External Heat Flow Research Articles

APPROXIMATE ANALYTICAL SOLUTION OF THE PROBLEM OF CONJUGATE HEAT TRANSFER BETWEEN THE BOUNDARY LAYER AND THE ANISOTROPIC STRIP

Optimization of technological processes in metallurgy related to transfer and use of heat energy makes more complicated demands for calculation of heat exchange. Therefore, the work, the approximate analytical method for solving the conjugate problems of viscous gas-dynamic boundary layer and thermal conductivity in the anisotropic strip, has been developed. The paper uses modern numerical methods for solving differential equations in partial derivative and analytic methods on the basis of an integral transform of Fourier and Laplace. Boundary equations have been solved analytically with certain simplifications, and the problem of anisotropic heat conduction has been solved analytically. The heat flows are determined analytically by the longitudinal variable at the interface boundary. It has been established that temperature increase of the external surface contributes to that all factors directly impacting on the magnitude of heat flows act towards their reduction. The analytical solution for the problem of thermal conductivity in the anisotropic strip with a general type of anisotropy when the heat flows from the boundary layer are determined at the boundaries is obtained. The conducted research for the temperature of external boundary and heat flow from gas to it demonstrates that with increasing the degree of longitudinal anisotropy the surface temperature of the strip downstream increases from increasing longitudinal heat conduction An original conjugation method using the continuous heat flows, and temperatures at the interface boundary is found. The numerical results for the heat flows and temperatures at the interface boundary have been obtained and analyzed.

Parabolic Profile in Heat-Conduction Problems. 3. Semi-Bounded Space with a Definite External Heat Flow

Some approaches to the optimization of the exponent n of the parabolic temperature profile for a semi-bounded space with the second-kind boundary condition, implying the prescription of an external heat flow, are considered. An optimization scheme involving the minimization of a new error norm estimating the absolute value of a temperature deviation, more efficient than the scheme of T. Myers with minimization of the Langford norm, as well as new hybrid integral methods are proposed. With the use of these methods, parabolic solutions have been obtained on the basis of the temperature momentum integral in different modifications and the heat-flow momentum integral, and these solutions define the surface temperature with a very high accuracy. As an illustrative example, the problem on the heat conduction of a semi-bounded space with a definite pulsating heat flow at its surface, whose time change is defined by a quadratic parabola, was considered. The approximation errors of the parabolic solutions obtained using the hybrid integral methods proposed comprise hundredths and thousandths of a percent, and they are smaller by one to two orders of magnitude than the errors of the analogous solutions obtained by the known heat balance integral method and refined integral method, involving the optimization of the exponent of a parabolic temperature profile on the basis of minimization of the Langford norm, and by the combined integral method.

Critical Heat Flux Determination of Electric Cable Insulation

Abstract Electric cables can contribute to the spread of fire through the insulating layer. This paper focuses on their properties characterizing the initiation of fire. Samples of ethylene-based cable insulation were tested using a cone calorimeter by exposing them to external heat flows of six different values (25 kW m−2 – 50 kW m−2). Time to initiate flame burning was observed. The critical heat flux (depending on the method of calculation was in the range 2.94 kW m−2 – 4.59 kW m−2) and the thermal response parameter (342 kW s−0.5 m−2) was calculated from the time of initiation and external heat flow dependence.

Расчет внешних тепловых потоков на космический аппарат методом z-буфера

Расчет внешних тепловых потоков на космический аппарат методом z-буфера

Experimental investigation of effective thermal conductivity of packed lithium-titanate pebble bed with external heat source and flow of helium

Packed pebble bed of ceramic solid breeder materials viz. lithium titanate or meta-titanate (Li2TiO3), lithium orthosilicate (Li4SiO4) etc. is being considered for fusion blankets. During the breeding of tritium, helium will be produced and also it is proposed to flow helium from outside through the bed to extract tritium, as well as to remove thermal energy from the bed. Experimental determination of thermal property data of the pebble bed under helium gas flow is important for the blanket design. Such study of determination of effective thermal conductivity with flowing helium condition was not performed previously. Though, it is known that with an increase in gas flow rate, the effective thermal conductivity of pebble bed increases, it is necessary to conduct experiments to know the exact values and the degree of variation. Model and experimental setup used in our previous study was used to for this present work. Effects of process parameters viz., helium gas flow rate, bed temperature, pebble size etc. on the effective thermal conductivity of the bed have been studied. Correlations have been developed from these experimental data to estimate effective thermal conductivity at different particle Reynold’s number and bed temperature. It was found that with the use of helium, the effective thermal conductivity of bed increases by many folds. Experimental details and results are discussed in this paper.

Liquid jet cooler-based liquid hydrogen fueling and thermostating launch system development

A variant of modernization is examined of the ground-based system for filling and thermostating by liquid hydrogen of fuel tank of superheavy carrier rocket (CR) “Energy” by using, instead of traditional surface heat exchangers, a jet liquid cooler. The filling systems that are in operation at present imply the filling of CR tanks with cryogenic components in the boiling state, which limits the period when a rocket is positioned in the launching pad. But the system that was created in the USSR way back in 1987 made it possible to fill the CR “Energy” fuel tanks with the components of rocket propellant in the supercooled state with subsequent thermostating of tanks. Filling the CR tanks with components in the supercooled state with heightened density provided for an increase in the mass of payload by 5 % and allowed prolonging the period, which would rather be used for eliminating the malfunctions in preparing CR for launch, without returning the components to the filling tanks. They used as heat exchange devices the heat exchanger-recuperators of the “boiling bath” type. In this case, the semiclosed scheme of the cryogenic component circulation was implemented. Using a compact jet liquid cooler as the heat exchange device makes it possible to eliminate bulky heat exchangers, as well as simplify the system through transition to the closed scheme of liquid hydrogen circulation while thermostating the CR fuel tank. Thus, the cryo-component saving is provided for in the process of cooling the structure elements in preparing for the filling. The procedure for calculating the process of thermostating a fuel tank based on the jet liquid cooler was developed for solving the task of modernization of the filling and thermostating system, which allowed us to calculate attainable parameters of the thermostating system and to determine the range of system working ability. The outlet of external heat flows with power to 1 MW is ensured at pressure at the JCL nozzle exit section 10–30 kPa and temperature of “hot” parahydrogen removal from the tank 18–20 K. In particular, the 73 % increase in the operation period of the system during standard system filling with liquid hydrogen or the savings in liquid hydrogen at the estimated time of CR launch might be regarded as the result of using the modernized filling and thermostating system. The technical solutions proposed may be applied when designing new and modernizing those existing systems of storage, cooling and filling with liquid hydrogen of the carrier rockets fuel tanks, including promising objects of heavy and superheavy CRs: SLS and Falkon Heavy (USA), CZ-5 and CZ-9 (C.P.R), Angara 5V (Russian Federation).

Size-dependent microstructures in rapidly solidified uranium[sbnd]niobium powder particles

The microstructures of rapidly solidified U-6wt%Nb powder particles synthesized by centrifugal atomization were characterized using scanning electron microscopy and transmission electron microscopy. Observed variations in microstructure are related to particle sizes. All of the powder particles exhibited a two-zone microstructure. The formation of this two-zone microstructure is described by a transition from solidification controlled by internal heat flow and high solidification rate during recalescence (micro-segregation-free or partitionless growth) to solidification controlled by external heat flow with slower solidification rates (dendritic growth with solute redistribution). The extent of partitionless solidification increased with decreasing particle size due to larger undercoolings in smaller particles prior to solidification. The metastable phases that formed are related to variations in Nb concentration across the particles. The microstructures of the powders were heavily twinned.

Development and evaluation of thermal model reduction algorithms for spacecraft

This paper is concerned with the topic of the reduction of thermal models of spacecraft. The work presented here has been conducted in cooperation with the company OHB AG, formerly Kayser-Threde GmbH, and the Institute of Astronautics at Technische Universität München with the goal to shorten and automatize the time-consuming and manual process of thermal model reduction. The reduction of thermal models can be divided into the simplification of the geometry model for calculation of external heat flows and radiative couplings and into the reduction of the underlying mathematical model. For simplification a method has been developed which approximates the reduced geometry model with the help of an optimization algorithm. Different linear and nonlinear model reduction techniques have been evaluated for their applicability in reduction of the mathematical model. Thereby the compatibility with the thermal analysis tool ESATAN-TMS is of major concern, which restricts the useful application of these methods. Additional model reduction methods have been developed, which account to these constraints. The Matrix Reduction method allows the approximation of the differential equation to reference values exactly expect for numerical errors. The summation method enables a useful, applicable reduction of thermal models that can be used in industry. In this work a framework for model reduction of thermal models has been created, which can be used together with a newly developed graphical user interface for the reduction of thermal models in industry.

Method for Calculating the Processes of Melting and Solidification of Heat-Accumulating Materials in Fine Capsules

We propose a new method for calculating the processes of melting and solidification of heat-accumulating materials placed inside small-sized elastic capsules of symmetric form. The specific feature of the method is the simultaneous solution of the heat conduction equation joined for both phases and the equation for the rate of change in the mass fraction of the new phase in the layer between two neighboring nodes of a stationary computational mesh. The method permits using explicit finite-difference schemes for continuous calculations of the nonlinear heat conduction equation without loss of stability at weak and alternating external heat flows, as well as in the presence of several phase transition zones in the capsule. The paper presents the results of a test solution of the two-phase Stefan problem for an unbounded plate and the results of calculations for spherical capsules with n-paraffin in one regenerative convective heat cycle.

Thermally hiding an object inside a cloak with feeling

A thermal cloak can hide an object inside the cloak from the detection by measuring external heat flow (outside the cloak); this behavior is called the cloaking effect. However, such a thermal cloak has a limitation. Namely, the object is always located inside the cloak’s central region with a homogeneous temperature distribution, and hence this object cannot feel the flow of heat. To overcome the limitation, we develop a coordinate transformation method to design a complementary material, and add it into the central region of the cloak. As a result of finite element simulations, in case of external heat flow, the temperature distribution in this central region becomes inhomogeneous and hence the object feels the flow of heat indeed. Meanwhile, the cloaking effect remains the same, or almost the same as the cloak without such a complementary material inside. Thus, it becomes possible to use complementary materials to design thermal devices where heat conduction can be controlled at will.

External heat losses in small turbochargers: Model and experiments

The behavior of small turbochargers is deeply affected by heat transfer phenomena. The external heat losses of these machines are studied and a simplified model that takes into account both radiation and convective mechanisms has been proposed. The model has been adjusted in a turbocharger test bench for two different turbochargers, later on it has been validated against experimental measurements on an engine test bench. Finally, the model has been used to estimate the most important external heat flows among the different elements of the turbocharger, showing the operative points in which external heat transfer in turbochargers cannot be neglected.

New Thermal Insulating Material Based on Geocement

A new thermal insulating material was developed on the basis of a geocement, formulated as Na2OAl2O36SiO220H2O. Ground limestone and aluminosilicate pellets were used as fillers for its production (composition: geocement 64.29 wt. [%]; fillers 35.71 wt. [%]). This material, which is applied having a thickness of 3.0-4.5 mm, swells when it is exposed to an external heat flow of 1273 K average temperature. Swelling is due to the matrix phases and filler dehydration, which include heulandite, ussingite, sodium zeolite and other phases. As a result, a finely porous glassy aluminosilicate frame of jadeite-albite composition is formed, which is characterized by low thermal conductivity (0.09-0.175 Wm-1K-1). The developed material can be used to protect and to insulate wooden, metal and concrete surfaces from an one-sided heat source.The paper is dedicated to the great scientist of the XXI century in the field of alkali-activated cements and materials based on them, Pavlo Kryvenko, in honor of his 75thbirthday anniversary.

Unconventional thermal cloak hiding an object outside the cloak

All the thermal cloaks reported in the literature can be used to thermally hide an object inside the cloak. However, a common limitation of this kind of thermal cloaks is that the cloaked object cannot feel the external heat flow since it is located inside the cloak; thus we call these cloaks “conventional thermal cloaks”. Here we manage to overcome this limitation by exploiting a class of unconventional thermal cloaks that enable the cloaked object to feel the external heat flow. Our finite-element simulations in two dimensions show the desired cloaking effect. The underlying mechanism originates from the complementary effect of thermal metamaterials with negative thermal conductivities. This work suggests a different method to design thermal devices where heat conduction can be controlled at will.

A mixed-integer nonlinear programming approach to the optimal design of heat network in a polygeneration energy system

A large number of heat flows at various temperature and pressure levels exist in a polygeneration plant which co-produces electricity and chemical products. Integration of these external heat flows in a heat recovery steam generator (HRSG) has great potential to further enhance energy efficiency of such a plant; however, it is a challenging problem arising from the large design space of heat exchanger network. In this paper, a mixed-integer nonlinear programming model is developed for the design optimization of a HRSG with consideration of all alternative matches between the HRSG and external heat flows. This model is applied to four polygeneration cases with different HRSG types, and results indicate that the optimized heat network mainly depends on the HRSG type and the model specification.

ROTARY HEAT PUMP

An original rotary heat pump suggested by the authors is described in the presented paper. All thermodynamic processes of working fluid (evaporation – compression – condensation) based on the principle of heat pipe are provided in one volume of mushroom form closed vessel. The transportation of external heat flows is combined in the unit as well. The heat necessary for evaporation is taken from external heat sources, and heat produced by working fluid at condensation is transferred by disk type ventilators (authorship certificate No 98544, 04.01.1981). The three components of a new type are applied in the suggested heat pump: hydrodynamic pump, disk ventilator, and regeneration heat exchanger.http://dx.doi.org/10.5755/j01.mech.17.5.732

Efficient control over heat transfer and hydrodynamics in closed regions due to optimal selection of materials for enclosure walls and external heat load

The effect of the external heat flow density and thermal-physical characteristics of the material of the enclosure walls on the modes of energy, mass, and momentum transfer in a standard component of electronics or electronic equipment with a local heat source is numerically analyzed. The distributions of the streamlines and isotherms in the solution region which reflect the possibility of controlling the thermohydrodynamics in the gas cavity by choosing the corresponding parameters of the external flow and the wall material are obtained. The dependences for the average heat transfer coefficient on the internal surface of one of the vertical walls in the case of variations in the thermal-physical characteristics of the material and the wall thickness are obtained.

Analysis of Thermo-Elastic Wear Problems

In the relative sliding motion of two elastic bodies on contact interfaces both frictional dissipation and wear process occur. The wear process induces shape evolution of contact surface and growth of contact zone. The temperature field is generated by external heat flow and by frictional dissipation on the contact surface. The wear and heat conduction processes are coupled and tend to their steady or quasi-steady states for which the stress and temperature fields are fixed on the translating contact zone and depend only on its size and shape parameters. The steady-state is characterized by the minimum of wear dissipation power for which the stationary conditions generate the contact pressure distribution. The related steady-state temperature field is next specified and the thermal distortion effect is analyzed. The contact shape attained in the steady wear state is optimal as it corresponds to minimal wear rate, and its form depends on both mechanical loading and temeperature field. Several specific examples are presented for translating and rotating punches.

Numerical Analysis of the Catalytic Combustion of Premixed Methane/Air Mixtures in Microtubes

AbstractThe combustion characteristics and extinction limits for the catalytic combustion of a methane/air mixture in a microtube are investigated computationally using the commercial CFD code FLUENT coupled to an external subroutine DETCHEM. The effects of the microtube dimensions, conductivities of wall materials, external heat losses and flow velocity on the combustion stability, are also studied. The numerical model is set as either adiabatic or non‐adiabatic with a fixed exterior heat transfer coefficient. Numerical results indicate that thermal conductivity and wall thickness are vital to preheat the methane/air mixture through the conducting wall. Two types of extinction occur, i.e., thermal quenching and blow out. These extinction limits are characterized by wall surface temperature in the microtube and the ratio of Pt(s)/O(s).

Energy Separation in the Wake of a Cylinder

Energy separation is a spontaneous redistribution of total energy (enthalpy) in a fluid without external work or heat flow, resulting in some portion of fluid having higher total energy (enthalpy) and another portion having lower energy (enthalpy) than the surrounding fluid. The present study investigates the mechanism of energy separation in the vortex field behind an adiabatic circular cylinder. Time-averaged velocity and temperature measurements are carried out in the wake of a cylinder in a cross flow of air. The measurements are performed at downstream locations of three, five, seven, and ten diameters, for a Reynolds number, based on upstream velocity and cylinder diameter, of 9.2×104 and freestream Mach number of 0.22. The measured velocity and recovery temperature data are expressed in nondimensional form as an energy separation factor. The distribution of energy separation factor indicates that the main cause of energy separation is the periodic vortex flow in the wake. The vortex strength and the separation effect decrease as the flow moves downstream. However, energy separation is observed even ten diameters downstream.

Analysis and Modeling for the in-Orbit Dynamics of Nano-Satellite's Thermal System

纳卫星热系统是在复杂空间热环境下实现纳卫星被动热控制任务的基本技术途径,对其进行在轨动态特性分析与建模对纳卫星的热控方法与效果研究及评价具有重要意义.通过详尽的机理分析建立了描述纳卫星热系统在轨飞行过程中瞬态温度变化的3维非线性动态特性模型,阐述了与之配套的外热流分析与轨道运动计算方法,并以一太阳同步轨道纳卫星为例,对其飞行过程中的外热流变化、在轨温度变化响应等动态特性进行了详细的分析和讨论,为进一步深入研究纳卫星热控制、热管理理论及技术提供了简便的分析计算模型与方法.