TOPICAL PROBLEMS OF FLUID MECHANICS 97 DOI: http://dx.doi.org/10.14311/tpfm.2016.014 DESIGN OF COMBUSTION CHAMBER FOR FLAME FRONT VISUALISATION AND FIRST NUMERICAL SIMULATION J. Kouba, J. Novotný, J. Nožička Department of Fluid mechanics and Thermodynamics, Faculty of Mechanical Engineering, Czech Technical University in Prague, Technická 4, 166 07 Praha 6, Czech Republic) Abstract This article deals with basic 2D simulation of flame front propagation in designed combustion chamber. It is designed for further research ignition and flame front propagation in gaseous mixtures. Flame front propagation is calculated for a stoichiometric mixture of methane and air at atmospheric pressure. For premixed combustion Zimont model were used. Figure of combustion chamber, a brief description of numerical calculation, and results, in graph form and sequence of figure shows the shape of a flame front, are presented. Keywords: Flame front propagation, premixed combustion 1 Introduction Due to growing demands for compliance with emission limits it is also necessary to follow up alternative fuels applicable in traditional internal combustion engines. Recently, there has been a strong increase in the number of CNG-combustion engines (Compressed natural gas). This fuel has comparing to other conventional fuels many advantages. Lower pollutant emissions (NOx, CHx and CO) and economical operation are among the most important. The natural gas is widely available in the gas pipeline network and its stocks are high. During the combustion of the natural gas solid particles do not form. It is ecologically the cleanest fuel, which also does not jeopardize atmosphere, does not contaminate soil and water resources in the event of leakage. As an alternative, a compressed biogas modified in accordance with CNG quality can be used. It is made of biomass and it is a renewable resource. CNG is currently avoiding price fluctuations typical for conventional petroleum products, such as gasoline or diesel. Due to its positive characteristics the development shall be geared towards this direction. In the internal combustion engines, the mixture burning rate is important. This work focuses on flame front propagation during CNG combustion in a simple chamber. 2 Chamber presentation The chamber (Figure 1.) intended for the experiment is designed in a cube shape with an edge of 70mm. It includes: Input of the fuel mixture Output of combustion gases Spark plug Input for temperature measurement Input for pressure measurement Optical inputs allowing for measurement with the help of LIF or PIV method Combusted CNG contains 98% of CH 4 (methane). Only dominant components in methane-air mixture shall be used for calculations in the combustion chamber. The following stoichiometric equation shall be applied: CH 4 + 2( O2 + 3,76 N 2 ) = 2H 2O + CO2 + 7, 52N 2 (1)
98 Prague, February 10-12, 2016 Figure 1: Experimental chamber for flame front visualization For stoichiometric mixture the fuel equivalence ratio Φ=1 is given by: where m is mass of components. m m Φ = m fuel m fuel oxidizer oxidizer stoich. (2) For this mixture, approximate value of laminar flame speed v L = 0.38m/s, was subtracted from graph in Figure 2. Figure 2: Experimental laminar flame velocity of methane/air mixture [1] 3 Numerical simulation For numerical calculation, a 2D structured mesh in Ansys ICEM was created. The creation of a simplified ignition point is also included in the model. Size of cell is on average 0.05mm. At the ignition point, where high velocity gradient values are assumed, the mesh refinement occurs.
TOPICAL PROBLEMS OF FLUID MECHANICS 99 Figure 3: 2D CAD model divided by blocks which are needful for structured mesh (left side) Mesh detail in ignition region (right side) 3.1 Model For numerical simulation, and post-processing Ansys Fluent 14.5 was used. The propagation of the flame front in the combustion chamber falls within the category of premixed turbulent flames. Therefore turbulent model is necessary to chose. The turbulent RSM (Reynolds stress model) in a non-stationary mode was chosen for the calculation of flow, supplemented by the premixed combustion model. RSM is the most complex RANS model offered in Fluent. Transport equations for Reynolds stress (3) and dissipation equation (4) are used [3,6]. The exact transport equations for Reynolds stresses can by written: Where C D T D L P G F Φ ε t ( ρu iu j ) +C = DT + DL, + P + G + F + Φ + ε is convection turbulent diffusion molecular diffusion stress production buoyancy production production by system rotation pressure strain dissipation., (3) The dissipation equation is written by: t x x µ ε σ ε x j 2 1 ε ε ii ε 3 ii Cε 2ρ k k t ( ρε ) + ( ρεu ) = µ + C [ P + C G ] i i j ε 1 2 Where C ε1, C ε2, σ ε1, are model constants (C ε1, =1,44, C ε2, =1,92, σ ε1 =1 ) C ε3 is evaluated as a function of the local flow direction to the gravitational vector. (4) In combustion model the flame is modelled as a progress variable c propagating at v L, which is the laminar speed of the flame propagation. The mixture is separated by the reaction zone into reactants (progress variable = 0) and products (progress variable = 1). In this model, the flame front has no thickness and is represented by a step change of the progress variable. As a transport equation for the progress variable a C-equation [4] was chosen, which is written in RANS in the following way: t µ S t ( ρc ) + ( ρvc ) = c +ρsc where c is reaction progress variable S ct turbulent Schmidt number for the gradient turbulent flux reaction progress source term S c Ct (5)
100 Prague, February 10-12, 2016 Condition for C-equation is relation ρs where ρ u is density of unburned mixture S t turbulent flame speed =ρ + S c c u t (6) A Zimont model was chosen for the calculation of the turbulent burning rate, used for most engineering applications. A prerequisite for the use of the Zimont model is that the flame propagation is dependent on the large scale turbulent structures. The Zimont model is given by [3] : where G is flame stretching factor A model constant u RMS velocity v l laminar flame speed α unburned thermal diffusivity turbulent length scale l t 3.2 Initial conditions Basic parameters of the calculation are summarized in Table.1 4 Results 3 1 1 1 ' S t = GA[ u ] 4 + [ vl ] 2 + [ α] 4 + [ lt ] 4 (7) reference pressure 101325Pa temperature of burned mixture 300 K temperature of unburned 2222K laminar flame speed 0,38 m/s density of unburned mixture 1,95 kg/m 3 energy at ignition 0,1 J radius of ignition area 0,02 mm duration of the ignition 2 ms Table. 1 The diagram in the Figure 4 shows the course of the progress variable depending on time. This variable indicates the course of burning in the combustion chamber. The course can be divided into four parts. In the first part, in time from 0 to 1.3ms, the mixture ignition and flame front propagation toward the edge of the spark plug are taking place. The flame front is limited here by the plug wall. In the range from 1.3 to 2.2ms the flame leaves the spark plug area and enters the space. From 2.2ms to 5.8ms the flame grows indefinitely and the curve shows linear character. In the last part the flame is limited by the chamber and the mixture is continuously burning out. The total burn time is 7.5ms.
TOPICAL PROBLEMS OF FLUID MECHANICS 101 Figure 4: Average value of progress variable in time Figure 5: Contours of progress variable showing flame front propagation 5 Conclusion This paper describes 2D simulation of flame propagation in a simple combustion chamber. The use of the premixed combustion model is not ideal for the planned comparison with the experimental LIF measurement. This model does not enable monitoring of the flame front with the help of OH radicals used in the LIF method. However, it is sufficient for initial estimation of the mixture burning time in the combustion chamber. This will contribute significantly to future adjustment of the experiment and further numerical simulations. This 2D simulation also serves as a basis for further numerical analysis including the calculation of chemical reactions and radiation impact. The planned calculations shall be done in 3D.
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