Transcript
Transient CFD simulation of wood log stoves with heat storage devices Claudia Benesch, Martina Blank, Manuel Kössl, Ingwald Obernberger
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Hedwig-Katschinka-Straße 4, A-8020 Graz, Austria TEL.: +43 (316) 481300; FAX: +43 (316) 4813004 E-MAIL:
[email protected] HOMEPAGE: http://www.bios-bioenergy.at
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Contents
Scope of work Methodology
Procedure concerning stationary and transient simulations
Stove geometry investigated + framework conditions
Model overview
Results and system optimisation Summary and conclusions
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Scope of work
Development of a CFD based methodology for the analysis and optimisation of wood log fired stoves and heat storage devices CFD aided development and optimisation of a wood log fired stove with heat storage device by applying the newly developed CFD methodology:
Stationary simulation of wood log stove and heat storage device for the basic evaluation and derivation of boundary conditions for transient CFD calculations
Transient simulation of the basic variant of the stove and the heat storage device
Transient simulation of the basic variant of the heat storage device alone Parameter study with regard to the heat storage device geometry and the storage material properties (transient simulations of the heat storage device alone)
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Scheme of stove and heat storage device geometry
BIOENERGIESYSTEME GmbH Hedwig-Katschinka-Straße 4, A-8020 Graz
insulation of heat storage device insulation of heat storage device opening of discharging air channels (can be closed) insulation of combustion chamber
heat storage device
heat storage device material discharging air channels charging flue gas channels
wood log stove
glass window
computational domain for the instationary CFD-simulation of the heat storage device alone
window air channel chamotte lining of combustion chamber wood logs
channels for convective air
grate
inlet for convective air
bed of embers primary air supply supply of combustion air
double jacket for convective air
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Nominal stove capacity: 10 kW fuel: 2 hard wood logs 33 cm long/ 2.77 kg per batch CFD simulations done with a steady-state operating case (at ~63% of batch time) Used also for transient simulations
Operating case and applied framework conditions parameter water C H O N gross calorific value (GCV) net calorific value (NCV) fuel power related to NCV
unit wt% w.b. ash-free wt% d.b. ash-free wt% d.b. ash-free wt% d.b. ash-free wt% d.b. ash-free
8.1 42.7 6.8 50.1 0.4
MJ/kg d.b. MJ/kg w.b. kW
17.7 14.7 10.3
flue gas in combustion chamber - total flue gas released from fuel
kg/h kg/h
mass flow of air
kg/h
26.3 4 2.51 23.8 3
[] vol% d.b.
2.03 10.7
total air ratio O2 fraction at stove outlet, dry
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Model overview
Hedwig-Katschinka-Straße 4, A-8020 Graz
Combustion of wood logs
Empirical wood log combustion model (in-house code)
Turbulence
Realizable k-e Model
Gas phase combustion
Eddy Dissipation Model (Amag = 0.8, Bmag = 0.5) / global methane 3-step mechanism (CH4, CO, CO2, H2, H2O und O2) + additional reaction step for wood volatiles
Radiation
Discrete Ordinates Model
Shell conduction model
Heat transport in metal sheets 6
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Stationary simulation of wood log stove and heat storage device (1)
Characterisation of basic variant of wood log stove and heat storage device
insulation material
air channels
flue gas channels
flue gas channels
Pathlines of combustion air and flue gas coloured by oxygen concentration [m³ O2/m³ wet flue gas] – side view (left); iso-surfaces of flue gas, air and material temperatures in the heat storage device (right)
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Stationary simulation of wood log stove and heat storage device (2)
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Derivation of the boundary conditions for transient simulations of the storage device alone
average temperatures at the bottom of the storage device mass fluxes and temperatures of the flue gas at the entrance to the charging channels Bottom of Flue gas Flue gas Flue gas storage channel channel channel device left center-left center-right
Flue gas channel right
parameter unit
mass flow of flue gas
g/s
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1.58
2.08
2.08
1.57
mean temperature
°C
388
512
574
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511
Iso-surfaces of the flue gas and material temperatures [°C] at the entrance to the storage device
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Transient simulation of wood log stove and heat storage device (1)
Operating mode of heat storage device (24 hour cycle):
Heating of wood log fired stove in a batch mode in order to charge the heat storage device (duration: 5 h, approximately 5 batches)
Heat storage/standstill (duration: 10 h/over night)
Discharge of heat storage via natural convection (duration: 9 h)
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heat-up (after 3 h)
Transient simulation of wood log stove and heat storage device (2) heat storage (after 10 h)
discharge (after 16 h)
Iso-surfaces of flue gas, air, and material temperatures [°C] in a vertical cross section through the rear part of the flue gas channels
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Transient simulation of wood log stove and heat storage device (3)
Total heating cycle of heat storage device + wood log stove
Profile of the mean temperature of the heat storage device and the stored heat inside the heat storage device
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Transient simulation of heat storage device alone (1)
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Comparison of transient simulation results of total and reduced system (heat-up phase after 3 h): heat storage device alone
heat storage device + wood log stove
start of storage device in this cross section
Iso-surfaces of flue gas-, air- and material temperatures [°C] in a vertical cross section through the rear part of the flue gas channels
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Design study for system optimisation (1)
Geometric variations (selection):
removal/enlargement of double jacket for convective air reduction of the cross-section of the flue gas exit from the storage device steeper inclination of the air channels/removal of air channels smaller air channels (increase of mass of heat storage material) better insulation of the heat storage device
Material property variations:
increased heat conductivity (2x) increased density (2x)
both increased heat conductivity and density (2x) 13
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Design Study (2) geometric variations
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Heat release during storage phase for the basic variant and two geometrical variants: basic variant variant without double jacket for convective air and better isolation of the air channels variant without discharging air channels and thus higher mass of storage material 12
stored heat [kWh]
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0 05:00
06:00
07:00
08:00
09:00
10:00 time [hh:mm]
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Design Study (2) variation of material properties
BIOENERGIESYSTEME GmbH Hedwig-Katschinka-Straße 4, A-8020 Graz
Heat storage during charging phase for the basic variant and three material variants: basic case variant base increased density (2x)
increased heat conductivity (2x) increased heat conductivity + density (2x)
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stored heat [kWh]
14 12 10 8 6 4 2 0 00:00
00:30
01:00
01:30
02:00
02:30 time [hh:mm]
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04:30
05:00
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Summary and conclusions
A CFD based methodology for the analysis and optimisation of a wood log stove with heat storage device was successfully developed and applied. Main results are:
The heat-up, heat storage and discharge behaviour of a stove + heat storage unit can be realistically evaluated.
The influence of air flow in the discharging channels and of flue gas flow in the charging channels can be identified.
The influence of geometry and material properties on the charging/discharging processes can be shown and assessed.
Transient CFD simulations of wood log fired stoves constitute an efficient process analysis and design tool. They allow a target-oriented and time saving method for the optimisation of wood log fired stoves + heat storage devices
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Thank you for your attention BIOS BIOENERGIESYSTEME GmbH Hedwig-Katschinka-Straße 4, A-8020 Graz, Austria TEL.: +43 (316) 481300; FAX: +43 (316) 4813004 E-MAIL:
[email protected] HOMEPAGE: http://www.bios-bioenergy.at
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