Experimental Investigation of Blending Acetylene with Iraqi LPG to Determine a Flame Stability Map

The difficult challenges facing the designers and engineers of combustion systems are the flame stability (flame stability map) represented by the limits of flashback and blow-off. In this study, acetylene gas was combined with Iraqi liquefied petroleum gas at rates (10% 50%). The reason for choosing these two components is the low cost and ease of access to it. Where the flashback limits (critical velocity gradient) were obtained from (40-485) 1/sec, while the blow-off limits were (265-2510) 1/sec with a diameter of 25 cm for the burning nozzle diameter only for Iraqi LPG without mixing acetylene. While in the case of mixing 10% acetylene, the flashback limits (critical velocity gradient) were from (30520) 1/sec and the blow-off limits (440-3985) 1/sec for the same diameter of the muzzle of 25 cm. Whereas, when mixing 30% of the acetylene, the flashback limits (critical velocity gradient) were from (55-575) 1/sec and the blow-off limits (570 4050) 1/sec. From the above three cases, noticed a relative expansion of the flame stability map for the flashback boundaries, while at the blow-off limits the amplitude was clear and large, which indicates the confidence in mixing acetylene with Iraqi LPG and obtaining a larger flame stability map. Thus, it stimulates its use in industrial fields and gas turbine power stations.


Introduction
Determining the type of fuel is the basic and economic criterion in the field of fossil energy, as well as pollution resulting from combustion reactions resulting from external combustion systems. Therefore, it is necessary to use environmentally friendly fuel that is low in cost and with high efficiency for use in industries that need to use thermal energy. The discovery of alternative energy and environmentally friendly fuels is one of the challenges necessary to face the decrease in the depletion of traditional fossil fuel sources [1,2] . Where researchers have made great efforts to find multiple types of alternative fuels, such as gaseous fuels, alcohol blends, and biofuels [3][4][5]. LPG and acetylene are a promising fuel in the field of internal combustion engines because they have unique advantages in combustion without the formation of any hindrance [6]. The characteristics of acetylene gas combustion are close to hydrogen in terms of high flame speed as well as in terms of flammability. Therefore, it is used in cutting and welding [7]. Many studies have been conducted in mixing diesel fuel with fuel for combustion engines that run on diesel fuel is highly efficient and low in pollution [10]. The ignition temperature of acetylene is high, so it has great properties that make it more reliable for diesel engines [11]. Through the above literature, acetylene gives advanced hope in combustion technology as a good alternative fuel mixed with diesel [12].
For gasoline combustion engines, acetylene mixed with ethanol alcohol can be used, as it can be obtained from non-petroleum sources for a four-stroke engine, where high efficiency and low emissions are observed from those found in gasoline [13]. The properties of acetylene gas is characterized by its high-speed flame and it needs small amounts of oxygen compared to other gases, but it is a colorless, odorless and highly flammable gas, thus giving it advantages in use in internal combustion engines [14]. Studies have shown that the use of liquefied petroleum gas is much better than the use of liquid fuels in terms of emission, efficiency, and cheapness in Iraq. However, the problems of using gases in external combustion systems represented in safety and control of flame stability made designers an obstacle in its use, but it is considered a reliable, future and promising source in the field of Modern energy [15]. In terms of properties with respect to acetylene, hydrogen and liquefied petroleum, we note a convergence of specifications in their use in external combustion systems, and thus they can be mixed without any hindrance as shown in the following Table (1) [16][17][18].

Table (1) Standard Specification for LPG Iraqi and Acetylene Fuel.
Flame instability is one of the important challenges in drawing a flame stability map represented by a flashback for the boundary layer that can be predicted [19]. Theoretical and experimental studies of the return ability of a pre-mixed flame to the airflow and fuel stream with a fixed equivalence ratio show a model for the propagation of the flame in the boundary layer [20]. Scientific investigations have shown that the inclination of the flashback has an important effect on the swirl number as when reducing the swirl from 0.66 to 0.53 it did not affect the blow-off and it has a clear effect on the inclination of the flashback [21]. The structure of the jet burner has a major role in influencing the flashback of mixed gases in turbulent streams [22]. Representation of flame instability in four mechanisms: (1) flame instability, (2) flashback boundary layer, (3) combustion vortex collapse (CIVB), and (4) core flow flashback through the use of a glass tube that shows the flame transfer to Burner nozzle [23]. The flashback mechanism of the boundary layer through the critical velocity gradient can be defined from the following equation [19]. (1) A modern technology has been devised to counter the flashback in the boundary layer by simulating engineering using fine surfaces (metal mesh) placed on the walls of the burner from the inside [24]. Vortex combustion technology has spread to stabilize combustion used in gas turbines where geometric shapes can play an important role in the flow field inside the combustion chamber that allows the flame to stabilize under pre-mixed conditions [25].
Flame instability often occurs due to two techniques, which is the flashback that occurs due to the collapse of the combustion vortex in the center of the flame nozzle and thus attacks the mixing chamber of the fuel, as well as the flashback occurs through the boundary layer of the walls of the burner where the speed of the flame prevails over the velocity of the fuel flow [26]. The geometry of the burner nozzle had a major role in drawing a flame stability map for using different length nozzles with a fixed diameter and determining the blowing and flashback limits using LPG [27][28][29]. It is necessary to draw a flame stability map for the burner, define the safety zone, define the flashback and blow boundaries, and in this study we highlight the flame stabilization zone by mixing acetylene in certain proportions with Iraqi liquefied petroleum gas.

Experimental Work
All tests were conducted under the supervision of a specialized scientific cadre at the Technical College of Engineering, Najaf, taking all occupational safety precautions using high-quality electrical control in the event of any emergency for the system, which consists of a cylindrical burner, rotometers and an air impeller with an electrical control panel, as well as using a high-precision camera. The burner was designed and manufactured in the workshops and laboratories of the College of Engineering Technology, Najaf, from welding, cutting, and examination using an old oil filter for the car and exploiting its cylindrical shape to make it a swirl burner by inserting two tubes from the bottom of the burner in two tangents with a diameter of 14 mm, one of which is used to enter compressed air and the other is used to enter acetylene gas and from the bottom a 6 mm tube was placed for the introduction of LPG, and the burner nozzle was made from the top with three nozzles with diameters (2.  The followed steps that were studied in this research were as follows: 1. Starting to supply LPG with a low flow rate for the purpose of first ignition.
2. Control the flow rates of air, acetylene, and liquefied petroleum, through rotometers and at specific mixing rates.
3. Increase the air flow rate slowly and continuously until the flame rises from the edge of the burner until blowing occurs and the flow rate is recorded.
4. Repeating the previous steps above from 1 to 3.  6. Repeating the previous steps with equivalence rates from (0.6 to 1.4) and mixing rates for acetylene from (0 -50%).

Journal of Petroleum Research and Studies
An analysis of the recorded values was used by repeating the experiment five times at least and taking the average values for the sake of accuracy of the information.

Results and Discussion
In this study experiments were conducted to determine the flame stability limits of flashback and blow-off using Iraqi LPG fuel with different mixing ratios of acetylene gas and air (equivalence ratios). Figure

Conclusions
This paper describes a set of experiments conducted to analyze and define a flame stability map for combustion mixed from Iraqi LPG with acetylene in a swirl burner using different diameters of a nozzle upstream for a set of acetylene mixing ratios and a set of equivalence ratios. The following important points were concluded: 1-The flame stabilization map (flashback and blow-off) mixed with LPG and air with mixing ratios of acetylene gas (0% -50%) for different diameters of the burner nozzle (2.5, 3.3 and 5) cm, and a set of equivalence ratios (0.6-1.4) that were measured. As all the tests were conducted in the conditions of atmospheric air and ambient temperature. Where it was found that the maximum flashback occurred at the stoichiometric ratio and decreased on both sides of the stoichiometric ratio.
2-Increasing the acetylene mixing ratio further improved the flame stability area and thus the flashback limits were slightly increased due to the high acetylene flame speed. As for the blow-off limits, it has been greatly increased and thus the flame stability zone has been expanded, which is an important indicator for the safe operation of the combustion system.
3-The large burner diameter (5 cm) nozzle improved the flame stabilization area by reducing the flashback limits compared to the lower diameter.