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In thermodynamics, it is essential to distinguish between state functions and process functions. The reason is that the simple compressible thermodynamic system is a bivariate-process system, and the change of internal energy, a state function, corresponds to two process functions, heat and work. Among the state functions in thermodynamics, entropy is a special one because it has to be defined through a process function, exchanged heat δ Q , and a unique factor of integration, 1/T. In heat transfer, it is shown that Fourier's law and the differential equation of heat conduction are both relations of state quantities alone, and process quantities appear when an integration with respect to time is applied. Moreover, an incompressible heat conduction medium element without conversion between heat and work is a univariate-process system governed by a single variable, temperature. In this case, the change of the thermal energy (“heat content”) stored in the system, a state quantity as a function of T alone, corresponds to only one process quantity, the transferred heat. Therefore, on the one hand, it is unnecessary to strictly distinguish between state quantities and process quantities in heat transfer, and on the other hand, there is no need to use a factor of integration to prove entransy a state quantity in heat transfer. Thermodynamics and heat transfer are two parallel sub-disciplines in thermal science. It is incorrect to deny entransy as a state quantity in heat transfer by the uniqueness of the factor of integration for entropy in thermodynamics, and entransy has significant physical meaning in the analysis and optimization of heat transfer processes.

Exploring the thermal transport properties of C2N under multiple regulatory mechanisms.

The thermal management of lithium-sulfur batteries with high specific energy has become one of the critical issues for their applications. Carbon-based nanotubes are widely used to construct composite sulfur cathodes. This paper focuses on the thermal transport properties of sulfur-coated and sulfur-embedded boron carbide nanotubes (BCNTs) and carbon nanotubes (CNTs) and their composites using molecular dynamics. It is found that phonon softening and localization play a role in making BCNT exhibit a lower thermal conductivity (TC) than CNT. Furthermore, it is discovered that the sulfur embedded inside the carbon-based nanotube has a greater negative impact on carbon-based nanotube phonon transport. Moreover, the effective medium theory model is not suitable for predicting the effective thermal conductivity of coated sulfur composites, in contrast to its good applicability to embedded sulfur composites. These findings provide an in-depth understanding of the thermal transport properties of composite sulfur cathodes in lithium-sulfur batteries.

The uniform distribution of certain physical quantities or functions, referred to as the corresponding equipartition principles, can be reflected in the optimization design of many physical processes. In this paper, three equipartition principles for the optimization of two- and three-stream heat exchangers are discussed, including the uniformity principle of thermodynamic force field, uniformity principle of the field of Δ T / T , and uniformity principle of temperature difference field. The main problems are highlighted for the first two principles. It is demonstrated that for a fixed heat transfer rate, the two principles cannot lead to the minimum entropy-generation rate or the highest heat exchanger effectiveness, and they cannot be satisfied when the best heat transfer performance is achieved. For the last principle, whether the heat transfer coefficient between cold and hot fluids is constant or not, it can be directly proved using the entransy theory, and theoretical derivation shows that for a fixed ratio of heat capacity flow rates and the number of heat transfer units, a larger uniformity factor of temperature difference field gives larger heat exchanger effectiveness. It is also demonstrated that when the temperature difference field is completely uniform, the entransy-dissipation-based thermal resistance has the minimum value. Therefore, the last principle can give the optimal heat transfer performance for heat exchangers. Some simple numerical examples for the application of the three principles are also presented, and the results directly verify the theoretical analyses. Theoretical analyses and simple examples show that the uniformity principles of the thermodynamic force field and the field of Δ T / T are obtained based on the entropy-generation concept and the uniformity principle of temperature difference field is derived with the entransy dissipation concept. The former concept describes the irreversibility of heat transfer from the viewpoint of the ability to do work, and the latter concept measures irreversibility from the viewpoint of heat transfer ability. This is why the first two principles are not consistent with heat transfer optimization, whereas the last principle is. Additionally, the heat transfer law does not conform to the actual heat transfer processes in the derivation of the first principle. Therefore, the first two principles are unsuitable for heat exchanger optimization, whereas the third principle is suitable; the statement that the uniformity principle of the temperature difference field is a duplicate of the uniformity principle of the field of Δ T / T for balanced counter-flow heat exchangers is unscientific and incorrect.

A newly developed frosting simulation method, pseudo-VOF (p-VOF) method, has applied to simulate the dynamic frost formation on the NACA0012 airfoil under strong convection. The p-VOF method is a pseudo volume of fraction simulation method of the multiphase flow with phase change. By solving a simplified mass conservation equation explicitly instead of the original volume fraction equations in CFD software, the efficiency and robustness of calculation are greatly improved. This progress makes it possible to predict a long-time frost formation. The p-VOF method was successfully applied to the simulation of dynamic frosting on the two-dimensional NACA0012 airfoil under strong convection conditions with constant frost physical properties. The simulation result shows that the average thickness of the frost layer increases, and the frost bulges and flow separation appear earlier, when the airfoil surface temperature decreases or the air humidity increases. The frost bulges and flow separation appear earlier, when the air velocity is faster, the growth rate of the frost layer at the early stage is greater, but the final frost layer is thinner.

We consider an ℝd-valued discrete time branching random walk in an independent and identically distributed environment indexed by time n ∈ ℕ. Let Wn(z) (z ∈ ℂd) be the natural complex martingale of the process. We show necessary and sufficient conditions for the Lα-convergence of Wn(z) for α > 1, as well as its uniform convergence region.

Because of its high specific surface area, metal foam is expected to further enhance the flow and thermal performance of a minichannel heatsink. In this study, fully developed flow in minichannels with porous fins was studied. The porous geometry was constructed based on Laguerre-Voronoi tessellation and simulated in a pore scale, which eliminated the influence of parameter selection in a macroscale method. The influence of porosity, filling ratio, and Reynolds number on velocity field and pressure drop was analyzed. The general relationship between the Poiseuille number and filling ratio was determined, and a correlation was obtained using the study results. The results of this study were compared with those of the macroscale method, and finally, the influence of effective viscosity was discussed.

The entransy theory is widely used and found to be effective in thermal analyses and optimizations. Some researchers considered the entransy variation due to viscous heating as part of entransy dissipation and analyzed convective heat transfer based on the differential relationship between entropy and entransy. However, it has been pointed out that the derivation of the differential relationship between entropy and entransy is incorrect. In this paper, the convective heat transfer processes with viscous heating is reconsidered and analyzed from the viewpoint of the energy conservation and the entransy balance equation. It is shown that the influence of the viscous heating is equivalent to that of an inner heat source. Therefore, the contribution of viscous heating to system entransy should not be treated as part of entransy dissipation, but entransy flow into the system. Two-stream parallel and counter flow heat exchangers with viscous heating and a thermal insulation transportation problem of heavy oil are taken as examples to verify the theoretical analyses intuitively. In the examples, the numerical results show that the entransy dissipation rates could be negative when the influence of the viscous heating on the system entransy is treated as part of the entransy dissipation. This is obviously unreasonable. Meanwhile, when the entransy contribution from the viscous heating to the system entransy is treated as entransy flow into the system, it is shown that the entransy dissipation rate is always positive, and the heat transfer processes can be well explained with the entransy theory.

Modeling and simulation of thermal-fluid systems are very important in industrial numerical simulation and play key roles in their design and control. In this paper, the modeling and simplification method of one-dimensional thermal-fluid network with variable-property are presented, including matrix representation of the network, simplification algorithm for series/parallel connection based on matrix operation and generation of flow equations based on system topology. This simplification is suitable for the simulation of thermal-fluid systems with arbitrary topological structure. The method to treat reflux during iteration is proposed. The outstanding features of the simplification algorithm are the significant reduction in the thermal-fluid network and therefore the number of the related governing equations, as well as the computation burden. The example in this paper shows that the number of the governing equations for flow is reduced by about 45% and the calculation time of flow calculation is reduced by an average of 32% after the simplification.

In this paper, the entransy functions for steady heat transfer are summarized and discussed based on the variational theory and the entransy theory. The entransy functions for steady convective heat transfer are derived for the first time. In steady heat transfer processes, it is shown that the steady distributions of heat flux and temperature (radiative thermal potential) should make the corresponding entransy functions reach their minimum values when the temperature (radiative thermal potential) or the heat flux of the boundary is given. The extremum entransy dissipation principles and the minimum entransy-dissipation-based thermal resistance principles are compared with the entransy functions It is shown that the entransy functions can describe a steady state, but cannot directly give a way to optimize heat transfer processes, while the extremum entransy dissipation principles and the minimum entransy-dissipation-based thermal resistance principles act in an opposite way.

Recently, the analogy between the processes of heating a solid body and charging an electric capacitor was deemed as “heat-work analogy” and argued that it was false and violated the second law of thermodynamics. It was claimed that the two processes were not analogous because the process of heating a solid body was classified as a purely thermal system, while that of charging an electric capacitor was classified as a purely mechanical system. In response, this note discusses the analogy between heat and electric conductions and that between heating a solid body and charging an electric capacitor. It is shown that the analogies have nothing to do with heat-work conversion, and never violate the second law of thermodynamics. The classification of charging an electric capacitor as a purely mechanical system is inappropriate. It is also shown that what we concern in the analogy analysis are the commonalities between the processes, rather than the differences.

Spay cooling is a complicated flow and heat transfer process affected by multi-factors among which the environmental pressure is extremely important. However the influence of pressure is not investigated sufficiently, especially the reduced pressure. In the present study, spray cooling under low initial environmental partial pressures and vapor partial pressures with R21 are investigated with a closed spray and condensation system. To study the influence of initial environmental partial pressure, different amounts of nitrogen are inflated into the vacuum flash chamber, while the vapor partial pressure is kept constant. To study the influence of vapor partial pressure, a cascade refrigerator is used to condense the vapor with different condensation temperatures so that the vapor partial pressure can be maintained or adjusted, while the initial environmental partial pressure is kept constant. The experimental results show that the spray cooling power increases monotonically with the increasing spray flow rate in the experimental range, while the cooling efficiency decreases with the increasing spray flow rate. The spray cooling power and cooling efficiency vary with the initial environmental partial pressure or the vapor partial pressure non-monotonously, which indicates there is an optimal pressure for the heat transfer performance. Besides, the mechanism of the non-monotonous variation trend is discussed based on the key aspects including flash evaporation, convection and boiling. Especially, the boiling heat transfer curve is applied to explain the trend.

The entropy generation minimization and the entransy theory are widely used in the optimization of heat transfer. In this paper, the two theories are applied to coupled steady heat transfer systems. The two different definitions of radiative entransy flux are discussed. It is found that the extremum principle of entransy dissipation for coupled heat transfer systems can be obtained only when temperature is treated as the driving force of radiative heat transfer and the definition of entransy flux for radiative heat transfer is the same as that in conductive and convective heat transfer. Taking temperature as the driving force of radiative heat transfer, we have analyzed three coupled heat transfer examples, which are the one-dimensional coupled conductive and radiative heat transfer, the coupled convective and radiative heat transfer and the coupled conductive, convective and radiative heat transfer. The results show that the extremum principle of entransy dissipation always leads to the best system performance, while the entropy generation minimization does not always. The definition of radiative entransy flux in which the blackbody emissive power is treated as the driving force is also used for the three examples. However, the results show that neither the extremum radiative entransy dissipation rate nor the extremum conductive/convective entransy dissipation rate results in the best system performance. Therefore, this definition is not suitable for coupled heat transfer systems.

Let ( N , A 1 , A 2 , … ) be a sequence of random variables with N ∈ N ∪ { ∞ } and A i ∈ R + . We are interested in asymptotic properties of non-negative solutions of the distributional equation Z = (d) ∑ i = 1 N A i Z i , where Z i are non-negative random variables independent of each other and independent of ( N , A 1 , A 2 , … ) , each having the same distribution as Z which is unknown. For a solution Z with finite mean, we prove that for a given α > 1 , P ( Z > x ) is a function regularly varying at ∞ of index − α if and only if the same is true for P ( Y 1 > x ) , where Y 1 = ∑ i = 1 N A i . The result completes the sufficient condition obtained by Iksanov & Polotskiy (2006) on the branching random walk. A similar result on sufficient condition is also established for the case where α = 1 .

Open-cell metal foam mini-channel evaporator (OMFME) is proposed to enhance heat transfer in electronics cooling. In present study, flow boiling in an OMFME is investigated using R134a. The OMFME has nine channels, which are 507 μm in width and 1097 μm in depth. Comparing with a mini-channel evaporator (ME) with the same footprint area, the heat transfer coefficient (HTC) is enhanced up to 1.5 times and the dry out is delayed. Flow patterns are presented and compared with an existed flow patterns map. HTCs generally increases with the quality, the heat flux and it is slightly influenced by the mass flow rate. Nucleate boiling and evaporation in the porous fins are the two heat transfer mechanisms in different quality range. A correlation is presented to calculate frictional pressure drop.

The transient heat transfer performance of spray cooling with R21 is studied experimentally. Three variation patterns of surface temperature depending on the initial surface temperature are observed. There is only one rapid temperature drop at lower initial surface temperature and two drops with a shoulder transition at higher initial surface temperature. Further increasing the initial surface temperature will deteriorate heat transfer. Based on the pool boiling heat transfer curve and one-dimensional transient heat conduction model, the temperature variation patterns and processes are discussed which relate to nucleate boiling, transition boiling and film boiling, respectively. The experimental results reveal that the deterioration temperature decreases with increase of the heating flux and nozzle-to-heater distance, while increases with the spray flow rate. Based on the experimental results a region map of temperature variation patterns in transient spray cooling is given.

In this paper, the performance of a concentrating photovoltaic/thermal solar system is numerically analyzed with a mathematical and physical model. The variations of the electrical efficiency and the thermal efficiency with the operation parameters are calculated. It is found that the electrical efficiency increases at first and then decreases with increasing concentration ratio of the sunlight, while the thermal efficiency acts in an opposite manner. When the velocity of the cooling water increases, the electrical efficiency increases. Considering the solar system, the surface of the sun, the atmosphere and the environment, we can get a coupled energy system, which is analyzed with the entropy generation minimization and the entransy theory. This is the first time that the entransy theory is used to analyze photovoltaic/thermal solar system. When the concentration ratio is fixed, it is found that both the minimum entropy generation rate and the maximum entransy loss rate lead to the maximum electrical output power, while both the minimum entropy generation numbers and the maximum entransy loss coefficient lead to the maximum electrical efficiency. When the concentrated sunlight is not fixed, it is shown that neither smaller entropy generation rate nor larger entransy loss rate corresponds to larger electrical output power. Smaller entropy generation numbers do not result in larger electrical efficiency, either. However, larger entransy loss coefficient still corresponds to larger electrical efficiency.

Experiments are carried out on the frost and defrost characteristics of fine tube bundles under coolant temperature between −20 and −58 °C. The diameter of the tube is 2 mm. The frost and defrost processes are observed by taking photos and measuring pressure drops. The results indicate that the frost layer thickness on the tube surface increases with decreasing coolant temperature, but decreases with increasing air humidity. A model is given to explain the variation of frost thickness. The experiments verified that spraying ethanol is effective on inhibiting the formation of the frost.

The analysis and the design of turbojet engines are of great importance to the improvement of the system performance. Many researchers focus on these topics, and many important and interesting results have been obtained. In this paper, the thermodynamic cycle in a turbojet engine is analyzed with the entransy theory and the T-Q diagram. The ideal thermodynamic cycle in which there is no inner irreversibility is analyzed, as well as the influences from some inner irreversible factors, such as the heat transfer process, the change of the component of the working fluid and the viscosity of the working fluid. For the discussed cases, it is shown that larger entransy loss rate always results in larger output power, while smaller entropy generation rate does not always. The corresponding T-Q diagrams are also presented, with which the change tendencies of the entransy loss rate and the output power can be shown very intuitively. It is shown that the entransy theory is applicable for analyzing the inner irreversible thermodynamic cycles discussed in this paper. Compared with the concept of entropy generation, the concept of entransy loss and the corresponding T-Q diagram are more suitable for describing the change of the output power of the analyzed turbojet engine no matter if the inner irreversible factors are considered.

The influence of gravity on flow boiling poses a challenge to the development of two-phase thermal systems for space applications. The present study investigates the heat transfer coefficient of flow boiling in a mini-channel evaporator in both horizontal and vertically downward flow directions to observe the effect of gravity. The evaporator has 23 channels that are 624 μm in width and 923 μm in depth. The experiments show that nucleate boiling dominates the heat transfer. The heat transfer and flow patterns for the two directions differ when the mass flow rate is less than 264.3 kg/m2 s and the heat flux is less than 3.0 W/cm2. The experimental results are compared with five general correlations. The outlet Froude number is more suitable for correlating the results than existing gravity-independent criteria. Gravity may influence the heat transfer through instabilities and changes in the flow patterns.

In this paper, the preconditions for the extensions of the entransy theory are summarized and discussed. As the physical meaning of entransy is the “potential energy” of thermal energy, the concepts and analyses method of the entransy theory can be extended based on the viewpoint of potential energy. The extension to microeconomics is taken as an example to show how the entransy theory is extended. The concept of economic entransy is defined based on the analogy analysis between heat transfer and microeconomics, and the indirect tax is found to be the “potential energy” of goods or services that the government takes from the market. With the extension, a new viewpoint is introduced to understand and analyze the microeconomic system. If the irreversibility in nature makes the potential energy in one system can never increase automatically, the entransy decrease principle and the corresponding equilibrium criteria can further be extended. Furthermore, if the mathematical expressions of the governing equations of the analyzed physical or social phenomena in the system are the same as those in heat transfer, the principle of least action and the optimization principles in the entransy theory can be extended. More similarities between heat transfer and the other phenomena result in more extensions of the entransy theory.

We prove necessary and sufficient conditions for the L p -convergence, p > 1 , of the Biggins martingale with complex parameter in the supercritical branching random walk. The results and their proofs are much more involved (especially in the case p ∈ ( 1 , 2 ) ) than those for the Biggins martingale with real parameter. Our conditions are ultimate in the case p ≥ 2 only.

Partial coalescence is a complicated flow phenomenon. In the present study, the coalescence process is simulated with the volume of fluid (VOF) method. The numerical results reveal that a downward high-velocity region plays a significant role in partial coalescence. The high-velocity region pulls the droplet downward continuously which is an important factor for the droplet turning into a prolate shape and the final pinch-off. The shift from partial coalescence to full coalescence is explained based on the droplet shape before the pinch-off. With the droplet impact velocity increasing, the droplet shape will get close to a sphere before the pinch-off. When the shape gets close enough to a sphere, the partial coalescence shifts to full coalescence. The effect of film thickness on the coalescence process is also investigated. With large film thickness, partial coalescence happens, while with small film thickness, full coalescence happens. In addition, the results indicate that the critical droplet impact velocity increases with the increase of surface tension coefficient but decreases with the increase of viscosity and initial droplet diameter. And there is a maximum critical Weber number with the increase of surface tension coefficient and initial droplet diameter.

With the aim to improve overall thermal-hydraulic and thermodynamic performance of parabolic trough solar receiver (PTR), a new configuration with partial insertion of gradient metal foam (GMF) is proposed. The effects of GMF with pore density gradient or porosity gradient on turbulent forced convection heat transfer, flow resistance, entropy generation rate and exergetic efficiency of PTR tube are numerically analyzed. The distribution of non-uniform heat flux on receiver tube wall is calculated by Monte Carlo Ray-Trace Method, and the local thermal non-equilibrium model is used, which considers the temperature difference between metal skeleton and fluid. The numerical method is validated by experimental data. The results show that GMF enhances the heat transfer of PTR tube, with Nusselt number being increased by 43.7%–812.6%; brings the increase of flow resistance, with friction factor being 4.2 to 16.7 times of empty tube; improves the overall thermal-hydraulic performance with performance evaluation criteria ranging from 1.5–3.6. The GMF causes maximum reduction of total entropy generation rate and maximum enhancement of exergetic efficiency to be 92.6% and 24.4%, respectively. The PTR tube partially filled with GMF can obtain higher thermal-hydraulic and thermodynamic performance than that with uniform metal foam.

Droplet impact onto thin film is a ubiquitous phenomenon in many industrial applications. In the present study, the VOF method is adopted to simulate the crown evolution during the impact process with different surface tension coefficient, viscosity and gravity. Especially, the crown behavior is investigated in the energy aspect. The kinetic energy, gravitational potential energy, surface energy and viscous dissipation are taken into consideration. The results reveal that higher surface tension coefficient leads to larger crown diameter. The evolution of gravitational potential energy can be divided into three phases: decrease phase, increase phase and collapse phase. Similarly, the evolution of surface energy can be divided into four phases: increase phase, drop phase, flat phase and collapse phase. The drop phase is caused by the film rupture in the center zone. In the case with zero gravity the flat phase is replaced by a second increase phase. In particular, it is found that larger viscosity will postpone the film rupture and the film even doesn't break at high viscosity.

Silicone rubber is widely used as a kind of thermal interface material (TIM) in electronic devices. However few studies have been carried out on the thermal conductivity mechanism of silicone rubber. This paper investigates the thermal conductivity mechanism by non-equilibrium molecular dynamics (NEMD) in three aspects: chain length, morphology, and temperature. It is found that the effect of chain length on thermal conductivity varies with morphologies. In crystalline state where the chains are aligned, the thermal conductivity increases apparently with the length of the silicone-oxygen chain, the thermal conductivity of 79 nm-long crystalline silicone rubber could reach 1.49 W/(m⋅K). The thermal conductivity of amorphous silicone rubber is less affected by the chain length. The temperature dependence of thermal conductivity of silicone rubbers with different morphologies is trivial. The phonon density of states (DOS) is calculated and analyzed. The results indicate that crystalline silicone rubber with aligned orientation has more low frequency phonons, longer phonon MFP, and shorter conducting path, which contribute to a larger thermal conductivity.

Entransy method is developed for the analyses of thermal processes with variable thermophysical properties. The entransy and enthalpy entransy balance equations for thermal processes with variable thermophysical properties are derived based on the definitions of differential entransy and enthalpy entransy. The definitions of the entransy and enthalpy entransy for an object are presented. The optimization principles for heat transfer and heat-work conversion processes with variable thermophysical properties are discussed with the concept of entransy. Entransy analyses for a two-stream heat exchanger and a closed Brayton cycle with variable thermophysical properties are conducted and compared with the entropy generation analyses. For the heat exchanger, the results show that both the minimum entransy-dissipation-based thermal resistance and the minimum revised entropy generation number correspond to the maximum heat exchanger effectiveness when the inlet condition of the heat exchanger is prescribed, but the minimum entropy generation rate and the minimum entropy generation number do not. For the closed Brayton cycle, the results show that both the maximum entransy loss rate and the minimum entropy generation rate correspond to the maximum output power and both the maximum entransy loss coefficient and the minimum entropy generation number correspond to the maximum heat-work conversion efficiency when the inlet conditions of the streams and the environmental temperature are prescribed. When the inlet temperature of the hot stream increases, larger entransy loss rate and larger entransy loss coefficient still correspond to larger output power and larger heat-work conversion efficiency respectively.

Solid–liquid systems widely exist in micro- and nanodevices, and it is necessary to study the heat transfer mechanism through solid–liquid interfaces. An asymmetrical sandwich structure of a solid–liquid system consisting of liquid argon and artificial solid walls that have the same FCC structure with argon but a different atomic masses is composed. Heat transfer characteristics are investigated by the molecular dynamics method. The interaction strength between a liquid and solid plays an essential role in heat transport at solid–liquid interfaces, and the thermal resistance length is inversely proportional to it. The mass arrangement of artificial solid walls also has a significant effect on heat transport as well. A maximum heat flux comes up due to the mismatch in phonon spectra with the increasing atomic mass of one solid wall. The asymmetrical liquid density profiles are obtained with various mass differences between solid walls. Especially, a thermal rectification effect is observed and the magnitude is inextricably bound up with asymmetry.

The advanced thermal management system (TMS) with electrically-controlled pumps, fan and valves for diesel engine was investigated with the aim of enhancing the engine performance by providing good thermal conditions and reducing its parasitic power consumption. A one-dimension, transient numerical model for the advanced TMS was established. Based on the model, the control strategies for the combined control of the coolant circuit and oil circuit in the advanced TMS were proposed and evaluated. A feedforward and feedback control strategy was proposed for the electrical pumps and fan, and a feedback control strategy was proposed for the electrical valves. In addition, a control strategy for the engine warm-up process was also proposed for the rapid warming up of the engine. Comparisons between the present advanced TMS and the conventional one under the European transient cycle (ETC) show that the advanced TMS with the present control strategies has an excellent ability to control the engine working temperature, and the total power consumption of the advanced TMS during the ETC can reduce 57.0% compared with the conventional system, which is due to the flexible control of the electrical components in the system.

Entransy balance equations and the corresponding optimization principles for heat conduction, heat convection and thermal radiation have been established. This paper develops the entransy balance equation for a low speed steady flow system with vaporization processes. The definition of enthalpy entransy for a substance from solid state to vapor state is proposed. The expressions of entransy and entransy dissipation are derived with differential volume element integration and the concept of enthalpy entransy. The heat transfer between a vaporization stream and a hot stream in a heat exchanger is also analyzed with the entransy approach. The theoretical results show that the minimum entransy-dissipation-based thermal resistance always leads to the maximum heat exchanger effectiveness. A numerical example compared with the entropy generation approach verifies the theoretical analyses. For a heat exchanger network undergoing vaporization processes, the minimum entransy-dissipation-based thermal resistance is proved to be applicable as the optimization criteria for the maximum heat exchange rate. The minimum entransy-dissipation-based thermal resistance principle is applied to two numerical cases for the thermal conductance distribution optimization to obtain the maximum heat exchange rate.

To study the concentration characteristics of mechanical vapor compression (MVC) technology on the wastewater concentration process, a single-effect MVC system for concentrating dimethylacetamide wastewater was investigated. The performance of the MVC experimental system was evaluated under different levels of distillation pressure and concentrations of the raw solution. We demonstrated that under conditions of atmospheric distillation (evaporating temperature was 100 °C), the COP and distilled water production rate of the experimental system reached 20.17 and 70.80 kg/h, respectively. In the distillation operating cycle, the DMAC solution was concentrated from 6% to 83.5%. In addition, when the system was under vacuum conditions (evaporating temperature was 55.8 °C), the COP and distilled water production rate were 5.44 and 18.2 kg/h, respectively, and the concentration of the DMAC dilute solution was only 0.2%.

The electrical power generation from low temperature heat source attracts more and more attentions but the temperature mismatching between the heat sources and working medium in the organic Rankine cycle (ORC) becomes an issue. The organic flash cycle (OFC) is an effective solution to this issue. In this paper, the OFC is analyzed by the concept of entransy loss and the T-Q (temperature-heat flow rate) diagram for the heat-work conversion. The equations for cycles of the basic OFC and the OFC whose heat source is the exhaust gas of the turbine in a Brayton cycle (the combined cycle) are derived theoretically and the results indicate that larger entransy loss rate leads to larger output power with prescribed inlet parameters of the hot stream in the discussed cases, which is displayed by the T-Q diagram intuitively. Two numerical examples demonstrate that the optimal mass flow rate of the working medium for the maximum entransy loss rate is the same as that for the maximum output power. The T-Q diagram analyses is in accordance with the numerical results. The concept of entransy loss can be used as the criteria for the OFC optimization.

Superlattices have great application potentials in thermo-electric materials and solid laser techniques. Their complicated heat transport mechanisms due to size effects, multi-interfaces and mini-band are very important issues for the prediction of thermal conductivity. In this paper, the short-period Si/Ge superlattices are investigated by a modified series model based on the Debye-Callaway description. The Lambert Law is used to describe the phonon emission within hemisphere space. In addition, the phonon interface transmission coefficients obtained from the phonon wave packet simulation are incorporated into boundary condition of the model, which removes the fitting parameters in the model. Better agreement with experiment is obtained. The effects of temperature, wavelength-dependent phonon transmission, superlattice periods, as well as the thickness of Ge layer are considered in this paper.

The analyses of thermal systems are necessary because of the energy situation. In this paper, two kinds of thermal systems with work, a common heat–work conversion system and a common heat pump system, are analyzed with the entransy theory. The entransy terms and variations for thermal system with heat–work interaction are discussed. The concept of entransy variation associated with work is proposed. The results show that the entransy variation associated with work is directly related to output work and has the same change tendency as that of the work.

The heat transfer during spray cooling was studied experimentally using deionized water to investigate the spray characteristics and the differences between spray cooling on a smooth silicon surface and micro-, nano- and hybrid micro/nano-structured surfaces. The spray cooling experiments show that the heat transfer rates were better for the nano-structured surface, followed by the smooth surface coated with the SiO2 film and the pure silicon surface since the contact angle was smallest on the nano-structured surface and increased on the latter two. The droplet parameter results show that most droplets were 40–60 μm in size. The heat transfer coefficient increased and the wall temperature decreased on the 25G × 25S surface coated with the SiO2 film compared with the 50G × 50S surface coated with the SiO2 film as the heat transfer moved into the partial dryout region due to the SiO2 film’s stronger hydrophilicity so the heated area was more fully utilized, while the CHF was larger for the 50G × 50S surface. Coating the micro-structured surfaces with carbon nano-tube (CNT) films having characteristic sizes smaller than the droplet size was more effective than on the surfaces with larger characteristic sizes. The CHF was largest on the 25G × 25S surface coated with 4 carbon nano-tube films with a 75.3% increase over the smooth surface. The wall temperature increase and the temperature fluctuations were small in the boiling regime as the power increases for the enhanced surfaces.

Due to the relative position invariance of L2 Libration point in the Earth-moon system, Halo orbits around LL2 adapt to the best mission trajectory for Lunar Relay Satellite, which can offer relay communication for space missions such as deep-space exploration and Moon outpost construction. In order to improve the payload launch capacity of Lunar Relay Satellite, the electric thrusters which have high specific impulse and low thrust can be used as main propulsion. In this paper, the Q-law control theory in optimal control theory is adopted to calculate the optimal transfer orbit of Lunar Relay Satellite using electric thrusters. In addition, the effect of initial orbit of transfer trajectory and invariant manifold has been considerate. The result shows the design rules and the algorithm is of great important engineering meaning.