Heat Fluctuations Research Articles

Energy performance of phase change materials integrated into brick masonry walls for cooling load management in residential buildings

Energy demand for space cooling in residential buildings is projected to witness rapid growth, primarily fueled by increasing household incomes in developing countries. To manage this ever-increasing cooling demand, integration of phase change materials (PCMs) in building walls is a potential solution that can reduce the buildings’ cooling energy consumption and peak cooling loads. However, to attain the proposed benefits from PCM integration, it is crucial to appropriately select PCM parameters such as its phase-change temperature and positioning in the wall. Thus, this investigation studied the energy performance of PCM integrated brick masonry walls for cooling load management in residential buildings under periodic steady-state conditions to identify the parameters that govern its performance and develop simple design guidelines.The research found that regardless of the amount of latent heat stored by the PCM, the daily heat gains and cooling loads were equal for wall configurations having equal thermal resistances under identical boundary conditions. Furthermore, even with the application of night ventilation, adding a PCM layer to a well-insulated wall did not reduce its cooling load; thus, PCM integration was ineffective in reducing the cooling load. However, the latent heat stored by the PCM reduced the fluctuations in the hourly heat gains and cooling loads; thus, PCM integration was found suitable for peak load management. For the PCM's proper utilization, its recommended position is on the inner side of the wall with sufficient insulation shielding it from outdoor conditions, and its melting temperature should be close to the indoor set-point temperature.

A general model of the thermal constraints on the world's most destructive locust, Schistocerca gregaria.

All terrestrial ectotherms are constrained to some degree by their thermal environment and the extent to which they can behaviorally buffer variable thermal conditions. New biophysical modeling methods (NicheMapR) allow the calculation of the body temperature of thermoregulating animals anywhere in the world from first principles, but require detailed observational data for parameterization and testing. Here we describe the thermoregulatory biology of marching bands of the desert locust, Schistocerca gregaria, in the Sahara Desert of Mauritania where extreme heat and strong diurnal fluctuations are a major constraint on activity and physiological processes. Using a thermal infrared camera in the field, we showed that gregarious nymphs altered the microhabitats they used, as well as postural thermoregulatory behaviors, to maintain relatively high body temperature (nearly 40°C). Field and laboratory experiments demonstrated that the preferred body temperature accelerated digestive rates. Migratory bands frequently left foraging sites with full guts before consuming all vegetation and moved to another habitat before emptying their foregut. Thus, the repertoire for behavioral thermoregulation in the desert locust strongly facilitates foraging and digestion rates, which may accelerate developmental rates and increase survival. We used our data to successfully parameterize a general biophysical model of thermoregulatory behavior that could capture hourly body temperature and activity at our remote site using globally available environmental forcing data. This modeling approach provides a stronger basis for forecasting thermal constraints on locust outbreaks under current and future climates.

Unsteady double diffusive convection inside a partial porous building enclosure subjected to time-periodic temperature boundary condition

Unsteady double diffusive convection under the influence of time-periodic sidewall temperature inside a rectangular enclosure with a porous wall is performed using numerical simulation and analytical prediction. The ranges of investigated physical parameters are listed as Darcy number (10−10 ≤ Da ≤ 101), thickness of porous wall (0 ≤ d ≤ 1), thermal Rayleigh number (103 ≤ Ra ≤ 107), buoyancy ratio (−1 ≤ N ≤ 1) and oscillating frequency of time-periodic sidewall temperature (0.01 ≤ ω ≤ 10). Results indicate that rates of heat and moisture transfer are impacted by time-periodic temperature and have a sinusoidal variation with time. The heat and moisture resonance phenomenon occurs when the convection effect is sufficiently stronger. Scale analysis has been performed to obtain the correlations of overall heat and moisture transfer rates in basic steady state and the resonant frequency, where the fluctuations of heat and moisture transfer are increased. A good agreement is observed from the analytical and numerical results. Present work could be applied to determine the heat and moisture transfer rates and resonant frequency in a thermal storage system, where the sidewall temperature is varying periodically with time due to the weather condition (solar radiation) and ambient temperature during a typical day.

Heat exchange and fluctuation in Gaussian thermal states in the quantum realm

The celebrated exchange fluctuation theorem—proposed by Jarzynski and Wójcik (2004 Phys. Rev. Lett. 92 230602) for heat exchange between two systems in thermal equilibrium at different temperatures—is explored here for quantum Gaussian states in thermal equilibrium. We employ the Wigner distribution function formalism for quantum states, which exhibits a close resemblance to the classical phase-space trajectory description, to arrive at a formal Jarzynski–Wójcik result. For two Gaussian states in thermal equilibrium at two different temperatures kept in contact with each other for a fixed duration of time, we show that the Jarzynski–Wójcik-like relation reduces to the corresponding classical result in the limit ℏ → 0.

A Q-learning based transient power optimization method for organic Rankine cycle waste heat recovery system in heavy duty diesel engine applications

In recent years, the organic Rankine cycle waste heat recovery (ORC-WHR) technology gains popularity in heavy-duty diesel engine applications. Drastic fluctuations of the waste heat caused by variable daily operation of mobile heavy-duty trucks bring an extreme transient power optimization challenge to ORC-WHR systems. Existing power optimization methods either neglect transient behavior of the Rankine cycle system or compromise model accuracy for computation efficiency. Different from literature, this study first time proposes a model-free reinforcement learning method to achieve online transient power optimization for the ORC-WHR system and explains the benefits of learning method in this application. A tabular Q-learning is formulated to optimize the net power on an experimentally validated ORC-WHR system. Q-learning is explained in detail using states, action, and policy information. To quantify the power optimization of the proposed method, Proper-Integral-Derivative method, state-of-art offline and online Dynamic Programming methods are implemented. The results showed that Q-learning generated 22% more cumulative energy than the energy Proper-Integral-Derivative method generated. Furthermore, Q-learning produces 96.6% of cumulative energy that the offline Dynamic Programming generates over a transient engine condition, while it requires less computation cost and is executed online. Additionally, the Q-learning produces 0.5% more cumulative energy than the machine learning-based online Dynamic Programming results and exhibits better vapor temperature robustness than the online Dynamic Programming method (4 °C-28 °C superheat by Q-learning vs. 5 °C-94 °C superheat by online Dynamic Programming). Given the excellent power production performance, low computation cost requirement and high robustness, the proposed Q-learning method has the potential to improve the power production of the ORC-WHR system with different configurations.

Thermodynamic uncertainty relations for bosonic Otto engines.

We study two-mode bosonic engines undergoing an Otto cycle. The energy exchange between the two bosonic systems is provided by a tunable unitary bilinear interaction in the mode operators modeling frequency conversion, whereas the cyclic operation is guaranteed by relaxation to two baths at different temperatures after each interacting stage. By means of a two-point-measurement approach we provide the joint probability of the stochastic work and heat. We derive exact expressions for work and heat fluctuations, identities showing the interdependence among average extracted work, fluctuations, and efficiency, along with thermodynamic uncertainty relations between the signal-to-noise ratio of observed work and heat and the entropy production. We outline how the presented approach can be suitably applied to derive thermodynamic uncertainty relations for quantum Otto engines with alternative unitary strokes.

Vapor condensation in Rayleigh–Bénard convection

In this work, the condensation process in the Rayleigh–Bénard convection is studied by a combination of theoretical analysis and numerical simulations. Depending on the domain size, three different patterns, namely, no condensation, critical condensation, and periodic condensation, are identified. By applying the order analysis to the energy equation, we show that the heat fluctuation is responsible to overcome the energy barrier of condensation and thus propose a new dimensionless number to describe the critical condition of condensation, which corresponds to zero value of the heat fluctuation. In addition, through the order analysis, a scaling law is established to quantify the condensation period when periodic condensation occurs. The scaling relations derived from the order analysis are well validated by the hybrid lattice Boltzmann finite difference simulations, where the Rayleigh number and the Prandtl number vary over the ranges of 104 ≤ Ra ≤ 106 and 1 ≤ Pr ≤ 10, respectively.

Investigating the conjugate heat transfer phenomena on various ducts for aircraft environmental control system

All commercial aircraft employ a system called Environmental control system (ECS) to ensure an environment that accomplishes the physiological and comfort demands by passengers and aircrew. This task requires functionalities like temperature control, humidity regulation apart from the governing of cabin pressure. The energy source for ECS is a pressurized hot air bleed (180 °C-250 °C, 2.75 bar) from the engine or Auxiliary Power Unit (APU) along with the ram air. During the functional operation, the unit of Heating, Ventilation, and Air Conditioning (HVAC) duct experiences repetitive temperature fluctuations in addition to the cyclic pressurization leading to thermal fatigue and cyclic stress over the duct material, promoting crack propagation at weld zones in the duct. Therefore, a comparative investigation on conjugate heat transfer analysis is done at welded joints annexed to the bypass valve and trim air valve using airflow bench set up and validated by computational fluid dynamics. During this investigation, the circular duct with spherical junction (D/d = 3.84 & L/d = 6) was the most efficient and economical design that could fit in for designing air conditioning duct undergoing high thermo-cyclic stress.

Optimizing thermal performance of a dry rigid clutch by varying groove pattern and friction material

A precise calculation of temperature distribution is necessary to avoid the heat fluctuation in clutch facing and this also acts as an aid during a clutch design to achieve improved heat transfer. This article presents a systematic way to design and analyze the thermal characteristics of a clutch facing during clutch engagement assuming uniform wear rate between the contact surfaces. Finite Element Analysis (FEA) has been used to study the effect of groove patterns on the thermal characteristics of a dry friction clutch disc, followed by a comparative analysis between the organic and inorganic materials. The clutches with different groove configurations are designed using SOLIDWORKS 2020 while the thermal simulations are carried out using ANSYS 2020 R2. In this research, the optimal design for better thermal stability is accomplished by considering different configurations and materials for the clutch facing. The acquired temperature results for each configuration depict that the heat dissipated by convection through the friction material can be regulated by altering the groove pattern and area. It is found that organic based friction linings are exhibiting about 30% lower temperatures than inorganic linings.

Advanced exergo-economic schemes and optimization for medium–low grade waste heat recovery of marine dual-fuel engine integrated with accumulator

Facing the multiple pressures of energy reserves decline, ecological environment deterioration and transportation industry prosperity, in order to enhance the marine performance, improve the enterprises economic benefits, and achieve the purpose of energy conservation and emission reduction, the revolution of green and efficient waste heat recovery technology is imperative. Therefore, the exergy-based configuration design of marine cascade waste heat recovery system is proposed, including supercritical Brayton cycle, organic Rankine cycle, organic flash cycle, organic trilateral cycle and absorption refrigeration cycle. Firstly, the mathematical models of three cycle-configurations are established based on thermodynamic equations, and the authority of simulation results is preliminarily verified. Then, the relationship between working-fluid, matching mode and operation parameters with cycle-configuration performance is analyzed in turn. After the multi-objective optimization, the thermodynamic performance and economic benefits of the cycle-configuration are comprehensively evaluated. Finally, the positive influence of the accumulator on the refrigeration system is explored. The numerical results prove that the proposed cycle-configuration has significant thermodynamic performance and economic benefits, reflects the good performance of energy saving and emission reduction, and can eliminate the negative impact of latent heat fluctuation on the refrigeration system. It provides a feasible idea for researchers and manufacturers who are committed to comprehensively improve the performance of cascade waste heat recovery, and fills the research gap in the latest investigation.

Обґрунтування створення електротепломеханічної системи перемішування та підігріву біомаси

To date, biomass fermentation in biogas plants is one of the most advanced, environmentally and economically viable solutions for energy production from waste. However, the process of anaerobic fermentation of waste is long, so one of the main ways to intensify biogas production is mixing and heating of biomass during fermentation. The article is devoted to the question of substantiation of creation of electrothermomechanical system for mixing and heating of biomass in a biogas reactor. The combination of two intensification processes in a combined system pays special attention to the energy efficiency of such a system, so the creation of the system requires in-depth study of heat fluctuations from speed and the presence of a contaminant layer on the heater surface and determine the optimal stirrer speed. The studies were performed for a cylindrical biogas reactor, assuming that the contaminant layer is evenly distributed on the surface of the blades and the shaft in which the electric heaters are installed. When determining the optimal frequency of biomass mixing, the criterion of optimality was taken to be the smallest value of the difference between the heat flux of the contaminated and uncontaminated surface of the heater. During the study it was found that at speed , the difference between the heat flux of the contaminated surface and uncontaminated is 40 %. At speed , the difference between the values is 26%. According to the selected optimality criterion, the optimal speed of the electrothermomechanical system taking into account the contaminant layer is in the range . The increase in heat flux from the stirring frequency is non-linear for both contaminated and non-contaminated heaters.

Power, efficiency, and fluctuations in steady-state heat engines.

We consider the quality factor Q, which quantifies the trade-off between power, efficiency, and fluctuations in steady-state heat engines modeled by dynamical systems. We show that the nonlinear scattering theory, in both classical and quantum mechanics, sets the bound Q=3/8 when approaching the Carnot efficiency. On the other hand, interacting, nonintegrable, and momentum-conserving systems can achieve the value Q=1/2, which is the universal upper bound in linear response. This result shows that interactions are necessary to achieve the optimal performance of a steady-state heat engine.

Escherichia coli small heat shock protein IbpA is an aggregation-sensor that self-regulates its own expression at posttranscriptional levels.

Aggregation is an inherent characteristic of proteins. Risk management strategies to reduce aggregation are critical for cells to survive upon stresses that induce aggregation. Cells cope with protein aggregation by utilizing a variety of chaperones, as exemplified by heat-shock proteins (Hsps). The heat stress-induced expression of IbpA and IbpB, small Hsps in Escherichia coli, is regulated by the σ32 heat-shock transcriptional regulator and the temperature-dependent translational regulation via mRNA heat fluctuation. We found that, even without heat stress, either the expression of aggregation-prone proteins or the ibpA gene deletion profoundly increases the expression of IbpA. Combined with other evidence, we propose novel mechanisms for the regulation of the small Hsps expression. Oligomeric IbpA self-represses the ibpA/ibpB translation, and mediates its own mRNA degradation, but the self-repression is relieved by sequestration of IbpA into the protein aggregates. Thus, the function of IbpA as a chaperone to form co-aggregates is harnessed as an aggregation sensor to tightly regulate the IbpA level. Since the excessive preemptive supply of IbpA in advance of stress is harmful, the prodigious and rapid expression of IbpA/IbpB on demand is necessary for IbpA to function as a first line of defense against acute protein aggregation.

Thermoacoustics and combustion instability analysis for multi-burner combustors

This paper presents a low-order thermoacoustic model for multi-burner combustors and proposes a control theoretic approach to linear combustion instability analysis in frequency domain, taking acoustic and flame interaction effects among burners into account under a non-zero mean flow condition. Our thermoacoustic model can deal with can-annular and annular combustors in a unified way and is given as a multi-input multi-output transfer function matrix from heat fluctuation input vector to velocity fluctuation output vector. We also propose a flame transfer function matrix description where off-diagonal components describe the effects of a local acoustic field of one burner location on the flame response at another burner location. The discrete rotational symmetry of burner arrangement in a combustor makes it possible to diagonalize both the aforementioned thermoacoustic and flame transfer function matrices analytically from a spatial discrete-time Fourier transformation. The decoupled thermoacoustic transfer function matrix provides a characterization of acoustic resonances, particularly the longitudinal-azimuthal coupled modes unique to multi-burner combustors. In addition, from the Bloch wave interpretation, we find general patterns in the mode shape of a multi-burner combustor and its relations to the mode shape of a single-burner combustor. Furthermore, from a diagonalized closed loop transfer function matrix, a simple combustion instability condition for a multi-burner combustor is developed. This stability condition shows how a single-burner combustor and a multi-burner combustor are different quantitatively, when it comes to combustion instability. Numerical examples are presented to validate our thermoacoustic model against three-dimensional FEM results, and to illustrate our developments and findings.

Quasiprobability Distribution for Heat Fluctuations in the Quantum Regime

The standard approach to deriving fluctuation theorems fails to capture the effect of quantum correlation and coherence in the initial state of the system. Here we overcome this difficulty and derive heat exchange fluctuation theorem in the full quantum regime by showing that the energy exchange between two locally thermal states in the presence of initial quantum correlations is faithfully captured by a quasiprobability distribution. Its negativities, being associated with proofs of contextuality, are proxys of non-classicality. We discuss the thermodynamic interpretation of negative probabilities, and provide heat flow inequalities that can only be violated in their presence. Remarkably, testing these fully quantum inequalities, at arbitrary dimension, is not more difficult than testing traditional fluctuation theorems. We test these results on data collected in a recent experiment studying the heat transfer between two qubits, and give examples for the capability of witnessing negative probabilities at higher dimensions.

Impact of differences in soil temperature on the desert carbon sink

An understanding of the global terrestrial carbon cycle is crucial to predict future climate change. Deserts are an important part of the terrestrial ecosystem, but their role in the terrestrial carbon cycle has long been neglected. Recent studies have shown that deserts may sequester enormous volumes of CO2 and play a pivotal role as a carbon sink. As the world’s second-largest shifting desert, the Taklimakan Desert (TD) contributes substantially to the global desert carbon sink. However, the contributions of the internal processes of the TD to its carbon sink and the long-term trend of the carbon sink under climate change are still unclear. Based on an observational study, we determined that both the expansion/contraction of soil air containing CO2 caused by heat fluctuation in shifting sand and the salt/alkali chemistry control the release/absorption processes of soil respiration in shifting sand. Besides, the mutual offset of these processes indicates that the shifting sand in the TD acts as a stable carbon sink at present, with an annual average CO2 uptake rate of 7.11 g m−2. However, an increase in soil temperature difference will stimulate soil air expansion and release more CO2 into the atmosphere under climate change, gradually lessening the carbon sink rate of shifting sand in TD in the future. These processes will be accelerated by a positive feedback loop, where this effect triggered by climate change will enhance regional warming. Our results highlight the status of deserts in the global carbon cycle and provide crucial information regarding the world’s missing carbon sink.

A new analytical approach for the research of thin‐film flow of magneto hydrodynamic fluid in the presence of thermal conductivity and variable viscosity

AbstractIn this endeavor thin‐film flow of magneto hydrodynamic (MHD) fluid in the presence of thermal conductivity and variable viscosity over a porous steady stretching surface with a magnetic field and radioactive heat fluctuation is studied. In this phenomenon the viscosity vary inversely and thermal conductivity directly with temperature. The nonlinear coupled differential equations for the velocity and temperature profiles are achieved and solved by using a new analytical approach, called 3rd form of Optimal Homotopy Asymptotic Method (OHAM‐3). The proposed technique consists of initial guess, embedding parameter, optimal convergence control parameters, auxiliary functions and homotopy. Numerical (ND‐Solve Method) solutions are also achieved and compared with the results gained by the proposed method. The fast convergence of the applied new method and the influence of different physical nondimensional parameters on the velocity and temperature profiles are mainly focused in this research.

Heat fluctuations in equilibrium

The characteristic function for heat fluctuations in a nonequilibrium system is characterised by a large deviation function whose symmetry gives rise to a fluctuation theorem. In equilibrium the large deviation function vanishes and the heat fluctuations are bounded. Here we consider the characteristic function for heat fluctuations in equilibrium, constituting a sub-leading correction to the large deviation behaviour. Modelling the system by an oscillator coupled to an explicit multi-oscillator heat reservoir we evaluate the characteristic function.

Large eddy simulation based multi-environment PDF modelling for mixing processes of transcritical and supercritical cryogenic nitrogen jets

Turbulent cryogenic nitrogen jets at transcritical and supercritical conditions are numerically investigated by LES-based multi-environment probability density function method together with the tabulated thermodynamic properties. To account for the non-ideal thermodynamic effects, the mixture properties are calculated by using Soave-Redlich-Kwong equation of state. The effect of sub-grid scale fluctuations of thermodynamic properties is modelled by the multi-environment PDF. Numerical results indicate that the present multi-environment PDF approach has the prediction capability to capture the essential features of cryogenic nitrogen jets under the supercritical pressure. Comparing with the supercritical jet, the transcritical jet features the longer and denser core which is relatively resilient to core destruction caused by turbulence. It is also found that the stronger pseudo-boiling strength and the larger density difference are directly tied with the longer cold-core length. In terms of the sub-grid scale scalar fluctuation, the transcritical jet shows the larger magnitude than the supercritical jet. In the transcritical nitrogen jet, it is identified that the strong sub-grid fluctuations of density and isobaric specific heat are closely tied with the pseudo boiling process. These numerical results suggest that the high-fidelity numerical modelling for transcritical flows must have the capability to realistically represent the combined effects of turbulence and thermodynamic nonlinearities, sub-grid scale fluctuations, and scalar conditional statistics.

Empirical investigation to explore potential gains from the amalgamation of phase changing materials (PCMs) and wood shavings

• Panels containing PCM, Wood-shaving and PCM/wood-shavings amalgams are created. • Panels are subjected to heat profiles and their thermal performances were compared. • Heat peak reduction, heat peak shift and fluctuation of temperatures are examined. • Amalgamation of wood-shavings did not improve the PCM's thermal performance. • The overall weight of the panels was reduced due to amalgamating wood-shavings. The reduction of gained heat, heat peak shifting, and the mitigation of air temperature fluctuations are some desirable properties that are sought after in any thermal insulation system. It cannot be overstated that these factors, in addition to others, govern the performance of such systems thus their effect on indoor ambient conditions. The effect of such systems extends also to Heating, Ventilation and Air-conditioning (HVAC) systems that are set up to operate optimally in certain conditions. Where literature shows that PCMs and natural materials such as wood-shavings can provide efficient passive insulation for buildings, it is evident that such approaches utilise methods that are of a degree of intricacy which require specialist knowledge and complex techniques, such as micro-encapsulation for instance. With technical and economic aspects in mind, an amalgam of PCM and wood-shavings has been created to be utilised as a feasible thermal insulation. The amalgamation was performed in the simplest of methods, through submerging the wood shavings in PCM. An experimental procedure was devised to test the thermal performance of the amalgam and compare this to the performance of the same un-amalgamated materials. Comparative analysis revealed that no significant thermal gains would be expected from such amalgamation. However, a significant reduction in the total weight of the insulation system would be achieved that, in this case, shown to be up to 20.94%. Thus, further reducing possible strains on structural elements due to the application of insulation on buildings. This can be especially beneficial in vernacular architectural approaches where considerably large amounts and thicknesses of insulations are used. In addition, cost reduction could be attained as wood shavings are significantly cheaper compared to the cost of PCMs.