Conditions Of Heat Exchange Research Articles

Numerical Study of the Two-Stage Process of Synthesis of a Composite Controlled by a Moving Source Under Conditions of Conjugate Heat Exchange

Numerical Study of the Two-Stage Process of Synthesis of a Composite Controlled by a Moving Source Under Conditions of Conjugate Heat Exchange

An integrated thermoelectric heating-cooling system for air sterilization— a simulation study

Semiconductor thermoelectric devices are broadly used for various heat-pump applications, where they function as either a heater or a cooler. In this article, an integrated system combining thermoelectric heating and cooling is introduced, which not only improves the overall energy efficiency but has a great potential for applications in air sterilization and purification. In the proposed system, the air is firstly heated up for air sterilization by fins thermally grounded to the hot side of multiple thermoelectric devices. The air is then cooled down by the cold side of the same thermoelectric devices. The heat exchange and flow conditions of the system are analyzed, and it is found that the Nusselt number ratio after adding the fins is increased by two times compared with the smooth tube. The air temperature in the system can reach up to more than 65°C using multiple thermoelectric devices. Such a temperature is able to inactivate many viruses. The coefficient of performance of the overall system can reach 5.4. This work demonstrates new applications of thermoelectric devices for energy-efficient air sterilization applications.

Analysis of Overcurrent Performance of YBCO Tape Considering Different Heat Exchange Conductions

The performance degradation of High Temperature Superconducting (HTS) tape, which may be influenced by various heat exchange conditions due to different cooling methods, is of significant for HTS devices under repeated overcurrent. In this article, considering different heat exchange conditions, a series of repeated overcurrent experiments were carried on YBCO tapes under 50Hz sinusoidal alternating current. The degradation of critical current of YBCO tapes after repeated overcurrent was recorded. the resistance characteristics of YBCO tapes under different cooling conditions were analyzed. Through finite element simulation, the temperature and current distribution of YBCO tape under different heat exchange conditions were simulated. The experimental results show that the overcurrent capability of the conductor under conduction cooling is far inferior to the liquid nitrogen immersion method.

Experimental study of the boiling process in a fire-tube boiler with different heat transfer fluids

Currently various boiler modifications differing in the types of heat transfer fluid used andin the heat exchange conditions in the boiler working area are presented on the market. To improve the boiler efficiency, it is necessary to take into account the peculiarities of the heat exchange process during convection or boiling depending on the properties and type of the heat transfer fluid used.

A Multiphysics Analysis of Coupled Electromagnetic-Thermal Phenomena in Cable Lines

The paper is focused on numerical modeling of multi-strand cable lines placed in free air. Modeling is carried out within the framework of the so-called multi-physics approach using commercial software. The paper describes in detail the steps undertaken to develop realistic, reliable numerical models of power engineering cables, taking into account their geometries and heat exchange conditions. The results might be of interest to the designers of multi-strand cable systems.

Thermocapillary deformation induced by laser heating of thin liquid layers: Physical and numerical experiments

Characterizing the profile of the laser-induced thermocapillary deformation of a thin liquid layer on a laser absorbing solid is of great fundamental and applied importance. The thermocapillary effect is the basis of several non-contact methods for measuring the physicochemical and thermal properties of liquids and solids, and methods of non-destructive testing in material science and thermal physics. The study of the layer rupturing caused by laser beam heating could contribute to solving the problem of the dry spots formation in thin–film heat exchangers. In this regard, the development of physical and numerical tools to determine the thermocapillary profiles of thin liquid layers is of great interest to researchers. In the present work, we developed a home-made setup to scan a deformed surface of a liquid layer with a laser sheet. Its accuracy was verified by scanning the surface of a solid standard specimen with a given Gaussian profile. By scanning the thermocapillary deformed layers of silicone oil, we found that for the Gaussian distribution of the radiation intensity in the heating laser beam, the thermocapillary deformations are significantly not the Gaussian. A modified Agnesi function, which is characterized by high accuracy and allows defining the surface profile using a minimum of experimental data, is shown to be an optimal function for approximating the thermocapillary deformation. Using Agnesi approximation can significantly reduce the time of the experiment and the processing of results. To validate new experimental data, an axisymmetric numerical model of the thermocapillary convection in a thin liquid layer was developed using commercial software Comsol Multiphysics. It helped calculating the surface profile and the temperature field on the substrate for two boundary conditions of radiative heat exchange in the system corresponding to maximum and minimum radiation heat loss. For the case of the ebonite-silicone oil system, when the laser beam is absorbed by the ebonite surface, it was numerically shown that the difference between the types of boundary conditions becomes noticeable only on very thin layers close to the pseudo-rupture. In general, a comparison of the stationary profiles of the thermocapillary surface deformation and temperature distributions obtained experimentally and numerically for the entire range of the studied layer thicknesses shows satisfactory agreement between the physical and numerical results.

Experimental performance analysis of a CO2 direct-expansion solar assisted heat pump water heater

Abstract In the past decades, combined solar energy with heat pump systems has been one of the very widespread ways to improve the efficiency of conventional heat pumps. In that way, this paper presents an experimental analysis of the influence of environmental conditions in a small size DX-SAHP operating with CO2 as refrigeration fluid to heat water. The variation of the water inlet temperature, the solar radiation flux and the relative humidity are analyzed and 88 different experimental results are presented. To support the analysis, a thermographic camera was also used. The results show that the gas cooler outlet pressure and temperature are strongly influenced by the water inlet temperature. As the water inlet temperature increases, the gas cooler outlet pressure and temperature increase, and consequently the COP decreases. The COP decreases in 45.8% for the variation of the water inlet temperature to 15–35 oC. The variation of the solar radiation of 876.9–30.17 W.m-2 presents a reduction of performance in about 30%. For the same conditions for the water heat exchange, the pressure ratio of the cycle decreases with an augment on the solar radiation flux. In addition, the COP is higher in 6% for a change in the mean relative humidity of 31.6–55.8 %.

Experimental Investigation on an Absorption Heat Transformer for Production of Hot Water or Steam Generation

The absorption heat transformer is widely used to utilize low-temperature waste heat in the field of distributed energy, industrial processing, and long-distance indoor heating, because it can upgrade energy level and deliver heat to heated medium. In this work, an experimental system of a vertical single-stage LiBr/H2O absorption heat transformer was established to study its performance in the case of producing high-temperature water or low-pressure steam generation under different heating water flow rates. The useful output heat, coefficient of performance, exergy coefficient of performance, and gross temperature lift of the single-stage heat transformer have all been tested. The results show that the absorber cannot directly generate low-pressure steam under the condition of counter-flow heat exchange but can obtain more useful output heat. The largest useful output heat is 20.3 kW, which is higher than that in the case of parallel-flow heat exchange. The generation of low-pressure steam has certain requirements on the mode of heat transfer. The largest internal gross temperature lift of 28.1°C corresponds to the smallest coefficient of performance of 0.22 when the heating water flow rate is 2.1 m3/h. The performance of the single-stage absorption heat transformer can be improved to some extent by increasing the heating water flow rate.

Numerical evaluation of finite length tubes effects in Stirling engines heaters

The performance of Stirling engines is directly related to the amount of energy that, in the form of heat, participates in the thermodynamic cycle. A peculiar characteristic of this type of engine is the closed circuit of the working fluid, implying a periodic admission and extraction, ideally involving the whole working fluid, of the heat exchanged during the cycle without any exchange of material with the external environment. Several correlations have been proposed in the literature to predict the heat exchange in the hot side heat exchangers, but most of them are based on nondimensional numbers, usually expressed in terms of an oscillatory Reynolds number and a non-dimensional length, that even if they take into account the amplitude and frequency of the oscillating flow inside the tube with respect to its diameter, do not include any dependency upon the length of the tube. Nevertheless, the length of the tube can have a great impact on the performance of a Stirling engine. The heater forms a significant part of the dead volume of the engine, making the optimization of the volume to surface ratio necessary. The friction losses increase by increasing the length of the tubes, determining a negative impact while the exchange surface increases. Even the Nusselt number in the inner side of the tubes changes along the length, achieving the largest values alternatively in the first portions of the tube lengths because of entrance effects (Graetz problem). The picture is made even more complex because of the special velocity profiles that develop in oscillating flows in tubes. One of the effects less investigated in previous studies, which we could call a “breathing effect”, regards the amount of working fluid that, in a real engine, can effectively travel from the hot side to the cold side, thus reaching the conditions of nominal heat exchange with the thermal source and sink of the cycle. An idealized configuration has been devised to investigate, using CFD simulations, these effects. Results reporting how the friction coefficient and the Nusselt number depend on the finite length of the tube will be illustrated.

ТЕПЛОВОЙ РЕЖИМ АНТЕННОГО БЛОКА

The article describes the procedure for calculating the thermal regime of a unit. The unit under study is part of a phased array. The calculation of the thermal regime allows you to verify the operability of the device under a steady thermal regime and to verify that the choice of the method of cooling the structure is correct. The work of the unit is analyzed under conditions of convective heat exchange without forced cooling at the maximum operating (permissible) ambient temperature. The calculation of the thermal regime of the unit is carried out in two stages. At the first stage, the temperature of the unit case is determined when the unit case overheats in the first approximation. The overheating value is determined from the graph. The calculation is carried out under conditions of natural air cooling. At the second stage, the average surface temperature of the heated zone is determined. The greatest heating in the unit is observed in the area occupied by the board with ERE. According to the results of the calculation, a problem was revealed, which consists in the fact that under the given operating conditions of the investigated device, the unit area is heated above the maximum operating temperature of some ERE. The value of the average surface temperature of the heated zone of the unit exceeds the value of the maximum allowable temperature of the least heat-resistant element of the heated zone. This means that it is necessary to use forced air cooling by air convection. The article proposes a solution to the problem of unit overheating, and describes the method of forced air cooling and its design.

Numerical Simulation of Conjugate Heat Exchange at Gas Flowing in a Well

A mathematical model is offered for conjugate heat exchange when gas flows in the section of a vertical well. The motion of the medium is described using a two-dimensional axisymmetric stationary formulation based on boundary-layer equations. The turbulent gas flowing due to the reservoir energy is considered. Natural gas is taken as a travelling medium. The numerical simulation results are presented in the form of the dependences of the flow temperature and gas density along the radius of the well on the external factors of the heat exchange changing along the well height. The results describe the thermobaric state of the well in the condition of the conjugate heat exchange of the produced natural gas flow and tubed well depending on the external thermal conditions.

Efficient realization of daytime radiative cooling with hollow zigzag SiO2 metamaterials* *Project supported by the Natural Science Foundation of Henan Educational Committee (Grant No. 21A140026).

A tunable selective emitter with hollow zigzag SiO2 metamaterials, which are deposited on Si3N4 and Ag film, is proposed and numerically investigated for achieving excellent radiative cooling effects. The average emissivity reaches a high value of 98.7% in the atmospheric window and possesses a high reflectivity of 92.0% in the solar spectrum. To reveal the enhanced absorptivity, the confined electric field distribution is investigated, and it can be well explained by moth eye effects. Moreover, tunable emissivity can also be initiated with different incident angles and it stays above 83% when the incident angle is less than 80°, embodying the excellent cooling performance in the atmospheric transparency window. Its net cooling power achieves 100.6 W⋅m−2, with a temperature drop of 13°, and the cooling behavior can persist in the presence of non-radiative heat exchange conditions. Therefore, high and tunable selective emitters based on our designed structure could provide a new route to realizing high-performance radiative cooling. This work is also of great significance for saving energy and environmental protection.

Optimization of Spatial Configuration of Multistrand Cable Lines

Skin and proximity effects have a considerable impact on current distribution in multistrand cable lines. Under unfavorable heat exchange conditions, some strands may be subject to excessive overheating, which may lead to serious malfunctions or even fires of the installation. The paper proposes a new criterion for a quick choice of spatial configurations, for which the effect might be minimized. A comprehensive analysis of literature cases is provided, including the recommendations of the U.S. National Code and the Canadian standard.

Thermodynamic Behaviors of Adsorbed Methane Storage Systems Based on Nanoporous Carbon Adsorbents Prepared from Coconut Shells.

The present work focused on the experimental study of the performance of a scaled system of adsorbed natural gas (ANG) storage and transportation based on carbon adsorbents. For this purpose, three different samples of activated carbons (AC) were prepared by varying the size of coconut shell char granules and steam activation conditions. The parameters of their porous structure, morphology, and chemical composition were determined from the nitrogen adsorption at 77 K, X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), and scanning electron microscopy (SEM) measurements. The methane adsorption data measured within the temperature range from 178 to 360 K and at pressures up to 25 MPa enabled us to identify the most efficient adsorbent among the studied materials: AC-90S. The differential heats of methane adsorption on AC-90S were determined in order to simulate the gas charge/discharge processes in the ANG system using a mathematical model with consideration for thermal effects. The results of simulating the charge/discharge processes under two different conditions of heat exchange are consistent with the experimentally determined temperature distribution over a scaled ANG storage tank filled with the compacted AC-90S adsorbent and equipped with temperature sensors and heat-exchanger devices. The amounts of methane delivered from the ANG storage system employing AC-90S as an adsorbent differ from the model predictions by 4–6%. Both the experiments and mathematical modeling showed that the thermal regulation of the ANG storage tank ensured the higher rates of charge/discharge processes compared to the thermal insulation.

Forced Vibration and Self-Heating of a Thermoviscoelastic Cylindrical Shear Compliant Shell with Piezoelectric Actuators and Sensors*

The problem of the forced resonant vibrations and dissipative heating of a hinged thermoviscoelastic cylindrical shell with piezoelectric actuators and sensors is stated. The transverse-shear strain and temperature dependence of the electomechanical characteristics of the material are taken into account. The effect of temperature dependence, transverse-shear strain, and heat exchange conditions on the surfaces on amplitude– and temperature–frequency characteristics of the forced vibration of the shell on the thermal failure of the system as well as the possibility of the active damping of the flexural vibration mode using piezoactuators and sensors are studied.

Hydrodynamics of Two-Phase Gas-Very Viscous Liquid Flow in Heat Exchange Conditions

This paper presents the results of analyses of the impact of heat transfer conditions on the hydrodynamics of downward co-current annular flow in vertical tubes of very viscous liquid and gas. The research was conducted within the range of gas velocities of 0–30.0 m/s and liquid velocities of 0.001–0.254 m/s, while the viscosity was in the unprecedented range of 0.046–3.5 Pas. The research demonstrates that the volume and nature of the liquid waves with various amplitudes and frequencies arising on the surface of the film are relative to the flow rate and viscosity of the gas phase. At the same time, we found that, under the condition of liquid cooling, an increase in viscosity resulted in the formation of a smooth interface whereas, under the conditions where the liquid is heated at the end of the channel section, a greater number of capillary waves were formed. This research resulted in the development of new dependencies which take into account the influence of selected thermal and flow parameters (including mass fraction) on the values of volumes specific to very viscous liquid film flows. These dependencies improve the accuracy of calculation by 8–10% and are fully applicable to the description of the performance of an apparatus with a hydraulically generated liquid film.

Freezing and freeze-drying of strawberries with an additional effect of micro-vibrations.

Our research focuses on the formation of ice crystals and evaluating the structure of preserved frozen and freeze-dried strawberries. Strawberries were frozen in two ways. One-half of strawberries were frozen at -30°C under conditions of convective heat exchange. The other half of strawberries were frozen under the same conditions with an additional effect on the strawberries of micro-vibrations created in the air of the freezer according to a specific program. A digital frequency synthesizer that generates 250W/m3 electromagnetic field rectangular pulse packets in the frequency bands of 2500-5000kHz creates micro-vibrations. The microstructure of strawberries, the number of cells that have retained their structure and firmness were determined in frozen strawberries. The strawberries retained 25-30% of the cell structure of their total number during traditional freezing, and 65-70% of the cell structure when frozen under micro-vibration. The data of the penetration and shear stress showed that the strawberries frozen under micro-vibration conditions were 10-15% stronger. Then researched strawberries were vacuum freeze-dried. The primary drying temperature was 30 + 1°C below zero and at the secondary drying the temperature was 38-40°C. The microstructure and firmness of strawberries were researched in dried samples also. Freeze-dried strawberries frozen under micro-vibration had small and evenly distributed capillaries and their firmness was 8-10% higher than freeze-dried strawberries frozen by the traditional method. Thus, freezing strawberries with the additional effect of micro-vibration have a positive effect on the firmness of both frozen strawberries and freeze-dried strawberries.

Identification of Dynamic Characteristics of Temperature Sensors

When submersible temperature sensors (TSs) are used to measure non-stationary temperatures or applied in systems for automatic control of non-stationary temperatures, it is necessary to know dynamic characteristics of the TSs. In Russian standards, dynamic characteristics of industrial TSs are specified only by the index of thermal inertia. Dynamic characteristics of some types of industrial TSs cannot be described with the required accuracy only by the thermal inertia indicator. Transmissive (transfer) functions of many TSs have an order of magnitude higher than the first and their time constants depend on the conditions of heat exchange with the measured medium. The aim of the work was to improve the accuracy of determination of dynamic characteristics of submergible TS for industrial and aviation purposes. A new method, based on spectral analysis of signal obtained from an experimental transient characteristic of TS, has been developed, in order to improve the accuracy of determination of dynamic characteristics of individual TS instances. This method makes it possible to determine the time constants of transfer functions that have an order higher than the first with high accuracy. To improve the accuracy of finding the dependence of TS nominal dynamic characteristics on the heat exchange conditions, a new method to construct the characteristic curves of thermal inertia has been developed. The technique is based on the use of two-dimensional regression analysis of experimental transient characteristics of individual instances of TSs, recorded at three or more heat transfer coefficients. Besides, a prediction technique was developed for nominal dynamic characteristics of TS for expected operating conditions, based on the use of established characteristic curves of thermal inertia of TS. Mathematical programs and computational procedures were developed to implement the proposed methods and techniques for determination of dynamic characteristics of TSs. The studies have shown that the proposed methods improve the accuracy of determination of dynamic characteristics of both submersible industrial TSs and aircraft gas temperature sensors.

The Effect of the Axial Heat Transfer on Space Charge Accumulation Phenomena in HVDC Cables

To date, it has been widespread accepted that the presence of space charge within the dielectric of high voltage direct current (HVDC) cables is one of the most relevant issues that limits the growing diffusion of this technology and its use at higher voltages. One of the reasons that leads to the establishment of space charge within the insulation of cables is the temperature dependence of its conductivity. Many researchers have demonstrated that high temperature drop over the insulation layer can lead to the reversal of the electric field profile. In certain conditions, this can over-stress the insulation during polarity reversal (PR) and transient over voltages (TOV) events accelerating the ageing of the dielectric material. However, the reference standards for the thermal rating of cables are mainly thought for alternating current (AC) cables and do not adequately take into account the effects related to high thermal drops over the insulation. In particular, the difference in temperature between the inner and the outer surfaces of the dielectric can be amplified during load transients or near sections with axially varying external thermal conditions. For the reasons above, this research aims to demonstrate how much the existence of “hot points” in terms of temperature drop can weaken the tightness of an HVDC transmission line. In order to investigate these phenomena, a two-dimensional numerical model has been implemented in time domain. The results obtained for some case studies demonstrate that the maximum electric field within the dielectric of an HVDC cable can be significantly increased in correspondence with variations along the axis of the external heat exchange conditions and/or during load transients. This study can be further developed in order to take into account the combined effect of the described phenomena with other sources of introduction, forming, and accumulation of space charge inside the dielectric layer of HVDC cables.

Keep Cool: Polyhedral ZnO@ZIF-8 Polymer Coatings for Daytime Radiative Cooling

Effective daytime radiative cooling materials with high solar reflectance and strong infrared emittance can greatly reduce the cooling demand of buildings without any energy consumption, which is a great application potential in the field of energy. Nanoparticle-based radiator is a simple and inexpensive solution for daytime radiative cooling. However, it is difficult for traditional oxide pigment nanoparticles to realize daytime radiative cooling under strong sunlight because of their low reflectance. Here, we report an inexpensive ZnO@ZIF-8 nanoparticle-based radiative cooling coating, which simultaneously achieves sunlight reflection and thermal radiation. ZnO@ZIF-8 polymer coatings can exhibit both high solar reflectance (0.90 ± 0.01) and infrared emittance (0.95 ± 0.01, 8–13 μm). The polyhedral morphology and the unoriented pores of the ZnO@ZIF-8 polymer coating significantly enhance the scattering of sunlight. Even under a nonradiative heat exchange condition, the temperature on the coating surface could be decreased by 7.6 °C when compared with that of the substrate under a solar intensity of 1000 W m–2. Owing to their superior optical capability and simple methods, ZnO@ZIF-8 polymer coatings are promising to be widely utilized and produced. This work is of great significance for designing advanced novel ZnO nanoparticle-based radiators, which provide daytime radiative cooling.