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•Showed that inclination dependence of cycle correlates with mass flow rate.•Proposed inclination dependence indicator for vapor compression cycle.•Showed risk of liquid flooding due to inclination to increase with refrigerant charge level.

Recently, linear compressor technology has gained increased attention in vapor compression systems due to its compactness and lower frictional losses compared with the conventional reciprocating compressors. Since the absence of the crank-mechanism eliminates the rotation and reduces the side loads on the piston centered inside the cylinder, it is possible to operate the linear compressors without oil lubrication. A number of advantages can be obtained in terms of the refrigerant compatibility, operation conditions and the system cost. In order to enable oil-free operation, while minimizing frictional losses and leakage flow between the piston and the cylinder wall, a gas bearing approach is applied. Little work was found in the open literature related to the numerical analysis of gas bearings coupled with a comprehensive dynamic linear compressor model. This paper presents a 1-D gas bearing model based on a Finite Volume Method (FVM) applied to a linear compressor. The dynamic characteristics of the gas bearings on the gas film pressure field are analyzed using the model. When integrated together with a comprehensive dynamic linear compressor model, it is possible to predict leakage and frictional losses within the compressor as well as the overall performance.

High-power small satellites (SmallSats) have the potential to provide new and advanced capabilities; however, significant challenges prevent wide-spread use. Of these challenges, thermal management of high heat loads is most significant. This paper evaluates vapor compression cycles (VCCs) as a thermal management solution to increase the maximum component heat load at a fixed radiator area. The VCC with radiative heat transfer as a heat sink is described with analytical equations that couple the required compressor work input for a desired additional component heat load for a given second law efficiency. A figure of merit is defined. In a case study, the results of the equations are compared to a numerically modelled vapor compression cycle. Challenges for a realization of vapor compression cooling on-board SmallSats are pointed out.

Single-screw type volumetric machines are employed both as compressors in refrigeration systems and, more recently, as expanders in organic Rankine cycle (ORC) applications. The single-screw machine is characterized by having a central grooved rotor and two mating toothed starwheels that isolate the working chambers. One of the main features of such machine is related to the simultaneous occurrence of the compression or expansion processes on both sides of the main rotor which results in a more balanced loading on the main shaft bearings with respect to twin-screw machines. However, the meshing between starwheels and main rotor is a critical aspect as it heavily affects the volumetric performance of the machine. To allow flow interactions between the two sides of the rotor, a non-symmetric modelling approach has been established to obtain a more comprehensive model of the single-screw machine. The resulting mechanistic model includes in-chamber governing equations, leakage flow models, heat transfer mechanisms, viscous and mechanical losses. Forces and moments balances are used to estimate the loads on the main shaft bearings as well as on the starwheel bearings. An 11 kWe single-screw expander (SSE) adapted from an air compressor operating with R245fa as working fluid is used to validate the model. A total of 60 steady-steady points at four different rotational speeds have been collected to characterize the performance of the machine. The maximum electrical power output and overall isentropic efficiency measured were 7.31 kW and 51.91%, respectively.

Thermoacoustic engine prototype design typically proceeds based on an intuitive, trial and error approach. This study investigates the benefits of performing a systematic optimization of design parameters in a thermo‐ acoustic engine for given operational requirements. The design optimization seeks to minimize a cost function of performance using the simplex method. This method allows constraints to be readily implemented without need for gradient evaluations. Parameter space maps are generated for visualization of solution sensitivity. System analysis is performed with the assistance of DeltaE, a commercially available software tool which solves the 1‐D lossy wave equation in resonator sections and Rott energy and wave equations in stack sections. The solution is therefore subject to the low‐amplitude (linear) flow assumptions and conduction mode heat transfer assumptions in heat exchangers. The optimization and space mapping tools systematically write input decks, execute DeltaE, and evaluate results. Optimization of up to 13 geometric, fluid, and material properties has been explored, producing encouraging performance predictions. Application of this strategy to design modifications of documented prototypes and resulting benefits of systematic optimization will be illustrated. It is anticipated that the optimization scheme will be useful in enhancing performance of future thermoacoustic engine prototypes.

A basic thermoacoustic cooler design was optimized with three different types of heat exchangers to obtain high COP predictions in a 200‐W refrigeration application. The heat exchanger designs employed: (1) a parallel array of copper‐alloy plates (conduction); (2) a single‐pass, single row of finned tubes, with water/ethylene glycol mixtures as the secondary fluids (finned tube); and (3) a parallel array of infinitely conductive plates (ideal). The optimization scheme utilized the Simplex algorithm, and the analysis was performed by DELTAE coupled with an original heat exchanger modeling tool which incorporated conventional straight fin analysis and semiempirical flow correlations. The external cooling and heat rejection temperatures were fixed at 260 and 310 K, for which the Carnot COP is 5.2. The best COP’s resulting from the optimization of around 25 design parameters for each case were approximately 2.0 (conduction), 2.5 (finned tube), and 3.0 (ideal). In the finned tube device, the COP was limited by the cold exchanger secondary fluid properties. The use of a phase change secondary fluid, in conjunction with a heat pipe, for example, may further improve the performance of finned tube heat exchangers. The ideal case performance represents a probable upper limit for the configuration considered.

Up to 25 thermoacoustic cooler design parameters were optimized to provide high COP predictions in selected air conditioning applications. Capacities ranging between 300 and 1500 W were considered. Two different configurations were investigated: (1) a single stack with a Helmholtz resonator termination, and (2) two drivers and two stacks. The optimization scheme utilized the Simplex algorithm, and the analysis was performed by DELTAE coupled with an original heat exchanger modeling tool which incorporated conventional straight fin analysis and semiempirical flow correlations. Various requirements were imposed for the external cooling and heat rejection temperatures, ranging between 270 and 287 K on the cold side and between 310 and 320 K on the hot side. Finned tube heat exchangers with water as the secondary fluid were considered. The predicted values of COP were greater than 3.8 (0.36, relative to the Carnot value) for an application analogous to a small window air conditioner. The performance and characteristics of the two different configurations were compared, and the loss distribution in each case was calculated. Furthermore, the impact of various constraints was examined. The performance predictions are encouraging and suggest that thermoacoustic air conditioning may be a viable proposition.