Regimes Of Heat Exchange Research Articles

Evaluation of Spatial and Temporal Variations in the Difference between Soil and Air Temperatures on the Qinghai–Tibetan Plateau Using Reanalysis Data Products

Many extreme meteorological events are closely related to the strength of land–atmosphere interactions. In this study, the heat exchange regime between the shallow soil layer and the atmosphere over the Qinghai–Tibetan Plateau (QTP) was investigated using a reanalysis dataset. The analysis was conducted using a simple metric ΔT, defined as the difference between the temperatures of the shallow soil and the air. First, the performance of 4 widely used reanalysis data products (GLDAS-Noah, NCEP-R2, ERA5 and ERA5-land) in estimating ΔT on the QTP at soil depths of 0~7 or 0~10 cm was evaluated during the baseline period (1981–2010); the ERA5-land product was selected for subsequent analysis, because it yielded a better performance in estimating the annual and seasonal ΔT and finer spatial resolution than the other datasets. Using the soil temperature at depths of 0~7 cm and the air temperature at 2 m above the ground, as provided by the ERA5-Land reanalysis dataset, the entire QTP was found to be dominated by a positive ΔT both annually and seasonally during the baseline period, with large differences in the spatial distribution of the seasonal values of ΔT. From 1950 to 2021, the QTP experienced a significant decreasing trend in the annual ΔT at a rate of −0.07 °C/decade, and obvious decreases have also been detected at the seasonal level (except in spring). In the southern and northeastern parts of the QTP, rapid rates of decrease in the annual ΔT were detected, and the areas with significantly decreasing trends in ΔT were found to increase in size gradually from summer, through autumn, to winter. This study provides a holistic view of the spatiotemporal variations in ΔT on the QTP, and the findings can improve our understanding of the land–atmosphere thermal interactions in this region and provide important information pertaining to regional ecological diversity, hydrology, agricultural activity and infrastructural stability.

Assessment of the volume and material container influence on juice cooling process

The quality of bottled beverages depends on the thermal process they undergo during the bottling process. The thermal history of the bottled beverage impacts the conservation of the product itself indeed. The temperature variation is related to specific aspects, such as the thermophysical properties, the heat exchange regime, and the possible simultaneous presence of more phases in the beverage itself. For instance, the behavior of the fluid inside the container might change according to the percentage of solid phases (e.g., pulp for juices). The present work addresses the cooling process on a processing line for beverage containers. This process aims to reduce the fluid temperature to preserve organoleptic characteristics and allows a stable and safe storage phase. The analysis has been carried out with a purpose-designed test bench to mimic the cooling process. Two juices with different percentages of solids (15 % and 50 %) are investigated for two container volumes (250 ml and 330 ml) and two container materials (glass and aluminum). The results show the influence of the pulp content on the cooling performance. The changes in the heat transfer performance determine different outlet temperature values and a different sensitivity to the container characteristics.

Influence of Lubricant Addition on Heat Exchange Regimes During Spray Cooling

Spray cooling is used in a variety of industries, including metallurgy, electronics, and medicine. Ease of spray generation, as well as the ability to cool relatively large surfaces or precise regions of interest, are among the key assets. Spray cooling can provide a relatively high heat flux, especially if it is accompanied by liquid evaporation. Additionally, it can be applied not only as a coolant but also as a transport medium for lubricating fluid in specific applications, such as hot die forging. Typically, the working fluid is a multi-component mixture with improved cooling and lubricating properties. Once the bulk liquid evaporates, particles or dissolved lubricant can settle on the hot solid substrate. Existence of components with various physico-chemical properties on the surface (binders, surfactants, dispersed particles, etc.) can significantly affect the spray impact, as well as the outcome of cooling regimes. Nevertheless, the accompanying thermal-hydraulic effects are still not well understood. An experimental facility for observing the impact of the multi-component spray, and characterizing heat transfer was designed for this study at the Institute for Fluid Mechanics and Aerodynamics. As a working fluid, varied mixtures of water and industrial white lubricant were used in different ratios. The visualization of spray impact and identification of the main hydrodynamic regimes were achieved using a high-speed video system. The inverse heat conduction problem was used to calculate the heat flux during continuous cooling from 445°C to 100°C taking into account temperature readings from within the substrate. Since the substrate was entirely insulated on all sides (except on the sprayed surface), the boundary conditions were well defined. The experimental data for pure water drops were compared with the results obtained using lubricating additives. It is discovered that even minimal volumes of lubricant augment the heat flux dramatically, particularly at rather high wall temperatures. The spray cooling process is accompanied by an extensive foaming near the wall and as a result, the Leidenfrost point is moved to a higher temperature, thus suppressing film boiling regime.

The dependence of the crystallization front shape on the heat exchange regime in the Bridgman-Stockbarger method

The growth of a silicon ingot in the Bridgman-Stockbarger crystal-melt-crucible system, similar to that used in real technology is studied numerically in the axisymmetric formulation by the finite element method. The simulation was carried out taking into account the heat of phase transition under conditions of unsteady thermal conductivity and natural convection at the initial heating of the melt up to 40 K and two rates of crucible lowering. The temperature gradient along the lower part of the side wall of the crucible is linear and equal to 35 K/cm or 70 K/cm. A comparison between the crystallization processes in the unsteady heat conduction regime and the natural convection regime is carried out. The dependence of the shape of the crystallization front on the heat transfer modes is investigated. It has been shown that in all heat conduction modes being considered, the crystallization front has a convex shape during the entire crystallization process. Moreover, the shape of the crystallization front in the examined range of parameters weakly depends on the crucible lowering rate, but is drastically influenced by the temperature gradient on the crucible walls. Under conditions of hermogravitational convection the shape of the crystallization front strongly depends on the crucible lowering rate and temperature gradient on the side walls of the crucible. At some parameters, a secondary convective vortex can be generated over the crystal during the crystallization process. Depending on the combinations of temperature gradient on the side walls and the lowering rate of the crucible, the shape of the crystallization front can be either concave or convex.

Peculiarities of determination of the heat transfer coefficient at boiling of non-azeotropic mixtures in tubes

In this work, the calculation techniques for determining the heat transfer coefficient for boiling non-azeotropic mixtures in tubes are described. In particular, the dependencies proposed by the authors of Shin et al. (1997) and Mezentseva et al. (2014). These dependencies were used to calculate and compare the experimental data on the heat transfer coefficients for the boiling of a non-azeotropic mixture R32/R134a (30/70%) in copper smooth tube with a diameter of 6.34 mm. The results showed that the dependence of Mezentseva et al. (2014) is in good agreement with the experimental data with an accuracy of ± 15%. In addition, this dependence considers the corresponding heat exchange regime and requires less thermodynamic properties of the mixture in calculations.

Microwave Heating of a Liquid Stably Flowing in a Circular Channel Under the Conditions of Nonstationary Radiative-Convective Heat Transfer

A class of nonlinear problems of nonstationary radiative-convective heat transfer under the microwave action with a small penetration depth is considered in a stabilized coolant flow in a circular channel. The solutions to these problems are obtained, using asymptotic procedures at the stages of nonstationary and stationary convective heat transfer on the heat-radiating channel surface. The nonstationary and stationary stages of the solution are matched, using the "longitudinal coordinate–time" characteristic. The approximate solutions constructed on such principles correlate reliably with the exact ones at the limiting values of the operation parameters, as well as with numerical and experimental data of other researchers. An important advantage of these solutions is that they allow the determination of the main regularities of the microwave and thermal radiation influence on convective heat transfer in a channel even before performing cumbersome calculations. It is shown that, irrespective of the heat exchange regime (nonstationary or stationary), the Nusselt number decreases and the rate of the surface temperature change increases with increase in the intensity of thermal action.

A simplified method for the numerical calculation of nonstationary heat transfer through a flat wall

The nonstationary operation modes of power equipment lead to the nonstationary regimes of heat exchange, in particular heat transfer. The transient processes, related to the accumulation of energy in the heat-transmitting surfaces, which manifest themselves in this case, may affect manageability of the work of equipment. Although in many cases, with a proper approximation, the heat exchange surfaces can be represented in the form of a simple variant of an infinite plate, the existing methods and tools for solving the problems on nonstationary heat transfer are built from the positions of universalism, introducing unjustified complications and hampering the generalization of numerical results obtained. We developed a simplified discrete analog to solve the one-dimensional problems on nonstationary heat transfer through an infinite plate. The realized approach allowed us to obtain the analog and results of calculations based on it in the dimensionless form, which substantially facilitates their generalization. A high stability of computational process is demonstrated relative to the selection of a number of nodes in computational grid and calculation step by time. The possibility of using the maximally small computational grids (3 nodes) makes it possible, at the current calculation step by time. To obtain an analytical solution for determining the temperatures at the surfaces of the plate at initial use of boundary conditions of the third kind. As a result, accumulated energy can be defined as a difference in the heat fluxes at the surfaces of the plate. Performing the calculations on the maximally small grids might be useful to solve the inverse problems on thermal conductivity.

Heat transfer monitoring between quenched high-temperature superconducting coated conductors and liquid nitrogen

High-temperature superconducting coated conductors (HTS-CCs) are good candidates for resistive superconducting fault current limiter (RSFCL) applications. However, the high current density they can carry and their low thermal diffusivity expose them to the risk of thermal instability. In order to find the best compromise between stability and cost, it is important to study the heat transfer between HTS-CCs and the liquid nitrogen (LN2) bath. This paper presents an experimental method to monitor in real-time the temperature of a quenched HTS-CC during a current pulse. The current and the associated voltage are measured, giving a precise knowledge of the amount of energy dissipated in the tape. These values are compared with an adiabatic numerical thermal model which takes into account heat capacity temperature dependence of the stabilizer and substrate. The result is a precise estimation of the heat transfer to the liquid nitrogen bath at each time step. Measurements were taken on a bare tape and have been repeated using increasing Kapton® insulation layers. The different heat exchange regimes can be clearly identified. This experimental method enables us to characterize the recooling process after a quench. Finally, suggestions are done to reduce the temperature increase of the tape, at a rated current and given limitation time, using different thermal insulation thicknesses.

Influence of the intensity of chemical heat-release source on the heat exchange at a flow of pseudo-plastic liquid in the initial interval of coaxial duct

The critical regimes of heat exchange at a laminar steady flow of a pseudo-plastic liquid in the initial interval of a coaxial duct are investigated with regard for both dissipative and chemical sources in the Arrhenius representation [5] under the conditions of an insignificant variation of the concentrations of reacting substances.

Temperature field of a solid body containing a spherical heating source with a uniformly moving boundary

The process of formation of a temperature field in an isotropic solid body containing a spherical heating source with a uniformly moving boundary for the case of realization of nonstationary heat-exchange regimes that lead to the time dependence of the heat-transfer coefficient have been investigated by mathematical-modeling methods.

Experimental investigation of natural convection heat exchange within a physical model of the manifold chamber of a thermosyphon heat-pipe evacuated tube solar water heater

The high capital costs associated with heat-pipe evacuated tube solar water heating systems can be reduced by replacing forced circulation with thermosyphon circulation. Currently research on thermosyphon heat-pipe evacuated tube solar water heaters is limited. An experimental investigation of the natural convective heat exchange regime that exists within the manifold chamber of a proprietary heat-pipe evacuated tube solar water was undertaken. This paper presents experimental data from a heat-pipe Evacuated Tube Solar Water Heater (ETSWH) subjected to the Northern Maritime Climate at the University of Ulster’s outdoor solar testing facility located at the Jordanstown campus. The thermal performance of this across solar noon (±30 min) was experimentally determined to be comparable to two physical laboratory 10 pin-fin model manifolds constructed to the same dimensions and geometry as the manifold chamber of the heat-pipe ETSWH when operated under steady laboratory conditions. When the surface temperatures of the pin-fins (simulated condensers) in the model manifold were normalised with respect to the lowest most pin-fin in the array the influence of buoyant flow was observed. Similarly to related studies in this field it was found that normalised surface temperatures on downstream pin-fins do not increase monotonically as would be expected if no interactions occur. It was found that at the pin-fin diameter to pitch used in the model manifold that normalised surface temperatures decrease at certain points in the array due to the action of buoyant flow generated from upstream pin-fins which increased heat transfer. Two-dimensional Particle Imaging Velocimetry (2D-PIV) was used to visualise the thermosyphon fluid flow regime. It was observed that the fluid flow regime varied across the model due to interactions between the fluid, chamber walls and pin-fins.

Heat Transfer Enhancement by Electric Fields in Several Heat Exchange Regimes

With the present article, the authors summarize over 15 years of work dedicated to studying the effects of the electrical and the gravitational force fields on two-phase and single-phase thermofluid dynamics. Results obtained on several microgravity platforms are presented and the role played by the electric field in the different heat transfer processes is analyzed. In particular, the regimes of nucleate boiling and film boiling are treated. Also, the parameters controlling the liquid-vapor interface instability and the main consequences regarding the critical heat flux (CHF) are outlined. In the final section, the promising technique of ion injection for efficient heat transfer enhancement in single-phase liquids is described, together with the main results obtained under different flow regimes and geometries. The following dielectrics were compared: R113, Vertrel XF, and FC-72 for the pool boiling research; again FC-72 and HFE-7100 for the single-phase convection heat transfer experiments.

Effects of gas type on the sensitivity and transition pressure of integrated thermal flow sensors

The effects caused on the response of integrated thermal flow sensors by operating pressure and gas type changes are investigated. The sensors are conventional differential temperature calorimeters fabricated by applying two post-processing steps to standard microelectronic chips. Differences in terms of sensitivity and linearity have been observed for Xe, Ar and N 2 in the flow rate interval 0–200 sccm. The progressive sensitivity reduction caused by decreasing the pressure from 100 to 12 kPa is ascribed to the onset of the transition to free molecular heat exchange regime. Estimation of the transition pressure for the three gases under test is presented.

Two regimes of heat exchange between a metal particle and a nonequilibrium plasma

A theoretical model is developed for the process of energy exchange between a spherical drop and a nonequilibrium plasma. It is demonstrated that, in a certain range of the plasma parameters, a quasistationary temperature of the drop can be maintained in two different heat exchange regimes. In one of these, the energy flux from plasma to drop is compensated by cooling due to the thermal emission of electrons; in the other, the energy supply is equilibrated by intensive evaporation of the drop material. The two regimes, characterized by certain quasistationary temperatures, are separated by a temperature interval featuring unstable states of the system. The particular realization of one or another quasistationary regime is determined by the initial temperature of the drop.

The effect of the reinforcement structure on the heat conductivity of shells of revolution with a system of tubes filled with a liquid heat-transfer agent

The initial boundary-value heat-conduction problem for shells reinforced by tubes filled with a flowing liquid heat-transfer agent is considered. The dependence of the coefficients in the heat-conduction equations on the thermophysical characteristics of the composition phases, reinforcement parameters, and shell geometry is studied. A comparative analysis of the stationary temperature fields in thin shells of revolution of different Gaussian curvature is performed for various reinforcement structures and heat-exchange regimes. It is shown that the temperature distribution in the shells depends strongly on the reinforcement structure and the shell geometry, which opens up new possibilities of designing optimal structures.

The peculiarities of interactions with water of hydrogen containing hydro-reacting compositions at high pressures

Experimental data concerning the peculiarities of chemical-kinetics processes near the surfaces of hydro-reacting compositions (HRC) with water are presented. It was found that the increasing pressure stimulates the reaction rate increases both on vertical as well as on horizontal surfaces. It has been shown that various operational limits on pressure of the samples on the basis of HRC depend upon the surface properties of the given HRC and upon the properties of their water interaction products. In this paper only the pressure dependencies were considered. The assumed analogy of HRC interaction processes with water in the yield of bubble gas generation regime involving the boiling and ebullition processes of inert gas through a fluid layer has been confirmed. Experiments have been carried out at up to 60 MPa for vertical and horizontal reacting surfaces. For a vertical surface the bubble departure diameter is greater than on a horizontal surface by 1.4 times (with 10% accuracy), and the frequency of departure thus is lower by 1.2 times with the same accuracy. The obtained experimental data concerning the gas evolving processes allow us to determine the heat exchange regime near the reactive surface and to exclude the heat explosive mode within the reactor that could be used as the hydrogen source.

Étude analytique des instabilités induites à la transition ébullition nucléée — ébullition en film

After a reminder of the static stability criterion of a system with non-linear characteristics, the thermal stability of a single-dimensional system was studied with the help of simple analytical methods. The influence of axial conduction, connected in particular with limit conditions, was highlighted and it was demonstrated that even if the static stability criterion is verified, the system can present second order instability over part of its intrinsic character. Multiple stationary states can also be superimposed on the instability. A more sophisticated analytical method allowed the existence of several temperature profiles to be verified on a heated conductor wire, dependent on the value of the amperage of the electric current. An experimental visualization confirms the existence of three temperature profiles, corresponding to three different heat exchange regimes.

Thermal multipoles for a jet in ambient spathermal multipoles for a jet in ambient space

A general solution of the thermal problem for a non-self-similar axisymmetric jet is determined on the basis of the multiple approach developed for problems of non-self-similar jets in ambient space [1, 2], and several problems of convective heat-transfer in simply- or doubly-connected domains are solved. The advantage of expanding the solution of the convective heat conduction equation in eigenfunctions of the problem is demonstrated. As an illustration, the solutions for a thermal dipole and quadrupole and for a jet flow in a heated tube are presented. The corresponding solutions are also obtained for a turbulent jet in ambient space. The most favorable heat-exchange regime for a jet in a heated tube is predicted on the basis of the particular behavior of the eigenfunctions of the thermal problem.

Cinematographic investigation of the cooling process of alloys in aqueous polymeric solutions

1. During the entire process of the cooling of cylindrical specimens in aqueous polymeric solutions (for example, PAA), there is a large temperature gradient along their vertical axis. This nonisothermal character is governed by the coexistence of different heat-exchange regimes in separate intervals of the cooling process. When an alloy D16 specimen 20 mm in diameter and 80 mm long is cooled for a period of approximately half its total cooling period, for example, approximately 90% of its surface is found in a film boiling regime. The second half of the cooling period is characterized by combined heat emission, which can be accomplished in accordance with three regimes; film, effervescent, and convective. In this case, the surface area encompassed by the film-boiling regime is reduced, and the effervescent regime progresses and is then, in turn, “supplanted” by the convective regime. 2. To determine the cooling capacity, or heat emission under nonstationary conditions, it is recommended to use long and thin specimens with a central thermocouple with the leads run through an axial channel. 3. The method of determining the cooling capacity of quenching media using surface thermocouples with their leads run through the medium should be eliminated from practice. 4. In calculating the temperature fields in real components, it is necessary to bear in mind the possible nonuniformity of boundary and initial conditions governing the existence of wave phenomena. The wave of an effervescent-boiling regime may come, for example, from the base of such structural elements as stiffeners, pins, etc.

Non-steady-state heat exchange of linear underground constructions with a thawing (freezing) soil

An approximate analytical method is presented for calculation of the temperature distribution of a heating agent over the length of underground channels of arbitrary form for arbitrary depth in thawing (freezing) surrounding soil in a non-steady-state heat-exchange regime.