Bimetallic Finned Tubes Research Articles

Convective Heat Exchange of Single-Row Bundles from Tubes with Rolled Aluminum Fins of Various Height at a Low Values of the Reynolds Number

The experimental study of the heat flow intensity of a single-row horizontal air-cooled tubular bundle of heat exchanger with spiral aluminum rolling fins at low Reynolds numbers (Re < 2000) is performed. The geometrical dimensions of the bimetallic finned tubes of the bundle, the following: the outer diameter of the fins d = 56.0 mm; the diameter of the tube at the base d0 = 26.8 mm; fin height h = 14.6 mm; pitch of fins s = 2.5 mm; the average fin thickness Δ = 0.5 mm; the coefficient of finned tubes φ = 19.3; heat transfer length l = 300 mm. The outer diameter of the load-bearing steel tube dн = 25 mm; wall thickness d = 2 mm. The research was carried out by the method of full thermal modeling at a specially designed experimental stand with electric heating of tubes and an exhaust shaft installed above the bundle. The air flow rate through the bundle was regulated by changing the height and cross-sectional area of the exhaust shaft. Calibration experiments were carried out and confirmed the reliability of the data obtained. Then the fins were sanded so to form new types of tubes, which were arranged in a single-row six-tube bundle with a constant relative cross-step σ1 = S1/d = 1.14 = const, and the thermal studies were repeated. As a result, a generalized criterion equation for heat transfer of a finned horizontal single-row bundle at small Reynolds numbers for various heights of the tube finning h = 0-14.6 mm was obtained. The effective height of the tube finning (h = 8 mm) for a single-row horizontal bundle was determined by dimensional and metal-intensive criteria.

Convective instability of air flows in the exhaust shaft above a four-row finned beam

Herein, multidirectional quasiperiodic air flows in an exhaust shaft above a four-order horizontal bundle consisting of bimetallic finned tubes used to remove heat in heat exchangers are considered. Modeling of the air movement is carried out on the basis of equations for thermogravitational convection in the Boussinesq approximation. It takes into account the viscosity of the air and the dependence of the air density on the temperature. An interpretation of quasiperiodic airstreams is proposed on the basis of Rayleigh – Bénard convection, as a result of which regular structures, called Rayleigh – Bénard cells, are formed in a liquid or gas. Rayleigh – Bénard cells are an analytical solution to the problem of the stability of hydrodynamics flows in the linear approximation. The appearance of two-dimensional (convective rolls) and threedimensional (rectangular cells) is possible. To estimate the number of emerging structures, the critical Rayleigh numbers were calculated, which characterizes the transition from an unstable mode of the convective fluid flow to a stable mode. For two experiments, the experimental Rayleigh numbers are compared with their critical values. The differences between the experimental conditions and the ideal boundary conditions used in the calculations and the partial destruction of quasiperiodic structures as a result of this are also discussed.

Research Treatment Studies for Cleanable Waste Water for Cleaning Pipes


 
 
 Currently, many methods of treating waste water are known using various lubricating coolant compositions, but there is no universal treatment method. The most suitable ways of reducing the harmful impact of waste water in the metallurgical industry on the water environment are 1) the local treatment of waste water of various compositions, 2) the removal or reduction of the total amount of waste treatment liquid emissions as a result of their regeneration, and 3) reuse of treated waste water in closed water circulation systems and technical water supply to enterprises. During production of bimetallic finned tubes by cold deformation method, alkaline cleaning solutions are used to clean the surface from lubricating-cooling liquid. As a result, there is a need to dispose of spent liquids. The spent detergent solution is a three-phase emulsion. The upper layer is oil-oil products, medium liquid with increased PH index, and aluminates precipitate. A spent emulsion, i.e. an end-of-life emulsion that has lost its functional and operational properties, needs to be decomposed and disposed of.
 Keywords: finned bimetallic tube, air cooler, lubricating coolant, oil – emulsion
 
 

Experimental studies of the efficiency of a single-row bundle of bimetallic finned tubes with different finning heights in free convective heat exchange with air

Considerer the influence of a change in the height of the height of the finning of bimetallic finned tubes of a single-row bundle on the thermal, overall and metal-capacity characteristics in the mode of free air convection at different angles of inclination (γ = 0, 15, 30, 45, 60 and 90°) to the horizontal plane. The investigated tubes have the following characteristics: the material of the supporting tube was carbon steel (outer diameter d out = 25 mm, wall thickness δ = 2 mm), the material of the rolling finned shell was aluminum alloy AD1M. Geometric parameters of the fins: d×d 0 ×h×s×Δ×φ× l = 56,0×26,8×14,6×2,5× 0,5×19,3×300 mm (type I). The studies were carried out by the method of complete thermal modeling on a specially designed experimental stand. Then the fins were ground by grinding with the formation of new types of tubes (II–VI), which were assembled into a single-row bundle with a constant relative transverse pitch σ 1 = S 1 / d = 1,14 = const. The article describes the criterion dependences for heat transfer of a finned single-row bundle of tubes with different finning heights at different angles of inclination to the horizontal plane, as well as a graphical dependenc e of the correction factor for the angle of inclination of bundle (γ = 0–90°) were obtained. It has been established that an increase in the angle of inclination and the height of the finning of tubes of a single -row bundle is generally accompanied by a decrease in heat transfer. In terms of overall and metal-capacity characteristics, the most effective is a single-row bundle at various angles of inclination to the horizontal plane with a tube finning height 2,0 mm.

Enhancing Energy Saving in Air Cooling Devices by Intensifying External Heat Transfer

The aerodynamic processes of heated air in the ventilation shaft installed above the heat transferring bundle of bimetallic finned tubes are studied. The regions of suppressed natural, steady, and unsteady mixed air convection in the bundle depending on the degree of narrowing of the outlet section of the ventilation shaft are determined. The maximum Nusselt numbers for multi-row bundles and their corresponding optimum parameters of the degree of narrowing of the outlet section of the ventilation shaft are determined. A method is proposed for determining the Reynolds numbers and the average air velocity during free convection for contracted smooth and finned tube bundles, which will allow optimization of heat transfer processes. The practical effectiveness of single- and two-row bimetallic finned tube bundles with a finning factor φ = 21 and an intertube spacing of 58 mm in air cooling and similar aircooled heat exchangers having a ventilation shaft is scientifically proved.

Features of Gravitational Flow of Heated Air in an Exhaust Shaft Above a Multirow Finned Bank

The authors have carried out investigations into the introduction of hot-air heating systems in buildings and structures, and also the applicability of mixed convection in calorifers of recirculating air heaters. The heat transfer of two- and four-row staggered air-heater banks consisting of bimetallic finned tubes in mixed convection has been studied. Experimental investigations into the characteristics of a gravitational flow of heated air in an exhaust shaft above a heat-exchange bank in a mixed-convection regime have been presented. An increase of 1.5–3 times in the heat transfer in the mixed-convection regime compared to the heat transfer under conditions of free convection of the air has been established. The dynamics of motion of a geyser flow of heated air in the exhaust shaft above the heatexchange bank under gravity has been described. A decline of 20–50% in the heat transfer of staggered two- and four-row banks because of the presence of geyser air flows has been established. A method to diminish the adverse impact of geyser air flows on the heat transfer of the bank has been proposed.

Comparative analysis of experimental investigations and numericalsimulation of single-row finned tube bundle at mixed convection

Mixed convective heat transfer is very important for a wide class of engineering tasks. However, the experimental study of mixed convection requires significant implementation costs, high-power equipment, as well as large time costs, so it is proposed to expand the scope of experimental studies using numerical simulation. Numerical simulation of the single-row bundle consisting of bimetallic finned tubes at mixed air convection conditions was performed and experimental data were compared. The formulation of the third-dimensional problem for numerical simulation was realized. The conjugated problem for heat exchange modeling from the tube fins to air was solved. In numerical simulation of air momentum it was taken into account that the Reynolds number based on tube diameter and velocity in the space between fins was varied from 100 to 720. Menter’s k–ω shear stress transport model in standard formulation was used to close the Reynolds equations. Flow visualization on the tube surface revealed the transient nature of the air flow. The temperature distribution visualization in the bundle and the exhaust mine made it possible to see the nature of cooling the finned bundle at mixed convection. Results of numerical simulation and experimental investigations are in good agreement and can be used for expansion of the scope of experiments. The experimental data and the numerical simulation results for the single-row bundle consisting of bimetallic finned tubes at mixed air convection are compared in this paper. Flow near tube surfaces was visualized, and the temperature and velocity distributions in a bundle and in the exhaust mine were obtained.

Intensified Convection Heat Transfer of Single-Row Bunch of Finned Tubes in an Air Stream: Experimental Study and Generalization of the Obtained Data

The article presents method and results of experimental study of the intensified heat transfer for the single-row bunch consisting of bimetallic pipes with spiral knurled ribs with the following parameters: outer diameter of a rib d = 56 mm; rib height h = 15 mm, rib pitch s = 2.5 mm, average thickness of a rib D = 0.5 mm; diameter of a rib at the bottom d0 = 26 mm, coefficient of fins j = 21. The pipes cross pitch S1 in a single-row bunch makes 58, 61, 64 and 70 mm. The intensification of a heat transfer has been arranged in a stream of heated air over an experimental bunch by means of two types of exhaust shaft i.e. the one with adjustable height and the one with the adjustable section through passage. The aim of the work was to perform an experimental study and to summarize the data of a heat transfer of the single-row bunches consisting of bimetallic finned tubes under the intensified (mixed) air convection and also to develop an engineering method of calculation of single-row recirculation air heaters. The results of experimental study of the intensified convective heat transfer of the single-row bunch consisting of bimetallic finned tubes in a stream of heated air are presented in the form of dependences of number of Nusselt on Grashof number and Reynolds number. As a result of generalization of experimental data, the generalized criteria equation for calculation of heat transfer of the single-row bunch consisting of bimetallic finned tubes when cross pitches of installation of tubes, the areas of exhaust outlets and heights of the exhaust shaft are various, has been obtained. The engineering technique for design calculation of the single-row recirculation air heater has also been developed.

Research of heat transfer of staggered horizontal bundles of finned tubes at free air convection

The study of free-convective processes is important because of the cooling problem in many machines and systems, where other ways of cooling are impossible or impractical. Natural convective processes are common in the steam turbine air condensers of electric power plants located within the city limits, in dry cooling towers of circulating water systems, in condensers cooled by air and water, in radiators cooling oil of power electric transformers, in emergency cooling systems of nuclear reactors, in solar power, as well as in air-cooling of power semiconductor energy converters. All this makes actual the synthesis of the results of theoretical and experimental research of free convection for heat exchangers with finned tube bundles. The results of the study of free-convection heat transfer for two-, three- and four-row staggered horizontal bundles of industrial bimetallic finned tubes with finning factor of 16.8 and equilateral tubes arrangement are presented. Cross and diagonal steps in the bundles are the same: 58; 61; 64; 70; 76; 86; 100 mm, which corresponds to the relative steps: 1.042; 1.096; 1.152; 1.258; 1.366; 1.545; 1.797. These steps are standardized for air coolers. An equation for calculating the free-convection heat transfer, taking into account the influence of geometrical parameters in the range of Rayleigh number from 30,000 to 350,000 with an average deviation of ± 4.8%, has been obtained. The relationship presented in the article allows designing a wide range of air coolers for various applications, working in the free convection modes.

THE METHOD OF CALCULATION AND ANALYSIS OF HEAT TRANSFER COEFFICIENT OF BIMETALLIC FINNED TUBES OF AIR COOLING UNITS WITH IRREGULAR EXTERNAL CONTAMINATION

The article focuses on a new method of calculating heat transfer coefficient of bimetallic finned tubes of air coolers taking into account external operational pollution. In contrast to wellknown methods that use the assumption of a uniform distribution of operational contamination layer with a constant thickness over the entire surface of the fins in the present method being introduced it is assumed that the thickness of the pollution layer during long-term operation is changed irregularly. Under such conditions the thickness of the pollution layer at the base of the fins becomes much greater than at the rest of the finned surface. The suggested method is based on a mathematical model developed with the use of the method of electrothermal analogy, whereby the heat flow through the wall of the finned tube is considered as divided into two components, viz. through the annular layer of outside contamination adjacent to the base of the ribs, and through the remaining part of the external ribbed surface covered with a thin layer of pollution. Within the framework of the developed methodology a new method for determining the thermal resistance of the pollution layer, which is based on analytical solution of two dimensional problem of heat conduction in the annular layer has been created. With the use of this technique the influence of the degree of contamination of the intercostal space of the industrially manufactured bimetallic finned tubes on the heat transfer coefficient has been studied taking into account the intensity of heat transfer of air and the properties and composition of the pollutant for industrial manufactured bimetallic finned tubes. It is established that a layer thickness of the pollutant at the base of the ribs has the greatest influence on the heat transfer coefficient. This is due primarily to the change of actual coefficient of the fins. It is demonstrated that the heat conductivity of the external pollutant has a significant impact on the heat transfer coefficient when the heat exchanger functions in the mode of forced convection of air.

A Generalized Dependence for Convective Heat Transfer in Horizontal Bundles of Finned Tubes with Free Air Convection

Results of a study of free convective heat transfer in two-, three-, and four-row horizontal in-line bundles of bimetallic finned tubes with coefficient of finning 16.8 are presented. The steps in the breakdown in the tubular sheets correspond to the applicable standardized steps in air-cooled heat exchangers with cross step S1 = 58–76 mm, and values of the longitudinal step S2 in the range 58–100 mm correspond to each value of S1. A generalizing equation for calculating the average convective heat-transfer coefficient, taking into account geometric parameters studied in the range of Reynolds numbers from 20000 to 70000, is obtained. Through the use of the dependence, it has become possible to design a wide range of gas–liquidtype heat exchangers for different purposes that function in free convection regimes.

Vibration Reliability of Finned Tube Banks in Air Coolers

A procedure based on the effective diameter proposed by the author is presented for analysis of vibration indicators of heat-exchange bimetallic finned tubes (BFT) in a forced transverse flow of air. It is indicated that in characteristic operating regimes, tube bundles formed from BFT have a significant reserve of vibration reliability, and there is no danger of their failure owing to eddy separation and formation of an aerodynamic wake beyond the tubes.

New designs of bimetallic finned tubes for air-cooled heat exchangers

Energy- and resource-conserving characteristics of heat-exchange tubes with wound-on helical aluminum fins, which are employed in air coolers, are presented. Designs of bimetallic finned tubes with intensified convective and contact heat exchange, enhanced thermal reliability, and high adaptability to fabrication are described. Characteristics are given for fabrication of the tubes.

Heat transfer and resistance of staggered banks of air-cooled heat exchangers formed from tubes with rolled aluminum fins of various height

Results are presented for experimental investigation of reduced heat transfer and aerodynamic resistance of five staggered six-row banks of bimetallic finned tubes in heat exchange sections of air coolers with a forced air flow passing across the finned tubes. The fins are rolled onto circular steel tubes with a diameter d 0 = 25 mm. The relative transverse spacing of the tubes in the lattices of banks is s 1 = S 1 /d = 1.136, and the longitudinal spacing s 2 = S 2 /d = 0.905, where d is the outside diameter of the spiral fin. The banks were assembled from tubes with a fin height h = 15.23, 11.57, 9.07, 5.56, and 3.57 m. The diameter of the fin along the base was constant d 0 = 25.87 mm. The quantitative effect of the relative height h/d 0 on the average heat transfer and resistance of a bank is determined. The experimental data are described by criterial equations of the form Nu = f(Re) and Eu = f(Re) in the interval of Reynolds numbers Re = 3000–30000.

Effect of blade pitch in an axial fan on the energy characteristics of an air cooler

The effect of the pitch of the blades in an axial fan with an UK-2M impeller 2.8 m in diameter on the characteristics of a horizontal three-section air cooler (AC), the six-tier heat-exchange sections of which consist of bimetallic finned tubes with rolled aluminum fins (tube-finning factor of 20.4; medium cooled – gasoline) is analytically investigated. It is found that for the same power consumed by the fan drive, the pitch of the fan blades in the interval from 15 to 25° will not affect the thermal flux of the AC. Recommendations are given for selection of energy-expedient pitches of the blades during AC operation, and also for thermal analyses of units under design.

Effects of repeated high-temperature cycles on the thermal contact resistance of bimetallic finned tubes

Measurements have been made on the thermal contact resistance (TCR) in repeated cycles of heating to 270°C and cooling to 50°C in experimental specimens of bimetallic finned tubes having roll and strip fins. Tenfold heating-cooling cycles increase the TCR of these tubes in the cold state by factors of 1.5–2.5.

An energy-saving nondestructive tester for the thermal performance of bimetallic finned tubes

The block diagram is given for a tester whose specific feature is the countercurrent of motion of the heat-bearing media. The heating medium is hot water, which is pumped within the monitoring tube in a closed loop, while the cooling fluid is atmospheric air, which is supplied by a fan to the ring gap formed by the cylindrical jacket and the finned shell of the bimetallic tube coaxial with the jacket. A method is presented for monitoring the quality of manufacture for bimetallic tubes.

Thermal and aerodynamic characteristics of banks of bimetallic finned tubes manufactured by the Oktyabr’skkhimmash plant

Results of investigation are presented for an experimental convective-heat-transfer unit and the aerodynamic resistance of tube banks formed from three types of bimetallic finned tubes (BFT). It is demonstrated that the most important indicators of the thermotechnical efficiency of BFT are the thermal contact resistance (TCR) and its variation with increasing temperature. We were the first to determine the TCR during a rise and fall in the temperature of a steel bearing tube.

Test-stand for determining the quality of contact in bimetallic finned tubes

Test-stand for determining the quality of contact in bimetallic finned tubes

Analysis and measurement of contact and gap resistances in extruded bi-metallic finned tubes

The problem of contact and gap resistance in an extruded bi-metallic finned tube is analysed. A new experimental method for determining these resistances is proposed. The procedure is particularly accurate in the case of good quality tubes in which the contact resistance is difficult to measure by most other methods. It furthermore tends to highlight resistances in poor quality tubes. Test results employing this method are compared to theoretical predictions and data obtained by other test equipment.