Fig. 1 Fuel cell powered notebook developed by NEC
Fig. 3 Thruster Cluster from EADS Space Transportation
Recently, there are the increasing demands on the developments of microdevices such as microsatellites, microaerial vehicles, micro reactors, and micro power generators. This project is first motivated by the development of small power generators with internal combustion and reaction for the replacement of traditional batteries. Small power generator has many advantages over batteries in that it has high energy density, it is light weight and portable, environmentally superior and inexpensive. Another example of microscale combustion /reaction system is micro fuel converter which has much higher conversion efficiency and can be used in poisonous gas disposal. Moreover, micro-chemical propulsion system developed for small spacecrafts can be used for primary thrust, orbit insertion, trajectory-control, and attitude control.
|
|
|
|
Fig. 1 Fuel cell powered notebook developed by NEC |
|
Fig. 3 Thruster Cluster from EADS Space Transportation |
In mesoscale combustion (length scale ~ quenching diameter), the increase of larger surface to volume ratio dramatically increases the wall heat loss and leads to flame extinction. On the other hand, the reduction of thermal inertia at small scale significantly reduces the response time of the wall and leads to strong wall flame coupling and extended burning limits. This flame-wall coupling can dramatically change the nature of flame propagation and yield different flame regimes. The flame bifurcation and the transition of flame regimes are dramatically affected by the channel width and flow velocity. In fact, all practical combustors have variable channel width in the flow direction. As a result, the simultaneous changes of channel width and flow rate will significantly modify the heat loss and flame-wall coupling. Therefore, it is of great interest to understand how the variation of channel width will affect the flame propagation and flame transition.
Heat recirculation mechanism (Fig. 4): Part of the heat loss from the flame to the wall can be pumped back to the preheat zone through wall heat conduction to preheat the premixture. As the scale goes down, this thermal feedback effect becomes significant and the flammability limit can be widely extended. New flame stabilization mechanism and instability phenomena can exist as well due to this flame-wall interaction.
 |
 |
| Fig. 4 Schematic of heat recirculation mechanism |
|
Fig. 5 shows the experimental setup to study the flame dynamics in a mesoscale channel. Due to the strong thermal coupling between the flame and the wall, different flame dynamics and flame regimes have been observed. Fig. 6 shows the spinning flame was observed inside a mesoscale diverging quartz tube for both methane and propane at equivalence ratio ranged from lean to rich. The spinning frequency ranges from 10 HZ to 70 HZ, highly depending on the equivalence ratio. The spinning flame is a result of wall-flame thermal coupling effect.Slow flames and fast flames coexist in a mesoscale straight quartz tube. A pulsating flame is also observed (Fig. 7). Both theory and experimental result show that there exist new bifurcations and new flame regimes in mesoscale combustion. The flammability limit can be extended due to the heat recirculation effect.
  |
 |
| Fig. 6 Spinning flames observed in a diverging mesoscale tube | Fig. 7 Timing history of pulsating flames in a mesoscale straight tube |
Measuring radical concentration is very important for understanding radical quenching effect on flame dynamics and instability in small scale combustion. Experimental setup of PLIF: A YAG laser was used to pump the Dye laser. The Dye laser was tuned to 283 nm wavelength. A laser beam sheet was formed using a telescope system and a cylindrical lens and passed through the flame. The fluorescence of OH radicals was captured by an ICCD camera.
 |
|
Fig. 8 Experimental setup of OH-PLIF. |
 |
|
Fig. 9
OH radical concentration distribution of a Benson
flame measured by PLIF. The first image is the flame front calculated
from the OH concentration gradient. |
Based on the understanding of flame dynamics in mesoscale combustion, a preliminary version of micro thruster is developed and the schematic of design is shown in Fig. 10. Both the liquid fuel and oxidizer are issued into the main combustor through the two outer shells to minimize heat loss and maximize the heat recirculation effect. The liquid fuel is heated up by the wall and vaporized before goes through the porous quartz. A pressurized millimeter scale catalytic tube is injected into the main combustor to supply constant radical pool to stabilize the combustion. The exit of the catalytic tube is chocked to generate high speed jet to enhance the mixing.
Fig. 10
Schematic of
the micro thruster design |
 |
| Fig. 11 Picture of the micro thruster |
Fig. 12 shows a testing case with liquid ethanol and air running in the main combustor and butene/air mixture running in the catalytic tube.
Fig. 12 Test conditions (1 atm); Main combustor: ethanol + air; Catalytic tube: butene + air