Faculty of Engineering
Mechanical Engineering
Study on Micro Scale Combustion Figure 2 shows that the flame has a higher temperature in the two-dimensional parallel plates
than the cylindrical tube, provided the tube diameter is the same as the plates spacing.
Combustion of hydrocarbon or hydrogen fuels provides a much higher
power density on a per unit mass or unit volume basis compared with
most conventional energy systems. Therefore, harnessing energy from
micro combustion is an ideal way to realize the miniaturization of a power
source package that will meet the requirements of micro-electro-
mechanical systems (MEMS). MEMS power devices such as micro
turbines, micro thermoelectric devices, micro rotary engines and micro
thermophotovoltaic systems have been prototyped in the past few years.
However, ensuring stable and efficient combustion remains a challenge.
The reduced combustor volume experiences significant heat loss and
radical destruction on the combustor wall. Understanding the fundamental
characteristics of combustion in the micro scale is the key to improving
the systems efficiency and optimizing the design. Fig. 3 Temperature (a) (both fluid zone and solid wall),
Fig. 2 Axis temperature
Analysis and Simulation (cylindrical tube vs. parallel plates) axial velocity (b) and H2 fraction (c) (fluid zone only)
Combustor I.D. = 0.8 mm and wall thickness = 0.2 mm
Analytical studies indicate that as the combustor size is Experimental investigation
reduced, the ratio of heat loss to heat generation in the
flame zone increases dramatically. Because of the Earlier studies show that the H2-air mixture may be burned in a tube with a diameter down to 0.8 mm within a wide
difficulty of accessing the small space using conventional range of equivalence ratio and flow velocity. Compared to H 2, CH4 is less active in terms of chemical reaction.
measuring devices, a numerical approach has been However, it is worth studying the combustion of CH4-air due to its lower cost and better safety. Experiments of the
employed to study the H2-air premixed flame in order to ignition and flame flash-back of CH4-air mixture have been conducted in a narrow channel with an adjustable width
obtain detailed information in the combustor. from 0 to 5 mm. Results indicate that under cold wall condition, the flame is able to flash back through the channel
successfully.
Fig. 5 Flame flashing back against a steady flow (Plate spacing is 2.5 mm, time
interval is 1/3 s for the first 6 columns and 1/15 for the last 2)
Fig. 1 Effect of combustor diameter on heat loss ratio Contact: S.K. Chou E-mail: mpecsk@nus.edu.sg
(Tf/Tfa is a dimensionless flame temperature) Fig. 4 Design features of the narrow channel