Condensation Heat Transfer Characteristics Research Articles

Influence of operating conditions on in-tube vapour condensation heat transfer characteristics in the presence of noncondensable gases at high pressure

The in-tube vapour condensation heat characteristics in the presence of noncondensable gas at high pressures are experimentally validated with an error of 20%. Numerical simulations are then conducted to explore heat transfer characteristics at different operating pressures, wall surface temperatures and mass flow rates. The flow and heat transfer characteristics are thoroughly discussed and the buoyancy effects induce a secondary circulation altering flow patterns, changing the water film and heat transfer coefficient distribution. In addition, increasing operating pressures is found to reduce the wall heat transfer rate. Even though the flow pattern is significantly different caused by the increased gas mixture density, the total condensate flow rate is approximately the same at different operating pressures due to the same inlet conditions and temperature difference. The difference in the wall heat transfer rate is attributed to the change of latent heat of vapourisation and falling film at different pressures and is evident at high pressures. At the high temperature difference, a nonuniform film height towards the bottom is emerged, sweeping the condensate upward. Similarly, increasing the mass flow rate can also increase the total wall heat transfer rate and total condensate flow rate.

Experimental study on heat transfer characteristics of steam underwater direct-contact condensation

Introduction: The direct-contact condensation (DCC) of steam under water injection is the basic thermodynamic process of the bubble deaerator. In order to understand the complex coupling behavior of strong turbulence and fast phase-change heat transfer involved in the process.Methods: This study uses a visualized method and convective heat transfer model.Results: Since the contact area is affected by steam injection flow and sub-cooled degree is affected simultaneously, the trend of the condensation heat-transfer coefficient depends on the degree of their respective effects under each condition, and the maximum variation of the coefficient exceeds 104 W/m2.°C. Moreover, they still effect the period of steam plume, and the maximum variation of the period was beyond 80 ms.Discussion: Calculated the average condensation heat transfer coefficient and then produces the variation law of heat transfer coefficient under various conditions in one steam plume evolution period.

Numerical simulation on heat transfer characteristics of unstable steam jet condensation under rolling condition

Direct contact condensation technology has been widely used in nuclear power, aerospace, chemical industry, and other fields due to efficient mass and heat transfer characteristics between two phases. Rolling motion applied to offshore cases typically significantly affects dynamic flow and heat transfer characteristics. Thus, investigating the heat transfer characteristics of unstable steam jet condensation is necessary. The present study numerically explored the heat transfer coefficient of main and detached bubbles under rolling and static conditions. The relationship between pressure and heat transfer coefficient was assessed to reveal the pressure generation mechanism. The peak value of pressure oscillation was mainly caused by the rapid collapse induced by intense condensation of detached bubble instead of main bubble. The heat and mass transfer intensity could be fortified under rolling conditions. It would increase as the maximum rolling amplitude rise and the rolling period decrease, while the maximum improvement was up to 30%.

Study on heat transfer characteristics of flue gas condensation in narrow gap heat exchangers

Flue gas after wet desulfurization contains a large quantity of water vapor, as well as a small amount of fine particulate matter, acid and other pollutants. Direct emissions will cause haze and environmental pollution. Flue gas condenser can effectively recover latent heat of vaporization and remove these pollutants. However, the traditional flue gas condenser has disadvantages such as large volume, low heat exchange efficiency and easy blockage. To overcome these problems, a new type narrow gap heat exchanger, which is easy to detach and clean, is proposed. Then, the flue gas condensation characteristic is experimentally investigated. The factors including velocity and temperature of flue gas, flow and temperature of cooling water are studied on heat transfer coefficient and the condensation rate. The results indicate that the overall heat transfer coefficient is seven times of convection heat transfer coefficient, and the condensation rate can reach more than 60%. As the cooling water temperature increases, the condensation rate and condensation heat transfer coefficient gradually decrease. With the increasing of the flue gas velocity, condensation rate gradually decreases and condensation heat transfer coefficient gradually increases. Finally, the dimensionless correlation of Nusselt number is carried out, and the error is within 10%.

Experimental study on condensation heat transfer and pressure drop characteristics of R32 flowing inside an alternating cross-section flattened tube

This work presents the experimental study of two-phase flow inside an alternating cross-section flattened (ACF) tube. The ACF tube was made from a copper circular tube with an inner diameter of 4.31 mm. We conducted the test under the following experimental conditions: a mass flux of 280, 430, and 580 kg/m2s; a saturation temperature of 40, 45, and 50 °C; a heat flux of 10, 15, and 20 kW/m2; and an average vapor quality of 0.1–0.8. Compared to the existing flow pattern maps, most fell into the annular and semi-annular flow regimes. However, the results were found in the transition and intermittent flow regions at low vapor quality. The results from the circular and ACF tubes were examined and discussed. We found that the ACF tube revealed a prominent condensation heat transfer coefficient and a reasonable pressure loss penalty. The heat transfer coefficient and frictional pressure drop of the ACF tube were higher than that of the circular tube by approximately 25.6–111.1% and 4.3–39.9%, respectively. New correlations for the Nusselt number and two-phase friction factor were proposed.

Effects of Sloshing Motions on Condensation Heat Transfer Characteristics of Integral-Fin Tubes Under Sea Conditions

Abstract Integral-fin tubes with high heat transfer capability are a promising solution for improving the compactness of condensers used in the exploitation of offshore natural gas at sea, and the sloshing motions including rolling and pitching would inevitably affect the performance of tubes. For applying integral-fin tubes offshore, the effects of rolling and pitching motions on condensation heat transfer characteristics of integral-fin tubes should be known. In this study, the condensation heat transfer characteristics of integral-fin tubes under rolling and pitching motions are experimentally investigated and the effects of motion angle and frequency are quantitatively analyzed. The results show that the sloshing motions have both positive and negative effects on the heat transfer of integral-fin tubes during a period, and the pitching motion has a greater influence than the rolling motion. As the sloshing angle increases from 0 deg to 12 deg, the maximum increase and reduction rates of the ratio of local wall subcooling temperature under pitching motion to that under static conditions are 10.9% and 12.5%, respectively, and the time-averaged condensation heat transfer coefficient (HTC) increases by 3.5% maximally. As the sloshing frequency increases from 0 to 0.25 Hz, the maximum increase and reduction rates of the ratio of local wall subcooling temperature under pitching motion to that under static conditions are 7.7% and 15.2%, respectively, and the increase rate of the time-averaged condensation HTC remains about 2%.

Condensation heat transfer on the outer surface of a horizontal annulus having surface enhancement

Condensation heat transfer characteristics on the outer annular walls of double-sided horizontal tubes having three different enhancement geometries are investigated experimentally. The enhancement geometries are (1) a fine fin array, (2) a rectangular convex platform array, and (3) a T-shaped column array. The inner surface geometries are threads of various pitches. The experiments are with various mass fluxes, average vapor qualities, and saturation temperatures. They were conducted with refrigerant R410A. The results show the effects of enhancement geometry. Six correlations are used to predict the heat transfer coefficients of smooth tube, and the correlation with the highest accuracy is selected and corrected to obtain a new correlation suitable for the enhanced tubes. The new correlation can predict 92% of the data points within a prediction error of +22% to -30%. Frictional pressure drops are correlated with mass flux, average vapor quality, and saturation temperature. The heat transfer performances of the three experimental tubes are compared using a performance evaluation factor and the enhancement geometries with better condensation heat transfer performances are identified.

Heat transfer and pressure drop of CO2/R32 mixture in mini-channel

In this study, we investigated the condensation heat transfer characteristics of the CO2/R32 mixture in a mini-channel with a diameter of 2 mm. The experimental conditions were as follows: mass flow rate of 50–400 kg·m−2·s−1, vapor quality of 0–1, and a condensation temperature of 35–45 °C. The experimental data were then compared with the heat transfer correlations. The higher the CO2 content, the smaller the annular flow area. The convective heat transfer coefficient decreases rapidly and then increases slowly with an increase in CO2. The minimum value was obtained when the CO2 content was approximately 55%. In addition, the convective heat transfer coefficient decreased with an increase in the condensation temperature. Higher vapor quality and mass flow rate conditions correspond to higher convective heat transfer coefficients. The prediction correlations applicable to the in-tube condensation heat transfer have low prediction results for CO2/R32, and the model of Shah is recommended as the prediction model. The experimental results of the condensation pressure drop were analyzed, and the results showed that the pressure drop increased linearly with the mass flow rate but decreased with an increase in the condensation temperature. The lower the CO2 content, the more clear the effect of the condensation temperature. The correlation of Kim and Mudawar is recommended for the CO2/R32 two-phase condensation friction pressure drop. This study provides theoretical support for the analysis of the condensation heat transfer mechanism of a CO2/R32 mini-channel.

Experimental study on condensation flow patterns and heat transfer characteristics of non-azeotropic and immiscible binary mixed vapors

The condensation phenomenon of non-azeotropic and immiscible mixed vapors always occurs in the waste heat recovery process of biomass gasification gas. During the condensation process, the immiscible condensation products, such as tar and water, have a different flow pattern from the pure working fluid, which can affect the waste heat recovery efficiency and the service life of the heat exchanger. However, the relationship between the flow pattern of immiscible condensates and condensation heat transfer is not clear at present. In this paper, water and cyclohexane were used as non-azeotropic and immiscible working fluids. The effects of different mixed vapors mass ratios, cooling water inlet temperatures, and vapor mass flow rates on the condensation heat transfer characteristics of the binary mixed vapors on the vertical wall were studied. And the condensate flow patterns were combined for analysis and discussion. The experimental results showed that the condensation on the wall surface of the mixed vapors presented a film-drop flow pattern comparing with the filmwise condensation of water vapor or cyclohexane vapor. The film-drop flow pattern was manifested as that the cyclohexane condensate was attached on the condensation wall in the form of liquid film, and the water condensate was attached on the cyclohexane film in the form of droplets. And the flow of water droplets on the cyclohexane liquid film can enhance the condensation heat transfer of the mixed vapors. In addition, different from water vapor and cyclohexane vapor, the heat transfer coefficient of the mixed vapors decreased with the increase of the cooling water inlet temperature, which was related to the film-drop condensation flow pattern.

Experimental and analytical investigation of CO2/R32 condensation heat transfer in a microchannel

Previous research has shown that CO2 mixtures can improve the thermal performance of CO2 refrigeration systems. Moreover, the use of a microchannel heat exchanger can reduce the refrigerant charge and improve the heat transfer coefficient. Therefore, in this experimental study, we investigated the condensation heat transfer characteristics of CO2/R32 in a porous flat microchannel with a hydraulic diameter of 0.715 mm. According to the results, the convective heat transfer coefficient decreased rapidly then increased slowly with an increase in the CO2 mass fraction. The minimum value was achieved when the CO2 mass fraction was approximately 55%. In addition, the convective heat transfer coefficient decreased with an increase in the condensation temperature. Higher vapour quality and mass flow rate conditions corresponded to higher convective heat transfer coefficients. The prediction correlations applicable to the in-tube condensation heat transfer exhibited low prediction results for CO2/R32, with a heat transfer coefficient prediction error for the newly established prediction model of 16.77%. This study provides theoretical support for analysis of the CO2/R32 condensation heat transfer mechanism in microchannels and the future design of microchannel condensers.

Experimental research on condensation heat transfer characteristics of low mass flow rate steam in a horizontal tube

The heat exchange character of pure steam flow inside horizontal vacuum tubes has been experimentally studied. The inside diameter and length of the tube are 38 mm and 3400 mm, respectively. The effects of steam mass flow rate, saturated temperature, and total temperature difference ΔTs,c on the flow process are analyzed. Based on the analysis of the heat exchange mechanism of stratified flow condensation, a calculation model of the thermal partition angle is established. The experimental results show that the thermal partition angle increases with the increase of the mass flow rate, and the rise of the heat transfer temperature difference. The saturated temperature has little impact on the local heat transfer coefficient and the thermal partition angle of condensation in the tube. By taking the thermal partition angle as the partition boundary, an empirical correlation formula for calculating the local heat transfer coefficients is established to better predict the results of local heat transfer coefficients.

Experimental study on condensation heat transfer characteristics of steam in rectangular channels rotating about a parallel axis

• Nu in rotating channel increases with Re but decreases with steam temperature. • Pressure drop in rotating channel increases with Re and steam temperature. • Propose a new heat transfer correlation for steam condensation in rotating channel. In the papermaking industry, a kind of new cylinder dryer with multiple small and rectangular channels are employed to enhance the drying efficiency of drying machine. In practical working conditions, the multi-channel cylinder dryer runs under rotating states that could affect the heat transfer characteristics of the inside two-phase flow. However, the fluid heat transfer characteristics in the multi-channel at rotating states have not been well explored. In this study, a set of rotating experimental system is established to investigate the condensation characteristics and pressure drop changes of steam in rotating rectangular channels. Firstly, the experimental apparatus and the data processing method are described in detail. Then the effects of rotation speed, mass flux and steam temperature on the heat transfer coefficients and pressure drop are studied through experiment. The results show that when Re is small, the heat transfer coefficient first increases and then decreases with the increased Re ω ; when Re is large, the heat transfer coefficient increases with the increased Re ω . Moreover, the heat transfer coefficient decreases as the steam temperature increases. In addition, the pressure drop decreases with the increased Re ω while increases with the increased Re and steam temperature. Finally, a heat transfer model of two-phase flow condensation under rotating states is developed to predict the heat transfer coefficient of steam in rotating multiple channels.

Study on condensation flow and heat transfer characteristics of multicomponent mixture in dimple tube

The flow and heat transfer characteristics of methane/propane and methane/ethane/propane have been numerically investigated in the dimple tube. The effect of mass flux, saturation pressure and composition ratio on the characteristics of flow, heat transfer and void fraction are examined. The results indicate that the heat transfer coefficient, friction pressure drop increase with the increase of mass flux and decrease with the pressure drop increased. The heat transfer coefficient of a binary mixture is higher than that of ternary mixture under the same condition. The friction pressure drop of the ternary mixture is greater than that of the binary mixture at high vapor quality. In addition, the influence of geometric parameters is discussed. Higher dimple depth and lower pitch will result in higher heat transfer efficiency. The dimple tube with r = 0.75 mm, p = 5.08 mm, e = 4.33 mm have the best heat transfer performance, and the surface performance factor PFS and the performance factor enhancement PFE are 1.041–1.304 and 0.885, respectively. Besides, the simulation result of void fraction, heat transfer coefficient and friction pressure drop are compared with some general models. Finally, an improved pressure drop model for the enhanced tube is proposed.

Numerical Study on R32 Flow Condensation in Horizontally Oriented Tubes with U-Bends

In this paper, the flow condensation heat transfer characteristics of R32 in a horizontally oriented tube with a horizontal U-bend (HUB tube) and a horizontally oriented tube with a vertical U-bend (VUB tube) were numerically investigated. The volume of fluid (VOF) multiphase model with the mass transfer assumption by Lee is adopted to simulate the flow condensation behavior of R32, which are validated by well-known empirical correlations. The influence of structural parameters, mass flux and vapor quality on the heat transfer coefficient (HTC) and liquid film distribution is investigated. Meanwhile, the local liquid film thickness (LLFT) and local heat transfer coefficient (LHTC) are also depicted. The phase transition at the U-bend section is captured. Results show that the U-bend has remarkable disturbance on the flow pattern and LHTC due to the effect of centrifugal force where the LLFT is changed, inducing strong secondary flow. The LHTC is increased by a maximum of 8.47% and 11.86% in VUB tube and HUB tube, respectively, when compared to the case in a horizontally straight oriented tube at the same operating conditions.

Numerical simulation for condensation in the presence of noncondensable gas in the condenser with a multi-channel structure

Condensation in the presence of noncondensable gas is of interest to the passive containment cooling system in the nuclear power plant. Most researches focus on the heat transfer characteristics of condensation in an individual vertical tube or a parallel tube bundle. However, the influence of the condenser structure on condensation heat transfer is rarely considered in previous works. Thus, a numerical simulation was conducted for condensation in the presence of NCG in a condenser composed of two headers and 158 condenser tubes. A steady three-dimensional model was developed to predict the NCG distribution and the heat transfer performance of the condenser when the PCCS loop was stably running. The fluid inside the condenser was regarded as a single-phase mixture composed of steam and air, and the liquid film was evaluated by the Eulerian Wall Film model. The results indicated that the air tends to accumulate in the middle of the condenser tubes rather than the headers. The fluid flow in most of the tubes was blocked, so the heat transfer coefficients of pure steam and the steam-air mixture were lower than expected. An orifice plate was further designed to optimize the pressure and flow field in the PCCS condenser. The effect of the optimization was quantitatively evaluated. The results showed that the heat transfer performance of the condenser was improved by up to 87.8% with the orifice plate.

Experimental research on refrigerant condensation heat transfer and pressure drop characteristics in the horizontal microfin tubes

The condensation heat transfer characteristics and pressure drop for R410A were investigated in the horizontal smooth and microfin tubes. The 6.35-mm-OD, 7-mm-OD and 8-mm-OD tubes with active length 2 m were selected for condensation test cooled by the heat transfer fluid (water) circulated in a surrounding annulus. A refrigerant superheating 2–3 °C was set at the test section inlet and the mass flux range was 300–1100 kg·m−2·s−1. The heat transfer coefficient (HTC) of the microfin tube was about 1.63–2.29 times of that of the smooth one. The microfin one yields ~28.9% higher pressure drop than the smooth one. Large helix angle and fin height were beneficial to enhance heat transfer due to the centrifugal force and surface tension, but increased the pressure drop. 6.35-mm-OD tube produced both the higher HTC and pressure drop than the 7-mm-OD and 8-mm-OD tubes due to the increased effect of the surface tension and the sheer stress at the gas-liquid interface. It was necessary to optimize finned structure to enhance heat transfer. Experimental data were also compared with previous correlations proposed for smooth and microfin tube. It was necessary to collect more data source and make a generally-accepted and accurate correlation in future.

Condensation Flow and Heat Transfer Characteristics of R410A in Micro-Fin Tubes and Three-Dimensional Surface Enhanced Tubes

Condensation heat transfer characteristics (using R410A as the working fluid) were studied experimentally to evaluate the heat transfer performance in copper and stainless-steel heat transfer tubes (smooth and enhanced). Experiments were carried out for a mass flux that varied from 250 to 450 kg m−2 s−1, at a saturation temperature of 318 K. Single-phase heat balance verification found that the heat loss is less than 6%, and the deviation between single-phase experimental results and various prediction correlations is less than 15%. Additionally, tube side condensation flow patterns were observed and recorded. Experimental results found that the enhancement ratio of the condensation heat transfer coefficient (enhanced tube/smooth tube) of the three-dimensional surface (1EHT) tube is in the range of 1.15~1.90, while the ratio of the micro-fin (HX) tube is in the range of 1.18~1.80. Heat transfer performance is affected by material conductivity, with the thermal conductivity of the smooth tube slightly affecting the heat transfer performance; larger heat transfer enhancements are produced in the enhanced tubes. At a low mass flow rates and vapor qualities, the flow pattern is a stratified wavy flow, while at higher mass flow rates and vapor qualities, the flow pattern is an annular flow (with the area in the enhanced tube being larger than the area of a smooth tube). Flow patterns in the smooth tube are consistent with the predicted values shown in previously reported flow pattern maps. A flow pattern diagram for condensation heat transfer in enhanced tubes is presented as part of this study. The condensation heat transfer coefficient increases with an increase in mass flow. When the mass flow rate increases, the turbulence of the liquid flow increases and the liquid film becomes thinner; thermal resistance is reduced and the heat transfer coefficient increases. Heat transfer values at lower mass velocities increase slightly with increasing mass flux values; however, at higher mass flux rates the heat transfer increase is larger than that at low mass flux values. Finally, tubes produced from high thermal conductivity materials produce larger heat transfer performance gains than the gains found in smooth tubes; small diameter tubes produce larger gains than larger diameter tubes.

Numerical study on the effect of CO2 on the heat transfer characteristics of steam condensation outside a vertical tube

Under severe accident conditions of a pressurized water reactor nuclear power plant, the reactor core may melt, and the molten corium even reacts with concrete in the containment, producing a large amount of CO2 into the entire containment gas space. This may have an obvious impact on the heat transfer characteristics of steam condensation in the containment. However, there are few studies focused on this phenomenon. In this paper, a commercial CFD software, STAR-CCM+, is used to study the effect of CO2 on steam condensation heat transfer. The calculations are based on 0.4 MPa gas pressure, 40 K wall sub-cooling, 0.66 or 0.2 steam mole fraction, and the CO2 mole fraction varies from 0 to 0.34 or 0.8. The results show that the CO2 demonstrates different influence laws at high and low steam concentrations, respectively. When the steam mole fraction is 0.66, as the proportion of CO2 in the non-condensable gas increases, the condensation heat transfer coefficient (HTC) will increase, and the enhancement can reach 30%. When the steam mole fraction is 0.2, the condensation HTC will first decrease and then increase, as the proportion of CO2 in the non-condensable gas increases. The effects of gas pressure and wall sub-cooling on the condensation HTC are also analyzed. The results show that the condensation HTC increases with the increase of the gas pressure and decreases with the increase of wall sub-cooling.

Numerical study on the condensation characteristics of various refrigerants outside a horizontal plain tube at low temperatures

The condensation of refrigerants outside the tubes of liquefied natural gas (LNG) at low temperatures is the main factor affecting the performance of the LNG intermediate fluid vaporizer (IFV). In this paper, a CFD model is established to investigate the condensation heat transfer characteristics of refrigerants outside a horizontal plain tube at low temperatures over a wide range of wall subcooling. The model, based on the combination of the VOF multiphase model and Lee phase change model, agrees well with the experimental data. The transient condensation heat transfer coefficient is obtained and the periodic dripping characteristics of the condensate are analyzed. Due to the decrease of the interfacial mass transfer with the pressure decreasing, there is a peak of the periodic-averaged heat transfer coefficient with the variation of the saturation temperature, which is different from that of the Nusselt's prediction. The saturation pressures corresponding to the peak values of the periodic-averaged heat transfer coefficients at the subcooling of 10 °C are all in the range of 0.35–0.65 MPa for the four different refrigerants. In addition, the surface tension plays a positive role in the heat transfer process as the condensate film is pulled towards the wall and thinned. In view of the heat transfer performance along the whole LNG tube, dimethylether (DME) and butane can be considered as the intermediate fluids in the future design of the IFV by taking account of the comparison with six different refrigerants.

Condensation heat transfer and pressure drop characteristics of low GWP R-404A alternative refrigerants in a multiport tube having axial microfins

Considering the phase out of the hydrofluorocarbons (HFCs), the search for an alternative refrigerants is of an urgent issue. In this study, four R-404A alternative refrigerants (R-448A, R-449A, R-455A, R-454C) were tested in a microfin multiport tube having 1.30 mm hydraulic diameter. The smooth mutiport tube of 0.8 mm hydraulic diamter was also tested for a comparison purpose. The results showed that, at a low mass flux and low vapour quality, the microfin tube yielded approximately equal heat transfer coefficients to those of the smooth tube. At a high mass flux and high vapour quality, however, the heat transfer coefficient trends were different depending on the refrigerants. For R-404A, which has a negligible temperature glide of 0.31 K (at 45 °C saturation temperature), the heat transfer coefficients of the two tubes were approximately the same. However, for R-454C and R-445A, which have the large temperature glides of 6.59 K and 9.46 K, the heat transfer coefficients of the microfin tube were smaller than those of the smooth tube. Based on the trends of the heat transfer coefficients of this study, three criteria for a possible good correlation are proposed. First, the correlation should predict the heat transfer coefficients of pure (or near azeotropic) refrigerants in a smooth tube adequately. Second, the correlation should predict the heat transfer coefficients of pure (or near-azeotropic) refrigerants in a microfin tube smaller than those in a smooth tube. Third, the underprediction of the microfin data should get weakened as the temperature glide increases. Based on the above criteria, it was found that some micro-channel or conventional channel correlations may be used to predict the condensation heat transfer coefficients in a multiport tube with/without microfins. The pressure drops were generally in the reverse order to the value of vapour density. Furthermore, the pressure drops of the alternative refrigerants were larger than those of R-404A.