channel with different wall heat transfer conditions for a stoichiometric propane−air mixture at Uinlet = 0.5 m/s. 3270

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varied from adiabatic to about h = 50 W/m2K, the flame loses heat to the walls and the stabilized flame front becomes flat. Further increase in the wall heat transfer coefficient results in the formation of a negatively stretched flame and at very high values of heat transfer coefficient (i.e., isothermal wall heat transfer condition), flame extinction occurs. However, for the case of the preheated channel wall conditions, the flame front is almost flat in depth direction as shown in Figure 6e. The flame does not stabilize for isothermal wall boundary conditions in a 2 mm channel. The flame front shape in a 3 mm channel with isothermal wall conditions is negatively stretched, as shown in Figure 6h. Similar flame structure observations have been reported by Kim and Maruta30 for adiabatic and isothermal boundary conditions with methane−air mixtures.

Table 2. Uncertainty Analysis for Reference Temperature of 300 K and Mixture Temperature of 413 K parameters reactant/avg. wall temp difference