Hydrocarbon Micro-heat Exchangers for Process Applications Figure 2: Double Spiral Type Heat Exchanger


Energy from a micro-combustor must be converted to a useful form. Energy for propulsion is obtained via the use of micro-turbines. Electricity may be produced using thermoelectric (TE) or thermophotovoltaic (TPV) conversion.13


of generating electricity on the milliwatt to watt scale.16 micro-combustors that are coupled to TE or TPV conversion are capable


Reactants Combustion chamber


The primary concern in micro- combustor design is that the residence time of the reactor be greater than the kinetic time required for the combustion reactor to take place.


Products micro-heat exchangers/combustors since 2001.13 These micro-heat


exchanger/combustor systems have also been referred to as power MEMS.


Combustion in micro-scale systems is subject to a number of design considerations. The primary concern in micro-combustor design is that the residence time of the reactor be greater than the kinetic time required for the combustion reactor to take place.7


Typical


residence time of a micro-combustor is about one millisecond.13 Micro-combustors have the advantage that high temperatures can be attained at near isothermal conditions, which ensures that the combustion reaction can proceed fully. Additionally, the high area-to- volume ratio differs significantly from traditional macro-scale combustors. The large wall area can increase losses to heat exchanger walls due to chemical quenching.13


The two-phase heat transfer


properties of hydrocarbons can be harnessed within micro-film combustors that keep the walls of the combustor cool due to the phase change, but maintain reaction due to the high area-to-volume ratio.7


Due to the ability to withstand high temperatures, ceramic materials such as silicon carbide (SiC) are excellent candidates for micro-scale heat exchangers/combustors.4,7,14,15


Several geometries have been formulated for application as micro- combustors. Some of these geometries are particularly relevant for catalytic combustion or other unique application. Micro-combustion


Some challenges in terms of the application of nano-fluids as coolant include problems due to surface fowling and channel blockage.


may be carried out with or without catalyst. Typical catalyst choices include noble metals, such as platinum (Pt) or palladium (Pd).


28


Heat losses through heat exchanger walls may quench the chemical reaction and thereby inhibit combustion. However, adiabatic walls may be generated by recursively stacking heat exchangers. This is known as a double spiral or ‘swiss-roll’ type heat exchanger.17 have also called heat-recirculating combustors.18


These devices A generalised view of


a double spiral heat exchanger is shown in Figure 2. The self-insulating double spiral heat exchanger concept uses no moving parts and enhances thermodynamic efficiency by pre-heating reactant fuel with combustion products. The reactants flow counter-current to the products. Thus, the pre-heating action is progressive and highly efficient. Double spiral heat exchangers have been designed with both rectangular and rounded micro-channels.


Continuous operation of micro-scale double spiral type devices has been demonstrated.18,19


The sustained combustion in these heat


exchanger devices has been described as ‘flameless’, the reaction occurs even at low Reynolds number and the devices have been demonstrated both with and without catalyst.18


The redundant walls,


even in laminar flow conditions, minimise losses within the double spiral heat exchanger.


The use of low-emissivity materials is essential to minimise radiation losses due to radiation heat transfer.17


There are generally two heat


Various hydrocarbon fuels have been tested in micro-scale double spiral heat exchanger designs, including propane, butane and methane.


loss mechanisms within double spiral MEMS heat exchangers: loss within the spiral flow channels and loss within the combustion chamber itself.17


of providing highly parallelised generation of electricity with relatively few moving parts, although the efficiency of these processes is about 10 %. Due to the catalyst the combustion occurs at relatively low temperatures and the micro-combustors are essentially MEMS heat exchanger geometries. Typical emergency diesel generator systems have many failure-prone moving parts and long start-up times. Thus, highly parallelised MEMS heat exchangers with TE or TPV conversion are a promising option for future application in emergency electricity generation systems.


TE or TPV conversion is capable Catalytic


Numerous recent designs have been developed for micro-combustion as thrust for aerospace vehicle propulsion.14


These designs include HYDROCARBON WORLD – VOLUME 6 ISSUE 2


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