Intensity Of Heat Exchange Research Articles

Thermal State of the Elements of Active and Combined Thermal Protection in Gasdynamic Tests

A complex experimental‐computational procedure of investigation of heat exchange on the exterior surface of a porous element of active thermal protection and along the surface of an element of combined thermal protection has been given; the procedure is based on the processing of results of thermogasdynamic tests by the methods of inverse heat‐conduction problems. Results of investigation of the influence of the parameters of porous cooling on the efficiency of thermal protection of the heated surface have been presented. Results of investigation of the intensity of heat exchange on the surface of a glass‐reinforced‐plastic plate in the curtain zone for different parameters of injection have been given, and conclusions on the efficiency of porous cooling as a function of the intensity of injection of a coolant have been drawn.

Investigation of Thermal Processes in Gas-Thermal Spraying of Metal-Ceramic Coatings

The features of the thermal regime of gas‐thermal deposition of metal‐ceramic coatings have been studied. The procedure of calculation of the temperature field of a multicomponent coating throughout the process of formation has been developed. The quantitative dependences of the character of heating of the deposited particles and melting of the substrate on the thermophysical properties and chemical composition of the material, the intensity of heat exchange, and the technological parameters of the process have been established. The thermal conditions of spraying of powder compositions, which ensure improved strength characteristics of the coating–substrate system, have been determined.

Chaotic Homogeneous Porous Media. 4. Heat Exchange in a Cell

An analytical study is made of a change in the intensity of heat exchange in a cell in passage from an ordered permeable system to a chaotic system.

Analysis of Nitrogen-Oxide Formation Mechanisms in Filtration Combustion of Methane–Air Mixtures

The kinetics of nitrogen‐oxide formation under conditions of filtration combustion of lean and rich methane–air mixtures is analyzed. Intense interphase heat exchange in the wave of filtration combustion of gases leads to a drastic decrease in the gas residence time in the high‐temperature region and also to a decrease in the maximum temperatures in the reaction zone. As a result, the processes of filtration combustion of gases are characterized by reduced emission index of nitrogen oxides as compared to diffusion flames. Under these conditions, the thermal mechanism does not make any significant contribution to NO formation, and the dominating channels are the NNH channels for lean mixtures and the Fenimore mechanism for rich mixtures. Nitrogen oxides in lean mixtures are mainly represented by NO and N2O, whereas nitrogen‐containing components HCN and NH3 are typical of rich mixtures. NO decomposition in rich mixtures occurs in “afterburning” reactions with participation of HCCO, CH3, and CH2 radicals.

Measuring local thermal fluxes during thermal exchange between a surface and a vortex structure

The effect of a vortex structure in a “dumped” flame on the intensity of heat exchange between this flame and a flat surface has been studied. An optical polarization technique is proposed for the visualization and measurement of an inhomogeneous temperature field at the surface and inside the heat exchanger wall. It is shown that the gas circulation in large vortex cells leads to an increase in the local thermal fluxes. In the range of Reynolds numbers 50 400, a thermal flux from the “dumped” flame to a flat heat exchanger surface is 20–30% greater than that for an oncoming laminar flame.

Modeling of Natural Thermogravitational Convection in Horizontal Channels with an Irregularly Shaped Cross Section

Results of a numerical investigation of the natural thermogravitational convection of air in horizontal channels with a Γ‐shaped cross section are presented. Two‐dimensional unsteady equations of convection in the Boussinesq approximation, which are written in the velocity vortex–current function–excess temperature variables, are employed as a mathematical model. In investigating, the range of Rayleigh numbers was bounded above by the value 106, which corresponds to the range of applicability of the Boussinesq approximation to air in the problem posed. The boundary‐value problem was solved numerically by the finite‐element method of Galerkin (weak formulation). The distinctive features of the temperature and flow fields were revealed on the basis of calculations and the intensity of heat exchange was evaluated.

Surface tension of a liquid under the conditions of a fast surface renewal during evaporation

A new capillary method of measuring surface tension σ under the conditions of a fast surface renewal due to evaporation is considered. At the highest rates reached for n-pentane and water evaporation (0.6 and 0.14 kg m–2 s–1, respectively), the deviations of σ from its equilibrium values may be explained by a change in the meniscus shape under the conditions of intense heat exchange.

Temperature profiles in microwave heated solid foods of slab geometry: Influence of process parameters

A simple 1-D mathematical model for prediction of local temperatures in a layer of solid material during microwave heating (Houšová et al. 1998) and a sensitivity analysis were used to evaluate the influence of process and material parameters on vertical temperature profiles in a layer of material during heating. The results of calculations are presented in graphs and discussed. The incident microwave power and heat capacity and density of heated material are parameters with great effect on all local and average temperatures and local and average heating rates. The shape of temperature profile is influenced only to a small extent by a change in the value of applied microwave power and also in the value of heat capacity or density of heated material. The whole profiles shift to higher or lower temperature values when the incident microwave power is changing. The distribution of applied microwave power between the upper and bottom layer surface very much influences the shape of the profile and the values and position of the highest and the lowest temperature in the layer. Depth of penetration and thermal conductivity of heated material influence on the shape of temperature profiles and the temperature spread in the layer (evenness of temperature distribution). Effect of penetration depth also depends on the relation to the layer thickness – its effect increases with the increasing layer thickness. At the low values of penetration depth relative to the layer thickness, an uneven temperature profile is to be expected. Effect of thermal conductivity value on temperature profile depends on the time of heating. Because of a short time of microwave heating, the effect of this parameter on temperature distribution is smaller compared to the conventional heating methods. At the beginning of heating its influence is quite negligible. Temperature of the air surrounding the layer and intensity of heat exchange between the air and layer surface are parameters with only small local effect on temperature distribution.

Heat exchange processes near the boundary of a film boiling nucleus

Results are presented of an experimental investigation of the change in temperature, heat flux to the liquid, and rate of displacement of the isotherms near a film boiling nucleus propagating over a plane surface. The experiment was carried out in a liquid nitrogen bath at atmospheric pressure on the saturation line. The heater was a sapphire plate 1.2 mm thick having a heat transfer surface area of 77×22 mm2. The following facts were established: 1) near the boundary of the film boiling nucleus a new heat exchange mechanism takes place caused by the instability of the liquid microlayer; 2) the maximum heat flux to the liquid is considerably greater than the critical heat flux; 3) the vapor film in the film boiling region grows gradually with increasing distance from the boundary, i.e., there is a smooth transition in terms of heat exchange intensity before the equilibrium film boiling level is reached.

Improvement of the performance of cw CO lasers by using externally ribbed wall cooled discharge tubes

We describe a modified design of a continuous-flow cw CO laser with longitudinal electric discharge using an externally ribbed wall cooled discharge tube. The transverse ribbed discharge tube provides a more intense heat exchange of the CO-laser gas with the walls. With this configuration it was possible to obtain about 50% more output power from an all-line CO Δv=1 laser. For a CO-overtone Δv=2 laser we were able to improve both the output power and the number of lines as compared to a conventional smooth tube.

The propagation of converging and diverging shock waves under intense heat exchange conditions

The motion of a gas with strong diverging shock waves, generated by supplying energy to the gas in accordance with a power law, and with converging shock waves in the cases of spherical and cylindrical symmetry is investigated. It is suggested that the intense heat exchange ensures that the temperature is the same over the whole volume of the moving gas and equal to the temperature of the particles directly behind the shock wave, on which the laws of conservation of mass, momentum and energy are satisfied.

Effect of Debye screening on heat transfer to a spherical particle from rarefied plasma

The methods of the kinetic theory tire applied for the description of charge and heat transfer front a rarefied plasma to a spherical particle for an arbitrary ratio between Dehye length and particle radius. Different models of the velocity distribution functions of the charged plasma species are considered. The results of the numerical analysis show that the intensity of plasma-particle heat exchange is greatly influenced by gas ionization, participation of electrons and ions in the transfer processes, particle charging, and .screening properties of the plasma. Even at a low degree of ionization, the electron and ion contribution to the heat transfer remains significant.

Heat transfer to nonspherical particles in a rarefied plasma flow

The interaction of a nonspherical metallic or nonmetallic particle with a rarefied thermal plasma flow is considered. Heat transfer to a particle of arbitrary shape with an extremely thin plasma sheath due to, respectively, gas molecules, electrons, and ions is described. Analytical expressions are derived for charge and heat fluxes in the particular case of a spheroidal metallic or nonmetallic particle in a subsonic plasma flow. It has been shown that the intensity of heat exchange is greatly influenced by gas ionization, charge transfer processes, and particle shape, velocity, and orientation in the plasma flow.

Identification of the level of heat exchange intensity in cavities of miniature cryogenic gas machines

If we represent the initial thin-walled structure as a three-layered structure with load bearing layers of zero stiffness, and a filler whose geometric characteristics and physicomechanical properties are identical with the characteristics of the thin shell, the minimal natural vibration frequency of such a structure differs by 1.1% from the natural vibration frequency of the thin shell. In accordance with'condition (2) the optimal step in this case is taken equal to At = 5.10 -6 sec; this is confirmed by the results of a comparative calculation for a thin and a three-layered shell (Fig. id).

Friction and heat exchange in flow over a permeable surface

The extremal character of the dependence of friction on suction velocity on a permeable surface immersed in an incompressible liquid flow is established. The suction value corresponding to maximum friction and the limiting heat exchange intensity are calculated.

Heat-exchange intensity in boiling at surfaces with porous claddings under capillary transport conditions

The results of an experimental investigation of the heat-exchange intensity in boiling at surfaces covered with porous, metal-fiber claddings under capillary transport conditions are described.

Local heat exchange of a cylinder in a slightly dusty flow

A study is performed of thermal resistances of convective heat exchange and thermal conductivity caused by reduction in the intensity of heat exchange when loose deposits are formed on the cylinder.

Using hot-gas sprayed aluminum coatings for evaporators of refrigerating machines

The authors investigated heat exchange in boiling of R-22 refrigerant on tubes made of 20 steel with an aluminum capillary-porous coating deposited on the tubes using the commercially manufactured EM-12M electric spraying machine. Tests were conducted on coatings produced at various pressures of air and with other technological parameters of the process being stable. The investigations show that the intensity of heat exchange in boiling of R-22 refrigerant on the steel surface with aluminum capillary-porous coatings is high.

Heat exchange between a laminar and pulsating plasma jet and a barrier

The influence of the parameters of the plasma stream and their time variation on the intensity of heat exchange near the stagnation point of a blunt body is analyzed.

Diffusion combustion of liquids with a free surface

This article develops an approximate (quasi-one-dimensional) approach to describe the combustion of liquids with a free surface. It is assumed that the distributions of characteristic quantities in the internal and external regions of the diffusion flame formed by combustion of the vapor jet are uniform. The combustion of liquids with a free surface is a complex process that consists of a number of sequential stages, in which liquid evaporates and vapor jets form above the surface, whereupon the vapor moves and reacts with the oxidizer contained in the surrounding medium. It is determined that the rate of combustion of a liquid layer of finite thickness located on a solid surface or the surface of a semiinfinite layer of inert liquid depends on the intensity of heat exchange with the phase boundary.