Condensation of a zeotropic CFC 114-CFC 113 refrigerant mixture in the annulus of a double-tube coil with an enhanced inner tube

Local heat transfer and pressure drop measurements were made during condensation of a zeotropic CFC114-CFCll3 refrigerant mixture in the annulus of a double-tube coil consisting of three U-bends and four straight lengths. The inner tube is a 19.1-mm O.D. corrugated copper tube with wire fins soldered onto the outer surface and the inner diameter of the outer duct is 25.0 mm. The vapor-phase mass transfer coefficient exhibited a sawtooth behavior with the U-bends showing higher coefficients than the straight lengths. The frictional pressure gradient data agreed well with a previously developed empirical equation for the condensation of pure refrigerants. A prediction method for the condensation heat transfer rate was proposed on the basis of the correlations of the vapor-phase mass transfer coefficient and heat transfer coefficient of the condensate film. The heat transfer data were correlated by the present method to a mean absolute deviation of 12.9%.     

Fouling resistance model for prediction of CaCO3 scaling in AISI 316 tubes

The term fouling is generally used to describe the deposition of unwanted (initially fluid) particles, which increases both resistance to heat transfer and pressure drop through the heat exchanger. CaCO₃ which is predominantly present in the cooling water, has inverse solubility characteristics i.e., it is less soluble in warm water, resulting in deposition of scales in heat transfer equipment. An experimental program is described in this paper to study the growth of fouling as a function of tube surface temperature, Reynolds number, tube diameter and the time for which the tube has been subjected to the scale forming solution. The data collected from the experiments are used to develop a fouling resistance model. In addition, the results obtained from the present study are also compared with those discussed earlier by several investigators with regard to CaCO₃ fouling.     

CaCO3 scaling in AISI 316 stainless steel tubes – effect of thermal and hydraulic parameters on the induction time and growth rate

Calcium Carbonate (CaCO₃) is predominantly present in cooling water which is commonly used as a coolant in many industrial processes. It has inverse solubility characteristics i.e., it is less soluble in warm water, resulting in the deposition of scale on heat transfer surfaces. An experimental study was carried out to determine the effect of tube surface temperature, Reynolds number, tube diameter and salt concentration on the induction time of CaCO₃ scaling. It was observed that tube surface temperature, Reynolds number and tube diameter had no effect on the onset time of scaling, whereas salt concentration and tube surface roughness had a profound influence on the induction period. The data collected from the experiments were used to develop dimensionless fouling resistance models for estimation and prediction purposes. Received on 22 December 1997     

Effect of a helical wire on mixed convection heat transfer to carbon dioxide in a vertical circular tube at supercritical pressures

Reactor core of a SCWR (supercritical water-cooled reactor) employs a tight lattice in order to efficiently remove heat from nuclear fuels. In the narrow sub-channels of a tight lattice reactor core, a helical wire instead of a complicated conventional spacer has been used as a turbulence generator and a space-keeper between the fuel rods.A series of experiments were performed in order to investigate an effect of a helical wire on heat transfer to upwardly flowing CO₂ in a electrically-heated circular tube with an inner diameter of 6.32 mm, where a helical wire with an outer diameter of 1.3 mm was tightly inserted inside the tube. The tube inner diameter corresponds to the equivalent hydraulic diameter of a sub-channel of a KAERI’s fuel assembly concept. The mass fluxes ranged from 400 to 1200 kg/m² s; the heat fluxes ranged from 30 to 90 kW/m²; and the pressures were 7.75 and 8.12 MPa. The corresponding Reynolds numbers at the test section inlet ranged from 1.8 × 10⁴ to 7.5 × 10⁴. The heat transfer rate reached almost twice the value obtained from the experiment with a plain tube of the same size near the pseudocritical temperature and the effect of a wire was attenuated as the temperature moved away from the pseudocritical temperature. The wall temperature distribution along the span between the contact points was a concave downward parabola. Near the pseudocritical temperature, the wall temperature showed relatively higher values, indicating a stagnant fluid around the wire. On the other hand, the wall temperature at the contact point showed a relatively lower value, indicating a fin function of a wire.     

Condensation heat transfer characteristics of R-22, R-134a and R-410A in a single circular microtube

The condensation heat transfer coefficients of R-22, R-134a and R-410A in a single circular microtube were investigated experimentally. The experiments are conducted without oil in the refrigerant loop. The test section is a smooth, horizontal copper tube of 1.77 mm inner diameter. The experiments were conducted at mass flux of 450-1050 kg/m² s, saturation temperature of 40 °C. The test results showed that in case of single-phase flow, the single-phase Nusselt Number measured by experimental data was higher than that calculated by Gnielinski correlation. In case of two-phase flow, the condensation heat transfer coefficient of R-410A was higher than that of R-22 and R-134a at the given mass flux. The condensation heat transfer coefficient of R-22 showed almost a similar value to that of R-134a. Most of the existing correlations which were proposed in the large diameter tube failed to predict condensing heat transfer. And also, recently proposed correlation in the single circular microtube is considered not adequate for small diameter tube. Therefore, it is necessary to develop accurate and reliable correlation to predict heat transfer characteristics in the single circular microtube.     

Experiments on the local heat transfer characteristics of a circulating fluidized bed

The role of particle diameter in the heat transfer of a gas–solid suspension to the walls of a circulating fluidized bed was studied for particles of uniform size. This work reports and analyzes new experimental results for the local bed to wall heat transfer coefficient, not including the radiation component, in a long active heat transfer surface length laboratory bed, which extend previous findings and clear up some divergences. The research included determining the effects of extension and location of the heat transfer surface, circulating solids mass flux and average suspension density. An experimental set-up was built, with a 72.5 mm internal diameter riser, 6.0 m high, composed of six double pipe heat exchangers, 0.93 m high, located one above the other. Five narrow sized diameter quartz sand particles − 179, 230, 385, 460 and 545 μm − were tested. Temperature was kept approximately constant at 423 K and the superficial gas velocity at 10.5 m/s. The major influence of suspension density on the wall heat transfer was confirmed, and contrary to other authors, a significant effect of particle size was found, which becomes more relevant for smaller particles and increasing suspension density. It was observed that the extension of the heat transfer surface area did not influence the heat transfer coefficient for lengths greater than 0.93 m.The heat transfer surface location did not show any effect, except for the exchanger at the botton of the riser. A simple correlation was proposed to calculate the heat transfer coefficient as a function of particle diameter and suspension density.     

Heat and mass transfer characteristics of a horizontal tube falling film absorber with small diameter tubes

This paper presents experimental results of the heat and mass transfer characteristics of a water–LiBr horizontal tube absorber made of small diameter tubes. The experimental set up includes a tube absorber, a generator, solution distribution system and cooling water system. Three different tube diameters of 15.88, 12.70 and 9.52 mm have been installed inside the absorber to investigate the effect of the tube diameter on the absorber performance. The experimental results show that the heat and mass transfer performance of the absorber increases as the tube diameter decreases. A comparison of the heat and mass transfer coefficients of the present study agree reasonable well with that of the previous studies.     

A parametric study of CaCO3 scaling in AISI 316 stainless steel tubes

 The formation of undesirable layer of deposits on the heat-transfer surface is defined as fouling. These deposits present a major problem in the operation and maintenance of heat exchangers, particularly in cooling-water systems. It has been generally observed that the deposits in such systems consist mainly of calcium carbonate (CaCO₃), which has inverse solubility characteristics. An experimental study was carried out to determine the effect of tube surface temperature, Reynolds number, tube diameter and salt concentration on the growth of CaCO₃ scale. In this paper, effects of some of these parameters on fouling growth are discussed. The effect of CaCO₃ concentration on the scale growth is compared with the ionic diffusion model presented by Hasson. The variation of the fouling thickness along the length of the heat exchanger is also illustrated. Furthermore, dimensionless parameters are introduced to present the fouling resistance data collected during the experimental study. Received on 14 June 2000     

Melting heat transfer along a horizontal heated tube immersed in a fluidized liquid ice bed

An experimental study was performed to determine the melting heat transfer characteristics along a horizontal heated circular tube immersed in a solid-air-liquid three-phase fluidized liquid ice bed. A mixture of fine ice particles and ethylene glycol acqueous solution was adopted as the liquid ice for the test. Measurements were carried out for a range of parameters such as airflow rate, heated tube diameter, and initial concentration of acqueous binary solution. It was found that the heat transfer coefficient for the fluidized liquid ice bed might be more than 20 times as large as that for the fixed liquid ice bed.     

Effect of EHD on heat transfer enhancement during two-phase condensation of R-134a at high mass flux in a horizontal smooth tube

In this study, effect of electrohydrodynamic (EHD) on the condensation heat transfer enhancement and pressure drop of pure R-134a are experimentally investigated. The test section is a 2.5 m long counterflow double tube heat exchanger with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth horizontal copper tubing of 9.52 mm outer diameter. The electrode is made from stainless steel wire of 1.47 mm diameter. The test runs are performed at average saturated temperatures ranging between 40 and 60°C, mass flux ranging between 200 and 600 kg/m² s, heat flux ranging between 10 and 20 kW/m² and applied voltage at 2.5 kV. For the presence of the electrode, the experimental results indicate that the maximum heat transfer enhancement ratio is around 30% while the maximum increase in pressure drop is about 25%.     

Natural convection heat transfer on surfaces of copper micro-wires

The natural convection heat transfer characteristics and mechanism for copper micro-wires in water and air were investigated experimentally and numerically. The wires with diameters of 39.9, 65.8 and 119.1 μm were placed horizontally in water inside of a sealed tube and in air of a large room, respectively. Using Joule heating, the heat transfer coefficients and Nusselt numbers of natural convection for micro-wires in ultra pure water and air were obtained. A three dimensional incompressible numerical model was used to investigate the natural convection, and the prediction with this model was in reasonable accordance with the experimental results. With the decrease of micro-wire diameter, the heat transfer coefficient of natural convection on the surface of micro-wire becomes larger, while the Nu number of natural convection decreases in water and air. Besides, the change rate of Nu number in water decreases apparently with the increase of heat flux and the decrease of wire diameter, which is larger than that in air. The thickness of boundary layer on the wall of micro-wire becomes thinner with the decrease of diameter in both water and air, but the ratio of boundary layer thickness in water to the diameter increases. However, there is almost no change of this ratio for natural convection in air. As a result, the proportion of conduction in total heat transfer of natural convection in water increases, while the convective heat transfer decreases. The velocity distribution, temperature field and the boundary layer in the natural convection were compared with those of tube with conventional dimension. It was found that the boundary layer around the micro-wire is an oval-shaped film on the surface, which was different from that around the conventional tube. This apparently reduces the convection strength in the natural convection, thus the heat transfer presents a conduction characteristic.     

Heat transfer enhancement in a tube with equilateral triangle cross sectioned coiled wire inserts

The heat transfer and pressure drop were experimentally investigated in a coiled wire inserted tube in turbulent flow regime. The coiled wire has equilateral triangular cross section and was inserted separately from the tube wall. The experiments were carried out with three different pitch ratios (P/D = 1, 2 and 3) and two different ratio of equilateral triangle length side to tube diameter (a/D = 0.0714 and 0.0892) at a distance (s) of 1 mm from the tube wall in the range of Reynolds number from 3500 to 27,000. Uniform heat flux was applied to the external surface of the tube and air was selected as fluid. The experimental results obtained from a smooth tube were compared with those from the studies in literature for validation of experimental set-up. The use of coiled wire inserts leads to a considerable increase in heat transfer and pressure drop over the smooth tube. The Nusselt number rises with the increase of Reynolds number and wire thickness and the decrease of pitch ratio. The highest overall enhancement efficiency of 36.5% is achieved for the wire with a/D = 0.0892 and P/D = 1 at Reynolds number of 3858. Consequently, the experimental results reveal that the best operating regime of all coiled wire inserts is detected at low Reynolds number, leading to more compact heat exchanger.     

Heat transfer in bubble layers at high pressures

An original experimental investigation of heat transfer with steam condensation on a surface of a horizontal cooled tube immersed in a bubbling layer was carried out. A copper test section 16 mm in diameter and 285 mm in length was placed in a bubbling column 295 mm in diameter. Experiments were made under a pressure of 0.72-3.8 MPa with volume steam content 0-0.18, steam superficial velocities 0-0.18 m/s, and liquid-wall temperature difference 38–106 K. The heat transfer process in a bubbling layer under high pressures is shown to be of considerably intensity; with moderate values of steam content heat transfer coefficients reach 10–12 kW/(m²·K). The use of the known correlations assumed for the case of air bubbling under atmospheric pressure results in systematically underestimating heat transfer by 30–80%. Data were obtained on heat transfer with film condensation of steam and natural convection of subcooled water at high temperature differences outside the range investigated earlier. Experimental data table is appended.     

Heat transfer characteristics of a new helically coiled crimped spiral finned tube heat exchanger

In the present study, the heat transfer characteristics in dry surface conditions of a new type of heat exchanger, namely a helically coiled finned tube heat exchanger, is experimentally investigated. The test section, which is a helically coiled fined tube heat exchanger, consists of a shell and a helical coil unit. The helical coil unit consists of four concentric helically coiled tubes of different diameters. Each tube is constructed by bending straight copper tube into a helical coil. Aluminium crimped spiral fins with thickness of 0.5 mm and outer diameter of 28.25 mm are placed around the tube. The edge of fin at the inner diameter is corrugated. Ambient air is used as a working fluid in the shell side while hot water is used for the tube-side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. The air-side heat transfer coefficient presented in term of the Colburn J factor is proportional to inlet-water temperature and water mass flow rate. The heat exchanger effectiveness tends to increase with increasing water mass flow rate and also slightly increases with increasing inlet water temperature.     

Numerical estimation of mixed convection heat transfer in vertical helically coiled tube heat exchangers

A numerical investigation of the mixed convection heat transfer from vertical helically coiled tubes in a cylindrical shell at various Reynolds and Rayleigh numbers, various coil‐to‐tube diameter ratios and non‐dimensional coil pitches was carried out. The particular difference in this study compared with other similar studies is the boundary conditions for the helical coil. Most studies focus on constant wall temperature or constant heat flux, whereas in this study it was a fluid‐to‐fluid heat exchanger. The purpose of this article is to assess the influence of the tube diameter, coil pitch and shell‐side mass flow rate on shell‐side heat transfer coefficient of the heat exchanger. Different characteristic lengths were used in the Nusselt number calculations to determine which length best fits the data and finally it has been shown that the normalized length of the shell‐side of the heat exchanger reasonably demonstrates the desired relation. Copyright © 2010 John Wiley & Sons, Ltd.     

Heat transfer and heat transfer fouling in Kraft black liquor evaporators

A large number of experiments have been performed with New Zealand Forest Products Kraft black liquor to measure heat transfer coefficients and fouling rates during convective and subcooled flow boiling heat transfer as a function of surface temperature, bulk temperature, velocity, and solids concentration. Results from experiments with two chemical fouling inhibitors, with Teflon surface coating and in plate and frame heat exchangers, also are presented. The fouling deposits are analyzed with respect to appearance, composition, and process conditions for which they were obtained. With the assumption of chemical reaction-controlled fouling, a deposition model is developed and compared with the experimental data.     

SMA扭力驱动器的响应速度分析及实验研究

介绍了马氏体逆相变的驱动力的形成原因以及和相变阻力的关系。研究了通电加热时形状记忆合金(SMA)丝的热平衡方程以及相变时间与丝直径的关系,并进行了实验验证。将SMA丝按与筒母线方向成某夹角缠绕并粘贴在外表面可以构成SMA扭力驱动器。利用SMA丝的本构关系和圆筒的扭转应力-应变关系给出了空载情况下最大驱动扭转角对应的缠绕方向;研究了模拟飞机尾气通过扭力管进行加热时的响应速度,并与实验结果进行了比较。     

Local heat transfer coefficients on the circumference of a tube in a gas fluidized bed

A heated horizontal heat transfer tube was installed 14.8 cm above the distributor plate in a square fluid bed measuring 30.5 × 30.5 cm. Four different Geldart B sized particle beds were used (sand of two different distributions, an abrasive and glass beads) and the bed was fluidized with cold air. The tube was instrumented with surface thermocouples around half of the tube circumference and with differential pressure ports that can be used to infer bubble presence. Numerical execution of the transient conduction equation for the tube allowed the local time-varying heat transfer coefficient to be extracted. Data confirm the presence of the stagnant zone on top of the tube associated with low superficial velocities. Auto-correlation of thermocouple data revealed bubble frequencies and the cross-correlation of thermal and pressure events confirmed the relationship between the bubbles and the heat transfer events. In keeping with the notion of a “Packet renewal” heat transfer model, the average heat transfer coefficient was found to vary in sympathy with the root-mean square amplitude of the transient heat transfer coefficient.     

Strömungs- und Wärmeübergangsverhalten von flüssigem Quecksilber bei Benetzung und Nichtbenetzung

In wetted and nonwetted tubes the hydrodynamics and heat transfer of liquid metal flow were investigated. Mercury was used as test fluid. For the laminar flow no hydraulics anomalies were indicated. In the case of the wetted tube a steady change of diameter and an increase of the surface roughness were observed. This is due to the formation and removing of a viscous amalgam layer at the tube wall. In the laminar region heat transfer measurements show very good agreement with theory. In the turbulent region the experimental results are comparable with heat transfer coefficients valid for the nonwetted tube published in literature. The influence of different surface contact conditions to heat transfer can be excluded. A thermal contact resistance could not be found.     

Condensation heat transfer characteristics of R-22, R-134a and R-410A in small diameter tubes

The condensation heat transfer of pure refrigerants, R-22, R-134a and a binary refrigerant R-410A flowing in small diameter tubes was investigated experimentally. The condenser is a countflow heat exchanger which refrigerant flows in the inner tube and cooling water flows in the annulus. The heat exchanger is smooth, horizontal copper tube of 1.77, 3.36 and 5.35 mm inner diameter, respectively. The length of heat exchanger is 1220, 2660 and 3620 mm, respectively. The experiments were conducted at mass flux of 200–400 kg/m² s and saturation temperature of 40°C. The main results were summarized as follows: in case of single-phase flow, the single-phase Nusselt Number measured by experimental data was higher than that calculated by Gnielinski and Wu and Little correlation. The new single-phase correlation based on the experimental data was proposed in this study. In case of two-phase flow, the condensation heat transfer coefficient of R-410A for three tubes was slightly higher than that of R-22 and R-134a at the given mass flux. The condensation heat transfer coefficient of R-22 showed almost a similar value to that of R-134a. The condensation heat transfer coefficient for R-22, R-134a and R-410A increased with increasing mass flux and decreasing tube diameter. Most of the existing correlations which were proposed in the large diameter tube failed to predict condensation heat transfer. Therefore, the new condensation heat transfer correlation based on the experimental data was proposed in the present study.