Nucleate pool boiling heat transfer over a bundle of vertical tubes

An experimental investigation has been carried out to determine the heat transfer coefficient during pool boiling of water over a bundle of vertical stainless steel heated tubes of 19.0 mm diameter and 850 mm height. The p/D of bundle was 1.66 and was placed inside a glass tube of 100 mm diameter and 900 mm length. The data were acquired for the heat flux range of 2–32 kWm^− 2.     

Enhanced boiling heat transfer in restricted spaces of a compact tube bundle with enhanced tubes

In flooded-type tube bundle evaporators with smooth tubes and general tube gaps, both wall superheat and heat flux are generally quite low and boiling cannot occur on the heated tubes. But when the tube gap is quite small or the enhanced heat transfer tubes are employed, the incipient boiling can occur at low heat flux levels and results in a significant heat transfer enhancement effect. This study investigates experimentally enhancement effects by the restricted space comprising the compact tube bundle and the enhanced tubes for boiling heat transfer of pure water and salt-water mixtures under atmospheric pressure. The experimental results show that the small tube gaps can greatly enhance boiling heat transfer for the compact enhanced tube bundle.     

紧凑满液型叉排管束蒸发换热器内水的沸腾强化换热特性

采用紧凑满液型蒸发换热器,利用水平传热管叉排管束狭窄空间内早期沸腾强化换热机理将中小热负 荷条件下的自然对流换热转化为旺盛核沸腾换热,换热性能大大优于传统的降膜式蒸发换热器。对水平传热管 管束在受限空间内沸腾强化换热进行实验研究,确认了紧凑满液式水平管蒸发换热器具有良好的换热性能,传 热管在管束中的位置对换热特性已经没有明显影响,随着压力增加,受限空间内沸腾强化换热强化效果显著增 加。     

满液型海水淡化蒸发器的换热特性研究

海水淡化装置，太阳能或余热吸收式制冷机中的蒸发换热器目前使用管排外降膜式蒸发方式。如将传热管束紧凑排列置于饱和状态液体中则变为满液式蒸发换热器，利用传热管束间受限空间内早期沸腾强化换热机理，将中小热负荷条件下的自然对流换热转化为核沸腾换热，在间隙尺寸适宜时，其换热性能可能优于降膜式蒸发换热器。该研究以盐水为实验工质，对紧凑传热管束受限空间的沸腾换热进行了实验研究，确认了满液式蒸发换热器也具有很好的换热性能，在中小热负荷条件下甚至超过降膜式蒸发换热器。     

Enhanced Boiling Heat Transfer of Water/Salt Mixtures in the Restricted Space of the Compact Tube Bundle

In desalinization devices and some heat exchangers making use of low-quality heat energy, both the wall temperature and the heat flux of the heated tubes are generally quite low, hence cannot cause boiling in flooded-type tube bundle evaporators with a large tube spacing. But when the tube spacing is quite small, incipient boiling can occur in the restricted space and results in higher heat transfer than that in a falling-film evaporator or during pool boiling at the same heat flux. This study experimentally investigates the effects of the tube spacing, the positions of tubes, and the salt-water concentration on bundle boiling heat transfer of salt water in the restricted space of the compact tube bundle evaporator under atmospheric pressure. The experimental results provide a restricted space boiling database for salt water in the compact tube bundle. Of particular importance is information concerning the influences of the tube spacing of the tube bundle and the concentration of salt water in desalination evaporators.     

Performance evaluation of flattened tube in boiling heat transfer enhancement and its effect on pressure drop

An experimental investigation has been carried out to study heat transfer and pressure drop characteristics of R-134a flow boiling inside a horizontal plain tube and different flattened tubes. Round copper tubes with an inner diameter of 8.7 mm were flattened into an oblong shape with an internal height of 6.6 mm, 5.5 mm, 3.8 mm, and 2.8 mm. The test apparatus was basically a vapor compression refrigeration system equipped with all necessary measuring instruments. Analysis of the collected data showed that, by flattening the round tube, the heat transfer coefficient and pressure drop increased simultaneously. The performance of these tubes from the point of view of heat transfer enhancement and pressure drop increasing were evaluated. It was concluded that, the tube with an internal height of 5.5 mm has the best performance compared with the other flattened tubes. Finally, based on the present experimental pressure drop data, a correlation was developed to estimate the pressure drop in flattened tubes. This correlation predicts the experimental data of the present study within an error band of ± 20%.     

Forced convective boiling in horizontal tube bundles

Experimental studies of crossflow boiling on a horizontal tube at various in Mass fluxes, local flow qualities and geometric arrangements are investigated. Since abundant information is available for the boiling on a single tube in a pool but it is still not clear whether this information in ay be applicable to tubes in bundles, the present study is therefore performed on three different conditions, namely: (1) a heated tube in a channel; (2) a heated tube in a non-heated, in-line tube bundle; and (3) a heated tube in a heated, in-line tube bundle. The different heat transfer results between a single tube in a channel and a tube in a non-heated bundle, and between a non-heated bundle and a heated bundle are discussed in terms of the different flow field geometry and thermal environment respectively due to the presence of different structures and the heating conditions near the tube. A modified Chen's correlation is established to predict the heat transfer of a single tube in a channel or in a bundle. The correlation is also in good agreement with other data in the literature.     

Enhancement of boiling heat transfer in restricted spaces in compact horizontal tube bundles

ln desalinization devices and some heat exchangers making use of low‐quality heat energy, both wall temperatures and heat fluxes of heated tubes are quite low and generally cannot cause boiling in flooded‐type tube bundle evaporators with a large tube spacing. But when the tube spacing is very small, boiling in restricted spaces can occur and induce a higher heat transfer than that of a falling film or pool boiling at the same heat flux. This study investigated experimentally the effects of tube spacing, positions of tubes, and heating status of tubes as well as surface status (smooth and roll‐worked) on boiling in restricted spaces in compact horizontal tube bundle evaporators under atmospheric pressure. The experimental results provide a restricted space boiling database for water in smooth and enhanced surface tube bundles. Of particular importance is information concerning the influence of tube spacing of flooded‐type tube bundle evaporators, especially for the case of zero pitch, when the neighboring tubes are contacting each other. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 394–401, 2001     

Effects of outer tube length on saturated pool boiling heat transfer in a vertical annulus with closed bottoms

To improve pool boiling heat transfer in an annulus with closed bottoms, the length of an outer tube has been changed between 0.2 m and 0.6 m. For the test, a heated tube of 19.1 mm diameter and water at atmospheric pressure has been used. To elucidate effects of the outer tube length on heat transfer results of the annulus are compared with the data of a single unrestricted tube. The change in the outer tube length results in much variation in heat transfer coefficients. As the outer tube length is 0.2 m the deterioration point of heat transfer coefficients gets moved up to the higher heat fluxes because of the decrease in the intensity of bubble coalescence.     

An experimental investigation on heat transfer characteristics of multi-walled CNT-heat transfer oil nanofluid flow inside flattened tubes under uniform wall temperature condition

An experimental investigation on heat transfer characteristics of MWCNT-heat transfer oil nanofluid flow inside horizontal flattened tubes has been carried out under uniform wall temperature condition. Nanoparticle weight fractions were 0%, 0.1%, 0.2%, and 0.4%. The copper tubes of 14.5 mm I.D. were flattened and used as the test section of oblong shape with inside heights of 13.4 mm, 11.7 mm, 10.6 mm, and 8.6 mm. The nanofluid flowing inside the tube was heated inside a steam chamber to keep the temperature of the tube wall constant. The required data were acquired for laminar hydrodynamically fully developed regime. The effects of different parameters such as volumetric flow rate, nanoparticle weight fraction, and hydraulic diameter on the heat transfer behavior of the tested systems have been investigated experimentally. For a given flattened tube at a constant nanoparticle weight fraction, increasing volumetric flow rate results in heat transfer enhancement. In addition, as the tube profile becomes more flattened and the hydraulic diameter decreases, the heat transfer coefficient goes up at constant volumetric flow rate. Utilizing nanofluids instead of the base fluid, the heat transfer rate enhances remarkably. The higher the nanoparticles weight fraction, the more the rate of heat transfer enhancement. Finally, the results show that the amount of increase in heat transfer coefficient caused by employing nanofluid instead of the base fluid is comparable to what caused by flattening the tube.     

Evaporating heat transfer and pressure drop of hydrocarbon refrigerants in 9.52 and 12.70 mm smooth tube

Experimental results of heat transfer characteristic and pressure gradients of hydrocarbon refrigerants R-290, R-600a, R-1270 and HCFC refrigerant R-22 during evaporating inside horizontal double pipe heat exchangers are presented. The test sections have one tube diameter of 12.70 mm with 0.86 mm wall thickness, another tube diameter of 9.52 mm with 0.76 mm wall thickness was used for this study. The local evaporating heat transfer coefficients of hydrocarbon refrigerants were higher than those of R-22. The average evaporating heat transfer coefficient increased as the mass flux increased. It is showed the higher values in hydrocarbon refrigerants than R-22. Comparing the heat transfer coefficient of experimental results with that of other correlations, the obtained results from the experiments had coincided with most of the Kandlikar’s correlation. Hydrocarbon refrigerants have higher pressure drop than R-22 in 12.7 mm and 9.52 mm. This results form the study can be used in the case of designing heat transfer exchangers using hydrocarbons as the refrigerant for the air-conditioning systems.     

Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998     

Heat and mass transfer characteristics of a helical absorber using LiBr and LiBr + LiI + LiNO3 + LiCl solutions

An experimental study has been performed to investigate the heat and mass transfer performance in a falling film absorber of a small-sized absorption chiller/heater. The components of the chiller/heater were concentrically arranged in a cylindrical form with a low temperature generator, an absorber and an evaporator from the center. The arrangement of such a helical-type heat exchanger makes the system more compact compared to a conventional one. As a working fluid, LiBr + LiI + LiNO₃ + LiCl solution is used to get improved heat transfer effect. The heat and mass transfer coefficients of the helical absorber provide similar values compared with the data obtained for horizontal absorbers at similar solution flow rates. The heat and mass transfer coefficients of LiBr + LiI + LiNO₃ + LiCl solution increase as the solution flow rate per unit length increases. However, if the solution flow rate is larger than 0.03 kg/m s, the heat and mass transfer increase is minimal. Thus, 0.03 kg/m s is recommended as an optimal solution flow rate. The heat and mass flux performance of LiBr + LiI + LiNO₃ + LiCl solution shows the tendency of 2-5% increase compared with that of LiBr solution.     

Surface effects in flow boiling of R134a in microtubes

The inner surfaces of microtubes may be influenced strongly by the process of making them due to manufacturing difficulties at these scales compared to larger ones, e.g. the surface characteristics of a seamless cold drawn tube may differ from those of a welded tube. Accordingly, flow boiling heat transfer characteristics may vary. In addition, there is no common agreement between researchers on the criteria of selecting tubes for flow boiling experiments. Instead, tubes are usually ordered from commercial suppliers, in many cases without taking into consideration the manufacturing method and its effect on the heat transfer process. This may explain some of the discrepancies in heat transfer characteristics which are found in the open literature. This paper presents a comparison between experimental flow boiling heat transfer results obtained using two different metallic tubes. The first one is a seamless cold drawn stainless steel tube of 1.1 mm inner diameter while the second is a welded stainless steel tube of 1.16 mm inner diameter. Both tubes have a heated length of 150 mm and the flow direction is vertically upwards. The tubes were heated using DC current. Other experimental conditions include: 8 bar system pressure, 300 kg/m² s mass flux, about 5 K inlet sub-cooling and up to 0.9 exit quality. The results are presented in the form of local heat transfer coefficient versus local quality and axial distance. Also, the boiling curves of the two tubes are discussed. The results show a significant effect of tube inner surface morphology on the heat transfer characteristics.     

Heat transfer augmentation in 3D internally finned and microfinned helical tube

Experiments are performed to investigate the single-phase flow and flow-boiling heat transfer augmentation in 3D internally finned and micro-finned helical tubes. The tests for single-phase flow heat transfer augmentation are carried out in helical tubes with a curvature of 0.0663 and a length of 1.15 m, and the examined range of the Reynolds number varies from 1000 to 8500. Within the applied range of Reynolds number, compared with the smooth helical tube, the average heat transfer augmentation ratio for the two finned tubes is 71% and 103%, but associated with a flow resistance increase of 90% and 140%, respectively. A higher fin height gives a higher heat transfer rate and a larger friction flow resistance. The tests for flow-boiling heat transfer are carried out in 3D internally micro-finned helical tube with a curvature of 0.0605 and a length of 0.668 m. Compared with that in the smooth helical tube, the boiling heat transfer coefficient in the 3D internally micro-finned helical tube is increased by 40-120% under varied mass flow rate and wall heat flux conditions, meanwhile, the flow resistance is increased by 18-119%, respectively.     

紧凑传热管束受限空间内沸腾强化换热特性

海水淡化装置以及太阳能或余热吸收式制冷机中的蒸发换热器,采用管排外降膜式蒸发方式,它具有很多优点,但管间距离较大,以致尺寸较大,供液方式较复杂。将传热管束紧凑排列置于饱和状态液体中,将其变为满液式蒸发换热器,利用传热管束间受限空间内早期沸腾强化机理,将中小热负荷条件下的自然对流换热转化为核沸腾换热,在间隙尺寸适宜时,其换热性能可能优于降膜式蒸发换热器。对紧凑传热管束在受限空间内沸腾强化换热进行实验研究,确认了满液式蒸发换热器具有良好的换热性能,在中小热负荷条件下甚至超过降膜式蒸发换热器。     

Pressure drop and thermal characteristics of CuO-base oil nanofluid laminar flow in flattened tubes under constant heat flux

An experimental investigation has been carried out to study the heat transfer and pressure drop characteristics of nanofluid flow inside horizontal flattened tubes under constant heat flux. The nanofluid is prepared by dispersion of CuO nanoparticle in base oil and stabilized by means of an ultrasonic device. Nanofluids with different particle weight concentrations of 0.2%, 0.5%, 1% and 2% are used. Copper tubes of 11.5 mm I.D. are flattened into oblong shapes and used as test sections. The nanofluid flowing inside the tube is heated by an electrical heating coil wrapped around it. Required data are acquired for laminar and hydrodynamically fully developed flow inside round and flattened tubes.The effect of different parameters such as flow Reynolds number, flattened tube internal height and nanofluid particle concentration on heat transfer coefficient and pressure drop of the flow is studied. Observations show that the heat transfer performance is improved as the tube profile is flattened. Flattening the tube profile resulted in pressure drop increasing. In addition, the heat transfer coefficient as well as pressure drop is increased by using nanofluid instead of base fluid. Furthermore, the performance evaluation of the two enhanced heat transfer techniques studied in this investigation shows that applying flattened tubes instead of the round tube is a more effective way to enhance the convective heat transfer coefficient compared to the second method which is using nanofluids instead of the base liquid.     

The Effect of Tube Flattening on Flow Boiling Heat Transfer Enhancement

The present study investigated the effect of smooth tube flattening on heat transfer enhancement in an evaporator. The tubes with internal diameter of 8.7 mm were flattened into an oblong shape with different inside heights. The test setup was basically a vapor compression refrigeration system equipped with all necessary measuring instruments. Refrigerant R-134a flowing inside the tube was heated by an electrical coil heater wrapped around it. The ranges of mass velocities were from 74 to 106 kg/m²-s and vapor quality varied from 25% to 95%. Analysis of the collected data indicated that the heat transfer coefficient elevates by increasing the mass velocity and vapor quality in flattened tubes just like the round tube. The flow boiling heat transfer coefficient increases when the flattened tube is used instead of the round tube. The highest heat transfer coefficient enhancement of 172% was achieved for the tube with the lowest inside height at mass velocity of 106 kg/m²-s and vapor quality of 85%. Finally, based on the present experimental results, a correlation was developed to predict the heat transfer coefficient in flattened tubes.     

Nucleate pool boiling characteristics from coated tube bundles in saturated R-134a

Nucleate pool boiling heat transfer from plasma coated copper tube bundles with porous copper (Cu) immersed in saturated R-134a was experimentally studied. The bundle is composed of 15 tubes (of which the number of heated/instrumented tubes was varied) arranged in four different configurations with a pitch-to-diameter ratio of 1.5. The influences of various parameters, for instance, bundle arrangements and heat flux were clarified. Tests were conducted with both increasing and decreasing the heat flux. The data presented indicated that at low heat fluxes, the vertical-in-line tube bundles have the highest bundle factor. A configuration factor was proposed which can be used to characterize the geometric arrangements of the bundles.