A. I. Leontiev

The effect of rearrangement of the vortex structure on heat transfer under conditions of increasing depth of a spherical dimple on the wall of a narrow channel

The results of numerical simulation of convective heat transfer in a spherical dimple located on the wall of a narrow square channel are used to analyze the effect of the hole depth on the vortex dynamics and intensification of heat transfer. It is demonstrated that the rearrangement of the separation flow structure from symmetric to monotornado is accompanied by a significant intensification of heat transfer both in the region of the spherical dimple (∼60%) and in the wake behind it (∼45%).

Heat transfer intensification for laminar and turbulent flows in a narrow channel with one-row oval dimples

One-row dimple reliefs for heat transfer intensification in narrow channels are designed based on the use of oval dimples for laminar and turbulent air flows. A comparative analysis of heat transfer in a channel with 22 one-row dimples and a periodic calculation module with one dimple under periodic boundary conditions at the flow-through boundaries is carried out. The focus is made on estimating the thermal and thermohydraulic effectivenesses of the channels under consideration.

The effect of intensifiers of heat transfer on the thermohydraulic properties of channels

The high energy-saving capacity of channels with transverse projections and the economic expediency of their extensive uses in industrial heat exchangers are confirmed. Models and methods of thermohydraulic calculation of discretely rough channels are analyzed. The problem of the effect of spacing between projections on the drag and heat-transfer coefficients of the channels is discussed in detail. The consequences of this effect are considered, such as the bistable mode of flow, self-induced oscillation, and resonance in the flow. Technical recommendations are given for design engineers. Problems for further investigations are formulated.

Compressible turbulent boundary layer on a permeable plate with injection of foreign gas

A numerical solution is obtained for equations of motion, diffusion, and energy using a three-parameter model of turbulence for boundary layer with different thermal boundary conditions on a permeable plate with injection of foreign gas. For an ideal gas with constant values of Prandtl and Schmidt numbers, the numerical solution is compared with limiting dependences for dimensionless temperature and enthalpy of gas on the wall. For helium/xenon mixtures with a low value of Prandtl number, the effect of intensity of injection and of the values of Reynolds and Mach numbers on the integral and local characteristics of flow and heat and mass transfer is investigated.

Numerical Modeling and Physical Simulation of Vortex Heat Transfer Enhancement Mechanisms Over Dimpled Reliefs

The paper presents a comprehensive analysis of conditions for numerical simulation and physical modeling of convective heat transfer in the vicinity of dimpled surface relief. Contradictory results, unreasonable assumptions, and non-justified conclusions are marked. Based on the analysis of physical experiments the correlation between the predictions and measured data is discussed. Detailed numerical study of turbulent air flow and heat transfer in the narrow channel with three types of dimples (spherical, conic and oval) was carried out. Various mathematical and discrete models, including, those based on solving Reynolds-averaged Navier-Stokes equations (RANS/URANS-SST), and also adaptive scale models (SAS-SST) are compared. The influence of flow parameters (Reynolds number) and geometric sizes (dimple diameter, depth, radius of rounding off of an edge, channel width and height) on local and integral characteristics of flow and heat transfer (total heat output and hydraulic losses) is determined. Special attention is given to reorganizing vortex structures and flow regime (with periodic fluctuations) with increasing relative dimple depth and Reynolds number. For the first time the influence of the scale factor of a constant cross-section channel is detailed. Thermal-hydraulic characteristics of various dimpled reliefs are compared, and the advantage of an oval dimple over a spherical one is shown.Copyright

Investigation of laminar-turbulent transition in supersonic boundary layers in an axisymmetric aerophysical flight complex and in a model in a wind tunnel in the presence of heat transfer and suction of air

Analysis is made of the problems associated with laminar-turbulent transition in wall boundary layers, as well as of scale effects observed in the investigation of laminar-turbulent transition in wind tunnels and laminarization of flow. Flight-performance data are given on the Reynolds number and on the gradient criterion of stability at the beginning of transition on the nose part of the Oblako aerophysical complex in the presence of heat transfer for the numbers ReL, ∞ ≤ 2 × 107, M∞ ≤ 2.0, and acceleration a ≤ 12g. Experimental data are given on laminarization of flow past a porous plate in a wind tunnel under the effect of suction for M∞ = 2.5. The theory of Emmons turbulent spots is generalized to the flight conditions of flow past the nose part of the Oblako aerophysical flight complex in the presence of heat transfer and to the case of laminar-turbulent transition on a porous plate for M∞ = 2.5 in the presence of suction of air.

Vortex Heat Transfer Enhancement in Narrow Channel by Oval Dimples Arrangement

There are many passive techniques of heat transfer enhancement ranging from surface (2D) to volumetric (3D) vortex generators, however only a few of them are capable to provide a reliable increase in a heat transfer rate overrunning the increase in pressure losses. One of such successful techniques is the profiling of a heat transfer surface with the regulated arrangement of 3D cavities (dimples). The authors explored that the deviation of the dimple geometry from the spherical shape affects the flow structure and thermal and hydraulic performance of the dimpled wall. Detailed numerical simulation of fluid flow and heat transfer has been performed in the narrow channel with the 2.5 × 0.33 cross section normalized by the equivalent diameter of the dimple footprint at the constant Reynolds number Re = 10,000 and the constant heat flux through the dimpled wall. The oval dimple geometry was varied by changing the aspect ratio of the dimple footprint from 1 to 4.5 keeping the same footprint area. In the course of the numerical study, the optimal geometry, the arrangement and the orientation of oval dimples on the heated surface to achieve the superior thermal and hydraulic performance over the spherical cavities are established. Numerical results of local and integral heat transfer characteristics enhanced with the visual representation of the generated vortices clearly illustrated the flow restructuring and an increase in the thermal and hydraulic performance.Copyright

Intensification of a Laminar Flow in a Narrow Microchannel with Single-Row Inclined Oval-Trench Dimples

A fully developed laminar air flow in a plane-parallel channel of width 6 and height 1 with single-row inclined oval-trench dimples on the walls are calculated using multiblock computing technologies at Re = 103. A periodic channel section of length 4 with one dimple of length 4.5, width 1, an angle of inclination to the flow of 45°, and a depth varying from 0 to 0.375 is considered. Intensification of a laminar flow in the flow core in a channel supplied with trench dimples of depth more than 0.25, such that the maximum velocity is 1.5 times higher than the maximum flow velocity in a smooth channel, is found.

Efficiency of Surface Heat Transfer Intensifiers for Laminar and Turbulent Flows in Heat Exchanger Channels

The present work presents the comparison of efficiency of surface-type intensifiers in a wide range of geometric parameters and in the range of Reynolds numbers corresponding to rated laminar and turbulent flow regimes in a channel on the basis of a single general criterion. It is also aimed to clarify objective thermal and hydraulic properties of intensifiers, to present a table of intensifier optimal parameters for the further theory and practice development, to formulate practical recommendations for selecting intensifier types and parameters in the design of promising energy-saving heat transfer equipment. The comparison of intensifiers efficiency was carried out under identical conditions for constricted and unobstructed channels containing intensifiers of various shapes. It is shown that intensifiers in the form of transverse annular ridges and systems of spherical holes show high thermal and hydraulic efficiency at specific flow operating parameters and design parameters of both, a channel and intensifier. The acquired results and recommendations on optimal parameters of surface-type intensifiers make it possible to calculate and design heat exchangers with intensifier optimal configurations and sizes. To confirm the obtained fundamental knowledge in the field of surface heat transfer augmentation based on a system of spherical holes/ridges several heat exchanger prototypes were developed.Copyright

Microlayer during boiling in narrow slot channels

An international space station Alpha will have a two-phase thermal control system. Boiling of a liquid ammonia will be a process of heat collection in evaporative heat exchangers. Unfortunately, only little data is available for boiling heat transfer in microgravity. Geometries of boiling channels working good in normal gravity are not appropriate in microgravity, and special means should be worked out to avoid some undesired events. From this point of view, the narrow slot channels may be assumed as a promising geometry for microgravity operation. During boiling in narrow slots, the vapor bubbles are flattened between the channel walls. The vapor phase and the channel wall are separated by a thin liquid film which is known as a microlayer. The paper presents the experimental results compared to the theoretical analysis, the paper also shows the narrow slot channels as a perspective configuration for microgravity applications.