Peculiarities Of Heat Transfer Research Articles

Investigation of heat transfer from inclined flat surface to aqueous foam

A lot of heat transfer surfaces have inclined orientation. The influence of an inclination on the intensity of heat transfer process is more significant in the case of two-phase coolants. Process of inclined flat surface cooling by aqueous foam flow has some peculiarities, such like drainage liquid film on the heated surface, differences in void fraction across and along foam flow and etc. All those factors make the heat transfer process more complicated in comparison with the single phase cooling.Main task of this work is to define the peculiarities of heat transfer between inclined flat surface and the aqueous foam flow. The relationship between drainage liquid film’s parameters, such like thickness of the film and the velocity of the liquid in the film, the rate of foam flow and volumetric void fraction is analysed. The analytical equations for the heat transfer process calculation are presented. Comparison with the results of the experimental investigation showed a good agreement.

Peculiarities of heat transfer in the hemispherical recesses streamlined by a pulsating flow

In this paper, we present the results of the experimental studies of the average heat transfer value in a hemispherical recess placed in the first, third, and fifth rows of the recess array when varying the frequency of the imposed air velocity fluctuations in the channel. The range of the Reynolds and Strouhal number variations corresponded to operating conditions of the turbine blade cooling systems.

Peculiarities of boiling in subcooled flow

Peculiarities of heat transfer during boiling in high-velocity flow subcooled relative to the saturation temperature are analyzed. Flow structure is analyzed. Boiling in swirled flow and boiling in flow heated on one side are considered.

Heat transfer peculiarities in separated flow past an oblique rib under different external turbulence

Results of an experimental study of turbulent flow past a flat rib installed at an angle to the free-stream direction are reported. In the experiments, external flows with two different turbulence numbers were used, and the angle of rib inclination to the free stream was varied from 50 to 90°. The experiments were performed for ribs of various heights under conditions with natural and high (13.4 %) free-stream turbulence levels. Visualization tests were performed to elucidate the vortex formation pattern and the direction of flow streamlines. Deformations of the recirculation region and secondary-vortex zone as well as enhanced effects due to 3D flow structure observed on decreasing the angle ϕ, and also notable restructuring of the flow at a high free-stream turbulence intensity, were identified. A comparison between pressure coefficients in different longitudinal sections of the channel is reported for ribs of various heights installed at various angles ϕ. The influence of rib inclination angle, rib height, and free-stream turbulence number on local heat-transfer coefficients and heat-transfer intensification is analysed.

Analysis of staggered tube bundle heat transfer to vertical foam flow

In general heat transfer intensity between solid surface and coolant (fluid) depends on three main parameters: heat transfer coefficient, size of heat exchange surface and temperature difference between surface and fluid. Sometimes the last two parameters (surface size and temperature difference) are strictly limited due to the process or technological requirements, and only increase of heat transfer coefficient is allowed. Simplest way offering sufficient increase in heat transfer rate (heat transfer coefficient as well) is to go from the laminar fluid flow regime to the turbulent one by increasing flow velocity. In many cases it helps despite such disadvantages like more complicated fluid supply system, rise of fluid flow mass rate and growth of energy usage for pumping. But in some cases, for example, in space application, in nuclear engineering, etc. there is not allowed to use high flow velocity of coolant – gas (due to vibration danger) or to apply high mass rate of coolant – liquid (due to limitation concerning weight or mass). One of the possible solutions of that problem could be the usage of two-phase flow as a coolant. An idea to use such two-phase coolant for heat removal from the solid surface is not new. Boiling liquid (water especially), gas flow with liquid droplets and other two-phase systems are widely used for heat and mass transfer purposes in various industries like food, chemical, oil, etc. An application of such two-phase coolants has lot advantages; high value of heat transfer coefficient is one of the most important. Unfortunately nothing is ideal on the Earth. Restrictions on vibration, on coolant weight (or mass rate); necessity to generate two-phase flow separately from the heat removal place; requirements on very low coolant velocities and other constraints do not allow using such type of two-phase coolant for purposes which were mentioned above (space application especially). As a possible way out can be usage of the statically stable foam flow produced from gas (air) and surfactant solutions in liquid (water). Our previous investigations [J. Gylys, Hydrodynamics and Heat Transfer under the Cellular Foam Systems, Technologija, Kaunas, 1998] showed the solid advantages of that type of two-phase coolant, including high values of heat transfer coefficient (up to 1000 W/m 2 K and more), low flow velocities (less than 1.0 m/s), small coolant density (less than 4 kg/m 3), possibility to generate foam flow apart from the heat removal place, etc. This article is devoted to the experimental investigation of the staggered tube bundle heat transfer to the vertical upward and downward statically stable foam flow. The investigations were provided within the laminar regime of foam flow. The dependency of the tube bundle heat transfer on the foam flow velocity, flow direction and volumetric void fraction were analyzed. In addition to this, the influence of tube position in the bundle was investigated also. Investigation shows that the regularities of the tube bundle heat transfer to the vertical foam flow differ from the one-phase (gas or liquid) flow heat transfer peculiarities. It was showed that the heat transfer intensity of the staggered tube bundle to the foam flow is much higher (from 25 to 100 times) than that for the one-phase airflow under the same conditions (flow velocity). The results of the investigations were generalized using criterion equations, which can be applied for the calculation and design of the statically stable foam heat exchangers with the staggered tube bundles.

The peculiarities of heat transfer in CO2 and N2O solids at low temperatures

The thermal conductivities of CO2 and N2O solids are investigated in the low-temperature range 1–40K. The thermal conductivities of CO2 and N2O are large compared with those of simple molecular crystals such as N2, CO, or O2 in the whole investigated temperature range. Analysis of the experimental data by the Callaway method shows that a relatively large size of the crystalline grains, low density of dislocations, and weak phonon–phonon interaction might be the reasons for the good thermal conduction in these crystals at temperatures near the maxima. A comparison between calculated values of the intensity of normal phonon scattering processes and experiment gives evidence that in N2O there is an additional (in comparison with CO2) giant scattering of phonons. This scattering is described in the frameworks of the soft potential model by the resonance phonon scattering on tunneling states and low-energy vibrations.

Interaction of the transfer processes in semitransparent liquid droplets

The study presents the mathematical model of unsteady heat transfer in evaporating semitransparent droplets of non-isothermal initial state and the numerical research method, evaluating selective radiation absorption and its influence on the interaction of transfer processes. The relation of the transfer processes inside droplets and in their surroundings and the necessity of thorough research of these processes are substantiated. When modeling the combined energy transfer in water droplets, the evaluation of thermoconvective stability in evaporating semitransparent liquid droplets is presented; the influence of the droplet initial state on its heating and evaporation process is investigated. The influence of heat transfer peculiarities on the change of the evaporating droplet state is indicated. Main parameters, which decide the peculiarities of the interaction of unsteady transfer processes in droplets and their surroundings, are discussed. The results of the numerical research are compared to the known results of the experimental studies of water droplet temperature and evaporation rate.

Development of the Theory of Boiling of Solutions with a Nonvolatile Component

The results of investigation which do not contradict the general methodology of modeling the nucleate boiling process of solution are presented. The development of the boiling model based on experimental results is given and the earlier hypothesis of the mechanism of nucleate boiling heat transfer is confirmed. The experimental results explain the peculiarities of heat transfer of solutions with a nonvolatile component at high-temperature difference.

Low-temperature heat transfer in nanowires.

A new regime of low-temperature heat transfer in suspended nanowires is predicted. It takes place when (i) only "acoustic" phonon modes of the wire are thermally populated and (ii) phonons are subject to the effective elastic scattering. Qualitatively, the main peculiarities of heat transfer originate due to the appearance of the flexural modes with high density of states in the wire phonon spectrum. They give rise to the T(1/2) temperature dependence of the wire thermal conductance. Experimental situations where the new regime is likely to be detected are discussed.

FORCED-CONVECTION HEAT TRANSFER FROM THE BOTTOM OF TRENCHES WITH RECTANGULAR OR INCLINED WALLS

Abstract Experimental results are presented on the heat transfer from the bottom of rectangular and trapezoidal trenches streamlined by a turbulent flow. Peculiarities of heat transfer are outlined upon varying both the width and height of rectangular trenches and the width of trenches with inclined walls. The mean heat transfer at the trench bottom is found to be determined, depending on the trench's geometry, either by turbulent diffitsional transfer, Nu ˜Re⅔ H or turbulent boundary layer, Nu ˜ Re0.8 H. A change from 45° up to 90° in the inclination angle of walls influences heat transfer in a complicated manner: at α = 60° the heat transfer is intensified, while at a α= 45° a delay in the transition from laminar to turbulent heat transfer takes place.

Heat transfer peculiarities in supersonic flows

A method of heat transfer and gas flow investigation based on the application of thermal sensitive coatings or thermocouple sensors and various visualization techniques is described. The thermal sensitive coatings and visualization reveal heat transfer peculiarities, and the complex nature of the method contributes to understanding the processes and generalization of quantitative results. Data concerning heat transfer on the leeward side of a blunt cone in the regions of the shock-wave boundary layer and bow wave interaction, in gaps and cavities of the orbiter's thermal insulation, and in the vicinity of them, are presented.

Peculiarities of heat transfer with hydrogen boiling in a thick heater made of AMg-6 alloy

The authors show that there is a mixed boiling regime for hydrogen in a heater of poorly conducting material.

Peculiarities of heat transfer in carbon and glass fibre materials at low temperature (in Russian)

Peculiarities of heat transfer in carbon and glass fibre materials at low temperature (in Russian)

Certain peculiarities of heat transfer in nucleate boiling in capillary tubes

Boiling cryogenic fluids in capillary tubes of varying geometry are used to study heat transfer. It was found that nucleate boiling in a capillary channel, when the heat flux density on the wall is increased, is followed by slug boiling. The optimal cross section for the system is also calculated.

Peculiarities of heat transfer in porous solid nitrogen

An experimental study was made concerning the peculiarities of heat transfer in porous solid nitrogen. It has been established that the rate of heat transfer in this medium is basically governed by the rate of mass transfer.