Single Tube Row Research Articles

Impact of a Natural Gas Cooler Design on the Cooling Performance

This article describes a methodology for the identification of cooling performance of a natural gas cooler relative to the shape of its heat-transfer surface and presents the outputs of numerical solutions for four different shapes of heat-transfer surfaces in coolers designated as C_A, C_B, C_C and C_D. Calculations were carried out for a cooler with a single row of tubes, and for coolers with two through six rows of tubes that were positioned above one another with an alternating arrangement. In all of the surface shapes, the boundary conditions were respected in order to facilitate the identification of the shape of the heat-transfer surface which is the most appropriate for achieving maximum cooling performance. Out of these four shapes, the best results were observed with the heat-transfer surface of the cooler designated as C_A. The cooling performance of a 1 m long tube with such a surface was 1,650 W.

A critical review of filmwise natural and forced convection condensation on enhanced surfaces

The use of enhanced surfaces is an efficient method to improve filmwise condensation. Due to their immense potential, many enhanced structures were developed and investigated with the aim of improving natural and forced convection condensation heat transfer coefficients. This has resulted in large collections of predictive models and experimental data being reported. In this review, the developments in this field of research in the past few decades and the recent advances are collated and examined. This paper focuses on the review of natural convection condensation on the external surfaces of enhanced flat plates and tubes and forced convection condensation in enhanced tubes. The various models predicting the heat transfer coefficients on plain and enhanced surfaces are evaluated. For natural convection condensation, the liquid film-based models, semi-empirical models and numerical modes are reviewed whereas, for forced convection condensation, the gravity-dominated and vapor shear-dominated models are discussed. The effects of these enhanced structures on the liquid film and two-phase flow characteristics are analyzed and the various types of enhanced tubes and flat plates investigated are categorized. The manufacturing techniques employed to fabricate these surfaces are identified. A detailed evaluation of the heat transfer and pressure drop performances of the various enhanced surfaces is performed. In addition, their thermal performances are summarized and compared, and their associated heat transfer mechanisms are elucidated. For external condensation on a single tube row, three-dimensional fin structures were found to provide better thermal performance than two-dimensional structures, with some three-dimensional fin structures exhibiting more than 6 times the heat transfer coefficients of a plain surface. However, in a tube bundle, the heat transfer coefficient of three-dimensional fin tubes decreases more significantly with increasing tube row as compared to two-dimensional fin tubes. For convective condensation in circular tubes, the herringbone and pin fin tubes demonstrated better thermal and pressure drop performances than other internally enhanced tubes. Their efficiency indices were between 1.25 and 1.28. Based on the literature surveyed, the various experimental results are compared, existing research gaps are identified and frameworks for future research work are provided.

Heat transfer enhancement of a single row of tube

A rod was positioned upstream of a circular cylinder to enhance its heat transfer and reduce its drag in air stream. The results are increasing the overall heat transfer by 40% over and reducing the drag by 30% the value for a single circular cylinder. In the present, this technique applied to a single row of tubes. Heat transfer enhancement and drag reduction are important factor for multi-tube heat exchanger. The present study investigated the heat transfer and surface pressure characteristics of each tube for single row of tubes under various numbers of tubes, pitch and Reynolds number. The tube diameter, D, was 20 mm, and the rod diameter was 5 mm. The distance between the center axes of the rod and the tube was 30mm. The Reynolds number based on D ranged from 5.3×103 to 2.1×104 . The result is that this technique is effective for heat transfer enhancement and drag reduction of row of tube.

Numerical Simulation of the Particle Deposition on a Tube with Coupled Lattice Boltzmann Method and Finite Volume Method

A coupling method between multiple-relaxation-time lattice Boltzmann method and finite volume method is used to simulate the 2D fouling process on a single row of tubes. The particle motion is simulated by the cellular automata model. The fluid flow and particle deposition process around the tubes is simulated by multiple-relaxation-time lattice Boltzmann method, while the downstream fluid flow is simulated by finite volume method to save the computational time. The restitution coefficient calculated by the energy balance and the critical impact angle are used as the criterions for particle deposition. The deposition processes for 5μm, 10μm particles with inlet velocity 5m/s and 10m/s are simulated with the method. The results show that the particle with small diameter and inlet velocity tends to deposit more rapidly. The vortices behind the tubes bring the particles back to the leeward side of the tube, and a concave shape of fouling continues growing on the entire leeward half of the tube. A cone shape fouling layer forms on the windward side of the tube. The cone shape changes the air flow and stops the deposition on the windward side. The proposed coupling method is suitable for the study of the fouling process on the tubes.

Falling-Film Evaporation on Tube Bundle with Plain and Enhanced Tubes—Part I: Experimental Results

The complexities of two-phase flow and falling-film evaporation on a tube bundle present important challenges in the design of heat exchangers and the understanding of the physical phenomena taking place. The development of structured surfaces to enhance nucleate boiling heat transfer and, thus, reduce the size of evaporators adds another level of complexity, requiring the characterization of the effect of enhanced surfaces on the boiling phenomena occurring in a falling-film evaporator. In this article, falling-film evaporation measurements on a single tube row bundle and a three-row tube bundle were performed to obtain local heat transfer coefficients. Four types of tubes were tested: a plain tube, an enhanced condensing tube (Gewa-C+LW, Wieland Werke, Ulm, Germany) and two enhanced boiling tubes (Turbo-EDE2, Wolverine, Hunstville, Alabama, USA; Gewa-B4, Wieland Werke, Ulm, Germany). Two refrigerants, R134a and R236fa, were tested at a saturation temperature of 5°C, liquid film Reynolds numbers ranging from 0 to 3,000, and heat fluxes between 20 and 60 kW/m2 in pool boiling and falling-film configurations. Finally, the understanding of the physical phenomena governing the falling-film evaporation of liquid refrigerants has been investigated with visualization of the process.

A numerical study of the fluid flow and heat transfer around a single row of tubes in a channel using immerse boundary method

The analysis of the time-dependent and two-dimensional fluid flow and heat transfer around a single row of tubes in a channel is performed numerically. Due to its fundamental significance and practical importance, aerodynamic and heat transfer characteristics of tube bundle have been paid great attention by many researchers. In the present study, the immersed boundary method is applied by using a Cartesian grid system. Numerical solution for the governing equations of mass, momentum and energy conservation is obtained with the finite volume method. To validate the numerical approach with the immersed boundary method, the results have been compared with published data. The generation and evolution of vortical structures, wake interactions and their effects on the drag, lift and heat transfer are analyzed at different Reynolds numbers. The effect of hydraulic boundary layer development on the fluid flow and heat transfer is also investigated.

Effect of fins on forced convection heat transfer around a tube in an aligned‐arranged tube bundle

AbstractThe effect of fins on heat transfer around a tube in an aligned‐arranged tube bundle was investigated experimentally, and the obtained results were compared for three arrangements, i.e., single tube, single tube row, and staggered‐arrangement. It was found from the experiment that the effect of fins begins to appear in an aligned‐arrangement with larger fin spacing than in a staggered‐arrangement. The degradation in the local heat transfer coefficient due to fins can be recognized not only on the rear region of the tube, as observed in other arrangements, but also on the frontal region. As a result of this phenomenon, the degradation in the average heat transfer coefficient in an aligned‐arrangement becomes larger than in other arrangements with the same fin spacing. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 555–563, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20091

Effect of Fin on Forced Convection Heat Transfer Around a Tube in an Aligned-Arranged Tube Bundle

Effect of fin on heat transfer around a tube in an aligned-arranged tube bundle was investigated experimentally, and the obtained results were compared with other arrangements, i.e. single tube, single tube row and staggered arrangement. It was clarified from the experiment, that the effect of fin begins to appear with larger fin spacing than the staggered arrangement, and the degradation in local heat transfer coefficient due to fin can be recognized not only in the downstream side of the tube, as observed in other arrangements, but also in the upstream side. As a result of this phenomenon, the degradation in average heat transfer coefficient becomes larger than the other three arrangements with the same fin spacing.

Effect of fins on forced convection heat transfer around a tube

AbstractEffect of fins on heat transfer around a tube was investigated experimentally. A test tube of 30 mm diameter was installed in a test section of an open‐type wind tunnel as a single tube, or as a center tube in a single tube row and in a tube bundle of staggered layout. Fins made of paper were put on the test tube having certain fin spacing. It was clarified from the experiment that the local heat transfer coefficient around the tube degrades with decreasing fin spacing, especially on the downstream side of the tube, and the minimum fin spacing where the effect of the fin begins to appear is the largest for the single tube and the smallest for the tube bundle. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 445–454, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10098

A numerical study of heat transfer in fin?tube heat exchangers using winglet-type vortex generators in common-flow down configuration

Numerical investigation of flow and heat transfer in a rectangular duct with a built-in circular tube in the presence of delta-winglet type vortex generators has been carried out for moderate Reynolds numbers and three different angles of attack of the winglets (24?, 30? and 34?). The investigation is necessitated by the need to enhance heat transfer on the gas side of the fin-tube heat exchangers through generation of streamwise longitudinal vortices. The strategy for heat transfer enhancement involves introduction of a strong swirling motion in the flow field. The swirling motion can be generated by the longitudinal vortices. In the present study, the longitudinal vortices have been created by the delta-winglet type vortex generators in common-flow-down configuration. The vortex generators are mounted behind the tubes. An element of a heat exchanger has been considered for detailed study of the flow structure and heat transfer analysis. The present numerical study is limited to isothermal fins, single row of tubes and laminar flow situations. The analysis can easily be extended to multiple rows of tubes. It has been separately verified that the flow situation retains its laminar nature for the range of Reynolds number considered in the present problem. Hence the heat transfer analysis does not use any turbulence model in the present investigation. The consequences of employing these vortex generators are a significant separation delay and removal of the zone of poor heat transfer from the near-wake of the tubes. The heat transfer in the near-wake region can be enhanced as high as 230%. Results also show a marked increase in overall heat transfer in the channel.

Effect of Fin on Forced Convection Heat Transfer Around a Tube.

Effect of fin on heat transfer around a tube was investigated experimentally. A test tube of 30 mm diameter was installed in a test section of an open type wind tunnel as a single tube, or as a center tube in a single tube row and in a tube bundle of staggered layout. Fins made by paper were put on the test tube having certain fin spacing. It was clarified from the experiment, that the local heat transfer coefficient around the tube degrades with decreasing fin spacing, especially on the down-stream-side of the tube, and the minimum fin spacing where the effect of fin begins to appear is the largest for the single tube and the smallest for the tube bundle.

Effect of Tube-to-Tube Ties on Fluidelastic Instability of Tube Arrays Exposed to Crossflow

Tube-to-tube ties and their arrangement within the tube array are shown to affect the onset of fluidelastic instability. The influence of tie arrangement for a single tube row and for an in-line tube array is obtained by numerical simulation using S. S. Chen’s unsteady flow theory. Maps of dimensionless critical velocities for groups of tubes consisting of two, three, four, and five tubes tied to each other are developed for several design configurations. It is shown that the stability limits can be raised by appropriate choice of tube group and tie location.

Particles flow pattern and local heat transfer around tube in moving bed

AbstractFlow visualization of flowing particles around a tube of various types of tube arrangements in a moving bed was studied using X‐ray video films to obtain a relation between particles behavior and local heat‐transfer coefficients. A stagnant part of solid particles was observed on the tube in the case of a staggered arrangement. This part did not appear in the case of the single tube and the single row of tubes. The measured local heat‐transfer coefficients around a tube was decreased in this stagnant part. Furthermore, influences of different tube arrangements both on flow patterns of particles and on local heat‐transfer coefficients between tubes and bed were examined.

Prediction of bubble size in a fluidized bed containing horizontal tubes

A freely-bubbling fluidized bed containing horizontal banks of tubes was observed by means of X-rays. The number of rows of tubes, the distances between them, the distance of the first row above the distributor and the fluidizing gas velocity were all varied, the size distribution of bubbles rising through the tube banks being recorded on cine film for subsequent frame-by-frame analysis. In many cases interaction between bubbles and tubes caused the bubbles to break up into two or more “daughter” bubbles, the sizes of which were determined by the ratio of the diameter of the tube to that of the parent bubble as well as by the horizontal separation between the tubes. Following break-up at a row of tubes bubbles grew by coalescence before reaching the next row above, whereupon further break-up occurred. From measurements with a single row of tubes a correlation was developed for the breakage rate and this was combined with an expression for bubble growth to give predictions of bubble size at any point in a bed fitted with tubes. The predictions were found to be in good agreement with the measurements made in the multitubular bed.

Heat Transfer and Pressure loss of a very shallow fluidized-bed heat exchanger Part 1. Experiment with a single row of tubes

A novel gas fluidized-bed heat exchanger with a very small static bed height has been developed for a heat-exchanging system using a low-pressure fan. This fluidized bed is composed of a multislit distributor, a single row of 8 mm diameter tubes, and glass beads 48–195 μm in diameter. The measured performance of heat transfer is excellent and that of fluidization is satisfactory, in spite of the static bed height being as small as 13 mm. In the best case, the test fluidized bed exhibited a heat transfer performance comparable to that of a conventional fluidized bed with a perforated plate distributor and a static bed height of 150 mm, and showed one-fourteenth the pressure loss.

Experimental study of the flow field downstream of a single tube row

An investigation of the flow behind a tube row in cross flow was carried out at a pitch-to-diameter ratio P/D = 1.3. A complex instability of the parallel jets emerging between the tubes, leading to a joining of groups of three or more jets, was observed by means of velocity traverses at various distances downstream of the tube row. The literature reports the existence of an instability leading to the joining of pairs of jets at P/D = 2.3 or less. Careful review of photographs and other evidence also supports the more complex instability involving groups of more than two jets when P/D = 1.5 or less.

An experimental investigation of fluid-elastic instability for a single row of tubes subjected to two-phase cross flow.

Interesting features, connected with fluid-elastic vibration of tube bundles in a cross flow, have been reported in many papers. Most of the experiments were done for single-phase (liquid or air) flow. This paper presents an experimental investigation on vibrations of a row of five circular tubes with P/D =1.33, subjected to single-phase water and two-phase (air-water) cross flow. Experiments covered mass-damping parameters from 0.9 to 5 by changing the damping ratio. Experimental results show differences in characteristics of vibration response between those for pure water and two-phase flows. Depending on conditions specified by mass-damping parameters, void fraction and reduced flow velocity, air bubbles contained in the flow destabilize the vibrations of the row of tubes or they suppress the vibrations. Further, critical flow velocity for two-phase flow induced fluid-elastic vibration has been made clear for mass-damping parameters ranging from 0.9 to 5.

Influence of surface roughness on the flow through a staggered tube bank

The influence of surface roughness on the cross-flow through a staggered tube bank is investigated in the range of Reynolds numbers 4 ×104 <Re, < 107. At two roughness conditions the total pressure drop, the static pressure distribution and the local skin friction were measured. In the subcritical flow regime no effect of surface roughness on the pressure drop could be observed. Exceeding a critical Reynolds number, which itself is depending on surface roughness, the curve splits in terms of surface roughness parameterks/D. In the transcritical range the flow becomes independent of the tube diameterD, while the roughness heightks is the new characteristic length. By means of the local pressure and skin friction distribution the boundary layer behaviour was studied. Particularly the boundary layer separation and the friction forces were considered. The experiments are completed by investigations on the flow past a single row of tubes.

Vortex Excited Vibration of a Heat Exchanger Tube Row

The results are presented of an experimental investigation into the possible modes of gas-excited vibration of a single row of tubes in a heat exchanger at typical practical spacings. The vibrations discussed are vortex excited and, for damping which permits large amplitudes, occur at values of the Strouhal number 0·26 &lt; ND/V &lt; 0·89. The critical range of ND/V is affected by both Reynolds number and spacing. Four distinct modes of vibration are found, all consisting of motion principally transverse to the flow direction. Preliminary studies of pairs of tubes, at varying spacing, indicate similarly large ranges of the parameter ND/V in which vibration will occur at typical practical spacings. The types of vibration described are distinct from, and should not be confused with, either the acoustically coupled type or the equally important modes, in which the motion is principally streamwise, occurring at much lower values of ND/V.