EXPERIMENTAL CHARACTERIZATION OF A EJECTOR PUMP

Transcription

1 EXPERIMENTAL CHARACTERIZATION OF A EJECTOR PUMP Capela, N. J. S. Department of Mechanical Engineering, Instituto Superior Técnico, Av. Rovisco Pais, , Lisbon, Portugal, 2012 ABSTRACT The option of future focuses on combustion processes under poor and with low levels of pollutant formation. In this context, this paper studies an ejector for a propane gas burner, fitted to a pot used for making food in the Portuguese Army, to understand the air entrainment mechanism, with a view to identifying the causes that can contribute to a decrease in the equivalence ratio of the air/fuel mixture. These burners are operating strictly mechanical devices and simple design that enable a range of heating powers from 8 and 22 kw. Allow a good stability of flame which is a fundamental requirement of security. The power setting is performed through a valve adjustment, containing three positions, allowing work on zero power, minimum and maximum. In order to achieve this goal, as a first step, has undergone the system comprising a ejector, the experimental characterization of the flow field of ejector air intake, from the Particle Image Velocimetry diagnostic technique (PIV). On that way it evaluate in which form the outside air flow influences the behavior of the flow of the air/fuel mixture at the outlet of the burner. In a second phase, the diagnostic technique Chemiluminescence, measurement of chemical species, to assess the behavior of various changes ejector parameters such as fuel flow, air intake area and output area of the diffuser. It is concluded that the ejector seems "insensitive" to change the input and output area, in relation to the average value of equivalence ratio. But it is observed that the air/fuel mixtures are more homogeneous when the areas of input, output and throat have close values. KEYWORDS: Particle Image Velocimetry, Chemiluminescence, burner, equivalence ratio, ejector. 1

2 1. INTRODUCTION The gas burners are intended to obtain thermal energy through combustion of a gaseous fuel (e.g. natural gas, propane, butane, etc.). The burners can be divided into three classes, depending on the type of calls, that is, as the fuel and oxidizer enters in contact. In a diffusion flame (or not aired), the reagents are initially separated and combustion occurs with the diffusion of oxygen from the surrounding atmosphere in emerging gas flow. In a pre-mixed flame, the fuel and oxidizer are mixed together and then that gives the ignition of the mixture. If the complete combustion with primary air supply, the flame takes the fully ventilated designation or fully premixed. If only part of the total air required is provided in primary air, then the flame takes the name partially aired, and the remaining air (known as secondary air) diffuses in the hot flue gas downstream of the flame front. Similarly, the Portuguese Army also accompanies this phenomenon worrying more and more with the economic factor as well as the functional factor. The work done in this dissertation aims to assess and understand the operating mechanism of a burner type "ejector pump" which team industrial-type stoves used in the armed forces. These burners work with flames partly aired. The ejectors are devices used to induce a secondary fluid, air, on impulse and energy transfer from a primary jet fuel, high-speed. 1.1 Characterization of ejector The figure 1 illustrates one of 38 burner ejectors and the table 1 its main dimensions, designed to work with propane. Are strictly mechanical operating devices and simple design that enable a range of heating powers from 8 and 22 kw. Must allow good stability of flame which is a fundamental requirement of security. The power setting is performed through a valve adjustment, containing three positions, allowing work on zero power, minimum and maximum. Figure 1- System burns. A- front view, B- side view, C- Schematic representation of the dimensions Table 1- Principal dimensions of the ejector 2

3 The mechanism of primary air inlet or air premix can be described with reference to figure 2. The gas flow emerges as free Jet gun, at a flow rate that depends on the gas pressure, the diameter of the gun and the gas composition. The transfer of momentum between the Jet and the surrounding air causes the entry and expansion at the beginning of the throat. On the other hand static pressure upstream must be higher to win the resistances of the fluid inside the burner. In this way the gas/air mixture reaches the burner head and exits by the air cap holes. Figure 2- Nomenclature zones characteristics of the burner used 1.2 Working principle of ejectors The working principle of ejectors can be described in three sections: 1- Nozzle Section: the primary nozzle is usually a convergent-divergent type nozzle. A high pressure fluid, known as primary fluid expands and accelerates through the main nozzle, flowing out of the nozzle with supersonic speeds to create a region of very low pressure in the nozzle exit plan and, subsequently, in the throat. Therefore, a differential pressure between the currents at the nozzle exit plan and the entry of the suction Chamber is established, which will induce a secondary fluid runoff. Since the speed is too high, the adiabatic and isentropic process consideration is applicable. 2- Mixing Section: based on the theory of Munday and Bagster [1], it is assumed that the primary fluid flows without mixing with the secondary fluid immediately. That expands and forms a convergent duct to the secondary fluid. In some cross-sections along the conduct, the secondary fluid speed increases to supersonic values and suffocates, were named as "hypothetical throat" (or "fictitious" or "sore throat aerodynamics" in some literature). Then begins the process of mixing. It is assumed that the pressure of the two fluids is uniform in the throat. In addition, this mix causes the primary fluid speed download while the secondary fluid is accelerated. At the end of the Canyon, the two streams are completely mixed and it is assumed that the static pressure remains constant. Due to a high pressure region downstream of the throat of the mixing chamber, the stream passes through a succession of normal shock waves and/or oblique accompanied by a corresponding rise in pressure. This shock causes an effect greater compression and a sudden drop of supersonic flow velocity to subsonic. In addition, the fluid mixture easily undergoes phase change and shock may occur. 3- Diffuser Section: the mixture of primary and secondary streams passing through the diffuser and converts the kinetic energy into pressure energy. At the exit of the diffuser, the speed is reduced to zero and the pressure is high enough to cause the discharge. 3

4 2. THEORETICAL MODEL The primary air entrainment characteristics of an ejector are studied based on simple principles of conservation of mass, momentum and energy. The following assumptions are calls to simplify the analytical formulation: 1- The flow is turbulent mixing tube such that there is no radial component of the velocity, temperature or composition; 2- Air is drawn perpendicular to the axis of the mixing tube and does not contribute, therefore, to any axial moment. This is not entirely true, however in [2], it is considered that the approximation is valid, because any increase in strength due to axial air entrainment is effectively compensated by the increase in load loss and for losses due to friction in the throat; 3- Flow is incompressible, that is, the pressure is not dependent on density. This is a fair approximation to the pressures used in domestic burners. The effect of temperature is also ignored; 4- the distribution of pressure in the burner head is uniform; 5- the combustion chamber is at atmospheric pressure; 6- Fluctuations of unburned gas/air mixture are omitted. This is a fair assumption, except for low flow rates in pipes of vertical mixing. The figure 3 shows a schematic drawing of the burner system with different sections used in air entrainment model. The model is composed of 3 equations [2,3]. Figure 3- Scheme entrainment of air in ejector The mass balance between the input and output of the burner is given by: Momentum balance between section 1 and section 2: + = = + Energy balance between section 2 and section 3: = Where k is a coefficient of loss between the throat and the diffuser expressed as a fraction of the kinetic energy at the beginning of the diffuser. 4

5 Energy balance between section 3 and section 4: = 1 2 To solve the system of equations above we need to introduce the perfect gas equation that is given by the equation: In a gas mixture, = =! = Replacing the two previous equations we get, = 1 # % " $ The can be determined by the manipulation of the previous equations. The equivalence ratio of the fuel-air mixture is given by equation: &= ' ( ' ( ) 3. EXPERIMENTAL RESULTS PIV AND CHEMILUMINESCENCE The aim of this study, initially, was to study, assess and understand the air entrainment mechanism. To achieve this end appealed to the experimental techniques of PIV and Chemiluminescence, changing the fuel flow, this value is known. The results allow obtaining the entrained air flow, the equivalence ratio of the mixture off the diffuser and classifying the mixture for its homogeneity. In a second phase, evaluate, through Chemiluminescence, the influence of the variation of three parameters: the fuel mass flow rate; the output area of the ejector; and the air intake area. With a view to identifying the causes that can contribute to a decrease in the equivalence ratio of the air/fuel mixture. 3.1 Visualization of the flames It is important to analyze the typical form of the flames in the ejector for fuel. The burner in question contains a valve with 3 operating positions: closed, minimum power consumption and maximum power. This subsection applies if these positions are within the bounds of flame stability, i.e. if there is no return flame, when the burner operates at the minimum power or separation of flame when the burner operates at full power, figure 4. 5

6 Figure 4- Evolution of the flame by increasing the fuel flow Fuel mass flow rate, charged by the ejector, minimum power, is next shown in Figure 4 with = / 123 h6, this value is fairly far from the return flow of calls, identified in Figure 4 with = / 123 h6. With regard to the operation of the burner at full power, the value shown in Figure 4 with = / 123 h6, can show that the flame is stable and clear the values for which begins to identify separation of calls in the middle of the output section of the diffuser, = / 123 h6. In this context, it can be said that the flame is inside the limits of stability for the burner operating ranges. 3.2 Evaluation of air entrainment mechanism The graph 1 represents the distribution of speeds at the entrance of the ejector, obtained in PIV, for different fuel flow. Graph 1- Velocity profiles at several measurements PIV 6

7 The values shown in table 2 are used in this section to determine the air flow rates dragged and equivalence ratio. Table 2- Values of stoichiometric ratio air / propane, densities of air and propane Taking the speed profiles obtained in the Graph 1, it is possible, through integration in the area and an increase in the density of air, obtain the mass flow rate through the holes and consequently the equivalence ratio. These results are presented in the following table: Table 3- Values of medium velocity and f of the flow around the burner. The system used to quantify the radiation emitted, Chemiluminescence, follows the code described by Trinity et al [4]. The results obtained by post-processing MATLAB use embedded information in digital images obtained by conventional CCD cameras. The Graph 2 shows the variation of the ratio of equivalence with the volumetric flow rate obtained in experimental trials and in the model. Graph 2- Evolution of f with the fuel mass flow At this stage, the model was calibrated, to make your answer more credible and close to the ejector operation conditions under study. To do this, moved the loss coefficient, k, between the throat and the diffuser until reaching the value, for the equivalence ratio, of This value has been used because it is the predominant in the obtained experimental results. A value of k = 0.17, as can be seen in the Graph 2. Using this value, the theoretical model, to confront all the results obtained experimentally in Chemiluminescence. Faced with these results we can validate the model, to the extent that the equivalence ratio does not vary with the fuel flow. There is, in the Graph 2, that there is a big difference between the values obtained in the PIV and those obtained in the theoretical model and Chemiluminescence. It was through the trials of PIV, the entrainment of air intake holes, is not symmetrical. It is observed through the profiles of speeds, greater acceptance by upper air inlet hole. This irregularity has grown for higher fuel flow rates. 7

8 3.3 VARIATION THE AREAS OF THE AIR INLET AND OUTLET DIFFUSER It is observed in Figure 5, the flame height decreases as we increase the output area. The appearance of yellow tips in the areas of lower output, petered out as it approaches the throat area and air intake, A ;<=>?; = B 1m 6, and disappearing completely for higher values. It turns out that, after reduction of the output area of the diffuser, the ejector is working on a range of equivalence ratio average of With the increased area occurs initially an ascent of f to a value of 1.22, constantly falling slightly to f = 1.18, at maximum aperture possible. Before these results, that the ejector is insensitive, in average terms, the alteration of the output area. However these changes have consequences on the quality of the mixture, making it more heterogeneous, Figure 5. Graph 3- Variation of f with the exit area The Graph 4 show two distinct phases, the reduction and the increased air intake area, in relation to the physical characteristics of the ejector, A DEFG; = B 1m 6. Observe that the reduction of the entrance area is reflected in the reduced ratio of equivalence, i.e. an increase of the aspirated air flow, passing the value f = 1.26, without modification, for f = 1.23, with two holes closed. On the other side, it turns out that increasing the entrance area is expressed in increased equivalence ratio, f = However these changes have consequences on the quality of the mixture, making it more heterogeneous, Figure 6. Graph 4- Variation of f with the air inlet 8

9 Figure 6- Variation of f with the exit area Figure 5- Variation of f with the air inlet 4. Conclusions In this present work understood and assessed the air entrainment mechanism of an ejector, which form part of a set of ejector 38, a burner of a Lunchbox. To this end it has become necessary to characterize the stability zone of the flame and to evaluate the response to changes, such as fuel flow, air intake area and output area of the diffuser. From the change of fuel flow, it was possible to see the flame stability zone, i.e. the area between the separation and return. It was found that the return of calls occurs to = / 123 h6. And the separation of calls to = / 123 h6. The burner, in the minimum and maximum power, debits = / 123 h6 and = / 123 h6, respectively, by ejector. It is concluded that the burner operates within the bounds of flame stability. It is concluded, through Chemiluminescence, which tests the ejector works with an equivalence ratio of From this value, set the template by changing the coefficient loss, k, between the throat and the exit of the diffuser, coming up to a value of k =

10 Thus, for low flows, the talcum powder particles are not good markers for getting results from PIV. It turns out that the small difference in densities between the talcum powder and the air has a high weight in the velocity values, i.e. there is a greater discrepancy between the air flow speed and talcum powder. It was through the trials of PIV, the entrainment of air intake holes, is not symmetrical. It is observed through the profiles of speeds, greater acceptance by upper air inlet hole. This irregularity has grown for higher fuel flow rates. It is concluded that the ejector seems "insensitive" to change the input and output area, in relation to the average value of equivalence ratio. Is observed a slight decrease of f with the increase of the output area and a slight increase of f with increased air intake area. However it is observed that the air/fuel mixtures are more homogeneous when the areas of input, output and throat have close values. REFERENCES [1] S. He, Y. Li, and R. Wang, Progress of mathematical modeling on ejectors, Renewable and Sustainable Energy Reviews, vol. 13, pp , [2] H. Jones, The application of combustion principles to domestic gas burner design, B. Gas, Ed. London, [3] F. M. White, Fluid Mechanics, McGraw-Hill, Ed.Fourth Edition. [4] T. P. Trindade, E. C. Fernandes, and J. M. Sanches, Color Image Processing as a Monitoring Tool in Gas Combustion Systems, I. S. de Engenharia de Lisboa (ISEL) and I. S. T. (IST), Eds.,