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TECHNICAL PAPERS: Internal Combustion Engines

# Modeling of MEMS-Type Rankine Cycle Machines

[+] Author and Article Information
Ling Cui

Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139

J. G. Brisson

Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139brisson@mit.edu

We arrived at this somewhat arbitrary upper limit for the HTHEX pressure drop through internal discussions with MIT micro gas turbine engine designers as an acceptable backpressure for a micro gas turbine engine.

J. Eng. Gas Turbines Power 127(3), 683-692 (Jun 25, 2004) (10 pages) doi:10.1115/1.1924400 History: Received October 23, 2003; Revised June 25, 2004

## Abstract

Preliminary design and performance calculations for a silicon-based micro Rankine machine are discussed. The designs considered draw heat from a high temperature air stream with inlet temperatures between 770 and $1000K$ and reject heat to an ambient air stream at $300K$. Most of the designs have a typical footprint of $6cm2$. Water and benzene are considered as working fluids. Effects of the limits of heat exchanger and turbomachinery performance are analyzed and discussed. The designs of two types of heat exchangers (hole type and fin type) are described in detail. Their respective performances are compared. The calculations indicate that a machine with a $6cm2$ footprint area is capable of delivering in excess of $40W$ of shaft power.

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## Figures

Figure 1

A block diagram (a) and T-s diagram (b) for the proposed micro Rankine cycle

Figure 2

A cross-sectional view of a first-cut design for a micro Rankine prime mover. Driving mechanism for the blower is not shown.

Figure 3

Exploded schematic view of the micro Rankine engine

Figure 4

Exploded view of the LTHEX. Each wafer is individually etched. The wafers are then bonded together to form the heat exchanger. A cover plate that ultimately bonds to and seals the passages of Wafer I is not shown. (Relative dimensions and number of passages do not correspond to the modeled device.)

Figure 5

Exploded view of the HTHEX. Each wafer is individually etched and then bonded together to form the heat exchanger. The air passages in Wafer I are closed by bonding a cover plate (not shown) to Wafer I or by bonding the lowest wafer in the combustor or gas turbine module to Wafer I. (Relative dimensions and number of passages do not correspond to the modeled device.)

Figure 6

An exploded diagram of a two-wafer hole-type heat exchanger structure. (Relative dimensions and number of passages do not correspond to the modeled device.)

Figure 7

The net work output and cycle thermal efficiency of a Rankine cycle with the hole-type LTHEX design specified in Table 2. Turbine inlet temperature equals 750K. LTHEX footprint is about 6cm2. Turbine pressure ratios are unrestricted.

Figure 8

Net work output and thermal efficiency as a function of LTHEX saturation temperature (T7) for the fin-type heat exchanger specified in Table 3. Turbine inlet temperature equals 750K. The turbine pressure ratios are unrestricted.

Figure 9

The net work output and thermal efficiency of a single cycle as a function of LTHEX discharge temperature. The turbine pressure ratio is set to a value of 4. The assumed LTHEX dimensions are listed in Table 3. Two cases are shown, one corresponds to an entering hot air temperature of 950K, the other corresponds to 1000K.

Figure 10

The net work output and thermal efficiency versus condenser discharge temperature in the LTHEX. The hot air mass flow rate is 0.1g∕s and steam mass flow rate is 0.011g∕s. The turbine inlet temperature is 750K and turbine pressure ratios are unrestricted. LTHEX and HTHEX dimensions are same in Tables  34 but with channel depths equal to 1mm.

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