Low-finned Tubes Research Articles

Pool boiling heat transfer of ammonia outside enhanced tubes with various fin structures

Pool boiling heat transfer of ammonia on a smooth tube and three enhanced tubes was investigated at the saturation pressure ranging from 6.15 to 11.7 bar, which corresponds to the saturation temperature from 10 to 30 °C. The test tube with a length of 796 mm was immersed in the liquid pool of ammonia inside a cylindrical chamber having an internal diameter of 254.4 mm, and convection or pool boiling heat transfer outside the test tube was generated by flowing hot water through the tube. Three enhanced tubes of a low-finned tube (Low-fin), a low-finned tube with oblique cuttings (Turbo-C), and a finned tube having re-entrant cavities (Turbo-E) were prepared to examine the pool boiling enhancement on various finned tubes. Boiling hysteresis on the smooth tube was observed between when the heat flux increases and when the heat flux decreases, which resulted in the deviation of the heat transfer coefficients from the previous correlations when the heat flux decreases. A suitable correlation was suggested to compensate for the deviation of the heat transfer coefficients in case of decreasing the heat flux by modifying the Stephan-Abdelsalam correlation. Three enhanced tubes exhibited considerable enhancement in the heat transfer coefficients compared with the smooth tube, and boiling performance on the Turbo-E tube having re-entrant cavities was highly affected by the variation of the bubble diameter according to the saturation pressure. Enhancement factors of the enhanced tubes were obtained from the ratio of thermal conductance on the enhanced tube to that on the smooth tube.

Experimental investigation of the impact of non-condensing gas on the condensation of n-propanol on a low-finned tube

This paper investigates the impact of a non-condensing gas on the condensation heat transfer of n-propanol on a low-finned and plain tube. Despite the potential of low-finned tubes to enhance outer heat transfer during condensation, the literature lacks measurement data on the condensation of solvents and bulk chemicals on these tubes. No experimental investigations on the impact of non-condensing gas on the heat transfer during the condensation of solvents on low-finned tubes are available in the literature. This paper addresses these gaps by presenting measurements of the heat transfer during the condensation of n-propanol in the presence of nitrogen on a horizontal GEWA-K28 low-finned tube and a plain tube using a novel test rig design. The results show that even a small amount of nitrogen significantly reduces the outer heat transfer coefficient. This effect is more pronounced on the low-finned tube than on the plain tube. The experimental data provided in this article can serve as a basis for developing calculation models for the condensation heat transfer of substances from the chemical industry on low-finned tubes.

Entropy generation and thermodynamic analysis of pool boiling heat transfer on doubly enhanced tubes

To improve the efficiency of the heat exchanger tube, one smooth tube and four doubly enhanced tubes (EX1, EX2, EX3 and EX4) have been investigated for the pool boiling heat transfer experiments in this paper. The indicate that the pool boiling heat transfer coefficient of the doubly enhanced tubes increased visibly with the augmentation of heat flux through the bubble behavior. Heat transfer reinforcement effect of the doubly enhanced tubes are significantly better than that of the smooth tube. Additionally, pool boiling heat transfer coefficient can be further enhanced by raising the saturate temperature. Entropy generation minimization analysis demonstrates that the heat transfer characteristics of like T-shaped tubes (EX1 and EX2) are superior to that of low fin tubes (EX3 and EX4). Particularly, tube EX1 exhibits higher pool boiling heat transfer efficiency. It is observed that a reasonable fin pitch is more advantageous for improving heat transfer characteristics. The utilization of entropy generation minimization analysis provides theoretical support for the design and optimization of doubly enhanced tubes.

Heat transfer of uncoated and nanostructure coated commercially micro-enhanced refrigeration tubes under pool boiling conditions

The heat transfer performance of commercially produced micro-enhanced tubes with and without a nanocoating was investigated under pool boiling of saturated refrigerant. These multiscale enhancements were on the outside of 19 mm horizontal copper tubes heated by water to determine the effectiveness of this multiscale enhancement technique on industrially relevant tubes and geometry. The tubes tested were a plain tube roughened by sandpaper, a low finned GEWA-KS tube and two micro-enhanced re-entrant cavity tubes, the GEWA-B5 and EHPII. The tubes were tested in R134a at saturation temperatures of 5 °C and 25 °C across a range of heat fluxes from 20 kW/m2 to 100 kW/m2 under pool boiling conditions. The nanocoating applied to the tubes produced a forest of copper oxide nanostructures on the surface, increasing wickability of the surface. A Scanning Electron Microscopy showed that copper oxide nanocoatings coated all micro-enhanced tubes evenly without impeding the surface features or significantly blocking the re-entrant cavities. For the uncoated tubes in pool boiling, the heat transfer coefficients of the EHPII was up to 519 % greater than those of the plain roughened tube, the GEWA-B5 was up to 539 % higher than the roughened tube and the GEWA-KS was at best 64 % higher than those of the plain roughened tube. Increases in the saturation temperature to 25 °C produced minor improvements in heat transfer coefficients. The application of the copper oxide nanocoating resulted in generally decreased heat transfer performance by approximately 40 % on average compared to the uncoated tubes, with the GEWA-B5 tube the worst affected. Degradation of the heat transfer is thought on plain surfaces to be due to the flooding of nucleation sites, while re-entrant cavity style enhanced surfaces were thought to experience degraded sensible and latent heat transfer due to impeded flow in the microstructure capillary channel, as bubbles were noted to be trapped in the channels. It is recommended that the heat transfer at much higher heat flux ranges be explored for possible HTC enhancement.

Multiscale enhancement of refrigerant falling film boiling by combining commercially enhanced tubes with nanostructures

Multiscale surface structures offer the opportunity to combine the heat transfer enhancement provided by microscale structures with the dryout benefits provided by some nanostructures, which is particularly attractive for falling film evaporators, who have to prevent dryout to ensure high heat transfer coefficients can be maintained.In this study commercially produced plain, low-finned and 3D enhanced commercially produced tubes were tested uncoated as well as coated with fibrous nanostructures that induce wicking on the surface to investigate the opportunity that multiscale enhancements might offer falling film boiling evaporators. Single tubes were heated internally by water and tested in the horizontal position with refrigerant R-134a drizzled over the outside of the tube surface at saturation temperatures of 5 and 25 °C, heat fluxes from 20 to 100 kW/m2 and film Reynolds numbers of up to 2000. The tubes tested were a roughened plain tube, a low-finned Gewa-KS tube and two different 3D enhanced tubes, an Gewa-B5 and an Turbo EHPII, that have networks of re-entrant cavities.The uncoated 3D enhanced tubes achieved the highest heat transfer coefficients, outperforming a roughened plain tube by up to 500 %. The uncoated low-finned tube had heat transfer coefficients that were up to 140 % higher than the roughened plain tube, despite having only an 80 % larger surface area, thought to be due to bubbles sliding down the channels between the fins, increasing the surface area of the bubbles in contact with the surface and thus allowing for increased microlayer evaporation.The application of the copper oxide coating benefitted the low-finned tube, with heat transfer enhancement of up to 60 %, possibly due to liquid wicked underneath the sliding bubbles and onto the fin tips. However, the nanocoating largely decreased the heat transfer of the roughened plain tube and the 3D enhanced tubes, thought to be due flooding of nucleation sites on the roughened tube and interference with the hydraulics of the re-entrant cavity network on the surface of the 3D enhanced surfaces.The low-finned tube also benefitted the most from the falling film boiling heat transfer mode compared to previous pool boiling results, with heat transfer increased by up to 80 and 460 % for the uncoated and coated low-finned tube respectively, again likely due to the bubbles sliding in the fin channels under falling film conditions.The Gewa-B5 tubes had the best dryout performance, followed by the Turbo EHPII tubes and then the low-finned Gewa-KS tube. The open pore structure of the Gewa-B5 likely aided it in dryout prevention, while the fins of the Gewa-KS are thought to have prevented lateral fluid distribution and thus worsen dryout.The dryout performance of the tubes we not markedly improved by the nanocoating, highlighting the difficulty in improving wettability on high wetting low surface tensions fluids such as refrigerants. Microscale enhancements were thus shown to have significant influence on dryout performance, while nanoscale enhancements had little effect in this study.

A new analytical model of condensation outside low-finned tubes for refrigerants at low temperatures and high heat flux

In this paper, a new analytical model is developed for the condensation of refrigerants outside low-finned tubes. It is based on the flow and heat transfer characteristics at low saturation temperatures and high heat flux, such as those in the LNG intermediate fluid vaporizer (IFV). Due to the increase of viscous effect at lower saturation temperature and higher heat flux, the previous condensate retention angle models are modified by adding the viscous term. At high heat flux and condensate rate, the condensate profile at the fin root is simplified to be arc-shaped and obtained directly by force analysis. Furthermore, considering the heat transfer enhancement caused by the circumferential flow at high heat flux, the convective mechanism is included for the heat transfer through the thick liquid film between the fins. The distributions of liquid film and fin surface temperature are also predicted. The present analytical model is in good agreement with different experimental and numerical results, while most deviations are within 10% for predicting the heat transfer coefficients at high heat flux. Because of the increased flow viscosity and surface tension, the optimal fin spacing and fin height increase as the heat flux increases and the saturation temperature decreases.

Crystallization fouling of turbo-structured tubes during nucleate pool boiling

The superiority of enhanced boiling on structured heat transfer surfaces over plain surfaces is based on excessive bubble generation and behavior as well as the latent heat of evaporation. The generated bubbles with turbulence of large scale would give rise to profoundly higher heat transfer coefficients. Meanwhile, the very same reasons of bubble formation and behavior may lead to accelerate the fouling of these surfaces for aqueous solutions with inverse solubility. The present study characterizes the fouling behaviour of turbo structured tubes under nucleate pool boiling. Several experiments were conducted for heat fluxes ranging from 50 to 250 kW/m2 and a concentration of 1.6 g/L of CaSO4 aqueous solution. The investigated structured tubes were re-entrant cavity tubes referred to as the Turbo-B type which were compared with plain tubes and low finned tubes of different fin heights and densities. Of these investigated tubes, the Turbo-B showed preferable anti-fouling behaviour. Based on a comparison of anti-fouling performance of the test tubes, the Turbo-B reduced fouling resistance up to ∼27 % and ∼83 % than the finned test tubes of 19 fpi (fins per inch) and 40 fpi, respectively. Most interestingly, the deposition of turbo tubes was of powdery and frail structure which could easily be cleaned. Moreover, the deposit was only formed on top of these tubes while no sign of deposition was observed within the cavities. The deposit structure for plain tubes, on the contrary, was a hard, thick and homogeneous layer under similar operating conditions as those for the turbo tubes.

An experimental study of the inundation effect on filmwise condensation heat transfer over horizontal smooth and enhanced tubes

Inundation effect on condensation heat transfer is ubiquitous and a crucial factor in designing a large-scale condenser. In the present study, the effect of condensation liquid inundation on heat transfer is experimentally studied for R134a on a plain tube and three kinds of enhanced tubes. A specially designed test method with a liquid distributor for studying the effect of inundation liquid has been adopted, by which the inundation liquid flow rate can be adjusted easily within a wide range with high accuracy of inundation liquid flow rate. It is found that the inundation liquid may enhance the heat transfer at low heat flux of 20 kW/m2 when ReΓ is less than 600 and 800 for the 3D finned tube and C9+ tube (a kind of 3D finned tube with smaller fins on each fin to break the liquid film), respectively, and for the 2D low fin tube within the entire test range of ReΓ, with the maximum enhancement factor being 1.21. At high heat flux of 80 kW/m2 the inundation liquid always deteriorate heat transfer for the three enhanced tubes tested. At middle heat flux of 40 kW/m2, the inundation usually has negative effect but for C9+ tube heat transfer is enhanced when ReΓ is less than 1100. It is also found that the advantage of C9+ tube in enhancing condensation heat transfer over the finned tubes is not so appreciable under the inundation case as it is for non-inundation case. Some flow visualization pictures are provided to deepen the understanding of physical process.

A dynamic model for predicting condensation heat and mass transfer characteristics in falling film condenser

A mathematical model of falling film condenser is developed to predict the dynamic response of shell-and-tube condenser having R1233zd(E) as refrigerant for high temperature heat pump systems. The model incorporates a vapor-mass-flux ratio, which includes the relative velocity slip between vapor and liquid to examine the performance of tube bundles in falling film condensers. Discretization of governing equations and control-volumes method are applied to construct the dynamic model which is first validated with the well-known HTRI software in steady-state performance. The mean relative deviations are within 14.8% for temperature and 2.0% for heating capacity in steady state. The established model can be used to predict the dynamic heat transfer response to the change of operating conditions and geometric configurations of tubing. When compared to the change of inlet pressure of vapor refrigerant, the response of the condensation temperature is much slower, yielding some 1.7 times longer time to reach stable state than that of pressure. Further, various heat exchange tubes (smooth tubes and low-fin tubes with 3 different fin pitches) are examined. It is found that the heating capacity and water outlet temperature increase with the rise of fin density. Yet higher fin density also tends to reach stable state earlier. The heat transfer enhancement ratio ranges from 2.7 to 2.0 for low-fin tube with 1.3 mm fin pitch, and the enhanced tubes are more effective with the low subcooling temperature.

POOL BOILING HEAT TRANSFER OF LiBr SOLUTION IN LOW-FIN TUBES: EFFECT OF FIN PITCH AND HEIGHT

POOL BOILING HEAT TRANSFER OF LiBr SOLUTION IN LOW-FIN TUBES: EFFECT OF FIN PITCH AND HEIGHT

Detailed CFD simulations of pure substance condensation on horizontal annular low finned tubes including a parameter study of the fin slope

The use of annular low-finned tubes in tube bundle condensers greatly increases the efficiency. This enables enhanced heat coupling within chemical plants, reducing the overall CO2 emission and power consumption. Due to the complex geometry of these tubes, no generalized condensation model is present so far. In this study, we use highly resolved computational fluid dynamics simulations to investigate pure substance condensation on said tubes with a condensation model which is independent of empirical parameters. Within these simulations, the condensate film is fully resolved and heat transfer coefficients are calculated providing the complete information about the condensation process on annular low-finned tubes for the first time. Additionally, a parameter study for the incline of the annular fin is provided. Therefore, computational fluid dynamics is used to predictively evaluate the influence of a single fin parameter, the incline of the fin, of annular low-finned tubes, for the first time. The simulations provide information about the film thickness along the fin flank and the flooding behavior of the tubes. An accurate fluid dynamic behavior of the two-phase flow is gained. Furthermore, the resulting heat transfer coefficients stand in excellent agreement to experimental data.

Numerical study on heat and mass transfer behavior of pool boiling in LiBr/H2O absorption chiller generator considering different tube surfaces

Investigating the pool boiling process in the absorption chiller generator by studying the valid parameters may enhance the chiller?s COP. In the present study, the transient 2-D numerical modelling of LiBr/H2O solution pool boiling in the generator of the absorption chiller was carried out using the two-phase Eulerian-Eulerian approach, extended Rensselaer Polytechnic Institute boiling model and renormalization group k-? turbulence model. The numerical model was applied on three types of the bare, notched fin, and low fin tubes to investigate the effect of using fin on the boiling heat transfer rate in the generator of the absorption chiller and comparing it with the bare tube. Moreover, the numerical results were compared with the data obtained from the previous experimental studies to validate numerical modelling. A good agreement was achieved between numerical and experimental results. The results showed the evaporation mechanisms in the microlayer, evaporation in the three-phase (liquid-vapor-solid) contact line, and transient conduction the superheat layer for constant thermal heat flux and the three surfaces of the copper tube within a specific period from the boiling point of LiBr/H2O solution. The results also showed that the use of a notched fin-tube and low fin tube increases the non-homogeneous nucleation rate, causes the solution boil earlier than the bare tube, and reduces the required thermal energy in the generator of an absorption chiller.

Investigation on Performance Evaluation of Corrugated Low Finned Tubes Oriented for Energy Saving

Heat exchangers are the energy facilities that exchange heat in process industries.Corrugated low finned tube is a kind of helically corrugated tube with trapezoidal shape groove. In this paper, the comparison approach is adopted to evaluate the overall performance of corrugated low finned tubes. The heat transfer and friction factor correlations were firstly introduced. Then the overall performance was evaluated for the identical pressure drop and identical pumping power. Finally, the performance evaluation criteria were fitted as the function of Reynolds number, Prandtl number, pitch to diameter ratio and rib height to diameter ratio. The results showed thatboth the evaluation factors at identical pumping power and at the identical pressure drop are greater than 1.0 over the entire Reynolds number for all the enhanced tubes, indicating that the enhanced tubes have the second best performance oriented from energy-saving.

Comparison of Heat Transfer Performance for Condensation in Corrugated Low Finned Tube and Smooth Tube with Air

The existence of air served as non-condensable gas makes it difficult to design effective titanium condenser in PTA plant. In this paper, the heat transfer characteristic for condensation inside corrugated low finned tube and smooth tube in presence of air was compared. The local heat transfer performance was studied by comparing bulk temperature and local heat transfer rate. The effect of non-condensable gas mass fraction on overall heat transfer performance was also analysed. The results indicated that both the bulk temperature and the local heat transfer rate decreased along the axial direction due to the reduction of steam partial pressure and temperature difference. The average heat transfer coefficient and total heat transfer rate decreased with the increase of inlet non-condensable gas mass fraction. And the corrugated low finned tube could reduce the impact of non-condensable gas by reducing the thickness of gas film and condensate film.

Mechanism of Condensation Enhancement by Corrugated Low Finned Tubes in Presence of Noncondensable Gas

Corrugated tubes are the most convenient reinforcement techniques to enhance intube condensation. In this paper, the average condensation heat transfer coefficients inside the corrugated low finned tube and smooth tube were firstly introduced. The distribution of local heat transfer coefficient was presented at lower and higher inlet noncondensable gas mass fractions. The results show that the major heat transfer resistance is the thermal resistance of liquid film at lower inlet gas fraction. So the mechanism of condensation enhancement by corrugated low finned tubes is the increase of liquid film condensation coefficient. But at higher inlet gas fraction gas layer mass transfer resistance becomes the control thermal resistance. The spiral ribs inside corrugated low finned tube can thinner not only the gas layer thickness but also the liquid film thickness. The mechanism of condensation enhancement is the increase of both gas layer and liquid film condensation coefficient.

Experimental Study on Flow Resistance Characteristic of Corrugated Low Finned Tubes

The heat transfer enhancement techniques have been developed to the third generation. The most popular and successful enhancement techniques are the artificial roughness surface and helically corrugated tube. Corrugated low finned tube is a kind of helically corrugated tube with trapezoidal shape groove. In this paper, the flow resistance characteristic of corrugated low finned tubes was studied experimentally. The results showed that all the enhanced tubes had a larger friction factor than the smooth tube due to the turbulence augmentation and rotational flow produced by helical rib. Moreover, the friction factor ratios slightly increased with the raise of Reynolds number and rib height presents more significant effect on flow resistance. Finally, all the experimental data were fitted as the function of Reynolds number, Prandtl number, tube pitch and rib height. The research results can provide a foundation for designing heat exchangers using this enhancement technique.

Experimental investigation on heat transfer performance of corrugated low finned tubes

With the outbreak of oil crisis, heat exchangers as the energy-saving devices have gained more and more attentions. The heat transfer enhancement techniques have been developed to the third generation. Corrugated low finned tube is a kind of helically corrugated tube with trapezoidal shape groove. In this paper, the performance of convective heat transfer inside corrugated low finned tubes was studied experimentally. The results showed that the heat transfer performance could be intensified compared with the smooth tube. The reason was that helical rib generated the secondary flow and longitudinal vortex, reducing the synergy angle between velocity vector and temperature gradient. Moreover, the effects of tube structural parameter and fluid Reynolds number were analyzed. Finally, all the experimental data were fitted as the function of Reynolds number, Prandtl number, tube pitch and rib height. The research results can provide a foundation for designing heat exchangers using this enhancement technique.

Numerical study on condensation of steam/air mixture inside corrugated low finned tubes

Corrugated low finned tubes are the specical spirally corrugated tubes, which can enhance the condensation heat transfer with and without noncondensable gas. In this paper, a numerical model on the basis of diffusion layer theory was established to investigate the local heat transfer characteristic inside corrugated low finned tubes. The local parameters contain bulk temperatures, gas mass fraction, heat transfer coefficient and heat flux. The results showed that the bulk temperatures decreased along the condenser tube but the noncondensable gas mass fraction displayed the opposite trend. The local heat transfer coefficient and heat flux decrease along the condenser tube especially near the outlet. Of the five enhanced tubes, #5 tube had the best overall heat transfer performance, but it also had the fastest rate of decline as condensation progressed, indicating that the rib was the main influence factor compared with pitch.

Dynamics of Bubble Behavior in Nucleate Boiling of R134a in an Enhanced Horizontal Tube

The design of tubes in heat exchangers has a significant influence on heat transfer coefficients (HTCs), affecting nucleate boiling at heat exchanger surfaces. Many researchers have explained heat transfer enhancement techniques applied to tubes by identifying empirical correlations, both dimensional and non-dimensional, for HTCs in terms of surface roughness factor, vapor density, and liquid density based on their experimental results. However, there has been a lack of research analyzing the mechanisms that affect the HTCs in depth. For example, the influence of bubble density, surface roughness factor, and the dynamics of bubble behavior have rarely been explained. In this regard, the present study focuses on mechanism analysis in terms of the value of HTC, specifically revealing the dynamics of bubble behavior. Additionally, this study reports the results of an experimental investigation of pool boiling heat transfer of R134a in a plain tube, a low-fin tube, and an enhanced (Thermoexcel-E) tube as well as results of examining the dynamics of bubble behavior observed on the surface of an enhanced tube. Ultimately, all these investigations aim to determine the heat transfer characteristics of the enhanced tube. In this study, however, circulated water flowing inside an evaporator is used as a heat source instead of a cartridge heater. The heat transfer enhancement ratio of the Thermoexcel-E tube is 2.69–4.13 in the range of heat fluxes tested when the values of HTC from the Thermoexcel-E tube are compared to counterparts from a plain tube. Meanwhile, analysis of the dynamics of bubble behavior observed on the surface of the enhanced tube allows determination of the enhanced tube’s heat transfer characteristics. The findings of this study can provide the refrigeration industry with a basic framework for better design using high-efficiency evaporators.

Methods of Heat-Exchange Intensification in NPP Equipment

The aims and problems of research on the means for intensifying heat exchange in the equipment of nuclear power plants are examined: nuclear reactors, steam generators, turbine condensers, heat-exchangers of dry water cooling towers, intermediate separators-steam superheaters of turbines, heat-exchange apparatus of safety systems, turbo-electro-generators, oil coolers, feed-water heaters. Methods of identifying heat-exchange are analyzed: different types of finning, turbulence generators and swirlers of one- and two-phase flows, coating on heating surfaces, systems with mixed flow. Computational recommendations are made for determining the intensity of heat emission from promising types of heat-exchange surfaces: bundles of lowfinned tubes and spiral coils.