Numerical study

Chapter 3 Experimental setup and Methodology

3.4 Data Processing

3.4.5 Numerical study

One-dimensional freely propagating simulations were performed for analysis in conjunction with experimental data. A freely propagating planar flame model in Ansys Chemkin [106] was used to estimate unstretched LBV. The 1D spherically expanding flame model in Cosilab [9] was used to estimate radius-time, stretched flame speed-time, stretch rate-time data, and later, unstretched flame speed and LBV. Apart from the above simulations, several transport parameters were obtained from the Transport property calculator by Colorado state university [114]. All simulations were performed using GRI Mech 3.0 (53 species and 325 elementary reactions) [108], FFCM-1 (38 species and 291 elementary reactions)[117] and USC Mech 2.0. model (111 species and 784 elementary reactions)[118].

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3.4.5.1 Freely propagating planar flame model in Ansys Chemkin

Planar flame considered in this model was a quasi-steady and adiabatic flame with uniform inlet flow conditions, specified pressure, and temperature. As a result governing equations were reduced to the following forms [119].

𝑚̇ = 𝜌𝑢𝐴 3.32

𝑚̇^𝑑𝑇

𝑑𝑥− ¹

𝑐_𝑝 𝑑

𝑑𝑥(𝜆𝐴^𝑑𝑇

𝑑𝑥) + ^𝐴

𝑐_𝑝∑ 𝜌𝑌_𝑘𝑉_𝑘𝑐_𝑝𝑘^𝑑𝑇

𝑑𝑥

𝐾𝑘=1 + ^𝐴

𝑐_𝑝∑^𝐾_𝑘=1𝜔̇_𝑘ℎ_𝑘𝑊_𝑘+ ^𝐴

𝑐_𝑝𝑄̇_𝑟𝑎𝑑= 0 3.33 𝑚̇^𝑑𝑌𝑘

𝑑𝑥 + ^𝑑

𝑑𝑥(𝜌𝐴𝑌_𝑘𝑉_𝑘) − 𝐴𝜔̇_𝑘𝑊_𝑘 = 0 3.34

𝜌 =^𝑃𝑊̅

𝑅𝑇 3.35

Where P was the pressure, x - the spatial coordinate, A - cross-sectional area of the stream tube, 𝑚̇ - mass flow rate (independent of x), 𝑄̇_𝑟𝑎𝑑 - heat loss due to gas and particle radiation, T - temperature, Vk - diffusion velocity of the k^th species, Yk - mass fraction of the k^th species, hk - specific enthalpy of the k^th species, u - velocity of the fluid mixture, 𝜔̇_𝑘 - molar rate of production by chemical reaction of k^th species per unit volume, 𝜌 - mass density, cpk - constant pressure heat capacity of the k^th species, Wk - molecular weight of the k^th species, cp - constant pressure heat capacity of the mixture, 𝑊̅ - mean molecular weight of the mixture, 𝜆 - thermal conductivity of the mixture, and R - universal gas constant. The mentioned continuity (Eq. 3.32), energy (Eq. 3.33), species (Eq. 3.34), and closure equation of state (Eq. 3.35) equations were solved by discretizing using implicit finite difference methods on a non-uniform grid. The molar rate of production by all species from all chemical reactions (represented by 𝜔̇_𝑘 for k^th species) as specified in the selected chemical kinetic mechanism was taken into account. In the present work, GRI Mech 3.0 and FFCM-1 were used. A mixture averaged transport model was used. The mass flow rate needed was determined as an eigenvalue in the current problem. This was achieved by fixing the flame in the computational domain at one location. The flame was tracked with a temperature of 500 K above the unburned gas temperature. Inlet boundary conditions were known mole fractions of reactants, initial temperature, and pressure.

Boundary conditions on the burned gas side were defined as the zero gradients of all the parameters. The grid adaptation technique was used to resolve the strong gradients as the

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conservation equations were stiff due to the inclusion of chemical kinetics. GRAD and CURV parameters were set to 0.02 and 0.2 and obtained grid-independent solutions.

TWOPNT steady-state solver was primarily used for solving the governing equations[119,120]. The simulation was initiated with a few grid points, and the final solution was achieved through different continuation. The accuracy of the obtained result was crosschecked with equilibrium flame temperature and mole fractions of product species obtained from equilibrium calculations using the equilibrium (EQUIL) model.

3.4.5.2 Freely propagating flame simulation in Cosilab

In this case, the flame front was considered propagating in the defined spatial domain (the beginning and end of the domain corresponding to the center and end boundary wall of the spherical chamber). The flame propagation and tracking were achieved by fixing the frame of reference to the flame front and adopting a Lagrangian formulation. This simulation assisted in generating essential data such as flame front radius, flame speed, and flame stretch with respect to time analogous to propagation of flame in experiments.

The flame simulations were performed under both constant volume and pressure configurations. The original governing equations were considered in a polar coordinate system to achieve a symmetrical domain to explore spherical flames. The transformation of governing equations into lagrangian form: The independent mass-weighted lagrangian variable (ψ) was used instead of the physical distance variable (r - the spatial coordinate).

𝜓(𝑟, 𝑡) = ∫ 𝜌(𝑟, 𝑡)𝑟_𝑟^{𝑟 𝛼}𝑑𝑟

0 3.36

Where r0 was a reference value of r, t was time, 𝜌 - mixture mass density, and α - geometrical parameter.

𝜕𝜓

𝜕𝑟 = 𝜌𝑟^𝛼 3.37

𝜕𝜓

𝜕𝑡 = ∫ ^{𝜕𝜌𝑟𝛼}

𝜕𝑡 𝑑𝑟

𝑟

𝑟0 = −𝜌𝑟^𝛼𝑣 + 𝑚₀(𝑡) 3.38

Where m0 was the value of 𝜌𝑟^𝛼𝑣 at reference coordinate r0 and 𝑣 was the flow velocity.

The chain rule was implemented for the above differential operators and transformed into the new coordinate system[(𝑟, 𝑡) → (𝜓, 𝜏)].

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3.4.5.2.1 Resultant differential operators in Lagrangian coordinates

𝜕( )

𝜕𝑟 = 𝜌𝑟^{𝛼 𝜕( )}

𝜕𝜓 3.39

𝜕( )

𝜕𝑡 = ℒ( ) − 𝜌𝑟^𝛼𝑣^{𝜕( )}

𝜕𝜓 3.40

ℒ( ) = ^{𝜕( )}

𝜕𝜏 + 𝑚₀^{𝜕( )}

𝜕𝜓 3.41

3.4.5.2.2 Resultant governing equations in Lagrangian coordinates

0 = ^𝜕𝑟

𝜕𝜓− ¹

𝜌𝑟^𝛼 3.42

ℒ(𝜌) = −𝜌^{2 𝜕𝑟𝛼𝑣}

𝜕𝜓 3.43

ℒ(𝑌_𝑖) = −^{𝜕(𝑟𝛼𝑗𝑖)}

𝜕𝜓 +^𝜔𝑖

𝜌 , 𝑖 = 1, … . , 𝐼 3.44

ℒ(𝑣) = −𝑟^{𝛼 𝜕𝑝}

𝜕𝜓+^4𝑟𝛼

3

𝜕

𝜕𝜓(𝜌𝜇^{𝜕𝑟𝛼𝑣}

𝜕𝜓 ) − 2𝛼𝑟^{𝛼 𝑣}

𝑟

𝜕𝜇

𝜕𝜓 3.45

ℒ(𝑇) −^ℒ(𝑝)

𝜌𝑐𝑝 = ¹

𝑐𝑝

𝜕

𝜕𝜓(𝜌𝑟^2𝛼𝜆^𝜕𝑇

𝜕𝜓) −^{∑ ℎ𝑖𝜔𝑖}

𝐼 𝑖=1

𝜌𝑐𝑝 − 𝑟^{𝛼 𝜕𝑇}

𝜕𝜓∑ ^𝑐𝑝𝑖

𝑐𝑝𝑗_𝑖

𝐼𝑖=1 + ^Φ

𝜌𝑐𝑝− ¹

𝑐𝑝

𝜕(𝑟^𝛼𝑞𝑅)

𝜕𝜓 + ^𝑄𝑒

𝜌𝑐𝑝 3.46

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Where Φ, 𝜏_𝑟𝑟, 𝜏_𝜃𝜃 and 𝐷_𝑣 were defined as presented below.

Φ = 𝜏_𝑟𝑟^𝜕𝑣

𝜕𝑟+ 𝛼𝜏_𝜃𝜃^𝑣

𝑟 3.47

𝜏_𝑟𝑟= 𝐷_𝑣(^𝜕𝑣

𝜕𝑟−^𝛼𝑣

2𝑟) 3.48

𝜏_𝜃𝜃 = −^𝐷𝑣

2 (^𝜕𝑣

𝜕𝑟− ^2𝑣

𝑚𝑟) 𝑓𝑜𝑟 𝛼 > 0; = 0 𝑓𝑜𝑟 𝛼 = 0 3.49

𝐷_𝑣 = 4𝜇 3⁄ 3.50

Where 𝑗_𝑖 = 𝜌𝑌_𝑖𝑉_𝑖 was the diffusion of i^th species, 𝑌_𝑖 - mass fraction of i^th species, 𝑉_𝑖 - diffusion velocity of i^th species, 𝜔_𝑖 - mass rate of production of i^th species, I - total number of species in the system, p - pressure, 𝜇 - dynamic viscosity of the mixture, 𝑐_𝑝- mass- based frozen specific heat at constant pressure, 𝜆 - thermal conductivity of the mixture, ℎ_𝑖 - mass-based enthalpy of the i^th species, 𝑇 - temperature of the mixture, 𝑐_𝑝𝑖 - specific heat at a constant pressure of the i^th species, 𝑄_𝑒 - the source of energy per unit volume and unit time and 𝑞_𝑅 - radiative heat flux. The above governing equations were solved with finite difference method discretization on a non-uniform-adaptive grid and associated boundary conditions, as discussed in the Cosilab theory manual[107]. Soret effect was included. GRAD and CURV, the grid adaptation parameters were set at 0.02 and 0.3, respectively, ensuring grid-independent solutions in Cosilab and Ansys Chemkin.