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1Challenge the future
COMPUTATIONAL MODELLING OF
A MICRO-GASIFIER COOKSTOVE
MSc thesis presentation
Anoop Asranna I.P.
Supervisors
Dr. Paul van der Sluis, Philips Research
Dr.Ir. Prof. Benediks Boersma, TU Delft

Contents
• Biomass as a cooking fuel – consequences
• Philips woodtsove – Operating principles , constructional features
• Motivation and objective
• Methodology, results and conclusion

Open fire cooking

2.7 billion
people rely on direct
biomass burning for
their cooking needs
Cooking
-90% of
total household
energy
consumption
The Cooking Conundrum
52% of total
population in
developing countries
using biomass

Climate
Health
Land usage
• 500g equivalent CO2
• 2% of total global emissions
• 21% of black carbon emissions
• 4.3 million annual deaths
• Biggest killer more than HIV,
TB and malaria combined
• Women & children,
most vulnerable
• Leading cause of
deforestation
in developing countries
The Impact

Solutions?
• Elimination of Household
Air Pollution needs
transition away from
solid fuels
• Transition to LPG,
electric stoves will
reduce emissions
• Significant share of
population reliant on
biomass until 2030

Philips Woodstove
‘For biomass cooking, only gasifier stoves (such as Philips)
approach the emission levels of LPG and hold the potential to
impact the deadliest HAP (household air pollution) linked
illnesses’ (World Bank, 2013)
• Biomass burning micro-gasifier cookstove.
• Highest efficiency, lowest emissions among wood burning
cookstoves.

MICRO-GASIFICATION

Primary air
Secondary air
Exhaust gases
Producer gases
Secondary combustion
zone
•Gas combustion
•Flame
•Pollutant formation
Primary combustion
zone
•Fuel bed
•Drying, pyrolysis
•Sub-stoichiometric
combustion
Heat transfer zone
•Convective heat
transfer
to the vessel
Fuel bed
Vessel

Constructional Features

Top view

Performance parameters
Performance indicator Value
Fuel use
Thermal efficiency (high power)

38.4%-39.4%
Emissions
CO emissions (g/MJ delivered) 0.98-2.71
PM 2.5 emissions (mg/MJ
delivered)
62.3-147.3
Indoor emissions
CO emissions 0.08-0.21 g/min

PM 2.5 emissions 4.70 -9.09 mg/min
Output
Firepower

4 -5 kW

• Experimental design
• Knowledge gaps –inadequate characterization of design parameters
• Traditional workflow – physical prototypes, time consuming and expensive.
• Local measurements of airflows, temperatures not possible
Design Philosophy
SIMULATIONS!

• Develop a CFD model that enables an investigation into the steady
state flow and temperature fields in the secondary combustion zone
of the Philips woodstove.
• Flaming mode operation, vessel on top
• Mixing analysis
• Aid and inform experimental design –qualitative validation
Objective

Computational domain

Insulation tile - dimensions

Boundary conditions
Inlet-2
Inlet-1
Volumetric
heat generation
T=373 K
Outlet
Ambient
Surface to
ambient
radiation
Central axis

Inlet 1- Annular chamber inlet
Property Value
Pressure (P) 30 Pa
Turbulence intensity 5%
Characteristic length (L) 7 mm
Turbulence length scale 4.9 *10-4
m

Inlet 2 – Fuel bed top
• Sub- stoichiometric combustion of volatiles with primary air
Value
Mass of volatiles/kg of wood 0.889 kg
Wood consumption rate 16.8 g/min
Producer gases (per segment) 0.541 g/min
Temperature (Varunkumar,2012) 1200K

Heat source
• Fire power of the stove - 4.5 kW
• Flaming mode -70%, 3.15 kW
• Net power in the domain = power generation + enthalpy of
producer gases
• Source term = 2.35 kW
• Specified – 900W

Grid independence study

Discretization order
A) P1 elements B) P2 elements
• Significant jump in DOF
• Flow field better resolved

Annular chamber +nozzles
• The preheating in the annular chamber is of the order of 100K. The
average temperature of air at the nozzle inlet is 440K
• Mass flow divided equally among the three nozzles
• Nozzles-flow transforms into a jet, rapid heating
• At the nozzle exit- Reynolds number =1000, Temperature = 580K

• Air jets suppress the
producer gas flow
• Collision of jets at the
center to set up a zone of
recirculation
Combustion chamber

Hotspot
K
K
Fuel bed
Vessel
Nozzle
• Existence of hotspots
• Consequence of uniform heat
generation
• Not a realistic approximation
• Massflows
Combustion chamber-
Temperature

Experimental values Simulation
Region Top (K) Bottom (K) Top (K) Bottom(K)
Insulation tile
holder
625-675 575-625 750 592
Inner shield 500-550 425 700 580
Outer shield 475-495 398 610 450
Housing 375-400 323 450 300
• Hotspot
• Inclusion of design details

Mixing analysis
• Mixture fraction analysis using passive scalars
• Prerequisites- temperature and reactant concentration
• Secondary combustion zone temperature -700°C
• Reactant concentration- Combustion equation
• Analysis conducted for two heights of the fuel bed.

Mixing analysis

Component Y (%) X (%)
CO 10.8 9.6
CO2 23.5 13.3
CH4 2.5 3.9
H 0.8 10.0
H2O 15.6 21.6
O 0.8 0.6
N2 46.0 41.0
Stoichiometric molar air fuel ratio - 0.809
Combustion equation

Fuel rich
wake
Fuel bed Fuel bed
Vessel Vessel
A B
Case A- Fuel mass fractions

Fuel bed
Vessel
Recirculation
zone
Vessel
Fuel bed
A B
Case B- Flow pattern

RICHER FUEL MIXTURE
RISING
LIMITED MIXING IN
THE RECIRCULATION
ZONE
VESSEL VESSEL
FUEL BED FUEL BED
Case B- Fuel mass fractions

Conclusions, Recommendations
• Good tool for flow visualization
• Uniform heat generation – hotspots
• Establishing the secondary airflow rate
• Influence of nozzle angle, diameter etc.
• Reactive flow modelling

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