Thermal Machines Research Articles

Experimental Study on Heat Transfer Characteristics on Intersecting Spiral Finned Tube Type on Heat Exchanger

Compact heat exchangers play an important role in the industrial world, one of their applications being in thermal machines to dissipate heat generated during mechanical processes. To improve the performance of heat exchangers, many studies have been conducted, including the addition of spiral fins, and the spacing of these fins on the outer surface. This study examines the heat transfer characteristics of the outer surface. The heat exchanger is made of galvanized pipe with an inner diameter of 20 mm and an outer diameter of 22 mm. It has a passage length of 300 mm with a sharp turn of 81 mm. The spiral fins are made of aluminum, with a thickness of 0.3 mm, a spiral fin height of 10 mm, and a distance of 30 mm between the spiral fins. The cross-section of the spiral fins varies, including options without spiral fins, without spiral fins, without intersecting spiral fins, with intersecting 2 mm spiral fins, intersecting 5mm spiral fins, and intersecting 7mm spiral fins. Heat exchangers are supplied with hot at a constant inlet temperature of 80C and a flow rate of 0.57 kg/s. The fan operates at speeds of 3.59 m/s, 4.45 m/s and 5.07 m/s. The results show that the highest heat transfer rate and heat transfer coefficient are produced by the heat exchanger with a cross-section of 5 mm intersecting spiral fins, specifically 11,682.7 W and 604 W/m2.K.

Probing coherent quantum thermodynamics using a trapped ion

Quantum thermodynamics is aimed at grasping thermodynamic laws as they apply to thermal machines operating in the deep quantum regime, where coherence and entanglement are expected to matter. Despite substantial progress, however, it has remained difficult to develop thermal machines in which such quantum effects are observed to be of pivotal importance. In this work, we demonstrate the possibility to experimentally measure and benchmark a genuine quantum correction, induced by quantum friction, to the classical work fluctuation-dissipation relation. This is achieved by combining laser-induced coherent Hamiltonian rotations and energy measurements on a trapped ion. Our results demonstrate that recent developments in stochastic quantum thermodynamics can be used to benchmark and unambiguously distinguish genuine quantum coherent signatures generated along driving protocols, even in presence of experimental SPAM errors and, most importantly, beyond the regimes for which theoretical predictions are available (e.g., in slow driving).

Catalytic Advantage in Otto-like Two-Stroke Quantum Engines.

We demonstrate how to incorporate a catalyst to enhance the performance of a heat engine. Specifically, we analyze efficiency in one of the simplest engine models, which operates in only two strokes and comprises of a pair of two-level systems, potentially assisted by a d-dimensional catalyst. When no catalysis is present, the efficiency of the machine is given by the Otto efficiency. Introducing the catalyst allows for constructing a protocol which overcomes this bound, while new efficiency can be expressed in a simple form as a generalization of Otto's formula: 1-(1/d)(ω_{c}/ω_{h}). The catalyst also provides a bigger operational range of parameters in which the machine works as an engine. Although an increase in engine efficiency is mostly accompanied by a decrease in work production (approaching zero as the system approaches Carnot efficiency), it can lead to a more favorable trade-off between work and efficiency. The provided example introduces new possibilities for enhancing performance of thermal machines through finite-dimensional ancillary systems.

The effects of common reservoirs on the performance of a quantum refrigerator

In this work, we study and find that the performance of an autonomous refrigerator can be improved by means of collective dissipations of common reservoirs. The refrigerator is based on a four-level system with two degenerate levels, coupled to three thermal reservoirs with different temperatures. Our study reveals that utilizing only one common reservoir can not significantly enhance the refrigerator’s performance. However, when two reservoirs are shared simultaneously, the performance improvement becomes more evident. Furthermore, we find that the refrigerator’s performance can be further enhanced by operating in a regime where the decoherence-free subspace is present. Our results indicate that careful engineering is essential to maximize the benefits of common reservoirs in enhancing the performance of quantum thermal machines.

Finite-Time Dynamics of an Entanglement Engine: Current, Fluctuations and Kinetic Uncertainty Relations.

Entanglement engines are autonomous quantum thermal machines designed to generate entanglement from the presence of a particle current flowing through the device. In this work, we investigate the functioning of a two-qubit entanglement engine beyond the steady-state regime. Within a master equation approach, we derive the time-dependent state, the particle current, as well as the associated current correlation functions. Our findings establish a direct connection between coherence and internal current, elucidating the existence of a critical current that serves as an indicator for entanglement in the steady state. We then apply our results to investigate kinetic uncertainty relations (KURs) at finite times. We demonstrate that there is more than one possible definition for KURs at finite times. Although the two definitions agree in the steady-state regime, they lead to different parameter ranges for violating KUR at finite times.

Entropy generation on the dynamics of volume fraction of nano-particles and coriolis force impacts on mixed convective nanofluid flow with significant magnetic effect

Recent advances in simulating entropy generation for dissipative cross-materials with a quartic auto catalyst are reported. Entropy generation can be used to generate entropy in any irreversible heat transfer process, which is important in thermal machines. This study examines the impact of entropy generation on the thermal transport and momentum of a continuous two-dimensional dusty fluid magnetohydrodynamic (MHD) migration across an inclined stretched sheet. We also conducted an entropy generation study to discuss the effects of viscous dissipation, the volume proportion of dust particles, and mixed convection. The two-phase models that address the fluid phase and the solid phase have been discussed. As part of the flow model’s innovation, the effect of raising particulate matter concentration on the dynamic behavior of the fluid is investigated. The described model transforms the prevailing partial differential equations (PDEs) for two phases into a nonlinearly associated, nondimensional set of equations by implementing appropriate similarity alterations. For graphical results, MATLAB programming incorporates the bvp4c mechanism. This study indicates how to examine the impact of appropriate factors on the dusty phase of fluid and the non-Newtonian fluid phase. Both the entropy generation ( E gen ) and the Bejan quantity (Be) are presented in relation to their associated dimensionless factors. Parametric research is used to evaluate the impact of different flow parameters on temperatures, entropy generation, and Bejan numbers. The heat transfer rate using several quadratic regression models, which provide more clarity in determining physical parameters crucial to engineering, is assessed. It is observed that the entropy ( E gen ) and Bejan number (Be) declines with the rising mass concentration ( β ν ) of dusty particles. Also demonstrated that for both cases, increasing the magnetic influence and dust volume fraction ( Φ D ) resulted in a decrease in velocity profiles at the same time as temperature increased.

Thermodynamic Geometry of Nonequilibrium Fluctuations in Cyclically Driven Transport.

Nonequilibrium thermal machines under cyclic driving generally outperform steady-state counterparts. However, there is still lack of coherent understanding of versatile transport and fluctuation features under time modulations. Here, we formulate a theoretical framework of thermodynamic geometry in terms of full counting statistics of nonequilibrium driven transports. We find that, besides the conventional dynamic and adiabatic geometric curvature contributions, the generating function is also divided into an additional nonadiabatic contribution, manifested as the metric term of full counting statistics. This nonadiabatic metric generalizes recent results of thermodynamic geometry in near-equilibrium entropy production to far-from-equilibrium fluctuations of general currents. Furthermore, the framework proves geometric thermodynamic uncertainty relations of near-adiabatic thermal devices, constraining fluctuations in terms of statistical metric quantities and thermodynamic length. We exemplify the theory in experimentally accessible driving-induced quantum chiral transport and Brownian heat pump.

Study of nucleate boiling growth regime on thin surfaces.

In a context of energy transition, heat exchanges are a key for energy sobriety. Among those exchanges, nucleate boiling is the preferred heat transfer method for high heat flux. Vaporisation plays an important role in the nuclear industry, but also in thermal machines such as cooling systems in data centers. In spite of decades studying boiling thermodynamics, modelling heat transfers in the nucleate boiling is still a major difficulty because of its numerous parameters. In the 1960s, scientists developed promising models called heat flux partitioning models. These models have been improved over the years, but they need closure laws about bubbles dynamics and nucleation site density. For the purpose of improving those models, we took part in an international collaboration (ANR TraThI) to study interface heat transfer. Project focuses on boiling with a controlled nucleation site density with a strong emphasis on studying isolated bubbles in water pool boiling, and later addressing multiple bubble interactions in pool and flow boiling. This work focuses on the bubble growth regimes on thin metallic foil observed in a water pool boiling experiment, with a distinction between microlayer and contact line growth regime. Pulsed nanosecond laser was used to create active nucleation site, while high-speed infrared thermography and shadowgraphy were implemented to record transient wall temperatures and bubble dynamics, respectively. This work shows that our experimental configuration induced two different bubble growth regimes, depending on the imposed heat flux and the recent past at the nucleation site and its vicinity. Study provides a framework for further in-depth investigations with different experimental configurations.

Steady-state entanglement production in a quantum thermal machine with continuous feedback control

Quantum thermal machines can generate steady-state entanglement by harvesting spontaneous interactions with local environments. However, using minimal resources and control, the entanglement is typically weak. Here, we study entanglement generation in a two-qubit quantum thermal machine in the presence of a continuous feedback protocol. Each qubit is measured continuously and the outcomes are used for real-time feedback to control the local system-environment interactions. We show that there exists an ideal operation regime where the quality of entanglement is significantly improved, to the extent that it can violate standard Bell inequalities and uphold quantum teleportation. In agreement with (Khandelwal et al 2020 New J. Phys. 22 073039), we also find, for ideal operation, that the heat current across the system is proportional to the entanglement concurrence. Finally, we investigate the robustness of entanglement production when the machine operates away from the ideal conditions.

Dissipation-induced collective advantage of a quantum thermal machine

Do quantum correlations lead to better performance with respect to several different systems working independently? For quantum thermal machines, the question is whether a working medium (WM) made of N constituents exhibits better performance than N independent engines working in parallel. Here, by inspecting a microscopic model with the WM composed by two non-interacting quantum harmonic oscillators, we show that the presence of a common environment can mediate non-trivial correlations in the WM leading to better quantum heat engine performance—maximum power and efficiency—with respect to an independent configuration. Furthermore, this advantage is striking for strong dissipation, a regime in which two independent engines cannot deliver any useful power. Our results show that dissipation can be exploited as a useful resource for quantum thermal engines and are then corroborated by optimization techniques here extended to non-Markovian quantum heat engines.

Anisotropy-assisted thermodynamic advantage of a local-spin quantum thermal machine.

We study quantum Otto thermal machines with a two-spin working system coupled by anisotropic interaction. Depending on the choice of different parameters, the quantum Otto cycle can function as different thermal machines, including a heat engine, refrigerator, accelerator, and heater. We aim to investigate how the anisotropy plays a fundamental role in the performance of the quantum Otto engine (QOE) operating in different timescales. We find that while the engine's efficiency increases with the increase in anisotropy for the quasistatic operation, quantum internal friction and incomplete thermalization degrade the performance in a finite-time cycle. Further, we study the quantum heat engine (QHE) with one of the spins (local spin) as the working system. We show that the efficiency of such an engine can surpass the standard quantum Otto limit, along with maximum power, thanks to the anisotropy. This can be attributed to quantum interference effects. We demonstrate that the enhanced performance of a local-spin QHE originates from the same interference effects, as in a measurement-based QOE for their finite-time operation.

Quantum Carnot thermal machines reexamined: Definition of efficiency and the effects of strong coupling.

Whether the strong coupling to thermal baths can improve the performance of quantum thermal machines remains an open issue under active debate. Here we revisit quantum thermal machines operating with the quasistatic Carnot cycle and aim to unveil the role of strong coupling in maximum efficiency. Our analysis builds upon definitions of excess work and heat derived from an exact formulation of the first law of thermodynamics for the working substance, which captures the non-Gibbsian thermal equilibrium state that emerges at strong couplings during quasistatic isothermal processes. These excess definitions differ from conventional ones by an energetic cost for maintaining the non-Gibbsian characteristics. With this distinction, we point out that one can introduce two different yet thermodynamically allowed definitions for efficiency of both the heat engine and refrigerator modes. We dub them excess and hybrid definitions, which differ in the way of defining the gain for the thermal machines at strong couplings by either just analyzing the energetics of the working substance or instead evaluating the performance from an external system upon which the thermal machine acts, respectively. We analytically demonstrate that the excess definition predicts that the Carnot limit remains the upper bound for both operation modes at strong couplings, whereas the hybrid one reveals that strong coupling can suppress the maximum efficiency rendering the Carnot limit unattainable. These seemingly incompatible predictions thus indicate that it is imperative to first gauge the definition for efficiency before elucidating the exact role of strong coupling, thereby shedding light on the ongoing investigation on strong-coupling quantum thermal machines.

Concave Version Printing of High-Performance Polymer THz Filters.

The development of high Q and tunable narrowband filters that can efficiently manipulate THz beams is critical for various THz applications, such as imaging and sensing. However, for filters made of metals and dielectrics, issues such as high losses, limited tunability, and lengthy process flows exist. Here, a scalable concave version reprinting technique to mass produce high-performance microstructured polymer filters is presented. The technique is extremely simple, eliminating the demand for the use of any large equipment including injection molding and thermal press printing machines, and is reliable; in the reprinted structures, there are no defects including gaps and air bubbles. The produced narrowband filters exhibit a high Q factor of 57 with wide tunability over the THz band from ∼80 to 160 μm in wavelength. The presented technique can be adopted to realize other devices as well using polymer materials with simplicity and high precision.

Simulation methodology and numerical comparison of boxer and opposed free piston linear generator configuration in 0D/1D environment

Free Piston Linear Generators (FPLGs) are thermal machines suitable for both stationary power generation unit (PU) and vehicle propulsion. They convert chemical energy from the fuel into electric energy to charge a battery using a linear electric generator. Due to the reduction of friction losses and the ability to work with different compression ratios, FPLGs are potentially characterized by high overall global efficiency with respect to a conventional alternative internal combustion engine. Like conventional internal combustion engines, FPLGs have several configurations: single piston, dual piston, opposed piston and boxer are the configurations considered in this work. Two configurations are compared in this paper from a functional and energetic perspective: opposed piston (Opposed) and Boxer. This is done with the aim to highlights pro and cons of these configurations. They are modelled using the 0D/1D modelling coupling GT-Power® and Simulink®. The comparison between the two configurations was done modelling the architecture with the same size and in two-strokes operating mode. The comparison of the two configurations highlights differences between them in terms of scavenging process, thermodynamic efficiency and global energy efficiency, putting in evidence what aspects need to be improved for each configuration and what are their advantageous features. Boxer configuration can provide less power due to lower volumetric efficiency compared to the Opposed configuration. Boxer configuration can provide 102.2 kW while Opposed can provide 110.0 kW at full load with a frequency of 20 Hz (equivalent to 1200 rpm). The Opposed configuration results in lower heat loss but higher energy lost at the exhaust. The final balance shows that Boxer configuration has a slightly higher brake efficiency (equal to 28.9 %) compared to the value found for Opposed (27.6 %).

Power Generation Limits in Thermal, Chemical and Electrochemical Systems

Power generation limits are evaluated via optimization for various energy converters, such like thermal, solar, chemical, and electrochemical engines, in particular fuel cells. Thermodynamic analyses lead to converters’ efficiencies, which help to solve problems of optimal upgrading and downgrading of resources. While methods of static optimization, i.e. differential calculus and Lagrange multipliers, are sufficient for steady processes, dynamic optimization applies the variational calculus and dynamic programming for unsteady processes. In reacting systems chemical affinities constitute prevailing components of an overall efficiency, thus flux balances are applied to derive power in terms of active parts of chemical affinities. Methodological similarity is observed when treating power limits in flow thermal machines and fuel cells. The examples show power maxima in fuel cells and prove suitability of a thermal machine theory to chemical and electrochemical systems. The main novelty of contribution in the fuel cell context consists in introducing an effective change of Gibbs free energy between products p and reactants s which takes into account lowering of voltage and power caused by the incomplete conversion of the overall electrochemical reaction.

Oscillating rotary electrical machine for Stirling cycle heat pump and other devices of renewable energy

The paper deals with oscillating rotary electrical machines, that is with periodical angular movement of their movable part. Expedient application of all oscillating electrical machines consists in possible avoidance of mechanical transformer in the drive, according to the direct drive principle. That is why oscillating electrical machines are suitable to use with positive displacement thermal machines. In the paper, principles of development of the Stirling cycle heat pump with oscillating rotary electrical motors according to the project EFFiHEAT are presented. Partners from Spain, Bulgaria and Lithuania are implementing the project. Tentative test of the oscillating rotary motor prototype confirms its working capacity and advantages. Principles of presented structure can be used in various other devices of renewable energy (e. g. solar power stations, geothermal power plants, heat and power cogenerators, generators with multi-fuel Stirling engine etc.).

Ice‐Bridging Frustration by Self‐Ejection of Single Droplets Results in Superior Anti‐Frosting Surfaces

AbstractSurfaces capable of delaying the frosting passively and facilitating its removal are highly desirable in fields where ice introduces inefficiencies and risks. Coalescence‐induced condensation droplets jumping (CICDJ), enabled on highly hydrophobic surfaces, is already exploited to slow down the frosting but it is insufficient to completely eliminate the propagation by ice‐bridging. The study shows here how the self‐ejection of single condensation droplets can fully frustrate all the ice bridges, resulting in a frost velocity lower than 0.5 µm s−1 and thus falling below the current limits of passive surfaces. Arrays of truncated microcones, covered by uniformly hydrophobic nanostructures, enable individual condensation droplets to grow and self‐propel toward the top of the microstructures and then to self‐eject once a precise volume is reached. The independency of self‐ejection on the neighbor droplets allows a fine control of the droplets size and distance distributions and thus the ice‐bridging frustration. The truncated microcones with the smallest heads area fraction maximize the percentage of self‐ejecting droplets and minimize the frost velocity. The ice bridges frustration also implies a small frost area coverage, highly desirable in aeronautics and thermal machines.

Formula omitted]-symmetry effects in measurement-based quantum thermal machines

Measurement-based quantum thermal machines are fascinating models of thermodynamic cycles where measurement protocols play an important role in the performance and functioning of the cycle. Despite theoretical advances, interesting experimental implementations have been reported. Here we move a step further by considering a measurement-based quantum thermal machine model where the only thermal bath is structured with PT-symmetric non-Hermitian Hamiltonians. Theoretical results indicate that PT-symmetric effects and measurement protocols are related along the cycle. Furthermore, tuning the parameters suitably it is possible to improve the absorbed heat and the extracted work, operating in the Otto limit for the efficiency, provided we consider a quasi-static cycle. Our model also allows switching the configuration of the cycle, engine, or refrigerator, depending on the strength of the measurement protocol.

Functionalization of All-Oxide Ceramic Matrix Composites Elements Using Three-Dimensional Braiding and Pressure Slip Casting for Composite Processing: Approaches to Reduce the Filter Effect of Dense Reinforcement Textiles

Abstract A key element for the transition to sustainable energy lies in the transformation of the power plant fleet, which is dominated by fossil fuels, toward sustainable energy production from renewable energy sources. An increase in efficiency and reduction of exhaust gas emissions, especially the minimization of CO2 emissions, is possible through the use of new turbine materials, which can withstand higher temperature levels. Oxide ceramics are well known for their high stability in aggressive environments, low density, high melting point, high stiffness, and great creep resistance, but their brittleness has strongly limited their number of applications. Therefore, the implementation of fiber reinforcement using the three-dimensional (3D) braiding process shows great potential to increase the damage tolerance of ceramic matrix composites (CMC) and consequently the performance of thermal machines significantly. Currently, the impregnation of 3D braids for the reinforcement of ceramic composites poses a challenge due to the high packing density of the textiles. In order to enable a homogeneous impregnation of the fiber structures using highly viscous ceramic slurries, the CMC research group at RWTH Aachen University's Institute of Textile Technology (ITA) is investigating the combination of 3D braiding and pressure slip casting for an economical production of all-oxide CMCs. To increase the impregnation quality of dense textiles, this paper describes approaches to reduce the filter effect of braids. The results of an initial investigation into the functionalization of two-dimensional braided reinforcement structures by using support structures and flow aids are described. The effectiveness of the impregnation ability is assessed by evaluating the residual porosity of generated green compacts via μCT analysis.

On the quality of stability and bifurcation sets in rotors with permanent shaft bow on nonlinear supports

The paper investigates the dynamics of simple and complex geometry rotors of various configurations with permanent bow, mounted on nonlinear bearings, in regards to nonlinear stability, quality of motion, and bifurcation set. Permanent shaft bow is a fault existing in most slender rotors in thermal machines, e.g. gas turbines, steam turbines, and turbopumps, and other applications of rotating machinery like transmission shafts and geared shafts. In this work, finite element models of multi-segment rotors are implemented, including permanent bow, and a generic rotor system of a uniform rotor with three masses is studied first on its stability characteristics for various bow values, and for different slenderness. A realistic rotor of complex geometry is used as the second application. The rotors are mounted on two plain short bearings to include the nonlinearity of impedance forces with analytical formulas; this is the unique source of nonlinearity in the systems.The rotor-bearing systems are found to produce the well-known oil whirl instability through a Hopf, or a secondary Hopf bifurcation, depending the unbalance grade, in speeds higher than the first critical speed, when bow is not included. Supercritical and subcritical Neimark-Sacker and period doubling bifurcations are produced in speeds lower than critical speed when bow is included above some threshold values. Stable and unstable limit cycles are generated both for unbalanced or balanced rotors with bow, and their periodicity is investigated. A bifurcation set of the rotor-bearing systems is evaluated utilizing the pseudo-arc length continuation method with embedded collocation method, for the evaluation of periodic limit cycles (where these appear). Neimark-Sacker bifurcations are found to collide and vanish as the bow magnitude changes from high to low values. Bow phase angle with respect to unbalance force is found not to influence the respective bifurcation sets.