Evaluation of a mesoscale thermal actuator in open and closed operating cycles

: Thermal-based actuators are known for generating large force and displacement strokes 1 at mesoscale (millimeter) regime. In particular, two-phase thermal actuators are found to beneﬁt 2 from the scaling laws of physics at mesoscale to offer large force and displacement strokes; but they 3 have low thermal efﬁciencies. As an alternative, a combustion-based thermal actuator is proposed 4 and its performance is studied in both open and closed cycle operations. Through a physics-based 5 lumped-parameter model, we investigate the behavior and performance of the actuator using a 6 spring-mass-damper analogy and taking an air standard cycle approach. Three observations are 7 reported: (1) the mesoscale actuator can generate peak forces of up to 400 N and displacement strokes 8 of about 16 cm suitable for practical applications; (2) an increase in heat input to the actuator results 9 in increasing the thermal efﬁciency of the actuator for both open and closed cycles; and (3) for a 10 speciﬁc heat input, both the open and closed cycle operations respond differently —different stroke 11 lengths, peak pressures, and thermal efﬁciencies. 12

Our present work on thermal actuators builds on our prior physics-based modeling efforts of a 36 resonant heat engine for portable power applications [7,13,14]. In this study, through a physics-based 37 lumped-parameter model, similar to our previously developed models on free piston engines

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[15-17], we will investigate the behavior and performance of a mesoscale thermal actuator using 39 a spring-mass-damper analogy and taking an air standard cycle approach. Here, using the model, we (1 → 2), heat addition (2 → 3), expansion (3 → 4), exhaust (4 → 5 → 6), and intake (6 → 7 → 1) 49 processes as depicted in Fig. 2 (Fig. 1b). The hollow cavity represents the control volume 55 CV and contains the working fluid (air). The combustion process is modeled as a constant volume heat 56 addition process to CV for a short duration t q →0 and the heat rejection at the end of the expansion process is modeled as a mass transfer process (for open cycle) or by applying a cooling pulse (for 58 closed cycle) for a short duration t q →0 (Fig. 3). The compliant cavity CV has a nominal volume V o 59 given by V o = LS, where L is the nominal cavity length and S is the cross-sectional area of the cavity.

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The cavity has a stiffness s which allows horizontal displacement x(t) of the piston m. Damper b 61 models energy conversion which is the sum of both friction work and useful work by the actuator,

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In the context of force and displacement generated, the actuator produces a peak force of up to 120 400 N and displacement stroke of up to 16 cm (Fig. 4a). Note that the force generated is not uniform 121 over the displacement range, but reduces in magnitude with the displacement.   Fig. 4a and 4b). The actuator performance metrics for the open and closed cycles are given in Table 1. intake processes. Furthermore, as can be seen in figure, an increase in the heat input or equivalence 148 ratio φ increases the volume ratio (ER or CR). This is because a higher heat input or equivalence ratio φ 149 will result in the higher pressure at the thermodynamic state 3 (Fig. 4), resulting in pushing the piston 150 further outward and causing higher volume ratios. In the context of power output, the actuator produces a peak output power of up to 1500 W for 163 an equivalence ratio of φ=1 (Fig. 7b). The power output is a product of energy output and operating 164 frequency of the actuator. Despite the operating frequency moderately increasing or decreasing, the 165 trend in output power is fairly increasing -because the efficiency of the actuator and the energy input 166 or output increases with the equivalence ratio φ (Figs. 5 and 6). (3) for a specific heat input, both the open and closed cycle operations respond differently -different stroke lengths, peak pressures, and thermal efficiencies.

Conflicts of Interest:
The author declares no conflict of interest.