Array Of Horizontal Tubes Research Articles

A numerical and experimental study of breakdown of falling film on horizontal circular tubes

Falling film horizontal tube heat exchangers are used in several industrial applications such as sea water desalination, multi effect distillation, chemical industries, ocean thermal energy conversion systems and refrigeration. Different flow patterns that occur during falling film evaporation on an array of horizontal tubes are droplet, droplet-columnar, columnar, columnar-sheet and sheet modes. From the heat transfer point of view, a sheet mode (continuous falling film spanning the intertube space) is the most desirable falling-film mode. Falling film breakdown leads to the occurrence of dry patches on tubes which cause heat transfer deterioration. In the present study, falling film breakdown on horizontal tubes under adiabatic condition has been investigated both numerically and experimentally. The study focuses on the influence of tube pitch to diameter ratio on the threshold film Reynolds number required to maintain continuous falling film. Results indicate that the threshold film Reynolds number increases with the increase in the pitch to diameter ratio for a given diameter of the tube. The study also shows that as the number of tubes in the column increases, the threshold film Reynolds number required to maintain continuous film over all the tubes increases. The study highlights the need for correlations for the column-sheet mode transition that take into account the effect of number of horizontal tubes in a vertical array.

Heat Transfer in a Falling Liquid Film of Freon R21 on an Array of Horizontal Tubes with Modified MAO Coatings

Heat Transfer in a Falling Liquid Film of Freon R21 on an Array of Horizontal Tubes with Modified MAO Coatings

Experimental study of heat transfer enhancement in a falling film of R21 on an array of horizontal tubes with MAO coating

The paper presents investigation results on heat transfer in falling films of refrigerant R21 on an array of horizontal tubes with a coating deposited by the method of microarc oxidation (MAO). Different MAO-coatings were applied on the surface of tubes of aluminum alloys AD31 and D16T with a diameter of 10 mm in silicate-alkaline, acidic, and phosphate electrolytes. Two coated test tubes were copper-plated additionally in a copper sulfate solution. The enhancement of heat transfer on the tubes with a porous coating depends in a complex manner on the parameters of MAO-coating surfaces defined by their treatment regimes. The highest 3-fold heat transfer enhancement in comparison with plain tube in the nucleate boiling regime was achieved on the surface of a tube made of alloy D16T with the MAO coating of 30-μm thickness, obtained in a silicate-alkaline electrolyte. Additional deposition of copper particles on the MAO-coatings leads to an enhancement of heat transfer by (20–30) % compared to plain tube. In the studied range of Reynolds number 600–1500, the effect of irrigation density on the heat transfer coefficients is not observed.

Heat Transfer at Film Cooling of an Array of Horizontal Tubes with an Enhanced Surface

The paper presents investigation results about the influence of various methods of surface treatment on heat transfer enhancement in falling films of R21 Freon on an array of horizontal tubes. The experiments were carried out on the aluminum alloy tubes with a ceramic coating obtained by micro-arc oxidation and on copper tubes with a structured surface, treated by deformational cutting. The experiments were carried out in a regime of turbulent film flow at Reynolds numbers from 400 to 1500. The results of measuring the heat transfer coefficients on the surface of samples with a developed surface and on the standard smooth tube are compared. The highest values of heat transfer coefficients in falling films during nucleate boiling were obtained on a structured surface with semi-closed cavities. Heat transfer enhancement on the surface of a tube made of alloy D16T with the MAO coating of 30 μm thick, obtained in a silicate-alkaline electrolyte, is comparable to heat transfer enhancement on the surface of copper tubes with a microstructure applied by the deformational cutting method.

Heat transfer in the falling liquid film on an array of horizontal tubes with MAO coating

The paper presents investigation results on heat transfer of falling liquid films of Freon R21 on an array of horizontal tubes with porous Al2O3 coating obtained by the method of microarc oxidation (MAO coating). The microcharacteristics of samples (porosity, thickness, surface roughness) were measured. The results of measuring heat transfer coefficients on the surface of an aluminum tube with a MAO coating and a reference smooth tube were compared. Heat transfer on the surface of the tube with a porous coating was lower than on a smooth reference tube. It was shown that to intensify heat transfer on a surface with a MAO coating, it is necessary to increase the porosity of the coating and decrease its the thermal resistance.

Effects of horizontal tube arrays on heat transfer in an external heat exchanger

Few studies have addressed the impact of horizontal tube arrays on the local bed-to-wall heat transfer coefficients. Therefore, in this paper, the local heat transfer coefficients were measured in an external heat exchanger with inline and staggered horizontal tube arrays. The corresponding gas–solid hydrodynamics on the tube surface was obtained by using the optical fiber probe. The lateral distribution of the heat transfer coefficients was found to be non-uniform, and the maximum values were achieved at the position of 0.64 times half-width. The impact of the tube arrays on the heat transfer coefficients varied with the lateral locations and the configuration of tube arrays. Compared with cases without tube arrays, the inline tube array reduced the heat transfer coefficients in the bed center but increased those near the column wall. Compared with the inline tube array, the staggered array increased the deviations of heat transfer coefficients both in the bed center and in the wall region. The modified packet renewal model could reflect the trend in the variation of the heat transfer coefficients along the bed width but slightly overestimated the values. The overall average heat transfer coefficients were also compared to the results predicted using common published correlations.

Falling film flow mode transitions on an array of horizontal tubes under nonuniform liquid distribution conditions

This study investigated the effect of nonuniform liquid distribution on the evolution of falling film flow mode on an array of horizontal tubes. A falling film flow mode observation system with water as the working fluid was designed and evaluated. In the test system, six liquid distributors, namely, DIS1, DIS2, DIS3, DIS4, DIS5 and DIS6, were designed to realise different liquid distribution conditions. The test tube array was composed of smooth copper tubes in which their outer diameter, tube length and tube spacing were 19.05, 280 and 10.0 mm, respectively. Results showed that the falling film flow mode transitions were strongly affected by liquid distribution conditions, and the falling film flow mode transitions under nonuniform liquid distribution conditions could not be predicted well by using the classic models that have been established under uniform liquid distribution conditions. For the droplet to droplet–column (D → DC), droplet–column to column (DC → C), column to column–sheet (C → CS) and column–sheet to sheet (CS → S) flow mode transitions, the transitional Reynolds number (Retr) for uniform condition (Retr,u) were 105, 145, 352 and 436, and the differences between Retr for the six nonuniform and uniform conditions were 50.4%, 38.7%, 18.1%, 5.93%, 23.4% and 37.1%; 22.1%, 37.0%, 23.3%, 55.7%, 51.9% and 55.3%; 10.7%, 3.53%, 10.7%, 12.4%, 16.7% and 29.6% and 94.1%, 58.2%, 13.7%, 26.2%, 29.6% and 96.8%, respectively. A semiempirical model was established to predict the evolution of falling film flow mode under nonuniform liquid distribution conditions. This study serves as reference for improving the flow pattern determination criteria of falling condensate on horizontal tube bundles.

Experimental study on fluid flow and heat transfer characteristics of falling film over tube bundle

The fluid flow characteristics, film thickness and heat transfer of falling liquid films over an array of horizontal tubes are experimentally studied. The effects of the flow rate, the heat flux and nozzles-holes arrangements on the averaged convective heat transfer coefficient are addressed. An experimental setup was constructed including a water circulation system, isothermal water bath, a feeder nozzle, three horizontal test tubes, aligned vertically, the setup structure frame and measuring system. The tubes wall temperatures were measured using K-type thermocouples embedded in the test tube walls. The fluid flow and the film thickness were captured using a high-speed camera. The measurements were performed at 20 °C for two kinds of nozzles-holes configurations of single space (nozzles pitch of 12 mm) and double space (nozzles pitch of 24 mm), three flow rates of 50, 100 and 150 mLPM and four heat fluxes of 5, 10, 15 and 20 kW/m2 °C. The measured results are reported in the form of flow-behavior photos in various time consequences, and the steady state film thickness in photos with marked measures. The tubes wall temperatures and heat flux are reported in tables and graphs. The measured results demonstrate that the arrangement of nozzles induces significant impact on the fluid flow and heat transfer characteristics of the test tubes. However, in the case of low flow rate (50 mLPM), the differences on the thermal performance are not notable. The results also show that the type and shape of film formation over the test tubes are very important on the heat transfer performance. When the liquid film extends over the test tube or covers it, the heat transfer rate increases. The heat transfer coefficient (h) for the test tube 1, just below the nozzles, is the highest; then test tube 2 in the middle receives better heat transfer rate, and test tube 3 at bottom is the last with lowest heat transfer rate. The nozzles pitch is a key parameter, which allows film bonding and can be considered as a controlling parameter for heat transfer enhancement and the flow structure.

Heat transfer characteristics of falling film evaporation on horizontal tube arrays

A falling film heat transfer test facility has been built for the measurement of falling film evaporation in a vacuum of about 1000 Pa. At this condition, only convective evaporation occurred in the liquid film. The Reynolds numbers of falling film over a range from 21.6 to 108.1 were tested on six-tube arrays made of enhanced or smooth tubes. Results show that the tubes with both enhanced outer and inner surfaces give high heat flux. Besides, as the Reynolds number increases, the heat transfer enhancement ratio of falling film evaporation decreases. A semi-analytical correlation is established to predict the heat transfer coefficients of falling film evaporation on smooth tube arrays, considering the contributions of partially dryout and fully wet regimes, respectively. For enhanced tubes, the heat transfer enhancement ratios to the smooth tubes were also correlated.

Film Condensation of R-134a and R-236fa, Part 1: Experimental Results and Predictive Correlation for Single-Row Condensation on Enhanced Tubes

New predictive methods for falling film condensation on vertical arrays of horizontal tubes using different refrigerants are proposed, based on visual observations revealing that condensate is slung off the array of tubes sideways and significantly affects condensate inundation and thus the heat transfer process. For two types of three-dimensional enhanced tubes, advanced versions of the Wolverine Turbo-C and Wieland Gewa-C tubes, the local heat flux is correlated as a function of condensation temperature difference, the film Reynolds number, the tube spacing, and liquid slinging effect. The proposed methods work best when using R-134a, as these tubes were designed with this refrigerant in mind.

Modeling and experimental study of nucleate boiling on a vertical array of horizontal plain tubes

An investigation of nucleate boiling on a vertical array of horizontal plain tubes is presented in this paper. Experiments were performed with refrigerant R123 at reduced pressures varying from 0.022 to 0.64, tube pitch to diameter ratios of 1.32, 1.53 and 2.00, and heat fluxes from 0.5 to 40 kW/m 2. Brass tubes with external diameters of 19.05 mm and average roughness of 0.12 μm were used in the experiments. The effect of the tube spacing on the local heat transfer coefficient along the tube array was negligible within the present range of experimental conditions. For partial nucleate boiling, characterized by low heat fluxes, and low reduced pressures, the tube positioning shows a remarkable effect on the heat transfer coefficient. Based on these data, a general correlation for the prediction of the nucleate boiling heat transfer coefficient on a vertical array of horizontal tubes under flooded conditions was proposed. According to this correlation, the ratio between the heat transfer coefficients of a given tube and the lowest tube in the array depends only on the tube row number, the reduced pressure and the heat flux. By using the proposed correlation, most of the experimental heat transfer coefficients obtained in the present study were predicted within ±15%. The new correlation compares reasonably well with independent data from the literature.

Falling Films on Arrays of Horizontal Tubes with R-134a, Part II: Flow Visualization, Onset of Dryout, and Heat Transfer Predictions

In tune with the falling film evaporation heat transfer test results described in Part 1, flow visualization of the boiling process and intertube flow mode transitions from droplet to column and sheet flows have been observed, and the onset of dryout results were obtained. A new empirical approach to describe falling film evaporation with a dominance of nucleate boiling that takes into account the onset of dryout has been proposed and is applicable to plain and enhanced tubes. The method predicts most of the current local measurements for R-134a to within ±20% for conditions without dryout (the desired design condition) and has also been extended to cover conditions with partial dryout. Furthermore, a criterion for predicting the onset of dryout as a function of heat flux has also been proposed for the four types of tubes.

Falling Films on Arrays of Horizontal Tubes with R-134a, Part I: Boiling Heat Transfer Results for Four Types of Tubes

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5°C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the film Reynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.

Visualization of R-134a Flowing on Tube Arrays with Plain and Enhanced Surfaces under Adiabatic and Condensing Conditions

Experimental studies were performed to investigate the flow patterns of liquid films of R-134a on vertical arrays of horizontal tubes under adiabatic test conditions at room temperature and during condensation at a saturation temperature of 304 K. Three arrays with tube pitches of 25.5, 28.6, and 44.5 mm and up to ten tubes of four commercially available copper tubes with a nominal diameter of 19.05 mm and 544 mm in length were tested: a plain tube, a 26 fpi/1024 fpm low finned tube (Turbo-Chil), and two tubes with three-dimensional enhanced surface structures (Turbo-CSL and Gewa-C). Flow pattern observations and transitions were obtained and compared to existing flow pattern maps. The effect of the flow pattern on heat transfer was also investigated. The ideal flow modes (droplet, column, and sheet) could strictly be observed on the tubes with 3D enhanced surface structures. On the low-finned tube and the plain tube, the flow was very unstable, causing a part of liquid to leave the array sideways. For the 3D enhanced tubes, a difference in the formation of the sheets could be seen. The Gewa-C tube tended to form small sheets, while the Turbo-CSL formed large sheets at low film Reynolds numbers. A cyclic forward and backward motion of the sheet was observed that increased in amplitude with an increasing film Reynolds number. At high film Reynolds numbers, part of the liquid left the array of tubes sideways due to this oscillation, the effect of which on heat transfer was found to be more significant than the effect of the flow modes themselves.

Film Condensation of R-134a on Tube Arrays With Plain and Enhanced Surfaces: Part I—Experimental Heat Transfer Coefficients

The aim of the present investigation was to study the effect of condensate inundation on the thermal performance of a vertical array of horizontal tubes with plain and enhanced surfaces. Refrigerant R-134a was condensed at a saturation temperature of 304K on tube arrays with up to ten tubes at pitches of 25.5,28.6,and44.5mm. Notably, local condensing heat transfer coefficients were measured at the midpoint of each tube, as opposed to mean values. Four commercially available copper tubes with a nominal diameter of 19.05mm(0.75in.) were tested: a plain tube, a 26fpi∕1024fpm low finned tube, and two tubes, with three-dimensional (3D) enhanced surface structures. At low liquid inundation rates, the tubes with 3D enhanced surface structures significantly outperformed the low finned tube. Increasing liquid inundation deteriorated the thermal performance of the 3D enhanced tubes, whereas it had nearly no affect on the low finned tube, resulting in a higher heat transfer coefficients for the low finned tube at high liquid film Reynolds numbers. All the tests were performed with minimal vapor shear.

Film Condensation of R-134a on Tube Arrays With Plain and Enhanced Surfaces: Part II—Empirical Prediction of Inundation Effects

New predictive methods for R-134a condensing on vertical arrays of horizontal tubes are proposed based on visual observations revealing that condensate is slung off the array of tubes sideways and significantly affects condensate inundation and thus the heat transfer process. For two types of three-dimensional (3D) enhanced tubes, the Turbo-CSL and the Gewa-C, the local heat flux is correlated as a function of condensation temperature difference, the film Reynolds number, the tube spacing, and liquid slinging effect. The measured heat transfer data of the plain tube were well described by an existing asymptotic model based on heat transfer coefficients for the laminar wavy flow and turbulent flow regimes or, alternatively, by a new model proposed here based on liquid slinging. For the 26fpi low finned tube, the effect of inundation was found to be negligible and single-tube methods were found to be adequate.

Flow Structures of a Liquid Film Falling on Horizontal Tubes

Patterns of a liquid film falling across a vertical array of horizontal tubes change from droplet mode at low flow rates to liquid sheet at high flow rates. Between these limits, liquid columns form as a further stable flow pattern. The transition from one flow mode to another occurs via unstable structures consisting simultaneously of droplets and columns or of merging columns. The boundaries of the flow modes can be obtained from relationships expressing the flow rate as a function of physical properties, that is, the Reynolds number as a function of the Kapitza number. Correlations for the pattern boundaries recommended in the literature are compared with each other and found to be in acceptable agreement for practical purposes.

An experimental study on heat transfer characteristics for a horizontal tubular array in a high-temperature fluidized bed

Experiments were performed with an array of horizontal tubes, arranged in a regular equilateral triangular pattern, immersed in a fluidized bed. Three different bed operating temperatures were used, these being 705, 761 and 812 K. Data are reported for heat transfer between the bed and a centrally-located tube in the array. Both total and radiative heat transfer rates were measured for three different sizes of particles and for superficial velocities spanning the range from packed bed conditions to over twice the minimum fluidization velocity. Local heat transfer values, measured around the tube periphery, and integrated averages are reported for all test conditions.

Tube array heat transfer in fluidized beds a study of particle size effects

AbstractExperiments were performed with an array of horizontal tubes, arranged in a regular equilateral triangular pattern, immersed in a fluidized bed operating at 812 K. Data are reported for heat transfer between the bed and a centrally‐located tube in the array. Both total and radiative heat transfer rates were measured for superficial velocities spanning the range from packed bed conditions to over twice the minimum fluidization velocity. Results are presented for five differentsize particles. Local heat transfer values, measured around the tube periphery, and integrated averages are reported for all test conditions.Comparisons are also made between the heat transfer behavior of a tube in an array and that for a single tube in a hot fluidized bed under the same overall operating conditions. The results of this comparison suggest that the two mechanisms, gas convection and radiation, are competing effects.

Expansion of coarse-grained fluidized beds impeded by horizontal tube arrays

Experimental data on the expansion of fluidized beds with immersed arrays of horizontal tubes are obtained and generalized.