Experimental and Analytical Investigation of a Counter-flow Reactor at Lean Conditions

ABSTRACT

Heat recirculating reactors have many potential applications as thermal oxidizers, combustors, and fuel reformers due to their extensive operating range. Low emissions and fuel flexibility make such devices highly desirable as heat sources as well as chemical reactors. The dependence on the solid/gas heat transfer implies that wall characteristics and operating conditions significantly influence the stable range. In this paper, the importance of various combustor parameters is examined through an analytical model, and an experimental reactor is fabricated from a new ceramic-metal composite using additive manufacturing. A full range of possible operation modes, from flashback to blow-off, was observed together with characteristic temperature distributions at various firing rates. The new combustor showed improved operational flexibility as compared to a traditionally assembled counterpart. Low CO and NO_x emission levels were observed together with an audible sound in the range between 825 Hz and 1000 Hz. The combustor operated for over 70 hours without visible damage to the material. The overall thermal performance, low emissions, and high power density make the heat recirculating reactor a viable solution for combustion applications.

KEYWORDS:

• Heat-recirculating reactor
• meso-scale combustion
• superadiabatic
• counter-flow
• additive manufacturing

Acknowledgments

Authors would like to acknowledge the generous help of the Laboratory for Freeform Fabrication at The University of Texas at Austin. Special gratitude goes towards Dr. David Bourell for sharing his extensive knowledge on material science and associated processing.

Declaration of Interest Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Nomenclature

′Dimensional value

_refReference value (calibration)

_gGas property

_wWall property

_iChannel indicator

_cCombustion position property

_∞Environment

_adAdiabatic flame

L′Reactor length [m]

aChannel width ≡ $\frac{2a{ }^{\mathrm{\prime }}}{L{ }^{\mathrm{\prime }}}$

bWall thickness ≡ $\frac{2b{ }^{\mathrm{\prime }}}{L{ }^{\mathrm{\prime }}}$

xAxial position ≡ $\frac{2x{ }^{\mathrm{\prime }}}{L{ }^{\mathrm{\prime }}}$

T′Temperature [K]

TTemperature ≡ $\frac{T{ }^{\mathrm{\prime }}}{T_{ref}^{\prime }}$

u′Flow velocity [m/s]

uVelocity ratio ≡ $\frac{u{ }^{\mathrm{\prime }}}{u_{ref}^{\prime }}$

k′Thermal conductivity [W/mK]

kRelative thermal conductivity ≡ $\frac{k_g^{\prime }}{k_w^{\prime }}$

h′Convective heat transfer coefficient [W/m²K]

α′Thermal diffusivity [m²/s]

D′Mass diffusivity [m²/s]

ν′Kinematic viscosity [m²/s]

E′_aActivation energy [J/mol]

R′Universal gas constant ≡ 8.3144 [J/molK]

LeLewis number ≡ $\frac{{\alpha }_g^{\prime }}{D_g^{\prime }}$

ReReynolds number ≡ $\frac{u{ }^{\mathrm{\prime }}L{ }^{\mathrm{\prime }}}{\nu }$

βModified Zeldovich number ≡ $\frac{E_a^{\prime }\mathrm{\Delta }{T}_{ad}^{\prime }}{R{ }^{\mathrm{\prime }}{T}_{ad}^{\prime }}\frac{T_{ref}^{\prime }}{\mathrm{\Delta }{T}_{ad}^{\prime }}$

ϵGeometry factor ≡ $\frac{2{\alpha }_g^{\prime }}{L{ }^{\mathrm{\prime }}{u}_{ref}^{\prime }}$

κConductivity factor ≡ $\frac{b{ }^{\mathrm{\prime }}}{2a{ }^{\mathrm{\prime }}}\frac{k_w^{\prime }}{k_g^{\prime }}$

μHeat transfer factor ≡ $\frac{h{ }^{\mathrm{\prime }}}{u_{ref}^{\prime }}\frac{{\alpha }_g^{\prime }}{k_g^{\prime }}$

χHeat loss factor ≡ $\frac{h_{\mathrm{\infty }}^{\prime }}{2h{ }^{\mathrm{\prime }}}$