Particularity of Heat Exchange of Twisted Heat Exchangers in External Flow

Perfecting the existing technologies and developing new ones require to rethink the processes in order to obtain qualitatively new results. Widespread use of cryogenic engineering in the chemical industry and medicine calls for a thorough analysis of both the efficiency of thermodynamic cycles and the hardware design of appropriate equipment. The power necessary to obtain low working medium temperatures is distributed between the cooling of the object and the losses in the various elements of the cryogenic setup. One of the best ways to increase the efficiency of the setup is to use the cold energy recovery. This is done by using various designs of recuperative heat exchangers, such as twisted heat exchangers. Existing methods of calculating the parameters of power equipment are based on empirical dependencies, which require some justification and clarification in order to be used for calculating cryogenic equipment parameters. The article describes the experimental setup, presents the research methods applied and analyses the results of the study on convective heat transfer in external flow past the tubular surface of the twisted heat exchanger. The obtained results for the laminar gas flow mode at Re < 2300 allowed determining the length of the initial heat section depending on the regime parameters of the contact phases and the geometric specifications of the twisted heat exchanger. The obtained dependence will make it possible to refine the method of calculating the parameters of the twisted heat exchanger in the annular channel.

Development of Highly Efficient Radiation-Convective Recuperator for Furnace of Secondary Aluminum Melting

The high efficient design of the radiation-convective recuperator with secondary emitters have been proposed, in which due to the rational arrangement of heating surfaces, as well as due to the installation of secondary emitters in flues, an increase in heat perception is transmitted to the secondary heat carrier – preheating air. High efficiency of air preheating is provided by two-stage heating: 1st stage of heating – the internal air ring channel with bilateral heating which is washed by combustion products from the parties of the central cylindrical and peripheral ring channels of combustion products; 2nd stage of heating – the external air ring channel in which unilateral heating by products of combustion from the peripheral ring channel of products of combustion is organized. Inner and outer annular air ducts (tanks), interconnected by bypass pipes. To increase the efficiency of heat transfer in the considered recuperator in the central channel of combustion products is placed emitter, which consisting of intersecting radial plates, and in the annular channel of combustion products are placed auxiliary emitters, which made in the form of flat radial edges. These emitters provide an increasing in total heat flux to the walls of the channels of the recuperator. On the basis of the conducted theoretical researches, engineering calculations and CFD – modelling the characteristics of operation of the recuperator for its installation on the furnace of secondary smelting of aluminium are defined. The main advantages of the new design of recuperator are high thermo-hydraulic efficiency, compactness and low metal consumption, ease of installation on the furnace and no need for placement in separate chimneys. It is established that the recuperator provides air heating ta,ex ~ 400 °C at an acceptable aerodynamic drag (pressure drop) on the air track (∆pa ~ 1000 Pa). Appropriate design documentation has been developed for the manufacture of the recuperator, which is installed on a pilot furnace of secondary aluminium smelting by California Die Casting (USA).

Shapes and Rise Velocities of Single Bubbles in a Confined Annular Channel: Experiments and Numerical Simulations

Shapes and rise velocities of single air bubbles rising through stagnant water confined inside an annular channel were investigated by means of experiments and numerical simulations. Fast video imaging and image processing were used for the experiments, whilst the numerical simulations were carried out using the volume of fluid method and the open-source package OpenFOAM. The confinement of the annular channel did not affect the qualitative behavior of the bubbles, which exhibited a wobbling rise dynamic similar to that observed in bubbles rising through unconfined liquids. The effect of the confinement on the shape and rise velocity was evident; the bubbles were less deformed and rose slower in comparison with bubbles rising through unconfined liquids. The present data and numerical simulations, as well as the data collected from the literature for use here, indicate that the size, shape, and rise velocity of single bubbles are closely linked together, and prediction methods that fail to recognize this perform poorly. This study and the limited evidence documented in the literature indicate that the confinement effects observed in non-circular channels of complex shape are more complicated than those observed with circular tubes, and much less well understood.

Numerical study of swirling flow in a steam generating element model with double-sided heating

The paper is devoted to numerical modelling of the swirling flow in an annular channel with an inner twisted pipe. The computational model is designed. The technique of swirling flow calculation is tested for CFD packages LOGOS and ANSYS CFX. The velocity and pressure calculated fields are obtained. Experimental and calculated velocity profiles over the channel cross section are presented. The loss coefficient values is obtained. Experimental and calculated comparison of results is made.

Intensification of heat transfer in the inlet of the convective stage of the recuperative-burner unit

The study of aerodynamics and heat transfer in the recuperator convective stage of the recuperative-burner unit with the jet leakage of the flue gas flow onto the outer cylindrical surface is carried out. Numerical modeling of the problem was carried out in a three-dimensional formulation using the ANSYS Fluent software package. It was found that in the original design of the recuperative-burner unit, there is a significant unevenness of heat transfer along the length and perimeter of the working surface of the convective stage. In the initial section of the annular gap, a stagnant zone with the lowest heat transfer rate is observed. To eliminate the stagnant zone and to intensify heat transfer on the surface in this area, it is proposed to make the entrance to the perforated pipe in the form of an inner quarter of a torus; to install smooth protrusions on its surface; to locate an annular flow divider on the inner surface of the heat transfer wall, which separates the front part of the annular channel with formation of a set of vortex chambers. The research results are presented.

Investigation of aerodynamics and heat transfer of the modular jet recuperator

A study of the aerodynamics and heat transfer of a jet modular recuperator with a change in its geometric characteristics has been carried out. The influence of the in-line and staggered arrangement of the blowing holes, as well as the diameter of the perforated pipe is considered. In all considered variants, the number of holes, their diameter and gas flow rate through the recuperator remained unchanged. Numerical modeling of the problem was carried out in a three-dimensional setting using the ANSYS Fluent 15.0 software package. It was found that with the in-line arrangement of the blowing holes, secondary flows are formed between their longitudinal rows in the form of swirling jets of opposite rotation directed towards the outlet section of the recuperative device, through which the main part of the heated air flows out. With the staggered arrangement of the blowing holes, the formation of spiral vortices is disturbed, the air flow is carried out along the entire cross section of the annular channel, increasing the drift effect of the flow on the impact jets, which leads to a decrease in the intensity of heat transfer and its uniformity along the length of the working surface. An increase in the diameter of the inner perforated pipe leads to a decrease in the drift effect of the cocurrent flow on the jets, an increase in the distribution uniformity of the heat flux along the length of the heat transfer surface, and an increase in the heat transfer coefficient.

Investigation of the influence of narrowing annular channel and Reynolds number on formation of Tayler-Gortler vortexes

Abstract. The article investigates the Taylor-Gortler vortices arising in a swirling flow when the annular channel narrows. Several options for the geometry of the narrowed annular channel are researched. In the first case, the outer cylinder with a constant diameter and the inner cone with a variable cone angle are considered. In the second case, on the contrary an inner cylinder with a constant diameter and an outer cone with a variable cone angle are considered. All geometries were tested at different Reynolds numbers Re. = 8.3-103…21•103. As a result, the analysis of the propagation of secondary vortices along the length of the annular channel is presented.