This paper presents the system-level and component design of a micro steam turbine power plant-on-a-chip which implements the Rankine cycle for micro power generation. The microfabricated device consists of a steam turbine that drives an integrated micropump and generator. Two-phase flow heat exchangers are also integrated on-chip with the rotating components to form a complete micro heat engine unit, converting heat to electricity. The system-level design includes cycle analysis and overall performance predictions, accounting for the expected performance of miniaturized components, thermal and structural integrity of the microsystem, and system-level trade-offs for optimal overall performance. Operating principles and design studies are also presented for the core component, with emphasis on a multistage, planar, radial microturbine and a spiral groove viscous pump. Design consideration for two-phase flow heat exchangers, microbearings, seals and micro-generators are also presented. Expected power levels range from 1–12 W per chip with energy conversion efficiency in the range of 1–11%. This suggests power density of up to 12 kW/kg for this technology, which is an order of magnitude greater than competing technologies, such as thermoelectrics. This study suggests the viability of a micro Rankine power plant-on-a-chip, but also identifies critical engineering challenges that must be met for practical implementation.

The presented work analyses the design space and performance potential of microfabricated Brayton cycle and Rankine cycle devices, accounting for lower component efficiencies, temperatures limited by the material properties and system implementation—constraints imposed by silicon microfabrication and miniaturization. By exploring the design space of such microsystems, their potential thermal efficiency and power density are defined. Results for both types of devices are shown graphically and design challenges and guidelines are determined and found to be different from their large-scale counterparts. Similar analysis was performed for Brayton and Rankine cycle devices, with more complete assessment of the latter by including, windage, generator, conductive and heat sink losses. In contrast to the Brayton cycle, the compression work of the Rankine cycle is minimal and the pump efficiency is not critical. The investigation suggests a higher potential for Rankine cycle devices than for Brayton cycle devices.

This two-part paper presents general methodologies for the evaluation of passive compressor stabilization strategies using tailored structural design and aeromechanical feedback control (Part I), and quantitatively compares the performance of several aeromechanical stabilization approaches which could potentially be implemented in gas turbine compression systems (Part II). Together, these papers offer a systematic study of the influence of ten aeromechanical feedback controllers to increase the range of stable compressor operation, using static pressure sensing and local structural actuation to postpone modal stall inception. In this part, the stability of aeromechanically compensated compressors was determined from the linearized structural-hydrodynamic equations of stall inception. New metrics were derived, which measure the level of aeromechanical damping, or control authority of aeromechanical feedback stabilization. They indicate that the phase between the pressure disturbances and the actuation is central to assess the impact of aeromechanical interactions on compressors stability.

This paper presents an experimental study of flow evaporation in non-uniform microchannels, demonstrating the ability to provide a stable flow of evaporated fluid for energy conversion and chip cooling applications. Two mechanisms are proposed to stabilize the internal flow evaporation. The first mechanism is to establish a temperature gradient along the channel to separate the room temperature inlet fluid from the steam exit flow. The second mechanism is to change the direction of the surface tension forces acting on the meniscus to fix its position along the channel. To achieve this, shaped channels are formed of contractions and expansions with varied wall angles. The device consists of a silicon wafer with through-etched complex microchannels, that is anodically bonded to a glass wafer on each side. Inlet and exit holes for the fluid are machined in the glass wafers. Water is forced through the chip while it is heated on the exit side of the three layer chip. The qualitative nature of the two-phase flow along the shaped channels is observed through the glass cover wafer, for different flow rates and wall temperatures. The temperature gradient achieved with different thickness of channel walls shows agreement with the modeling results. Also, the benefit of having multiple expansions in the channels was demonstrated. By using these two mechanisms the onset of water evaporation was fixed along the channel. This will lead to the development of adequate two-phase flow micro heat exchangers.

In a recent study, an effective means of mixing a low Reynolds number pressure-driven flow in a micro-channel was reported by Stroock et al. [10] using trenches on the lower wall that form a staggered herringbone pattern. In the present work numerical results are reported that indicate enhanced mixing using a similar herringbone pattern in the context of an electro-osmotically driven flow in microchannels. Instead of trenches, all walls are flush, making microfabrication easier. The lower wall would have lithographically deposited polymer coatings that exhibit a zeta potential of a sign opposite to that at the other walls. These coatings are chosen to form a herringbone pattern. If mixing can be achieved using purely electro-osmotic flows, then it becomes easier to scale the channel dimensions to smaller values without the penalty of a dramatic increase in pressure drop. Moreover, the possibility of mixing with purely electro-osmotic flows that do not require time varying electric fields leads to a simpler system with fewer moving parts. With current micro-fabrication techniques, it is difficult to produce periodic patterned coatings on all four walls of a rectangular microchannel. For this reason, this study limits its scope to coatings applied only on the lower surface of the microchannel, with a rectangular cross-section. Numerical simulations are used in order to elucidate the dominant mechanism responsible for mixing, which is identified as the blinking-vortex [3]. The flow regime chosen to illustrate these effects is the same as that used by Stroock et al. [10], characterized by Reynolds numbers that are O(10−2) and Pe´clet numbers that are of O(105). The presence of patterned zeta potentials in a microchannel violates conditions of ideal electro-osmosis [4] and hence the flows are necessarily three-dimensional. The efficiency of mixing is quantified by examining particle tracks at several downstream sections of the microchannel and averaging their concentration over boxes of finite size to model diffusion. It is found that the standard deviation of the concentration decays exponentially, and that the rate of decay is independent of the Pe´clet number when the latter is sufficiently large, indicating that chaotically-enhanced mixing is occurring.