Ideal Heat Research Articles

A new prediction equation of compressed liquid isochoric heat capacity for pure fluids and mixtures

In this paper, a new prediction equation for compressed liquid isochoric heat capacity (cv) was developed based on the corresponding states principle (CSP). By knowing acentric factor (ω), critical parameters (Tc, pc and vc) and ideal gas isochoric heat capacity (cv,0), the new equation could predict cv for pure fluids and mixtures (including polar and nonpolar fluids) at 0.25 < Tr < 1 and 0.5 < pr < 10. The overall average absolute relative deviation (AARD) of the new equation for 25 pure fluids and 10 mixtures are 2.21% and 1.62%, respectively. A comparison was conducted between the new equation and other models (PR EoS, Helmholtz energy EoSs, generalized equation (GE) of Zhong et al. [1] and corresponding states equation (CSE) of Sheng et al. [2]). The new equation significantly improves the prediction performance for the heat capacity of important fluids, such as halogenated hydrocarbons, carbon dioxide, water and so on, compared with the existing models. Although the prediction performance of the new equation is slightly worse than that of the Helmholtz energy EoS, the new equation has a simpler form and more powerful prediction performance than the Helmholtz energy EoS for mixtures.

Prediction of HAZ Microstructure and Hardness for Q960E Joints Welded by Triple-Wire GMAW Based on Thermal and Numerical Simulation.

Since heat affected zone (HAZ) is the weak area of welded joints, this article proposes a method to predict the HAZ microstructure and hardness for the triple-wire gas metal arc welding (GMAW) process of Q960E high strength steel. This method combines welding thermal simulation and numerical simulation. The microstructures and hardness of Q960E steel under different cooling rates were obtained by thermal simulation and presented in a simulated HAZ continuous cooling transformation (SH-CCT) diagram. The cooling rate in HAZ were obtained by numerical simulation with ANSYS software for the triple-wire welding of Q960E thick plates. By comparing the cooling rate with the SH-CCT diagram, the microstructure and hardness of the HAZ coarse-grained region were accurately predicted for multiple heat input conditions. Further, an ideal heat input was chosen by checking the prediction results. This prediction method not only helps us to optimize the welding parameters, but also leads to an overall understanding of the process-microstructure-performance for a complex welding process.

The Local Distribution of Temperatures and Entropy Generation Rate in an Ideal Counterflow Heat Exchanger

The process of heat exchange between two fluids of different temperatures and separated by a solid wall occurs in many engineering applications. Log mean temperature difference and effectiveness-NTU methods are widely used to assist in the design of heat exchangers. However, the two methods focus on overall analysis and cannot show the local temperature distributions. This paper obtains the mathematical solutions to the temperature profiles in an ideal counterflow heat exchanger. The aim of this research is to explain the phenomenon called the “entropy generation paradox”, which indicates a discrepancy between effectiveness and optimal entropy generation. The theoretical analysis reveals that the temperature curves are exponential functions when the heat capacity rates of the two streams are different; otherwise, the curves are linear functions. A heat exchanger is demonstrated to draw the temperature profiles under different working conditions. Local entropy generation rates are determined by the ratio of local stream temperatures in the form of a hook function. To realize a certain heat duty, there are many stream flow rate couples, and each couple results in a different entropy generation profile and obtains a corresponding total entropy generation. The helical steam generator of a high-temperature gas-cooled reactor is analyzed in this article and the principle of equipartition of entropy generation is confirmed. This principle indicates that, among the many working conditions to achieve a certain heat duty, a heat exchanger characterized by a nearly constant entropy production gives the best second law efficiency possible in order to achieve the best energy conversion.

Performance analyses and optimizations of desiccant wheel-assisted atmospheric water harvesting systems based on ideal thermodynamic cycles

This study proposes a novel atmospheric water harvesting system that uses desiccant wheels to transfer vapor from one air stream (Adeh) to another (Ahum), and the humidified Ahum is cooled to obtain water. First, the ideal air handling processes of desiccant wheels are introduced, and the heating temperature and ideal humidification efficiency are compared among the three humidification configurations. The system with multi-stage desiccant wheels and independent Adeh performs best, with an ideal humidification efficiency that is 0.4 to 2.2 times higher than that of the other two systems. The ideal water harvesting efficiency of the system is much higher than that of the direct cooling water harvesting method. A two-stage humidification system using actual desiccant wheels is then designed. Driven by ideal heating and cooling systems, water harvesting rates and water harvesting efficiencies are calculated under typical dry and mild working conditions. When the heating temperature is from 40 °C to 90 °C, and the cooling source temperature ranges from 0 °C to 12 °C, diagrams are drawn to assist the selection of the heating temperature and cooling source temperature to obtain the highest water harvesting efficiency for different water harvesting rates. Finally, vapor compression cycles are used as heating and cooling sources, and the performance of the system is compared with that of the direct cooling method. Although this system has higher water harvesting rates, water harvesting efficiencies can be higher when the ambient relative humidity ratio is lower than 60%. With solar thermal energy, water harvesting efficiencies are higher than those of the direct cooling method, especially when the relative humidity ratio is high.

Machine learning prediction of ORC performance based on properties of working fluid

In order to develop machine learning methods for performance prediction of basic ORC (BORC) and regenerative ORC (RORC), thermodynamic properties of working fluids are used to separately model thermodynamic processes including compression, expansion, evaporation and regeneration by artificial neural network (ANN), based on REFPROP calculation data of 106 working fluids. These properties include critical temperature, critical pressure, acentric factor and ideal gas heat capacity. Other cycle performances can be derived from the predictions of established ANNs. Furthermore, for RORC, considering that dry or isentropic working fluid is usually preferred, vapor slope of temperature-entropy saturation curve is modeled by ANN to determine the working fluid behavior. Meanwhile, for ensuring occurrence of heat exchange in regenerator, temperature difference between outlets of turbine and pump is modeled to judge whether temperature difference is larger than the assumed pinch point temperature difference. Based on the obtained predictions for BORC, average absolute deviations (AADs) of net work and thermal efficiency are 1.3928% and 1.5461%, respectively. As for RORC, AADs of vapor slope, temperature difference, net work and thermal efficiency are 2.3659%, 1.2020% 1.8277% and 2.1047%, respectively. Based on the above models, cycle performances of BORC and RORC can be accurately predicted from thermodynamic properties of any working fluid.

주기적으로 작동하는 평행류 열교환기의 해석해와 그 응용

The objective of this study is to present an analytical model for a parallel-flow rotary wheel heat exchanger. The model is based on mathematical solution for an ideal parallel-flow heat exchanger with a periodic inlet temperature condition. Air and wheel matrix temperatures are given in an infinite series of the product of trigonometric and exponential functions. An analytical expression is also presented for the effectiveness of the heat exchanger, which has been validated using literature and numerical data. The model is confirmed to give an exact result when the NTU ratio between hot and cold region is unity. It tends to give a higher error for smaller heat capacity rate ratio as it deviates from the unity. Ranges of parameters are presented with distribution of the maximum absolute error. This study shows that the model is applicable to cross-flow heat exchangers when the split ratio approaches zero.

Experimental Investigation on the Heat Dissipation Performance of Bismuth-Based Alloy Thermal Conductive Sheet

At present, the existing thermal interface materials (TIMs) cannot meet the heat dissipation requirements of some high-power density electronic devices. In this study, Bi-based low melting point alloy was made into a thermal conductive sheet to reduce the interface thermal resistance. The thermal conductivity of a thermal conductive sheet was found to be 37.83 W/(m·K), 10 times higher than Dow Corning 5021 thermal grease. In addition, the surface morphology of the Bi-based alloy thermal conductive sheet was changed in this experiment, which was divided into textured and planer type, and the measured interface thermal resistance values lower than Dow Corning 5021 thermal grease by approximately 30% and 27%, respectively. The results prove this Bi-based alloy thermal conductive sheets have the ideal heat dissipation performance and their wide application prospects in high-power density electronic devices.

Review on sodium acetate trihydrate in flexible thermal energy storages: Properties, challenges and applications

Future energy systems with a large share of fluctuating renewable energies demand thermal energy storages that are flexible and reliable. Sodium acetate trihydrate (SAT) has been investigated for many years as heat storage materials but the focus of the investigations were mostly on short-term applications. SAT has a high energy storage density and a large supercooling degree which make it an ideal flexible heat storage material. Heat storages utilizing stable supercooling of SAT can store heat almost heat loss free in both short-term and long-term, which offers great benefits for the energy system. A review of SAT heat storages is presented in this article. The investigations show that flexible heat storages using SAT have clear advantages over a traditional water based heat storage in both short-term and long-term heat storage. Significant decrease of the storage size (at least 60%) is considered a merit for applications in single family houses where there is a spatial constrain. Moreover, the SAT heat storages offers flexibility in system operation, which is considered beneficial for renewable energy such as solar heating systems. The main challenges for the heat storages, for instance, low heat exchange capacity rate, phase separation and spontaneous crystallization, are addressed. Remedies to overcome the challenges are discussed. The paper will also investigate stability of supercooling and reliability of triggering methods for controllable release of the latent heat stored in supercooled sodium acetate trihydrate. Recommendations for further investigations are proposed.

Pool Boiling Heat Transfer Coefficients in Mixtures of Water and Glycerin

Heat transfer coefficients were investigated for saturated nucleate pool boiling of binary mixtures of water and glycerin at atmospheric pressure in a wide range of concentrations and heat fluxes. Mixtures with water mass fractions from 100% to 40% were boiled on a horizontal flat copper surface at heat fluxes from about 25 up to 270kWm−2. Experiments were carried out by static and dynamic method of measurement. Results of the static method show that the impact of mixture effects on heat transfer coefficient cannot be neglected and ideal heat transfer coefficient has to be corrected for all investigated concentrations and heat fluxes. Experimental data are correlated with the empirical correlation α=0.59q0.714+0.130ωw with mean relative error of 6%. Taking mixture effects into account, data are also successfully correlated with the combination of Stephan and Abdelsalam (1980) and Schlünder (1982) correlations with mean relative error of about 15%. Recommended coefficients of Schlünder correlation C0=1 and βL=2×10−4ms−1 were found to be acceptable for all investigated mixtures. The dynamic method was developed for fast measurement of heat transfer coefficients at continuous change of composition of boiling mixture. The dynamic method was tested for water–glycerin mixtures with water mass fractions from 70% down to 35%. Results of the dynamic method were found to be comparable with the static method. For water–glycerin mixtures with higher water mass fractions, precise temperature measurements are needed.

Infrared radiation and thermal properties of Al-doped SrZrO3 perovskites for potential infrared stealth coating materials in the high-temperature environment

The Al-doped SrZrO3 perovskite powder with low infrared emissivity at high temperatures was prepared. The infrared radiation performance and thermophysical properties of the perovskites at high temperatures were discussed. As a result, the infrared emissivity of the Al-doped SrZrO3 perovskite powder is associated with Al3+-doping content, phase composition and particle morphology. The flaky particles SrZr0.85Al0.15O2.925 formed by heat treatment at 1000 °C for 6 h have the lowest infrared emissivity of 0.245 in 3–5 μm wavebands at 590 °C. The perovskite powder's infrared emissivity is positively correlated with its electrical resistivity and has no apparent change after heating over 800 °C for long-term. The SrZr0.85Al0.15O2.925 perovskite ceramic formed by pressureless sintering still maintains ideal heat insulation performance with the thermal conductivity from 1.17 to 2.21 W m−1 K−1 below 1400 °C. The Al-doped SrZrO3 perovskite tablet exhibits significant weak radiation intensity due to its characteristics of both low infrared emissivity and thermal conductivity at high temperatures.

Heat Capacities and Enthalpies of Normal Alkanes in an Ideal Gas State

The reference literature contains data on the ideal gas-phase heat capacities and enthalpies of substances obtained experimentally. In this brief report, the highly accurate and simple unified analytical dependences of the heat capacity and enthalpy of normal alkanes larger than propane in the ideal gas state on temperature and the number of carbon atoms in a molecule are derived based on the analysis of the structure of chemical groups in the molecules and on the reference values of heat capacity and enthalpy for any two selected normal alkanes at one temperature. The dependences include a single set of coefficients for all normal alkanes and can be applied even to normal alkanes with the number of carbon atoms larger than 20, for which there is no data available. The dependences are thoroughly checked against two different sets of experimental data and two different sets of computational data obtained by low- and high-level theoretical methods: PM6 and G4. The error in calculating the heat capacity and enthalpy in the temperature range 300–1500 K is about 0.1% or less, which is comparable with the error of reference experimental data. Due to the generality of these dependences, one can treat the arising deviations as the indication of possible errors in the sets of reference data used for analysis. Such dependences are useful for estimating the heat capacities and enthalpies of vapors of large n-alkanes with scarce or lacking data. In addition, they can be readily used in computational fluid dynamics applications and provide the accurate interpolation/extrapolation of the tabulated experimental data without using semi-empirical methods of chemical thermodynamics and high-order polynomials.

Investigation of waste EPS foams modified by heat treatment method as concrete aggregate

The method for the recycling of expanded polystyrene foam (EPS) in waste state, which causes a significant environmental pollution, is presented in this study. With this method, modified EPS aggregate was produced by heat treatment applied on 10 × 10 × 10 mm EPS aggregates and the production of a new type of lightweight concrete was aimed by means of being used instead of sand. This disadvantage of EPS aggregate concretes with low compressive strength values has been turned into an advantage through heat treatment method. In this method, the heat treatment process temperatures were determined as 75 °C, 100 °C and 125 °C, while the durations of processes were determined to be 15, 30, 45 and 60 min. The ideal heat treatment temperature and time is determined to be 15 min at 125 °C. 16 samples with various aggregate rates were produced, and the following results were obtained from the tests carried out: i) During 15 min of heat treatment at 125 °C, while the volume change decreased by 12 times, the unit weight increased by 21 times and turning into the form of a synthetic lightweight aggregate. ii) Modified EPS aggregated concretes can be used in building parts where semi-carrier lightweight concrete is required with its strength (with values of 12.66–15.83 MPa up to 40% volumetric ratio. iii) Thermal conductivity coefficients of new concretes vary between 0.154 and 0.481 W/mK. It is conferred that modified EPS aggregate concretes can be used both for insulation purposes and as semi-carrier lightweight concrete.

Extraction Conditions determination for Oryzanol extracted from Rice bran

The extracted Oryzanol under the optimum conditions were determined , Five temperatures were used which are 30, 40, 50, 60, and 70 degrees Celsius and the results showed that the ideal temperature for extraction is 40 Cº and Five heating times were used which are 10, 20, 30, 40, and 50 minutes and the results showed that the ideal heating time for extraction is 20 minutes . As for the volume of the solvent mixture used in the extraction, five sizes were used, which are 15, 35, 55, 75, and 95 ml, and it was found that the best volume for extraction is 95 ml in addition fife ratios of Solvent Hexane : Isopropanol were used for extraction 1:1 , 1:3 , 3:1 , 3.5:0.5 , 0.5:3.5 V/V and the ideal ratio for mixing the hexane solvents: isopropanol 1: 3 volume : volume.

Experimental study on heating performance of pure electric vehicle power battery under low temperature environment

For effective solutions to performance problems, such as discharge capacity attenuation of the pure electric vehicle (EVs) power battery under low temperatures, the heating module of the pulsating heat pipe (TiO2CLPHP) for the power battery was constructed with TiO2 nanofluid as working medium. Moreover, heating performance experiments under different heating strategies were carried out through consideration of the higher efficient bidirectional thermal transfer and thermal conductivity characteristics of the pulsating heat pipe, along with thermal management technology. The results demonstrated that the optimal heat transfer performance of TiO2CLPHP could be achieved with working medium concentration of 2% and heat pipe filling ratio of 50%. In addition, TiO2CLPHP was utilized for the preheating and heat preservation of power batteries, which could highly reduce the fluctuation of voltage during discharge, as well as improve the low-temperature discharge capacity of power batteries, followed by the effective surface temperature uniformity improvement of the battery (maximum temperature difference on power battery surface did not exceed 2.6°C). Furthermore, when TiO2CLPHP was utilized for the power battery heating under continuous heat preservation, the discharge capacity of the power battery reached 58.69Ah, while the capacity enhancement ratio reached up to 56.6%. Therefore, on the basis of the current research, through comprehensive consideration of preheating, heat preservation (preheating time, discharge voltage and discharge capacity) and energy consumption, it proved that the preheating coupled continuous heat preservation was an ideal heating strategy. Through this strategy, rapid heating along with discharge voltage and discharge capacity increase to the maximum extent, the performance of the EVs power battery under low-temperatures was improved.

Flexible Thin Carbon Nanotube Web Film for Curved Heating Elements Under High Temperature Conditions.

Heating elements need a rapid heating property and long-term cycle stability when subjected to extreme temperatures. Carbon nanotube-based films can be used as ideal heating units owing to their superior electrical and thermal properties. However, carbon nanotube polymer composites are not appropriate for extreme conditions such as high temperatures (300 °C) due to the poor thermal stability of the polymer matrix. In this study, we fabricated a carbon nanotube web film, comprising heating elements consisting of pure carbon nanotubes, through the direct spinning method. The carbon nanotube web film has a microscale thickness. The carbon nanotube web film showed flexibility at high temperatures, while a fracture occurred in the case of the carbon nanotube polymer composite. We conducted electrical heating experiments on the curved carbon nanotube web film to observe the heating uniformity and flexibility. The heating test is conducted on various curved form heaters. The carbon nanotube web film showed rapid heating properties and a uniform heat distribution (temperature departure of less than 3%) without thermal aggregation. The curved heating units can be utilized in various applications such as functional clothes and de-icing systems having curved surfaces.

Relationship between the vaporization enthalpies of aromatic compounds and the difference between liquid and ideal gas heat capacities

Understanding connection between physicochemical parameters, molecular structure and heat capacity of liquids remains limited, especially for complex organic molecules. One of possible ways to resolve this problem is analysis of difference between the liquid and ideal gas heat capacities. This difference also defines the temperature dependence of vaporization enthalpy.This work is based on the assumption that the difference between the liquid and gas phase heat capacity is primarily related to intermolecular interactions in the liquid. As the measure of intermolecular interactions, the vaporization enthalpy at 298.15 K was considered. The relationship between the difference of liquid and gas phase heat capacities and the vaporization enthalpy was analyzed for a wide range of aromatic and heteroaromatic compounds. An excellent correlation was observed (residual standard deviation 3.7 J·K−1·mol−1, N = 99). This correlation appeared to be useful for estimation of the vaporization enthalpies of aromatic compounds at elevated temperatures solely from the value at 298.15 K. The calculated and literature vaporization enthalpies measured between 360 and 650 K of 80 compounds from 134 sources were compared. An average absolute deviation of 1.5 kJ·mol−1 was observed, being competitive with the experimental errors of vaporization enthalpy determination.

Machinable boron-doped diamond as a practical heating element in multi-anvil apparatuses.

Being refractory and X-ray transparent, a boron-doped diamond (BDD) heater is considered an ideal heating element in a multi-anvil apparatus under diamond-stable pressures. However, the extremely high hardness of diamond makes it difficult to manufacture a BDD tube, which, in turn, hinders the wide application of BDD heaters in multi-anvil apparatuses. Here, I sintered a machinable BDD (MBDD) from a mixture of BDD powder and pitch (CnH2n+2) by its annealing in Ar at 1273K for 5 h. The BDD powder was bound by a small amount of graphite (<10 wt. %) during the sintering process. Tubes (such as 1.2/0.7/4.0mm in outer/inner diameter/length) can be manufactured from the MBDD block using a lathe or a computer numerical control machine. Due to the low content of graphite in MBDD, the graphite-diamond conversion has a small effect on heating performance. The MBDD heater shows a comparable performance in ultrahigh temperature generation with a high-pressure synthesized BDD heater by generating a temperature higher than 3300K and melted Al2O3 under a pressure of 15GPa. With good heating performance and excellent machinability, MBDD is a practical heating element in multi-anvil apparatuses. The achievement of stable temperature generation over 3300K by the MBDD heater enables various measurements on the physicochemical properties of melts under the Earth's mantle conditions.

Numerical calibration for thermal resistance in discrete element method by finite volume method

Discrete element method (DEM) is a powerful tool for solid granular, while the related heat transfer model requires more calibrations due to the over-simplified heat transfer path. In the present study, the calibration of overall thermal resistances (Rp) was conducted between a particle and the wall, where heat conduction in the particle interior and gap is more remarkable. It was found that, the existing DEM models should be improved in a wide range of the solid-to-gas ratio of thermal conductivity (ks/kg) and the dimensionless body distance (Sx/dp). The ideal heat transfer path may result in larger deviations as ks/kg > 250 or Sx/dp > 0, which can be only mathematically modified by the empirical factor. The general descriptions for modification are still required. To overcome the limit, an effective thermal conductivity model is proposed with certain formulas, where Rp is divided into two parts in the mixing phase zone and the gas zone.

Regeneration units for thermolytic salts applications in water & power production: State of the art, experimental and modelling assessment

Thermolytic solutions are often proposed as high salinity or “draw” stream to generate a chemical potential driving force in Salinity Gradient Power (SGP) and Forward Osmosis (FO) technologies. Depleted “draw” solutions exiting the process can be regenerated by a thermal process powered at very-low grade heat, which is able to decompose the salt into gaseous ammonia and carbon dioxide, which can be stripped and then reabsorbed in the draw solution, restoring its initial concentration. In this work, two different experimental prototypes for the regeneration of ammonium bicarbonate aqueous solution were designed, built and tested. The effect of several operating parameters on the regeneration efficiency was experimentally investigated also identifying technological limitations and relevant solutions. A process simulation tool has been developed, and for the first time in the literature, successfully validated against original experimental results. Results from modelling analysis suggest that among the investigated processes, only the vapour stripping is viable for such applications. Models were used to evaluate the performance of ideal forward osmosis desalination and ideal SGP heat engines, finding, in the case of forward osmosis desalination, specific thermal consumptions between 180 and 250 kWh/m3 and, in the case of SGP heat engines, exergy efficiency up to almost 5%.

Optimum heat treatment to enhance the weak-link response of Y123 nanowires prepared by Solution Blow Spinning

Although the production of YBa2Cu3O7-δ (Y123) has been extensively reported, there is still a lack of information on the ideal heat treatment to produce this material in the form of one dimension nanostructures. Thus, by means of the Solution Blow Spinning technique, metals embedded in polymer fibers were prepared. These polymer composite fibers were fired and then investigated by thermogravimetric analysis. The maximum sintering temperatures of heat treatment were chosen in the interval 850 °C–925 °C for 1 h under oxygen flux. SEM images allowed us to determine the wire diameter as approximately 350 nm for all samples, as well as to map the evolution of the entangled wire morphology with the sintering temperature. XRD analysis indicated the presence of Y123 and secondary phases in all samples. Ac magnetic susceptibility and dc magnetization measurements demonstrated that the sample sintered at 925 °C/1 h is the one with the highest weak-link critical temperature and the largest diamagnetic response.