Peculiarities Of Heat Transfer Research Articles

Pressure drop and heat transfer characteristics in 60° Chevron plate heat exchanger using Al2O3, GNP and MWCNT nanofluids

PurposeThe study aims to use nanofluids as coolants for improving heat transfer peculiarities of plate heat exchangers (PHE). The experimental and numerical investigations are thoroughly performed using distilled water-based Al2O3, graphene nanoplatelet (GnP) and multi-walled carbon nanotubes (MWCNT) nanofluids.Design/methodology/approachThe numerical simulation based on Single Phase Model (SPM) was performed on a realistic 3 D model of PHE having similar dimensions as of the actual plate. The standard k-epsilon turbulent model was used to solve the problem. The concentration and flow rate of nanofluids were ranging from 0.1 to 1 Vol.% and 1 to 5 lpm, respectively, at 30°C. Whereas, hot side fluid is distilled water at 2 lpm and 80°C. The heat transfer characteristics such as bulk cold outlet temperature, heat transfer rate (HTR), heat transfer coefficient (HTC), Nusselt number (Nu), pressure drop, pumping power, effectiveness and exergy loss were experimentally evaluated using nanofluids in a PHE.FindingsThe experimental results were then compared with the numerical model. The experimental results revealed maximum enhancement in an average heat transfer rate of 9.86, 14.86 and 17.27% using Al2O3, GnP and MWCNT nanofluids, respectively, at 1 Vol.%. The present computational fluid dynamics model accurately predicts HTR, and the results deviate <1.1% with experiments for all the cases. The temperature and flow distribution show promising results using nanofluids.Originality/valueThe study helps to visualise heat transfer and flow distribution in PHE using different nanofluids under different operating conditions.

Improving the thermal insulation of nitrogen cryocontainers using the loop-shaped evacuation process

The improvement of serial cryobiological Dewar containers with the use of the lowest thermal conductivity in calorimetric samples of screen-vacuum thermal insulation (SVTI) during storage of liquid nitrogen became possible after the development of fundamental methods for determining heat fluxes for each element of cryocontainers structure and the study of the peculiarities of heat transfer in SVTI. It is established that the deterioration of thermal insulation on cryocontainers by 7–8 times occurs from its gluing when they are heated during the manufacturing process. An additional decrease in the quality of thermal insulation by ∼2 times during operation is due to the formation of cryocondensate from the pumped-out gas separation products in the cold layers of SVTI and an increase in radiant heat transfer. Eliminating the adhesion of the SVTI, as well as changing the direction of evacuating gas separation products towards the warm wall of cryocontainers through 35 perforated SVTI layers using an open loop-shaped evacuation process allowed to increase their service life by ∼2 times (up to 145–148 days). However, it remains practically unchanged over 15 years of operation.

Title High-Power Heat Release in Supercritical Water: Insight into the Heat Transfer Deterioration Problem

The investigation is focused on the phenomenon of heat transfer deterioration (HTD), inhibiting the use of supercritical fluids in the processes in which high-power local heat release is possible. The ordinary discussion is based solely on the data of quasi-stationary measurements. In order to clarify the intrinsic nature of HTD as an essentially non-stationary phenomenon, it is worthwhile to take into account the peculiarities of heat transfer confined in time and space. In the present paper, we briefly discuss the characteristic features of such heat transfer in supercritical water. The characteristic heating time was $$10^{-3}\div 10^{-2}$$ s and the heat flux density through the wall increased up to 15 MW/m$$^{2}$$. The results provide the heat transfer pattern under conditions of predominance of the heat conduction mode. Exactly this mode most closely corresponds to the regime of high-power local heat release in a viscous sublayer. We also propose that the effect of threshold decrease in the heat transfer intensity (typical of pulse processes in supercritical transitions) may be a fundamental factor for occurrence of the HTD mode.

The effect of water traces on heat transfer in liquid hydrocarbons under pulse heating mode

The peculiarities of heat transfer and spontaneous boiling-up of pure hydrocarbons (n-hexane, n-decane, and n-hexadecane) and those with water traces have been studied under the pulse heating mode. The pulse lengths were from 5 to 25 ms; the pressure was elevated up to the critical one. The phenomenon of strong sensitivity of heat transfer intensity to the presence of water traces has been revealed in the vicinity of spontaneous boiling-up temperature. The hypothesis on the initiating action of water traces on macroscopic phase transitions is assumed.

Flow structure and heat transfer around the cube

In this paper we present the results of a numerical study of convective heat transfer from a single cube, located on a plane surface perpendicular to the incoming flow. The Reynolds numbers, calculated from the height of the cube and the average velocity, are Re = 4200 and 46400. Numerical simulation is performed using the RANS and LES methods. The working fluid is air. Thermal condition on a flat surface and on the cube faces is a constant heat flux. A three-dimensional picture of the structure of the horseshoe-shaped vortex around the cube is clearly shown. Areas with maximum pressure loads on the surface of the cube are identified. The main attention is paid to the peculiarities of heat transfer in the flow around a cube with a turbulent flow. Areas on the cube surface where the most effective heat transfer occurs, as well as low-efficiency stagnant zones with low fluid velocity and high temperatures are shown. The obtained data on aerodynamics and heat transfer are compared with the experimental data of other authors.

Peculiarities of Heat Transfer in Flow of a Liquid Metal in Vertical Duct in Coplanar Magnetic Field

The hydrodynamics and heat transfer at downward and upward flows of liquid metal in the rectangular duct with the height-to-width aspect ratio of ~3/1 in a coplanar magnetic field were studied. The problem simulates the flow in head-transfer ducts of the liquid metal test module of the blanket of a fusion reactor of tokamak type. The experiments were conducted on the basis of the mercury magnetohydrodynamic test bench. The probe technique was used for measurements in the flow. The averaged profiles of velocity, temperatures, temperature oscillations of flow, and duct wall temperatures were measured in the duct cross section located far from the beginning of heating in the region of uniform magnetic field. The magnetic field suppresses the turbulent transfer owing to the decrease in the heat transfer coefficients and the increase in the wall temperature. However, under the conditions of downward flow, a significant influence of the counter thermogravitational convection (TGC) on the flow was discovered. The interaction of TGC with an external magnetic field in a number of modes leads to the appearance and development of secondary flows. The largescale vortex structures of TGC generate low-frequency temperature oscillations of very high intensity. In this case, the heat emission is improved.

Peculiarities of heat transfer at the liquid metal flow in a vertical channel in a coplanar magnetic field

The research of hydrodynamics and heat transfer at the liquid metal (LM) downward flow and upflow in a vertical duct of a rectangular cross section with a ratio of sides ∼1/3 in a coplanar magnetic field (MF) under conditions of bilateral symmetrical heating is performed. The problem simulates the LM flow in the heat exchange channels for cooling the liquid metal module of the blanket of the thermonuclear reactor (TNR) of the TOKAMAK type. The experiments were carried out on the basis of the mercury magnetohydrodynamic test-bed (MHD) Moscow Power Engineering Institute (MPEI) – Joint Institute for High Temperatures of the Russian Academy of Sciences (JIHT RAS). The probe measurement technique was used in the flow. Profiles of averaged velocity and averaged temperature, as well as profiles of temperature pulsations in the axial planes of the channel cross-section, are obtained; the distribution of the dimensionless wall temperature along the perimeter unfolding of the channel in the section and along the length of the channel. A significant effect of thermogravitational convection (TGC), which leads to unexpected effects, is found. At the downflow in a magnetic field, in some modes, low-frequency pulsations of anomalously high intensity occur.

Active thermal testing of hyperthermoconductive panels

Application of active infrared thermography to assessing the internal structure and functioning of hyperthermoconductive panels used in on-board electronics is described. Effective thermal- diffusivity maps of hyperthermoconductive panels, obtained using the pulsed Parker method, are presented. The peculiarities of heat transfer in hyperthermoconductive panels are illustrated using experimental modeling in which a local thermal-load source is placed on the surface of hyperthermoconductive panels.

Heat transfer during quenching of high temperature surface by the falling cryogenic liquid film

Contact between a liquid and an extremely superheated solid surface is restored during quenching. The cryogenic experiment and numerical simulation were performed to study the peculiarities of heat transfer during quenching of hot vertical copper plate being much higher than the Leidenfrost temperature by the falling film of liquid nitrogen. An anomalous behavior of heat transfer has been found in the vicinity of the quench front. The results revealed that the maximum heat flux into the liquid during quenching is significantly higher than the average in the quasi-stationary conditions. The dynamic pattern of quench front propagation has been obtained in the numerical experiment. It was found that the quench front initialization occurs after lowering the surface temperature to the thermodynamic limit of superheat for liquid nitrogen. The correlation to determine the time of quench front stagnation is proposed. The numerical model allows us to quantify the quench front velocity and temperature fields in the heater, which are variable in space and time. The reliability of the simulation results has been confirmed by direct comparison with the experimental data on the quench front geometry and velocity as well as on the surface temperature change over time.

FEATURES OF HEAT TRANSFER PROCESS IN ENERGY ACTIVE FENCES FOR BUILDINGS

Calculations have been performed to determine the heat flow and temperature distribution within the energyactive fence (EAF). The peculiarities of heat transfer in EAF during the summer are considered when implementing two modes: heat-insulating and with heat removal in the presence of ventilation. It is known that the presence of convection in the gaps leads to an intensification of the process of heat transfer through it. The efficiency of the functioning of energy supply systems using alternative energy sources depends on the choice of the design of energy-active fences, which are used as an element of conversion of incoming energy. With the optimal choice of the design of energy-efficient fences, the use of an innovative system allows a 3-fold decrease in energy costs for hot water supply.

Effect of Heat Transfer Peculiarities on Ignition and Combustion Behavior of Al Nanoparticles

Nanoenergetic materials have some advantages against micrometric and bulk materials. This is due to enhanced surface area and intimacy between reactive components that leads to increase in the reaction rate and decrease in the ignition delay. However, till now there is very limited understanding of fundamental physical processes that control reaction and combustion wave propagation. The heat transfer in the case of nanoparticles is characterized some specifi c features which determine the sometime unusual ignition and combustion behavior. The paper is focused on discussing the ignition and combustion of nano Al particles in conditions of a shock tube and in a plastic tube. It is shown that tiny metal particles at high temperatures and pressures become “thermally isolated” from ambient gas environment and experimentally observed ignition delays may be two order magnitudes longer of those calculated without accounting real energy accommodation and sticking coeffi cients. When going to conditions of reaction propagation in a plastic tube, some different ways for heat transfer have to be carefully analyzed. Actually, there are no evidences for unique dominant process which may provide propagation of combustion wave with observed speed through the loose Al/CuO particles mixture. It can be stated that the process comprises 2 stages with very fast ignition, releasing large amount of heat and propelling hot gas and condensed material in direction of unreacted mixture followed by more slow reaction of remaining metal with evolved in oxide decomposition oxygen. Common conclusion is that further detailed studying the fundamental properties of nanoenergetics materials and their reaction behavior may open ways for purposed control of the combustion behavior and for effective use of nanoenergetics in practical applications.

Heat transfer under high-power heating of liquids. 4. The effect of water admixtures on the heat transfer in superheated hydrocarbons

The a priory unknown peculiarities of heat transfer in superheated hydrocarbons with water traces have been studied experimentally. The approach was based on the relative version of the method of controlled pulse heating of a wire probe at constant current mode. The pulse duration was varied in the range of 1–120ms. A region of probe temperature rise was limited from above by the value of spontaneous boiling-up temperature of a substance. The objects of study were n-hexane, n-decane, and n-hexadecane. The water content was varied in the range from 5ppm (initial sample) to 35ppm (watered sample). A sharp increase in the heat transfer intensity for watered sample with respect to that of initial one has been revealed. The observed effect was manifested solely at temperatures close to the values of boiling-up temperature of watered sample.

Peculiarities of heat transfer in water droplets with a solid inclusion during heating in a high-temperature gas medium

A physical model and a mathematical model of heat transfer in the conditions of inhomogeneous (with a solid inclusion—a carbon particle) liquid droplet evaporation while moving through high-temperature (800–1500 K) gases are formulated. Numerical investigations were performed using, as an example, a spherical inhomogeneous water droplet during heating in the air medium. The most probable mechanism of phase transitions in a water–carbon particle–heated air system is considered (the initial droplet size, radius, varied in the range from 0.5 mm to 1.5 mm, the inclusion radius was 0.1–1 mm). It has been found that in certain conditions, besides water evaporation from the outer (free) droplet surface, intensive vaporization is possible at the liquid–solid inclusion interface. Conditions of realization of these phase transitions in inhomogeneous water droplet–high-temperature gas medium systems are identified.

Peculiarities of heat transfer in a flat disk-shaped evaporator of a loop heat pipe

Two models of a flat disk-shaped evaporator of a loop heat pipe have been developed for analyzing the effect of the convective component on heat transfer in a wick, and also on heat-exchange processes in an evaporation zone. Simulation data were obtained for two evaporators. The first evaporator was made entirely of copper, and the second had a body of stainless steel and a nickel wick. The geometrical dimensions of the evaporators were equal. The diameter of the heating zone was 30mm. Calculations were made for heat fluxes from 2.8⋅104 to 4.2⋅105W/m2. Water was used as a working fluid. An analysis of the results has shown that the contribution of the convective component to the overall heat transfer is small. The higher the thermal conductivity of the wick, the smaller the effect that the convection has on the temperature distribution in the evaporator.

Heat transfer under high-power heating of liquids. 3. Threshold decrease of heat conduction in supercritical region

The peculiarities of heat transfer in isopropanol in the course of rapid transition from the state of compressed liquid to supercritical state along the isobar have been studied experimentally. The temperature plateau mode, as a specific case of the technique of controlled pulse heating of a wire probe was used. The characteristic heating time was of the order of tens milliseconds, the characteristic heating layer thickness was of the order of micrometers. The pressure range was from 1pc to 6pc, where subscript “c” corresponds to the critical point of a substance. A sharp decrease in the heat transfer intensity for supercritical isopropanol with respect to that of subcritical one has been revealed. The similar effect, which was found previously by the technique of constant heating power as well, is uncharacteristic for well-known stationary measurements. A discussion of the origin of discrepancies between the results from pulse and stationary measurements has been initiated by us.

Heat transfer under high-power heating of liquids. 2. Transition from compressed to supercritical water

The peculiarities of heat transfer in water in the course of rapid transition from the state of compressed fluid to supercritical state along the isobar has been studied experimentally. The constant heating power mode for the technique of controlled pulse heating of a wire probe was used. The characteristic heating time was of the order of milliseconds, the characteristic heating layer thickness was of the order of micrometers. The pressure range was from 1pc to 2pc, where subscript “c” corresponds to the critical point of water; the heat flux density through the probe surface was of the order of 10 MW/m2. A sharp decrease in the heat transfer intensity for supercritical water with respect to that of subcritical one uncharacteristic for well-known stationary measurements has been revealed.

Peculiarities of heat transfer at anodic plasma electrolytic treatment of cylindrical pieces

Heat transfer in a three-phase electrochemical system consisting of a metal cylindrical anode (a workpiece), a vapor-gas envelope, and an electrolyte solution is considered. The purpose of the study is to determine the effect of the length of the sample part contacting with the electrolyte on the distribution of the heating characteristics on its surface. It is established that the increase of the length of the part of the anode submerged into the electrolyte causes a decrease in the average current density, reduction in the vertical current density gradient, and a decrease in the density of the heat flux from the envelope to the sample. A model is proposed to calculate the stationary cylindrical anode temperature and the current in the system with the evaluation of the convective heat flux from the anode portion protruding above the electrolyte into the atmosphere. The influence of the electrolyte flow along the sample on its temperature and the average current density in the system is determined and attributed to the variation of the vapor-gas envelope thickness.

Effect of Peculiarities of Heat Transfer, Diffusion and Phase Transformation on Joint Formation During Welding of Dissimilar Materials by High Power Fiber Laser

Abstract The article describes mathematical models of diffusion and thermal processes for welding of dissimilar materials and kinetic model of diffusion-controlled deposition and growth of intermetallic inclusions in the weld. Developed models were combined and implemented in the model of weld joint formation for dissimilar materials. To verify a model the microstructure analysis of weld joints and elemental analysis in the diffusion zone by SEM has been made for welding of systems Fe-Cu, Al-Ti, Fe-Al. The good agreement between calculated and experimental data has been obtained. Examples of developed technologies of welding of dissimilar materials using high-power fiber lasers were discussed also.

Heat transfer and wave characteristics of a binary freon film flowing over a three-dimensional textured surface

The results of the study of heat transfer, crisis phenomena, and wave characteristics of laminar-wavy liquid films falling over a vertical three-dimensional texture surface have been considered. The R21/R114 freon mixture is served as a working fluid. Some peculiarities of heat transfer, hydrodynamics, and development of crisis phenomena in a binary liquid film falling over heat-releasing surfaces of different geometry have been revealed. The dependence of the heat-transfer coefficient on the flow rate for a three-dimensional texture surface in the regime of evaporation is shown to be similar to the dependence for a smooth surface with slightly intensified heat transfer in the region of inlet film Reynolds numbers of 100–500. The heat transfer coefficients of nucleate boiling on the studied types of structured surfaces are much lower than for a smooth tube. In the regime of undeveloped nucleate boiling, the critical heat flux for all surface types is deter-mined by the dependence obtained by considering the appearance of the heat transfer crisis in an evaporating wave liquid film.

Peculiarities of heat transfer in thermoacoustic converters

The problems of heat transfer in thermoacoustic converters are discussed in the framework of analysis of studies of heat exchangers of thermoacoustic converters with a view of summarizing the concepts of physical processes of heat transfer in parameters of the theory of heat transfer, fluid dynamics and wave theory.