NUMERICAL VALIDATION OF PRODUCER GAS CARBURETOR - PROJECT REPORT
Chapter-i
1.1 Introduction:
In
the present situation emission affects a lot to the environment, so we need to
reduce emission effect on the environment as much as possible. Current state of
technological advances, it is recognized that Biomass is one of the viable and
sustainable renewable resources and new technologies emerging out of biomass
based gasification systems find a significant role in bridging the energy
crisis. The advanced biomass gasification systems are known to generate producer
gas as the combustible fuel that is clean enough to be used in Direct Injection
gas engines. However in order to adapt standard gas engines few of its
components need modifications before they are used in the biomass power plants.
Since this area is an emerging one and the technology has not been disseminated
to the scale of driving market, it is essential that specialized components
that require modification need be studied. Carburetor is one of the important
components in such Category and it is identified that additional research work
is to be carried out in establishing a design procedure for this application.
In the recent times, gaseous fuels
are gaining prominence as cleaner fuels for power generation via internal
combustion engine route; the power generation package including both
reciprocating engines and gas turbine machinery. Complete combustion with
minimal emission is the key feature of gaseous fuels and this feature is
currently being exploited the world-over for power generation purposes. Among
the clean sources of fuel for power generation, natural gas has been exploited
largely due to significant availability in specific locations.
Similarly,
there is also an impetus on using gas generated from industrial and municipal
wastes, namely diluted natural gas – biogas and land-fill gas. As distinct from
gas generation from biological/organic wastes by biological conversion process,
which is limited to non-loganiaceous matter, the thermo chemical conversion
route (also termed gasification) can process any solid organic matter. The
range of biomass includes agro-residues like rice husk, sugarcane trash and
bagasse in compact or briquetted form. The resultant gas known as ‘Producer
gas’ can be used for fuelling a compression ignition (CI) engine in dual-fuel
mode or a spark ignition (SI) engine in gas alone mode. Harnessing of energy
from biomass via gasification route is not only proving to be economical but
also environmentally benign friendly.
The thermo-chemical conversion of biomass leads to a gas
generally termed as producer gas. The process is termed gasification implying
that a solid fuel is converted to a gaseous fuel. In the recent times, there is
a renewed interest in biomass gasification technology, which has stimulated
interest in producer gas operated engines.
1.1.1Gasification
PROCESS:
Gasification is the incomplete
combustion of biomass resulting in production of combustible gases consisting
of Carbon monoxide (CO), Hydrogen (H2) and traces of Methane (CH4).
This mixture is called producer gas. However in gasification where there is a
surplus of solid fuel (incomplete combustion) the products of combustion are
combustible gases like Carbon monoxide (CO), Hydrogen (H2) and
traces of Methane and non useful products like tar and dust. Biomass is
basically composed of carbon, hydrogen and oxygen represented approximately by
CH1.4O0.6. A proximate analysis of biomass indicates the volatile matter to be
between 60% - 80% and 20% – 25% carbon and rest, ash. Gasification is a
two-stage reaction consisting of oxidation and reduction processes. These
processes occur under sub-stoichiometric conditions of air with biomass. The
first part of sub stoichiometric oxidation leads to the loss of volatiles from
biomass and is exothermic; it results in peak temperatures of 1400 to 1500 K
and generation of gaseous products like carbon monoxide, hydrogen in some proportions
and carbon dioxide and water vapor which in turn are reduced in part to carbon
monoxide and hydrogen by the hot bed of charcoal generated during the process
of gasification. Reduction reaction is an endothermic reaction to generate
combustible products like CO, H2 and CH4 as indicated
below.
C+CO2 2CO2
c+h2O co+h2o
C+2H2 CH4
Since char is generated during the gasification process
the entire operation is self-sustaining.
The temperature of gas exiting the reactor is about 600–900
K. Typical composition of the gas after cooling to ambient temperature is about
18-20% H2, 18-20% CO, 2-3% CH4, 12% CO2, 2.5%
H2O and rest, N2. The
lower calorific value of the gas ranges is about 5.3 + 0.3 MJ/Nm3, with a
stoichiometry requirement of 1.2 to 1.4 kg of air for every kg of producer gas.
One of the pre-requisites for the producer gas to be suitable for internal
combustion application is the cleanliness of the gas apart from the
composition. Conventionally, the gas purity is specified by quantifying the
contaminant levels in terms of particulate and tar matter. The permissible
levels of gas quality also differ with the nature of the engine’s induction
process.
Producer gas can either be used in mono or dual-fuel mode
in reciprocating engines. In case of mono-fuel mode of operation, the gas is
fuelled to a Spark Ignition (SI) engine, whereas in the dual-fuel mode it is
operated along with small quantity of liquid fuel (high-speed diesel, furnace
oil or bio-diesel) in a compression ignition (CI) engine. The choice of mode of
operation is entirely dictated by the economics of operation, and of course on
the availability of appropriate engines.
Conventionally, gasifiers can be classified as fixed bed
and fluidized bed gasifiers. In a fixed bed gasifier, the charge is held
statically on a grate and the air moving through the fuel bed leads to
gasification in the presence of heat. In a fluidized bed system, the charge is
suspended using air as the fluidizing media. The fluidized bed system generates
excessively large tar-laden gas and external cracking using dolomite bed is
necessary to bring down the tar to acceptable levels and hence the approach is limited
to large power level systems. There are again variations in fluidized bed
system known as the circulating fluidized bed system designed to make the
system more compact. It is well recognized that for power levels of 1 MWe or
less, fixed bed systems offer excellent performance at lower capital costs.
1.1.2 Different Types of Gasifiers:
Fixed bed gasifiers are classified
depending upon the flow path of feedstock (biomass) and the generated gas
(producer gas) as updraft, cross draft and downdraft systems. The updraft
system is of counter current design, wherein the biomass and resultant gas flow
path are in opposing directions as shown in Fig1.( a) In this case, the
volatiles released from biomass in the upper region of the reactor do not pass
through the hot char bed and therefore exit the reactor without cracking along
with the producer gas. This gas is therefore less amenable for engine operation
than thermal applications. In a cross draft system the flow path of biomass and
resultant gas are normal to each other as shown in Fig1(b). Even this system
produces tar-laden gas and is therefore not amenable for engine operations.
Figure 1: Gasifier Types – (a) Updraft, (b) Crossdraft
(a)
(b)
Figure 2: Downdraft Gasifier – (a) Closed Top, (b) Open Top
Re-burn
The downdraft system shown in Fig.2 is
a co-current design wherein biomass and the resultant gas flow path are in the
same (downward) direction. It is known from literature that among the fixed bed
gasifiers, the downdraft design generates less of tarladen gas and is amenable
for thermal and engine applications. This happens by design wherein tar
cracking occurs within the reactor (the gases generated in the upper regions of
the reactor pass through the hot bed char). These allow for simpler gas clean-up
system for usage of gas in internal combustion engines. In the design shown in
Fig.2 (a) the reactor top is normally kept closed and hence referred as ‘closed
top’. This design has a barrel shaped reactor with a provision for opening the
top for feedstock charging and a narrow region called the ‘throat’ for tar
cracking, a feature very vital for wood based systems. The gasification media
i.e. air is drawn through the air nozzles/tuyures located at the oxidation
zone.
The open top re-burn design shown in
Fig.2 (b) has concepts that can be argued to be helpful in reducing the tar
levels in the resultant gas. This design has a long cylindrical reactor with
air entry both from the top and the oxidation zone. The principal feature of
the design is related to residence time of the reacting mixture in the reactor
so as to generate a combustible gas with low tar content at different
throughputs. This is achieved by the combustible gases generated in the
combustion zone located around the side air nozzles to be reburnt before
passing though a bottom section of hot char. Also the reacting mixture is
allowed to stay in the high temperature environment along with reactive char
for such duration that ensures cracking of higher molecular weight molecules.
1.1.3 Producer Gas Fuel:
Producer gas obtained from
incomplete combustion of biomass, typically contains 18-20% each of H2 and CO,
2% CH4 and, rest inert like CO2 and N2. The lower calorific value varies
between 4.5 – 4.9 MJ/kg, with stoichiometric air-to-fuel ratio being 1.25 +
0.05 on mass basis. Some of the fundamental data relating to producer gas are
compared with pure gases. The comparison
of producer gas with methane is more vital with regard to the internal
combustion engine operation. This is because most of the engines operating on
gaseous fuels are either close to pure methane (natural gas) or diluted methane
(bio-gas, land-fill gas). The fuel-air equivalence ratio (actual fuel to air
ratio)/(stoichiometric fuel to air ratio) at the flammability limits compares
closely for both the gases, but the laminar burning velocity for producer gas
at the lean limits is much higher. The laminar burning velocity for producer
gas (at 0.1MPa, 300K) is about 0.5 m/sec, which is about 30% higher than
methane.
This feature is argued to demand
lower advancement in the ignition timing and needs consideration while arriving
at the optimum ignition timing for the producer gas fuel.
Like any other gaseous fuel, producer gas can be used for
internal combustion engine operation provided the gas is sufficiently clean
such that contaminant does not accumulate in the intermediary passages to the
engine cylinder. But this fuel has largely been left unexploited due to additional
perceptions, namely auto-ignition tendency at higher compression ratio (CR), large
de-rating in power due to energy density being low. However, these perceptions
need re-examination and clarification. The arguments against the classical view
in favour of better knock resistivity are as follows. Firstly, with the laminar
burning velocity being high due to the presence of hydrogen (more so, with the
gasifier system adopted in this work) might reduce the tendency for the knock.
Secondly, the presence of inert in the raw gas (CO2 and N2) might suppress the pre-flame
reactions that are responsible for knocking on account of increased dilution.
Also the maximum flame temperature attainable with the producer gas being lower
compared to conventional fuels like methane, one could expect better knock
resistivity.
1.1.4 Producer
Gas Carburetor:
A carburetor is a device that blends air and fuel for an internal combustion engine. The fuel in this case would be Producer Gas generated from biomass gasification systems after having been modified for some of its components. Producer gas consists of carbon monoxide and hydrogen with some amount carbon dioxide, methane and nitrogen.
The
blend of air and producer gas can then be fed into Direct Injection
Engines for combustion purposes.
Figure 3: Experimental setup of PG Carburettor
Mixing
devices for gases used in gas engines generally referred to as carburetor, for
mixing air and gaseous fuels are commonly attached to the intake manifold of an
internal combustion engine. In gas carburetor the mixing of air and gaseous
fuels needs to be in a proper ratio for a particular demand of the engine. In
designing the producer gas carburetor, simplicity and ruggedness have always
been considered as a basic requirement to achieve easy adjustment and
reproducible performance. The effective area reduction of gas and air entry
holes is considered by taking a suitable coefficient of discharge. The air and
fuel flow is through orifices into the mixing chamber of the carburetor which
enables proper mixing of air and fuel. The producer gas carburetor is being
designed to have air and fuel flow at ambient conditions to be stoichiometry.
The
producer gas carburetor is as shown in the Figure 3 has orifices placed at air
and gas inlets such that the A/F ratio at ambient flow condition should be
stoichiometry for a engine suction pressure of a 25 kWe engine. The amount of
fuel flow inside the carburetor is controlled by a butterfly valve which is
located prior to the air and fuel inlet orifices. The pressure balancing
electronic controller drives suitably the butterfly valve with the help of a
motor that brings the valves for a null pressure differential across the
manifolds for the fuel and air attached upstream to the main engine manifold
and works in suction pressures. If the differential pressure at both the
carburetor manifolds are maintained at zero, with the manifolds tuned for their
effective flow areas to match the ideal mixture condition, then the mixture
flow what we get at engine intake manifold will be stoichiometry.
The
Figure 3 shown above is the geometric model of the producer gas carburetor
designed and analyzed for optimal pressure drop with good mixing ability. In
order to overcome the problems associated with the use of zero pressure
regulators and to maintain the stoichiometry A/F mixture, carburetor uses the
orifices at both air and gas lines. Orifices are designed based on the mass
flow rate of producer gas required for IC engine.
1.2 LITERATURE
REVIEW
- “EXPERIMENTS
AND MODELLING STUDIES OF PRODUCER GAS BASED SPARK-IGNITED RECIPROCATING
ENGINES”
Sridhar Guru Raja Rao
In this author discussed about
producer gas based spark-Ignited Reciprocating engines, gaseous fuels are
gaining prominence as cleaner fuels for power generation via internal
combustion engine route, the power generation package including both
reciprocating engines and gas turbine machinery. Complete combustion with
minimal emission is the key feature of gaseous fuels and this feature is
currently being exploited the world-over for power generation purposes.
Producer gas derived from biomass is one such eco-friendly gaseous fuel. While
natural gas is the most use done, next one being the land fill gas or biogas,
which is diluted natural gas (with 20-30%CO2) biomass based producer gas is
perhaps the least used fuel,
- “EXPERIMENTAL
ANALYSIS OF FEEDBACK CONTROL SYSTEM FOR LAMBDA SENSOR BASED PRODUCER GAS
ENGINE CARBURETOR”
Sumer B. Dirbude, Sheshagiri
G.S and Dr.N .K.S.Rajan
The study reported in this paper forms initial
experimentation efforts for the development of the lambda sensor based Producer
gas carburetor for frequent load changes and throw off conditions using
feedback control system. This paper aims on the experimentations carried out
for the design confirmation of the control and actuation parts and discuss the
results obtained.
Currently no Producer gas carburetors are being sold
commercially. So the development of the carburetor which will fulfill all the
requirements of low energy density fuels is a need of the time.
·
“CFD
ANALYSIS OF A MIXTURE FLOW IN A PRODUCER GAS CARBURETOR”
T.R.Ani1,
S.D.Ravi, M.Shashikanth, P.G.Tewari and N.K.S.Rajan
The study reported in this paper on a
specially designed producer gas carburetor is comprehensively analyzed for its
mixing performance and response with a CFD modeling. The model is made up of a
mixer chamber that has the essential orifices for air and fuel (producer gas)
inlets to generate stable stoichiometric mixture at near to ambient conditions
using the induction of the engine as the driving pressure differential for the
flow, and tested for a case of engine of 25 kWe capacity. The CFD simulations
are carried out followed with experimental studies to validate the analysis.
The results show a consistency in the experimental data and the modeling has
provided a good insight into the flow details and has paved way in optimization
in the geometrical design to get a good mixing efficiency.
- “NUMERICAL AND
EXPERIMENTAL STUDY OF AIR AND FUEL FLOW IN SMALL ENGINE CARBURETORS”
Diego Alejandro Arias
The study contains the CFD analysis of the most important
parts found in the sensitivity analysis: the main fuel orifice and the
carburetor venturi. The CFD studies allowed gaining a better understanding of
the flow characteristics in these elements, and their results were used to
develop engineering correlations that may be implemented in the one-dimensional
model.
It was found that the flow in the small metering orifices
behaves like a small pipe, which may be characterized with an inlet pressure
loss coefficient and a Darcy friction factor. The analysis of the carburetor
venturi showed that the flow may be considered isentropic from the inlet of the
venturi to the venturi throat. Besides the throttle plate, the fuel tube is the
most important part controlling the flow field and the pressure losses
downstream of the venturi throat.
- “BIOMASS
GASIFICATION”
Anil K. Rajvanshi
In
this author told how the biomass fuel useful for producing very low emission.
Modern agriculture is an extremely energy intensive process. However high
agricultural productivities and subsequently the growth of green revolution has
been made possible only by large amount of energy inputs, especially those from
fossil fuels1. With recent price rise and scarcity of these fuels there has
been a trend towards use of alternative energy sources like solar, wind, and
geothermal etc.2 however these energy resources have not been able to provide
an economically viable solution for agricultural purpose.
One biomass energy based system,
which has been proven reliable and had been extensively used for transportation
and on farm systems during World War II is wood or biomass gasification4.
Biomass gasification means
incomplete combustion of biomass resulting in production of combustible gases
consisting of Carbon monoxide (CO), Hydrogen (H2) and traces of Methane (CH4).
This mixture is called producer gas.
Producer gas can be used to run internal combustion engines (both compression
and spark ignition), can be used as substitute for furnace oil in direct heat
applications and can be used to produce, in an economically viable way,
methanol – an extremely attractive chemical which is useful both as fuel for
heat engines as well as chemical feedstock for industries5. Since any biomass
material can undergo gasification, this process is much more attractive than
ethanol production or biogas where only selected biomass materials can produce
the fuel.
·
“BIOMASS
DERIVED PRODUCER GAS AS A RECIPROCATING ENGINE FUEL - AN EXPERIMENTAL ANALYSIS”
G.
Sridhar, P.J. Paul and H.S. Mukunda
This
paper uncovers some of the misconceptions associated with the usage of producer
gas, a lower calorific gas as a reciprocating engine fuel. This paper
particularly addresses the use of producer gas in reciprocating engines at high
compression ratio (17:1), which hitherto had been restricted to lower
compression ratio (up to 12:1). This restriction in compression ratio has been
mainly attributed to the auto-ignition tendency of the fuel, which appears to
be simply a matter of presumption rather than fact. The current work clearly
indicates the breakdown of this compression ratio barrier and it is shown that
the engine runs smoothly at compression ratio of 17:1 without any tendency of
auto-ignition. Experiments have been conducted on multi-cylinder spark ignition
engine modified from a production diesel engine at varying compression ratios
from 11:5:1 to 17:1 by retaining the combustion chamber design. As expected,
working at a higher compression ratio turned out to be more efficient and also
yielded higher brake power.
A
maximum brake power of 17:5 kWe was obtained at an overall efficiency of 21% at
the highest compression ratio. The maximum de-rating of power in gas mode was
16% as compared to the normal diesel mode of operation at comparable
compression ratio, whereas, the overall efficiency declined by 32.5%. A careful
analysis of energy balance revealed excess energy loss to the coolant due to
the existing combustion chamber design. Addressing the combustion chamber
design for producer gas fuel should form a part of future work in improving the
overall efficiency.
1.3
COMPUTATIONAL FLUID DYNAMICS
Computational
Fluid Dynamics (CFD) is the analysis of systems involving fluid flow, heat
transfer and associated phenomena such as chemical reactions by means of
computer-based simulation. The technique is very powerful and spans a wide
range of industrial and non – industrial application areas. Some examples are:-
Ø Aerodynamics of aircrafts and vehicles: Lift and drag.
Ø Hydrodynamics of ships.
Ø Power Plants: combustion in IC engines and gas turbines
Ø Turbo-machinery: Flow inside rotating passages, diffusers
etc.
Ø Electrical and electronics engineering: cooling of equipment
including microcircuits.
Ø Chemical process engineering: mixing and separation, polymer
moulding.
Ø Marine engineering: loads on off-shore structures
Ø Biomedical engineering : blood flows through arteries and
veins.
From 1960s
onwards the aerospace industry has integrated CFD techniques into the design, R
& D and manufacture of aircraft and jet engines. More recently the methods
have been applied to the design of internal combustion engines, combustion
chambers of gas turbines and furnaces. Increasingly CFD is becoming a vital component
in the design of industrial products and processes.
There are several
unique advantages of CFD over experimental based
Approaches to
fluid systems design:-
Ø
Substantial reduction of lead times
and cost of new designs.
Ø
Ability to study systems where controlled
experiments are difficult or impossible to perform (e.g., very large systems).
Ø
Ability to study systems under
hazardous conditions at and beyond their normal performance limits (e.g.,
safety studies and accidents scenarios).
Ø
Practically unlimited level of
detail of results
1.3.1 How does a CFD code works
CFD codes
are structured around the numerical algorithms that can tackle fluid flow
problems. In order to provide easy access to their solving power all commercial
CFD packages include sophisticated user interfaces to input problem parameters
and to examine the results. Hence all codes containing three main elements: (i)
a pre-processor, (ii) a solver and (iii) a post-processor. We briefly examine
the function of each of these elements within the context of a CFD code.
A) Pre-processor
Pre-Processor
consists of the input of a flow problem to a CFD program by means of an
operator – friendly interface and the subsequent transformations of this input
into a form suitable for use by the solver. The user activities at the
preprocessor stage involve:-
Ø
Definition of the geometry of the
region of interest: the computational domain
Ø
Grid generation – the sub-division
of the domain into a number of smaller, non overlapping sub-domains: a grid (or
mesh) of cells (or control volumes or elements).
Ø
Selection of physical and chemical
phenomena that need to be modeled
Ø
Definition of fluid properties
Ø
Specification of appropriate
boundary conditions at cells which coincide with or touch the domain boundary.
The solution
of a flow problem (velocity, pressure, temperature etc) is defined at nodes
inside each cell. The accuracy of a CFD solution is governed by number of cells
in the grid. In General, the larger the number of cells, the better the
solution accuracy. Both the accuracy of a solution and its cost in terms of
necessary computer hardware and calculation time are dependent on the fineness
of the grid.
B) Solver:
There
are three distinct streams of numerical solution techniques: finite difference,
finite element and finite volume method. In outline the numerical methods that
form the basis of the solver perform the following steps.
·
Approximation of the unknown flow
variables by means of simple functions
·
Discreatisation by substitution of
the approximations into the governing flow equations and subsequent
mathematical manipulations
·
Solution of the algebraic equations.
The main difference between the
three streams is associated with the way in which the flow variables are
approximated and with the discretisation processes
C) Finite difference method
Finite
difference method describes the unknown’s Φ of the flow problem by means of point samples at the node
points of a grid of co-ordinate lines. Truncated
D) Finite element method
Finite
element methods use simple piece wise functions (e.g linear or quadratic) valid
on elements to describe the local variations of unknown flow variables Φ. The governing equation is
precisely satisfied by the exact solution Φ. If the piece wise approximating functions for Φ are substituted into the equation it
will not hold exactly and a residual is defined to measure the errors. Next the
residuals (and hence the errors) are minimized in some sense by multiplying
them by a set of weighing functions and integrating.
As a result
we obtain a set of algebraic equations for the unknown coefficients of the
approximating functions. The theory of finite elements has been developed
initially for structural stress analysis
E) Finite volume method
The finite
volume method was originally developed as a special finite difference
formulation. There are five main commercially available CFD codes: FLUENT,
FLOWED, PHOENICS and STAR-CD. The numerical algorithm consists of following
steps.
Ø Forma integration of the governing equations of fluid flow over
all the (finite) control volumes of the solution domain
Ø Discretisation involves the substitution of a variety a
finite difference type approximations for the terms in the integrated equations
representing flow processes such as convection, diffusion and sources. This
converts integral equations in to a system of algebraic equations.
Ø Solution of the algebraic equations by an iterative method
F) Post – Processor
As in
pre-processing a huge amount of development work has recently taken place in
the post – processing field. Owing to the increased popularity of engineering
workstations many of which have outstanding graphics capabilities, the leading
CFD packages are now equipped with versatile data visualization tools, these
include:-
Ø
Domain geometry and grid display
Ø
Vector plots
Ø
Line and shaded contour plots
Ø
2D and 3D surface plots
Ø
Particle tracking
Ø
View manipulation (translation,
rotation, scaling, etc.)
Ø
Colour postscript output
More
recently these facilities may also include animation for dynamic result display
and in addition to graphics all codes produce trustly alphanumeric output and
have data export facilities for further manipulation external to the code as in
many other branches of CAE other graphics output capabilities of CFD codes have
revolutionized the communication of ideas to the non – specia.
G) Numerical formulation of
methodology
Governing equations
For a
general flow problem involving heat transfer there are seven unknowns:
pressure, temperature, velocities along the three co-ordinate axes and internal
energy. Therefore, Six governing equation are required to solve the problem.
These governing equations are continuity, momentum equations along three
coordinate axes, energy equation and two equations of state. Governing
equations for unsteady, time-dependent, three dimensional flow of a compressible
Newtonian fluid are given by
(i) Continuity equation
(ii)
Momentum Equation: (Navier-stokes equation)
x-momentum
y-momentum
z-momentum
Where SMx, SMy, SMz are source terms, include contribution due to body forces only (for example the body force due to gravity would be modeled by SMz=-ρg) and τ is viscous stress.
(iii)
Energy equation
Where
is the source of energy
per unit volume per unit time and E is the sum of internal energy and
gravitational potential energy.
1.3.2 Introduction to FLUENT
CFD is
primarily used as a design aid for predicting the performance. Characteristics
of equipment involving fluid flow and heat transfer. The ability to Simulate
heat transfer and fluid flow problems numerically even before a prototype being
built, reduces the cost and most essentially time of development to a greater
extent. Obviously, CFD results have to be continuously checked experimentation
ensuring the numerical predictions are reliable. Thus a cycle is formed
involving theoretical predictions, CFD and experimentation. FLUENT is one of the
most widely used computational fluid dynamics (CFD) software package to
simulate fluid flow related problems. It uses the finite – volume method to
solve the governing equations for a fluid. It provides the capability to use
different physical models such as incompressible or compressible, in viscid or
viscous, laminar or turbulent, etc.
Once a grid
has been read into FLUENT, all remaining operations are performed within the
solver like setting boundary conditions, defining fluid properties, iterating the
solution and post processing the results.
1.3.3 Discretization of Governing Equations
Discretion
is the process whereby the continuous governing differential equations are
replaced by their counterparts. The differential equations are transformed to algebraic
equations, which should correctly approximate the transport properties of the
physical process. Discreatization identifies the node locations used to model
the physical problem configuration. In order to solve these algebraic equations
it is necessary to calculate the transport property at the cell faces, which
the upwinding differencing scheme does. The important steps involved in the
discretization procedure are listed below:-
Ø Division of the computational domain into discrete control
volumes using a general curvilinear grid
Ø Integration of the governing equations on the individual
control volumes to construct the algebraic equation for discrete unknowns
Upwinding
Upwinding
difference scheme is simple and has been widely used in CFD calculations. This
scheme takes into account the flow direction while determining the value at the
cell face. The scheme is based on the backward differencing formula. So the
accuracy is only first order on the basis of the Taylor Series Truncation Error
(TSTE). The use of the upwinding quantities ensure that the schemes are very
stable and obey the transportiveness requirement but the first order accuracy
makes them prone to numerical diffusion errors. Employing higher order schemes
can minimize such errors. Higher order discretization schemes involve more
neighbor points and reduce the discretization errors by bringing in a wide
influence
1.3.4 Modeling of turbulence
The flow
through diffuser is turbulent. Turbulence is the most complicated kind of fluid
motion. Turbulent flows are unsteady, irregular, three dimensional and
dissipative. Turbulent velocity fluctuations can generate rates of momentum
transfer, which are far greater than those due to molecular diffusion
The flow
coming from the combustion chamber is turbulent in nature and therefore the
turbulence model is an important component of CFD analysis. For present
analysis, the K-ε and K-ω two equation model is used.
In turbulent flows, the velocity at
the point is considered as the sum of the mean (time averaged) and fluctuating
components:-
Substituting
the expressions of his form into the basic momentum equations (and dropping the
over – bar on the mean velocity, u) yields the ensemble-averaged momentum
equations, for predicting turbulent flows.
The above
equation has the same form as the fundamental momentum balance. Equation of
velocities now representing time averaged (or mean flow) values and the effect
of turbulence through the “Reynolds Stress”,
The K-ε Turbulence model
The K-ε turbulence model is an eddy
viscosity model in which the Reynolds Stresses are assumed to be proportionate
to the mean velocity gradients, with the constant of proportionality being the
turbulent viscosity, the assumption is known as Boussineq hypothesis, provides
the following expression for the Reynolds Stresses:-
Where
K is the turbulent kinetic energy given by the expression:
The forms
of turbulent momentum equations remain identical to the form of laminar
Momentum equations expect that is replaced by an effective viscosity (μeff)
Equations of the turbulent viscosity
The turbulent viscosity μt is obtained by assuming that it is
proportional to the product of a turbulent viscosity scale and length scale. In
the K-ε turbulence
model, These velocity (K) and dissipation rate (ε). The velocity scale is taken to be K and the scale
is taken to be
Where Cμ is an empirical derived constant of proportionality (Cμ=0.09)
Transport equation for K and ε
The value of K and ε required are obtained by solution of
conservation
equations, Turbulent energy (k)
Turbulence dissipation rate (ε)
Where C1 and C2 are empirical constants and σk and σε are prandtl numbers
giving Pk generation of K
σk is the turbulent prandtl numbers μtCp/kt The coefficient C1, C2,
Cμ, σε, are empirical constants which have the following empirically derived
values. C1 = 1.44, C2 =1.92, Cμ=0.09, σε=1.3
1.4 Problem
definition:
The producer gas
carburetor used for fairly well mixing of air and fuel before combustion
chamber for SI engine to decrease the emission produced from exhaust. It is because homogeneous mixing of producer
gas and air takes place in
the carburetor. Due to this homogenous mixing, complete combustion of fuel
takes place inside the combustion chamber. Hence as the combustion efficiency
increases, therefore engine efficiency also increases and due to this complete
combustion of fuel the emissions emitted by the engine will reduce.CFD
simulations are performed for different set of air and fuel ratios.
1.5 Objectives:
The Objective of the present
research will focus on developing an advanced Producer
Gas Carburetor for SI engine. This system will make the well mixing of
air and fuel before enter into the combustion chamber. Because of Producer Gas
properties and this well mixed air-fuel enter into combustion chamber and the exhaust
gas produces less emission.
1.5.1 Qualitative objectives:
The flow field will be described
by
plotting the following on
3 different Planes for the geometric configurations
at one design point only.
- Velocity and pressure
contours
- Vector plots and path lines
1.6
Assumptions:
The following assumptions are used in order to simplify
the problem
·
The
Working fluid considered is air.
·
The
Producer Gas Carburetor used in this project is of its inlet
cross‐section area for air and
PG used is of 50x50x50x50mm respectively.
·
The
flow is steady and incompressible.
Chapter-2
2.1 GEOMETRIC
MODELING
The below fig.4 shows designing of the producer gas carburetor,
simplicity and ruggedness
have always been considered as a basic requirement to achieve easy adjustment and reproducible performance.
The effective area reduction of
gas and air entry holes is considered by taking a suitable coefficient of discharge. The air and fuel flow is through orifices into the mixing chamber of the carburetor which enables proper mixing of air and fuel. The producer gas carburetor is being designed to have air and fuel flow at ambient conditions
to be stoichiometry.
Figure 4: Simplified model of PG Carburettor
The producer gas carburetor
has
orifices placed at air and gas inlets such that the A/F ratio at ambient flow condition should be stoichiometry for an engine suction pressure
of
a 25 kWe engine. The amount of fuel flow inside the carburetor is controlled by a butterfly valve which is located prior to
the air
and fuel
inlet orifices.
The pressure balancing
electronic
controller drives suitably the
butterfly valve with the
help of a motor that brings the valves for a null pressure differential
across the manifolds
for the fuel and air attached upstream to the main engine manifold
and works
in
suction pressures. If the differential
pressure at both the carburetor manifolds is maintained at zero, with the manifolds tuned for their effective flow areas to match the ideal mixture condition, then the mixture flow obtained at engine intake manifold will be stoichiometry. The geometric model of the producer gas carburetor designed and analyzed
for
optimal pressure drop
with
good mixing ability has been
reported in the
literature.
The
air
and producer
gas
passes
through inlets
of
50
mm x 50 mm. The air
inlet is kept tangential to the mixing chamber whereas producer gas inlet is radial to the mixing chamber.
The outlet diameter is 35 mm
based on the inputs for Reynolds number and velocity at the outlet assuming air at Standard
Temperature Pressure.
Air and producer gas enter into mixing chamber through an orifice of 28.0mm and
26.5mm diameter respectively. This will not be considered in the CFD analysis.
The experimental tests done on the geometry described in Figure 3 have
been tabulated
below
in table 1.
Table 1: Experimental Description of Model
|
Sl no |
∆p across venturi Meter (air side) g/s |
∆P across venture meter (PG side) g/s |
∆P across air orifice in mm of water column |
∆P across PG orifice in mm of water column |
Absolute pressure (carburetor outlet) mm of
water column |
Air flow rate g/s |
Fuel flow rate g/s |
A/F ratio |
|
1 |
68 |
10 |
25 |
6 |
102 |
27.62 |
9.423 |
2.931 |
|
2 |
68 |
20 |
25 |
10 |
102 |
27.62 |
13.32 |
2.074 |
|
3 |
68 |
30 |
25 |
13 |
99 |
27.62 |
16.32 |
1.692 |
|
4 |
68 |
40 |
25 |
19 |
99 |
27.62 |
18.85 |
1.465 |
|
5 |
68 |
50 |
25 |
22 |
97 |
27.62 |
21.07 |
1.311 |
|
6 |
68 |
60 |
25 |
25 |
95 |
27.62 |
23.08 |
1.196 |
|
7 |
68 |
70 |
25 |
30 |
95 |
27.62 |
24.93 |
1.108 |
In the present analysis, we will be considering a geometrical modification for taking
advantage of the low pressure generated at the core of the vortex generated by the tangential air inlet. The final geometry considered for this analysis will have the following geometrical description
which is given in table 2.
Table
2: Geometrical description of the Model
|
|
Geometry |
|
Mixing Section cylinder diameter |
200mm |
|
Mixing Section cylinder Length |
300mm |
|
Outlet Diameter |
70mm |
|
Outlet
Cylindrical Section length |
70mm |
|
Air inlet duct
cross-section |
50mm X 50mm X
50mm X 50mm |
|
Air inlet duct
length |
127mm |
|
Air inlet duct
location |
Tangential but 200
angle w. r. t the direction of the redial direction at 45mm form the end of
the carburetor and 72mm offset from the axis of the mixing section
cylinder |
|
PG inlet duct
cross section |
50mm X 50mm X
50mm X 50mm |
|
PG inlet duct
length |
40mm |
|
PG inlet duct
location |
Radially inward
at 45mm from the end of the carburetor. |
2.2: Grid Generation:
Figure
5: Mesh Model of Geometry
Analysis will be done to determine the flow field, air/producer gas distribution
for geometrical configurations at different air and fuel flow rates as reported
in the experimental set-up. ANSYS ICEM CFD will be used for generating multi-block structured mesh for the
geometries and ANSYS FLUENT will be used for doing the single phase multi
species flow simulation.
2.3 Domain Conditions:
The CFD simulation will not include
the orifices preceding the
inlets but the geometry as described in Figure 1.
A single phase, multispecies simulation will be carried out under atmospheric isothermal
operating conditions for the geometry for the different set of air and PG boundary
conditions. Turbulence will be modeled according to the ke RNG turbulence model
with wall functions for near wall treatment. A total of 7 transport equations will be solved:
1continuity, 3 momentum, 2 turbulence and 1 species equation for producer gas. The
mass fraction of air being abundant will be calculated from the constraint relationship
of (mass fractions of species) = 1.
2.4 Material properties:
Air ideal gas
and Producer gas are considered to be
different species with their averaged properties based on
their individual constitution. The following volume percentage has been considered based on the
experimental data available in the literature.
Table 3: Material
Properties
|
Component |
Air (%) |
Producer Gas (%) |
|
Carbon
dioxide |
0.03 |
12.5 |
|
Carbon
Monoxide |
0.01 |
21 |
|
Hydrogen |
0 |
21 |
|
Argon |
0.86 |
0 |
|
Methane |
0 |
3.5 |
|
Oxygen |
21 |
0 |
|
Nitrogen |
78 |
42 |
|
Total |
100 |
100 |
The transport and specific properties of air and Producer Gas therefore used
for or isothermal flow simulation are the following:
Table 4: The Transport and Specific Properties of Air and Producer Gas
|
Property |
Air |
Producer Gas |
|
Density (Kg/m3) |
1.175 |
0.978 |
|
Viscosity (Pa.S) |
1.179 x 10-5 |
1.452 x 10-5 |
|
Specific Heat (J/kg-k) |
1005.148 |
3838.358 |
|
Thermal Conductivity (W/m-K) |
0.0240 |
0.0535 |
2.5 Boundary Conditions:
The problem set-up has static pressure at the inlets and
mass flow at outlets. As ANSYS FLUENT will be used in the present analysis, we
will be using the experimental mass flow rates at the air and PG inlets and
pressure outlet at the outlet boundary. For air inlet boundary
condition mass and momentum, static pressure equivalent to domain reference
pressure is set with flow condition being subsonic. The initial condition of
flow through the air inlet with air ideal mass fraction as 1 is considered. The
initial boundary condition for fuel inlet is same as the air inlet except for
the flow of producer gas mass fraction being 1 at the inlet. The boundary
condition for carburetor outlet is of different mass flow rate which is to be
simulated is considered.
2.6
METHODOLOGY:
To simulate the turbulence parameters, a standard k-ε RNG
model has been chosen with isothermal heat transfer condition at 300 K. The
Solver uses k-ε RNG model with two new variables.
2.7 Numerical Scheme
The numerical scheme of FLUENT
solver is based on finite volume technique, and involves the following steps
1. Integration of the governing equations of
fluid flow over all the control volumes of the solution domain.
2. Discretization involves substitution of
the terms in the integrated equation representing flow processes such as
convection, diffusion and sources with
finite-volume type approximation and thereby converting into algebraic
equations.
3. The algebraic equations are solved
iteratively.
Computation uses a coupled solver, which solves
the hydrodynamic equations (for u, v, w, p) as a
single system. This solution approach uses a fully implicit discretization of
the equations at any given time step. Second-order Backward Euler (SBE) method
used for discretizing the transient term and high resolution method for
discretizing the convection terms. Time step of 10CA and RMS
residual type with a residual target of 1e-05 is used for simulation.
3.1 RESULTS AND DISCUSSION:
In this section, mass fraction plot,
contour plot of pressure and velocity across the different cross sectional
plane is presented, for different air and producer gas ratios. Furthermore mass
fraction, pressure and velocity graphs against different length are presented.
3.1.1 CASE I:
Air fuel Ratio (2.931)
Ø
Mass Fraction Plot:
Table 5: Quantitative Objectives
|
PG mass flow
rate |
0.00942300
[kg s^-1] |
|
Air mass flow
rate |
0.02762000
[kg s^-1] |
|
MF Spread at
outlet |
0.00057634 |
|
Pressure drop |
36919.55 [Pa] |
Graph 1: Mass Fraction V/s Length of Carburetor:
From the above graph it is observed that Mass fraction is
high at air inlet and low at PG inlet. PG and air fairly
well making its mass fraction variation nearly 0.00057. This mass fraction is to
be considered as a good enough for premixed combustion. This proper mixed fuel
and air will enter into the combustion chamber. Because of this there will be a
complete combustion and the exhaust gas produces low emissions.
Graph 2: Pressure V/s Length of Carburetor:
From the
above graph it
is observed that pressure will be high at the inlets of producer gas and the
air inlet. And it will be low at the middle section of the carburetor and at
the outlet of the carburetor. The pressure drop observed here is 36919.55 [Pa].
Graph 3: Velocity V/s Length of Carburetor:
From the above graph it is observed
that velocity is very low at inlet of producer gas and air. And in the middle section it will be high.
And at the outlet velocity is low which is around 8 m/s. which is considered to
be good for SI engine.
3.1.2
CASE II: Air fuel ratio (2.074)
Ø
Mass fraction Plot:
Figure 11: Mass fraction Plot
Ø Vector Plot:
Figure 12: Vector Plot
Ø
Pressure plot:
Figure 13: Pressure plot
Ø
Velocity contours:
Figure 14: Velocity contour
Ø
Velocity Path lines:
Figure 15: Velocity Path lines
Table 6: Quantitative Objectives
|
PG mass flow rate |
0.01332000 [kg s^-1] |
|
Air mass flow rate |
0.02762000 [kg s^-1] |
|
MF Spread at outlet |
0.00081813 |
|
Pressure drop |
29884.05 [Pa] |
Graph 4: Mass Fraction V/s Length of Carburetor:
From the above graph it is observed that Mass fraction is
high at air inlet and low at PG inlet. PG and air fairly
well making its mass fraction variation nearly 0.00081813. This mass fraction is to be considered as a
good enough for premixed combustion. This proper mixed fuel and air will enter
into the combustion chamber. Because of this there will be a complete
combustion and the exhaust gas produces low emissions.
Graph 5: Pressure V/s Length of Carburetor:
From the above graph it is observed that pressure will be high at the inlets of
producer gas and the air inlet. And it will be low at the middle section of the
carburetor and at the outlet of the carburetor. The pressure drop observed here
is 29884.05 [Pa].
Graph 6: Velocity V/s Length of Carburetor:
From the above graph it is observed that velocity is very
low at inlet of producer gas and air and in the middle section it will be high.
And at the outlet velocity is low which is around 5 m/s. which is considered to
be good for SI engine.
3.1.3 cASE III: Air fuel ratio (1.692)
Ø Mass Fraction Plot:
Figure 16: Mass Fraction Plot
Ø Vector Plot:
Figure 17: Vector Plot
Ø Pressure plot:
Figure 18: Pressure Plot
Ø Velocity plot:
Figure 19: Velocity Plot
Ø
Velocity path lines:
Figure 20: Velocity path lines
Table 7: Quantitative Objectives
|
PG mass flow rate |
0.01632000 [kg s^-1] |
|
Air mass flow rate |
0.02762000 [kg s^-1] |
|
MF Spread at outlet |
0.00088840 |
|
Pressure drop |
31426.9 [Pa] |
Graph 7: Mass Fraction V/s Length of Carburetor:
From the above graph it is observed that Mass fraction is
high at air inlet and low at PG inlet. PG and air fairly
well making its mass fraction variation nearly 0.00088840. This mass fraction is to
be considered as a good enough for premixed combustion. This proper mixed fuel
and air will enter into the combustion chamber. Because of well mixing fuel
entering into the combustion chamber there will be a complete combustion and
the exhaust gas produces low emissions.
.
Graph 8: Pressure V/s Length of Carburetor:
From the above graph it is observed
that pressure will be high at the inlets of producer gas and the air inlet. And
it will be low at the middle section of the carburetor and at the outlet of the
carburetor. The pressure drop observed here is 31426.9 [Pa].
Graph 9: Velocity V/s Length of Carburetor:
From the above graph it is observed that velocity is very
low at inlet of producer gas and air and in the middle section it will be high.
And at the outlet velocity is low which is around 9 m/s. which is considered to
be good for SI engine.
Graph 10: Comparison of Pressure Drop for Different PG
Ratio:
The graph plotted here is the pressure drop against the
producer gas inlet for different air fuel ratios. The pressure is increasing as
the producer gas mass rate increases.
Graph 11: Comparison
of Mass Fraction for Different PG Ratio:
The graph plated here is the mass fraction against the
producer gas inlet for different air fuel ratios. It is observed from the graph
the mass fraction is in the range of 0.0005 to 0.001. which is consistency with
result obtained in the experimental data from literature considered to be good
for premixed combustion.
CHAPTER-4
4.1 CONCLUSION
Turbulent model based on k-ε RNG
theory with a RANS code has been used for the CFD predictions of the producer
gas mass fraction has been evaluated leading to bringing out of an optimal
design of the Producer Gas Carburetor. It is observed that mass fraction is around
0.0005 to 0.001 inside the Producer Gas Carburetor which is considered to be
good for premixed combustion as it is consistency in the experimental data obtained from literature, so there will be a proper mixing of air and fuel lead
to a stoichiometric mixture inside
the Carburetor. This well mixed fuel will enter into the combustion chamber and
there will be a complete combustion. Because of properties of producer gas fuel
and the well mixing of air and fuel, the emission produced will be low at the exhaust.
4.2 SCOPE FOR FUTURE
WORK:
Simulation can be continued for
different set of air and producer gas fuel ratios. And also we can do the simulation by modifying
the geometry of producer gas carburetor by changing the position of the inlet
of the producer gas for different angles from 0 to 90 degrees.
References
1. Klimstra
J “Carburetors for Gaseous Fuels –on Air to Fuel ratio, Homogeneity and Flow
restriction. SAE paper 892141.
2. Versteeg, H.K. and Malalasekara, w. An
introduction to CFD-The finite volume method, 1995 (Longman Scientific and
Technical).
3. T.R.Anil, P.G.Tewari N.K.S.Rajan An Approach
for Designing of Producer Gas Carburetor for Application in Biomass based Power
Generation Plants proceedings of the national conference of Natcon 2004
4. Yoshishige Ohyama, “Air /fuel ratio control
using upstream models in the intake system.” SAE paper 0857 vol1 1999.
5. T.R.Anil, S.D.Ravi, M.Shashikanth, N.K.S.Rajan
P.G.Tewari. “CFD Analysis of a Mixture Flow in a Producer Gas Carburettor”
International Conference On Computational Fluid Dynamics, Acoustics, Heat
Transfer and Electromagnetics CFEMATCON-06, July 24-25, 2006,
6.
Anil
K. Rajvanshi, “ Biomass Gasification”,
Published as a Chapter (No. 4) in book “Alternative Energy in
Agriculture”, Vol. II, Ed. D. Yogi Goswami, CRC Press, 1986, pgs. 83-102.
7. G. Sridhar _,
P.J. Paul, H.S. Mukunda, “Biomass derived producer gas as a reciprocating
engine fuel—an experimental analysis”, Biomass and Bioenergy 21 (2001) elsevier. Pages:
61–72.
8.
G
Sridhar, H V Sridhar,
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