Heat-Accumulation Stoves: Numerical Simulations of Two Twisted Conduit Configurations *Scotton P., *Rossi D., **Barberi M., **De Toni S. *University of Padova, Department of Geosciences ** Barberi Srl, Trento (Italy) Rotterdam, Oct. 2013 1
INDEX Description of the Physical Problem; Hydrodynamic and Heat Transfer Equations; Results of twisted refractory pipe on a vertical plane refractory pipe spacially twisted Rotterdam, Oct. 2013 2
Heat storage and release P [W] Thermal Power [KW] Description of the Physical Problem Components of a heat accumulation stove Burning Process of Woody Material V, T V, T time HEAT STORAGE in the refractory HEAT RETURNED to the environment time [h] Rotterdam, Oct. 2013 3
Description of the Physical Problem Examples of heat accumulation stoves Modern stove Classical stove Historical heat accumulation stove Sfruz (Valle di Non, Trentino, Italy). Contemporary stove Rotterdam, Oct. 2013 4
Description of the Physical Problem The driving force acting on the flue gases Z = H air supply Z = 0 P P P A B A P P P H H B ρ ρ a f (T,P a (T,P f a a ) g ΔH ) g ΔH ρ (T,P ) -ρ (T,P ) gδh ρ -ρ gδh a a a f f a a f Rotterdam, Oct. 2013 5
curve Resistance number [-] E [m] Description of the Physical Problem Hydrodynamic flow aspects Temporal evolution of Reynolds number Sharp Curve Turbulent motion IRe = 28400 x/d = 1.4 1.3 1.0 0.7 0.4 0.1 DP/g -5 15 35 x/d [-] A Apii DE Rotterdam, Oct. 2013 6
Rotterdam, Oct. 2013 7 Physics and Equations Transport Equations, k- model Reynolds-averaged Navier Stokes eq. F u u I p u u u u t u T ' ' 0 ) ( u t k k T P k k u t k k C P k C u t k T 2 2 1 Turbulent energy eq. Turbulent Dissipation energy eq. where 2 k C T +
Physics and Equations Wall Functions Wall d w u d dw w dw 11.06 influence of choosen mesh on the results Rotterdam, Oct. 2013 8
Physics and Equations Heat Transfer conduction q i k T x i Heat transfer is guaranteed by three terms: convection q h A T s T radiation q A 4 T s heat flux by conduction Equation of heat transfer T Cp : t T T p t u T q S u p Q p Equation of mass conservation t v 0 viscous heating.. the conserved property is the total energy not the heat H k T u qr u 0 heat flux by radiation total energy flux Rotterdam, Oct. 2013 9
Physical and numerical experiments 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 Rotterdam, Oct. 2013 Thermotechnical characteristics refractory calcespan density [kg/m 3 ] 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 Inlet section Radiation in participating media Discrete ordinate method: S2 Scattering = 0; absorption k = 1.524 E-3 [1/cm] Modest, 83 Mass flow rate and temperature at the inlet section. Boundary conditions: 1. Mass flow rate + Temperature at the inlet face; 2. Pressure at the outlet face; 3. Convective cooling on the outer surface: ; 4. Surface to Ambient Radiation: ; Rotterdam, Oct. 2013 11
Physical and numerical experiments Twisted Refractory Pipe on a vertical plane: numerical results t= 76 min. Residual combustion activity inside the first vertical stretch? Rotterdam, Oct. 2013 12
Physical and numerical experiments Twisted Refractory Pipe on a vertical plane: numerical results Sp Sx CENTER Dx St Temperature gauges distribution inside an instrumented section S1 S2 S3 Rotterdam, Oct. 2013 13
Physical and numerical experiments Twisted Refractory Pipe on a vertical plane: numerical results MASS BALANCE ENERGY BALANCE ERROR 15% Rotterdam, Oct. 2013 14
Physical and numerical experiments Refractory Pipe spacially twisted: physical model Measurement positions of the temperature at the section 13. Thermotechnical properties of the materials. Temperature and mean velocity of the combustion air (pipe diameter = 0.16 m). Calculated flue gas mass discharge at section 01. Rotterdam, Oct. 2013 15
Physical and numerical experiments Refractory Pipe spacially twisted: numerical model OUTLET Tethraedical meshes INLET Radiation in participating media Discrete ordinate method: S2 Scattering = 0; absorption k = 1.524 E-3 [1/cm] Boundary conditions: 1. Calculated mass flow rate + Measured temperature at the inlet face; 2. Pressure at the outlet face; 3. Convective cooling on the outer surface; 4. Surface to Ambient Radiation; Rotterdam, Oct. 2013 16
Physical and numerical experiments Refractory Pipe spacially twisted: numerical results t= 62 min. Mesh M2 Mesh M3 Mesh M3 Rotterdam, Oct. 2013 17
Physical and numerical experiments Refractory Pipe spacially twisted: numerical results MASS BALANCE ENERGY BALANCE Mesh M3 Mesh M3: ERROR 30% Mesh M2: ERROR 17% Mesh M3 Rotterdam, Oct. 2013 18
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. Rotterdam, Oct. 2013 19
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 Rotterdam, Oct. 2013 20
Description of the Physical Problem General Features of global system flue gas rad. heat exchanges conv. heat exchanges rad. heat exchanges conv. heat exchanges refractory walls of room air supply combustion Barberi Srl Rotterdam, Oct. 2013 21
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. Rotterdam, Oct. 2013 22
Temperature gauges distribution inside an instrumented section Rotterdam, Oct. 2013 23
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 Rotterdam, Oct. 2013 24
Velocity measurement Propeller anemometer Hot wire anemometer Pitot Rotterdam, Oct. 2013 25
Pressure measurement Furness FC0332 Analogical piezometer Testo 521 Rotterdam, Oct. 2013 26
Temperature measurement Thermocouples k; Pt100; Pt1000. Rotterdam, Oct. 2013 27