Surface Of Horizontal Tube Research Articles

REWETTING AND TRANSIENT HEAT TRANSFER ON THE HEATED HORIZONTAL TUBE SURFACE DURING THE AIR-ATOMIZED SPRAY COOLING

The purpose of this study is to investigate the cooling and rewetting of a heated horizontal tube surface with an air-atomized water spray impingement. Rewetting and transient heat transfer are crucial in nuclear reactor safety during a postulated accident, such as cooling of hot calandria tubes (CT) during the large-break loss of coolant accident (LOCA). The rewetting velocity in the circumferential direction and the rate of cooling of the heated tube surface were determined as a function of nozzle operating parameters. To estimate the local spray impingement density on the tube surface, an in-house mechanical patternator was designed and developed. To record the flow state during cooling, a high-speed video camera was used. The rewetting velocity on the tube surface was determined using the outcome of thermocouples mounted on the heated tube wall and an imaging system used to record the video picture during the runs. The two techniques of calculating rewetting velocity are compared. The highest heat flux removed from the tube surface was estimated as 3.7 MW/m<sup>2</sup>, and the maximum rewetting velocity was found to be 5.58 mm/s. An excellent agreement regarding rewetting velocity has been reported utilizing thermocouples and a high-speed camera.

Experimental and Numerical Study of Hydrodynamic and Heat Transfer Characteristics of Falling Film Over Metal Foam Layered Horizontal Tube

AbstractA novel nonintrusive technique based on an air-coupled ultrasonic transducer was used to study the hydrodynamic behavior of falling film over metal foam layered horizontal tube. Copper foam having a porosity of 90.5%, brazed over a copper tube of 25.4 mm diameter was used in this study. Falling film thickness distribution in the circumferential direction and the dynamic characteristics of falling film were studied in the falling film Reynolds number range of 356–715, and at a tube spacing of 5 mm and 15 mm. The falling film characteristics over metal foam layered horizontal tubes were compared with that over a plain horizontal tube surface. Heat transfer studies of falling film over metal foam layered tube were studied in an evaporator of a multi-effect desalination system by experiment. It was observed that the falling film heat transfer coefficient was enhanced 2.7 times by the application of metal foam over the plain horizontal tube. The measurements obtained from hydrodynamic and heat transfer studies were compared with the predictions made by a computational model and were found to be in good agreement. Metal foam properties required for the computational model were obtained using a microcomputed tomography-based study.

An investigation on the heat transfer model for immersed horizontal tube bundles in a pressurized fluidized bed

In order to predict the heat transfer coefficient between the immersed horizontal tube bundles and the pressurized fluidized bed, different types of heat transfer models research process for the horizontal tube surface are presented. The accuracy of these models has been analyzed with the data of pressurized experiments, which indicates the limitation of these models in the application of pressurized fluidized bed. A new semi-empirical emulsion renewal model is proposed which takes the advantages of thermal contact resistance model and heat penetration model into account and modifies the correlation for gas convection heat transfer calculation at elevated pressure. The total experimentally reported 89 points from different sources with pressures ranging 0.1–1.6 MPa, particle diameter 240–700 μm are predicted within the absolute average deviation of 10.25%, which is much lower compared with other models for the same data. The satisfactory description for the influence rule of bed pressure, temperature, gas velocity and particle diameter on the heat transfer coefficient was also given from the model.

Asymmetric distribution of falling film solution flowing on hydrophilic horizontal round tube

A two-dimension two phase flow model was established to simulate the falling film flow of LiBr solution on a horizontal hydrophilic tube with different solution sprinkle density and tube surface wettability, and the latter has been an overlook factor. The transient characteristics of solution spreading as well as steady film thickness were analyzed. The results show that a continuous film can only be obtained at sufficiently greater sprinkle density with real surface wettability, the liquid coverage of tube surface increases with the increase of sprinkle density or the decrease of static contact angle. The obvious asymmetric distributions of film thickness and film velocity over the horizontal tube surface are demonstrated in a steady state. The thinnest film thickness or maximum film velocity takes place at circumferential angle around 120°. A modified Nusselt equation for predicting the film thickness is suggested and verified by available both simulation and experimental data.

Calculation of the film flowing over horizontal tube surface

Hydrodynamic processes taking place during the motion of liquid droplets and the formation of a liquid film flowing over surface of a horizontal tube are considered. Mathematical modelling of motion of two-phase medium is based on the Lagrangian approach. The results of numerical calculations of the liquid film thickness over surface of the tube are presented.

Research on a new type waste heat recovery gravity heat pipe exchanger

The industrial waste heat carrier, such as exhaust gas, mostly contains oil, particles, fibers and other impurities. If the conventional heat transfer enhancement techniques are applied in the gas side to recovery waste heat, the gas side flow channels are easily to be blocked, which not only greatly reduces the waste heat recovery efficiency but some time also makes the heat exchanger out of work. In this paper, a new type waste heat recovery heat pipe exchanger has been designed and applied to recover thermal energy in high temperature exhaust gas emitted from setting machine in the dyeing and printing industry. Its major feature is that clean air passes though fin-enhanced vertical tubes whose inner side is a condenser while dirty gas passes though inner smooth surface of horizontal tubes whose outside is an evaporator. The new type heat pipe exchanger has a big boiling chamber. The condensed water falls down to the chamber by gravity. Three-month continuous operation of recovering dirty exhaust gas waste heat shows that the new type heat pipe exchanger can save 15% natural gas without any blockage of the gas side channel.

An investigation on the falling film thickness of sheet flow over a completely wetted horizontal round tube surface

An investigation was performed to analyze the liquid film distributions of sheet flow on the surface of a horizontal round tube in a falling film evaporator using the commercial computational fluid dynamic code Fluent 6.3.26. A two-dimensional multi-phase flow model was developed under adiabatic condition. The temporal variation of flowing process of sheet flow and the steady film thickness distribution in the circumferential direction were analyzed in detail. The main objective has focused on the effects of multiple factors on the film thickness distribution. It was found that the whole process of liquid film flowing along the horizontal tube surface includes the transient sub-process and steady-state sub-process, and the former can be divided into five stages to be analyzed. The obviously asymmetric distribution of film thickness, which is contradictory to the Nusselt theory, is captured by the simulation results. Multiple factors have effects on the film thickness distribution. The film thickness increases with the increase in film Reynolds number. The big inter-tube spacing is capable of inducing high momentum and increasing the impact velocity of liquid film on the top of the tube, which results in the thinner distribution of liquid film layer around the perimeter of the horizontal tube. The film thickness becomes thinner as tube diameter increases. The effect of tube diameter is minor and the further increment of tube diameter shows few values to the improvement of heat and mass transfer.

Numerical study of falling film thickness over fully wetted horizontal round tube

The numerical investigation was performed to analyze the film thickness characteristics over fully wetted horizontal round tube in a falling film evaporator using commercial CFD code Fluent 6.3.26. A two-dimension multi-phase flow model for numerical simulation was developed under adiabatic condition. The temporal variation characteristics of film flow process and the steady film thickness distribution over horizontal tubes were analyzed in detail. It was found that the whole process of liquid film flowing along the horizontal tube surface includes the transient sub-process and steady state sub-process, and the former can be divided into five stages to be analyzed. The obviously asymmetric distribution of film thickness is captured by the simulation results. The film thickness increases with the increase in film Reynolds number. An obvious zone without liquid is formed very closely to the lower stagnation point, and the size of the zone is bigger when the Reynolds number is higher.

Neural network optimization by comparing the performances of the training functions -Prediction of heat transfer from horizontal tube immersed in gas–solid fluidized bed

This paper describes the selection of training function of an artificial neural network (ANN) for modeling the heat transfer prediction of horizontal tube immersed in gas–solid fluidized bed of large particles. The ANN modeling was developed to study the effect of fluidizing gas velocity on the average heat transfer coefficient between fluidizing bed and horizontal tube surface. The feed-forward network with back propagation structure implemented using Levenberg–Marquardt’s learning rule in the neural network approach. The objective of this work is to compare performances of five training functions (TRAINSCG, TRAINBFG, TRAINOSS, TRAINLM and TRAINBR) implemented in training neural network for predicting the heat transfer coefficient. The comparison is shown on the basis of percentage relative error, coefficient of determination, root mean square error and sum of the square error. The predictions by training function TRAINBR found to be in good agreement with the experiment’s values.

The Desalination Device Using the Rising Liquid Film on the Microscale Fluted Surface of Horizontal Tubes

With the development of marine economy in our country, many islands are faced with the bottleneck of lack of fresh water. The purpose of this paper is to calculate the efficiency parameters by designing the multi-effect desalination device and procedure, which is based on the parameters from the single effect desalination experiment of the rising liquid film on the microscale fluted surface of horizontal tubes. Compared with the vertical tube falling film desalination in the same working condition, the desalination using the rising liquid film on the microscale fluted surface of horizontal tubes can reduce the initial evaporation pressure and steam temperature. This improvement will be more conducive to the application of marine equipment, and provide technical support to resolve the lagging problem of islands infrastructure. Nomenclature

Experimental investigation of horizontal tube immersed in gas–solid fluidized bed of large particles using artificial neural network

The average heat transfer coefficient is determined between the fluidizing bed and horizontal tube surface immersed in the bed of large particles. The mustard (dp=1.8mm), raagi (dp=1.4mm) and bajara (dp=2.0mm) were used as particles in the bed. The effect of fluidizing gas velocity on the heat transfer coefficient in the immersed horizontal tube is discussed. The results obtained by experiment are compared with correlations and artificial neural network modeling. The parameters particle size, temperature difference between bed and immersed surface were used in the neural network modeling along with fluidizing velocity. The feed-forward network with back propagation structure implemented using Levenberg–Marquardt’s learning rule in the neural network approach. The network’s performance tested with regression analysis. The predictions of the artificial neural network were found to be in good agreement with the experiment’s values, as well as the results achieved by the developed correlations.

Theoretical investigation of stable dropwise condensation heat transfer on a horizontal tube

The heat transfer model of stable dropwise condensation for saturated vapor on a horizontal tube is developed based on previous theoretical models. Through a comprehensive analysis of all the contributing thermal resistances, the convection effect inside the droplet itself is taken into consideration in the model. For the stable dropwise condensation process in dynamic conservation, a method of double integration of heat flux through numerous inclined plates with different inclination angles is introduced to obtain the overall heat flux through the horizontal tube surface. The model can predict the variation of heat transfer of stable dropwise condensation with different contact angles outside a horizontal tube. The influences of contact angle, temperature difference, and other typical parameters on both a single droplet and the whole condensation process are discussed. The results indicate that a high contact angle can cause a size reduction of falling droplets from condensing surface and thus taking more heat away. The adsorbed condensate film adds an extra thermal resistance and its thickness plays a significant role on the dropwise condensation heat transfer.

Mathematical modeling and experimental verification of an absorption chiller including three dimensional temperature and concentration distributions

One of the main drawbacks in modeling absorption chillers is the lack of justified hypotheses of heat and mass diffusion in an annular flow on the outer surface of horizontal tubes. Heat and mass transfer in diffusion is a three-dimensional problem with vector characteristics. This paper introduces the characterization of vapor–absorbent heat and mass transfer phenomena in three dimensional space to obtain steady state simulation results for single effect LiBr–H2O absorption chillers.It is thus possible for the first time to ascertain in the simulation that the heat and mass transfer characteristics are not uniform throughout the solution film. The diffusion boundary layer gradually becomes thicker towards the downstream tubes, which has previously been assumed but never confirmed in simulations. The radial component of the concentration field exhibits a potential for determining an optimum film thickness which enhances the system performance. The ability to use the proposed model with various absorbents and different cooling capacities increases its applicability. The model also offers the possibility to analyze different firing techniques at different temperatures. All the simulation results are consistent with the experimental data found in the literature.

Investigation of coated tubes in cross-flow boiling

The boiling in cross-flow is investigated for coated tubes (low-porosity, flame-sprayed) in this paper. The effect of surface roughness on flow boiling heat transfer for a horizontal tube surface in cross-flow is studied for saturated boiling of water at atmospheric pressure. The parameters varied were for flow velocity up to 3.24 kg/s ( G = 258.49 kg/m 2 s), heat flux from 12 to 45 kW/m 2, surface roughness ( Ra) from 0.3296 to 4.731 μm. Nominal enhancement in heat transfer coefficient at higher mass flux may be attributed to the continued nucleation at the uppermost surfaces (in the wake region of the flow) of the rougher tubes thereby increasing the overall heat transfer rate. The flow boiling data was found to best fit the Kutateladze asymptotic equation h = h l[1 + ( h npb/ h l) n ] 1/ n with the value of n = 2.258 (which is close to the value of n = 2 suggested by Kutateladze).

The local heat transfer mathematical model between vibrated fluidized beds and horizontal tubes

A dimensionless mathematical model is proposed to predict the local heat transfer coefficients between vibrated fluidized beds and immersed horizontal tubes, and the effects of the thickness of gas film and the contact time of particle packets are well considered. Experiments using the glass beads (the average diameter d ¯ p = 1.83 mm ) were conducted in a two-dimensional vibrated fluidized bed (240 mm × 80 mm). The local heat transfer law between vibrated fluidized bed and horizontal tube surface has been investigated. The results show that the values of theoretical prediction are in good agreement with experimental data, so the model is able to predict the local heat transfer coefficients between vibrated fluidized beds and immersed horizontal tubes reasonably well, and the error is in range of ±15%. The results can provide references for future designing and researching on the vibrated fluidized beds with immersed horizontal tubes.

Experimental investigation of capillary-assisted evaporation on the outside surface of horizontal tubes

Capillary-assisted evaporation is a typical heat transfer method in heat pipes which is characterized by high evaporation coefficient due to extremely thin liquid film. This paper introduces such a micro-scale heat transfer method into normal-scale applications. A series of enhanced heat transfer tubes with circumferential rectangular micro-grooves on the outside surfaces have been experimentally investigated. The aim is to investigate the influence of the tubes’ geometries and operating parameters on the evaporation heat transfer coefficients. In the experiment, the tested tubes are hold horizontally and the bottom surfaces are immersed into a pool of liquid. The heat is added to the thin liquid film inside the micro-grooves through the heating fluid flowing inside the tubes. The factors influencing the capillary-assisted evaporation performance, such as the immersion depth, evaporation pressure, superheating degree, etc. are considered. The experimental results have indicated that there is a positive correlation between the evaporation heat transfer coefficient and evaporation pressure, and negative for the superheating and immersion depth. For water, under the evaporation saturated temperature of 5.0±0.1°C, the superheating of 4.0±0.1°C and the dimensionless liquid level of 1/2, the film side evaporation heat transfer coefficients are 3100–3500W/m2K, which are equivalent to those of the falling film evaporator in LiBr–water absorption machine (2800–4500W/m2K [Y.Q. Dai, Y.Q. Zheng, LiBr–water Absorption Machine, first ed., Chinese National Defence Industry Press, Beijing, China, 1980.]).

Film condensation in presence of non-condensable gases over horizontal tubes with progressively increasing radius of curvature in the direction of gravity

A theoretical study is executed to investigate the simultaneous influences of reduction in heat transfer rate on account of condensation in presence of non-condensables and its augmentation by the geometry of an horizontal tube surface with increasing radius of curvature in the direction of gravity. The tube surface profile considered for the present work is an equiangular spiral described in a polar form as: R = ae mθ ( a and m being parametric constants). It is observed that a very small bulk concentration (even less than 1%) of the non-condensable gas reduces considerably the heat transfer coefficient. However, there is an enhancement in heat transfer coefficient for condensation over a polar tube surface, as compared to that over a circular tube surface. This enhancement in heat transfer coefficient, with an increase in the value of m (a surface profile parameter), in presence of non-condensables is more than the corresponding proportional enhancement in the same in absence of any non-condensable species.

Local Pool Boiling Coefficients on the Outside Surface of a Horizontal Tube

Changes in local pool boiling heat transfer coefficients on the outside surface of a 51mmdia horizontal tube have been investigated experimentally in water at atmospheric pressure. The local values were determined at every 45deg from the very bottom to the uppermost of the tube periphery, and a representative value was suggested. The maximum and the minimum local coefficients were observed at the azimuthal angles of 45 and 180deg, respectively, from the tube bottom. The coefficient at 90deg can be suggested as the representative of the coefficients. The major mechanisms affecting heat transfer on the surface were liquid agitation and bubble coalescence. Moreover, it is identified that the data can be curve fitted as hb=C1+C2q″.

Characteristics of thin film desalination plants

The hydrodynamics and heat exchange during sea water and distillate boiling in a thin film on the heated horizontal and vertical surface of tubes over wide range parameters are described. The experimental data illustrate the boiling sea water on the horizontal tube surface and variation coefficient of hydrodynamic resistance for thin film upflow.

Heat transfer in condensation on the external surface of horizontal tubes

A relation taking account of the intensification of heat transfer on account of waves at the lower and lateral surfaces of tubes is obtained.