Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments

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Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments Vipin Dhyani, K.A. Subramanian* Engines and Unconventional Fuels Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, 110016, India

highlights
Backfire in hydrogen fuelled SI engine was characterized using CFD and experiments.
Hot spot’s minimum temperature for backfire occurrence was 950 K.
Hot spot’s location does not influence backfire except the timing of its origin.
Backfire propagation was characterized as deflagration.
Spark plug tip/ex valves’ temperature shall be below 900 K for controlling backfire.

article info

abstract

Article history:

Backfire, an abnormal combustion phenomenon, in a hydrogen fuelled spark ignition (SI)

Received 27 March 2019

engine was analyzed using computational fluid dynamics (CFD) and experimental tests.

Received in revised form

One of the main causes of backfire origin is the presence of any high temperature heat

16 July 2019

source including hot spot in the combustion chamber of the engine during intake process.

Accepted 10 October 2019

A CFD based parametric study was carried out by varying the temperature of hot spot and

Available online xxx

its location in the combustion chamber of the engine in order to analyze their effects on backfire origin and its propagation in the intake manifold of the engine. The temperature of

Keywords:

hot spot was varied from 800 K to till the temperature of backfire occurrence. The mini-

Hydrogen fuelled spark ignition

mum temperature of hot spot at which backfire occurred was observed as 950 K and

engine

beyond. The probability of backfire occurrence increases with increase in hot spot tem-

Backfire

perature. The CFD simulations were also carried out by varying the location of hot spot

Hot spot

(spark plug tip and exhaust valve) and the results indicate that the location of hot spot does

Computational fluid dynamics (CFD)

not influence the characteristics of backfire but it affects the timing of its origin. The

Transparent intake manifold

average backfire velocity is 230 m/s based on the average turbulent flame velocity during backfire propagation in the intake manifold and the value agreed reasonably well with the experimental observations of backfire propagation on the engine with the transparent intake manifold. Backfire propagation is under the category of deflagration based on its velocity (subsonic), and the maximum pressure gradient (<0.3 bar). The backfire phenomenon is characterized into three stages namely ignition delay for backfire, backfire propagation and its termination. The study results provide a better in-depth understanding of backfire origin and its propagation and would be helpful for developing a robust control strategy. Based on this study, it is recommended that the spark plug and exhaust valves of

* Corresponding author. E-mail address: [email protected] (K.A. Subramanian). https://doi.org/10.1016/j.ijhydene.2019.10.077 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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hydrogen fuelled SI engine should be customized in such a way that the temperature of spark plug tip and exhaust valves should not exceed 900 K during suction process in order to eliminate backfire occurrence. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Nomenclature aBDC AIT AMR aTDC bBDC bTDC CA CFD CFL CHR CNG EGR EV EVO H2O2 HRR HS1000 HS950 IC IVC IVO MIE MP OH SI SIT SP URANS

after bottom dead center auto-ignition temperature adaptive mesh refinement after top dead center before bottom dead center before top dead center crank angle computational fluid dynamics Courant-Friedrichs-Lewy cumulative heat release compressed natural gas exhaust gas recirculation exhaust valve exhaust valve opening hydrogen peroxide heat release rate hot spot of 1000 K hot spot of 950 K internal combustion intake valve closing intake valve opening minimum ignition energy monitor point hydroxyl radical spark ignition spontaneous ignition temperature spark plug unsteady Reynolds Averaged Navier-Stokes

Introduction Internal combustion engines are currently powering almost entire transportation (more than 99.9%), and it is projected to not be lower than 90% even by 2040 [1,2]. However, various initiatives have been taken worldwide to improve the efficiency and reduce the emissions coming out from these engines. One of such initiatives is to use alternative fuels. Among various alternative fuels, hydrogen can promise the lowest carbon footprint with higher engine efficiency if the source of hydrogen is based on renewable energy such as tidal, wind and solar. Hydrogen as a fuel can be used in compression ignition engines under dual fuel mode and it decreases engine vibration along with carbon based emissions [3e6] but neat hydrogen is more suitable for spark ignition (SI) engines due to some of its unique properties including higher octane number

[7e9]. However, low minimum ignition energy (MIE), wide flammability range and low quenching distance of hydrogen result in backfire during the intake process of port/manifold injection type hydrogen fuelled SI engines. Backfire in a hydrogen fuelled SI engine is a pre-ignition phenomenon (abnormal combustion) which takes place during intake process when freshly inducted hydrogen gets ignited by any high temperature source (hot spot) present in the combustion chamber of the engine and thus causes a sudden pressure rise in the cylinder. As the intake valves are still open, the developed flame propagates into the intake manifold and burns the hydrogen present in the intake manifold and thus can damage the intake system, fuel supply system that subsequently result in stalling of the engine operation and in case of strong backfire intensity, it may lead to explosion also. Therefore, it is critical to in-depth understanding of backfire for developing a robust control strategy. Backfire is a pre-ignition phenomenon of hydrogen-air mixture during intake process of hydrogen fuelled SI engine. Thus, the origin of backfire mainly depends on the reactant temperature, pressure, MIE, equivalence ratio, type of ignition source and the residence/contact time of the mixture with the ignition source during intake process. Van et al. [10] carried out a study on the ignition of various combustible gas-air mixtures such as hydrogen, methane, ethane, propane, butane etc. It was reported that the ignition of a combustible gas mixture depends on the type of ignition source available such as electrical source (electrostatic and inductance spark), hot surface (frictional or impact sparks, exhaust manifold, heated walls) and heated gas (generated by shock waves or adiabatic compression or jet of hot gases). In case of the concentrated heat source (e.g. electrostatic spark), which produces very high local temperature within a small volume of the combustible gas mixture, the critical factor on which ignition depends is an extensive property (energy) of the source (called MIE). If the heat/ignition source is not concentrated (such as heated surface (heated vessel containing combustible gas mixture)), the energy supplied is very large but the critical factor on which ignition depends is an intensive property (temperature) called ignition temperature. Another important factor on which flame sustainability or propagation depends is the ignition delay, which mainly depends on the contact duration between the combustible gas mixture and the ignition source. If the contact duration is very short even with a high temperature source (higher than the AIT/spontaneous ignition temp (SIT)), the ignition cannot be initiated or flame cannot be sustained. In most of the cases of heated gas or hot gas, the critical factor on which ignition depends is the temperature of the hot gas (combustion product or residual gas in case of IC engine) and the sufficient contact time with the high temperature which must be maintained up to the period exceeding the ignition delay. The

Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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hot gas ignition temperature is always higher than the hot surface ignition temperature; for hydrogen, it is around 755  C and 520  C respectively. One interesting point in case of H2 is that the difference between these two temperatures is the lowest as compared to that of with any other fuel. Wolfhard and Burgess [11] also reported the similar criteria for ignition of fuel-air mixture. Holleyhead [12], in his review work was also reported that the AIT of hydrogen is a strong function of the type of ignition source. The ignition temperature is in the range of 632  C to 670  C if the ignition source is hot air whereas the range is of 750  C to 900  C for hot surface. It was pointed out that the hot surface temperatures required for ignition of hydrogen were much higher than the largely accepted AIT (580  C) of hydrogen. Neer [13] carried out experiments on a shock tube in order to understand the low temperature ignition of hydrogen-air mixture. It was concluded that the ignition temperatures were very different from the AIT under various initial pressure and temperature conditions. The ignition temperature of hydrogen was found as 438  C at 1.3 bar pressure. The main reason of low temperature ignition of flowing hydrogen-air mixture was originated in the boundary layer due to the friction induced ionization of hydrogen molecule at the surface of the tube. It is reported in NASA Technical Paper 1457 [14] that the selfignition or auto-ignition of hydrogen is very sensitive to local mixture temperature and thus wall or temperature of engine components is very important from the ignition point of view. For auto-ignition of flowing hydrogen-air mixture, four conditions are necessary to meet namely reactant temperature, pressure, equivalence ratio and residence time. Generally, the probability of ignition increases with increasing reactant temperature and pressure, increasing equivalence ratio (approaches stoichiometric) and higher residence time. Elbe [15] also reported that the ignition could be obtained with the sources which are at much cooler than that of flame temperature if the contact time or the heating period is high. It is mentioned in NASA-Glenn Safety Manual [16] that the cooler objects (about 317  C) can also cause ignition under prolonged contact at less than atmospheric pressure. The importance of contact time or residence time of combustible gas with ignition source on ignition was also mentioned by other researchers [12,17,18]. The AIT of hydrogen-air mixture as a function of equivalence ratio is reported in the literature [19]. The AIT decreases with increasing equivalence ratio, and it is minimum near/at stoichiometric condition [12,16,19,20]. There is ambiguity in the AIT at stoichiometric condition which could be due to its sensitivity towards the experimental apparatus, procedure and its definition used while conducting experiments [9]. Backfire in a hydrogen fuelled SI engine was studied by various researchers. However, most of these studies were mainly focused on the various control strategies such as delaying the hydrogen injection [21e27], varying valves timings [28e31], increasing the compression ratio [32,33], retarding the ignition timing [27,34], using water cooled spark plug [35], reducing piston-cylinder crevice volume [36], using charge dilution, exhaust gas recirculation (EGR) [31,33,37] and water injection [7,37e40]. The main causes of backfire were reported as high temperature heat source present in the combustion chamber such as hot spark plug, hot exhaust

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valves, lubricating oil deposits, hot residual gas [7,9,23,26,34,41], hot gases trapped in crevice volume [36] and abnormal electric discharge [35]. Apart from this, backfire and knocking are interlinked with each other at high equivalence ratio [27] which makes backfire very critical to understand. Most of the above mentioned studies are based on the experimental work of the respective researchers, and the experimental based analysis of an abnormal combustion phenomenon such as backfire has various limitations thus there is a need of computational fluid dynamics (CFD) based analysis of backfire. Liu et al. [42] carried out CFD simulations for analyzing the effects of injection timing of hydrogen on mixture formation in the intake manifold with varying speed and equivalence ratio. The concentration of residual hydrogen in the intake manifold was co-related with the possibility of backfire occurrence. Duan et al. [25] were also carried out similar kind of CFD based analysis of backfire by varying injection pressure along with injection timing. The mass of residual hydrogen in the intake manifold and in-cylinder temperature distribution before intake valve opening (IVO) were the parameters used as the indication of the probability of backfire in a hydrogen fuelled engine by Yang et al. [43] with different injection modes in their CFD work. A CFD based analysis of backfire is required to carry out a better (in-depth) understanding of backfire due to the complexities involved in the phenomenon. Although some CFD based analysis of backfire is reported in the literature, but these studies provide only qualitative information of the probability of backfire occurrence. A study pertaining to the characteristics of backfire is scanty. Hence, this study is aimed at characterization of backfire in hydrogen fuelled spark ignition engine using CFD simulation and experiments. In the present study, backfire origin due to a hot spot and its propagation in the intake manifold of a hydrogen fuelled SI engine was studied using CFD. The experiments were conducted on a single cylinder SI engine with transparent intake manifold in order to validate some of the CFD results. The characteristics of backfire in terms of its intensity (maximum pressure and rate of pressure rise) and timing of its origin were discussed and further, backfire velocity was characterized based on the average turbulent flame velocity and the maximum pressure gradient during backfire. Finally, some recommendations were made based on this study to eliminate backfire in a hydrogen fuelled SI engine.

Computational modelling details A single cylinder spark ignition research engine was selected for the present work (Fig. 1a). The details of the engine including experimental setup are given in Section Experimental details. The three dimensional CFD analysis was carried out using CONVERGE, a commercial CFD code. CONVERGE generates mesh during the simulation run time. The engine geometry was divided into four regions namely intake, injector, cylinder, and exhaust. The grid independence study was carried out with different base grid size (3 mm, 4 mm and 6 mm). Based on the accuracy, computational time and resource consumption, the base grid size of 4 mm with AMR (adaptive mesh refinement) and fixed embedding was

Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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selected for the study. The AMR of level 3 (minimum grid size ¼ base grid size/(2^AMR level)) was permanently used in intake and cylinder regions for capturing the velocity and temperature gradients, which locally reduces the grid size to 0.5 mm. The fixed embedding of various refinement levels was also used for various regions and boundaries along with AMR in order to capture the process with high resolution. The fixed embedding reduces the cell size to 1 mm in the cylinder and valves angle regions and 0.125 mm in the region around the spark discharge during ignition. The total cell count varies between 0.25 and 2 million. The generated mesh with AMR and fixed embedding during simulation run time is shown in Fig. 1b. Each simulation of abnormal combustion took about 20 days on Ubuntu 16.04 server with 20 cores. The gas simulation was carried using the CFD software with Redlich-Kwong gas equation. The reaction mechanism for hydrogen combustion was taken from Connaire et al. [44] which contains 5 elements, 10 species and 21 chemical reactions. The variable time step algorithm based on CourantFriedrichs-Lewy (CFL) numbers for convection, diffusion and Mach number were used throughout the simulation. The CFL numbers estimate the number of cells through which the related quantity will move in a single time-step. In order to precisely capture the phenomena, the maximum values of the CFL numbers were decided after running multiple simulations. The maximum convective CFL number was taken as 1 for cylinder region and intake region (during suction process), and it was taken as 5 for rest of the regions/cycle. The maximum diffusive CFL number was taken as 2 throughout the simulations. The maximum Mach CFL number was taken as 50 for normal combustion and 3 for abnormal combustion (backfire) simulations. The pressure-velocity coupling was achieved using modified Pressure Implicit with Splitting of Operator (PISO) algorithm developed by Issa [45]. The intake and exhaust valve profiles were obtained from the

experiments and incorporated with the code. Unsteady Reynolds Averaged Navier-Stokes equations (URANS) were applied to model the flow field. Turbulence effects were considered using Renormalization group (RNG) k-ε model developed by Han and Reitz [46]. O’Rourke and Amsden model [47] was incorporated for wall heat transfer. The ignition process was modelled using high temperature thermal source (energy) of spherical shape with two phases, arc and glow discharge between the electrodes of the spark plug. SAGE detailed chemical kinetics solver was used for modelling combustion [48].

The governing equations The fluid flow dynamics is governed by a set of equations and this subsection provides the details of various governing equations used in the CFD analysis.

Mass and momentum equations The partial differential equations of continuity (mass) and momentum in conservative form are given in Eqs. (1) and (2) respectively. vr vðruÞ þ ¼0 vt vx

(1)

vðruÞ vðruvÞ vp vtxy þ ¼ þ þ Sx vt vy vx vy

(2)

Where, r is density, t is time, u; v; w are the Cartesian velocity components in x, y and z direction respectively, p is pressure, txy is stress tensor, which is given in Eq. (3), and Sx is the momentum source component. txy ¼ m




 vu vv 2 vw 0 þ þ m  m dxy vy vx 3 vz

(3)

Fig. 1 e (a) Engine geometry with the location of various monitor points (MP) and (b) Illustration of grid generation during simulation. Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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0

Where, m is the viscosity, m is dilatational viscosity and dxy is Kronecker delta. In the presence of any turbulence model, m is replaced by total viscosity (mtot ) as given in Eq. (4) mtot ¼ mmol þ Cm r

k2 ε

(4)

Where, mmol , Cm , k and ε are the molecular viscosity, turbulence model constant, turbulent kinetic energy and turbulent dissipation respectively.

Energy equation The energy conservation equation is given in Eq. (5) !


 X vYm vðreÞ vðrevÞ vv vu v vT v þ ¼ p þ txy þ k þ rD hm þS vt vy vy vy vy vy vy vy m

equations of turbulent kinetic energy (k) and turbulent kinetic energy dissipation (ε) are given in Eqs. (10) and (11) respectively. vðrkÞ vðrukÞ vu v m þ mt vk Cs þ ¼ txy þ  rε þ Ss vt vx vy vy Prk vy 1:5
 vðrεÞ vðruεÞ v m þ mt vε vu þ ¼ þ Cε3 rε vt vx vy vx Prε vy
 ε vu Cε1 txy  Cε2 rε þ Cs Ss þ S  rR þ k vy

m kt ¼ k þ cp t Prt

(6)

Where, cp , mt , and Prt are the specific heat, turbulent viscosity and turbulent Prandtl number. The energy equation (Eq. (5)) has four extra terms (source term, S, pressure work term,  p vv vy, viscous dissipation term,
 P vYm v txy vu hm vy ) apart from vy, and species diffusion term, vy rD m

 vðreÞ vðrevÞ v transient ( vt ), convection ( vy ) and diffusion vy k vT vy terms. The source term accounts the user specified energy sources and turbulent dissipation. The pressure work term accounts the compression and the expansion. The viscous dissipation term accounts the kinetic energy viscously dissipating into heat and the species diffusion term accounts the energy transport due to species diffusion.

Species transport equation The species conservation equation is given in Eq. (7)
 vrm vðrm vÞ v vYm ¼ rD þ þ Sm vy vy vt vy

(7)

R¼


 Cm h3 ε2 1  hh0

rm ¼ Ym r

(8)

n Sc

(9)

D¼

Where, n is kinematic viscosity and Sc is Schmidt number.

(12)

kð1 þ bh3 Þ

Where, h is given in Eq. (13) k qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2Sxy Sxy ε

h¼

(13)

Where, Sxy is mean strain rate tensor given in Eq. (14) Sxy ¼


 ~ vv ~ 1 vu þ 2 vy vx

(14)

~ is the Favre average for velocity component, u In Eq. (14), u and is given in Eq. (15) ~¼ u

ru r

(15)

In Eq. (15), “” denotes the ensemble mean.

Combustion model The combustion was modelled using SAGE detailed chemistry solver developed by Senecal et al. [48]. The reaction rates of each elementary reaction available in the chemical reaction mechanism of hydrogen combustion are calculated by SAGE as explained below. A multi-step chemical reaction can be expressed in the form of Eq. (16) M X

0

vm; r Xm 4

m¼1

Where, rm is species density calculated by Eq. (8), D is mass diffusion coefficient calculated by Eq. (9), Ym is the mass fraction of species m and Sm is source term which accounts for the chemical reactions (combustion), evaporation and other sub-models.

(11)

Where, Ss and S are the source terms, prior is the userspecified and later represents the interactions with discrete phase (spray), the C terms are the model constants and R is given in Eq. (12).

(5) Where, e is specific internal energy, k is conductivity, T is temperature, D is coefficient of mass diffusion, hm is species enthalpy, Ym is mass fraction of species m and S is source term. In the presence of any turbulence model, k is replaced by turbulent conductivity (kt ) as given in Eq. (6)

(10)

M X

00

vm;

r

Xm for r ¼ 1; 2;…R

(16)

m¼1

Where, M is the total number of species present in the chemical reaction mechanism, R is the total number of 0 00 elementary reactions, vm; r and vm; r are the stoichiometric coefficients of the reactants and products respectively, for the mth species in the rth reaction and Xm is the chemical symbol of species m. The net production rate of each species in a multi-step chemical reaction is given in Eq. (17) u_ m ¼

R X

vm;r qr

(17)

r¼1

Turbulence model Renormalization group k-epsilon (RNG k-ε) type of URANS turbulence model was used in the study. The governing

00

0

Where, vm;r is the coefficients difference (¼vm; r -vm; r ) and qr is the rate-of-progress variable for the rth elementary reaction and is given in Eq. (18)

Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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qr ¼ kfr

YM m¼1

0

½Xm vm;

r

 krr

YM m¼1

00

½Xm vm;

r

(18)

Where, kfr and krr are the elementary forward and reverse rate coefficients for rth reaction (determined by Arrhenius equation), and ½Xm  is molar concentration of species m. These rate equations are solved by SAGE while the transport equations are solved by the CFD solver to model combustion with detailed chemistry.

Case setup for backfire A three-dimensional CFD code, CONVERGE was used for engine cycle simulations. The simulations were performed in two steps, normal combustion and abnormal combustion (backfire). The operating conditions of the normal combustion simulation are based on the experiments carried out on a single cylinder SI engine and are shown in Table 1. The results of normal combustion cycle simulations were compared with the experimental data in order to validate the computational approach adopted in the CFD code. Once the validation was done, a map.dat file, which preserves all the initial conditions at the particular crank angle, was generated in normal combustion simulation cycle and this file was used for initializing the backfire simulation cycle at the crank angle. For simulating backfire, a spherical energy source of 1 mm radius with varying temperature was introduced inside the engine cylinder as a hot spot. A similar approach was also used by Mubarak et al. [49] for simulating knocking due to hot spot inside the engine cylinder. The temperature of hot spot was varied from 800 K to till the temperature of backfire occurrence. Simulations were also carried out by varying the location of the hot spot in the combustion chamber of the engine. The details of selecting the temperature and location of hot spot are explained in subsection Details of hot spot. In order to precisely capture the timing of backfire occurrence, the Mach CFL number was taken as 3 for the backfire simulation cycle, and it was 50 for normal combustion simulations. Various researchers [49e52] used low Mach CFL number for capturing the local pressure oscillation due to knocking. The mass of OH along with pressure and temperature were used to visualize the backfire occurrence and its propagation in the intake manifold of the engine. Pal et al. [52] and Pan et al. [51] used OH whereas Mubarak et al. [49] used H2O2 as the indicator to visualize knocking. The above mentioned parameters were also captured locally at various monitoring points (MP) placed at different locations in the geometry as shown in Fig. 1a. Some of the results of backfire simulations were validated with experimental data obtained from a single cylinder SI engine. The details of the experimental setup are given in section Experimental details.

Table 1 e Operating conditions of the engine. Parameter Engine speed Equivalence ratio Spark timing Start of Injection Injection duration

Values 2000 rpm 0.59 4  bTDC TDC 155  CA

Details of hot spot The tip of spark plug and exhaust valves are the hottest components in an IC engine, and thus these two components were chosen as the location of hot spot for analyzing their effects on backfire origin and its propagation. The temperature of hot spot was varied from 800 K to till the temperature of backfire occurrence. The reason for selecting 800 K is based on the auto-ignition temperature of hydrogen-air mixture and the minimum temperature of possible hot spot present in the engine cylinder such as spark plug and exhaust valves. The minimum temperature of the tip of spark plug is limited due to the self-cleaning mechanism required for avoiding fouling whereas, the maximum temperature is limited for preventing pre-ignition/knocking [53,54]. It is recommended that the tip temperature should be in the range of 500  Ce850  C during engine operation [53]. Heywood [54] reported the range as 350  Ce950  C. A similar range of minimum temperature of exhaust valves of an IC engine is reported in the studies [55,56]. The hot spot of 800 K may serve as an ignition source for hydrogen-air mixture during suction process however, the same hot spot may not serve as an ignition source for compressed natural gas (CNG)-air mixture and gasoline-air mixture due to peculiar combustion properties of hydrogen such as low minimum ignition energy requirement, wide flammability range and high flame speed as compared to CNG and gasoline. So based on these facts, it is logical to select tip of spark plug and exhaust valve as the locations of hot spot and vary its temperature from 800 K to till the temperature of backfire occurrence in CFD simulations.

Experimental details The experimental tests were conducted on a single cylinder SI research engine fuelled with hydrogen. The technical specifications of the engine and a photographic view of the experimental setup are given in Table 2 and Fig. 2 respectively. Electronic control unit (ECU) was used to control ignition and injection parameters of the engine. Combustion characteristics of the engine were analyzed using combustion analyzer. A flashback arrestor was placed in the hydrogen supply line in order to avoid any hazard caused by backfire. Hydrogen flow rate and air flow rate were measured by Coriolis based gas flow meter and thermal based air flow meter respectively. The experiments were conducted on the engine at 2000 rpm. The equivalence ratio of the engine was varied till the occurrence of backfire. The operating conditions of the engine corresponding to backfire limiting equivalence ratio are given in Table 1. Once the backfire conditions are known, the conventional intake manifold of the engine was replaced by the specifically fabricated transparent intake manifold in order to visualize backfire propagation. A high speed camera (Phantom VEO 410) was used to capture backfire propagation in the transparent intake manifold. The specifications of the high speed camera are given in Table 3. The frame rate of the camera was synchronized with the crank angle of the engine. For 2000 rpm, frame rate of 11976 fps was used (1CA ¼ 1 frame ¼ 0.0835 ms). Based on the images captured with respect to time, the backfire velocity in the intake manifold

Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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was determined, and the corresponding CFD results were compared and validated.

Results and discussion CFD based simulations to analyze backfire were carried out on a hydrogen fuelled SI engine. The simulations were carried out in two steps namely normal combustion and abnormal combustion (backfire). The normal combustion simulation was carried out in order to access the validation of CFD simulation with our experimental data. Once the validation was done, a map.dat file, which preserved the data at the given time, was generated. This data file was used as one of the inputs for simulating backfire during intake process. The temperature of hot spot and its location in the combustion chamber of the engine were varied in order to analyze their effects on backfire origin and its propagation in the intake manifold of the engine. Some CFD simulations results of backfire origin and its propagation were compared with the experimental data obtained from the hydrogen fuelled single cylinder engine with transparent intake manifold. The results obtained from this study are discussed as follows.

Validation of CFD simulation Fig. 3 shows the in-cylinder pressure variation with crank angle for experiment and CFD simulation of normal combustion. The experimental data shown in the figure is an average of 100 cycles. The difference in peak in-cylinder pressure is less than 5% between experiment and CFD simulation. Thus, it can be concluded that the CFD simulation predicts the normal combustion within reasonable accuracy. Some of the results of abnormal combustion (backfire) were validated with experimental data and found the results are within reasonable accuracy.

Effects of varying hot spot temperature on backfire The mechanism of backfire origin was analyzed by considering different temperatures of the hot spot in the combustion chamber of the engine. In a first phase, the tip of spark plug was considered as the location of hot spot, and its temperature was varied from 800 K to till the temperature of backfire occurrence (950 K and 1000 K). The reason for the selection of hot spot location and its temperature is explained with the detailed manner in the methodology section. Backfire is a preignition (abnormal combustion) phenomenon that takes place during engine intake (when intake valves are open). During

Table 2 e Technical specifications of the engine. Description Bore Stroke Connecting rod length Compression ratio Intake valve open and close Exhaust valve open and close

Values 86 mm 86 mm 143 mm 10.5:1 34  bTDC and 54  aBDC 74  bBDC and 14  aTDC

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backfire, there is a sudden rise in in-cylinder pressure and temperature due to combustion caused by any high temperature source (hot spot) present in the combustion chamber of the engine. As the intake valves are open, the flame propagated from engine cylinder to intake manifold of the engine resulted in pressure and temperature rise in the intake manifold. Fig. 4a shows the variation of in-cylinder and intake manifold pressures with hot spot temperatures during engine intake. The intake valves of the engine open (IVO) at 34 0bTDC (394 0CA) and close (IVC) at 54 0aBDC (126 0CA). It is observed from Fig. 4a and b and that the minimum temperature of hot spot at which the backfire occurred is 950 K. The peak in-cylinder pressure and temperature are 3.75 bar and 2240 K and peak intake manifold pressure and temperature are 1.76 bar and 1650 K during backfire. A notable point observed from the figures is that the timing of backfire origin caused by hot spot of 950 K (HS950) during upward movement of the piston (after BDC). Based on previous study conducted by the same authors [27] and other researchers including Kondo et al. [35], Verhelst and Wallner [9], Verhelst [30] and Luo and Sun [57], it was observed that in most of the cases the timing of backfire origin is during downward movement of the piston. In order to confirm backfire simulation more realistic, the temperature of the hot spot was increased to 1000 K, and it can be clearly seen from the figure that the timing of backfire occurrence is before BDC with the hot spot of 1000 K (HS1000). With HS1000, the peak in-cylinder pressure and temperature are 2.88 bar and 1980 K and the corresponding variables for intake manifold are 2.42 bar and 1680 K. These values of the pressures have good agreement with the experiments and are very close to the values reported in the literature pertaining to backfire in a hydrogen fuelled SI engine [9,27,35,57]. Fig. 4c provides an overview of the characteristics of backfire (maximum in-cylinder pressure, maximum intake manifold pressure during backfire and their respective timing of origin) reported in literature along with the cases studied in the present work. As the characteristics of backfire such as timing of its origin and peak pressure with HS1000 are very similar to that of reported in the literature and the experiments carried out in the present study (Fig. 4c), further analysis was mainly focused on the backfire occurrence and propagation due to HS1000. The variation of heat release rate (HRR) and cumulative heat release (CHR) during backfire with HS950 and HS1000 are shown in Fig. 5a. It can be observed from the figure that the HRR with HS950 is lower than that of with HS1000 however, the duration of heat release is higher with HS950 than HS1000 which indicates a comparatively slower rate of chemical reaction with HS950 than with HS1000. The rate at which a chemical reaction proceeds, reaction rate (RR), depends on the rate coefficient (kðTÞ), given by Arrhenius equation (Eq. (19)), concentration of reactants and catalyst (if present) [54]. 
Ea (19) kðTÞ ¼ ATb exp  Ru T where, A and b are the empirical parameters, Ea is the activation energy and T is the reactant temperature. It can be seen from the equation that the reaction rate is also directly proportional to the temperature of reactants. The rate of a chemical reaction is a strong function of temperature of

Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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Fig. 2 e Photographic view of the experimental setup.

reactants than their concentration [58]. In case of HS1000, the reactant temperature at the crank angle of backfire origin is higher (417 K) than that of with HS950 (401 K) which results in lower reaction rate with HS950 though the concentration of reactants is higher with HS950 than with HS1000 (Fig. 5b). Another observation which can be drawn from the figure is that the in-cylinder CHR with HS950 is higher than with HS1000 whereas the CHR in the intake manifold with HS950 is lower than with HS1000. The reason for these trends is due to the amount of hydrogen available for burning in the cylinder

is higher with HS950 compared to with HS1000 as the backfire occurrence is delayed with HS950 and most of the hydrogen reached to the cylinder (Fig. 5b). It is also be noted that the total CHR (in-cylinder þ intake manifold) is almost same in both the cases. The physics behind the backfire origin due to hot spot is illustrated in Fig. 6a. Any high temperature source in the

Table 3 e Specifications of the high speed camera. Make/Model Sensor Sensor’s full resolution Frames per Second (fps) at full resolution Maximum fps at minimum resolution Pixel size Bit Depth CAR Straddle time Exposure time ISO color

Phantom VEO 410 CMOS 1 Mega-pixel (1280  800) 5200 fps 600000 fps 20 mm 12 Bit 64  8 480 ns 1 ms (minimum) 2000 D

Fig. 3 e In-cylinder pressure for experiment and CFD simulation.

Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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Fig. 4 e (a) Variation of pressure, (b) Temperature during backfire in suction stroke and (c) Comparison of backfire characteristics between present work and reported in the literature.

engine cylinder such as hot spot, residual exhaust gas of previous cycle, traces of lubricating oil etc. transfers heat to its surroundings, fresh charge (hydrogen þ air) which is at lower temperature, during suction stroke. If the temperature of the source is high enough (>900 K) to overcome the heat loss to the surroundings than the backfire originates and subsequently propagates. Fig. 6b provides a close view of the phenomenon of backfire origin due to hot spot with respect to crank angle. Although the state (Fig. 4a and b and ) and concentration of

reactants (Fig. 5b) are identical for both the cases, HS950 and HS1000, at the crank angles shown in Fig. 6b, it is the heat generation that results in backfire origin with HS1000. It can be clearly observed from the figure that the heat generation is always higher with HS1000 due to high temperature associated with it compared to HS950 because the heat generation has to overcome the heat loss to its surroundings for occurrence of backfire. In case of HS950, the heat generation is not enough to overcome the heat loss to its

Fig. 5 e (a) Heat release rate (HRR) and cumulative heat release (CHR) during backfire and (b) Variation of in-cylinder concentrations during suction stroke. Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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Fig. 6 e (a) Illustration of backfire origin and (b) Backfire origin due to hot spot during suction stroke.

surroundings and thus backfire does not take place during the downward movement of the piston. However, during upward movement of the piston, the backfire originated with HS950, which could be due to the longer contact time of the hot spot with the surroundings that results in the increased temperature of the surroundings and reduced heat loss. Thus, as soon as the heat generation exceeds the heat loss, backfire originates with HS950 after BDC. The evolutions of backfire due to HS950 and HS1000 during suction process are shown in Fig. 7a and b and respectively. Apart from this, various monitoring points were considered at various locations in the engine in order to capture the local fluctuations in the pressure caused by backfire as explained in the methodology section. Fig. 7c shows the variation of pressure at some of the monitor points (MP) with HS1000. It can be clearly seen from the figure that there is negligible fluctuation in the pressure at various monitor points in the cylinder (MP1, MP2 and MP4) and the average in-cylinder pressure (In-cyl_avg) provides an almost similar variation of pressure alike the monitor points. Similarly, the average intake manifold pressure (Man_avg) shows an almost similar variation of pressure like the monitor point in the intake manifold (MP6). Therefore,

it was concluded that monitoring various points is not necessary for simulating backfire unlike simulating any other abnormal combustion phenomena such as knocking and super knocking [51,59]. Hence, backfire is computationally less expensive than knocking.

Effects of varying hot spot locations on backfire As explained earlier in the methodology section that the tip of spark plug (SP) is one of the hottest components of an SI engine. Apart from SP, exhaust valve (EV) is another hottest component of the engine, and thus it can serve as a hot spot for backfire. CFD simulations were carried out to analyze the effects of the locations of hot spot on the characteristics of backfire. Fig. 8a shows the variation of in-cylinder and intake manifold pressures with two locations of HS1000. It can be clearly seen from the figure that all the characteristics of backfire such as its intensity (peak pressure, rate of pressure rise) and duration are almost same in both the cases except the timing of backfire origin. A similar variation can be seen in in-cylinder and intake manifold temperatures with two locations of HS1000 (Fig. 8b).

Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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Fig. 7 e Evolutions of backfire due to (a) HS950, (b) HS1000 during suction stroke and (c) Variation of pressure at various monitor points (MP) with HS1000.

Characterization of backfire propagation Backfire velocity and rate of pressure rise are two very important parameters in order to characterize backfire propagation, whether deflagration or detonation. As the temperature changes across the turbulent flame thickness from unburnt temperature to adiabatic flame temperature (nearly),

any temperature between the unburnt and adiabatic flame temperature that matches best with the OH mass fraction can be a representation of the flame front. The iso-thermal surface of 1700 K, which matches appropriately with the OH mass fraction, was chosen as the representation of the flame front. The velocity of backfire propagation was determined based on the propagation of the turbulent flame front in the intake

Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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Fig. 8 e Variation of (a) Pressure and (b) Temperature during suction stroke.

manifold of the engine. Fig. 9a and b shows the propagation of backfire from the combustion chamber to the upstream of the intake manifold for HS950 and HS1000 respectively. It can be seen from the figures that in case of HS950, the backfire origin takes place when the piston is at BDC, and the flame enters into the intake manifold around 16 0aBDC (16 0CA after its origin) and subsequently propagates into the intake manifold; in case of HS1000, backfire origin advances to 66 0bBDC and the flame enters into the intake manifold around 52 0bBDC (14 0 CA after its origin). This shows that the flame velocity is slightly higher with HS1000 than HS950. The experiments were conducted on the engine with transparent intake manifold in order to visualize and subsequently characterize the backfire propagation. As backfire is an abnormal combustion, its intensity varies from cycle to cycle [27], and thus it is difficult to get similar backfire velocity whenever it occurred. In most of the cases, the backfire velocity was in the range of about 160 m/s to 236 m/s though in few cases it was as low as 80 m/s, which may be the case of

low intensity backfire. Fig. 10a shows the simulation (HS1000) result of backfire propagation, which agreed well with experimental visualization of backfire propagation (of 236 m/s) in the intake manifold. Hence, it can be concluded that the CFD simulation predicts backfire propagation within a reasonable accuracy. Fig. 10b shows the experimental visualization of backfire propagation in the transparent intake manifold with respect to time. The velocity of backfire propagation corresponding to the figure is about 160 m/s. Based on the CFD measurements of average turbulent flame speed, the average backfire velocity was determined as 230 m/s (Fig. 11a). From the experimental visualization of backfire propagation, the maximum backfire velocity was determined around 236 m/s, as this value of the velocity is very close to the simulations, only this value was considered in the figure. It can be noted that this velocity is under the category of subsonic, as the Mach number is less than one in each case. Along with velocity, the rate of pressure rise was also determined as shown in Fig. 11b. It can be observed from

Fig. 9 e Visualization of backfire propagation due to (a) HS950 and (b) HS1000 with iso surface of 1700 K. Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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Fig. 10 e (a) Comparison between simulation and experimental results of backfire propagation in intake manifold and (b) Backfire Propagation in the transparent intake manifold. Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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Fig. 11 e (a) Average turbulent flame speed during backfire propagation and (b) Rate of pressure rise during backfire.

the figure that the maximum rate of pressure rise during backfire is below 0.3 bar/0CA which is below the deflagration limit (0.5 bar). A notable conclusion emerged from these two parameters is that backfire can be characterized as a deflagration combustion.

Characterization of backfire phenomenon

Fig. 12 e Various stages of backfire phenomenon.

The backfire phenomenon in a hydrogen fuelled SI engine can be characterized based on this study. The pressure history during the intake process can be an ease to characterize different stages of backfire phenomenon as shown in Fig. 12. There are mainly three stages of the phenomenon namely ignition delay for backfire, backfire propagation and its termination. Ignition delay for backfire is the time duration between the start of hydrogen injection and backfire origin (start of combustion). The backfire origin can be determined from the pressure-crank angle data, at which there is an appreciable rise in the pressure observed during the intake process. The backfire propagation is the time period during

Table 4 e Main characteristics of backfire. Parameters Temperature of hot spot

Values 950 K

Maximum pressure

In-cylinder: 2.88e3.75 bar (CFD) Intake manifold: 1.76e2.42 bar (CFD) In-cylinder: 2.72e3.3 bar (experimental) Intake manifold: 1.82e2.54 bar (experimental) Maximum rate In-cylinder: 0.16e0.25 bar/ CA (CFD) of pressure Intake manifold: 0.1e0.23 bar/ CA (CFD) In-cylinder: 0.21e0.29 bar/ CA (experimental) rise Intake manifold: 0.14e0.26 bar/ CA (experimental) Maximum In-cylinder: 1980e2240 K (CFD) temperature Intake manifold: 1650e1680 K (CFD) Velocity Mach number

215e233 m/s (CFD) 160e236 m/s (experimental) 0.61e0.66 (CFD) 0.46e0.67 (experimental)

Remarks The CFD results show that the minimum temperature of hot spot present in the combustion chamber of the engine at which backfire occurred is 950 K, though the hot spot of 1000 K provides the characteristics of backfire very similar to the experiments. The CFD results of maximum pressure during backfire agreed reasonably well with the experimental values and the literature.

The CFD results of maximum rate of pressure rise during backfire agreed reasonably well with the experimental values.

The maximum in-cylinder temperature is higher than the maximum intake manifold temperature during backfire due to the burning of more fuel in the combustion chamber of the engine. The CFD results of backfire velocity in the intake manifold agreed reasonably well with the experimental values. The CFD results of Mach number associated with backfire propagation in the intake manifold agreed reasonably well with the experimental values.

Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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which the flame propagates from the combustion chamber to the upstream of intake manifold. This stage of backfire phenomenon can be determined as the time period between backfire origin and maximum pressure. Based on the results obtained from experiments and CFD simulation of backfire propagation in the intake manifold with respect to crank angle, the point at which the maximum pressure occurred can be taken as the end of backfire propagation. Subsequently, the backfire gets terminated, and this stage can be taken as the time duration between the maximum pressure and minimum pressure as shown in the figure.

Control measures of backfire Based on this study it is recommended that the temperature of any hot spot present in the combustion chamber of hydrogen fuelled SI engine should be kept below 900 K during suction process in order to control backfire. As the tip of spark plug and exhaust valves are the hottest components of an engine, their temperature should be maintained below 900 K during suction process. Therefore, cold-rated spark plug and cooled exhaust valves are recommended in hydrogen fuelled SI engines. Similar recommendations were also made in ref. [9]. Along with this, various other strategies can be used to reduce the temperature of hot spot during suction process such as delaying hydrogen injection, using cooled EGR and water injection [27,37].

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➢ The peak pressure at in-cylinder and intake manifold while occurring backfire during suction stroke is up to 3.75 bar and 2.42 bar respectively. ➢ The location of hot spot does not influence the characteristics of backfire but it affects the timing of its origin. ➢ The average turbulent flame velocity of backfire is 230 m/s which is agreed well with the experimentally measured backfire velocity (236 m/s) on the engine with transparent intake manifold. The maximum pressure gradient is less than 0.3 bar during backfire. Based on these observations, backfire can be categorized as deflagration (subsonic). ➢ The backfire phenomenon is characterized into three stages namely ignition delay for backfire, backfire propagation and its termination. This study provides a better understanding of the backfire phenomenon in a hydrogen fuelled SI engine and would be helpful for developing a robust control strategy of the phenomenon. Based on this study, it is recommended that the spark plug and exhaust valves of hydrogen fuelled SI engine should be customized in such a way that the temperature of spark plug tip and exhaust valves should not exceed 900 K during suction process in order to eliminate backfire occurrence. For this purpose, cold-rated spark plug and cooled exhaust valves could be used in hydrogen fuelled SI engines.

references

Summary of backfire characteristics As backfire is an abnormal combustion phenomenon, characterization of the phenomenon is generally difficult. However, an effort was made in the present study to characterize backfire in a hydrogen fuelled SI engine using CFD and experiments. Based on the study, the main characteristics of backfire are summarized in Table 4.

Conclusions A CFD and experimental based analysis of backfire origin and its propagation due to hot spot were carried out on a single cylinder hydrogen fuelled SI engine. Backfire was analyzed during intake process of the engine by varying the temperature of hot spot and its location in the combustion chamber of the engine. Backfire was then characterized based on the timing of its origin, intensity (maximum pressure, rate of pressure rise), and its velocity. The following conclusions are emerged from this study. ➢ The probability of backfire occurrence increases with increase in hot spot temperature. ➢ The minimum temperature of hot spot at which backfire occurred was 950 K. Therefore, the hot spots of temperature more than 900 K should be avoided during the intake process for backfire free engine operation. ➢ The backfire characteristics with hot spot of 1000 K have good agreement with CFD simulation and experiments.

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Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077

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Please cite this article as: Dhyani V, Subramanian KA, Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.077