Review of heat transport properties of solar heat transfer fluids

The present article reviews the test techniques for some of the important heat transport properties of oils such as viscosity, density, specific heat capacity and thermal conductivity mainly used for characterization of heat transfer fluids. It can be seen that while density of oils can be tested at higher temperatures, the other heat transport properties of oils like viscosity, specific heat capacity and thermal conductivity have a limitation of being tested at low temperatures below 100–150 °C. While quite a few number of researchers have reported evaluation of heat transfer properties like specific heat capacity and thermal conductivity of oils by different methods, there remains a huge scope of debate and discussions on the repeatability and reproducibility of such tests, especially in case of oils used in high-temperature applications. A lot of insight has been gathered with respect to testing of thermal conductivity of oils, and several common test methods have been compared with each other. Lastly, two mathematical models, reported in the literature in open domain, have been reviewed and compared with each other. If the oils are to be used at elevated temperatures, like heat transfer fluids used in concentrated solar power generation where temperatures go as high as 400 °C and beyond, there is an urgent need to standardize a laboratory test method for performance evaluation of heat transport properties, which can help in formulating new generation oils based on novel chemistries and technologies like nanofluids, synthetic oils of novel chemistries, molten salts and molten metals.     

Iteration-free calculation of heat transfer coefficients in heat exchangers

If a heat transfer coefficient is dependent on the wall temperature it is usually calculated by an iterative technique. Starting from an earlier iteration-free method which was developed for air-cooled heat exchangers and the cooling of viscous oils, an improved procedure is derived for convective cooling or heating of liquids. Furthermore, it is shown how the local heat transfer coefficient can also be calculated without iteration in the corresponding cases of combined radiation and convection.     

Determination of heat of mixing and heat of vaporization with a differential scanning calorimeter

A simple method is presented for measuring the heat of mixing and the heat of vaporization of volatile liquids at temperatures below their boiling point. It consists in introducing liquids by a microsyringe into a nearly closed cell of the DSC. The relative standard deviation for 4 to 5 runs is ca. 5% for heat of mixing and ca. 2% for heat of vaporization.     

Solar heat storage using chemical reactions

As an alternative to storage of sensible heat in liquids or solids or as latent heat of fusion, heat storage by means of reversible chemical reactions is under investigation. By this method, a chemical is separated into two components by heating and heat absorption, following which the components are stored in separate vessels and are recombined to generate heat when it is needed. The attractiveness of this concept of heat storage is not only higher energy density, but the capability to store energy as long as desired at ambient temperature, the option of transporting the chemicals to generate heat at another location, and the high temperatures characteristic of some of the reactions which result in high efficiency when the stored heat is used to generate electricity. Many reactions have been proposed and analyzed. Experimental work is in progress on inorganic hydroxide/oxide reactions, the decomposition of ammoniated salts, sulfur trioxide decomposition, ammonium sulfate decomposition, and others. The problems to be solved and potential applications are illustrated by the results of work in progress on Mg(OH)₂ and Ca(OH)₂ decomposition.     

The heat capacity of NdPO4

The heat capacity of neodymium orthophosphate has been measured in the temperature range (0.6 to 1570) K. From the results the excess (electronic) heat capacity has been derived by subtracting the lattice heat capacity derived from the heat capacity of the isostructural LaPO₄ and GdPO₄, reported earlier. The calculated excess heat capacity thus obtained is in reasonable agreement with that calculated from the crystal field energies.     

Generalized solvation heat capacities

The partial molar heat capacity associated with a constant-pressure solvation process is extended to define a total of six generalized solvation heat capacities, each of which contain unique physical information. These arise from all the possible cross derivatives of the reversible heat of solvation (with respect to T and N), each evaluated at either constant pressure or constant volume. The resulting quantities may be interconverted using expressions that depend on the solvent equation of state and the solute partial molar volume. Moreover, contributions to each of the solvation heat capacities arising from the temperature dependence of the solute-solvent interaction energy and the solvent-reorganization energy (at either constant pressure or constant volume) are formally identified. For the self-solvation of a molecule in its own pure fluid, the latter quantities may be extracted directly from experimental data, while for more general solvation processes additional input is required, either from computer simulation or from theoretical approximations. The results are used to experimentally quantify the generalized heat capacities pertaining to the self-solvation of xenon, difluoromethane, n-hexane, and water, as well as the hydration of xenon, cyclohexane, and three hard sphere solutes (of about the same size as water, xenon, and cyclohexane).     

Effective waste heat recovery from engine exhaust using fin prolonged heat exchanger with graphene oxide nanoparticles

Waste heat recovery is an important alternative to reduce the energy consumption in industrial processes. Heat Exchangers are used effectively for heat recovery. Thus, the role of heat exchangers for waste heat recovery system is crucial. The exclusive of heat transmission of a heat exchanger can be improved by many methods such as by modifying the geometries and using nano-additives of different concentration. In this continuation, a modified geometry of finned heat exchanger is developed with CFD analysis. Modified heat exchanger includes the fins in the internal pipe to improve heat transfer. Nanoparticles of graphene oxide with various concentrations are introduced in working fluid. A steady numerical study is performed by using ANSYS Fluent with k-omega turbulence model for exhaust flow. Variation at inlet velocities of exhaust gas and water, particles concentration and internal fin geometry are considered. The reduction in hot fluid temperature from 6 m/s to 2 m/s enhanced the effectiveness by approximately 33.3%. The decrease in hot fluid velocity to 2 m/s and 6 m/s can reduce its outlet temperature by 100 K and 14 K at 0.03 m/s cold fluid temperature. The inclusion of nanoparticles at 0.1% can enhance the effectiveness by maximum of 7%.     

The specific heat of surface layers

Summary The well-known relation ofW. Thomson for the latent heat of surface expansion was obtained by applying the second law to a cyclic surface expansion and heating process. A simple derivation of an expression for the excess heat capacity of the surface layer is obtained by applying the first law to the same cyclic process.If we do not include the amount of potential energy converted to heat,A /TdT in the definition of the excess heat capacity, the resultant relation between the specific heat of the surface layers and their excess heat capacity is of the same form as that expressing the latent heat of vaporization in terms of the differential heat capacities of the liquid and vapor phases.Numerical calculation shows that the excess heat capacity for the surface layer of water amounts to 24 percent of the heat capacity of the molecular layer in the bulk liquid phase.     

Waste heat driven solid sorption coolers containing heat pipes for thermo control

This paper provides a focus on the R&D of solid sorption coolers and heat pumps made in the Luikov Heat & Mass Transfer Institute (CIS Countries Association Heat Pipes) under Thermacore, Inc. Agreement.Commercial and space applications of sorbent systems offer an attractive alternative to compression systems and liquid sorption systems for cooling, heating and air conditioning.MgA zeolites solid sorption systems are analyzed. Some new results are presented.Solid sorption heat pump technology utilizing heat pipe heat recovery with a condensing/evaporating refrigerant holds considerable promise for bivariant (space and domestic) applications due to the variable temperature and variable load capabilities of such machines.     

A method for the determination of the specific heat and heat of decomposition of composite materials

A method has been developed to determine the specific heat of a material during thermal decomposition using a combination of DSC and TGA data obtained at the same heating rate. The heat of decomposition is calculated simultaneously using the same data. This technique was used to determine the specific heat and heat of decomposition of a widely used fiberglass-filled phenol—formaldehyde resin and a fiberglass-filled acrylonitrile—butadiene (AB) copolymer. Experimental data are presented for the specific heat of both the virgin and char components for temperatures between 60 and 730°C. The calculated specific heat of the mixture during decomposition for both materials is also presented.     

Thermodynamic evaluation of heat recovery through a Canopus heat exchanger for vapor compression refrigeration (VCR) system

This communication deals with the waste heat recovery from the industrial refrigeration and air-conditioning system by introducing Canopus heat exchanger. There is a considerable amount of low-grade heat available in large-capacity systems. To recover this low-grade heat, a Canopus heat exchanger is introduced between compressor and condenser components. The system feasibility is studied with various operating parameters and its effect on heat recovery factor and overall COP of the system. The parametric results obtained for different eco-friendly working fluids, such as R-134a and R-507a, have been presented. It is found that, in general, overall COP of the system is improved without affecting the actual performance of the system. The potential of low-grade heat availability is increased with increasing cooling capacity.     

Electrochemical heat flow calorimeter

A device designed for research of heat phenomena occurring in chemical power sources (CPS) is described. The device includes two functional blocks: electrochemical and calorimetrical, operating under single control, which allows simultaneously performing electrochemical and calorimetric measurements. The calorimetric block is a heat flow calorimeter. The calorimetric chamber design provides the possibility of studying thermal processes in laboratory electrochemical cells and CPS of planar, disk, and prismatic design. The absolute measurement error of the heat flow is ±50 μW at the resolution of 1 μW. The operating temperature range of the calorimetric chamber is 0–90°C. The basis of the electrochemical block is a module of a four–range potentiostat–galvanostat. The maximum polarizing current of the potentiostat is ±200 mA at the maximum voltage on the auxiliary electrode of ±10 V. Multiuser remote access from the user computers over Ethernet to the device is provided for control and treatment of experimental data. Digital deconvolution filters allowing to compensate the response rate of the heat flow meter are used for processing primary data of calorimetric measurements.     

Limiting efficiency of adsorption heat pump

Based on the concept of unrecoverable energy losses in small-scale dissipative structures, the upper limit of the heat pump efficiency has been estimated. This limiting efficiency is achieved in the absence of energy losses due to heat exchange with the heat carrier but when nonequilibrium nonisothermal vapor transfer is taken into account. The limiting efficiency is smaller than the thermodynamically equilibrium efficiency and serves a better reference point in the search for optimal process layout.     

灰污热流探针模拟锅炉受热面灰沉积的研究

基于傅里叶导热定律,设计了一简单实用的灰污热流探针,以神木煤、黄陵煤、新汶水煤浆和新汶黑液水煤浆为研究对象,在0.25MW热态实验炉上用灰污热流探针模拟了灰沉积过程,研究了四种燃料灰沉积过程中的热流变化特性和灰沉积机理。结果表明,灰污探针能很好地模拟不同燃料的灰污形成过程,模拟结果与实际情况相吻合;灰粒的沉积速率和吸收热流的衰减速率主要取决于燃料本身特性,同时也受烟气温度的影响;通过对探针上灰污的表观物理特性、微观结构、元素构成和矿物相的分析,发现四种燃料的灰沉积机理是不同的,黑液水煤浆灰污中Na、K含量较高,主要物相为熔融温度很低的富Na霞石和无水芒硝,黄陵煤灰污含有较高的Fe、Ca、S,而水煤浆燃烧时Fe的沉积和富集是灰污形成的主要因素;四种实验燃料中,黑液水煤浆和黄陵煤的结渣趋势强于神木煤和水煤浆。     

State of the art on flow and heat transfer performance of compact fin-and-tube heat exchangers

Journal of Thermal Analysis and Calorimetry - The need for better thermal–hydraulic performance of heat exchangers remains the primary reason for further improving the design of heat...     

Theoretical investigations into heat transfer in laser-welded steel sheets

This study contains mathematical modelling and numerical analysis of heat transfer in laser beam welding process. The temperature field was obtained on the basis of numerical solution into unsteady heat transfer equation with convective term and volumetric heat sources taken into account. Volumetric heat source model describing laser beam power distribution in combined truncated cone?Ccylinder volume was developed. Due to the wide range of temperatures appearing in the process latent heat of fusion, evaporation as well as latent heat of phase transformations in solid state were taken into account in the solution algorithm. On the basis of developed numerical algorithms an analysis of heat transfer in laser butt-welded steel sheets as a three-dimensional initial-boundary problem was performed.     

Anomalously high heat capacity of core-softened liquids

Heat capacity is among the most important thermodynamic characteristics of a substance. The behaviour of the heat capacity is well understood in gases and crystals, but not in liquids. A common view on the heat capacity of liquids is that it should be close to the solid-like values close to the melting line and it should approach the gas values in the limit of high temperatures. However, some liquids can show anomalously high magnitudes of isochoric heat capacity. In the present paper, I show that core-softened liquids can demonstrate unusually high magnitude of the heat capacity induced by a structural crossover of the liquid.     

Convective flow in the liquid crystal heat switch

The 'heat switch' is based on the fact that in some liquid crystal (LC) materials heat transfer depends on the voltage applied between two parallel electrodes containing the sample. The rate of heat transfer depends on the voltage rather than the electric field intensity, but is not understood. Since the heat switch can involve electric field intensities up to at least the breakdown field of air, it is important to understand the mechanism responsible for heat transfer. Results are presented indicating that a mechanism described earlier is involved. A proposal for a refrigerator using LC heat switches is also made.     

Deuterium isotope effect on molar heat capacities and apparent molar heat capacities in dilute aqueous solutions: A multi-channel heat-flow microcalorimeter study

The molar heat capacities of chloroform, dichloromethane, methanol, acetonitrile, acetone, dimethyl sulfoxide, benzene, dimethylformamide, toluene, and cyclohexane, as well as their deuterated isotopologues, were measured using a multi-channel heat conduction TAM (Thermal Activity Monitor) III microcalorimeter. In addition, the apparent molar heat capacities of some of the associated dilute aqueous solutions (0.0039 < solute mole fraction, x_i < 0.0210) were also measured. A temperature drop method from (298.15 to 297.15) K at 0.1 MPa was employed. The corresponding heat capacities were determined from the integration of the measured heat flow. The heat capacity results are shown to be in good to very good agreement with the available literature values. In addition, good correlations were obtained for the effect of isotopic substitution on both molar heat capacity and apparent molar heat capacity in aqueous solutions. These correlations should be useful in the prediction of the molar heat capacities or the apparent molar heat capacities of other deuterated compounds. Since these measurements were conducted with ampoules, the effects of heat of condensation and/or vapor space on the accuracy of the heat capacity determinations are discussed. The overall results from this study demonstrate the utility of a multi-channel heat conduction microcalorimeter in obtaining good reproducibility and good accuracy for molar heat capacities as well as apparent molar heat capacities from simultaneous samples.     

Numerical modelling of the spiral plate heat exchanger

The spiral plate heat exchanger (SHE) is widely used in plenty of industrial services in full counter current flow liquid-liquid heat exchange. We have produced a thermal modelling of the heat exchanges in both steady-state and time dependent cases with 2D spiral geometry, allowing computation with different materials, forced convective heat transfer models in turbulent flow and geometrical parameters options. We will display here some results in steady-state conditions in order to improve the exchanger performances.