Local Condensation Heat Transfer Coefficients Research Articles

Numerical simulation on the condensation heat transfer and flow characteristics outside smooth and enhanced tubes

Most of the current studies only conducts two-dimensional simulations of condensation heat transfer and flow outside enhanced tubes, which cannot accurately reflect the flow and heat transfer characteristics of liquid film outside enhanced tubes. In order to provide theoretical support for the design optimization of enhanced tubes, this study adopts the VOF multiphase model and Lee condensation model to simulate the condensation heat transfer(HTC) of refrigerants outside smooth and enhanced tubes. The instantaneous film flow characteristics of the liquid film outside the horizontal smooth and enhanced tubes are analyzed, and the thickness of the liquid film and the relationship with the condensation heat transfer coefficient of the smooth and enhanced tubes under different working conditions are calculated. The refrigerants are compared under the same working conditions, and the results are shown: The simulation results are in good agreement with the experimental data and the Nusselt analytical solution, and the error is all within 15%; the distribution of liquid film is highly sensitive to the magnitude of local condensation heat transfer coefficients, and the condensation HTC outside the enhanced tube is approximately six times of that of smooth tube. Combining the distribution of the liquid film and the thermophysical properties of the refrigerants, it was determined that R410A has the highest HTC, while R1234yf has the lowest HTC.

NUMERICAL AND EXPERIMENTAL INVESTIGATIONOF STEAM FILM CONDENSATION ON A VERTICAL TUBE

NUMERICAL AND EXPERIMENTAL INVESTIGATIONOF STEAM FILM CONDENSATION ON A VERTICAL TUBE

Heat transfer characteristics of R-1234yf, an eco-friendly alternative refrigerant in condenser for semiconductor chiller

This study aims to optimize a semiconductor chiller condenser that uses R-1234yf refrigerant, focusing on its flow pattern and heat transfer characteristics during condensation while considering the tube diameter. The heat transfer coefficient (HTC) decreases as the pressure of R-1234yf increases. This decline is attributed to changes in its physical properties, such as the density ratio between the liquid and gas phases and the thermal conductivity of the liquid phase, according to the change in saturation pressure. The HTC of R-1234yf increases with the mass flow rate (MFR) because the Reynolds number in the tube increases as MFR increases, enhancing heat transfer. Furthermore, the local condensation HTC (CHTC) of R-134a exceeds that of R-1234yf across the entire vapor quality range, primarily due to the higher liquid-phase density of R-1234yf. The correlations by Dobson–Chato [21] and Bashar et al. [10] were found to be suitable for predicting CHTC values for R-1234yf. The predicted values closely align with the experimental values, with an error range of less than ± 10 %, thus indicating that the correlations by Dobson–Chato and Bashar et al. can be directly applied to the design of condensers that use R-1234yf refrigerant.

Steam entrainment and heat transfer characteristic of steam submerged jet condensation within steam-water mixture layer

Steam-water interface and mixture layer are the key window for understanding the heat and mass transfer mechanism of steam submerged jet condensation. In this paper, the steam-water two-phase mixture layer is captured and the interfacial vortex is observed with a quasi-two-dimensional submerged steam jet visualization experimental platform. Then, the volume heat transfer capacity of steam-water mixture layer and heat transfer coefficient with steam-water inner interface are obtained. Furthermore, a new turbulent eddy entrainment model is developed to calculate the local entrainment mass rate and local condensation heat transfer coefficient. Both the local entrainment mass rate and local condensation heat transfer coefficient increases along steam jet direction, and reaches the maximum value at the tail of the mixture layer. The calculated condensation heat transfer coefficient ranges from 0.88 to 2.94 MW·m−2·K−1, which is about the half of the value of hypothetical heat transfer coefficient with inner interfacial area.

Experimental Investigation of Coolant Side Characteristic on the Performance of Air-Cooled Condenser Structured by Horizontal Flattened Tube

The steam condensation process has been experimentally investigated in an air-cooled condenser (ACC). The ACC has been designed and built using a flattened cross-section horizontal tube. The flattened tube has an internal dimension of 102 mm x 12 mm with 4030 mm length. A range of vacuum operating conditions are applied to operate the ACC. In the experiments, parameters such as vacuum pressure, saturated temperature, wall tube temperature, rate of heat transfer, and local average steam heat transfer coefficient have been considered along the flow direction with the variation of cooling water temperature. The experimental results revealed that the steam saturated temperature and the related pressure decrease with the reduction of the cooling water temperature, and the temperatures of the upper and lower parts of the horizontal flattened tube. The results also showed that the local steam condensation heat transfer coefficient decreases along with the direction of the flow, but it there is incrementing with the decreasing of saturated steam temperature at a certain range of cooling water temperature.

Thermal-hydraulic test facility for nuclear reactor containment: Engineering design methodology and benchmarking

The paper describes detailed thermal and mechanical design procedure of a CONtrolled FLOw (CONFLO) facility and a complimentary Thermal HYdraulic test facility for CONtainment (THYCON), to investigate the post-severe accident scenarios in Nuclear Power Plant (NPP) containments, involving condensation of steam in the presence of non-condensable gases (NCGs), such as air and hydrogen/helium. The former setup allows the study of flow condensation of steam on flat surfaces kept inside a rectangular flow section of 250 mm × 200 mm, under different inlet conditions of mixture concentration, pressure and temperature. The latter is a large cylindrical chamber of total height = 3.6 m and diameter = 0.96 m (vessel volume of 2.7 m3), mimicking a containment, wherein steam condensation and mixing transport of non-condensable gases can be experimentally simulated, as per conditions close to accident scenarios. Parameters of interest are local and average condensation heat transfer coefficient, NCG concentrations, flow Reynolds number, volumetric Richardson number, operating gas/vapour pressure, temperature and inclination angle of condensing surfaces. After due benchmarking of both the setups, the generated experimental data is being used for developing the design equation(s) to model steam condensation in containment environments and for suggesting locations for condensers and passive autocatalytic recombiners essential for containment safety. Ongoing experiments are also providing data for the validation of computer codes. The thermal-hydraulic experiments suggest that the condensation heat transfer inside NPP containments is primarily governed by several interlinked phenomena, such as stratification, mixing, turbulence and condensation.

Experimental investigation of the two-phase local heat transfer coefficients for condensation of R134a in a micro-structured plate heat exchanger

Plate heat exchangers are widely used for two-phase heat transfer in the industrial applications, and recently more attention has been paid to the plate heat exchangers with enhanced surface due to their better heat transfer performance. In this paper, the local condensation heat transfer coefficients are studied using R134a in a micro-structured plate heat exchanger. In order to obtain a more accurate prediction model, a series of measurements are conducted under various operating conditions. The mass flux of R134a varied from 47 kg/m2s to 77 kg/m2s, the saturation pressure in the condenser ranged from 6.32 bar to 8.95 bar, and the value of the heat flux was between 13 kW/m2 and 22 kW/m2. The local two-phase Nusselt number increases with the increase of the mass flux. As the saturation pressure increases, the local two-phase Nusselt number increase at the beginning of the condensation and decrease at the end of the condensation. However, the effect of heat flux on local heat transfer is irregular, due to the interaction of these parameters in the experiment. Comparing with the unstructured plate heat exchanger, R134a condenses faster at the beginning of the process in the micro-sturctured plate heat exchanger, and the local heat transfer performs better when the vapor quality is lower. Combing with the phenomenon that the overall heat flux in micro-structured plate is larger under the same working conditions, it shows that the overall heat transfer of the micro-structured plate is improved, but the local heat transfer uprades only at lower vapor qualities. A new correlation is developed, it predicts all the experimental data within the root mean square error 10%, and a new correlation for the waterside is suggested as well.

EXPERIMENTAL STUDY TO DETERMINE THE LOCAL CONDENSATION HEAT TRANSFER COEFFICIENTE FOR R134A FLOWING THROUGH A 4.8 MM INTERNAL DIAMETER SMOOTH HORIZONTAL TUBE

Refrigerant fluid R134a is commonly one of the most utilised invapour compression cycles wordlide, wheter in dommestic HVAC orautomotive regrigeration systems. This paper’s goal is toexperimetnally determine the fluid local condensation Heat TransferCoefficient (HTC), in several flor regimes. In this work, the mass fluxwas equal to 200, 250 and 300 kg/(m2s) and the fluid flowedthrough a smooth, horizontal 4.8 mm internal diameter aluminiumpipe, during which its vapour quality varied along the entire qualityrange. A purpose built test rig was developed, in which fluidconditions were constantly monitored and controlled. Throughmeasurements in temperature and pressure, an energy balance wasused to calculate experimentally the local heat transfer coeeficient.Average results for the unit quality range equalled to 3781 , 3459 and3944 W/m2K for saturation temperature equal to 30 C and theaforementioned mass velocities. Likewise, at 35C the averages HTCfound were 2903, 3141 and 3898 W/m2K at the same mass fluxrates. Later on, the experimental results were compared to tencommonly used HTC correlation found in relevant references,with Chato’s correlation returning the best fitting.

PREDICTING OF STEAM CONDENSATION HEAT TRANSFER COEFFICIENT IN HORIZONTAL FLATTENED TUBE

The heat transfer coefficient of steam condensation has a significant role in the performance of air-cooled heat exchangers. The purpose of this work is to predict the local/average local steam condensation heat transfer coefficient inside the horizontal flattened tube under vacuum conditions using numerous correlations that were developed by some researches which have been conducted under specified conditions. The results from these correlations have been compared with experimental data of Davies, therefore more investigate for the values are necessary to improve or/and validate the existing correlations. The effect of such parameters like the uniform heat flux and saturation temperature also have been studied on the local steam condensation heat transfer coefficient as the results show that the heat transfer coefficient decrease as the heat flux increase, while it increases as the steam saturated temperature increase.

Experimental investigation on condensation heat transfer for bundle tube heat exchanger of the PCCS (Passive Containment Cooling System)

To provide a passive cooling system for the reactor containment, Passive Containment Cooling System (PCCS) was adopted in the design of i-POWER nuclear power plant. This study focused on validation tests for condensation heat transfer of the PCCS heat exchanger using the CLASSIC facility. The tests include investigation of the condensation heat transfer in prototypic single tube and bundle tubes. From the single tube experiments, condensation heat transfer model was proposed to reflect the PCCS heat exchanger tube geometry. Experimental results in the bundle tube show consistent trend compared to the proposed heat transfer model from the single tube test. The local condensation heat transfer coefficient of inside tubes was smaller than the average value due to a shadow effect by a larger mass fraction of non-condensable gas, so that design of the PCCS should take into account degradation of the condensation heat removal in the bundle geometry.

Experimental analysis of condensation heat transfer and frictional pressure drop in a horizontal circular mini channel

The present study aims to investigate the effect of working fluid, mass flux, vapour quality and saturation temperature on local condensation Heat Transfer Coefficient (HTC) and local Frictional Pressure Drop (FPD) in a horizontal circular mini-channel. The refrigerants HFO-1234yf and HFC-R134a have been used as a working fluids. The mass flux was varied from 200 to 800 kg/m2 s whereas two distinct saturation temperatures were used: 35 °C and 40 °C. The experimental analysis was carried out using horizontal circular single port mini-channel of hydraulic diameter 1 mm. During experiment, inlet condition of refrigerant was maintained to be saturated vapour. The results show that HTC and FPD increases with increase in vapour quality and mass flux whereas decreases with increase in saturation temperature. The experiment results of present study and previously published results are compared with various predictive models. Models which show minimum deviation from experimental results were used to develop new modified model to predict HTC with MARD of ± 15 % and FPD with MARD of ± 10 %.

A numerical study on condensation flow and heat transfer of refrigerant in minichannels of printed circuit heat exchanger

Printed circuit heat exchanger (PCHE), with the core comprising many mini/microchannels, have been extensively studied, but few works involved two-phase flow. A simplified channel model of PCHE hot side was herein established to numerically explore the heat transfer and condensation flow performance of R22 in minichannels under different conditions. The model was a semicircular minichannel with 0.91 mm hydraulic diameter and 50 mm full length. The heat flux was kept at −77,195 Wm−2, the inlet pressure was 0.63 MPa and the mass fluxes ranged from 58 to 93 kgm−2s−1. The condensation flow characteristics along the channel were analyzed. Four flow patterns, i.e., smooth-annular, wavy-annular, slug and bubbly, were noticed at different refrigerant mass fluxes and plotted as a flow pattern map. The effects of import vapor quality from 0.5 to 1 on pressure drop and heat transfer performance were evaluated. The heat transfer performance was optimal at the import vapor quality of 0.7. The impacts of refrigerant mass flux on pressure drop and local condensation heat transfer coefficient were assessed. Intermittent flow enhanced heat transfer. In addition, the correlations for local Nusselt number and Fanning friction factor were presented and compared with numerical data.

Study on high-speed condensation heat transfer of steam/nitrogen mixture in horizontal rectangular channel

In this paper, the steam condensation mixed with nitrogen is experimentally investigated to examine the effect of non-condensable gas on the steam condensation process at high speed. The mass flux of steam/nitrogen mixture is in the range of 159.2–464.6 kg/(m2·s), the nitrogen mass fraction is up to 10%, and the inlet pressure ranges between 0.15 and 0.47 MPa. The mass flow rate of cooling water varies from 300 to 900 kg/h. The effects of mass flow rate of cooling water, nitrogen mass fraction, mass flux of steam/nitrogen mixture and channel configuration are studied. It is found that the increase of mass flux of mixture increases the condensation heat flux, the condensation heat transfer coefficient and the overall heat transfer coefficient. Including the nitrogen into steam deteriorates the condensation process, but the influence of nitrogen on the condensation process at high speed is much smaller compared to that at low speed. The increase of mass flow rate of cooling water promotes the condensation heat flux and the overall heat transfer coefficient, but decreases the local condensation heat transfer coefficient. The channel configuration and the gravity has little effect on the high-speed condensation. Finally, a new fitting correlation is proposed to predict the high-speed steam condensation in the presence of nitrogen based on Shah’s correlation.

Dropwise Condensation on Superhydrophobic Microporous Wick Structures

Previous research in dropwise condensation (DWC) on rough microtextured superhydrophobic surfaces has demonstrated evidence of high heat transfer enhancement compared to smooth hydrophobic surfaces. In this study, we experimentally investigate the use of microporous sintered copper powder on copper substrates coated with a thiol-based self-assembled monolayer to attain enhanced DWC for steam in a custom condensation chamber. Although microtextured superhydrophobic surfaces have shown advantageous droplet growth dynamics, precise heat transfer measurements are underdeveloped at high heat flux. Sintered copper powder diameters from 4 μm to 119 μm were used to investigate particle size effects on heat transfer. As powder diameter decreased, competing physical factors led to improved thermal performance. At consistent operating conditions, we experimentally demonstrated a 23% improvement in the local condensation heat transfer coefficient for a superhydrophobic 4 μm diameter microporous copper powder surface compared to a smooth hydrophobic copper surface. For the smallest powders observed, this improvement is primarily attributed to the reduction in contact angle hysteresis as evidenced by the decrease in departing droplet size. Interestingly, the contact angle hysteresis of sessile water droplets measured in air is in contradiction with the departing droplet size observations made during condensation of saturated steam. It is evident that the specific design of textured superhydrophobic surfaces has profound implications for enhanced condensation in high heat flux applications.

Simple and general correlation for heat transfer during flow condensation inside plain pipes

This work proposes a new general and simple model to determine the local flow condensation heat transfer coefficient inside plain pipes. The model considers two regimes corresponding to high mass fluxes and/or high thermodynamic qualities and low mass fluxes and/or low thermodynamic qualities. For each region, a new model is suggested which resembles the single-phase heat transfer coefficient model but defining an equivalent Reynolds number in terms of the sum of the superficial liquid and vapour Reynolds numbers. The models consider that the superficial vapour Reynolds number plays a major role in controlling the heat transfer coefficient. The model is able to predict the heat transfer coefficient from channels with a hydraulic diameter of 67 μm up to pipes with a hydraulic diameter of 20 mm for several fluids. No noticeable effect of the diameter of the channel, shape or fluid properties on the heat transfer coefficient has been observed for the studied cases.

Condensation flow patterns and heat transfer in horizontal microchannels

An experimental investigation was carried out to study the effect of refrigerant mass flux, local vapour quality, coolant flow rate and inlet coolant temperature on the local condensation heat transfer coefficient. Flow visualization was also conducted to capture flow patterns during flow condensation using a high-speed camera integrated with a microscope. HFE-7100, a dielectric and eco-friendly refrigerant was used in rectangular multi-microchannels with a hydraulic diameter of 0.57mm. Experiments were performed at a saturation temperature of 60°C, mass flux range 48–126kg/(m2s), coolant flow rate range 0.5–1.1L/min and inlet coolant temperature range 20–40°C. The results showed that the local condensation heat transfer coefficient increases with increasing mass flux and decreases with decreasing local vapour quality. A negligible effect of the coolant side conditions, saturation-to-wall temperature difference, on the local condensation heat transfer coefficient was found. The main flow regime was annular flow, while slug and bubbly flow were found at some operating conditions. The experimental results were compared with the existing correlations for heat transfer rates. Also, two existing flow pattern maps, for conventional and mini/microchannels, were used to compare the current flow pattern results.

An experimental study and development of new correlation for condensation heat transfer coefficient of refrigerant inside a multiport minichannel with and without fins

This paper is dedicated to the experimental study and development of new correlation for condensation heat transfer in horizontal rectangular multiport minichannel with and without fins using R134a. A new experimental apparatus has been fabricated in order to obtain explicit local condensation heat transfer coefficient measurements over a range of test conditions. The test section is an 852mm long horizontal rectangular multiport minichannel with and without fins having 20 channels with a hydraulic diameter of 0.64mm and 0.81mm. The measurements were done over a range of saturation temperature from 30 to 35°C with mass fluxes ranging from 50 to 200kg/m2s. The effects of vapor quality, mass fluxes, channel geometry, and saturation temperature on the heat transfer coefficient have been clarified and analyzed. The experimental results were compared with the well-known condensation heat transfer models available in the open literature. All of the existing correlations were failed to capture the present experimental heat transfer coefficient with a high degree of accuracy. Asa consequence, a new condensation heat transfer correlation was developed based on the present experimental data and validated with 750 data points collected from the available journal.

Local Condensation Heat Transfer Characteristics of Refrigerant R1234ze(E) Flow Inside a Plate Heat Exchanger

In the present study, local condensation heat transfer coefficients of the R1234ze(E) inside a vertical plate heat exchanger (PHE) were investigated experimentally. In the experiment, three vertical flow channels are formed in the test section where refrigerant flows downward in the middle channel and cooling water flows upward in other two channels. The test section consists of eight plates: two of them form a channel of chevron type PHE for refrigerant flow channel, other two flat plates are set for heat transfer measurements, and another consist on cooling water flow channel. Down flow of the condensing refrigerant R1234ze(E) in the center channel releases heat to other channels of cooling water. In order to measure local heat transfer characteristics, a total of 60 thermocouples were set at middle of flow direction and also in the right and left sides of plates in test section. Experiments were conducted for mass fluxes ranging from 10[Formula: see text]kg/m2s to 50[Formula: see text]kg/m2s. The measurement results show that local heat transfer coefficients decrease with increase of wetness with different values in horizontal direction. Further, characteristics of local heat flux and wall temperature distribution as a function of distance from inlet to outlet of refrigerant channel were explored in detail.

Effect of air on condensation in a non-vacuum gravity heat pipe

An experimental and theoretical investigation was performed to show the effects of air on condensation in a non-vacuum gravity heat pipe at heat loads of 0.8–5.3kW. Parameters involving local condensation heat transfer coefficients, air mole fractions, and air storage capacity in a reservoir mounted below the condensation tube, were calculated. A degradation factor method was applied to solve vapor condensation length. Results showed that local air mole fraction increased to over 95% in the condensation tube downstream, where a saturated moist air column was formed. The reservoir effectively alleviated the adverse effects of air on the condensation section. At an operating pressure of 0.24MPa, 68% of the reservoir was filled with air. In the condensation tube downstream with a low heat load, air seriously affected condensation heat transfer, and the average degradation factor was only 0.26. By contrast, air slightly affected condensation heat transfer in the condensation tube upstream with a high heat load, and the average and local degradation factors were 0.7 and 0.76, respectively. Vapor condensation length with air was 1.32 times as much as pure vapor condensation length at a vapor mass flux of 1.8g/s and an operating pressure of 0.36MPa.

Experimental investigation of filmwise and dropwise condensation inside transparent circular tubes

In this study, we conducted air-cooled condensation experiments inside bare and tetrafluoroethylene (TFE)-coated Pyrex glass tubes. Using infrared thermometry and side-view visualization with a charge-coupled device (CCD) camera, high-resolution local wall temperature distributions on the tubes and the formation of filmwise and dropwise condensation inside the tubes could be determined. In bare-tube experiments, the condensation flow patterns belonged to a fully stratified flow regime. Dropwise condensation phenomena inside the TFE-coated tube were clearly observed at all locations in the condenser tube. Inside the TFE-coated tube were distributed several condensate droplets with sizes from 10μm to 1mm order. The local heat flux values in the TFE-coated tube were higher than those in the bare tube in the upstream region of the tube. In the downstream region, the local heat flux values on the TFE-coated surface were similar to or slightly lower than those of the bare tube. This may explain the distributions of condensation droplets in different local positions. The local temperatures on the outer wall of the TFE-coated tube were significantly higher than those of the bare tube. These results indicated that the local condensation heat transfer coefficients in dropwise mode were much larger than those in filmwise mode. As the vapor quality inside the TFE-coated tube decreased, the dropwise condensation heat transfer capability was degraded. This may be the result of the uniform distributions of relatively larger droplets and the reduced vapor shearing effect in the lower quality region.