Numerical Simulation of Wood –Volatiles & Air Combustion in Differentially Heated Diffuser Tube under Free Convection

7/27/2019 Numerical Simulation of Wood Volatiles & Air Combustion in Differentially Heated Diffuser Tube under Free Conve

1/7

I nternational Journal of Engineering Trends and Technology (I JETT) - Volume4 I ssue7- Jul y 2013

ISSN: 2231-5381 http://www.ijettjournal.org Page 2751

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 )


2/7



(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


3/7



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


4/7



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)


5/7



Fig. (III)

Fig. (IV)

Fig. (V)

Fig. (VI)

Fig. (VII)

Fig. (VIII)


6/7



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.


7/7



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.

Numerical Simulation of Wood –Volatiles & Air Combustion in Differentially Heated Diffuser Tube under Free Convection

Documents