7/27/2019 Numerical Simulation of Wood Volatiles & Air Combustion in Differentially Heated Diffuser Tube under Free Conve
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I nternational Journal of Engineering Trends and Technology (I JETT) - Volume4 I ssue7- Jul y 2013
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Numerical Simulation of WoodVolatiles &
Air Combustion in Differentially Heated
Diffuser Tube under Free ConvectionGopal Kumar Deshmukh1, Rajesh Gupta 21 Post Graduate Student, Department of Mechanical Engineering, M.A.N.I.T, Bhopal, India 462051
2 Associate Professor, Department of Mechanical Engineering, M.A.N.I.T, Bhopal, India 462051
Abstract- Simulation of combustion phenomenon in a
vertical diffuser in the presence of buoyancy-inducedairflow and radial fuel inflow is presented. Thiscombustion problem is similar to the combustion ofpyrolysis gases in an annular vertical diffuser which iswidely used in rural areas for cooking. The analysis ofthe problem is complicated owing to the couplingamongst natural convection flow, heat transfer and
combustion. A simple finite reaction rate model for a
single component fuel is presented for the combustionprocess. Then, the fuel is taken to be a mixture of
combustible gases and an Arrhenius-type single stepreaction is assumed between each of the combustiblegases and oxygen. In each stage of modelling,temperature and species profiles are generated forvarious tube wall temperatures and fuel inflow rates.Results indicate that the maximum flame temperature is
located close to the radial diffuser. The flame tends tomove away from the wall with higher volatiles flow rateand higher wall temperatures also. It was also seen thatmaximum flame temperature increases with increase intube wall temperature and power input, maximumflame temperature region spread in radial diffuser.
Keywords: wood-volatiles, vertical diffuser, natural
convection, mixture fraction formulation.
1. INTRODUCTIONResearch and development of cook stoves and
vertical combustors have been receiving its due share
of attention at least in the developing world where alarge chunk of the population still resides below a
standard of living. This part of the population
generally resorts to the use of wood, wood volatiles
or any other bio mass fuel available easily for itscooking needs. The issue of stove or combustor is
therefore quite sensitive and has direct implications
on the socio-economic developments of the poorest
of the poor human beings. A general layout of
vertical diffuser combustor has shown in figure 1. A
combustion zone is sustained in the central passage
by a buoyancy-induced upward flow of air and a
radial inflow of combustible volatile gases from the
vertical combustor. The hot flue gases transfer heat to
the pan placed above the conical tube. Scientific
investigation of such combustor is difficult becauseof the coupled phenomena of natural convection
flow, heat transfer, and flaming combustion. In this
work a certain radial flow rate of combustible
volatiles has been prescribed from the inner surface
of a vertical tube. In the present work, the
combustion phenomenon of fuel gas in a buoyancy-
induced airflow in a vertical tube is modeled undertheassumptions of a finite reaction rate. The volatileflow rate and the inner wall temperature were
parametrically varied over a restricted range for
studying their effects on combustion.
Fig. 1 vertical diffuser combustor
2. MATHEMATICAL FORMULATIONA typical vertical diffuser is shown in figure 1. The
simulation becomes relatively simplified and well
defined if the vertical portion of the hole considered.
The phenomena involved in the combustion process
in vertical diffuser are
(1) Natural convection flow and heat transfer(buoyancy induced flow )
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(2) CombustionNatural convection flow methodology from existingmodel is adopted for the present work uniform fuel
rate is assumed from the wall and taken as input to
replace pyrolysis model. Present work mainly
concentrates on combustion modeling. The fuel
mixture is taken as CH4, C2H6, CO, H2, CO2, and
H2O from pyrolysis of wood. Composition of
volatiles for CH4, C2H6, CO, H2, CO2, and H2O are
taken as 0.0489, 0.10756, 0.4756, 0.0187, 0.249 &
0.1 respectively.
Assuming one step reaction, four single step reactions
are possible for this mixture.
1. CH4 + 2O2 CO2 + 2H2O2. C2H6 + 3.5 O2 2CO2 + 3H2O3. CO + 0.5 O2 CO24. H2+ 0.5 O2 H2O
Assumptions of the proposed model
The vertical hole is used in the diffuser. Theassumption has been made to make the
model axi-symmetric.
The viscous dissipation is negligible The flow will be Turbulent and K &
models are used.
Process assumed to be Steady state.Governing Equations for Reactions
Conservation of mass Conservation of momentum Conservation of species (CH4, C2H6, CO2,
H2, CO, and H2O)
Conservation of energyMass Conservation Equation:
0
z
1
z
V
r
rVr
r
(1)
Radial Momentum Equation:
z
rV
zr
zV
z
zV
r
r
rV
rrV
rr
rV
rrr
z
zV
rrrr
rV
rrr
Dar
Vr
p
z
Vr
V
r
rVr
r
z
3
2
3
22
3
4
1
3
2
1
3
2
1
3
4
z
2
1
(2)
Axial Momentum Equation:
zr
Vr
rrrzV
rrr
rVr
rz
z
zV
DazVGrp
z
zV
r
zVr
Vr
r
1
r1
13
2
z3
4
z
2
1
(3)
Energy Equation:
genQzTekrrrrTekrrzTzVrTrVpC
1
r
1
Pr
1
(4)
The symbols Gr, Pr and Da indicate the Grashof
number, Prandtl number and Darcy numbers
respectively. Qgen refers to the source term
representing volumetric heat generation during
combustion. The species equation, which will be
presented in the latter section, is coupled to theenergy equation through these source terms.
Species Equation:
The combustion model consists of
conservation of species equations. The fuel mixture
contains species namely CH4, C2H6, CO2, H2, CO,
H2O and O2, N2.
These species are assumed to undergo a
finite rate, single-step irreversible reaction kinetics.
izi
Y
Dzr
iY
DrrrLei
YVzi
Yr
Vrrr
1
Pr
1
z
1
(5)
Le and D refer to the Lewis number and
diffusion coefficient respectively while Y i and irepresent mass fraction and the rate of generation or
depletion of the ith species respectively. The rate of
generation or depletion non-dimensional i for eachindividual species mentioned above can be expressed
in terms of Arrhenius equation.
3. BOUNDARY CONDITIONS At the fuel inlet:
At fuel inlet the stagnation pressure is taken
equal to the ambient pressure at the same elevation.
Then, neglecting the viscous losses due to
acceleration of the fluid from surroundings to thestove inlet. Further, the fluid is considered entering
the stove radially thus; axial velocity at the fuel inlet
is taken to be zero. Then, continuity equation dictates
the normal gradient of axial velocity is also zero. The
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temperature at the stove inlet can be assumed to the
ambient temperature,
0,10for0,0
,0,2
2
1
zr
z
zV
zVzVp
(6)
At the axis of symmetry:Due to assumed axi-symmetry of the flow,
computations may be carried out for only one axi-symmetric plane of the domain as shown in figure 1.
Symmetry, then, dictates the radial velocity, normalgradients of the axial-velocity and the temperaturerespectively to be zero,
HszrT
r
zV
rV 0,0for0
r
,0
,0
(7)
At the stoves exit:The exhaust gases leaving the combustion
chamber can be treated as a jet entering a still
medium i.e. static pressure is equal to the local
ambient pressure. Also, assuming that the flow leaves
the exit radially, consequently, axial velocity is zero.Thus, boundary conditions at the exit can be written
as;
0
,0
,0,
200
21)00(
r
T
r
rV
zVrgzp
p
(8)
At solid walls:Ceramic liner is a solid wall boundary at all
locations other than the primary and secondary air
slots. At solid walls, the no slip boundary condition
would apply, i.e., both radial and axial velocities are
zero. For temperature, the convective boundary
condition is imposed,
HzTNuw
qr
TzV
rV 0(z),
maxrrfor
k,0,0
(9)
At the air inlets:The air can be assumed to enter in the
combustor axial direction and stagnation pressure is
taken equal to the ambient pressure. The temperatureat port can be taken as the ambient temperature. Air
will flow due to free convection.
0,0
,0,2
2
1
T
r
rV
rVrVp
(10)
Equation for Property Variation:The fluid properties such as viscosity and
thermal conductivity are assumed to be varyingaccording to the Sutherlands relation as follows
0
56.110,
0
44.194,
12
3,
12
3
TS
TkS
S
ST
kS
kS
Tk
(11)
The variation of specific heat in terms of temperature
is expressed by a cubic polynomial as:
9987.00691.02
3408.03
1291.0 TTTpC
4 SOLUTION PROCEDURE
A finite volume method was employed to discretise
the governing equations. The SIMPLER algorithm,
Steady state, & K- Turbulence model were used.The discretised sets of algebraic equations were
solved by Ansysfluent 13.0. To predict the effect
near the wall clearly, uniform grids are selected insuch a way that more number of grid points are near
the wall.
5 RESULTS AND DISCUSSION
The variation in Wall temperature and power input
gives different combustion temperature. The species,
temperature profiles and contours are generated of800 K, 900K, and 1000K for 0.5KW, 1.0KW & 1.2
KW for each case. The temperature profile in the
domain shows that maximum temperature region
away from wall. This region corresponds to the flamelocation. This also matches with maximum velocity
region. Maximum combustion temperature obtained
is 1263 K. The maximum combustion temperaturesshown in the table 1.1 for various cases.
Kelvin.inisT;1000
TwhereT
7/27/2019 Numerical Simulation of Wood Volatiles & Air Combustion in Differentially Heated Diffuser Tube under Free Conve
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Figure a, b, & c shows temperature profile for
0.5KW, 1KW, &1.2KW respectively. The effect of
wall temperature and power on combustion
temperature is studied by varying power between 0.5
KW to 1.2 KW and wall temperatures between 800 to
1000 K. the simulations results show in figures (I-IX)that with increase in wall temperature, by keeping
same power shows increase in combustion
temperature. It is clear that with increase in wall
temperature flame region is increasing and flame ismoving away from the wall and spread in radial
diffuser.
Similar effects are obtained with increase of power
with constant wall temperature. It is clearly indicatesincrease in maximum combustion temperature with
increase in power and increase in wall temperature.
The contours below show to temperature variation in
vertical combustor. The contours arranged in the
manner of case id according to Table 1.1.
TABLE 1.1
S.N
.
CaseID
WallTemperature
(K)
PowerMaximum
CombustionTemperature
(K)
1 P1T1 800 0.5 KW 1190
2 P2T1 800 1 KW 1200
3 P3T1 800 1.2 KW 1206
4 P1T2 900 0.5 KW 1219
5 P2T2 900 1 KW 1230
6 P3T2 900 1.2 KW 1232
7 P1T3 1000 0.5 KW 1249
8 P2T3 1000 1 KW 1262
9 P3T3 1000 1.2 KW 1263
Fig. (I) Fig. (II)
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Fig. (III)
Fig. (IV)
Fig. (V)
Fig. (VI)
Fig. (VII)
Fig. (VIII)
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Fig. (IX)
Fig. (a)
Fig. (b)
Fig. (c)
6 CONCLUSIONS
In present work, the results of the combustion
simulation are summarized as follows:
(1) The velocity, species profiles andtemperature profiles are predicted by the
present combustion model. Results indicatemovement of the flame away from the wall
for higher volatiles flow rates and higherwall temperatures.
(2) The region of maximum combustiontemperature also corresponds the zone where
the velocities are maximum.
(3) In general, maximum temperature in theflame region tends to increase with increasein wall temperature as well as power input.
Maximum flame temperature region tends tomove in radial diffuser.
ACKNOLEDGEMENT
I would like to express my deepest sense of gratitude
and sincere thanks to my highly respected and
esteemed guide Dr. RAJESH GUPTA (Associate
Professor), Department of Mechanical Engineering,MANIT, Bhopal, for suggesting the subject of the
work and providing constant support, great advice
and supervision during the work of this thesis, which
appointed him as a backbone of this work. His
cooperation and timely suggestions have been
unparalleled stimuli for me to travel eventuallytowards the completion of this thesis.
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I nternational Journal of Engineering Trends and Technology (I JETT) - Volume4 I ssue7- Jul y 2013
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REFRENCES
1. S. Kohli & M.R. Ravi, Biomass stove: A Review, Journal ofSolar Energy Society of India, 6 (1996): 101-145.
2. S. Bhandari, S. Gopi, A.W. Date, Investigation of CTARAwood burning stove: Part I. experimental investigation,
Sadhana, 13(4) (1988): 271-293.
3. S. Kohli, Buoyancy induced flow and heat transfer inbiomass stoves, PhD thesis, Indian Institute of Science,Bangalore.
4. S. Karthikeyan, Development of a combustion model for asawdust stove, M.Tech thesis, Department of Mechanical
Engineering, IIT-Delhi, (2000):12-24.
5. G. Kaur,Modelling of carbon monoxide emissions in sawduststove, M.Tech thesis, Department of Mechanical
Engineering, IIT-Delhi, (2000):
6. A.F. Roberts, Problems associated with theoretical analysisof burning wood, Combustion Flames, 14(1971): 261.7. R. Gupta, Heat transfer & Fluid flow modeling of a single
pan wood stove, PhD thesis Department of AppliedMechanics MANIT Bhopal (2010): 4312-5088.
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