Heat-Accumulation Stoves: Numerical Simulations of Two ... · Temporal evolution of Reynolds number r [-] •Description of the Physical Problem . Rotterdam, Oct. 2013 7 •Physics

Rotterdam, Oct. 2013 1

*Scotton P., *Rossi D., **Barberi M., **De Toni S.

*University of Padova, Department of Geosciences

** Barberi Srl, Trento (Italy)

COMSOL CONFERENCE ROTTERDAM 2013

Heat-Accumulation Stoves: Numerical

Simulations of Two Twisted Conduit

Configurations



• Description of the Physical Problem;

• Hydrodynamic and Heat Transfer Equations;

• Results of twisted refractory pipe

on a vertical plane

refractory pipe spacially

twisted

INDEX



Components of a

heat accumulation

stove

Burning Process of Woody Material

time T

he

rma

l P

ow

er

[KW

]

V, T ↑↑ V, T ↓↓

He

at sto

rag

e

an

d r

ele

ase

P [

W]

time [h]

HEAT STORAGE

in the refractory

HEAT RETURNED

to the environment

• Description of the Physical Problem


Examples of heat accumulation stoves

Historical heat accumulation

stove “Sfruz” (Valle di Non,

Trentino, Italy).


Classical stove

Contemporary

stove

Modern stove




The driving force acting

on the flue gases

Z = 0

Z = H

ΔHg)P,(TρPP

ΔHg)P,(TρPP

affHB

aaaHA

ΔHgρ-ρΔHg)P,(Tρ-)P,(TρPP faaffaaaBA

air supply



Hydrodynamic flow aspects

Sharp Curve – Turbulent motion

IRe = 28400 x/D = 1.4

0.1

0.4

0.7

1.0

1.3

-5 15 35

x/d [-]

E [m

]

A

Apii

curv

e

DE

DP/g


Temporal evolution of Reynolds number

Resis

tance n

um

be

r

[-]



• Physics and Equations

Transport Equations, k- model

Reynolds-averaged Navier–Stokes eq.

FuuIpuuuut

u T

''

0)(

u

t

k

k

T Pkkut

k

kCP

kCu

tk

T2

21

Turbulent energy eq.

Turbulent Dissipation energy eq.

where

2kCT

+



Wall Functions

Wall dw

06.11

ddd w

ww

u

influence of

choosen mesh

on the results

COMSOL CONFERENCE ROTTERDAM 2013 • Physics and Equations


Heat Transfer

Heat transfer is guaranteed by three terms:

i

ix

Tkq

conduction

TTAhq sconvection

4sTAq radiation

Qput

p

T

TSqTu

t

TC

p

p

:

0

v

t

Equation of mass conservation

Equation of heat transfer

heat flux by conduction

.. the conserved property is

the total energy not the heat rquTkHu 0 heat flux by radiation

COMSOL CONFERENCE ROTTERDAM 2013 • Physics and Equations

viscous heating

total energy flux

Rotterdam, Oct. 2013

Straight Steel Pipe


Thermotechnical characteristics

stainless

steel

black

steel

thickness [mm] 0.2 2.0

emissivity [-] 0.1 0.95

conductivity [W/mK] 17 50

C-shaped refractory pipe

Thermotechnical characteristics

refractory calcespan

density [kg/m3] 2550 600

heat cap. [J/kgK] 859 1000

conductivity [W/mK] 3.16 0.15

emiss. [–] 0.95 0.70

• Physical and numerical experiments


Twisted Refractory Pipe on a vertical plane: numerical model

Mesh properties

Boundary conditions: 1. Mass flow rate + Temperature at the inlet face;

2. Pressure at the outlet face;

Mass flow rate

and temperature

at the inlet

section.

Inlet

section

3. Convective cooling on the outer surface: ;

4. Surface to Ambient Radiation: ;

COMSOL CONFERENCE ROTTERDAM 2013 • Physical and numerical experiments

Radiation in participating media – Discrete ordinate method: S2 Scattering = 0; absorption k = 1.524 E-3 [1/cm]

Modest,

‘83


Twisted Refractory Pipe on a vertical plane: numerical results

t= 76 min.

Residual combustion

activity inside the first

vertical stretch ?




S1 S2 S3

CENTER

St

Sp

Dx Sx

Temperature gauges distribution

inside an instrumented section




MASS BALANCE

ENERGY BALANCE

ERROR 15%



Measurement positions

of the temperature at the

section 13.

Temperature and mean velocity

of the combustion air (pipe

diameter = 0.16 m) .

Calculated flue gas mass

discharge at section 01.

Thermotechnical properties of the

materials.

Refractory Pipe spacially twisted: physical model



Refractory Pipe spacially twisted: numerical model

Boundary conditions:

3. Convective cooling on the outer surface;

1. Calculated mass flow rate + Measured temperature at the

inlet face;

2. Pressure at the outlet face;

4. Surface to Ambient Radiation;

INLET

OUTLET Tethraedical

meshes

Radiation in participating media – Discrete ordinate method: S2 Scattering = 0; absorption k = 1.524 E-3 [1/cm]



Refractory Pipe spacially twisted: numerical results

t= 62 min.

Mesh M2

Mesh M3 Mesh M3



Refractory Pipe spacially twisted: numerical results

MASS BALANCE

ENERGY BALANCE

Mesh M3: ERROR 30%

Mesh M2: ERROR 17%

Mesh M3

Mesh M3



Conclusions

•The numerical results have been obtained considering the thermal- and the hydrodynamic

equations, including buoyancy forces (Boussinesque approximation). Radiation has been

considered both outside and inside the pipe;

• The software is able to describe correctly the mass balance until the time of occurrence of a

numerical instability;

• The energy balance depends strongly on the mesh refinement. A refined mesh, realized

with the use of a boundary layer, can induce an anticipated instability;

• The pressure variations are not significantly far from the measured ones (this experimental

measure is very difficult);

• The mean velocity values along the pipes are coherent with the mean temperature values;

• The calculated mean temperature values along the pipes are generally much higher than

the measured ones; the measured temperature variation inside the pipes are much larger

than the measured ones; temperature variations inside the pipe, both measured and

calculated, decrease increasing the distance along the pipe;

• The use of a boundary layer is not always the best choice, having an influence on the

reduction of the stability period.



What’s more …

• Use of Comsol in the final stove

configuration, where a radiant

covering envelops the twisted pipe;

• Need of a higher computing

power (now Dell T7500 24GB Ram)

to reduce the energy loss and to

hope to give technical importance

in the design process. At the

moment, due to the large

calculation time, of the order of a

week, is almost useless;

• Another road: decoupling the hydrodynamic equations relevant to the pipe from the thermal

equations relevant to the refractory;


Thank you for your

attention



General Features of global system

combustion air supply

refractory

flue gas

rad. heat exchanges

rad. heat

exchanges

conv. heat

exchanges

conv. heat exchanges

walls of room

Barberi Srl




Software version: Comsol 4.3

Computing machine: Workstation Dell T7500 equipped with

two Intel R Xeon R Processor X5550 and

24GB DDR3 1333MHz ECC-RDIMM.


Temperature gauges distribution inside an instrumented section



THE MASS FLOW RATE HAS BEEN DETERMINED

BY MEANS OF VELOCITY AND TEMPERATURE

MEASURED UPSTREAM DEL COMBUSTION

CHAMBER.


The technical regulation

prEN15544 has been used.

prEN 15544 – “One off

tiled/kachelofen stoves - Calculation

method”



Velocity measurement

Propeller anemometer

Pitot

Hot wire anemometer



Analogical piezometer

Furness FC0332

Testo 521

Pressure measurement



Thermocouples k;

Pt100;

Pt1000.

Temperature measurement

Heat-Accumulation Stoves: Numerical Simulations of Two ... · Temporal evolution of Reynolds number r [-] •Description of the Physical Problem . Rotterdam, Oct. 2013 7 •Physics

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