Radiative Heat Exchange Research Articles

Dynamics of magnetic flux tubes in accretion discs of T Tauri stars

Abstract Dynamics of slender magnetic flux tubes (MFTs) in the accretion discs of T Tauri stars is investigated. We perform simulations taking into account buoyant, aerodynamic, and turbulent drag forces, radiative heat exchange between MFT and ambient gas, and magnetic field of the disc. The equations of MFT dynamics are solved using Runge–Kutta method of the fourth order. The simulations show that there are two regimes of MFT motion in absence of external magnetic field. In the region r < 0.2 au, the MFTs of radii $0.05 \le a_0 \le 0.16\, H$ (H is the scale height of the disc) with initial plasma beta of 1 experience thermal oscillations above the disc. The oscillations decay over some time, and MFTs continue upward motion afterwards. Thinner or thicker MFTs do not oscillate. MFT velocity increases with initial radius and magnetic field strength. MFTs rise periodically with velocities up to 5–15 km s−1 and periods of 0.5–10 yr determined by the toroidal magnetic field generation time. Approximately 20 per cent of disc mass and magnetic flux can escape to disc atmosphere via the magnetic buoyancy over characteristic time of disc evolution. MFTs dispersal forms expanding magnetized corona of the disc. External magnetic field causes MFT oscillations near the disc surface. These magnetic oscillations have periods from several days to 1–3 months at r < 0.6 au. The magnetic oscillations decay over few periods. We simulate MFT dynamics in accretion discs in the Chameleon I cluster. The simulations demonstrate that MFT oscillations can produce observed IR-variability of T Tauri stars.

Performance of Unglazed Photovoltaic-Thermal Collectors for Cooling Purpose

Unglazed photovoltaic-thermal (PVT) collectors for combined usable heat and electricity production are still expanding on the market. Besides the summer heat production and whole year electricity production the unglazed PVT collectors can be used also for night cooling (sky radiation and wind convection heat exchange). Different unglazed PVT collectors have been experimentally tested for verification of reliable model for night cooling operation. Based on the results, the model has been implemented into simulation software TRNSYS. Simulation analyses of the cooling performance for different climate and operation conditions have been performed. While best of tested unglazed PVT collectors can achieve average specific cooling power around 110 W/m2 (80 kWh/m2.season) at chilled water temperature 20 °C in the climate conditions of central and northern Europe, in the climate conditions of south Europe these unglazed PVT collectors achieve not more than 38 W/m2 (27 kWh/m2.season) during summer season (June–August).

Development of the Compositions of High-Temperature Coatings with a High Emissivity Factor to Intensify Radiant Heat Exchange

The emissivity factors of Co2O3, NiO, and Fe2O3 are studied in the temperature range 293–1100 K. The application of Co2O3 and NiO in coatings is shown to increase the emissivity factor of a heat-exchange surface in the wavelength range λ = 0.75–12.0 μm and the given temperature range.

Three-dimensional numerical simulation of conduction, natural convection, and radiation through alveolar building walls

Numerical simulation of the coupled heat transfers by conduction, convection, and radiation through two kinds of the alveolar structures used in the construction are numerically investigated. Owing to the low-temperature differences involved, the tridimensional model is based on the Boussinesq approximation and constant thermophysical fluid properties at mean temperature. The alveolar walls are assumed grey and diffuse. The symmetric and periodic boundary conditions for flows governed by the incompressible Navier-Stokes equations are considered. Equations governing the natural convection in the alveolars, the radiative heat exchanges between their internal surfaces and the heat conduction in the surrounding walls are solved using a finite volume method. The pressure-velocity coupling is solved by SIMPLE algorithm. The objective of this study is to improve the effects of wall conduction and radiation heat exchange between surfaces on the decrease of the thermal resistance in the alveolar walls. Two kinds of concrete blocks with three and six-hole numbers are chosen and their thermal resistances are numerically predicted. The comparison between the thermal resistances, specified in terms of thermal regulations for buildings and numerical solutions, is presented.

Three-Dimensional Frequency-Dependent Thermal Model for Planar Transformers in LLC Resonant Converters

In this paper, a thorough thermal analysis is carried out in order to find an analytical three-dimensional (3-D) frequency-dependent thermal model of planar transformers (PTs) used in high-frequency power converters. Because of the tendency of having higher power density as well as miniaturization of power converters, the components have to work near their thermal limit margins. This issue is more critical for planar magnetics in LLC converters due to their variable high-frequency operation, making thermal modeling and heat transfer analysis tools necessary. The proposed 3-D frequency thermal model is obtained based on the lumped parameters network (LPN) according to different thermal resistances of the PT including convection, conduction, and radiation heat exchanges. It should be noted that not only the 3-D geometry effect of the core is applied to the proposed model, but thermal surface resistances are also considered to model the high-frequency operation effect. Moreover, the lumped capacitance thermal model is used to analyze the transient thermal variation of the transformer. The LLC resonant converter with two PTs, including EE58/11/38 and ER51/10/38 planar cores, is considered to show the compatibility of the proposed 3-D-frequency LPN model. In addition, the temperature distribution of different parts of the transformers is evaluated using finite element method modeling and compared with the proposed 3-D frequency-dependent LPN model. Experimental results confirm the improved accuracy of the proposed LPN model and show the proposed model predicts the temperature distribution in PTs with an error of less than 3%. Considering the improved accuracy and low computational time of the method, the proposed 3-D frequency-dependent model is a powerful and fast design tool to evaluate the temperature distribution for different designs, and so can be used to effectively optimize the transformer from thermal point of view.

Principles of energy conversion and noise characterization in air ventilation ducts exposed to solar radiation

The aim of this paper is to present a design tool with its guiding principles for energy conversion and noise characterization of an exterior rectangular duct often installed on building envelopes for pre-conditioning of fresh air. The energy conversion in an exterior rectangular duct is a function of solar irradiation, air gap width, mass flow rate and pressure, wall and air temperatures. A generalized two dimensional thermal analysis of an outdoor duct is presented by placement of surface and air nodes into two adjacent stacks of control volumes representing outer and inner walls of duct. A matrix solution procedure is adopted by constituting conjugate heat exchange of conduction, convection, radiation and ventilation heat transport. The rectangular duct model is built by a metallic exterior wall exposed to a steady heat flux generation due to solar heat gain and a well-insulated back panel as its counterpart wall. The model improves the results of traditional thermal models for the cases where: (i) there are sections of conjugate heat exchange; and (ii) stack effect due to thermal buoyancy necessitates 2-D nodal analysis for distant composite nodes. The design tool is supported with some numerical and experimental results of an airflow window with a photovoltaic (PV) solar wall installed in an outdoor test-room. Furthermore, some examples of noise characterization calculations are illustrated using devised noise measurement equations.

Experimental and theoretical investigation on a radiative flat heat pipe heat exchanger

Abstract Large amounts of heat are rejected by hot steel when it is cooling during the manufacturing process. In an earlier investigation a flat heat pipe (FHP) was constructed and tested in a factory for recovering this heat loss from steel wires by radiation and convection. In this paper, the performance of the FHP was examined by testing it at different configurations. In parallel a theoretical study was conducted. The FHP consisted of 14 stainless steel tubes connected by a bottom collector and a shell and tube condenser top header. The heat transfer area was increased by attaching a stainless steel at the back of the tubes. The effect of the back panel on heat recovery was examined by testing the FHP with the back panel and without it. In addition, the effects of the emissivity and absorptivity of the FHP surface on the thermal performance were investigated by testing the FHP both painted with high temperature black paint and with it unpainted. A theoretical modelling tool based on thermal network modelling was built. The theoretical prediction of the thermal performance of the FHP represented by the amount of heat recovery was compared with the experimental findings. The results of the FHP thermal performance were compared for four cases. The results indicated that heat recovery was greatly increased by painting the surface with thermal black paint. Furthermore, the back panel has a significant influence on the magnitude of heat recovery.

Modeling of Energy Conversion and Noise Characterization in Outdoor Ducts exposed to Solar Radiation

The energy conversion and noise characterization in an exterior rectangular duct is important, for example, in modeling PV solar wall and transpired unglazed structures. The energy conversion in an exterior rectangular duct is a function of solar irradiation, air gap width, mass flow rate and pressure, wall and air temperatures. A generalized two dimensional thermal analysis of an outdoor duct is presented by placement of surface and air nodes into two adjacent stacks of control volumes representing outer and inner walls of duct. A matrix solution procedure is adopted by constituting conjugate heat exchange of conduction, convection, radiation and ventilation heat transport. Noise characterization due to energy conversion in outdoor ducts is also presented. The energy conversion and noise characterization is supported with some experimental results of a photovoltaic (PV) solar wall installed in an outdoor test-room. Furthermore, some examples of noise characterization calculations are illustrated using devised noise measurement equations.

Problem of radiation heat exchange with boundary conditions of the Cauchy type

In this paper, a quasi-static problem of radiative-conductive heat exchange in a three-dimensional domain is considered in the framework of diffusion P1 approximation of the radiation transfer equation. The peculiarity of the problem statement is that the boundary values for the radiation intensity are not prescribed. Instead of that, the heat flux is additionally prescribed for the temperature field on the boundary. The unique, nonlocal in time, solvability of the problem is proven. Theoretical results are illustrated by numerical examples.

Study the Effect of Diameter and Depth of Parabolic Dish Collector on the Concentration Ratio and Temperature Amount of Solar Tower Receiver

This work introduces three models of parabolic dish collectors with different dimension to study the effect of change diameter and depth of the dish on the position of focus point and concentration ratio and the temperature of outlet hot water or steam by using different receivers. The present work deals with a new system consists of dish and receiver to produce hot water and steam from solar energy. The parabolic dish solar collector fabricated from iron with different dimension the first model with diameter (82 cm) and depth (6 cm), this gives focus length (70cm) and Concentration ratio (25.6). The second model with a diameter (100cm) and depth (3 cm), this give focus length (208cm) and concentration ratio (15). The third model with a diameter (150cm) and depth (12cm), This give focus length (100cm) and concentration ratio (399) and the front side covered by nickel sheet metal to focus the solar radiation upon the receiver for all models . Three different shapes of receiver tanks used with the third model ; namely case one which used rectangular receiver tank (100×50×5) cm3 filled with 25 liter of water. Case two used helical copper coil (12.5 mm× 3 m) inside aluminum cylindrical vessel (0.2 m ×0.6 m). Case three used radiator heat exchanger receiver tank (37×47×4) cm3 filled with 3 liter of water. Use low cost and available materials to manufacture the experimental part. The dish, used as part of the solar reflector, covered by several strips of nickel sheet metal. When the parabolic dish collector was operational, the temperature of water was 60 oC in rectangular receiver tank, 75 0C within the copper coil and 125 oC in radiator device receiver. For rectangular receiver tank, hot water obtained within time (2 h), for copper coil receiver tank, hot water obtained within time (30 min) and for radiator heat exchanger receiver, steam obtained within time (20 min).

Developing a model of tank cooling by water jets from hydraulic monitors under conditions of fire

The main danger of a fire at an oil storage tank farm lies in its cascade spreading to neighboring tanks. This happens due to heating metal structures to the temperature of self-ignition of vapors of petroleum products. That is why cooling tanks is a priority in the localization of such fires. One of the most reliable methods of cooling is water feeding onto the tank walls using hydraulic monitors that are stationed outside the banking. In this case, the problem is to calculate the cooling action of water and to determine such parameters of its supply that would ensure cooling tank structures to a safe temperature. The model of the water jet motion after exiting the nozzle of a firefighting barrel was constructed. The algorithm of water supply by using a hydraulic monitor, which provides consistent alternation of the water jet motion on the tank wall in the horizontal and vertical direction was proposed. The model of cooling action of the water film formed after water jet hitting the wall of the tank was constructed. The model is based on heat balance equations for a tank wall and a water film and takes into consideration the periodic water jet motion on a tank wall. When constructing heat balance equations, we took into consideration convective and radiant heat exchange with a fire and the environment. It was shown that the temperature distribution on the tank wall and the water film is described by the system of two nonlinear differential equations of the first order. The findings obtained in the study make it possible to determine the parameters of water supply, which provide tank cooling to a safe temperature

Research on Thermal Protective Clothing Based on Temperature Distribution

This paper studThis paper studies the design of professional clothing for high-temperature operation. Based on the temperature distribution inside the garment and outside the human skin, a mathematical model is established to calculate the temperature distribution. For a three layers fabric material, the heat transfer equation for the fabric layer is established. The heat transfer of the air layer between the fabric and the skin is dominated by radiant heat exchange, effectively interpreting the heat transfer mechanism of the air layer, thereby constructing the energy heat balance equation of the air layer. Combined with the initial conditions and boundary conditions, the spatial and temporal dispersion of the finite explicit difference format is performed.

Outflows and particle acceleration in the accretion disks of young stars

Magneto-gas-dynamic (MGD) outflows from the accretion disks of T Tauri stars with fossil large-scale magnetic fileld are investigated. We consider two mechanisms of the outflows: rise of the magnetic flux tubes (MFT) formed in the regions of efficient generation of the toroidal magnetic fileld in the disk due to Parker instability, and acceleration of particles in the current layer formed near the boundary between stellar magnetosphere and the accretion disk. Structure of the disk is calculated using our MGD model of the accretion disks. We simulate dynamics of the MFT in frame of slender flux tube approximation taking into account aerodynamic and turbulent drags, and radiative heat exchange with external gas. Particle acceleration in the current layer is investigated on the basis of Sweet-Parker model of magnetic reconnection. Our calculations show that the MFT can accelerate to velocities up to 50 km s-1 causing periodic outflows from the accretion disks. Estimations of the particle acceleration in the current layer are applied to interpret high-speed jets and X-rays observed in T Tauri stars with the accretion disks.

Heat exchange processes and productivity of carbon environment transformations of the water-plasma energy stream

The analysis of the process of thermal transformations of carbon environment at high temperature (T = 2000 K) processing in a chamber of a plasma-jet reactor with the use of a water vaporizer as an oxidizer is done. The influence of heat exchange parameters on the heating of coal particles of different sizes is established. The method of mathematical modeling of the process of conversion of dispersed carbon raw materials in a stream of water plasma at a temperature in the reaction chamber Tg = 2000 - 5000 K was developed. The technique allows to determine the influence of thermal and kinetic parameters on the process of heat exchange between coal particle and a steam-plasma environment. A conversion time of a coal particle into a gaseous state is determined. It is the time that laid the foundation for calculating the main geometric and regime parameters of the reactor. The expediency of increasing the temperature of the gases in the reactor Tg = 3000 K is proved. This determines the minimum time for conversion of carbon under the combined influence of convective and radiation heat exchanges.

Study of marine waters hydrological characteristics near the Zmiinyi Island in 2016-2017

Purpose. Results of hydrological characteristics observations in the Zmiinyi Island coastal waters carried out by research group of the Regional Center for Integrated Monitoring and Environmental Studies (Odessa National I.I. Mechnikov University) in 2016-2017 have been summarized. Methods. Primary data on transparency, temperature and salinity of marine water have been collected using standard methods. Data processing, calculation of statistics, producing of graphs and maps have been carried out using Excel and ArcGIS software. Results. Temporal and spatial distributions of transparency, temperature and salinity of marine waters near the Zmiinyi Island for the period from April 2016 to December 2017 have been presented and analyzed. Using the results of analysis of the accumulated experimental data on transparency, temperature and salinity of sea water the peculiarities of those characteristics’ seasonal changes have been revealed. Conclusions. Impact of desalinated water masses advection from the Danube River mouth and from the North Western part of the Black Sea has been revealed. Well-visible seasonal variation was revealed in the sea water temperatures distribution near the Zmiinyi Island in 2016-2017, which reflected seasonal changes in radiation balance and heat exchange processes at the sea / atmosphere boundary, as well as the influence of water masses advection from other areas of the sea. It was shown that salinity seasonal variation in the sea water near the Zmiinyi Island, which ranged from 7.48 to 18.34 PSU, in 2016-2017 had the disturbance as the result of migration of the frontal zone of the Danube River waters distribution in the Zmiinyi Island area, as well as advection of desalinated water masses from the NWBS. Forming of stable two-layer vertical density stratification of the Zmiinyi Island coastal waters in spring-summer periods of 2016-2017 has been traced. An earlier (compared to Odessa Bay) spring vertical thermal stratification of water (May) and smoother, till late autumn, seasonal temperature decrease were revealed.

Engineering solutions for heart efficient exterior walls in climatic condition of the Republic of Bashkortostan

Problems and disadvantages of some technical solutions concerning modern heat efficient exterior walls are considered herein under experience of engineering, construction and operation of residential and civil buildings in the Republic of Bashkortostan. Special attention is paid to defects in engineering and erection of three-layer walls, and functioning principles of all components of the wall are strictly fixed. Recommendation to increase maintenance-free service life by meeting special engineering requirements are given herein. Effect to increase buildings heat insulation is achieved by: reducing heat loss through separate enclosing structures and heat insulating shells of the building thereby enabling to reduce thermal power requirements; improving heat acceptability in rooms by decreasing rate of radiative and convective heat exchange in outer surface of enclosures; reducing environmental pollution due to air emissions reduction. Exterior wall insulation is the important element of the building heat balance. Many researchers in this field currently focus on analysis of series of residential buildings with different number of storeys made by engineering solutions for exterior walls to increase heat homogeneity thereof. Subject: main engineering solutions for heat efficient exterior walls. Objectives: three-layer wall constructed of masonry units; exterior three-layer wall within 121u panel series; three-layer wall of “Ventilated façade” system; façade heat insulation with plaster on lath; wall made of load-bearing structural insulating materials. Materials and methods: calculation of reduced total thermal resistance in exterior walls. Results: one of the most efficient methods to meet specific heat-shielding performance requirements is to increase heat homogeneity of the exterior wall. Conclusions: in the short term structural components of three-layer exterior walls are to be upgraded according to construction rules and regulations SNiP 23-02-2003 “Buildings Heat Insulation” thereby enabling the engineering solutions to meet the appropriate norms and rules.

Моделирование процессов радиационного теплообмена

Моделирование процессов радиационного теплообмена

Monitoring the Effective Ambient and Sky Temperature Based on Infrared Sensor for Advanced Thermal Calculations

Building performance simulations and advanced thermal analysis are becoming the basis for a well-established practice of a building sector. This approach requires many input data which are typically not available on site. Apart from already well-practiced climate variable quantities, such as ambient temperature, solar radiation and parameters of wind, more complex data are needed for advanced building thermal analysis. One of those is based on longwave radiation level. A pyrgeometer is a device that measures longwave radiation part of whole thermal radiation phenomena. This can be determined based on sky or effective ambient temperature monitoring. Secondly, both variables might be approximated by infrared sensors as an applicable option in the calculation of longwave radiation heat exchange between the external surface and the ambient building environment. The paper presents data obtained both by pyrgeometer and infrared sensor corresponding to their mutual comparison to demonstrate its application when longwave radiation exchange needs to be calculated or analyzed in advanced. Integrating of the infrared sensor with aim to monitor the effective ambient and/or sky temperature enables its applicability as an alternative, integrated and less cost consuming method towards the monitoring by commercial pyrgeometer.

Entropy generation analysis and thermal characteristics of radiation heating system

This article presents the developed methodology for the numerical simulation of radiation heat transfer from water panel heaters and optimization results of water temperature in the supply pipeline from the mass flow rate of the heat-carrier and the surface area for a given thermal power of the panel system. A numerical mathematical model is developed in the assumption that heat transfer occurs by means of radiation heat exchange by longitudinal ribs and pipes, which are thermally insulated on top. It is assumed that the temperature of the rib base is equal to the temperature of the outer wall of the pipe. The irregularity of the radiation density in different directions depends on the angle and distance to the irradiated area. The aim of the work is to develop a methodology to simulate the heat transfer processes of a radiation panel water heating system and optimization of design and operating parameters. The radiation intensity is determined by a numerical method using the MATLsoftware package. Our results of experimental studies of the radiation flux density are presented and compared with the results of numerical ones. The thermodynamic efficiency of a panel heating system is analyzed using the entropy production method (exergy destruction). The multicriteria optimization of water temperature in the supply pipeline is performed by LP?-search. It is found that the unevenness of surface temperature of panels reaches 24.4% as well as for the panels of about 50m in length a decrease in water temperature to 20K is observed, which leads to the unevenness of radiation flux density over the heated area. The area of the cooling system as a function of water temperature and the conditions under which the entropy production in the system is minimal is determined.

Fine structural spectrometry and spectroradiometry of combustion products of energy fuels

The results of experimental studies of the fine structure of the spectra of molecular absorption and emission of combustion products of energetic fuels and their application in solving problems of radiative heat transfer in structurally inhomogeneous multicomponent media are considered. The methods of determining the parameters of the spectral absorption lines from the experimental emission spectra of the flame in the combustion products of gas fuel the identification of optically active ingredients in the combustion products and anthropogenic emissions into the atmosphere are analyzed. Attention is drawn to the fact that the emission of gas components is selective and the methods for calculating the radiative heat exchange in the combustion chambers of power units must take into account the acute selection of the molecular absorption spectra of radiation.