Film Condensation Heat Transfer Research Articles

Modeling of film condensation flow in oval microchannels

A theoretical film condensation heat transfer model was developed for refrigerant flowing through oval-shaped microchannels in annular flow regime with laminar liquid film flowing along the channel walls. The model considered the effects of surface tension, disjoining pressure, interfacial shear stress and interfacial thermal resistance. Study was conducted for mass fluxes in a range of 300–1500 kg/(m2 s) with two refrigerants, R134a and R1234ze(E). The local heat transfer coefficient and circumferential mean heat transfer coefficient along the flow direction were obtained. The liquid film thickness profiles at different locations are presented. Due to the effect of surface tension, the liquid film accumulates towards the semi-circular corner. The circumferential mean heat transfer coefficient is very high after entering the channel inlet, decreases along the flow direction and reaches a constant value. R134a has a slightly higher circumferential mean heat transfer coefficient than that of R1234ze(E) due to its higher liquid conductivity.

Research on film condensation heat transfer of the shell side of the spiral coil heat exchanger

The theoretical basis for the calculation of shell side condensation of the spiral coil heat exchanger is weak. In order to solve the problem of pure steam film condensation heat transfer of the shell side of the spiral coil heat exchanger, the theoretical model of the homogeneous flow was established. Considering the centrifugal force, two different mathematical models were established and the heat transfer characteristics were studied. The solution formula of the heat transfer coefficient and thickness of first layer liquid film, meanwhile, was obtained. The theoretical formula of the membrane-like condensation heat transfer of the heat exchanger was worked out, based on the calculation model of the heat transfer coefficient of one-component working fluid outside the tube bundle, considering the effect of tube bundle. The two calculated results in this paper and the theoretical result of Bays and McAdams were compared with experimental values respectively. The results show that the relative error between the calculated values obtained by the two theoretical models and experimental values is 7.48% (ignoring centrifugal force), 3.05% (considering centrifugal force), while the relative error between Bays and McAdams’s result and the experimental value is 61.3%. The correctness of the theoretical models proposed in this paper are verified. The theoretical models can be used to calculate the film condensation heat transfer of the spiral coil heat exchanger.

Experimental study on film condensation heat transfer characteristics of R1234ze(E) and R1233zd(E) over horizontal plain tubes

In this study, experimental investigations were carried out to evaluate the film condensation heat transfer characteristics of R1234ze(E) and R1233zd(E) over horizontal plain tubes. Experiments were conducted at saturation temperatures of 36, 38 and 40 °C with different surface sub-cooling temperatures in the range of 3∼18 °C. Experimental results showed that the film condensation heat transfer coefficient decreased with increase in the surface sub-cooling temperature. The effect of tube diameter on film condensation heat transfer coefficient was also examined, and it was found that the smaller the tube diameter, the higher the film condensation heat transfer coefficient. In the present study, experimental correlations of the Nusselt number for R1234ze(E) and R1233zd(E) were also proposed respectively, with ±20 % of error band.

Vapor free convection film condensation heat transfer in the presence of non-condensable gases with smaller molecular weights than the vapor

Non-condensable gases seriously reduce the free convection film condensation heat transfer on surfaces with greater reductions when the gas molecular weight is smaller than that of the vapor (MNG < Mv) than when MNG > Mv. Thus, existing approximate heat transfer models for MNG > Mv cannot accurately predict the heat transfer for MNG < Mv. In this study, R11 free convection film condensation heat transfer on a horizontal tube outside surface was experimentally investigated in the presence of three non-condensable gases, N2, CO2 and R22, whose molecular weights are all smaller than that of R11. The results show that the heat transfer is sharply reduced at very low gas concentrations with smaller reductions at increasing gas concentrations. The condensation heat transfer reduction due to the gases with MNG < Mv is much stronger than that with MNG > Mv at low gas concentrations. The reductions for the two types of gases get closer with increasing gas concentrations. A heat transfer correlation was developed with 95% of the predictions within ±20% of the experimental data.

Approximate equations for film condensation in the presence of non-condensable gases

Non-condensable gases greatly influence vapor condensation, resulting in a substantial reduction in the condensation heat transfer coefficient. Although extensive analytical and numerical investigations of condensation heat transfer in the presence of non-condensable gases have been done, most of the solutions are quite complicated. Based on a thermodynamics analysis, when the vapor is not close to its critical state and the mass fraction of the non-condensable gas in the main stream is less than 0.1, an equation which relates the vapor/gas-liquid interface parameters and the main stream parameters was developed in the present work. For forced convection film condensation heat transfer on the outside surface of a horizontal tube, the present equation combining with an existing analytical solution as well as a heat transfer correlation given by previous investigators, gives the heat flux and the interfacial parameters of the water vapor-air mixture. The results show that the predicted heat flux is in good agreement with experimental data available in the literature and that even a small amount of air substantially reduces the heat flux. An algebraic equation set is given to calculate free convection film condensation on a vertical flat surface, which associates the interfacial and main stream parameters, an integral solution and an analytical solution given by previous investigators. The calculated results are in good agreement with experimental data in the literature.

Effects of Tube Diameter and Surface Sub-Cooling Temperature on R1234ze(E) and R1233zd(E) Film Condensation Heat Transfer Characteristics in Smooth Horizontal Laboratory Tubes

Effects of Tube Diameter and Surface Sub-Cooling Temperature on R1234ze(E) and R1233zd(E) Film Condensation Heat Transfer Characteristics in Smooth Horizontal Laboratory Tubes

The investigation on condensation performance of gas/steam mixtures with the fine coal particles in a vertical channel

Theoretical and experimental study is conducted to investigate the condensation heat transfer characteristics of mixed gas with fine coal particles in this paper. The visualization experiments of fine coal particles striking the film and mixed gas condensation heat transfer experiments in a vertical channel are carried out. Mixed gases contain different proportions of N2, steam and fine coal particles. The theoretical model of particles flicking condensate film is established. Results show that fine coal particles can penetrate the condensate film and flow inside the film when the particle diameter and the incidence velocity meet the conditions of penetrating judgment. The Nu number of heat transfer and the mass of the condensation decrease with the increase of the diameter and the concentration of the particles in the mixed gases.

Experimental study on condensation heat transfer characteristics of R410A in short vertical tubes

An experimental study on condensation heat transfer of R410A in short vertical tubes (8.02 mm ID and 10.7mm ID) was presented. Experiments were performed in eight short copper tubes length varied from 300mm to 600mm at mass fluxes range of 58–246 kg m−2s−1 and saturation temperature of 38°C. Effects of mass flux, tube length on condensation heat transfer coefficient were investigated. The distribution of temperature, thickness of condensate film and local condensation heat transfer coefficient along the tube were also analyzed. It is indicated that the entrance effect played an important role in condensation heat transfer of vertical tube, and the influence of entrance effect on average condensation heat transfer coefficients will weaken with the length of tube in the experimental condensation. The experimental results were compared with four well known correlations available in literatures, and the Chen correlation shows good agreement with the experimental data but with ±40% deviation. A new modified condensation heat transfer correlation with 12.7% mean deviation was developed to predict the condensation heat transfer coefficients in short vertical tube based on the experimental data.

Heat and mass transfer in binary film evaporation and condensation in vertical channel

Evaporation and condensation in the presence of binary liquid film flowing on one of two parallel vertical plates by mixed convection have been studied numerically. The first plate is adiabatic and wetted by a binary liquid film while the second one is dry and isothermal. The results concern the effects of the inlet parameters on the ethylene glycol evaporation and on the water condensation. Results obtained show that the increase of the inlet vapor concentration of water benefits its condensation and the increase of the inlet vapor concentration of ethylene glycol inhibits its evaporation.

Experimental and theoretical investigations on condensation heat transfer at very low pressure to improve power plant efficiency

This paper presents experimental investigation on the influence of very low pressure on local and average condensation heat transfer in a vertical tube. Furthermore, this paper develops an analytical study for film condensation heat transfer coefficient in the presence of non-condensable gas inside a vertical tube. The condensate film thickness is calculated for each location in a tube using mass and heat transfer analogy. The effects of interfacial shear stress and waves on condensate film surface are included in the model. The comparative studies show that the present model well predicts the experimental data of Khun et al. [1]for local condensation of steam air mixture at high pressure. Different correlations defined for condensation heat transfer are evaluated. It is found that the correlations of Cavallini and Zecchin [2] and Shah [3] are the closest to the calculated steam condensation local heat transfer coefficient. The model gives a satisfactory accuracy with the experimental results for condensation heat transfer at very low pressure. The mean deviation between the predictions of the theoretical model with the measurements for pure saturated vapor is 12%. Experimental data show that the increase of air fraction to 4% deteriorates condensation heat transfer at low pressure up to 50%.

수직관 내 순수 증기의 층류 액막 응축 모델

In this study, a new model for calculating the liquid film thickness and condensation heat transfer coefficient in a vertical condenser tube is proposed by considering the effects of gravity, liquid viscosity, and vapor flow in the core region of the flow. In order to introduce the radial velocity profile in the liquid film, the liquid film flow was regarded to be in Couette flow dragged by the interfacial velocity at the liquid?vapor interface. For the calculation of the interfacial velocity, an empirical power ?law velocity profile had been introduced. The resulting liquid film thickness and heat transfer coefficient obtained from the proposed model were compared with the experimental data from other experimental study and the results obtained from the other condensation models. In conclusion, the proposed model physically explained the liquid film thinning effect by the vapor shear flow and predicted the condensation heat transfer coefficient from experiments reasonably well.

Condensation of FC-72

Heat transfer coefficients for condensing FC-72® vapor on vertical copper and Teflon plates are reported as a function of sub-cooling at one atmosphere. Results include the evolution of the average heat transfer coefficient with time and a visual record of droplet formation and coalescence owing to non-condensable gas. Experiments are run for a Reynolds number of 15.1, wall heat flux of 0.92–2.88W/cm2, and 6–44K sub-cooling. Film condensation heat transfer coefficients compare reasonably well with those of prior studies run under different convective conditions. High resolution video captures evolution of droplet size (average diameter) and number density. A correlation is shown to exist between overall heat transfer coefficients and droplet size and number density. When droplet number density exceeds 10cm−2 and droplet area exceeds ∼1.5mm2, average heat transfer coefficients approach a limiting value.

Numerical modeling of the effects of oil on annular laminar film condensation in minichannels

This paper presents a numerical model to predict laminar film condensation heat transfer in small channels of different internal geometries for miscible refrigerant-oil mixtures. The model includes the contributions of surface tension, axial shear stresses induced by the vapor to film interface, gravitational forces, wall conduction and the oil concentration dependency on the liquid's dynamic viscosity. For the same operative conditions and fluid, the presence of the oil has a significant negative impact on the thermal performance at high vapor qualities, with the degradation depending on the channel's shape. Presently, the performance of different channel shapes (circular and flattened shapes) are simulated and compared. It is concluded that the presence of oil has slightly less effect on capillary-dominated regimes (i.e. when the surface tension has a strong effect on the film dynamics) than on gravity-dominated regimes (i.e. annular stratified regime).

New experimental method for inundation effect of film condensation on horizontal tube bundle

Theory of film condensation heat transfer (FCHT) for vapor condensed on horizontal tube bundle (HTB) is vital to many industry processes. Meanwhile, the inundation effect is the key to model the film condensation heat transfer coefficient (CHTC) on HTB. This paper proposed a new experimental method, homologous method, to obtain the inundation effect precisely. Based on the requirements of the new test method, a new test facility was designed and established. Then, the superiority of homologous method for inundation effect was investigated based on experiment result and theoretic analysis. The results showed that the homogenous method can effectively control the experimental error of inundation effect, which is less than 50% of the error of CHTC, and less than 30% of the error of the inundation effect gained by routine method. The new test facility built for the homogenous method is excellent in obtaining the accurate inundation effect of film condensation on HTB. All the result is a foundation of the theoretical development of the FCHT on HTB.

Flow pattern modulation in a horizontal tube by the passive phase separation concept

A passive phase separation concept was proposed to modulate flow pattern in a condenser tube. An empty mesh cylinder is suspended in the condenser tube. The miniature mesh pores prevent gas bubble entering the mesh cylinder but capture liquid into the mesh cylinder, ensuring largest possibility for cold tube wall directly contacted with gas to form the perfect thin liquid film condensation heat transfer. We performed the air–water two-phase flow experiment. It was found that for a relatively higher liquid height in the horizontal tube, all liquid can be captured by the mesh cylinder to form the “gas-floating-liquid” mode. If the liquid height is small in the horizontal tube, partial liquid can be sucked by the mesh cylinder, the contact area between tube wall and gas is increased. When plug flow reaches the mesh cylinder surface, elongated saddle bubbles are formed in the annular region to envelop the mesh cylinder surface. When bubbly flow in the horizontal tube approaches the mesh cylinder area, miniature bubbles can merge to form large bubbles in the annular region. For the later two cases, all the gas flow rate is flowing in the annular region and the inside mesh cylinder is the liquid.

Numerical Modeling of the Conjugate Heat Transfer Problem for Annular Laminar Film Condensation in Microchannels

This paper presents numerical simulations of annular laminar film condensation heat transfer in microchannels of different internal shapes. The model, which is based on a finite volume formulation of the Navier–Stokes and energy equations for the liquid phase only, importantly accounts for the effects of axial and peripheral wall conduction and nonuniform heat flux not included in other models so far in the literature. The contributions of the surface tension, axial shear stresses, and gravitational forces are included. This model has so far been validated versus various benchmark cases and versus experimental data available in literature, predicting microchannel heat transfer data with an average error of 20% or better. It is well known that the thinning of the condensate film induced by surface tension due to gravity forces and shape of the surface, also known as the “Gregorig” effect, has a strong consequence on the local heat transfer coefficient in condensation. Thus, the present model accounts for these effects on the heat transfer and pressure drop for a wide variety of geometrical shapes, sizes, wall materials, and working fluid properties. In this paper, the conjugate heat transfer problem arising from the coupling between the thin film fluid dynamics, the heat transfer in the condensing fluid, and the heat conduction in the channel wall has been studied. In particular, the work has focused on three external channel wall boundary conditions: a uniform wall temperature, a nonuniform wall heat flux, and single-phase convective cooling are presented. As the scale of the problem is reduced, i.e., when moving from mini- to microchannels, the results show that the axial conduction effects can become very important in the prediction of the wall temperature profile and the magnitude of the heat transfer coefficient and its distribution along the channel.

A Comparative Study of Heat Transfer Coefficients for Film Condensation

Film condensation heat transfer has wide applications in a variety of industrial systems. A number of film condensation heat transfer correlations (FCHTCs) have been proposed. However, their predictions are often inconsistent. This paper presents a comparative study of existing FCHTCs. Totally 1214 experimental data points are obtained from 10 published papers, and 14 FCHTCs are reviewed, among which four correlations are used for horizontal flow outside smooth tubes, three for flow on vertical surfaces of plates or tubes, two for flow inside smooth tubes either vertically or horizontally, and five for horizontal flow inside smooth tubes. 13 FCHTCs are compared with the experimental data. There are three FCHTCs for horizontal flow inside smooth tubes having a mean absolute relative deviation (MARD) less than 26%, among which the best one has an MARD of 22.2%. More efforts should be made to develop better correlations. Key words: Correlation; Heat transfer; Film; Condensation

Film condensation heat transfer on a horizontal tube in presence of a noncondensable gas

The double boundary layer model has been developed to study the behavior of film condensation heat transfer outside a horizontal tube in presence of air treated as a noncondensable gas. And the coupled heat and mass transfer on a smooth horizontal tube is numerically solved with the finite difference method. The local mass concentration of the noncondensable gas, the distributions of velocity and temperature in the boundary layers are presented and discussed. The numerical results have shown that the mass concentration and velocity of the noncondensable gas increase from the bulk mixture to the interface while the temperature decreases from the bulk mixture to the interface. Although the mass concentration of the noncondensable gas in the bulk mixture could be small, the reduction in average heat transfer coefficient is obvious. The comparisons of heat transfer coefficient show that the numerical predictions agree well with available experimental data. ► We develop the double boundary layer model. ► We examine film condensation heat transfer in presence of noncondensable gas. ► We present local mass concentration, velocity and temperature in boundary layers. ► Low noncondensable gas causes large reduction of condensation heat transfer. ► Good agreement for heat transfer is achieved between numerical and experimental data.

Simple heat transfer model for laminar film condensation in a vertical tube

A new physical model for estimating the liquid film thickness and condensation heat transfer coefficient in a vertical tube, considering the effects of gravity, liquid viscosity, and vapor flow in the core region, is proposed. In particular, for calculating the velocity profile in the liquid film, the liquid is assumed to be in Couette flow forced by the interfacial velocity at the liquid–vapor interface. The interfacial velocity is calculated using an empirical power-law velocity profile. The film thickness and heat transfer coefficient from the new model are compared with existing experimental data and the original Nusslet condensation theory. The new model describes the liquid film thinning effect due to the vapor shear flow and predicts the condensation heat transfer coefficient from experiments reasonably well.

Pure steam condensation model with laminar film in a vertical tube

A new physical model for calculating the liquid film thickness and condensation heat transfer coefficient in a vertical condenser tube is proposed by considering the effects of gravity, liquid viscosity, and vapor flow in the core region of the flow. To estimate the velocity profile in the liquid film, the liquid film was assumed to be in Couette flow forced by the interfacial velocity at the liquid–vapor interface. For simplifying the calculation procedures, the interfacial velocity was estimated by introducing an empirical power-law velocity profile. The resulting film thickness and heat transfer coefficient from the model were compared with the experimental data and the results obtained from the other condensation models. The results demonstrated that the proposed model described the liquid film thinning effect by the vapor shear flow and predicted the condensation heat transfer coefficient from experiments reasonably well.